1-Methyl-3-butylimidazolium Decatungstate in Ionic Liquid: An Efficient Catalyst for the Oxidation of Alcohols

Abstract

An excellent system for the selective oxidation of ­benzylic and secondary alcohols with hydrogen peroxide catalyzed by 1-methyl-3-butylimidazolium decatungstate in [bmim][BF₄] ionic liquid is described. The catalytic system is reusable and products are obtained in excellent yield under environmentally benign conditions.

Key words

ionic liquid - oxidation - sodium tungstate - 1-methyl-3-butylimidazolium decatungstate

References

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15

Synthesis of Catalyst [bmim] ₄ [W ₁₀ O ₂₃ ]: The boiling solutions of Na₂WO₄·2H₂O (16 g) in 100 mL H₂O and 35 mL of 3 M HCl were mixed in a Erlenmeyer flask and further heated for 5 min to get yellow solution. To this was added 1-methyl-3-butylimidazolium bromide [bmim][Br] (4.4 g) in 50 mL H₂O with stirring. A puff greenish white precipitate separates out instantly. Washed the solid with H₂O thoroughly and then with MeOH (50 mL) and Et₂O (50 mL). The solid was dried under air at r.t. to obtain puff white product.

16

Representative Procedure for the Oxidation of Alcohols: Pre-saturated catalyst [bmim]₄[W₁₀O₂₃] (100 mg) was dissolved in [bmim][BF₄] ionic liquid (2 mL) by heating. Then added diphenyl carbinol (184 mg, 10 mmol) and heated the reaction mixture to 90 °C on oil bath, followed by drop-wise addition of hydrogen peroxide 30% solution (172 µL) with fast stirring. Then, 10 µL of reaction mixture were taken out at different time interval, extracted with Et₂O and monitored by TLC and GC. The conversion percentage and purity of product was checked by gas chromatography (Table [2] ). On completion of reaction, the reaction mixture was cooled to r.t. and extracted with Et₂O (3 × 3 mL). Finally, the solvent was evaporated under reduced pressure to obtain diphenyl ketone (175 mg, 96% yield).