Bottom–Up Design of Photoactive Chiral Covalent Organic Frameworks for Visible-Light-Driven Asymmetric Catalysis

Abstract

The development of chiral covalentorganic framework catalysts (CCOFs) to synthesize enantiopure organic compounds is crucial and highly desirable in synthetic chemistry. Photocatalytic asymmetric reactions based on CCOFs are eco-friendly and sustainable while they are still elaborate. In this work, we report a general bottom–up strategy to successfully synthesize several photoactive CCOFX (X = 1–5 and 1-Boc). The photoactive porphyrin building blocks are selected as knots and various secondary-amine-based chiral catalytic centers are immobilized on the pore walls of CCOFX through a rational design of benzoimidazole linkers. The porphyrin units act as light-harvesting antennae to generate photo-induced charge carriers for the activation of bromide during the photocatalytic asymmetric alkylation of aldehydes. Meanwhile, various aldehydes are activated by the chiral secondary amine to form the target products with a high yield (up to 97%) and ee value (up to 93%). The results significantly expand the scope to predesign CCOF photocatalysts for visible-light-driven asymmetric catalysis.

Robust Nitro-Functionalized {Zn3}-Organic Framework for Excellent Catalytic Performance on Cycloaddition Reaction of CO2 with Epoxides and Knoevenagel Condensation

“…Due to the economic benefits of recyclability in practical application, consecutive experiments under the optimized reaction conditions were carried out by using benzaldehyde (10 mmol) and malononitrile (20 mmol) as the model substrates. The yield results of BN product catalyzed by repeatedly used NUC-80a are shown in Figure S33, which exhibited that the yield of BN product had a slight downward trend with the increase of cycle time, which might be caused by the loss and contamination of the catalyst in the process of centrifugal separation, cleaning, drying, and transfer. , However, the very slow catalytic deactivation performance of NUC-80a during performed five-cycle experiments proved that it could be recovered and used repeatedly. In addition, the PXRD and the FTIR diagrams for recovered and as-synthesized NUC-80a in Figures S34–S35 exhibited that the link mode between Zn 2+ and organic ligand in the host framework remained intact because of the consistence for main peaks.…”

Visible Light-Driven Flexible Synthesis of α-Alkylated Glycine Derivatives Catalyzed by Reusable Covalent Organic Frameworks

“…Photoactive covalent organic frameworks (COFs), possessing long π-conjugated systems, highly ordered layer structures, and considerable stability, have become a qualified heterogeneous photocatalyst for visible light-driven organic transformations, featuring additional sustainability and workup simplicity of these processes. − Various functional organic monomers bestow unlimited design ability on photoactive COFs, extending their catalytic power to various visible light-driven organic processes. , To date, many organic reactions, including oxidation, − reduction, , radical cyclization, − C–H bond functionalization, − E – Z isomerization, and cross-coupling, − were compatible with photoactive COFs, proceeding elegantly and fruitfully under visible light irradiation. COFs always played the following three primary roles in the above catalytic cycles.…”

Observation of Chiral Channels in Helical Covalent Organic Frameworks

“…Chiral COFs , inherit the periodic, porous, and hierarchical structure of COFs and exhibit great potential in asymmetric catalysis, chiral separation, chiral optics, etc. Elegant examples have been developed for constructing chiral COFs either from chiral monomers via postmodification , /direct reticulation − or from achiral precursors via chiral induction , /catalytic asymmetric synthesis , /chiral memory . In most cases, however, the chirality of these COFs is expressed only at the molecular level.…”