Asymmetric Binary Acid Catalysis: Switchable Enantioselectivity in Enantioselective Conjugate Hydride Reduction.

The exchange of the metal ion from Zr(IV) to Fe(III) leads to a switch in the enantioselectivity of binary acid-catalyzed conjugate hydride reductions. In the presence of Hantzsch ester, γ-indolyl β,γ-unsaturated α-keto esters could be reduced to the desired ( S )- or ( R )-products, respectively, with good to excellent enantioselectivity (up to 98% ee).