Effect of π-Linkages in Covalent Organic Framework-Catalyzed Light-Harvesting Thioesterification Reaction.

Abstract

Covalent organic frameworks (COFs) serve as an outstanding platform for heterogeneous photocatalysis. We synthesized two analogous pyrene-based two-dimensional COFs with π-conjugated networks, one linked by C═N bonds and the other by C═C bonds, through Schiff base and Knoevenagel condensation reactions, respectively. We investigated the impact of these linkages on the photocatalytic activity of these COFs, using visible-light-mediated thioesterification as a model reaction. It was found that the olefin-linkage COF outperformed the imine-linkage COF as a photocatalyst. The developed protocol demonstrated a broad substrate scope, including 35 diverse carboxylic acids, 14 drug molecules, and several disulfide coupling partners, achieving up to a 95% yield of thioesters. The practical utility of this strategy is further demonstrated by its successful application in gram-scale reactions. The photocatalyst is robust and was successfully reused for multiple cycles without any loss of catalytic activity. The COF backbone facilitated enhanced electron transfer upon light irradiation, enabling the cross-coupling of carboxylic acid and disulfide through a reductive photocatalytic cycle.