Photochemistry of 9-Benzoylanthracene

Article abstract of DOI:10.1021/jo00075a038

Photoexcitation of 9-benzoylanthracene (1) in toluene solution under argon results in head-to-tail dimerization by 4? + 4? cycloaddition to give dibenzoyl-substituted dianthracene in about 60percent yield.The concomitant formation of both anthracene and 9,10-dibenzoylanthracene (ca. 4percent yield) suggests that intermolecular benzoyl group/hydrogen exchange may be an inefficient mode of deactivation the intermediate excimer.Irradiation of crystalline 1 gave the head-to-tail dimer, without byproducts, in a maximal yield of 50percent.It was established by X-ray diffraction that theasymmetric unit of 1 consists of two molecules, 1A and 1B, in which the carbonyl group is twisted out of the plane of the anthracene by 67.4 deg and 86.5 deg, respectively.Investigation of the packing pattern revealed that only parallel overlapping head-to-tail oriented molecules of 1A, characterized by an interplanar spacing of 3.35 Angstroem, can undergo photochemical dimerization by 4? + 4? cycloaddition.The spatial relation of adjacent molecules of 1B is such as to preclude their involvement in the photochemical dimerization.