A Kinetic Study of the Electron-Transfer-Initiated Carbon-Silicon Bond Cleavage Reactions of Benzyltrialkylsilanes Promoted by 12-Tungstocobalt(III)ate Ion

Article abstract of DOI:10.1021/jo00043a028

The reactions of some ring-substituted benzyltrimethylsilanes (1Z), (p-methylbenzyl)triethylsilane (2), (p-methylbenzyl)triisopropylsilane (3), and (p-methoxybenzyl)triisopropylsilane (4) with potassium 12-tungstocobalt(III)ate, K5IIIW12O40> (Co(III)W), have been studied in AcOH-H2O.In all cases desilylated products, benzyl acetates, and benzyl alcohols have been obtained, with a 2:1 oxidant to substrate stoichiometry.A kinetic investigation has shown that the reactions of 1Z and 2 are strictly first order in Co(III)W and first order in the substrate.Added salts (NaClO4, NaOAc) decrease the rate, whereas no retarding effect by Co(II)W has been noted.In contrast, the reaction of 4 is no longer first order in Co(III)W and the rate is significantly decreased by added Co(II)W.These data have been interpreted in terms of an electron-transfer mechanism, in which a benzyltrialkylsilane radical cation is formed and then undergoes a C-Si bond cleavage to form a benzyl radical.Reaction of this radical with Co(III)W leads to the products.In the reactions of 1Z and 2 the rate-determining step is the transfer of the electron from the substrate to Co(III)W, whereas in the reaction of 4 the slow step is the one where the C-Si bond cleavage takes place.The mechanistic changeover is probably related to the much lower rate of the C-Si bond cleavage in 4^+* than in 1Z^+* and 2^+*, due to the fact that in the former cation radical nucleophilic attack to the silicon is made difficult by the bulky isopropyl groups.The kinetic data for the reactions of 1Z and 2 fit the Marcus equation for a reorganization energy λ of 43 +/-1 kcal mol^-1.From this value a λ value of 61 kcal mol^-1 for the couple ArCH2SiMe3^+*/ArCH2SiMe3 has been evaluated, which is somewhat higher than that found for a number of ArCH3^+*/ArCH3 systems.The possible reasons for this difference are discussed on the basis of ab initio calculations (STO-3G) concerning bond distances and angles in PhCH3^+* and PhCH2SiH3^+*.