Gaseous [ t-C 4H 9+ α,ω-diphenylalkane] complexes: methyl substituent effects on the intracomplex proton transfer and regioselective hydride abstraction

Abstract

The loss of isobutane and isobutene from a series of long-lived protonated 1-(4- tert-butylphenyl)-3-phenylpropanes bearing two or three methyl substituents on the aromatic rings have been studied by mass-analyzed ion kinetic energy spectrometry and deuterium labelling. Both processes occur via intermediate ion/neutral complexes consisting of the tert-butyl cation and the corresponding di- or trimethyl-substituted 1,3-diphenyl-propanes, and the competition between these intracomplex proton and hydride transfer processes is governed by the (overall) gas-phase basicity of the 1,3-diphenylpropane, on the one hand, and by the localized activation of the benzylic C–H bonds acting as hydride donors, on the other. In addition to the benzylic α- and ω-methylene groups, ortho-sited methyl substituents are also involved in the hydride transfer, in contrast to those at the meta and para positions, and their fractional contributions to the overall isobutane loss strongly depends on the electronic influence of the other methyl group(s) at the same ring. Regioselectivities of the intracomplex hydride abstraction by t-C 4H 9 + from the α and γ or α, γ and ortho positions of the 1,3-diphenylpropanes have been evaluated assuming the kinetic isotope effect k H/ k D that is found to operate ubiquitously in these complexes. While acting as hydride donors on their own, ortho-methyl groups attenuate the hydride donor ability of the adjacent benzylic methylene group by steric hindrance, in particular in the case of twofold (2,6-dimethyl) substitution. Comparison with the fragmentation behavior of the 2-methyl- and 2,6-dimethyl-substituted mononuclear 4-( tert-butyl)ethylbenzenium ions corroborate the kinetic isotope effect and, in particular, the activating effect of the second (“spectator”) ring on the hydride abstraction within the [ t-C 4H 9 + α,ω-diphenylalkane] complexes.