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 °C in hydrocarbon solvents than in acetonitrile in all cases, but the Hammett ρ-values defined by the data for the substituted 1,1-diphenylsilenes are larger in isooctane (ρ ≈ +1.5) than in acetonitrile (ρ ≈ +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 Ea in acetonitrile an...