Benzofurazan ring

More recent studies on the structure of o- dinitroso derivatives of benzene indicate they may have the benzofurazan ring (I) which was described for the first time by Forster and Fierz [2],... 

The nitro group valence vibration bands of nitrobenzofurazans are in the frequency ranges 1475-1540 (vj and 1270-1350 cm-1 (vs) [762, 763, 769, 773, 1106-1109]. Splitting of the N02 symmetric vibration band of 7-nitrobenzofurazan and 4-amino-7-nitrobenzofurazan found by the authors [1104] may be connected with nonequivalence of the N-O bonds in the nitro group which forms a weak intramolecular hydrogen bond with the proton NH and by that is located in the benzofurazan ring plane in the two molecules. 

The benzofuroxtin [benzofurazan oxide, 3,4-benzo-l,2,6-oxa-diazole-2-oxide, or 2,1,3-benzoxadiazole-l-oxide (1)] ring system has been reviewed briefly on several occasions, notably by Kaufman and Picard,Boyer, and Behr. The mqst recent of these covers the literature until 1959, and since that date there have been many advances in the subject. This, we feel, justifies the field being covered once more, and its separation from the monocyclic 1,2,5-oxadiazole oxides—the furoxans. We consider also other furoxano-fused compounds in this chapter, subject to the limitation that the ring adjacent to the furoxan is aromatic and six-membered. 

Benzofurazan (8), on irradiation in benzene solution, undergoes a complex sequence of ring openings and rearrangements to yield, in situ, the cyanobutadienylcarbonylnitrene 9 which is trapped by the solvent to give ultimately the 1 //-azcpinc 10 as the major product.125... 

Reactivity of Fully Conjugated Rings 5.05.4.1 Furazans and Benzofurazans... 

Decomposition of 1,2,3,4-benzotetrazine 1-oxides 282 involves opening of the tetrazine ring to afford ortho-azidonitroso derivatives, followed by their cyclization with the evolution of the N2 molecule to give benzofurazans 283 (Equation 56) <2002EJO3435>. 

The oxidative ring-closure method is well known. Halo-substituted benzofurazans are obtained by reaction of aqueous sodium hypochlorite with potassium salts of the corresponding 2-nitroanilines (Equation 79) <2001SC2329, 2002BML233, 2003HCA1175, 2003JFC(121)171, 20030PD436>. 

Table 2 NMR chemical shifts ( H, C, N, O) for the 1,2,5-oxadiazole ring atoms of furazan, benzofurazan and their A -oxides.

The fundamental chemical reactivity of the 1,2,5-oxadiazole ring was already well established and documented in 1984 at the time CHEC-I was published and this section is therefore intended to provide a resume and an update unless otherwise stated the content is cited in CHEC-I or in the reviews listed in the Introduction to this chapter. In view of the distinctive influence imparted by the presence of the V-oxide moiety on the reactions of the heterocycle separate sections are devoted to furazans (including benzofurazans) and their furoxan counterparts. 

The furoxan ring is notably resistant to electrophilic attack and reaction normally takes place at the substituents. Thus aryl groups attached to monocyclic furoxans and the homocyclic ring of benzofuroxans are nitrated and halogenated without disruption of the heterocycle. Reaction with acid is also slow protonation is predicted to occur at N-5 <89KGS1261> and benzofuroxans have pKj, values of ca. 8, similar to those of benzofurazans. Monosubstituted furoxans are, as expected, less stable and can be hydrolyzed to the corresponding carboxylic acid. Treatment of the parent furoxan (3) with concentrated sulfuric acid results in rearrangement to (hydroxyimino)acetonitrile oxide (HON=CHC=N —O ) and subsequent dimerization to bis(hydroxyiminomethyl)furoxan... 

Furazane mit einem anellierten Benzol-Ring werden als Benzofurazane I bczcichnet auch Na-men wie 3,4-Benzo-l, 2,5-oxadiazol und 2,1,3-Benzoxadiazol sind (insbesondere in der alteren Literatur) verwendet worden. 

Wird ein Benzol-Ring an ein Furoxan anelliert, erhalt man Benzofurazan-l-oxid (Benzofuro-xan, I). Nach Chemical Abstracts wird dieser Heterocyclus als Benzofurazan-1-oxid bezeichnet zudem findet man die Benennungen 3,4-Benzo-l,2,5-oxadiazol-2-oxid, 2,1,3-Benzoxadiazol-1-oxid und 2,1,3-Benzoxadiazol-N-oxid2. 

Benzofurazan-l-oxide reagieren mit Nitriloxiden am aromatischen Ring unter Bildung von Mono- und Bis-Addukten daneben wurden - in Analogic zur Beirut-Reaktion (s.S. 799) -aiich die gelbgefarbten l,2,4-Benzotriazin-l,2,4-tris-oxide isoliert340 341. 

Benzofurazans and benzofuroxans have been included in this chapter because of their heteroaromatic character as lOx-electron ring systems. However, their interactions with nucleophilic reagents seem to be exclusively confined to attachment of the nucleophilic atom to the carbon atoms of the 6-membered ring away from the fused 5-membered heterocyclic moiety, which exerts a strong electron-withdrawing influence. 

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