ELECTRON-TRANSFER-INDUCED PHOTOREARRANGEMENTS AND PHOTOOXYGENATIONS OF 1,1,2,3-TETRAARYLCYCLOPROPANES

1,1-Diaryl-trans-2,3-diphenylcyclopropanes 3a-e and trans-2,3-diaryl-1,1-diphenylcyclopropanes 3f-i were prepared by di-π-methane rearrangement of 1,1-diaryl-3,3-diphenylpropenes 2a-e and 3,3-diaryl-1,1-diphenylpropenes 2f-i, respectively. In nonpolar and in polar solvents, tetraarylcyclopropanes 3a-i are inert toward singlet oxygen. In the presence of 9,10-dicyanoanthracene (DCA), irradiation of cyclopropanes 3a-e in O2-saturated acetonitrile (MeCN) solutions yield oxygenation products: benzophenone and benzaldehyde as main products from 3a, 1,2-dioxolanes 4b-e from 3b-e. Cyclopropanes 3f-i, however, rearrange to 3,3-diaryl-1,1-diphenylpropenes 2f-i under these conditions. When irradiated in the presence of DCA or 1,8-dihydroxyanthraquinone (1,8-AQ) in deoxygenated MeCN solutions, cyclopropanes 3a-e remain unchanged, whereas cyclopropanes 3f-i rearrange to propenes 2f-i. By using biphenyl and lithium perchlorate as additives in DCA-sensitized reactions, photooxygenations of 3a-e as well as photorearrangements of 3f-i are shown to proceed as electron-transfer-induced reactions via 1,3-radical cations formed by ring opening, occurring in all cases exclusively between carbon atoms C-1 and C-2 of these cyclopropanes. Small amounts of benzoquinone quench the DCA-sensitized oxygenation of cyclopropanes 3a-e completely, and 1,8-AQ is incapable of sensitizing the oxygenation of these cyclopropanes, indicating that electron-transfer-induced oxygenations of 1,1,2,3-tetraarylcyclopropanes need superoxide radical anion, O2.+, as the oxygenating species. A mechanism is proposed according to which the cationic sites of the 1,3-radical cation intermediates determine whether the respective radical cation rearranges rapidly by 1,2-aryl migration to the corresponding 1,1,3,3-tetraarylpropene or is long lived enough to undergo a bimolecular reaction with O2.+ to yield the corresponding 1,2-dioxolane. © 1990 American Chemical Society.