Photochemical isomerizations of 2‐vinyl‐2‐phenylcyclohexanone; ketene, alkenal and 1,3‐acyl shift product formation from the excited singlet state

The photochemistry of 2‐vinyl‐2‐phenylcyclopentanone (1) and 2‐vinyl‐2‐phenylcyclohexanone (2) has been studied under conditions of both direct (λ 300 nm) and triplet‐sensitized irradiation. Whereas compound 1 is photostable, compound 2 yields three photochemical isomerization products upon direct irradiation, viz. the ring‐expanded phenylcyclooctenone 8 (ϕ = 0.43), (E)‐and (Z)‐phenyloctadienals 9 (ϕ = 0.06 for each geometric isomer) and a ketene, which, in 2‐propanol, affords the corresponding ester 10 (ϕ = 0.08). In these rearrangements, the intermediacy of a common singlet diradical, formed by initial α‐cleavage, is suggested. This diradical yields the alkenals and ketene by intramolecular 1,5‐H shifts and cyclizes to the 1,3‐AS 8 and the starting material.