Insertion reactions, nitrenes intersystem crossing

It is clear that the ortho and para substituted diarylamines 16 and 17 are derived from capture of the singlet nitrene and the product of benzylic CH insertion 15 can be formed from either the triplet or singlet state of the nitrene. Decafluoroazobenzene is derived from a dimerization reaction of the triplet nitrene and pentafluoroaniline is formed by hydrogen atom abstraction reactions of triplet pentafluorophenyl nitrene, and possibly by some photoreduction of an excited state of the azide. It is clear from this data that the singlet and triplet nitrene are not rapidly interconverting, and there is no evidence for uphill intersystem crossing from the triplet to the singlet nitrene. 

Scheme 3 summarizes reaction pathways of 9-azidoacridine photolysis in different reaction conditions. The first photochemical step is the azide photodissoeiation in a singlet excited state with the formation of the singlet nitrene. Among the variety of the subsequent reaction pathways of aromatic singlet nitrenes, the following two main reactions can be mentioned [8] intramolecular insertion at the ortho position to form aziridine and then dehydroazepine and intersystem crossing to a triplet state, whieh is the ground state for nitrene. 

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