2-Aminoimidazoles, amino tautomer

The solid-state IR spectra (KBr discs) have been recorded for several simple 5-aminoimidazoles (180 R1, R2 = alkyl) and are consistent with these compounds existing as amino tautomers (92JCS(P1)2779). Typically, the two NH stretching frequencies of the amino group are observed between 3300 and 3400 cm-1. The frequencies of the NH deformations are observed in the range 1590-1615 cm-1. 

Aminoimidazoles are rather unstable and have not been studied extensively, but despite the absence of a specific study of their tautomerism it is possible to gather some evidence in favour of the amino tautomer of types (35 X = NH) (39 X = NH) or (47 X = NH). Imino structures (e.g. 34, 38, 44 X = NH) require the appropriate CH signals in the NMR spectrum these are absent, and the IR spectra display characteristic bands for NH2. The X-ray structure for 4-acetamido-2-bromo-5-isopropyl-1-methylimidazole confirms the structure (65) rather than the tautomer (66) (Scheme 21). On the basis of CNMR evidence, 2-arylamino-2-imidazolines prefer to exist in the arylimino forms (B-77MI40600). 

Aminoimidazoles exist as the amino tautomers. Carbonyl compounds condense normally with C-aminoimidazoles, - which can also be diazotized although some amines are rather unstable. The diazonium fluoroborates have been transformed into fluoroimid-... 

Amino-l,3-azoles exist as the amino tautomers. 2-Amino-l,3-azoles tend to be more stable than other isomers. All amino-1,3-azoles protonate on the ring nitrogen. 2-Aminothiazole has a pA an of 5.39, which compares with the value for 2-aminoimidazole of 8.46, reflecting, in part, the symmetry of the resonating guanidinium type system in the latter. 

Iminoimidazolin-4-ones 227b are the only tautomers detected for 4-hydroxy-2-aminoimidazoles (R = H, Ph R = H) (91KGS62). Similar tautomeric forms are also characteristic of l-methyJ-2-imino-5-benzylideneim-idazole-4-one 228 (91KGS62) and 3-amino-l,2,4-triazol-5-ones 229 (R = R = H, COMe) (Scheme 78) [76AHC(S1), p. 482]. 

Using a semiempirical SCF MO it approximation, the tautomerism of several amino heterocycles has been studied (70JA2929). From the theoretical heats of atomization of the tautomers of 4-aminoimidazole (181a-d), it was suggested that there would be a slight predominence of 1-//-4-... 

Although it has been suggested that some of the reactions of 2-ami-noimidazole can best be explained by its existence as the imino tautomer in dimethylformamide, this result was based on incorrectly interpreted chemical evidence. In fact, a UV study of 2-amino-l-methyl-4,5-di-phenylimidazole provides convincing evidence for the amino form with Kj = 3 X 10". Calculations of heats of combustion show that in the 4-aminoimidazole 5-aminoimidazole equilibrium there is a slight predominance of the 4-amino form [a conclusion which is inconsistent with Charton s hypothesis since (7m(NH2) = —0.161]. The compounds, however, exist as amino rather than imino structures. ... 

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