Organocatalysis promoted by 1,2,3-triazolylidenes (MICs): carbenes which make a difference

Abstract

N-Heterocyclic carbenes (NHCs) hold a unique significance in organometallic catalysis and are powerful organocatalysts for a variety of organic transformations involving crucial intermediates such as Breslow intermediates (BIs), deprotonated BIs (BI⁻s), ketyl radicals (KRs), and acyl azoliums (AAs). To address the remaining challenges facing reactions catalyzed by NHCs, non-classical stable carbenes, namely 1,2,3-triazolylidenes (MICs), cousins of NHCs, have shown great potential. MICs share similar features with typical NHCs but possess unique characteristics, such as enhanced σ-donor ability and absence of dimerization. Consequently, they have emerged as effective metal-free carbene catalysts for various chemical transformations. This review provides an overview of the fundamental characteristics of 1,2,3-triazolylidenes, along with the significant organic synthetic applications of these carbenes.