Electronic and Steric Control of α- versus β-Naphthyl Migratory Aptitudes in Enone Photochemistry. Mechanistic and Exploratory Organic Photochemistry

Article abstract of DOI:10.1021/jo00270a022

The photochemistry of 4-α-naphthyl-4-β-naphthylcyclohexenone was investigated.Photolysis afforded endo and exo isomers of 5-α-naphthyl-6-β-naphthylbicyclo<3.1.0>-2-hexanone and 5-β-naphthyl-6-α-naphthylbicyclo<3.1.0>-2-hexanone as well as 3-β-naphthyl-4-α-naphthyl-2-cyclohexenone.The reaction was highly stereoselective with the 6-endo-naphthyl product being preferred both in the 6-α-naphthyl and the 6-β-naphthyl cases.Quantum yields were determined for the direct and sensitized reactions.In direct irradiations a ratio of 48:52 was observed for α- to β-naphthyl migration products while in sensitized photolyses the ratio was 41:59.Despite the differences between direct and sensitized runs, quenching studies revealed that the reacting species were triplets throughout.Excited-state triplet decay and reaction rates were determined.Interestingly, evidence was adduced for differential local excitation of α- and β-naphthyl moieties depending on the mode of energy transfer A stereoselective triplet transfer from enone triplet to axial naphthyl in the direct irradiations and a statistical triplet excitation of axial and equatorial naphthyl groups in sensitization accounts for the results.Conformational information bearing on the photochemistry was obtained from molecular mechanics calculations.