Partial Oxidation of Methane Enabled by Decatungstate Photocatalysis Coupled to Free Radical Chemistry

Abstract

The decatungstate anion, [W10O32]4– or DT, is a useful photocatalyst for organic transformations involving C–H functionalization. Herein, we leverage the unique photoredox properties of DT to generate a chlorine radical from chloride ion for the photochemical partial oxidation of methane. Under optimized conditions, the DT–chloride–iodine ensemble achieves methane to methyl trifluoroacetate conversion with >350 photocatalyst turnovers at ∼60% yield based on methane in trifluoroacetic acid solvent. Methyl trifluoroacetate exhibits excellent stability under reaction conditions with minimal amounts of degradation (<6%) detected after 41 h. Based on density functional theory calculations, we propose a mechanism that involves synergistic relationships among the DT, chloride, and iodine species with the following key features: (1) photoredox electron transfer reaction of DT with Cl– to generate Cl•, (2) reaction of photoexcited DT with methane to generate methyl radicals via net hydrogen atom abstraction, (3) a Cl/I radical-based pathway in which methane is converted to MeTFA, and (4) reoxidation of reduced DT species by dioxygen. This mechanism takes advantage of the unique redox potential of DT and the ability of DT to mediate both electron transfer and hydrogen atom transfer reactions, ultimately generating an efficient pathway for aerobic methane partial oxidation.