.
\nChemists are helping to reduce heat-trapping carbon dioxide (CO2)
\nemissions, which are a global concern. For example, they are devising
\nnew catalytic systems that would enable waste CO2 to be recycled as
\na non-toxic and practically free source of carbon for organic synthetic
\nreactions. However, current CO2 conversion techniques require expensive
\nmetal catalysts or drawn-out procedures.
\n.
\nNow, Zhaomin Hou and colleagues from the RIKEN Advanced Science
\nInstitute in Wako have found a way to insert CO2 directly into the framework
\n of aromatic molecules, turning them into carboxylic acid derivatives that
\nare widely used as pharmaceuticals, agrichemicals, and dyes1. Importantly,
\n this transformation can be achieved economically and with negligible
\nenvironmental impact, thanks to a low cost copper complex bearing an
\n organic ligand.
\n.
\nN-heterocyclic carbenes (NHCs) are molecules with near metal-like reactivity
\nbecause of an electron-deficient carbon center. For the past two decades,
\n scientists have used NHCs as organic replacements for metal catalysts
\nand as \u2018spectator\u2019 ligands that attach to metal centers and influence their
\ncatalytic behavior. Hou and colleagues recently discovered that adding NHCs
\n to copper, one of the most abundant metals in nature, created a complex
\nthat catalyzed CO2 addition to boron esters2\u2014a trick the team hoped to
\nrepeat with aromatic hydrocarbons.
\n.
\nThe most efficient way to incorporate CO2 into benzene-like molecules is by
\nreplacing one of the carbon\u2013hydrogen (C\u2013H) bonds on the outer ring; unfortunately,
\n these bonds are notoriously unreactive. To overcome this problem, the researchers
\nturned to benzoxazole: this double-ringed aromatic compound has a C\u2013H bond
\nsituated between nitrogen and oxygen atoms, making it easier to chemically
\nactivate this position.
\n.
\nWith just a pinch of the NHC\u2013copper catalyst complex, the team found they could
\n convert a mixture of CO2 and several different benzoxazole-based molecules into
\nsolid carboxylic acids and esters in excellent yields. Carefully characterizing
\n the crystal structures of several intermediate compounds revealed that CO2 inserted
\n in between a copper\u2013carbon bond formed at the benzoxazole C\u2013H site, followed
\n by a dissociation step that regenerated the catalyst.
\n.
\nAccording to Hou, the NHC ligand was essential in enabling CO2 capture. \u201cThe
\nelectron-donating ability of NHC could make the C\u2013H activation and CO2 insertion
\n steps easier, while its steric bulk brings stability to the active catalyst species,\u201d
\nhe notes. The researchers now hope to extend this technique to less reactive C\u2013H
\nbonds by fine-tuning the catalyst complex and optimizing reaction conditions.
\n.<\/p>\n","protected":false},"excerpt":{"rendered":"
. Chemists are helping to reduce heat-trapping carbon dioxide (CO2) emissions, which are a global concern. For example, they are devising new catalytic systems that would enable waste CO2 to be recycled as a non-toxic and practically free source of carbon for organic synthetic reactions. However, current CO2 conversion techniques require expensive metal catalysts or … <\/p>\n