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

Herein, we report the synthesis of two-dimensional chiral ZnII 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 ZnII 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 ZnII Salen COFs (2) with Mn(OAc)2·4H2O to synthesize chiral MnIII 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 MnIII Salen COF (3) could easily be separated by centrifugation and reused up to four recycles with an observable loss in activity without impairing enantioselectivity.