Confinement-Driven Enantioselectivity in 3D Porous Chiral Covalent Organic Frameworks.

Covalent organic frameworks (COFs) show great potential in heterogeneous catalysis, but the confinement effect of frameworks on molecular catalysts has yet to be explored. Here we demonstrate the utilization of 3D COFs with well-defined porous channels capable of inducing chiral molecular catalysts from non-enantioselective to highly enantioselective in catalyzing organic transformations, sharply different from the typical methods for tailoring enantioselectivities by varying steric and electronic properties of molecular catalysts. By condensations of a tetrahedral tetraamine and two linear dialdehydes derived from enantiopure 1,1'-binaphthol (BINOL), two chiral 3D COFs with a 9-fold or 11-fold interpenetrated diamondoid framework are prepared. Obviously enhanced Bronsted acidity was observed for the chiral BINOL units that are uniformly distributed within the tubular channels compared to the non-immobilized acids. This facilitates the Bronsted acid catalysis of cyclocondensation of aldehydes and anthranilamides to produce 2,3-dihydroquinazolinones. While homogeneous BINOL controls display no enantioselectivity and/or low activity, constraint of their conformations in CCOFs leads to up to 91% isolated yield with 97% ee. DFT calculations show COF catalyst provide preferential secondary interactions between the substrate and framework to induce enantioselectivities that are not achievable in homogeneous systems.