Fast Kinetics Study of the Reactions of Transient Silylenes with Alcohols. Direct Detection of Silylene−Alcohol Complexes in Solution

The kinetic behavior of dimethyl-, diphenyl-, and dimesitylsilylene in hexanes solution in the presence of methanol (MeOH), tert-butanol (t-BuOH), and the respective O-deuterated isotopomers has been studied, with the goal of elucidating a detailed mechanism for the formal O−H insertion reaction of transient silylenes with alcohols in solution. The data are in all cases consistent with a mechanism involving the intermediacy of the corresponding silylene−alcohol Lewis acid−base complexes, which have been detected directly for each of the SiMe₂−ROL and SiPh₂−ROL (L = H or D) systems that were studied. Complexation proceeds effectively irreversibly (K_eq ≥ 2 × 10⁵ M⁻¹) and at close to the diffusion-controlled rate in these cases. In contrast, the kinetic and spectroscopic behavior observed for SiMes₂ in the presence of these alcohols indicates the SiMes₂−ROL complexes are involved as steady-state intermediates, formed reversibly and 10−100 times more slowly than is the case with SiMe₂ and SiPh₂. Product formation from the silylene−alcohol complexes is shown to proceed via catalytic proton transfer by a second molecule of alcohol, the rate of which exceeds that of unimolecular intracomplex H-migration in all cases, even at submillimolar alcohol concentrations. The catalytic rate constants range from 10⁹ to 10¹⁰ M⁻¹ s⁻¹ for the SiMe₂−ROH and SiPh₂−ROH complexes, sufficiently fast that the isotope effect ranges from ca. 2.5 to close to unity for all but the SiPh₂−t-BuOL complex, where it is remarkably large (k_HH/k_DD = 10.8 ± 2.4). The value is consistent with a mechanism for catalysis involving double proton transfer within a cyclic five-membered transition state. The isotope effects on the ratio of the rate constants for catalytic proton transfer and dissociation of the SiMes₂−MeOH and SiMes₂−t-BuOH complexes suggest that a different mechanism for catalytic proton transfer is involved in the case of the sterically hindered diarylsilylene.