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

Abstract 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.