NaCl-Promoted Cobalt-Catalyzed Dioxygen-Mediated Methane Oxidation to Methylene Bis(trifluoroacetate) with a Dramatic Salt Effect

Abstract

Although O2 is an ideal oxidant in the transformation of methane, very limited studies realize this process in homogeneous solvent systems with high efficiency under mild conditions. Herein, we present a NaCl-promoted cobalt-catalyzed O2-mediated methane oxidation with turnovers over 700 at 120 °C with a dramatic salt effect. The unexpected methylene bis(trifluoroacetate) can be produced with a selectivity up to 90%. This product is used in the reaction with TsNH2 and 1,2-diimine to efficiently form triazinane and imidazole salt, respectively. The Co-NaCl catalytic system is also applied in the oxidation of other hydrocarbons such as adamantane, 9,10-dihydrophenanthrene, 9,10-dihydroanthracene, and 9,10-dihydroacridine. Mechanistic studies indicate that NaCl can promote the decomposition of trifluoroacetic anhydride (TFAA) in the presence of Co(II), and a highly reactive fluorine radical might be formed to react with methane, generating a methyl radical, which will further react with O2 and then with trifluoroacetic anhydride to form methylene bis(trifluoroacetate).

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

“…Of particular interest was the possibility that photoexcited DT could also generate chlorine atoms, Cl•, under these reaction conditions due to the very high reduction potential of DT•• . There have been a number of recent advances in the photogeneration of Cl•, including photoreduction of metal chlorides ,,,,− and photoredox oxidation of chloride ion (Cl – ). ,− As noted above, the generation of chlorine has been used to functionalize unactivated hydrocarbons. − , DT is robust under acidic conditions and has a high quantum yield for photoexcitation, but it has not been employed for the generation of Cl• as a C–H activation mediator. Herein, we report experimentally and validate computationally the use of DT as a photocatalyst for methane partial oxidation in the presence of chloride and iodine to form methyl trifluoroacetate (MeTFA) and methyl chloride (MeCl or CH 3 Cl).…”

“…42,84−86 As noted above, the generation of chlorine has been used to functionalize unactivated hydrocarbons. [42][43][44][45][46]87 DT is robust under acidic conditions and has a high quantum yield for photoexcitation, but it has not been employed for the generation of Cl• as a C−H activation mediator. Herein, we report experimentally and validate computationally the use of DT as a photocatalyst for methane partial oxidation in the presence of chloride and iodine to form methyl trifluoroacetate (MeTFA) and methyl chloride (MeCl or CH 3 Cl).…”

Pyridine N-Oxide-Promoted Cobalt-Catalyzed Dioxygen-Mediated Methane Oxidation

“…18,20 We recently reported a NaCl-promoted cobalt-catalyzed methane oxidation with O 2 as the oxidant to form methylene bis(trifluoroacetate) (MBTFA), which could be isolated by distillation and used in the synthesis of triazinane and imidazole salt (Scheme 1B). 21 The reaction is very efficient with productivity around 90 mol ester mol metal −1 h −1 but became violent when the temperature reached 140 °C. In this reaction, chloride can promote the decomposition of TFAA into trifluoromethyl radical (CF 3…”

“…• ) followed by a highly reactive fluoride radical in the presence of Co(II). 21 A more efficient catalytic system for generating CF 3…”

“…It should be noted that more TFAA in the reaction would increase the efficiency (entries 16 and 17), and the reactions proceeded smoothly, which was different from our previous study. 21 Too much TFAA (more than 25/30) will inhibit the reaction, probably because of the low solubility of trifluoroacetoxy pyridinium. When the temperature was increased to 140 °C, the yield of MBTFA increased to 11.38 mmol with almost the same selectivity of MBTFA.…”