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 [FeIII(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 [FeV(O)(dpaq)]2+ from proton-coupled electron-transfer (PCET) oxidation of the iron(III) aqua complex [FeIII(dpaq)(H2O)]2+ was evidenced by cyclic voltammetry analysis; the iron(V)-oxo species [FeV(O)(dpaq)]2+ was recently detected using transient absorption spectroscopy in water oxidation reactions. Mechanistic studies revealed that electrochemical oxidation of alcohols catalyzed by FeIII(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.