(nBu4N)4W10O32-catalyzed selective oxygenation of cyclohexane by molecular oxygen under visible light irradiation

Abstract

The development of mild and efficient process for the selective oxygenation of organic compounds by molecular oxygen (O2) can be one of the key technologies for synthesizing oxygenates. Here, the photo-oxygenation of cyclohexane to cyclohexanol and cyclohexanone over tetrabutylammonium decatungstate (W10O324−) was carried out under visible light irradiation and pure O2 atmosphere. The W10O324− was found to be active to this photo-oxygenation in a pure acetonitrile (MeCN), which can achieve ca. 8.1% cyclohexane conversion with ca. 64.3% selectivity for cyclohexanone under sustained visible light irradiation of 12h. Notably, the above-described photo-catalysis oxygenation was improved to some extent in the presence of some acidic additives such as10M HCl, H2SO4, or H3PO4 aqueous solution and benzenesulfonic acid, providing ca. 12.4–14.3% cyclohexane conversion with ca. 59.7–69.4% cyclohexanone selectivity. Moreover, this acid-promoted effect was further magnified in the case of adding a suitable amount of water, which can lead to enhancing conversion (16.8–20.0%) and improving cyclohexanone selectivity (68.2–78.3%). This acidic aqueous solution-promoted effect was also observed upon the W10O324−-photocatalyzed oxygenation of other substrates like toluene, ethylbenzene and butanone. This is likely due to the acidic aqueous solutions playing important roles in improving photo-redox cycling of W10O324− and preserving its stability, as supported by the UV–vis spectra and cyclic voltammetry measurements.