Synthesis of Cu(II) quinoxolinol Complexes and their Catalytic Applications

Global energy demand and industrial production have risen rapidly, and this trend is expected to continue over the next 20 years. A shift toward more sustainable practices has emerged, exemplified by the “Green Chemistry”, a set of guidelines aimed at minimizing waste and potential hazards while improving the safety and efficiency of chemical processes, serving as a framework for more sustainable chemistry. In this work, Cu(II) complexes are shown to exhibit potential as sustainable and practical alternatives to toxic precious metals. Employing a solution-phase parallel synthesis method, a salophen-like ligand containing a 2-quinoxolinol backbone, named “salqu” for its salicylaldehyde and quinoxaline precursors, has been reported in the Gorden group. These Cu(II)-salqu complexes, in combination with tert-butyl hydroperoxide (TBHP) as an oxidant, were used in the C-H activation reactions for allylic, benzylic, and propargylic substrates. In this work, a novel copper(II) imidazolium functionalized quinoxolinol complex has been prepared based on modified previous literature. The resulting ionic Cu(II)-salqu together with TBHP functioned as a powerful catalytic system for the oxidation of benzylic and heterocyclic alcohols to aldehydes, achieving excellent yields (up to 99%) within a short reaction time and demonstrating broad functional group tolerance. The Cu(II)-salqu complexes were examined as efficient catalysts for the direct synthesis of imines via oxidative coupling of alcohols and amines under mild conditions. Various aromatic alcohols were effectively coupled with anilines or benzylamines to afford the corresponding imines in good to excellent yields, with water produced as the only byproduct over two steps. This approach represents a greener and more sustainable strategy, as alcohols are readily available starting materials, enabling the synthesis of a diverse range of imine products. The substrate scope and reaction conditions were systematically investigated to optimize efficiency and selectivity. In summary, these works represent a significant step toward greener, more sustainable chemistry by exploring copper-based catalysts, which provide a promising strategy for oxidation and, accordingly, for oxidative C-N coupling reactions, due to their lower cost, greater availability, and reduced environmental impact.