Unexpected, Latent Radical Reaction of Methane Propagated by Trifluoromethyl Radicals

Unexpected, Latent Radical Reaction of Methane Propagated by Trifluoromethyl Radicals
Nima Zargari,† Pierre Winter,‡ Yong Liang,§ Joo Ho Lee,† Andrew Cooksy,*,‡ K. N. Houk,*,§ and Kyung Woon Jung*,†
†Loker Hydrocarbon Research Institute, Department of Chemistry, University of Southern California, 837 Bloom Walk, Los Angeles, California 90089, United States
‡Department of Chemistry and Biochemistry, San Diego State University, 5500 Campanile Drive, San Diego California 92182, United States
§Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095, United States

J. Org. Chem. 2016, 81, 9820-9825.
http://pubs.acs.org/doi/pdf/10.1021/acs.joc.6b01903

Abstract:
 

Thorough mechanistic studies and DFT calculations revealed a background radical pathway latent in metal-catalyzed oxidation reactions of methane at low temperatures. Use of hydrogen peroxide with TFAA generated a trifluoromethyl radical (•CF3), which in turn reacted with methane gas to selectively yield acetic acid. It was found that the methyl carbon of the product was derived from methane, while the carbonyl carbon was derived from TFAA. Computational studies also support these findings, revealing the reaction cycle to be energetically favorable. 

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Catalytic Mechanism and Efficiency of Methane Oxidation by Hg(II) in Sulfuric Acid and Comparison to Radical Initiated Conditions

Catalytic Mechanism and Efficiency of Methane Oxidation by Hg(II) in Sulfuric Acid and Comparison to Radical Initiated Conditions

Jack T. Fuller, III, Steven Butler, Deepa Devarajan, Austin Jacobs, Brian G. Hashiguchi, Michael M. Konnick, William A. Goddard, III, Jason Gonzales, Roy A. Periana, and Daniel H. Ess

ACS Catal. 2016, 6, 4312-4322
http://pubs.acs.org/doi/pdf/10.1021/acscatal.6b00226

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Abstract: 

Methane conversion to methyl bisulfate by HgII(SO4) in sulfuric acid is an example of fast and selective alkane oxidation catalysis. Dichotomous mechanisms involving C−H activation and electron transfer have been proposed based on experiments. Radical oxidation pathways have also been proposed for some reaction conditions. HgII is also of significant interest because as a d10 transition metal it is similar to d10 main-group metals that also oxidize alkanes. Density- functional calculations are presented that use both implicit and a mixture of implicit/explicit solvent models for the complete HgII catalytic cycle of methane oxidation to methyl bisulfate. These calculations are consistent with experiment and reveal that methane is functionalized to methyl bisulfate by a C−H activation and reductive metal alkyl functionalization mechanism. This reaction pathway is lower in energy than both electron transfer and proton-coupled electron transfer pathways. After methane C−H functionalization, catalysis is completed by conversion of the proposed resting state, [HgI(HSO4)]2, into Hg0 followed by Hg0 to HgII oxidation induced by SO3 from dehydration of sulfuric acid. This catalytic cycle is efficient because in sulfuric acid the HgII/Hg0 potential results in a moderate free energy barrier for oxidation (∼40 kcal/mol) and HgII is electrophilic enough to induce barriers of <40 kcal/mol for C−H activation and reductive metal alkyl functionalization. Comparison of HgII to TlIII shows that while C−H activation and reductive metal alkyl functionalization have reasonable barriers for TlIII, the oxidation of TlI to TlIII has a significantly larger barrier than Hg0 to HgII oxidation and therefore TlIII is not catalytic in sulfuric acid. Comparison of HgII to CdII and ZnII reveals that while M0 to MII oxidation and C−H activation are feasible for these first-row and second-row transition metals, reductive metal alkyl functionalization barriers are very large and catalysis is not feasible. Calculations are also presented that outline the mechanism and energy landscape for radical-initiated (K2S2O8) methane oxidation to methanesulfonic acid in sulfuric acid.