Equilibria amino/imino tautomeric

However, for derivatives of 2,4-diaminothiazole 214 evidence exists for the amino-imino tautomeric equilibrium (Scheme 71) [76AHC(S1), p. 460]. 

Fig. 15.4. Keto/enol and amino/imino tautomeric forms of uracil, cytosine, adenine, guanine. The keto and amino forms predominate, the enol and imino forms are only present in equilibrium at 0.01 Vo or less...

Unsubstituted and N-substituted bipyridyls 76 (R = H, Me, Ph, MeCO, 4-MeC6H4S02) were investigated for amino-imino tautomerism using basicity measurements and UV spectral data. The bipyridyls 76 (R = H, Me, MeCO) were shown to exist predominantly in the amino form both in aqueous solution and in non-polar media. The tautomeric equilibrium of 76 (R = 4-McC6H4S02), however, depends on the solvent polarity whereas the imino tautomer predominates in water and ethanol, significant amounts of the amino tautomer were observed in dioxane and cyclohexane (78JCS(P2)1215). 

Photochromic isomerization of salicylaldehyde 2-quinolylhydrazone 80 involves both amino-imino tautomerism and E-Z isomerizations. When an ethanolic solution of 80a is irradiated in the UV region between 250 and 400 nm at room temperature, it is readily converted into the colored isomer 80b, which is stable in both protic and non-protic solvents at ambient temperature. This conversion presumably occurs via an intramolecular hydrogen transfer from the phenolic group, which precedes the double bond isomerization. In the dark, the reverse isomerizations take place (75JOC2512). No substituent effect on this tautomeric equilibrium has been observed (75JCS(P1)2036). 

Aminoacridines 84 and 85 (R1 = H, F, Br, Cl R2 = Me, CH2Br, OMe, etc. R3 = H, OMe) exist in amino imino tautomeric equilibrium in DMSO-<76 solution at ambient temperature however, increasing the temperature to 370 K results in the complete shift toward the imino tautomers (03MRC549). The tautomeric equilibria in five-membered heterocycle-fused 9-aminoacridines are completely shifted toward the imino tautomers in CDC13 and DMSO-4, solution (01MRC225). 

In a letter to the author, dated December 10, 1971, Professor Postovskii stated I am of the opinion that an amino-imino tautomerism obviously takes place in this case (i.e., 42 43). According to the general views of Kabachnik78 the first form (42) is more available in solutions because the acidity of form (43) is probably stronger than that of (42). Only form (42) is probable in the solid state. I believe that the equilibrium (42) (43) depends on the polarity of a solvent with the increasing of form (43) in more polar solvents. ... 

The equilibrium between the amino and imino forms of cytosine has also been studied by Cieplak et al.9 Their results agree with those of Colominas et al. In this study, the related 2-oxopyridine and 2-oxopyrimidine molecules were also treated. In both these molecules the amine group of cytosine is not present, and in oxopyridine only one ring nitrogen is present. This enabled the keto-imino tautomerism to be studied in isolation. In both cases the imino form dominates in the gas phase, but the keto form is stabilised by solvation, and dominates in solution, in agreement with experiment. 

The keto/enol or amino/imino equilibrium can be shifted, however, if the bases are chemically modified as in isoguanine (2-hydroxy,4-amino-purine). In this molecule, the tautomeric equilibrium depends largely on the dielectric constant of the solvent [525]. 

At present no systematic study of the factors which influence tautomeric behaviour in 1,3-oxazines and thiazines has been undertaken. All three types of 1,3-oxazines are known and both 2-amino-4-thioxo- and 2-imino-4-mercapto-l,3-thiazines exist in equilibrium with each other (74MI22700). When, however, the 4-position is disubstituted the amino isomer predominates (71KGS946). 

Thiazoles substituted in the 2-, 4- or 5-position with an amino, hydroxy or mercapto group are in tautomeric equilibrium with the corresponding imino-, oxo- or thioxo-thiazo-lines (Scheme 57). A similar protomerism has been established for (4-phenylthiazol-2-yl)acetone (109 Scheme 58). 

Among pyrimidine bases, cytosine shows significant tautomeric acitivity. In argon and nitrogen matrices, it exists as a combination of amino-oxo (N1H) and amino-hydroxy forms the tautomeric equilibrium being shifted towards the latter form [66, 67], In microwave studies, the amino-oxo, imino-oxo, and amino-hydroxy forms of cytosine are revealed [68] and in the aqueous solution only amino-oxo forms (N1H and N3H) are present [69], In a recent REMPI study of laser desorbed jet cooled... 

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