Mechanistic insights into methane oxidation by molecular oxygen under photoirradiation: Controlled radical chain reactions

The selective oxidation of CH₄ using O₂ is one of the most attractive subjects as an elusive target reaction. Ohkubo and Hirose recently reported that chlorine dioxide radical (ClO₂^•), which is generated by mixing NaClO₂ and HCl in an aqueous solution, acts as an efficient oxidant in the oxidation of CH₄ to CH₃OH and HCOOH under photoirradiation in the two-phase system of perfluorohexane and water (Angew. Chem., Int. Ed. 2018, 57, 2126). The reaction system gives CH₃OH and HCOOH without further oxidation products. They proposed that methoxy radical (CH₃O^•) plays an important role as an intermediate in the oxidation of CH₄. In the present work, we focus on the reactivity of CH₃O^• to CH₄ in detail to propose a reasonable radical mechanism for the oxidation of CH₄ using DFT calculations at the M06-2X/6-311+G** level of theory and UCCSD(T)/6-311+G** calculations. Our reaction analysis suggests that the reaction of CH₃O^• with CH₄ and the disproportionation of CH₃O^• take place as CH₄ + CH₃O^• ¼ CH₃^• + CH₃OH and 2CH₃O^• ¼ CH₃OH + HCHO, respectively. In contrast, the isomerization from CH₃O^• to CH₂^•(OH), suggested by Ohkubo and Hirose, is unlikely to occur under ambient conditions, due to the high activation barrier for this reaction. A better understanding of the well-controlled radical chain reactions is useful for reaction design of the hydroxylation of methane.