Catalytic Properties and Stability of the Heteropolytungstate [P2W21O71(H2O)3]6– in H2O2‐Based Oxidations

Abstract

AbstractCatalytic properties of the sandwich‐type heteropolytungstate [P2W21O71(H2O)3]6– (1) in the selective oxidation of three representative organic substrates, methyl phenyl sulfide, cyclohexene, and cyclohexanol, with aqueous H2O2 have been studied in acetonitrile. With one equivalent of hydrogen peroxide, methyl phenyl sulfide readily produced the corresponding sulfoxide with 92–94 % selectivity at 95–97 % conversion. The substituent effect on the oxidation rates of the set of aryl methyl sulfides followed the Hammett free‐energy relationship (ρ = –0.85). The catalytic activity of heteropolyacid H‐1 in thioether oxidation was several times higher than the activity of its tetrabutylammonium salt, TBA‐1. In contrast, H‐1 was completely inactive in the oxidation of cyclohexanol, while TBA‐1 catalyzed this reaction effectively to give cyclohexanone with 94 % yield. Neither H‐1 nor TBA‐1 was active in the oxidation of cyclohexene. The catalytic performance of 1 in H2O2‐based oxidations in MeCN is similar to that of the Keggin heteropolyanion [PW12O40]3– (PW12) and differs significantly from the performance of the Venturello complex {PO4[W(O)(O2)2]4}3–. The stability of 1 and PW12 towards solvolytic destruction under turnover conditions (50–100 equiv. H2O2, 25–70 °C, 2–72 h) was confirmed by using 31P NMR, IR, and Raman spectroscopic techniques.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)