Selectivity of Hydrosilylative Ether Cleavage by Cationic Bis(phosphine) Iridium Complexes

Abstract

Catalysts capable of heterolytic silane activation have been utilized for the conversion of alkyl ethers to silyl ethers via C-O bond cleavage. Previously reported systems for this transformation show poor selectivity. In this work, bis(phosphine) iridium complexes are shown to be competent for the regioselective and chemoselective reduction of benzyl ethers in the presence of reductively-labile alkyl and aryl halides. Mechanistic work suggests a tetrahydriosilyliridium complex as the catalyst resting state, which serves as the hydride source for the cleavage of silyloxonium ions generated in situ. Selectivity in cyclohexyl methyl ether cleavage is shown to depend on relative rates of SN1 and SN2 processes, which are subject to both catalyst and substrate control. The extension of this approach for C-O bond cleavage to protected aryl glycosides allows for the selective demethylation of the C3 position. These experiments show that bis(phosphine) iridium complexes are capable of selective C-O bond cleavage in complex substrates, overcoming limitations of previously reported catalysts.