Rhodium(I)-Catalyzed Asymmetric 1,4-Addition of Arylboronic Acids to α,β-Unsaturated Amides

Abstract

The conjugate addition of arylboronic acids to α,β-unsaturated amides was carried out in the presence of a chiral rhodium catalyst and an aqueous base. The catalyst prepared in situ from Rh(acac)(CH₂CH₂)₂ and (S)-binap provided (R)-N-benzyl-3-phenylbutanamide with 93% ee in the addition of phenylboronic acid to N-benzyl crotonamide. The reaction suffered from incomplete conversion resulting in moderate yields, but addition of an aqueous base, such as K₂CO₃ (10−50 mol%) was found to be highly effective to improve the chemical yields. The role of the base giving a RhOH species active for transmetalation with arylboronic acids was discussed.