Synthesis and Characterization of Cu@Hal, an Effective Heterogeneous Catalyst for Aqueous Multicomponent Azide-Alkyne [3+2] Cycloaddition Reactions

The synthesis of a nanocomposite material consisting of Cu nanoparticles encapsulated in halloysite nanotubes (Cu@Hal) was achieved by the reduction of Cu(NO₃)₂ ⋅ 3H₂O with sodium ascorbate/sodium borohydride in an aqueous suspension of trisodium citrate and halloysite. The nanocomposite was found to be an effective heterogeneous catalyst for the multicomponent copper catalyzed azide-alkyne cycloaddition reaction (CuAAC). A variety of terminal alkynes reacted with benzyl halides and sodium azide in the presence of Cu@Hal in water. In situ formation of the organic azides afforded the corresponding 1,4-disubstituted 1,2,3-triazoles regioselectivily, in excellent yields. The catalyst was easily recovered and recycled without loss of activity with low metal leaching.