Researchers at RIKEN in Japan have developed a new copper catalyst that allows carbon dioxide to be converted into a variety of functionalized carboxylic acid derivatives—versatile synthetic chemicals. A paper on their work was published earlier this year in the journal Angewandte Chemie International Edition.
Industry has used carbon dioxide as a chemical building block—in the manufacture of aspirin, for example—but its use is limited by the difficulty of breaking open its strong carbon-oxygen double bonds. Carbon compounds activated by lithium or magnesium are often needed to attack and incorporate carbon dioxide successfully, but these reagents are extremely reactive and quite hazardous on a large scale.
Milder, boron-based alternatives require a rhodium catalyst to speed up the reaction. Unfortunately, this catalyst tends to break down particularly sensitive chemical groups in the product.
Zhaomin Hou, of RIKEN’s Advanced Science Institute, Wako, along with colleagues Takeshi Ohishi and Masayoshi Nishiura, has now developed a copper catalyst that helps the boron compounds for the carboxylation of aryl- and alkenylboronic esters with CO2.
This makes the reaction particularly useful for building complex molecules containing several different types of chemical group, something that would not be possible with the harsh lithium reagents.
We have tried many different metal compounds, among which the copper catalyst was the best.—Zhaomin Hou
The team was also able to study exactly how the catalyst works, by isolating key molecules at various intermediate stages of the reaction. They found that the active copper catalyst first displaces the boron group from the starting molecule, forming a new copper–carbon bond. Carbon dioxide then inserts itself into this bond before the copper catalyst is finally removed, leaving behind a carboxylic acid (-CO2H) group.
Various forms of the boron compounds, known as boronic esters, are commercially available, and can also easily be prepared in the lab.
Hou notes that their method is also amenable to large-scale, commercial synthesis.
Since CO2 is a renewable carbon resource, exploration of new reactions and catalysts for its efficient use is of great importance. One of our goals is to find a catalyst that can transform CO2 in exhaust gasses of automobile vehicles or chemical plants into useful materials.—Zhaomin Hou
Ohishi, T., Nishiura, M. & Hou, Z. (2008) Carboxylation of organoboronic esters catalyzed by N-heterocyclic carbene copper(I) complexes. Angewandte Chemie International Edition 47, 5792–5795 doi: 10.1002/anie.200801857