Abstract
Synthetic control over chirality and function is the crowning achievement for metal-organic frameworks, but the same level of control has not been achieved for covalent organic frameworks (COFs). Here we demonstrate chiral COFs (CCOFs) can be crystallized from achiral organic precursors by chiral catalytic induction. A total of nine two-dimensional CCOFs are solvothermally prepared by imine condensations of the C3-symmetric 1,3,5-triformylphloroglucinol (Tp) with diamine or triamine linkers in the presence of catalytic amount of (R)- or (S)-1-phenylethylamine. Homochirality of these CCOFs results from chiral catalyst-induced immobilization of threefold-symmetric tris(N-salicylideneamine) cores with a propeller-like conformation of one single handedness during crystallization. The CCOF-TpTab showed high enantioselectivity toward chiral carbohydrates in fluorescence quenching and, after postsynthetic modification of enaminone groups located in chiral channels with Cu(II) ions, it can also be utilized as a heterogeneous catalyst for the asymmetric Henry reaction of nitroalkane with aldehydes.
Highlights
Synthetic control over chirality and function is the crowning achievement for metal-organic frameworks, but the same level of control has not been achieved for covalent organic frameworks (COFs)
Covalent organic frameworks (COFs) are an emerging class of crystalline polymers that allow precise integration of organic building blocks into two-dimensional (2D)[18,19,20,21,22,23,24,25] or threedimensional (3D)[26,27,28,29] networks with tunability of composition, structure, and function, similar to those found in metal-organic frameworks (MOFs)[30,31]
We showed that the chiral COFs (CCOFs)-TpTab could be utilized as a chiral solid fluorescent receptor for a wide range of disaccharides with high binding affinity and enantioselectivity, and after metallation, as a heterogeneous catalyst for asymmetric Henry reactions
Summary
Synthetic control over chirality and function is the crowning achievement for metal-organic frameworks, but the same level of control has not been achieved for covalent organic frameworks (COFs). We demonstrate chiral COFs (CCOFs) can be crystallized from achiral organic precursors by chiral catalytic induction. A total of nine two-dimensional CCOFs are solvothermally prepared by imine condensations of the C3-symmetric 1,3,5-triformylphloroglucinol (Tp) with diamine or triamine linkers in the presence of catalytic amount of (R)- or (S)-1-phenylethylamine. Homochirality of these CCOFs results from chiral catalyst-induced immobilization of threefold-symmetric tris(N-salicylideneamine) cores with a propeller-like conformation of one single handedness during crystallization. We demonstrated homochiral crystallization of a total of nine CCOFs with controlled handedness by imine condensations of Tp with achiral diamine or triamine linkers in the presence of chiral catalyst (Fig. 2). We showed that the CCOF-TpTab could be utilized as a chiral solid fluorescent receptor for a wide range of disaccharides with high binding affinity and enantioselectivity, and after metallation, as a heterogeneous catalyst for asymmetric Henry reactions
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