Lewis-acids-promoted Oxidations By Maganese Complexes

Redox-inactive metal ions have been recognized to be able to participate in redoxmetal-ion-mediated biological and chemical oxidative events; however, their roles are stillelusive. This work presents how the redox inactive metal ions affect the oxidativereactivity of a well-investigated manganese(II) having a cross-bridged cyclam ligand. Indry acetone, the presence of these metal ions can greatly accelerate stoichiometricoxidations of triphenylphosphine and sulfides by the manganese(IV) complexes throughelectron transfer or catalytic sulfoxidations by the corresponding manganese(II)complexes with PhIO. Significantly, the rate enhancements are highly Lewis-acid strengthdependent on added metal ions. These metal ions like Al3+can also promote thethermodynamic driving force of the Mn(IV) OH moiety to facilitate its hydrogenabstraction from ethylbenzene having a BDEC-Hvalue of85kcal/mol, while it isexperimentally limited to80kcal/mol for Mn(IV) OH alone. Adding Al3+may alsoimprove the manganese(II)-catalyzed olefin epoxidation with PhIO. The existence of theinteraction between Lewis acid and the manganese(IV) species was evidenced by UV-Visabsorbance and EPR signal of the manganese(IV) species. Cyclic voltammograms of themanganese(IV) complexes reveal that adding Lewis acid would substantially shift itspotential to the positive direction, thus enhancing its oxidizing capability.