Catalytic oxidation of hydroxyarenes over silica-supported bimetallic noble metal catalysts Tumisang Lekgetho 1 , Mabuatsela V. Maphoru 1 , Letlhogonolo F. Mabena 1 , Matshwandile Tukulula 2 1 Department of Chemistry, Tshwane University of Technology, South Africa, 2 School of Chemistry and Physics, University of KwaZulu Natal, South Africa Catalytic oxidative coupling of hydroxyarenes is an important goal in fine chemical synthesis 1 . These reactions are essential for the formation of biologically active compounds that serve as central building blocks for many synthetic and natural products 2. The products obtained from these reactions, quinones and binaphthones, are a class of natural and synthetic compounds with pharmaceutical applications due to their biological activities 3 . Conventional methods such as the use of stoichiometric oxidants have served as primary synthesis routes for numerous years to obtain quinones and binaphthones. However, these methods have become synonymous with low yields, poor oxidation and coupling selectivity, production of byproducts and reduced oxidants that have adverse effects on the environment 4 . Therefore a need to substitute these conventional methods with greener, efficient and environmentally friendly methods is very important. In this work, the oxidation of phenols and naphthols were carried out using heterogeneous Ru-Bi/SiO 2 , Au-Ru/SiO 2 and Au-Bi/SiO 2 catalysts. The catalysts were prepared by microwave-assisted loading (MW) and deposition (DP) methods and characterized for their physicochemical properties by N 2 physisorption, XRD, SEM-EDX (Figure 1), TEM-EDX and XPS analysis. The catalysts were tested for the oxidation of trimethylhydroquinone and 4-methoxy-1-naphthol in methanol and/ or nitromethane at room temperature and under reflux with hydrogen peroxide as an oxidant. In the oxidation of trimethylhydroquinone (1) in MeOH at r.t and 60ºC over MW-1Ru-1Bi catalyst, yields of 78.9% and ≥99% for trimethyl-1,4-benzoquinone (2) were obtained, respectively. However, no reaction was observed when MW-5%Ru- 5%Bi catalyst was used under the same reaction conditions. The oxidation of 4-methoxy-1-naphthol (3) over MW-5%Au-5%Ru catalyst gave 47.1% conversion (Yield: 24.5% of 4 ) in MeNO 2 at r.t, while the reaction carried out under refluxing conditions over the same catalyst achieved ≤99.0% conversion (Yield: 54.5% of 4 ). The yield, selectivity and conversion of this reactions depends on the solvent, temperature and catalyst used
References 1. Adeyini, A.A. & Ajibade, P.A. Bioinorg. Chem. Appl ., 2018 , 2018: 5796287 2. Soto-Hernandez, M., Palma-Tenanango, M. & Garcia-Mateos, M. London Tech, 2017. 3. El-Najjar, N., Gali-Muhtasib, H., Ketola, R.A., Vuorela, P., Urtti, A. & Vuorela, H. Phytochem. Rev. , 2011, 10: 353-370. 4. Dockrey, S.A.B., Lukowski, A.L., Becker, M.R. & Narayan, A.R.H. Chem. , 2018, 10(2): 119-125
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