Reprinted from The Journal of the American Chemical Society, 118, 8971-8972 (1996).
© American Chemical Society, 1996.
Christine J. Bradaric and William J. Leigh*
Contribution from the Department of Chemistry, McMaster University, Hamilton, Ontario, Canada L8S 4M1
Abstract: The temperature dependences of the bimolecular rate constants for addition of methanol, t-butanol, and acetic acid to 1,1-diphenylsilene in acetonitrile solution have been studied over the -20-55 ºC temperature range using nanosecond laser flash photolysis techniques. Addition of methanol and t-butanol to yield the corresponding alkoxymethyldiphenylsilane proceeds with Arrhenius activation energies of -2.5±0.2 and -0.4±0.1 kcal/mol, respectively, and strongly negative entropies of activation (~ -25 e.u. at 296K). Addition of acetic acid, on the other hand, exhibits Ea = +1.9±0.2 and delta-S ~ -13 e.u. The results are consistent with a mechanism in which addition proceeds by fast, reversible formation of a complex between the silene and the nucleophile, followed by proton transfer from oxygen to carbon as the complex collapses to product. The latter is rate-determining in the cases of methanol and t-butanol, while complexation is rate-determining in the case of acetic acid.