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

Abstract

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 (DPQN2,4-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 DPQN2,4-di-OMe. Simply upon protonation, DPQN2,4-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 DPQN2,4-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 DPQN2,4-di-OMe could be minimized to 0.025 mol % (TON = 3360), and a polymer-supported photocatalyst, DPQN2,4-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).