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

摘要

A novel sustainable hydrogel catalyst based on the reaction of sodium alginate naturally extracted from brown algae Laminaria digitata 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) via 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 via 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 ( ω _(k) ) at the electrophilic reagent and the local nucleophilicity ( N _(k) ) at the nucleophilic confirms the polar character of CuAAC. This catalyst has the main advantage of being sustainably ligand-free and recyclable.