Silicon Chemistry, in press.

W.J. Leigh

The Photochemistry of a Potential Diphenylsilylene Precursor. Time-resolved Spectroscopic Studies of the Reactivity of a Transient Vinylsilirane.

William J. Leigh,* Lawrence A. Huck, Erica Held, and Cameron R. Harrington

Contribution from the Department of Chemistry, McMaster University, 1280 Main Street West, Hamilton, ON Canada L8S 4M1.

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-methylpropenyl-1,1-diphenylsilirane (9; ca. 75%), the product of formal photochemical [1,3]-sigmatropic rearrangement. Attempts to detect the silylene by laser flash photolysis were unsuccessful, but the vinylsilirane could be easily detected as a long-lived transient exhibiting lmax = 285 nm and lifetime t ~ 1.3 s in hexane at 25 oC. The same spectrum is observed in neat methanol solution, where the lifetime of the species is shortened to 36 ms and it is quenched by sodium methoxide and methanesulfonic acid with absolute rate constants of kMeO- = 2.0 x 106 M-1s-1 and kH+ = 2.6 x 103 M-1s-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 photoproduct from 1 in solution, is the exclusive product of photolysis of 1 in a 3-methylpentane glass at 78K, where it exhibits an essentially identical UV/vis spectrum to that of 9. The lifetimes of the homologous vinylmetalliranes (9 and 11a) presumably reflect the rates of their unimolecular isomerization to the corresponding 1-metallacyclopent-3-enes (2 and 1, respectively), and differ by three orders of magnitude in hydrocarbon solution at 25 oC.

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