J. Amer. Chem. Soc., 120, 9504-9512 (1998)

© American Chemical Society, 1998

Substituent Effects on the Reactivity of the Silicon-Carbon Double Bond.   Resonance, Inductive and Steric Effects of Substituents at Silicon on the Reactivity of Simple 1-Methylsilenes

W.J. Leigh,* R. Boukherroub and C. Kerst

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

Abstract: The reactivities of a series of substituted 1-methylsilenes RMeSi=CH₂ (R = H, methyl, ethyl, t-butyl, vinyl, ethynyl, phenyl, trimethylsilyl and trimethylsilylmethyl) in hydrocarbon solvents have been investigated by far-UV (193-nm) laser flash photolysis techniques, using the corresponding 1-methylsilacyclobutane derivatives as silene precursors. Each of these silacyclobutanes yields ethylene and the corresponding silene, which can be trapped as the alkoxysilane RSiMe₂OR’ cleanly and efficiently upon 193 or 214-nm photolysis in solution in the presence of alcohols. Absolute rate constants for reaction of the silenes with methanol, ethanol, and t-butanol have been determined in hexane solution at 23^oC. They vary from a low of 3 x 10⁷ M^-1s^-1 for reaction of 1-methyl-1-trimethylsilylsilene with t-BuOH to a high of 1 x 10¹⁰ M^-1s^-1 for reaction of 1-ethynyl-1-methylsilene with MeOH. In several cases, rate constants have been determined for addition of the deuterated alcohols, and for addition of methanol over the 0-55 ^oC range. Invariably, small primary deuterium kinetic isotope effects and negative Arrhenius activation energies are observed. These characteristics are consistent with a mechanism involving reversible formation of a silene-alcohol complex which collapses to alkoxysilane by unimolecular proton transfer from oxygen to carbon. Silene reactivity increases with increasing resonance electron-donating and inductive electron-withdrawing ability of the substituents at silicon, and is significantly affected by steric effects within this series of compounds. This is suggested to be due a combination of effects on both the degree of electrophilicity at silicon (affecting the rate constant for complexation) and nucleophilicity at carbon (affecting the partitioning of the complex between product and free reactants). Two 1-methyl-1-alkoxysilacyclobutanes were also investigated, but proved to be inert to 193-nm photolysis.