Polyhydride Osmium-Mediated Hydrosilylation of Carbonyl Compounds: Intermediates and Mechanism

OsH6(PiPr3)2 (1) reacts with phenylsilane to
afford OsH5(SiH2Ph)(PiPr3)2 (2), which reacts with additional phenylsilane
to give OsH4(SiH2Ph)2(PiPr3)2 (3). The reaction of 1 with
diphenylsilane
affords OsH5(SiHPh2)(PiPr3)2 (4), which in the presence of diphenylsilane
and traces of water leads to OsH4{κ2-Si,Si-(Ph2Si–O–SiPh2)}(PiPr3)2 (5). Complex 4 promotes
the hydrosilylation of aldehydes with H2SiPh2 to give silyl ethers, while for ketones mixtures of hydrosilylation
and dehydrogenative silylation products are obtained. The reactions
of 4 with benzaldehyde and acetone afford OsH5{Si(OR)Ph2}(PiPr3)2 (R
= CH2Ph (6), iPr (7)), which undergo a metathesis between a Si–Ph bond and a
C(sp3)–H bond of one methyl group of one phosphine
to give OsH4{κ1-P,η2-SiH-[iPr2PCH(Me)CH2Si(OR)PhH]}(PiPr3) (R = CH2Ph (8), iPr (9)). The combination of experimental findings and
density functional theory calculations has permitted to establish
the mechanism for the hydrosilylation processes. The key intermediate
is the tetrahydride-silylene OsH4(=SiPh2)(PiPr3)2, formed by an outer-sphere hydrogenation
of the carbonyl group promoted by a trihydride(dihydrogen) isomer
of 4. For enolyzable ketones, this pathway competes with
one where the enol form directly attacks the Si atom of 4, affording silyl enol ethers and a tetrahydride(dihydrogen) isomer
of 1, that reacts with diphenylsilane to release hydrogen
and close the cycle.