Substituent Effects on the Reactivity of the Silicon−Carbon Double Bond. Substituted 1,1-Dimethylsilenes from Far-UV Laser Flash Photolysis of α-Silylketenes and (Trimethylsilyl)diazomethane

• Published in: Journal of the American Chemical Society Vol. 121; no. 20; pp. 4744 - 4753
• Main Authors: Leigh, William J., Kerst, Corinna, Boukherroub, Rabah, Morkin, Tracy L., Jenkins, Stephen I., Sung, Kuangsen, Tidwell, Thomas T.
• Format: Journal Article
• Language: English
• Published: American Chemical Society 26.05.1999
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja984277z

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-(trimethylsilyl)silene, 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 (trimethylsilyl)diazomethane 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 σR°, affording ρR 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 simple silenes toward nucleophiles is enhanced by resonance electron-acceptor substituents at carbon. To probe for the possible intermediacy of (trimethylsilyl)carbene in the formation of 1,1,2-trimethylsilene from these compounds, flash photolysis experiments with (trimethylsilyl)ketene, -diazomethane, and -diazirine in the presence of pyridine have been carried out. 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 (trimethylsilyl)carbene in hydrocarbon solvents at room temperature.