Ethyl 5-Bromo-2-Phenyl-Thiazole-4-Carboxylate

What Are the Consequences of Ethyl Ester Hydrolysis During Pd-Catalysed Aminocarbonylation?

Aminocarbonylation of aryl bromides with primary amines at 500 psi carbon monoxide in a 316 stainless steel autoclave—using 1 mol% PdCl₂(PPh₃)₂ and 2.0 eq of DABCO as base in dimethylacetamide at 90 °C—affords the corresponding amides without detectable debromination. However, the ethoxycarbonyl group at position 4 is susceptible to nucleophilic attack by traces of water and the amine reactant, generating 5–12% of the 4-carboxylic acid impurity and the corresponding ethyl amide by transesterification. The competing hydrolysis pathway is suppressed by pre-drying the DMAc over 4 Å molecular sieves to a water content below 50 ppm (determined by ASTM E1064) and by employing anhydrous diisopropylethylamine (1.5 eq) in place of DABCO, which coordinates water less strongly. This protocol is incorporated into the production of a GMP advanced intermediate intended for a neuropeptide Y5 receptor antagonist program; the amide product must meet the criteria of ICH Q3A for new drug substance impurities, with the hydrolytic acid specified as a qualified impurity at ≤0.15%. Scale-up runs in a 50 L Hastelloy autoclave fitted with a gas-entrainment impeller demonstrate robustness only when the charging order strictly follows: bromo-ester, catalyst, ligand, amine, then base, with the CO atmosphere established prior to heating to avoid premature Pd(0) aggregation. The isolated yields plateau at 78% due to an intractable tarry by-product that partitions into the organic phase, removed by flash chromatography on 230–400 mesh silica gel eluting with ethyl acetate:heptane (1:3 v/v). A downstream treatment with activated carbon Darco KB-B in refluxing ethyl acetate reduces palladium leaching to ≤5 ppm, required before Chiralpak AD-H separation of enantiomers if a chiral amine was employed.

When the 5-Position Must Remain Intact: Orthogonal Elaboration via Halogen Dance and Directing Group Assistance

Direct functionalisation of the 5-bromo substituent is not always desirable; in certain divergent library syntheses the bromine atom must be temporarily retained while the phenyl ring or the ester group is elaborated. Lithium 2,2,6,6-tetramethylpiperidide (LiTMP) in THF at –40 °C induces a halogen dance, migrating the bromine from the 5-position to the thiazole 4-position and generating the 4-bromo-5-lithio intermediate; quenching with D₂O or a mild electrophile installs a new substituent at C-5 and leaves bromine at C-4, an isomer not directly accessible via esterification. The reaction is capricious above 50 mmol scale owing to competitive ring-opening of THF, so a solvent switch to 2-methyltetrahydrofuran at –30 °C is recommended. Alternatively, the ester group can be reduced to the primary alcohol using 2.2 eq of LiAlH₄ in diethyl ether at 0 °C to RT without touching the C–Br bond; the resulting (5-bromo-2-phenylthiazol-4-yl)methanol is then tosylated and displaced to introduce tertiary amine motifs, presenting a handle for subsequent Suzuki coupling. The dibrominated isomer arising from over-dance (3–5%) and the des-bromo impurity are separated by normal-phase silica chromatography; however, published data for the chromatographic separation of these regioisomers is limited, and preparative SFC (supercritical fluid chromatography) with a 2-ethylpyridine column is under evaluation. For the purpose of regulatory starting material designation, the halogen-dance product would be considered a furcation step according to ICH Q11 example 4, placing the burden of upstream impurity control on the supplier of the original 5-bromo ester. A typical specification requires ≥99.0 area% GC purity, bromine content by Schöniger flask combustion (Ph.Eur. 2.5.5), and ≤0.10% 5,4-dibromo analogue.

Screen-Printable n-Type Polymer Semiconductors from Direct Arylation Polymerization

Ethyl 5-bromo-2-phenylthiazole-4-carboxylate, when paired with a bis-thiophene co-monomer, undergoes C–H activation polymerization (CHAP) to yield a low-bandgap donor–acceptor copolymer with electron mobility reaching 0.12 cm²/V·s in bottom-gate top-contact OFET devices. The polymerization is conducted in anhydrous DMF at 110 °C using 2 mol% Pd(OAc)₂, 4 mol% P(2-MeC₆H₄)₃, catalytic pivalic acid (30 mol%), and 2.5 eq of K₂CO₃ over 48 h. The ethyl ester on the acceptor unit acts as an internal solvating group, preventing premature aggregate precipitation that would arrest chain growth; the number-average molecular weight (Mn) obtained under optimized conditions is 21 kg/mol with Đ of 2.3. Reducing the bromo monomer loading to 0.95 eq relative to the bis-thiophene yields a bromine-terminated chain end, which effectively caps molecular weight and improves shelf-life of the ink formulation by suppressing Pd-catalysed oxidative degradation. To meet IEC 61249-2-21 halogen-free requirements for printed electronics, residual bromide ions in the precipitated and Soxhlet-extracted polymer are verified below 900 ppm by ion chromatography. Formulated inks in anhydrous tetralin (10 mg/mL) are screen-printed onto octadecyltrichlorosilane-treated SiO₂/Si wafers; after annealing at 180 °C for 30 min under nitrogen, the semi-crystalline film exhibits a π–π stacking distance of 3.8 Å by GIWAXS. The terminal device configuration is a flexible logic gate circuit used in NFC sensor tags, where the polymeric semiconductor’s ester side chains are later saponified to potassium carboxylates to afford water-processable semiconductors, an emerging green-chemistry compliance pathway under REACH.

Monomer feed ratio (Br:bis-thiophene)

Mn (kg/mol)

μ_e (cm²/V·s)

Comment

1.00 : 1.00

31.2

3.1

0.08

Significant gel fraction

0.98 : 1.00

24.5

2.5

0.12

Optimum mobility

0.95 : 1.00

21.0

2.3

0.10

Best ink stability

A 2000 L glass-lined reactor equipped with a retreat-curve impeller and a jacket circulating 30% ethylene glycol brine is charged with 68.0 kg (213 mol) of ethyl 5-bromo-2-phenylthiazole-4-carboxylate, 49.5 kg (256 mol) of 4-fluorophenylboronic acid pinacol ester, and 520 kg of degassed toluene. The mixture is sparged with argon through a sintered dip-tube for 45 minutes, then 0.85 kg (1.2 mol, 0.56 mol%) of Pd(dppf)Cl₂·CH₂Cl₂ is added, followed by a filtered solution of 72.5 kg potassium carbonate in 210 kg deionized water. The coupling proceeds at an internal temperature of 65 °C with vigorous agitation (120 rpm) for 12–14 hours, during which the aqueous potassium carbonate is introduced via a dip-tube at a rate maintaining the internal pH of the aqueous phase at 9.5–10.5, verified by in-line Raman spectroscopy. Exceeding pH 11.5 expedites ester saponification, generating levels of the 4-carboxylic acid derivative beyond 0.5% within 4 hours and compromising batch purity. After phase separation at 50 °C, the organic phase is treated with 2.4 kg activated carbon (Norit SX Plus) and filtered through a 0.5 µm in-line PTFE membrane. The filtrate is concentrated under vacuum (50 mbar, bath 45 °C) and the crude product recrystallized from 2-propanol/water (80:20 v/v) to yield off-white crystals of the 5-(4-fluorophenyl) adduct. This intermediate is a supply-chain-critical starting material for an oral SGLT2 inhibitor in Phase III trials, triggering full ICH Q11 designation and requiring a comprehensive control strategy for mutagenic impurities. Despite the absence of structural alerts in the bromo-ester or its boronic ester partner (assessed by DEREK Nexus and Sarah, ICH M7 supplemental), both compounds are controlled as Class 5 non-mutagenic impurities with a limit of 0.15% each, while residual palladium is regulated to ≤10 ppm under ICH Q3D oral permitted daily exposure. The final product specification is monitored batch-to-batch as follows.

Test

Method

Acceptance Criterion

Appearance

Visual, Ph.Eur. 2.2.1

White to off-white crystalline powder

Assay (anhydrous basis)

HPLC, USP 〈621〉

98.5–101.5%

Any single impurity

HPLC area%, ICH Q3A

≤ 0.10%

Hydrolytic acid (byproduct)

HPLC (C18, UV 254 nm)

≤ 0.15%

Palladium content

ICP-MS, USP 〈233〉

≤ 10 ppm

Residual toluene

GC-headspace, USP 〈467〉

≤ 890 ppm

Water (Karl Fischer)

USP 〈921〉 Method Ic

≤ 0.5%

Why the Bromo Substituent Determines Catalyst Turnover Frequency in High-Throughput Experimentation Campaigns

A library of 96 Pd-precatalysts was screened in 1 mL argon-sparged vials using a Chemspeed robotic platform to benchmark C–C bond formation between ethyl 5-bromo-2-phenylthiazole-4-carboxylate and 3-thiopheneboronic acid. The oxidative addition step is the rate-limiting manifold because the thiazole ring's electron-withdrawing effect lowers the LUMO of the C–Br bond, increasing its electrophilicity relative to phenyl bromide by a factor of 3–5 (based on DFT computed activation energies at the B3LYP-D3/def2-SVP level). Turnover number (TON) varied from 50 (P(t-Bu)₃, slow transmetallation) to 980 (Pd-XPhos G2 precatalyst) in THF/water (4:1 v/v) with 2 eq of K₃PO₄ at 40 °C over 18 h. Crucially, the 5-position bromine, when replaced by chlorine or iodine, shifts the selectivity profile: the chloro analogue requires higher Pd loading (5 mol%) and yields 22% homocoupling, while the iodo substrate generates nearly quantitative conversion but is prone to photolytic deiodination under laboratory lighting, making the bromo compound the optimal balance of reactivity and shelf stability for distributed parallel synthesis in medicinal chemistry. The data generated are integrated into a Design-of-Experiments (DoE) model, and the predictive power is validated on a 250 g pilot-scale batch, achieving 88% isolated yield after recrystallization from heptane/MTBE. No specific regulatory framework governs this screening use except standard laboratory safety practices (e.g., OSHA 1910.1450), but the resultant catalyst system is directly transferred to the manufacture of a Phase II clinical intermediate, triggering ICH Q11 starting material specification requirements.

Ethyl 5-bromo-2-phenyl-thiazole-4-carboxylate—C₁₂H₁₀BrNO₂S, molecular weight 312.18 g/mol—functions as a densely functionalized heterocyclic intermediate within pharmaceutical, agrochemical, and materials discovery programs. The electron-withdrawing ester at position 4, in combination with the C5 bromine and C2 phenyl substituent, establishes a highly regioselective scaffold for orthogonal metal-catalyzed cross-coupling sequences. In practice, the compound is predominantly consumed in Suzuki–Miyaura, Negishi, and Buchwald–Hartwig amination cascades targeting kinase insert domain receptor inhibitors, transient receptor potential channel modulators, and strobilurin-analogue fungicides. Direct production-scale monitoring via HPLC–UV at 254 nm using a C18 column (mobile phase acetonitrile:water 70:30 v/v, 0.1% trifluoroacetic acid) confirms typical elution windows with retention time reproducibility within ±0.05 minutes across serial batches produced in 500 L glass-lined reactors operating at −5°C to 0°C during the bromination step to suppress dibromo impurity formation.

What Distinguishes C5 Bromine Reactivity in Palladium(0) Oxidative Addition?

The bromine atom positioned at the thiazole C5 occupies a unique electronic environment distinct from simple aryl bromides. Thiazole π-deficiency increases the carbon–bromine bond polarity, rendering oxidative addition with Pd(PPh₃)₄ or Pd₂(dba)₃/XPhos systems approximately 1.5–2× faster than the corresponding 4-bromophenyl analogue at 60°C in anhydrous tetrahydrofuran, as inferred from competition experiments monitored by ¹⁹F NMR when a fluorinated internal standard is employed. However, this gain in reactivity carries a defined processing risk: competing debromination via β-hydride elimination from the oxidative addition complex can become significant if the reaction mixture exceeds 70°C or if triethylamine is introduced as a base without rigorous degassing. Production-scale Suzuki couplings executed in Hastelloy C-276 reactors with overhead agitation at 150 rpm routinely maintain internal temperature ≤65°C using jacketed cooling with −10°C brine, achieving >99.5% conversion while limiting the protodebrominated byproduct to <0.3 area% by GC-FID calibrated against an external standard of ethyl 2-phenylthiazole-4-carboxylate.

Table 1: Typical specification profile for ethyl 5-bromo-2-phenyl-thiazole-4-carboxylate (technical grade)
Parameter	Method/Standard	Acceptance Criterion
Appearance	Visual, against standard N9 white card	Off-white to pale yellow crystalline powder
Assay (anhydrous basis)	HPLC–UV, 254 nm; column: C18, 250×4.6 mm, 5 µm	≥98.5% area
Moisture content	Karl Fischer coulometric titration, ASTM E203-16	≤0.50% w/w
Residual volatiles (single impurity)	Headspace GC–MS, EPA 5021A guideline	≤0.10% each; total ≤0.40%
Heavy metals (as Pb)	ICP–MS after microwave digestion, ICH Q3D Guideline for Elemental Impurities	≤20 ppm
Chloride ion	Ion chromatography, suppressed conductivity detection, ASTM D4327	≤100 ppm
Isomeric/dimethyl ester impurity	HPLC under specified conditions; relative retention time 1.12 to main peak	≤0.30%
Melting range	Differential scanning calorimetry, heating rate 10°C/min, nitrogen purge	78–82°C

Stability Boundaries Under Humid Load

Prolonged exposure to relative humidity exceeding 60% at 25°C initiates reversible hydration at the ester carbonyl, forming a gem-diol intermediate detectable by FTIR as a broad O–H stretch centered at 3420 cm⁻¹ and a concomitant reduction in the carbonyl band at 1715 cm⁻¹. Following 72 h at 25°C/75% RH inside an ICH-compliant stability chamber, the HPLC assay decreased by 1.8–2.2% absolute, with the hydrate eluting as a poorly retained shoulder peak. Therefore, storage is specified in double polyethylene-lined fiber drums under active nitrogen flush, with in-process handling in areas where dew point is maintained below −20°C by desiccant rotor dryers. Manufacturing facilities located in tropical climates have adopted vacuum-assisted transfer from dryer to glovebox isolators rated for <1 ppm O₂ and H₂O.

Operational incompatibilities include strong amines—especially N,N-dimethylaminopyridine in concentrations above 0.1 mol%—which accelerate transesterification in alcoholic solvents and generate ethyl 4-(dimethylamino)benzoate-related artifacts. The compound is insoluble in water at neutral pH but undergoes slow hydrolysis in aqueous sodium hydroxide at pH ≥12, with a half-life of approximately 48 h at 25°C; this hydrolysis pathway is exploited deliberately in preparative-scale synthesis of the corresponding carboxylic acid for subsequent amide coupling, using ethanol/water 1:1 with 2.5 equivalents NaOH and monitoring by LC–MS (ESI negative mode, m/z 296/298 [M–H]⁻ for the acid).

Reactivity Divergence from the Chloro and Iodo Homologs

The homologous 5-chloro derivative (ethyl 5-chloro-2-phenylthiazole-4-carboxylate) exhibits oxidative addition rates too slow for low-temperature cross-coupling with electron-neutral boronic acids, demanding reaction temperatures of >100°C and microwave irradiation at 150 W in a Biotage Initiator+ system to reach 90% conversion in 2 h. Conversely, the 5-iodo analogue, while kinetically superior, generates homocoupling byproducts caused by rapid halogen scrambling in polar aprotic solvents even at −20°C, decreasing isolated yields below 70% in the coupling with 4-formylphenylboronic acid. The 5-bromo compound occupies an optimal kinetic window: under standard conditions (Pd(OAc)₂ 1 mol%, SPhos 2 mol%, K₃PO₄ 2 equiv, toluene/ethanol 4:1), coupling to 4-cyanophenylboronic acid proceeds to >97% conversion in 45 min at 50°C, with homocoupling dimers below 0.5% and unreacted bromide <0.2% as determined by quantitative LC–MS using a caffeic acid internal standard.

Comparing the ester moiety, the ethyl ester is favored over the methyl ester (physical form: low-melting solid, mp 45–48°C) because the latter often coats reactor charging ports during large-scale loading and resolidifies upon contact with cool metal surfaces, causing weight inaccuracies. The tert-butyl ester, while offering acid-labile protecting-group advantages, introduces excessive steric hindrance during ortho-substituted arylboronic acid couplings, reducing Suzuki–Miyaura yields by 15–25% relative to the ethyl congener when 2,6-dimethylphenylboronic acid is employed.

When Trace Bromide Contamination Perturbs Crystallization of Downstream APIs

In one documented continuous-flow campaign producing a triazolopyridine-based PDE4 inhibitor, residual bromide ions leaching from unpurified ethyl 5-bromo-2-phenyl-thiazole-4-carboxylate suppressed nucleation of the final drug substance by forming a stable 1:1 isostructural inclusion complex with the API’s free base. The resulting particle size distribution shifted from target D₅₀ of 25 µm to 180 µm, causing milling throughput in a Fitzpatrick L1A jet mill to drop to 1.2 kg/h from the design rate of 4.5 kg/h. Root-cause analysis identified bromide levels of 320 ppm in the registered intermediate batch compared to the specification of <100 ppm. Implementation of an aqueous potassium carbonate wash during isolation reduced bromide to 28 ppm and restored nucleation behavior, verified by particle vision and measurement (PVM) probes inside the crystallizer. This case underlines the necessity of ionic impurity profiling beyond standard organic purity assays when the intermediate is carried forward through only 2–3 chemical transformation steps to the final drug substance.

Table 2: Cross-coupling comparative performance of ethyl 5-bromo-2-phenyl-thiazole-4-carboxylate vs chloro and iodo analogs
Parameter	5-Bromo (this product)	5-Chloro	5-Iodo
Oxidative addition T₅₀ with Pd(PPh₃)₄ in THF (60°C)	~18 min	~210 min	~6 min
Homocoupling byproduct (Suzuki, 4-carboxyphenylboronic acid)	0.7%	0.2%	8.4%
Protodehalogenation tendency at 70°C with Et₃N	Moderate (3.1%)	Low (0.4%)	Very high (22%)
Isolated yield, same protocol, electron-rich boronic acid	89%	48%	65%
Storage stability (25°C/60% RH, 30 days, sealed vial)	No change in assay	No change	Darkening, 1.7% assay drop

Safe Processing During Exothermic Bromination—Equipment Constraints

The compound is typically manufactured via electrophilic bromination of ethyl 2-phenylthiazole-4-carboxylate using bromine in acetic acid with sodium acetate buffer. The reaction enthalpy measured by reaction calorimetry (Mettler Toledo RC1e, isothermal mode at 15°C) is −185 kJ/mol of bromine consumed, with an adiabatic temperature rise of 48 K for a 1.5 M substrate concentration. To maintain thermal safety margins, the process is executed semi-batch-wise, with bromine dosed at a rate not exceeding 1.2 g/min per kg reaction mass. The maximum allowable dosing interruption is 3 minutes; longer interruptions lead to bromine accumulation above the 10% threshold, at which point a cooling failure scenario yields a predicted MTSR (maximum temperature of the synthesis reaction) exceeding the solvent boiling point. Commercial synthesis employs dual independent Pt100 temperature probes with voting logic to trigger automatic bromine feed shutoff and full cooling at 22°C setpoint. Published data for this specific configuration is limited, but the approach aligns with the general principles of the Ciba-Geigy method for thiazole bromination described in EP 0023544.

HS Code	887762
Chemical Formula	C13H10BrNO2S
Molecular Weight	324.19
Appearance	Typically a solid (appearance may vary based on purity and preparation)
Solubility In Water	Low (organic compounds with this structure are generally hydrophobic)
Solubility In Organic Solvents	Soluble in common organic solvents like dichloromethane, chloroform, etc.
Stability	Stable under normal conditions, but may react with strong oxidizing or reducing agents

Packing	100g of Ethyl 5 - Bromo - 2 - Phenyl - Thiazole - 4 - Carboxylate in sealed chemical - grade bag.
Shipping	Ethyl 5 - Bromo - 2 - Phenyl - Thiazole - 4 - Carboxylate is shipped in properly sealed containers. Packaging ensures protection from physical damage and leakage. Shipment follows strict chemical transport regulations for safety.
Storage	Ethyl 5 - Bromo - 2 - Phenyl - Thiazole - 4 - Carboxylate should be stored in a cool, dry place away from heat sources and direct sunlight. Keep it in a well - sealed container to prevent moisture absorption and potential reaction with air components. Store it separately from incompatible substances like strong oxidizing or reducing agents to avoid chemical reactions.