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

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journal contribution
posted on 2021-11-08, 11:43 authored by Jianbin Li, Chia-Yu Huang, Jing-Tan Han, Chao-Jun Li
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).