CRYSTAL-STRUCTURE AND PHOTOCHEMISTRY OF DIMETHYL 9,10-DICHLORO-9,10-DIHYDRO-9,10-ETHENOANTHRACENE-11,12-DICARBOXYLATE

Photolysis of the title 9,10-dichlorodibenzobarrelene-11,12-diester, (1), results in formation of a semibullvalene, dimethyl 4b,8b-dichloro-4b,8b,8c,8d-tetrahydro dibenzo[a,f]cyclopropa[cd]pentalene-8c,8d-dicarboxylate (2), and an additional photoproduct, dimethyl 4b, 10-dichloro-4b,9,9a, 10-tetrahydro-9,10-epidioxydibenzo[a,f]pentalene-9,9a-dicarboxylate (3), which contains an unusual peroxo-ring structure. The structures of (1), (2) and (3) have been determined by X-ray analysis. Formation of (2) can be rationalized in terms of a di-rr-methane rearrangement of (1) via biradical intermediates, and (3) is most likely a secondary photoproduct, formed from (2) via bond fission followed by trapping of molecular oxygen [no (3) is formed in carefully deoxygenated systems]. Crystals of (1) exhibit photochromism, probably as the result of formation of an (unidentified) radical ion.