PUB - Publikationen an der Universität Bielefeld

The reactivity of alpha,omega-diphenylalkanes (1 less than or equal to n less than or equal to 4) toward Me(3)C(+) ions has been investigated with the radiolytic technique at 720 Torr, at temperatures of 47 and 120 degrees C. The intramolecular isotopic discrimination of Me(3)C(+), favoring attack at the unlabeled ring of C6H5(CH2)(2)C6D5 by a factor of 1.5 at 120 degrees C, contrasts with the lack of intermolecular isotopic discrimination, reflected by the same reactivity toward (C6H5CH2)(2) and (C6D5CH2)(2). Competition experiments show an appreciably higher reactivity of Ph(CH2)(n)Ph (n = 2-4) relative to toluene and diphenylmethane (DPM), but, when Ph(CH2)(n)Ph (n = 3, 4) or (3-CH3C6H4)(CH2)(2)C6H5 compete with Ph(CH2)(2)Ph, relative reactivities level off. 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 1 from Me(3)C(+) and diphenylalkanes. The additional (''spectator'') ring of the higher homologues Ph(CH2)(n)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(3)Si(+) at 120 degrees C show that this electrophile attacks preferably the unlabeled ring of C6H5(CH2)(2)C6D5 by a factor of 1.5, consistent with the 1.4 times higher reactivity toward (C6H5CH2)(2) 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.