Benzofurazanes, reduction

Catalytic reduction of 1,2,4-oxadiazoles also breaks the N—O bond e.g. (264) gives (265). Benzofuroxan can be reduced under various conditions to benzofurazan (266), the dioxime (267) or o-phenylenediamine (268) (69AHC(10)l). Reduction by copper and hydrochloric acid produced o-nitroanilines (Scheme 30) (69AHC(lO)l). 

Benzofurazan, 7-chloro-4-nitro-, 6, 394 as fluorigenic agents, 6, 410, 426 Benzofurazan, 4-chloro-7-sulfo-ammonium salt properties, 6, 426 Benzofurazan, 4-nitro-synthesis, 6, 408 Benzofurazans, 6, 393-426 Beckmann fragmentation, 6, 412 biological activity, 6, 425 bond angles, 6, 396 bond lengths, 6, 396 diazo coupling, 6, 409 dipole moments, 6, 400 electrochemical reduction, 5, 73 electrophilic reactions, 6, 409-410 ESR spectroscopy, 6, 400... 

The reduction of benzofuroxans can lead to a variety of products, depending upon the conditions. Deoxygenation to benzofurazans (40) can be effected either directly, using trialkyl phosphites, -tributyl or triphenyl - phosphine, or indirectly, via o-quinone dioximes (41), using methanol and potassium hydroxide, or hydroxyl-amine and alkali. - - The dioximes may be isolated, but... 

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. 

Some nitro derivatives of benzofurazan have been investigated for their explosive properties. 4-Amino-5,7-dinitrobenzofurazan (56) has been prepared by a number of routes including (1) the thermally induced cyclodehydration of l,3-diamino-2,4,6-trinitrobenzene (55), (2) the nitration of 4-amino-7-nitrobenzofurazan and (3) the reduction of 4-amino-5,7-dinitrobenzofuroxan with triphenylphoshine. The isomeric 5-amino-4,7-dinitrobenzofurazan (57) has been prepared along similar routes. ... 

The UV PE spectra of furazan and benzofurazan have been recorded and interpreted with the aid of ab initio MO calculations. X-ray photoelectron spectroscopic studies for benzotrifuroxan support the NMR assignment of structure as (15) rather than hexanitrosobenzene. The radical anions of benzofurazan and benzofuroxan, generated by alkali metal or electrochemical reduction have been studied by ESR spectroscopy. 

Various nitrosoarenes have been utilized as benzofurazan precursors including o-azido derivatives generated from the o-chloro analogues <66JCS(B)1004>, and 1-amino-2-nitrosoarenes which can be oxidized with ferricyanide or hypochlorite. Treatment of o-nitrosophenols with hydroxylamine also affords the furazan, presumably via an oximation-dehydration pathway involving the tautomeric o-quinone monooxime. Other related approaches involve the reduction of o-dinitroarenes with borohydride, and the thermolysis of o-nitroanilines and o-nitroacetanilides. 

The partial reduction of benzofuroxans, discussed in Section 4.05.5.2.4, represents an effective route to numerous benzofurazans. The conversion may be achieved either directly by deoxygention, for example, with sodium azide in ethylene glycol or acetic acid <75Ci(M)243> or using phosphites, or in two stages via the dioxime with subsequent dehydration as described above. 

The utility of the method depends on the availability of the dioxime precursors. When the parent quinone is readily accessible, direct oximation provides a straightforward approach which has been used for the acenaphtho- and phenanthro-furazans, (22) and (94 n= 0), although in the former case mild conditions (SOCl2, 20 °C) are required for the subsequent cyclodehydration in order to avoid fragmentation of the product (73JOC1054) (see Section 4.22.3.1.1). o-Quinone dioximes can be generated by reduction of benzo-furoxans, but in many such cases direct deoxygenation to the benzofurazan is also possible. 

Cationic rings are reduced with the uptake of one electron (e.g., electrochemically) to give neutral radicals. Examples of radicals which have been detected by ESR are 373 and 374. Such radicals, e.g., those 376 from benzothiazolium ions, can dimerize (to 377) or undergo further reduction (to 378) in practice, a mixture of the dihydro derivative 378 and the dimer 377 is obtained. Benzofurazan is reduced polarographically in a six-electron reaction to o-phenylenediamine. 

Benzofurazans, except for nitrated ones, at reduction on a dropping mercury electrode (in acetonitrile or DMF) on polarogram, have the one-electron wave, which corresponds to reduction of furazan cycle (Scheme 3.42) [1005] ... 

Electrochemical reduction (see also Section V,D,5) of furoxans produces radical-anions derived from the reduction products (e.g., the benzofurazan radical-anion, from benzofuroxan), as shown by the electron spin resonance (ESR) spectra.138 Benzotrifuroxan (19) gives a radical-anion showing a... 

In the mid 1960s, Issidorides and Haddadin addressed some of the limitations in substitution met in earlier reported procedures by developing a synthesis of heteroaromatic A/ oxides that, upon reduction, afford heteroaromatic compounds like phenazine. The Beirut reaction, named after its origin from the capital of Lebanon, involves condensation of benzofurazan oxide (benzo[1,2,5]oxadiazole-1-oxide) and, e.g., enamines, dienes, aldehydes, a,/ -unsaturated ketones, or enolates to form heterocycles... 

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