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Synthesis

Methane Yields To Supercritical CO2

Carbon dioxide solvent, coupled with a silver catalyst, provides the right conditions for a successful C–H functionalization

by Carmen Drahl
May 16, 2011 | APPEARED IN VOLUME 89, ISSUE 20

Methane is a molecule that rarely bows to chemists’ will, but researchers in Spain and France have coaxed it to do their bidding by using a silver catalyst and supercritical carbon dioxide solvent (Science, DOI: 10.1126/science.1204131). Few successful functionalizations of methane have been reported, so Gregorio Asensio of Spain’s University of Valencia, Michel Etienne of France’s University of Toulouse, Pedro J. Pérez of Spain’s University of Huelva, and colleagues decided to apply their electron-poor silver catalysts to the problem. Designed with heavily halogenated versions of so-called scorpionate ligands, which grip silver like a scorpion’s pincers, the catalysts were known to insert carbenes into relatively inert alkane C–H bonds. At first, however, the chemistry was problematic for methane because of side reactions and solubility issues. The researchers solved the problems by switching their solvent to supercritical CO2, which doesn’t react with intermediates and is miscible with methane. They show that a carbene generated from ethyl diazoacetate inserts into a methane C–H bond to make a new C–C bond, forming ethyl propionate, a chemical commonly used in the food and flavor industry, albeit in only 19% yield.

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