Synergistic effects of CHCOH and Ca on C-H bond activation by MnO.

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