Photochemistry of non-conjugated dienones. Part V. Photolysis of (E)-β-ionone and its isomeric α-pyran
Direct photolysis of (E)-β-ionone at 30° leads to the formation of (Z)-retro-γ-ionone (5) and the α-pyran (2), whereas the triplet sensitized reaction only yields the latter. It is suggested that the formation of (2) proceeds through the triplet state of (E)-β-ionone via(Z)-β-ionone (4) as intermediate and that (Z)-retro-γ-ionone is formed from the excited singlet state. The rate of α-pyran formation and of (E)-β-ionone disappearance is less at –70 than at 30° due to a higher activation free energy for the formation of (Z)- than (E)-β-ionone from the (E)-β-ionone excited triplet state. Photoisomerization of the α-pyran (2) at 30° leads to the formation of (E)-β-ionone and (Z)-retro-γ-ionone. Triplet photosensitization experiments revealed that the former product results from the excited triplet and the latter from the excited singlet state of the α-pyran. (E)-β-Ionone is not a product at –70°, due to a substantially higher activation free energy for the conversion of excited triplet α-pyran into (E)-β-ionone than into ground state α-pyran. In the photoisomerization of both (E)-β-ionone and the α-pyran (2) eventually a tricyclic oxetan (6) is formed as a photo-product of (Z)-retro-γ-ionone. Thermal isomerization of the α-pyran (2) yields (Z)-retro-γ-ionone and (E)-β-ionone as primary and secondary product respectively.