Fluorinated 4H-1,3-diazepines by reaction of difluorocarbene with 2H-azirines

5,7-Diaryl-2-fluoro-4H-1,3-diazepines have been synthesized from 3-aryl-substituted 2H-azirines and difluorocarbene. The reaction involves isomerization of azirinium ylide into a 2-aza-1,3-diene which undergoes [4+2]-cycloaddition with the starting azirine followed by ring expansion and dehydrofluorination.