Competing pathways in the phototransposition of pyrazoles

Abstract

Cyano-substituted pyrazoles transpose photo-chemically into imidazoles by two concurrent paths: (a) 1,5-interchange, probably by 2,5-bonding to a diazabicyclopentene which isomerises by nitrogen ‘walk’ before rearomatisation, and (b) 2,3-interchange, probably via an intermediate azirine; in sharp contrast, 1,5-dimethyl-3-trifluoromethylpyrazole phototransposes exclusively by the former path.