Discovery and characterization of catalysts for azide-alkyne cycloaddition by fluorescence quenching.

Copper-based catalysts for the 1,3-dipolar cycloaddition of azides and alkynes were screened in parallel fashion using a fluorescence quenching assay. The method was designed to identify systems able to accelerate the coupling of reactants at micromolar concentrations in aqueous mixtures and to obtain quantitative comparisons of their activities. In addition to the tris(triazolylamines) previously reported, two types of compounds (bipy/phen and 2-pyridyl Schiff bases) were found to exhibit significant ligand-accelerated catalysis, with one complex showing especially dramatic rate enhancements. Preliminary explorations of the dependence of reaction rate on pH, ligand:Cu ratio, and Cu concentration are described.