An Efficient Solvent-Free Route to Silyl Esters and Silyl Ethers

Dinuclear metal complexes, especially (p-cymene)ruthenium dichloride dimer {[RuCl2(p-cymene)](2)}, have been found to exhibit high catalytic performance for the dehydrosilylation of various kinds of carboxylic acids and alcohols. The dehydrosilylation with [RuCl2(p-cymene)](2) proceeded efficiently with only one equivalent of silane with respect to substrate (carboxylic acids or alcohols) under solvent-free conditions to give the corresponding silyl esters and ethers in excellent yields with a high turnover number (TON) and frequency (TOF). The H-1 NMR spectrum of a toluene-d(8) solution of [RuCl2(p-cymene)](2) and a silane showed a signal assignable to the ruthenium hydride species. In contrast, no new signals were detected in the H-1 NMR spectrum of a toluene-d(8) solution of [RuCl2(p-cymene)](2) and a carboxylic acid or an alcohol. Therefore, the ruthenium metal in [RuCl2(p-cymene)](2) activates a silane to afford the hydride intermediate, possibly a silylmetal hydride species. Then, the nucleophilic attack of a substrate (carboxylic acid or alcohol) to the hydride intermediate proceeds to give the corresponding silylated product. The present dehydrosilylation with an optically active silane proceeded exclusively under inversion of stereochemistry at the chiral silicon center, suggesting that the nucleophilic attack of a substrate to the hydride intermediate occurs from the backside of the ruthenium-silicon bond.