2-(4-Bromo-Phenyl)-Thiazole-4-Carboxylic Acid Ethyl Ester

How Does the Para-Bromo Substituent Enable Versatile Cross-Coupling Chemistry?

Catalog product PBT-401, 2-(4-bromophenyl)thiazole-4-carboxylic acid ethyl ester (CAS 885460-84-4), is supplied as a white to off-white crystalline solid with a minimum HPLC purity of 98.0% (area%, UV 254 nm). The molecular formula is C₁₂H₁₀BrNO₂S, and the molecular weight is 312.18 g/mol. Standard packaging comprises 1 g, 5 g, and 25 g units under inert atmosphere. The batch release specifications are collected in the adjacent table.

Release Specifications for 2-(4-Bromophenyl)thiazole-4-carboxylic Acid Ethyl Ester

Parameter	Method	Specification
Appearance	Visual (white background)	White to pale yellow crystalline powder
Melting Point	DSC (10°C/min, N₂)	96–99°C
HPLC Purity	RP‑C18, gradient MeCN/H₂O + 0.1% TFA, 254 nm	≥ 98.0% area
Water Content	Karl Fischer, coulometric	≤ 0.5% w/w
Residual Solvents	GC‑FID, headspace	EtOAc < 500 ppm, DMF < 100 ppm

The ethyl ester moiety is retained across a wide operational window but becomes susceptible to nucleophilic cleavage under strongly alkaline aqueous media or at elevated temperatures. In a 5 L Hastelloy reactor, an initial Suzuki–Miyaura protocol employing 2.5 equiv K₂CO₃ in a 4:1 DME/H₂O mixture at 78°C led to 7–12% saponification over 6 h, generating the free carboxylic acid that subsequently retarded catalytic turnover through carboxylate‑palladium coordination. Switching the base to finely powdered K₃PO₄ (3.0 equiv) in anhydrous dioxane and maintaining a jacket temperature of 72°C suppressed hydrolysis to ≤2% while preserving a coupling yield above 85%. The oxidative addition step benefits from the electron‑withdrawing thiazole ring, which lowers the energy barrier compared with unsubstituted bromobenzene. Using catalyst Pd(PPh₃)₄ at 1.0 mol% loading, complete consumption of the aryl bromide occurs within 90 min under reflux. For Buchwald–Hartwig animations, a Pd₂(dba)₃/XPhos system (1.5 mol% Pd, L:Pd = 2:1) in toluene with NaOt‑Bu (1.2 equiv) at 100°C provides secondary amine adducts in 78–92% isolated yield; the ethyl ester remains intact under these strictly anhydrous conditions. Trace‑level debromination can occur during prolonged storage in solution or upon exposure to UV light. In a 200‑g lot manufactured via Hantzsch condensation, LC‑MS revealed 0.18% des‑bromo impurity (2‑phenylthiazole‑4‑carboxylic acid ethyl ester) alongside 0.05% homo‑coupled biaryl byproduct. The impurity profile is routinely quantified with a linearity range of 0.02–2.0% and an R² of 0.9998. A thermal stability screen run on a Mettler‑Toledo RC1e reaction calorimeter showed no exotherms below 250°C, but a gradual mass loss (0.8%) commencing at 160°C corresponds to traces of occluded DMF. Degassing under high vacuum (0.1 mbar) for 24 h at 40°C removes the entrapped solvent, after which DSC becomes featureless until decomposition onset at 278°C. Differences from regioisomeric and halogen‑variant analogs are mechanistically decisive. The corresponding 2‑(4‑chlorophenyl) ester (CAS 885460-85-5) exhibits oxidative addition rates that are 15‑ to 20‑fold slower under identical palladium conditions, forcing reaction temperatures above 110°C and requiring biaryl ligand systems such as SPhos to reach a turnover number exceeding 2000. In contrast, the 4‑iodo derivative (CAS 1053657-30-2) reacts almost instantaneously but is plagued by light‑promoted deiodination and homocoupling side reactions even under amber‑glass storage; measured homocoupling content in a fresh batch can exceed 3.2%. The bromo substitution therefore occupies a reactivity‑stability niche that makes it the preferred intermediate in iterative Pd‑catalyzed sequences. Without an explicit header, the batch‑to‑batch consistency of the product warrants attention. Over 47 consecutive commercial lots produced at the 500 g scale, the melting point ranged from 96.1°C to 98.8°C, with a mean of 97.4°C and a standard deviation of 0.7°C. ¹H NMR (DMSO‑d₆, 400 MHz) signals always appear within these tolerances: δ 1.35 (t, J = 7.1 Hz, 3H), 4.35 (q, J = 7.1 Hz, 2H), 7.72 (d, J = 8.6 Hz, 2H), 7.91 (d, J = 8.6 Hz, 2H), 8.45 (s, 1H). Integration ratios of aryl and ester protons are kept between 0.98–1.02. One batch exhibiting a discolored off‑white appearance traced to residual 1.2 ppm of iron introduced from a glass‑lined reactor repair; the contamination was eliminated by implementing a chelating EDTA wash (0.1 M) during workup, returning color to the standard white without altering purity.

Stabilizing the Ester Group Against Ambient Moisture During Long-Term Storage

The ethyl ester is moderately sensitive to humidity. At 25°C and 80% relative humidity, a 1 g sample of PBT-401 stored in an open vial accumulated 1.1% of the free acid within 48 h, as quantified by HPLC (RRT 0.78 against the main peak). In a desiccator over phosphorus pentoxide, acid accumulation remained below 0.05% after 30 d. Accordingly, unopened septum‑sealed vials maintain ≥ 98.0% purity for 24 months when kept at −20°C in the dark. Once the container has been breached, the material should be handled inside a glovebox (H₂O <1 ppm, O₂ <1 ppm) or in a dry‑air‑purged balance enclosure. Storage under argon with activated molecular sieves (4 Å) is advisable for quantities above 50 g. Direct contact with primary or secondary aliphatic amines leads to rapid aminolysis, making the ester incompatible with amine‑based quenching strategies unless immediate extraction into an organic solvent is performed. Exposure to sodium hydride or lithium aluminum hydride results in vigorous decomposition; reduction to the primary alcohol must be carried out with DIBAL‑H at ‑78°C in dichloromethane, where the reaction proceeds with 95% conversion and minimal ring‑opening of the thiazole.

When the Ethyl Ester Is Replaced by a Methyl or tert-Butyl Analog

Alkyl ester interchange drastically alters the hydrolytic stability and the scope of downstream deprotection. Table 2 summarizes comparative data generated on the same thiazole core. The ethyl ester occupies a midpoint in the stability sequence, offering sufficient resilience to survive aqueous workups and mild basic conditions while still being cleavable with LiOH in THF/H₂O at 25°C within 4 h. The methyl ester hydrolyzes faster under identical conditions, diminishing yields when aqueous‑based peptide couplings are attempted. The tert‑butyl ester withstands the alkaline conditions routinely encountered in cross‑couplings but requires trifluoroacetic acid (50% v/v in CH₂Cl₂, 2 h) for removal, which may not be compatible with acid‑sensitive downstream functionality. The benzyl ester is readily hydrogenolyzed but its accelerated base‑promoted hydrolysis precludes any protocol involving hydrous alkoxides.

Comparative Hydrolytic Stability and Reactivity of 2-(4-Bromophenyl)thiazole-4-carboxylic Acid Esters

Alkyl Group	t₁/₂ (min) in 0.1 M NaOD/D₂O, 25°C	Solubility in THF (mg/mL)	Relative Suzuki Yield (%)a	Cleavage Conditions
Methyl	18	> 200	82	LiOH, THF/H₂O, 2 h
Ethyl (PBT-401)	52	> 250	85	LiOH, THF/H₂O, 4 h
tert-Butyl	stable (>72 h)	180	91	TFA/CH₂Cl₂ (1:1), 2 h
Benzyl	7	100	45	H₂, Pd/C, 1 atm

^a Suzuki coupling with 4‑fluorophenylboronic acid, 1.0 mol% Pd(PPh₃)₄, K₃PO₄, dioxane, 72°C, 4 h. Yield of isolated ester. The brominated thiazole‑4‑carboxylic acid ethyl ester also differentiates itself from the corresponding acid, which in its free form requires activation with EDC/HOBt for amidation and exhibits poor solubility in low‑polarity solvents. The ester serves directly as a masked acid, enabling one‑step transesterification or aminolysis under tightly controlled conditions. In contrast, 2‑(4‑bromophenyl)thiazole‑4‑carbonitrile—another common building block—carries the cyano group meta to the thiazole ring and participates in an orthogonal set of cycloaddition and reduction chemistries; the ester is selected when a latent carboxylic acid functionality is strategically required late in a synthetic route. No part of this document serves as a concluding remark. The technical data presented reflects product PBT-401 as manufactured and handled under the described protocols, and operational boundaries are reported without extrapolation beyond the tested ranges.