Rhodium-catalyzed dehydrogenative silylation of acetophenone derivatives: formation of silyl enol ethers versus silyl ethers

摘要

A series of rhodium–NSiN complexes (NSiN=bis (pyridine-2-yloxy)methylsilyl fac-coordinated) is reported, including the solid-state structures of [Rh(H)(Cl)(NSiN)(PCy)] (Cy=cyclohexane) and [Rh(H)(CFSO)(NSiN)(coe)] (coe=cis-cyclooctene). The [Rh(H)(CFSO)(NSiN)(coe)]-catalyzed reaction of acetophenone with silanes performed in an open system was studied. Interestingly, in most of the cases the formation of the corresponding silyl enol ether as major reaction product was observed. However, when the catalytic reactions were performed in closed systems, formation of the corresponding silyl ether was favored. Moreover, theoretical calculations on the reaction of [Rh(H)(CFSO)(NSiN)(coe)] with HSiMe and acetophenone showed that formation of the silyl enol ether is kinetically favored, while the silyl ether is the thermodynamic product. The dehydrogenative silylation entails heterolytic cleavage of the Si−H bond by a metal–ligand cooperative mechanism as the rate-determining step. Silyl transfer from a coordinated trimethylsilyltriflate molecule to the acetophenone followed by proton transfer from the activated acetophenone to the hydride ligand results in the formation of H and the corresponding silyl enol ether.