Photochemistry of α-oxo-oximes. Part 3. Photoisomerization of some α-oxo-oxime ethyl ethers
The photoisomerization of four α-oxo-oxime ethyl ethers (1)–(4) has been investigated. The (E)-isomers of (1)–(3) exist in the s-trans conformation. By spectroscopic analysis and by comparison of the triplet sensitized photoisomerization of (1) and (2) it was established that (Z)-(1) has a (possibly slightly twisted) s-cis conformation and (Z)-(2) a (probably non-planar) s-trans conformation. Also with (Z)-(3) there are indications for an s-trans conformation. Oxime ether (4) with the fixed s-cis structure undergoes only (E)–(Z) isomerization. An energy diagram for the various states of (E)- and (Z)-(1) has been constructed. Both upon direct and triplet sensitized irradiation (E)–(Z) isomerization takes place from T1(π–π*). Upon direct irradiation also photodecomposition occurs which proceeds from S1(n–π*). From the quantum yield data it was concluded that all excited molecules of (Z)-(1) reach the common triplet state T, whereas with (E)-(1) only 37% of the excited molecules yield the common triplet state.