Kinetic isotope effect study on methyl participation in solvolysis of neopentyl-type arenesulfonates

Carbon-14 and deuterium kinetic isotope effects at various positions have been measured in the solvolysis of neopentyl arenesulfonates. In the acetolysis of neopentyl nosylate (6) at 100/sup 0/C, carbon isotope effects (k/sup 12//k/sup 14/) are 1.073 at ..cap alpha.., 1.019 at ..beta.., and 1.046 at ..gamma... Deuterium effects (k/sub H//k/sub D/) are 1.187 at ..cap alpha.. (D/sub 2/) and 1.016 at ..gamma.. (D/sub 9/). In the trifluoroacetolysis of neopentyl brosylate (7) at 65/sup 0/C, k/sup 12//k/sup 14/ are 1.073 at ..cap alpha.., 1.023 at ..beta.., and 1.026 at ..gamma.. and k/sub H//k/sub D/ are 1.190 at ..cap alpha.. (D/sub 2/) and 1.012 at ..gamma.. (D/sub 9/). These positive isotope effects at all the positions are compatible with the concerted nature of the solvolytic rearrangement. Kinetic isotope effects of only a migrating methyl group have been measured in the solvolysis of 2-methyl-2-adamantanemethyl brosylate (9), a model system of neopentyl solvolysis; k/sup 12//k/sup 14/ and k/sub H//k/sub D/ (D/sub 3/) are 1.037 and 1.123 in the acetolysis at 100/sup 0/C, 1.037 and 1.135 in the trifluoracetolysis at 25/sup 0/C, and 1.035 and 1.12 in the trifluoracetolysis at 45/sup 0/C, respectively. Combination of the results for 6 and 7 with those for 9 revealed that the carbon-14 isotope effects at ..gamma.. primarily arise from the migrating methyl group and that the negligibly small D/sub 9/ effect at ..gamma.. is brought about by cancellation of a normal effect of the migrating methyl group and inverse effects of the two nomigrating methyl groups. The nature of sigma participation as a composite of two formally conflicting modes, namely, hyperconjugation and bridging, is discussed on the basis of these results.