Electrochemically driven nonheme iron complex-catalyzed oxidation reactions using water as an oxygen source

Abstract

High-valent metal-oxo species have been invoked as key intermediates in enzymatic and biomimetic oxidation reactions. The generation of high-valent metal-oxo species using water (H₂O) as an oxygen source represents one of the most environmentally friendly approaches in developing biologically inspired oxidation catalysis. Herein, we report the electrochemical oxidation of benzylic C−H bonds and alcohols utilizing a mononuclear nonheme iron(III)-monoamidate complex [Fe^III(dpaq)(H₂O)]²⁺ (dpaq = 2-[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-yl-acetamidate) as a catalyst and H₂O as an oxygen source. Selective benzylic C−H bond oxidation of alkanes to ketones was achieved in 43–85 % yields, and primary and secondary alcohols were converted to the corresponding aldehydes and ketones, respectively, in 46–95 % yields. The generation of an iron(V)-oxo species [Fe^V(O)(dpaq)]²⁺ from proton-coupled electron-transfer (PCET) oxidation of the iron(III) aqua complex [Fe^III(dpaq)(H₂O)]²⁺ was evidenced by cyclic voltammetry analysis; the iron(V)-oxo species [Fe^V(O)(dpaq)]²⁺ was recently detected using transient absorption spectroscopy in water oxidation reactions. Mechanistic studies revealed that electrochemical oxidation of alcohols catalyzed by Fe^III(dpaq) is a two-electron oxidation process, hydrogen-atom transfer (HAT) from the α-C−H bond of alcohols by iron(V)-oxo species is the rate-determining step, and there is a remarkable charge transfer from the highly electrophilic iron(V)-oxo species to the alcohols in the HAT step. This research paves a significant groundwork aimed at developing electrochemically driven biomimetic asymmetric oxidation reactions catalyzed by nonheme metal complexes supported by chiral ligands.