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Light Alkanes, Heavy Metals
Hydraulic fracturing, or fracking, has rapidly increased the supply of natural gas and has motivated methods to convert its constituents into commodity chemicals. Hashiguchi et al. (p. 1232) have found that lead and thallium salts are both efficient and selective oxidants, not only for methane, but for ethane and propane as well. In trifluoroacetic acid solvent, the alkanes are cleanly oxidized to the trifluoroacetate esters of their respective alcohols and 1,2-diols. Building on earlier discoveries, this work paves the way to developing methods that reduce our dependence on petroleum for industrial feedstocks.
Much of the recent research on homogeneous alkane oxidation has focused on the use of transition metal catalysts. Here, we report that the electrophilic main-group cations thallium(III) and lead(IV) stoichiometrically oxidize methane, ethane, and propane, separately or as a one-pot mixture, to corresponding alcohol esters in trifluoroacetic acid solvent. Esters of methanol, ethanol, ethylene glycol, isopropanol, and propylene glycol are obtained with greater than 95% selectivity in concentrations up to 1.48 molar within 3 hours at 180°C. Experiment and theory support a mechanism involving electrophilic carbon-hydrogen bond activation to generate metal alkyl intermediates. We posit that the comparatively high reactivity of these d10 main-group cations relative to transition metals stems from facile alkane coordination at vacant sites, enabled by the overall lability of the ligand sphere and the absence of ligand field stabilization energies in systems with filled d-orbitals.