Catalytic Properties and Stability of the Heteropolytungstate [P 2 W 21 O 71 (H 2 O) 3 ] 6– in H 2 O 2 ‐Based Oxidations

Catalytic properties of the sandwich‐type heteropolytungstate [P 2 W 21 O 71 (H 2 O) 3 ] 6– ( 1 ) in the selective oxidation of three representative organic substrates, methyl phenyl sulfide, cyclohexene, and cyclohexanol, with aqueous H 2 O 2 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 H 2 O 2 ‐based oxidations in MeCN is similar to that of the Keggin heteropolyanion [PW 12 O 40 ] 3– (PW 12 ) and differs significantly from the performance of the Venturello complex {PO 4 [W(O)(O 2 ) 2 ] 4 } 3– . The stability of 1 and PW 12 towards solvolytic destruction under turnover conditions (50–100 equiv. H 2 O 2 , 25–70 °C, 2–72 h) was confirmed by using 31 P NMR, IR, and Raman spectroscopic techniques.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)