Presence of Protonic and Lewis Acids

Early studies on the decompositions of aryl azides in the presence of protonic acids have been reviewed. It is difficult to draw conclusions from this early work regarding nitrene involvement, as azides were heated in acids, and it 

Heating aryl azides in a mixture of a carboxylic acid and polyphosphoric acid affords a useful, fairly general method of fusing an oxazole ring to aromatics and heterocycles.The method suffers from the limitation that bicyclic azides in which the azido substituent is a- to the ring junction give 0-diacetyl derivatives of the 1,4-aminohydroxy compound as, for example, the formation of 98 from a-naphthyl azide  

Azides that are a- to a pyridine-like nitrogen usually exist predominantly in the tetrazole form and so difficulties are often encountered in their photolysis. Photolysis of tetrazoles has been found to be facilitated by the use of trifluoroacetic acid as solvent which converts the tetrazole to azide before nitrogen is lost  

Derivatives of the new 6H-indolo[2,3-h]-l,8-naphthyridine ring system 99, which resemble the tumor inhibitor ellipticine, have been prepared by this approach.  

Decomposition of aryl azides in the presence of Lewis acids does not involve arylnitrenes arylnitrenium-Lewis acid complexes are much more likely intermediates. The reaction of phenyl azide with aluminum chloride and aromatic substrates gives fair yields of diarylamines. This reaction is not undergone by phenyl azide alone on thermolysis or photolysis in aromatic solvents (Section II.l.A)  

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