Investigation of Base-free Copper-Catalysed Azide–Alkyne Click Cycloadditions (CuAAc) in Natural Deep Eutectic Solvents as Green and Catalytic Reaction Media

The click cycloaddition reaction of azides and alkynes affording 1,2,3-triazoles is a transformation widely used to obtain relevant products in chemical biology, medicinal chemistry, materials science and other fields. In this work, a set of Natural Deep Eutectic Solvents (NADESs) as “active” reaction media has been investigated in the copper-catalysed azide–alkyne cycloaddition reactions (CuAAc). The use of these green liquids as green and catalytic solvents has shown to improve the reaction effectiveness, giving excellent yields. The NADESs proved to be “active” in this transformation for the absence of added bases in all the performed reactions and in several cases for their reducing capabilities. The results were rationalized by DFT calculations which demonstrated the involvement of H-bonds between DESs and alkynes as well as a stabilization of copper catalytic intermediates. The green experimental conditions, namely the absence of a base, the low temperatures, the lowering

of reagents and the possibility of recycling of these green solvents, outline the great potential of NADESs for CuAAc and in general, for green organic synthesis.