Aci tautomer

Nitromethane was proposed as a probe molecule of basic sites on catalyst surfaces. The nitro form is in equilibrium with a small amount of the aci tautomer. Kheir and Haw (165) demonstrated that the aci tautomer was deprotonated on MgO and CaO to form chemisorbed aci anion (Scheme... 

For the spontaneous decomposition of nitramide (NH2N02) the transition state is assumed to have formula 8, on the supposition that the rate-limiting step is base-catalysed removal of a proton from the (less stable) aci-tautomer (NH N02H). This suggestion is made in the... 

Aci tautomer A tautomeric form, R2C=N+(-0H)0, of a nitro compound, R2CH- N02, hence the nitro/aci tautomerism. 

Nitronic ester The result of O-alkylation of the carbanion of the aci tautomer of a nitro compound and having the general formula R2C=N+(0-)0R. 

These are usually named according to the structure of the more stable tautomer, that is the keto and nitro versions, respectively. If reference needs to be made to the other tautomer, it is just called the enol or aci tautomer of the corresponding keto or nitro compound. 

Similarly, titanium tetrachloride or stannic tetrachloride induces addition of silyl enol ethers to nitroalkenes. The initial product is a complex of the aci tautomer of the nitro group. Hydrolysis of this complex generates the new carbonyl group. ... 

In this context the recently documented, specific incorporation of l-nitro-2-phenylethane (85) into benzylglucosinolate in T. majus, comparable with that of DL-phenylalanine, deserves attention. Definite proof that the nitro-com-pound, or maybe rather its aci-tautomer (86), plays the role as a true intermediate, foreshadowed several years ago, is still outstanding and will supposedly require studies at the enzymic level. If confirmed, the subsequent steps, (86) -> (87) -> (88), appear unexceptional and not without in vitro analogies. What remains a fact is that phenylacetaldoxime in the intact plant gives rise to the production of l-nitro-2-phenylethane, present in shoots of T. majus, albeit in concentrations as low as SO pg per 100 g of fresh weight. ... 

The aci-form of the nitro group is frequently claimed in pure chemistry. However, only ad-nitromethane appears to have been comprehensively studied as an isolated species15. Ionized keto-enol systems are characterized by reversal of the relative stabilities of the single species compared to their neutral counterparts. Thus, the ionized enols are generally the thermodynamically more favoured tautomers by approximately 15-20 kcalmol-1, the... 

A second and related consequence in aliphatic nitro compounds is the acidification of the directly bonded CH unit through the attendant stabilization of the derived conjugate bases (5,6). As with all delocalized anions, reprotonation gives rise to tautomers, the original C-nitro compound (I) and the oci-nitro or isonitro form (II), Eq. 2.1. The aci-nitro tautomers are typically present in very minor concentrations, with equilibrium constants (A eq) between 10 and 10 (7). Alkylation of the delocalized anion leads to both a-substituted nitro compounds and the regioisomeric nitronic esters (nitronates). Nitronates were described as early as 1894 (8), however, the first isolated nitronic ester was obtained several years later upon the addition of diazomethane to phenylazonitromethane (1), Eq. 2.2 (9). 

At high buffer concentrations, positive curvature may be observed in buffer dilution plots, indicating that the general acid and base are simultaneously participating in the rate-determining step.27 In such a case, the rate law must be expanded by third-order terms. Furthermore, plots of buffer slopes versus xHb may be nonlinear, when the unstable tautomer is a diprotic acid as, for example, the aci-nitro tautomer of nitrobenzene.28... 

Yim and Liu [145] used a combined ab initio molecular dynamics (MD) study to reveal several new mechanistic paths that are energetically favored over dissociation of the C-N02 bond. Their study indicated dissociation of NTO via an aci-nitro tautomer followed by ring scission to a ketinimine intermediate as the most favorable pathway (Scheme IX), requiring only 38 kcal/mol as compared to the 62.5 kcal/mol needed for cleaving the C-N02 bond. Kohno et al. [151] conducted a theoretical study on the decomposition of the NTO dimer. They reported that HONO elimination is the last step of a cascade of reactions with a total barrier of 88 kcal/mol. 

Evidently the a-H is likely to be involved in the early stages of the decomposition process [16], perhaps moving to an adjoining nitro group to form a nitronic acid tautomer, 4 [35,45,46]. These are known to be reactive and unstable [47], The transfer or loss of a proton to yield a nitronic acid or a nitronate (aci) anion has also been invoked as the initial step in the decompositions of other energetic molecules, e.g. picric acid [35,48] and amine-sensitized nitromethane [49-51]. 

Here again a double bond has been broken this time a nitrogen/oxygen bond, and a new carbon/nitrogen double bond has been formed in its place. This is called the nitro/aci tautomerism, and again each tautomer may be isolated, because each tautomer represents a real compound and not just a hypothetical model. These tautomers are shown below. 

Schworer, M., Wirz, J., Photochemical Reaction Mechanisms of 2 Nitrobenzyl Compounds in Solution I. 2 Nitrotoluene Thermodynamic and Kinetic Parameters of the aci Nitro Tautomer, Helv. Chim. Acta 2001, 84, 1441 1458. 

Anhydrous 5-nitrobarbituric acid (9) exists in the monohydroxy form (8a),27 but the trihydrate is in the aci-nitro form (9b)28 in the crystalline state as shown by X-ray analysis. IR and UV investigations29 and dipole moment measurements30 are consistent with those conclusions. The results of MO calculations show that the monohydroxy structure (9a) is the most stable. UV spectra31 as well as 14N-NQR data32 also support tautomer 9a in water. 

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