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

Photocatalytic C(sp3) radical generationviaC–H, C–C, and C–X bond cleavage

“…In 2021, Li's group reported a quinolinium/cobaloxime co-catalysed Minisci alkylation of alkyl trifluoroborates without external chemical oxidant (Scheme 48B). 210 With detailed mechanistic studies, they conceptualised a proton-activation mode of N -heteroaromatics and developed a novel 2,4-bis(4-methoxyphenyl)quinoline organophotocatalyst (DPQN 2,4-di-OMe ) with an extensive oxidation window ( = +1.96 V vs. SCE in MeCN). 62 SET with different alkyl trifluoroborates broke the C–B bonds and turned them into R˙, which was enrolled in the typical Minisci alkylation mechanism.…”

Dual Photoredox and Cobalt Catalysis Enabled Transformations

“…Recently, Li and coworkers [28] reported a dual photoredox and cobaloxime catalyzed Minisci alkylation of heterocycles using newly designed novel class of quinoline based photocatalyst with highly oxidizing excited state under visible light irradiation. Potassium alklytrifluoroborates were used as alky radical precursor along with trifluoroacetic acid and catalytic amount of Co(III) salt Co(dmgH) 2 (Py)Cl.…”

“…Moreover, an array of products like alkenyl (hetero)arenes, alkenyl sulfides, enamides, and phenol derivatives can be directly synthesized in high yields under mild condition from chemical feedstocks. Indeed, based on the results of control experiments, plausible mechanism was proposed (Figure 11 Recently, Li and coworkers [28] reported a dual photoredox and cobaloxime catalyzed Minisci alkylation of heterocycles using newly designed novel class of quinoline based photocatalyst with highly oxidizing excited state under visible light irradiation. Potassium alklytrifluoroborates were used as alky radical precursor along with trifluoroacetic acid and catalytic amount of Co(III) salt Co(dmgH) 2 (Py)Cl.…”

Deoxygenative Functionalizations of Aldehydes, Ketones and Carboxylic Acids

“…In modern synthetic chemistry,t he carbon-centered radical is ah ighly versatile intermediate,a nd its addition to unsaturated functionalities has become arobust strategy to to form aC ÀClinkage between two desired organic motifs. [81][82][83][84][85][86][87][88][89][90][91] Thec arbonyl unit represents one of the ideal sources of carbon-centered radicals,a si ti saprevalent native functionality in many chemical feedstocks.M ore importantly,t he generation of radicals from carbonyl compounds such as aldehydes and ketones can often be realized without prefunctionalization (Scheme 12 a). [38,92] Despite this theoretical feasibility,ad irect deoxygenative conversion of aldehydes and ketones into alkyl radicals or their equivalents with the newly generated radical center on the carbonyl carbon remains rare.…”