Light-Driven C-H Oxygenation of Methane with Chlorine Dioxide (2020)

Extensive efforts have been devoted towards the development of methods for the direct conversion from methane (CH 4 ), ethane (CH 3 CH 3 ), 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 CH 4 into liquid products without the concomitant formation of CO 2 and CO has served as an elusive target reaction. The one-step transformation of CH 4 into methanol (CH 3 OH) is carried out in nature using methane monooxygenases. However, under chemical conditions, the selective oxygenation of CH 4 to CH 3 OH with molecular oxygen (O 2 ) has been unknown because the oxidation of oxygenated products, CH 3 OH and formic acid (HCOOH) is much easier than that of CH 4 , leading to over-oxidation products such as CO and CO 2 . Here we show that chlorine dioxide radical (ClO 2 • ) acts as an efficient oxidizing agent in the selective oxygenation of methane under photoirradiation. A fluorous solvent, perfluorohexane (PFH, n -CF 3 (CF 2 ) 4 CF 3 ) was chosen as an ideal solvent for CH 4 oxygenation for the following reasons. First, PFH is more inert than CH 4 . It is only comprised of strong C–F bonds and does not contain any C–H bonds; therefore, PFH does not react with CH 3 • and Cl • intermediates in the oxygenation of CH 4 . Notably, PFH can dissolve gaseous substrates such as CH 4 , CH 3 CH 3 , and O 2 very well. Additionally, oxygenated products such as CH 3 OH and HCOOH, as well as water, are insoluble in PFH. Thus, if a two-phase PFH/water system was used for the oxygenation of CH 4 , the reaction would occur in the fluorous phase, and the products would be transferred into the aqueous phase without further oxygenation to CO and CO 2 . Towards that end, herein, we report the two-phase photooxygenation of CH 4 by molecular oxygen (O 2 ) with chlorine dioxide radical (ClO 2 • ) in PFH/H 2 O under ambient conditions to produce oxygenated products such as CH 3 OH and HCOOH. The oxygenation of CH 4 (1.0 mM) with ClO 2 • (1.0 mM) did not proceed in the dark. In contrast, the photochemical oxygenation of CH 4 with O 2 occurred to form CH 3 OH 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 CH 3 OH and HCOOH were 5% and 25%, respectively, as determined by 1 H NMR and gas chromatography-mass (GC-MS) spectroscopies. The conversion of CH 4 was determined to be 30% by 1 H NMR. The selectivity of the formation of CH 3 OH and HCOOH was &gt;99%, based on the consumption of CH 4 . 2 Further improvement of the product yield by employing a two-phase system instead by one-phase aqueous system, a CH 4 -saturated solution of PFH (1.0 mL) was added to an oxygen-saturated aqueous solution containing ClO 2 • (1.0 mM) to prepare a two-phase system; oxygenation of CH 4 (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. CH 4 dissolved in the aqueous and fluorous phases was completely consumed following photoirradiation for 15 min. The product yields of CH 3 OH and HCOOH were improved to 14% and 85%, respectively, as determined by the 1 H NMR analyses of the aqueous phase. The selectivity of the CH 3 OH and HCOOH formation was &gt;99%, based on the initial concentration of CH 4 . The formation of further oxygenated products such as CO and CO 2 , was not observed under the present reaction conditions. The quantum yield of the photo-driven CH 4 oxygenation was130%, as determined by actinometric measurements using monochromatized light. When CH 4 was replaced with ethane (CH 3 CH 3 ), light-driven oxygenation also occurred to yield CH 3 CH 2 OH (19%) and CH 3 COOH (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 ClO 2 • , 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 ClO 2 • reported herein could be generalized to provide novel chemical reactions, which may have significant implications in synthetic, pharmaceutical and polymer chemistry. 3 Ohkubo, K.; Hirose, K.; Shibata, T.; Takamori, K.; Fukuzumi, S. Phys. Org. Chem. 2017 , 30 , e3619. Ohkubo, K.; Hirose, K. Chem. Int. Ed. 2018, 57 , 2126-2129. Ohkubo, K.; Asahara, H.; Inoue, T. Chem. Commun. 2019 , 55 , 4723-4726.