Internal Solvation Effects on the Reactivity of apDiphenylalkanes toward Me$+ Ions

The reactivity of a,w-diphenylalkanes (1 I n I 4) 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 C6Hs(CH2)2C6Ds by a factor of 1.5 at 120 “C, contrasts with the lack of intermolecular isotopic discrimination, reflected by the same reactivity toward ( C ~ H S C H ~ ) ~ and (C&CH2)2. Competition experiments show an appreciably higher reactivity of Ph(CH2),Ph (n = 2-4) relative to toluene and diphenylmethane (DPM), but, when Ph(CH2),Ph (n = 3,4) or (3-CH3Cdb)(cH2)2C6Hs compete with Ph(CH2)2Ph, relative reactivities level off. The lack of substrate selectivity, in contrast to an intramolecular discrimination in the tert-butylation of (3-CH3C6&)(CH2)2C& of a factor of 2, together with the related variation of kinetic isotope effects, points out the kinetic role of the collision complex 1 from Me$+ and diphenylalkanes. The additional (“spectator”) ring of the higher homologues Ph(CH2),Ph (n = 2-4) prevents dissociation of 1, 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 Me&+ at 120 “C show that this electrophile attacks preferably the unlabeled ring of C6H5(CH2)2C6D5 by a factor of 1.5, consistent with the 1.4 times higher reactivity toward ( C ~ H S C H ~ ) ~ with respect to (C6D5CH2)2. In this case, the isotopic discrimination is traced to the competition between desilylation and deprotonation of intermediate ipso-silylated arenium ions.