Efficient homogeneous radical-anion chain reactions initiated by dissociative electron transfer to 3,3,6,6-tetraaryl-1,2-dioxanes.

A series of 3,3,6,6-tetraaryl-1,2-dioxanes (TADs) have been investigated at an inert electrode by using cyclic voltammetry, constant potential electrolysis and digital simulations. The series consists of the phenyl-substituted TAD (1 a), p-methoxy-aryl TADs (1 b, 1 c) and the p-methoxy/nitro-bearing TAD (1 d). The heterogeneous electron-transfer (ET) reduction is dissociative, causing rupture of the oxygen-oxygen bond, which generates a distonic radical-anion that reacts competitively either by beta-scission fragmentation or ET. Fragmentation of the distonic radical anion yields an alkene, a substituted benzophenone, and a benzophenone radical anion. The benzophenone radical-anion propagates an efficient homogeneous ET-fragmentation chain reaction that accounts for the potential dependence of the product ratios and the low charge consumption observed in the controlled potential electrolysis experiments. Digital simulation of the experimental cyclic voltammograms allowed for estimates of the rate constants of the heterogeneous ET to the O--O bond, and for the rate constants for the beta-scission fragmentation of the distonic radical-anions. Density functional theory calculations corroborate the differences in the heterogeneous kinetics of the initial dissociative ET. The endoperoxides 1 a-1 c react predominantly by a concerted dissociative ET mechanism, although the data suggests a stepwise dissociative pathway is also competitive. Bearing a nitro-aryl substituent, 1 d provides a rare example of an endoperoxide that proceeds by a stepwise dissociative ET mechanism. Irrespective of the initial mechanistic details, we find a propagating radical-anion cycle is a general mechanistic feature.