Hydride-transfer reactions in the gas phase .2. Anchimeric assistance in the H- transfer from 1,1-dimethylcyclopentane to alkyl cations

The gas-phase hydride transfer from the title compound to several ionic accepters GA(+) = CH5+, C2H5+ s-C3H7+, and t-C4H9+ was studied by mass spectrometric and radiolytic methods in the pressure range 1.1 x 10(-8)-700 Ton. 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 alpha-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 interpreted in terms of a transition state, which is placed rather late along the reaction coordinate, and wherein the assistance of the neighboring (methyl or methylene) group to the departing hydride increases by decreasing the strength of the GA(+) acceptor. The results obtained from the present gas-phase investigation are discussed and compared with those of related gas-phase and solution data.