Asymmetric Boron Conjugate Additions to Enones in Water Catalyzed by Copper(0)

The use of copper(0) powder in water enables chiral induction as well as high catalytic activity. Water plays a prominent role to determine both the catalytic activity and enantioselection. Elevated temperature is indispensable for the regular reaction pathway, and an active species tends to aggregate after the completion of the reaction. Selective deuteration implies enantiofacial differentiation of the deuteron toward an O-enolate intermediate. Strict substrate specificity suggests the vital interaction between copper(0) and an enone. A very weak isotope effect is indicative of instantaneous protonation. Accordingly, a redox copper cycle (0 -> II) is tentatively proposed, in which transmetalation of a cyclometalated enone with diboron would furnish a boron enolate, followed by hydrolysis and reductive elimination. However, a single electron transfer process cannot be excluded on the available evidence and a more detailed mechanistic study is under way.