Mechanistic insights into Cu(I)-catalyzed azide-alkyne "click" cycloaddition monitored by real time infrared spectroscopy.

A designed ligand-accelerated Cu(I)-catalyzed cycloaddition (CuAAC) reaction was monitored for the first time by real time infrared analysis technique based on ATR-FTIR principles. Principal components analysis (PCA) and two-dimensional correlation spectroscopy (2Dcos) results showed that the consumption of alkyne and azide took place successively followed by the formation of the product 1,2,3-triazole, and a 1:1 complex of two reactants would be formed in the reaction process. The rate-determining step of the CuAAC reaction was also experimentally confirmed for the first time to be the transition of azide-alkyne 1:1 complex to the preproduct 1,2,3-triazole. Our results are in good conformity with the current catalytic mechanism proposed by Sharpless et al. according to DFT calculation results.