Singlet-Triplet Reactivity of a β,γ-Unsaturated Ketone: Mechanistic Studies in Photochemistry

Article abstract of DOI:10.1021/ja00373a021

The photochemistry of bicyclo<3.2.1>oct-2-en-7-one (1) and four halogen-substituted derivatives is reported.The parent ketone 1 undergoes a 1,3-acyl shift (1,3-AS) with an efficiency of 0.42 on direct irradiation.Introduction of a chlorine atom at the bridgehead (ketone 9) or on the double bond (ketone 5) reduces the 1,3-AS efficiency to 0.11 and 0.14, respectively.Bromine atom substituents (ketones 8 and 4) lower the 1,3-AS efficiencies even farther to 0.088 and 0.062.However, this loss in singlet reactivity does not result in successively more efficient intersystem crossing to the ketone triplet state, for no oxadi-?-methane (ODPM) products, the triplet rearrangement products, are found on direct irradiation for any of the ketones examined.The oxadi-?-methane products are formed upon sensitized irradiations with the triplet sensitizers acetone and acetophenone.As with the direct irradiations, the efficiencies of the sensitized ODPM reactions decrease monotonically with heavy-atom introduction, i.e., ODPM: 0.08 (H),; 0.0062 and 0.03 (Cl); 0.0033 and 0.007 (Br); triplet sensitized 1,3-AS: 0.048 (H); 0.0041 and 0.009 (Cl); 0.0036 and 0.0029 (Br).The disappearance efficiencies reflect the same trend for heavy-atom substitution.It is concluded that introduction of the heavy atom increases the radiationless decay rate for the triplet state.Acetophenone and benzophenone phosphorescence quenching studies indicated that energy fransfer from bromo ketone 4 was slower than the diffusion-controlled rate, indicating excited state complex formation between the sensitizer and 4.Furthermore, a combined acetophenone sensitization-piperylene quenched reaction gave a nonlinear Stern-Volmer plot from which was derived a 6-ns lifetime for the complex.It was concluded that the heavy-atom effect was to increase the importance of the radiationless decay processes for the triplet complex.