2-Pyridylnitrene and 3-pyridazylcarbene and their relationship via ring-expansion, ring-opening, ring-contraction, and fragmentation

Photolysis of triazolo[1,5-b]pyridazine isolated in Ar matrix has generated diazomethylpyridazines and diazopentenynes as detected by IR spectroscopy. Three possible mechanisms of ring-contraction in arylnitrenes are identified by ring-opening to dienylnitrenes, concerted ring-contraction and by spiroazirenes.