Oxygen Vacancy-Mediated Reactivity: The Curious Case of Reduction of Nitroquinoline to Aminoquinoline by CuO

Reduction of nitroarenes is a useful strategy for the preparation of aromatic amines. In this work, CuO was used as a reusable solid reagent for the reduction of nitroquinolines to aminoquinolines using hydrazine monohydrate as the hydrogen donor under mild reaction conditions. The reaction of 0.5 mmol of 5-nitroquinoline to 5-aminoquinoline was completed in 10 min with 100% conversion and selectivity. The reagent CuO could be easily collected and reused for up to four cycles, after which it is completely transformed into Cu2O and Cu. The high activity and selectivity of CuO were attributed to the facile formation of oxygen vacancies on the CuO surface under the reducing environment, as confirmed by X-ray photoelectron spectroscopy. Further, density functional theory simulations showed that oxygen vacancies led to higher adsorption (Eads = -174.4 kJ/mol) and selective activation of the nitro group (Ea = 43 kJ/mol) coupled with a reaction involving hydrogen atoms activated on the CuO surface. However, the presence of an additional pyridine ring in 5-nitroquinolines slightly impeded the rate of complete reduction (t = 10 min).