Development of a Quinolinium/Cobaloxime Dual Photocatalytic System for Oxidative C-C Cross-Couplings via H-2 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-methoxyphenyOquinoline (DPQN(2,4-di-OMe)), that could be assembled via the facile aldehyde-alkyne-amine (A(3)) 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).