N-Heterocyclic carbenes (NHCs) are a special type of carbenes in which the carbene carbon atom is part of the nitrogen heterocyclic ring. Due to the simplicity of their synthesis and the modularity of their stereoelectronic properties, NHCs have unquestionably emerged as one of the most fascinating and well-known species in chemical science. The remarkable stability of NHCs can be attributed to both kinetic as well as thermodynamic effects caused by its structural features. NHCs constitute a well-established class of new ligands in organometallic chemistry. Although initially NHCs were regarded as pure σ-donor ligands, later experimental and theoretical studies established the presence of a significant back donation from the d-orbital of the metal to the π* orbital of the NHC. Over the last two decades, NHC–metal complexes have been extensively used as efficient catalysts in different types of organic reactions. Of these, NHC–Cu(I) complexes found prominence for various reasons, such as ease of preparation, possibility of structural diversity, low cost, and versatile applications. This article overviews applications of NHC–Cu(I) complexes as catalysts in organic synthesis over the last 12 years, which include hydrosilylation reactions, conjugate addition, [3 + 2] cycloaddition, A³ reaction, boration and hydroboration, N–H and C(sp²)–H carboxylation, C(sp²)–H alkenylation and allylation, C(sp²)–H arylation, C(sp²)–H amidation, and C(sp²)–H thiolation. Preceding the section of applications, a brief description of the structure of NHCs, nature of NHC–metal bond, and methods of preparation of NHC–Cu complexes is provided.

A simple method involving microwave-assisted reaction of 3-benzylthiazolium/benzothiazolium bromides with cuprous oxide has been developed for the synthesis of four new N-heterocyclic carbene (NHC): thiazolylidene/benzothiazolylidene–Cu(I) complexes. Structures of the complexes could be established on the basis of the IR, ¹H, and ¹³C NMR studies and elemental analysis. A representative NHC:benzothiazolylidene–Cu(I) complex was used as catalyst for the microwave-assisted A3 reaction of phenylacetylene with a secondary amine and aldehyde to obtain new substituted propargylamines in excellent yields. A theoretical investigation of the model A3 reaction at the DFT level reveals initial coordination of the alkyne with the NHC–Cu(I) complex followed by its attack on the iminium ion generated from the reaction of secondary amine with aldehyde. In the presence of the catalyst, the activation free energy barrier (ΔG^#) is lowered by ca. 3 kcal mol⁻¹. Graphical Abstract