Directing Group Enables Electrochemical Selectively <i>Meta</i>-Bromination of Pyridines under Mild Conditions

Wu, Yanwei; Xu, Shanghui; Wang, Hong; Shao, Dongxu; Qi, Qiqi; Lu, Yi; Zhou, Junhu; Hu, Wei; Gao, Wei; Chen, Jianbin

doi:10.1021/acs.joc.1c00923

Cited by 23 publications

(12 citation statements)

References 49 publications

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“…Based on the above results and previous reports ( Yuan et al, 2019 ; Gou et al, 2021 ; Wu et al, 2021 ), a plausible reaction mechanism for the electrochemical bromination of glycals was depicted in Scheme 4 . A bromine anion was oxidized to the bromine radical on the anode and subsequently molecular Br 2 .…”

Section: Resultssupporting

confidence: 63%

Electrochemical Bromination of Glycals

Luo

Liu

et al. 2021

Front. Chem.

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Herein, the convenient one-step electrochemical bromination of glycals using Bu4NBr as the brominating source under metal-catalyst-free and oxidant-free reaction conditions was described. A series of 2-bromoglycals bearing different electron-withdrawing or electron-donating protective groups were successfully synthesized in moderate to excellent yields. The coupling of tri-O-benzyl-2-bromogalactal with phenylacetylene, potassium phenyltrifluoroborate, or a 6-OH acceptor was achieved to afford 2C-branched carbohydrates and disaccharides via Sonogashira coupling, Suzuki coupling, and Ferrier rearrangement reactions with high efficiency. The radical trapping and cyclic voltammetry experiments indicated that bromine radicals may be involved in the reaction process.

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Section: Resultssupporting

confidence: 63%

Electrochemical Bromination of Glycals

Luo

Liu

et al. 2021

Front. Chem.

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“…Steady-state photolysis of PTH and TBACl was performed in the presence of a halogen atom trap, 1,1-diphenylethene (DPE). , Continuous irradiation with 390 and 532 nm light through two sides of the sample yielded the photoproduct where one chlorine atom was added to DPE, identified by gas chromatography–mass spectrometry (GC–MS) (Figure S7). Control experiments indicated that irradiation with exclusively 390 nm light generated some chlorine-substituted DPE but with a significantly diminished yield, which is consistent with trace chlorine-atom formation from the aforementioned disproportionation mechanism.…”

Section: Resultsmentioning

confidence: 99%

Chloride Oxidation by One- or Two-Photon Excitation of N-Phenylphenothiazine

Deetz

et al. 2022

J. Am. Chem. Soc.

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Chloride oxidation has tremendous utility in the burgeoning field of chlorine-mediated C–H activation, yet it remains a challenging process to initiate with light because of the exceedingly positive one-electron reduction potential, E° (Cl•/–), beyond most common transition-metal photooxidants. Herein, two photocatalytic chloride oxidation pathways that involve either one- or consecutive two-photon excitation of N-phenylphenothiazine (PTH) are presented. The one-photon pathway generates PTH• + by oxidative quenching that subsequently disproportionates to yield PTH2+ that oxidizes chloride; this pathway is also accessed by the electrochemical oxidation of PTH. The two-photon pathway, which proceeded through the radical cation excited state, 2PTH• +*, was of particular interest as this super-photooxidant was capable of directly oxidizing chloride to chlorine atoms. Laser flash photolysis revealed that the photooxidation by the doublet excited state proceeded on a subnanosecond timescale through a static quenching mechanism with an ion-pairing equilibrium constant of 0.36 M–1. The PTH photoredox chemistry was quantified spectroscopically on nanosecond and longer time scales, and chloride oxidation chemistry was revealed by reactivity studies with model organic substrates. One- and two-photon excitation of PTH enabled chlorination of unactivated C(sp3)–H bonds of organic compounds such as cyclohexane with substantial yield enhancement observed from inclusion of the second excitation wavelength. This study provides new mechanistic insights into chloride oxidation catalyzed by an inexpensive and commercially available organic photooxidant.

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“…Considering the potential application prospects of Ce catalysts and in continuation of our interest in the generation of heteroatom radicals (Cl·, , Br·, , and N· − ) via a sustainable approach, we envisioned to replace the chemical oxidants with traceless electricity to enable the LMCT of Ce IV providing the Cl· species. Hence, a sustainable photoelectrochemical CeCl 3 -mediated protocol to construct the α,α-dichloroketones from aryl acetylenes was established (Scheme d).…”

Section: Introductionmentioning

confidence: 99%

Decoupled Photoelectrochemical Cerium-Catalyzed Oxydichlorination of Alkynes: Slow Releasing of Chloride Ions and Chlorine Radicals

Shao

et al. 2022

ACS Sustainable Chem. Eng.

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Synthetic photoelectrochemistry shows significant advantages in handling of reactive intermediates via a sustainable way and has developed to be the frontier of single electron transfer chemistry. The exploration of new photoelectrocatalysts, especially uncommon metal-based ones, will contribute to the exploitation of new synthetic strategies. Benefiting from reversible redox ability and photoactivity, the cerium salt is expected to become a new photo-electro-response medium to generate active species. In this work, with a green and sustainable reaction system (neutral conditions and renewable light and electricity as energy sources), we developed a novel type of generation of Cl• via electrooxidation and photoinduced ligand-to-metal charge transfer by using CeCl 3 as the photoelectrocatalyst in the oxydichlorination of aryl acetylene. Importantly, we found that the slow releasing of chloride ions and chlorine radicals played an important role in radical chlorination. In addition, several analytical methods including cyclic voltammetry, X-ray photoelectron spectroscopy, electron paramagnetic resonance, and control experiments were conducted to investigate the reasonable mechanism.

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Directing Group Enables Electrochemical Selectively Meta-Bromination of Pyridines under Mild Conditions

Cited by 23 publications

References 49 publications

Electrochemical Bromination of Glycals

Electrochemical Bromination of Glycals

Chloride Oxidation by One- or Two-Photon Excitation of N-Phenylphenothiazine

Decoupled Photoelectrochemical Cerium-Catalyzed Oxydichlorination of Alkynes: Slow Releasing of Chloride Ions and Chlorine Radicals

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