RIKEN chemists have developed a catalyst that should allow carbon dioxide to be used as a versatile synthetic chemical.
Carbon dioxide (CO2) is produced whenever fossil fuels are burned, and it is a powerful greenhouse gas that traps heat in our atmosphere, contributing to global warming. As such, turning the gas into a chemical feedstock, rather than allowing it to escape into the atmosphere, is an extremely appealing idea.
In fact, industry has long used carbon dioxide as a chemical building block—in the manufacture of the painkiller 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.
Chemists have recently developed milder, boron-based alternatives, which 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 to react with carbon dioxide without destroying sensitive chemical groups.
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,” says 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, says Hou. “And they can also be easily prepared in the lab.”
Hou adds 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,” he says. “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.”
- Ohishi, T., Nishiura, M. & Hou, Z. Carboxylation of organoboronic esters catalyzed by N-heterocyclic carbene copper(I) complexes. Angewandte Chemie International Edition 47, 5792–5795 (2008).