The annular tautomerism of imidazoles and pyrazoles: The possible existence of nonaromatic forms

Abstract

DFT (B3LYP/6-31G*) and G3//B3LYP (usually referred as G3B3 in the literature) calculations have been carried out on annular tautomers of C- and/or N-functionalized imidazoles (both the aromatic 1H- and nonaromatic 2H-) and pyrazoles (the aromatic 1H- and 2H-, and nonaromatic 3H- and 4H-). The aromaticity of 1H-imidazole and 1H- and 2H-pyrazole results in these species being more stable than their nonaromatic tautomers. However, this stability is reversed when the hydrogen on the azole nitrogen or methylene carbon is substituted by OH or by F, e.g., in increasing order of stability we find 1-fluoro-1H-imidazole <2-fluoro-2H-imidazole (<2-fluoro-1H-imidazole). These results are related to a recent report of a highly substituted imidazole that exists in the “nonaromatic” 2H-tautomeric form, and discussed subsequently in a purely thermochemical context.