Mechanistic Borderline of One-Step Hydrogen Atom Transfer versus Stepwise Sc3+-Coupled Electron Transfer from Benzyl Alcohol Derivatives to a Non-Heme Iron(IV)-Oxo Complex

The rate of oxidation of 2,5-dimethoxybenzyl alcohol (2,5-(MeO)(2)C6H3CH2OH) by [Fe-IV(O)(N4Py)](2+) (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)-methylamine) was enhanced significantly in the presence of Sc(OTf)(3) (OTf- = trifluoromethanesulfonate) in acetonitrile (e.g., 120-fold acceleration in the presence of Sc3+). Such a remarkable enhancement of the reactivity of [Fe-IV(O)(N4Py)](2+) in the presence of Sc3+ was accompanied by the disappearance of a kinetic deuterium isotope effect. The radical cation of 2,5-(MeO)(2)C6H3CH2OH was detected in the course of the reaction in the presence of Sc3+. The dimerized alcohol and aldehyde were also produced in addition to the monomer aldehyde in the presence of Sc3+. These results indicate that the reaction mechanism is changed from one-step hydrogen atom transfer (HAT) from 2,5-(MeO)(2)C6H3CH2OH to [Fe-IV(O)(N4Py)](2+) in the absence of Sc3+ to stepwise Sc3+-coupled electron transfer, followed by proton transfer in the presence of Sc3+. In contrast, neither acceleration of the rate nor the disappearance of the kinetic deuterium isotope effect was observed in the oxidation of benzyl alcohol (C6H5CH2OH) by [Fe-IV(O)(N4Py)(2+) in the presence of Sc(OTf)(3). Moreover, the rate constants determined in the oxidation of various benzyl alcohol derivatives by [Fe-IV(O)(N4Py)](2+) in the presence of Sc(OTf)(3) (10 mM) were compared with those of Sc3+-coupled electron transfer from one-electron reductants to [Fe-IV(O)(N4Py)](2+) at the same driving force of electron transfer. This comparison revealed that the borderline of the change in the mechanism from HAT to stepwise Sc3+-coupled electron transfer and proton transfer is dependent on the one-electron oxidation potential of benzyl alcohol derivatives (ca. 1.7 V vs SCE).