222. K. Li, A. Alexakis,
     “Asymmetric conjugate addition to α-halo enones : Dramatic effect of styrene on the enantioselectivity”
     Angew. Chem. Int. Ed. 2006, 45, 7600-7603.

A simple trick could prove generally valuable for asymmetric copper-catalyzed conjugate addition reactions. It was found that the enantioselectivities of such reactions of dialkyl zinc reagents with α-halo enones in the presence of chiral phosphorimidite ligands were dramatically improved upon the addition of styrene, which acts as a radical scavenger to suppress the competitive non-asymmetric radical pathway (see example).

DOI : 10.1002/anie.200602417 

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