Synergistic effects of CH 3 CO 2 H and Ca 2+ on C-H bond activation by MnO 4 .

The activation of metal-oxo species with Lewis acids is of current interest. In this work, the effects of a weak Brønsted acid such as CH 3 CO 2 H and a weak Lewis acid such as Ca 2+ on C-H bond activation by KMnO 4 have been investigated. Although MnO 4 - is rather non-basic (p K a of MnO 3 (OH) = -2.25), it can be activated by AcOH or Ca 2+ to oxidize cyclohexane at room temperature to give cyclohexanone as the major product. A synergistic effect occurs when both AcOH and Ca 2+ are present; the relative rates for the oxidation of cyclohexane by MnO 4 - /AcOH, MnO 4 - /Ca 2+ and MnO 4 - /AcOH/Ca 2+ are 1 : 73 : 198. DFT calculations show that in the active intermediate of MnO 4 - /AcOH/Ca 2+ , MnO 4 - is H-bonded to 3 AcOH molecules, while Ca 2+ is bonded to 3 AcOH molecules as well as to an oxo ligand of MnO 4 - . Our results also suggest that these synergistic activating effects of a weak Brønsted acid and a weak Lewis acid should be applicable to a variety of metal-oxo species.