INVOLVEMENT OF OXIRANE INTERMEDIATES IN THE ELECTRON-TRANSFER PHOTOOXYGENATION OF 1,1-DIARYLETHYLENES AND 1,2-DIARYLETHYLENES SENSITIZED BY 2,4,6-TRIPHENYLPYRYLIUM TETRAFLUOROBORATE

The photooxygenation of trans- and cis-stilbene (2a,b) by means of 2,4,6-triphenylpyrylium tetrafluoroborate (TPT) led to benzaldehyde (5) and diphenylacetaldehyde (6). Under the same conditions trans- and cis- 1,2-diphenyloxirane (4a,b) afforded 6 as major product, together with minor amounts of 5 benzophenone (8) and, in the case of 4b, 1,2-diphenylethanone (7). These data support the intermediacy of the oxiranes in the photooxygenation of the corresponding stilbenes and reveal that rearrangement of the radical cations 4a,b(+.) occurs by two alternative pathways: phenyl migration and H migration. The TPT-photosensitized oxygenation of 5-methylene-10,11-dihydro-5H-dibenzo[a,d]cycloheptene (9) produced mainly dibenzosuberone (10) and dibenzosuberane (11), together with trace amounts of products arising from dehydrogenation of the dimethylene bridge (14,15,16) and solvent-derived products (12,13,18). The formation of 10 and 11 is justified via the radical cation of the 1,1-diphenylethylene moiety, generated by electron transfer from the olefin to the excited sensitizer. The epoxide 20, aldehyde 17 and dioxetane 19 appear to be key intermediates. Formation of the minor compounds 14, 15 and 16 can be explained through the radical cation of the 1,2-diphenylethane substructure.