Photocyclisations of 1,4-diarylbut-1-en-3-ynes. Part III. Scope and limitations of the reaction

The photoreactivity of a large series of diarylbutenynes has been studied in order to establish the scope of this photocyclisation reaction. The conditions for an optimal yield of photocyclisation products are discussed. On addition of iodine to the butenyne solution, irradiation gives rise to iodo-derivatives of the normal cyclisation products, whereas in the presence of iodine and with benzene as the solvent the corresponding phenyl substitution products are found. The cyclisations are regiospecific and ring closure always takes place between C-4 and an ortho-position of the aryl group at C-1. Only low yields are obtained when the aryl residue at C-1 is a phenyl group because of its low reactivity in radical additions. Overcrowding of aromatic groups in the product formed in this type of photocyclisation has no great effect on its yield. Factors that sometimes interfere with the cyclisation are easy intersystem crossing due to substituents with heteroatoms and the occurrence of side reactions such as dimerisation. In other cases the yields are mostly better than 50%.