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A Mechanistic Change Results in 100 Times Faster CH Functionalization for Ethane versus Methane by a Homogeneous Pt Catalyst

Konnick, Michael M., Bischof, Steven M., Yousufuddin, Muhammed, Hashiguchi, Brian G., Ess, Daniel H., Periana, Roy A.
Journal of the American Chemical Society 2014 v.136 no.28 pp. 10085-10094
catalysts, energy, ethane, ethanol, ethylene, methane, oxidation, platinum, shale, solvents, sulfuric acid, temperature
The selective, oxidative functionalization of ethane, a significant component of shale gas, to products such as ethylene or ethanol at low temperatures and pressures remains a significant challenge. Herein we report that ethane is efficiently and selectively functionalized to the ethanol ester of H₂SO₄, ethyl bisulfate (EtOSO₃H) as the initial product, with the Ptᴵᴵ “Periana-Catalytica” catalyst in 98% sulfuric acid. A subsequent organic reaction selectively generates isethionic acid bisulfate ester (HO₃S-CH₂-CH₂-OSO₃H, ITA). In contrast to the modest 3–5 times faster rate typically observed in electrophilic CH activation of higher alkanes, ethane CH functionalization was found to be ∼100 times faster than that of methane. Experiment and quantum-mechanical calculations reveal that this unexpectedly large increase in rate is the result of a fundamentally different catalytic cycle in which ethane CH activation (and not platinum oxidation as for methane) is now turnover limiting. Facile Ptᴵᴵ-Et functionalization was determined to occur via a low energy β-hydride elimination pathway (which is not available for methane) to generate ethylene and a Ptᴵᴵ-hydride, which is then rapidly oxidized by H₂SO₄ to regenerate Ptᴵᴵ-X₂. A rapid, non-Pt-catalyzed reaction of formed ethylene with the hot, concentrated H₂SO₄ solvent cleanly generate EtOSO₃H as the initial product, which further reacts with the H₂SO₄ solvent to generate ITA.