Mixed-Linker Chiral 2D Covalent Organic Frameworks with Controlled Layer Stacking for Electrochemical Asymmetric Catalysis.

ABSTRACT

Covalent organic frameworks (COFs) have undergone extensive research as heterogeneous catalysts for a wide range of significant reactions, but they have not yet been investigated in the realm of electrochemical asymmetric catalysis, despite their recognition as an economical and sustainable strategy for producing enantiopure compounds. Here, we report a mixed-linker strategy to design multicomponent two-dimensional (2D) chiral COFs with tunable layer stacking for highly enantioselective electrocatalysis. By crystallizing mixtures of triamines with and without the MacMillan imidazolidinone catalyst or aryl substituent (ethyl and isopropyl) and a dialdehyde derivative of thieno-[3,2-b]thiophene, we synthesized and structurally characterized a series of three-component homochiral 2D COFs featuring either AA or ABC stacking. The stacking modes that can be synthetically controlled through steric tuning using different aryl substituents affect their chemical stability and electrochemical performance. With the MacMillan catalyst periodically appended on their channels, all three COFs with conductive thiophene moieties can be highly enantioselective and recyclable electrocatalysts for the asymmetric α-arylation of aldehydes, affording alkylated anilines with up to 97% enantiomeric excess by an anodic oxidation/organocatalytic protocol. Presumably due to their higher charge transfer ability, the ABC stacking COFs exhibit improved reactivity compared to the AA stacking analogue. This work therefore advances COFs as electrocatalysts for asymmetric catalysis and may facilitate the design of more redox-active crystalline organic polymers for electrochemical enantioselective processes.