5 -Substituted 4-nitrobenzofuroxans

Nitrobenzofuroxan (355) undergoes a rearrangement (recognizable as an isomerization in unsymmetrically substituted derivatives) which is an example of this general rearrangement (Scheme 45) (64AG(E)693) see Table 10. 

Several examples of nucleophilic displacement of nitro-activated leaving groups have been recorded. 5,6-Dinitrobenzofuroxan with aniline and p-bromoandine gives the corresponding substitution product (50). Azide ion displaces chloride from both 5-chloro-4-nitro- and 4-chloro-7-nitrobenzofuroxan (51 and 52) the product from the former loses nitrogen spontaneously to give furoxanobenzo-furoxan (benzobisfuroxan, 17), which is also formed, although in poor... 

Reduction of benzofuroxans is usually the most convenient route to benzofurazans and o-quinone dioximes (see Section VI, C). Reduction of 4-nitrobenzofuroxan would seem to be a method of synthesis of 1,2,3-triaminobenzene, while the haloalkoxy-substitution reaction (Section VTT,B) and the rearrangements of Section VIII provide compounds accessible only with difficulty by other methods. Apart from these reactions, the benzofuroxans appear to be of limited synthetic utility. 

One of the nitro groups in furazan (78) can be substituted by chloride and azide, and displacement of sulfonyl groups has also been reported. Nucleophilic reactions in the homocyclic ring of benzofuroxans may be accompanied by deoxygenation of the A-oxide 4-nitrobenzofuroxan with dialkyl-amines affords 4-dialkylamino-7-nitrobenzofurazans as the major product. Substitution reactions... 

The principal methods for forming the heterocyclic ring of benzofuroxans involve oxidation of o-quinone dioximes, thermolysis of o-nitroaryl azides, and oxidation of o-nitroanilines (Scheme 25). Ring chain tautomerism (Section 4.05.5.2.1) for the A -oxides of asymmetrically substituted benzofuroxans is more facile than for monocyclic analogues and mixtures of isomers may result. Benzofuroxans are also formed by Boulton-Katritzky rearrangement of 7-nitro-2,l-benzisoxazoles and 4(7)-nitrobenzofuroxans (Section 4.05.5.2.5). 

Substitution in the homocyclic ring may be accompanied by deoxygenation of the furoxan. 4-Nitrobenzofuroxan with dialkylamines gives 4-dialkylamino-7-nitrobenzofurazan 5-trifluoromethyl and 5-ethoxy carbonyl compounds are reported to react similarly at C(6). 

In recent years, particular attention focuses on reactivity of nitrobenzofuroxans and nitrobenzofurazans [531], The latest are represented as a class of neutral 10-ii-elec-tron-deficient heteroaromatic substrates that exhibit an extremely high electrophilic character in many covalent nucleophilic addition and substitution processes. 

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