MECHANISTIC STUDIES OF THE REACTIONS OF SILICON-CARBON DOUBLE-BONDS -ADDITION OF ALCOHOLS TO 1,1-DIPHENYLSILENE

The addition of water, aliphatic alcohols, and acetic acid to 1,1-diph enylsilene (generated by photolysis of 1,1-diphenylsilacyclobutane) ha s been studied in polar solvents using steady-state and nanosecond las er flash photolysis techniques. Absolute rate constants and (selected) deuterium kinetic isotope effects for the addition of water, methanol , ethanol, 2-propanol, tert-butyl alcohol, and acetic acid have been d etermined at 23 degrees C in acetonitrile solution. Silene quenching f ollows a linear dependence on quencher concentration over the range in vestigated in all cases and proceeds with rate constants which vary ov er a range 4.1 x 10(8)-1.6 x 10(9) M(-1) s(-1). The rate constants exh ibit small primary deuterium kinetic isotope effects in all cases exce pt acetic acid. Rate constants for addition of methanol, tert-butyl al cohol, and acetic acid have also been determined in hexane and THF sol ution. The transient absorption spectrum of the silene is broadened an d red-shifted markedly in the latter solvent compared to that in aceto nitrile and hexane, consistent with the formation of a solvent complex . Steady-state competition experiments have been carried out with vari ous pairs of alcohols and water. The product ratios agree with the cor responding relative rate constants for water, methanol, and ethanol. T hose for methanol/tert-butyl alcohol are significantly different from the rate constant ratio but approach it at very low total alcohol conc entrations. The results are consistent with a two-step mechanism invol ving reversible formation of a silene-alcohol complex, followed by int racomplex proton transfer. The latter is rate-determining in all cases but acetic acid, for which it is proposed that complexation is the ra te-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 tert-butyl alcohol an d acetic acid.