Tungstoborates as Highly Active Catalysts for Cycloalkane Oxygenation Using Hydrogen Peroxide

Abstract

Keggin-type tungstoborates [BW₁₂O₄₀]^5-, H_x[BW₁₁O₃₉]^9-x)- and [BM(H₂O)W₁₁O₃₉]^6-, M = Fe^III, Mn^III or Ru^III proved to be highly active catalysts for the H₂O₂ oxidation of cyclooctane and cyclohexane to the corresponding ketone, alcohol and alkyl hydroperoxide derivatives. High turnover numbers and high selectivity for the hydroperoxide were observed.

polyoxometalates - cycloalkanes - oxidation - hydrogen peroxide - tungstoborates

References

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The reactions were typically carried out by heating a solution of 1 mmol of the cycloalkane and 1.5 µmol of the catalyst in 1.5 mL of acetonitrile at 80 ºC. The oxidant used was 30% aqueous H₂O₂. Aliquots were withdrawn from the reaction mixture and injected directly into a GC-MS (fused silica Supelco capillary column, SPB-5, with 30 m × 0.25 mm i.d.; 0.25 µm film thickness). The percentages of each compound in the reaction mixture were estimated directly from the corresponding chromatographic peak areas.

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<A NAME="RD15703ST-25">25</A>

For BFe an aqueous solution of BW₁₁ was added to a solution of Fe(NO₃)₃·9H₂O (order of addition reversed in comparison with BMn). TBA₄H₂BFe(H₂O)W₁₁O₃₉·H₂O Yield, 91%. Anal. Found: W, 55.0; Fe, 1.48; C, 20.53; H, 4.09, N, 1.56; hydration H₂O, 0.51. Calcd: W, 54.4; Fe, 1.50; C, 20.60; H, 4.03, N, 1.50; hydration H₂O, 0.48; IR (cm^-1): 997 (m), 956(vs), 900(vs), 825(vs), 757 (s, sh), 528 (m). For preparation of BRu RuCl₃·H₂O was used. TBA₄H₂BRu(H₂O)W₁₁O₃₉·2H₂O Yield, 89%. Anal. Found: W, 54.2; Ru, 2.56; C, 20.58; H, 4.11, N, 1.72; hydration H₂O, 1.02. Calcd: W, 53.5; Ru, 2.69; C, 20.30; H, 4.02, N, 1.48; hydration H₂O, 0.95; IR (cm^-1): 998 (m), 950 (vs), 902 (vs), 825 (vs), 743 (s, sh), 530 (m). µ = 1.99 µ_B.

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<A NAME="RD15703ST-27">27</A>

The decomposition of H₂O₂ during the reaction was monitored by titration of aliquots with ceric sulphate. At the end of the reactions, the total of unused H₂O₂ and hydroperoxide produced was estimated by titration with ceric sulphate. From the yields of hydroperoxide (and cyclohexanol and cyclohexanone) determined by gas chromatography, the amount of H₂O₂ used in the reaction could be determined.

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