Photochemistry of 1-Phenyl-1,2-dihydronaphthalene in Methanol

Article abstract of DOI:10.1021/jo00007a033

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 ?₄a+ ?₂a-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.On irradiation of 1 with a broad-spectrum lamp, the ratio of the photoproducts exo-2 and 8 is correlated with the ratio of the pe and pa conformers of 1.The novel compound 1-(o-benzylphenyl)allene (6) is also formed under these conditions.Irradiation of 1 at 254 nm yields 8 as the only photoproduct.Photochemical addition of methanol was observed to complete with the photoprocess, leading to exo-2, though at a comparatively low level.A photodecomposition mode of exo-2, presumably involving heterolytic cleavage of the cyclopropane ring, presents the chief source of methoxylated photoproducts at low temperatures in methanol.