Abstract

Semi-empirical molecular orbital conformational analyses of benzocyclooctatetraene, 1, dibenzo [ a, c] cyclooctatetraene, 2, dibenzo [ a, e] cyclooctatetraene, 3, tribenzo[ a, c, e] - cyclooctatetraene; 4, tetrabenzo[ a, c, e, g] cyclooctatetraene, 5, octafluorocyclooctatetraene, 6A, and cyclooctatetraene, 6B, have been done at the MNDO and MINDO/3 (excluding 4 and 5) levels of approximation. Comparison of these results with the X-ray structural data for 1 (as determined in the present work) and the other benzannelated cyclooctatetraenes showed that, while MNDO is vastly superior to MINDO/3, both fail to account for the subtle effects of benzannelation on the conformation of the cyclooctatetraene ring. The experimental data indicate that the tub conformation of cyclooctatetraene is only slightly perturbed by the successive fusion of one and two non-adjacent benzene rings, but steric repulsion between adjacent annelated benzene rings, when present, predominates and gives rise to more folded configurations. Compound 1 crystallizes in space group P2 1 n with a = 11.856(2), b = 6.327(1), c = 12.470(3) A ̊ , β = 110.03(2)° , and Z = 4. The structure has been solved by direct methods and refined to R = 0.049 for 1435 observed Mo- K α diffractometer data.

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