Anion effect on enantioselective oxidative NHC catalysis: highly efficient kinetic resolution of tertiary alcohols and beyond†
The effect of the counter-anion on enantio-control in oxidative NHC catalysis is demonstrated systematically for the first time. For kinetic resolution of oxindole-derived tertiary alcohols through NHC-catalyzed esterification, the identity of anions, either in the form of an additive or as the counter-anion in the azolium pre-catalyst, showed a significant effect on the enantioselectivity of the catalytic system. With the suitable choice of anions, highly efficient kinetic resolution (with krel between 30 and >200) was achieved for a wide range of oxindole-derived tertiary alcohols bearing alkyl, aryl and functionalized substituents. The anion effect on other oxidative NHC-catalyzed processes such as formal cycloaddition is also demonstrated.