A method of cis,trans-isomerisation of non-conjugated olefins without double-bond migration

It is shown that isomerisation of non-conjugated olefins by irradiation in the presence of iodine, or by heating at 200° in the presence of selenium or benzenethiyl radical-producing compounds, is accompanied by extensive migration of the double bond. Irradiation in the presence of iodine gives noticeable migration long before any cis,trans-equilibrium can be established, but with suitably filtered light in the presence of diphenyl sulphide or diphenyl disulphide the thermodynamic cis,trans-equilibrium is obtained without migration or side reactions. Irradiation at > 320 mµ in the presence of acetophenone as a photosensitiser gives cleanly a photostationary state which usually comes fairly close to the thermodynamic equilibrium, but some migration (2–3%) occurs.