Gaseous [t-C4H9+ alpha,omega-diphenylalkane] complexes: methyl substituenteffects on the intracomplex proton transfer and regioselective hydride abstraction

The loss of isobutane and isobutene from a series of long-lived protonated 1-(4-tert-butylphenyl)-3-phenylpropanes bearing two or three methyl substit uents on the aromatic rings have been studied by mass-analyzed ion kinetic energy spectrometry and deuterium labelling. Both processes occur via inter mediate ion/neutral complexes consisting of the tert-butyl cation and the c orresponding di- or trimethyl-substituted 1,3-diphenyl-propanes, and the co mpetition 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-II bond s acting as hydride donors, on the other. In addition to the benzylic alpha -and omega-methylene groups, ortho-sited methyl substituents are also invol ved in the hydride transfer, in contrast to those at the meta and para posi tions, and their fractional contributions to the overall isobutane loss str ongly depends on the electronic influence of the other methyl group(s) at t he same ring. Regioselectivities of the intracomplex hydride abstraction by t-C4H9+ from the alpha and gamma or alpha, gamma and ortho positions of th e 1,3-diphenylpropanes have been evaluated assuming the kinetic isotope eff ect k(H)/k(D) that is found to operate ubiquitously in these complexes. Whi le acting as hydride donors on their own, ortho-methyl groups attenuate the hydride donor ability of the adjacent benzylic methylene group by steric h indrance, in particular in the case of twofold (2,6-dimethyl) substitution. Comparison with the fragmentation behavior of the 2-methyl- and 2,6-dimeth yl-substituted mononuclear 4-(tert-butyl)ethylbenzenium ions corroborate th e kinetic isotope effect and, in particular, the activating effect of the s econd ("spectator") ring on the hydride abstraction within the [t-C4H9+ alp ha,omega-diphenylalkane] complexes. (Inc J Mass Spectrom 199 (2000) 155-187 ) (C) 2000 Elsevier Science B.V.