51525-44-1Relevant articles and documents
Fast kinetics study of the reactions of transient silylenes with alcohols. Direct detection of silylene-alcohol complexes in solution
Leigh, William J.,Kostina, Svetlana S.,Bhattacharya, Adroha,Moiseev, Andrey G.
scheme or table, p. 662 - 670 (2010/05/15)
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 (Keq ≥ 2 x 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 (kHH/kDD = 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.
