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An extremely rare intermediate reveals the secrets of boron-based catalysis

Hydroboration catalysis by complexes of transition metals, such as iridium, makes it possible to create bonds sought after in organic chemistry, such as carbon-carbon bonds, thanks to the activation of a boron-hydrogen bond. This method is particularly popular among the pharmaceutical industry, as it replaces catalysts that are more difficult to handle and store. Hydroboration also earned its inventor, Herbert C. Brown, the Nobel Prize in Chemistry in 1979. However, the process of hydroboration catalysis is still poorly understood. It is generally accepted that it goes through an oxidative addition, where a hydrogen-boron bond breaks into two fragments that each retain one electron. Researchers from the Institut de chimie de Strasbourg (IC, CNRS/Université de Strasbourg), the Chinese universities of Zhejiang and Westlake and the University of Zurich (Switzerland) have proven that hydroboration catalysis can also take place via hydride transfer. This does not change the chemical composition of the products obtained, but shows that their electronic structures are more varied than previously assumed.

To achieve this, chemists demonstrated the formation, during a reaction in the presence of an iridium complex and a boron cation called borenium, of a reaction intermediate that is not detectable by crystallographic methods typically used in this type of work . Instead, the researchers combined theoretical studies with techniques of nuclear magnetic resonance, X-ray diffraction and X-ray photoelectron spectrometry. They also showed the existence of a bond between borenium and a transition metal. , here iridium, which had never been isolated and formally characterized. This better understanding of hydroboration catalysis may lead to the synthesis of more stable and efficient catalysts. This work also made it possible to establish an effective methodology for studying these questions.

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