An Efficient Tandem Oxidation of Cyclohexanol to ε-Caprolactone with Peroxyacids and TEMPO Catalyst in Ionic Liquids as Solvents

Abstract

The new one-pot tandem oxidation of cyclohexanol to ε-caprolactone with potassium peroxomonosulfate or m-chloroper­oxybenzoic acid as oxidation agents and ionic liquids as solvents is described. A 2,2,6,6-tetramethylpiperidine-1-oxyl radical with tetrabutylammonium bromide as the co-catalyst was used. A solution of KHSO₅ in the ionic liquid 1-butyl-3-methylimidazolium tetra­fluoroborate [bmim]BF₄ facilitates the tandem oxidation of alcohol to lactone. In classic solvents, this reaction can only be carried out to the ketone formation step. This is most probably due to the ability of [bmim]BF₄ to dissolve both alcohol and KHSO₅. In the case of using m-chloroperoxybenzoic acid, ionic liquids provide an efficient medium for this reaction. This new method enables ε-caprolactone formation with high yields (75-80%).

Key words

ionic liquids - oxidation - alcohols - tandem reaction - lactones

References and Notes

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13

Standard Experimental Procedure for KA Oil Oxidation
Into the round-bottom flask cyclohexanol (1 mmol), cyclohexanone (0.5 mmol), TEMPO (0.03-0.05 mmol), TBAB (0.01-0.03 mmol), MCPBA or KHSO₅ (5 mmol) and IL (5 mL) were introduced at r.t. The reaction mixture turned orange upon addition of the peroxy acid. The reaction mixture was stirred at 40 ˚C for 5-10 h. The progress of the reaction was monitored by GC. After this time, the orange colour had faded away, and the post reaction mixture was dissolved in CH₂Cl₂ and filtered from Oxone^® inorganic salts. Next, the filtrate was concentrated. The IL was then extracted with Et₂O (6 × 5 mL); the extract was washed with a sat. aq NaHCO₃ to remove all residual peroxy compounds and was concentrated. The yields of ε-caprolactone after the purification by column chromatography with hexane-EtOAc (4:1) as the eluent were 75-80%.