Dynamics of Proton Transfer from Radical Cations. Addition−Elimination or Direct Proton Transfer?

Abstract

Kinetic studies including the evaluation of deuterium kinetic isotope effects and Arrhenius activation energies implicate an addition−elimination mechanism for the proton transfer reaction between 9-methylanthracene radical cation and 2,6-lutidine in acetonitrile−Bu4NPF6 (0.1 M) and in dichloromethane−Bu4NPF6 (0.2 M). Isotopic substitution of D for H at the 10-position results in inverse deuterium kinetic isotope effects (kH/kD) equal to 0.83 due to nucleophilic attack on the radical cation by 2,6-lutidine. Primary kH/kD of 3.5−5.9 were observed for D3 substitution in the 9-methyl group. The addition−elimination mechanism involves unimolecular rearrangement of the initially formed adduct to give the product of proton transfer, 9-anthracenylmethyl radical. Oxidation of the latter followed by reaction with 2,6-lutidine affords N-(9-anthracenylmethyl)-2,6-lutidinium ion, which was isolated as the perchlorate salt. A comparison of kinetic data from reactions of both 9-methyl and 9,10-dimethylanthracene radical cations with pyridine and 2,6-lutidine results in the conclusion that in the absence of severe steric effects, radical cation−nucleophile combination is kinetically favored over direct proton transfer for these radical cations.