N-Heterocyclic carbene (NHC) organocatalysis using aliphatic aldehydes

Catalysis using N-heterocyclic carbenes (NHCs) has been widely employed for the umpolung of aldehydes, and these reactions proceed via the generation of nucleophilic Breslow intermediates. These acyl anion equivalents can be added to aldehydes, ketones, imines, Michael acceptors, and even unactivated carbon-carbon multiple bonds. In a majority of the cases, aromatic aldehydes are used in NHC catalysis. However, the use of aliphatic aldehydes is relatively less explored because of the low electrophilicity of the aliphatic aldehydes compared to aromatic aldehydes for the nucleophilic addition of carbenes, and the presence of α-acidic protons leading to side reactions under basic conditions. There are several NHC-catalyzed reactions that engage aliphatic aldehydes, and this Highlight summarizes the advances in NHC catalysis using aliphatic aldehydes as substrates.