Organometallics, 29, 662-670 (2010).
© American Chemical Society
William J. Leigh*, Svetlana S. Kostina, Adroha Bhattacharya, and Andrey G. Moiseev
Contribution from the Department of Chemistry and Chemical Biology, McMaster University, 1280 Main Street West, Hamilton, ON Canada L8S 4M1.
Dimethyl-, diphenyl-, and dimesitylsilylene have been generated in hexanes solution by laser flash photolysis and their kinetic behavior 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 × 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-ROH 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 sub-millimolar alcohol concentrations. The catalytic rate constants range from 109 M-1s-1 to 1010 M-1s-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 5-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.
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