4-Aminopyrimidine, tautomers

AP the canonical 4-aminopyrimidine tautomer of ThDP or its C2-substituted derivatives... 

UV, IR, and H NMR spectroscopic studies of 2-(phenylamino)pyrimidine showed that this compound exists as a tautomeric mixture containing only a small amount (ca. 0.2%) of the imino tautomer in an aqueous ethanolic solution (Kt=5. 5 x 102, amino/imino tautomer), but the Ky value is considerably smaller than that of 2-(phenylamino)pyridine (80BCJ717). The amino tautomer was also found to be highly predominant in solutions of 2-(phenylamino)-4,6-dimethylpy-rimidine (84H(22)2591) and 2-[/V-(2,4,6-trinitrophenyl)amino]pyrimidine in both polar and apolar solvents (92JHC1461). For cyclohexane-fused 2-aminopyrimidines 54 (X = NH2 R = CF3) and 55 (X = NH2 R = CF3 Ar = 4-Et2NC6H4), both amino and imino tautomers are present in the solid state however, only amino tautomers were observed in CHC13 solution (00S1738). 

Both semiempirical and ab initio calculations of 4-aminopyrimidine predict the highest stability of the amino tautomer (86JST(148)45,96JST(376)375,99JST(458)217)). The next stable tautomer is 3//-imino tautomer 89b, which is less stable than 89a by... 

All spectroscopic and studies of aminopyrimidines and aminoquinazolines show the amino form to an overwhelming extent . Spectroscopic studies of 2-aminoperimidines indicate about 2% of the imine tautomer in equilibrium with the amine. In I-substituted 2-acylaminoperimidines and aceperimidines, the equilibrium is almost entirely in favor of the imino form, but in the absence of a substituent in the 1-position, the amino form persists. In both cases the preferred tautomer is stabilized by intramolecular H-bonding <81RCR816>. 

The notion that the 4 -aminopyrimidine could exist in the l, 4 -iminopyrimidine tautomeric form was suggested earlier by models attempting to mimic the reactivity of such a tautomer. The N1 -methyl analog of both the... 

These compounds exist as tautomers. Hydroxypyrimidines adopt the lactam form whereas aminopyrimidines prefer the enamine form (see p 413). 

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