Reaction mechanism of ruthenium-catalyzed azide-alkyne cycloaddition reaction: A DFT study

The ruthenium catalyzed reaction of alkynes with azides (RuAAC) have enabled a facile route to obtain substituted triazoles, in contrast to thermal 1,3-dipolar cycloaddition which requires high activation barrier and is slow with limited regioselectivity. In this study, ruthenium catalyzed azide-alkyne cycloaddition reaction mechanism has been modeled by quantum mechanical methods. The reactions of benzyl azide and substituted alkynes have been modeled by following model mechanism with quantum mechanical calculations at the B3LYP/6-31G* level of theory with LANL2DZ on Ru. The reaction has been investigated for terminal and internal alkynes, with Cp (cyclopentadiene) and Cp* (pentamethylcyclopentadiene) ligand and compared with the experimental results. The calculations in this study have reproduced the experimental regioselectivity and allowed us to account on the electronic and steric effects. Both thermodynamic and kinetic parameters appeared to be important in these reactions. The thermodynamic stability of the Ru-azide-alkyne complexes and the relative ease of the complex to undergo reaction determined the product distribution. (C) 2012 Elsevier B. V. All rights reserved.