CRYSTAL-STRUCTURE AND PHOTOCHEMISTRY OF DIMETHYL -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 zo[a,f]cyclopropa[cd]pentalene-8c,8d-dicarboxyl ate (2), and an additional photoproduct, dimethyl 4b, 10-dichloro-4b,9 ,9a, 0-epidioxydibenzo[a,f]pentalene-9,9a-dicarboxylate (3), which con tains an unusual peroxo-ring structure. The structures of (1), (2) and (3) have been determined by X-ray analysis. Formation of (2) can be r ationalized in terms of a di-rr-methane rearrangement of (1) via birad ical intermediates, and (3) is most likely a secondary photoproduct, f ormed from (2) via bond fission followed by trapping of molecular oxyg en [no (3) is formed in carefully deoxygenated systems]. Crystals of ( 1) exhibit photochromism, probably as the result of formation of an (u nidentified) radical ion.