Substituent effects on the reactivity of the silicon-carbon double bond. Substituted 1,1-dimethylsilenes from far-UV laser flash photolysis of alpha-silylketenes and (trimethylsilyl)diazomethane

Photolysis of (trimethylsilyl)-, bis(trimethylsilyl)- and pentamethyldisila nylketene in hydrocarbon solution in the presence of aliphatic alcohols aff ords alkoxysilanes from trapping of silene reactive intermediates. The sile nes (1,1,2-trimethylsilene, 1,1,2-trimethyl-2-(trimethylsilyl)silene, and 1 ,1-dimelhyl-2-trimethylsirene) can be viewed as the products of [1,2]-methy l or -trimethylsilyl migration in the silyl carbene formed by photodecarbon ylation of the ketene, although a competing direct excited-state pathway ca nnot be ruled out. Far-UV (193 nm) laser flash photolysis of the compounds in hydrocarbon solution affords transients which are formed during the simi lar to 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-Trimethyl silene 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 corres ponding 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 sigma(R)degrees, affording rho(R) values o f +8.0 +/- 2.2 and +6.5 +/- 2.6 for MeOH and t-BuOH, respectively. This all ows the conclusion that the reactivity of simple silenes toward nucleophile s is enhanced by resonance electron-acceptor substituents at carbon. To pro be for the possible intermediacy of (trimethylsilyl)carbene in the formatio n of 1,2,2-trimethylsilene from these compounds, flash photolysis experimen ts with (trimethylsilyl)ketene, -diazomethane, arid -diazirine in the prese nce of pyridine have been carried out,Absorptions assignable to the carbene -pyridine ylide were observed from the ketene and the diazirine, allowing a n estimate of between 0.1 and 0.4 ns for the lifetime of singlet(trimethyls ilyl)carbene-in hydrocarbon solvents at room temperature.