Nitroso-oxime tautomerism,

Nitroso-oxime tautomerism was reported to occur in compounds 266-268 [76AHC(S1), pp. 436,443,452],... 

Nitroso-Oxime Tautomerism. This equilibrium lies far to the right, and as a rule nitroso compounds are stable only when there is not a hydrogen. 

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. 

The diastereomeric ratio is high only for ( )-disubstituted alkenes and norbornene, but low for cyclic and (Z)-alkenes, reflecting the order of stability of the intermediate complexes. The loss of stereochemical purity is probably due to base-catalyzed nitroso-oxime tautomerization during the reduction. However, the use of diisobutylaluminum hydride, before adding lithium aluminum hydride, allows (3/ , 4S )-3,4-hexanediamine to be obtained from (Z)-3-hexene with d.r. [(3R, 4S )IQR AR )] 90 10. 

Mechanism The reaction of and A -steroids with nitrosyl fluoride to form ni trimines is best discussed in conjunction with the nitrosyl chloride reaction leading tothe5a-chloro-6 -nitro steroids (33). Since nitroso alkanes are oxidized with nitrosyl chloride to nitro alkanes it is believed that 5a-chloro-6j5-nitro steroids are formed in this way from an initially formed 5a-chloro-6 -nitroso adduct. The same is true for nitrosyl fluoride up to the stage of the nitroso fluoride (56). Since NOF is a weaker oxidizing agent than NOCl the nitroso fluoride tautomerizes to the fluoro oxime (57) at a rate... 

There are three possible products when NOCl is added to alkenes. 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 ... 

Primary and secondary nitroso compounds tautomerize to isonitroso compounds - oximes of aldehydes and ketones, respectively. Their reductions are dealt with in the sections on derivatives of carbonyl compounds (pp. 106,132). 

Heating 3-nitroso-2-phenyl-l//-indole with excess phosphoryl chloride or tosyl chloride in sulfolane at 200 C for 1 h, followed by dilution with water, produces 2-phenylquinazolin-4(3//)-one in 90 or 68% yield, respectively. The ring expansion proceeds via the second-order Beckmann rearrangement of the imino-oxime tautomeric form of 3-nitroso-2-phenyl-l//-indole. °... 

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. 

Die Nitrosierung ist auf elektronenreiche 1,3-Thiazole beschrankt, die in 2-Position eine Amino-, Hydroxy- oder Mercapto-Gruppe tragen (vgl. Bd. X/3, S. 58). Die 5-Nitroso-Produkte liegen haufig als Oxim-Tautomere vor. 

After the formation of tautomeric anions A=A. the anion A a rearranges to give the anion B, which reacts with the second nitroso acetal molecule to form a mixture of stereoisomers of silyl derivative 509a. After desilylation of 509a, oxime 510a is isolated. The reaction with the fluoride anion proceeds at low temperature, whereas the use of triethylamine is efficient only at room temperature. The yield of oxime (510a) is virtually independent of the reaction conditions, whereas the diastereomeric ratio varies substantially. 

There appears to be little reported work on S-nitrosation reactions of simple thioke-tones. Thiocamphor when treated with /50-amyl nitrite in fact gives the oxime58 (formerly called a isonitroso compounds), presumably via the tautomeric form of the thione, i.e. the enethiol. In this respect the reaction is very similar to the reactions of ketones59 which give oximes or C-nitroso compounds via the enol intermediates60. 

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. 

The reduction of aliphatic nitrocompounds in acid solution proceeds in two steps. First the nitrosocompound is formed. A low steady state concen ation of 2-methyl-2-nitrosopropane has been detected during the reduction of 2-methyl-2-nitropropane [13]. At the cathode potential necessary to attach the first electron to a nitro group, the nitroso intermediate undergoes further reduction to the hydroxyla-mine. When the nitrocompound has one a-hydrogen substituent, tautomerism of the nitroso intermediate to an oxime is in competition with further reduction. Both temperature and proton availability affect the rate of this isomerisation. Reduction of aliphatic nitrocompounds to the hydroxylamine is usually carried out in acid solution at 0-5° C to minimise oxime formation [14, 15], The hydroxylamine is stable towards further reduction in acid solution. Oximes in acid solution are reduced... 

Nitroso compounds (e.g. 116, equation 85) that are unable to tautomerize to oximes undergo an ene reaction with aikenes 117 giving Af-aUylhydroxylamines 118 (equation 85). Both trifluoromethyl and aryl nitroso compounds react with aikenes although in many cases the resulting Af-allylhydroxylamines are prone to subsequent chemical transformations. If allylzinc compounds are used as the aikene components, the chemos-electivity of the reaction is reversed and O-allylation products are preferably formed . 

Meldmm s acid 5-oxime, prepared by reaction of Meldrum s acid and NO followed by tautomerism of the nitroso group, thermolyzes at elevated temperatures analogously to give the highly reactive nitrosoketene <1997H(46)503, 1998H(47)383>, which has been characterized spectroscopically (Equation 15) <2002JRS443>. 

Since the question of the necessary conditions for isomerization of nitroso-cyclohexane to cyclohexanone oxime is of considerable commercial importance, it has been shown that this tautomerization is favored by the presence of gaseous hydrogen chloride, particularly at a wavelength of 300 mp. [57]. When cyclohexane is saturated with hydrogen chloride, treated with nitric oxide, and exposed to a source of ultraviolet radiation, the oxime forms along with a trace of 1-chloro-l-nitrosocyclohexane [58]. Cyclooctane seems to form the corresponding oxime and the chloronitroso compound, but under no circumstances nitrosocyclooctane [58]. 

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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