N-heterocyclic carbene-copper(I) complexes in homogeneous catalysis

N-Heterocyclic carbenes (NHCs) have become increasingly popular as ligands for transition metals, thanks to their remarkable successes in metathesis and cross-coupling areas. Important advances have also been achieved in organocopper chemistry. To date, the reported neutral N-heterocyclic carbene-copper(I) complexes have shown outstanding catalytic activity. In this account, we describe the application of such complexes in a variety of organic transformations. 1 Introduction 2 Copper-Hydride-Mediated Reactions 2.1 Reduction of Carbonyl Compounds 2.1.1 Activity and Scope 2.1.2 Mechanistic Considerations 2.2 Related Transformations 3 Carbene Transfer Reactions 3.1 Cyclopropanation and Insertion Reactions 3.2 Olefination Reactions 4 [3+2] Cycloaddition of Azides and Alkynes 4.1 'Click Chemistry' 4.2 Use of Internal Alkynes: Mechanistic Implications 5 Miscellaneous Reactions 6 Concluding Remarks.