Reprinted from the Journal of the American Chemical Society, 115, 5332-5333 (1993).
©American Chemical Society, 1993.
William J. Leigh,* Christine J. Bradaric, and Gregory W. Sluggett
Department of Chemistry, McMaster University, Hamilton, Ontario, Canada L8S 4M1
Abstract: Nanosecond laser flash photolysis of 1,1,2-triphenylsilacyclobutane (1) in acetonitrile or isooctane solution gives rise to a readily detectable transient absorption with spectral maximum at 323-nm, which is assigned to 1,1-diphenylsilene (2). Silene 2 decays on the microsecond timescale with mixed first- and second-order kinetics in deoxygenated isooctane solution (tau < 13000 ns), and with first-order kinetics (tau ~ 800 ns) in dried acetonitrile. Direct photolysis of 1 in the presence of a series of carbonyl compounds affords high chemical yields of the corresponding methyldiphenylsilyl enol ethers by formal ene-addition. Silene 2 is relatively unreactive toward oxygen (k < 5 X 10^6 /M.s), but is quenched by alkanones, ethyl acetate, and 2,3-dimethyl-1,3-butadiene with rate constants in the 10^6 - 10^9 /M.s range. The rate constants are generally about a factor of two greater in isooctane than in acetonitrile solution. A kinetic deuterium isotope effect of 1.9 has been measured for the reaction of 2 with acetone in isooctane. The magnitude of the KIE is smaller in acetonitrile (KIE = 1.34 +/- 0.17). The kinetic data for the reactions of 2 with carbonyl compounds are consistent with a concerted, non-pericyclic mechanism, in which Si-O bond formation and H-transfer proceed asynchronously. Pulsed laser irradiation (337-nm) of 2 within 300 ns of its formation by laser photolysis of 1 in isooctane solution results in no detectable decrease in the transient absorption due to 2, suggesting that the silene is photostable under these conditions in fluid solution at room temperature.