4.6 Article

Asymmetric Carboxycyanation of Aldehydes by Cooperative AlF/Onium Salt Catalysts: from Cyanoformate to KCN as Cyanide Source

Journal

CHEMISTRY-A EUROPEAN JOURNAL
Volume 25, Issue 6, Pages 1515-1524

Publisher

WILEY-V C H VERLAG GMBH
DOI: 10.1002/chem.201804388

Keywords

aluminum; bifunctional catalysis; fluorine; nitriles; trimolecular reaction

Funding

  1. Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) [404194277, PE 818/8-1]
  2. European research council (ERC) [646717]
  3. Carl-Zeiss-Stiftung
  4. DFG within the Cluster of Excellence in Simulation Technology (EXC 310/2) at the University of Stuttgart
  5. European Research Council (ERC) [646717] Funding Source: European Research Council (ERC)

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Asymmetric 1,2-additions of cyanide yield enantioenriched cyanohydrins as versatile chiral building blocks. Next to HCN, volatile organic cyanide sources are usually used. Among them, cyanoformates are more attractive on technical scale than TMSCN for cost reasons, but catalytic productivity is usually lower. Here, the development of a new strategy for cyanations is described, in which this activity disadvantage is overcome. A Lewis acidic Al center cooperates with an aprotic onium moiety within a remarkably robust bifunctional Al-F-salen complex. This allowed for unprecedented turnover numbers of up to 10(4). DFT studies suggest an unexpected unique trimolecular pathway in which the ammonium bound cyanide attacks the aldehyde, which itself is activated by the carbonyl group of the cyanoformate binding to the Al center. In addition, a novel practical carboxycyanation method was developed that makes use of KCN as the sole cyanide source. The use of a pyrocarbonate as carboxylating reagent provided the best results.

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