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

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.