Recent advances in NHC–palladium catalysis for alkyne chemistry: versatile synthesis and applications

During the past few decades, alkyne chemistry has attracted considerable attention owing to alkenes' unique and idiographic nucleophilic and electrophilic properties in transition-metal-catalyzed chemical transformations. Among the various metal catalysts, palladium complexes show remarkable catalytic activity for the assembly of structurally diverse organic functional molecules from readily available feedstocks. Impressively, the N-heterocyclic carbene–palladium catalytic system has been recognized as one of the most efficient and precise synthetic methodologies due to its strong σ-donating and weaker π-accepting abilities. In this review, NHC–palladium catalyzed transformations of alkynes have been summarized and discussed in detail with focus on the Sonogashira type coupling reactions, difunctionalization reaction, cyclization reaction, hydrogenation reaction, semihydrogenation reaction, dimerization reaction and alkynylation reaction. We also highlight some representative synthetic strategies along with reaction mechanisms involved in the reaction. Moreover, the literature has been surveyed from 2014 to the end of 2020.