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 the NHC‐organocatalysis.

Boosting N-Heterocyclic Carbene Radical Organocatalysis with Nickel Chemistry: A Rational Mechanistic Study-Based Approach

“…−2 V vs SCE). − Very recently, primary and secondary unactivated alkyl iodides and bromides ( E red < −2 V) have been acylated via NHC organocatalysis. , Note that in the case of these substrates, an SN 2 reaction with Breslow intermediates is also possible, and it is likely that both ionic and radical pathways coexist. In any case, tertiary alkyl halides remain out of reach and therefore represent the new redox frontier for NHC-catalyzed radical activation of electrophiles. − …”