Nitromethane tautomerism

The 8N chemical shift can be used in studies of tautomeric equilibrium for optically active Schiff bases, however, it depends not only on the position of equilibrium but also on the types of substituents. Usually values higher than —200 ppm (relative to nitromethane) indicate the presence of pure NH-form or equilibrium strongly shifted towards NH tautomer. Lower values of 815N imply the presence of tautomeric equilibrium. ... 

The tautomerism of />-amino- and p-dimethylamino-azobenzene cations has also been studied recently in nitromethane and acetonitrile (Korolev and Titova, 1971), and it has been shown that azonium forms are much more favoured in these solvents than in aqueous acid. The results are shown in Table 3. The estimates are based on the assumption that the extinction coefficient of the ammonium ion [48] may be teiken to be equal to that of p-tri-methylammoniumazobenzene and that of the azonium ion to be equal to that of the conjugate acid of 4-amino-3,5-di-t-butylazo-benzene (Yeh and Jaffe, 1959c), which is assumed to exist only in... 

Nitroparaffins such as nitromethane, nitro-ethane, nitropropane, are mild oxidizers and should not be heated with easily oxidized hydrocarbons. With water, they form nitronic acids as tautomeric forms, which form explosive salts with inorganic bases (Ref 83)... 

Tautomerism in nitromethane is the simplest example of nitro == ac/ -nitro tautomerism. Xc/ -nitromethane has been generated and characterized by Schwarz and co-workers using neutralization-reionization mass spectrometry [29, 30]. Formation of stable ac/ -nitromethane ions has been shown to occur via the elimination of ethylene from the molecular ion of 1-nitropropene [31]. Formation of ac/ -nitromethane by a concerted acid-catalyzed ring opening of nitrocyclopropanes has been shown to occur by Cao et al. [7c], The unsaturated character of ac/ -nitromethane has been used for trapping various free radical... 

Aliphatic nitro compounds show a number of reactions which parallel those of carbonyl chemistry. Primary and secondary nitro compounds exhibit tautomerism paralleling keto-enol tautomerism (Scheme 3.94a). Aliphatic nitro compounds dissolve in aqueous sodium hydroxide with the formation of sodium salts. The resultant anions behave as carban-ions and will condense with aldehydes. An example involves the formation of m-nitrostyrene from nitromethane and benzaldehyde (Scheme 3.94b). 

Because of their kinship to syntheses of nitro sugars, some syntheses of simpler, cyclic, aliphatic nitro compounds will be mentioned. Tetrahydropyran-2-ol, which is in tautomeric equilibrium with 5-hydroxypentanal (126), was found to undergo nitromethane addition catalyzed by sodium hydroxide to give120 tetrahydro-2-(nitromethyl)-pyran (128). Undoubtedly, the first reaction-product is the diol ni-tronate (127), which, under the conditions of processing (acidification and steam distillation), forms the cyclic ether 128. 

Aliphatic nitro compounds are characterized by another interesting and mechanistically important property. Based on tautomeric and protolytic equilibria the stable form of nitromethane in basic solution is that of a double bonded ac/-nitromethane anion, CH2=N02 , which becomes ready target for electrophilic radical additions. An example, studied by pulse radiolysis, is its reaction with hydroxyl radicals as formulated in eq. 3."... 

In another review on a related subject, Alkorta and Elguero have summarised the GIAO calculations of chemical shifts and absolute shieldings and their correlation with experimentally measured chemical shifts for aromatic hetero-cycles. Automatic assignment, conformational analysis, E/Z isomerism, and, in particular, tautomerism, have been discussed in detail. In addition, solid state and solvent effects, as well as the problem of heteroaromaticity have been examined. The review ends with the discussion of some methodological aspects with a special emphasis on the calculation of reference chemical shifts, such as those for TMS and nitromethane. 

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