Precise Control over the Number of Pyrrolidine Catalytic Centers in Chiral COFs for Boosted Asymmetric Catalysis

Chiral covalent organic frameworks (COFs) hold significant potential in the field of asymmetric catalysis. However, currently reported chiral COF catalysts often contain limited chiral catalytic sites, leading to prolonged reaction times and low enantiomeric excess (ee) values of the catalytic products. Therefore, precisely regulating the number of chiral catalytic centers in chiral COF frameworks to enhance asymmetric catalytic performance remains challenging. In this study, through symmetry control of chiral monomers, three chiral COFs with varying numbers of pyrrolidine catalytic centers were precisely constructed, and their applications in heterogeneous asymmetric catalysis were investigated. Notably, the asymmetric catalytic performance of the chiral COFs was closely related to the number of pyrrolidine catalytic centers within the pores. A high density of catalytic sites significantly improved both the activity and enantioselectivity of the catalytic reaction. Remarkably, L-DATDP-Tz COF, containing 6 pyrrolidine catalytic centers per pore channel, exhibited excellent catalytic performance in the reaction between cyclohexanone and p-nitrobenzaldehyde, achieving a 91% yield and 88% ee within only one day. This performance far surpassed that of the counterparts with only 3 or 4.5 pyrrolidine catalytic centers per pore channel, as well as most other reported chiral COFs. This study provides key insights into the precise regulation of catalytic site density in chiral COFs, offering a new perspective for designing COF catalysts with high-efficiency asymmetric catalytic performance.