DI-PI-METHANE REARRANGEMENT OF 1-METHYLENE-4,4-DIPHENYL-2-CYCLOHEXENE AND RELATED PHOTOCHEMICAL PROCESSES . MECHANISTIC AND EXPLORATORY ORGANIC PHOTOCHEMISTRY .45.

A photochemical transformation was uncovered in which a 1,2 shift of a phenyl group to the terminal carbon of a 1,3-diene occurs. Thus irradiation of 1-methylene-4,4-diphenyl-2-cyclohexene (2) in cyclohexane was found to afford the stereoisomeric 2-methylene-5,6-diphenylbicyclo[3.1.0]hexanes. The reaction provides a new variation of the di-π-methane rearrangement and is formally analogous to the n-π* rearrangement of 4,4-diaryl-cyclohexenones. However, in contrast to the enone reaction, the present rearrangement was shown to proceed via a singlet excited state. In the present instance, the triplet excited state of the 4,4-diphenyldiene (2) may be generated but is unreactive, in sharp contrast with the di-π-methane rearrangement of benzobarrelene which reacts via a triplet. The present reaction proved to be highly stereoselective with preferential formation of the trans stereoisomer of 2-methylene-5,6-diphenylbicyclo[3.1.0]hexane (ca. 125:1). Also, the reaction efficiency was found to be enhanced in solvent methanol compared to cyclohexane. The bicyclic products themselves were found to be photochemically interconvertible and also to revert to the 4,4-diphenyldiene. However, the cis-bicyclic compound returned to 4,4-diphenyldiene only inefficiently and, in addition, afforded 1-methylene-3,4-diphenyl-2-cyclohexene. Quantum yields were determined for the reactions studied. Molecular details of the reactions, including stereochemistry, are discussed from a mechanistic viewpoint. © 1969, American Chemical Society. All rights reserved.