| Abstract
| - Photolysis of (trimethylsilyl)-, bis(trimethylsilyl)-, and pentamethyldisilanylketene in hydrocarbonsolution 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 byphotodecarbonylation 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 whichare formed during the ∼20 ns laser pulse and are assignable to the silenes on the basis of their UV absorptionspectra and reactivity toward alcohols. 1,1,2-Trimethylsilene has also been generated by laser flash photolysisof (trimethylsilyl)diazomethane and -diazirine in hexane and acetonitrile solution, and its spectrum and rateconstants for reaction with MeOH, MeOD, and t-BuOH have been determined. Those in hexane agree withthose obtained using the corresponding ketene as the precursor. The rate constants for addition of ROH tothese 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 theconclusion that the reactivity of simple silenes toward nucleophiles is enhanced by resonance electron-acceptorsubstituents at carbon. To probe for the possible intermediacy of (trimethylsilyl)carbene in the formation of1,1,2-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 0.4ns for the lifetime of singlet (trimethylsilyl)carbene in hydrocarbon solvents at room temperature.
|
