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
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Standard Experimental
Procedure for KA Oil Oxidation
5 (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 CH2 Cl2 and filtered from Oxone® inorganic salts.
Next, the filtrate was concentrated. The IL was then extracted with
Et2 O (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%.
