4‐Substituted‐Phenyl(bisoxazoline)‐Rhodium Complexes: Efficiency in the Catalytic Asymmetric Reductive Aldol Reaction

Electronic effects of the substituents on the phenyl ring of the phenyl(bisoxazoline) ligand skeleton of rhodium catalysts was examined in the asymmetric reductive aldol reactions of acrylates and aldehydes. The electron‐withdrawing NO2 group of Rh(4‐NO2‐Phebox‐R)(OAc)2 showed an increase in the enantioselectivity of the products in the reductive reaction of methyl acrylate but not in the reactions of tert‐butyl‐ and trimethylsilyl acrylates. Theoretical calculations of the corresponding model rhodium‐Phebox complexes indicate that the remote electron‐withdrawing substituent slightly shortens the Rh–Cphebox bond. Also, the Rh–C bonds of Rh(4‐NO2‐Phebox‐iPr)(OAc)2 and the corresponding Rh(4‐H‐Phebox‐iPr)(OAc)2 were compared on the basis of X‐ray analysis. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)