Electronic Effects in Asymmetric Catalysis: Structural Studies of Precatalysts and Intermediates in Rh-Catalyzed Hydrogenation of Dimethyl Itaconate and Acetamidocinnamic Acid Derivatives Using C(2)-Symmetric Diarylphosphinite Ligands.

Enantioselectivity of Rh(I)-catalyzed asymmetric hydrogenation of dehydroamino acid derivatives and dimethyl itaconate can be enhanced by the appropriate choice of substituents on the aromatic rings of vicinal diarylphosphinites derived from carbohydrates as well as trans-cyclohexane-1,2-diol. For example, the use of phosphinites with electron-donating bis(3,5-dimethylphenyl) groups at phosphorus provide high ee's in these reactions whereas electron-withdrawing aryl substituents decrease the enantioselectivity. In this paper, an attempt is made to clarify the origin of these remarkable electronic effects at two levels.