Reaction mechanisms of transition-metal-catalyzed azide-alkyne cycloaddition click reactions: A DFT investigation

DFT calculations at the PBE0/LANL2DZ level have been performed on model compounds to investigate the reaction mechanism of two recently reported metal-catalyzed alkyne azide cycloaddition (MAAC). The first one, that involves a [Cu(tren)]Br-+(-) catalyst, is shown not to proceed through a metal alkynyl intermediate, but, after precomplexation of the alkyne in an eta(2)-mode, directly to the 1,4-disubstituted 1,2,3-triazole product, through a metallacyclic transition state. The other system, involving a rare-earth Ln[N(SiMe3)(2)](3) complex, is found to proceed through an alkynyl-azide complex which produces an eta(2)-coordinated heterocyclic ligand before protonation by an incoming alkyne molecule. Our results are discussed with respect to other computational data from the literature. (C) 2015 Elsevier B.V. All rights reserved.