posted on 2024-02-20, 18:04authored byKai Zhang, Xihao Tang, Xi Yang, Jialin Wu, Baoying Guo, Rui Xiao, Yao Xie, Shengrun Zheng, Huawei Jiang, Jun Fan, Weiguang Zhang, Yi Liu, Songliang Cai
Chiral covalent organic frameworks (COFs) hold considerable
promise
in the realm of heterogeneous asymmetric catalysis. However, fine-tuning
the pore environment to enhance both the activity and stereoselectivity
of chiral COFs in such applications remains a formidable challenge.
In this study, we have successfully designed and synthesized a series
of clover-shaped, hydrazone-linked chiral COFs, each with a varying
number of accessible chiral pyrrolidine catalytic sites. Remarkably,
the catalytic efficiencies of these COFs in the asymmetric aldol reaction
between cyclohexanone and 4-nitrobenzaldehyde correlate well with
the number of accessible pyrrolidine sites within the frameworks.
The COF featuring nearly one pyrrolidine moiety at each nodal point
demonstrated excellent reaction yields and enantiomeric excess (ee)
values, reaching up to 97 and 83%, respectively. The findings not
only underscore the profound impact of a deliberately controlled chiral
pore environment on the catalytic efficiencies of COFs but also offer
a new perspective for the design and synthesis of advanced chiral
COFs for efficient asymmetric catalysis.