Azines tautomerism

Azinyl-ylidene tautomerism of azinylmethanes and general problems of azine tautomerism 90UK456. 

Figure 9. Bis(hydrazone)/bis(azine) tautomerism of a 14-membered macrocycle upon complexation of metal salts.

Kartritzky AR, Wariing AJ (1962) Tautomeric azines. Tautomerism of 1-methyluracil and 5-bromo-l-methyluracil. J Chem Soc (1962) 1540-1544... 

Table 11 summarizes the main results on the tautomerism of mono-hydroxy-, -mercapto-, -amino- and -methyl-azines and their benzo derivatives, in water. At first sight the equilibrium between 2-hydroxypyridine (71) and pyridin-2-one (72) is one between a benzenoid and a non-benzenoid molecule respectively (71a 72a). However, the pyridinone evidently... 

Table 11 Tautomeric Equilibria of some Monofunctional Azines and Benzazines ...

Another analogy with the behavior of the azidoazoles is the influence of substituents in various positions of the azine rings on the tautomeric equilibria. Electron-withdrawing substituents favor azidoazine and electronreleasing substituents favor tetrazoloazine forms (Table X). 

An illustrative example of the Michael reaction is that of the thiirene dioxide 19b with either hydroxylamine or hydrazine to give desoxybenzoin oxime (87) and desoxybenzoin azine (88), respectively, in good yields6 (see equation 29). The results were interpreted in terms of an initial nucleophilic addition to the a, j8-unsaturated sulfone system, followed by loss of sulfur dioxide and tautomerization. Interestingly, the treatment of the corresponding thiirene oxide (18a) with hydroxylamine also afforded 86 (as well as the dioxime of benzoin), albeit in a lower yield, but apparently via the same mechanistic pathway6. 

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. 

Table 11 summarizes the main results on the tautomerism of mono-hydroxy-, -mercapto-, -amino- and -methyl-azines and their benzo derivatives, in water. At first sight the equilibrium between 2-hydroxypyridine (71) and pyridin-2-one (72) is one between a benzenoid and a non-benzenoid molecule respectively (71a 72a). However, the pyridinone evidently has a continuous cyclic p- orbital system, containing six it- electrons, the usual aromatic count, if the carbonyl group contributes none. This assumption implies the formula (72b), from which by redistribution of electrons we arrive at (72c), which has the same benzenoid system as (71a). Further canonical forms (71b, 71c) can be drawn of (71) which correspond to the non-benzenoid forms of (72). The elusive property of aromaticity is therefore possessed by both tautomers, although not necessarily by both equally. When the carbonyl oxygen of (72) is replaced by less electronegative atoms, as in the imine tautomers of amino heterocycles, or the methylene tautomers of methyl derivatives, the tendency towards polarization in forms corresponding to (72b) and (72c) is considerably less, and the amino and methyl tautomers are therefore favoured in most instances. 

In the consideration of the tautomerism of pyridines and azines there is usually little need to consider the transfer of hydrogen to a ring carbon atom. In other words, in the set of formulae (73)-(75) the non-aromatic form (75) is not significantly populated. However, there are exceptions in, for example, polyhydroxy-pyridines and -azines. 

The most well-known tautomeric systems of this type involve a-azidoazoles. Unlike their azine counterparts normally existing as condensed tetrazoles (Sec. 2.2.5.4), they exist as a rule in an open-chain structure, e.g. (255). However, in alkaline media N-anion (256) has a strong tendency to cyclize into tetrazole (257) which often becomes predominant in an equilibrium mixture. 

As well as the previous chapter on the Hammett equation,1 the importance of the approach has found expression in specific areas in other contributions to these volumes side chain reactivities of thiophenes 17 tautomerism 18 reactions of azines with nucleophiles 19,20 formation of benzofuroxans by decomposition of o-nitrophenyl azides 21 acid dis-... 

The tautomerism of functional derivatives of azines (e.g., pyridones) and other more complex systems (guanines, porphyrins, etc.) has been much studied both experimental and theoretically [40, 76-82], Our contribution to this field has been modest using a combination of 13C and 15N NMR in solution and in the solid state, together with GIAO//B3LYP/6-311++G calculations, the tautomerism of 2-pyri-done, 4(37/)-pyrimidone, uracil, and cytosine were determined [83],... 

The tautomerism of 2-pyridones 25 that are favored over 2-hydroxypyridines 24 (and conversely, 2-aminopyridines 26 that are favored over 2-imino-derivatives 27) plays a central role in the chemistry and biochemistry of all azines [146-148], A comparison between A-methyl-2-pyridone and 2-methoxypyridine shows that the magnetic susceptibility of the former is about 20% greater than that of the latter [149],... 

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