Cu nanoparticles immobilized on montmorillonite by biquaternary ammonium salts: a highly active and stable heterogeneous catalyst for cascade sequence to indole-2-carboxylic esters

Copper nanoparticles immobilized on montmorillonite (MMT) by biquaternary ammonium salts (N1,N6-dibenzyl-N1,N1,N6,N6-tetramethylheptane-1,6-diaminium bromide, Q) were prepared by cation-exchange and impregnation–reduction and designated Cu-Q-MMT. The material was extensively characterized by various characterization techniques such as FTIR, XRD, XPS, SEM, TEM, and N2 adsorption–desorption. The Cu-Q-MMT could be used as a highly active heterogeneous catalyst for cascade sequence to indole-2-carboxylic esters from ortho-bromobenzaldehydes with ethyl acetamidoacetate. Even for inactive chlorobenzaldehydes, a good yield could be obtained. In addition, the catalyst can be reused six times without any significant loss of activity. The high activity and stability of the Cu-Q-MMT catalyst is mainly attributed to the excellent synergistic effects of biquaternary ammonium salts, Cu nanoparticles and the nanospace structure of MMT.