Organometallics 24, 2307-2318 (2005).
© American Chemical Society, 2005
Thomas R. Owens, Joanna Grinyer, and William J. Leigh*
Contribution from the Department of Chemistry, McMaster University, Hamilton, ON Canada L8S 4M1
The reactivity of the transient silene 1,1-diphenyl-2-neopentylsilene (4) has been studied in various solvents by laser flash photolysis methods, using trans-1,1,3,3-tetraphenyl-2,4-dineopentyl-1,3-disilacyclobutane as the precursor. Silene 4 exhibits a lifetime of ca. 250ms and a UV absorption maximum of lmax = 335 nm (e= 10000 ± 2900 M-1cm-1) in dry, deoxygenated hexane, where it undergoes head-to-tail dimerization and reacts with oxygen with absolute rate constants of kdim = (5 ± 2) x 108 M-1s-1 and kO2 = (6.5 ± 0.8) x 105 M-1s-1, respectively. Identical absorption maxima are exhibited by 4 in 1,2-dichloroethane, tetrahydrofuran, and acetonitrile solution, indicating that the silene does not form detectable Lewis acid-base complexes with these solvents, and thus making it possible to study the effects of solvent polarity on silene reactivity for the first time. Accordingly, absolute rate constants for reaction of 4 with acetone, methanol, acetic acid, 2,2,2-trifluoroethanol, and n-butylamine have been determined as a function of temperature in two or more of the four solvents. The results are compared to previously reported data for 1,1-diphenylsilene (2a), and show that the 2-neopentyl substituent in 4 enhances the reactivity of the Si=C bond with oxygen, but reduces its reactivity with nucleophilic reagents and toward 2+2-dimerization by as much as a factor of 103. As with 2a, the Arrhenius parameters for the reactions of 4 are consistent with stepwise mechanisms, initiated by reversible complexation between the nucleophile and the silene, followed by H-transfer. The absolute rate constants at 25oC vary by only a factor of 3-10 with solvent in each case, but there is a general trend toward increasingly positive Arrhenius activation energies and pre-exponential factors with increasing solvent polarity. The mechanistic ramifications of these results are discussed.
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