Cation-π Interactions in the Gas Phase Methylation of α, ω-Diphenylalkanes .

The methylation of alfa,omega-diphenylalkanes (C6H5(CH2)nC6H5, 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 deltaEa 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.