Cu(<scp>ii</scp>)-alginate-based superporous hydrogel catalyst for click chemistry azide–alkyne cycloaddition type reactions in water

Abstract

A novel sustainable hydrogel catalyst based on the reaction of sodium alginate naturally extracted from brown algae <i>Laminaria digitata</i> residue with copper(ii) was prepared as spherical beads, namely Cu(ii)-alginate hydrogel (Cu(ii)-AHG). The morphology and structural characteristics of these beads were elucidated by different techniques such as SEM, EDX, BET, FTIR and TGA analysis. Cu(ii)-AHG and its dried form, namely Cu(ii)-alginate (Cu(ii)-AD), are relatively uniform with an average pore ranging from 200 nm to more than 20 μm. These superporous structure beads were employed for the copper catalyzed [3 + 2] cycloaddition reaction of aryl azides and terminal aryl alkynes (CuAAC) <i>via</i> click chemistry at low catalyst loading, using water as a solvent at room temperature and pressure. The catalytic active copper(i) species was generated by the reduction of copper(ii) by terminal alkyne <i>via</i> the oxidative alkyne homocoupling reaction. The prepared catalysts were found to be efficient (85-92%) and regioselective by affording only 1,4-disubstituted-1,2,3-triazoles. They were also recoverable and reused in their dried form for at least four consecutive times without a clear loss of efficiency. A mechanistic study was performed through density functional theory (DFT) calculations in order to explain the regioselectivity outcome of Cu(ii)-alginate in CuAAC reactions. The analysis of the local electrophilicity (<i>ω</i> _k) at the electrophilic reagent and the local nucleophilicity (<i>N</i> _k) at the nucleophilic confirms the polar character of CuAAC. This catalyst has the main advantage of being sustainably ligand-free and recyclable.