Synergistic activating effects of a strong Lewis acid (metal ion) and a strong BrØnsted acid enable KMnO4 to oxidize CH4 at room temperature

Abstract

The search for reagents that can oxidize methane (CH₄) under mild conditions has long been a challenge for chemists. In this work, we report the synergistic effects of a strong Lewis acid Sc(OTf)₃ and a strong Bronsted acid CF₃CO₂H (TFA) to activate KMnO₄ towards the oxidation of CH₄ at room temperature. KMnO₄ in the presence of 0.5-2 mol equiv. of Sc(OTf)₃ in TFA/TFAA (1: 4 by vol., TFAA = trifluoroacetic anhydride) is able to oxidize CH₄ (30 atm) at 22 °C to produce methyl trifluoroacetate (CF₃CO₂CH₃) in 17±2 % yield (based on KMnO₄). The yield is increased to 34% when the temperature is raised to 40 °C. No product is observed using Sc(OTf)₃ alone, and only 2% of methyl trifluoroacetate is produced using TFA alone. A kinetic isotope effect of 2.2 is found using a mixture of CH₄ and CD₄ as substrate. The use of BrCCl₃ as a radical trap results in the formation of BrCH₃, indicating that CH₃• radical is an intermediate in CH₄ oxidation. These results suggest the Sc(OTf)₃ and TFA form an active intermediate with MnO₄−, which undergoes hydrogen-atom abstraction (HAT) from CH₄ to give CH₃•, which is further oxidized to CH₃OH. CH₃OH is prevented from further oxidation by trapping with TFAA to produce CF₃CO₂CH₃. DFT calculations show that Sc(OTf)₂⁺ and CF₃CO₂H combine together with MnO₄− to produce an active intermediate that undergoes HAT with CH₄ with a low barrier of 12.5 kcal mol−¹.