Dinuclear copper intermediates in copper(I)-catalyzed azide-alkyne cycloaddition directly observed by electrospray ionization mass spectrometry.

The mechanism of the CuAAC reaction has been investigated by electrospray ionization mass spectrometry (ESI-MS) using a combination of the neutral reactant approach and the ion-tagging strategy. Under these conditions, for the first time, putative dinuclear copper intermediates were fished out and characterized by ESI(+)-MS/MS. New insight into the CuAAC reaction mechanisms is provided and a catalytic cycle is proposed.