Photochemistry of dienones—X : Photochemistry of (e)-β-ionone oxime ethyl ether

Abstract

( E)-β-ionone oximc ethyl ether [( E, E)- 4] upon direct irradiation with λ either 254or 313 nm yields the geometrical isomer ( E, Z)- 4 and ( Z)- retro-γ-ionone oxime ethyl ether ( Z,E)-5 as the sole primary products, illustrating ( E)-( Z) isomerization (φ 313 =0.49) and a 1, 5-hydrogen shift (φ313 =0.15) respectively. From studies with triplet photosensitizers and with ethyl iodide (to enhance the singlet-triplet intersystem crossing) it is concluded that these two products in the direct irradiation result only from the singlet excited state, and that the inter-system crossing quantum yield is relatively low. Upon prolonged irradiation of (E,E)- 4 with λ 313 nm the eventual products are ( Z,E)- 5 and ( Z,Z)- 5, whereas with λ 254 nm they are ( E,E)- 5 and [( Z,E)- 5 and/or ( E,Z)- 5]. Upon triplet photosensitization ( E,E)- 4 undergoes only ( E)-( Z) isomerization, leading to a mixture of all the four geometrical isomers of 4. From the dependence of the geometrical isomer distribution in the photostationary state on the triplet energy of the sensitizer the triplet energies of ( E,E)- 4, ( E, Z)- 4, ( Z, E)- 4, and ( Z, Z)- 4 have been determined to be ca 55, < 55,57, and 57 kcal mol respectively.