Development of a Quinolinium/Cobaloxime Dual Photocatalytic System for Oxidative C–C Cross-Couplings <i>via</i> H<sub>2</sub> Release
Jianbin Li, Chia‐Yu Huang, Jing‐Tan Han, Chao‐Jun Li
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).