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 (pK(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.