Light-Driven C-H Oxygenation of Methane with Chlorine Dioxide
Authors: Ohkubo Kei 
Published: December 2020

Extensive efforts have been devoted towards the development of methods for the direct conversion from methane (CH4), ethane (CH3CH3), or other abundant natural gasses into useful products, such as the corresponding alcohols, aldehydes, ketones, and carboxylic acids, as liquid fuels and precursors of chemical and pharmaceutical products. Selective aerobic oxygenation of CH4 into liquid products without the concomitant formation of CO2 and CO has served as an elusive target reaction. The one-step transformation of CH4 into methanol (CH3OH) is carried out in nature using methane monooxygenases. However, under chemical conditions, the selective oxygenation of CH4 to CH3OH with molecular oxygen (O2) has been unknown because the oxidation of oxygenated products, CH3OH and formic acid (HCOOH) is much easier than that of CH4, leading to over-oxidation products such as CO and CO2.

Here we show that chlorine dioxide radical (ClO2
•) acts as an efficient oxidizing agent in the selective oxygenation of methane under photoirradiation. A fluorous solvent, perfluorohexane (PFH, n-CF3(CF2)4CF3) was chosen as an ideal solvent for CH4 oxygenation for the following reasons. First, PFH is more inert than CH4. It is only comprised of strong C–F bonds and does not contain any C–H bonds; therefore, PFH does not react with CH3
• and Cl• intermediates in the oxygenation of CH4. Notably, PFH can dissolve gaseous substrates such as CH4, CH3CH3, and O2 very well. Additionally, oxygenated products such as CH3OH and HCOOH, as well as water, are insoluble in PFH. Thus, if a two-phase PFH/water system was used for the oxygenation of CH4, the reaction would occur in the fluorous phase, and the products would be transferred into the aqueous phase without further oxygenation to CO and CO2. Towards that end, herein, we report the two-phase photooxygenation of CH4 by molecular oxygen (O2) with chlorine dioxide radical (ClO2
•) in PFH/H2O under ambient conditions to produce oxygenated products such as CH3OH and HCOOH.

The oxygenation of CH4 (1.0 mM) with ClO2
• (1.0 mM) did not proceed in the dark. In contrast, the photochemical oxygenation of CH4 with O2 occurred to form CH3OH and HCOOH under photoirradiation with a xenon lamp (500 W) in an aqueous solution at 298 K and 1 atm for 2 h. The yields of CH3OH and HCOOH were 5% and 25%, respectively, as determined by 1H NMR and gas chromatography-mass (GC-MS) spectroscopies. The conversion of CH4 was determined to be 30% by 1H NMR. The selectivity of the formation of CH3OH and HCOOH was >99%, based on the consumption of CH4.2

Further improvement of the product yield by employing a two-phase system instead by one-phase aqueous system, a CH4-saturated solution of PFH (1.0 mL) was added to an oxygen-saturated aqueous solution containing ClO2
• (1.0 mM) to prepare a two-phase system; oxygenation of CH4 (1.0 mM) in an aqueous liquid-liquid two-phase solution (1:1 v/v) comprised of PFH and distilled water was carried out under the same conditions. CH4 dissolved in the aqueous and fluorous phases was completely consumed following photoirradiation for 15 min. The product yields of CH3OH and HCOOH were improved to 14% and 85%, respectively, as determined by the 1H NMR analyses of the aqueous phase. The selectivity of the CH3OH and HCOOH formation was >99%, based on the initial concentration of CH4. The formation of further oxygenated products such as CO and CO2, was not observed under the present reaction conditions. The quantum yield of the photo-driven CH4 oxygenation was130%, as determined by actinometric measurements using monochromatized light. When CH4 was replaced with ethane (CH3CH3), light-driven oxygenation also occurred to yield CH3CH2OH (19%) and CH3COOH (78%) under the otherwise same reaction conditions. The photochemical oxygenation of methane is initiated by generation of chlorine radical and singlet oxygen from photoexcited state of ClO2
•, leading to the final products by aerobic radical chain processes. Thus, the present study provides an environmentally benign approach towards the photooxidation of organic compounds. The photochemical oxygenation using ClO2
• reported herein could be generalized to provide novel chemical reactions, which may have significant implications in synthetic, pharmaceutical and polymer chemistry.3

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