A radical-anion chain mechanism initiated by dissociative electron transfer to a bicyclic endoperoxide: Insight into the fragmentation chemistry of neutral biradicals and distonic radical anions

摘要

The electron-transfer (ET) reduction of two diphenyl-substituted bicyclic endoperoxides was studied in N,N-dimethylformamide by heterogeneous electrochemical techniques. The study provides insight into the structural parameters that affect the reduction mechanism of the O-O bond and dictate the reactivity of distonic radical anions, in addition to evaluating previously unknown thermochemical parameters. Notably, the standard reduction potentials and the bond dissociation energies (BDEs) were evaluated to be -0.55 +/- 0.15 V and 20 +/- 3 kcal mol(-1), respectively, the last representing some of the lowest BDEs ever reported. The endoperoxides react by concerted dissociative electron transfer (DET) reduction of the O-O bond yielding a distonic radical-anion intermediate. The reduction of 1,4-diphenyl-2,3dioxabicyclo[2.2.2]oct-5-ene (1) results in the quantitative formation of 1,4-diphenylcyclohex-2-ene-cis-1,4-diol by an overall two-electron mechanism. In contrast, ET to 1,4-diphenyl-2,3dioxabicyclo[2.2.2]octane (2) yields 1,4diphenylcyclohexane-cis-1,4-diol as the major product; however, in competition with the second ET from the electrode, the distonic radical anion undergoes a beta-scission fragmentation yielding 1,4-diphenyl-1,4-butanedione radical anion and ethylene in a mechanism involving less than one electron. These observations are rationalized by an unprecedented catalytic radical-anion chain mechanism, the first ever reported for a bicyclic endoperoxide. The product ratios and the efficiency of the catalytic mechanism are dependent on the electrode potential and the concentration of weak non-nucleophilic acid. A thermochemical cycle for calculating the driving force for beta-scission fragmentation is presented, and provides insight into why the fragmentation chemistry of distonic radical anions is different from analogous neutral biradicals.