Laser flash photolysis of various organosilicon compounds such as aryl-, vinyl-, and alkynyldisilanes, silacyclobutanes and silacyclobutenes, and alpha-silylketenes and -diazomethanes leads to the formation of reactive silenes which can be detected directly in solution, allowing detailed studies of the kinetics and mechanisms of their reactions with nucleophiles. Over 30 transient silenes have now been studied by these methods, providing the opportunity to systematically assess the effects of substituents at silicon and carbon on the reactivity of the Si=C bond.

No 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-(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.

Irradiation of biphenyl encapsulated in the cavities of a NaZSM-5 zeolite framework has been reported to result in the formation of an extremely long-lived radical cation. Here, we show that such zeolite encapsulated radical cations can act as irreversible one-electron oxidants for simple alkenes and dienes, in a solid-state analogue to solution-phase cosensitization. Compared to the well-known semiconductor photooxidizers, such as titanium dioxide, the NaZSM-5 zeolite-based solid photooxidants exhibit enhanced selectivity based on oxidation potential, molecular size and shape, and Lewis base character.