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

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 investig ated by far-UV (193-nm) laser flash photolysis techniques, using the c orresponding 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 upo n 193- or 214-nm photolysis in solution in the presence of aliphatic a lcohols. UV absorption spectra and absolute rate constants for reactio n of the silenes with methanol, ethanol, and t-butyl alcohol have been determined in hexane solution at 23 degrees C. The rate constants var y from a low of 3 x 10(7) M-1 s(-1) for reaction of 1-methyl-1-trimeth ylsilylsilene with t-BuOH to a high of 1 x 10(10) M-1 s(-1) for reacti on of 1-ethynyl-1-methylsilene with MeOH. In several cases, rate const ants have been determined for addition of the deuterated alcohols, and for addition of methanol over the 0-55 degrees C range. Invariably, s mall 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 c omplex which collapses to alkoxysilane by unimolecular proton transfer from oxygen to carbon. Silene reactivity increases with increasing re sonance electron-donating and inductive electron-withdrawing ability o f the substituents at silicon and is significantly affected by steric effects within this series of compounds. This is suggested to be due t o a combination of effects on both the degree of electrophilicity at s ilicon (affecting the rate constants for formation and reversion of th e complex) and nucleophilicity at carbon (affecting the partitioning o f the complex between product and free reactants). Two 1-methyl-1-alko xysilacyclobutanes were also investigated, but proved to be inert to 1 93-nm photolysis.