Elucidation of the Electron Transfer Reduction Mechanism of Anthracene Endoperoxides

The homogeneous and heterogeneous reductions of the endoperoxides 9,10-diphenyl-9,10-epidioxyanthracene (DPA-O2) and 9,10-dimethyl-9,10-epidioxyanthracene (DMA-O2) were investigated, and
they were found to undergo a dissociative electron-transfer reduction of the O−O bond to yield a distonic
radical anion, with no evidence for C−O bond dissociation. A number of thermochemical parameters for
each were determined using Savéant's model for dissociative electron transfer (ET), including E°, Δ

,
and bond dissociation energies. The products of the ET are dependent on the mode of reduction, namely
heterogeneous or homogeneous, and on the electrode potential or standard potential of the homogeneous
donor, respectively. The dissociative reduction of DMA-O2 under heterogeneous and homogeneous
conditions yields the corresponding 9,10-dihydroxyanthracene DMA-(OH)2, quantitatively, in an overall two-electron process. In the case of DPA-O2, ET reduction also yields the corresponding 9,10-dihydroxyanthracene DPA-(OH)2 from reduction of the distonic radical anion, but in competition with this reduction, an
O-neophyl-type rearrangement occurs that generates a carbon radical with a minimum rate constant of
5.9 × 1010 s-1. In the presence of a sufficiently reducing medium, the carbon-centered radical is reduced
(E° = −0.85 V vs SCE) and ultimately yields 9-phenoxy-10-phenyl anthracene (PPA). The observation of
this product is remarkable. In the heterogeneous ET, the yield of DPA-(OH)2/PPA is 97:3 and allows an
estimate of the rate constant for ET to the distonic radical anion. In homogeneous reductions, the O-neophyl
rearrangement is quantitative, but the yield of PPA depends on the redox properties of the donor. A unified
mechanism of reduction of DPA-O2 is presented to account for these observations.