Construction of a spherical chiral covalent organic framework for efficient asymmetric aldol catalysis

Abstract

Developing chiral covalent organic frameworks (COFs) with high catalytic activity, stereoselectivity, and recyclability is challenging. Here, we designed a chiral building block (L-DTP-Boc) from natural proline, which was reacted with 1,3,5-tris(4-aminophenyl) benzene (TAPB) at room temperature to construct a spherical chiral COF (L-DTP-TAPB-Boc). After removing the Boc group, the resulting L-DTP-TAPB COF retained high crystallinity, spherical morphology, and abundant chiral catalytic sites, rendering it an effective catalyst for asymmetric aldol reactions. It achieved a 96 % yield and 87 % enantiomeric excess (ee) in the reaction between p-nitrobenzaldehyde and cyclohexanone within 3 days, outperforming most reported chiral COFs and metal-organic frameworks (MOFs). Additionally, it demonstrated excellent recyclability with minimal loss in performance after five cycles. We also proposed a potential mechanism for the aldol reaction catalyzed by chiral L-DTP-TAPB COF. This work not only offers new insights into the design of spherical chiral COFs but also uncovers their vast potential as effective heterogeneous chiral catalysts for asymmetric aldol reactions.