The mechanism of copper-catalyzed azide-alkyne cycloaddition reaction: A quantum mechanical investigation

In this study, the mechanism of CuAAC reaction and the structure of copper acetylides have been investigated with quantum mechanical methods, namely B3LYP/6-311 + G(d,p). A series of possible copper-acetylide species which contain up to four copper atoms and solvent molecules as ligand has been evaluated and a four-copper containing copper-acetylide, MIA, was proposed more likely to form based on its thermodynamic stability. The reaction has been modeled with a representative simple alkyne and a simple azide to concentrate solely on the electronic effects of the mechanism. Later, the devised mechanism has been applied to a real system, namely to the reaction of 2-azido-1,1,1-trifluoroethane and ethynylbenzene in the presence of copper. The copper catalyst transforms the concerted uncatalyzed reaction to a stepwise process and lowers the activation barrier. The pre-reactive complexation of the negatively charged secondary nitrogen of azide and the positively charged copper of copper-acetylide brings the azide and the alkyne to a suitable geometry for cycloaddition to take place. The calculated activation barrier difference between the catalyzed and the uncatalyzed reactions is consistent with faster and the regioselective synthesis of triazole product. (C) 2012 Elsevier Inc. All rights reserved.