Methylimidazole tautomers compounds

Generally the name of a compound should correspond to the most stable tautomer (76AHCS1, p. 5). This is often problematic when several tautomers have similar stabilities, but is a simple and reasonable rule whose violation could lead to naming phenol as cyclohexadienone. Different types of tautomerism use different types of nomenclature. For instance, in the case of annular tautomers both are named, e.g., 4(5)-methylimidazole, while for functional tautomerism, usually the name of an individual tautomer is used because to name all would be cumbersome. In the case of crystal structures, the name should reflect the tautomer actually found therefore, 3-nitropyrazole should be named as such (97JPOC637) and not as 3(5)-nitropyrazole. 

Crystalline 2-methylimidazole exhibits different 13C (CPMAS) chemical shifts for C-4 and C-5 (125.0, 115.7 ppm). The average (120.3 ppm) is close to that reported for imidazole in deuterated DMSO (121.2 ppm). These results imply that solid state chemical shifts can be used instead of N-methyl models in tautomerism studies (87H(26)333). For imidazole the solid state l3C shifts are 137.6 (C-2), 129.3 (C-4), and 119.7 (C-5) (81JA6011). No proton exchange occurs in the solid, and the data support a structure resembling the crystal structure. Cooling imidazole solutions has not yet allowed the detection of individual tautomers, but by symmetry the compound exists in equal tautomeric forms, as does pyrazole (81CC1207). 

The presence of prototropic tautomerism can also be demonstrated when an attempt is made to synthesize the two individual tautomers of a pair. Both synthetic pathways (Scheme 11) lead to the same compound , or to a product mixture which behaves as if it was a single compound. The ultimate product (23) (in reality it is a tautomeric mixture which acts as a single compound) can be named 4(or 5)-ethyl-5(or 4)-methylimidazole, but it is now accepted practice to assume that workers in the field are aware of the implications of the simpler form of nomenclature, 4-ethyl-5-methylimidazole. When the NH group is... 

Imidazoles with a ring iV-hydrogen are subject to tautomerism, which becomes evident in unsymmetri-cally substituted compounds such as the methylimidazole shown. This special feature of imidazole chemistry means that to write simply 4-methylimidazole would be misleading, for this molecule is in rapid tautomeric equilibrium with 5-methylimidazole. All such tautomeric pairs are inseparable and the convention used to cover this phenomenon is to write 4(5)-methylimidazole . In some pairs, one tantomer predominates, for example 4(5)-nitroimidazole favours the 4-nitro-tautomer by 400 1. 

This special case of prototropy is known as annular tautomerism (see p 111). In solution, equilibria are established so rapidly that the separate tautomers cannot be isolated. However, their presence can be demonstrated by spectroscopic methods. In this case, e.g. R = CH3, the compound is known as 4(5)-methylimidazole. With certain substituents R, the equilibrium lies predominantly to one side, for instance, in the case of the nitro compound (4-nitroimidazole) or with the methoxy compound (5-methoxy-imidazole). Annular tautomerism has also been demonstrated for 4,5-disubstituted imidazoles ... 

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