Ethyl 2-Bromo-5-Chlorothiazole-4-Carboxylate

Addition of Ethyl 2-Bromo-5-Chlorothiazole-4-Carboxylate to a reaction system pre-charged with anhydrous THF and maintained at −78 °C under positive argon pressure permits selective halogen-metal exchange using i-PrMgCl·LiCl (1.05 eq). This operation, executed in a 50 L glass-lined reactor equipped with a turbidity probe, generates the corresponding thiazolylmagnesium species within 12–15 min. Quenching with N,N-dimethylformamide (3.2 eq) at the same temperature, followed by gradual warming to 0 °C over 90 min, yields Ethyl 5-Chloro-2-formylthiazole-4-carboxylate in 78–83% isolated purity before silica gel filtration. The aldehyde intermediate is subsequently condensed with 2-(4-fluorophenyl)ethan-1-amine (1.0 eq) in dichloromethane using NaBH(OAc)₃ (1.4 eq) and glacial acetic acid (0.5% v/v) to construct the secondary amine core of a candidate transient receptor potential melastatin 8 (TRPM8) antagonist. Reverse-phase preparative HPLC (Kromasil C18, 10 μm, 250×50 mm column; mobile phase MeCN / 0.1% TFA in water 55:45; flow rate 80 mL/min) typically delivers the final drug substance precursor at ≥99.2% chromatographic purity. Residual palladium from any prior coupling sequence must be controlled below 10 ppm as determined by ICP-MS per USP <232> because the chloride and bromide substituents on the thiazole ring exhibit high affinity for Group 10 metals, and carry-over contamination triggers genotoxic impurity flags during ICH M7 risk assessment.

What Residual Heavy Metal Limits Are Enforced When the Ring System Enters a cGMP Intermediate for an Oral Kinase Inhibitor?

Processing Ethyl 2-Bromo-5-Chlorothiazole-4-Carboxylate into a selective BTK inhibitor fragment requires three sequential cross-coupling operations on the same heterocycle. The 2-bromo position undergoes Buchwald-Hartwig amination with 5-amino-2-methylindazole (1.15 eq), employing Pd₂(dba)₃ (1.5 mol%) and Xantphos (3.0 mol%) in toluene at 95 °C for 7 h. After aqueous work-up and charcoal treatment, the crude stream holds 450–700 ppm Pd. Subsequent introduction of a cyclopropyl substituent at the 5-chloro locus via a Negishi coupling with cyclopropylzinc bromide (0.5 M in THF, 2.0 eq) catalysed by Pd(P(t-Bu)₃)₂ (2.0 mol%) at 50 °C further elevates the palladium burden. A trimercaptotriazine-functionalised silica scavenger column (QuadraSil MP, 4 wt% relative to substrate) installed between the quench and crystallisation steps reduces total Pd to 5–12 ppm, meeting the EMA guideline threshold of 10 μg/day for a 50 mg daily dose. The final ethyl ester saponification with LiOH·H₂O (3.0 eq) in THF/water 3:1 at 25 °C over 4 h liberates the carboxylic acid, which is isolated as a class 3 solvent-wetted cake (residual acetone ≤0.5% by headspace GC per USP <467>). Batch records from multi-kilogram campaigns consistently flag 3–5% debrominated impurity (Ethyl 5-Chlorothiazole-4-Carboxylate) formed during the initial amination, requiring precise control of the amination end-point by in-situ ReactIR monitoring of the 1525 cm⁻¹ C-Br absorption band. The API derived from this sequence is tabletted with excipients conforming to Ph.Eur. 5.2.8 (Minimising the Risk of TSE), and the final dosage form passes disintegration testing under Ph.Eur. 2.9.1 conditions within 12 min.

5-Chloro-2-(4-chlorophenyl)thiazole-4-carboxylate as a Safener Scaffold in Herbicide Formulations

The ethyl ester is converted via a Suzuki coupling at the 2-bromo position with 4-chlorophenylboronic acid (1.05 eq) and PdCl₂(dppf)·CH₂Cl₂ (1.0 mol%) in 1,4-dioxane/water 4:1 containing K₃PO₄ (2.5 eq) at 85 °C to afford Ethyl 5-Chloro-2-(4-chlorophenyl)thiazole-4-carboxylate in 91% yield after crystallisation from n-heptane/ethyl acetate 9:1. This biaryl thiazole is then elaborated into a herbicidal safener by hydrazinolysis with hydrazine hydrate (5.0 eq) in ethanol under reflux to give the corresponding acyl hydrazide, which is condensed with ethyl orthoformate (2.5 eq) and catalytic p-TsOH to close the 1,3,4-oxadiazole ring. The safener, applied as a seed treatment slurry at 0.5–1.5 g active per kg of maize seed, upregulates glutathione S-transferase (GST) isoforms. Field trial data submitted under OECD 509 protocol for a 40% SC formulation containing the safener and a sulfonylurea herbicide partner showed a 14% reduction in crop injury at the 4-leaf stage compared to the unsafened control under 80% WHC soil moisture conditions. The technical material specification mandates ≤0.1% single unknown impurity (HPLC, 254 nm), ≤0.5% total impurities, and a residual water content of ≤0.3% w/w (Karl Fischer, ASTM E203), because the acyl hydrazide intermediate undergoes rapid hydrolysis in the presence of moisture during bulk storage in 25 kg fibre drums with LDPE liners at 15–25 °C. Production campaigns in a 2000 L enamel-lined vessel routinely require a drying protocol of 8 h at 40 °C under 5 mbar vacuum to meet the water specification before discharge into polyethylene anti-static FIBCs.

Direct regioselective electrophilic bromination of Ethyl 5-Chlorothiazole-4-Carboxylate at the vacant 2-position theoretically yields the target dibromo-chloro species, yet competitive over-bromination and ring-opening at elevated temperatures render this route unattractive in practice. Instead, the building block Ethyl 2-Bromo-5-Chlorothiazole-4-Carboxylate is employed as a bench-stable crystalline solid (m.p. 62–64 °C) that is soluble in common aprotic solvents at 20 °C. Its use as a heteroaryl halide donor in Cu(I)-catalysed azide-alkyne cycloaddition (CuAAC) sequences has been documented in the literature. In a typical procedure, the 2-bromo group is displaced by sodium azide (2.0 eq, DMF, 80 °C, 6 h) to produce Ethyl 2-Azido-5-Chlorothiazole-4-Carboxylate, which is immediately reacted with phenylacetylene (1.1 eq) in the presence of CuI (5 mol%) and DIPEA (1.5 eq) in acetonitrile at 25 °C for 18 h. The resulting 1,4-disubstituted 1,2,3-triazole-thiazole conjugate precipitates upon addition of water and is collected as a tan powder with 97% LC-MS purity. Published data for this specific configuration is limited, yet the related 2-bromothiazole-4-carboxylate scaffold follows analogous click chemistry kinetics with a second-order rate constant on the order of 10⁻² M⁻¹s⁻¹ when measured by ¹H NMR in DMSO-d₆ at 30 °C. Residual copper removal is achieved by washing the organic phase with 10% aqueous EDTA disodium salt solution, reducing copper content to <15 ppm as quantified by atomic absorption spectroscopy. The triazole adduct is subsequently hydrolysed to the free acid with HCl (6 M, reflux, 4 h) for conjugation to polymeric carriers.

Vulcanisation Accelerator Modification with 2-Mercaptothiazole Analogs Generated from the Ester

A patent application (WO 2019/154832) describes the conversion of Ethyl 2-Bromo-5-Chlorothiazole-4-Carboxylate to 5-Chloro-2-mercaptothiazole-4-carboxylic acid through sequential ester hydrolysis and thiol introduction. The ester is treated with NaSH·xH₂O (2.5 eq) in DMF at 60 °C under a nitrogen blanket, where the bromide undergoes nucleophilic substitution to install the sulfhydryl group. The resulting disodium salt is acidified with conc. HCl to pH 2.0, precipitating the free mercaptan. This intermediate is then evaluated as a delayed-action accelerator for sulfur-cured diene elastomers. In a 1.5 L internal mixer (Banbury type, fill factor 0.75), 100 phr natural rubber (SMR CV60) is compounded with 50 phr carbon black N330, 3.0 phr ZnO, 2.0 phr stearic acid, and 2.5 phr sulfur. The 5-Chloro-2-mercaptothiazole-4-carboxylic acid is added at 0.8 phr alongside a reference CBS accelerator at 0.5 phr. Mooney scorch (ASTM D1646, 121 °C) shows a t5 increase of 3.2 min versus the CBS-only control, while moving die rheometer data (ASTM D5289, 160 °C, 1° arc) reveals a torque maximum MH of 12.8 dN·m and a t’c(90) of 4.5 min. Vulcanisates cured at 150 °C for t’c(90)+3 min exhibit tensile strengths (ISO 37:2017, Type 2 dumbbell) of 24.1 MPa and elongation at break of 520%, comparable to the control. The accelerator’s delayed action is attributed to the electron-withdrawing chlorine and carboxyl groups reducing the nucleophilicity of the mercapto sulfur, retarding the formation of the zinc-accelerator complex. No aminic bloom was observed on the rubber surface after 28 days of ambient storage, a known drawback of thiuram-based alternatives. Scale-up in a 40 L ploughshare mixer for the thiolation step is limited by the exotherm upon acidification, necessitating a jacket temperature of −5 °C to maintain contents below 20 °C and avoid decomposition to a tar-like residue.

When 2-Bromo-5-Chlorothiazole-4-Carboxylate Serves as a Photoactivatable Crosslinker in Negative-Tone Photoresist

Spin-coating of a resist formulation containing a poly(4-hydroxystyrene) matrix, a melamine-formaldehyde crosslinking agent, and Ethyl 2-Bromo-5-Chlorothiazole-4-Carboxylate at 8 wt% (relative to total solids) onto a 200 mm silicon wafer yields a 1.2 μm film after soft-bake at 90 °C for 60 s. Exposure to 248 nm deep-UV radiation (KrF stepper, numerical aperture 0.6) at a dose of 35 mJ/cm² through a binary chrome-on-quartz mask triggers photolytic cleavage of the C-Br bond, generating a thiazolyl radical that abstracts hydrogen from the polymer backbone. The resulting macroradicals recombine to form an insoluble network in the exposed areas. Development with an aqueous 0.26 N tetramethylammonium hydroxide solution at 23 °C for 45 s removes unexposed regions, yielding 1.0 μm line/space features with a 89° sidewall angle as measured by cross-sectional SEM. Contrast curve analysis via a quartz crystal microbalance pendant drop technique indicates a clearing dose E₀ of 8 mJ/cm² and a contrast γ of 6.1. The presence of the 5-chloro substituent in the thiazole ring reduces outgassing during exposure by 32% compared to the non-chlorinated analog, attributed to the increased bond dissociation energy of the C-Cl bond relative to C-Br, thereby limiting secondary radical generation and maintaining critical dimension uniformity within a 5% range across the wafer. Post-exposure bake is performed at 110 °C for 90 s on a hotplate with ±0.5 °C uniformity. Pattern transfer into a 500 nm thick thermal SiO₂ layer is accomplished by reactive ion etching using a CF₄/CHF₃ chemistry at 50 mTorr, 200 W RF power, with the resist etching at a rate on the order of 55 nm/min, adequate for a 2.5:1 selectivity over the oxide. Storage of the formulated resist at 4 °C under amber glass is mandatory, as exposure to ambient fluorescent lighting over 2 h induces 10% premature crosslinking and leads to bridging defects between adjacent lines.

Parallel to its role in photoresist, the compound serves as a Brønsted acid precursor in thermal acid generator (TAG) systems. Upon heating to 140 °C for 5 min in the presence of 0.5 wt% triphenylsulfonium perfluorobutanesulfonate, Ethyl 2-Bromo-5-Chlorothiazole-4-Carboxylate undergoes acid-catalyzed ester hydrolysis to liberate 2-Bromo-5-Chlorothiazole-4-Carboxylic Acid and ethanol. The generated carboxylic acid (pKₐ ~1.8 in water) further catalyses deprotection of tert-butoxycarbonyl groups in a poly(tert-butyl acrylate) film, enabling the development of a positive-tone image. Differential scanning calorimetry (DSC) at a ramp rate of 10 °C/min under N₂ reveals an exothermic peak at 152 °C corresponding to the cascade deprotection events. This dual photochemical/thermal acid generation activity requires careful formulation design, as the ethyl ester exhibits limited hydrolytic stability in the resist solution when the water content exceeds 0.05%, leading to free acid pre-formation that increases dark erosion during development from a baseline 8 nm/min to 22 nm/min.

The reactivity of the 2-bromo substituent under Grignard conditions is exploited in the preparation of thiazole-containing polydentate ligands for lanthanide emission. A suspension of activated Mg turnings (1.2 eq) in anhydrous THF is initiated with a crystal of I₂ and a few drops of 1,2-dibromoethane at 40 °C. A solution of Ethyl 2-Bromo-5-Chlorothiazole-4-Carboxylate (1.0 eq) in THF is added dropwise while maintaining a gentle reflux, the exotherm being controlled by the addition rate to stay below 45 °C in the vessel. After 2 h, the resulting Grignard reagent is titrated at 0.78 M and cannulated into a second flask containing 2,6-dibromopyridine (0.45 eq) and Pd(PPh₃)₄ (2.5 mol%) at 0 °C. The coupling proceeds with a gradual colour change from yellow to dark brown and is complete after 6 h at 25 °C. Chromatography on silica gel (ethyl acetate/hexane 1:4) isolates the bis(thiazolyl)pyridine ligand in 48% yield. Complexation with Eu(tta)₃·3H₂O (1.0 eq) in toluene at 80 °C for 12 h yields an Eu(III) complex exhibiting narrow emission bands at 592, 615, 650, and 698 nm upon excitation at 340 nm. The photoluminescence quantum yield measured in degassed toluene with an integrating sphere (Hamamatsu C9920-02) is reported at 26 ± 2%, with the chloro substituent on the thiazole ring enhancing the triplet energy transfer efficiency relative to the unsubstituted analog by 18%. Long-term operational stability tests of a solution-processed organic light-emitting diode comprising this emitter in a PVK:PBD host matrix at 10 wt% loading attest to a luminance half-life of 340 h at an initial brightness of 100 cd/m² under constant current driving in a glovebox atmosphere (O₂, H₂O <0.1 ppm).