Benzofuroxans structure

Green and Rowe [3] suggested a benzofuroxane structure (II) for o- dinitroso-benzene and its derivatives. This view was further developed by Hammick,... 

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... 

Further benzofuroxan spectra are reported by Gaughran, Picard, and Kaufman, who compare them with benzofurazans, by Boyer et al., who find similarities with furoxans and nitroso compounds, and by others. Hexanitrosobenzene was shown by IR and Raman spectroscopy to exist in the benzotrifuroxan structure. ... 

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... 

Intermediates involved in the tautomerization of furoxans gave rise to speculations about their structure. Systematic calculations on possible intermediates being involved in the reaction of furoxan [92JCC177] and benzofuroxan 79 showed that dinitrosoethylene and dinitrosobenzene 81 are the most likely ones (Scheme 50) [94JOC6431, 94JPC12933]. 

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). 

Benzofuroxans (2), benzodifuroxan (37), and benzotrifuroxan (38) are systems that have long been known [6-8,10,11]. Early studies showed that 37 is a potent in vivo and in vitro vasodilator, as are its furazanobenzofuroxan and thiazolobenzofuroxan analogues [26]. Structural comparison between 37 and the extended glyceryl trinitrate (GTN) conformation has shown a close similarity in the relative position between 1 and 3 positioned NO2 moieties in GTN and the 0-N+-0 substructures in the furoxan... 

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. 

Carbon-13 NMR spectroscopy has also been utilized to probe the structure, isomers, and conformational aspects of 1,2,5-oxadiazole chemistry. The dc values for monocyclic furazans vary from 140 to 160 ppm depending on the substituent, and C- H couplings of ca. 200 Hz are observed for the parent furazan and monosubstituted derivatives (92KGS1 lOl >. For furoxans the most noteworthy feature of their carbon spectra is the large displacement of the C(3) resonance to lower chemical shift. While the C(4) peak remains in the range 140-160 ppm that for C(3) is shifted to 100-125 ppm. At low temperatures benzofuroxan shows two peaks at 113.7 (C(3)) and 152.2 (C(4)) which coalesce on warming. The parent furoxan has C- H couplings of 211 Hz for C(3) and 202 Hz for... 

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. 

Similar uncertainty surrounded the benzofuroxan series. Indeed as late as 1961 the mesomeric system (10) + (12) was being considered and at that time frequent reference was made to o- dinitrosobenzene . Thereafter incontrovertible evidence for a rapid interconversion of isomeric JV-oxides was provided by NMR spectroscopy (see Section 4.22.2.3) and the benzofuroxan formula (4) was finally confirmed by X-ray crystallography. The history of the problem is contained in previous reviews and a summary of the development of the major theories through to final proof of the Wieland-Werner structure is presented in the recent account by Gasco and Boulton (8lAHC(29)25i). 

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). 

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... 

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