Solvent Effects on the Solvolysis of Benzyl Tosylates. Behavior of Derivatives with Electron-Donating Substituents

Abstract

Abstract Solvolysis rates of p-methylbenzyl and 3-bromo-4-(methylthio)benzyl p-toluenesulfonates were measured in a wide variety of solvents and the solvent effects were compared with those for less activated benzyl tosylates and other related benzylic substrates. Plots of the dual parameter Winstein–Grunwald Equation failed to give satisfactory correlations using the ordinary 2-adamantyl YOTs parameter. m Values close to unity for both derivatives were obtained but the linearity of the correlations was unsatisfactory. Although the high response (m value) to the solvent polarity and the low l value may indicate a nearly limiting mechanism, the pattern of dispersion observed in binary solvent mixtures cannot be interpreted in terms of nucleophilic solvent assistance. The use of the polarity scale YΔ derived from 2-methyl-2-(p-methoxyphenyl)propyl tosylate solvolysis instead of YOTs significantly improved the correlations. The dispersion can be ascribed to the varying solvation-desolvation interactions characteristic of highly delocalized cationic centers in binary solvent mixtures. We used extended similarity comparison analysis, log (k/k80E) = mcYOTs + mΔYΔ + (lN). Based on the resulting characteristic susceptibility parameters, the mechanisms of benzyl solvolysis and the structures of relevant benzylic solvolyses transition states are discussed.