5-Bromopyrazine-2-carboxylic Acid: Handling & Synthesis Protocol

5-Bromopyrazine-2-carboxylic acid is a halogenated heteroaromatic carboxylic acid that serves as a versatile bifunctional building block in medicinal chemistry, enabling selective cross-coupling at the C-5 bromide position and amide or ester bond formation at the carboxylic acid group.

CAS Number

923482-27-3

Molecular Formula

C₅H₃BrN₂O₂

Molecular Weight

202.99 g/mol

Appearance

White to off-white crystalline solid

Recommended Storage

Room temperature, sealed, away from moisture and light

Key Functional Groups

Aryl bromide (C-5), carboxylic acid (C-2), pyrazine nitrogen atoms (N-1, N-4)

SKU

B177857

Overview and Chemical Properties

5-Bromopyrazine-2-carboxylic acid (SKU B177857) is a bifunctional aromatic heterocyclic compound featuring a pyrazine ring substituted with a bromine atom at position 5 and a carboxylic acid group at position 2. Its molecular formula is C₅H₃BrN₂O₂ with a molecular weight of approximately 202.99 g/mol.

The compound possesses two orthogonal reactive handles: (1) the aryl C–Br bond, which is amenable to transition-metal-catalyzed cross-coupling reactions such as Suzuki–Miyaura, Buchwald–Hartwig, and Sonogashira couplings; and (2) the carboxylic acid moiety, which can be activated toward amide bond formation, esterification, or reduction. The electron-deficient pyrazine ring further enhances the reactivity of the aryl bromide toward oxidative addition into palladium(0) catalysts.

These features make 5-Bromopyrazine-2-carboxylic acid a valuable building block in the synthesis of pharmaceutically active molecules, agrochemicals, and functional materials.

Safety, Handling, and Storage

Personal Protective Equipment (PPE): Handle 5-Bromopyrazine-2-carboxylic acid wearing nitrile gloves, a laboratory coat, and safety goggles. Work in a well-ventilated fume hood to avoid inhalation of dust or aerosols.

Skin and Eye Contact: In case of skin contact, wash the affected area thoroughly with soap and water for at least 15 minutes. For eye contact, flush immediately with copious amounts of water and seek medical attention.

Storage Conditions: Store the compound at room temperature in a tightly sealed container, protected from light and moisture. Prolonged exposure to humid conditions may promote hydrolysis or physical degradation of the solid. Keep away from strong oxidizing agents and strong bases.

Disposal: Dispose of waste material in accordance with local institutional, regional, and national regulations governing halogenated organic compounds. Do not dispose of down drains.

Note: Consult the Safety Data Sheet (SDS) supplied with SKU B177857 for complete hazard and first-aid information.

Protocol: Suzuki–Miyaura Cross-Coupling at the C-5 Bromide

This protocol describes a representative palladium-catalyzed Suzuki–Miyaura coupling of 5-Bromopyrazine-2-carboxylic acid with an arylboronic acid to install an aryl group at the C-5 position while preserving the free carboxylic acid for subsequent transformations.

Materials Required:

• 5-Bromopyrazine-2-carboxylic acid (SKU B177857), 1.0 equiv.
• Arylboronic acid, 1.5 equiv.
• Pd(PPh₃)₄ or Pd(dppf)Cl₂, 3–5 mol%
• K₂CO₃ or Cs₂CO₃ (base), 2.0–3.0 equiv.
• Solvent: DMF/H₂O (4:1 v/v) or 1,4-dioxane/H₂O (4:1 v/v)
• Inert atmosphere: N₂ or Ar

Procedure:

1. Charge a dry Schlenk flask or round-bottom flask equipped with a magnetic stir bar with 5-Bromopyrazine-2-carboxylic acid (1.0 equiv.) and the arylboronic acid (1.5 equiv.).
2. Add the palladium catalyst (3–5 mol%) and the base (2.0–3.0 equiv.).
3. Add degassed solvent (DMF/H₂O, 4:1) and purge the reaction vessel with N₂ or Ar for 10 minutes.
4. Heat the reaction mixture at 80–90 °C with stirring for 12–16 hours, monitoring progress by TLC or LCMS.
5. Upon completion, cool the reaction to room temperature. Dilute with ethyl acetate and wash with 1 M HCl (to acidify and precipitate the product carboxylic acid if applicable), saturated NaHCO₃, and brine.
6. Dry the organic layer over anhydrous Na₂SO₄, filter, and concentrate under reduced pressure.
7. Purify the crude product by column chromatography (silica gel, ethyl acetate/hexane or ethyl acetate/DCM gradient) or recrystallization as appropriate.

Expected outcome: The coupled product bearing the new aryl group at C-5 and the intact carboxylic acid at C-2. Yields and conditions should be optimized for each specific boronic acid partner.

Protocol: Amide Bond Formation at the Carboxylic Acid

The carboxylic acid group of 5-Bromopyrazine-2-carboxylic acid can be converted to an amide using standard coupling reagents. This protocol retains the C-5 bromide intact for downstream cross-coupling, enabling sequential diversification strategies.

Materials Required:

• 5-Bromopyrazine-2-carboxylic acid (SKU B177857), 1.0 equiv.
• Amine coupling partner (primary or secondary amine), 1.2 equiv.
• HATU or EDC·HCl / HOBt, 1.2–1.5 equiv.
• DIPEA or Et₃N (base), 2.0–3.0 equiv.
• Solvent: DMF or DCM, anhydrous
• Reaction temperature: 0 °C to room temperature

Procedure:

1. Dissolve 5-Bromopyrazine-2-carboxylic acid (1.0 equiv.) in anhydrous DMF (0.1–0.5 M) under N₂.
2. Add HATU (1.2 equiv.) and DIPEA (2.0 equiv.) and stir at 0 °C for 10 minutes to pre-activate the carboxylic acid.
3. Add the amine (1.2 equiv.) and allow the reaction to warm to room temperature. Stir for 2–12 hours, monitoring by TLC or LCMS.
4. Dilute with ethyl acetate, wash with saturated NH₄Cl, saturated NaHCO₃, and brine.
5. Dry over Na₂SO₄, filter, and concentrate under reduced pressure.
6. Purify by column chromatography or trituration as appropriate.

Note: The C-5 bromide remains intact under these mild conditions and is available for subsequent palladium-catalyzed cross-coupling reactions.

Troubleshooting and Optimization Tips

Low coupling yield in Suzuki reaction: Ensure rigorous exclusion of oxygen; even trace O₂ can deactivate the palladium catalyst. Use freshly degassed solvents and high-quality boronic acid. Increase catalyst loading to 5 mol% if necessary. Consider switching from Pd(PPh₃)₄ to more active Pd(dppf)Cl₂ or XPhos-Pd-G3 for electron-deficient heterocycles.

Incomplete amide coupling: If conversion is incomplete after 12 hours, add an additional 0.3 equiv. of HATU and 1.0 equiv. of DIPEA. Ensure the amine is free-base form; amine salts require additional equivalents of base.

Solubility issues: 5-Bromopyrazine-2-carboxylic acid has limited solubility in non-polar solvents. Use DMF, DMSO, or aqueous base (0.1 M NaOH) to dissolve the starting material prior to reaction setup. For aqueous washes, acidify to pH 2–3 with HCl to ensure recovery of the product carboxylic acid into the organic layer.

Product identification: Characterize products by ¹H NMR, ¹³C NMR, and HRMS. The pyrazine ring proton resonances (typically δ 8.8–9.2 ppm for H-3 and H-6) and the absence of the aryl bromide signal in the coupled product serve as diagnostic markers.

Frequently asked questions

What is the recommended catalyst system for Suzuki–Miyaura coupling of 5-Bromopyrazine-2-carboxylic acid?

Pd(PPh₃)₄ (3–5 mol%) in DMF/H₂O with K₂CO₃ at 80–90 °C is a reliable starting point. For challenging substrates, Pd(dppf)Cl₂ or third-generation palladacycle precatalysts (e.g., XPhos-Pd-G3) may provide improved yields. Rigorous deoxygenation of the solvent is essential to prevent catalyst deactivation.

Can the carboxylic acid and aryl bromide of 5-Bromopyrazine-2-carboxylic acid be functionalized sequentially?

Yes. A common strategy is to first perform amide bond formation at the carboxylic acid using HATU/DIPEA under mild conditions, preserving the C-5 bromide intact. The resulting amide intermediate is then subjected to palladium-catalyzed cross-coupling to install the desired aryl or heteroaryl group at C-5. This sequential approach allows efficient access to diversely functionalized pyrazine scaffolds.

How should 5-Bromopyrazine-2-carboxylic acid (SKU B177857) be stored to maintain stability?

Store the compound at room temperature in a tightly sealed container, protected from light and moisture. Avoid prolonged exposure to humid air, which may cause physical degradation. No refrigeration is typically required, but keeping the container tightly closed after each use and under dry conditions will help maintain purity over time.

Is 5-Bromopyrazine-2-carboxylic acid suitable for Buchwald–Hartwig amination?

In principle, the C-5 aryl bromide of 5-Bromopyrazine-2-carboxylic acid can undergo Buchwald–Hartwig amination using palladium catalysts with bulky phosphine ligands (e.g., BINAP, XPhos) and a suitable base. However, the free carboxylic acid may interfere with catalyst performance; protecting or pre-converting the acid to an ester or amide before conducting the amination is advisable. Reaction conditions should be optimized empirically for each amine substrate.

References

1. Miyaura, N.; Suzuki, A. Palladium-Catalyzed Cross-Coupling Reactions of Organoboron Compounds. Chem. Rev. 1995, 95, 2457–2483. doi:10.1021/cr00039a007
2. Buchwald, S. L.; Mauger, C.; Mignani, G.; Scholz, U. Industrial-Scale Palladium-Catalyzed Coupling of Aryl Chlorides and Amines — A Personal Account. Adv. Synth. Catal. 2006, 348, 23–39.
3. El-Faham, A.; Albericio, F. Peptide Coupling Reagents, More than a Letter Soup. Chem. Rev. 2011, 111, 6557–6602. doi:10.1021/cr100048w

Products are supplied for research and development use only. Not for use in humans, animals, diagnosis, or therapy.