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 + ) CH 5 + , C 2 H 5 + , s-C 3 H 7 + , and t-C 4 H 9 + 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 R-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-C 3 H 7 + or t-C 4 H 9 + , amount to 2.70 (s-C 3 H 7 + ) and 1.89 (t-C 4 H 9 + ) and to 1.77 (s-C 3 H 7 + ) and 1.15 (t-C 4 H 9 + ), 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.

Diastereomeric differentiation of peptides with CuII and FeII complexation in an ion trap mass spectrometer

“…An axial/equatorial equilibrium of the Ala methyl group conformation generates the a 4 form, from which a stepwise dissociation can occur via the formation of an ion–dipole complex (a 5 form). Its production proceeds through a hydride transfer68–71 from the Ala methyl group to the neighboring N ‐terminal α‐carbon, concomitantly with benzyl cleavage assisted by a 1–2 proton shift. From this ion–dipole complex, the negative charge of the p ‐methylphenoxide moiety could yield the reduction of Cu II into Cu I to promote the loss of the [CH 3 C 6 H 4 O] • radical.…”

质子化脯氨酸三肽的产物离子在电喷雾质谱中的碎裂机理

“…该肽链中不含精氨酸, 则可以确定该肽链中含有脯 氨酸 [16,17] . [35] 、电子迁移 [37] 、酰 基转移反应 [38,39] 等, 而这些反应在由共价键结合的 体系中是很难发生的. 在 PBC 中, 两部分中性分子…”

Benzyl anion transfer in the fragmentation of N-(phenylsulfonyl)-benzeneacetamides: a gas-phase intramolecular SNAr reaction

“…[4][5][6][7] In gaseous ionic reactions, ion-neutral complex (INC) intermediates are extensively invoked to rationalize the generation of some special product ions. 13 Several other transfer reactions were also observed including transacylation, 15 hydride transfer, [16][17][18][19] electron transfer, 20,21 benzyl cation transfer (BCT), [22][23][24] etc. In such a temporary system, various intriguing chemical reactions occur prior to final separation of the INC partners.…”