Electron transfer photoinduced degenerate rearrangement

The three methylenecyclobutanes, 51,52, and 53, do not interconvert under direct irradiation or SET-photosensitized conditions. However, 3,3-diaryl-4,4-dideuterio-l-methylenecyclobutane (d -Sd) does undergo electron-transfer photoinduced degenerate methylene-cy-clobutane rearrangement when either DCN, DCA, or 2,6,9,10-tetracy-anoanthracene is used as sensitizer. These processes involve the allylically stabilized 1,4-cation radical intermediates dj-SS and d -58/-+28 tjjg 1 4-cation radical 58 is efficiently captured by molecular oxygen, giving rise to 73 as shown in Scheme 15. 

The Electron Transfer Photoinduced Degenerate Methylenecyclopropane Rearrangement... 

A study of the photochemical Cope reaction of the hexadienes 40 has been carried out under photoinduced electron-transfer conditions. Evidence was gathered for the formation of a chair cyclohexane-1,4-radical cation 41 °. In snch systems, where the radical cation is formed using DCA as the sensitizer, a degenerate Cope process is operative. Thus when the tetradeuterio derivative 42 is used, rearrangement affords a (52 48) mixture of the two dienes 42 and 43. Related to this general problem, DCA-sensitized reactions of the isomeric dienes 44 and , -45 and the cyclization prodnct, the bicyclohexane 46, have been studied in considerable detail. At low conversions, the irradiation of 46 affords a mixture of the dienes 44 and , -45 in ratios that are independent of temperature. The influence of the position of the aiyl groups on the diene skeleton has also been studied. This does not appear to affect the conversion to a cyclic radical cation. Thus the SET-induced reaction of the diene 47 has shown that the open chain radical cation of the diene 48 cyclizes preferentially to the radical cation 49. ... 

Energetics and Mechanisms of Photoinduced Electron Transfer Degenerate Methylenecyclopropane Rearrangements... 

The most characteristic feature of the photoinduced electron transfer degenerate methylenecyclopropane rearrangement is die intervention of both cation radical and biradical intermediates. In this case, highly endothermic recyclization of d2-S2a tod -TIa and d -TIeT allows the operation of the important back electron transfer. Nevertheless, such a back electron transfer process may operate generally if a highly stabilized cation radical intermediate is formed in a highly exothermic process. In fact, the photoinduced electron transfer degen-... 

Ikeda, H., Minegishi, T., Abe, H., Konno, A., Goodman, J.L., and Miyashi, T., Photoinduced electron-transfer degenerate Cope rearrangement of 2,5-diaryl-l,5-hexadienes a cation radical cyclization-diradical cleavage process,/.Am Chem. Soc., 120, 87,1998... 

---