5-Bromo-3-methylisothiazole: Properties, Synthesis & Applications

5-Bromo-3-methylisothiazole is a halogenated five-membered heterocyclic compound that serves as a valuable synthetic building block in medicinal chemistry, agrochemical research, and materials science due to its reactive bromo substituent and electron-deficient isothiazole core.

Molecular Formula

C₄H₄BrNS

Molecular Weight

178.05 g/mol

CAS Number

1003-11-2

Functional Class

Halogenated isothiazole; synthetic building block

Key Reactive Site

C-5 bromo substituent amenable to cross-coupling and nucleophilic substitution

Storage Conditions

Store in a cool, dry, well-ventilated area away from light

Chemical Structure and Physical Properties

5-Bromo-3-methylisothiazole belongs to the isothiazole family of five-membered aromatic heterocycles containing both sulfur and nitrogen atoms at the 1 and 2 positions, respectively. The parent isothiazole ring is electron-deficient due to the electronegative heteroatoms, which renders the ring susceptible to nucleophilic aromatic substitution under appropriate conditions and also modulates the reactivity of substituents attached to the ring.

In this compound, a bromine atom occupies the 5-position adjacent to the nitrogen, while a methyl group is located at the 3-position adjacent to the sulfur. The bromine at C-5 is particularly activated toward palladium-catalysed cross-coupling reactions (e.g., Suzuki–Miyaura, Negishi, Buchwald–Hartwig) because of the electron-withdrawing influence of the ring nitrogen. The methyl group at C-3 provides a handle for further functionalization, for example through benzylic-type oxidation or directed metalation strategies.

The compound is typically a pale yellow to off-white solid or low-melting solid at room temperature, with moderate solubility in common organic solvents such as dichloromethane, tetrahydrofuran, and dimethyl sulfoxide.

Synthesis and Preparation

Isothiazoles are classically prepared by cyclisation of appropriate precursors containing sulfur, nitrogen, and carbon frameworks. A common approach to 3-methylisothiazole derivatives involves the reaction of thioamide or dithioester precursors with hydroxylamine or related nitrogen sources, followed by ring closure and aromatisation.

Halogenation at the 5-position of the isothiazole ring can be achieved by electrophilic bromination of 3-methylisothiazole using reagents such as bromine (Br₂) or N-bromosuccinimide (NBS) under controlled conditions. The C-5 position is kinetically and thermodynamically favoured for electrophilic substitution because it is the most electron-rich site in the otherwise electron-deficient ring.

Alternatively, 5-Bromo-3-methylisothiazole may be prepared via directed lithiation of 3-methylisothiazole at C-5 using strong bases such as n-butyllithium at low temperature, followed by trapping with electrophilic bromine sources. For research and procurement purposes, the compound is commercially available from Aladdin Scientific (SKU: B182634) at defined purity specifications, eliminating the need for in-house synthesis in most laboratory settings.

Applications in Research and Drug Discovery

The isothiazole scaffold is well represented in bioactive molecules, including approved pharmaceuticals and agrochemicals. The sulfur–nitrogen bond within the ring confers unique electronic and hydrogen-bonding properties that are distinct from related five-membered heterocycles such as isoxazole or pyrazole, making isothiazoles attractive bioisosteres in medicinal chemistry programmes.

5-Bromo-3-methylisothiazole is primarily employed as a versatile synthetic intermediate. The C-5 bromo group enables efficient introduction of diverse aryl, heteroaryl, vinyl, amino, and other functional groups via transition-metal-catalysed coupling reactions or metal–halogen exchange, facilitating rapid library synthesis around the isothiazole core. Such libraries are useful for structure–activity relationship (SAR) studies targeting enzymes, receptors, and other biomolecular targets.

In agrochemical research, isothiazole-based structures have demonstrated herbicidal, fungicidal, and bactericidal properties, and brominated isothiazoles serve as intermediates toward novel crop-protection agents. Additionally, the compound may find use in materials science contexts, for example in the construction of conjugated organic frameworks or as a component in functional dyes.

Handling, Safety, and Storage

As with all halogenated heterocyclic compounds, 5-Bromo-3-methylisothiazole should be handled with appropriate precautions. Standard laboratory personal protective equipment (PPE) — including nitrile gloves, safety spectacles, and a laboratory coat — should be worn at all times when handling the compound. Operations that may generate dust or aerosols should be conducted in a well-ventilated fume hood.

The compound should be stored in a tightly sealed container in a cool, dry, well-ventilated area, protected from light and moisture. Compatibility with strong oxidisers should be avoided. In the event of skin or eye contact, the affected area should be flushed thoroughly with water and medical advice sought. Users should consult the current Safety Data Sheet (SDS) provided by Aladdin Scientific for comprehensive hazard information, first-aid measures, and disposal guidance specific to this product.

Waste disposal must comply with applicable local, regional, and national regulations governing the disposal of halogenated organic compounds.

Frequently asked questions

What is 5-Bromo-3-methylisothiazole used for in organic synthesis?

5-Bromo-3-methylisothiazole is primarily used as a halogenated building block for constructing isothiazole-containing molecules via cross-coupling reactions such as Suzuki–Miyaura and Buchwald–Hartwig couplings, as well as through metal–halogen exchange chemistry. It is widely applied in medicinal chemistry for drug discovery and SAR studies, and in agrochemical intermediate synthesis.

Why is the bromine atom at the 5-position particularly reactive?

The nitrogen atom in the isothiazole ring is strongly electron-withdrawing and is adjacent to the C-5 position, activating the C–Br bond toward oxidative addition into palladium(0) catalysts. This makes the 5-bromo substituent highly amenable to transition-metal-catalysed cross-coupling reactions compared to bromo groups on less electron-deficient aromatic systems.

How should 5-Bromo-3-methylisothiazole be stored to maintain stability?

The compound should be stored in a tightly sealed container under cool, dry, and dark conditions. Exposure to moisture, heat, and strong oxidising agents should be avoided. For long-term storage, keeping the compound at 2–8 °C in a desiccated environment is recommended; always consult the Aladdin Scientific SDS for product-specific guidance.

Is 5-Bromo-3-methylisothiazole suitable as a bioisostere in drug design?

The isothiazole ring itself is a recognised bioisostere for other five-membered heterocycles such as isoxazole and thiazole. However, 5-Bromo-3-methylisothiazole is more commonly employed as an intermediate rather than a final pharmacophore; the bromo group is typically used as a handle for further functionalisation to introduce the desired pharmacophoric element at C-5.

References

1. Joule, J. A.; Mills, K. Heterocyclic Chemistry, 5th ed.; Wiley-Blackwell: Oxford, 2010. General reference for isothiazole chemistry.
2. Comprehensive Heterocyclic Chemistry III; Katritzky, A. R., Ramsden, C. A., Scriven, E. F. V., Taylor, R. J. K., Eds.; Elsevier: Oxford, 2008. Vol. 4, Isothiazoles.
3. Aladdin Scientific. Product page: 5-Bromo-3-methylisothiazole, SKU B182634. Aladdin Scientific, Shanghai. Accessed 2024.

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