The photochemistry of a potential diphenylsilylene precursor. Time-resolved spectroscopic studies of the reactivity of a transient vinylsilirane

The photochemistry of 3,4-dimethyl-1,1-diphenylsilacyclopent-3-ene (2), a potential precursor to diphenylsilylene (SiPh2), has been studied in hydrocarbon and methanol solution by steady state and laser flash photolysis techniques. The results are consistent with the formation of three major photoproducts: SiPh2 (ca. 20%), 2,3-dimethyl-1,3-butadiene (DMB; ca. 20%), and 2-methyl-2-(methylethenyl)-1,1-diphenylsilirane (9; ca. 75%), the product of formal photochemical (1,3)-sigmatropic rearrangement of 2. Attempts to detect the silylene by laser flash photolysis were unsuccessful, but the vinylsilirane could be easily detected as a long-lived transient exhibiting λmax=285 nm and lifetime τ=1.3 s in hexane at 25 ^C. The same spectrum is observed in neat methanol solution, where the lifetime of the species is shortened to τ=36 ms and it is quenched by sodium methoxide and methanesulfonic acid with absolute rate constants of k MeO-=2.0×106 M−1 s−1 and kH+ =2.6×103 M−1 s −1, respectively. The photochemistry of 2 is compared to that of the corresponding germanium homologue (1), whose photolysis in solution has been previously shown to afford diphenylgermylene and DMB cleanly and in high yields. The corresponding vinylgermirane (11a), though not detectable as a primary photoproduct from 1 in solution, is the exclusive product of photolysis of 1 in a 3-methylpentane glass at 78 K, where it exhibits an essentially identical UV/vis spectrum to that of 9. The rate constant for its unimolecular thermal isomerization to regenerate 1 is three orders of magnitude larger than that of the corresponding process in the homologous vinylsilirane (9) in hydrocarbon solution at 25 ^C.