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 C 3 -symmetric 1,3,5-tris[(5- tert -butyl-3-formyl-4-hydroxyphenyl)ethynyl]benzene 1 with (1 R ,2 R )-1,2-diaminocyclohexane in excellent yields. The synthesized chiral Zn II Salen COF ( 2 ) showed a 454 m 2 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 ·4H 2 O 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.