[(NHC)₂Cu]X Complexes as Efficient Catalysts for Azide–Alkyne Click Chemistry at Low Catalyst Loadings

La click, c'est chic! A catalytic system based on an [(NHC)2Cu]X complex (NHC=N-heterocyclic carbene) was developed for the [3+2] cycloaddition of azides with alkynes under click conditions (see scheme). This system is broad in scope and highly efficient (turnover frequencies up to 5000 h−1) even at very low catalyst loadings (down to 40 ppm). Preliminary mechanistic studies suggest a specific precatalyst-activation pathway.