Oxidation of cyclic ketones by cerium(IV) in presence of iridium(III) chloride

Interestingly IrCl3, which is considered to be a sluggish catalyst in alkaline media, was found to surpass the catalytic efficiency of even osmium and ruthenium in acidic media. Kinetic data, in iridium(III) chloride catalyzed oxidation of cyclopentanone and cycloheptanone by cerium(IV) perchlorate in aqueous perchloric acid medium, suggest the formation of complex C1 between cerium(IV) and organic substrate in the first equilibrium step, which in turn gives rise to another complex C2 with the catalyst. This second complex in the rate-determining step gives rise to the intermediate products. Rate decreases in the beginning at low acid concentrations, but after reaching to a minimum it becomes directly proportional to acid concentrations. Probably on increasing the acid concentration hydrolyzed species of ceric perchlorate gradually converts into the un-hydrolyzed species, which then accelerates the rate at higher [H+] resulting in the observed peculiar effect of hydrogen ions on the rate. Initial concentrations of cerium(IV) and acid determine the extent of reduction of cerium(IV) by water. Order of the reaction shows direct proportionality with respect to the low concentrations of oxidant and ketone, but tends to become zeroth order at their higher concentrations. Rate of the reaction shows direct proportionality with respect to [IrCl3] while change in ionic strength of the medium does not affect the reaction velocity. 2-Hydroxy cycloheptanone was confirmed spectrophotometrically as a reaction intermediate in the case of cycloheptanone. Energy of activation, free energy of activation and entropy parameters suggest that cyclopentanone forms the activated complex more easily compared to cycloheptanone.

Graphical abstractIrCl3 in acidic medium was found more effective catalyst than RuCl3 or OsO4 in the oxidation of cyclic ketones by ceric perchlorate. Conversion of hydrolyzed to un-hydrolyzed species of cerium with increasing acid concentrations, its reduction by water and spectral evidence for 2-hydroxy ketone as intermediate in the oxidation was verified.Figure optionsDownload full-size imageDownload as PowerPoint slide