Turning CO2 into useful carbon–boron synthetic reagents
While some groups search for ways to lower and store carbon dioxide (CO2) gas emissions, there are efforts worldwide to convert this molecule into a chemical feedstock. Researchers at RIKEN Advanced Science Institute in Wako, Japan, developed the first protocol for attaching both CO2 and boron atoms to unsaturated carbon–carbon triple bonds. This protocol uses inexpensive organic–copper catalysts under mild conditions to construct building blocks that can be used by chemists.
The strong double bonds inside CO2 make this molecule inert and hard to use in most chemical reactions. Other efforts to convert CO2 into something more useful have resulted with generation of simple carboxylic acids. This method employs transition metals to catalyze addition of electron-rich organic nucleophiles to CO2’s central carbon atom, however, production of more complex substances containing non-hydrocarbon atoms has remained mostly out of reach.
Zhaomin Hou and his managed to turn CO2 into organoboron reagents – valuable synthetic compounds able to perform various transformations of carbon–boron bonds. After turning alkynes (molecules with carbon–carbon triple bonds) into nucleophiles, the researchers figured that usage of N-heterocyclic carbene (NHC) copper complexes aids in control of resulting nucleophilic species. While nucleophilic species are highly reactive with many types of chemical groups, they are pretty difficult to control. NHC copper complexes are a hybrid organic/inorganic system with a great ability to catalyze the addition CO2.
X-ray experiments revealed that the NHC–copper complexes could indeed catalyze the addition of CO2 and diborane molecules to alkynes through a three-step catalytic insertion process. This reaction generates a final product previously unknown cyclic structure which is composed out of a boron atom, a carbon–carbon double bond and a carboxyl group researchers named boralactone.
By tweaking the structure of the NHC–copper catalyst, RIKEN researchers were able to apply the technique to a wide range of alkyne-type molecules with no side reactions. The catalyst is able to deliver the same geometric arrangement no matter which substituents were attached to the carbon triple bond. Hou explains that this process occurs due to selectivity of boralactone and because the diborane–catalyst complex always attacks the alkyne bond from a specific direction due to electronic interactions.
“Our reaction may serve as an attractive method for the synthesis of multifunctional alkenes, as it uses CO2 and easily available alkynes as building blocks with a relatively cheap copper catalyst”, concludes Hou, who leads the Organometallic Chemistry Laboratory, RIKEN Advanced Science Institute.
For more information, read the article published in the Journal of American Chemical Society: “Catalytic Boracarboxylation of Alkynes with Diborane and Carbon Dioxide by an N-Heterocyclic Carbene Copper Catalyst