Photocyclisation and photoisomerisation of 1,3,4- and 1,4,5-triphenylpyrazole
Abstract
Irradiation of 1,4,5-triphenylpyrazole in the presence of iodine gives 1-phenyl-1H-phenanthro[9,10-c]pyrazole, identical with a specimen prepared by an independent route. 1,3,4-Triphenylpyrazole does not react under these conditions; this is thought to be because the intermediate non-aromatic polyene would be dipolar. Irradiation of 1,4,5-triphenylpyrazole in the presence of benzophenone gives 3-anilino-2,3-diphenylacrylonitrile, which is itself inert towards irradiation. 3-Aminoacrylonitriles are postulated as intermediates in some rearrangements of pyrazoles to imidazoles.