Efficient Homogeneous Radical‐Anion Chain Reactions Initiated by Dissociative Electron Transfer to 3,3,6,6‐Tetraaryl‐1,2‐dioxanes

Abstract

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.

Electrochemical behavior of N‐oxyphthalimides: Cascades initiating self‐sustaining catalytic reductive N―O bond cleavage

“…The key feature of PI‐ORs examined in the present work is the presence of the relatively weak N―O bond, which is susceptible to cleavage via reductive 1‐electron transfer . These compounds provide an interesting counterpart to organic peroxides, a related class of biologically active antiparasitic and antitumor compounds containing a weak O―O bond.…”

Dissociative Electron Transfer to Diphenyl-Substituted Bicyclic Endoperoxides: The Effect of Molecular Structure on the Reactivity of Distonic Radical Anions and Determination of Thermochemical Parameters

“…The heterogeneous reduction of aliphatic peroxides [10,11,12,13] and peresters [14,15,16,17] in aprotic solvents, such as acetonitrile and N , N -dimethylformamide (DMF), has been shown to occur by a dissociative electron transfer (ET) mechanism. Similarly, the heterogeneous reduction of endoperoxides has also been consistent with a dissociative ET mechanism [18,19,20,21,22,23,24,25] and including photo-initiated examples [26,27]. The first step of the mechanism involves O-O bond cleavage by ET to form a distonic radical-anion, a reactive intermediate with a spatially separated radical and anion (Equation (1)).…”

Upconversion of Reductants

“…Examples of reductant upconversion where a labile chemical bond and a stable coproduct are combined include the decomposition of radical anions of cyclic and bicyclic peroxides (Table ) . In this case, the peroxide bond is the labile bond, whereas the ketone and ethylene (or another alkene) eliminated by this chemical system are the stable coproducts.…”