Oxidation of Tertiary Silanes by Osmium Tetroxide

Abstract

In the presence of an excess of pyridine ligand L, osmium tetroxide oxidizes tertiary silanes (Et(3)SiH, (i)Pr(3)SiH, Ph(3)SiH, or PhMe(2)SiH) to the corresponding silanols. With L = 4-tert-butylpyridine ((t)Bupy), OsO(4)((t)Bupy) oxidizes Et(3)SiH and PhMe(2)SiH to yield 100 +/- 2% of silanol and the structurally characterized osmium(VI) mu-oxo dimer [OsO(2)((t)Bupy)(2)](2)(mu-O)(2) (1a). With L = pyridine (py), only 40-60% yields of R(3)SiOH 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 OsVI products, and similar osmium "over-reduction" is observed with PhSiH(3), Bu(3)SnH, and boranes. The pathway for OsO(4)(L) + R(3)SiH 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)(OSiR(3)). 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 sigma-bond oxidation, though such species have previously been implicated in reactions of H-H and C-H bonds with OsO(4)(L) and RuO(4).

Selective Manganese‐Catalyzed Oxidation of Hydrosilanes to Silanols under Neutral Reaction Conditions

“…Whilst less bulky silanols can be synthesized by hydrolysis of chlorosilanes under strictly controlled conditions, it is difficult to synthesize sterically hindered silanols, silanediols, and silanols with functional groups sensitive to hydrolytic conditions . Silanols can also be synthesized by oxidation of organosilanes, with strong oxidants, such as silver salts, permanganate, osmium tetroxide, dioxiranes, peracids, oxaziridines, and ozone ,. Apart from generating stochiometric amounts of waste, these methods are often accompanied with problems of selectivity, for example, producing undesired siloxanes by condensation of two molecules of silanol, or byproducts resulting from silanol decomposition .…”

Facile Oxy‐Functionalization of a Nucleophilic Metal Alkyl with a cis‐Dioxo Metal Species via a (2+3) Transition State

“…One intriguing possibility is the use of cis-metal dioxo compounds that could react with MÀR s bonds in a potentially facile addition reaction via a (3+2)-type transition state (TS, see Figure 2). Related mechanisms are well known for the cis-dihydroxylation of alkenes by OsO 4 [5] and the oxidation of the s bonds of hydrogen, [6] silanes, [7] and alkanes. [8] Herein we report the quantitative and instantaneous oxyfunctionalization of MTO to methanol by reaction with OsO 4 in aqueous basic media at room temperature [Eq.…”

“…Important goals are to identify reactions that do not involve free radicals, are compatible with CH-activation reactions with electropositive metals, and are sufficiently fast and selective to intercept and convert nucleophilic M À R dÀ intermediates into oxygenated products.One intriguing possibility is the use of cis-metal dioxo compounds that could react with MÀR s bonds in a potentially facile addition reaction via a (3+2)-type transition state (TS, see Figure 2). Related mechanisms are well known for the cis-dihydroxylation of alkenes by OsO 4[5] and the oxidation of the s bonds of hydrogen, [6] silanes, [7] and alkanes. [8] Herein we report the quantitative and instantaneous oxyfunctionalization of MTO to methanol by reaction with OsO 4 in aqueous basic media at room temperature [Eq.…”

“…[5] and the oxidation of the s bonds of hydrogen, [6] silanes, [7] and alkanes. [8] Herein we report the quantitative and instantaneous oxyfunctionalization of MTO to methanol by reaction with OsO 4 in aqueous basic media at room temperature [Eq.…”

Catalytic Oxidation of Silanes by Carbon Nanotube–Gold Nanohybrids

“…The selective oxidation of silanes has attracted wide interest as silanols are key synthons for the production of siliconcontaining materials [1] and nucleophilic partners in organometallic cross-coupling reactions. [2] Silanes are classically converted into silanols using strong oxidizing agents such as osmium tetroxide, [3] permanganate, [4] ozone, [5] peracids, [6] or peroxides. [7] However, under these reaction conditions, significant amounts of siloxanes and toxic by-products are formed.…”