Elegant Experiments With Light

Expansion of the chemical toolbox thanks to special catalysts

Researchers working with Prof. Benjamin List have developed a new method of photoredox catalysis. They have now published their results in the renowned scientific journal Science.

Researchers at the Max Planck Institute for Coal Research have expanded the molecular tool kit for efficient, targeted syntheses: They use a very special catalyst - and the energy of light. The scientists have now published the results of their work in their paper "Asymmetric counteranion-directed photoredox catalysis" in the renowned journal Science.

Typically, in a so-called photoredox reaction, the light is absorbed by a photocatalyst. Electron transfer between the catalyst and the substrate then takes place, creating a highly reactive “radical ion” that can undergo various desired reactions.

However, the problem of these “radicals” has so far been the lack of selectivity. A second mode of activation was often necessary to obtain a desired product in the required purity. This had restricted stereoselective photoredox reactions to specific substrates that could be activated by this second mode.

"Photocatalysis enables chemical reactions with the help of light - for example in the leaves of plants, but also in the manufacture of medicines," explains Benjamin List. Photocatalytic reactions proceed via high-energy intermediates, and controlling the selectivity of their reactions has been difficult with chemical catalysts. "We are now providing a general concept for carrying out these reactions with high stereoselectivity, in which we can produce mirror-image molecules," says Benjamin List.

A molecular precision tool
In concrete terms, this means that the photocatalyst absorbs light and accepts an electron from the substrate, i.e. from a reaction partner. Since an electron is negatively charged, this substrate now automatically becomes positive and combines with a counter anion. This ion pair is now involved in another reaction step – the one that actually interests the scientists. The original photocatalyst is no longer part of this step - and can be regenerated and reused to absorb light and accept an electron.

With this new method, the scientists from the Max Planck Institute for Coal Research have succeeded in developing another molecular precision tool. "We haven't really thought about concrete application examples yet," reveals Dr. Sayantani Das, postdoc at Benjamin List and also involved in the project. But asymmetric photoredox catalysis can certainly be useful in the field of synthesis - for example in the production of medicines or fragrances.