Original Article:
Organic Dye-Catalyzed, Visible-Light Photoredox Bromination of Arenes and Heteroarenes Using N-Bromosuccinimide
David A. Rogers, et al.
ACS Omega 2018, 3, 10, 12868-12877
10.1021/acsomega.8b02320
Aryl bromides are valuable end targets in a variety of synthetic applications, including pharmaceuticals, natural products, agrochemicals, and advanced materials. Furthermore, brominated arenes serve as important precursors in various synthetic transformations. In this study, the authors used N-bromosuccinimide (NBS) to brominate various arenes and heteroarenes under mild and practical conditions, and developed a new catalytic mode for NBS activation to catalyze arenes and heteroarenes. This strategy utilizes the organic dye erythrosine B as a visible-light photo-oxidative catalyst to amplify the electrophilicity of NBS.
Screening of Reaction Conditions of NBS Brominated Naphthalene
Naphthalene as a representative aromatic substrate was brominated by NBS, and a variety of organic dyes were screened as catalysts. According to the experimental results, erythrosine B was selected as the most effective catalyst for the production of 1-bromonaphthalene. Subsequently, using erythrosine B as the VLPC catalyst, the reaction conditions for the production of 1-bromonaphthalene were further optimized, including the stoichiometric ratio of naphthalene and NBS substrate, catalysts, solvents, reaction concentration, and light source. The optimal reaction conditions are 1 equiv arene, 1.1 equiv NBS, 1%mol erythrosine B, 2h of reaction time, acetonitrile as solvent, (NH4)2S2O8 as additive, and white LED as light source.
Substrate Scope of NBS Bromination
After optimizing the reaction conditions, the researchers investigated a series of reaction substrates, including aromatic reagents and nitrogen-containing heteroaromatics. The findings demonstrate the potential utility of the photocatalytic system. By oxidizing NBS and increasing the electrophilicity of the bromine atom, the photo-oxidative approach appears to shift the potential side reaction (light-promoted radical bromination of benzyl C-H bonds) to the more reactive electrophilic aromatic bromination (Br+) pathway.
Reaction Mechanism of NBS Bromination
Based on the experimental results, the authors proposed a reasonable reaction mechanism as shown in the figure. The photoexcited state of erythrosine B can induce the oxidation of the nitrogen atom of NBS to cationic radical B, thereby amplifying the positive polarization on the bromine atom. After electrophilic aromatic bromination with arene substrates, external oxidants [such as O2, (NH4)2S2O8, or H2O2] can act as oxidants to restore erythrosine B to the ground state. Alternatively, the resulting charged succinimide species C can catalyze the oxidation of the reduced state of erythrosine B to form the succinimide anion D.
Plausible Mechanism
General Procedure for NBS Bromination
1. Add a magnetic stir bar, erythrosine B, ammonium peroxodisulfate, arene/heteroarene, acetonitrile, and NBS to a dry flask.
2. Stir the reaction mixture in air at room temperature in a white LED chamber for the corresponding time.
3. After completion of the reaction, the crude mixture was evaporated under pressure and the brominated product was isolated by silica gel column chromatography.
Chemicals Related in the Paper:
| Catalog Number | Product Name | Structure | CAS Number | Price |
|---|---|---|---|---|
| ACM128085 | N-Bromosuccinimide (NBS) |  |
128-08-5 | Price |
