Synthesis of 9-cyanophenanthrene-furan and -benzene cyclodimers and their reversion

In contrast to the previous report, the irradiation of 9-cyanophenanthrene (9-CP) and furan through a uranium filter yielded an anti-[2+2] adduct (1) as well as the syn-[2+2] cyclodimer (2) of 9-CP. A mechanistic study indicated that the photocycloaddition of furan occurred in the triplet state of 9-CP. The anti-[2+2] adduct (1) was prepared in the consumed yield of 69% by a photosensitized cycloaddition with Michler's ketone. The anti-[2+2] cyclodimer (5) of 9-CP and benzene was synthesized by the triplet-sensitized photocycloaddition of a substituted cyclohexa-1,3-diene to 9-CP followed by dehydroxylation. The thermoreversion and photoreversion of 1 and 5 prepared were studied. The photodissociation of 1 and 5 was found to be adiabatic with the efficiencies of 0.11 and 0.33.