Chitosan-supported copper salt and copper metal nanoparticles/copper (I) oxide microcrystals: Efficient and recyclable heterogeneous catalysts for the synthesis of bis(indolyl)methanes

Abstract

Chitosan (CTS)-supported-CuSO4 (CTS-CuSO4) and CTS-supported-Cu metal nanoparticles/Cu2O microcrystals (CuNPs/Cu2OMCs) heterogeneous catalysts were developed through a simple, eco-friendly, efficient, and homogeneous immobilization methodology, by exploiting the chelation capacity of CTS. Notably, the CTS-CuSO4 and CTS-CuNPs/Cu2OMCs catalysts lead to process intensification for the synthesis of bis(indolyl)methanes (BIMs) through the promotion of catalyst recovery and reusability in up to five catalysis/recovery cycles, solvent-free reactions under mild conditions, high product yields, low amounts of catalysts, and no metal waste, owing to catalyst recovery and reuse. Different crystal structures of Cu2OMCs are obtained in combination with CuNPs through the chemical reduction of CuSO4 using ascorbic acid as a reducing agent, which is a simple procedure that can be conducted under mild reaction conditions. Moreover, we establish that the CTS-CuNPs/Cu2OMCs heterogeneous catalyst is an effective alternative to CTS-CuSO4 in BIM synthesis when aliphatic aldehydes are used. This is the first report on the use of CTS-CuSO4 and CTS-CuNPs/Cu2OMCs catalysts for the synthesis of BIMs through a low-cost, simple, eco-friendly, and sustainable approach.