Synergistic effects of CH₃CO₂H and Ca²⁺ on C-H bond activation by MnO₄

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 CH₃CO₂H and a weak Lewis acid such as Ca²⁺ on C-H bond activation by KMnO₄ have been investigated. Although MnO₄ ^- is rather non-basic (pK _a of MnO₃(OH) = -2.25), it can be activated by AcOH or Ca²⁺ to oxidize cyclohexane at room temperature to give cyclohexanone as the major product. A synergistic effect occurs when both AcOH and Ca²⁺ are present; the relative rates for the oxidation of cyclohexane by MnO₄ ^-/AcOH, MnO₄ ^-/Ca²⁺ and MnO₄ ^-/AcOH/Ca²⁺ are 1 : 73 : 198. DFT calculations show that in the active intermediate of MnO₄ ^-/AcOH/Ca²⁺, MnO₄ ^- is H-bonded to 3 AcOH molecules, while Ca²⁺ is bonded to 3 AcOH molecules as well as to an oxo ligand of MnO₄ ^-. 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.