Tautomerism, nitro compounds nitroso-oxime

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. 

Aliphatic nitro compounds exhibit rather different behavior from nitroaromatic compounds. Secondary and primary nitro compounds tend to produce oximes because the intermediate nitroso compound quickly tautomerizes to the oxime (equation 1). Under aprotic conditions the radical anions of primary and secondary nitro compounds are relatively stable those derived from tertiary nitro compounds, on the other hand, eject nitrite ion relatively readily (equation 2)8. 

There are three possible products when NOCl is added to alkenes, a (3-halo nitro-so compound, an oxime, or a (3-halo nitro compound.The initial product is always the (3-halo nitroso compound, but these are stable only if the carbon bearing the nitrogen has no hydrogen. If it has, the nitroso compound tautomerizes to the oxime, H—C-N=0 C=N-OH. With some alkenes, the initial (3-halo nitroso compound is oxidized by the NOCl to a (3-halo nitro compound. Many functional groups can be present without interference (e.g., COOH, COOR, CN, OR). The mechanism in most cases is probably simple electrophilic addition, and the addition is usually anti, although syn addition has been reported in some cases.Markovnikov s mle is followed, the positive NO going to the carbon that has more hydrogens. 

Since oximes are reduced to imines with buffered aqueous TiCb, one can assume that reduction of nitronate anions might progress through a similar mechanism. With regard to the reduction of nitro compounds themselves, there is some evidence for the intermediacy of a nitroso compound (178 Scheme 43). Indeed, at low pH tautomerization of (178) to oxime (179) would be rapid, whereas with TiCb-NH4OAC at pH 6 the nitro group is trapped to give only pyrroline (183), due to slow tautomerism. 

The mechanism of the Nef reaction has been extensively studied. Under the original reaction conditions, the nitronate salt is first protonated to give the nitronic acid, which after further protonation is attacked by a molecule of water. The process is strongly dependent on the pH of the reaction medium. Weakly acidic conditions favor the regeneration of the nitro compound and by-product formation (oximes and hydroxynitroso compounds), whereas strongly acidic medium (pH 1) promotes the formation of the carbonyl compound. The most popular reductive method (TiCb) proceeds via a nitroso compound that tautomerizes to form an oxime and finally upon work-up the desired product is obtained. 

All functional groups having nitrogen attached to the pyrazolinone nucleus are considered in this section except nitroso, nitro and some oximes. Only those oximes which cannot be considered as tautomeric with the nitroso group are discussed here. Hydrazones are in this section. These compounds are listed in Tables XXII and XXIII. 

Some OYEs have the novel biocatalytic activity of reducing aliphatic sec-nitro compounds to carbonyl compounds instead of the expected amines. This process is the biocatalytic equivalent to the Nef reaction, and it spares the use of strong acids (like H2SO4) and N2O production of the chemical alternative. The bioreduction mechanism presumably proceeds by reduction of the nitro group to the nitroso group, which subsequently tautomerizes to the more stable oxime that is further reduced to an imine derivative, which spontaneously hydrolyzes to the carbonyl compound and ammonia (Scheme 2.15). 

The nitrosation of aliphatic carbon atoms, particularly of carbon atoms activated by adjacent carbonyl, carboxyl, nitrile, or nitro groups, has been reviewed in great detail [2]. Judging from this review, with few exceptions, nitrosation of active methylene compounds leads to the formation of oximes (unfortunately termed isonitroso compounds in the older literature). The few exceptional cases cited in which true nitroso compounds (or their dimers) were formed involved tertiary carbon atoms in which no hydrogen atoms were available to permit tautomerism to the oxime or involved a reaction which was carried out under neither acidic nor basic conditions. 

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