Photochemical Reinvestigation of a 5-Phenyl-2-pyrazoline and Its Product Azocyclopropanes

Article abstract of DOI:10.1021/jo00375a008

Irradiation of 1-methyl-5-phenyl-2-pyrazoline (1) affords the previously reported azocyclopropanes 2t and 3t; however, the major product of this reaction is β-(methylamino)-β-phenylpropionitrile (4).Although the azo linkage in 2t and 3t has the trans configuration, irradiation of either isomer causes azo trans -> cis isomerization, slower interconversion of the ring isomers, reversion to pyrazoline 1, and cleavage to styrene.Thermolysis of 2t proceeds twice as fast as that of 3t to interconvert the ring isomers and ultimately to afford exclusively 1.The rapid thermolysis rate of 2t and 3t relative to a model phenylvinylcyclopropane is interpreted in terms of an unusually high facile formation of the α-azo (hydrazonyl) radical.