Clicking Azides and Alkynes with Poly(pyrazolyl)borate-Copper(I) Catalysts: An Experimental and Computational Study

Abstract

The synthesis of 1,4-disubstituted-1,2,3-triazoles under a copper(I)-catalyzed azide–alkyne cycloaddition (CuAAC) regime was accomplished in high yields and a regioselective manner by using two homoscorpionate poly(pyrazolyl)borate anions: tris(pyrazolyl)hydroborate (HB(pz)3−) and bis(pyrazolyl)hydroborate (H2B(pz)2−), which stabilized in situ the catalytically active copper (I) center. The [3+2] cycloaddition (32CA) reactions took place under strict click conditions, including room temperature and a mixture of environmentally benign solvents such as water/ethanol in a 1:1 (v/v) ratio. These click chemistry conditions were applied to form complex 1,2,3-triazoles-containing sugar moieties, which are potentially relevant from a biological point of view. Computational modeling carried out by DFT methodologies at the B3LYP/6-31G(d) level showed that the coordination of poly(pyrazolyl)borate-copper(I) to alkyne groups produced relevant changes in terms of generating a high polar copper(I)-acetylide intermediates. The analysis of the global and local reactivity indices explains correctly the role of poly(pyrazolyl)borate ligands in the stabilization and activation of the copper(I) catalyst in the studied 32CA reactions.