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) ₂C₆H₃CH₂OH) 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)₂C₆H₃CH₂OH 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)₂C₆H₃CH₂OH 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 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 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).