Maud Bollenbach

Patrick Wagner

Pedro Aquino

Jean-Jacques Bourguignon

Frédéric Bihel

Christophe Salomé

Martine Schmitt

d -Glucose: An Efficient Reducing Agent for a Copper(II)-Mediated Arylation of Primary Amines in Water

Keywords: C-N formation, Ullmann-type reaction, primary amines, D-glucose as reducing agent, micellar conditions

come   L'archive ouverte pluridisciplinaire

Introduction

Transition metal-catalyzed reactions have a prominent place in the arsenal of synthetic chemists. [1] These reactions have traditionally been carried out in conventional organic solvents. Currently, the design of sustainable protocols involving green catalysis is emphasized. [START_REF] Sheldon | Green Chemistry and Catalysis[END_REF] The use of water as a solvent has many advantages because it is the most economical, safe and environmentally friendly solvent. [3] The presence of micelles formed via non-ionic amphiphile polyoxyethanyl-α-tocopheryl succinate (TPGS-750-M) helps solubilize the organic reagents. [4a] The viability of this surfactant-promoted chemistry was demonstrated by Lipshutz et al. Indeed, since 2008, they reported the use of this new methodology for metathesis [4] and important cross-coupling reactions such as Suzuki-Miyaura, [4a,d-e,5] Heck, [4d- f,6] C-H activation, [4a,e] Sonogashira [4a,7] and Buchwald-Hartwig reactions. [4a,e,8] Recently, our group further investigated the Buchwald-Hartwig reaction [9] since the formation of the C-N bond is one of the most useful reaction types in medicinal chemistry and materials science. [10] Thus we reported a universal catalytic system for Buchwald-Hartwig reactions acting under micellar conditions. [11] However, industrial use of these methods may be limited by the high cost, rarefaction and toxicity of palladium. As an alternative, copper-based Ullmann aminations attracted our attention due to their low cost, low toxicity and the high abundance of copper. Copper-mediated arylation was discovered several decades before the Buchwald-Hartwig reaction, but its use was limited for a long time due to several drawbacks that restrict its industrial application. Indeed, originally, it required stoichiometric amounts of copper, high reaction temperatures and high-boiling point polar solvents. [START_REF] Reichardt | Solvents and Solvent Effects in Organic Chemistry[END_REF] Moreover, the scope of the reaction was also restricted to electron-poor aromatic substrates. [13a-c] In 2001, the discovery of new copper/bidentate ligand systems led to a spectacular renewed interest in Ullmann reactions. [10b,13] The milder conditions of the new copper-catalyzed aminations enlarged the scope in terms of substrate tolerance, chemoselectivity and enantioselectivity. [13e, 14] Despite the emergence of green chemistry, the copper-catalyzed coupling reactions using water as solvent have been rarely described. Only few examples of copper-catalyzed N-arylation in aqueous media have been reported with heterocyclic amines, [15a-l] aliphatic amines, [15g-h,l,n] amino acids [15h,m] or ammonia [15n-o] . To be efficient, these reactions required temperatures between 80 °C and 130 °C. Herein, we describe an efficient and highly selective coppercatalyzed N-arylation or heteroarylation with primary amines under micellar conditions at much lower temperature (room temperature (rt = 25 °C) or 50 °C).

Results and Discussion

In our initial screening experiments, 3-iodotoluene 1a and 3phenylpropylamine 2 were used as a prototypical substrates to establish the most suitable conditions (Figure 1). Seven commercially available classical ligands such as -diketones, diamines or phenanthroline were tested using CuI (10%) as the catalyst. NaOt-Bu was selected as a suitable base, and the reactions were carried out in water supplemented with the TPGS-750-M (2 wt%) as the surfactant. In the absence of ligand (W/L), no reaction was observed, while the nitrogen-based ligands (L3-L7) were poorly active or inactive. Both β-diketones L1 and L2 showed a promising effect with about 25% yield. To explain this modest yield, we could consider the possible oxidation of copper(I) leading to an inactive copper(II) in water. To validate this hypothesis, a reducing agent was added to regenerate the copper(I) catalyst in situ. Sodium ascorbate (NaAsc) was chosen for its ability to reduce copper(II) and its well-known use in the "click" reaction. [16] It is also often used for cross-coupling arylations with ammonia, [15o] amides, [17] and disulfides. [18] A significant improvement in yields (50%) was obtained in the presence of NaAsc with the -diketone ligands L1 and L2 (Figure 1). This result encouraged us to favor the use of copper(II) catalysts in the presence of reducing agents. The influence of eight different copper(II) salts and leaving groups were studied in combination with either dipivaloylmethane (THMD) L1 or 2-isobutyrylcyclohexanone L2 (Figure 2). Surprisingly, using L1, bromo derivatives appeared systematically more reactive than the corresponding iodo ones. This result was quite unexpected because bromoaryls are generally less reactive in Ullmann-type reactions than iodo derivatives. In contrast, L2 appeared systematically more effective with the 3-iodotoluene 1a versus 3-bromotoluene 1b.

However, it is important to note that the yields using L2 never exceeded 50%, while L1 led to yields higher than 70%. Further explorations revealed the non-efficacy of chloro, O-triflate and Otosylate as leaving groups. While copper(II) oxide showed a modest activity, the other copper(II) salts afforded good yieldsup to 75% yield with Cu(OTf)2, Cu(NO3)2.3H2O and CuBr2. Based on these results, we selected 3-bromotoluene 1b with the dipivaloylmethane L1 in combination with Cu(OTf)2, Cu(NO3)2.3H2O or CuBr2 to continue our study. We have previously highlighted the importance of reducing agents in this copper-catalyzed amination under micellar conditions. Without a reducing agent, no reaction was observed using the copper(II) catalyst (SI Figure S2). To optimize this parameter, three classical reducing agents were used (sodium ascorbate, hydroquinone and tris (2-carboxyethyl)phosphine (TCEP)) as well as three reducing sugars (D-glucose, D-fructose and D-galactose), at two different temperatures (room temperature and 50 °C, Figure 3 and SI Figure S1). In presence of Cu(OTf)2, no reaction was observed with TCEP at both temperature, nor with NaAsc at room temperature. All the other reducing agents led to the desired product 3b in moderate to excellent yieldseven at room temperature. In particular, Dglucose afforded the desired product with 84% yield at room temperature. Similar results were obtained with Cu(NO3)2.3H2O and CuBr2 in presence of sodium ascorbate and D-glucose. However, the catalytic system Cu(OTf)2/L1/D-glucose was the best combination for this green-Ullmann-type amination. To the best of our knowledge, D-glucose has rarely been used as reducing agent in organic chemistry. It has been reported for the one-pot assembly of N-fused imidazoles [19] , the reductive homocoupling of aryl halides [20] and the formation of quinolines involving a three-steps cascade. [21] Therefore, this seems to be the first report of D-glucose used as a reducing agent in a coppercatalyzed reaction in aqueous medium. Its abundance and nontoxicity make D-glucose the perfect additive for sustainable chemistry approaches. This aspect is strengthened by the efficiency of the catalytic system Cu(OTf)2/L1/D-glucose at room temperature, which is a positive parameter in terms of energy saving.

Several other parameters were further evaluated including different bases (SI Figure S3), various β-diketones (SI Figure S4), Cu/ligand ratio (SI Figure S5), catalyst loading (SI Figure S5), reducing agent molar ratio (SI Figure S2), surfactants (SI Figure S6), quantity of surfactant (SI Figure S7) and the atmosphere (SI Figure S8). Based on all these experiments, a combination of Cu(OTf)2 (10 mol%), L1 (20 mol%), D-glucose (10 mol%) and NaOt-Bu (2 equiv.) was selected as the best catalytic system to perform this Ullmann-type amination at room temperature in an aqueous medium supplemented with TPGS-750-M (2 wt%).

With the optimal conditions in hand, the scope of the reaction was investigated using a diverse set of substituted amines and Reaction conditions : Cu(OTf)2 (10 mol%), L1 (20 mol%), D-glucose (10 mol%), NaOt-Bu (2 equiv.), ArBr (1 equiv.), RNH2 (1.2 equiv.), TPGS-750-M (2 wt%), rt, 24 h. a Yields refer to isolated product. b Unpublished products were fully characterized by NMR and HR-MS data. c Use of TPGS-750-M (5 wt%). d Reaction carried out at 50 °C. e RNH2 (5 equiv.). f RNH2 (3 equiv.). g Presence of 22% of 3mb. n.r.: no reaction.

aryl or heteroaryl bromides. The results are summarized in Table 1. The reaction was efficient with aryl bromide having either electron-donating groups in ortho (3ca), meta (3a, 3b, 3cb and 3n) and para (3cc, 3d and 3e) position or electron-withdrawing groups (3f-m, o-q). In presence of a sterical hindrance at the ortho-position of the aryl-bromide (3ca), the reaction needs to be heated at 50 °C to obtain a yield of 48%. When using an aryl ring bearing both bromine and chlorine (3f, 4e) substituent, the reaction only occurred on the brominated position.

It is particularly noteworthy that even in the presence of sensitive substituents (i.e. CN 3i, CONH2 3j, CO2tBu 3k, MeCO 3l, CHO 3m or nitro 3o) the reaction proceeds successfully to provide the desired products in moderate to high yields without protection of the functional group. Starting from an acetal moiety, the aminated product 3n was isolated as a sole product when the reaction was performed at rt. However, at 50 °C a partial deprotection of the acetal was observed with the formation of 22% of the corresponding aldehyde 3mb. We then turned our attention to various bromoheterocyclic systems, such as pyridine (4a-e), pyrimidine (4f-g), pyrazine (4h) and thiophene (4i) which were efficiently N-arylated with numerous primary amines (Table 1). When the bromine atom is in the meta-position, the reaction proceeded under the classical condition (2 wt% of TPGS-750-M at rt) with good to excellent yield even with a strongly hindered amine (4c-d). Good yields were also obtained using 2bromopyrazine, 2-bromopyrimidine and 3-bromothiophene (4h, 4f and 4i). Unfortunately no reaction was observed starting from an iminochloride such as 2-chloropyridine. Next, a representative series of cyclohexyl or piperidinyl-containing amines were used with different steric hindrances at the amino group (3t-x). As illustrated in Table 1, the reaction worked even with bulky αbranched primary amines, but it is necessary to work at 50 °C to reach 60% yields (3x-y). Interestingly, when starting from a substrate bearing both primary and secondary amine moieties, only the primary amine reacted (3za, 3zb). As shown in Table 1, our methodology is compatible with free hydroxyl function. Starting from 4-bromobenzyl alcohol the Ncoupling product with phenyl-propylamine was formed in 74% yield at rt (3d, 74%). The yield could be slightly increased at 50 °C. Unfortunately, no reaction was observed with 3-aminopropanol, even in presence of an excess of amine. The lack of reaction may be explained by the chelation of copper to the aminoalcohol. To verify this hypothesis the same reaction was performed with 6aminoalcohol, a higher homologue, and we were pleased to isolate the expected coupling product in a satisfactory yield (3sb, 60%). When the alcohol is protected as an ether group, the desired product 3sc was obtained with a moderate yield of 61%. No reaction was observed with other amines, like anilines, secondary amines, or benzamides. Starting from 4bromobenzonitrile, the corresponding product 3h was formed with a yield of only 29%, due to the poor solubility of the reactant. We overcame this issue by using either 5 wt% of TPGS-750-M at rt or 2 wt% of TPGS-750-M at 50 °Cthis improved the yield to 87 and 71% respectively. The same conditions were applied with 4-(4',5',6'-trifluorophenyl)propylamine (3f), 2-bromopyridine (4a-b), and 5-bromopyrimide (4g) to overcome the solubility issues. In contrast, starting from a highly water soluble amine, like npropylamine, an excess of amine was needed (5 equiv.) to reach a yield of 67% (3r). In the absence of surfactant, our model reaction was still successful with 75% yield against 85% with TPGS-750-M (2%) (see SI Figure S7). This important result shows that this coppercatalyzed reaction is likely to occur in water rather than inside the micelle. However, with crystalline educts, the surfactant was crucial to help solubilize the reactants in the aqueous media and enhance the yield of the reaction. This is particularly true for the following examples (3i, 4a-b, 4g) where 5% of TPGS-750-M was required for optimal yield. To demonstrate the efficacy of our catalytic system and thereby its potential industrial application as a "sustainable process", the reaction was next attempted on a multigram scale. Thus, five mmoles of 3-bromotoluene 1b and 3phenylpropylamine 2 in TPGS-750-M (2 wt%) successfully lead to 3b in 86% yield under our conditions. Finally, the kinetic parameters of this reaction were monitored (Figure 4). Firstly, at 50 °C, the reaction using D-glucose as reducing agent was faster compared to the one using NaAsc. Indeed, after 30 min, 20% conversion was already observed (purple) and the conversion reached 60% after 2 hours. In contrast, with NaAsc, a latency time of 4 hours was necessary before the formation of 3b (orange). It is notable that after this latency time, the conversion rate is similar to the rate observed for the reaction using D-glucose. Interestingly, a latency time of 6 hours was also observed (green) when the reaction was performed with D-glucose at room temperature. We hypothesized that this period is required to reduce the inactive Cu(II) into the active Cu(I). This induction period might also suggest the formation of nanoparticles as the active catalysts for 2 reasons: -i) C-N cross coupling reaction was achieved recently with copper nanoparticles 22 as catalyst.

-ii) D-glucose is known to be an excellent reducing agent for the formation of copper nanoparticles in water. 23 However, different experiments performed in our laboratory invalidate these hypotheses (see SI page S-12-13).

Conclusions

In summary, we developed the first Ullmann-type reaction at low temperature (rt or 50 °C) under micellar conditions for primary aliphatic amines. These mild conditions are simple yet robust and cheap. The use of renewable feedstocks (D-glucose and TPGS-750-M composed of vitamin E), the absence of organic solvent and the economy of energy make these conditions environmentally friendly and adaptable to industrial production. This easy-to-handle procedure is extremely competitive compared to palladium-based reactions and may be used to synthesize N-containing molecules such as drugs or OLED.

Experimental Section

General procedure: Amine (1.2 equiv.) and aryl or heteroaryl halide (1 equiv.) were added to an aqueous solution of TPGS-750-M (2 wt %, 1 mL/mmol). The mixture was degassed by bubbling Argon in through (5 min). NaOt-Bu (97% purity, 2 equiv.), Cu(OTf)2 (10%), dipivaloylmethane (20%) and D-glucose (10%) were added and the resulting mixture was stirred (at 1200 rpm) at room temperature or 50 °C (24 h). The mixture was extracted twice with 3 mL of ethyl acetate. The combined organic layers were evaporated and the crude residue was purified by chromatographic column on silica gel using n-heptane and ethyl acetate as eluent.

N-Benzyl-3-methylaniline 3a

Prepared following the general procedure, using Cu(OTf)2 (18.0 mg, 0.050 mmol), dipivaloylmethane (20.9 µL, 0.100 mmol), D-Glucose (9.0 mg, 0.050 mmol), 3-bromotoluene (60.6 µL, 0.500 mmol), benzylamine (65.6 µL, 0.600 mmol) and NaOt-Bu (99 mg, 1.00 mmol) in aqueous TPGS-750-M (2 wt %, 0.50 mL), followed by purification using column chromatography (SiO2) and nheptane/ethyl acetate (95:5), yielded 3a as a yellow oil (81 mg, 82%). 1 H NMR (400 MHz, CDCl3) δ 2.30 (s, 3H), 3.90 -4.06 (br s, 1H), 4.34 (s, 2H), 6.46 -6.59 (m, 3H), 7.09 (t, J = 7.5 Hz, 1H), 7.27 -7.41 (m, 5H) ppm; 13 C NMR (100 MHz, CDCl3) δ 21.7, 48.4, 110.0, 113.6, 118.6, 127.2, 127.6, 128.6, 129.2, 139.1, 139.6, 148.3 ppm.

Figure 1 .

 1 Figure 1. Copper catalyzed amination of 3-iodotoluene in TPGS-750-M: Impact of various ligands in presence or absence of NaAsc. a Reaction conditions: CuI (10 mol%), ligand (20 mol%), NaOt-Bu (2 equiv.), 3-iodotoluene (1 equiv.), 3phenylpropylamine (1.2 equiv.), TPGS-750-M (2 wt%), 50 °C, 24 h. b Average yield of 2 experiments. c Yields were determined by HPLC/UV using benzophenone as an external standard. W/L: without ligand.

Figure 2 .Figure 3 .

 23 Figure 2. Impact of copper(II) salts and leaving groups in combination with either L1 or L2. a Reaction conditions: Copper(II) (10 mol%), L1 or L2 (20 mol%), NaAsc (10 mol%), NaOt-Bu (2 equiv.), ArX (1 equiv.), 3-phenylpropylamine (1.2 equiv.), TPGS-750-M (2 wt%), 50 °C, 24 h. b Average yield of 2 experiments. c Yields were determined by HPLC/UV using benzophenone as an external standard.

Figure 4 .

 4 Figure 4. Kinetic of the reaction at rt and 50 °C with D-glucose and sodium ascorbate. a Reaction conditions: Cu(OTf)2 (10 mol%), L1 (20 mol%), Reducting agent (10 mol%), NaOt-Bu (2 equiv.), 3-bromotoluene (1 equiv.), 3phenylpropylamine (1.2 equiv.), TPGS-750-M (2 wt%), 50 °C or rt. b Average yield of 2 runs. c Yields were determined by HPLC/UV using benzophenone as an external standard.

Figure 5 .

 5 Figure 5. Proposed mechanism of the copper cross-coupling reaction in presence of D-glucose.

Finally a reasonable

  mechanism has been proposed to explain the different observations described in SI (see page S-14-15). Cu(I) was generated in situ from Cu(II) with D-glucose. Nucleophilic substitution of Cu(I) by amine leads to the formation of [Cu(I)Nu, intermediate A]. Then oxidative addition of the aryl halide to the intermediate A generates a transient Cu(III) species (B). Finally, the C-N bond is formed by reductive elimination from B leading to the aim compound and regenerating the initial copper(I) specie (Figure 5).

Table 1

 1 Expanded scope of amination with arylbromidesa,b


Maud Bollenbach

Pedro G V Aquino

João Xavier De Araújo-Júnior

Jean-Jacques Bourguignon

Frédéric Bihel

Christophe Salomé

Patrick Wagner

Martine Schmitt

Efficient and mild Ullmann-type N-arylation of amides, carbamates, and azoles in water

A simple, sustainable, efficient, mild, and low-cost protocol was developed for D-glucose-assisted Cu-catalyzed Ullmann reactions in water for amides, carbamates, and nitrogen-containing heterocycles. The reaction was compatible with diverse aryl/heteroaryl iodides, giving highly substituted pyridine, indole, or indazole rings. This method offers an attractive alternative to the existing protocols, because the reaction proceeds in aqueous media, occurs at or near ambient temperature, and provides the N-arylated products in good-to-high yields.

Introduction

The formation of transition-metal-catalyzed C-N bonds via crosscoupling reactions plays an important role in the preparation of numerous important products in biological, pharmaceutical, and material sciences. [1] Formation of C-N bonds by Ullmann and Goldberg coupling reactions provides a powerful tool to prepare these nitrogen-containing compounds on both laboratory and industrial scales. [2] However, the classic Ullmann and Goldberg coupling reactions suffer from drawbacks such as harsh reaction conditions, including high reaction temperatures (160-200 C), extended reaction times, and the use of large amounts of Cu, and low tolerance of functional groups. [3] In 2001, an important breakthrough was achieved by two research groups with the discovery of efficient and versatile Cu/ligand systems that enable these cross-coupling reactions to occur under much milder conditions (90-110 C). [4,5] Since the last decade, the formation of C-N bonds by Cu-mediated cross-coupling approaches has led to a spectacular resurgence of interest. [6][7][8] From economic and environmental point of view, water as a solvent is a good alternative to organic solvents; [9] wide applications of N-arylated compounds in many fields have stimulated the development of new strategies for their synthesis in water. However, reactions in water suffer from the poor solubility of organic reagents in aqueous media. A micellar system can improve the solubility of components, thereby accelerating the reaction rate owing to the aggregation of reactants in the unique micellar environment. [10] Very recently, our group highlighted the efficiency of D-glucose to generate Cu(I) species from a Cu(II) source for conducting the N-arylation of primary amines in water at 25 C. [11] However, this new catalytic system (Cu(OTf)2/D-glucose, dipivaloylmethane, NaOt-Bu) failed when other classes of amines were used. N-arylamides represent an important class of compounds found in numerous bioactive products [13] such as top-selling medicines (e.g., Lipitor®, Liloderm®, and Vyvanse®) [14] as well as biological and synthetic polymers (e.g., proteins and nylons); [15,16] they also serve as versatile building blocks in the preparation of pharmaceuticals, agrochemicals, polymers, etc. [17] Therefore, we continued our efforts to develop Cu-catalyzed cross-couplings in aqueous media for that class of amine. [12] Herein, we report a simple, practical, and efficient D-glucose-assisted Cu-catalyzed Narylation of amides, carbamates, and nitrogen-containing heterocycles. Our approach includes i) use of an inexpensive Cu(II) catalyst, ii) presence of renewable feedstocks (D-glucose and TPGS-750-M composed of vitamin E), iii) absence of organic solvents, and iv) broad substrate scope of both nucleophiles and aryl or heteroaryl iodides.

Results and Discussion

First, 3-iodotoluene 1 and 4-methoxybenzamide 2a were used as the prototypical substrates to establish the most suitable conditions for our new catalytic system. CuSO4 was selected as the Cu source as it was successfully used by Wang in combination with sodium ascorbate (NaAsc) in DMSO at 100 C in a ligand-free N-arylation. [18] However, the target product was not observed at 50 C when the reaction was performed in water supplemented with TPGS-750-M (2 wt%) in combination with either NaAsc or D-glucose (Figure 1). Different reaction parameters were investigated to improve this coupling reaction in water (Figures 1234). In particular, different commercially available ligands were tested using 10 mol% CuSO4 as the Cu(II) catalyst, 10 mol% of NaAsc or D-glucose as the reducing agent, and 2 equiv of NaOt-Bu as the base in TPGS-750-M (2 wt%) at 50 C.

As reported previously, the linear β-keto ester (THMD) was inactive. [11] Reactions promoted by different bidentate N, O (L2, L7) or O, O (L3, L5, and L6) chelators gave only traces of compound (Figure 1). Phenanthroline (L4) did not effectively help the Cu salts to improve the yield of 3a. However, bidentate N, Nligands such as dimethylethylenediamine (DMEDA, L8) or racemic (L9) showed a promising effect with Next, we evaluated the lower limit of ligand/catalyst ratio compatible with high yields. This ratio significantly affected the amidation cross-coupling reaction (Figure 2). In presence of NaAsc, reducing the catalyst/ligand ratio from 1:2 to 1:1 improved the formation of 3a with either ligand L9 or L10. D-glucose was found to be independent of the ligand/metal ratio as comparable yields were obtained in both the cases. In contrast to L8, a significant decrease in the catalytic activity was observed in the presence of both the reducing agents. Finally, a decrease in the amount of CuSO4 to 5% led to an incomplete consumption of 3iodotoluene, or the reaction was inefficient (SI Figure S1).

Then, the effect of temperature was evaluated. Attempts to couple the iodohalides with amide 2a at room temperature failed when L9 was used in combination with either NaAsc or D-glucose. However, when the reaction was conducted using L10 in the presence of D-glucose, this coupling was achieved even at room temperature. The use of NaAsc gave a lower yield under the same conditions.

The reaction was also carried out using different Cu sources as shown in Figure 3. Among them, CuSO4, CuBr2, CuNO3, and Cu(OTf)2 were found to have the same efficacy with a yield of 80%.

Other Cu salts such as CuO, CuCl2, and Cu(OAc)2 provided slightly lower yields, while a significant drop of 20% conversion was observed with amorphous CuFe2O4. CuBr2 was selected for this study because of its cost-effectiveness.

Further investigation revealed the nonefficacy of aryl bromide and triflate (SI Figure S2). Various surfactants were also tested under the optimized conditions, but none of them showed a higher performance than TPGS-750-M 2% (Figure 5).

In the absence of surfactant, no reaction was observed. Only moderate yields were obtained when Brij 30 (45% yield), tween (53% yield), triton (57% yield), and TPGS 1000 (65% yield) were used. Surprisingly, the third-generation sterol-based surfactant Nok [19] was inefficient under those conditions. This may be explained by the difference in the geometry of the micelles obtained using Nok (worm-like micelles) compared to those obtained using TPGS-750-M (spherical particles).

Lastly, it is important to notice that NaOt-Bu can be replaced by NaOH with only a slight decrease of yields (5-10%) for the Narylation of 3a and 3k (see Table S1). However, in order to get the best yield, we choose to run the reaction in presence of NaOt-Bu.

Scope and limitations

Under the optimized set of conditions (10 mol% CuBr2, 10 mol% racemic trans-dimethyl 1,2-cyclohexyldiamine (L10), 10 mol% Dglucose, 2 equiv. NaOt-Bu in TPGS-750-M (2 wt%, 1 M substrate concentration) at 25 C), the scope and versatility of the reaction were investigated using variously substituted aryl iodides 1 and acetanilides 2 to prepare the corresponding benzanilides 3 at 25 or 50 C depending on the solubility of the reactant. The results are shown in Table 1. Acetanilides substituted with electronwithdrawing as well as electron-donating groups such as MeO, SMe, Cl, Br, CF3, NH2, and NO2 were well tolerated in the reaction, giving the corresponding benzanilides 3 in good yields. We were particularly pleased to find that 3l could be efficiently prepared by this protocol as trifluoromethyl-substituted amides serve as important building blocks in the preparation of diverse agrochemicals and pharmaceuticals. [20] Finally, the reaction of 2aminobenzamide with 3-iodotoluene proceeded in an excellent yield and with complete selectivity for the reaction of the amide group (3r). This N-arylation was also amenable to diverse iodoaryl coupling partners including different electronic and steric substituents. The reaction tolerated diverse functional groups such as ester (3t), ketone (3l), trifluoromethyl (3n), and nitrile (3f and 3k). Moreover, the use of an unprotected hydroxymethyl group enabled the arylation (3q, 63%). Particularly noteworthy is the reactivity difference exhibited by arylbromide versus aryliodide towards oxidative addition. The reaction of benzamide with 4-bromoiodobenzene at 50 C occurred on the iodine moiety with complete regioselectivity (3m, 79%). To further expand the scope of substrates, alkyl (3s, 3u) and aralkyl (3v, 3w) acetanilides were subjected to the standard reaction conditions; the target products were obtained in satisfactory yields (54-79%).

Starting from the highly soluble acetamide, an excess of the amide was needed (5 equiv) to achieve a yield of 60% at 50 C (3s). The reaction was also successfully extended to Reaction conditions : CuBr2 (10 mol%), L10 (10 mol%), D-glucose (10 mol%), NaOt-Bu (2 equiv.), ArI (1 equiv.), RCONH2 (1.2 equiv.), TPGS-750-M (2 wt%), 25 °C, 24 h. a Yields refer to isolated, chromatographically, purified materials. b Unpublished products were fully characterized by NMR and HR-MS data. c Reaction carried out at 50 °C. d Use of TPGS-750-M (5 wt%). e Amide (3 equiv.). f Amide (5 equiv.). g Quenched with NH3. h Product isolated by precipitation.

heteroaromatic amides. In the case of the reaction with nicotinamide, only 11% of the target product 4a was observed under standard conditions. Interestingly, when the reaction was carried out with 3 equiv of the amide, the yield became 57%. Under the same conditions, N-aryl pyrazine-(4b) and furan-(4c) carboxamide derivatives were obtained in 47% and 69% isolated yields, respectively.

To further evaluate our methodology, we attempted reactions with heteroaryl iodides. As shown in Table 1, generally the desired coupling products 5 were obtained in moderate-to-good yields depending on i) the position of the iodide and ii) the reaction conditions. N-arylation of 3-iodothiophene was conducted at 25°C, and the target product 5a was isolated in 89% yield. Similarly, the catalytic system was suitable for the amidation of 3(4)-iodopyridine derivatives, providing 5d and 5e in moderate yields (40%). A significant improvement (+20% in yield) was achieved by quenching the reaction mixture with ammonia (67% for 5d and 58 % for 5e). The N-arylation of 2-iodopyridine or thiophene tedious even at 50 C (5f, 41%, 5c, 59%). The presence of a second halogen in the pyridine moiety increased the efficiency of the coupling reaction as shown by 5h (71%) and highly functionalized pyridine derivatives 5j (70%) [21a] and 5k [21b] (63%). Finally, the cross-coupling reaction was also effective when two heterocyclic partners were used. In particular, starting from 2-thienylamide and 3-iodotoluene, the N-arylated compound 5l was isolated in 69% yield.

The reaction was affected by the steric hindrance of the nucleophile, because although cyclic aliphatic secondary amides such as pyrrolidine-2-one or its homolog (n = 0) afforded the coupling product 3x in good yields, no reaction was observed starting from the acyclic N-methylbenzamide. Of particular interest was the reaction with tert-butylcarbamate, a protected ammonia equivalent. The reaction of 3-iodotoluene at room temperature afforded the target compound 3y in 57% isolated yield. The use of 5 wt% TPGS-750-M at 25 C or 50 C improved the yield of 3y to 64% and 71%, respectively. Similar results were obtained starting from 3-iodothiophene 5b. Next, the N-arylation of oxazolidin-2-one with iodobenzene was performed. N-aryloxazolidin-2-ones have gained significant interest in the recent decade as illustrated by the FDA-approved drugs:

Toloxatone® [22] and Rivaroxaban® [23] . Under the micellar conditions, the five-membered ring cyclic carbamate (3z) was isolated in 68% yield.

To broaden the scope of our arylation protocol, the efficiency of our catalyst-ligand systems was evaluated for reactions involving weak nucleophilic nitrogen atoms such as azoles. The coupling of azoles is of great importance to the pharmaceutical industry. Pharmaceuticals incorporating this privileged motif display diverse biological activities such as anti-inflammatory, antiviral, antimicrobial, and anticancer activities. [24] Therefore, the development of efficient methods to access highly functionalized azole derivatives with respect to sustainable chemistry is highly desirable.

As shown in Table 2, representative couplings involving pyrrole, pyrazole, indole, and indazole ring-containing substrates afforded the desired N-arylated coupling products 6 in moderate-to-good yields (43-95%) at 25 C or 50 C in TPGS-750-M 5 wt%. Attempts to N-arylate 1,2,4-triazole with 3-iodotoluene failed at 50 C even with a higher loading of ligand (Cu:L = 1:2 and 1:4). The arylating agent was recovered after 20 h heating. The efficiency of the reaction was affected by the steric hindrance of an adjacent substituent as no reaction was observed starting from 2-or 7substituted indoles and 7-substituted indazoles. However, displacement of the substituent from positions 2 to 3, or 7 to 6 restored the efficiency of the reaction, and 6fa, 6fb and 6rb were obtained in respectively 77%, 90% and 91% isolated yields.

Azoles bearing both electron-donating and -withdrawing substituents coupled smoothly, affording the target products 6 in good yields and with a high tolerance of several functional groups (CF3, CN, CHO, NO2, F, Cl, Br, and OMOM). Unfortunately, as shown in Table 2, the methodology is not compatible with free phenol. However, when the phenol is protected (6n, OMOM), the desired product was obtained in a moderate yield of 44%. Notably, halogen substituents such as Cl and Br were well tolerated (6c, 6h-i, 6q, and 6u-x), thus providing handles for postcoupling transformations.

Scheme 1: Synthesis of an advances intermediate for the preparation of AZD 7594.

Scheme 2: Gram-scale experiments Notably, our method provided a straightforward way to synthesize an advanced intermediate for the preparation of AZD-7594 actually under active development (Phase II) for chronic obstructive pulmonary diseases (COPD). [25] Unfortunately, under our standard conditions, only 54% of the expected N1-Aryl indazole 6y was isolated after the purification.Beside the remaining starting materials, we also the formation of a small amount of his regioisomer, the N2-Aryl indazole 6z with a N /N 2 -selectivity of 1:6. Addition of an organic solvent (THF) to the reaction mixture, as recently reported by Gallou and coworkers, [26] was the key to success. The presence of a cosolvent channeled the reactive components of the system between water and the lipophilic micellar interior, playing both the roles of slightly increasing the solubility and rate of dissolution. To our pleasure, under these new conditions the target product 6y was isolated in 87% yield (6z: 13%).

To demonstrate the usefulness of this novel N-amidation protocol, two gram-scale experiments were carried out under the standard conditions. Using 3-iodobenzonitrile (10 mmol) and benzamide, the corresponding N-(3-cyano-phenyl)benzamide 3k was isolated in 78% yield. In addition, the reaction with indole and 3iodotoluene also proceeded smoothly, affording product 6e in 76% yield.

All the final compounds 3-6 were purified by flash chromatography using a step gradient algorithm to i) optimize the separation of the target compound and ii) use a minimal amount of solvent. However, to avoid the column chromatography step, we also developed a greener procedure to recover the final solid compounds. The strategy was based on a convenient purification procedure based on precipitation upon the addition of water. The micellar media was first extracted with isopropyl acetate and then passed through a cotton-wool filter to remove traces of Cu. The filtrate was concentrated under vacuum and then diluted with isopropanol. After the precipitation from cold water and filtration, this procedure allowed the isolation of 3a and 5l in 67% and 60% yields (Table 1), respectively, and the purity was higher than 98% (determined by HPLC). For these two examples, the reaction was run on a small scale (see SI). Scale-up of the synthesis from laboratory scale to pilot scale will probably maximize the precipitation of the expected final compounds and in fact improve the product yields.

In order to investigate the greener and economical aspects of the developed catalytic system, a recyclability study was carried out for the N-arylation of indole with 3-iodotoluene. As illustrated in Figure 6 the yield drops from 95% to 88% for the second cycle and to 79% for the third cycle. These results validated the possibility to recycle CuBr2. After each cycle, the aqueous reaction mixture was extracted using diethyl ether to remove all traces of the product or reactants. The resulting aqueous solution containing the CuBr2 catalyst was directly used for the next catalytic cycle.

Conclusions

We developed a facile, convenient and mild protocol for aqueous Ullmann cross-coupling catalysis. We successfully demonstrated that the catalytic system (CuBr2, D-glucose, L10, and NaOt-Bu) allowed the high-yielding arylation of diverse nitrogen nucleophiles at relatively low temperatures (25-50 C). Diverse substituted aryl or heteroaryl iodides were readily coupled with several amides, carbamates, and azoles. With respect to azoles, the catalytic system was particularly efficient for condensations with substituted 3-, 4-, 5-and 6-indazole and indole derivatives. The method enables the selective conversion of polyhalogenated aromatics and provides access to differentially substituted pharmacologically relevant motifs. Mildness, low-cost, experimental simplicity, and ability to use water and D-glucose as the reducing agent are the features of the developed methodology that make it particularly well suited for industrial-scale syntheses.

Experimental Section

General procedure

Nucleophile (1.2 equiv.) and aryl or heteroaryl halide (1 equiv.) were added to an aqueous solution of TPGS-750-M (2 wt %, 1 mL/mmol). The mixture was degassed by bubbling Argon (5 min). NaOt-Bu (97% purity, 2 equiv.), CuBr2 (10 mol%), trans-N,N'-dimethylcyclohexane-1,2-diamine (10 mol%) and D-glucose (10 mol%) were added and the resulting mixture was stirred (at 1200 rpm) at 25 °C or 50 °C (20 h). The mixture was extracted twice with 3 mL of isopropyl acetate. The combined organic layers were evaporated and the crude residue was purified by chromatographic column on silica gel using n-heptane and ethyl acetate as eluent.

4-Methoxy-N-(3-methylphenyl)benzamide 3a

Prepared following the general procedure, using CuBr2 (11.2 mg, 0.050 mmol), trans-N,N'-dimethylcyclohexane-1,2-diamine (7.9 µL, 0.050 mmol), D-Glucose (9.0 mg, 0.050 mmol), 3-iodotoluene (64.2 µL, 0.500 mmol), 4methoxybenzamide (90.7 mg, 0.600 mmol) and NaOt-Bu (99 mg, 1.000 mmol) in aqueous TPGS-750-M (2 wt %, 0.50 mL), followed by purification using column chromatography (SiO2) and n-heptane/ethyl acetate (7:3), yielded 3a as a white solid (98 mg, 0.405 mmol, 81%). mp = 117-119 °C; 1 H NMR (400 MHz, CDCl3) δ ppm 2.34 (s, 3H), 3.87 (s, 3H), 6.95-6.97 (m, 3H), 7.25 (t, J = 7.7 Hz, 1H), 7.41 (d, J = 7.7 Hz, 1H), 7.51 (s, 1H), 7.80 (d, J = 8.9 Hz, 2H), 7.87 (br s, 1H); 13 C NMR (101 MHz, CDCl3) δ ppm 21.5, 55.4, 113.9, 117.2, 120.8, 125.1, 127.2, 128.8, 128.9, 138.0, 138.9, 162.4, 165.2.

Figure 1 .Figure 2 .

 12 Figure 1. Copper catalyzed amidation of 3-iodotoluene in TPGS-750-M at

Figure 3 .

 3 Figure 3. Copper catalyzed amidation of 3-iodotoluene in TPGS-750-M at 25 or 50 °C: Impact of ligands (L9, and L10) in presence of NaAsc or D-Glucose. a Reaction conditions: CuSO4. 5H2O (10 mol%), ligands (10 mol%), NaOt-Bu (2 equiv.), 3-iodotoluene (1 equiv.), 4-methoxybenzamide (1.2 equiv.), TPGS-750-M (2wt%), 25 or 50 °C, 20 h. b Average yield of 2 runs. c Yields were determined by HPLC/UV using benzophenone as an external standard.

Figure 4 .Figure 5 .

 45 Figure 4. Impact of copper(II) salts : a Reaction conditions: Copper(II) (10 mol%), L10 (10 mol%), NaOt-Bu (2 equiv.), 3-iodotoluene (1 equiv.), 4methoxybenzamide (1.2 equiv.), TPGS-750-M (2wt%), 25 °C, 20 h. b Average yield of 2 runs. c Yields were determined by HPLC/UV using benzophenone as an external standard.

Figure 6 .

 6 Figure 6. Recycling of the copper catalyst and the micellar phase

Table 1 :

 1 Scope of amides and carbamates derivatives

Table 2 :

 2 Scope of NH heterocycles

Reaction conditions : CuBr2 (10 mol%), L10 (10 mol%), D-glucose (10 mol%), NaOt-Bu (2 equiv.), ArI (1 equiv.), HetNH (1.2 equiv.), TPGS-750-M (5 wt%), 50 °C, 24 h. a Yields refer to isolated, chromatographically, purified materials. b Unpublished products were fully characterized by NMR and HR-MS data.

c

Reaction carried out at 25 °C. d Use of TPGS-750-M (2wt%). e Nucleophile (3 equiv.). f Nucleophile (5 equiv.). g 18% on N2. h 12% on N2. i Preformulated.

Acknowledgements

We thank Dr Frédéric Leroux for the gift of 4-Iodo pyridine trifluoromethyl ether derivative. Financial supports from MNERT (M.B.) and CAPES-Br (P.G.V. A) are gratefully acknowledged with thanks. We thanks Dr Gallou (Novartis) for the gift of TPGS-750-M and ind(az)ols derivatives and Isochem for the gift of TPGS-1000.

Keywords: amination • amides • NH heterocycle • copper • micelles

Entry for the Table of Contents

Naturally efficient! A protocol for the smooth N-arylation of amides, carbamates and azoles is described in water at almost rt with the help of cheap and commercial catalysts. The method enables the selective conversion of polyhalogenated aromatics and provides access to pharmacologyrelevant motifs.

M. Bollenbach, P. G. V. Aquino, J. X. De Araújo Jr., J.-J. Bourguignon, F. Bihel, C. Salomé, P. Wagner, M. Schmitt*


Mark F Nüesch

Miloš T Ivanović

Jean-Benoît Claude

Daniel Nettels

Robert B Best

Jérôme Wenger

Benjamin Schuler

Single-molecule detection of ultrafast biomolecular dynamics with nanophotonics

Single-molecule Förster resonance energy transfer (FRET) is a versatile technique for probing the structure and dynamics of biomolecules even in heterogeneous ensembles. However, due to the limited fluorescence brightness per molecule and the relatively long fluorescence lifetimes, probing ultrafast structural dynamics in the nanosecond timescale has thus far been very challenging. Here we demonstrate that nanophotonic fluorescence enhancement in zero-mode waveguides enables measurements of previously inaccessible low-nanosecond dynamics by dramatically improving time resolution and reduces data acquisition times by more than an order of magnitude. As a prototypical example, we use this approach to probe the dynamics of a short intrinsically disordered peptide that were previously inaccessible with single-molecule FRET measurements. We show that we are now able to detect the low-nanosecond correlations in this peptide, and we obtain a detailed interpretation of the underlying distance distributions and dynamics in conjunction with all-atom molecular dynamics simulations, which agree remarkably well with the experiments. We expect this combined approach to be widely applicable to the investigation of very rapid biomolecular dynamics.

Investigating the entire spectrum of biomolecular dynamics is essential for understanding the mechanisms of biological processes at the molecular scale. A powerful class of methods for probing a broad range of timescales is single-molecule fluorescence spectroscopy [START_REF] Selvin | Single-Molecule Techniques: A Laboratory Manual[END_REF][START_REF] Sauer | Handbook of Fluorescence Spectroscopy and Imaging: From Single Molecules to Ensembles[END_REF][START_REF] Schuler | Single-molecule spectroscopy of protein folding dynamics-expanding scope and timescales[END_REF][START_REF] Dimura | Quantitative FRET studies and integrative modeling unravel the structure and dynamics of biomolecular systems[END_REF][START_REF] Juette | Single-molecule imaging of non-equilibrium molecular ensembles on the millisecond timescale[END_REF][START_REF] Chung | Protein folding transition path times from single molecule FRET[END_REF][START_REF] Lerner | Toward dynamic structural biology: Two decades of single-molecule Forster resonance energy transfer[END_REF][START_REF] Mazal | Single-molecule FRET methods to study the dynamics of proteins at work[END_REF] . The application of single-molecule Förster resonance energy transfer (FRET), e.g., is particularly common for obtaining information on molecular distances and dynamics from milliseconds to minutes. However, monitoring more rapid intramolecular distance dynamics often remains a challenge.

Timescales down to the sub-microsecond regime can be reached with fluorescence correlation spectroscopy 2 (FCS) combined with single-molecule FRET between a donor and acceptor fluorophore attached to specific sites on biomolecules [START_REF] Schuler | Single-molecule spectroscopy of protein folding dynamics-expanding scope and timescales[END_REF][START_REF] Dimura | Quantitative FRET studies and integrative modeling unravel the structure and dynamics of biomolecular systems[END_REF] . One important application of this nanosecond fluorescence correlation spectroscopy (nsFCS) has been the investigation of the rapid chain dynamics of unfolded polypeptides and intrinsically disordered proteins [START_REF] Nettels | Ultrafast dynamics of protein collapse from single-molecule photon statistics[END_REF][START_REF] Schuler | Single-Molecule FRET Spectroscopy and the Polymer Physics of Unfolded and Intrinsically Disordered Proteins[END_REF] . By globally analyzing the donor-donor, acceptor-acceptor, and donoracceptor fluorescence intensity correlations, distance dynamics can be quantified and distinguished from other contributions, such as fluorescence quenching or rotational motion [START_REF] Schuler | Single-Molecule FRET Spectroscopy and the Polymer Physics of Unfolded and Intrinsically Disordered Proteins[END_REF][START_REF] Haenni | Intramolecular distances and dynamics from the combined photon statistics of single-molecule FRET and photoinduced electron transfer[END_REF] . Moreover, subpopulations can be separated and their dynamics extracted even for heterogeneous mixtures of conformational states [START_REF] Schuler | Single-molecule spectroscopy of protein folding dynamics-expanding scope and timescales[END_REF][START_REF] Dimura | Quantitative FRET studies and integrative modeling unravel the structure and dynamics of biomolecular systems[END_REF][START_REF] Nettels | Ultrafast dynamics of protein collapse from single-molecule photon statistics[END_REF] . Disordered proteins of 50 to 100 residues typically exhibit chain reconfiguration times of tens to hundreds of nanoseconds. [START_REF] Schuler | Single-Molecule FRET Spectroscopy and the Polymer Physics of Unfolded and Intrinsically Disordered Proteins[END_REF] However, nsFCS currently suffers from two major limitations. First, data acquisition times of ten hours or more are often required for reliably quantifying dynamics in the sub-microsecond regime, which limits sample throughput. Given typical photon count rates below 10 6 s -1 , the probability of detecting photons separated by nanosecondsthe key requirement for retrieving information on this timescaleis intrinsically low. Second, the shortest accessible times are limited by the fluorescence lifetimes of the fluorophores, typically a few nanoseconds. If the distance dynamics of interest are not well separated from this range, distinguishing the two contributions becomes difficult and has limited the use of nsFCS to distance dynamics above ~20 ns. [START_REF] Schuler | Single-Molecule FRET Spectroscopy and the Polymer Physics of Unfolded and Intrinsically Disordered Proteins[END_REF] Here we use nanophotonics in zero-mode waveguides (ZMWs) to overcome these limitations. ZMWs are subwavelength apertures (Figure 1a, Figure S1) that were first introduced as a means of reducing the observation volume to the attoliter range and thus enabling single-molecule detection at much higher concentrations than previously possible [START_REF] Levene | Zero-mode waveguides for singlemolecule analysis at high concentrations[END_REF][START_REF] Zhu | Zero-Mode Waveguides for Single-Molecule Analysis[END_REF] . However, another major advantage of ZMWs for single-molecule spectroscopy is the enhancement of fluorescence emission by the local environment in the nanocavity [START_REF] Novotny | Principles of Nano-Optics[END_REF][START_REF] Enderlein | Modification of Forster Resonance Energy Transfer Efficiency at Interfaces[END_REF][START_REF] Punj | Plasmonic antennas and zero-mode waveguides to enhance single molecule fluorescence detection and fluorescence correlation spectroscopy toward physiological concentrations[END_REF] : The fluorescence brightness is increased, which improves the signal-to-noise ratio and reduces data acquisition times. Simultaneously, fluorescence lifetimes are decreased, which enables access to a broader range of dynamics in the nanosecond regime.

Figure 1 shows the effect of ZMWs produced by focused ionbeam milling in a 100-nm aluminum layer on the confocal singlemolecule detection of the commonly used Alexa Fluors 488 and 594 attached as FRET donor and acceptor, respectively, to the termini of the short, disordered peptide G(AGQ)6AGC. As expected from the smaller observation volume in the 120-nm ZMWs, the translational diffusion time of this AGQ peptide is much shorter than in confocal measurements without ZMW (Figure 1d, Figure S2). More importantly, however, the fluorescence lifetimes of the dyes are reduced by about twofold in the ZMWs (Figure 1b), and the average count rate per molecule (CRM) is enhanced by about six-to eightfold (Figure 1c, see Methods). This nanophotonic enhancement of the electric field and the connected effects on the decay and transfer rates of donor and acceptor in the ZMWs are expected to cause a change in the distance dependence of the FRET efficiency [START_REF] Baibakov | Extending Single-Molecule Forster Resonance Energy Transfer (FRET) Range beyond 10 Nanometers in Zero-Mode Waveguides[END_REF] . The small shift in efficiency we observe (<0.1, Figure 1e) indicates only a moderate effect on the energy transfer process [START_REF] Punj | Plasmonic antennas and zero-mode waveguides to enhance single molecule fluorescence detection and fluorescence correlation spectroscopy toward physiological concentrations[END_REF] that can be accounted for by calibration (see Methods, Figure S3) and shifts sensitivity towards slightly shorter distances. Singlemolecule FRET and nsFCS are thus feasible in ZMWs with this commonly used dye pair.

We use the Alexa488/594-labeled disordered AGQ peptide to assess the enhancement of nsFCS by ZMWs (Figure 1d, Figure 2). In 7.4 M urea, where the peptide is more expanded than in the absence of denaturant (Figure S3), its rapid end-to-end distance fluctuations result in a correlation time of only about twice the natural fluorescence lifetimes of the fluorophores (Figure 2a, Figure 1d). This situation is challenging for conventional nsFCS, [START_REF] Nettels | Unfolded Protein and Peptide Dynamics Investigated with Single-Molecule FRET and Correlation Spectroscopy from Picoseconds to Seconds[END_REF] because the correlation caused by conformational dynamics overlaps with the pronounced drop of the curve at short times caused by photon antibunching (Figure 2a, Figure S2). With the ZMWs, however, the reduced fluorescence lifetimes lead to a narrower photon antibunching component, and the rapid conformational dynamics can be clearly resolved in the correlation functions (Inset Figure 1d, Figure 2b, Figure S2). We extract a characteristic relaxation time corresponding to conformational dynamics of 8.8 ± 0.3 ns (Figure 2b). Even in the absence of urea, where the AGQ peptide is more compact (Figure S3) and the conformational dynamics are faster still, nsFCS in ZMWs can resolve the contribution from chain dynamics (Figure 2d). The observed correlation time of 4.2 ± 0.5 ns is very close to the natural fluorescence lifetimes of the dyes and thus beyond the reach of measurements without ZMWs (Figure S4). Furthermore, the nanophotonic enhancement of the fluorescence emission rates enables a marked reduction in the data acquisition time required for a quantitative analysis of rapid chain dynamics (Figure 2b), even in mixtures of different proteins (Figure S5). Instead of the typical measurement times of ten hours or more, adequate statistics can be achieved in tens of minutes (Figure 2a,b; Figure S6).

The rapid dynamics accessible with nsFCS measurements in ZMWs enable a direct comparison with molecular dynamics (MD) simulations to gain a detailed picture of the underlying polypeptide chain dynamics. We thus performed all-atom explicit-solvent MD simulations of the AGQ peptide, including explicit donor and acceptor dyes [START_REF] Vaiana | Fluorescence quenching of dyes by tryptophan: Interactions at atomic detail from combination of experiment and computer simulation[END_REF][START_REF] Graen | AMBER-DYES: Characterization of Charge Fluctuations and Force Field Parameterization of Fluorescent Dyes for Molecular Dynamics Simulations[END_REF] , using a force field and water model suitable for disordered proteins, Amber99SBws/TIP4P2005s 22 , and a previously optimized parameterization of the fluorophores [START_REF] Best | Quantitative Interpretation of FRET Experiments via Molecular Simulation: Force Field and Validation[END_REF] and urea [START_REF] Zheng | Empirical Optimization of Interactions between Proteins and Chemical Denaturants in Molecular Simulations[END_REF] (Figure 2e and Video S1). Extensive sampling of peptide conformations was ensured with 16 μs of total simulation time (Figure S7). To enable a direct comparison with the measurements rather than with derived quantities, we simulated photon emission based on the MD trajectories, using the fluorescence lifetimes, excitation rates, and detector crosstalk corresponding to the experiments, including the influence of the ZMWs on the photophysics. In the simulation, the exact dye configuration is known for every simulation frame, so both the distance-and the orientation-dependence of Förster transfer can be taken into account explicitly (see Methods for details). The resulting fluorescence correlation functions (Figure 2c) are in remarkable agreement with the experimental data (Figure 2b), suggesting a faithful representation of the peptide dynamics in the simulations. In the absence of urea, the more rapid chain relaxation is also reproduced accurately, but an additional slower component points towards a slight overestimate of dye interactions (Figure S8). The agreement between the average transfer efficiencies from simulations (0.80 ± 0.01 with urea, 0.92 ± 0.01 without urea) and experiments (0.82 ± 0.03 with urea, 0.94 ± 0.03 without urea) provides an additional indication that MD simulations using current force fields with appropriately balanced proteinwater interactions [START_REF] Best | Balanced Protein-Water Interactions Improve Properties of Disordered Proteins and Non-Specific Protein Association[END_REF][START_REF] Robustelli | Developing a molecular dynamics force field for both folded and disordered protein states[END_REF] can provide a realistic picture of the dynamics and conformational ensembles of disordered peptides and proteins [START_REF] Shrestha | Generation of the configurational ensemble of an intrinsically disordered protein from unbiased molecular dynamics simulation[END_REF][START_REF] Graen | Transient Secondary and Tertiary Structure Formation Kinetics in the Intrinsically Disordered State of alpha-Synuclein from Atomistic Simulations[END_REF][START_REF] Zerze | Diffusive Dynamics of Contact Formation in Disordered Polypeptides[END_REF] . Moreover, by comparing simulations of the AGQ peptide in urea with and without fluorophores, we can infer that the end-toend distance correlation time of the unlabeled peptide is ~30 % lower than that of the labeled one (see SI Methods). This difference is not unexpected given the size of the probes compared to this relatively small peptide (the relative contribution will be less for larger IDPs), but an accurate value cannot easily be inferred from the experimental data alone.

In summary, nanophotonics-enhanced nsFCS provides new opportunities for probing rapid biomolecular dynamics. The extended range of accessible timescales in the low nanosecond regime together with the pronounced reduction in measurement time eliminate previous limitations of nsFCS. The method will thus enable the investigation of peptides, disordered protein regions, nucleic acids, and other biomolecules in a time range that has so far been difficult to monitor, with much higher throughput than previously possible, and with less stringent demands on long-term sample stability. The method is complementary to the structural information and dynamics available from techniques such as NMR [START_REF] Rezaei-Ghaleh | Local and Global Dynamics in Intrinsically Disordered Synuclein[END_REF][START_REF] Milles | Characterization of intrinsically disordered proteins and their dynamic complexes: From in vitro to cell-like environments[END_REF] , and we expect it to be directly transferable to other single-molecule fluorescence techniques, such as photo-induced electron transfer [START_REF] Sauer | PET-FCS: probing rapid structural fluctuations of proteins and nucleic acids by singlemolecule fluorescence quenching[END_REF] . Finally, results from nanophotonics-enhanced nsFCS can provide stringent benchmarks for the further optimization of molecular dynamics force fields, and in synergy with the resulting simulations, they will help to obtain an increasingly detailed understanding of biomolecular dynamics over the full range of relevant timescales.

ASSOCIATED CONTENT

Figure 1 .

 1 Figure 1. Enhancing nsFCS with zero-mode waveguides (ZMWs). (a) Schematic depiction of confocal measurements of the AGQ peptide labeled with Alexa 488 and 594 in a ZMW. Inset: scanning electron micrograph of a 120-nm aluminum ZMW nanoaperture (Figure S1). (b) Polarization-resolved fluorescence lifetime decays of donor and acceptor measured confocally without (gray) and with ZMW (color; darker: parallel polarization, lighter: perpendicular polarization; instrument response function shaded in gray; average lifetimes from the fits (solid lines) are indicated). Donor decays are from the donor-only population, acceptor decays from the FRET population after direct acceptor excitation (see Methods). (c) Time-binned (0.2 ms) fluorescence traces of donor and acceptor emission without (gray) and with ZMW (green, red), from measurements with similar average numbers of molecules in the observation volume (confocal: ~0.2, ZMW: ~0.24); average count rates per molecule (CRM, see Methods) are indicated. (d) Normalized correlation functions from picoseconds to milliseconds of the labeled peptide measured without (gray, 40 h acquisition time) and with ZMW (color, 7 h acquisition time). Global fits with Eq. 5 (see Methods) are shown as black solid lines. Insets show magnified views of the time range where distance dynamics dominate the correlations. The timescales of photon antibunching and translational diffusion with (colored, black) and without ZMW (gray) are indicated as dashed vertical lines. (e) Transfer efficiency histograms (see Methods for corrections) observed without (top) and with ZMW (bottom); black: measured, shaded: shot noise-limited photon distribution analysis 9 . All measurements shown are in 50 mM sodium phosphate, 7.4 M urea, pH 7.4

Figure 2 .

 2 Figure 2. Probing the rapid dynamics of a disordered peptide with enhanced nsFCS. (a,b,c) Overlay of donor (green) and acceptor (red) fluorescence autocorrelations and donor-acceptor crosscorrelations (blue) from measurements of the disordered AGQ peptide in 7.4 M urea without ZMW (a) and with ZMW (b) after 7 h of data acquisition (inset in (a) after 20 h and in (b) after 0.2 h of data acquisition) with 100ps time binning, and comparison with fluorescence correlation curves simulated based on all-atom molecular dynamics (MD) trajectories (c) using photophysical parameters corresponding to measurements in ZMWs (black solid lines are fits with Eq. 8, see Methods, with resulting fluorescence intensity correlation times, τcd, as indicated, with uncertainties estimated from three independent measurements; see Figure S7 for uncertainties of τcd in (c)). (d) nsFCS measurement with ZMW of the AGQ peptide without urea. The slight additional correlation decay visible for longer times is caused by contact quenching between donor and acceptor. (e) Representative snapshots from MD simulations of the disordered G(AGQ)6AGC peptide with explicit donor and acceptor dyes, Alexa Fluors 488 (green) and 594 (red) and various inter-dye distances. (f) Distance distributions, P(r), from MD simulations in 7.4 M urea (gray histogram: complete distribution; blue histogram: emissive conformations excluding dye-dye contacts) with corresponding root-mean-square inter-dye distance, r 2  1/2 (dashed vertical line)

ACKNOWLEDGMENT

We thank Andrea Sottini for a sample of labeled prothymosin α, and we thank Mikhail Baibakov, Satyajit Patra, Erik Holmstrom, and Hervé Rigneault for stimulating and helpful discussions. This work was supported by the Swiss National Science Foundation (B.S.), the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (grant agreement 723241, J.W.), the Agence Nationale de la Recherche (ANR) under grant agreement ANR-17-CE09-0026-01 (J.W.), the Intramural Research Program of the NIDDK at the National Institutes of Health (R.B.B.), a grant from the Swiss National Supercomputing Centre (CSCS) under project 334, and by PRACE with access to Joliot-Curie at GENCI@CEA, France.

AUTHOR INFORMATION

Corresponding Authors * robert.best2@nih.gov, * jerome.wenger@fresnel.fr, * schuler@bioc.uzh.ch

Notes

The authors declare no competing financial interests.


Sreevardhan Reddy Sanapureddy

Laurent Plasseraud
email: laurent.plasseraud@u-bourgogne.fr

(n-Bu2Sn)2O(CO3): An active, robust and recyclable organotin(IV) for the direct synthesis of linear organic carbonates from carbon dioxide and alcohols

Keywords: Carbon dioxide utilization, Alcohols conversion, Linear organic carbonates, Diorganotin(IV), Carbonato complex

Organotin(IV) compounds are known to promote the direct synthesis of organic carbonates from carbon dioxide and alcohols. In the past, structural studies have highlighted that the carbonato moiety is a recurring ligand of tin species collected during the CO2 pressurized reactions. In a mimetic approach and in order to achieve an available and recyclable precursor, the title compound, (n-Bu2Sn)2O(CO3) (1) was prepared in a single step by reacting commercial di-n-butyltin dichloride with an aqueous solution of sodium carbonate. 1 was characterized by infrared spectroscopy, thermogravimetric and elemental analysis.

Multinuclear NMR investigations in solution were also conducted. 1 was then evaluated for the direct carbonation of alcohols (methanol, ethanol, n-butanol and isopropanol) under CO2 pressure. Recycling experiments were performed showing the efficient reuse of 1 without loss of activity. Furthermore the infrared fingerprint of 1 was preserved even after several runs demonstrating a good stability. The effects of pressure and of reaction time on the DMC formation were also studied.

Introduction

The past two decades have witnessed a steadily growing interest in the chemical transformation of carbon dioxide into chemicals with higher value-added. The large number of review articles published on this field in recent years [START_REF]Non-exhaustive list of review articles published since 2012: a) I. Omae[END_REF], as well as the periodically organization of several international events [START_REF]ICCDU XIV -International Conference on Carbon Dioxide Utilisation / Sheffield, 7 th Carbon Dioxide Utilisation Summit / Lyon; 5 th Conference on Carbon Dioxide as Feedstock for Fuels, Chemistry and Polymers / Cologne; Closing the Carbon Cycle[END_REF] reflect the strong attention and involvement of the scientific community worldwide. Considered initially as a waste, in particular partially responsible for the greenhouse effect, carbon dioxide is increasingly seen as a potential valuable raw-material.

Thus, with the depletion of fossil fuels, CO2 can be considered as an alternative and abundant source of carbon, acting as C1-building blocks. To date a wide range of CO2-based syntheses have been identified leading to several class of commodity chemicals, some of them have been produced at an industrial scale for a long time.

Organic carbonates, (RO)2CO, also called carbonic esters represent historical target molecules of the concept of CO2 utilization, often considered as a case study. They are classified in three groups. Depending on the nature of R, they can be linear, cyclic or polymeric. Highly oxygenated and polar, organic carbonates are considered as environmentally friendly and used for different types of applications. Dialkyl carbonates are increasingly used as solvent in organic synthesis and homogeneous catalysis, taking advantage of their green character [3].

Conventionally organic carbonates are prepared from hazardous reactants such as phosgene and carbon monoxide. The alternative routes involving CO2 are particularly attractive in terms of green chemistry and atom economy. While the synthesis of cyclic carbonates by reacting oxiranes with CO2 has been widely studied and is now well documented [4], the direct synthesis of linear organic carbonates from alcohols and CO2 still arouses a strong interest from academic groups and continues to be a real challenge. In this context and from an organometallic chemistry point of view, organotin(IV) compounds are recognized from a long time as efficient precursors for the transformation of carbon dioxide into linear alkyl carbonates (Eq. 1).

Since the preliminary work of Kizlink during the 90s [5], several groups have reported the beneficial use of tin complexes for the carbonate synthesis, directly in solution [6] or grafted on various supports [7]. Furthermore, organotin(IV) compounds are also known to be effective for the CO2-based synthesis of 2-oxazolidones [8] and carbamates [9]. In our group, we especially focused on diorganotin(IV) alkoxides R2Sn(OR')2 (R = n-Bu, tert-Bu, Bz; R' = Me, Et, i Pr). We studied in particular the CO2 insertion into the Sn-OR bond and the assistance role of these complexes for the direct carbonation of alcohols (Eq. 1) [10]. Recently we supported mechanistic insights by high-pressure NMR spectroscopy studies in a sapphire tube [11].

Interestingly, when dialkoxide diorganotin(IV) compounds are used as precursors, we characterized by X-ray diffraction analysis and during recycling experiments, unexpected structures of polynuclear oxodiorganotin(IV) complexes bearing carbonato ligands [12]. Two of them obtained for the di-tert-butyl and di-n-butyl derivatives, respectively, are depicted in scheme 1. Moreover, solid state 119 Sn NMR investigations conducted on insoluble organotin residues collected from high pressure reactions have led to suggest the formation of cross-linked networks based on a central contribution of the carbonato ligands (Scheme 2) [13]. Thus, organotin carbonates occupy a preponderant role in tin-catalyzed organic carbonate synthesis. Based on these previous experimental results and with the aim of a mimetic approach, we seek to develop a simple, robust, and recyclable precursor for the direct synthesis of linear dialkyl carbonates from alcohols and carbon dioxide. We report in this paper the preparation, the characterization and the activity evaluation of (n-Bu2Sn)2O(CO3) for the direct carbonation of various primary alcohols.

Results and discussion

Synthesis and characterization of (n-Bu2Sn)2O(CO3) (1)

The preparation and characterization of carbonates derivatives of organotin complexes were initially studied from the 60s to 80s [14]. Since then, and due to the growing interest for the CO2 activation, several compounds were isolated as single crystals and their structures have been fully resolved. In general the carbonato ligand exhibits 2-O,O' [15] and 3-O,O',O" [16] coordination modes, linking distinct tin atoms and leading to the formation of polynuclear and polymeric structures. Rare examples of organotin carbonates with terminally CO3 moiety have been recently reported by Jambor and co-workers for mononuclear tin complexes [17]. In 1976, Goel and co-workers reported the synthesis and the characterization of (Me2Sn)2O(CO3) by reacting an aqueous solution of potassium carbonate with an acetone solution of dimethyltin dichloride [14b]. Interestingly, previously we concluded on the basis of solid-state NMR experiments that the tin-residue collected from the direct carbonation of alcohols was related to the oxycarbonate structure advanced by Goel [13]. Therefore, we decided to prepare the n-butyl analogue to (Me2Sn)2O(CO3) by adding an aqueous solution of sodium carbonate to a methanolic solution of di-n-butyltin dichloride (Eq. 2).

The initial Goel's procedure was modified by keeping for 24 h the reaction mixture under CO2 pressure (see experimental part). Indeed, we observed experimentally that the conditions used for the synthesis of (Me2Sn)2O(CO3) (CO2-saturated acetone solution) were insufficient for the preparation of (n-Bu2Sn)2O(CO3). Thus, the infrared spectra recorded from the material synthesized according to the Goel's conditions showed that the carbonate bands (asCO3 and sCO3) exhibited much lower intensities than expected (Fig. S1a). In addition, the decarbonation occurs rapidly from 100 °C (Fig. S1b). A CO2 pressure treatment was found to be beneficial leading to IR data consistent with the Goel's description. Finally, (n-Bu2Sn)2O(CO3) (1) was obtained as a white solid which turns out insoluble in most common organic solvents. Several batches were synthesized with yields in the range of 35-45%. The elemental analyses confirm the reproducibility of 1 and are in agreement with the expected theoretical formula (Table 1). The tin content of one of the batches was also determined by ICP-AES and matches satisfactorily with C17H36O4Sn2 (found: Sn = 45.95%; calculated: Sn = 43.82%). The infrared spectrum of a fresh sample of 1 (ATR mode), dried under vacuum at room temperature (Figure S2a), also corroborates the organotin oxycarbonate structure. The n-butyl chains are distinctly identified by the vibration bands located in the range of 3000-2900 cm 1

Tableau

(CH). The carbonate function is recognizable by the two large bands at 1507 and 1371 cm 1 , and corresponding to asymmetric and symmetric CO3 stretching elongations, respectively. In the past, comparable values which characterize bridging carbonate groups were reported by Goel for (Me2Sn)2O(CO3) [14b]. The bands in the 650-500 cm 1 region reflect Sn-O bonds.

Increasing the temperature during the drying stage does not induce significant changes on the structure of 1, the infrared fingerprints remain strictly comparable (Figures S2b andS2c).

However at 150 °C the spectrum undergoes substantial transformations. The characteristic absorptions of the carbonate function disappeared leading unambiguously to the formation of di-n-butyltin oxide (Figure S2d).

The thermogravimetric analysis (TGA) of 1 was carried out under nitrogen atmosphere at a heating rate of 5 ºC min 1 (Figure 1). A major weight loss of 8 % is recorded in the range of 149-160ºC, it can be attributed to the release of carbon dioxide. The CO2 wt% calculated for (n-Bu2Sn)2O(CO3) ( 1) is 8.12 which is very close to the loss experimentally observed. This analysis support also the formula of 1. At the end of each run, the infrared fingerprint of the tin-based residues was recorded and compared to 1. Interestingly, as demonstrated on Figure S6, the spectra are almost similar.

Thus, (n-Bu2Sn)2O(CO3) does not undergo significant changes during the successive runs and is recovered without alteration. Elemental analyses also support the preservation and the stability of 1.

Tableau. In the presence of DMP, infrared spectra of the residues highlight a noteworthy evolution (Figure S7a and S7b). In addition to the spectrum of 1, three additional bands appear at 2804, 1539 and 1062 cm 1 and can be attributed to OCH, CO3 and OC absorptions, respectively. They remarkably match with the signature of the deca-tin compound, (n- Such reminiscent methoxy groups are observable in bridging positions in the structure of (n-Bu2SnO)6[(n-Bu2SnOCH3)2(CO3)]2. Moreover, in absence of dehydrating agent, the initial spectrum of 1 is fully recovered (Figure S7c). The characteristic infrared bands of the decanuclear tin species disappear. Thus, these observations point a direct link existing between (n-Bu2Sn)2O( CO3) and (n-Bu2SnO)6[(n-Bu2SnOCH3)2(CO3)]2, which implies a reversible process.

The stannane complex n-Bu2Sn(OCH3)2 is recognized as one of the most active and selective molecular organometallic species for DMC synthesis [19]. Often considered as a reference, its reactivity towards CO2 was widely studied, experimentally and by modelling [20].

To assess the activity of 1, n-Bu2Sn(OCH3)2 was used as precursor under the same reaction conditions (150 °C, 200 bar). Using four mmoles of n-Bu2Sn(OCH3)2, a DMC/Sn ratio of 0.85 was found after a reaction time of 24 h. The activity of 1 is thus lower (almost divided by two) but (n-Bu2Sn)2O(CO3) has the advantage to be easily manipulated in ambient air, which is not the case with n-Bu2Sn(OCH3)2, easily hydrolyzable. Moreover, the examination by FT-IR of the collected residual material (white powder) at the end of the experiment conducted with n-Bu2Sn(OCH3)2, exhibits very strong similarities with the fingerprint of 1 (Figure S8). The two spectra are very close demonstrating a strong relationship between the two compounds. A possible pathway of transformation of n-Bu2Sn(OCH3)2 into 1 is reported in equation 3, involving the in situ formation of n-Bu2SnO.

In parallel of these experiments, the optimization of reaction conditions was conducted.

The influence of pressure and reaction time on DMC formation were successively investigated.

As shown in Figure 3, the DMC yield was found to increase linearly with the pressure.

Previously, this strong effect of the pressure on the reaction was also observed by Sakakura and co-workers when using Bu2Sn(OCH3)2 as tin precursor. Pressures above 1000 bar were then tested [19]. In our case and because of the limitations of our equipment it was not possible to continue beyond 200 bars. However the impact is already significant. The influence of reaction time on the amount of DMC was also studied (Figure 4). From 15 h, the activity slows and reaches a plateau beyond 20 h of reaction. Thus, a duration of 24 h has been selected as the optimized reaction time. The initial aim of this work was also to design a versatile tin precursor for the synthesis of linear dialkyl organic carbonates. Therefore the activity of 1 was extended to the direct carbonation of higher alcohols: (i) ethanol, (ii) n-butanol and (iii) isopropanol. The corresponding amounts of diethyl carbonate (DEC), di-n-butyl carbonate (DBC) and di-isopropyl carbonate (DIPC) are reported in Figure 5 and compared to that obtained for the DMC. 1 is active for the conversion of ethanol into DEC but it is less effective than for the synthesis of DMC. Similarly, 1 can be efficiently recycled several times for the synthesis of DEC (Figure 6). However the activity decreases drastically for the synthesis of DBC and is almost nil for the DIPC. In previous studies on the synthesis of DMC [12b] we showed that the yield of carbonates was closely related to the phase diagram of the CO2/methanol binary mixture and was closely linked to the supercritical properties of the reaction medium. Indeed, we determined experimentally and by simulation that the optimal conditions were obtained when the pressure-temperature parameters are above the critical point of the mixture (monophasic medium). One hypothesis to explain the fall in activity of 1 could be that the P-T conditions are not sufficient in cases of n-butanol and isopropanol. These parameters should be optimized to approach the expected supercritical conditions of the mixtures. Work in this direction are currently in progress.

Conclusion

We have shown that complex 1, (n-Bu2Sn)2O(CO3), prepared in one step from a commercial precursor, is active under carbon dioxide pressure for the conversion of methanol into dimethyl carbonate. Recyclable several times without loss of activity, 1 has the advantage to be robust, undergoing few deterioration even after high pressure runs. Compared to the diorganotin dialkoxide complexes, which are recognized as being the most effective promoters for the direct carbonation of alcohols, 1 is air-stable and can be stored for several months without notable modifications. Moreover, 1 is a versatile precursor and can also be used in the presence of ethanol and n-butanol leading to the synthesis of diethyl carbonate and di-n-butyl carbonate, respectively. However, 1 is not active in the presence of isopropanol. From a mechanistic point of view, the use of 2,2-dimethoxypropane have highlighted an interconnection between 1 and the decameric oxocluster (n-Bu2SnO)6[(n-Bu2SnOCH3)2(CO3)]2. The formation of 1 was also clearly observed when n-Bu2Sn(OCH3)2 is used as precursor. Thus, (n-Bu2Sn)2O(CO3) (1) can be viewed as a key-intermediate. Further work is currently underway to deepen the links existing between these species. TA Instruments TGA Q500 thermoanalyzer using aluminum pans. Samples were heated at a rate of 5 °C min -1 under flowing nitrogen gas. The gas flow rate at the sample was 60 mL min -1 , while the balance flow rate was 40 mL min -1 . Weight loss percentages and onset temperatures were determined using the TA Universal Analysis 2000 software dedicated to the instrument.

Experimental Section

Elemental analyses and ICP-AES measurements were performed at the ''Plateforme d'Analyse

Chimique et de Synthèse Moléculaire de l'Université de Bourgogne (PACSMUB)'' on a Fisons EA 1108 CHNS-O apparatus and a Thermo Scientific iCAP 7400 Series analyser, respectively.

Preparation of (n-Bu2Sn)2O(CO)3 (1)

A methanolic solution of di-n-butyltin dichloride (3.09 g, 0.01 mol, 15 mL CH3OH) was mixed slowly with 15 mL of an aqueous solution of sodium carbonate (1,06 g, 0.01 mol). The white solid, which precipitated immediately, was then transferred to a 125 mL stainless steel reactor and the mixture was heated under CO2 atmosphere at 150 ºC for 24 h (Ptotal = 60 bar).

After the reaction, the excess of CO2 was removed slowly and the resultant solid was filtered and washed several times with de-ionized water, then dried under vacuum at 100 ºC for 3 h.

The dry material was finally stored under argon atmosphere for future use. 28.29, 28.49, 28.58, 28.72, 160.12; 119 Sn{ 1 H} NMR (CD3OD, 300 K, ppm):  173.5, 205.8.

IR (cm -1 ): 2956, 2924, 2871, 2857, 1507, 1464, 1371, 1287, 1156, 1078, 869, 826, 675, 617. Anal. Calc. For C17H36O4Sn2 (541.88): C, 37.68;H, 6.70. Found: C, 37.69.; H, 7.02%.

High Pressure organic carbonates synthesis

Safety Warning: Experiments involving pressurized gases can be hazardous and must only be conducted with suitable equipment and following appropriate safety considerations [20].

The reaction was carried out in a 125 mL stainless steel reactor equipped with a magnetic stirrer. The reactor was purged with argon and a 20 mL suspension of 1 in the required alcohols (methanol, ethanol, n-butanol or isopropanol) was introduced by syringe. Then, CO2

was admitted to the desired amount (32 g). The reaction temperature was controlled by an internal thermocouple. After a reaction time of 24 h the reactor was cooled down to 0 °C, the pressure was gently released and the liquid phase was transferred to a Schlenk tube. Trap-totrap distillation under vacuum at ambient temperature allowed separation of volatile compounds that were quantitatively analyzed by GC (DEC external standard, Thermo Scientific FOCUS GC, TR-Wax 30 m capillary column, FID detector). The tin-based residue was characterized by IR spectroscopy and elemental analyses.

Scheme 2 .

 2 Scheme 2. Possible arrangements of oxycarbonate-based networks (R = n-Bu)[13].

Figure 1 .

 1 Figure 1. TG-DTG analyses of 1 focusing the loss of CO2.

Figure 2 .

 2 Figure 2. Recycling experiments. Reaction conditions: 20 mL of MeOH, 4 mmol of 1 (run 1), reaction temperature 150 °C, pressure 200 bar, reaction time 24 h. a addition of 10 mmol of DMP. b addition of 10 mmol of DMP, reaction time 48 h.

Bu2SnO) 6 [

 6 (n-Bu2SnOCH3)2(CO3)]2, depicted in Scheme 1 and fully characterized in the past[12b, 13]. (n-Bu2SnO)6[(n-Bu2SnOCH3)2(CO3)]2 consists of two pentameric ladders, (n-Bu2Sn)5O5, bridged by two carbonato ligands. Four bridging methoxy groups are located at each corners of the framework and the alkyl chains are located on either side of the inorganic plane. Tin residues collected in run 5 and 6 contain both species, 1 and (n-Bu2SnO)6[(n-Bu2SnOCH3)2(CO3)]2. In terms of DMC produced, the mixture leads to a concomitant benefit. A better activity is observed (Fig.2, run 5 and 6). In addition to the dehydration action of DMP, this can be explained by the in situ formation of Sn-OCH3 bonds which are known to exhibit a great reactivity towards CO2 and leading to hemicarbonate moieties (Sn-OC(O)OCH3)[10].

Figure 3 .

 3 Figure 3. Effect of pressure on the amount of DMC. Reaction conditions: 20 mL of MeOH, 4

Figure 4 .

 4 Figure 4. Effect of reaction time on the amount of DMC. Reaction conditions: 20 mL of MeOH,

Figure 5 .

 5 Figure 5. Comparison of activity of 1 for the synthesis of higher dialkyl carbonates.

Figure 6 .

 6 Figure 6. Recycling experiments. Reaction conditions: EtOH (20 mL), 4 mmol of 1 (run 1),

  . 1. Elemental analyses of several batches of 1.

	%	Batch No.	1	2	3	Theoretical values a
		Carbon	37.69	37.68	37.61	37.68
		Hydrogen	7.02	6.70	7.50	6.70

a Calculated for C17H36O4Sn2

2 .

 2 Elemental analyses of tin residues.

	%	Run No.	2	3	Theoretical values a
		Carbon	37.61	37.48	37.68
		Hydrogen	6.63	7.53	6.70

a Calculated for C17H36O4Sn2

  H NMR (CD3OD, 300 K, ppm):  0.96 (t, 3 JH-H = 6.7 Hz, 12H),  1.49 (m, 16H),  1.71 (m, 8H); 13 C{ 1 H} NMR (CD3OD, 300 K, ppm):  14.02, 14.07, 24.03 ( 1 JCSn = 600 Hz), 28.09,

1

Acknowledgments

We gratefully acknowledge the Institut de Chimie of the Centre National de la Recherche Scientifique (CNRS-INC, France) and the Agence Nationale de la Recherche (ANR, France, Grant ANR-08-CP2D-18) for support of this work and a postdoctoral grant to S.R.S. We thank also Mr. Marcel Soustelle, the Dr. Myriam Heydel, and the Dr Lucie Chupin for elemental analyses, ICP-AES measurements and improvement of the manuscript, respectively. We are also grateful to the anonymous reviewers for their helpful and relevant comments which have significantly contributed to enhance the content of this manuscript.