Can. J. Chem. 86, 1105-1117 (2008).

© National Research Council of Canada

Substituent Effects on Silene Reactivity. Reactive Silenes from Photolysis of Phenylated Tri- and Tetrasilanes

William J. Leigh*, Andrey G. Moiseev, Eugenie Coulais, Farahnaz Lollmahomed, and Mohammed S. Askari

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

Laser flash photolysis of 2-phenylheptamethyltrisilane (5d), 2,2-diphenylhexamethyltrisilane (5e), and phenyltris(trimethylsilyl)silane (5f) in hexane and acetonitrile solution affords strong, long-lived transient absorptions centered in the 440-470 nm range, which are assigned to the transient silenes formed via [1,3]-trimethylsilyl migration into the ortho-position of a phenyl ring on the basis of their UV/vis spectra and kinetic data for their reactions with methanol (MeOH), acetic acid (AcOH), acetone, 2,3-dimethyl-1,3-butadiene (DMB), carbon tetrachloride (CCl₄), and oxygen. The silene derivatives are formed along with the corresponding silylenes (SiMePh, SiPh₂, and Si(SiMe₃)Ph, respectively) upon photolysis of these compounds in solution, and indeed, weak short-lived transient absorptions assignable to the silylenes can also be detected in laser photolysis experiments with the three compounds in hexane, superimposed on the much more prominent absorptions due to the silenes. The silylene absorptions are quenched by MeOH and triethylsilane (Et₃SiH) with absolute rate constants varying over the narrow ranges of (1.1 – 1.8) x 10¹⁰ M^-1s^-1 and (2.5 – 3.6) x 10⁹ M^-1s^-1, respectively, in excellent agreement with previously reported values for SiPh₂ and SiMe₂ under the same conditions. The kinetic data obtained for the silenes are compared to previously reported data for the reactions of the same substrates with the related silenes (6a-c) formed by photolysis of pentamethylphenyl-, 1,1,1,2-tetramethyl-2,2-diphenyl-, and 1,1,1-trimethyl-2,2,2-triphenyldisilane (5a-c, respectively) under similar conditions. The comparison provides the first systematic, quantitative assessment of the stabilizing effects of trialkylsilyl substitution at the silenic silicon atom in silene derivatives.

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