Mechanism of the Superoxide Anion Radical (O₂^- •) Mediated Oxidation of Diarylmethanes

Variously substituted diphenylmethanes (2a-l) were prepared and reacted competitively with O₂^- • (generated from K0₂/18-crown-6 polyether) in benzene. The relative rate constants (k_rel) correlated best (r = 0.993) with σ^-, giving a p value of 3.96 ± 0.16. For the corresponding oxygenation mediated by tert-butoxide, the p obtained was 1.77 ± 0.41 (r = 0.950). The primary deuterium isotope effects (KH/KD) on the superoxide reaction of diphenylmethane and its 4,4';-dichloro analogue were 2.36 and 2.14, respectively. The rate of reaction was found to be linearly proportional to the crown ether concentration, and no reaction occurred in its absence. These results indicate that the reaction is homogeneous and is first order in superoxide and diphenylmethane. The correlation with σ^- and the magnitudes of p and the primary isotope effect are interpreted as requiring a reaction sequence in which a proton is first transfered from substrate to superoxide in the rate-determining step, with the resulting benzylic anion undergoing subsequent oxygenation to the corresponding ketones 1. A Bronsted analysis of the deprotonation reaction yields an a value of 0.69, suggesting a late transition state. The discrepancy between these results obtained in benzene and those of others for Me₂SO studies raises the possibility of a solvent-dependent duality of mechanism.