Silver(I)-Catalyzed Azide-Alkyne Cycloaddition Reaction in Water. Scope
and Mechanistic Aspects
Abstract
A combined experimental work and molecular electron density theory
(MEDT) analysis was performed to reveal the strict click of
1,2,3-triazole derivatives by Ag(I)-catalyzed azide-alkyne cycloaddition
(AgAAC) reaction and its corresponding mechanistic pathway. Such
straightforward protocol for the click formation of
1,4-disubstituted-1,2,3-triazoles makes use of AgCl as catalyst in water
as solvent under ambient conditions., with excellent yields and simple
experimental work-up. MEDT study was performed by using DFT calculations
at the B3LYP/6-31G(d,p) (LANL2DZ for Ag) level in order to understand
the observed regioselectivity in AgAAC reactions, and to delineate the
number of silver(I) species and their roles in this clickable
1,2,3-triazole formation. The comparison of the mononuclear
Ag(I)-acetylide and binuclear Ag(I)-acetylide in the AgAAC reaction
paths concerning the AgAAC reactions, shows that the values of the
energy barriers for the binuclear processes are smaller than that of the
mononuclear one. The intramolecular nature of these AgAAC reactions
accounts for the regioselective formation of the 1,4-regiosisomeric
triazole derivatives. The ionic nature of the starting metallated
species is revealed for the first time, ruling out any covalent
interaction involving the silver(I) complexes throughout the reaction as
supported by the ELF topological analysis of the electronic structure of
the stationary points, reaffirming the zw-type mechanism of the AgAAC
reactions.