Photochemistry of 1-phenyl-1,2-dihydronaphthalene in methanol

The photochemistry of 1-phenyl-1,2-dihydronaphthalene (1) in methanol and hexane has been investigated. The lack of influence of solvent polarity and of the presence of sulfuric acid on the rate of formation of exo-4-phenylbenzobicyclo [3.1.0] hex-2-ene (exo-2) from the primary, ring-opened photoproduct cZt-3a of 1 precludes a sudden-polarized, zwitterionic excited state from being involved in the intramolecular [4+2] photocycloaddition reaction leading to exo-2. The experimental results can be rationalized by assuming that this photoprocess involves a concerted π 4 a+π 2 a-type electrocyclic reaction. cis-Dibenzobicyclo [3.3.0] octa-2,7-diene (8) is proposed to arise by photocyclization of cZc-3b, followed by a thermal rearrangement