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
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Synthesis of Catalyst [bmim]
4
[W
10
O
23
]: The boiling solutions of Na2 WO4 ·2H2 O (16 g) in 100 mL H2 O 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 H2 O with stirring. A puff greenish white precipitate separates out instantly. Washed the solid with H2 O thoroughly and then with MeOH (50 mL) and Et2 O (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 [W10 O23 ] (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 Et2 O and monitored by TLC and GC. The conversion percentage and purity of product was checked by gas chromatography (Table
[2 2 O (3 × 3 mL). Finally, the solvent was evaporated under reduced pressure to obtain diphenyl ketone (175 mg, 96% yield).
