CHEMISTRY OF NITRENES GENERATED BY THE PHOTOCLEAVAGE OF BOTH AZIDES AND A 5-MEMBERED HETEROCYCLE

Pyrazolo[1,2-a]benzotriazole and some of its derivatives (2a-c) are photochemically cleaved to form 2-(1-pyrazolyl)phenylnitrenes, which are also obtained through a more conventional path by photodecomposition of the pyrazolylphenyl azides (1a-c). Product studies and laser flash photolysis analysis of nitrenes from both sources show that there is competition between fast intramolecular reclosure to heterocycles 2 and rearrangement to dehydroazepines. The latter species are trapped by nucleophiles, or otherwise slowly rearrange back to the nitrenes (conformational factors affect these processes). The (quite unreactive) triplet nitrenes are identified spectroscopically and possibly are in equilibrium with the previous transients.