Ru-catalyzed dehydrogenative coupling of carboxylic acids and silanes - a new method for the preparation of silyl ester

Ru3(CO)12/EtI has been found to be an efficient catalyst system for the dehydrosilylation of carboxylic acids with silanes. In the presence of 1 mol% Ru3(CO)12 and 4 mol% EtI, dehydrosilylation reactions in toluene afforded the corresponding silyl esters at 100 °C in good and high yields.


Introduction
Polymers composed of nucleophilically-labile silyl ester bonds in the main chain are being studied as a new type of degradable functional polymers with the potential for an extremely broad range of degradation behavior through variation in the functionalities attached to the silicon atom. In the design of degradable materials, the physical and mechanical properties must be considered for performance in serving the expected function, while degradation rate and degradation products are also very important. Since the lability of a silyl ester linkage is dramatically affected by the substituents attached to the silicon atom, poly(silyl ester)s were found to be an ideal family of degradable polymers [1]. Also, multifunctional silyl esters have been found to be ideal cross-linking agents since they require only mild reaction conditions, especially for silicone elastomers. The demand for degradable poly(silyl ester)s has been increasing greatly due to biomedical field and environmental concerns [2][3][4]. Obviously, silyl esters are very important intermediates for the preparation of easily degradable functional poly(silyl ester)s, widely utilized as gene delivery carriers, matrices for drug delivery, biodegradable surgical devices, and recyclable materials [2][3][4][5][6][7][8][9][10][11][12]. To develop simple, economical and practical protocols for the conversion of carboxylic acids into silyl esters is not only required in normal organic synthesis procedure, but is also a prerequisite for the accurate performance of gas-chromatographic analyses in organic and biological chemistry [13,14]. From the viewpoint of synthetic chemistry, the ideal protecting group for an active-hydrogen moiety such as carboxylic acid should be attached in high yield, be stable towards severe reaction conditions and, at the same time, be selectively removable in the presence of other functional groups Scheme 1: Dehydrogenative silyl ester synthesis with Ru 3 (CO) 12 /EtI. carrying different protecting groups. Indeed, silylation of carboxylic acids is a useful method for their protection because deprotection of silyl esters is easily achieved under mild reaction conditions [15][16][17][18].

Results and Discussion
In this communication, we wish to report the first finding that a catalytic system of dodecacarbonyltriruthenium and ethyl iodide [Ru 3 (CO) 12 /EtI] effectively promotes the dehydrogenative coupling of carboxylic acids with silanes, yielding the corresponding silyl esters selectively. The results are summarized in Scheme 1 and Table 1-Table 4.
Dehydrogenative coupling reactions were carried out by heating a mixture of carboxylic acid, silane and a catalytic amount of Ru 3 (CO) 12 /EtI in solvents under a nitrogen atmosphere for several hours (Scheme 1, Table 1-Table 4, dehydrocoupling reaction was monitored by GC). The transformation of  propionic acid with triethylsilane was employed as a model to optimize the reaction conditions.

Conclusion
In conclusion, we have demonstrated that Ru 3 (CO) 12 /EtI is an efficient catalytic system for the dehydrogenative cross-coup-ling of carboxylic acids with silanes. The dehydrogenative cross-coupling reactions proceed efficiently to give the corresponding silyl esters in good and excellent yields. No overreduced silyl esters are formed in the case of coupling nitro-, bromo-, and chlorobenzoic acid with silanes. We believe that the Ru 3 (CO) 12 /EtI-catalyzed dehydrosilylation of carboxylic acids with silanes provides another important protocol for a one-step, highly selective, atom-economical and efficient synthetic method. We are currently broadening the scope of this dehydrosilylation of carboxylic acids and silanes in our laboratory and the results will be published elsewhere.

Experimental
To a mixture of propionic acid (40 mmol, 2.96 g), and triethylsilane (40 mmol, 4.64 g) in toluene (20 ml) was added Ru 3 (CO) 12 (0.4 mmol, 0.01 equiv) and EtI (2.0 mmol, 0.05 equiv) at room temperature under a nitrogen atmosphere. The reaction mixture was stirred at 100 °C for 8 hours (monitored by GC). The desired triethylsilyl propionate was obtained as a colourless oil (yield: 95%) after distillation under reduced pres- All of the silyl esters are known compounds and were compared with authentic samples [prepared by cross-coupling of carboxylic acids and chlorosilanes in the presence of a base such as triethylamine or imidazole (tert-butylsilyl esters) in dichloromethane] and were identified on the basis of their IR, 1 H NMR, 13 C NMR and GC-MS spectral data.