Thursday, March 31, 2011

Recycling Of Carbon Dioxide Emissions Gets Significantly Easier Thanks To A Novel Copper–Organocatalyst Complex

Schemes for storing carbon dioxide (CO2) emissions underground often make the headlines, but chemically ‘fixing’ this molecule onto the frameworks of other compounds is a potentially more lucrative proposition. Transforming CO2 into useful products like polymers and pharmaceuticals could simultaneously reduce greenhouse gases and boost manufacturers’ profits. Unfortunately, the expensive metal catalysts and hot temperatures typically needed to break CO2 apart and rearrange the bonds render this technology too pricy for most applications.

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Yugen Zhang and Dingyi Yu at the A*STAR Institute of Bioengineering and Nanotechnology1 have now discovered a way to make CO2 fixation more economical than ever before. The researchers developed a mixed copper–organocatalyst system that can convert almost any molecule bearing terminal alkyne groups—outward-facing carbon–carbon triple bonds—into carboxylic acids via CO2 insertion at room temperature. Alkynyl carboxylic acids have extensive applications in synthetic chemistry, so the discovery could have wide-reaching impact.

Adding CO2 onto a hydrocarbon is an energy-intensive process that generally requires the creation of a new carbon–carbon bond. Consequently, chemists have been hunting for metal catalysts that can lower these energy barriers to cost-effective levels.

Recent studies have identified copper complexes as promising catalysts because they could activate a variety of substrates by bonding to the carbon atom and then incorporating CO2 in between the carbon and its neighboring metal atoms. However, copper-catalyzed additions of CO2 to terminal alkynes have had limited success so far.

Zhang and Yu discovered that previous experiments were simply too hot to stabilize the important copper–alkyne intermediates. By mixing a copper catalyst containing a basic ligand with alkyne molecules and COat room temperature, instead of the usual 100 °C, the researchers obtained several types of alkynyl carboxylic acids in excellent yields. “We could achieve broad tolerance for different substrates because we use mild reaction conditions—no strong base or acid, no heating, no oxidant or reductant,” says Zhang.

However, alkynes containing deactivating ‘electron-withdrawing’ substituents remained stubbornly inert with copper catalysts. To resolve this, the researchers synthesized N-heterocyclic carbenes (NHCs)—molecules with proven CO2-activating behavior—into a large, robust polymer. When used as a copper catalyst ligand, the unique structure of poly-NHC enabled it to surround the metal and enhance the chances of CO2 conversion, boosting yields from 2% to 70% for a typical electron-withdrawing alkyne. Furthermore, the solid structure of the poly-NHC–copper catalyst makes it compatible with industrial systems, a technological advantage that Yu and Zhang are currently investigating.
The A*STAR-affiliated researchers contributing to this research are from the Institute of Bioengineering and Nanotechnology

  1. Yu, D. & Zhang, Y. Copper- and copper–N-heterocyclic carbene-catalyzed C–H activating carboxylation of terminal alkynes with CO2 at ambient conditions. Proceedings of the Natural Academy of Sciences 107, 20184–20189 (2010). | article

Source: A*STAR

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