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

Abstract

The rate of oxidation of 2,5-dimethoxybenzyl alcohol (2,5-(MeO)(2)C(6)H(3)CH(2)OH) 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 Sc(3+)). Such a remarkable enhancement of the reactivity of [Fe(IV)(O)(N4Py)](2+) in the presence of Sc(3+) was accompanied by the disappearance of a kinetic deuterium isotope effect. The radical cation of 2,5-(MeO)(2)C(6)H(3)CH(2)OH was detected in the course of the reaction in the presence of Sc(3+). The dimerized alcohol and aldehyde were also produced in addition to the monomer aldehyde in the presence of Sc(3+). These results indicate that the reaction mechanism is changed from one-step hydrogen atom transfer (HAT) from 2,5-(MeO)(2)C(6)H(3)CH(2)OH to [Fe(IV)(O)(N4Py)](2+) in the absence of Sc(3+) to stepwise Sc(3+)-coupled electron transfer, followed by proton transfer in the presence of Sc(3+). In contrast, neither acceleration of the rate nor the disappearance of the kinetic deuterium isotope effect was observed in the oxidation of benzyl alcohol (C(6)H(5)CH(2)OH) 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 Sc(3+)-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 Sc(3+)-coupled electron transfer and proton transfer is dependent on the one-electron oxidation potential of benzyl alcohol derivatives (ca. 1.7 V vs SCE).