Reprinted from Organometallics, 15, 2246-2253 (1996).

© American Chemical Society, 1996.

Mechanistic Studies of the the Reactions of Silicon-Carbon Double Bonds. Addition of Alcohols to 1,1-Diphenylsilene.

William J. Leigh,* Christine J. Bradaric, Corinna Kerst, and Jo-Ann H. Banisch

Contribution from the Department of Chemistry, McMaster University, Hamilton, Ontario, Canada L8S 4M1

Abstract: The addition of water, aliphatic alcohols, and acetic acid to 1,1-diphenylsilene (generated by photolysis of 1,1-diphenylsilacyclobutane) has been studied in polar solvents using steady state and nanosecond laser flash photolysis techniques. Absolute rate constants and (selected) deuterium kinetic isotope effects for the addition of water, methanol, ethanol, isopropanol, tert-butanol and acetic acid have been determined at 23°C in acetonitrile solution. Silene quenching follows a linear dependence on quencher concentration over the range investigated in all cases, and proceeds with rate constants which vary over a range of 4.1 X 10⁸ - 1.6 X 10⁹ M^-1s^-1. The rate constants exhibit small primary deuterium kinetic isotope effects in all cases except acetic acid. Rate constants for addition of methanol, t-butanol, and acetic acid have also been determined in hexane and THF solution. The transient absorption spectrum of the silene is broadened and red-shifted markedly in the latter solvent compared to that in acetonitrile and hexane, consistent with the formation of a solvent complex. Steady state competition experiments have been carried out with various pairs of alcohols and water. The product ratios agree with the corresponding relative rate constants for water, methanol, and ethanol. Those for (methanol / t-butanol) are significantly different from the rate constant ratio, but approach it at very low total alcohol concentrations. The results are consistent with a two-step mechanism involving reversible formation of a silene-alcohol complex, followed by intracomplex proton transfer. The latter is rate-determining in all cases but acetic acid, for which it is proposed that complexation is the rate-determining step for reaction. Proton transfer from the complex to a second molecule of alcohol competes with the intracomplex pathway at higher alcohol concentrations for all cases but t-butanol and acetic acid.