Structure of benzofuroxans

The 1-oxide 3-oxide tautomerism [Eq. (3), p. 4] has been discussed earlier (Sections II and III,C) in connection with the problem of the structure of benzofuroxan. A second type of rearrangement involves the furoxan ring and an adjacent substituent group, and arose out of a suggestion of Bailey and Case that 4-nitro-benzofuroxan might be a resonance hybrid of type (57)-(-> (58), rather than 57. NMR ruled out this possibility the three protons present in... 

Proton resonance techniques provided the hrst clear-cut evidence for the unsymmetrical structure of benzofuroxans and for their tautomerism. The unsymmetrical (ABCD) spectrum observed at low temperature (-40 °C) becomes well-resolved and symmetrical (A2B2) above... 

The structure of benzofuroxan (1) has been confirmed by X-ray crystallography. Crystals of 5-chloro- and 5-bromobenzo-furoxan consist of molecules of the 5-halo-tautomer, and not the... 

The reactions of 2-(2-benzylaminophenyl)benzimidazole derivatives 247 with electrophilic 7-chloro-4,6-dinitro-benzofuroxan afforded a new bipolar spiro-rr-complex 248. The structure of the complex was established by X-ray diffraction analysis (Equation 48) <2002CHE1428, 2004RCB2075>. 

Benzo-flw-triazine tri-N-oxides 384 (R = H, Me R1 = mesityl, 2,6-ClC6H3) are formed from the reaction of nitrile oxides R CNO with benzofuroxans 385. The structure of 384 (R = Me, R1 = mesityl) has been confirmed by X-ray crystal structure analysis (431). 

In the benzofuroxan series ab initio calculations at the MP2 level give qualitatively correct predictions of structure, whereas semiempirical AMI, PM3 treatment is less satisfactory showing a preference for the dinitroso tautomer <94jpci2933>. The most likely intermediate in the interconversion of (9a) and (9b) is considered to be anri-l,2-dinitrosobenzene which is predicted to have an energy 50 kJ mol above that of benzofuroxan itself <94JOC643i>. Other calculated properties include diagmagnetic susceptibilities and IR, UV, and PE data. 

Similar chemical shift differences (ca. 0.25 p.p.m.) are observed for the protons at the 4- and 7-positions of benzofuroxans 5-H and 6-H show a smaller separation (ca. 0.15 p.p.m.). In each case the proton conjugated to the N-oxide is at higher held. For benzofurazan the vicinal proton-proton coupling constants (/4i5=/6,7 = 9.3 and Js,6 = 6.4 Hz) indicate substantial bond localization, in accord with the results of X-ray crystal structure analysis. A similar pattern emerges for benzofuroxan (63JCS197). 

Although the possibility of anthranil-A-oxide formation may be recognized, there are certain difficulties in the way of accepting at least one of the foregoing examples. The two generalized systems (181 and 182) differ, in certain conformations, only in their electron distribution, and interconversion would be expected to occur so readily as to preclude isolation of other than the energetically more stable form. Several examples may be cited to bear this out. Thus, 2-phenyl-benzotriazole-1-oxide (181, X = N, Y = NPh) is well-known,320 but no evidence for the independent existence of o-nitrosoazobenzene (182, X = N, Y=NPh) is available. The rapid tautomerism of benzofuroxan (181, X = N, Y = 0) from the 1- to the 3-oxide structure, through... 

With symmetrical 1,3-diketones and benzofuroxans 1 with identical 5,6-substituents, the structure of the reaction product is unequivocal. 

Benzofuroxan reacts with benzo[6]furan-3(2//)-oncs to give a product at an oxidation level lower than the expected di-lV-oxide in a process where benzo[A]furan-3(2//)-one plays the dual role of substrate as well as reductant during the reaction, and redox processes determine the structure of the products,This is a general, one-step synthesis of 2-(2-hydroxyaryl)-quinoxaline 4-oxidcs, c.g. formation of 3. ... 

Aromatic nitro compounds which in ortho position have azido group can readily be transformed into benzofuroxanes (31, 33). An excellent review on furoxanes was written by Kaufman and Picard [206]. Recent works on the structure of benzotrifuroxane were also published on X-ray analysis [207, 208), vibrational and Raman spectra [209 210], nitrogen NMR [211], carbon 13 NMR [212]. Explosive properties were also discussed [213]. 

The literature on benzofuroxans is considerably complicated by uncertainties concerning their structure, which were resolved only in 1960. Thus, Beilstein s Hauptwerk includes them in Volume 7 (as derivatives of o-quinones, depicted as dioxadiazines, and named as... 

Since the earlier reviewers, and Mallory and Wood, and, for the earlier literature, Hammick el al. have summarized the history of the structure problem fairly fully, and since it cannot in any case be treated properly in isolation from that of the monocyclic furoxans, we shall present only a brief outline here. The earliest formulas to be suggested (3 and 4), in connection with 1,2-naphthofuroxan, discovered by Koreff and von Ilinski in 1886, were open to the objections that the first is a peroxide structure, whereas the benzofuroxans... 

The tautomeric structure leads to ambiguities in the nomenclature of compounds in this series. Thus, 5-methyl- and 6-methylbenzo-furoxan denote two different molecules which, because of their interconversion, cannot be isolated separately at normal temperatures. Throughout this review, when we intend to refer to the ambiguous mixture, we shall use the system employing the lowest numbers. The above methyl derivative, for example, will be described as 6-methyl-benzofuroxan regardless of the form adopted in the crystal. When a... 

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