Kinetics of hydride transfer reactions from hydrosilanes to carbenium ions. Substituent effects in silicenium ions

Article abstract of DOI:10.1021/ja00034a044

Rates of hydride transfer from hydrosilanes HSiR¹R²R³ with widely varying substitution to para-substituted diarylcarbenium ions have been measured in dichloromethane solution. Generally the reactions follow a second-order rate law, -d[Ar₂CH⁺]/dt = k₂[Ar₂CH+][HSiR¹R²R³], and k₂ is independent of the degree of ion-pairing and the nature of the counterion (exceptions are reported). The reaction rates are almost independent of solvent polarity. Kinetic isotope effects exclude an SET-type mechanism and are in accord with a polar mechanism with rate-determining formation of silicenium ions. The reactivities of para-substituted aryldimethylsilanes are linearly correlated with σ_p (ρ = -2.46), not with σ_p⁺. In the series H₃SiHex, H₂SiHex₂, HSiHex₃, the relative reactivities are 1.00:155:7890, and in the corresponding phenyl series the reactivity increase is much smaller (H₃SiPh:H₂SiPh₂:HSiPh₃ = 1.00:17.2:119). As a consequence, trihexylsilane is approximately two orders of magnitude more reactive than triphenylsilane though hexylsilane and phenylsilane show similar reactivities. Tris(trimethylsilyl)silane is just slightly more reactive than trimethylsilane. Replacement of hydrogen by chlorine reduces the reactivity by one order of magnitude. Variation of the electrophilicities of the hydride abstractors does not affect the relative reactivities of the silanes, i.e., constant selectivity (Ritchie-type) relationships are encountered. Correlation equations are given, which permit the calculation of hydride transfer rates from hydrosilanes to any carbenium ion on the basis of pk_R⁺ values or the ethanolysis rate constants of the corresponding alkyl chlorides.