Sustainable and Versatile CuO/GNS Nanocatalyst for Highly Efficient Base Free Coupling Reactions

Abstract

A CuO nanoparticles (CuO NPs)/graphene nanosheet (GNS) hybrid was prepared by a very simple method and employed as a nanocatalyst (CuO/GNS) for base free coupling reactions, namely, A³-coupling and <i>aza</i>-Michael reactions. TEM shows that CuO NPs of below ∼35 nm size are homogeneously dispersed on the GNS. Strong adhesion between CuO NPs and GNS was acknowledged by a high Raman <i>I</i>_D/<i>I</i>_G ratio and XPS result. The BET surface area of CuO/GNS was found to be 66.26 m² g^–1. The EDS and XPS investigations revealed that the weight percentage and chemical state of Cu in CuO/GNS were 4.46% and +2, respectively. Under mild reaction conditions, CuO/GNS exhibited an outstanding catalytic activity in terms of yield, turnover number (TON) and turnover frequency (TOF) toward A³-coupling reaction. A small amount of catalyst (10 mg, 0.7 mol % of Cu) is enough to carry out the reactions with a wide range of substrates. The CuO/GNS is stable, heterogeneous in nature and reusable for at least five times without any significant loses of yield. N-oxidation of tertiary amines was also carried out to explore further the activity of CuO/GNS, and the results are found to be excellent. Versatility of the CuO/GNS was realized from the superior catalytic activity of used CuO/GNS in <i>aza</i>-Michael reaction. Finally, GNS (∼95%) and CuO (as CuCl₂) were successfully recovered from the used CuO/GNS and confirmed by TEM, Raman, XPS, XRD and SEM-EDS analyses