Oxidation of Tertiary Silanes by Osmium Tetroxide

In the presence of an excess of pyridine ligand L, osmium tetroxide oxidizes tertiary silanes (Et3SiH, iPr3SiH,
Ph3SiH, or PhMe2SiH) to the corresponding silanols. With L = 4-tert-butylpyridine (tBupy), OsO4(tBupy) oxidizes
Et3SiH and PhMe2SiH to yield 100 ± 2% of silanol and the structurally characterized osmium(VI) μ-oxo dimer
[OsO2(tBupy)2]2(μ-O)2 (1a). With L = pyridine (py), only 40−60% yields of R3SiOH are obtained, apparently because
of coprecipitation of osmium(VIII) with [Os(O)2py2]2(μ-O)2 (1b). Excess silane in these reactions causes further
reduction of the OsVI products, and similar osmium “over-reduction” is observed with PhSiH3, Bu3SnH, and boranes.
The pathway for OsO4(L) + R3SiH involves an intermediate, which forms rapidly at 200 K and decays more slowly
to products. NMR and IR spectra indicate that the intermediate is a monomeric OsVI-hydroxo-siloxo complex, trans-cis-cis-Os(O)2L2(OH)(OSiR3). Mechanistic studies and density functional theory calculations indicate that the
intermediate is formed by the [3 + 2] addition of an Si−H bond across an OOsO fragment. This is the first
direct observation of a [3 + 2] intermediate in a σ-bond oxidation, though such species have previously been
implicated in reactions of H−H and C−H bonds with OsO4(L) and RuO4.