Donor–Acceptor-Conjugated Porous Polymer for Photopromoted N–H Formylation of Secondary Amines: Efficient In Situ N-Heterocyclic Carbene Catalysis in Radical Manner

The photogenerated reactive oxygen species (ROSs) in N-heterocyclic carbene (NHC)-based heterogeneous photocatalysis hold promise for addressing the challenge of fostering Breslow-derived radicals. In this study, two conjugated porous polymers (CPPs), namely, Im-PCPP-1 and Im-PCPP-2, decorated with donor–acceptor-type porphyrin and imidazole/imidazolium moieties, are synthesized via Suzuki–Miyaura coupling reaction. Importantly, the ionization of the imidazole monomer effectively tunes the push–pull effect of D–A building blocks and the band structure of Im-PCPPs. On this basis, Im-PCPP-2 with ionized imidazolium exhibits good catalytic activity toward oxidative N-formylation of diverse secondary amines through the cooperation of photoredox and in situ NHC catalysis in a radical manner, where acetaldehyde serves as the carbon source of formylation. Experimental and calculation results suggest that photogenerated multiple ROS play crucial roles: hydroxide OH– as in situ base to generate NHC; hydroxyl radical ·OH as hydrogen atom transfer abstractors to produce aza-Breslow-derived radicals and as radical partners for cross-coupling; singlet oxygen 1O2 as oxidant to convert aza-Breslow-derived radical into cyclic peroxide. It is noteworthy that the photochemical process has achieved efficient conversion of acetaldehyde and its derivatives to high-value-added N-formamides.