Quinone monooximes

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. 

Nitrosophenols have a pronounced tendency to tautomerize to the corresponding quinone monooximes, the two resonance forms of which are represented below (17 and 18). In view of the above, it would be appropriate to give their account along with carbonyl oximes.13... 

In the case of 2-nitrosophenols, the main issue is the question of whether the species exist as an nitrosophenol (71) or as the quinone-monooxime tautomer (72). Both forms are stabilized by intramolecular hydrogen bonding. A similar tautomerism, but without intramolecular hydrogen bonding, is relevant for 4-nitrosophenols. According to IR spectroscopic data , 2-nitrosophenol is present in the quinonoid form while 4-nitrosophenol exists in equilibrium between both tautomeric forms. l-Nitroso-2-naphthol exists in quinonoid form only, while the existence of both forms has been suggested for 2-nitroso-l-naphthol. For recent reviews, see the publications by Ivanova and Enchev and Krzan and coworkers . 

Phenol, p-nitroso- Quinone monoxime p-Quinone monooxime Quinone oxime. Yellow crystals mp = 144° (dec) = 1,479 >.m = 228, 311 nm (e = 7610, 11000, MeOH) slightly soluble in H2O, CeHe, CS2 solubie in CHCI3, C7H8, C5H5N, very soluble in EtOH, Et20, Me2CO. 

Quinone monooximes resulted from photolyses of equimolar amounts... 

Benzofuroxan may be obtained by oxidation of o-quinone dioxime. The first benzofuroxan derivative, 1,2-naphthofuroxan, was obtained by this method. Suitable oxidizing agents include alkaline ferri-cyanide, bromine water, chlorine, and nitric acid. The method is of practical value only when the o-quinone or its monooxime (o-nitrosophenol) is readily available, and since this is not generally the case, other routes, e.g., the oxidation of o-nitroanilines and the thermal decomposition of o-nitrophenyl azides/ are more commonly used. 

However, in the majority of cases these wells are of very different energies, so that essentially only one of the two states is found. Even in cases where there are measurable concentrations of both tautomers in solution, it may well be that only one occurs in the solid. This is so for the quinone oxime-nitrosophenol system already discussed (34,91), for the monooximes of 1,2- and 1,4-naphthoquinone (91), and for dibenzoylmethane (33) and other 1,3-diketones. Similarly, the furopyridone 42 and the pyranopyridone 43 are in the keto form in the solid, whereas in solution different amounts of enol are also found (92). Compound 44 has recently been shown to exist in the solid only in the 1,4-dihydro form, a (93). 

Benzofuroxans can be prepared by oxidation of o-quinone dioximes with, for example, fer-ricyanide or hypohalite in a process which closely parallels the formation of monocychc furoxans from glyoximes. Its utility is restricted by the availability of the starting materials which are themselves often best made by reduction of the furoxan. However, it is a valuable approach when the parent quinone or its monooxime is accessible by other means. It was, for example, the route originally used for naphtho[l,2-c]furoxan, the first aromatic-fused derivative <1886CB176>, and it is the method of choice for acenaphthofuroxans (11). In other cases oxidation of o-nitroanilines or thermolysis of o-nitroaryl azides are more suitable. 

Benzofuroxans are formed from o-quinone dioximes by oxidation with, for example, alkaline ferricyanide, nitric acid, bromine water and chlorine, While the reaction is usually straightforward and high yielding the method is not generally applicable since the dioximes themselves are not readily obtainable and are often best prepared via reduction of the furoxan (see Section 4.22.3.1.3). However it can be used when the parent quinones or their monooximes (o-nitrosophenols) are available from other sources. Thus it is the method of choice for the acenaphtho- and phenanthro-furoxans, (18) and (94 n = 1), respectively. In other cases alternative routes, such as the oxidation of o-nitroanilines or the thermolysis of o-nitroaryl azides, are more commonly utilized. 

In a continuing study(20), these experiments were applied to five fulvic and humic samples. In addition, the DEPT pulse sequence was used. This technique allows for a further discrimination between singly and doubly protonated nuclei. They found that all five samples react in the same manner. Reaction products of hydroxylamine with esters were observed. Other resonances discovered were attributable to the tautomeric forms of the nitrosophenol and monooxime derivatives of quinones. Thus, this study provides indirect evidence for the presence of quinones in humic material and also suggests the possible presence of cyclic... 

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