Internal Solvation Effects on the Reactivity of alpha,omega-Diphenylalkanes towards Me3C+ Ions

The reactivity of alfa,omega-diphenylalkanes toward Me3C+ ions has been investigated with the radiolytic technique at 720 Torr, at temperatures of 47 and 120 “C. The intramolecular isotopic discrimination of Me3C+, favoring attack at the unlabeled ring of C6H5(CH2)2C6D5 by a factor of 1.5 at 120 “C, contrasts with the lack of intermolecular isotopic discrimination. Competition experiments show an appreciably higher reactivity of Ph(CH2),Ph (n = 2-4) relative to toluene and diphenylmethane (DPM). The lack of substrate selectivity, in contrast to an intramolecular discrimination in the tert-butylation of (3-CH3C6H4)-(CH2)2-C6H5 of a factor of 2, together with the related variation of kinetic isotope effects, points out the kinetic role of the collision complex from Me3C+ and diphenylalkanes. The additional (“spectator”) ring of the higher homologues Ph(CH2)nPh (n = 2-4) prevents dissociation of the complex, making its formation irreversible and causing tert-butylation to occur at the encounter rate, in contrast to DPM, whose second ring appears essentially inert. Experiments involving substitution by Me3Si+ at 120 “C show that this electrophile attacks preferably the unlabeled ring of C6H5-(CH2)2-C6D5 by a factor of 1.5. In this case, the isotopic discrimination is traced to the competition between desilylation and deprotonation of intermediate ipso-silylated arenium ions.