Reprinted from Organometallics 17, 645-651 (1998).
© 1998, American Chemical Society.
Christine J. Bradaric and William J. Leigh*
Contribution from the Department of Chemistry, McMaster University, Hamilton, Ontario, Canada L8S 4M1
Abstract: 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 isooctane and acetonitrile solution using nanosecond laser flash photolysis techniques. The reaction (which affords the corresponding silyl enol ether) proceeds significantly faster in isooctane solution than in acetonitrile in all cases, but the Hammett r-values defined by the data for the substituted 1,1-diphenylsilenes are larger in the non-polar solvent at 23 oC (r ~ +1.5 in isooctane; r ~ +1.1 in acetonitrile). Deuterium kinetic isotope effects and Arrhenius parameters have been determined for the reactions of 1-methyl-1-phenyl-, 1,1-diphenyl-, 1,1-di-(4-methylphenyl)- and 1,1-di-(4-trifluoromethylphenyl)silene in both solvents. All but 1,1-di-(4-trifluoromethylphenyl)silene exhibit negative activation energies for reaction. The trifluoromethyl derivative, the most reactive in the series, exhibits a positive Ea in acetonitrile and a curved Arrhenius plot in isooctane. 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 isooctane are rationalized as due to the effects of strong solvation of the free silene by the nitrile solvent.