Direct detection of 1,1-diphenyl-2-neopentylsilene and the effects of solvent polarity on its reactivity with nucleophiles

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. 250 μ s and a UV absorption maximum of λ(max) = 335 nm (ε = 10000 ± 2900 M-1 cm(-1)) in dry, deoxygenated hexane, where it undergoes head-to-tail dimerization and reacts with oxygen with absolute rate constants of k(dim) = (5 ± 2) x 10(8) M-1 s(-1) and k(O2) = (6.5 ± 0.8) x 10(5) M-1 s-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, 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 10(3). 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 25 ° C 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.