N-Heterocyclic Carbene Catalyzed the Umpolung of Aldehyde Compounds

Aldehyde compounds are a class of important organic compounds, which are widely used in organic synthesis. The typical reaction of aldehydes is the addition reaction with various nucleophiles as electrophiles. However, the electrophilicity of aldehyde carbonyl group limits the further application and development of aldehydes in synthesis. Since chemists discovered that cyanogen ions can change the reaction polarity of benzaldehyde, the strategy of umpolung has attracted much attention. N-Heterocyclic carbene (NHC) is a kind of important small organic molecule catalyst, which plays a key role in the field of organic catalysis. A series of important advances have been made in the study of the umpolung of aldehyde compounds catalyzed by NHC. The umpolung of aldehydes catalyzed by NHCs is introduced, and the positions of carbon atoms and the reactions of umpolung are summarized with the purpose of attracting much attention to this research field, and the development of new models and reactions within NHC catalysis.