Tautomerism and Structure of Azoles

This review provides comprehensive data (from the author's work and the literature) on tautomerism of pyrazoles, imidazoles, 1,2,3- and 1,2,4-triazoles, tetrazoles, benzimidazoles, benzimidazolones, benzotriazoles, and related compounds by multinuclear 1H, 13C, 15N nuclear magnetic resonance (NMR) spectroscopy. Emphasis is given to studies of azoles by multinuclear dynamic NMR spectroscopy because prototropic transformations of almost all azoles in solution proceed so quickly. Quantum-chemical investigations of tautomeric azoles are covered in detail. Critical analysis of literature data on the tautomerism of azoles is presented. Theoretical studies of structure and reactivity of azoles are most intriguing because correct interpretation of their chemical behavior and biological activity depends on an understanding the factors determining relative stability of their tautomers. 15N NMR spectroscopy is the most reliable and fast technique for structural and tautomeric determination of nitrogen-containing heteroaromatic compounds.