Journal of the American Chemical Society, 121, 4744-4753 (1999).

1999 American Chemical Society.

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Substituent Effects on the Reactivity of the Silicon-Carbon Double Bond. Substituted 1,1-Dimethylsilenes from Far-UV Laser Flash Photolysis of a-Silylketenes and Trimethylsilyldiazomethane

William J. Leigh,* Corinna Kerst, Rabah Boukherroub, Tracy L. Morkin, Stephen I. Jenkins, K. Sung, and T.T. Tidwell

Contribution from the Department of Chemistry, McMaster University, Hamilton, ON  Canada L8S 4M1

and the Department of Chemistry, University of Toronto, Toronto, ON Canada

Abstract: Photolysis of trimethylsilyl-, bis(trimethylsilyl)- and pentamethyldisilanylketene in hydrocarbon solution in the presence of aliphatic alcohols affords alkoxysilanes from trapping of silene reactive intermediates. The silenes (1,1,2-trimethylsilene, 1,1,2-trimethyl-2-trimethylsilylsilene, and 1,1-dimethyl-2-trimethylsilene) can be viewed as the products of [1,2]-methyl or -trimethylsilyl migration in the silyl carbene formed by photodecarbonylation of the ketene, although a competing direct excited state pathway cannot be ruled out. Far-UV (193-nm) laser flash photolysis of the compounds in hydrocarbon solution affords transients which are formed during the ~20-ns laser pulse, and are assignable to the silenes on the basis of their UV absorption spectra and reactivity toward alcohols. 1,1,2-Trimethylsilene has also been generated by laser flash photolysis of trimethylsilyldiazomethane and -diazirine in hexane and acetonitrile solution, and its spectrum and rate constants for reaction with MeOH, MeOD, and t-BuOH have been determined. Those in hexane agree with those obtained using the corresponding ketene as the precursor. The rate constants for addition of ROH to these and three other 2-substituted 1,1-dimethylsilenes correlate with the resonance substituent parameter sRo, affording rR-values of +8.0 2.2 and +6.5 2.6 for MeOH and t-BuOH, respectively. This allows the conclusion that the reactivity of 1,1-dimethylsilene is enhanced by resonance electron-acceptor substituents at carbon, due to an increase in the polarity of the Si=C bond. Flash photolysis experiments with trimethylsilylketene, -diazomethane, and -diazirine in the presence of pyridine have been carried out, in order to probe for the possible intermediacy of trimethylsilylcarbene in the formation of 1,1,2-trimethylsilene from these compounds. Absorptions assignable to the carbene-pyridine ylide were observed from the ketene and the diazirine, allowing an estimate of between 0.1 and 0.4 ns for the lifetime of singlet trimethylsilylcarbene in hydrocarbon solvents at room temperature.

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