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.
[(NHC)2Cu]X complexes as efficient catalysts for azide–alkyne click chemistry at low catalyst loadings
Angew. Chem. Int. Ed. 2008, 47, 8881-8884.