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Oxidative transformation of phenolic compounds on metal oxides supported precious metal catalysts Mabuatsela V Maphoru*, Thandiwe P Mntambo and Tumisang Lekgetho Department of Chemistry, Tshwane University of Technology, Pretoria, South Africa Selective oxidation of phenols and naphthols offers efficient access to the production of functionalized quinones, naphthofurans, binaphthones and phenoxazinones that serve as central building blocks for a wide variety of biologically active compounds [1,2]. These compounds play a very significant role in the field of chemistry, biochemistry, pharmaceutical, medical and cosmetic sciences to name a few [3]. For example, quinones derivatives such as mitomycin C, RH1, indoloquinone and streptonigrin, are part of a huge group of antibiotics used for the treatment of diseases such as gastro-intestinal tumors, anal cancer, breast cancer, leukamia and bladder tumors [3]. In the past, the industrial production of important oxygenated compounds such as quinones was mainly through the use of stoichiometric oxidants and homogeneous catalysts. The main challenge with these methods is that they often lead to low yields, poor selectivities and generates large amounts of toxic waste which has adverse consequences on the environment [4]. Separation of products from homogeneous catalysts is also one of the major problems. The need to substitute these traditional methods with more efficient and environmentally friendly catalytic methods is of high importance. For the sake of sustainable chemistry, these methods need to be replaced with catalytic methods that involve the use of stable and active noble metal heterogeneous catalysts and cheap oxidants under mild and green conditions.
Figure 1: TEM images of MW-1%Ru-1%Pd/TiO 2 and MW-5%Ru-5%Au/SiO 2 In this study, noble metal nanocatalysts supported on metal oxides, Ru-Au/TiO 2 , Ru-Pd/TiO 2 , Ru-Au/ SiO 2 , Ru-Bi/SiO 2 and Au-Bi/SiO 2 catalysts, were used as catalysts in the oxidation of various phenols and naphthols. The use of supported noble metal catalysts have, in the past, proved to have numerous advantages such as ease of separation, recycling, thermal stability and high catalytic activities as compared to their homogenous counterparts. The catalysts were prepared by microwave-assisted loading (MW) and deposition (IM) methods and characterized for their physicochemical properties using N 2 physisorption, XRD, SEM-EDX, TEM (Figure 1) and XPS analysis. It was observed that the type of solvent, type of catalyst and temperature affect the outcome of the reaction. For example, in the oxidation of trimethylhydroquinone carried out in MeOH under reflux and at r.t, MW-5%Au-5%Bi/SiO 2 and MW-5%Ru- 5%Bi/SiO 2 catalysts, it was observed that the catalysts were not effective in activating this substrate. However, when MW-5%Au-5%Ru catalyst was used for the same reaction, a 100% yield and selectivity of trimethylquinone was realised in both MeOH and MeNO 2 under reflux. In the reaction of 2-aminophenol, a high yield of 94% for 2-amino-3 H -phenoxazin-3-one was obtained in methanol using MW-1%Ru-1%Pd/ TiO 2 catalyst at room temperature, whereas MW-1%Ru-1%Au/TiO 2 gave only 54%. MW-1%Ru-1%Au/TiO 2 , however, showed better catalytic activity in nitromethane as compared to methanol resulting in 68% yield. Based on the catalyst testing data obtained, these catalysts have a high potential in activating phenol- based substrates and can therefore be used for the green production of quinones, binaphthones and phenoxazines. 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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