Substituent effects on the reactivity of the silicon-carbon double bond. Mechanistic studies of the ene-addition of acetone to reactive arylsilenes

Absolute rate constants for the reaction of acetone with phenylsilene, 1-methyl-1-phenylsilene, and a series of ring-substituted 1,1-diphenylsilene derivatives have been determined in polar and nonpolar solvents using nanosecond laser flash photolysis techniques. The reaction (which affords the corresponding silyl enol ether) proceeds significantly faster at 23 degrees C in hydrocarbon solvents than in acetonitrile in all cases, but the Hammett rho-values defined by the data for the substituted 1,1-diphenylsilenes are larger in isooctane (rho approximate to +1.5) than in acetonitrile (rho approximate to +1.1). Deuterium kinetic isotope effects and Arrhenius parameters have been determined for the reactions of 1-methyl-1-phenyl-, 1,1-diphenyl-, 1,1-bis(4-methylphenyl)-, and 1,1-bis(4-(trifluoromethyl)phenyl)silene in hexane and acetonitrile. All but 1,1-bis(4-(trifluoromethyl)phenyl)silene exhibit negative activation energies for reaction. The trifluoromethyl derivative, the most reactive in the series, exhibits a positive E-a in acetonitrile and a curved Arrhenius plot in hexane. The results are consistent with a mechanism involving initial, reversible formation of a silene-ketone complex which collapses to product by rate-controlling proton transfer. The trends in the data can be rationalized in terms of variations in the relative rate constants for reversion to reactants and hydrogen transfer as a function of temperature, substituent, and solvent. The differences between acetonitrile and hydrocarbon solvents are rationalized as due to the effects of the strong solvation of the free silene by the nitrile solvent.