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

1997; American Chemical Society; Volume: 119; Issue: 47 Linguagem: Inglês

10.1021/ja973031b

1943-2984

Vernon D. Parker, You Tein Chao, Gang Zheng,

Molecular Sensors and Ion Detection

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.