Dramatic improvement of the enantiomeric excess in the asymmetric conjugate addition reaction using new experimental conditions.
The asymmetric conjugate addition of dialkylzincs is usually performed with Cu(OTf)(2) in toluene. We show that by using a copper carboxylate in Et(2)O, THF, or EtOAc, we strongly improve the enantioselectivity with a given ligand. Ee values up to 99.1% could be reached with new ligands based on the induced atropisomerism of a simple biphenol unit. In addition, we show that the Lewis acid effect of Cu(OTf)(2) is not a significant.
Alexakis
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Department of Organic Chemistry, University of Geneva, 30, Quai Ernest Ansermet, 1211 Geneva 2, Switzerland.
Benhaim
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C
Rosset
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S
Humam
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M