Boosting Enantioselectivity of Chiral Molecular Catalysts with Supramolecular Metal-Organic Cages

Abstract

The search for new methodologies to tune and control the enantioselectivities of molecular catalysts is of great importance in the field of asymmetric catalysis. Here, we have illustrated that chiral molecular catalysts can be boosted from very low enantioselectivity to high enantioselectivity when installed in supramolecular metal-organic cages. By deliberately designing two optically active 1,1′-spirobiindane-7,7′-diol (SPINOL)-based dipyridine linkers, we synthesized two chiral Pd3L6 cages featuring chiral dihydroxyl or dimethoxymethyl auxiliaries in the nanosized cavities. After treatment with metal ions, the cage featuring dihydroxyl groups could serve as efficient catalysts for asymmetric conjugate addition of styrylboronic acids to α,β-enones to produce γ,δ-unsaturated ketone and the asymmetric addition of diethylzinc to aldehydes to afford secondary alcohols. While the molecular SPINOL display very low enantioselectivity, restriction of its freedom in the cages led to 91−99.6% and 80−99.9% enantiomeric excess (ee) of products, respectively, which were increased by up to 35% and 78% ee, compared with the molecular control. Thus, our present work paves a way of utilizing supramolecular porous assemblies to manipulate the enantioselectivities of molecular catalysts.