The Use of Ligands in Copper-Catalyzed [3+2] Azide-Alkyne Cycloaddition: Clicker than Click Chemistry?

The last decade has witnessed a tremendous increase of the studies concerning [3+2] cycloaddition reactions of azides and alkynes. The recent discovery of copper(I) species as efficient catalysts for this transformation is at the origin of this true explosion. In contrast with classical thermal reactions, copper catalysis leads to the regioselective formation of 1,4-disubstituted 1,2,3-triazoles in high yields and under simple reaction conditions. This transformation has indeed become the most prominent example of the principles of Click chemistry, postulated in 2001 by Sharpless and co-workers. Despite the ever-increasing interest in this process, the use of ligands in this context remains minor when compared to ligandless systems. Nevertheless, besides overcoming the inherent limitations of ligandless systems, the use of ligands in this context has been shown to increase and, more importantly, modulate the catalytic activity of the metal center. Herein, the catalytic activities of ligated copper systems are reviewed in a way intended inspirational for future developments in this field.