Heterogeneous Chiral Covalent Organic Framework for the Enantioselective Epoxidation of Styrenes and Chromenes

Herein, we report the synthesis of two-dimensional chiral Zn^II Salen covalent organic frameworks (COFs) (2) via rapid microwave-promoted condensation of C₃-symmetric 1,3,5-tris[(5-tert-butyl-3-formyl-4-hydroxyphenyl)ethynyl]benzene 1 with (1R,2R)-1,2-diaminocyclohexane in excellent yields. The synthesized chiral Zn^II Salen COF (2) showed a 454 m² g^-1 BET surface area with excellent crystallinity and thermal stability. Further, the post-synthetic metal exchange reaction was performed for chiral Zn^II Salen COFs (2) with Mn(OAc)₂·4H₂O to synthesize chiral Mn^III Salen COFs (3) and utilized as an effective heterogeneous catalyst for the enantioselective epoxidation of styrenes and chromenes to afford chiral epoxides up to 72% ee. Chiral Mn^III Salen COF (3) could easily be separated by centrifugation and reused up to four recycles with an observable loss in activity without impairing enantioselectivity.