Nitroso-oxime tautomerism, also

The nitro form is much more stable than the aci form in sharp contrast to the parallel case of nitroso-oxime tautomerism, undoubtedly because the nitro form has resonance not found in the nitroso case. Aci forms of nitro compounds are also called nitronic acids and azinic acids. 

Upon reaction with nitrous acid, indole produces a complex mixture of products. In addition to 3-oximino-3H -indole (16), which is the stable tautomeric form of 3-nitrosoindole (17), dimeric products of the type (18) and (19) are also formed. In contrast, (16) appears to be the sole product of the nitrosation of indole with amyl nitrite and sodium ethoxide (72HC(25-2)537). Studies of the nitrosation of pyrrole are somewhat indecisive. The mononitrosopyrrole, obtained from the reaction of pyrrole with nitrous acid, has not been fully characterized, but there is some evidence that nitrosation of pyrrole with amyl nitrite and sodium ethoxide leads to the sodium salt of the 3-nitroso derivative. However, upon the addition of acid, the product rearranges to give the oxime of 3-formylisoxazole (20) (B-77MI30502). 

The tautomerism of other pyrazoles has also been studied. The examination of the tautomerism, protonation, and E/Z isomerism of a 4-nitroso-5-aminopyrazole and its salts shows that this compound exists as a mixture of two amino/nitroso isomers 50a and 50b the imino/oxime tautomer 50c was not present [66], The salt is a mixture of isomers 51a and 51b resulting from the protonation on the oxygen of the nitroso tautomers. The functional tautomerism of l,5,6,7-tetrahydro-4//-inda-zol-4-ones (a pyrazole derivative) has been studied theoretically and experimentally [67], In the case of imidazoles 2-substituted by Se, there is experimental evidence that the selenone is more stable than the selenol tautomer [68],... 

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. 

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. 

Studies on the tautomeric equilibria of 3,5-diamino-4-hydroxyimino-l,2,6-thiadiazine 1,1-dioxides (79 R = H) and 5-amino-4-hydroxyimino-1,2,6-thiadiazin-3(2//)-one 1,1-dioxides (80) are also available <88Mi 6i6-oi>. On the basis of H, C, and N NMR studies it is concluded that in all instances, the oxime tautomers are preferred over the 4-nitroso- forms, and that the 3-ones exist as the lactams rather than the 3-hydroxy tautomers. In addition, for the diamino-oxime (79 R = H) and its 0-methyl ether (79 R = Me), the amino rather than the imino tautomer is dominant. In contrast, with the 5-amino-1,2,6-thiadiazinone 1,1-dioxides (80 R = H or PhCHj) there appear to be unquantified amounts of the imino tautomers (81) present. 

Several other types of tautomerization are shown in Figure 3.7. In each case, the equilibrium lies well to the right. For keto-phenol tautomers, the favored phenol form is aromatic whereas the keto form is not. Enamines that have at least one hydrogen on the nitrogen are generally not stable and revert to the imine form. Aliphatic nitroso compoimds with a hydrogen attached to the adjacent carbon are also usually not stable and exist primarily as oximes. 

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