Abstract
This chapter discusses the five-membered rings with two or more heteroatoms. This chapter describes the distinction between tautomeric rearrangements involving only ringnitrogen centers (annular tautomerism) and those accompanied with transformations in ring-side groups and the arrangement of the material according to the compounds' main structural types. Four principal types of tautomeric processes relevant to the heterocyclic compounds are addressed. It includes (1) tautomerism involving annular centers, (2) side-chain tautomerism associated with transformations involving functional group(s) attached to the rings, (3) ring-chain tautomerism as a special case of the side-chain tautomerism accompanied by ring closing, and (4) valence tautomerism. This chapter focuses on tautomeric reactions of azoles and their derivatives in electronically excited states and stabilization of certain tautomers in their metal complexes.