Hydride-Transfer Reactions in the Gas Phase. 2. Anchimeric Assistance in the H- Transfer from 1,1-Dimethylcyclopentane to Alkyl Cations

Abstract

The gas-phase hydride transfer from the title compound to several ionic acceptors GA+ = CH5+, C2H5+, s-C3H7+, and t-C4H9+ was studied by mass spectrometric and radiolytic methods in the pressure range 1.1 × 10-8−700 Torr. Analysis of the irradiated mixtures points to the exclusive formation of rearranged products, with those with methyl migration prevailing over those with ring expansion. Thermochemical considerations, isotope labeling experiments, and deuterium isotope effect measurements concur in defining the detailed mechanism of the gas-phase hydride transfer, which involves significant anchimeric assistance of the methyl and methylene groups adjacent to the reaction center. The primary and the secondary α-D kinetic isotope effects, measured in the hydride transfer from the methylene moieties adjacent to the quaternary carbon of the title compound to either s-C3H7+ or t-C4H9+, amount to 2.70 (s-C3H7+) and 1.89 (t-C4H9+) and to 1.77 (s-C3H7+) and 1.15 (t-C4H9+), respectively. These values are interpre...