The γ-Silicon Effect. I. Solvent Effects on the Solvolyses of 2,2-Dimethyl-3-(trimethylsilyl)propyl and 3-(Aryldimethylsilyl)-2,2-dimethylpropyl p-Toluenesulfonates

Abstract

Abstract The solvolysis rates of 2,2-dimethyl-3-(trimethylsilyl)propyl and 3-(aryldimethylsilyl)-2,2-dimethylpropyl p-toluenesulfonates were measured in a wide variety of solvents at 45 °C. The solvent effects were analyzed by using the Winstein–Grunwald equation. The solvent effects observed did not give simple linear correlations with the 2-adamantyl YOTs parameter, but showed dispersion behavior in a series of binary solvents. The m values of 0.59—0.67 were remarkably lower than unity for the limiting kc solvolysis of 2-adamantyl p-toluenesulfonate. The deviation patterns could not be interpreted in terms of nucleophilic assistance by the solvent. The dispersion behavior with reduced m values was found to be more significant for the 3-(aryldimethylsilyl) than for the 3-(trimethylsilyl) derivatives and was compatible with the delocalization of the incipient cationic charge by participation of the Si–Cγ bond in the rate-determining step. An extended dual-parameter treatment, log (k/k80E) = mcYOTs + mΔYΔ, successfully correlated such γ-silyl assisted solvolyses. The MΔ values of 0.24—0.49 so obtained, where MΔ = 0.51mΔ/(mc + 0.51mΔ), are a measure of the extent of charge delocalization, suggesting that the γ-silyl group in the percaudal interaction is more effective in delocalizing the cationic charge than the alkyl group in C–C σ-participation, but less so than π-assisted interaction by the β-aryl group.