4.8 Article

Highly Active Cooperative Lewis Acid-Ammonium Salt Catalyst for the Enantioselective Hydroboration of Ketones

Journal

ANGEWANDTE CHEMIE-INTERNATIONAL EDITION
Volume 60, Issue 10, Pages 5544-5553

Publisher

WILEY-V C H VERLAG GMBH
DOI: 10.1002/anie.202012796

Keywords

ammonium salts; asymmetric catalysis; chiral alcohols; cooperative catalysis; hydroboration

Funding

  1. Deutsche Forschungsgemeinschaft (DFG) [PE 818/8-1, 404194277]
  2. Studienstiftung des Deutschen Volkes (German National Academic Foundation)
  3. Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany's Excellence Strategy [EXC 2075-390740016]
  4. state of Baden-Wrttemberg through bwHPC
  5. German Research Foundation (DFG) [INST 40/467-1 FUGG]
  6. Projekt DEAL

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Enantiopure secondary alcohols are high-value synthetic building blocks, and a new concept for catalytic hydroboration has been proposed with exceptional catalytic turnover numbers. The catalyst, accessible in few steps, is stable, recyclable, and efficient for at least 10 times of reuse.
Enantiopure secondary alcohols are fundamental high-value synthetic building blocks. One of the most attractive ways to get access to this compound class is the catalytic hydroboration. We describe a new concept for this reaction type that allowed for exceptional catalytic turnover numbers (up to 15 400), which were increased by around 1.5-3 orders of magnitude compared to the most active catalysts previously reported. In our concept an aprotic ammonium halide moiety cooperates with an oxophilic Lewis acid within the same catalyst molecule. Control experiments reveal that both catalytic centers are essential for the observed activity. Kinetic, spectroscopic and computational studies show that the hydride transfer is rate limiting and proceeds via a concerted mechanism, in which hydride at Boron is continuously displaced by iodide, reminiscent to an S(N)2 reaction. The catalyst, which is accessible in high yields in few steps, was found to be stable during catalysis, readily recyclable and could be reused 10 times still efficiently working.

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