PHOTOREACTIONS OF SOME MONOARYL-BENZOQUINONES AND DIARYL-P-BENZOQUINONES IN SOLUTION - DEPENDENCE OF DIMERIZATIONS, CYCLIZATIONS AND REARRANGEMENTS ON THE SUBSTITUENTS AND SOLVENTS

Phenyl- and 2,6-diphenyl-p-benzoquinone give the respective cyclobutan e-type dimers on irradiation in non-polar solvents, such as benzene, ( cycle) alkanes and Freon-113. In polar solvents, such as alcohols, ace tic acid and acetonitrile, however, intramolecular photocyclization to the respective 2-hydroxydibenzofurans occurs. Photodimerization is no t observed in these polar solvents. Moreover, the cyclobutane-type dim ers dissolved in methanol are quantitatively converted into the respec tive 2-hydroxydibenzofurans on irradiation. Substituents (Cl, Br, CH3) in the aromatic ring of phenyl-p-benzoquinone and (Cl) in the quinone ring of 2,6-diphenyl-p-benzoquinone considerably affect both the cour se and efficiency of the photoreactions. 2-tert-Butyl-6-phenyl-p-benzo quinone gives a 2-hydroxydibenzofuran, and the usual photorearrangemen ts involving the tert-butyl group. An alkoxy group in the quinone ring of 2,6-diphenyl-p-benzoquinone leads to yet another type of photoreac tion: cyclization to the 6H-dibenzo[b.d] pyran system. The reaction me chanisms are discussed. (C) 1997 Elsevier Science S.A.