Understanding the regioselectivity of the copper(I)- and ruthenium(II)- catalyzed [3 + 2] cycloadditions of azido derivative of ribose with terminal alkyne: a theoretical study

Abstract

In the present work, the uncatalyzed, the copper(I)-catalyzed and the ruthenium(II)-catalyzed [3 + 2] cycloadditions (32CA) of azido derivative of ribose with terminal alkyne leading to 1,4- and/or 1,5- 1,2,3-triazole regioisomers have been studied at the B3LYP level of theory in combination with the LanL2DZ basis set for Cu, Ru and Cl atoms and the standard 6-31G(d) basis set for other atoms. The obtained results reveal that the uncatalyzed reaction requires high and similar activation energies, namely 18.29 and 18.80 kcal/mol for the 1,4 and 1,5 regioisomeric pathways, respectively, indicating a very limited regioselectivity in agreement with the experimental outcomes. Interestingly, for the copper(I)-catalyzed azide–alkyne cycloaddition (CuAAC), investigated using the Fokin stepwise mechanism involving two copper atoms, the 1,4 regioisomeric reaction path found to be kinetically more favored than the 1,5 regioisomeric reaction path by 9.13 kcal/mol. By contrast, for the ruthenium(II)-catalyzed azide–alkyne cycloaddition (RuAAC), investigated using the Fokin mechanism using the pentamethylcyclopentadienyl ruthenium chloride [Cp * RuCl] complex, the 1,5 regioisomeric reaction path is more favored than the 1,4 regioisomeric reaction path by 3.48 kcal/mol. The present work puts in evidence the determinant role of Cu/Ru catalysts in the regioselectivity of this click reaction.