Rearrangement of 1,3-Diradicals. Arylcyclopropane Photochemistry

Article abstract of DOI:10.1021/jo00005a018

The photochemistry of a series of aryl-substituted cyclopropanes was investigated as part of our continuing investigations of these systems.The literature held a puzzling discrepancy in which several similar reactants exhibited differing photochemistry.A series of 3-aryl-1,1,2,2-tetramethylcyclopropanes was found to rearrange photochemically to give primarily two types of products, the anticipated 4-aryl-2,3,3-trimethyl-1-butenes and, additionally, 1-aryl-2,3,3-trimethyl-1-butenes.The latter arise from regioselective methyl migration of intermediate singlet 1,3-diradicals.Also, the usual Griffin carbene fragmentation was encountered as a minor pathway.Biphenylyl-, p-cyanophenyl-, and p-anisyl-substituted cyclopropanes were studied.Also, the photochemistry of 3-phenyl-1,1,2,2-tetramethylcyclopropane was reinvestigated and found to conform to the general pattern of reactivity.Throughout, it was the singlet excited states responsible for the observed reactivity, and the triplet counterparts were found to be unreactive.In addition, the photochemistry of 3-biphenylyl-2,2-dimethyl-1,1-diphenylcyclopropane was studied.Again, the triplet was unreactive.The singlet gave rise to 4-biphenylyl-2-methyl-3,3-diphenyl-1-butene exclusively.The differing behavior of the various arylcyclopropanes is discussed from a mechanistic viewpoint.In the case of the 3-aryl-1,1,2,2-tetramethylcyclopropanes, the regioselectivity of the 1,3-diradical intermediate favors migration toward the less delocalized odd-electron center.This selectivity is understood on a quantum mechanical basis.Finally, quantum yields are reported.