Cation-π interactions in the gas phase methylation of α,ω-diphenylalkanes

The methylation of alpha,omega-diphenylalkanes (C6H5(CH2)(n)C6H5, n = 1-6) has been performed in the gas phase using Me2Cl+ ions as alkylating species and toluene as reference substrate. Both in radiolytic experiments at atmospheric pressure and in FT-ICR measurements at 10(-8) Torr, the selected diphenylalkanes reacted faster than toluene, the highest reactivity displayed by 1,3-diphenylpropane. The kinetic pattern of the reaction, conforming to the established scheme of an electrophilic alkylation reaction, is consistent with a rate-determining formation of the sigma-complex intermediate, at variance with the tert-butylation of the same series of compounds by Me3C+ ions, occurring at the collisional encounter rate. The kinetic features are explained by a marked effect due to the presence of the second aryl ring, providing additional stabilization of both the ion-neutral collision complex and the sigma complex with respect to toluene. Both factors contribute to the deltaE(a) of ca. 8 kcal mol(-1) for the competition of 1,3-diphenylpropane and toluene found in the temperature dependence study of the Me2Cl+ reaction.