Two-Silicon Cycle for Carbonyl Hydrosilylation with Nikonov’s Cationic Ruthenium(II) Catalyst

An experimental analysis proves that Nikonov’s carbonyl hydrosilylation proceeds through a two-silicon cycle rather than the originally proposed one-silicon cycle. The intermediate ruthenium(II) monohydride is not sufficiently hydridic to transfer its hydride onto the silylcarboxonium ion. However, that hydricity is enhanced by oxidative addition of another hydrosilane molecule to afford the corresponding ruthenium(IV) silyl dihydride as the actual hydride donor. The present study also demonstrates that the acetonitrile ligands in Nikonov’s ruthenium(II) catalyst are not innocent. That complex is able to hydrosilylate its own ligand(s), and the resulting N,N-disilylated amine base accounts for competing dehydrogenative silylation of enolizable carbonyl compounds, explaining the formation of a silyl enol ether in substantial quantities next to the expected silyl ether. Both findings lead to a revised mechanistic picture that provides the basis for the development of more efficient and chemoselective catalysts.