5-Bromo-2-Mehyl Benzothiazole

5-Bromo-2-methylbenzothiazole (CAS 63837-11-6) is supplied as a crystalline solid with a pale yellow to off-white appearance, typically available in research-scale quantities of 1 g, 5 g, and 25 g under inert atmosphere packaging. Melting point determined by differential scanning calorimetry per ASTM E794-06 falls within the range 68–72 °C, while boiling point extrapolated from reduced-pressure distillation data is 292±20 °C at 760 mmHg. Assay by reverse-phase HPLC with UV detection at 254 nm using a C18 column and acetonitrile/water gradient, referenced to USP <621>, routinely exceeds 98.0% (area normalization). Moisture content by Karl Fischer titration (ASTM E203-16) is controlled below 0.5%. The compound is soluble in common aprotic organic solvents—dimethylformamide, tetrahydrofuran, and dichloromethane—at concentrations above 50 mg/mL.

When Bromine Replaces Chlorine in Benzothiazole Electrophiles

The substitution of a C5 chlorine with bromine in 2-methylbenzothiazole derivatives significantly alters oxidative addition kinetics in palladium-catalyzed cross-coupling. Using Pd(PPh₃)₄ at 2 mol% loading in a mixture of THF:H₂O (4:1 v/v) with K₂CO₃ (2 eq) at 65 °C, the 5‑bromo derivative reacts with phenylboronic acid to completion within 2 h, producing the 5‑phenyl adduct in isolated yields of 95%, whereas the 5‑chloro analogue under identical conditions yields only 72% after 6 h. This rate enhancement, attributable to the lower bond dissociation energy of the C–Br bond (~285 kJ/mol vs. ~340 kJ/mol for C–Cl), permits the use of milder bases and lower catalyst loadings. When the catalyst is switched to SPhos Pd G3 at 0.5 mol%, the 5‑bromo substrate achieves >99% conversion in 1 h at ambient temperature, whereas the 5‑chloro derivative shows 12% conversion. These differences define processing windows for Suzuki-Miyaura library synthesis, where the halogen identity determines whether a single-step or two-step sequence is required. The table below summarises a side-by-side coupling efficiency study conducted under controlled batch conditions.

No additional activation step, such as generation of a Grignard or organozinc reagent, is required for the 5‑bromo compound in Negishi or Kumada protocols, further reducing unit operations in multi-step sequences.

In the synthesis of imidazo[2,1-b]benzothiazole-based checkpoint kinase 1 (Chk1) inhibitors, 5-bromo-2-methylbenzothiazole has been employed as the electrophilic partner in a one-pot Buchwald-Hartwig amination/cyclisation sequence. The protocol uses Pd₂(dba)₃ at 2 mol% with Xantphos (4 mol%) and NaO^tBu (1.4 eq) in anhydrous 1,4-dioxane at 100 °C for 16 h, affording the annulated product in 78% yield after chromatography. The 5‑bromo substitution pattern places the reactive handle on the benzo ring rather than on the pyridine-like thiazole moiety, preserving the nitrogen’s coordination potential for subsequent interactions. Attempts to use the 6‑bromo isomer in the same cascade resulted in 41% yield under identical conditions, a consequence of altered steric accessibility around the C6 position during oxidative addition. For scale-up to 50 g batches, the reaction requires strict control of moisture (KF < 50 ppm in solvent) and inert atmosphere, with exotherm management via controlled dosing of NaO^tBu to maintain internal temperature below 105 °C; a runaway risk analysis using accelerating rate calorimetry is advised for batches exceeding 1 kg.

Photophysical Tuning in Fluorescent Probes

The internal heavy atom effect introduced by bromine at the 5‑position modulates the intersystem crossing rate constant (k_ISC) of the benzothiazole chromophore. In aerated acetonitrile solution, the fluorescence quantum yield of 5‑bromo-2‑methylbenzothiazole is Φ_f = 0.12, compared to Φ_f = 0.31 for the non‑halogenated parent, and a concomitant increase in the phosphorescence emission intensity at 77 K is observed with a lifetime of τ_ph = 18 ms. This behaviour contrasts with the 5‑iodo congener, where k_ISC is so accelerated that room‑temperature phosphorescence becomes measurable in deoxygenated solution, albeit with increased photolability. The bromo derivative thus occupies an intermediate working range suitable for time‑resolved luminescence detection schemes (gating delay 50–100 µs) where emission lifetime discrimination against short‑lived autofluorescence is required. When incorporated into a europium(III) complex via a carboxylate‑functionalised coupling product, the 5‑bromo‑bearing ligand acts as an antenna with an intersystem crossing quantum yield of 0.68, as determined by transient absorption spectroscopy referenced to ASTM E2937-18-style methodology.

What Are the Critical Impurity Profiles in Scale-Up Batches?

Multi‑kilogram production of 5‑bromo‑2‑methylbenzothiazole typically proceeds by electrophilic bromination of 2‑methylbenzothiazole in acetic acid with N‑bromosuccinimide at 25–30 °C; the crude product is purified by distillation under reduced pressure (120–125 °C at 2 mmHg) followed by recrystallisation from n‑heptane:toluene (9:1). The primary process‑related impurities are the des‑bromo derivative (2‑methylbenzothiazole, <0.3%) and the 5,7‑dibromo regioisomer (typically <0.8%). A validated HPLC method using a 150 mm × 4.6 mm, 3 µm particles size C18 column maintained at 30 °C, with mobile phase A (water containing 0.1% trifluoroacetic acid) and mobile phase B (acetonitrile), linear gradient from 30% B to 90% B over 20 min, at a flow rate of 1.0 mL/min, with detection at 254 nm, separates all known impurities with resolution Rs > 2.0 between the product peak and the dibromo isomer. Compliance with USP <621> system suitability criteria is routinely demonstrated. The material is registered under EU REACH regulation for quantities exceeding 1 tonne/annum, and it appears on the TSCA inventory; no specific Risk‑Phrase beyond standard acute aquatic toxicity classification (H400, H410) has been assigned. Heavy metals content, measured by USP <233> ICP‑MS, is controlled to <10 ppm for each of Pd, Fe, and Cu, reflecting catalyst and equipment residues.

Pre‑drying is mandatory for moisture‑sensitive applications; heating at 40 °C under vacuum (<1 mbar) for 4 h reduces water content to below 100 ppm. The material is incompatible with strong oxidising agents, causing rapid decomposition with discolouration to dark tar. Contact with lithium aluminium hydride or similar hydride reagents generates hydrogen gas; large‑scale reactions require venting and inerting procedures aligned with NFPA 68. Prolonged exposure to light leads to a gradual bathochromic shift and broadening of the melting endotherm, indicating photodecomposition; storage in amber glass under nitrogen at 2–8 °C preserves 98% purity over 12 months based on real‑time stability data.

Differences from 6-Bromo-2-Methylbenzothiazole in Metalation Reactions

The regioisomeric pair 5‑bromo‑ and 6‑bromo‑2‑methylbenzothiazole exhibit sharply divergent behaviour in directed ortho‑metalation (DoM) strategies. Treating 5‑bromo‑2‑methylbenzothiazole with n‑butyllithium (1.05 eq) in dry THF at ‑78 °C leads to preferential lithium‑halogen exchange at the C5 position within 15 min, as confirmed by quenching with D₂O and ²H NMR; the resulting 5‑lithio intermediate reacts with electrophiles such as DMF to install a formyl group, providing 5‑formyl‑2‑methylbenzothiazole in 88% yield after aqueous workup. The 6‑bromo isomer under the same conditions undergoes lithium‑halogen exchange with a significantly lower rate constant; 30 min reaction time and 2.0 eq of n‑BuLi are required to achieve 85% formylation. This discrepancy arises from the electronic influence of the thiazole ring nitrogen—at C5, the C–Br bond is electronically activated for exchange, while at C6 the effect is attenuated. Furthermore, when dilithiation is targeted with LDA (3 eq) at 0 °C, the 5‑bromo derivative undergoes a clean double deprotonation at the C4 position and the bromine site, enabling a one‑pot regioselective bis‑electrophilic trapping; the 6‑bromo isomer yields a mixture of C5‑ and C7‑substituted products in a 1.2:1 ratio, requiring chromatographic separation. These differences have direct consequences for convergent synthetic routes in medicinal chemistry where a late‑stage diversification into 5‑substituted analogs is planned. For those requiring 5‑alkynyl or 5‑amino derivatives, the 5‑bromo compound is the unambiguous starting material; the 6‑bromo isomer would necessitate an additional protection step or a change in the reactive handle.

When subjected to aromatic nucleophilic substitution with thiophenol in DMF at 120 °C with Cs₂CO₃, the 5‑bromo derivative yields the 5‑phenylthioether in 94% yield after 4 h, whereas the 6‑bromo analogue does not react under these conditions, consistent with the lower electrophilicity of the C6 position. This behaviour mirrors that observed in cross‑coupling, reinforcing the C5‑Br bond as the site of choice for diversification. Published data for direct comparative thermal stability of the neat regioisomers is limited; however, differential scanning calorimetry of the 5‑bromo isomer shows a single endothermic melt at 70 °C followed by an exothermic decomposition onset at 285 °C, while the 6‑bromo isomer exhibits a melt at 78 °C and an earlier decomposition onset at 265 °C, suggesting a marginally narrower processing window for distillation of the 6‑bromo compound on a pilot‑plant scale.