M Alejandro Perez-Luna

2'-bipyridyl HOMO: Highest Occupied Molecular Orbital HRMS: High Resolution Mass Spectrometry

Keywords: 6]: 1, 4, 7, 10, 13, 16-hexaoxacyclooctadecane aryl BDE: Bond Dissociation Energy Bn: benzyl Boc: tert-butoxycarbonyle bpy: 2, 2'-bipyridyl bpz: 2, 2'-bipyrazine COD: 1, 5-cyclooctadiene Cp: cyclopentadienyl doublet DBU: 1, 8-diazabicyclo[5.4.0]undec-7-ene ESI: Electro-Spray Ionisation EWG: Electron Withdrawing Group dppe: ethylenebis(diphenylphosphine)

Je souhaite particulièrement remercier mes encadrants, Cyril Ollivier et Louis Fensterbank pour m'avoir permis de réaliser mon doctorat au sein de l'équipe MACO.

Merci pour votre disponibilité, vos conseils et remarques, ainsi que pour votre compréhension. Bien que le début de doctorat ait pu être compliqué, le chemin parcouru pendant ces trois années sur le plan professionnel et personnel a été très enrichissant et je souhaite une nouvelle fois vous remercier pour cela.

Je remercie aussi les membres de l'équipe MACO, membres permanents ainsi qu'étudiants présents ou passés, pour la bonne ambiance et les bons moments passés à vos côtés.

Enfin, un grand merci aux personnes de mon entourage, famille et amis, qui ont su me soutenir pendant ces trois années.

Résumé en français

Bien que les réactions ioniques occupent une place importante en chimie de synthèse, les processus impliquant des espèces radicalaires ont réussi à s'imposer comme des alternatives de choix. Le premier radical de synthèse a été mis en évidence en 1900 par Moses Gomberg.

Il s'agit du radical triphénylméthyle. 1 Malgré cette découverte, l'intérêt pour la chimie radicalaire de synthèse a mis du temps à émerger. Grâce aux travaux de plusieurs chimistes, dont notamment ceux de Paneth, Hey et Kharasch, et la découverte de la RPE dans les années 40, l'étude des réactions faisant intervenir des radicaux a été simplifiée et a permis d'élargir les applications de la chimie radicalaire à la synthèse organique. Parmi les processus mettant en jeu des espèces radicalaires, la mise au point de processus en chaines médiés par l'hydrure d'étain, les réactions de transfert d'atome ou de groupe d'atomes, mais aussi les réactions de cyclisation ainsi que les processus en cascade ont en partie fortement contribués au développement de la chimie radicalaire de synthèse.

Malgré tous ces efforts, leurs utilisations dans des procédés industriels restent limités. La polymérisation radicalaire ou l'oxydation du cumène pour la production de phénol méritent toutefois d'être citées. En effet, les radicaux ont longtemps été considérés comme des espèces très réactives hors de contrôle par une partie des chimistes. Pourtant, les contributions de Kochi, Davis, Giese, Minisci, Curran, Hart et d'autres ont pu démystifier la réactivité des radicaux et en faire des candidats valables pour la chimie de synthèse. A l'opposé des réactifs organométalliques de type Grignards, organolithiens, cuprates,… qui doivent être manipulés avec précautions, les précurseurs radicalaires permettent d'engendrer les radicaux en solvant partiellement hydraté. De plus, les réactions radicalaires ont l'avantage de ne pas être sensibles à la nature du solvant, d'être chimiosélectives et peuvent être réalisées sans avoir à utiliser de groupes protecteurs. De fait, plusieurs options pour la formation de radicaux sont utilisables. Pour des réactions en chaîne, plusieurs initiateurs (trialkylboranes, peroxydes, composés azo,…) et médiateurs radicalaires (stannanes, silanes, thiols,…) sont à disposition.

La formation des espèces radicalaires peut aussi se faire par transfert monoélectronique à l'aide de sels métalliques en quantité stoechiométriques, en oxydation (manganèse, cérium, argent,…) ou en réduction (samarium, titane, zinc, nickel,…). Toutes ces alternatives sont autant d'outils à disposition du chimiste pour la formation de liaisons carbone-carbone ou carbone-hétéroatome. 2 Bien qu'elle possède des avantages, la chimie radicalaire a aussi ces limitations. En effet, les réactions radicalaires doivent être réalisées en milieu dilué, ce qui limite la possibilité de les réaliser sur large échelle. De plus, afin de piéger de façon efficace les radicaux, un excès d'accepteur radicalaire est souvent nécessaire. A noter que, les initiateurs radicalaires présentent des risques d'explosion. Mais l'aspect le plus négatif de cette chimie est l'utilisation trop récurrente de médiateurs à base d'étain, toxiques pour l'Homme et difficiles à éliminer. Avec l'avènement du concept de chimie verte à la fin des années 1990, 3 de nombreux efforts ont été réalisés pour éviter l'utilisation des dérivés de l'étain dans les méthodes de synthèse.

Progressivement, des alternatives ont été proposées pour remplacer les médiateurs stannylés. Des méthodes utilisant des quantités catalytiques de ces dérivés ou bien des supports solides à base d'étain ont d'abord été considérées. L'utilisation d'autres médiateurs radicalaires tels que des silanes, phosphines ou thiols moins toxiques se sont révélés de bon réactifs mais moins efficaces. Les métaux mettant en jeu des transferts monoélectroniques ont aussi été considérés comme alternatives mais doivent être utilisés en quantités stoechiométriques ou sur-stoechiométriques, de plus ils sont spécifiques d'une fonction chimique. D'un point de vue développement durable et écologique, ces alternatives aux dérivés de l'étain sont limitées. Pour autant, un domaine de recherche amorcé à la fin des années 1970, permettant de former des radicaux par transfert monoélectronique grâce à un complexe métallique photoactivé à travers le développement d'une catalyse photorédox en lumière visible, a attiré depuis une dizaine d'années de nombreux groupes de recherche.

Après activation par un photon suivie de la promotion d'un électron dans une orbitale de plus haute énergie, certains complexes métalliques ou photocatalyseurs, ont la faculté de réduire ou oxyder par transfert monoélectronique un substrat, engendrant ainsi une espèce radicalaire. Les radicaux ainsi formés peuvent être employés de la même façon qu'avec les méthodes précédentes dans des processus radicalaires unitaires, incluant la recombinaison, le transfert d'électrons, la β-fragmentation, l'addition homolytique et la substitution homolytique. Les processus d'oxydation et de réduction sont en partie régis par des facteurs thermodynamiques et plus particulièrement par les potentiels redox des espèces mises en jeu.

Afin de moduler le potentiel d'oxydation ou de réduction des photocatalyseurs, le métal (Cu, Ru, Ir,…) ou les ligands peuvent être modifiés, ceci permettant ainsi d'obtenir une large gamme de potentiels redox utilisable selon les substrats à réduire ou oxyder (Schéma 1). 4 Même si la plupart des photocatalyseurs sont des complexes métalliques, un nombre croissant Corriu, 13 peuvent être formés à partir de trialkoxysilanes ou trichlorosilanes et de catéchol.

Ces espèces anioniques possèdent des potentiels d'oxydation relativement faible (~+0.3-+0.9V vs ESC). 14 Une version ammonium ainsi qu'une version potassium de ces espèces ont pu être obtenu. Dans notre cas, les potassiums alkyles bis-catécholato silicates ont été le coeur de notre étude. Cependant, il a été remarqué que la version potassium des silicates se décompose progressivement. L'ajout d'un éther couronne, le [18-C-6], permet de complexer le potassium et d'éviter la dégradation observée. [Ir(dF(CF 3 )ppy) 2 bpy](PF 6 ) et 3 mol% d'un complexe de nickel (0) [Ni(dtbbpy)]. 17 17 C. Lévêque, L. Chenneberg, V. Corcé, J.-P. Goddard, C. Ollivier and L. Fensterbank, Org. Chem. Front., 2016, 3, 462-465. Schéma 9. Catalyse duale photoredox/nickel en présence de silicates Dans un souci de rendre toujours plus ''vertes'' les conditions réactionnelles, nous avons par la suite envisagé de remplacer le photocatalyseur d'iridium (très coûteux et non recyclable après catalyse) par un photocatalyseur organique. Cependant, ces derniers ont des propriétés moins favorables pour la catalyse photoredox que les analogues métalliques. Les temps de vie à l'état excité étant trop cours, la probabilité de pouvoir effectuer des transferts monoéléctroniques est donc diminuée. Parmi les chromophores ayant prouvé leur efficacité en catalyse photorédox, nous avons sélectionné la fluorescéine, l'Eosin Y et le catalyseur de Fukuzumi. Lors d'expériences de piégeage avec le TEMPO, seul le catalyseur de Fukuzumi a montré une activité modérée vis-à-vis du silicate de benzyle. En effet, les alkyles silicates non activités n'ont pas pu être converti de façon efficace dans ces conditions. 18 Schéma 10. Photooxydation des alkyles bis-catécholato silicates par des photocatalyseurs organiques En 2012, le groupe d'Adachi, en quête de nouvelles molécules pour la préparation de diodes organiques électroluminescentes (OLEDs), a décrit une famille de N-carbazolyl dicyanobenzène qui avait des propriétés particulièrement performantes pour ce type d'applications. 19 Quelques années plus tard, après avoir déterminé les potentiels rédox de ces composés, Zhang a montré que le 1,2,3,5-tetrakis-(carbazol-yl)-4,6-dicyanobenzène (4CzIPN) pouvait être utilisé en tant que photocatalyseur 20 dans des conditions de catalyse duale photorédox/nickel en reproduisant les réactions décrites par Molander et MacMillan. 19 H. Uoyama, K. Goushi, K. Shizu, H. Nomura and C. Adachi, Nature, 2012, 492, 234-238. 20 J. Luo and J. Zhang, ACS Catal., 2016, 6, 873-877. Les conditions de catalyse duale développées nous ont par la suite permis de coupler des bromures éthyléniques avec le [18-Crown-6] bis(catécholato)-acétoxypropylsilicate de potassium. Les bromures éthyléniques non activés ont pu être convertis en produits de couplage avec des rendements modérés, même si les bromures éthyléniques activés ont donné de meilleurs résultats. L'utilisation de β-bromo/chlorostyrènes diastéréomériquement purs en tant qu'électrophiles a permis d'obtenir des produits de couplage avec conservation de la géométrie de la double liaison lorsque le 4CzIPN a été utilisé. 21 Schéma 14. Extension de la méthode de couplage aux halogénures éthyléniques En outre, nous avons observé que les dérivés styryles engagés en catalyse pouvaient s'isomériser sous irradiation lumineuse en présence du photocatalyseur organique. Au vu de ces résultats, nous avons pu conclure que le 4CzIPN peut être un photooxydant mais aussi un simple photosensibilisateur. Toutefois, l'oxydation des silicates s'est révélée être plus rapide que le processus de photosensibilisation conduisant à l'isomérisation. 21 Schéma 15. Photosensibilisation vs. photooxydation par le 4CzIPN

Après avoir étudié le couplage croisé de type C(sp 3 )-C(sp 2 ) en conditions de catalyse duale photoredox/nickel entre des alkyles silicates et des halogènures éthyléniques ou aromatiques, nos travaux se sont tournés vers la formation de liaisons C(sp 3 )-C(sp 3 ) en utilisant cette fois des halogénures d'alkyles. 22 L'optimisation du système permettra de développer de nouveaux systèmes catalytiques mettant en jeu les silicates comme précurseurs de radicaux carbonés.

Chapter I. But some examples can be mentioned like the radical polymerization or the production of phenol from oxidation of cumene. Although radicals have remained mysterious and considered as out of control by a part of the chemists, the contribution of Kochi, Curran, Giese, Hart and others6 demystified the radicals and turned this species as alternatives of usual anionic reactions. Indeed, contrary to organometallic reagents, radical precursors can provide highly reactive neutral radicals in smooth conditions, under air atmosphere and nondistilled solvents. In addition, radical reactions are highly chemoselective and can be performed without protected functions. Indeed, several conditions for the generation of radicals are available. Radical initiators (Et 3 B, peroxides, azo compounds …) and mediators (stannanes, silanes, thiols…) can provide radicals for chain reactions. Single electron transfers from stoichiometric metallic oxidants (manganese, cerium) or reductants (samarium, titanium, zinc, nickel) are as many alternatives for the formation of radicals.

Photoredox catalysis, an opportunity for

Challenges in radical chemistry

Radical chemistry offers many advantages (vide supra) and promising features compared to anionic reactions. However, it suffers of several drawbacks which limit their use in synthesis. Because the reactions are performed in diluted conditions, the scale up is quite complicated. Some of the processes also require an excess of radical acceptor. But, the most problematic aspects are the use of explosive initiators (peroxides, azo compounds) or toxic mediators like the tin (IV) derivatives which are also difficult to remove from the product. In this context, solutions have been proposed to escape from ''the tyranny of tin'' and to offer the opportunity of using sustainable methods.

In order to progressively substitute the use of tin reagents, methodologies involving only catalytic amounts of these reagents or tin-supported surfaces have been reported. 7 Processes employing stoichiometric mediators such as silanes, phosphines or thiols are also potential alternatives. Organoboranes, as substrates or radical initiators for chain-reactions, showed promising results as well. 6b,8 Less toxic metals responsible of one-electron transfers were also considered as a possible solution. However, methodologies involving metal complexes based on iron, copper, manganese, titanium, samarium… require excess amounts.

Unfortunately, in terms of sustainability and eco-compatibility, it is important to develop alternatives even more efficient. At the end of the 70's, some pioneering works 9 mentioned the generation of radicals thanks to a single electron transfer (SET) mediated by a photoactivated metal complex.

The photoredox catalysis as an alternative

The radical chemistry allowed chemists to develop a wide range of processes for the carbon-carbon bond formation. Many efforts, to avoid the use of toxic tin reagents and to find others alternatives have been done so far, but many reactions are still performed under these conditions. Since the end of the 2000s, photoredox catalysis has emerged as a powerful and versatile eco-compatible approach for the generation of radicals.

Nature as a source of inspiration

The evolution has allowed living organism to develop sustainable and highly sophisticated process. Among them, the photosynthesis of plants has early interested scientists. The chlorophyll present in plant cells absorbs the sunlight in the visible range and initiates the transformation of CO 2 and water to saccharides and oxygen. This natural synthetic process highlights the fundamental and efficient conversion of energy from sunlight into chemical energy.

Taking advantage of this process, increasing efforts from the radical chemistry community has been realized to develop new methodologies using the visible light as a promotor of redox reactions for the generation of radicals. Visible-light photoredox catalysis has emerged as a powerful methodology for radical formation in terms of selectivity and sustainability. Since the pioneering work on this field, more and more groups have incremented the photoredox catalysis as a part of their research and the number of publications on this topic demonstrates its growing popularity (Figure 2). Inspired by the photoredox process of natural photosynthesis, chemists were interested on the development of photocatalysts absorbing light in the visible region and with a large range of potential values to perform efficient redox transformations. The first man-made photocatalyst mainly reported and still frequently used is the photoactive complex Ru(bpy) 3 Cl 2 . First reported as a photoredox catalyst for organic synthetic purposes by Kellog, 9 Pac 10 and Deronzier, 11 this complex has been essentially used in inorganic chemistry for devices applications 12 or transformation of small molecules 13 (CO 2 , H 2 O).This is only from 2008 and the advances of MacMillan, 14 Yoon, 15 and Stephenson, 16 that organic photoredox catalysis has definitely taken off.

Artificial redox photocatalysts

The photosynthesis led to the development of catalytic processes involving chromophores as light harvesters to promote electron transfers for photoelectrochemical cells, photocatalytic water splitting systems or photobioreactors. In this context, chemists tuned photocatalysts which could act as chlorophyll. The complex Ru(bpy) 3 Cl 2 first synthesized by Burstall 17 in 1936 has shown photophysical and redox properties to perform single electron processes under visible light activation. Thus, many efforts have been done to widen the range of transition metal-based photocatalysts. In order to get closer to the principles of green chemistry, organic dyes have also attracted interest. In terms of efficiency, a valuable photocatalyst has to absorb the visible light, get fitting redox properties and a long lifetime at the excited state to enable electron transfers. Several polypyridyl complexes of metal from the fourth to sixth periods showed these properties. Most of them are ruthenium, 12 rhenium 18 or

Photophysical properties

The This MLCT transition is actually the most important one since the absorption is in the visiblelight region spectra and it allows to reach a 3 MLCT after an Inter-System Crossing (ISC) thanks to a strong spin-orbit coupling, usually observed with the heavy metal atom (Scheme 2 b)). The consequence is a longer luminescence lifetime (10 -7 -10 -6 s) 12a,19b compared to 3d metal complexes and so the possibility of electrons transfers. Constantly trying to evolve toward a greener chemistry, many photoredox processes involve organic dyes as photocatalyst such as fluorescein, Eosin Y or rose bengal. 24 As organic molecules, the strongest light absorption results in the promotion of an electron of a π orbital to a π* orbital. Because of the lack of heavy atoms and so the inefficient intersystem crossover, the light emission is mainly fluorescence. The consequence for these photocatalysts is a very short lifetime of the S 1 excited state (2 to 20 ns for most of the organic photocatalysts). The envisaged electron transfers must be at least as fast as the deexcitation of the chromophore.

Principle of photoredox catalysis

As we have seen above, photoexcited chromophores (PC*) can transfer one electron to substrates providing radicals which can be involved in radical processes. However, to render the overall process catalytic, the photocatalyst (PC However, regarding a triplet state, the spin multiplicity must change during the BET which is less favored. Therefore, free radicals are more likely to be formed in the reaction once the photocatalyst is in a triplet excited state (Scheme 8). Metal complexes reach easily the triplet excited state thanks to the heavy metal atom which increases the spin-orbit coupling and so the inter-system crossover (ISC). For organic molecules, the most populated excited state is most of the times the singlet state which limits their efficiency in photoredox catalysis.

Despite this property, some organic dyes have already attested to be used as photocatalysts in redox processes. 28 At the moment, no explanation can rationalize these observations inconsistent with the theory.

Scheme 8. Cage escape and BET in singlet and triplet ion pairs for photooxidation of a substrate

Since the triplet excited state is more likely to provide radical processes, organic dyes reaching efficiently this state would be more promising for photoredox catalysis. Organic

Thermally Activated Delayed Fluorescence (TADF) materials have begun to be exploited in this field. As mentioned in the name category of these molecules, they are chromophores with a very long luminescence lifetime compared to other common organic molecules. Usually, organic dyes or complexes reaching a triplet state after ISC lose their excess of energy by phosphorescence or vibrational relaxation. In the case of TADF materials, a reverse intersystem crossover is possible (RISC) thanks to the thermal activation by the surrounding environment (Scheme 9 a)). This pathway is possible only if the energy difference between S 1 and T 1 (∆E ST ) is enough low and particularly in the range of the thermal energy. Thanks to a quantum mechanical analysis, Adachi 25b found that the reduction in the overlap between the HOMO and the LUMO results in a small ∆E ST . In this context, molecules displaying a donor(red)-acceptor(green) scaffold are unavoidable candidates such as phenoxazinetriphenyltriazine (PXZ-TRZ), 29 4,6-bis[4-(9,9-dimethyl-9,10- For TADF materials, the kinetics values of ISC, RISC and fluorescence processes are crucial to obtain the enhancement of luminescence lifetime. The fluorescence process is usually in the range of nanoseconds. In order to avoid this direct desexcitation pathway, the ISC kinetics must be in the range of the fluorescence. Also, the luminescence lifetime would be increased if the RISC is slower than the ISC. Actually, for this kind of molecules, the ISC process is rather efficient. The spin conversion efficiency is correlated to the first-order mixing coefficient between singlet and triplet states (λ). This parameter is inversely proportional to ∆E ST and proportional to H SO which is the spin-orbit interaction (Scheme 9 c)). 31 Because the ∆E ST is low, the overall spin-orbit coupling is important. In consequence, TADF materials can exist in their triplet excited state and perform electron transfers.

Particularly, the 4CzIPN proved to be an efficient photocatalyst in photoredox/nickel dual catalysis. 26,32 Scheme 9. a) Energy diagram for TADF materials. b) Examples of Organic Donor(red)-Acceptor(green) TADF materials. c) First-order spin-orbit coupling parameter

Formation of carbon centered radicals

In terms of synthetic interests, the photoredox catalysis found opportunities to generate various types of radicals by photoreduction or photooxidation of different chemical functions.

Radicals centered on carbon, nitrogen, 33 sulphur 34 or phosphorus 35 can be obtained by photoredox catalysis and engaged in the formation of carbon carbon bonds or heteroatom carbon bonds. The wide diversity of carbon-centered radicals generated (aryl, alkenyl, alkyl) makes this method of great interest. Thus, many works on the generation by photocatalysis of such intermediates have been realized, starting with the formation of aryl radicals.

Formation of aryl radicals by photoreduction

The pioneering example developed by Deronzier was the reduction of arenediazonium salts with Ru(bpy) 3 Cl 2 as photocatalyst. The excited ruthenium complex enables the formation of an aryl radical by photoreduction of the diazonium salt to perform a Pschorr type reaction (Scheme 10 a)). 36 More recently, König extended this catalytic process to intermolecular Meerwein type arylation reactions with Eosin Y as photocatalyst (Scheme 10 b)). 37 Aryl radicals were further obtained by photoreduction of sulfonium 38 or iodonium 39 salts and engaged in intermolecular radical allylation reactions. Although aryl radicals are obtained efficiently, alkyl radicals are more of interest for the formation of molecular scaffolds. Halides are substrates of choice for reduction processes to generate alkyl radicals.

Fukuzumi 40 was the first to report the photoreduction of alkyl halides. Recently, inspired by this work, MacMillan 14 managed to merge photoredox catalysis and organocatalysis, and to perform enantioselective α-alkylation of aldehydes with bromoalkanes as radical precursors.

In the presence of a chiral imidazolidinone, an enamine is formed. Reduction of the bromoalkane by [Ru(bpy) 3 + ] leads to an alkyl radical which adds to the enamine double bond.

The resulting α-amino radical is then oxidized by [Ru(bpy) and formic acid. 43 The presence of formic acid favored the formation of an excellent donor of hydrogen, the ammonium salt (H + NEtiPr 2 .HCO 2 -) In similar conditions(Et 3 N instead of DIPEA), radicals obtained from the reduction of bromomalonate could also be engaged in radical cyclizations with alkenes, alkynes 44 , indoles and pyrroles 45 moieties (Scheme 12 a)).

More interestingly, polyene substrates gave polycyclic compounds after cascade cyclizations.

Scheme 11. Enantioselective α-alkylation of aldehydes

Although the processes look efficient, the main limitation is the nature of the halide.

Indeed, reductive potentials of non-activated alkyl iodides or bromides are lower than [Ru 2+ ] * or [Ru + ] and measured between -1.61V and -2.5V vs SCE 46 . Stephenson showed that they are reduced with the highly reductant Ir(ppy) 3 (-1.73V vs SCE at the excited state) and the generated radical can be engaged in 5-exo-trig and 5-exo-dig cyclization processes (Scheme In 2011, MacMillan mentioned a reaction between photogenerated α-amino radicals and benzonitrile. 60 The radicals were obtained by oxidation of N-alkyl anilines and added at the ipso position of the cyano arene releasing cyanide and the α-amino arene (Scheme 18).

The reaction performed with the Ir(ppy) 3 photocatalyst was suitable for (non)-cyclic anilines and various cyano (hetero)arenes. Photoexcitation of the iridium photocatalyst leads to the reduction of the cyano arene. The [Ir IV ] intermediate then oxidizes the aniline, resulting in the formation of the α-amino radical which couples with the reduced arene. The newly formed anion provides the final formation of the product and cyanide.

Scheme 18. Photoredox amine α-arylation reaction

• Oxidation of α α α α-aminocarboxylates
Anionic species are substrates of choice for oxidation reactions. MacMillan showed the ability of α-aminocarboxylates to undergo photooxidation, generating an α-amino radical by the loss of CO 2 . As he showed before, the radicals were able to be coupled with dicyanobenzene in similar conditions. 61 . Even if the amine must be protected (Boc or Cbz) to avoid the oxidation of the nitrogen atom, secondary and tertiary amines could be engaged in the process because, these radicals' precursors are quite easy to oxidize TEMPO adducts. However, secondary and primary alkyl trifluoroborates were not converted to the expected product. Only (triol)borates of 2-(hydroxymethyl)-2-methylpropane-1,3-diol which showed lower oxidation potentials and could provide TEMPO adducts in moderate yields (Scheme 20). Synthetic applications were also extended to Giese type reactions.

(E 1/2 (R-CO 2 - /R- CO 2 . )= +0.95 -+1.16 V vs SCE).

Scheme 20. Photooxidation of organoborates

Few years after, Molander and co-workers extended the synthetic applications of organotrifluoroborates. Scheme 21. Organic photooxidation of alkyl trifluoroborates

Conclusion

The photoredox catalysis has emerged as a powerful alternative method for the formation of radicals. The diversity of photocatalyst (ruthenium/iridium based complexes or organic dyes) with various redox potentials allows to promote single electron transfers selectively. Many reactions performed in classical conditions, could be realized in a photocatalytic version. The appeal of this field grows year after year and demonstrates its importance in the chemistry community and particularly for the development of a greener radical chemistry.

Chapter II

Merging photoredox and organometallic catalysis for cross-coupling reactions

2 Chapter II Merging photoredox and organometallic catalysis for crosscoupling reactions

Context

Photoredox catalysis has shown its efficiency to accomplish radical reactions. Methods merging organocatalysis and photoredox catalysis have been also developed. 14,41,42,[START_REF] Hamilton | For examples of photoredox-organo dual catalysis[END_REF] Recently, photoredox catalysis has proved to be compatible with transition metal catalysis. In this type of catalysis, the photocatalyst is necessary to generate a radical and the organometallic catalyst performs the cross-coupling steps between an electrophile and the generated radical. Therefore, metal complexes able to trap radicals are catalysts of choice for such transformations.

Radical trapping by transition-metals

Transition metal catalyzed reactions have become essential tools for the elaboration of new molecular building blocks. For instance, palladium catalyzed cross-coupling reactions and C-H functionalization reactions are processes well-established and efficient. The mechanisms relying on two electron transfer steps are inherent to the nature of fifth period transition metal. However, concerning 3d transitions metals, although they are known to perform ioxidative addition in carbon-halogen or carbon-hydrogen bonds, they can also promote single electron transfer 94 and generate radicals. These ones can be further trapped by the metal and involved in organometallic processes.

• Cobalt Some cobalt complexes showed to be able to reduce alkyl halides to alkyl radicals which could undergo cyclizations. The resulting radical intermediates lead the formation of an

Scheme 30. Proposed mechanism for Photoredox/palladium C-H arylation catalysis

This mild approach for palladium catalyzed-C-H bonds arylation is the first photoredox/transition metal dual catalytic process which demonstrates the feasibility of photoredox mediated cross-coupling reactions and pointed the way towards new synthetic opportunities.

Towards photoredox/transition-metal dual catalysis processes

Since the seminal work of Sandford, many photoredox/transition-metal dual catalysis processes have been developed. One of the most important features of this kind of catalysis is the opportunity offered by the photoredox transformations to modify the oxidation state of organometallic intermediates. This allows tuning the reactivity of the metallic centers for synthetic applications. Two tandem processes can be distinguished: -Catalytic reactions which do not involve the formation of radicals (except superoxide anion [O 2 ]· -). In this case, the SET takes place only between the photocatalyst and the transition metal. -Reactions in which a photogenerated radical is trapped by an organometallic complex. In this case, a second electron transfer between the organometallic catalyst and the photocatalyst occurs to maintain the redox balance. Each process will be therefore named ''Catalysis of Redox

Steps'' and ''Catalysis of Downstream Steps'' respectively (Figure 4). as photocatalyst under visible-light irradiation. The authors showed that the gold catalyst Ph 3 AuNTf 2 was the most effective complex in methanol for tandem 5-exo-trig cyclizationarylation reactions of 4-penten-1-ol. The scope could be extended to 5-penten-1-ol and 5penten-1-tosylamide. Various aryldiazoniums bearing substituents such as halogens, esters or methoxy groups led to the product formation of products in moderate to good yields (Scheme

35)

.

Scheme 38. Photoredox/Gold catalyzed ring expansion-arylation reaction

Gold catalyzed transformation of allenes under photoredox conditions has also been reported by the group of Shin. 125 In the same way, allenoates could be converted to arylated furanone in the presence of a cationic gold complex, aryldiazonium salts and Ru(bpy) 3 (PF 6 ) 2 .

In the presence of a cationic gold(I) catalyst ([Ph 3 PAu]OTf), allenoates are converted to a gold(I)-furanone intermediate. Then, the aryl radical addition and the oxidation step would generate the gold(III) complex which after reductive elimination liberates the arylated furanone and the starting gold(I) catalyst. (Scheme 39).

Scheme 39. Photoredox/Gold catalyzed cross-coupling of allenes with diazonium salts

The photoredox/gold dual catalysis has demonstrated its efficiency towards crosscoupling reactions with aryldiazonium salts. Moreover, the ability of gold to promote cyclizations or rearrangements and perform cross-coupling reactions depends mainly on the electrophilicity of the catalyst. Two mechanisms have been proposed depending on the nature of the starting gold catalyst (neutral or cationic).

Copper-mediated catalysis

As mentioned in paragraph2.2, copper is also an efficient radical trapping agent. After her pioneering works with palladium on dual catalysis, Sanford developed a copper-catalyzed trifluoromethylation of arylboronic acids. This known reaction usually required expensive trifluoromethylating agents or stoichiometric amounts of copper. 126 For these reasons, they choose CF 3 I which can be reduced in the presence of Ru(bpy) 3 (PF 6 ) 2 as photoredox catalyst to a CF 3 • radical, 41,127 and a copper catalyst (20 mol% of Cu(OAc)) to obtain the cross-coupling products. 128 Various aryl and heteroaryl (pyridine, furane, quinoline) boronic acids could be trifluoromethylated (Scheme 40). Electron-rich and electron-poor electrophiles were tolerated and converted to the expected products in moderate to excellent yields. In addition, perfluoroalkyliodides were also suitable partners. However, with these materials, the loading of copper must be drastically increased to get satisfactory yields.

Conclusion

The combination of photoredox catalysis and organometallic catalysis has shown to be a highly versatile approach for the construction of a wide range of molecular scaffolds. The overall methodology involves either only electron transfers between both catalysts, or with an additional radical trapping step by the organometallic catalyst. Various transition metals including palladium, gold and copper can be used to perform cyclizations or cross-coupling reactions via C-H or C-X bond activation. Nickel complexes have also widely participated to such type of transformations. These processes will be detailed in chapter four.

Chapter III

Oxidation of alkyl bis-catecholato silicates: a mild way for the formation of carbon centered radicals

Definition of silicates

Silicon is the most abundant element by mass in earth's crust. Even if pure silicon ( 0)

is widely used for its application in electronics, as semiconductor, it is commonly found combined with oxygens in minerals called ''silicates'' which contains the silicate ion ((SiO 4 ) 4-) associated with metal oxides. However, in some minerals, the silicon-oxygen ratio is different than 1:4 due to the oligomerization of ((SiO 4 ) However, since silicon can reach penta-and hexavalency with alkoxy or fluoride ligands, this definition is biased. Consequently, hypervalent silicon species bearing such groups are considered as silicates as well. Thus, pentacoordinate silicon species with 4 alkoxy ligands and 1 organic substituent are so called silicates (Figure 5).

Oxidation of hypercoordinate silicon compounds

Hypercoordinated silicon compounds containing one, two or three alkyl residues have shown various reactivities for the formation of synthetic scaffolds. Indeed, these silicon 129 https://goldbook.iupac.org/html/O/OT07579.html derivative substrates have been especially engaged with nucleophiles, 130 used as Lewis acids 131 for the activation of carbonyl compounds, or involved in Hiyama type cross-coupling reactions. 132 Also, the in situ formation of such compounds led to the same kind of reactivity.

Furthermore, anionic penta-and hexacoordinated silicon compounds display a negative charge which gives them a reductant feature which has been investigated Finally, the proposed mechanism starts with a direct oxidation of the silicate by the copper(II) halide giving the radical and a copper(I). Another equivalent of copper(II) halide is required to generate the final alkyl halide (Scheme 44 a)).

Metal mediated oxidation of organopentafluorosilicates

The methodology was then extended to radical conjugate addition processes. To limit the formation of alkyl halide, copper(II) acetate was used as oxidant. The radicals were involved in Giese-type reactions with various α,β unsaturated carbonyl compounds (Scheme 44 b)).

Scheme 44. Mechanism of the oxidation of organopentafluorosilicates

Copper(II)-mediated oxidation of alkylpentafluorosilicates is the first process involving the formation of a carbon centered radical after fragmentation of the carbon-silicon bond of silicates. During the following twenty years, no other evidence about the oxidation of hypercoordinated silicon species was reported.

Photon-induced electron transfers with alkyl bis-catecholato silicates

Among the variety of hypercoordinated silicon compounds, the alkyl bis-catecholato silicates have shown interesting features for synthetic applications. bis-catecholato silicates. Among the bases used for the synthesis, he also demonstrated that pyridine or quaternary ammonium hydroxide were also effective (Scheme 48 Eq.1). At the end of the 80's Corriu,136b,143 inspired by the work of Frye, reported the synthesis of alkaline alkyl bis-catecholato silicates. Instead of using amines as base he proposed to use methanolic solution of sodium or potassium methoxide to get the corresponding sodium and potassium silicates (Scheme 48 Eq.2).

Scheme 48. First synthesis of alkyl bis-catecholato silicates

In our laboratory, we decided to apply these methods with modifications. Indeed, we found that the potassium version of these silicates is not stable. In less than one week, we observed a slow decomposition of the silicates. In order to stabilize them, we envisioned to add a [18-C-6] crown ether to intensify the hypervalent bond by charge separation 144a (Scheme 49). With this modification, we avoided the introduction of water in the solid and we managed to synthesize a wide range of primary (2-18), secondary (1, 1' and 22), tertiary (21) and aryl (19-20) silicates and store them on the bench for months. 32a,144 About the ammonium version, we decided to develop a straightforward synthesis of tetraalkylammonium silicates. So, starting from trichlorosilane, catechol and a tertiary amine in THF, the triethylammonium silicates were obtained. Then a metathesis performed with tetraethylammonium bromide in acetonitrile gave the tetraethylammonium silicates (21 and

22).

With this easy synthetic approach, primary, secondary and tertiary alkylsilicates as well as arylsilicates were obtained in moderate to excellent yields. Scheme 49.Modified synthesis of alkyl bis-catecholato silicates and scope synthesis

All the silicates can be categorized into 4 groups: the activated silicates, secondary silicates, primary silicates and aryl silicates. Tertiary, benzyl and allylsilicates and silicates substituted in α position by a hetero atom will be all considered as activated silicates.

It has to be noted that the group of Molander reported the synthesis of ammonium silicates without performing any metathesis step. 145 The alkoxysilanes were treated by catechol and triethylamine or diispropylamine in THF or dioxane according to the Frye's 145 M Jouffroy, D N. Primer and G A. Molander, J. Am. Chem. Soc., 2016, 138, 475-478. procedure (Scheme 50). They were able to get primary and secondary alkyl silicates in good yields but with questionable stabilities.

Scheme 50. Molander's Ammonium silicates synthesis

Structural analysis and properties

Hypercoordination of the silicon atom in alkyl bis-catecholato silicates

Like the element carbon, silicon is tetracoordinated in a tetrahedral geometry respecting the octet rule. Nevertheless, silicon can extend its coordination to 5 and 6 with a trigonal bipyramidal or pyramidal square base structure, and octahedral structure respectively around the silicon. We will not consider the hexacoordinated silicon species in this part.

In the case of a pentacoordinated silicon atom, a p orbital is engaged in a hypervalent The [18-C-6] potassium alkyl bis-catecholato silicates was crystallized and analyzed by X-ray diffraction. We observed that the stability of these silicates depends on the interaction with the potassium. Even if the crown ether, modify de near environment of the potassium, the bond lengths of the Si-O and Si-C are not importantly modified (Table 1). The effect is more noticeable for the C-Si-O and O-Si-O angles. According to the values, the potassium cyclohexyl bis-catecholato silicate crystallize in a trigonal bipyramidal structure whereas the [18-C-6] cyclohexyl bis-catecholato silicate ( 1) is almost in a pyramidal square base structure. Table 1. Crystallographic data around the silicon for the cyclohexyl silicates Silicate

Redox properties

Even if the redox potential is a thermodynamic value and does not give any information about the kinetic of the reaction, it is important to know in photocatalysis the redox properties of the substrates. Indeed, these data inform us about the feasibility of a considered reaction.

Some electrochemical experiments were realized on all the synthesized silicates. Activated silicates showed to be the easiest to oxidize (E ox = +0,34 +0,72 V vs SCE) followed by the secondary silicates (E ox = +0,69 +0,76 V vs SCE), primary silicates (E ox = +0,74 +0,90 V vs SCE) and the aryl silicates (E ox > +0,88V vs SCE) (Figure 7). If we envisage that the silicates may undergo homolytic fragmentation of the C-Si bond to provide the carbon centered radicals, the stability of the radicals seem linked to the oxidation facility of the corresponding silicates.

Figure 7. Scale of the oxidation potential of the silicates

Regarding the oxidation process, DFT calculations revealed that the oxidation likely occurs on a catechol ring of the silicates. The resulting catechol radical is then highly delocalized and so stabilized. Prokof'ev and co-workers146 showed by ESR experiments of penta-coordinated bis- (3,6-ditertbutylcatecholato) silicatesderivatives that the radical of this species is especially localized on the four oxygen atoms (Figure 8). The presence of the tertbutyl group on the catechol ring was chosen to increase the kinetic stability of the radicals and to simplify the ESR spectrum. Nevertheless, in their case and ours, the radical may also be delocalized on both rings, increasing the number of mesomeric form and so the stability of the radical. Moreover, the calculations revealed that the BDE of the C-Si bond becomes low enough to observe the homolytic fragmentation (Table 2) and give the free radical by an irreversible radical substitution on the silicon of the phenoxyl radical. Even though these silicates are easy to oxidize, the difference of redox potentials is not well rationalized. Since the oxidation is considered to occur on the catechol ring, the oxidation potential must be only depending on the electron density on the catechol which must be the same for all the silicates. Indeed, no electron withdrawing or donating group was added on the ligand. Considering an orbital approach, the oxidation process takes out an electron of the HOMO of the silicate localized on the catechol rings. The higher is the energy level of the HOMO the lower is the oxidation potential. Actually, the HOMO-1 is representative of the C-Si bond. In term of energy, if the HOMO-1 energy level increases, the one of the HOMO increases also. In consequence, different kind of alkyl group would not have the same effect on the oxidation potential of the silicates. In addition, Nishigaishi reported that coordinating solvent would facilitate the oxidation of the silicates. 138 Coordination of the DMF to the silicon is therefore possible. Probably a combination of both effects would drastically modify the oxidation potential of the silicates.

Studies on photooxidation of alkyl bis-catecholato silicates

Bis-catecholato silicates offer many advantages in terms of preparation and stability.

They also display interesting low oxidation potentials (~0.3 -0.9V vs SCE). The very mild and more sustainable conditions, offered by photoredox catalysis, were strongly appealing.

We have realized some investigations about the generation of radicals by photooxidation of alkyl bis-catecholato silicates. 144a

Photooxidation by polypyridine transition metal photocatalysts

Prior to engage the [18-C-6] potassium alkyl bis-catecholato silicates in radical carbon-carbon bond forming reactions, the generation of alkyl radicals was proved by spintrapping experiment with TEMPO. It is known that TEMPO is a good radical scavenger that quickly reacts with carbon centered radicals. 147 To get a representative result, benzylsilicate5

was chosen due to its low oxidation potential. It was found that the benzylsilicate reacts with TEMPO (2.2 equivalent) in the presence of 2 mol% of Ir[(dF(CF 3 )ppy) 2 (bpy)](PF 6 ) in DMF for 24 hours under light irradiation (477 nm) at room temperature to afford the benzyl-TEMPO adduct 23 in 95% yield (Table 3). This result demonstrates that the benzylsilicate is an excellent precursor of a benzyl radical under visible-light photooxidative conditions.

147 V. W. Bowry and K. U. Ingold, J. Am. Chem. Soc., 1992, 114, 4992-4996.

To a metal free oxidation

On the basis of this work, we envisioned to develop a metal free oxidation of silicates for eco-compatible reasons. We also compared their reactivities with the respective alkyl trifluoroborates.

Stoichiometric oxidation

Inspired by the work of Kumada on stoichiometric oxidation of pentafluorosilicates, 134 previously known reaction 151 but not exploited in radical chemistry. Post functionalization of the resulting radicals was achieved by TEMPO spin trapping, allylation 152 or conjugate addition reactions.

Our purpose was to avoid the use of metallic oxidants by replacing with pure organic oxidants. We previously found that the Dess-Martin periodinane (DMP) could efficiently oxidize trifluoroborates in Et 2 O. 150 So we considered Et 2 O as an optimized solvent for the oxidation of trifluoroborates with non-metallic oxidants. Owing to the facile oxidation of alkyl bis-catecholato silicates in DMF with iridium photocatalyst, we also performed the study in this solvent. Among the organic oxidants, tritylium tetrafluoroborate, an underexplored oxidant, 153 was tested first with a series of alkyl trifluoroborates and alkyl biscatecholato silicates. 154 For the reasons previously mentioned, 147,150 we engaged respectively benzyltrifluoroborate and the benzylsilicate in spin-trapping experiments with TEMPO. We observed that a good yield of TEMPO adduct was obtained in Et 2 O for the oxidation of benzyltrifluoroborate and DMF did not appear as the best solvent, while benzylsilicate did not react efficiently with the tritylium in both solvents (Table 4).

We also considered the Ledwith-Weitz aminium salt as organic oxidant for both radical precursors. The use of this radical cation oxidant as a SET oxidative agent (oxidation potential: + 1.06V vs SCE) 155 had never been tested. The reactions were performed in DMF and Et 2 O as well. For benzyltrifluoroborate and benzylsilicate, the benzyl-TEMPO adduct was not obtained in satisfactory yield. However, DMF had a surprising effect on the reaction and the expected product was isolated in 69% and 86% yield respectively. The low yields obtained in Et 2 O for benzylsilicate might be due to their low solubility in this solvent. The difference of yield obtained with both oxidants is not obvious to rationalize. Less stabilized trifluoroborates and silicates were further engaged in the optimized conditions to see the synthetic opportunities of these oxidants. Good yields were obtained with the secondary (25) and primary series (24/24') of alkyl trifluoroborates using Et 2 O and tritylium conditions (Scheme 54). Only tert-butyl precursor gave low yield of product 26, (25%) presumably for steric reasons. Only 27% yield of 25 with the secondary substrate and no TEMPO adduct in the primary alkyl for the aminium salt oxidation version.

Scheme 54. Oxidation of organotrifluoroborates by organic reagents 5-Hexenyl ( 3) and cyclohexyl silicates (1) were involved in this process. The aminium salt proved to be ractive in DMF for secondary and primary alkyl substrates giving, 44% of 27 and 61% of 24/24' respectively (Scheme 55). Tritylium can also be used as a reliable oxidant for the silicates. In fact, tritylium conditions gave also a moderate yield of 53% for 27. In the case of the 5-hexenyl silicate, a mixture of linear (24) and cyclized products (24') with a ratio of 10:1 was observed demonstrating once again the radical character of these transformations.

Scheme 55. Screening of stoichiometric oxidation of silicates

Then we were able to oxidize the potassium ((1S,2R,3S,6S)-3methylbicyclo [4.1.0]heptan-2-yl)trifluoroborate salt leading the generation of secondary radical, the trifluoroborate salt subsequently underwent a ring opening of the cyclopropane to give 28 (Scheme 56). This time, the tertiary radical was trapped in good yield (67%).

Interestingly, the tritylium in diethyl ether conditions proved to be also compatible with conjugate addition since the methyl vinyl ketone (MVK) adduct29 was also isolated in satisfactory yield (63%). No reaction was observed with silicates. Even if we succeed to oxidize moderately the alkyl bis-catecholato silicates with stoichiometric organic oxidants, the next step was to improve these conditions to a catalytic version. The solution was to use an organic photocatalyst.

Scheme 56. Generation of tertiary radical and conjugate addition

Organic dyes as photooxidant

Organic dyes have already attested their efficiency in photoredox processes. 156 We envisioned using some dyes for which the redox potentials (excited state reduced photocatalyst) match with those of silicates and TEMPO. Fluorescein, Eosin Y and Fukuzumi catalyst (9-Mesityl-10-methylacridinium perchlorate) were first selected. The main drawback of these photocatalysts is their short excited state lifetime (< 6 ns) (Scheme 57). The experiments were performed with benzyltrifluoroborate and benzylsilicate to see the viability of the methodology and its applications in other radical processes.

Scheme 57. Selected organic dyes for the photooxidation of silicates

We choose DMF as solvent and a 10 mol% of photocatalyst loading to start the study.

Fluorescein and Eosin Y showed to be not convenient photocatalyst. Only small amount of benzyl-TEMPO adduct was obtained when the reaction was performed with benzyltrifluoroborate. Fukuzumi catalyst allowed us to get the expected product with benzyltrifluoroborate and benzylsilicate in 92% and 66% yield respectively (Scheme 58). We then considered this catalyst for further substrates. In this photooxidative conditions, allyl, 5hexenyl and cyclohexyl type substrates were tested. For both kinds of substrates, the same trend was observed: the less stabilized is the radical, the lower is the yield. However, the yields were rather better for trifluoroborates than for silicates, especially for the primary and secondary radical precursors.

Scheme 58. Photooxidation of silicates and trifluoroborates with organic photocatalysts

At this stage, the metal-free photooxidation of silicates showed to be possible but not highly efficient. These photocatalysts may suffer from a short excited state lifetime. Even if the thermodynamic data enable the reaction, the kinetic of the oxidation step is probably too slow compared to the other deexcitation pathways of the excited state. In 2012, Adachi et al.

described a family of carbazolyl dicyanobenzenes as light-harvesters for organic lightemitting diodes. 25e Among them, 1,2,3,5-tetrakis-(carbazol-yl)-4,6-dicyanobenzene (4CzIPN) displayed promising photophysical properties for photoredox catalysis: a high photoluminescence quantum yield (94.6%) and a long life-time in the excited state (5.1 µs)

which is a really huge value for an organic photosensitizer.

The group of Zhang 26 calculated the redox parameters of this chromophore. Thanks to photophysical analysis and electrochemical studies, they found the following values: In the same conditions as before, we were pleased to see the formation of the benzyl-TEMPO adduct in 92% yield using 4CzIPN as photocatalyst. Moreover, decreasing the photocatalyst loading to 1 mol% showed the same efficiency (Scheme 59).

Scheme 59. Spin-trapping experiments with 4CzIPN as photocatalyst

In order to explore the potential of this dye, various alkyl silicates were engaged in a series of radical addition reactions (Scheme 60). The first selected radical acceptor was the activated allylsulfone 31 which previously proved to be a convenient radical trap. 144a,149 Allylation adducts were obtained in excellent yields for stabilized radicals (α-aminyl and tertiary) and secondary radicals (32-34), and in moderate yields for primary radicals (35-37).

Interestingly, 2-(diphenylphosphineoxide)ethylsilicate gave allylation adduct (36), without fragmentation of the Ph 2 PO radical. 157 The kinetic rate of β-fragmentation is probably lower than the addition of the carbon-centered radical on the double bond of the acceptor.

Cyclohexylsilicate (1) was further chosen as radical precursor for alkynylation 39,158 vinylation 158 41 and Giese-type reaction 43. In each case, the expected product was obtained in good yield.

Scheme 60. Radical addition reaction catalyzed by 4CzIPN

Conclusion

Ammonium and [18-C-6] potassium alkyl bis-catecholato silicates can be synthesized quantitatively and efficiently. Oxidation of such species led to the formation of (non)stabilized carbon-centered radicals which can be trapped by acceptors. Stoichiometric organic oxidants demonstrated to be a valuable oxidant, but photocatalysts (metal based or organic) are more performing for the generation of the radicals. It is the first evidence for the generation of non-stabilized alkyl radicals by photooxidation. Development of new synthetic combining photooxidation of silicates and transition metals catalysis can be therefore envisioned to develop other types of cross-coupling processes.

Chapter IV

Combining photooxidation of alkyl bis-catecholato silicates and

Nickel catalysis: functionalization of eletrophiles

Nickel is the first element of group 10, above palladium, platinum and darmstadtium.

Named after the goblin-like sprite of Germanic mythology Nickel, this element has been extremely used for its corrosion resistance when it is employed in alloys. Also, nickel's compounds are involved in electrochemical processes, especially in rechargeable cells.

Unfortunately, evidence of allergic reactions and toxicity made the use of nickel restricted.

Despite its presumed hazards for health, nickel has found applications in efficient catalytic industrial processes (oil, pharmaceutical, food), and has recently received significant development in organic synthesis for cross-coupling reactions.

Progress in Nickel catalysis

As a fourth-period transition metal, nickel is able to do SET processes (2.2) like its neighbor, cobalt and copper. Also, located just above palladium, it has similar reactivity and can promote the same elementary steps suchs as oxidative addition, reductive elimination or C-H activation (Scheme 61). In contrary to Hiyama, Sonogashira, Stille or Suzuki coupling and Heck reaction where a palladium catalyst is almost always used, nickel proved to be an alternative to palladium to Negishi cross-coupling reactions. Moreover, nickel is much cheaper (4.5 $/mmol) than palladium (6.200 $/mmol) 159 which makes it more attractive for economic reasons. Due to its specific properties, nickel has proven to be an efficient catalyst involved in various transformations 160 thanks to the large oxidation states accessible (Table 5). +4 are also accessible. 102,162 This ability of nickel allows different kinds of reactivities and radical pathways. Thus, catalytic cycle involving 1, 2, 3 or 4 different oxidation states are possible. Thanks to these properties, new approaches for the formation of the C-C bond have been envisaged and the studies are still going on.

One major progress in nickel catalysis has been brought recently by the work of Weix.

In 2010, he reported the cross-electrophile coupling of aryl halides with alkyl halides. 163 Instead of using an organometallic nucleophile with an electrophile like in the common crosscoupling reactions, his group succeeded to perform the cross-coupling reaction of two electrophiles using a nickel catalyst and a reductant in a stoichiometric amount. In situ formation of a Ni(0) complex with 4,4'-di-tert-butyl-2,2'-bipyridine and 1,2bis(diphenylphosphino)benzene as ligands led to the cross-coupling products bearing various functional groups on both partners like boronic esters, carbamates or esters for instance (Scheme 62). In order to render the process catalytic, they found that a Mn(0) was required as a reductant.

Scheme 64. First oxidation step determination

Regarding the next steps of the mechanism, they envisioned the formation of radicals.

Radical clock experiments proved the generation of radicals: when cyclopropylmethyl bromide was used as alkyl electrophile, only the homoallyl product was obtained and performing the reaction with an enantiopure alkyl bromide provided a racemic mixture of cross-coupling products (Scheme 65). Both results are consistent with the formation of an alkyl radical after the oxidative addition step of the aryl halide. This nickel-mediated radical formation may occur before the addition of the radical on the same nickel center. As a proof, they used 5-hexenyl iodide because the 5-hexenyl radical is known to cyclize slower (k = 2.3

x 10 5 ) than the ring-opening of the cyclopropylmethyl radical (k = 7 x 10 7 ). 6a A mixture of cyclized (A) and linear (B) products should be observed. Variation of the concentration of the catalyst changed the ratio of cyclized and linear products (Figure 9) which is contrary to the idea of a direct radical formation radical trapping by the same nickel center. Indeed, if the radical formation and the radical addition happen on the same nickel center, the ratio must be unchanged, whatever the concentration. But, in the case of a high concentration, the radical should not have time to cyclize and should be directly trapped by a nickel center (the same or not).

Scheme 65. Radical clock experiments

Development of visible-light photoredox/nickel dual catalysis

With the dynamic initiated by Sanford 110 on photoredox/transition metal dual catalysis and the work of Weix on nickel catalysis, the idea of merging photoredox catalysis and nickel arose with two distinct reports from Molander169 in one hand and MacMillan and Doyle170 in the other hand.

Pioneer works

• Benzyltrifluoroborates in photoredox/nickel dual catalysis

The work of Molander was inspired by the report of Akita 66 who demonstrated the photooxidation of alkyl trifluoroborates to generate stabilized alkyl radicals in the presence of [Ir(dF(CF 3 )ppy) 2 bpy](PF 6 ) as photocatalyst under blue light irradiation. His idea was to engage the formed radical in a nickel catalytic process. Among the alkyl trifluoroborates, the benzyl precursor was selected due to its lowest oxidation potential (E 1/2 ox = +1.13V vs SCE in MeCN). In the presence of 2 mol% of the iridium photocatalyst, 3 mol% of Ni(COD) 2 and 4,4'-di-tert-butyl-2,2'-bipyridine (as the ligand) they managed to couple the benzyl radical with (hetero)aryl bromide derivatives in moderate to excellent yields (Scheme 67). The reaction could be performed with benzyltrifluoroborates bearing electron donating or electron withdrawing groups with high yields. Various aryl bromides could be coupled with excellent yields. Moreover, bromo-pyridine, pyrimidine, indole, quinoline and thiophene have proved to be also suitable substrates.

Scheme 67. Photoredox cross-coupling of benzyltrifluoroborates with aryl bromides

About the mechanism of this reaction, the authors proposed the direct oxidative addition of the aryl bromide on the nickel( 0) complex (Scheme 68). For its part, the photoexcited iridium complex (E

Advances on photoredox/nickel dual catalysis

• Tuning of alkyl trifluoroborates About the alkyl trifluoroborates, only activated radical precursors could be engaged.

The limitation of the process is due to the photooxidation step. Indeed, generation of unstabilized radicals from trifluoroborates is tricky, caused by the high oxidative potential of such species. Nevertheless, good yields of cross-coupling products can be obtained once the generated radical is stabilized such as secondary, 172 benzyl 173 and α-amino or oxy 174 radicals are valuable entities for such coupling reactions. In order to promote, the formation of the radicals. The oxidation potential can be decreased from 1.50V vs SCE for the secondary trifluoroborates to 0.91V for (2,2,2-trifluoroethyl)benzene trifluoroborates (Scheme 72).

Scheme 72.Scale of organotrifluoroborates oxidation potential

Concerning the electrophiles, aryl and heteroaryl halides were the most suitable substrates (Scheme 73). Recently, acid derivatives such as acyl chloride 175 and anhydride176 also revealed to be potent coupling partners. Non-activated primary alkyl trifluoroborates did not afford cross-coupling product so far, that is one of the drawbacks of such radical precursors.

Scheme 73. Organotrifluoroborates in photoredox/nickel dual catalysis

• Decarboxylative processes

MacMillan demonstrated all the potential of his methodology based on the generation of alkyl radicals by photooxidation of carboxylic acids followed by decarboxylation. Among the substrates susceptible to undergo CO 2 extrusion, α-amino acids, 170 anhydrides, 177 keto acids 178 and oxalates 179 revealed to be excellent radical precursors.

After in situ formations of anhydride, from a carboxylic acid and an acyl chloride, a Interestingly, benzyl alcohols gave good yields but these ones dramatically dropped when non-activated primary alcohols were tested.

Scheme 76. Photoredox/nickel catalyzed cross-coupling reactions from oxalates various aryl electrophiles. 144b We found that 2 mol% of photocatalyst and 3 mol% of nickel catalyst under blue light irradiation, gave the cross-coupling product 45 between the acetoxypropylsilicate 17 and 4'-bromoacetophenone 44 (Table 6). Control experiments showed that each reagent is necessary for the reaction as well as the light irradiation.

Moreover, the reaction was performed with the acetoxypropylsilicate 17' without the crownether (entry 6). No effect of the chelating agent was observed on the yield of the reaction. We then limited our studies to primary [18-C-6] Using the same conditions, a series of arylbromides with various substituents was engaged with acetoxypropylsilicate 17 (Scheme 81). We first studied the selectivity towards the substitution of halogen atoms on the aryl ring. Bromobenzenes substituted at the para position with other halogens were screened. Fluoro and chloro substituted bromobenzenes were converted to the cross-coupling product 58 and 59 in good yields. The reaction is highly selective as no substitution of the fluorine or chlorine atom was observed. However, when the reaction was started with 1-bromo-4-iodobenzene, a mixture of products 60 and 61 was obtained in moderate yield in a ratio 10:1. The functionalization of the iodo position was preferred over the bromo position. Indeed, oxidative addition is more likely at the C-I bond184 that explains the selectivity observed. 1,4-Dibromobenzene gave largely the monofunctionalized product 60 and the di-functionalized product 62 in low yield.

Scheme 81. Cross-coupling reactions of silicate 17 with various arylbromides

This cross-coupling methodology was very efficient for substrates bearing electron withdrawing groups such as trifluoromethyl 63 and 64 or acetyl 45 and 65 moieties at the ortho, meta or para position. We further investigated electron enriched aryl substrates.

Electrophiles substituted by slightly donor groups such as methyl 68 and 69 or trimethylsilyl 66 and 67, were converted in good yields. Various bromoanisoles were then engaged in the reaction conditions. Cross-coupling products were obtained for meta-bromoanisoles 71 and 72 in moderate yields. However, only the starting material was recovered with orthobromoanisole and para-bromoanisole. Using 1-bromo-4-iodoanisole instead afforded the expected adduct 70 in moderate yield (46%). These results highlight again the importance of the oxidative addition in the process. Aryl bromides substituted by strongly donor groups are not reactive enough. In this case, the corresponding iodo compound should be privileged.

Remarkably, a pinacol boronate function could also be tolerated under these conditions as illustrated by the formation of 73 that could be utilized for further coupling reactions. Since pinacol boronates are sensitive to acidic conditions and to purification by silica column chromatography, a direct oxidation to the phenol was performed (NaOH, H 2 O 2 ). Higher yields were obtained in phenol product 74 (69% vs 53% for 73). A similar procedure was applied to give product 75.

This transformation was then extended to heterocyclic bromides. 2-Fluoro-4bromopyridine was selected as electrophile in the same catalytic conditions and coupled with silicates (Scheme 82). A small library of new alkyl-fluoropyridines was obtained 77-81 in satisfactory yields (59-86%). As before, silicate 17' without the crown-ether gave adduct 79 in a similar yield (79% vs 81%). Other heteroaryl bromide derivatives were screened. Pyridyl systems proved to be compatible with these cross-coupling reaction conditions.

Methylpyridine 82, trifluoromethylpyridine 84 and methylnicotinate 83 adducts were obtained in 40-75% yields. Others heterolytic systems such as pyrimidine, quinoline, indole, benzofuran and thiophene were converted to the corresponding cross-coupling products 85-89 in low to fair yields.

Ni(III) complex which would be the result of two successive steps: oxidative addition of the aryl halide and addition of the radical on the nickel. The order of the steps is still not elucidated. Some computations on benzyltrifluoroborates suggest a complex mechanism with a thermodynamically favored radical addition whether to a Ni(0) or a Ni(II). 185 According to the report, both pathways involving first the oxidative addition of ArX to the Ni(0) followed by the radical addition or the opposite can occur. Some of our results (59-62) showed nevertheless the importance of the oxidative addition step.

Toward a green dual catalytic system

We demonstrated first the ability of potassium ([18-C-6]) bis-catecholato silicates of being good precursors of alkyl radicals for cross-coupling reactions. We also proved that photooxidation of such species can be mediated by as well ruthenium or iridium polypyridine complex or more interestingly by organic dyes and especially by 4CzIPN. Zhang et al.

reported 26 the first examples of the use of 4CzIPN as photocatalyst in photoredox/nickel dualcatalyzed cross-coupling reactions of aminocarboxylates and benzyltrifluoroborates with arylbromides. Similarly, we planned to extend this mixed organic photoredox/metallic dualcatalysis to alkyl silicates and compare its efficiency to our previous conditions. 32a Thus, we selected acetoxypropylsilicate (1.5 equiv.) 17 and 4'-bromoacetophenone 44 as crosscoupling partners (Table 7). We were expecting to find Ni(COD) 2 as a workable nickel source but no product formation was observed after 24 hours (entry 1). Switching to NiCl 2 .dme as a precatalyst (3 mol%) with 4CzIPN (2 mol%) and 4,4'-di-tert-butyl-2,2'-bipyridine, these new conditions provided the cross-coupling product 45 in 77% yield. We were able to decrease the catalysts loading to 2 mol% of the nickel and 1 mol% of photocatalyst for a 83% yield of the expected product (Entry 4). Unfortunately, decreasing the amount of alkyl silicate affected the yield of the reaction (Entry 5). Yield (%) coupling process. This last result highlights once again the radical character of the transformation.

1 1.5 2 Ni(COD) 2 , 3 0% 2 1.5 2 NiCl 2 .dme, 3 77% a 3 1.5 2 NiCl 2 .dme, 5 81% a 4 1.5 1 NiCl 2 .dme, 2 79% a , 83% b 5 1.2 1 NiCl 2 .dme, 2 57% a a
Scheme 84. Screening of alkyl silicates for organic photoredox/nickel dual catalysis with 44

Again, we screened several aryl and heteroaryl bromides with acetoxypropylsilicate 17

(Scheme 85). A better yield was observed for the electron neutral substrate 92 and the electron poor electrophile 45 than for para-bromoanisole 96. Polyaromatic substrates also gave the expected products 93-95 in moderate yields. In these cases, the polyarylated bromides may act as quencher of luminescence of the photocatalyst through an energy transfer. The oxidative process would be then in competition. Moreover, when 4'chloroacetophenone was engaged, the yield of cross-coupling adduct decreased dramatically (from 83% to 14% of 45). This highlights the importance of the oxidative addition step, in accordance with the postulated mechanism. Heteroaryl bromides can also act as electrophilic partners. Pyridines 79 and 83, quinoline 86 or benzofuran derivatives 87 provided coupling products in moderate to good yields.

Scheme 85. Screening of (hetero)aryl bromides for organic photoredox/nickel dual catalysis with 17

In order to illustrate the efficiency of 4CzIPN as a photocatalyst for this type of dual catalysis, a scale-up experiment was accomplished using acetoxypropylsilicate 17 and 4'bromoacetophenone 44. The reaction was performed on 3 mmol by using the usual experimental conditions except for the reaction time. Indeed, after 24 hours of reaction, only 63% yield of 45 was obtained, which could be further improved to 76% by increasing the reaction time to 48 hours (Scheme 86).

Scheme 86. Scale-up reaction

We then considered extending this methodology to alkenyl halides. These ones were already reported to be valuable electrophiles in such processes 171 and particularly with silicates. 188 Therefore, we engaged alkenyl bromides with acetoxypropylsilicate 17 under these modified conditions. Unactivated alkenyl bromides were converted to their corresponding products in moderate yields 96-98. Using activated alkenyl halides such as styryl bromide, styryl chloride and β-gem-dichlorostyrene gave fair to good yields of coupling [Ir(dF(CF 3 )ppy) 2 (bpy)](PF 6 ) (Table 8). Previously reported conditions 144b,188 led to the expected products in 75% (E/Z: 85/15) yield with the ruthenium photocatalyst (entry 2) and 90% (E/Z: 67/33) with the iridium photocatalyst (entry 3). In order to verify that the reaction does not follow a pure radical vinylation mechanism involving an addition-β-elimination tandem, the reaction was performed without nickel catalyst but only traces of products were observed. This result rules out the exclusive radical pathway. In addition, isomerization of βbromostyrene in the presence of a photosensitizer has been already reported. 189 Control experiments were realized in the absence of the nickel catalyst and silicate and showed that the bromide slightly isomerized in the presence of 4CzIPN (E/Z: 65/35) (entry 5) compared to an E/Z ratio of 40/60 with the iridium complex (entry 7). The bromide atom has brought a heavy atom effect and so an easy access to the triplet state190 which then promotes the isomerization. The photoexcited Ru(bpy) 3 2+ * did not provide any isomerization of the styryl bromide, probably due to its lower energy triplet state (entry 6). Therefore we can assume that the 4CzIPN can act as photosensitizer of styryl derivatives but the photooxidation is faster than the photoisomerisation, opening opportunities for stereoselective alkenyl-alkyl crosscoupling reactions. In conclusion, this organic dye has proved its efficiency in photoredox/nickel dual catalysis with alkyl silicates. Other works reported later the use of the 4CzIPN with alkyl silicates for the functionalization of imines,191 borylated bromoarenes, 32b and also with benzyltrifluoroborates. 32c

Alkyl bis-catecholato silicates in photoredox/nickel dual catalysis: formation of C(sp 3 )-C(sp 3 ) bonds

Alkyl bis-catecholato silicates have demonstrated to be powerful substrates for the generation of primary alkyl radicals which can be involved in radical addition reactions and nickel-catalyzed C(sp 3 )-C(sp 2 ) bonds formation. The remaining challenge is to develop a catalytic version for the C(sp 3 )-C(sp 3 ) bonds formation.

C(sp 3 )-C(sp 3 ) cross-coupling reactions involving alkyl carboxylates

In 2016, the group of MacMillan reported new reaction conditions for cross-coupling reactions between alkyl carboxylates and alkyl bromides. 192 The reported methodology involves the use of [Ir(dF(CF 3 )ppy) 2 (dtbbpy)](PF 6 ) as oxidizing photocatalyst, NiCl 2 .dme as nickel source and 4,4'-dimethoxy-2,2′-bipyridine (4,4'-dOMe-bpy) as ligand. The carboxylates obtained by reaction of the corresponding carboxylic acids with potassium carbonate reacted with various alkyl bromides under these conditions. Decarboxylation of αamino or α-oxo carboxylates gave stabilized alkyl radicals which could be coupled with primary or secondary alkyl bromides in good yields (Scheme 88). The reaction conditions were tolerated by electrophiles bearing a free alcohol, carbonyl or oxirane function. In addition, unactived primary and secondary carboxylic acids were engaged in the process, giving the coupling products in moderate yields.

Silicates as nucleophilic partners

When MacMillan and co-workers reported their advances on C(sp 3 )-C(sp 3 ) crosscoupling photoredox/nickel dual catalysis, we were developing our own catalytic system involving alkyl bis-catecholato silicates.193

Optimization of the cross-coupling reaction

We started our studies on the coupling of [18-C-6] n-hexylsilicate 2 with ethyl 4bromobutyrate 103. The catalytic system was composed with 2 mol% of [Ir(dF(CF 3 )ppy) 2 (bpy)](PF 6 ) as photocatalyst and 5 mol% of Ni(COD) 2 and 2,2'-bipyridine to produce the organometallic catalyst. After 24 hours in DMF under light irradiation with 1.2 equivalents of silicate and 1 equivalent of the electrophile 103, we were pleased to see the formation of cross-coupling product 104 in 29% yield (Table 9 entry 4). Contrary to MacMillan's results, we observed the dimerization product of the electrophile 105 in 52% yield. This byproduct has been previously reported by Weix and co-workers during their studies on the nickel-catalyzed dimerization of alkyl halides. 194 They used the same conditions as for the aryl-alkyl electrophilic cross-coupling reactions. 163,164 Based on their mechanism, they proposed (Scheme 66) the alkyl halide may furnish an alkyl radical.

Therefore two kinds of radicals would be in competition in the reaction mixture. Then, the aim was to improve the reaction conditions in order to avoid the formation of the homocoupling product and favor the cross-coupling adduct. Increasing the photocatalyst loading to 5 mol% slightly improved the 104/105 ratio (entry 1). Switching to other photocatalysts (Ru(bpy) 3 PF 6 and 4CzIPN) did not give better results (entry 2 and 3).

However, starting with 1.5 equivalents of silicates gave the best ratio of products 104/105 42/38 (entry 6) whereas changing the number of equivalents of the silicate (to 2 equiv.) (entry 7) or using a nickel(II) precatalyst proved to be less effective (entry 8). The nature of the ligand was analyzed. Indeed, the efficiency of the process has been proved to be dependent of the ligand. 192,195 Several bidentate ligands were screened (Scheme 90). Modified bipyridyl type ligands did not enhance the yield of cross-coupling product 104 and limit the formation of adduct 105. In particular, the ligand 4,4'-dOMe-bpy revealed to be ineffective. Bioxazole derivatives were also engaged as ligand but again, the reaction was less productive. Tridentate ligands were also tested. Terpyridine proved to be an effective ligand leading to 104 in 42% yield but did not avoid the dimerization (50% of 105). 2,6-Bis(oxazoline)-pyridine (pybox) gave both products in lower yields.

Scheme 90. Screening of ligands

The electrophile partner was also modified (Scheme 91). Ethyl 4-chlorobutyrate was not converted as well as the tosyl derivative while the iodo analogous afforded both products in poor yields. The triflate derivative was also tested but showed to react with the solvent DMF.

Scheme 91. Screening of electrophiles

Under the optimized reaction conditions, we envisioned that the formation of the desired product 104 might be promoted at lower concentration. Thus, we performed the crosscoupling reaction with slow addition of ethyl 4-bromobutyrate 103. The electrophile was added over 18 hours thanks to a syringe pump. Unfortunately, the homocoupling and the cross-coupling products were obtained in similar yields.

Scope of the reaction and limitations

After optimisation, various silicates were engaged in our reaction conditions. As for the aryl-alkyl cross-coupling reactions, silicates bearing a nitrile group 106 and 107, a halogen 108 and 109 or an oxirane 110 could be coupled with compound 103 (Scheme 92). Activated silicates such as benzyl or anilinomethyl silicates afforded the cross-coupling products 112 and 113. Conversely, acetoxymethyl and chloromethyl silicates did not provided any coupling product 115 and 116. Secondary cyclohexyl silicate showed also to be reactive leading to 111.

With triethylammonium cyclohexyl and cyclopentyl silicates, the reaction revealed to be poorly effective to give 111 and 114 and so, the yields of cross-coupling products were low to moderate. It should be noted that the formation of the homocoupling product observed in the optimization study, was obtained in each case with yields close to those of the cross-coupling product. It suggests that the formation of both products is simultaneous, and so two catalytic pathways are occurring at the same time with possibly a nickel intermediate involved in both processes. Finally, we also engaged different bromides with acetoxypropylsilicate 17. Primary 117-120 and secondary alkyl bromides(121-123 reacted more or less efficiently (27-37 and 16-35%, respectively). 5-Hexenyl bromide provided a mixture of direct C(sp 3 )-C(sp 3 ) cross-

Extension to other catalytic systems

The efficiency of alkyl bis-catecholato silicates to generate alkyl radicals is now well established. We expected to engage these precursors in other catalytic systems.

Concept of non-innocent ligands

In our group and in the context of the Ph.D. thesis of Jérémy Jacquet,196 it has been demonstrated that some non-innocent ligands can catalyze redox processes through the generation of radicals. In 1966, Jørgensen defined the concept of innocent ligand: ''ligands are innocent when they allow oxidation state of the central atoms to be defined' '. 197 For instance, ligands in palladium-catalyzed cross-coupling reactions are innocent.In each step of the catalysis, the oxidation state of the metal is well-defined. At the opposite, iridium and ruthenium complexes in photoredox catalysis are catalyst with non-innocent ligands because they are metal-ligand coupled redox system. In addition, some complexes exist with almost pure ligand centered redox processes. This is the case of the bis-iminosemiquinonato copper complex Cu(L SQ ) 2 125, which displays a Cu(II) and two iminosemiquinonate radical ligands (Scheme 93).

This complex can undergo two successive single electron reductions leading to the formation of the bis-anionic bis-aminophenolate copper(II) complex Cu(L AP ) 2

2-

, or two successive single electron oxidations to give the bis-cationic bis-iminobenzoquinone copper(II) complex Cu(L BQ ) 2 2+ 127 (Scheme 93). 198 This ability of single electron redox processes can be exploited to generate radicals by SET between one of the copper species and a radical precursor.

Scheme 93. Redox properties of Cu(L SQ ) 2

It was found that complex Cu(L SQ ) 2 125 can reduce electrophilic CF 3 sources to produce radicals. Spin-trapping experiments with TEMPO and stoichiometric amounts of the trifluoromethylating agent and the copper complex were realized (Scheme 94). In DCM after 3 hours, the adduct CF 3 -TEMPO can be obtained in 67% yield from the Umemoto's reagent.

The same product was obtained after 3 hours in 69% yield from the Togni II reagent in NMP.199 Inspired by these catalytic processes, we envisioned to develop a copper-mediated catalytic oxidation of alkyl bis-catecholato silicates.

Preliminary results

We first considered two properties of the alkyl bis-catecholato silicates to perform the oxidation. In one hand, these substrates are easy to oxidize given that the range of oxidation potential is +0.34-+0.90 V vs SCE in DMF. On the other hand, the oxidation process is more efficient in DMF than other solvents. With these two parameters, we expected to realize efficiently the oxidation with the complex Cu(L BQ ) 2 (OTf) 2 (E 1/2 = +0.83V vs SCE in DCM).

In addition, these types of copper complexes showed efficient in NMP, 199 which displays a similar polarity to DMF. So for this reason, we opted for this last solvent. We first looked at the possible formation of alkyl radicals from the corresponding bis-catecholato silicates by oxidation with the copper complex 127. Benzylsilicate 4 was engaged in spin-trapping experiments with TEMPO in the presence of a stoichiometric amount of complex 127. The benzyl TEMPO adduct23 was isolated in 15% yield after 24 hours in DMF (Scheme 96). The same reaction was also performed with 10 mol% of catalyst and two equivalents of TEMPO.

In this case, only 7% yield of the product was obtained. We also tested allylsulfone 31 as radical scavenger. However, with a catalytic loading or a stoichiometric amount of copper complex 127, we never observed the formation of product 128.

Conclusion

The alkyl radicals generated by photooxidation of alkyl bis-catecholato silicates could be engaged in dual-catalytic processes involving photoredox/nickel dual catalysis. Thanks to the nickel catalysis, formation of C(sp 2 )-C(sp 3 ) bonds from alkyl silicates and substituted aryl or heteroaryls could be realized with an iridium based photocatalyst. A dual-catalytic system was also developed with 4CzIPN, an organic dye as photocatalyst. These new reaction conditions showed to be as efficient compared with those previously exposed. Moreover, the methodology was extended to the formation of alkyl substituted alkenes. Finally, a dualcatalytic process for the C(sp 3 )-C(sp 3 ) bonds formation was developed with a limited efficiency due to the formation of a homocoupling adduct as side product. Finally, the ability of alkyl bis-catecholato silicates to undergo radical formation upon oxidation may also be extended to other catalytic processes such as the use of copper complexes with non-innocent ligands which require more investigations.

5 Experimental Section

General informations

Unless otherwise noted, reactions were carried out under an argon atmosphere in oven-dried glassware. THF and diethyl ether were distillated over sodium/benzophenone, triethylamine over potassium hydroxide. Catechol was purchased from commercial source and purified by crystallization from toluene followed by sublimation.

Synthesis of photocatalysts

Synthesis of 4CzIPN

The 4CzIPN has been synthesized following a previous reported procedure. [7] To a 100 mL round-bottom-flask was added NaH (60% in mineral oil) (15 mmol, 600 mg). THF (40 mL) was added followed by the slow addition of carbazole (10 mmol, 1.67 g). After 30 min of stirring at room temperature the tetrafluoroisophtalonitrile (2 mmol, 400 mg) was added and the mixture was stirred at room temperature for 20 hours. A yellow precipitate progressively appeared. Water (1 mL) was added to neutralize the excess of NaH and the mixture was evaporated to give a yellow solid. The solid was successively washed with water and ethanol.

The crude product was dissolved in the minimum of DCM and crystallized by addition of pentane to give the pure 4CzIPN (1.13g, 71% yield). The spectroscopic data are in agreement with those reported in the literature. and hexanes (~30 mL) to afford iridium µ-Cl-dimer (58%).

The spectroscopic data are in agreement with those reported in the literature.202

1 H NMR (400 MHz, CDCl 3 ): δ9.52 (s, 1H), 8.46 (dd, J = 8.8, 3.1 Hz, 1H), 8.05 (d, J = 8.3 Hz, 1H), 6.50 -6.36 (m, 1H), 5.08 (d, J = 8.9 Hz, 1H).

To a 25 mL round-bottom flask equipped with a magnetic stir bar was added iridium dimer (302 mg, 0.2 mmol) and 2,2'-bipyridine (66 mg, 0.42 mmol). The flask was attached to a reflux condenser and the flask were placed under an inert atmosphere by three evacuation/purge cycles. The reaction components were dissolved in degassed ethylene glycol (13 mL) and heated with stirring at 150 °C for 16 h. Upon cooling to rt, the reaction mixture was diluted with water (120 mL) and transferred to a separatory funnel. The aqueous phase was washed three times with hexanes (3x 60 mL), then drained into an erlenmeyer flask and heated to 85 °C for 15 min to remove residual hexanes. 8.36-8.26 (m, 2H), 8.14 (ddd, J = 5.5, 1.6, 0.8 Hz, 1H), 7.75 (dt, J = 1.9, 0.9 Hz, 1H), 7.69 (ddd, J = 7.7, 5.5, 1.3 Hz, 1H), 6.88 (ddd, J = 12.8, 9.4, 2.3 Hz, 1H).

Synthesis of silicates

General procedure A for [18-C-6] potassium silicate synthesis

To a stirred solution of catechol (2 eq.) in dry methanol (0.25 M) was added 18-C-6 (1 eq.). After dissolution of the crown ether, the trialkoxy organosilane (1 eq.) was added, followed by a solution of potassium methoxide in methanol (1 eq.). The reaction mixture was stirred for 3 hours and the solvent was removed under reduced pressure. The residue was dissolved in the minimum volume of acetone and diethyl ether was added until a cloudy solution was obtained (scrapping on the edge of the flask could be done to induce crystallization). The flask was placed at -20°C overnight. The crystals were collected by filtration, washed with diethyl ether and dried under vacuum to afford [18-C-6] potassium silicate.

General procedure B for tetraethylammonium silicate synthesis

To a stirred solution of catechol (2 eq.) in dry THF (0.1 M) was added triethylamine (4 eq.). The reaction mixture was cooled to 0°C with an ice bath and the organotrichlorosilane (1 eq.) was added dropwise. The mixture was stirred for an hour at 0°C and an additional hour atroom temperature. The triethylamine hydrochloride salt was filtered off and the filtrate was evaporated under reduced pressure. The residue was taken up in acetonitrile (0.3 M) and tetraethylammonium bromide (1 eq.) was added. The mixture was stirred for an hour and the solvent was evaporated under reduced pressure. The solid was taken up in water, filtered, washed with water and dried under high vacuum to afford the tetraethylammonium silicate.

Potassium [18-Crown-6] bis(catecholato)-cyclohexylsilicate (1)

Following the general procedure A with cyclohexyltriethoxysilane (2.5 mmol, 0.5 mL), catechol (5.0 mmol, 550.6 mg), 18-Crown-6 (2.5 mmol, 660 mg) and potassium methoxide (2.5 mmol, 700 µL of a 3.56 M solution in methanol) in 10 mL of dry methanol at room temperature. The crude product was purified according the general procedure to afford 1 (1.23 g, 78%) as a white solid. , 2844, 1486, 1454, 1351, 1267, 1100, 1011, 963, 893, 816, 739,593 cm -1 .

mmol, 700 µL of a 3.56 M solution in methanol) in 10 mL of dry methanol. The crude product was purified according the general procedure to afford 3 (800 mg, 50%) as a white solid.

1 H NMR (400 MHz, Methanol-d4): δ 6.71 -6.62 (m, 4H), 6.58 -6.52 (m, 4H), 5.69 (ddt, J = 17.0, 10.2, 6.7 Hz, 1H), 4.89 -4.82 (m, 1H), 4.80 -4.75 (m, 1H),3.54 (s, 24H), 1.96 -1.85 (m, 2H), 1.39 -1.20 (m, 4H), 0.71 -0.62 (m, 2H). 3063, 2898, 1598, 1485, 1452, 1350, 1283, 1267, 1246, 1102, 1010,963, 816, 739, 587 cm -1 .

Potassium [18-Crown-6] bis(catecholato)-benzylsilicate (4)

Following the general procedure A with benzyltriethoxysilane (2.5 mmol, 642 µL), catechol (5 mmol, 550.6 mg), 18-Crown-6 (2.5 mmol, 660 mg) and potassium methoxide (2.5 mmol, 700 µL of a 3.56 M solution in methanol) in 10 mL of dry methanol. The crude product was purified according the general procedure to afford 4 (1.35 g, 84%) as a white solid. 54.6, 54.1, 53.6, 53.4, 36.2, 33.5, 32.5, 32.5, 31.7, 31.7, 27.5, 26.4, 25.3, 24.4, 15.4, 15.1. 29 3044, 2906, 2871, 1591, 1488, 1450, 1349, 1245, 1183, 1095, 1037, 1013, 948, 822, 795, 730, 698, 596 cm -1 .

Following the general procedure B with tert-butyltrichlorosilane (10 mmol, 1.91g), catechol (20 mmol, 2.20 g), triethylamine (40 mmol, 1.4 mL) and tetraethylammonium bromide (10 mmol, 2.10 g) in 100 mL of dry THF. The crude product was purifies according the general procedure to afford 21 (1.54 g, 39%) as a white solid. , 2951, 2928, 2840, 1596, 1485, 1393, 1361, 1246, 1174, 1100,1012, 1000, 890, 815, 738, 696, 626, 595 cm -1 .

Tetraethylammonium bis(catecholato)-cyclopentylsilicate (22)

Following the general procedure B with cyclopentyltrichlorosilane (5 mmol, 830 µL), catechol (10 mmol, 1.1 g), Et 3 N (20 mmol, 2.79 mL) in 50 mL of dry THF. The counter anion metathesis was performed with Et 4 NBr (5 mmol, 1.05 g) in 20 mL of acetonitrile. The crude product was purified according the general procedure to afford 22 (1.25 g, 56%) as a white solid.

Synthesis of electrophiles 1-Bromocyclooctene

To a 50 mL round-bottom-flask was added cyclooctene (31.5 mmol, 3.5 g) and 7 mL of DCM. At 0°C was added dropwis bromine (31.5 mmol, 1.65 g) and the mixture stirred 1 hour at room temperature. The solvent was removed under reduced pressure to give 1,2dibromocyclooctane. The compound was added in 14 mL of piperidine without purification.

The mixture was heated at reflux temperature and stirred overnight. The solution was filtrated and the solid washed with pentane. The organic phase was then washed with HCl (1M, 2x70 mL), saturated NaHCO 3 (2x70 mL) and brine (100 mL), and dried over magnesium sulphate.

The solvent was removed under reduced pressure to give 1-bromocyclooctene (3.15g, 37%).

The spectroscopic data are in agreement with those reported in the literature. 8, 125.0, 35.3, 30.0, 28.8, 27.6, 26.6, 25.6.

((2-Bromoallyl)oxy)(tert-butyl)dimethylsilane

To a 50 mL round-bottom-flask was added tert-butyldimethylsilyl chloride (5.25

(E)-(2-bromovinyl)benzene

To a solution of cinnamic acid (10 mmol, 1.48 g) in methylene chloride triethylamine (0.5 mmol, 70 µL) was added at room temperature and stirred for five minutes. Nbromosuccinimide (12.0 mmol, 2.13 g,) was added in one portion and the mixture stirred for 30 minutes. The solvent was removed under reduced pressure. The crude was purified by flash column chromatography (pentane) to afford (E)-(2-bromovinyl)benzene (1.06 g, 99%).

(

2-Chlorovinyl)benzene

To a solution of (E)-3-(4-methoxyphenyl)acrylic acid (20 mmol, 3.56g) 5, 59.3, 40.1, 34.3, 32.6, 25.7, 24.8, 19.9, 17 48.2, 43 .7, 42.3, 42.0, 38.8, 36.1, 34.8, 34.5, 34.1, 30.0, 28.1, 23.0, 21.7. 3, 167.3, 140.6, 124.8, 64.4, 60.8, 31.6, 28.3, 25.0, 21.1, 14.4. IR (neat): 29953, 167.3, 140.6, 124.8, 64.4, 60.8, 31.6, 28.3, 25.0, 21.1, 14.4. IR (neat): , 29493, 167.3, 140.6, 124.8, 64.4, 60.8, 31.6, 28.3, 25.0, 21.1, 14.4. IR (neat): , 17413, 167.3, 140.6, 124.8, 64.4, 60.8, 31.6, 28.3, 25.0, 21.1, 14.4. IR (neat): , 17103, 167.3, 140.6, 124.8, 64.4, 60.8, 31.6, 28.3, 25.0, 21.1, 14.4. IR (neat): , 16303, 167.3, 140.6, 124.8, 64.4, 60.8, 31.6, 28.3, 25.0, 21.1, 14.4. IR (neat): , 13683, 167.3, 140.6, 124.8, 64.4, 60.8, 31.6, 28.3, 25.0, 21.1, 14.4. IR (neat): , 12343, 167.3, 140.6, 124.8, 64.4, 60.8, 31.6, 28.3, 25.0, 21.1, 14.4. IR (neat): , 11873, 167.3, 140.6, 124.8, 64.4, 60.8, 31.6, 28.3, 25.0, 21.1, 14.4. IR (neat): , 11463, 167.3, 140.6, 124.8, 64.4, 60.8, 31.6, 28.3, 25.0, 21.1, 14.4. IR (neat): , 1028, 944, 813cm -1 , 944, 813cm -1 Following general procedure E with 1-phenyl-2-p-toluenesulfonylethyne (1.2 mmol, 307.6 mg).The crude product was purified by flash column chromatography (pentane) to afford 39 as a colorless oil (44 mg, 78%).The spectroscopic data are in agreement with those reported in the literature. 116a

1-(Tert

1 .92 -1.85 (m, 2H), 1.79 -1.74 (m, 2H), 1.58 -1.52 (m, 3H), 1.40 -1.33 (m, 3H). 13 C NMR (75 MHz, CDCl 3 ): δ 131.7 (2 C), 128.3 (2 C), 127.5, 124.3, 94.6, 80.7, 32.9, 29.8, 26.1 (2 C), 25.1 (2 C).

(2,2-Dichlorovinyl)cyclohexane (41)

Following general procedure E with trichloroethylene (1.2 mmol, 108 µL).The crude product was purified by flash column chromatography (pentane) to afford 41 as a colorless oil (39 mg, 70%).The spectroscopic data are in agreement with those reported in the literature. 2, 118.7, 39.3, 31.8 (2 C), 25.9, 25.7 (2 C).

Dimethyl 2-cyclohexylsuccinate (43)

Following general procedure Fwith dimethyl maleate (1.2 mmol, 150 µL). The crude product was purified by flash column chromatography (pentane/ethyl acetate, 95/5) to afford 40 as a colorless oil (84 mg, 78%). The spectroscopic data are in agreement with those reported in the literature. General procedure H for photoredox/nickel cross-coupling dual catalysis with aryl/heteroaryl halide or vinyl bromide To a Schlenk flask was added aryl, heteroaryl halide or alkenyl halide (1 eq., 0.3 mmol), appropriate silicate (1.5 eq., 0.45 mmol), 4CzIPN (1 mol%, 3 µmol, 2.4 mg), and 4,4'-di-tert-1 H NMR (400 MHz, CDCl 3 ): δ 7.90 (d, J = 8.4 Hz, 2H), 7.28 (d, J = 8.4 Hz, 2H), 5.95 (ddt, J = 17.1 Hz, 10.5 Hz, 6.7 Hz, 1H), 5.13-5.08 (m, 2H), 3.45 (d, J = 6.7 Hz, 2H), 2.59 (s, 3H).

13 C NMR (100 MHz, CDCl 3 ): δ 197. 8, 145.8, 136.3, 135.3, 128.8 (2 C), 128.6 (2 C), 116.7, 40.1, 26.6. IR (neat): 3050, 1680, 1604, 1356, 1266 cm -1 .

4'-(Anilinomethyl)acetophenone (48)

Following general procedure G with anilinomethylsilicate 8 (0.45 mmol, 294 mg) and 4'-bromoacetophenone 44 (0.3 mmol, 60 mg). The crude product was purified by flash column chromatography (pentane/diethyl ether, 80/20) to afford 48as a colorless oil (62 mg, 91%).

The spectroscopic data are in agreement with those reported in the literature. 212

1 H NMR (400 MHz, CDCl 3 ): δ 7.93 (d, J = 8.4 Hz, 2H), 7.47 (d, J = 8.4 Hz, 2H), 7.17-7.15 (m, 2H), 6.75-6.71 (m, 1H), 6.66 -6.60(m, 2H) 0, 149.0, 135.1, 128.7 (2 C), 128.6 (2 C), 36.1, 31.8, 31.2, 29.0, 26.7, 22.7, 14.2. IR (neat): 2900, 1681, 1605, 1265 cm -1 .

4'-(Isobutyl)acetophenone (51)

Following general procedure G with isopropylsilicate 5 (0.45 mmol, 272 mg) and 4'bromoacetophenone 44 (0.3 mmol, 60 mg). The crude product was purified by flash column chromatography (pentane/diethyl ether, 98/2) to afford 51 as a colorless oil (39 mg, 75%).

The spectroscopic data are in agreement with those reported in the literature. 0, 147.7, 135.1, 129.4 (2 C), 128.4 (2 C), 45.5, 30.2, 29.7, 22.5 (2 C).

IR(neat): 2909, 1680, 1605, 1265 cm -1 .

214 E. V. Bellale, D. S. Bhalerao and K. G. Akamanchi, J. Org. Chem., 2008, 73, 9473-9475.

4-Acetoxypropylphenylboronic pinacol ester (73)

Following general procedure G with acetoxypropylsilicate 17 (0.45 mmol, 292 mg) and 4-bromophenylboronic acid pinacol ester (0.3 mmol, 85 mg. The crude product was purified by flash column chromatography (pentane/EtOAc, 90/10) to afford 73as a brown oil (49 mg, 53%). Following general procedure G with acetoxypropylsilicate 17 (0.45 mmol, 292 mg) and 4-bromophenylboronic acid pinacol ester (0.3 mmol, 85 mg). After 24h of reaction, the crude reaction mixture was filtered through a plug of celite, washing with THF (15 mL). The filtrate was concentrated by rotary evaporation. The resulting solution of DMF was diluted with THF (10 mL) and cooled to 0°C in an ice water bath. To the cold stirring solution was added 1M

NaOH (1.5 mL, 5 equiv.) and 30% aq. H 2 O 2 (171 µl, 5 equiv.). After 30 min the mixture was diluted with water (10 mL) and diethyl ether (10 mL) and neutralized by addition of 1M HCl (2.5 mL). The organic layer was collected and washed with water (2 x 10mL), brine (2 x 10 mL), dried over MgSO 4 and evaporated under reduced pressure. The crude product was purified by flash column chromatography (pentane/EtOAc, 90/10) to afford 74 as a brown oil (41 mg, 69%).

The spectroscopic data are in agreement with those reported in the literature. 5, 153.9, 133.2, 129.4 (2 C), 115.3 (2 C), 63.9, 31.2, 30.4, 21.0. IR (neat): 3356, 2978, 1707, 1595, 1514, 1227, 1035 cm -1 .

3-(2-Hydroxyphenyl)propylacetate (75)

Following general procedure G with acetoxypropylsilicate 17 (0.45 mmol, 292 mg) and 2-bromophenylboronic acid pinacol ester (0.3 mmol, 67 µl). After 24h of reaction, the crude reaction mixture was filtered through a plug of celite, washing with THF (15 mL). The filtrate was concentrated by rotary evaporation. The resulting solution of DMF was diluted with THF (10 mL) and cooled to 0°C in an ice water bath. To the cold stirring solution was added 1M

NaOH (1.5 mL, 5 equiv.) and 30% aq. H 2 O 2 (171 µl, 5 equiv.). After 30 min the mixture was diluted with water (10 mL) and diethyl ether (10 mL) and neutralized by addition of 1M HCl (2.5 mL). The organic layer was collected and washed with water (2 x 10mL), brine (2 x 10 mL), dried over MgSO 4 and evaporated under reduced pressure. The crude product was

4-(3-Chloropropyl)-2-fluoropyridine (81)

Following general procedure G with 3-chloropropylsilicate 12 (0.45 mmol, 281 mg) and 4-bromo-2-fluoropyridine 76 (0.3 mmol, 31µL). The crude product was purified by flash column chromatography (pentane/diethyl ether, 99/1 then 95/5) to afford 81 as a colorless oil (42 mg, 81%).

3-(2-methylpyridin-3-yl)propylacetate (82)

Following general procedure G with acetoxypropylsilicate 17 (0.45 mmol, 292 mg) and 3-bromo-2-methylpyridine (0.3 mmol, 35 µl). The crude product was purified by flash column chromatography (pentane/diethyl ether, 50/50) to afford 82 as a colorless oil (39 mg, 67%).

3-(Thiophen-3-yl)propyl acetate (89)

Following general procedure G with acetoxypropylsilicate 17 (0.45 mmol, 292 mg) and 3-bromothiophene (0.3 mmol, 29 µl). The crude product was purified by flash column chromatography (pentane/diethyl ether, 90/10) to afford 89 as a colorless oil (28 mg, 50%). 1, 143.9, 126.8, 124.5, 123.2, 63.5, 30.5, 26.3, 20.9 7.74 -7.72 (m, 1H), 7.55 -7.46 (m, 2H), 7.42 -7.39 (m, 1H), 7.34 -7.32 (m, 1H), 4.17 (t, J = 6.5 Hz, 2H), 3.18 -3.15 (m, 2H), 2.14 -2.07 (m, 2H), 2.09 (s, 3H). 13 C NMR (100 MHz, CDCl 3 ): δ 171. 2, 137.2, 133.9, 131.8, 128.8, 126.9, 126.1, 125.9, 125.5, 125.5, 123.6, 64.1, 29.5, 29.3, 21.0. IR (neat): 20522, 137.2, 133.9, 131.8, 128.8, 126.9, 126.1, 125.9, 125.5, 125.5, 123.6, 64.1, 29.5, 29.3, 21.0. IR (neat): , 29532, 137.2, 133.9, 131.8, 128.8, 126.9, 126.1, 125.9, 125.5, 125.5, 123.6, 64.1, 29.5, 29.3, 21.0. IR (neat): , 19302, 137.2, 133.9, 131.8, 128.8, 126.9, 126.1, 125.9, 125.5, 125.5, 123.6, 64.1, 29.5, 29.3, 21.0. IR (neat): , 17332, 137.2, 133.9, 131.8, 128.8, 126.9, 126.1, 125.9, 125.5, 125.5, 123.6, 64.1, 29.5, 29.3, 21.0. IR (neat): , 15922, 137.2, 133.9, 131.8, 128.8, 126.9, 126.1, 125.9, 125.5, 125.5, 123.6, 64.1, 29.5, 29.3, 21.0. IR (neat): , 13602, 137.2, 133.9, 131.8, 128.8, 126.9, 126.1, 125.9, 125.5, 125.5, 123.6, 64.1, 29.5, 29.3, 21.0. IR (neat): , 12342, 137.2, 133.9, 131.8, 128.8, 126.9, 126.1, 125.9, 125.5, 125.5, 123.6, 64.1, 29.5, 29.3, 21.0. IR (neat): , 1040, 738 cm -1 , 738 cm -1 .

HRMScalc.for [C 15 H 16 NaO 2 ] + 251.1043; found 251.1041.

3-(Phenanthren-9-yl)propylacetate (94)

Following general procedure H with acetoxypropylsilicate 17 (0.45 mmol, 292 mg) and 9-bromophenanthrene (0.3 mmol, 77 mg). The crude product was purified by flash column chromatography (pentane/diethyl ether, 95/5) to afford 94 as a colorless oil (38 mg, 45%). 1 H NMR (400 MHz, CDCl 3 ): δ 4.12 (q, J = 7.1 Hz, 2H), 2.30-2.26 (m, 2H), 1.65 -1.58 (m, 2H), 1.35 -1.25 (m, 12H), 1.25 (t, J = 7.1 Hz, 3H), 0.88 (t, J = 6.8 Hz, 3H). 13 C NMR (100 MHz, CDCl 3 ): δ 174. 1, 60.3, 34.6, 32.0, 29.6, 29.4, 29.4, 29.3, 25.2, 22.8, 14.4, 14.2.

NOESY of 102

Diethyl octanedioate (105)

Following general procedure I with ethyl 4-bromobutyrate 103 (0.3 mmol, 43 µl) and hexylsilicate 2 (0.45 mmol, 284 mg). The crude product was purified by flash column chromatography (pentane/diethyl ether, 90/10) to afford 105 as a colorless oil (26 mg, 38%).

The spectroscopic data are in agreement with those reported in the literature. The spectroscopic data are in agreement with those reported in the literature. 224 resulting dark-red solution was stirred for 1 h at room temperature. After evaporation of the solvent, the residue was triturated in hexane and filtered to afford a red-brown solid (quantitative). This Cu(L BQ ) 2 Br 2 complex (249 mg, 0.31 mmol, 1 equiv.) and silver triflate (157 mg, 0.62 mmol, 2 equiv.) were introduced into a Schlenk flask under an argon atmosphere. Degassed acetonitrile (5 mL) was added and the resulting mixture stirred for 2 h at room temperature. The suspension was filtered, and the filtrate was evaporated. The residue was dissolved in DCM and filtered. The filtrate was concentrated to give complex 127 (245 mg, 83%).

The UV-vis data were consistent with those reported in the literature. 106

UV-vis [CH 2 Cl 2 ; λ nm (ε, M -1 .cm -1 )]: 290 (14100), 434 (6970), 504 (7370) of silicates by a bis-iminobenzoquinone copper(II) complex are promising but still require more investigations.

The alkyl bis-catecholato silicates previously used as Lewis-acid proved to be excellent radical precursors upon photooxidation and thus versatile substrates for synthetic applications. Photoredox catalysis in visible light has succeeded in establishing itself as a gentle and ecocompatible method of formation of radical species, and more particularly of carbon radicals.

Although this catalysis has proved to be efficient for the formation of carboncarbon/heteroatom bonds, the generated carbon centered radicals are oftenverystabilized.Conversely, bis-catecholato silicates have shown to be capable of generating alkyl radicals that are not stabilized by photooxidation using polypyridine complexes of transition metals (Ru, Ir) that are photoactive in visible light but also organic photocatalysts. The radicals formed can thus be trapped by various radical acceptors. In addition, the bis-catecholato silicates can be employed in cross-coupling reactions with alkene halides and (hetero)aromatic halides under photoredox/nickel dual catalysis conditions for the formation of C(sp 2 ) -C(sp 3 ) bonds. The methodology can also be extended to C(sp 3 ) -C (sp 3 ) with some limitations. On the other hand, a study comparing the silicates and the "atecomplex" of boron for the formation of radicals by oxidation process is presented. Finally, promising works on the oxidation of silicates by copper complexes bearing non-innocent ligands have been initiated. Key words: Photoredox catalysis, Dual catalysis, photooxidation, photocatalyst, silicate, radical, cross-coupling

5 Schéma 1 .Schéma 2 .Schéma 3 .

 5123 photoréduction ont rapportés la formation de radicaux alkyles stabilisés, notamment par réduction de α-halo-esters, 6 de dérivés d'acides 7 ou d'époxydes 8 par exemple (Schéma 2).

Schéma 4 .

 4 Photooxydation d'α-amino acides et d'organotrifluoroborates Dans notre groupe, nous avons montré que des espèces hypervalentes du silicium en conditions de photooxydation peuvent former des radicaux carbonés non stabilisés. Les alkyles bis-catécholato silicates dont les premières synthèses ont été effectuées par Frye 12 puis

Schéma 5 . 14 Schéma 6 . 16 Schéma 8 .

 5146168 photoréduction, des travaux ont ensuite montré la possibilité de coupler des électrophiles aromatiques avec des radicaux engendrés par photooxydation, grâce à des complexes de nickel. Les premiers travaux sur ce type de couplage ont été réalisés de façon simultané par Molander en utilisant des benzyltrifluoroborates15 comme source de radicaux d'une part, et d'autre part à partir de α-amino carboxylates et anilines grâce aux études de MacMillan et Doyle.16

Schéma 11 . 21 Schéma 12 . 21 Schéma 13 .

 1121122113 photocatalyseur organique.21

  Photoredox catalysis, an opportunity for sustainable

Figure 1 .

 1 Figure 1. Bio-catalyzed dehydroxylation of alcohols

Figure 2 .

 2 Figure 2. Number of publications on photoredox catalysis since 2000

  Scheme 2. a) Simplified molecular orbital diagram for Ruthenium and Iridium photocatalysts. b) Simplified energy diagram of absorbing and emitting processes.

Figure 3 .

 3 Figure 3. Electronic absorption spectrum of [Ru(bpy) 3 ](PF 6 ) 2 in EtOH.

Scheme 10 . 1 . 2 . 4 . 2

 101242 Scheme 10. a) Photocatalyzed Pschorr reaction. b) Photocatalyzed Meerwein arylation

Scheme 23 . 2 . 3

 2323 Scheme 22. Cobalt-mediated radical reactions

Figure 4 . 2 . 4 . 1

 4241 Figure 4. Modes of photoredox/transition metal catalysis

Scheme 40 .

 40 Scheme 40. Photoredox/copper-catalyzed trifluoromethylation and perfluoroalkylation of boronic acids

Figure 5 .

 5 Figure 5. General representation of organic silicates

1

 1 Photo-induced Electron Transfer (PET) between the benzil and the bis-catecholato allylsilicate. To have a better mechanistic insight of the reaction, he performed solvent studies 138 and found that coordinating solvents such as DMF or methanol or additives (pyridine, imidazole, n-butylamine, DMF) drastically improved the yield of the reaction.

  144 (a) V. Corcé, L.-M. Chamoreau, E. Derat, J.-P. Goddard, C. Ollivier and L. Fensterbank Angew.Chem. Int.Ed. 2015, 54, 11414-11418. (b) C.Lévêque, L. Chenneberg, V. Corcé, J.-P. Goddard, C. Ollivier and L.Fensterbank, Org. Chem. Front., 2016, 3, 462-465.

  Scheme 51. a) Hybridization of silicon orbitals. b) Molecular orbital diagram of 3c-4e interaction.

Figure 6 .

 6 Figure 6. Crystal structures of potassium cyclohexyl bis-catecholato silicates with acetone (left 1') and [18-C-6] (right 1)

Figure 8 .

 8 Figure 8. Valence tautomerism of bis-(3,6-ditertbutylcatecholato) silicate radicals

E 1 / 2 (

 12 photooxidation of the silicates.

Scheme 62 .

 62 Scheme 62. Nickel-catalyzed cross-electrophile coupling of aryl halides and alkyl halides

Figure 9 .

 9 Figure 9. Evolution of cyclized/linear products depending on the concentration of nickel catalyst

  NMR yield.b Isolated yield At first, the compatibility of several types of alkyl silicates was explored with 4'bromoacetophenone (Scheme 84). As previously reported conditions, α-amino and α-oxo alkyl silicates provided the products 48 and 49 in good yields. When the chloromethyl silicate was used as radical precursor, no product formation was observed. Aliphatic silicates bearing a phosphine oxide, an ester or an alkene function were as well tolerated. Concerning the phosphine oxide silicate 16, and as previously mentioned no product resulting from the generation of a P-centered radical was observed. 5-Hexenyl silicate gave rise a mixture of products 91, 91' and 91'' in a 10/13/77 ratio and 88% overall yield. Here, the intermediate 5hexenyl radical would directly react to provide products 91 and 91' resulting from postisomerization of 91.187 Interestingly, the formation of the major product 91'' is attributed to a 5-exo-trig cyclization of the radical prior to the addition to the nickel intermediate in the 187 (a) R. G. Miller, P. A. Pinke, R. D. Stauffer, H. J. Golden and D. J. Baker, J. Am.Chem. Soc., 1974, 96, 4211-4220. (b) F. Weber, A. Schmidt, P. Ro¨se, M. Fischer, O.Burghaus and G. Hilt, Org. Lett., 2015, 17, 2952-2955.

products 99 -

 99 Scheme 87. Reaction of silicates with alkenyl halides

Scheme 88 .

 88 Scheme 88. Dual photoredox/nickel catalyzed sp 3 -sp 3 coupling reaction between alkyl carboxylates and alkyl bromides

  195 F. Lima, M. A. Kabeshov, D. N. Tran, C. Battilochio, J. Sedelmeier, G. Sedelmeier, B. Schenkel, S. V.Ley, Angew. Chem. Int. Ed., 2016, 55, 14085-14089.

Scheme 94 .Scheme 95 .

 9495 Scheme 94. Spin-trapping of CF 3 ·radical experiments with TEMPO

  Scheme 97. Envisioned mechanism for the copper-mediated oxidation of silicates

(

  round-bottom flask equipped with a magnetic stir bar was added 2-(2,4difluorophenyl)-5-(trifluoromethyl)pyridine (400 mg, 1.50 mmol) and IrCl 3 hydrate (209 mg, 0.70 mmol). The flask was equipped with a cold water condenser and evacuated and purged with argon three times. A degassed mixture of methoxyethanol/water (2/1) (12 mL) was added and the resulting mixture stirred at 120°C for 20 h, during which time a yellow precipitate was observed to form. After cooling to rt, 10 mL of water were added and the precipitate was collected by vacuum filtration. The solid was washed with water (2x20 mL)

  116a

1 H

 1 NMR (300 MHz, CDCl 3 ): δ 5.70 (d, J = 9.2 Hz, 1H), 2.43 -2.32 (m, 1H), 1.77 -1.62 (m,5H), 1.37 -1.04 (m, 6H). 13 C NMR (75 MHz, CDCl 3 ): δ 135.

  116a

  photoredox/nickel cross-coupling dual catalysisTo a Schlenk flask was added aryl or heteroaryl halide (1 eq., 0.3 mmol), silicate(1.5 eq., 0.45 mmol),Ir[(dF(CF3)ppy)2(bpy)](PF6) (2 mol %, 6 µmol, 6 mg), and 4,4'-di-tert-butyl-2,2'bipyridine (3 mol %, 9 µmol, 2.4 mg). The Schlenk flask was taken into a glovebox and Ni(COD)2 (3 mol %, 9 µmol, 2.5 mg) was added. The Schlenk flask was sealed with a rubber septum, removed from the glovebox, and evacuated / purged with vacuum / argon three times.Degassed DMF (3 mL) was introduced (followed by the aryl or heteroaryl halide if liquid) and the reaction mixture was irradiated with blue LEDs (477 nm) for 24 hours. The reaction mixture was diluted with diethyl ether (50 mL), washed with saturated NaHCO3 (2 times), brine (2 times), dried over MgSO4 and evaporated under reduced pressure. The residue was purified by flash column chromatography on silica gel to afford the cross-coupling product.

  214

1 H

 1 NMR (400 MHz, CDCl 3 ): δ 7.87 (d, J = 8.3 Hz, 2H), 7.23 (d, J = 8.3 Hz, 2H), 2.58 (s, 3H), 2.53 (d, J = 7.2 Hz, 2H), 1.93 -1.87 (m, 1H), 0.91 (d, J = 6.6 Hz, 6H). 13 C NMR (100 MHz, CDCl 3 ): δ 198.

  223

1 H 9 (

 19 NMR (400 MHz, CDCl 3 ): δ 4.12 (q, J = 7.1 Hz, 4H), 2.28 (t, J = 7.5 Hz, 4H), 1.66-1.58 (m, 4H),1.35 -1.31 (m, 4H), 1.25 (t, J = 7.1 Hz, 6H). 13 C NMR (100 MHz, CDCl 3 ): δ 173.Following general procedure I with ethyl 4-bromobutyrate 103 (0.3 mmol, 43 µl) and cyanoethylsilicate 13 (0.45 mmol, 271 mg). The crude product was purified by flash column chromatography (pentane/diethyl ether, 80/20) to afford 106 as a colorless oil (10 mg, 20%).

  Les premières expériences d'optimisation nous ont permis d'obtenir le produit de couplage entre le 4-bromobutyrate d'éthyle et le n-Une fois encore les rendements en produits de couplage croisé sont demeurés faibles. En outre, le bromure de hex-5-ényle a conduit à la formation de trois produits de couplage croisé dont un produit de cyclisation. Ce dernier est probablement le résultat d'une cyclisation radicalaire 5-exo-trig. Ce résultat nous a laissé penser qu'un radical hex-5-ényle a pu être formé durant la réaction, ce qui signifie que nos conditions réactionnelles permettent alors la formation d'un radical à partir des bromures d'alkyles. Ainsi, deux radicaux sont en compétition lors de la réaction de couplage, ce qui expliquerait la formation du produit d'homocouplage. Des études mécanistiques plus approfondies permettraient de mieux rationaliser ces résultats. Dans notre cas, nous avons tenté d'utiliser ce type de complexe pour engendrer des radicaux alkyles par oxydation des alkyles bis-catécholato silicates. Les silicates possèdent un potentiel d'oxydation relativement bas (~0.3 -0.9V vs SCE). Par rapport à ces valeurs, le Actuellement, des travaux sont à l'étude sur l'oxydation des silicates par des complexes métalliques avec des ligands non-innocents. Des résultats préliminaires ont montré que le complexe de cuivre Cu(L BQ ) 2 (OTf) 2 en quantité catalytique peut former un radical α-aminé à partir du silicate correspondant et que ce radical peut être piégé en réaction d'allylation.

	complexe Cu(L BQ ) 2 (OTf) 2 (L BQ = iminobenzoquinone) a donc été choisi (+0.83V vs SCE). Des
	premières expériences de piégeage par le TEMPO ont été réalisées. En conditions catalytiques
	hexylsilicate avec un rendement modéré (34%). Ce résultat s'explique par la formation d'un ou stoechiométriques dans le DMF, de faibles de rendements d'adduit benzyl-TEMPO ont été
	produit secondaire qui s'est avéré être le produit d'homocouplage du bromure utilisé avec un obtenus. De plus, la présence d'allylsulfone à la place du TEMPO n'entraine aucune formation
	rendement similaire (38%). Plusieurs axes d'optimisation ont été explorés pour favoriser la de produit d'allylation.
	formation du produit de couplage croisé par rapport au produit d'homocouplage. Les
	modifications apportées sur la nature du ligand complexé au nickel, du photocatalyseur, des

charges catalytiques ou encore du solvant de réaction, n'ont pas permis de former exclusivement le produit de couplage croisé. Dans les meilleures conditions, le produit attendu a pu être obtenu avec un rendement de 43% et le produit secondaire avec un rendement de 38%. Schéma 16. Conditions optimisées de catalyse duale photoredox/nickel pour la formation de liaisons C(sp 3 )-C(sp 3 ) Après optimisation des conditions réactionnelles, plusieurs alkyles silicates ont été engagés en présence du 4-bromobutyrate d'éthyle comme partenaire électrophile. Malheureusement, pour l'ensemble des silicates engagés dans le processus catalytique, le produit d'homocouplage a toujours été isolé avec un rendement proche de ceux des produits de couplage croisé, lesquels restent encore faibles. Par la suite, le partenaire électrophile a été modifié. Plusieurs bromures primaires et secondaires ont été couplés avec l'acétoxypropylsilicate. Schéma 17. Catalyse duale photoredox/nickel: couplage croisé entre des alkyles silicate et le 4-bromobutyrate d'éthyle Schéma 18. Catalyse dual photoredox/nickel: couplage croisé entre l'acétoxypropyl silicate et divers bromures d'alkyles Après avoir démontré tout le potentiel des alkyles bis-catécholato silicates pour la formation de liaisons carbone-carbone en catalyse photoredox ainsi qu'en catalyse duale photoredox/nickel, nous nous sommes tournés vers un autre système catalytique mettant aussi en jeu des transferts monoélectroniques. Une thématique développée au sein de notre laboratoire, concerne la catalyse par des métaux de transition portant des ligands noninnocents. Un ligand est considéré non-innocent lorsque le degré d'oxydation du métal, sur lequel il est complexé, ne peut être clairement défini. En particulier, le complexe bis(iminosemiquinonate) de cuivre (Cu(L SQ ) 2 ) possède la particularité d'être un complexe sur lequel les deux ligands ont un caractère radicalaire. Chaque ligand pouvant être oxydé ou réduit, le complexe peut exister sous cinq formes différentes. 23 Schéma 19. Propriétés redox du complexe Cu(L SQ ) 2 Des travaux réalisés au sein de notre équipe ont montré que le complexe Cu(L SQ ) 2 utilisé en quantité catalytique pouvait former des radicaux CF 3 · par réduction d'une source de CF 3 électrophile tel que le réactif d'Umemoto ou le réactif de Togni II. Ce dernier a notamment été utilisé pour former ces radicaux CF 3 · et les engager dans des processus d'addition radicalaire sur des alcènes, des alcynes, des éthers d'énols silylés ainsi que sur des hetérocyles tels que le pyrrole ou le furane. 24 Schéma 20. Piégeage des radicaux CF 3 · formés par réduction du réactif de Togni II avec le complexe Cu(L SQ ) 2 . Schéma 21. Oxydation du benzylsilicate par le complexe Cu(L BQ ) 2 (OTf) 2 Par la suite, nous avons remplacé de solvant DMF par le dichlorométhane ainsi que le précurseur radicalaire par l'anilinométhyle silicate, étant plus facile à oxyder. En présence d'allylsulfone comme accepteur radicalaire, nous avons pu cette fois-ci observer la formation du produit d'allylation avec un rendement moyen de 48%. Après quelques étapes d'optimisation (augmentation de la charge catalytique en complexe de cuivre, de la quantité d'accepteur et de la concentration), un rendement de 66% en produit d'addition radicalaire a pu être atteint. Même si l'optimisation et l'exploration de cette nouvelle voie d'oxydation des silicates restent encore à développer, ces résultats préliminaires sont prometteurs. Schéma 22. Résultats préliminaires d'addition radicalaire sur l'allylsulfone En conclusion, les travaux réalisés au cours de cette thèse se sont centrés sur la transformation de l'énergie lumineuse en énergie chimique avec la formation d'espèces radicalaires à partir de substrats organiques. Dans notre cas, nous nous sommes intéressés plus particulièrement à la formation de radicaux alkyles primaires par photooxydation d'alkyles bis-catécholato silicates. Ces espèces présentant un atome de silicium hypervalent possèdent plusieurs avantages en terme de synthèse, de stabilité mais aussi de faibles potentiel d'oxydation, ce qui en fait des précurseurs de choix pour la génération de radicaux par photooxydation. En effet, l'oxydation de ces espèces par des photocatalyseurs métalliques d'iridium ou de ruthénium photoactivés a permis de former des radicaux carbonés stabilisés ou non. Des radicaux primaires, secondaires et tertiaires ont pu être engagés dans des réactions d'addition radicalaire en conditions catalytiques. Par la suite, les études ont montré que ces radicaux pouvaient être piégés par des complexes de nickel et couplés avec des électrophiles aromatiques ou hétéroaromatiques en conditions de catalyse duale, avec de bon rendements. Afin de rendre ce type de catalyse plus éco-compatible, nous avons donc cherché à remplacer le photocatalyseur d'iridium par un organophotocatalyseur. Le 4CzIPN a démontré qu'il pouvait photooxyder les silicates avec la même efficacité que les complexes métalliques. En catalyse duale, cet organophotocatalyseur nous a permis d'obtenir des produits de couplage croisé avec des bromures aromatiques ou hétéroaromatiques mais aussi avec des bromures éthyléniques. Enfin, le couplage alkyl-alkyl en conditions de catalyse duale avec les silicates a pu être réalisé, mais malheureusement avec de faibles rendements.

1.2.2.2 Molecular orbital approach and redox potentials

	excited state lifetimes of few micro-secondes have shown optimal features for photoredox E([M]/[M -]) and E λ λ λ λem (Scheme 4. left). Also, the energy necessary to oxidize the
	catalyzed processes. 26 photocatalyst at the excited-state E
	In this part, we will focus on the description of metal-based photocatalysts and
	especially on the Ru(bpy) 3	2+ which is the most studied. However, the mentioned statements
	are applicable to homo-and heteroleptic iridium photocatalysts. As previously detailed,
	ruthenium and iridium photocatalysts reach a triplet state ( 3 MLCT) after visible-light
	absorption. About the Ru II (bpy) 3	2+ , it corresponds to the promotion of an electron from the t 2 g
	orbitals of the metal to a π* orbital of the ligand. The excited complex can be formally written
	as ''[Ru III (bpy) 2 (bpy) -] 2+ '', equivalent to [Ru II (bpy) 3	2+]* where respectively the metal becomes
	oxidant and a ligand bpy reductant (Scheme 3).
	Scheme 3. Simplified orbital diagram of Ru(bpy) 3	2+ during light absorption
	Recent work on the development of OLEDs has shown new perspective in The photoexcited complex [M] can then oxidize or reduce a substrate by a SET giving
	terms of organic photocatalysts design. 25 Thanks to a short gap between singlet and triplet the opportunity to generate radicals. In order to have an idea on the tolerant substrates for the
	state and efficient spin conversion processes, a series of tetracarbazolyl dicyanobenzene with SET, the redox potentials of photocatalysts at the excited-state have to be determined. From
	electrochemical and fluorescence data, a qualitative estimation of the excited-state redox
	potentials can be given. The minimum difference of energy between the ground state of the
	catalyst and its excited state corresponds to the wavelength of the emission maximum E λ λ λ λem .
	Therefore, the required energy to reduce the photocatalyst in the excited state E([M]*/[M -])
	equals the sum of the required energy to reduce the photocatalyst in the ground state

([M] + /[M*]) equals

  the difference between the energy necessary to oxidize the photocatalyst in the ground state E

([M] + /[M]) and E λ

	substrate D· + and Ru(bpy) 3	+ (Scheme 5). In the case of an acceptor A, if the LUMO is in the
	same energy range, the electron transfer would form the radical anion A· -and Ru(bpy) 3	3+
			λ λ λem (Scheme 4.
	right). However, an electrostatic work term w r , describing the charge generation and
	separation within the electron-transfer complex, must be taken in account but is difficult to
	estimate. Thus the calculated potentials are slightly over-or underrated, but experimental
	values can be measured 27 by phase-modulated voltammetry, showing a good match with the
	calculated ones. Fine tuning of the redox potential can be realized changing the ligand
	(bipyridine, bipyrazine, phenanthroline…) or modifying the metal. In this context,
	heteroleptic iridium complexes are particularly interesting due to their two ligands type
	(cyclometalating ligand and bidentate ligand) which can be both modified.
	Scheme 5. SET between excited Ru(bpy) 3	2+ and donor or acceptor.
	Scheme 4. Calculation of the redox potential of photocatalysts at the exited-state and
		comparison with the measured values.
	Therefore, the photocatalyst excited state may pick up or give out an electron to a
	substrate if the redox potentials match. In terms of orbital, if the HOMO of a donor D is
	between the t 2 g and π* orbitals of the Ru(bpy) 3	+2 , a SET can happen giving the oxidized

• Photocatalyzed Barton-McCombie deoxygenation

	Under these conditions, tertiary and secondary alcohols could be deoxygenated in moderate
	yields (Scheme 13 d)).
	A famous reaction generating alkyl radicals is the Barton-McCombie deoxygenation
	reaction. Secondary or tertiary alcohols are converted to thiocarbonate, thiocarbamate or
	xanthate derivatives which are afterwards reduced by a tin-hydride reagent (nBu 3 SnH).
	During the process, the C-O bond of the modified alcohol undergoes fragmentation, to give an
	alkyl radical. After a hydrogen abstraction step from the tin-hydride reagent, the
	deoxygenated product and the reactive tin mediator (nBu 3 Sn·) are obtained. 48 In order to
	avoid the use of tin reagents, some groups have tried a photocatalytic version. Similarly to the Scheme 15. Evolution of N-centered radical cations
	classical conditions of the Barton-McCombie deoxygenation, alcohols have to be activated as In 2012, Zheng and co-workers showed that an N-centered radical cation can evolve to
	an ester, an oxalate or a thiocarbamate (Scheme 13 a)).
	In 2013, inspired by the N-(acyloxy)phthalimides photocatalyzed decarboxylation of
	Okada, 49 Overman 50 reported the efficient deoxygenation of aliphatic tertiary alcohols by
	photoreduction of the corresponding N-(acyloxy)phthalimide oxalates derivatives in the
	presence of photocatalyst Ru(bpy) 3 (PF 6 ) 2 , Hantzsch ester and i-Pr 2 NEt.HBF 4 as a sacrificial
	electron donor and a H-atom donor respectively. The generated tertiary alkyl radicals could
	then be then engaged in Giese-type reactions (Scheme 13 b)). Although no example involving
	secondary or primary radicals were mentioned, Reiser et al. reported the photoreduction of
	3,5-bis(trifluoromethyl)benzoate esters. 51 Starting with the reduction of the photoexcited
	[Ir(ppy) 2 (dtbbpy)](PF 6 ) by DIPEA, benzyl, α-carbonyl and α-cyano esters were reduced by
	the [Ir(II)] complex to give the deoxygenated products in good yields (Scheme 13 c)). At the
	same time in our group, a procedure involving the reduction of O-thiocarbamates was
	developed. 52 The high reductive potential of such derivatives (-1.56V to -1.73V vs SCE)
	required the use of the highly reducing photoexcited fac-Ir(ppy) 3 photocatalyst Scheme 13. Photoreductive Barton-McCombie type deoxygenation reactions.
	• Reduction of α α α α-ketoepoxides and α α α α-ketoaziridines
	Our group reported in 2011 the photoreduction of alpha-ketoepoxides and
	ketoaziridines 53 to generate an alpha carbonyl radical intermediate that can be trapped by
	allylsulfones. This transformation requires the use of [Ir(ppy) 2 (dtbbpy)](PF 6 ) that absorbs in

the formation of carbon-carbon bonds. After oxidation of Ν-cyclopropylanilines, the cyclopropane ring opening led to an intermediate bearing an iminium moiety and a primary alkyl radical. The radical then adds to an alkene or an alkyne substituted by an aryl or an ester moiety.

57

The resulting stabilized radical cyclizes to the iminium to give the N-cyclopentyl (or N-cyclopentenyl) aniline (Scheme 16).

Scheme 16. Photocatalyzed intermolecular [3+2] cycloaddition of N-cylopropylanilines

57 (a) S. Maity, M. Zhu, R. S. Shinabery and N. Zheng, Angew. Chem.

Int. Ed., 2012, 51, 222-226. (b)

T. H. Nguyen, S. A.

Morris and N. Zheng, Adv. Synth. Catal., 2014, 356, 2831-2837. (c)

T. H. Nguyen, S. Maity and N. Zheng, Beilstein J.

Org. Chem., 2014, 10, 975-980.

  61,62 They were also engaged in vinylation reactions.

63

Oxidation of the carboxylate by photoexcited

[Ir(dF(CF 3

)ppy) 2 (dtbbpy)](PF 6 ) catalyst gives the carbon-centered radical which is directly trapped by a vinylsulfone. After the C-SO 2 Ph bond fragmentation, the sulfonyl radical (E 1/2 (PhSO 2 •/PhSO 2 Na)=+0.50V vs SCE)

64

participates to the regeneration of the photocatalyst via a SET with the Ir II intermediate (Scheme 19).

Scheme 19. Photooxidation of α-aminocarboxylates • Oxidation of organoborates

In the same line, alkyl trifluoroborates have proved to be suitable substrates to generate radicals upon oxidation.

65

Akita and Koike reported their work on the photooxidation of organoborates with the [Ir(dF(CF 3 )ppy) 2 (bpy)](PF 6 ) catalyst.

66

In order to prove the photooxidation feasibility, they performed spin-trapping experiments with TEMPO and found that allyl, benzyl and tertiary alkyl trifluoroborates could form the corresponding alkyl-65 (a) Y. Nishigaichi, T. Orimi and A. Takuwa, J.

Organomet. Chem., 2009, 694, 3837-3839. (b)

G. Sorin, R. Martinez Mallorquin, Y. Contie, A.

Baralle, M. Malacria, J.-P. Goddard and L. Fensterbank, Angew. Chem. Int. Ed., 2010, 49, 8721-8723. (c) G. A. Molander, V. Colombel and V. A. Braz, Org. Lett., 2011, 13, 1852-1855. 66

Y. Yasu, T.

Koike and M. Akita, Adv. Synth. Catal., 2012, 354, 3414-3420.

2 Processes with radical formation: Catalysis of Downstream Steps

  Based on an article of van Leeuwen 112 who reported the coupling reaction between anilidines and olefins with Pd(OAc) 2 as catalyst and benzoquinone as oxidant, they chose the photooxidizing catalyst [Ir(ppy) 2 (bpy)](PF 6 ) to perform this catalytic process. Starting from (Z)-(phenylamino)but-2-enoate derivatives in the presence of Pd(OAc) 2 and potassium carbonate, they could obtain the formation of indoles in good to excellent yields. Control experiments showed that no reaction occurred under oxygen atmosphere. However, the presence of potassium superoxide proved to be essential. Based on these informations, the authors proposed a mechanism in which Pd(OAc) 2 did two consecutive C-H insertions. After reductive elimination, the indole is obtained and a Pd(0) complex re-oxidized simultaneously by superoxide and the photoexcited iridium complex.Others photoredox/transition-metal dual catalysis processes involving two SETs between both catalysts were developed. Each SET allows the organometallic catalyst to reach the specific oxidation state required for the catalysis. Tunge and co-workers reported a decarboxylative allylation of para-amino allyl 2-phenylacetate in the presence of Pd(PPh 3 ) 4 as transition metal catalyst and [Ir(ppy) 2 (bpy)](BF 4 ) as photocatalyst.116 They could obtain parahomoallylaniline products in moderate yields. Alternatively starting the reaction from carboxylic acids and allyl methyl carbonate in stoichiometric amounts provided the products in the same range of yields. During the optimization, the authors noted the formation of a Regarding the mechanism, they proposed an oxidative addition of the aryl bromide on the Ni(0)(dtbbpy) active catalyst. A ligand exchange with the alcohol would then generate the Ni(II) aryl alkoxide. At this stage, the crucial oxidation to Ni(III) was carried out by the photoexcited iridium complex (E 1/2 (Ni III /Ni II+ ) = +0.71V vs Ag/AgCl 119 and E 1/2 (Ir III */Ir II ) = +1.21V vs SCE). The Ni(III) aryl alkyloxide complex then underwent reductive elimination to give the aryl ether and a Ni(I) complex reduced by the Ir(II) intermediate to regenerate both catalysts.Over the last years, gold catalysis has emerged as a powerful tool for the formation of

	• Two successive SETs catalysis 2.4.Catalysis of downstream steps involves radical formation by photoredox catalysis.
	This chemical species adds on the organometallic transition metal, generating a reactive
	metallic intermediate which can undergo reductive elimination or participate to electrophilic
	activation of multiple bonds for instance.
	2.4.2.1 Gold mediated catalysis
	Scheme 34. Photoassisted nickel catalyzed C-O bond formation
	Scheme 31. Photocatalyzed Fujiwara-Moritani reaction

• C-H Functionalization Among the photoredox/transition metal dual catalytic reactions reported in the literature, C-H functionalization has been particularly studied. Rueping and co-workers developed another approach to oxidative Heck Reaction so called Fujiwara-Moritani reaction under photoredox conditions.

111

The Ir(II) photocatalyst transfers its excess of electron to a molecule of oxygen, regenerating the photocatalyst Ir(III) and providing a superoxide anion (Scheme 31).

dibenzyl product suggesting the formation of a benzyl radical in the process. In addition, they observed the formation of a 1,5-hexadiene resulting from the presence of an allyl radical. So they suggested a plausible mechanism involving a benzyl-allyl radical coupling. They proposed the oxidative addition of the allyl ester to Pd(0) to form a Pd-π-allyl complex and a carboxylate anion (Scheme 33). The photoactivated iridium catalyst oxidizes the carboxylate which undergoes decarboxylation to furnish the benzyl radical. Thereafter, the Pd-π-allyl cationic complex is reduced by the Ir(II) intermediate to provide the allyl radical and regenerate both catalysts. 116 S. B. Lang, K. M. O'Nele and J. A. Tunge, J. Am. Chem. Soc., 2014, 136, 13606-13609. Scheme 33. Photocatalyzed decarboxylative allylation of phenylacetate derivatives Nickel also proved to be a valuable candidate to photoredox/transition metal dual catalysis. MacMillan developed an efficient intermolecular cross-coupling reaction between arylbromides and alcohols involving photoredox/nickel dual catalysis. Studies on the nickel catalysis revealed that the C-O bond formation using this metal is tricky. Reductive elimination of the C-Ni II -O is endothermic 117 and does not favor the C-O bond formation except for nickel metallacycles. 118 However, reductive elimination from a nickel(III) intermediate was suspected to be much easier. In order to reach this oxidation state, the idea of this group was to use a photocatalyst to oxidize the [C-Ni II -O] intermediate. Using NiCl 2 .dme, a bipyridine type ligand and [Ir(dF(CF 3 )ppy) 2 (dtbbpy)]PF 6 as photocatalyst, they could obtained the expected aryl ether products in excellent yields (Scheme 34). cyclic systems, functionalization of alkenes, alkynes or allenes by nucleophilic addition reactions 120 and cross-coupling reactions. The latter involves a reductive elimination step from a Au(III) intermediate. In 2006, García and co-workers observed that benzyl and aryl radicals react with gold(I) complexes to afford organogold(III) species. 121 This property has been exploited by Glorius to develop the first photoredox/gold dual catalysis. 122

Aryldiazonium tetrafluoroborate salts were used as aryl radical precursors and Ru(bpy) 3 (PF 6 ) 2

  . The successive radical addition and oxidation steps would happen directly on the Ph 3 PAuCl complex (Scheme 37). Then, the cationic Au(III) intermediate may promote the cyclization step by electrophilic activation of the triple bond and the reductive elimination leading to the C-C bond formation.

120

For recent reviews on Gold catalysis see:

(a)

L.

Fensterbank and M. Malacria, Acc. Chem. Res., 2014, 47, 953-965. (b)

R. Dorel and A. M.

Echavarren, Chem. Rev., 2015, 115, 9028-9072. 121 C. Aprile, M. Boronat, B. Ferrer, A. Corma and H. García, J. Am. Chem. Soc., 2006, 128, 8388-8389. 122 B. Sahoo, M. N. Hopkinson and F. Glorius, J. Am. Chem. Soc., 2013, 135, 5505-5508.

Scheme 35. Ru/Au catalyzed oxy/aminoarylation of alkenes Regarding the mechanism, the starting cationic Au(I) coordinates to the alkene which gives a cyclic alkylgold(I) intermediate. The photoexcited Ru(II)* reduces the aryldiazonium to generate the aryl radical which adds on the alkylgold(I). The resulting Au(II) complex is then oxidized by Ru(III) to afford the key Au(III) intermediate. A reductive elimination step provides the final product and regenerates the initial Au(I) complex (Scheme 36). Scheme 36. Proposed mechanism for the Ru/Au dual catalysis On the same line, our group realized a similar tandem process with o-alkynylphenols. 123 A neutral gold catalyst was used suggesting another possible pathway for the C-C bond 123 Z. Xia, O. Khaled, V. Mouriès-Mansuy, C. Ollivier and L. Fensterbank, J. Org. Chem., 2016, 81, 7182-7190. formationScheme 37. Dual Photoredox/Gold catalysis arylative cyclization of o-alkynylphenols This kind of pathway was first proposed by Toste for the arylative ring expansion cascade of alkenyl and allenyl cycloalkanols. Upon treatment with aryldiazonium salts, Ph 3 PAuCl and Ru(bpy) 3 (PF 6 ) 2 as catalysts, aryl substituted cyclic ketones were obtained in low to good yields.

124

Regarding the mechanism (Scheme 38), the photoredox process is similar to the one proposed by Glorius, except that the neutral complex Ph 3 PAuCl undergoes an addition of an aryl radical followed by oxidation with Ru(III). The resulting alkylgold(III) intermediate was proposed. Its formation was supported by the observation of [Ph 3 P-Ph] + .BF 4 -as a side product formed by reductive elimination. In the case of a fast coordination of the complex to the alkene or allene, the ring expansion may occur. Then, the reductive elimination affords the corresponding ketone and regenerates the starting Au(I) catalyst.

124 X. Shu, M. Zhang, Y. He, H. Frei and F. D. Toste, J. Am.

Chem. Soc., 2014, 136, 5844-5847.

  It is well established that the fluoride ion has the strongest affinity with silicon. Taking advantage of this property, the synthesis of hypercoordinated silicon compounds containing several fluorine atoms can be easily prepared by treatment of chlorosilane with a fluoride salt. Synthesis and copper mediated oxidation of organopentafluorosilicates Some of the synthesized silicates were engaged with stoichiometric amount of copper halide salts (CuCl 2 and CuBr 2 ). They showed that upon treatment in ether, THF or methanol, the alkylpentafluorosilicates were converted into the corresponding alkyl halides in low to good yields, depending on the nature of the starting copper salt (Scheme 42 Eq. 2).Investigating the mechanism of the reaction, they evidenced of a radical pathway. When the reaction was performed with octylpentafluorosilicate and copper chloride in methanol under an oxygen atmosphere, they observed the formation of octanal. The octanylperoxyde radical obtained from reaction of the octanyl radical with molecular oxygen, would produce octanal and octanol after dismutation (Scheme 43 Eq. 1). Spin trapping experiments with nitrosobenzene derivatives revealed the formation of an N-oxyl radical confirmed by ESR

						O 2 , MeOH	C 7 H 15	O O	C 7 H 15	O	Eq. 1
	C 7 H 15	SiF 5	2	2 K	CuX 2 (2 equiv.)	C 7 H 15		H
					Solvent	ArNO	C 7 H 15	N O	Ar	Eq. 2
					2
						2 K
					SiF 5
						CuBr 2	Eq. 3
								Br
						2 K	exo/endo: (70-87)/(30/13)
					SiF 5

Inspired by the work of

Tansjö, 133

Kumada and co-workers 134 reported in 1982 the first efficient synthesis of organopentafluorosilicates. By treating a large panel of trichlorosilanes by potassium fluoride in aqueous conditions, they could synthesize a wide range of organopentafluorosilicates (Scheme 42 Eq. 1).

Scheme 42.

(Eq. 2). Finally, treating pure exo-and endo-2-norbornylpentafluorosilicates with copper(II) bromide gave a mixture of exo-and endo-2-norbornyl bromide in an exo/endo ratio of (70-87)/(30-13) (Eq. 3). This epimerization suggests the formation of a planar radical intermediate. Moreover, the obtained ratios were similar to the literature on the formation of a 2-norbornyl radical. 135

2 Scheme 43. Investigation on the mechanism of oxidation of organopentafluorosilicates 135 (a) E. C. Kooyman and G. C. Vegter, Tetrahedron, 1958, 4, 382-392. (b) F. D. Greene, C.-C. Chu and J.

Walia, J. Org. Chem., 1964, 29, 1285-1289.

Table 2 .

 2 Calculated BDE of C-Si bond (before/after oxidation)

	Carbon chain	C-Si BDE* (kcal/mol)
	n-hexyl	88.8/23.8
	cyclohexyl	86.0/20.3
	benzyl	79.8/13.8
	PhNHCH 2	86.3/17.0
	phenyl	104.1/37.5

Table 3 .

 3 Optimization of photooxidation of benzylsilicate and spin-trapping experiments with TEMPO All the screened photocatalysts were competent for the photooxidation of the benzylsilicate including Ru(bpy) 3 (PF 6 ) 2 , Ir[(dF(CF 3 )ppy) 2 (dtbbpy)](PF 6 ) andIr[(dF(CF 3 )ppy) 2 (bpy)](PF 6 ). This last photocatalyst was chosen for the next studies. This photocatalyst showed a redox potential at the excited state (E°(Ir(III)*/Ir(II)) = + 1.21V vs SCE) appropriated to oxidize the benzylsilicate (E°(Bn-[Si] -/Bn . ) = + 0.61V vs SCE) and liberate the TEMPO adduct with an excellent 95% yield. Indeed, fluorescence experiments demonstrated that the luminescence of the photocatalyst is quenched by the silicate. However, it is also known that TEMPO is also a luminescence quenching agent 66 of this iridium photocatalyst, which can provide the Noxoammonium cation (E°(TEMPO + /TEMPO) ≈ +1.00V vs SCE in water 148 ). However, comparing the oxidation potentials of TEMPO and benzylsilicate 23, it is reasonable to admit that the excited photocatalyst directly react with the silicate instead of TEMPO.About the influence of the solvent, we observed the same trend reported by Nishigaichi.138 Solvent with a strong donor effect (DMF and methanol) gave better yields of benzyl-TEMPO adduct than acetone or acetonitrile which are weak donating solvents.In summary and based on the work of Akita on photooxidation of alkyl trifluoroborates, 66 it was proposed that benzylsilicate is oxidized by the excited Ir(III)*. The benzyl radical is directly trapped by the TEMPO and the Ir(II) intermediate transfers an electron to another equivalent of TEMPO (Scheme 52). The methodology was then extended to various radical acceptors. The scope of the reaction proved to be quite large from primary to tertiary silicates. Particularly, we can mention the generation of primary non-activated radical, a new example of aminomethylation and this never reported . The cyclohexyl silicate could be engaged in Giese-type reactions, alkenylation and alkynylation reactions, giving the radical adducts in moderate to good yields.

	Entry	Photocatalyst	Solvent	Yield a
	1	Ru(bpy) 3 (PF 6 ) 2	Acetone	34%
	2	Ir[(dF(CF 3 )ppy) 2 (dtbbpy)](PF 6 )	Acetone	34%
	3	Ir[(dF(CF 3 )ppy) 2 (bpy)](PF 6 )	Acetone	38%
	4	Ru(bpy) 3 (PF 6 ) 2	Acetonitrile	25%
	5	Ir[(dF(CF 3 )ppy) 2 (dtbbpy)](PF 6 )	Acetonitrile	20%
	6	Ir[(dF(CF 3 )ppy) 2 (bpy)](PF 6 )	Acetonitrile	23%
	7	Ru(bpy) 3 (PF 6 ) 2	MeOH	52%
	8	Ir[(dF(CF 3 )ppy) 2 (dtbbpy)](PF 6 )	MeOH	86%
	9	Ir[(dF(CF 3 )ppy) 2 (bpy)](PF 6 )	MeOH	77%
	10	Ru(bpy) 3 (PF 6 ) 2	DMF	90%
	11	Ir[(dF(CF 3 )ppy) 2 (dtbbpy)](PF 6 )	DMF	95%
	12	Ir[(dF(CF 3 )ppy) 2 (bpy)](PF 6 )	DMF	95%

a NMR yields, butadiene sulfone was used as internal standard.

Scheme 52. Mechanism of formation of benzyl-TEMPO adduct under photooxidative conditions

To explore the reactivity of silicates as carbon centered radical precursors, the synthesized

[18-C-6]

potassium alkyl bis-catecholato silicates were engaged in radical allylation reactions with allylsulfone 27.

149

With the previously optimized photooxidative conditions, all categories of silicates (stabilized or not) were tested. The corresponding allylated compounds were isolated in moderate to good yields (Scheme 53). 149 (a) C.-P. Chuang, Tetrahedron, 1991, 47, 5425-5436. (b) M. H. Larraufie, R. Pellet, L. Fensterbank, J. P. Goddard, E. Lacôte, M. Malacria, C. Ollivier, Angew. Chem. Int. Ed., 2011, 50, 4463-4466. chloromethylationScheme 53.Examples of intermolecular radical additions

  our group reported studies on copper(II)-mediated oxidation of alkyl trifluoroborates, 150 a

150 G. Sorin, R. Martinez Mallorquin, Y. Contie, A. Baralle, M. Malacria, J.-P.

Foddard and L. Fensterbank, Angew. Chem. Int. Ed., 2010, 49, 8721 -8723.

Table 4 .

 4 Spin-trapping experiments with organic oxidants

	Benzyl precursor	Oxidant	Solvent	Yield
	Bn-BF 3 K	Ph 3 CBF 4	Et 2 O	65%
	Bn-Si(catechol) 2 K [18-C-6]	Ph 3 CBF 4	Et 2 O	16%
	Bn-BF 3 K	Ph 3 CBF 4	DMF	35%
	Bn-Si(catechol) 2 K [18-C-6]	Ph 3 CBF 4	DMF	10%
	Bn-BF 3 K	Ar 3 N.SbCl 6	Et 2 O	2%
	Bn-Si(catechol) 2 K [18-C-6]	Ar 3 N.SbCl 6	Et 2 O	16%
	Bn-BF 3 K	Ar 3 N.SbCl 6	DMF	69%
	Bn-Si(catechol) 2 K [18-C-6]	Ar 3 N.SbCl 6	DMF	86%

Table 5 .

 5 Comparison of characteristics of nickel and palladium

		Nickel Palladium
	Oxidation states	-1 0 +1 +2 +3 +4 0 +1 +2 +3 +4
	Atomic radius	135 pm 140 pm
	Electronegativity	1.91 2.20
		Harder Softer
		Facile oxidative addition Facile reductive elimination
		β-migratory insertion β-migratory elimination
		Radical pathways more
		accessible

  1/2 red (Ir III */Ir II ) = +1.21V vs SCE inMeCN) oxidizes the benzyltrifluoroborate by SET to give the benzyl radical and an Ir(II) intermediate. As envisaged by Weix, the radical At the same time, MacMillan and Doyle reported a similar approach with the participation of α-amino radicals. These radicals were obtained by photooxidation of α-amino carboxylic acids in the presence of cesium carbonate61,63 or by photooxidation of dimethylaniline.60 Both radicals were coupled with aryl halides through a nickel catalysis. The radicals were obtained by a photooxidative process with[Ir(dF(CF 3 )ppy) 2 (dtbbpy)](PF 6 ) as photocatalyst and the cross-coupling reactions realized with NiCl 2 .dme as precatalyst and 4,4'-di-tert-butyl-2,2'-bipyridine as ligand. Chloro, bromo and iodo arenes, and heteroaryl bromides could be cross-coupled with cyclic α-amino carboxylates with good yields (Scheme

	• α α α α-amino radicals in photoredox/nickel dual catalysis This complex may undergo oxidative addition with the aryl halide to form the Ar[Ni(II)]X
	intermediate. Oxidation of the carboxylate (E 1/2 (R-CO 2	-/R-CO 2 . ) = +0.95 -+1.16V vs SCE)
	or dimethyl aniline (E 1/2 (PhNMe 2 /PhN +• .Me 2 )= +0.71 vs SCE) by the photoexcited Ir(III)
	(E 1/2	red (Ir III */Ir II ) = +1.21V vs SCE in MeCN) leads to the corresponding α-amino radical
	which adds on the nickel complex to form the intermediate Ar[Ni(III)]RX. After reductive
	elimination, the benzylamine and complex [Ni(I)]X are obtained. A final SET with the Ir(II)
	regenerates both catalysts.
		Scheme 70. Proposed mechanism of photoredox/nickel dual catalysisα-amino arylation
		This photoredox/nickel dual catalysis was then extended to the coupling of α-oxy
	carboxylic acids with vinyl iodide as electrophile. The α-oxy carboxylates are as easy as
	the α-amino carboxylate (E 1/2	ox = +1.08 V vs SCE in MeCN) 171 to be photoxidized and so
	able to provide α-oxy radicals after decarboxylation which can be engaged in dual-
	Scheme 68. Proposed single-electron transmetalation in photoredox/nickel cross-catalysis. Following the same mechanism, allyl ethers were obtained in good yields with
	coupling only 1 mol% of iridium photocatalyst and 2 mol% of nickel catalyst. It is interesting to
	mention that the process showed to be stereoconvergent. No erosion of the E:Z ratio was
	observed during the process. Moreover, α-amino and simple alkyl carboxylic acids could

may react with the complex Ar

[Ni(II)

]X to give a Ni(III) intermediate which after reductive elimination affords the cross-coupling product. Both catalysts would be regenerated from Ni(I) and Ir(II) by a SET and completing both photoredox and organometallic catalytic cycles.

69).

As efficiently, non-cyclic α-amino and α-anilino radicals were engaged with aryl bromides.

Scheme 69. Photoredox cross-coupling of α-amino radicals with aryl halides

The authors proposed the same mechanism as Molander (Scheme 70). They envisaged the reduction of the

[Ni(II)

] complex by two SET event to give the

[Ni(0)

] active catalyst. be engaged in a similar dual catalysis (Scheme 71). 171 A. Noble, S. J. McCarver and D. W. C. MacMillan, J. Am. Chem. Soc., 2015, 137, 624-627. Scheme 71. Decarboxylative cross-coupling of carboxylic acids with vinyl Iodides

Table 6 .

 6 Control experiments and influence of the crown ether

	Entry	Conditions	Yield
	1	Standard conditions	85%
	2	No [Ir]	0%
	3	No Ni(COD) 2	0%
	4	No ligand	0%
	5	No light	0%
	6	Silicate without [18-C-6]	86%

  A comparison of reactivity was made with the benzyltrifluoroborate in our conditions. It was found a better reactivity of the benzylsilicate.

	potassium alkyl bis-catecholato
	silicates. First of all, the silicates were screened with 4'-bromoacetophenone as electrophile
	(Scheme 80). Silicate precursors of stabilized benzyl, allyl and α-amino radicals gave
	excellent yields of cross-coupling products 46, 47, 48 with 88%, 86% and 91% yields
	respectively. A lower yield was obtained with the acetoxymethylsilicate 49. Synthetically
	more attracting, non-stabilized radicals could also be involved furnishing the corresponding
	coupling products. Aliphatic silicates such as hexyl or isobutyl silicate gave product 47 and

51 in good yields. The reaction conditions were tolerated by various silicates bearing functional groups such as an ester 45 and 49, a nitrile 52 and 53, an oxirane 54 and 55 or a halogen 56 and 57. Scheme 80. Cross-coupling reactions of alkyl silicates with 4'-bromoacetophenone

Table 7 .

 7 Screening of reaction conditions for organic photoredox/nickel dual-catalyzed cross-coupling of acetoxypropylsilicate and 4'-bromoacetophenone

	Entry	Silicate	4CzIPN loading	Nickel source/loading
		(nb equiv.)	(mol%)	(mol%)

Table 8 .

 8 Comparison of experimental conditions for photoredox/nickel dual-catalysis with β-bromostyrene

	Entry	Photocatalyst (mol%)	Nickel source (mol%)	Yield (E/Z)
	1	4CzIPN (1)	NiCl 2 .dme (2) a	73% b (90/10)
	2	Ru(bpy) 3 (PF 6 ) 2 (2)	NiCl 2 .dme (3) a	75% b (85/15)
	3	[Ir(dF(CF 3 )ppy) 2 (bpy)](PF 6 ) (2)	Ni(COD) 2 (3) a	93% b (67/33)
	4	4CzIPN (1)	No nickel	<5% b
	5	4CzIPN (1)	No nickel	S.M. b,c (65/35)
	6	Ru(bpy) 3 (PF 6 ) 2 (2)	No nickel	S.M. b,c (85/15)
	7	[Ir(dF(CF 3 )ppy) 2 (bpy)](PF 6 ) (2)	No nickel	S.M. b,c (40/60)

Table 9 .

 9 Optimization of the reaction conditions

	Entry Silicate (equiv.) Photocatalyst (mol%) Ni source (5 mol%)	Yield
					(104/105)
	1	1.2	[Ir] (5)	Ni(COD) 2	34/38
	2	1.2	Ru(bpy) 3 (PF 6 ) (5)	Ni(COD) 2	22/36
	3	1.2	4CzIPN (5)	Ni(COD) 2	22/25
	4	1.2	[Ir] (2)	Ni(COD) 2	29/52
	5	1.5	[Ir] (2)	Ni(COD) 2	42/50
	6	1.5	[Ir] (5)	Ni(COD) 2	43/38
	7	2	[Ir] (5)	Ni(COD) 2	44/42
	8	1.5	[Ir] (5)	NiCl 2 .dme	26/30

Table 10 .

 10 Optimization of copper mediated oxidation of anilinomethylsilicate

	Entry	Concentration	Loading [Cu]	Yield
		1	0.1M	10 mol%	52%
		2	0.05M	10 mol%	48%
		3	0.2M	10 mol%	56%
		4	0.1M	5 mol%	12%
		5	0.1M	2 mol%	6%
		6	0.1M	10 mol%	66% a
	a 6 equivalents of allylsulfone 27 used.
	About the mechanism of the reaction, copper catalyst 127 would oxidize silicate 8 and
	provide the anilinomethyl radical and complex 126. After addition of the radical on the
	acceptor followed by fragmentation of the C-S bond, product 32 would be obtained with a
	tosyl radical. This one might oxidize complex 126 regenerating the catalyst 127 with the
	formation of a sulfinate anion (Scheme 97). However, analysis of the redox potentials of
	these species involved in the last step reveals that the electron transfer is not
	thermodynamically favored (E 1/2 (PhSO 2 •/PhSO 2 Na) = +0.50V vs SCE and
	E 1/2 (Cu(L BQ ) 2	2+ /Cu(L BQ )(L SQ ) + ) = +0.83V vs SCE).

  13 CNMR (100 203 D. Hanss, J. C. Freys, G. Bernardinelli, O. S. Wenger, Eur. J.Inorg. Chem., 2009, 4850-4859.

Potassium [18-Crown-6] bis(catecholato)-acetoxymethylsilicate (18) Following

  Si NMR (119 MHz, Methanol-d4): δ -75.75. HRMS calc. for [C 20 H 21 O 5 Si] -369.1164; found 369.1176. .5 mmol, 700 µL of a 3.56 M solution in methanol) in 10 mL of dry methanol at room temperature. The crude product was purified according the general procedure to afford 18 (1.36 g, 87%) as a white solid.

	Potassium bis(catecholato)-acetoxypropylsilicate (17')
	1 H NMR(600 MHz, Methanol-d 4 ): δ 6.68 (dd, J = 5.6, 3.4 Hz, 4H), 6.57 (dd, J = 5.8, 3.5 Hz,
	4H), 3.82 (s, 2H), 3.53 (s, 24H), 1.83 (s, 3H). 13 C NMR(151 MHz, Methanol-d 4 ): δ 174.1,
	150.9 (4 C), 119.5 (4 C), 111.7 (4C), 71.2 (12 C), 58.1, 20.7. 29 Si NMR (119 MHz, Methanol-
	d 4 ): δ -85.8 (t, J = 5.7 Hz). HRMS calc. for [C 15 H 13 O 6 Si] -317.0487; found 317.0495. M.p.
	110°C. IR (neat): 3028, 2901, 2868, 1719, 1599, 1487, 1348, 1243, 1102, 963, 830, 737 cm -1 .
	Following the general procedure A with acetoxypropyl trimethoxysilane (5 mmol, 1.05 Potassium [18-Crown-6] bis(catecholato)-4-methoxyphenylsilicate (19)
	mL), catechol (10 mmol, 1.10 g) and potassium methoxide (5 mmol, 1.4 mL of a 3 .56 M
	solution in methanol) in 20 mL of dry methanol. The crude product was purified according
	the general procedure to afford 17' (1.55 g, 70%*) as a white solid.
	5 (4 C), 68.6,
	24.9, 20.8, 13. 7. HRMS calc. for [C 17 H 17 O 6 Si] -345.0800; found 345.0813. M.p. 160°C. IR
	(neat): 3016, 2950, 2882, 1735, 1597, 1486, 1351, 1242, 1105, 955, 819, 749, 725 cm -1 .
	Following the general procedure Awith 4-methoxyphenyltriethoxysilane (2.5 mmol,
	656 µL), catechol (5 mmol, 550.6 mg), 18-Crown-6 (2.5 mmol, 660 mg) and potassium
	methoxide (2.5 mmol, 700 µL of a 3.56 M solution in methanol) in 10 mL of dry methanol.
	The crude product was purified according the general procedure to afford 19 (1.4 g, 85%) as a
	white solid.
	1 H NMR (400 MHz, Methanol-d 4 ): δ 7.53 (d, J = 8.8 Hz, 2H), 6.90 -6.66 (m, 6H), 6.62 -
	Following the general procedure A with 3-chloropropyltrimethoxysilane (5 mmol, 910 6.45 (m, 4H), 3.69 (s, 3H), 3.52 (s, 24H). 13 C NMR (100 MHz, Methanol-d 4 ): δ 161.4, 151.1
	µL), catechol (10 mmol, 1.10 g), 18-Crown-6 (5 mmol, 1.32 g) and potassium methoxide (5 (4 C), 137.6 (2 C), 132.8, 119.4 (4 C), 113.6 (2 C), 111.6 (4 C), 71.2 (12 C), 55.3. 29 Si NMR
	mmol, 1.4 mL of a 3.56 M solution in methanol) in 20 mL of dry methanol. The crude (119 MHz, Methanol-d 4 ): δ -87.50. HRMS calc. for [C 19 H 15 O 5 Si] -351.0694; found 351.0700.
	product was purified according the general procedure to afford 12 (2.96 g, 95%) as a white
	solid.

M.p.197°C. IR

(neat): 2895, 1588, 1485, 1453, 1350, 1246, 1228, 1096, 1012, 949, 821, 743,657, 585

cm-1. Potassium [18-Crown-6] bis(catecholato)-3-chloropropylsilicate (12) 1 H NMR (400 MHz, Methanol-d 4 ): δ 6.68 (dd, J = 5.6, 3.5 Hz, 4H), 6.56 (dd, J = 5.6, 3.5 Hz, 4H), 3.88 (t, J = 7.0 Hz, 2H), 2.15 (s, 6H), 1.92 (s, 3H), 1.66 -1.56 (m, 2H), 0.7 -0.65 (m, 2H). 13 C NMR (101 MHz, Methanol-d 4 ): δ 173.3, 150.8 (4 C), 119.4 (4 C), 111.*Silicate without [18-Crown-6] cristallyzes with a molecule of acetone. the general procedure A with 3-acetoxypropyltrimethoxysilane (2.5 mmol, 590 µL), catechol (5 mmol, 550.6 mg), 18-Crown-6 (2.5 mmol, 660 mg) and potassiummethoxide (2M.p. >250°C. IR (neat):

  mmol, 791 mg), 2-bromoallyl alcohol (5 mmol, 685 mg) and 25 mL of DCM. After 15 minutes of stiring, imidazole (5.25 mmol, 374 mg) was added and the mixture stirred overnight. Thee mixture was filtrated over a pad of silica and eluted with Et 2 O. The solvent was removed under reduced pressure to give ((2-bromoallyl)oxy)(tert-butyl)dimethylsilane(1.25 g, 99%).The spectroscopic data are in agreement with those reported in the literature.204

1 H NMR (400 MHz, CDCl 3 ): δ5.85 (d, J = 1.8 Hz, 1H), 5.43

(d, J= 1.6 Hz, 1H),4.11-4.10

(m, 2H), 0.82 (s, 9H), 0.00 (s, 6H). 13 C NMR (100 MHz, CDCl 3 ): δ132.

0, 114.8, 67.6, 25.9 (3 C), 18.5, -5.2 (2 C)

. IR

(neat): 2958, 2854, 1637, 1463, 124, 1085, 838, 774 cm -1 .

  .1.

	Following general procedure C with potassium potassium ((1R,2R,3R,5S)-2,6,6-
	trimethylbicyclo[3.1.1]heptan-3-yl)trifluoroborate(1 mmol, 244 mg), methyl vinyl ketone (5
	mmol, 0.4 mL) and triphenylcarbenium tetrafluoroborate (1 mmol, 330mg). The crude
	product waspurified by flash column chromatography (pentane) to afford 29 as a colorless oil
	2,2,6,6-Tetramethyl-1-((2-((1R,4S)-4-methylcyclohex-2-en-1-yl)propan-2-(131 mg, 63%). The spectroscopic data are in agreement with those reported in the
	yl)oxy)piperidine (28) literature. 150
	1 H NMR (400 MHz, CDCl 3 ): δ 2.46 (ddd, J = 16.5, 10.1, 5.9 Hz, 1H), 2.34 (ddd, J =
	16.5, 10.4, 5.1 Hz, 1H), 2.30-2.22 (m, 1H), 2.12 (s, 3H), 2.15-2.06 (m, 1H), 1.91 -1.85 (m,
	1H), 1.85 -1.70 (m, 2H), 1.68 -1.50 (m, 2H), 1.45 -1.34 (m, 2H), 1.16 (s, 6H), 0.99 (d, J =
	7.0 Hz, 3H), 0.96 (s, 3H), 0.70 (d, J = 9.7 Hz, 1H).
	Following general procedure C with (1S,2R,3S,6R)-3,7,7-trimethylbicyclo[4.1.0]
	heptan-2-trifluoroborate(0.3 mmol, 65 mg) and triphenylcarbenium tetrafluoroborate (0.3
	mmol, 99 mg) in DMF. The crude product waspurified by flash column chromatography
	(pentane/diethyl ether, 99/1) to afford 28 as a colorless oil (48 mg, 67%).The spectroscopic
	data are in agreement with those reported in the literature. 150
	1 H NMR (400 MHz, CDCl 3 ): δ 5.76 (m, 1H), 5.67 (m, 1H), 2.53 (m, 1H), 2.18 (m, 1H),
	1.74-1.65 (m, 2H), 1.57-1.40 (m, 6H), 1.30-1.24 (m, 2H), 1.21 (s, 3H), 1.20 (s, 3H), 1.12
	(s, 6H), 1.10 (s, 3H), 1.09 (s, 3H), 0.98 (d, J = =7.2 Hz, 3H). 13 C NMR (100 MHz, CDCl 3 ):
	δ 133.9, 128.8, 81.1, 59.5 2 C), 47.2, 41.2 (2 C) 35.3, 35.1, 29.3, 29.0, 24.0, 23.8, 21.2, 21.0 (2
	C), 20.7, 17.3.
	4-((1R,2S,3R,5R)-2,6,6-Trimethylbicyclo[3.1.1]heptan-3-yl)butan-2-one (29)

13

C NMR (100 MHz, CDCl 3 ): δ 209.5,

  HRMS calc. for [C 20 H 23 NaO 3 P] + 365.1277; found 365.1268, for [(C 20 H 23 O 3 P) 2 Na] + 707.2662; found 707.2366 .

	1437, 1176, 1105, 717, 694 cm -1 . Ethyl 6-acetoxy-2-methylenehexanoate (37)
	Following general procedure D with 17 (0.3 mmol, 194.6 mg). The crude product was
	purified by flash column chromatography (pentane/diethyl ether, 90/10) to afford 37 as a
	colorless oil (24 mg, 37%).
	Thepotassium allyl trifluoroborate (0.3 mmol, 44 mg) and 9 mesityl-10-
	methylacridinium perchlorate (0.03 mmol, 12.4 mg) and TEMPO (0.66 mmol, 103 mg) were
	added to a Schlenk flask. The Schlenk flask was evacuated / purged with vacuum / argon
	three times. Degassed DMF (3 mL) was introduced followed by two freeze-pump-thaw
	cyclesand the reaction mixture was irradiated with blue LED (477 nm) at room temperature
	for 24h under an argon atmosphere. The reaction mixture was diluted with diethyl ether (50
	mL), washed with saturated aqueous NaHCO 3 (2 times), brine (2 times), dried over MgSO 4
	and evaporated under reduced pressure. The crude product was purified by flash column
	chromatography to afford30 as a colorless oil (19 mg, 32%). The spectroscopic data are in
	agreement with those reported in the literature. 66

-butoxy)-2,2,6,6-tetramethylpiperidine (30) 1 H NMR (400 MHz, CDCl 3 ): δ 6.15 (d, J = 1.5 Hz, 1H), 5.52 (d, J = 1.4 Hz, 1H), 4.20 (q, J = 7.1 Hz, 2H), 4.07 (t, J = 6.5 Hz, 2H), 2.35 -2.31 (m, 2H), 2.04 (s, 3H), 1.69 -1.62 (m, 2H), 1.58 -1.50 (m, 2H), 1.30 (t, J = 7.1 Hz, 3H). 13 C NMR (100 MHz, CDCl 3 ): δ 171.

Cyclohexylethynyl)benzene (39)

  . HRMS calc. for [C 11 H 18 NaO 4 ] + 237.1097;

	found 237.1097.
	(

  , 4.42 (s, 2H), 4.16 (s, 1H), 2.59 (s, 3H). 13 C NMR (100 MHz, CDCl 3 ): δ197.7, 147.7, 145.2, 136.2, 129.3 (2 C), 128.7 (2 C), 127.3 (2 C), 117.9, 112.9 (2 C), 47.9, 26.6. IR (neat): 3321, 1669, 1597, 1510 cm -1 . NMR (400 MHz, CDCl 3 ): δ 7.87(d, J = 8.2 Hz, 2H), 7.26 (d, J = 8.2 Hz, 2H), 2.68-2.64 (m, 2H), 2.58 (s, 3H),1.67-1.58 (m, 2H), 1.38-1.26 (m, 6H), 0.88 (t, J = 6.8 Hz, 3H). 13 C NMR (100 MHz, CDCl 3 ): δ 198.

212 G.-N. Wang, T.-H.Zhu, S.-Y.Wang, T.-Q.Wei and S.-J. Ji, Tetrahedron 2014, 70, 8079-8083.

1 H

3-(4-Hydroxyphenyl)propylacetate (74)

  NMR (400 MHz, CDCl 3 ): δ 7.74 (d, J = 8.0 Hz, 2H), 7.20 (d, J = 8.0 Hz, 2H), 4.08 (t, J = 6.6 Hz, 2H), 2.72 -2.68 (m, 2H), 2.05 (s, 3H), 1.97 -1.94 (m, 2H), 1.34 (s, 12H). 13 C NMR (100 MHz, CDCl 3 ): δ171.1, 144.6, 135.0 (2 C), 127.8 (2 C), 83.7 (2 C), 63.8, 32.4, 30.1, 24.9 (4 C), 21.0. 11 B NMR (128 MHz, CDCl 3 ): 30.6. IR (neat):2960, 1737, 1611, 1357, 1235, 657 cm -1 . HRMS calc. for [C 17 H 25 BNaO 4 ] + 327.1741; found 327.1754

1 H

  218 1 H NMR (400 MHz, CDCl 3 ): δ 7.04 (d, J = 8.5 Hz, 2H), 6.76 (d, J = 8.5 Hz, 2H), 5.19 (s, 1H) 4.08 (t, J = 6.6 Hz, 2H), 2.63 -2.59 (m, 2H), 2.06 (s, 3H), 1.95 -1.89 (m, 2H). 13 C NMR (100 MHz, CDCl 3 ): δ171.

  NMR (400 MHz, CDCl 3 ):δ 8.12(d, J = 5.1 Hz, 1H), 7.02 (dt, J = 5.0, 1.6 Hz, 1H), 6.85 - 6.68 (m, 1H), 3.53 (t, J = 6.3 Hz, 2H), 2.83 (dd, J = 8.4, 6.8 Hz, 2H), 2.19 -2.02 (m, 2H). 13 C NMR (100 MHz, CDCl 3 ):δ 164.1 (d, J = 239.5 Hz), 155.7 (d, J = 7.7 Hz), 147.6, 121.7, 109.3 (d, J = 38.0 Hz),43.6, 32.5, 31.9 (d, J = 2.8 Hz).19 F NMR (376 MHz, CDCl 3 ): δ -68.6. IR(neat): 2926, 1613, 1558, 1411, 1275, 1148, 908, 728 cm -1 . HRMS calc. for [C 8 H 9 ClFNNa] + 196.0300; found 196.0306.

1 H

  NMR (400 MHz, CDCl 3 ): δ 7.13 (dd, J = 5.2 Hz, J = 1.2 Hz, 1H), 6.92 (dd, J = 5.1 Hz, J = 3.4 Hz, 1H), 6.80 (s,1H), 4.12 (t, J = 6.4 Hz, 2H), 2.95 -2.90 (m, 2H), 2.06 (s, 3H), 2.04 -1.99 (m, 2H). 13 C NMR (100 MHz, CDCl 3 ): δ171.

1 H

  . IR (neat): 2942, 1734, 1232, 1040, 697 cm -1 . HRMS calc.for [C 9 H 12 SLiO 2 ] + 191.0713; found 191.0710. Following general procedure F with 2-(diphenylphosphine oxide)ethylsilicate 16 (0.45 mmol, 350 mg) and 4'-bromoacetophenone 44 (0.3 mmol, 60 mg). The crude product was purified by flash column chromatography (dichloromethane/ethyl acetate, 60/40 to 40/60)) to afford 90 contaminated with 15% of ethyldiphenylphosphine oxide as a white solid (79 mg, Following general procedure H with acetoxypropylsilicate 17 (0.45 mmol, 292 mg) and 1-bromonaphthalene(0.3 mmol, 42 µl). The crude product was purified by flash column chromatography (pentane/diethyl ether, 95/5) to afford 93 as a colorless oil(43 mg, 63%). NMR (400 MHz, CDCl 3 ): δ 8.03 (dt, J = 7.8, 0.9 Hz, 1H),7.87 (dd, J = 8.2, 1.3 Hz, 1H),

	3-(Naphthalen-1-yl)propylacetate (93)
	68%).

1-(3-(2-(Diphenylphosphoryl)ethyl)phenyl)ethan-1-one (90)

1 H NMR (400 MHz, CDCl 3 ): δ 7.82

(d, J = 8.3 Hz, 2H), 7.77 -7.72(m, 4H), 7.53 - 7.43 (m, 6H), 7.23 (d, J = 8.3 Hz, 2H), 3.06 -2.87 (m, 2H), 2.60 -2.54 (m, 2H), 2.53(s, 3H)

. 1 H

Photooxydation de silicates hypervalents pour la génération de radicaux carbonés: processus radicalaires et catalyse duale

  La catalyse photoredox en lumière visible a réussi à s'imposer comme une méthode douce et éco-compatible de formation d'espèces radicalaires, et plus particulièrement de radicaux carbonés. Bien que cette catalyse ait su prouver son efficacité pour la formation de liaisons carbone-carbone/hétéroatome, les radicaux carbonés formés sont très souvent stabilisés. A l'inverse, les alkyles bis-catécholato silicates ont montré leur capacité à engendrer des radicaux alkyles non stabilisés par photooxydation à l'aide de complexes polypyridine de métaux de transition (Ru, Ir) photoactifs en lumière visible mais aussi de photocatalyseurs organiques. Les radicaux formés peuvent ainsi être piégés par différents accepteurs radicalaires. En outre, les alkyles bis-catécholato silicates sont engagés en présence d'électrophiles comme des halogénures éthyléniques ou (hétéro)aromatiques dans des conditions de catalyse duale photoredox/nickel afin de former des liaisons C(sp 2 ) -C(sp 3 ). La méthodologie a été étendue au couplage C(sp 3 ) -C(sp 3 ) avec toutefois quelques limitations. D'autre part, une étude comparant les silicates et les « ate-complexes » de bore pour la formation de radicaux par processus d'oxydation est présentée. Enfin, des travaux prometteurs sur l'oxydation des silicates par des complexes de cuivre portant des ligands noninnocents ont été amorcés. Mots clés: Catalyse photoredox, photooxydation, photocatalyseur, silicate, radical, couplage croisé, catalyse duale, catalyse au nickel

Photooxidation of hypervalent silicon species for the formation of carbon centered radicals: radical processes and dual catalysis

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d C-Si (Å) d O-Si (Å) α C-Si-O (°) α O-Si-O (°)With acetone 1.880 O 1 : 1.778 O 2 : 1.737 O 3 : 1.776 O 4 : 1.735 O 1 : 93.48 O 2 : 116.35 O 3 : 105.75 O 4 : 105.98 O 1 -O 3 : 160.64 O 2 -O 4 : 137.40 With [18-C-6] 1.883 O 1 : 1.749 O 2 : 1.769 O 3 : 1.743 O 4 : 1.743 O 1 : 103.61 O 2 : 103.61 O 3 : 106.88 O 4 : 106.88 O 1 -O 3 : 149.51 O 2 -O 4 : 149.51

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H NMR (300 MHz, CDCl 3 ): δ 7.42 -7.39 (m,

2H),7.31 -7.24 (m, 3H), 2.64 -2.55 (m,1H),

H NMR (300 MHz, CDCl 3 ): δ 3.67 (s, 3 H),3.64 (s, 3 H),

2.76 -2.65 (m, 2 H), 2.48 -2.38 (m, 1 H), 1.74 -1.85 (m, 6 H), 1.29 -0.97 (m, 5 H).13 C NMR (100 MHz, CDCl 3 ): δ 175.0, 173.0, 51.80, 51.6, 47.1, 40.03, 33.3, 30.7, 30.2, 29.8, 26.4, 26.2.

H NMR (400 MHz, CDCl 3 ): δ 7.22 -7.18 (m, 1H), 3H), 4.09 (t, J = 6.6 Hz,

Y. M. Chiang, H. K. Liu, J. M. Lo, S. C. Chien, Y. F. Chan, T. H. Lee, J. K. Su, Y. H. Kuo, J. Chin.Chem. Soc., 2003, 50, 161-166.

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H NMR (400 MHz, CDCl 3 ): δ 4.13 (q, J = 7.1 Hz,

2H), 2.35 (t, J = 7.1 Hz, 2H), 2.32 (t, J =

Remerciements

Abbreviations

On the same line, they developed a methodology for the cross-coupling of aryl bromides/iodides with keto acids to provide aryl ketones. Aryl electrophiles bearing electron withdrawing groups or electron donating groups afforded the expected products in good yields. Aliphatic and aryl keto acids were also valuable coupling partners. Interestingly, vinyl bromides and secondary alkyl bromides could be engaged in the process (Scheme 75). The authors proposed a photooxidation of the keto carboxylate which after loss of CO 2 provides the acyl radical. The postulated mechanism is then similar to those reported above (Scheme 70) except the nature of the generated radical.

• Formation of radicals by atom abstraction

Once the nickel catalyst undergoes oxidative addition of RX to give the intermediate R [Ni(II)]X and a radical R'• adds upon it to furnish the transient complexe RR' [Ni(III)]X.

The way to generate the carbon-centered radical is, therefore an essential point. The direct oxidation of radical precursors proved to be effective toward these dual/catalytic processes.

Recently, the formation of radicals by atom abstraction further involved in this kind of synthetic approach has been reported. MacMillan and co-workers could realize the crosscoupling reaction of alkyl bromides with aryl bromides. 180 This type of reaction previously reported by Weix could be performed at room temperature by using as radical mediator TTMSSH (Scheme 77 a)). The authors proposed the formation of a bromine radical in the process which can abstract a hydrogen atom to the TTMSSH. Then, the silyl radical abstracts the bromine of the alkyl bromide and liberates the reactive alkyl radical.

The group of Doyle proposed a hydrogen atom abstraction by a chlorine radical. In their system, the cross-coupling was achieved between an aryl chloride and an α-oxy radical (Scheme 77 b)). They proposed that a chlorine radical can be formed from the intermediate Ar [Ni(III)]Cl. Actually, after oxidative addition, the Ar [Ni(II)]Cl is oxidized by the photoexcited catalyst. Then, the intermediate Ar [Ni(III)]Cl would release the chlorine radical which abstract a hydrogen to the solvent (THF 181 or dioxolane 182 ). The generated carboncentered radical reacts in the same manner as above to give the cross-coupling product. In the case of dioxolane, treatment of the product by HCl provided benzaldehyde derivatives. 180 P. Zhang, C. "Chip" Le and D. W. C. MacMillan, J. Am. Chem. Soc., 2016, 138, 8084-8087. 181 B. J. Shields and A. G. Doyle, J. Am. Chem. Soc., 2016, 138, 12719-12722. 182 M. K. Nielsen, B. J. Shields, J. Liu, M. J. Williams, M. J. Zacuto and A. G. Doyle, Angew. Chem., 2017, 129, 7297-7300. Scheme 77. Photoredox/nickel dual catalysis with the generation of radicals by atom abstraction

• Formation of C-P/C-S bonds

Radicals are not only carbon-centered. The formation of C-X bonds by photoredox/nickel dual catalysis has been also envisioned. In 2015, the group of Xiao reported an efficient method for the C-P bond formation under mild conditions. Thanks to a ruthenium photocatalyst and an in-situ formed nickel catalyst, they could realize the crosscoupling reaction between (hetero)aryl iodides and phosphine oxides in high yields. The authors proposed the same catalytic cycles as above except that a phosphinyl radical is at stake. This radical is obtained by a base-assisted photooxidation of the phosphinous acid (Scheme 78). More recently, another group engaged phosphonates as radical precursors and aryl/alkenyl tosylate, mesylate or sulfamate as electrophiles.

Scheme 78. Photoredox/nickel dual-catalyzed C-P bond formation Scheme 79.Photoredox/nickel dual-catalyzed C-S bond formation.

Alkyl bis-catecholato silicates in photoredox/nickel dual catalysis:

formation of C(sp 2 )-C(sp 3 ) bonds

Process involving a metal based photocatalyst

We have recently reported that potassium and ammonium alkyl bis-catecholato silicates valuable source of C-centered radicals upon visible-light photooxidation using [Ir(dF(CF 3 )ppy) 2 (bpy)](PF 6 ) as catalyst. 144a The silicates display several advantages compared to the alkyl trifluoroborates in terms of synthesis, byproducts and stability of the generated radicals. In addition, they are easier to oxidize than the analogous trifluoroborates or carboxylates. 14 Due to the ability of these substrates to be engaged in photoredox/nickel dualcatalysis reactions, we envisioned to use the silicates as radical source in such processes.

Preliminary results revealed that silicates can be coupled with 4-bromobenzonitrile in the presence of iridium photocatalyst, Ni(COD) 2 and dtbbpy under visible-light irradiation. 144a In order to gain more insight of these early results, we engaged a wide range of silicates with The latter presumably indicates the generation of the 5-hexenyl radical and its further interception by the nickel complex to enter the cross-coupling catalytic cycle. This radical generation from the alkyl bromide partner may also be at the origin of the homocoupling product.

We succeed to extend the photoredox/nickel dual catalysis process with alkyl silicates and electrophiles to the formation of C(sp 3 )-C(sp 3 ) bonds. Unfortunately, the yields of crosscoupling products are low to moderate due to the formation of the homocoupling product.

Based on the reports on the nickel electrophilic cross-coupling reaction and photoredox/nickel catalysis, we could not propose a mechanism taking account of the formation of each product.

This mechanism poses intriguing questions and required to be more investigated.

Scheme 92. Screening of alkylsilicates and alkyl bromides for the photoredox/nickel C(sp 3 )-C(sp 3 ) cross-coupling reactions

Scheme 96. Copper mediated trapping experiments of benzyl radical

We next switched to DCM which proved to be also efficient with this family of copper complexes. 199 Anilinomethyl silicate 8 was selected due to its lowest oxidation potential (E ox = +0.34 V vs SCE in DMF), in order to render the reaction as thermodynamically favorable as possible. In this case, we did not engage the silicate with TEMPO because it should lead to an unstable hemiaminal. Allylsulfone 31 was used as radical acceptor with 10 mol% of complex 127 to give product 32 in 52% yield after 24 hours of reaction (Table 10, entry 1). Several parameters of the reaction were modified to optimize the system. The catalyst loading was first analyzed. Decreasing the amount of copper complex from 10 mol% to 5 mol% dropped dramatically the yield (12%) of the reaction and the worst case was observed with 2 mol% (entry 4 and 5). We then considered the influence of the concentration. Dividing or multiplying by two, the concentration had not significant impact on the yield (entry 2 and 3).

Increasing the number of equivalents of acceptor from 4 to 6 improved the yield to 66% (entry 6).

Experimental Section

Potassium [18-Crown-6] 1350, 1298, 1246, 1104, 1013, 949, 911, 893,866, 818, 751, 738, 586 cm -1 .

Potassium [18-Crown-6] bis(catecholato)-hex-5-enylsilicate (3)

Following the general procedure A with hexenyltrimethoxysilane (2.5 mmol, 511 mg), catechol (5 mmol, 550.6 2890, 1599, 1487, 1352, 1246, 1209, 1191, 1142, 1105, 1012, 953, 884, 825, 769, 736, 700, 647, 596 cm -1 .

Potassium [18-Crown-6] bis(catecholato)-3-chloropropylsilicate ( 7)

Potassium [18-Crown-6] bis(catecholato)-allylsilicate (10) Following the general procedure A with allyltriethoxysilane (2.5 mmol, 538 µL), catechol (5 mmol, 550.6 mg), 18-Crown-6 (2.5 mmol, 660 mg) and potassium methoxide (2.5 mmol, 700 µL of a 3.56 M solution in methanol) in 10 mL of dry methanol at -20°C. The crude product was purified according the general procedure to afford 10 (1.25 g, 84%) as a white solid. 3035, 2892, 2870, 1486, 1350, 1240, 1166, 1100, 1009, 964,888, 819, 746, 732,689, 603, 588 cm -1 .

Potassium [18-Crown-6] bis(catecholato)-3-cyanopropylsilicate ( 14)

Following the general procedure A with 3-cyanopropyltriethoxysilane (5 mmol, 1.16 mL), catechol (10 mmol, 1.10 g), 18-Crown-6 (5 mmol, 1.32 g) and potassium methoxide ( 5mmol, 1.4 mL of a 3.56 M solution in methanol) in 20 mL of dry methanol. The crude product was purified according the general procedure to afford 14 (2.76 g, 90%) as a white solid. 3039, 2952, 2870, 2236, 1702, 1599, 1484, 1353, 1245, 1227, 1098, 1011, 953, 820, 737 cm -1 Potassium [18-Crown-6] bis(catecholato)-3,3,3-trifluoropropylsilicate ( 15)

Following the general procedure A with 3,3,3-trifluoropropyltrimethoxysilane (5 mmol, 956 µL), catechol (10 mmol, 1.10 g), 18-Crown-6 (5 mmol, 1.32 g) and potassium methoxide

(5 mmol, 1.4 mL of a 3.56 M solution in methanol) in 20 mL of dry methanol. The crude product was purified according the general procedure to afford 15 (2.56 g, 80%) as a white solid.

1 H NMR (600 MHz, Methanol-d 4 ):δ 6.70 (dd, J = 5.6, 3.5 Hz, 4H), 6.59 (dd, J = 5.7, 3.4 3040, 2907, 2871, 1597, 1485, 1353, 1245, 1201, 1098, 1057, 820, 739 cm -1 .

Following the general procedure A with 2-(diphenylphosphine oxide)ethylsilane (2.5 mmol, 981 mg), catechol (5 mmol, 550.6 mg), 18-Crown-6 (2.5 mmol, 660 mg) and potassiummethoxide (2.5 mmol, 700 µL of a 3.56 M solution in methanol) in 10 mL of dry methanol at room temperature. The crude product was purified according the general procedure to afford 16 (1.41 g, 72%) as a white solid.

Tetraethylammoniumbis(catecholato)-tertbutylsilicate (21)

The spectroscopic data are in agreement with those reported in the literature.

(Z)-(2-bromovinyl)benzene

To a solution of cinnamic acid (50 mmol, 8.9 mmol) in AcOH (25 mL) was added bromine (55 mL, 2.85 mL) at rt. When the solution turns yellow the reaction is over. The mixture was quenched with an aqueous solution of sodium thiosulfate (1 M, 25 mL). The precipitate was filtered and washed with water. The dibromo intermediate compound was directly engaged in a solution of triethylamine (100 mmol, 5.8 mL) in DMF at 0°C. The resulting mixture was warmed to room temperature and stirred for 5 h. The reaction was quenched by addition of water, the two phases were separated and the aqueous phase was extracted with pentane (2x40 mL). The combined organic layers were washed with water (50 mL), then dried over anhydrous magnesium sulfate and concentrated under reduced pressure to afford the (Z)-(2-bromovinyl)benzene (15 g, 97%).

The spectroscopic data are in agreement with those reported in the literature. filtrated over a pad of celite and the solvent removed under reduced pressure to give the pure product (950 mg, 100%).

The spectroscopic data are in agreement with those reported in the literature.

Synthesis of ethyl 4-hydroxybutanoate

To a 50 mL round-bottom-flask was added butyrolactone (40 mmol, 3.44 g) and 9 mL of distilled methanol. To the mixture was slowly added a solution of potassium hydroxyde (40 mmol, 2.24 g) in the minimium of methanol. After 4 hours of reaction, the solvent was removed under reduced pressure. The resulting white solide was washed with AcOEt and pentane. The solide was dissolved in DMF (25 mL) and bromoethane (40 mmol, 4.5 mL) was added to the solution. The mixture was stirred overnight at room temperature and then diluted with water (75 mL). The aqueous phase was extracted with AcOEt (3x50 mL). The combined organic phase were washed with water (2x50mL), NaHCO 3 (2x50 mL and brine (2x50 mL), and dried with magnesium sulfate.The solvent was removed under reduced pressure to give the product (4.2 g,79%).

Synthesis of ethyl 4-(tosyloxy)butanoate

To a 100 mL round-bottom-flask was added ethyl 4-hydroxybutanoate (7.6 mmol, 1.01 g), pyridine (3 mL) and 30 mL of distiled DCM. At 0°C was slowly added tosyl chloride (7.6 mmol, 2.43 g). The resulting yellow mixture was stirred overnight. The solution was washed with a saturated solution of CuSO 4 . The aqueous phase was extracted with DCM (2x30 mL)

and the combined organic phases dried magnesium sulfate. The solvent was removed under

Radical additions reactions

General procedure C for stoichiometric oxidation of organotrifluoroborate and organobis(catecholato) silicate.

To a Schlenk flask was added the appropriate trifluoroborate salt or silicate salt (0.3 mmol), the oxidizing agent (0.3 mmol) and TEMPO (0.9 mmol, 1 41 mg ). The Schlenk flask was sealed with a rubber septum, and evacuated/purged with vacuum/argon three times.

Degassed DMF or diethyl ether (3 mL) was introduced followed by two freeze-pump-thaw cycles. The reaction mixture was stirred at room temperature for 24h under an argon atmosphere. The reaction mixture was diluted with diethyl ether (50 mL), washed with water (2 times), brine (2 times), dried over MgSO 4 and evaporated under reduced pressure. The reaction residue was purified by flash column chromatography on silica gel.

General procedure D for addition of silicates to allylsulfone

To a Schlenk flask was added the appropriate silicate (1 eq., 0.3 mmol), allyl sulfone (4 eq., 1.2 mmol, 322 mg) and 4CzIPN (1 mol %, 3 µmol, 2.4 mg). Degassed DMF was added (3 mL) and the reaction mixture was irradiated with blue LED (477 nm) at room temperature for 24h under an argon atmosphere. The reaction mixture was diluted with diethyl ether (50 mL), washed with saturated aqueous NaHCO 3 (2 times), brine (2 times), dried over MgSO 4 and evaporated under reduced pressure. The crude product was purified by flash column chromatography to afford the adduct.

General procedure E for vinylation and alkynylation reactions of cyclohexylsilicate 1

To a Schlenk flask was added potassium [18-C-6] bis(catecholato) cyclohexylsilicate ( 1eq., 0.3 mmol, 189.3 mg), 4CzIPN (1 mol %, 3 µmol, 2.4 mg) and the desired acceptor (4 eq., 1.2 mmol) (liquid alkenes were added with the solvent). Degassed DMF was added (3 mL).

The reaction mixture was irradiated with blue LED (477 nm) for 24 hours. The reaction mixture was diluted with diethyl ether (50 mL), washed with saturated aqueous NaHCO 3 (2 times), brine (2 times), dried over MgSO 4 and evaporated under reduced pressure. The crude product was purified by flash column chromatography to afford the adduct.

General procedure F for addition of cyclohexylsilicate 1 to activated alkenes

To a Schlenk flask was added potassium [18-C-6] 44.5, 41.6, 40.8, 40.6, 38.5, 34.7 (3 C), 33.1, 27.6, 23.9, 22.0, 20.5 (2 C), 17.5. 2, 139.1, 126.8, 60.6, 53.4, 44.5, 31.5, 29.2 (3 C), 14.2.

1-(Tert

Ethyl 2-(cyclohexylmethyl)acrylate (34)

Following general procedure D with 1 (0.3 mmol, 189.3 mg). The crude product was purified by flash column chromatography (pentane/diethyl ether, 95/5) to afford 34 as a colorless oil (52 mg, 88%).The spectroscopic data are in agreement with those reported in the literature. 139.8, 125.6, 125.5, 60.6, 40.1, 36.8, 33.4, 33.2, 26.7, 26.4, 14.3.

The spectroscopic data are in agreement with those reported in the literature. 6, 145.3, 135.6, 128.8 (2 C), 128.6 (2 C), 119.1, 34.3, 26.5, 26.5, 16.4. IR (neat): 2905, 2258, 1675, 1605, 1266 cm -1 .

4'-(2-(7-Oxabicyclo-[4.1.0]hept-3-yl)ethyl)acetophenone (54)

Following general procedure G with 2- (7-oxabicyclo-[4.1.0] 8 (197.8), 148.4 (148.3), 135.0 (135.0), 128.5 (128.5), 128.5 (128.5), 53.0 (52.5), 51.8 (51.7), 38.1 (37.8), 33.3 (33.0), 32.1 31.8), 30.6 (29.3), 27.0 (25.1), 26.5, 24.3 (23.5) 8, 146.5, 135.5, 128.8 (2 C), 128.7 (2 C), 44.1, 33.6, 32.8, 26.6. IR (neat):

2935, 1678, 1605, 1358, 1265 cm -1 .

4'-(3,3,3-Trifluoropropyl)acetophenone(57)

Following general procedure G with 3,3,3-trifluoropropylsilicate 13 (0.45 mmol, 290 mg) and 4'-bromoacetophenone 44 (0.3 mmol, 60 mg). The crude product was purified by flash column chromatography (pentane/diethyl ether, 99/1 then 95/5) to afford 57 as a colorless oil (52 mg, 80%). 6, 144.4, 135.8, 128.8 (2 C), 128.5 (2 C), 126.5 (q, J = 275 Hz), 35.1 (q, J = 28 Hz), 28.2 (q, J = 3 Hz), 26.5. 19 F NMR (376 MHz, CDCl 3 ):δ -66.57. IR (neat): 2871, 1677, 1607, 1266 cm -1 .

217 X.-Q. Li, W.-K. Wang, C. Zhang, Adv. Synth. Catal.,2009, 351, 2342-2350. purified by flash column chromatography (pentane/EtOAc, 90/10) to afford 75 as a colorless oil (34 mg, 58%).

The spectroscopic data are in agreement with those reported in the literature. 219

1 H NMR (400 MHz, CDCl 3 ): δ 7.12 -7.06 (m, 2H), 6.88 -6.84 (m,1H), 6.77 -6.75 (m, 1H) 5.43 (s, 1H) 4.12 (t, J = 6.5 Hz, 2H), 2.72 -2.68 (m, 2H), 2.07 (s, 3H), 1.99 -1.94 (m, 2H),

13 C NMR (100 MHz, CDCl 3 ): δ171. 6, 153.7, 130.3, 127.4, 127.3, 120.7, 115.4, 64.2, 28.6, 26.3, 21.0. IR (neat): 3355, 2999, 1707, 1491, 1236, 1032 cm -1 .

4-Anilinomethyl-2-fluoropyridine (77)

Following general procedure G with anilinomethylylsilicate 8 (0.45 mmol, 294 mg) and 4-bromo-2-fluoropyridine 76(0.3 mmol, 31 µl). The crude product was purified by flash column chromatography (pentane/EtOAc, 80/20) to afford 77as a colorless oil (53 mg, 86%). 129.34, 119.7(d, J = 3.9 Hz), 118.3, 112.8, 107.7 (d, J = 37.8 Hz) 3015, 2895, 2257, 1904, 1674, 1601, 1437, 1264, 1174, 1111, 747, 725 The spectroscopic data are in agreement with those reported in the literature.

Compound 91 (characterictic signals) 2, 141.3, 128.5 (2C), 128.5 (2C), 126.1, 63.9, 32.3, 30.3, 21.1. 220 R. Ortiz and M. Yus, Tetrahedron, 2005, 61, 1699-1707. 221 B. Karimi, H. M. Mirzaei, A. Mobaraki and H. Vali, Catal. Sci. Technol., 2015, 5, 3624-3631. 3, 139.6, 124.5, 64.6, 33.7, 30.1, 29.0, 28.9, 27.1, 26.7, 26.4, 26.4, 21.2. 2, 137.5, 131.4, 130.0, 128.8 (2 C), 128.3 (2 C), 126.8, 64.0, 28.9, 25.1, 21.0. 2, 64.7 , 40.0, 35.8, 33.7, 32.7, 28.9, 28.6, 25.9, 25.2, 21.0.

4-(((tert-Butyldimethylsilyl)oxy)methyl)pent-

5-Phenylpent-

Synthesis of non-inocent ligand complexes

Synthesis of complex Cu(L SQ ) 2 (125)

To a colorless solution of 2-(3,5-dimethoxyanilino)-4,6-di-tert-butylphenol, prepared following a previously reported procedure, 106 (3.56 mmol, 2 equiv.) in acetonitrile (40 mL), were added at 40°C, CuCl (176 mg, 1.78 mmol, 1 equiv.) and triethylamine (992 µL, 7.12 mmol, 4 equiv.). The resulting dark green mixture was refluxed for 2 h under air. After cooling down to romm temperature, the dark green complex formed was filtered and washed with cold acetonitrile.

The UV-vis data were consistent with those reported in the literature. 106

UV-vis [CH 2 Cl 2 ; λ nm (ε, M -1 .cm -1 )]: 307 (21000), 460 (4610), 795 (7270).

Synthesis of complex Cu(L BQ ) 2 (OTf) 2 (127)

To a flame-dried Schlenk flask was introduced complex 125 (200 mg, 0.31 mmol, 1 equiv.). The flask was back-filled three times with argon and degassed DCM (9 mL) was added. A 2.8M solution of bromine in DCM (111 µL, 0.31 mmol, 1 equiv.) was added and the

The visible-light photoredox catalysis has proved to be an efficient alternative to toxic tin-mediated radical transformations and redox processes involving a stoichiometric amount of organometallic complexes, for the generation of radical species. This catalytic approach and the use of visible-light offers several advantages in terms of mild, selective and more ecocompatible process.

In this manuscript, we first reported the formation of non-stabilized alkyl radicals under photooxidative conditions. Photooxidation of alkyl bis-catecholato silicates, simply obtained from organosilanes, which are substrates of sol-gel processes, by a photoactive iridium complex under visible-light irradiation can provide alkyl radicals. Electrochemical studies of bis-catecholato silicates showed lower oxidation potentials than alkyl trifluoroborates or carboxylates. Spin-trapping experiments revealed that the silicates are good radical precursors. Further experiments demonstrated the ability of the generated radicals to be engaged in radical addition reactions such as allylation, vinylation or Giese-type reaction. In addition, the organic dye 4CzIPN (1,2,3,5-tetrakis-(carbazol-yl)-4,6-dicyanobenzene), proved to be an efficient photocatalyst as well.

The second part of this Ph. D. thesis focuses especially on a promising and growing application of photoredox catalysis: the merge of photoredoxandorganometallic catalysis. In conjuction with photooxidative processes, nickel catalysis showed to be a suitable candidate to perform cross-coupling reactions. With an iridium based photocatalyst and a nickel complex, alkyl bis-catecholato silicates were coupled to (hetero)aryl bromides via the formation of alkyl radicals. A ''greener'' version was further developed with 4CzIPN without loss of performance. The conditions for C(sp 3 )-C(sp 2 ) cross-coupling reactions were compatible to aryl and alkenyl halides. Subsequently, the C(sp 3 )-C(sp 3 ) bond formation in photoredox/nickel dual catalysis was studied. However, the methodology developed suffers of a side reaction of homocoupling which affects the yield of the reactions.

Finally, the ability of alkyl bis-catecholato silicates to be easily oxidized provides an opportunity to engage these precursors in others redox catalytic systems. Organometallic complexes with non-innocent ligands can participate to the generation of radicals by a single electron transfer through oxidation or reduction. Preliminary results concerning the oxidation