Photochemistry of dienones-VIII : The low temperature photochemistry of (z)-β-ionone and its photo-isomers

Abstract

The isomerizations of (E)-β-ionone 1, and of mixtures of the isomeric pyran 2 and (Z)-β-ionone 3 in CD 3OD as solvent on direct irradiation with λ 254 nm and on triplet photo-sensitization have been studied at temperatures ⩽ - 50°, where the thermal isomerization between 2 and 3 is fully inhibited. The direct irradiation of 1 at -60° leads to 3 and (Z)-retro-γ-ionone 4 as primary products; 3 is subsequently rapidly photo-converted into mainly 2. Evidence is presented that 4 is also a primary photoproduct from both 2 and 3. The quantum yield ratio φ 2→4:φ 3→4 ⋍0.50. On starting with either 1 or mixtures of 2 and 3 the same photo-stationary equilibrium ratio of 1-3 is eventually obtained, viz 1:2:3 ⋍17:72:11. 4 is photostable relative to 1–3. The perdeuterobenzophenone triplet photo-sensitization with λ 366 nm at -50° of 1 leads to 3 as the sole primary product, which isomer on triplet sensitization yeilds both 1 and 2. The triplet sensitized conversion is much faster for (Z)- than (E)-β-ionone. On starting with either 1 or mixtures of 2 and 3, eventually the same photo-stationary state is obtained, viz 1:2:3⋍39:46:15. (Z)-retro-λ-ionone 4 is not formed in the triplet sensitized irradiations of 1,2 and 3 and in the direct irradiation it apparently results from the singlet excited state of the three substrates. The UV spectrum of the (unstable) (Z)-β-ionone 3 has been indirectly determined; its absorption occurs at lower wavelength and is of lower intensity than that of the (E)-isomer 1.