Generation of azolium dienolates as versatile nucleophilic synthons via N-heterocyclic carbene catalysis

N-Heterocyclic carbene (NHC) organocatalysis has emerged as a powerful tool in the field of modern organic synthesis especially in asymmetric construction of various cyclic skeletons. As emerging NHC-bound nucleophilic intermediates, azolium dienolates have been widely investigated and applied in NHC-catalyzed transformations due to their unique reactivities and versatile structures. Particularly, four representative kinds of reactive azolium dienolates (I-IV) have been explored as versatile nucleophilic synthons via either alpha-functinalization or remote gamma-functionalization to achieve efficient construction of complex skeletons. Moreover, these NHC-bound intermediates had wide-ranging applications in asymmetric synthesis of diversely enantioenriched centrally chiral molecules. In this review, the recent advances in NHC-catalyzed generation of azolium dienolates from different precursors and their applications in the synthesis of various valuable molecules are summarized comprehensively.