NEIGHBORING GROUP PARTICIPATION IN SOLVOLYSES. 11. KINETIC ISOTOPE EFFECT STUDY OF THE SOLVOLYSIS OF NEOPHYL ARENESULFONATES

Carbon-14 and deuterium kinetic isotope effects (KIE) have been determined in the solvolysis reactions of para-substituted neophyl arenesulfonates. Large ..cap alpha..-carbon KIE (1.094 in acetic acid (AcOH), 75/sup 0/C; 1.141 in trifluoroacetic acid (F/sub 3/AcOH), 0/sup 0/C) and medium Ph-1-carbon KIE (1.023 in AcOH; 1.035 in F/sub 3/AcOH) were observed for the unsubstituted neophyl brosylate; larger ..cap alpha..-carbon and smaller Ph-1-carbon KIE were observed for a substrate with a more electron-donating substituent, and the reverse trend was found for electron-withdrawing substituents. These results, together with small ..beta..-carbon KIE (1.014 in AcOH and in F/sub 3/AcOH) and medium ..cap alpha..-deuterium KIE (1.214 in AcOH; 1.247 in F/sub 3/AcOH, both per D/sub 2/), suggest that the reaction proceeds via a k/sub ..delta../ mechanism in which the neighboring phenyl group participates in the rate-determining ionization step and that the transition state (TS) shifts to a more reactant-like structure as the electron-donating character of the substituent increases. The calculations of the KIE were carried out within the framework of transition state theory, in order to test these mechanistic conclusions, assuming a bridged TS structure with the neighboring phenyl group acting as an internal nucleophile displacing the leaving group. Cutoff models containing 10 atoms weremore » used for both the reactant and TS, with structural parameters and force constants of the TS related to those of the reactant by empirical expressions relating geometry and force constants to bond orders. The bond orders of the five reacting bonds and a reaction coordinate consistent with S/sub N/2 character at C/sub ..cap alpha../ constitute the independent parameters; consequently, families of solutions are found for which calculated KIE reproduce experimental values. 10 figures, 9 tables.« less