Mechanistic and exploratory organic photochemistry. LXXXVIII. Stereochemistry at the methane carbon in the di-.pi.-methane rearrangement (original) (raw)

The preferential migration of p-cyanophenyl relative to phenyl, previously reported in the photochemical transformation of 4-p-cyanophenyl-4-phenylcyclohexenone (3) to give isomeric 5,6-diarylbicyclo[3.1 .O]hexan-2-ones, was studied in mechanistic detail. The selectivity favoring p-cyanophenyl over phenyl migration was shown to arise from greater ease of p-cyanophenyl migration rather than from reluctance of p-cyanophenyl to remain behind. In an intermolecular comparison, the quantum efficiency of p-cyanophenyl migration in 4-p-cyanophenyl-4-phenylcyclohexenone (3) was found to be 0.177 DS. 0.022 per phenyl group migrating in the case of 4,4-diphenylcyclohexenone (1).