Diazole, Triazole, and Tetrazole N-Oxides

Application of N-oxides in chemistry and biology has attracted increasing interest. General trends in chemical properties are emerging but further investigations are needed to exploit the full potential of these multifaceted and useful compounds. This review constitutes the first comprehensive description of the preparation, properties, and reactions of 5-membered heteroaromatic N-oxides derived from pyrazole, imidazole, 1,2,3-triazole, 1,2,4-triazole, and tetrazole. The azole N-oxides display enhanced reactivity and complementary regioactivation as compared to the parent azoles. They can undergo unique reactions making them highly versatile by regiocontrolled functionalization of 5-membered azaheterocycles, when functionalization is followed by removal of the N-oxide oxygen atom. O-Alkylation or acylation further activates both ring and lateral positions for subsequent transformations in one pot into substituted azoles. The N-oxides can be divided into two families characterized by their reactivity that can be rationalized and predicted by analysis of charge distribution.