Formation of 5,6- and 7,8-Dihydrohexahelicene: Mechanistic Details of the Rearrangement of the Primary Photocyclization Product of 2-Styrylbenzophenanthrene in the Presence of a Base

Article abstract of DOI:10.1021/jo00276a034

Irradiation of 2-styrylbenzophenanthrene (1) in alkylamines or basic alcoholic solution results in the formation of a mixture of two dihydrohexahelicenes (5,6- and 7,8-dihydrohexahelicene, 5 and 6).The ratio of 5 and 6 depends on the kind of solvent.In alkylamine 6 is the favored dihydrohexahelicene.In basic alcoholic solution 5 is the preferred product.Deuteration of the solvent causes a change in the ratio of 5 and 6 in favor of 5.The reaction starts with the deprotonation of the primary formed, unstable 16d,16e-dihydrohexahelicene (2), followed by a protonation step.The site of this protonation determines the ratio of 5 and 6 and depends upon the acidity of the protonating agent, an alkylammonium cation or solvent molecule, and the electron densities at the various possible sites for protonation in the intermediate.Irradiation of 1 in several chiral alkylamines yielded optically enriched 6.