Product-Determining Steps in Gas-Phase Bronsted Acid-Base REactions. Deprotonation of 1-Methylcyclopentyl Cation by Amine Bases

Article abstract of DOI:10.1021/ja00279a007

Experimental results are presented for deprotonation of 1-methylcyclopentyl cation (1) by ammonia, trimethylamine, and 1-methylcyclopentylamine at pressures below 10^-3 Torr, which yields two neutral products, 1-methylcyclopentene (2) and methylenecyclopentane (3).The isomer ratio varies from 2:3 = 2.74 for trimethylamine, consistent with previous reports.FTMS measurement of ion-molecule reaction rates of 1 plus ammonia gives an efficiency of 0.34 when 1 is generated by a 20-70 eV electron impact on bromocyclohexane.REaction of the predeuterated cation 1-d₁₁ with ammonia yields products that reveal only a very low level of hydrogen scrambling between acid and base.Isotope effects on the reaction rate and the neutral product ratio, determined from experiments using 1-d₁₁ or ND3, are found to be small.Two mechanistic alternatives are discussed: Lewis (in which covalent acid-base complexes are formed) and Eigen (in which hydrogen-bonded intermediates are formed).Reaction of 1-d₁₁ with 1-methylcyclopentylamine shows only a small amount of interchange of deuterated and undeuterated alkyl groups, which argues against a Lewis mechanism as an obligatory pathway.Experimental results are interpreted in terms of orbiting intermediates, in which acidic C-H bonds and the basic nitrogen experience large deviations from collinearity.More than one subsequent step is required for the reactants to engage chemically.In order for Eigen mechanisms to operate, a C-H bond must point toward the nitrogen, which requires loss of some internal degrees of freedom.For proton transfer to be completed, the nitrogen lone pair must be pointed along the C-H...N axis, which requires loss of additional degrees of freedom.Progress of the reaction beyond the initially formed orbiting intermediate is described by a scheme in which these stages of approach to reactive orientaion represent discrete steps.