Ring cleavage nitrenes

The ring cleavage of 3-aryl-2-substituted-2//-azirines by molybdenum hexacarbonyl has been described earlier in regard to the synthesis of pyrroles, pyrazoles and isoxazoles. In contrast to this behavior, analogous reactions of 2-unsubstituted derivatives lead to the formation of mixtures of 2,5-diarylpyrazines (139) and isomeric 3,6- and 1,6-dihydropyrazine derivatives (140,141) (Scheme 163).47,53 It is possible that the pyrazine products are formed by an intermolecular nitrene mechanism akin to the intramolecular processes described earlier (see Scheme 22 in Section IV,A,1). 

Oxidation of heteroaromatic primary amines can be an alternative route to nitrenes. One example of such a reaction which leads to ring-cleavage is oxidation of the pyrrolopyrimidine 103 by lead(IV) acetate (71JCS(C)3237). 

The photochemistry of 3-aryl-substituted l,4,2-dioxazol-2-in-5-ones (79) can be interpreted in terms of ring cleavage and loss of carbon dioxide with the formation of an acyl nitrene (80) such nitrenes are also formed by the photolysis of acid azides. In dimethyl sulfide, therefore, the 3-phenyl derivative itself (79 Ar=Ph) is converted into the photoproduct (81).05 When the phenyl group is substituted in... 

Heteroaryl nitrenes can be produced by thermolysis and photolysis, however, the reaction pathways are fundamentally different [17]. Under thermolytic conditions, ring cleavage is obtained, however, photolytically... 

The cyclohexylpyrazole (376) and the azirine (377) are formed by irradiation of 3-diazo-4-methyl-5-phenylpyrazolenine (378) in cyclohexane (Scheme 35) (77JA633). The former is the result of carbene insertion into cyclohexane followed by a [1,5] hydrogen shift, whereas the latter arises by ring cleavage of nitrene (379) or by a concerted pathway. 

The synthesis of metal-coordinated 1-azirines and the reactions of azirines induced by metals have opened a new area in the chemistry of this small ring heterocycle. Many of the reactions encountered bear resemblance to previously discussed thermally and photo-chemically induced reactions of 1-azirines. The reaction of a series of diiron enneacarbonyls in benzene results in coupling and insertion to give diimine complexes and ureadiiron complexes as well as pyrroles and ketones (76CC191). A mechanism for the formation of these products which involves initial 1,3-bond cleavage and generation of a nitrene-iron carbonyl complex as an intermediate was proposed. 

The 1,3-diazines have also been prepared by a reaction of enamines and S, S -dimethyl-A (A arylbenzimidoyl)sulfimides 360 in boiling tetraline220,221. The mechanism of the ring formation probably involves a thermal cleavage of the imidoylsulfimides 360 into imidoylnitrenes 361 and dimethyl sulfide. The nitrene can then react with the enamine to give an aziridine intermediate 362 which rearranges to a 3,4-dihydroquinazoline 363. Subsequent elimination of amine yields the quinazoline 364220. 

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