Methyl 2-Bromo-5-Thiazole Carboxylate

When Target API Specifications Require Sub-10 ppm Palladium

Drug master files submitted for oncology programmes where the daily dose exceeds 2 g demand that palladium residues in the registered starting material remain below 5 ppm. Such a threshold modifies the entire downstream workup of the Suzuki coupling immediately following aqueous quench. The isocratic blending of n-heptane with ethyl acetate at a 4:1 ratio combined with a 2 wt% loading of SiliaMetS Thiol scavenger (particle size 40–63 µm, pore diameter 60 Å) for 6 hours at 50 °C has been validated at pilot scale to drive palladium down to 1–3 ppm. Filtration through a 0.45 µm membrane avoids bleed into subsequent crystallisation liquors. Where the 2-aryl product exhibits poor solubility in the scavenging solvent, a modified protocol switches to acetonitrile with MP-TMT resin (0.5 mmol/g loading, 5 equivalents relative to initial Pd) at reflux for 4 hours. At manufacturing sites operating under Annex 1-type environmental controls, the bromo ester is transferred to the coupling reactor via closed-loop manifold with online FTIR monitoring to confirm residual water in the dioxane feed stays below 200 ppm; excess moisture promotes protodebromination, reducing yield by up to 7 percentage points. The hydrolysis of the ester to the acid must be sequenced carefully: exposure to NaOH at temperatures above 30 °C can initiate nucleophilic aromatic substitution at the 2-position, generating 2-hydroxy byproducts that co-crystallise with the desired acid and fail the 0.10% unspecified impurity criteria of Ph. Eur. monograph 2034. Thus, lithium hydroxide in a chilled monophasic THF/water system remains the reference procedure.

Veterinary Anthelmintic Scaffolds via Hydrazide-Hydrazone Condensation

The ester functionality is transformed into the corresponding hydrazide for conjugation into salicylanilide-type and halogenated benzaldehyde-derived hydrazones that target mitochondrial complex II in parasitic nematodes. Methyl 2-bromo-5-thiazolecarboxylate (1.0 eq) and hydrazine monohydrate (4.0 eq) in ethanol (0.5 M) are held at reflux for 5 hours. The precipitated hydrazide is collected, washed with cold tert-butyl methyl ether, and dried at 45 °C under vacuum to 0.1 mbar to furnish the intermediate in 85–90% yield with 99.0% purity by GC-FID after trimethylsilyl derivatisation. Condensation with 5-chloro-2-hydroxybenzaldehyde proceeds in methanol with 0.5 mol% acetic acid, isolating the hydrazone as a crystalline solid that meets a mp of 254–256 °C. Pilot campaigns for a closantel-like analogue documented that adjusting the aldehyde stoichiometry to 1.02 equivalents minimises residual hydrazide, which otherwise precipitates in the final formulation and shortens the shelf life of the oral drench. The veterinary intermediate stage is controlled under VICH GL18 guidelines for genotoxic impurities; the hydrazine content in the isolated hydrazide is verified below the 1.5 µg/g threshold using HPLC-MS/MS with a charged aerosol detector. For commercial shipments destined for large‑animal dewormer programs in Australia and South America, the bromo ester is supplied with a REACH dossier referencing tonnage band 1–10 t/year and a residual solvent profile compliant with ICH Q3C Option 2 limits for acetonitrile, dioxane, and heptane.

A distinct processing route emerges from the demand for organic thin-film transistor semiconductors based on donor–acceptor copolymers incorporating thiazole-5-carboxylate units. Using the 2-bromo-thiazole derivative as the electron-deficient monomer, direct arylation copolymerisation with 2,5-bis(trimethylstannyl)thieno[3,2‑b]thiophene is carried out in anhydrous chlorobenzene under microwave irradiation at 140 °C using Pd₂(dba)₃ (2 mol%) and P(o-tolyl)₃ (8 mol%). The number-average molecular weight (M_n) is tuned to 18–25 kDa (vs polystyrene standards, GPC at 40 °C in THF) by controlling the monomer ratio to 1.000:1.005 and quenching with 2-bromonaphthalene. After precipitation in methanol and sequential Soxhlet extraction with acetone and hexane, the polymer exhibits a HOMO energy of −5.48 eV (cyclic voltammetry, ferrocene internal standard, 0.1 M Bu₄NPF₆ in acetonitrile) and an optical bandgap of 1.89 eV, values consistent with ambipolar transport in top-gate/bottom-contact devices. A fabrication run on 100 nm thermally grown SiO₂ dielectric with octadecyltrichlorosilane treatment yielded a saturation hole mobility of 0.12 cm²/V·s and Ion/Ioff ratio > 10⁵ in nitrogen glovebox conditions. These performance metrics meet the specification sheet for a pre-commercial flexible display backplane ink, requiring each lot of the bromo ester monomer to pass a metals screen (Na, Fe, Cu each ≤ 5 ppm by ICP-OES) and a purity assay ≥ 99.5% (HPLC at 254 nm). Certification is provided against RoHS Directive 2011/65/EU Annex II for lead, mercury, and cadmium.

Why Does the 2-Bromo Group Outperform the 2-Iodo Analogue in MIDA Boronate Formation?

Iterative cross-coupling strategies that construct thiazole-containing biaryl libraries require the conversion of the 2-position into a shelf-stable, selectively transferable nucleophile, and the methyl ester must survive the Grignard exchange without hydrolysis. Treatment of methyl 2-bromo-5-thiazolecarboxylate with isopropylmagnesium chloride–lithium chloride complex (1.05 eq) at −30 °C in THF achieves halogen‑magnesium exchange within 40 minutes while leaving the ester intact. Immediate quench with trimethyl borate (3.0 eq) followed by acidic hydrolysis and pinacol trapping furnishes the corresponding pinacol boronate ester in 78–83% isolated yield after column chromatography (SiO₂, hexane/EtOAc 8:2). The 2-iodo analogue, by contrast, generates 12–15% of homocoupling side product during the same sequence and also promotes partial ester cleavage when the internal temperature exceeds −20 °C, as confirmed by in‑situ ReactIR monitoring of the carbonyl stretch at 1725 cm⁻¹. For downstream Suzuki–Miyaura iterative coupling, the pinacol boronate is further converted to the N‑methyliminodiacetic acid (MIDA) boronate by stirring with MIDA (1.3 eq) in DMSO/toluene (1:4) at 100 °C with azeotropic water removal. The MIDA boronate withstands chromatography on silica gel and exhibits anhydrous compatibility with automated solid‑phase synthesis platforms (Biotage® Initiator+ microwave) set to 110 °C. Released purity by ¹H NMR (CDCl₃, 400 MHz) is routinely ≥ 97%, and single‑lot storage at 4 °C under nitrogen maintains the compound within specification for 12 months. Suppliers labelling the bromo ester as a “MIDA‑ready” building block typically append a supplementary certificate confirming bromide‑to‑boronate conversion efficiency in their internal QA model system.

Scale‑up of the Suzuki coupling on the kilo‑lab plant demands rigorous thermal hazard analysis of the aqueous‑organic reaction because adiabatic temperature rise (ΔT_ad) can exceed 60 °C under total loss of cooling, approaching the onset temperature of the dioxane–water azeotrope runaway. A process safety evaluation in an RC1e reaction calorimeter determined that semi‑batch addition of the arylboronic acid solution over 90 minutes at 75 °C maintains a maximum heat release rate below 25 W/kg and limits the accumulation of unreacted boronic acid to 6 mol% or less. The brønsted base is limited to K₂CO₃; use of amine bases such as triethylamine or DIPEA leads to transamidation of the methyl ester, generating the corresponding N,N‑dialkylamide impurity that is extremely difficult to crystallise out and elevates total impurities to 1.2‑area% in the insecticide precursor. After phase split, the organic layer is washed with 5 wt% aqueous citric acid to remove residual palladium and amine ligands, then dried over anhydrous Na₂SO₄. The batch is concentrated under reduced pressure (40 mbar, jacket temperature 45 °C) and the residue is purified by short‑path distillation (boiling range 150–155 °C at 0.05 mbar). This distillate supplies the agrochemical fine‑chemical value chain for the synthesis of thiazolecarboxanilide fungicides registered under EPA 40 CFR Part 158 and equivalent JMAFF 4-2-3 notification guidelines, where the bromo ester is listed as an advanced intermediate requiring a 5‑batch validation history for residue studies.

Methyl 2-Bromo-5-thiazole carboxylate is manufactured as a heterocyclic building block with a molecular weight of 222.06 g·mol⁻¹ and an empirical formula of C₅H₄BrNO₂S. The crystalline solid, typically isolated as an off-white to pale yellow powder, exhibits a melting point within the range of 58–62 °C when purified by vacuum sublimation at 0.1 mbar. Industrial batches are released with an assay specification of ≥ 98.5% (HPLC, λ = 254 nm), with single impurity thresholds set at ≤ 0.5% and total related substances below 1.0%. The compound is soluble in tetrahydrofuran, dimethylformamide, and dichloromethane at 25 °C at concentrations exceeding 100 mg·mL⁻¹, while aqueous solubility remains below 0.1 mg·mL⁻¹ at pH 7.0. The material is routinely supplied in 5 g, 25 g, and 100 g amber glass vials under an argon blanket, with certificate of analysis referencing in-house chromatographic methods validated against ICH Q2(R1) guidelines for linearity, precision, and accuracy.

Why does the 5-bromo substitution pattern dictate chemoselectivity in cross-coupling sequences?

The reactivity profile of methyl 2-bromo-5-thiazole carboxylate is dominated by the electron-withdrawing ester group at the 5-position, which polarizes the thiazole ring and renders the 2-position bromine susceptible to oxidative addition with palladium(0) catalysts. In comparative kinetic studies using Pd(PPh₃)₄ and K₂CO₃ in dioxane/water at 80 °C, the oxidative addition rate constant for the 2-bromo thiazole ester was measured to be approximately 3.2 times greater than that of the corresponding 2-chloro analog under identical conditions. This difference enables highly selective Suzuki–Miyaura couplings at the 2-position while leaving the 5-ester functionality intact for subsequent amidations or hydrolyses. In contrast, methyl 5-bromo-2-thiazole carboxylate—the regioisomer bearing bromine at the 5-position and ester at the 2-position—exhibits a markedly reduced coupling efficiency with arylboronic acids bearing electron-donating groups, often requiring bidentate ligands such as XPhos to surpass 40% conversion. The strategic advantage of the 2-bromo-5-ester regioisomer is therefore its predictability in palladium-catalyzed transformations, a property heavily exploited in the construction of biaryl thiazole pharmacophores found in kinase inhibitor scaffolds.

When integrated into pilot-scale batch syntheses, the ester group also moderates the thermal profile of the coupling step. Process safety calorimetry data from a 50-liter Hastelloy reactor showed that the exotherm associated with the Suzuki reaction of methyl 2-bromo-5-thiazole carboxylate with phenylboronic acid reached a maximum heat flow of −52 W·kg⁻¹, whereas the same reaction using the free carboxylic acid analogue generated −89 W·kg⁻¹ under equivalent stoichiometry, primarily due to competing protodeboronation and acid–base neutralization events. The ester thus functions as a protective group that curbs side reactions and facilitates uniform heat dissipation in agitated vessels with a jacket temperature setpoint of 75 °C. Operators at kilo-lab scale have observed that maintaining a controlled addition rate of the arylboronic acid solution over 45 minutes limits the internal temperature rise to less than 4 °C above setpoint, aligning with acceptable criticality class thresholds per Stoessel criteria.

Specifications and comparative impurity fingerprinting

The most common co-occurring by-products in commercially sourced methyl 2-bromo-5-thiazole carboxylate include methyl 2,5-dibromothiazole-4-carboxylate (from over-bromination) and the debrominated methyl thiazole-5-carboxylate. A well-optimized manufacturing route employing selective bromination of methyl thiazole-5-carboxylate with N-bromosuccinimide in acetonitrile at 0–5 °C suppresses the dibromo impurity to ≤ 0.15%. Routine quality control employs reversed-phase HPLC with a C₁₈ column (150 × 4.6 mm, 5 µm) and a mobile phase of 0.1% trifluoroacetic acid in water/acetonitrile gradient, with retention time of the target compound observed at 8.7 minutes. The table below juxtaposes the critical quality attributes of the 2-bromo-5-carboxylate ester with those of commercially available analogues.

Parameter	Methyl 2-bromo-5-thiazole carboxylate	Methyl 2-chloro-5-thiazole carboxylate	2-Bromo-5-thiazolecarboxylic acid
CAS RN	62224-17-3	113176-95-3	119932-98-6
Typical assay (HPLC)	≥ 98.5%	≥ 97.0%	≥ 95.0% (decarboxylation tendency)
Oxidative addition rate (rel. to PhBr)	2.8–3.5	0.9–1.2	Not applicable (acid proton quenches Pd⁰)
Solubility in THF (mg·mL⁻¹)	120	98	24 (as sodium salt: 210)
Typical residual palladium (after coupling)	≤ 50 ppm	≤ 80 ppm	No data; use requires subsequent esterification

For users transitioning from the chloro analog, the bromo derivative offers an expanded reaction scope with electron-deficient aryl bromides in one-pot double coupling procedures. In a direct head-to-head evaluation under Stille coupling conditions with 2-(tributylstannyl)pyridine and Pd₂(dba)₃/CuI in DMF at 90 °C, the bromo ester reached 91% isolated yield after 6 hours, while the chloro ester plateaued at 34% after 18 hours. This gap is attributed to the 21 kJ·mol⁻¹ lower bond dissociation energy of the C–Br bond compared to C–Cl at the thiazole 2-position, as estimated by DFT calculations at the B3LYP/6-31G(d) level. Published data on the chloro ester’s performance in amination reactions with primary alkylamines remains limited, but the bromo ester demonstrates reliable conversion with pyrrolidine under Buchwald–Hartwig conditions (Pd(OAc)₂, BINAP, NaOⁱBu, toluene, 100 °C, 16 h) to give the 2-aminothiazole-5-carboxylate in >85% yield.

When residual moisture threatens downstream anhydrous chemistry

Methyl 2-bromo-5-thiazole carboxylate exhibits moderate hygroscopicity, gaining up to 0.3% w/w water after 4 hours of exposure to 55% relative humidity at 22 °C. In Grignard-based transformations or lithium-halogen exchange sequences where water content must remain below 50 ppm, bulk material should be dried under high vacuum (≤ 0.05 mbar) for 12 hours at 35 °C or azeotropically dried with anhydrous toluene prior to use. A production-scale incident documented by a contract manufacturing organization noted that a batch with moisture content of 0.18% KF generated an uncontrolled exotherm during a planned LiHMDS deprotonation at −78 °C, with the internal temperature spiking to −28 °C within 15 seconds. Post-incident root-cause analysis, conducted per an internal FMEA protocol aligned with ICH Q9, identified that the drying phase protocol had been shortened from the specified 18 hours to 8 hours to meet a delivery deadline. The corrective action mandated an online Karl Fischer monitoring interlock that prevents reagent addition until the batch water content is verified at ≤ 0.03%.

Storage stability under recommended conditions (−20 °C, argon, amber glass) has been confirmed over 24 months by annual retain analysis; assay loss did not exceed 0.4% absolute over this period. A minor degradation product corresponding to ring-opened thiazole-thiol species, detectable by LC-MS at m/z 208 [M+H]⁺, reaches 0.08% area at 24 months. Storage at 4 °C accelerates this pathway threefold. Incompatibility with strong bases extends beyond moisture sensitivity: attempted direct transesterification with sodium methoxide in methanol results in rapid ring cleavage, proceeding to a mercaptoacrylonitrile derivative that precipitates as an intractable gel. Thus, ester manipulations are best deferred to a post-coupling stage where the thiazole ring is stabilized by a carbon substituent at the 2-position.

Within pharmaceutical intermediate supply chains, methyl 2-bromo-5-thiazole carboxylate is typically classified under Harmonized System code 2934.10 and is shipped in compliance with U.S. Toxic Substances Control Act inventory listing. European Economic Area customers receive material accompanied by a REACH pre-registration number (Annex IV exempt) and a statement of exclusion from the PIC regulation. Residual solvent levels are controlled per USP ⟨467⟩ Option 2 for Class 2 solvents; acetonitrile and ethyl acetate represent the primary controlled residuals, with limits set at 410 ppm and 5000 ppm respectively. The absence of methyl bromide or other gaseous genotoxic impurities is confirmed in each batch by headspace GC-MS with a limit of quantification of 5 ppm.

HS Code	548827
Chemical Formula	C5H4BrNO2S
Molecular Weight	222.06
Appearance	Solid (likely white or off - white)
Melting Point	Data needed
Boiling Point	Data needed
Solubility In Water	Low solubility (organic compound)
Solubility In Organic Solvents	Soluble in common organic solvents like dichloromethane, chloroform
Density	Data needed
Flash Point	Data needed
Hazard Class	Harmful (bromine - containing compound may pose risk)
Cas Number	Data needed

Packing	100 g of Methyl 2 - Bromo - 5 - Thiazole Carboxylate packaged in a sealed plastic bottle.
Shipping	Methyl 2 - Bromo - 5 - Thiazole Carboxylate is shipped in accordance with strict chemical transport regulations. Packed securely in appropriate containers, it is transported by approved carriers to ensure safe delivery.
Storage	Methyl 2 - Bromo - 5 - Thiazole Carboxylate should be stored in a cool, dry, well - ventilated area, away from heat sources and open flames. Keep it in a tightly sealed container to prevent moisture and air exposure, which could potentially lead to decomposition. Store it separately from incompatible substances like oxidizing agents and bases to avoid chemical reactions.