Hydride ion transfer reactions in the gas phase. Pressure dependence of reaction efficiency as a criterion for the recognition of anchimeric assistance

Abstract

The efficiency (kclb$kcoll) of the gas phase hydride ion transfer from 2,2-dimethylbutane to tert-butyl cation displays a pronounced posir.ive pressure dependence, which suggests significant anchimeric assistance of the methyl groups of 2,2-dirnethylbiitane adjacent to the departing CH2 hydride in the transition state.

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

“…1 atm (stationary radiolysis). 1 This pressure-dependence criterion is based upon the consideration that the efficiency of a gas-phase ion-molecule reaction ()k f /(k f + k r )), involving the formation of a stable, electrostatically bonded ion-neutral complex (INC) is determined by the competition between the evolution of INC to products (k f ), through an internal activation barrier E*, and its back-dissociation to reactants (k r ), through the pseudobarrier E°. This competition depends on both the features of the free energy profile and the experimental conditions.…”

“…The actual isolation and characterization of the neutral reaction products provide the necessary information on structure and stereochemistry of the ionic intermediates involved . A demonstration of this capability is provided by the observation that few unambiguous pieces of evidence of gas-phase anchimerically assisted ionic processes are reported in the literature and most of them based upon high-pressure kinetic investigations using radiolytic methods. , Among these, the most recent one provided the first unambiguous evidence of methyl-group anchimeric assistance in the gas-phase hydride transfer from the CH 2 of 2,2-dimethylbutane to tert -butyl cation (eq 1a; R = CH 3 ), on the grounds of the over 100-fold increase of the efficiency of the H - transfer by increasing the system pressure from 4.0 × 10 -8 Torr (FT-ICR) to ca. 1 atm (stationary radiolysis) …”

“…At the collision-free limit (approached in the FT-ICR kinetic experiments), the INC is characterized by a constant total energy and angular momentum. Its evolution to reactants or products is essentially governed by entropic, rather than by enthalpic factors.…”

“…Under such conditions, its crossing of the internal barrier E * is rather unlikely and thus, the reaction efficiency is generally low. An increase in the reaction pressure (radiolytic experiments) 1 favors the disposal of the excess energy E ° of INC by unreactive collisions with the buffer gas (present at 160−700 Torr) . Its lifetime increases and its evolution tends to be progressively governed by enthalpic, rather than by entropic factors and, therefore, to follow the lowest activation energy pathway.…”

“…In the gas-phase hydride transfer from the CH 2 of 2,2-dimethylbutane to tert -butyl cation (eq 1a; R = CH 3 ), the positive pressure dependence of the reaction efficiency excludes the stepwise mechanism 1b, involving formation of an INC between isobutane and the 3,3-dimethyl-2-butyl cation at energy levels protruding above that of the starting reactants ( E * − E ° > ca. 1−2 kcal mol -1 ).…”