Nitrenes small rings

The synthesis of metal-eoordinated 1-azirines and the reaetions of azirines indueed by metals have opened a new area in the ehemistry of this small ring heteroeyele. Many of the reaetions eneountered bear resemblanee to previously diseussed thermally and photo-ehemieally indueed reaetions of 1-azirines. The reaetion of a series of diiron enneaearbonyls in benzene results in eoupling and insertion to give diimine eomplexes and ureadiiron eomplexes as well as pyrroles and ketones (76CC191). A meehanism for the formation of these produets whieh involves initial 1,3-bond eleavage and generation of a nitrene-iron earbonyl eomplex as an intermediate was proposed. 

Photolysis (245 nm) of tetrazolo[l,5-/ ]pyridazine 21 in an argon matrix at 16 K leads to nitrogen extmsion and ring opening to form (2Z)-4-diazobut-2-enenitrile 22. Further photolysis produces predominantly cycloprop-2-ene-1-carbonitrile 23 and small amounts of l,3,7-triazacyclohepta-l,2,4,6-tetraene 24. No formation of triplet pyrid-azine-3-nitrene 25 is observed (Scheme 5) <2003PCP1051>. 

The mechanism of insertion of 2-alkylphenylnitrenes into a 1,5-related CH bond was studied by three methods 80 determination of isotope effects, stereochemistry, and radical clock. During the formation of indolines, a kn/kD of 12.6-14.7 was observed coupled with complete loss of stereochemical integrity at the CH carbon. When the CH insertion carbon bore a cyclopropane group, ring-opening products were observed. These observations suggest a mainly radical H-atom abstraction mechanism. The sensitivity of the isotope effects to solvent was taken to imply a small concerted nitrene insertion contribution. 

Alkyl-2-phenylquinazolines 9 are readily available by reaction of 5,S -dimethyl-A-(A-aryl-benzimidoyl)sulfimides 7 with enamines 8 in refluxing Tetralin (1,2,3,4-tetrahydronaphthalene). The mechanism of quinazoline ring formation probably involves a thermal cleavage of the imidoylsulfimides into imidoyl nitrenes, nitrene addition to the enamine double bond and subsequent rearrangement of the aziridine intermediate thus formed to the final product 9. Small amounts (10-15%) of 4-unsubstituted quinazolines 10 are obtained as byproducts. The formation of these byproducts involves a known intramolecular rearrangement of the benz-imidoylsulfimides employed. ... 

A new type of rearrangement is described by Rees and Yelland186,187 the 1-aminopyridone 60 when oxidized with lead tetraacetate gives the pyridazine 61 as the main product and a small amount of 62 and 63. It is supposed that the reaction proceeds via a nitrene, followed by ring expansion, valence isomerization, and extrusion of carbon monoxide. 

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