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

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.