Substituted Benzofuroxans Reactions of Substituents

A table listing the benzofuroxans Imown to the authors, from the literature or otherwise, with their melting points, appears in Section X, at the end of this chapter. The present section presents a brief summary of the presently available types of substituent groups on the benzene nucleus, and of the reactions they undergo. 

A number of alkyl and dialkyl (mainly methyl) benzofuroxans are known. The 4-methyl compound resisted attempts to brominate the methyl group and to oxidize it to the carboxylic acid.  

4-Phenylbenzofuroxan (22) is formed on pyrolysis or photolysis of 2-azido-3-nitrobiphenyl the alternative mode of cyclization—to 4-nitroearbazole—was not observed.  

Hetero-substituted 4,7-diarj lbenzofuroxans (Section V,C) are known. The only representative 5-arylbenzofuroxan in Table I is 5,5 -bisbenzofuroxanyl (23), prepared from 4,4 -diamino-3,3 -dinitro-biphenyl by oxidation with hypochlorite. 

5-benzo-fused compound, 1,2-naphthofuroxan (24) was the first compound in the benzofuroxan series to be prepared the 

There are several instances of heterocyclic ring annellation, including 4,5-furoxano and 4,5-furazano ring fusion (17 and 18), 

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The most reliable method of preparing benzofuroxans is by decomposition of o-nitrophenyl azides. Decomposition can be achieved by irradiation, or more usually by pyrolysis temperatures between 100° and 1.50° are commonly used. Refluxing in glacial acetic acid is the recommended procedure for 4- or 5-sub-stituted 2-nitrophenyl azides, but with 3- or 6-substituted compounds higher boiling solvents are usually necessary. Quantitative studies on the reaction rate have been made, and a cyclic transition state invoked, an argument which has been used to account for the greater difficulty of decomposition of the 6-substituted 2-nitrophenyl azides. Substituent effects on the reaction rate have also been correlated with Hammett a constants, ... 

The reactions of the homocyclic ring of benzofuroxans, which are described in detail in Section 4.22.3.3, provide access to numerous derivatives. Nucleophilic displacement of halides is facile when activating nitro groups are present, allowing alkoxy, aryloxy, thio and amino groups to be introduced. Electrophilic substitutions, e.g. nitration, are also valuable. Further transformations may also be performed on benzo-ring substituents. Such modifications include acetoxy to hydroxy acetamido to amino and acyl halides to esters and amides. Some reactions of the substituents of monocyclic furoxans allow hetero-substituted analogues of benzofuroxans to be prepared. For example, pyridazinofuroxans are formed by condensation of diacylfuroxans with hydrazine. 

In the reaction of benzofuroxan with unsymmetrical 1,3-diketoncs, or of unsymmetrically substituted benzofuroxan,however, two different isomeric products can be formed. Stehc and electronic effects influence the regioselectivity of this reaction and determine which group forms the acyl and which the quinoxaline substituent in the product. ... 

Quinoxaline di-iV-oxides without substituents in the pyrazine ring have been prepared by reaction of benzofuroxans with synthons of the type RCH==CHX, where R = COMe or 4-substituted benzoyl and X = ONa, or where R = H and X = NR2 or OAc. - Two cases are known of mono-N-oxide rather than di-N-oxide formation using benzofuroxan. Thus reaction with benzofuran-3(2H)ones gives 3-(o-hydroxyaryl)quinoxaline... 

Substituents on the benzene rings exert their usual influence on the orientation and ease of electrophilic substitution reactions. For example, further nitration (H2S04/S03/HN03) of 4-nitro-benzofuroxan (496) gives the 4,6-dinitro derivative, the first nitro group direct meta as expected. 

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