Oxidation of tertiary silanes by osmium tetroxide

In the presence of an excess of pyridine ligand L, osmium tetroxide oxidizes tertiary silanes (Et3SiH, (Pr3SiH)-Pr-i, Ph3SiH, or PhMe2SiH) to the corresponding silanols. With L = 4-tert-butylpyridine ((t)Bupy), OsO4((t)Bupy) oxidizes Et3SiH and PhMe2SiH to yield 100 +/- 2% of silanol and the structurally characterized osmium(VI) mu-oxo dimer [OsO2((t)Bupy)(2)](2)(mu-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)(2)py(2)](2)(mu-O)(2) (1b). Excess silane in these reactions causes further reduction of the Os-VI 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 Os-VI-hydroxo-siloxo complex, trans-cis-cis-Os(O)(2)L-2(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 OOsO fragment. This is the first direct observation of a [3 + 2] intermediate in a sigma-bond oxidation, though such species have previously been implicated in reactions of H-H and C-H bonds with OsO4(L) and RuO4.