Interannular Proton Transfer in Thermal Arenium Ions from the Gas-Phase Alkylation of 1,2-diphenylethane

The first demonstration of thermal interannular proton shifts in gaseous bicyclic arenium ions and the evaluation of their Arrhenius parameters have been achieved with the radiolytic technique. The model ions have been obtained by alkylation of C6D5CH2CH2C6H5 with radiolytically formed Me3C+ ions in isobutane at sufficiently high pressure (630-1730 Torr) to ensure that the processes of interest obey thermal kinetics. From the ''tent of interannular H/D scrambling in the alkylated products, measured as a function of the arenium ions' lifetime, the rate constant for the H+(D+) ring-to-ring transfer in the para-substituted arenium ions has been estimated to be (1.3 +/- 0.4) x 10(7) ((2.9 +/- 0.6) x 10(6)) s-1 at 47-degrees-C. A temperature-dependence study covering the range from 47 to 150-degrees-C has allowed the evaluation of the Arrhenius parameters for the interannular H+(D+) transfer, giving E(a) = 6.3 +/- 0.2 (8.0 +/- 0.2) kcal mol-l and log A = 11.4 +/- 0.4 (11.9 +/- 0.3). Further mechanistic insight into the detailed mechanism of ps-phase alkylation is provided by other kinetic results. In particular, the significant interannular H/D discrimination of Me3C+, which favors the unlabeled ring of C6D5CH2CH2C6H5 by a factor of 1.7 at 47-degrees-C, provides further evidence for the reversible character of aromatic tert-butylation suggested by previous radiolytic and mass spectrometric studies on monocyclic arenes. The higher tert-butylation rate (2.9:1 at 120-degrees-C) of 1,2-diphenylethane than of toluene supports recent mass spectrometric results pointing to the formation of stable complexes between Me3C+ and alpha,omega-diphenylalkanes.