CYCLOOCTATETRAENE FORMATION IN THE PHOTOLYSES OF DIBENZOBARRELENE DIESTERS

A study of cyclooctatetraenes (COTs) derived from the photolyses of di benzobarrelene derivatives indicates the formation of two types of COT with different substitution patterns; the structures of one example o f each type have been determined by X-ray crystal analyses. Crystal da ta: T = 294 K, Cu Kalpha, lambda = 1.5418 angstrom. (1), dimethyl ldib enzo[a,e]cyclooctatetraene-5,11-dicarboxylate, C2]H18O4, M(r) = 334.37 , orthorhombic, Pna2(1), a = 18.364 (4), b = 10.485 (3), c = 9.031 (4) angstrom, V = 1739 (2) angstrom3, Z = 4, D(x) = 1.28 g cm-3, F(000) = 704, mu = 6.8 cm-1, R = 0.054 for 1303 reflexions. (2), 11-methyl 5-( 2-propyl) dibenzo[a,e]cyclooctatetraene-5,11-dicarboxylate, C22H20O4, M(r) = 348.40, monoclinic, P2(1)/n, a = 9.121 (1), b = 20.486 (1), c = 10.028 (1) angstrom, beta = 100.91 (1)-degrees, V = 1839.9 (3) angstr om3, Z = 4, D(x) = 1.26 g CM-3 , F(000) = 736, mu = 6.6 cm-1, R = 0.04 3 for 2688 reflections. COT (2) has a structure consistent with format ion via a normal [2pi + 2pi] intramolecular cycloaddition, while forma tion of (1) provides another example of abnormal behaviour, with react ion via fragmentation of a bis-benzylic 1,4-biradical. The difference in reaction pathways probably results from intramolecular steric effec ts, which are more severe in the dibenzobarrelene from which (1) is de rived (three substituents), in comparison with the formation of (2) (t wo substituents).