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

作者：Leigh William J^*; Kostina Svetlana S; Bhattacharya Adroha; Moiseev Andrey G

来源：Organometallics, 2010, 29(3): 662-670.

DOI：10.1021/om9009747

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 off lie corresponding silylene-alcohol Lewis acid-base complexes, which have been detected directly for each of the SiMe(2)-ROL and SiPh(2)-ROL (L = H or D) systems that were studied. Complexation proceeds effectively irreversibly (K(cq) >= 2 x 10(5) M(-1)) and at close to the diffusion-controlled rate in these cases. I it contrast, the kinetic and spectroscopic behavior observed for SiMes(2) in the presence of these alcohols indicates the SiMes(2)-ROL complexes are involved its steady-state intermediates. Formed reversibly and 10-100 times more Slowly than is the case with SiMe(2) and SiPh(2) Product formation from the silylene-alcohol complexes is shown to proceed via catalytic proton transfer by it 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 10(9) to 10(10) M(-1) s(-1) for the SiMe(2)-ROH and SiPh(2)-ROH complexes, sufficiently fast that the isotope effect ranges from ca. 2.5 to close to unity for all but the SiPh(2)-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 it cyclic five-membered transition state. The isotope effects on the ratio of the rate Constants for Catalytic proton transfer and dissociation of the SiMes(2)-MeOH and SiMes(2),-t-BuOH complexes suggest that a different mechanism for catalytic proton transfer is involved in the case of the sterically hindered diarylsilylene.

• 出版日期2010-2-8