An Assortment of Highly Unusual Rearrangement in the Photochemistry of Vinylcyclopropanes. Mechanistic and Exploratory Organic Photochemistry

Abstract

The singlet and triplet photochemistry of 2,2-dimethyl-1,1-diPhenyl-3-(2,2-diphenylvinyl)cyclopropane and 3-(2,2-dicyanovinyl)-2,2-dimethyl-1,1-diphenylcyclopropane was investigated. In the (diphenylvinyl)cyclopropane case, direct irradation led via the singlet excited state to three isomers along with Griffin fragmentation products. Interestingly, the three isomers arose from fission to afford the less stable of two alternative allylic diradicals. Two of the isomers arose from unique rearrangements of the allylic diradical wherein the allylic moiety closes 1,3 and C-2 of this group either bonds to a second odd-electron center or abstracts a hydrogen. The third isomer resulted from ring opening and hydrogen transfer. In the case of the (dicyanovinyl)cyclopropane direct irradiation gave rise to a reverse di-π-methane rearrangement involving bicycling of a divalent carbon species along a π-system. Additionally, the hydrogen transfer process observed for the (diphenyvinyl)cydopropane was again encountered. As in the case of the (diphenylvinyl)cyclopropane, there was a preference for products derived fom the less stable of two alternative diradicals. The partition between the reverse di-π-methane rearrangement and the hydrogen transfer process proved solvent dependent with the latter being favored in polar solvents. The triplet of the (dicyanovinyl)cyclopropane again opened to the less stable of two diradicals and this underwent an unprecedented regioselective phenyl migration. Quantum yields were obtained for the reactions of interest. © 1989 American Chemical Society.