Hydrophobic sulfonated iron oxide: A new magnetically recyclable heterogeneous nanocatalyst for the efficient one-pot four-component synthesis of hexahydroquinolines

  A new heterogeneous sulfonated magnetic nanocatalyst decorated with hydrophobic tag denoted as hydrophobic-NBS-γ-Fe2O3, is successfully synthesized and characterized by different methods such as FT-IR, SEM, TEM, TGA and elemental analysis. The catalyst worked well for the efficient synthesis of hexahydroquinolines (HHQs) via the Hantzsch one-pot four-component reactions under solvent-free conditions. The enhanced catalytic activity of hydrophobic-NBS-γ-Fe2O3 could be attributed to the synergistic effects of the anchored sulfonic acid groups and hydrophobic tags on the catalyst surface. The presence of hydrophobic tags well circumvents the problem of water-poisoning of acidic sites in the synthesis of HHQs as a water generating reaction and also facilitates the mass transfer of hydrophobic starting materials to the active acidic sites of the catalyst. Using the solvent-free conditions, good to high yields of the products, use of reusable catalyst, short reaction times and simple work-up procedure make this method favorable from the environmental and economic viewpoints for the synthesis of HHQs. This study represents the first example of using a hydrophobic sulfonated magnetic nanocatalyst towards the one-pot four-component synthesis of HHQs.