1,3,4-Trisubstituted-1,2,3-Triazol-5-ylidene 'Click' Carbene Ligands: Synthesis, Catalysis and Self-Assembly

This review examines the use of the Cu(I)-catalyzed 1,3-cycloaddition of organic azides with terminal alkynes (the CuAAC 'click' reaction) for development of a novel family of abnormal/mesoionic N-heterocyclic carbenes and their corresponding metal complexes. These 1,3,4-trisubstituted-1,2,3-triazol-5-ylidenes have donor properties that are intermediate between the traditional Arduengo-type imidazol-2-ylidenes and more highly sigma-donating abnormal carbenes, such as imidazol-4-ylidenes or pyrazolin-4-ylidenes. Metal complexes of the 1,3,4-trisubstituted-1,2,3-triazol-5-ylidenes have been used as catalysts for a variety of reactions including the CuAAC cycloaddition, Pd cross-couplings, and ring closing/ring opening metathesis. Additionally, 'click' carbene ligands have been used to generate self-assembled metallo-macrocycles and novel photosensitizers. The mild, modular CuAAC approach to these ligands should allow the rapid generation of libraries of 1,3,4-trisubstituted-1,2,3-triazol-5-ylidenes that can be further exploited to generate novel catalysts, metallo-pharmaceuticals and materials.