NHC-catalysed Reactions of Ketenes

Due to the unique bivalent carbene and diversity of the N-heterocyclic motif, NHCs have been demonstrated to be efficient organocatalysts for various enantioselective reactions. In addition to the traditional thioazolium and imidazolium NHCs, triazolium NHCs have become the most successful organocatalysts. Recently, NHC/Lewis acid cocatalysis and bifunctional NHCs have shown a very promising future. Beyond the classic NHC-catalysed umpolung of aldehydes, the extended umpolung of functionalised aldehydes are extremely successful. A series of NHC-catalysed reactions of ketenes have been developed for the synthesis of various enantioriched heterocycles. Esters, anhydrides, carboxylic acids and even Michael acceptors are useful alternative substrates for NHC-catalysed reactions. With increasing interest and rapid development of NHC catalysis, new structures of the catalysts, new reaction modes, and synthetic applications can be expected in the near future. 

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