Development of a Quinolinium/Cobaloxime Dual Photocatalytic System for Oxidative C–C Cross-Couplings via H₂ Release

Designing molecular photocatalysts for potent photochemical reactivities ranks among the most challenging but rewarding endeavors in synthetic photochemistry. Herein, we document a quinoline-based organophotoredox catalyst, 2,4-bis­(4-methoxyphenyl)­quinoline (DPQN^{2,4‑di‑OMe}), that could be assembled via the facile aldehyde–alkyne–amine (A³) couplings. Unlike the reported photocatalysts, which impart their photoreactivities as covalently linked entities, our mechanistic studies suggested a distinct proton activation mode of DPQN^{2,4‑di‑OMe}. Simply upon protonation, DPQN^{2,4‑di‑OMe} could reach a highly oxidizing excited state under visible-light irradiation (E*_1/2 = +1.96 V vs a standard calomel electrode, SCE). On this basis, the synergistic merger of DPQN^{2,4‑di‑OMe} and cobaloxime formulated an oxidative cross-coupling platform, enabling the Minisci alkylation and various C–C bond-forming reactions with a diverse pool of radical precursors in the absence of chemical oxidants. The catalytic loading of DPQN^{2,4‑di‑OMe} could be minimized to 0.025 mol % (TON = 3360), and a polymer-supported photocatalyst, DPQN^2,4-di-OR@PS, was prepared to facilitate catalyst recycling (at a 0.50 mmol % loading and up to five times without significant loss of photosynthetic efficiency).