The greenhouse gas CO₂ could be used as a sustainable chemical feedstock. It can, for example, be used to convert organic compounds into carboxylic acid derivatives. However, the direct carboxylation of C(sp³)–H bonds is still challenging.

Burkhard König, University of Regensburg, Germany, and colleagues have developed a visible-light-mediated carboxylation of benzylic C–H bonds with CO₂ to give 2-arylpropionic acids (pictured). The team used 2,4,5,6-tetra(carbazol-9-yl)isophthalonitrile (4CzIPN) as a photosensitizer, triisopropylsilanethiol as a hydrogen atom transfer catalyst, and N,N-dimethylformamid (DMF) as the solvent. They converted a variety of compounds with benzylic C–H bonds into the corresponding carboxylic acids under visible light in moderate to excellent yields.

The proposed mechanism involves a single-electron oxidation of triisopropylsilanethiol, which produces a radical. This radical abstracts hydrogen from the substrate, giving a benzyl radical. The benzyl radical can then be reduced to give a carbanion, which reacts with CO₂. The overall reaction is metal-free, atom-economic, and can, e.g., be used to synthesize commonly used drugs.

• Photocarboxylation of Benzylic C–H Bonds,
  Qing-Yuan Meng, Tobias E. Schirmer, Anna Lucia Berger, Karsten Donabauer, Burkhard König,
  J. Am. Chem. Soc. 2019.
  https://doi.org/10.1021/jacs.9b05360