Imino-amino equilibrium

Physicochemical studies on aminothiazoles are mainly centered on two problems the position of imino-amino protomeric equilibrium and IsRR substitution effects on the thiazole nucleus. 

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

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). 

The intramolecular and dimensionless equilibrium constant K-i = [imino]/[amino] was estimated as being on the order of 10 to 10, which leads to values for the acidity constant of 10 to 10 [112],... 

Angyal and Angyal measured the pK of 2-aminothiazoles and 2-imino-4-thiazolines to obtain the protomeric equilibrium constants (Scheme l4) (74). The higher pK values of the imino derivative (9.65) compared with that of 2-aminothiazole (5.68) prove that the amino form is highly predominant, Xp = 2x 10" (72), The limitations of such a method of Kp determination are discussed by Elguero. Marzin and Katntzky in a review of protomeric equilibria in heterocycles (75). 

Earlier studies of 4-aminopyridine 1-oxide were less conclusive. The solid-state infrared spectrum could be interpreted to indicate the existence of both the imino structure and/or, more probably, the amino structure. Comparison of the actual pKa value of 4-aminopyridine 1-oxide wdth the value calculated using the Hammett equation was considered to indicate that the compound existed as such or as an equilibrium mixture with l-hydroxypyrid-4-onimine, the latter possibility being considered the less likely on the basis of resonance and bond energies/ Resonance energy and ultraviolet spectral considerations have been advanced to support the 4-aminopyridine 1-oxide structure/ The presence of an infrared absorption band at... 

The fact that the equilibrium for aminopyridine 1-oxides is displaced further in favor of the amino form than is the equilibrium for aminopyridines is in accord with the mesomerism of these compounds. The stabilization of the amino forms (e.g., 241) by structures of type 240 is more effective than the corresponding stabilization in the pyridine series since the negative charge is associated with the oxygen atom. The stabilization of the imino form (e.g., 242) by structures of type 243 is less than in the pyridine series because of the adverse inductive effect of the oxygen atom. ... 

The predominance in the gas phase of the CH tautomers in the case of several pyrazolinones including one 3-aminopyrazolin-5-one was demonstrated by PES [88JCS(P2)641] this result is consistent with previous findings (76AHCS1, p. 325). The same technique was applied to the study of the amino/imino equilibrium in 9-amino and 9-(methylamino)acridines [91MI(61)166] although both tautomers have similar stabilities, the experimental data are in better agreement with the amino tautomer. 

By means of and NMR spectroscopy it was found that 2-(meth-ylamino)-l-pyrroline (R = Me) is present in solution mainly in the amino form. For the corresponding A -phenyl derivative an equilibrium between 2-aminopyrroline, imino, and 2-amino-4,5-dihydropyrrole tautomers is found (Scheme 20) (87MI5). 

Tlie amino form 84a exists in equilibrium with the imino structure 84b and not with 85 as concluded from the structure of products of deuterium exchange (87MRC757 91H329). 

The amino form is usually much more favored in the equilibrium between amino and imino forms than is the hydroxy form in the corresponding keto-enol equilibrium. Grab and XJtzinger suggest that in the case of a-amino- and a-hydroxy-pyrroles, structure 89 increases the mesomeric stabilization and thus offsets the loss of pyrrole resonance energy, but the increase due to structure 90 is not sufficient to offset this loss. Similar reasoning may apply to furans and... 

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. 

Pyridinedicarboxaldehyde (37) and tetraethyleneglycol bis(2-aminophe-nyl)ether (38) react to form macrocycle 39. NMR studies indicate that the three components in this mixture are in equilibrium since the imino groups, water and aldehydo/amino groups are constantly interconverting. Addition of bis(3,5-dimethoxybenzyl)ammonium hexafluorophosphate (40) species has a... 

Intramolecular formation of an imino group is possible in compounds that contain properly positioned carbonyl and amino groups (primary or secondary). The equilibrium between the open-chain parent and the cyclic Schiff base product will be pH-dependent, as explained in Sect. 11.6.1. 

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