Fragmentation and rearrangement in nitrogen heterocyclic chemistry.

The main methods of generating heterocyclic amino-nitrenes are summarised and the subsequent fate of the nitrenes is considered. The photochemistry of three membered ring systems is reviewed. Certain 1-phthalimido- and 1 - (2-methylquinazol- 4-on-3-yl)-aziridines, when irradiated in the presence of an olefin, undergo an exchange reaction which produces a new aziridine incorporating the added olefin. Evidence is presented for a mechanism involving amino- nitrene formation by concerted cleavage of the aziridine C-N bonds. The scope of this reaction is delineated. Vapour phase flash pyrolysis of N-heterocyclic sulphoximides is used to generate amino-nitrenes under conditions such that nitrogen extrusion occurs. For example, phthalimido-nitrene gives benzocyclobutenedione and indazolyl-nitrene gives 1-cyanohexa-1, 3-dien-5-yne. Sulphoximides containing active protons ? to the sulphur atom and involving amide or imide heterocycles undergo a competing fragmentation to the parent amide or imide. Isoindolinyl-aziridines, prepared by reduction of the corresponding phthalimido-aziridines, are fragmented readily, possibly via isoindolinyl-nitrene. The possibility of intramolecular insertion of phthalimido-nitrene to give phthalaz-l,4-dione is upheld since the generation of the latter species by vapour phase flash pyrolysis of its Diels Alder adducts with dienes or its [2+2] adduct with indene is found to give benzocyclobutenedione. The retro Diels Alder reaction is also used to generate other reactive ?-carbonyl azo compounds in the vapour phase such that unimolecular decomposition then occurs. Thermolysis of 1-(quinazol-4-on-3-yl) -2-vinyl-aziridines gives the corresponding 3-(trans-but-2-en-1-ylideneamino) quinazol-4-ones in a novel rearrangement which is in competition with formation of 3-pyrrolines. At higher temperatures a fragmentation involving cleavage of the N-N bond also competes and this occurs to the exclusion of rearrangement in substituted vinyl-aziridines. An analogous fragmentation of trans-2,3-diphenyl-1-phthalimidoaziridine gives 2,3-diphenyl-2H-azirine, which in turn rearranges to 2-phenylindole. Reaction mechanisms are discussed.