Concomitant Hydride and Proton Transfer: An Essay on Competing and Consecutive Key Reactions Occurring in Gaseous Ion/Neutral Complexes

Abstract

The interplay of proton transfer and hydride transfer reactions in alkylbenzenium ions and related protonated di- and oligophenylalkanes is presented and discussed. While intra- and inter-annular proton exchange has been recognised to be a ubiquitous feature in protonated arenes, hydride abstraction is much less obvious but can become a dominating fragmentation channel in metastable ions of tert-butyl-substituted alkylbenzenium ions and related carbocations. In such cases, proton-induced release of the tert-butyl cation gives rise to ion/neutral complexes as reactive intermediates, for example, [(CH3)3C+ ṫ arylCH2α(CH2) nCH2ωaryl ′] with n ⩾ 0 and highly regioselective intra-complex hydride transfer occurs from all of the benzylic methylene hydride ion donor groups (α-CH2 and ω-CH2) to the tert-butyl cation acting as a Lewis acid. Substituent effects on the individual contributions to the overall hydride transfer from different donor sites, including ortho-methyl groups, in particular, and the concomitant intra-complex proton transfer from the tert-butyl cation to the neutral diarylalkane constituent corroborate the view of “bisolvated” complexes as the central intermediates, in which the carbenium ion is coordinated to both of the aromatic π-electron systems. The role of cyclisation processes, converting the benzylic [M – H]+-type ions into the isomeric benzenium, [M + H]+-type ions prior to fragmentation, is demonstrated for several cases. This overall scenario, consisting of consecutive and/or competing intra-complex hydride abstraction and proton transfer, intra-annular proton shifts (H+ ring walk) and interannular proton transfer, hydrogen exchange (“scrambling”) processes, and cyclisation and other electrophilic substitution reactions, is of general importance in this field of gas-phase ion chemistry and more recent examples concerning protonated ethers, benzylpyridinium and benzylammmonium ions are discussed in which these recurring features play central and concerted mechanistic roles as well.