Tony Cousin

Gregory Chatel

Nathalie Kardos

Bruno Andrioletti
email: bruno.andrioletti@univ-lyon1.fr

Micheline Draye
email: micheline.draye@univ-smb.fr

Recent trends in the development of sustainable catalytic systems

Introduction

Oxidation reactions are known to be part of the most important processes in chemical industry. [START_REF] Teles | Ullmann's Encyclopedia of Industrial Chemistry[END_REF][START_REF] Cavani | [END_REF][START_REF] Weissermel | Industrial organic chemistry[END_REF][START_REF] Strukul | Catalytic Oxidations with Hydrogen Peroxide as Oxidant[END_REF] Their ability to convert carbon compounds into functionalized molecules enables the production of a wide variety of everyday life compounds. Among the products obtained through oxidation processes, dicarboxylic acids show a great interest for the production of value added polymers. They represent important feedstock in the manufacture of polyesters and polyamides and find applications in various fields: fibers, lubricants, pharmaceuticals, pesticides, cosmetics and plasticizers.

5,6

The main processes developed for the manufacture of dicarboxylic acids lie on oxidative cleavage.

5,6

This reaction based on the cleavage of a carbon-carbon double bond using an oxidant leads to various oxygenated compounds according to the functional groups borne by the double bond and the experimental conditions: ketones, aldehydes and carboxylic acids (Scheme 1). Scheme 1: General scheme for oxidative cleavage reaction.

Industrially, dicarboxylic acids are produced through oxidative cleavage of two kinds of molecules: cyclic olefins or unsaturated carboxylic acids. For instance, the oxidative cleavage of oleic acid by ozonolysis leads to azelaic acid (Scheme 2 a)). The two-step process, firstly patented at the end of the 50's by Emery Industries 7 , allows the production of up to 10,000 tons/year of azelaic acid along with pelargonic acid to deserve the fields of resins, The current production of hydrogen peroxide is based at 95% on the anthraquinone auto oxidation process (AO process) allowing a production of 4.5 million ton/year in 2014 [START_REF] Ciriminna | [END_REF] mainly for bleaching in the pulp and paper industry and the Hydrogen Peroxide Propylene Oxide Process (HPPO). 31,33, [START_REF] Sanderson | Handbook of Green Chemistry and Technology[END_REF] One the one hand, AO process has to face safety and economic issues to make hydrogen peroxide more competitive with usual oxidants. Indeed, with a selling price varying from 700 to 1200 $/ton, H 2 O 2 is 3 to 5 times more expensive Cl 2 used for the production of propylene oxide and up to 30 times expensive than pure oxygen. [START_REF] Wilson | Catalysis[END_REF][START_REF] Ciriminna | [END_REF]36 The decreasing of H 2 O 2 cost by developing new routes for its synthesis is thereby a preliminary step to be implemented for its large scale production and replacement of current oxidants. [START_REF] Strukul | Catalytic Oxidations with Hydrogen Peroxide as Oxidant[END_REF][START_REF] Jones | Applications of hydrogen peroxide and derivatives[END_REF] On the other hand, some processes for the production of key compounds, such as propylene oxide reveal the huge potential of hydrogen peroxide thanks to the possibility of using crude H 2 O 2 resulting from the AO process. 31, [START_REF] Ciriminna | [END_REF] Historically, the reaction has long been undertaken with chlorine through epichlorhydrin process (Scheme 3 a)). It has been later improved by combining the heterogeneous titanosilicate catalyst (TS-1) along with hydrogen peroxide allowing a one-step epoxidation (Scheme 3 b)).

35,37

Since 2008, the HPPO Process has proven its efficiency by allowing the production of up to 300,000 metric ton/year of propylene oxide with 95% selectivity. Improvements of the environmental aspects such as wastewater lowering and energy consumption led BASF-Dow Process to be awarded the USEPA Presidential Green Chemistry Challenge in 2010.

35,38

Scheme 3: Industrial processes for propylene oxide production: a) through epichlorhydrin process, b) through Hydrogen Peroxide Propylene Oxide Process (H2O2 amount calculated from Refs 35 and 38). [START_REF] Cavani | [END_REF][START_REF] Horwitz | Innovations in Green Chemistry and Green Engineering[END_REF]37,[START_REF] Bassler | [END_REF]39 Over the past few decades, because of the improved performances of the HPPO process, the use of hydrogen peroxide attracted the interest of researchers for olefin oxidative cleavage. Its growing availability and non-toxicity enabled hydrogen peroxide to be particularly well studied for this reaction. While lots of studies have been carried out and reviews published on the H 2 O 2 -mediated oxidative cleavage of fatty acids 29,42,43 , to the best of our knowledge, no review dealing with the oxidative cleavage of cycloalkenes by hydrogen peroxide have been published to date. [44][45][46][47][48] Thus, the present contribution aims at bringing a critical view on the development of catalytic systems in combination with hydrogen peroxide for the cleavage of various cycloalkenes.

More specifically, a focus on different transition metal-based catalytic systems will be addressed. Also, the current trends on the oxidative cleavage of cyclic olefins with H 2 O 2 and tungstenbased catalysts will be outlined with a particular emphasis on the catalyst design regarding sustainable development criteria. Finally, the limits of the reaction of the oxidative cleavage of medium-sized rings will be discussed.

Current trends in sustainable oxidative

Copper and osmium-based catalysts

Among the realm of the catalysts investigated for the cycloalkenes oxidative cleavage, transition metal-based ones are among the most efficient. Indeed, their interaction with hydrogen peroxide produces oxo-and peroxo-complexes, which are very active toward the oxidation of carbon-carbon double bonds. Hence, the copper complex of tetraazatetradecatetraene proved to be more efficient than OsO 4 to convert styrenes to benzaldehyde derivatives in the presence of H 2 O 2.

50,51

However, both of them resulted in low oxidative cleavage results. The only efficient osmium-based systems for cycloalkenes oxidative cleavage are based on waste generating Oxone® (KHSO 5 , 0.5 KHSO 4 , 0.5 K 2 SO 4 ).

Titanium-based catalysts

Among the transition-metal based compounds studied for the oxidative cleavage of cycloalkenes, titanium substituted materials were particularly investigated as heterogeneous catalysts. These insoluble modified materials can be recovered at the end of the reaction and their titanium sites present interesting catalytic activity and selectivity. 52, 53 TAPO-5 was the first example of titanium-based catalyst used in the cycloalkene oxidative cleavage. This titanium-incorporated aluminophosphate-framework presents acidic Brønsted sites favouring key steps of the reaction. However, even after 72 h of reaction with 25% H 2 O 2 , TAPO-5 allowed the formation of adipic acid in only 30% yield (Scheme 5). 54 Its low catalytic activity was attributed to both the hydrophilic character of alumino-phosphate structure and the progressive inhibition of the catalyst microporosity by the water formed from H 2 O 2 .

55

Similarly, the Ti-MMM titanium-based catalyst has been synthesized from mesoporous silica to study its activity for the cyclohexene cleavage under solvent-free conditions. In spite of its high specific area and enhanced stability, the adipic acid yield never exceed 30%. The authors explained these low results by a partial loss of the mesoporous structure and destruction of the active sites because of reaction medium aggressiveness (Scheme 5).

Scheme 5:

Oxidative cleavage of cyclohexene with H2O2 mediated by titaniumsubstituted heterogeneous materials.

54,56

The hypothesis of the progressive water poisoning of these catalysts is further supported by the only successful results reported with tert-butyl hydroperoxide (TBHP) as the oxidant. Indeed, with an aluminium and titanium-modified mesoporous silica Ti-AlSBA- Please do not adjust margins Please do not adjust margins Adipic acid was produced in 84% yield, without leaching of titanium species grafted on the silica.

57

However, as the reaction involves 4 equivalents of TBHP, the development of the technology based on this oxidant appeared not economically viable due to its high cost.

Chromium-based catalysts

Based on historical excellent results obtained with the use of CrO 3 for double bond oxidative cleavage [START_REF] Zaldman | [END_REF] , chromium has been used for the design of Metal-Organic Frameworks (MOFs). These microporous crystalline materials are three-dimensional structures made up of a metal ions or clusters linked to bi-or multipodal organic linkers. Because of its high valence and coordination ability, chromium has been widely used for designing MOFs displaying the largest pore-sized metal catalysts. [START_REF] North | Royal Society of Chemistry[END_REF] Chromium-based MOFs are used for various applications such as gas adsorption, separation processes and even catalysis due to their high porosity and large surface area. [START_REF] Farrusseng | [END_REF] They present a structure and a microporosity that are comparable to those of zeolites leading to high specific areas. Contrary to zeolites that have 178 known structures, MOFs exhibit a wider variety of shape and size porosity due to their ability to tune metals and organic linkers. 61 Recently, the same group described the deposition of MIL-101 on magnetic nanoparticles of Fe 3 O 4 to make the catalyst more easily recovered. 62 Similar oxidative cleavage results were observed under similar conditions and the deposited catalyst proved its activity after 5 successive runs with yields exceeding 90%. 62 In both studies, the use of MIL-101 led to high dicarboxylic acid yields but its synthesis requires hazardous conditions. Indeed, it involves dangerous compounds such as hydrofluoric acid and the toxic DMF, severe synthetic conditions and lengthy overall purification.

60,61

A large excess of hydrogen peroxide is also necessary to reach high dicarboxylic acids yields. Moreover, the use of chromium compounds is no longer favoured in the industry due to the high toxicity of Cr(VI) salts used or generated during the reaction.

Rhenium-based catalysts

One of the oldest system patented for the oxidative cleavage of cycloalkenes with H 2 O 2 goes back to the 70's and reports the use of rhenium-based catalyst. More specifically, a patent from Dupont describes the use of Re 2 O 7 as the catalyst for the cleavage of cyclododecene at 120 °C with acetic acid, yielding to only 30% of dicarboxylic acid. [START_REF][END_REF] Next, MethylTriOxorhenium (MTO) and related organo-rhenium compounds were well studied for various oxidation reactions. 65, 66 Recently, the combination of quaternary ammonium perrhenates with ionic liquids have been reported for the cyclooctene oxidative cleavage. Advantageously, this system offers the possibility to recover the amphiphilic active catalyst at the end of the reaction. 67 Then, using [HMIm][HSO 4 ] loaded with 5% of catalyst, 75% yield of suberic acid have been obtained after 6 h at 80 °C in the presence of 5 eq. of H 2 O 2 (Scheme 7). 67 It also afforded 88% yield of adipic acid from cyclohexene. Despite their high activity, these systems present some drawbacks, especially their high cost and ionic liquid degradation, to be competitive with other transition metal-based catalysts. The scaling-up to 1 mol for the oxidative cleavage of cyclohexene and cyclooctene allowed the production of the corresponding adipic and suberic acids in 85 and 91% yields respectively, after 12 h of reaction. Nevertheless, ruthenium-based complexes still have to face important drawbacks before being integrated into sustainable oxidation systems. Especially, ruthenium is the sixth rarest metal in earth crust due to its low abundance making its complexes and catalysts very expensive.

Molybdenum-based catalysts

Among the transition-metal based catalysts described for H 2 O 2 -mediated oxidative cleavage, molybdenum complexes have been very well studied along with their tungsten analogues. Their physico-chemical properties allowed them to be used for various oxidation reactions such as alkene epoxidation, alcohol, amine and sulfide oxidation, and oxidative cleavage. They were also studied for the characterization of active peroxocompounds formed during reactions with hydrogen peroxide. [74][75][76] Only few studies highlight benefits of using molybdenum compounds for adipic acid synthesis. In spite of similar reactivity, molybdenum-based complexes seldom showed higher performances to tungsten-based ones. Indeed, the literature on H 2 O 2 -mediated cycloalkene oxidative cleavage describes the almost exclusive use of tungsten-based catalysts because of their numerous advantages compared to other transition metal-based catalysts.

Tungsten-catalysed oxidative cleavage

Over the past 30 years, studies dedicated to the development of greener conditions for the oxidative cleavage of cycloalkenes with H 2 O 2 mainly dealt with the use of tungstenbased catalysts. Indeed, the latter benefit from the numerous advantages of tungsten 79 such as its non-toxicity

91-93

Additional contributions from Venturello et al. allowed considering other intermediates in this multi-step reaction. In fact, the formation of hydroperoxyalcohol 7 arose from the detection of the mono-aldehyde 8. 89 Hypothesis of oxidation of the latter compound to the dicarboxylic acid has been then supported by further studies on aldehyde oxidation.

94,95

Combinations of Noyori's and Venturello's mechanisms have also been suggested to complete the reaction pathway. In fact, a study on the oxidative cleavage of 1,2-cyclohexanediol and its derivatives carried out by Fujitani et al. led to consider the presence of both Venturello's hydroperoxy-alcohol 7 and semialdehyde 8 and Noyori's Baeyer-Villiger product 5.

96

In spite of these numerous propositions, no complete mechanism has been established yet for explaining the transformation of the hydroxy-ketone 4 to the final dicarboxylic acid. However, these different studies evidenced some other common points such as the absence of possible 1,2-dione through hydroxy-ketone oxidation and the identification of epoxide hydrolysis as the rate determining step of the reaction.

75,85,100

Hence, a large variety of heterogeneous and homogeneous tungsten-based catalysts have been studied to improve the environmental performances of cycloalkenes oxidative cleavage in the presence of aqueous H 2 O 2 .

Tungsten-based heterogeneous catalysts

Tungsten-based heterogeneous materials have been investigated due to the low environmental impact and high activity of peroxo-tungsten compounds. Their catalytic action as heteropolyanions towards various liquid organic reactions have even been comprehensively reviewed recently. Please do not adjust margins Please do not adjust margins multiple runs. First catalysts designed for the reaction were mesoporous silica-based materials. First catalytic oxidative cleavage was reported with SBA-15type catalyst, comprising a mesoporous silica-based structure containing 0.06 wt% oxotungsten sites. However, the highest adipic acid production was observed after 30 h reaction with 45% yield thanks to a catalyst reused from a previous run.

102 Few years later, the outcome of the reaction was improved with a similar mesoporous silica displaying surface-integrated oxo-tungsten sites. Holmberg et al. reported the use of a modified silica fully characterized. This catalyst showed better performances than the previously described one. Indeed, adipic acid was synthesized in 95% yield after 24 h reaction at 80 °C under solvent-free conditions in the presence of H 2 SO 4 .

103 It is worth noting that the surfacemodified materials led to larger amounts of dicarboxylic acid than the reactions that used a mixture of silica and soluble tungsten catalyst. Unfortunately, despite the recyclability of the catalyst and a shorter reaction time, a progressive contamination of the catalyst prevented additional runs.

Moreover, the hydrophilic nature of these mesoporous materials often constitutes a limitation for the oxidation of lipophilic substrates under biphasic conditions. Finally, the use of 2 weight equivalents of catalyst compared to cyclohexene, makes the process non-economically-viable.

Modified organic-inorganic hybrid silica have been then designed to enhance the solubility of ionic tungsten species.

104

These heterogeneous materials are composed of a silica matrix covalently functionalized with terminal alkyl ammonium that can interact with soluble tungstate ions (Scheme 12). Such structure combines the advantages of both heterogeneous and homogeneous catalysis. Indeed, the organic tails enhance the solubility of the catalyst in lipophilic substrates while the inorganic shell ensures its recovery. Thus, alkyl ammonium groups play the role of quasi-homogeneous phase transfer catalysts due to the amphiphilic nature of the organic chain they contain. The first use of these particular materials was described by Hashemi et al.. In their study, the catalyst was prepared from a silica gel functionalized by sodium tungstate and 2-[2-(3-trimethoxysilylpropylamino) ethylamino] ethylamine.

104 By mixing 43 wt% of the catalyst, 5.5 mol% of ptoluenesulfonic acid and 4.4 equivalents of 30% aqueous H 2 O 2 , cyclohexene was transformed to the corresponding diacid in 89% yield, after 20 h of reaction. This kind of organic-inorganic structure has also been studied for the design of hybrid Zr-SBA 15 mesoporous silica. The synergy of the zirconium ions with the SBA-15 structure is described as particularly significant to form strong bonds with amino-based molecules. Active hybrid catalyst has been functionalized by PentaEthyleneHexAmine (PEHA) and pre-synthesized H 3 PW 4 O 24 .

105

This semihomogeneous catalyst led to adipic acid with 89% yield in only 8 h with a slightly higher amount of H 2 O 2 (30%). Both of these materials can be recycled and reused over 4 cycles, allowing to reach a dicarboxylic acid yield above 80%. Nevertheless, these specific materials suffer from being involved in very long and energy consuming syntheses. Indeed at least three reaction steps are required to form these catalysts with reaction times ranging from 4 h to 24 h at temperatures exceeding 90 °C. Besides, harmful compounds such as concentrated HCl, H 3 PO 4 or CF 3 SO 3 H and toxic solvents such as Et 2 O, CH 2 Cl 2 or toluene are usually involved.

104,105

Scheme 12: Schematic representation of organic-inorganic hybrid silica material.

Recently, the idea of performing reactions under homogeneous catalysis conditions using a heterogeneous catalyst has been reported under a "catch and release" catalytic cycle. Here, a supported catalyst made up of WO 3 /Zn-SnO 2 is described for the oxidative cleavage of cyclohexenes leading to a minimum of 75% yield over 10 reaction cycles.

106

Under these conditions, tungsten catalyst is released into the media as soluble peroxotungstates and catalyses the reaction while hydrogen peroxide is present. As soon as H 2 O 2 is completely consumed, tungstates are "caught" back onto the heterogeneous support under the WO 3 form and can be reused over multiple cycles. This example combining the advantages of homogeneous and heterogeneous catalysis illustrates a promising system for sustainable organic synthesis. Nevertheless, solvents are still needed for the syntheses of both catalyst and adipic acid and long reaction times make these processes non-productive and economically unattractive.

Heteropolyacids and polyoxometalates for oxidative cleavage

As heterogeneous systems require non-environmentallyfriendly synthetic pathways and long reaction times to perform oxidative cleavages, water-soluble forms of tungsten catalysts have been also investigated. Among the developed homogeneous catalysts, polyoxometalates, known as POM's, have been well studied because of their multiple features allowing to perform the different stages of oxidative cleavage. These catalysts are composed of three dimensional molecular structures constituted by multiple early transition metal atoms bridged to each other via oxygen atoms (Scheme 13).

107, 108 In combination with hydrogen peroxide, these complexes display interesting features such as a strong oxidizing ability, a high thermal stability and a wide range of physico-chemical properties conferred by their tunable features. Please do not adjust margins

Please do not adjust margins with lipophilic quaternary ammoniums or pyridiniums-type to react with alkenes in organic medium. and a cationic long lipophilic chain possessing a phase transfer function as the catalyst. Peroxotungsten anion-containing catalysts were particularly efficient, leading to a very good 78% yield for the cyclohexene oxidative cleavage (Table 2, Entries 1 and 2). Indeed, tungsten-based polyoxometalates led to better results than their molybdenum counterparts due to their ability to preserve H 2 O 2 from decomposition. 113 However, they require at least 20 h reaction times to perform the reaction. Among the literature describing the catalytic oxidations with POMs, heteropolyanions have been most studied because of their high stability and activity. These complexes are constituted of p or d block "heteroatoms" in addition to metal and oxide ions. Most common tungsten-based POMs used for H 2 O 2 -mediated oxidations are the ones with so-called "Keggintype" structure (Fig. 1). While studying their mode of action in the oxidative cleavage of 1,2-cyclopentanediol, yields as high as before were also observed for the cleavage of cyclopentene and cyclohexene under similar conditions (Table 2, Entries 8 and 9). 120 Besides, both of these catalysts are particularly interesting as they are no longer soluble in the media at the end of the reaction and can be easily recovered. , to achieve cyclohexene cleavage in a "catalytic duck reactor". In the presence of a lower amount of hydrogen peroxide than usual, i.e. 2 eq. instead of 4.4 eq., and a shorter time of reaction, i.e. 3 h instead of 24 h, the olefin was not fully converted but yielded in 72% of adipic acid. ( The substitution of POM catalysts becomes highly desirable in the design of environmentally-friendly oxidation conditions due to the impact of their synthesis on the environment. Indepth analysis of the species formed during the reaction between POMs and H 2 O 2 reveal that these catalysts act as precursors of catalytically active species. Indeed, it has been shown that mono-and polymeric tungsten species resulting from POMs could be produced from simpler tungstate precursors in a similar way (Scheme 15). Please do not adjust margins Please do not adjust margins Please do not adjust margins Among them, sodium tungstate and tungstic acid were well studied for the oxidative cleavage due to their ease of use, low cost and low toxicity. Yet, development of these catalysts require the use of Phase Transfer Catalysts (PTC) in order to carry out the oxidations under solventless conditions. Indeed, phase transfer agents are essential to allow species present in different non-miscible phases to react, as mass transfer constitutes a key limitation in biphasic conditions.

First uses of PTC in oxidation reactions were published by Starks describing the oxidative cleavage of 1-octene with aqueous neutral KMnO 4 under biphasic conditions. 122 The reaction, which did not proceed at room temperature under vigorous stirring for several hours, was instantaneous after addition of a little amount of a quaternary ammonium catalyst. C. Starks describes PTC as onium salts comprising a cationic moiety able to extract anions from the aqueous phase to the organic phase. . As observed in previous studies with POMbased catalysts, the complex results from the reaction between Na 2 WO 4 , H 3 PO 4 and H 2 O 2 . However, 15 equivalents of 30% H 2 O 2 and 24 h were required to reach 99% yield of adipic acid, making the reaction non-economically attractive (Table 3, Entry 1).

126

In order to enhance the reaction in biphasic conditions, Perez et al. firstly described the use of H 2 O 2 microemulsions as the oxidizing media for the oxidative cleavage of cyclohexene.

127

This particular medium composed of oil droplets in nanodispersed in aqueous phase displays beneficial aspects for the reaction. Indeed, it allows performing organic synthesis in water, leads to fine dispersion of reactant within the oxidant and then increases local concentrations and reaction rates. Nevertheless, microemulsions require very high amounts of reactants to be stabilized. Specifically, 8:77:15 weight-ratios of cyclohexene, hydrogen peroxide and benzalkonium chloride respectively are needed to carry out the reaction. Compared to cyclohexene, this corresponds to 7 molar equivalents of hydrogen peroxide and almost 2 weight equivalents of the onium salt. In this case, the onium salt must be used in stoichiometric excess to stabilize microemulsions and thus plays the role of surfactant rather than PTC. Under these conditions, the reaction is completed in 18 h in the presence of Na 2 WO 4 and H 2 SO 4 . (Table 3, Entry 2). 127 The initial moderate 56% yield observed with microemulsions have been later

improved by optimizing the reaction temperature rates.

128

Indeed it has been shown that a step by step increase of the temperature from 75°C to 90°C is beneficial in many aspects of the reaction such as lower volatilization and faster conversion of cyclohexene and better H 2 O 2 consumption. Such conditions lead to 80% adipic acid with reaction time shortened to 8 h (Table 3, Entry 3). In spite of these good results, microemulsions could hardly be implemented at the industrial scale. Indeed, very large amounts of reactants are required, the catalytic system is difficult to recycle and the reaction products are tediously extracted.

127,128

Hence, continuous-flow microreactors have attracted the researchers' attention for the development of multiphasic reactions. Indeed, the microchannels of these continuous-flow reactors promote mass and heat transfer owing to the important surface to volume ratios (up to 10,000 m²/m 3 ) reached by the micrometric size of the inner cavity (10 to 500 µm).

129,130

These features enable continuous-flow microreactors to afford multiple benefits for oxidations reactions that mainly occur in multiphasic conditions. They allow intense mixing of non-miscible reactants, high mass transfer, fast dissipation of the heat produced during exothermic oxidation steps and working in explosive regimes by reason of low residence times of the reactants. Among the studies using Na 2 WO 4 and a PTC, the Oct 3 MeN + HSO 4 -firstly proposed by Noyori and coworkers generally leads to better results than other studied quaternary ammoniums (Table 3). Excellent results for dicarboxylic acid syntheses are explained by the combination of two physico-chemical properties of this PTC. Firstly, the lipophilicity of the cationic part allows bringing the active tungsten complex into the organic phase in order to oxidize the hydrophobic olefin. In addition, the acidity brought by hydrogensulfate anion is very important to maintain the active catalyst obtained at pH < 3 but also to carry out the different acidic steps of oxidative cleavage (Scheme 11 and 17).

13,88,134

However, despite the versatility of Oct 3 MeN + HSO 4 -to perform a large variety of oxidations under eco-friendly conditions

88

, it cannot be considered sustainable because of a nonenvironmentally friendly synthesis. Indeed, toxic toluene, long reaction times and substantial amount of corrosive H 2 SO 4 are required to produce this PTC.

134 Thus, overall environmental aspect of oxidative cleavages is impacted by the toxic and harmful reactants used to produce this essential compound.

134

Moreover, its high cost due to a multistep synthesis is also an obstacle for large scale applications.

® = [(C13-C17-CONHC2H4)N(C2H4OH)CH3][SO3(OCH3)].
e Benzalkonium chloride = BnMe2RN + Cl -(R = C12: 60% -C14 : 40%). f Aliquat 336 ® = Majority of Oct3MeN + Cl -/ Minority of (C10H23)3MeN + Cl -.

g Synthesized from Aliquat 336 ®.

Thus, in spite of environmentally friendly conditions offered by such catalytic systems, problems of recovery of phase transfer material and its co-precipitation with solid diacids make them hardly applicable for large scale synthesis.

103 For these reasons, researchers also investigated biphasic conditions for the oxidative cleavage of cycloalkenes but without addition of any phase transfer material.

Phase Transfer Catalyst-free systems

Among the PTC-free catalytic systems studied for oxidative cleavages with H 2 O 2 , ionic liquids (IL) have attracted much attention for many years. Indeed, these salts display interesting physico-chemical properties such as a low melting point (below 100 °C), low vapour pressure and high chemical and thermal stability for applications in organic chemistry. This journal is © The Royal Society of Chemistry 20xx

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Recently, a strong interest in the use of ionic liquids for catalysed reactions has emerged due to their versatile properties affecting the rate of some reactions and their ability to immobilize catalysts. 137,138 This latter feature allows catalysts to be recycled when combined with ionic liquids. Accordingly, application of these particular solvents in wide variety of oxidation reactions has been comprehensively reviewed.

139,140

Among the different ionic liquids studied for the oxidative cleavage of alkenes, IL based on imidazolium cations have been the most described in the literature followed by pyridinium and ammonium-based ones. Hashemi et al. engaged pioneering studies on ionic liquids for adipic acid synthesis by developing various specific systems. The first system describes a silica supported acid catalyst obtained by impregnation with task-specific ionic liquid (Scheme 18, left). Here, the system combines numerous interesting features and benefits for the reaction such as the recyclability of the heterogeneous support, the Brønsted acidity of the sulfamic acid group grafted on the chain, the thermal and chemical stability of [BMIm] 2 WO 4 and the immobilization of the active tungsten-based catalyst. After 18 h of reaction at 75 °C with 4.4 eq. H 2 O 2 , this semi-heterogeneous system leads to 87% yield of adipic acid. 141 A recyclability test gave 79% yield after a second run. However, no more recycling test were implemented and the slight decrease of yield has been attributed to a loss of ionic liquid during catalyst washing steps. Thus, catalyst recovery constituting a major issue in homogenous catalysis, a system combining a new task-specific ionic liquid with silver tungstate nanorods has been developed. In this work, 1,2-dimethyl substituted imidazolium is believed to prevent the ionic liquid from progressive oxidation during reaction (Scheme 18, right). 142 The presence of the dodecyl chain on the imidazolium moiety enhanced the miscibility of the catalytic system with organic substrates. Moreover, the hydrogensulfate anion of the ionic liquid allowed reaching the low pH values required for the reaction. Such properties led the designed ionic liquid to act as a surfactant-type Brønsted acid. Finally, the system was more easily prepared than the silica-supported one, and afforded adipic acid in 85% yield after 18 h at 75 °C. 142 Nevertheless, its recyclability was demonstrated through only a second reuse of the concentrated mother liquor from the first run.

As semi-heterogeneous catalytic systems require long reaction times to perform the reaction, imidazolium-based task-specific ionic liquids have been studied alone. For instance, the [BMIm] 2 WO 4, previously reported in combination with silicabased support 141 (Scheme 18, left) was synthesized and used for cyclopentene oxidative cleavage with 30% H 2 O 2 . 143 Such conditions would enhance the environmental character of the reaction that commonly involves toxic cyanides and releases poisonous hydrogen cyanide HCN into the environment.

143

After 11 h of reaction between 50 and 80 °C in the presence of [BMIm] 2 WO 4 and p-toluenesulfonic acid, glutaric acid was obtained in 87% yield. 143 Reusability of the ionic liquid-based system also proved to be efficient over 4 cycles. However, progressive catalyst loss is often observed along catalyst recycling and is probably responsible for the yield decrease from 87% to 70%.

143

Another "dimethyl" substituted imidazolium-based ionic liquid has also been investigated by Hashemi et al. for the cyclohexene oxidative cleavage. This particular ionic liquid displays a strong acidity brought by the sulfonated alkyl chain on the imidazolium cation and an oxidizing activity due to the tungstate anion (Scheme 18, center). This homogeneous system afforded adipic acid in 85% yield after 14 h of reaction between 70 and 90 °C. 144 The oxidative cleavage of 1,4cyclooctadiene and cyclopentene were also performed under these conditions leading to 88% and 79% yields of succinic and glutaric acids respectively. 146 Indeed, while a maximum 42% yield is obtained with neutral ionic liquids based on chloride and tetrafluoroborate anions, sulfonic acid containing ILs allowed to reach more than 80% yields (Scheme 19). For instance, 94% yield of adipic acid was obtained after 10 h reaction at 85 °C in the presence of 2.5 mol% [(CH 2 ) 4 SO 3 HMIm][Tos].

146

Unfortunately, even if ionic liquids are promising solvents because of their tunable physico-chemical properties, their implementation at the industrial scale still faces major issues. Indeed, their non-environmentally friendly multi-step synthesis and consecutive purifications make the generalization of their use not-economically viable. As an illustration, ionic liquids used for H 2 O 2 -mediated oxidative cleavage require 2 to 4 reaction steps in toxic solvents (toluene, dichloromethane) and long reaction times each (up to 48 h) to be produced. 142,144 In addition, the possibility of their re-usability has not been proved via multiple recyclability tests.

In order to substitute quaternary ammonium-based compounds that are usually polluting, some organic complexing agents have been proposed and tested with tungstate and peroxotungstate catalysts. Among them, bidendate ligands are well known for H 2 O 2 -mediated organic oxidations. Indeed, alcohol oxidation with picolinate chelated peroxotungstate catalyst was the first example described by Jacobson and Mares in the late 70's (Scheme 20).

147 Scheme 20: Secondary alcohol oxidation with hydrogen peroxide involving peroxotungstate complex chelated by picolinate ligand.

147 On this basis, different families of organic chelating agents for tungsten have been developed to perform the oxidative cleavage of cyclohexene. For instance, nitrogen-based ligands such as 8-hydroxyquinoline or 10-phenantroline have been used for the preparation of oxodiperoxo complex

[W(O)(O 2 ) 2 (L) 2 ]
-. More specifically, chelated peroxotungstate complexes were obtained by mixing WO 3 , H 2 O 2 and the organic ligands prior to their use. Such catalysts afforded adipic acid with yields over 80% (Scheme 21).

148,149

Nevertheless, the reaction required a higher H 2 O 2 amount than the one used in previous works (5.4 eq. instead of usual 4.4 eq.) and 20 h reaction to achieve yields above 80%. Similar catalyst preparation procedure was applied with pyridine as a monodendate ligand. The catalytic complex obtained proved its efficiency for the oxidative cleavage of chiral dibenzylester derivative of cyclohexene within only 4 h reaction with 79% yield. These excellent results can be explained by key features. Firstly, a mild to strong acidity appeared to be essential to reach high dicarboxylic acid yields by favouring the acidic steps.

135,145

However, the chelating activity of the ligand was shown to be more important than its acidity to enhance the activity of the catalyst. 145 Moreover, the formation of single chelate ring would also lead to better oxidative cleavage results. A strong affinity of the ligands with the organic phase would increase the lipophilicity of the active tungsten species hence ensuring the compatibility with organic substrates.

135,145

Interaction of the peroxotungsten species with electrondonating atoms has also been pointed out as an important factor to increase the oxidizing power of the catalyst. In fact, for some dimeric tungsten species such as [W 2 O 3 (O 2 ) 4 ] 2-, the presence of two asymmetrically bounded η 2 -peroxide ligands was shown to weaken the W-O bond and make it easier to break (Scheme 13). 85 In a similar way, the chelating effects of nitrogen or oxygen containing ligands would lower the bond energy between W and the peroxide function making it more reactive toward olefins.

Oxidative cleavage of cycloolefins has also been studied without any organic additive, by only mixing the substrate, a tungsten salt as catalyst and hydrogen peroxide. In these conditions, tungsten salts were mainly described as they enable the production of carboxylic acids while tungsten oxides led to the formation of aldehydes. Indeed, after 20 h at 35 °C with 2 eq. H 2 O 2 and WO 3 as the catalyst, oxidative cleavage of cyclohexene and cyclopentene afforded adipaldehyde and glutaraldehyde in 50 to 80 % yield respectively.

152

In the development of organic additive-free conditions for adipic acid synthesis, Ishii engaged pioneering works by improving its first system based on cetylpyridinium catalyst. Future challenges for such homogeneous catalytic systems will rely on their recyclability for consecutive runs. Interestingly, Zheng, Wang and Li et al. developed a simple pH-controlled method to precipitate selectively tungsten oxides from mother liquors isolated at the end of reactions. 158 This easy method exhibits high recovery rates and allows recovered WO 3 to display the same activity as fresh catalyst for a single run of adipic acid synthesis. Nevertheless, it could be interesting to combine this method with strategies using reaction mother liquors for several consecutive runs. As soon as the performances of the catalytic system begin to decrease along reactions, tungsten catalyst could be recovered and integrated again in a fresh oxidizing system for new consecutive syntheses.

Limits of hydrogen peroxide-mediated oxidative cleavage of cycloalkenes

The case of medium-size cycloalkenes

As illustrated previously, catalytic systems for cycloalkene oxidative cleavage with hydrogen peroxide have mainly been studied with cyclohexene as the model compound. This starting material is of particular interest due to the challenges offered by the current HNO 3 -based process for adipic acid production. More specifically, the possibility of using H 2 O 2 as primary oxidant in a one-step reaction would represent a successful example of integration of a green strategy into an industrial process.

However, the opportunity to improve the actual KA oil process only represents the tip of the iceberg. As previously illustrated, the high reactivity of the cyclohexene double bond and its oxidation products in the presence of H 2 O 2 have also to be taken into account when choosing this model compound.

But one of the issue of oxidative cleavage of cycloalkenes is to limit the formation of undesired shorter diacids. Often observed along with terminal keto-acids, their formation has not been fully understood to date. Functional Theory calculations have explained their formation through specific intermediates, they haven't been detected during analyses yet. 93 In most of cases, these by-products are formed in low quantities (from 1 to 5%) within studies with small-sized cycloalkenes. While very efficient with strained cycloalkenes, the tungstenbased systems also reach their limit when they are used for the oxidative cleavage of medium-sized cycloalkenes. Indeed, as illustrated in the The main explanation for the poor results observed with the oxidation of medium-size cycloalkenes lies on the side reactions involving the epoxide intermediate.

The first observation of a transannular reaction during an oxidation process was made during cis-cyclooctene hydroxylation by performic acid. After saponification of the produced formates, the expected trans-1,2-cyclooctanediol had been produced along with non-negligible amounts of an unexpected cis-1,4 diol (Scheme 23).

159, 161 The formation of this by-product originated from the solvolysis with formic acid of the cyclooctene oxide intermediate. Here, the particular conformation of cyclooctene oxide is responsible for the spatial proximity of non-adjacent hydrogen atoms. More specifically, the presence of the hydrogen atoms adjacent to the oxirane ring would explain the direct reaction on the epoxide mainly through a 1,5 hydride shift following the simultaneous solvent addition on the C 6 position (Schemes 23 and 24). This trend is also enhanced by the solvent nature and its acidity. Indeed, more detailed studies on the cyclooctene oxide solvolysis showed that the more acid the solvent, the more transannular products formed. 159,161 This observation can be attributed to a decreased nucleophilic nature of the solvent letting the transannular hydride ion migration to proceed. Such observations on transannular reactions on medium-sized cycloalkenes are consistent with the information obtained during their oxidative cleavage with the W-H 2 O 2 system. For instance, a study by Venturello et al. revealed that 1,4cyclooctanedione is the major neutral by-product detected during the cyclooctene oxidative cleavage. 90 This compound is an indirect evidence of the formation of undesired 1,4-diol, which is further oxidized into the 1,4-dione by H 2 O 2 . Moreover, the observed low yields for the long chain dicarboxylic acids may be explained by the classically used very acidic conditions required to form the active form of the tungstate catalyst that also favour the formation of transannular products.

Hence, the main challenge chemists have to address for the tungsten-based oxidative cleavage of medium-sized cycloalkenes with H 2 O 2 is to avoid the formation of problematic epoxide intermediate. Such synthetic route is illustrated with the use of alternative oxidizing systems such as RuO 4 /NaIO 4 or OsO 4 /Oxone. 22 Scheme 25: Example of successful RuO4 and OsO4-mediated cyclooctene oxidative cleavages. 164,165 Indeed, in the presence of these specific catalytic systems, the efficient oxidative cleavage of medium-sized rings to the corresponding dicarboxylic acid was performed in excellent yields under mild reaction conditions (Scheme 25).

164,165 It is worth noting that, from a mechanistic point of view, the common point of ruthenium and osmium oxides is the formation of cyclic peroxometalate ester during the reaction. This complex enables the direct formation of cis-1,2 diols or dialdehydes, avoiding the epoxide intermediates.

164,166-168

Future trends in the developments of W-H 2 O 2 systems would thus focus on the direct formation of the diol intermediate in order to replace efficient but harmful RuO 4 /NaIO 4 or OsO 4 /Oxone systems to reach high dicarboxylic acid yields from medium-sized cyclooolefins.

Conclusion and perspective

The oxidative cleavage of cycloalkenes by hydrogen peroxide has witnessed a growing interest for the sustainable synthesis of dicarboxylic acids. Thanks to the low environmental impact of this oxidant, the H 2 O 2 -mediated oxidation reaction has been widely studied in order to enhance the environmental performances of the current oxidative cleavage processes that involve energy-consuming conditions and harmful oxidants. To This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins Please do not adjust margins this aim, various transition metal-based catalysts have been investigated to perform the reaction in the presence of H 2 O 2 .

Transition metals such as titanium, chromium, rhenium, ruthenium, manganese and molybdenum have been reported in the literature but most studies relate tungsten-based catalysts as the most environmentally-friendly and efficient ones.

A great emphasis on tungsten-based catalysts led to the development of a large variety of W-H 2 O 2 oxidizing systems to perform eco-friendly oxidative cleavages: from tungsten supported heterogeneous catalysts, to homogeneous ones based on polyoxometalates or tungstates, in combination with Phase Transfer Catalysts, ionic liquids or organic acids. Over time, their design gradually integrated the 12 principles of green chemistry to lead to simpler and more efficient systems: progressive removal of solvents, design of simple catalytic systems of low toxicity and decrease of both reaction time and amount of hydrogen peroxide used. Among the developed systems, the ones involving tungstate-based catalysts and catalytic amounts of inorganic acid additives were proven the most efficient in terms of yield, and were up-scaled up to 5,000 L.

As disclosed in this review, despite the high efficiency exhibited by the W-H 2 O 2 catalytic systems, their excellent performances are restricted to small-sized cycloalkenes. Indeed, low oxidative cleavage results are observed when tungsten-based catalysts are applied to 8 to 12-membered cycloolefins. Indeed, in the latter cases, the unavoidable epoxide intermediate formation allows transannular reactions to proceed leading to high amounts of non-desirable byproducts. Hence, the main challenge of such systems will require to prevent the epoxide intermediates formation. Today, systems based on ruthenium and osmium proved to overcome the limits reached with medium-sized cycloolefins by directly forming 1,2-diol derivative. Unfortunately, they still involve expensive and toxic catalysts with waste generating oxidants.

Non-conventional activation techniques would thus constitute a new opportunity to allow switching W-based oxidative cleavage mechanism and improve previous results. Some of them such as enzymatic catalysis or supercritical CO 2 were already investigated but only led to aldehydes or lack of selectivity.

28,169,170

Among non-conventional techniques, ultrasound is part of the most promising ones. Indeed, the cavitation phenomenon occurring through propagation of ultrasonic waves into an elastic media leads to modifications of different nature. Subsequent nucleation, growth and brutal collapse of micrometric gaseous bubbles under ultrasonic irradiation create extreme local conditions with extremely high temperatures (up to 5,000 K) and pressures (up to 1,000 atm). Such implosions result in both important physical (micro-jets, shockwaves, shearing forces) and chemical effects (solvent sonolysis, radical production).

171,172

Ultrasound-assisted syntheses highlighted the multiple benefits of this technique to better understand and improve H 2 O 2 -mediated oxidations.

173-177

To date, the only example of ultrasound-mediated oxidative cleavage involve the use of the RuCl 3 /NaIO 4 system in combination with low frequency ultrasound. Physical effects produced under 20 kHz irradiation proved to promote mixing between non-miscible reactants for the biphasic oxidative cleavage of cyclohexene and cyclooctene. Excellent dicarboxylic acid yields were reported under only 30 min of low frequency ultrasound (Scheme 25). The potential of such reactive species for the selective oxidation of organic compounds have been recently reported for the aerobic oxidation of glucose to glucuronic acid under ultrasound and catalyst-free conditions. Indeed, the formation of HO • and HOO • radicals from H 2 O and O 2 under ultrasound is shown to be very important for the the selective oxidation of primary alcohol of glucose with conversion and selectivities above 95%.

Combination of W-H 2 O 2 catalytic systems with ultrasound thus constitutes a promising approach paving the way to a more efficient and sustainable synthesis of dicarboxylic acids through oxidative cleavage.

cleavage of cycloalkenes with H 2 O 2 2. 1 Scheme 4 :

 214 Scheme 4: Transition metal-based catalysts studied for the oxidative cleavage of cycloalkenes with H2O2 (adapted from Ref 49).

51

 51

63 Scheme 6 :

 636 Scheme 6: Schematic representation of MIL 101 (left, reproduced with the permission of Ref 57) and oxidative cleavage of various cycloalkenes with H2O2 mediated by chromium based Metal-Organic Framework MIL-101 (right).

61

 61

101

  Generally, heterogeneous tungsten-based systems are based on inorganic frameworks resistant to strong oxidizing conditions and allowing to reuse the catalytic system over This journal is © The Royal Society of Chemistry 20xxCatal. Sci. Technol., 2019, 00, 1-3 | 7

Scheme 13 :

 13 Scheme 13: Structures of some POM used for the oxidative cleavage of C=C double bonds with H2O2 (upward,representations adapted from Refs 85,110,111 ), and picture of phosphotungstate anion (downward, reproduced from Ref 112 with the permission of the International Union of Crystallography). POM-based complexes can be composed of only one type of early transition metal and are so-called isopolyanions. Among the publications describing H 2 O 2 -mediated oxidative cleavage of C=C double bonds, very few studies relate on the preparation and isolation of polyoxotungstates as oxidation catalysts. To the best of our knowledge, such complexes were only described by Li et al. 113 who succeeded in the synthesis and characterization of surfactant-type polyoxometalates constituted of a dimeric anion [M 2 O 3 (O 2 ) 4 ] 2-(M = W and Mo)

Fig. 1 :Scheme 14 :

 114 Fig. 1: Three dimensional structure of Keggin-Type polyoxometalate. The general formula is XM12O40 x-8 where X is typically P or S (orange atom), M is Mo or W (non visible atoms at the center of blue octahedras) and x the metal oxidation state (reproduced from the permission of Ref 40). Preliminary studies made for the design of tungsten-based complexes were undertaken in the 80's by Venturello et al.. The group described the combination of Na 2 WO 4 as the tungsten source, H 3 PO 4 and a Phase Transfer Catalyst (PTC) in 1,2-dichloroethane to convert various terminal alkenes into their epoxide derivatives. 70 to 90% yields were reported on the basis of the H 2 O 2 charged but these results did not exceed 52% when calculated on olefin concentration, as an excess of alkene is required to perform the reaction (Scheme 14). 97 Advanced analyses on the active species succeeded in the first complete isolation and characterization of a polyperoxometalate complex known today as the "Ishii-Venturello's complex". It is composed of the active anion {PO 4 [W(O)(O 2 ) 2 ] 4 } 3-coupled with its quaternary ammonium moiety (Scheme 14).110

144 Scheme 18 : 17 Please do not adjust margins Please do not adjust margins Scheme 19 :

 181719 Scheme 18: Ionic liquid-based systems developed for the oxidative cleavage of cyclohexene with H2O2. 141,142,144

159 Scheme 23 :

 15923 Scheme 23: Formation of cis-1,4 diol product upon formic acid solvolysis of cyclooctene oxide (upward) and mechanism of responsible transannular reactions through 1,3 and 1,5 shift (downward) (adapted from Ref 159 with permission from the Royal Society of Chemistry).

  165 Hence, the synergistic use of hydrogen peroxide and ultrasound opens new perspectives to oxidative cleavage thanks to the chemical effects brought by ultrasound. Indeed, submitting an aqueous solution of hydrogen peroxide to ultrasound would result in splitting H 2 O 2 into HO • radicals. Such reactive species would thus avoid the formation of problematic epoxide by forming directly the desired 1,2-diol intermediate through addition on C-C double bond (Scheme 26).

Scheme 26 :

 26 Scheme 26: Hypothesis of H2O2-mediated oxidative cleavage of medium-sized cycloalkenes under ultrasound: cis-cyclooctene example.

  care must be applied to implement H 2 O 2 within the design of oxidation processes.

31,35,36 Besides, as H 2 O 2 is subjected to thermal and catalytic decomposition, Please do not adjust margins Please do not adjust margins safety

29,40,68,70 In

	Scheme 8: Oxidative cleavage of various cycloalkenes with H2O2 mediated by Ru-Me3tacn complex. 69
	2.1.6 Manganese-based catalysts
	Long-time known for the oxidation of alkenes with H 2 O 2
	metallocomplexes, manganese catalysts have been rarely
	described for the alkene oxidative cleavage. An exception to
	this is the Y-zeolite-encapsuled manganese bipyridyl complex
	[c-Mn(bpy) 2 ] 2+ -Y (bpy = (C 6 H 4 N) 2 ). Indeed, this intrazeolite
	manganese complex proved its efficiency at 20 °C by reaching
	dicarboxylic acids yields over 80% in 40 h from cyclohexene
	and cyclododecene (Scheme 9). 72 Appart from these results,
	very few studies relate the H 2 O 2 -mediated oxidative cleavage
	with manganese species. Recently, a promising manganese-
	based ecocatalyst "Eco-Mn®" was synthesized from Mn-
	hyperaccumulating plants; unfortunately, using this catalyst,
	the oxidative cleavage of styrene stops at the aldehyde
	intermediate stage.

addition, ruthenium is involved in long and tedious pathways for the synthesis of the catalytic complexes 69 and harmful oxidants are required for the recycling of ruthenium-based catalysts. This journal is © The Royal Society of Chemistry 20xx Catal. Sci. Technol., 2019, 00, 1-3 | 5 Please do not adjust margins Please do not adjust margins 73 Scheme 9: Oxidative cleavage of cyclohexene with H2O2 mediated by [c-Mn(bpy)2] 2+ -Y.

72

80,81 and its availability. 40,82 Furthermore, this transition metal is very active toward hydrogen peroxide affording active peroxo- complexes. 83-85 To determine the critical parameters of the reaction, several reaction mechanisms involving tungsten- based catalysts have been suggested both on experimental observations and theoretical considerations. 2.2.1 Mechanistic and experimental considerations

	Please do not adjust margins
	at least epoxide, diol and α-hydroxy-ketone intermediates
	(Scheme 11, compounds 2
	Common characteristics on reaction conditions have been
	highlighted throughout the studies made on the cycloalkene
	oxidative cleavage with tungsten-based catalysts. More
	specifically, the need of acidic conditions, of a minimum
	amount of 4 equivalents of H 2 O 2 and of a minimum
	temperature of 80 °C are the necessary conditions leading to
	high oxidative cleavage yields (70-95%). Besides, most studies
	highlight the presence of 3 main by-products along with
	dicarboxylic acids: cycloalkenes derived epoxides, 1,2-diols and
	α-hydroxyketones.	86-90 Previous experimental observations
	and considerations on hydrogen peroxide activity suggested
	that the reaction occurs via a multi-step mechanism involving
	This journal is © The Royal Society of Chemistry 20xx
	Please do not adjust margins

, 3, 4). Scheme 11: Hypothetic

  mechanism for the oxidative cleavage of cycloalkenes with H2O2 and a tungsten-based catalyst.

29,30,42 Even if the formation of intermediates 2 to 4 have been

	13,88-90
	One of the most cited mechanism to rationalize the hydrogen
	peroxide-mediated oxidative cleavage of cycloalkenes was
	proposed by Noyori et al. (Scheme 11). 13 In this scheme, the
	oxidative cleavage involves 6 distinct oxidation and hydrolysis
	steps: alkene epoxidation, epoxide hydrolysis, 1	st alcohol
	oxidation, Baeyer-Villiger reaction, 2	nd alcohol oxidation and
	cyclic anhydride hydrolysis. This mechanism is a little different
	from the one known for the oxidative cleavage of linear olefins
	involving a C-C bond cleavage of 1,2-diol intermediate into two
	aldehydes that are further oxidized into carboxylic acids.
			well
	explained, up to date, no studies highlights the presence of the
	Baeyer-Villiger ring enlargement during the oxidative cleavage.
	Only indirect studies propose the presence of products 5 and 6
	in the mechanism.

13,89,96

  The use of an acidic source, since best oxidative cleavage yields were obtained in the 0-3 pH range. These very acidic conditions are essential to form the active catalyst,

	Among the various mechanisms reported in the literature, all
	of them require 2 key experimental conditions, which are:
	(i) The use of a minimum of 4 equivalents of H 2 O 2 . Indeed, this
	minimal amount required is supported by the four distinct
	oxidation steps taking place from the cycloolefin to the diacid
	production. Experimentally, 4.4 equivalents are commonly
	used to ensure a sufficient amount of oxidant that could be
	thermally or catalytically decomposed.
	(ii)

88,97,87,98 perform the acidic steps of the reaction 13,90 but also to prevent H 2 O 2 from thermal and catalytic decomposition.

4,32,33 Indeed, at pH < 3 the disproportionation reaction of H 2 O 2 into H 2 O (Eq. 1) and O 2 (Eq. 2) is slow. In addition, according to the E h -pH equations of hydrogen peroxide, low pH values also increase the H 2 O 2 redox potential (Eq. 3. and 4). 99 (Eq. 1) (Eq. 2) (Eq. 3) (Eq. 4) Moreover, the action of hydrogen peroxide on tungstate species leads to water soluble complexes which are stabilized under acidic conditions. Firstly described by Pourbaix as pertungstic acid H 2 WO 5 .H 2 O 2 99 , more recent studies by Griffith et al. revealed that these complexes are actually polyperoxotungstates that are formed under pH < 3.

Table 2 ,

 2 Entries 4 to 6). For instance, Ishii's previous conditions were adapted by Dai et al. to perform the solventless oxidative cleavage of cyclopentene in the presence of PW 12 O 40 3-in only 8 h. Through a progressive and controlled increase of the temperature of the medium and the use of a more concentrated H 2 O 2 solution, the yield was improved up to 86% (Table 2, Entry 7).

	97,110
	From the discovery of Venturello's active catalyst
	{PO 4 [W(O)(O 2 ) 2 ] 4 } 3-, many studies were undertaken to identify
	the mono-and polynuclear species formed during the
	oxidation of the catalyst by H 2 O 2 . 75,85,98,111 Venturello's system
	was later improved for olefin epoxidation	116	while
	progressively considering environmental aspects of the
	reaction 117 transposed to olefin oxidative cleavage.

This journal is © The Royal Society of Chemistry 20xx

Catal. Sci. Technol., 2019, 00, 1-3 | 9

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Table 2 ,

 2 Entry 10). To date, the conditions disclosed by Venturello for the oxidative cleavage of various olefins by the same catalyst are still the most promising. Indeed, the group described the use of [(C 8 H 17 ) 3 NCH 3 ] 3 PO 4 [W(O)(O 2 ) 2 ] 4 and proved that optimization of H 2 O 2 addition into the media led to enhanced results through a better temperature control. 90 Thus, glutaric and adipic acid were isolated with yields above 80% under biphasic conditions in less than 7 h (Table 2, Entries 11 to 14). As a conclusion, surfactant-type POMs have been particularly studied due to the possibility to select both anion and cation moieties, in order to adjust the solubility and the activity of the catalyst. Nevertheless, among the above cited studies, only very few papers describe the recyclability of such complexes. For instance, only Dai et al. achieved to reuse the catalyst [π-C 5 H 5 N(C 16 H 33 )] 3 PO 4 [WO 3 ] 4 over 5 cycles with high yields for glutaric acid synthesis. 119 Unfortunately, recovery issues and sharp loss of activity were observed generally after several recyclings. Moreover, surfactant-type POMs have to face some environmental issues to be obtained. They need non-negligible amounts of toxic organic volatile solvents such as benzene or dichloromethane and corrosive acids such as H 3 PO 4 and H 2 SO 4 , in order to be produced via multiple steps of syntheses.

85,

86,97,110,119,120 2.2.4 Oxidative systems based on tungstates 2.2.4.1 Combination with Phase Transfer Catalysts (onium salts)

  Anionic peroxo species issued from the oxidation of POM or tungstate salts where isolated peroxo-compounds are illustrated in bold blue (adapted from Ref 111).

75,111 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins Scheme 15: This journal is © The Royal Society of Chemistry 20xx Catal. Sci. Technol., 2019, 00, 1-3 | 11 Please do not adjust margins Please do not adjust margins Please do not adjust margins

Table 2

 2 Homogeneous catalytic systems based on polyoxometalates reported for the oxidative cleavage of cycloalkenes in the presence of H2O2 Synthesized from Aliquat 336® (PTC composed of a majority of Oct3MeN + Cl -and a minority of (C10H23)3MeN + Cl

	Entry	Polyoxometalate (POM)	Substrate	Catalytic system	PTC	a		Reaction conditions	Product	Yield	b	Ref.
	1	W2O3(O2)4 2-	Cyclohexene	POM + H2O2 30% (5.4 eq.)	[π-C5H5N(C16H33)]2 W2O3(O2)4	90 °C, 20 h	Adipic acid	78%	113
	2		Cyclohexene	POM + H2O2 30% (5.4 eq.)	[C16H33N(CH3)3]3 W2O3(O2)4	90 °C, 20 h	Adipic acid	78%	113
	3	PW4O22 3-	Cyclohexene	POM + H2O2 35% (4.4 eq.)	[(C8H17)3NCH3]3 PW4O22		Dichloroethane, 70 °C, 16 h	Adipic acid	71%	118
	4	PW12O40 3-	Cyclopentene	POM + H2O2 35% (4.4 eq.)	[π-C5H5N(C16H33)]3 PW12O40	t-BuOH, 82 °C, 24 h	Glutaric acid	66%	86
	5		Cyclohexene	POM + H2O2 35% (4.4 eq.)	[π-C5H5N(C16H33)]3 PW12O40	t-BuOH, 82 °C, 24 h	Adipic acid	70%	86
	6		1-Methyl-1-cyclohexene	POM + H2O2 35% (4.4 eq.)	[π-C5H5N(C16H33)]3 PW12O40	t-BuOH, 82 °C, 24 h	acid 5-Oxohexanoic	90%	86
	7		Cyclopentene	POM + H2O2 30% (or 50 %) (4.4eq.)	[π-C5H5N(C16H33)]3 PW12O40	85 °C, 8 h	Glutaric acid	77% 86% (or	d d )	119
	8	{PO4[WO3]4} 3-	Cyclopentene	POM + H2O2 30% (or 50 %) (4.4 eq.)	[π-C5H5N(C16H33)]3 PO4[WO3]4	85 °C, 8 h	Glutaric acid	80% 83% (or	d d )	119
	9		Cyclohexene	POM + H2O2 50% (3.3 eq.)	[π-C5H5N(C16H33)]3 PO4[WO3]4	85 °C, 5 h	Adipic acid	85%	d	120
	10	{PO4[W(O)(O2)2]4} 3-	Cyclohexene	POM + H2O2 30% (2 eq.)	[(C8H17)3NCH3]3 PO4[W(O)(O2)2]4	e	90 °C, 3 h	Adipic acid	72% 85%) (Conv. c	121
	11		Cyclopentene	POM + H2O2 40% (4.6 eq.)	[(C8H17)3NCH3]3 PO4[W(O)(O2)2]4	e	85 °C, 7 h	Glutaric acid	84%	90
	12		Cyclohexene	POM + H2O2 40% (4.4 eq.)	[(C8H17)3NCH3]3 PO4[W(O)(O2)2]4	e	85 °C, 6 h	Adipic acid	87%	90
	13		1-Methyl-1-cyclohexene	POM + H2O2 40% (3.5 eq.)	[(C8H17)3NCH3]3 PO4[W(O)(O2)2]4	e	85 °C, 6 h	5-Oxohexanoic acid	82%	90
	14		Cycloheptene	POM + H2O2 40% (4.6 eq.)	[(C8H17)3NCH3]3 PO4[W(O)(O2)2]4	e	85 °C, 7 h	Pimelic acid	68%	90
	a Phase Transfer Catalyst.

b Unless otherwise stated, refers to isolated yield. c Yield based on GC analysis. d Yield based on acid-base titration. e -) This journal is © The Royal Society of Chemistry 20xx Catal. Sci. Technol., 2019, 00, 1-3 | 12

  Different families of PTC were investigated in the presence of Na 2 WO 4 to perform the oxidation reaction under solventless conditions. For instance, the oxidative cleavage of cyclohexene was studied in the presence of Varisoft® 222 [Bis(tallowamidoethyl)polyethoxymethylammonium methosulfate]. Under these conditions, the introduction of phosphoric acid led to the production of the active {PO 4 [W(O)(O 2 ) 2 ] 4 }

	123,124 Among the different classes of PTC,
	quaternary ammonium compounds are particularly efficient
	for oxidation reactions.	88,125
	3-

Table 3 ,

 3 2 WO 4 was investigated for adipic acid synthesis. For instance, Reedijk and coworkers patented the use of both Na 2 WO 4 and H 2 WO 4 with Aliquat 336® (containing Oct 3 MeN + Cl -) and ClCH 2 CO 2 H. Under these conditions and in the presence of 5 equivalents of 30% H 2 O 2 , 90% yield of adipic acid are obtained in only 4 h (Table 3, Entry 12). Entry 6). Despite longer reaction times than those reported in previous works (Table 3, Entry 5), this catalytic system is the only example able to perform oxidative cleavages of various cycloalkenes and arenes at a 100 g scale. Using these conditions, cyclohexene derivatives have been converted to carboxylic acids with good to excellent yields (Table 3, Entries 7-9). Cyclopentene required 13 h of reaction to produce glutaric acid in 90% yield (Table 3, Entry 10).

	2 SO 4 and

131 Within this context, adipic acid synthesis has been recently reported by Hessel et al. under micro-flow conditions. After only 20 minutes of reaction at 100 °C in the presence of H As catalytic systems involving sodium tungstate need low pH ranges (< 3) to prevent H 2 O 2 decomposition and reach high dicarboxylic acid yields, the combination of Na 2 WO 4 and This journal is © The Royal Society of Chemistry 20xx Catal. Sci. Technol., 2019, 00, 1-3 | 13 Please do not adjust margins Please do not adjust margins H Scheme 16: : Halide and solvent free oxidative cleavage of cyclohexene by hydrogen peroxide. 13,88

  Schematic representation of Phase Transfer Catalysts used for cycloalkene oxidative cleavage with hydrogen peroxide. Framed in grey: used separately with the tungsten-based catalyst. Framed in blue: used to produce new tungsten-based PTC when reacted with POM. a) Methyltrioctylammonium Hydrogensulfate , b) Benzalkonium Chloride, c) Varisoft 222 ®, d) Aliquat 336®, e) Cetylpyridinium Chloride, f) Hexadecyltrimethylammonium Chloride.

102,135 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins Scheme 17: This journal is © The Royal Society of Chemistry 20xx Catal. Sci. Technol., 2019, 00, 1-3 | 15 Please do not adjust margins Please do not adjust margins Please do not adjust margins

Table 3

 3 Homogeneous catalytic systems based on Na2WO4 or H2WO4 and PTC reported for the oxidative cleavage of cycloalkenes with H2O2

	Entry	Tungsten oxide	Substrate	Catalytic system	PTC	a		Reaction conditions	Product	Yield	b	Ref.
	1	Na2WO4	Cyclohexene	Na2WO4 + H3PO4 + H2O2 30% (15 eq.)	Varisoft 222®	d	90 °C, 24 h	Adipic acid	99%	c	126
	2		Cyclohexene	Na2WO4 + H2O2 30% (7 eq.) + H2SO4	Benzalkonium Chloride e	70 °C, 18 h	Adipic acid	(after 5 runs) 56% (76% c ), up to 97%	127
	3		Cyclohexene	Na2WO4 + H2O2 30% (7 eq.) + H2SO4	Benzalkonium Chloride e	75-85 °C, 8 h	Adipic acid	runs) 70%, up to 79% (after 4	128
	4		Cyclohexene	Na2WO4 + H2O2 50% (4.8 eq.) + H2SO4	Oct3MeN + HSO4 -g	100 °C, time) 20 min (residence	Adipic acid	50% (based on reactant) charged 30% of	130
	5		Cyclohexene	Na2WO4 + H2O2 27.5% (4.3 eq.)	Oct3N.H2SO4		80-95 °C, 5 h	Adipic acid	94%	133
	6		Cyclohexene	Na2WO4 + H2O2 30% (4.4 eq.)	Oct3MeN + HSO4 -	75 -90 °C, 8 h	Adipic acid	90% (93% c )	13,88
	7		Cyclohexene-4,5-dicarboxylic anhydride	Na2WO4 + H2O2 30% (4.4 eq.)	Oct3MeN + HSO4 -	75 -90 °C, 8 h	Meso-1,2,3,4-acid butanetetracarboxylic	91%	13,88
	8		Cyclohexene-4,5-dicarboxylic acid	Na2WO4 + H2O2 30% (4.4 eq.)	Oct3MeN + HSO4 -	75 -90 °C, 8 h	acid Meso-1,2,3,4-butanetetracarboxylic	96%	13,88
	9		1-Methyl-Cyclohexene	Na2WO4 + H2O2 30% (4.4 eq.)	Oct3MeN + HSO4 -	75 -90 °C, 8 h	6-Oxoheptanoic acid	59%	13,88
	10		Cyclopentene	Na2WO4 + H2O2 30% (4.4 eq.)	Oct3MeN + HSO4 -	70 -90 °C, 13 h	Glutaric acid	90%	13,88
	11		Phenanthrene	Na2WO4 + H2O2 30% (4.4 eq.) + NH2CH2PO3H2	Oct3MeN + HSO4 -	75 -90 °C, 8 h	2,2'-Biphenyl-dicarboxylic acid	41%	13,88
	12	Na2WO4 + H2WO4	Cyclohexene	ClCH2CO2H Na2WO4 + H2WO4 + H2O2 30% (5 eq.) +	Aliquat 336®	f	90 °C, 4 h	Adipic acid	90%	132
			a Phase Transfer Catalyst.

b Unless otherwise stated, refers to isolated yield. c Yield based on GC analysis. d Varisoft 222

  150 Interestingly, Ford et al. proposed the addition of acetic acid into the media to reach milder pH conditions than Noyori hence preventing ester functions from hydrolysis. Nevertheless, despite shorter reaction times, this methodology has not been applied for the oxidative cleavage of cyclohexene or other cycloalkenes. Some oxygen-based ligands were also investigated as chelating agents in combination with sodium tungstate. Deng et al. described the first solvent and PTC-free synthesis of adipic acid with a system involving oxalic acid (Scheme 21).

	Scheme 21: Organic ligands used in phase transfer catalyst-free oxidative cleavage of cyclohexene. 135,145,148,149,151
			135 Later, Jiang
	et al. improved these conditions by decreasing the reaction
	time from 24 h to 8 h. Among the 46 ligands tested,
	sulfosalycilic acid used in 2.5 mol% afforded the best results,
	leading to 88% adipic acid yield (Scheme 21). 145 Once more,
	even if no complete characterization of the active specie was
	described in this work, the authors highlighted the need of
	mixing catalyst, ligand and H 2 O 2 together to form an active
	peroxotungstate species prior to the addition of the
	substrate.	135,145
	More recently, the association of a polyoxometalate with an
	amino-acid resulted in the formation of a new
	heteropolysalt.	151 This Keggin-type catalyst comprising a
	protonated glycine and phosphotungstate ions has been well-
	characterized and showed similar behaviours as previous
	surfactant-type polyoxometalates. Emulsifying effect and
	strong acidity of this complex led Wang et al. to perform the
	cyclohexene oxidative cleavage with excellent 95% yield after
	12 h of reaction in the presence of only 0.2 mol% catalyst
	(Scheme 21).

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153

  Thus, the research group reported the first PTC-free oxidative cleavage of cyclohexene mediated by tungstic acid. However, the reaction required 5 wt% of H 2 WO 4 and t-BuOH as solvent to reach a moderate 62% yield after 24 h at 95 °C.As acidity plays a pivotal role within reaction mechanisms, some inorganic acids were also investigated to study their influence on the outcomes of the reaction. Tang and Wang et al. studied the product distribution following the conditions of cyclohexene oxidative cleavage. Among the parameters tested, the influence of the amount of added H 2 SO 4 on adipic acid yield and by-products formation was studied. An optimal amount of 6.5 mol% of H 2 SO 4 in the presence of 2 mol% of Na 2 WO 4 and 4.4 eq. of 30% H 2 O 2 were proven optimal to afford adipic acid in 95% yield in only 6 h.156 On the basis of this work and previous studies, the same research group reported the first large-scale continuous-flow oxidative cleavage of cyclohexene with hydrogen peroxide. Combined use of sulfuric acid and phosphoric acid was described as essential to Optimisation of the reaction conditions with the H 2 WO 4 -H 2 SO 4 -H 3 PO 4 -H 2 O 2 system allowed to scale up the reaction up to four 5,000 L continuous stirred tank reactors. For the first time, the group succeeded to develop the large scale synthesis of 99% pure adipic acid without any PTC with a 95% yield (Scheme 22). Example of large scale production of adipic acid through H2O2mediated oxidative cleavage: Scheme of the reaction (up) and schematic representation of continuous flow process (reproduced from Ref 157 with permission from The Royal Society of Chemistry).

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	promote the acidic steps of the reaction and to prevent H 2 O 2
	decomposition	during	successive	runs	respectively.
						153 Similar
						moderate yields reaching 48% and 72% were also observed
						from cyclopentene and 1-methyl-cyclohexene, respectively.
	157 Efficiency of this large scale process was evidenced with 350 h continuous production of adipic acid associated to excellent yields and limited H 2 O 2 decomposition. This unique PTC and solvent-free process illustrates the relevance of W-H 2 O 2 oxidizing systems at industrial scale for the production of carboxylic acids of interest under environmentally-friendly conditions.	Tungstophoshoric acid was proposed by Fujitani et al. for the biphasic oxidative cleavage of various cycloalkenes in the presence of 60% H 2 O 2 . Under these conditions, cyclohexene derivatives were successfully converted into their corresponding polycarboxylic acids with yields from 86 to 95% in 8 h to 15 h without any solvent and phase transfer 154 material. The H 2 WO 4 -H 2 O 2 oxidizing system also proved its efficiency for
						adipic synthesis in micro-flow reactors. The reactions were
						carried out in a system consisting of two glass syringe pumps, a
						T-mixer, a residence tube, and an adjustable stainless steel
						back pressure regulator. Using this system, cyclohexene
						oxidative cleavage was performed at 140 °C over an hour with
						20 min residence time and resulted in 73% isolated yield.	155 In
						spite of safe operations conditions allowed by continuous
						micro-flow systems under high temperatures, such systems
						still need to be improved. Indeed, side-reactions are observed
						and H 2 O 2 decomposition occurs during the reaction due to
						reactor-wall effects coming from the high surface-to-volume
						ratio.	129,130,155 H 2 WO 4 -H 2 O 2 system was also used for the
						selective oxidation of cyclopentene to glutaric acid under
						solvent and PTC-free conditions. In the presence of 1.5 mol%
						of catalyst and 30% or 50% H 2 O 2 , yields over 80% were
						reached after 8 h of reaction.
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119 This journal is © The Royal Society of Chemistry 20xx Catal. Sci. Technol., 2019, 00, 1-3 | 19 Scheme 22:

13,141,143,144,157

13,29,90,93,96 Even if Density

Table 4

 4 Transannular reactions are described by Cope as "the formation of a full bond between atoms on opposite sides of the ring, or reactions which occur on an atom activated only by the proximity in space of a functional group located across the ring".159 They have been extensively studied byCope et al. for

	, the few reports published on the
	oxidative cleavage of medium-sized cycloalkenes showed that
	the larger the cycle, the lower the results. At reasonable
	reaction time and H 2 O 2 amounts (< 8 h and 4.4 eq. H 2 O 2 ),
	cleavage of 8 to 12-membered cycloalkenes afforded
	dicarboxylic acids in less than 30% yield. Very few exceptions
	report on the use of larger amounts of H 2 O 2 or extended
	reaction times to improve these yields (Table 4, Entries 1,2 and
	6). Unfortunately, none of these examples provide clear
	explanations on the low performances observed with medium-
	sized cycles. While the nature of the problem was pointed out
	by Noyori et al. to come from the acid-catalyzed resistance of
	epoxide intermediates, 13,88 transannular reactions have been
	proposed for the first time by Venturello as a probable cause
	for obtained low oxidative cleavage yields.

90,159

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Catal. Sci. Technol., 2019, 00, 1-3 | 21

MIL-101

Acknowledgements

Authors are grateful to the Région Auvergne Rhône-Alpes for the financial support through an ARC Environnement doctoral research grant (ARC 2016). They also thank Dr. Marion Chevallier for her help on the design of molecules of the graphical abstract.

Notes and references

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Conflicts of interest

There are no conflicts to declare.