N-Heterocyclic Carbenes (NHCs)-Catalyzed Coupling Reactions of Aldehydes and Organic Halides

This concept aims to summarize the direct coupling of aldehydes and organic halides (including activated and unactivated organic halides) catalyzed by N-heterocyclic carbenes (NHCs). The key factor of this transformation is the formation of Breslow intermediates, which are either nucleophilic or reductive. Additionally, the activation of inert C-H bond also was reviewed in this performance via the processes of single electron transfer (SET) and subsequent intramolecular 1,n- hydrogen atom transfer (HAT) or intermolecular HAT. Finally, the key differences and reactivity of various Breslow intermediates, generated from different NHCs, have been discussed to give a deeper understanding of the ongoing popularity of NHC-organocatalysis.