Cationic Copper Iminophosphorane Complexes as CuAAC Catalysts: A Mechanistic Study

We
have combined Cu K-edge X-ray absorption spectroscopy with NMR
spectroscopy (1H and 31P) to study the Cu-catalyzed
azide–alkyne cycloaddition (CuAAC) reaction under operando conditions. A variety of novel, well-defined CuI iminophosphorane
complexes were prepared. These ligands, based on the in situ Staudinger reduction when [Cu­(PPh3)3Br] is
employed, were found to be active catalysts in the CuAAC reaction.
Here, we highlight recent advances in mechanistic understanding of
the CuAAC reaction using spectroscopic and kinetic investigations
under strict air-free and operando conditions. A
mononuclear Cu triazolide intermediate is identified to be the resting
state during catalysis; cyclization and protonation both have an effect
on the rate of the reaction. A key finding of this study includes
a novel group of highly modular CuI complexes that are
active in the base-free CuAAC reaction.