Base-free oxidative coupling of 2-aminophenol to questiomycin A over microwave-synthesized titania-supported Ru Au and Ru Pd catalysts

This study focuses on the application of highly dispersed noble metal catalysts in green oxidative coupling of 2-aminophenol to biologically active questiomycin A. Owing to its environmental friendliness and cost effectiveness, microwave-assisted polyol method (MW) was used in the preparation of titania-supported monometallic 1Ru, 1Pd, 1Au and bimetallic 1Ru1Pd and 1Ru1Au catalysts. The characteristics of the catalysts were examined by SEM, TEM, N2-physisorption, XPS and XRD analysis. The perfomance of the catalysts were tested in the oxidative coupling of 2-aminophenol (1) with H2O2 at room temperature (r.t) and under reflux. The coupling of 2-aminophenol (1) yielded questiomycin A (2-amino-3H-phenoxazin-3-one, 2) as a major product. A high yield of 94 % for 2 was obtained over 1Ru1Pd catalyst in MeOH at r.t, whereas 54 % yield of 2 was obtained over 1RuAu under the same reaction conditions. However, 1Ru1Au displayed an improved catalytic activity in MeNO2 at r.t with a yield of 68 % for 2. An increasing trend in the yield of 2 was observed for both catalysts when the reaction temperature was increased. When compared to the monometallic catalysts, bimetallic catalysts were found to be highly active and selective in the oxidative coupling of 1. Microwave-assisted catalyst preparation methods yielded highly dispersed Ru Pd and Ru Au nanoparticles on the titania support. The materials were used as catalysts for the base-free oxidative coupling of 2-aminophenol to questiomycin A with hydrogen peroxide and product yields of up to 99 % were opened. The outcome of the reaction depends on the catalyst, solvent and temperature used.