4-Nitrosophenol, tautomerism

Nitroso dyes are metal-complex derivatives of o-nitrosophenols or naphthols. Tautomerism is possible in the metal-free precursor between the nitrosohydroxy tautomer (76) and the quinoneoxime tautomer (77). 

Readily available copper(II) complexes derived from o-nitrosophenols react with dimethyl acetylenedicarbonxylate to give the 1,4-benzoxazine products that would be expected from formal [4 + 2] cycloaddition across the diheterodiene system (Scheme 168).239 No such reaction is observed in blank experiments with uncomplexed tautomeric nitrosophenols hence the copper may cause sufficient electronic perturbation within the heterodiene complex to allow reaction to occur. 

Tautomerism exists in the case of o- and p-nitrosophenols and naphthols which exist mainly as the quinone oximes, and which also give high field shifts. The values for the oxime groups in the 1,2-naphthoquinones, for example, were 229 and 265 ppm for the 1-oxime and the 2-oxime respectively. 

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. 

Particularly in the older literature, the structure of the nitrosophenols was not clearly established. Thus, for example, the dinitroso derivative of resorcinol is presumed to be the tautomeric dioxime [32] shown in Eq. (20). 

Earlier attempted interpretations of the hydrogen bond with the help of resonance or delocalization forces, e.g. in the case of the dimeric carboxylic acids (Illa-d) or, in particular, of substances containing intramolecular hydrogen bonds such as o-nitrosophenol (IVa-d), were shown to be untenable by unambiguous experimental evidence. Thus, in the dimer carboxylic acids the proton is not in the centre of the 0. O distance and the C=0 and C—OH distances are not identical [7], and derivatives of ortho-hydroxyazobenzene and of ortho-nitrosophenol have been shown to exist as solvent-dependent tautomeric equilibria (II), in spite of the presence of the internal hydrogen bond in both tautomers [3, 4]. 

In the course of treating lignin with nitric acid, especially with dilute acid, hydrocyanic acid is evolved. This is. explained by the formation of nitrosophenols that undergo a tautomeric conversion into quinone oximes, the latter then being oxidized and hydrolysed. During these reactions HCN is split off in a manner resembling that reported by Seyewetz [64] (compare Vol. I). 

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... 

This reaction is frequently applied to the preparation of dialkylamines. The dimethylamine evolved in the above reaction may be absorbed by leading through hydrochloric acid from the latter solution the hydrochloride is obtained by evaporation. p-Nitrosophenol, it is to be noted, is tautomeric, in some reactions behaving as quinone monoxime. 

Heating o-nitrosophenols with hydroxylamine is reported to give furazans, naphtho[l,2-c]furazan (95) being formed from both l-nitroso-2-naphthol and 2-nitroso-l-naphthol, presumably by oximation of the tautomeric o-naphthoquinone monooximes and subsequent dehydration. Compound (95) has also been prepared by oxidation, using alkaline ferri-cyanide or hypochlorite, of l-amino-2-nitroso- and 2-amino-l-nitroso-naphthalene. This latter approach is suitable for heterocyclic fused furazans thus 4,6-diamino-5-nitrosopyrimidine is converted into the furazanopyrimidine (96) by oxidation with lead tetraacetate (71JOC3211). In a similar reaction alkaline hypochlorite oxidizes o-nitrosoacetaniiide to benzofurazan in quantitative yield. 

Cycloadditions have also been used to form benzoxazines . The reactants are typically an alkene such as 350 and a l-nitroso-2-naphthol 349 (the tautomeric form of a 1,2-benzoquinone monoxime) (Scheme 189) <1981TL3945> microwave heating was found to improve yields of this process <2004SC315>. A similar, but metal-catalyzed cycloaddition of a 2-nitrosophenolate 352 with DMAD gives two isomeric products 353 and 354 (Scheme 190) <1995JCM454>. 

Of the methods for determining lignin in solution based on a specific chemical reaction, that involving nitrosation, the so-called Pearl-Benson method, has found the widest application. In this procedure, reaction of the phenolic units in lignin with acidified sodium nitrite leads to the formation of a nitrosophenol which, upon addition of alkali, is tautomerized to an intensely colored quinone mono-oxime. The absorbance of the latter structure is measured at 430 nm and related to lignin concentration by calibration with a standard lignin. The procedure described below is essentially that developed by Barnes et al. (1963), who modified the original Pearl-Benson method (Pearl and Benson 1940) to improve its sensitivity. 

In the case of 2-nitrosophenols such as 2,4-dialkyl-6-nitrosophenols, the tautomeric equilibrium is shifted towards the 1,2-benzoquinone monoxime form with increasing polarity of the solvent [76a]. 

Nitration takes place easily and again there is a tendency for polysubstitution. Phenols are readily nitrosated by nitrous acid. The resultant nitrosophenols are tautomeric with the corresponding quinone monoximes (Scheme 4.16). 

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... 

The o- and p-nitrosophenols enjoy the possibility of resonance stabilization by jt-electron donation from the phenolic hydroxyl group to the nitroso group, and the o-isomer could also be stabilized by an intramolecular hydrogen bond. These species are also tautomeric with benzoquinone oximes. All of this could confound interpretation of enthalpy of formation values if only they were available—there are seemingly no measured enthalpy of formation values for o-nitrosophenol. The value for p-nitrosophenol will be discussed later in Section VI because of tautomeric ambiguity. The m-species lacks the stabilizing conjugate NO/OH interaction, and so the monomer-dimer equilibrium as found in other nitroso compounds becomes problematic—should the measurement of enthalpy of combustion be available. 

The o- and p-nitrosophenols are tautomeric with o- and p-benzoquinone oxime, respectively. Some of the nitrosonaphthols are tautomers of naphthoquinone oximes. A recent publication summarizes the current knowledge of tautomeric equilibria in solution, the composition of the solid phase and the results of theoretical smdies . While tautomeric composition in solution is very much dependent on compound structure and solvent polarity, various nitrosophenols, 2-nitrosonaphthol, l-nitroso-2-naphthol and 4-nitrosonaphthol exist exclusively as quinone oximes in the solid state. 

Tautomeric equilibria have been studied in nitrosophenols. An early study of 4-nitroso-phenol showed an intermolecular tautomerism catalysed by slight traces of water. In dry dioxane both the Af-oxide and the oxime form could be observed . ... 

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 . 

Primary and secondary nitroso compounds usually stabilize themselves, even under the conditions of their preparation, by passing irreversibly into their isomers, the oximes. In tertiary aliphatic nitroso compounds this transformation is accompanied by fission of the neighboring carbon-carbon bond by hydrolysis or decarboxylation. Aromatic nitroso compounds are more stable, and when oxime formation does occur this is usually reversible within the framework of a true tautomeric equilibrium for instance, the tautomeric pair -nitrosophenol/p-benzoquinone monooxime is obtained both by nitrosation of phenol and by oximation of />-benzoquinone. 

Certain nitroso phenols and polyhydroxy compounds are exceptions where the exchange takes place under relatively mild conditions and sometimes without the need for a catalyst. For example,/7-nitrosophenol and ammonium salts, when heated on a steam-bath for 0.5 hour, give 50% of /7-nitrosoaniline, 1049.cf.1050 and l-nitroso-2-naphthol and aqueous methylamine solution at 35° give 90% of iV,iV-dimethyl-1 -nitroso-2-naphthylamine.1051 The easier reaction s due to the tautomerism of these phenols with the ketonic isomers, e.g.9 for nitrosophenol ... 

Bruciquinone forms a reddish-yellow monosemicarbazone and a yellowTsh-green monoxime (both isolated as perchlorates (159)), while at higher temperatures (80°) a reddish-violet hydrate of the monoxime is formed (probable structure XCIV). The formation of a monoxime, even ivith an excess of hydroxylamine, suggests that this derivative may exist as the tautomeric nitrosophenol. Reduction of either the monoxime or the monosemicarbazone by tin and hydrochloric acid yields aminohydroxy-strychnine (XCV), while the dihydro derivative of XCV results from the uptake of six atoms of hydrogen over PtOa (159). 

The C-N frequency couples with vibrations of the rest of the molecule, generally resulting in a band about 1100 and at 860-750 cm" Nitrosophenols are in tautomeric equilibrium with the quinone oximes. Nitrosoanilines are resonance hybrids. 

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