O-Nitrosophenols

Condensation of an o-nitrosophenol with an amine hydrochloride in acetic acid (10) Nile Blue [2381-85-3] is formed from the hydrochloride of 1-naphthyl amine. 

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). 

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. 

Very little is known about the parent benzoxazine analogue,1 due to difficulties in the preparation of o-nitrosophenol. Synthetic procedures and practical application of spironaphthooxazines can be found in the patent literature and have been reviewed.72... 

Arylbenzoxazoles have been prepared in moderate yields by allowing aromatic aldehydes to react with copper complexes of o-nitrosophenols (Scheme 108).172 The role of the copper in reactions of this type is unclear but it may be noted that the uncomplexed nitrosophenols are relatively labile.173 Copper complexes of o-nitrosophenols have also been used for the synthesis of benzoxazines (see Section V,D). 

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. 

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. 

An interesting reaction, the Baudisch reaction, involves the formation of nitrosophenols by the action of hydroxylamine hydrochloride and hydrogen peroxide, in the presence of metallic ions or certain Werner complexes, on aromatic compounds. The products are primarily o-nitrosophenol complexes of the metallic ion. Unfortunately, this reaction requires further development before it can be considered a reliable preparative procedure. 

Although the references cited mention the preparation of a large number of new o-nitrosophenols by this method, this work had been done primarily to study the reaction mechanism or the properties of the complexes. Unfortunately, detailed directions for typical preparations, yield data, and properties of the final product are lacking. We believe that a study of this reaction from the preparative standpoint would make valuable contributions to synthetic chemistry. 

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 Table 7, data for the B- and K-bands of o-hydroxybenzaldehyde and o-nitrosophenol, for the R-bands of the latter substance and of o-hydroxythiobenzophenone, and for the corresponding bands of them methyl ethers in hexane (or benzene) solution are shown. 

Again, the appreciable electron-shifts towards the hydrogen accepting 0 and S atoms in o-nitrosophenol and o-hydroxythiobenzo-phenone should account for the strong displacements to shorter wavelengths of the R-bands (—785 and —650 A respectively). 

AH-1,4-Benzoxazines are available by reaction of the copper complexes derived from o-nitrosophenols with alkynic dienophiles (338 — 339). 

Technical grade of p-nitrosophenol may explode on storage.1 o-Nitrosophenol explodes on heating while p-nitrosophenol may ignite spontaneously if nitrates were present in the sodium nitrite used for its preparation. 

Acids. Contact with concentrated acids causes o-nitrosophenol to explode.3 The addition of sulfuric acid to p nilrosophenol containing nitrate impurities results in effervescence or ignition and can be used as a test for the presence of these impurities.4... 

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. 

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. 

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 addition of enamines to l-nitroso-2-naphthol results in A -hydroxy-l,4-oxazines 380 (equation 156) but the reaction could not be extended to o-nitrosophenols. ... 

The two carbon atoms required to form this ring from an o-nitrosophenol may be supplied by a phosphorane (cf. Chapter 35, Section I.l). 

A reactive group in a ligand can be protected by metal chelate formation. For example, in the compound (XCV) one of the amino groups is protected and the terminal amino group is free to react (124, 126). The reactive nitroso group in o-nitrosophenol can be protected by the formation of a stable and water-insoluble copper(II) chelate (159). It is possible that other reactive groups, such as the mercapto group in 8-mercaptoquinoline, could be protected by metal chelation in the course of a synthetic procedure. 

Sodium pentacyuno-a in wine-ferrate o-Nitrosophenols from hydrocarbons Baudisch reaction s. 3, 298 Nas[Fe(CN)zNHa ... 

The oxidation of o-quinone dioximes to benzofuroxans has been known since the early years,15 and it is an efficient route, but not often a practical one, since the most convenient way to prepare an o-quinone dioxime is usually by reduction of the benzofuroxan (see Section V,D). Other methods— from o-quinones or o-nitrosophenols with hydroxylamine—are known these were well established at the time of the earlier reviews, and the reaction needs no further mention here. Ferricyanide oxidation of the trioxime 63 gives the fused furoxan 64.102 Benzofuroxan appeared as a by-product in the reaction of o-benzoquinone dioxime with diselenium dichloride.320... 

The Baudisch reaction295 makes it possible simultaneously to introduce a nitroso and a hydroxyl group in the ortho-position to one another. In this reaction NOH radicals are produced by oxidation of hydroxylamine or reduction of nitrous acid these radicals, in conjunction with an oxidizing agent and in the presence of a copper salt as catalyst attack the aromatic nucleus. Yields are seldom very high, but few o-nitrosophenols are easily prepared in other ways. Cronheim296 has reported the first preparation of fifty mono-and di-substituted n-nitrosophenols by the Baudisch reaction. 

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