Solvolysis of endo- and exo-bicyclo[3.2.1]octan-3-yl toluene-p-sulphonates. Part 2. Rates and deuterium kinetic isotope effects

Abstract

Rates and secondary deuterium kinetic isotope effects for the solvolysis of endo-and exo-bicyclo[3.2.1]octan-3-yl toluene-p-sulphonates [(4a and c) and (5a and c)] have been measured. Rate constants, rate ratios, and apparent m values, when compared with results for model compounds, suggest that the solvolyses of (4a) and (5a) are close to the limiting (S_N1) extreme in all solvents. The α-deuterium kinetic isotope effects are high (ca. 1.19—1.20 at 25 °C) for (5c) in formic and acetic acids, aqueous 50, 80, and 98% ethanol, and aqueous 97% 2,2,2-trifluoroethanol, and for (4c) in all these solvents except aqueous 98% ethanol (1.141 at 60.4 °C). The β-²H₄ kinetic isotope effects are also high for both (4a) and (5a). The results are interpreted in terms of a principal mechanism involving rate-determining formation of intimate ion-pair intermediates. For the solvolysis of (4a) in aqueous 98% ethanol some direct S_N2 reaction of solvent with covalent tosylate is also invoked. Mechanisms of solvolysis generally are discussed including the roles of intimate and solvent-separated ion-pairs, and solvent-induced S_N2.