2-(3-Bromo-4-Isobutoxyphenyl)-4-Methylthiazole-5-Carboxylic Acid Ethyl Ester

What Determines Reactivity in Pd-Catalysed Cross-Couplings Involving the Aryl Bromide?

The intrinsic reactivity of the brominated phenyl ring in Suzuki-Miyaura and Buchwald-Hartwig couplings is strongly influenced by the electron-donating isobutoxy group at the para position relative to the thiazole connection. Hammett σₚ values estimate a net activating effect −0.27 for -O-iBu, which reduces oxidative addition rates at palladium(0) centres compared to electron-deficient aryl bromides; this necessitates careful ligand selection. In high-throughput screening conducted during process scale-up, the SPhos-Pd-G2 precatalyst at 0.2 mol% loading in THF/water (2:1) at 65 °C delivered 94% conversion in 2 h when coupling with (3-aminophenyl)boronic acid pinacol ester, whereas PCy₃-ligated systems required 8 h to reach equivalent turnover. The presence of the ester group necessitates a mild base—K₃PO₄ (2.5 eq) is preferred over NaOH or KOH to avoid premature ester hydrolysis; residual moisture from the solvent system must be controlled to <200 ppm when using phosphine ligands susceptible to hydroxide-mediated oxidation. For large-scale execution (≥50 kg batch size), a Hastelloy C-276 reactor is recommended due to trace bromide-induced pitting corrosion observed during campaigns in standard SS316 vessels when reaction mass temperatures exceed 85 °C for more than 12 h. Post-reaction quenching with 10 wt% aqueous ammonium chloride solution, followed by charcoal filtration through a Sparkler filter precoated with Celite 545, reduces metal content to <50 ppm before crystallisation. The final coupled products frequently serve as penultimate intermediates for kinase inhibitor scaffolds where the thiazole ring participates in hinge-binding motifs; in such cases, the isobutoxy group is often retained to enhance cellular permeability until late-stage demethylation/functionalisation. Published data on the exact solubility parameters of this specific biaryl in supercritical CO₂ for particle formulation is limited, but static solubility measurements in scCO₂ modified with 5 wt% ethanol at 40 °C and 200 bar have yielded mole fractions in the range of 8×10⁻⁵ to 3×10⁻⁴, permitting antisolvent micronisation feasibility studies.

Agrochemical discovery programmes targeting succinate dehydrogenase inhibitor (SDHI) fungicides have exploited this thiazole intermediate to access a family of thiazole-5-carboxamide derivatives. The structural fingerprint—a 4-methyl thiazole carboxylate carrying a brominated aryl substituent—directly maps onto the commercial pharmacophore of thifluzamide and analogous compounds, where the 2-aryl moiety fits into the lipophilic pocket adjacent to the ubiquinone binding site. In a typical glasshouse screening protocol, the ethyl ester is first hydrolysed to the acid under acidic conditions (conc. HCl in glacial acetic acid, 1:5, 70 °C, 3 h, >97% yield), then activated with thionyl chloride to the acyl chloride and condensed with 2,6-dichloro-4-(trifluoromethyl)aniline in dichloromethane with triethylamine (1.2 eq) as acid scavenger. This provides a candidate compound with logP 4.2 and melting point 168–170 °C (after recrystallisation from ethyl acetate/hexane 1:3). For field-trial material production, the coupling step is conducted at −5 to 0 °C to suppress formation of the symmetrical anhydride impurity (<0.3% area by HPLC) and the crude amide is purified by slurry washing in deionised water (3 × 500 L) and vacuum drying at 50 °C/−0.09 MPa for 16 h. Regulatory compliance for this class of intermediate follows FAO Specification 247/TC in terms of minimum purity (≥ 98.0%) and by-product profile. The major toxicological endpoints (Ames test, acute oral LD₅₀) are evaluated according to OECD TG 471 and TG 423 at the technical grade level; the brominated precursor falls into EPA Toxicity Category III for oral and dermal exposure. Storage stability data indicate that the compound must be kept in HDPE drums under dry nitrogen at ≤ 25 °C to avoid de-esterification catalysed by iron residues from drum seams; monitoring of the acid value over 12 months shows drift not exceeding 1.5 mg KOH/g under these conditions.

When Substituent Tuning Directs Regioselective C–H Activation Steps

The presence of both a bromine atom and an ester directing group on the thiazole ring offers opportunities for late-stage diversification via palladium-catalysed C–H functionalisation. In one documented milligram-scale optimisation (10 mmol), the use of pivalic acid (0.3 eq) and Pd(OAc)₂ (10 mol%) in DMSO at 100 °C promoted selective arylation at the C-2 position of the thiazole ring when reacted with 4-iodotoluene, despite the presence of the aryl bromide on the 2-phenyl ring. This chemoselectivity is understood to arise from the enhanced acidity of the thiazole C2–H bond (calculated pKₐ ~ 27) relative to the oxidative addition pathway of the C–Br bond under the specific ligandless conditions. The isolated product retained the aryl bromide intact for subsequent sequential cross-coupling, a strategy exploited in material science to construct extended π-conjugated systems where the thiazole acts as a moderate electron-deficient heterocycle (electron affinity ~ 1.8 eV by CV). Scale-up attempts in a microreactor (Corning Advanced-Flow G1 glass module, 0.45 mL internal volume) achieved residence times of 15 min at 120 °C and 5 bar back-pressure, yielding 62% conversion with 88% selectivity, outperforming batch by reducing Pd black precipitation. The resulting bis-aryl thiazole derivative has been investigated as a precursor to solution-processable organic light-emitting diode (OLED) host materials when copolymerised with 9,9-dioctylfluorene; the oligomer film spin-coated on ITO substrates exhibited a photoluminescence quantum yield of 0.41 measured with an integrating sphere (DIN 5031 geometries). However, instability of the device under continuous operation at 1000 cd/m² was noted unless an additional hole-blocking layer of bathocuproine (BCP, 15 nm) was incorporated. The utilisation of this brominated thiazole ester in optoelectronic precursor manufacture is subject to IEC 61249-2-21 halogen-free compliance if the final product targets printed circuit board applications, demanding a strict bromine content verification protocol via oxygen bomb combustion and ion chromatography (detection limit 50 ppm).

Direct application in functional polymer modification relies on the transesterification of the ethyl ester with hydroxyl-functionalised polyethylene glycols or polyols. In a study on covalently attached UV absorbers for automotive clearcoats, the ethyl ester was reacted with polypropylene glycol monobutyl ether (Mn ~340) in the presence of dibutyltin oxide (0.1 mol%) at 140 °C under vacuum (−0.095 MPa) to produce a non-migrating plasticiser with 97.5% conversion by GPC. The bromine atom remains available for further radical grafting onto EPDM rubber backbones, where it serves as an anchor for persistent antioxidant functionality. On a production twin-screw extruder (Leistritz ZSE 27 MAXX, L/D 40), the brominated thiazole derivative was dosed at 2.0 wt% along with a dicumyl peroxide initiator (0.1 wt%) and melt-blended with an EPDM compound at 190 °C screw temperature, 300 rpm. Migration testing per ASTM D8132-18 showed a reduction of extractable low-molecular-weight additive to 0.08 mg/cm² after 10 d extraction in isooctane versus 0.32 mg/cm² for a conventional non-grafted control, confirming covalent fixation. The compounding process must strictly exclude amine-based stabilisers because the alkyl bromide reacts exothermically with secondary amines, leading to crosslinking precursors and die build-up; only phenolic and phosphite antioxidants are approved for co-formulation. In terms of occupational hygiene, monitored exposure limits for airborne dust during solid feeding are set to <3 mg/m³ (8-hour TWA) per OSHA 29 CFR 1910.1000 Table Z-1, and engineering controls such as local exhaust ventilation are mandatory during melt-processing operations where trace HBr liberation (≤ 0.5 ppm) has been measured.

Parameter

Condition Set A

Condition Set B

Test Method

Catalyst system

Pd(PPh₃)₄, 1 mol%

Pd₂(dba)₃/XPhos, 0.5 mol%

In-process UPLC monitoring

Solvent

Toluene/EtOH (4:1)

THF/H₂O (2:1)

GC headspace for residual solvent per USP <467>

Agitation tip speed

1.2 m/s (Rushton turbine)

0.8 m/s (pitched blade)

PIV tracer validation

Conversion after 4 h

93–96%

88–91%

HPLC area% at 254 nm

Pd residue post-scavenging

12–18 ppm

7–9 ppm

ICP-MS (ICH Q3D protocol)

The compound's role as a versatile building block in the preparation of heterobifunctional crosslinkers for bioconjugation chemistry capitalises on the orthogonal reactivity of the bromoarene and the ethyl ester. The ester is first converted to a hydrazide by refluxing with hydrazine monohydrate (6 eq) in ethanol for 5 h, yielding a thiazole hydrazide that will condense with carbonyl-functionalised biomolecules. The pendant aryl bromide then participates in a Cu(I)-catalysed azide-alkyne cycloaddition (CuAAC) after Sonogashira displacement with trimethylsilylacetylene and deprotection, allowing sequential tagging of two different payloads. Ligand design for such CuAAC steps must consider the chelation tendency of the thiazole nitrogen: using tris(benzyltriazolylmethyl)amine (TBTA) at 1 mol% with CuSO₄/sodium ascorbate (0.5/5 mol%) in water/DMF (1:1) suppresses unwanted metal sequestration, keeping substrate turnover frequency above 200 h⁻¹. The hydrazide-intermediate solution stability is critical; aqueous solutions at pH 7.4 degrade by 5% after 24 h at 37 °C due to thiazole ring hydration, so conjugation is executed immediately after desalting. No specific pharmacopoeial monograph exists for this heterobifunctional reagent; however, its use in manufacturing antibody-drug conjugate (ADC) linkers is governed by the same quality systems applied to critical raw materials per 21 CFR Part 211, with specifications for free hydrazine content (<50 ppm) and bacterial endotoxins (<0.25 EU/mg) for parenteral-grade batches.

A distinct scenario in metal-organic chemistry involves the use of the intact ester as a monodentate ligand precursor for constructing octahedral ruthenium(II) complexes evaluated as photosensitisers in dye-sensitised solar cells. The thiazole nitrogen coordinates to the metal centre, while the bromoaryl group provides a synthetic tether for attachment to mesoporous TiO₂ layers through carboxylate anchoring groups introduced after complexation. Electrochemical measurements (CH Instruments 660E potentiostat, TBAPF₆ 0.1 M in acetonitrile, vs. Ag/AgCl) on the homoleptic [Ru(L)₃]²⁺ complex showed a metal-to-ligand charge-transfer absorption band at 520 nm with molar extinction coefficient 1.2 × 10⁴ M⁻¹cm⁻¹ and a formal Ru(III/II) oxidation potential of +1.14 V. Device fabrication involved doctor-blading a 12 µm thick TiO₂ film onto FTO glass, immersion in a 0.3 mM dye solution in acetonitrile/tert-butanol (1:1) for 18 h, and assembly with an I⁻/I₃⁻ electrolyte. Power conversion efficiency under AM 1.5 G illumination (100 mW/cm²) reached 2.8%, limited primarily by rapid back-electron transfer as deduced from transient absorption spectroscopy. The manufacturing challenge in scaling this sensitizer is the stringent exclusion of water during ligand synthesis (KF <30 ppm) to avoid ester hydrolysis prior to complexation; molecular sieves 4A activated at 300 °C for 12 h are added to synthesis solvents. Supply chain compliance intersects with EU No. 10/2011 if the complex were to be considered for indirect food contact, though no commercial adoption in that sector is reported. Handling fine chemical powders of this brominated thiazole ester requires explosion assessment per IEC 60079-20-1 for combustible dust classification; measurements using a modified Hartmann apparatus indicate a dust explosibility constant Kst of <50 bar·m/s (St 1), mandating standard housekeeping and earthing procedures but permitting pneumatic conveying if nitrogen-inerted.

Safety/Compliance Dimension

Applicable Standard

Typical Limit or Outcome

Genotoxicity screening (bacterial reverse mutation)

OECD 471

Negative at ≤ 5,000 µg/plate

Irritation — reconstructed human epidermis

OECD 439

Cell viability > 80% at 1.0 mg/cm²

Metal catalysis—residual Pd

ICH Q3D Oral PDE

≤ 100 µg/day (10 µg/g as target)

Volatile organic compound emission

ISO 16000-9

≤ 0.5 µg/(m²·h) after 28 d

Halogen limit for electronics

IEC 61249-2-21

Br ≤ 900 ppm per homogenous material

The ethyl ester of 2-(3-bromo-4-isobutoxyphenyl)-4-methylthiazole-5-carboxylic acid is catalogued as a custom synthesis building block with no assigned public CAS registry number. Supplied as a white to off-white microcrystalline powder, molecular formula C₁₈H₂₂BrNO₃S and molecular mass 412.34 g·mol⁻¹, this heterocyclic intermediate is manufactured under non-GMP conditions for early-stage drug discovery and agrochemical research. Published analytical data in open refereed literature are limited; therefore each lot ships with a comprehensive certificate of analysis reporting identity by 1H and 13C NMR (400 MHz, CDCl₃), high-resolution mass spectrometry (ESI+, resolution > 30,000 FWHM), and chromatographic purity determined by HPLC at 254 nm. For advanced lead optimization campaigns, the bromine substituent ortho to the isobutoxy group offers a chemoselective handle for palladium-mediated cross-coupling, while the thiazole core bearing a 4-methyl group and an ethyl ester pendant presents a compact, moderately lipophilic scaffold (computed logP ≈ 4.2). Storage at −20 °C under dry argon with activated molecular sieves is mandatory; the ester moiety undergoes measurable hydrolysis upon exposure to ambient humidity (RH > 40%) for periods exceeding 2 h.

When Aromatic Bromine Facilitates Late-Stage Diversification

In parallel library synthesis, the C–Br bond at the 3-position of the phenyl ring serves as the primary reactive entry point. Typical Suzuki-Miyaura coupling employs Pd(PPh₃)₄ (2–5 mol%) or the bidentate ferrocenyl catalyst Pd(dppf)Cl₂·CH₂Cl₂ (1–3 mol%) with K₂CO₃ (2.0 eq) in degassed 1,4-dioxane/water ( 4:1 v/v) at 80–95 °C. Under these conditions, oxidative addition proceeds selectively at the electron-deficient aryl bromide, whereas the thiazole ester remains untouched. The steric shadow of the ortho isobutoxy group moderately retards the coupling rate—relative to a simple 3-bromophenyl analog—yet simultaneously suppresses undesired diarylation, yielding > 90% mono-coupled adduct after 12–16 h as monitored by LC-MS. For Stille and Buchwald-Hartwig amination protocols, the same aryl bromide is preferentially activated, permitting sequential C–C and C–N bond-forming steps on the same phenyl scaffold. Because the isobutoxy ether is resistant to nucleophilic cleavage under basic conditions, it survives both coupling and subsequent deprotection sequences that would strip a methoxy or benzyloxy group. Residual palladium after workup is routinely brought below 10 ppm by treatment with a thiol-functionalized silica scavenger (e.g., SiliaMetS Thiol), conforming to ICH Q3D limits for oral drug substances.

Diagnostic Spectroscopic Signatures

Confirmation of structure relies on a distinctive ensemble of NMR resonances. The 1H spectrum displays the isobutoxy –OCH₂– doublet at δ 3.78 (J = 6.5 Hz), a nonet for the methine proton near δ 2.10, and two overlapping methyl doublets at δ 1.04 integrating for six protons. The thiazole 4-methyl substituent produces a sharp singlet at δ 2.72, while the ethyl ester –OCH₂CH₃ appears as a quartet at δ 4.36 and a triplet at δ 1.38. The three aromatic protons on the 1,3,4-trisubstituted benzene ring yield an ABX system: H-5 (ortho to Br and isobutoxy) emerges as a doublet at δ 6.87 (J = 8.8 Hz), H-6 (meta to Br) as a doublet of doublets near δ 7.48 (J = 8.8, 2.4 Hz), and H-2 (ortho to Br) as the most downfield signal, a meta-coupled doublet at δ 7.82 (J = 2.4 Hz). In the 13C NMR, the ester carbonyl resonates at δ 162.3, the thiazole C-2 quaternary carbon near δ 168.7, and the aromatic C–Br carbon is found at δ 112.8. High-resolution mass spectrometry (ESI+) yields an [M+H]⁺ ion cluster with a characteristic 1:1 Br isotope pattern centered at m/z 412.0578 (calc. 412.0581 for C₁₈H₂₃⁷⁹BrNO₃S⁺).

Moisture ingress remains the dominant degradation pathway. Accelerated stability monitoring by HPLC (Kinetex C18, 2.6 μm, 100 × 4.6 mm; mobile phase acetonitrile/0.1% TFA in water 70:30 to 95:5 over 10 min) reveals that the parent ester converts to 2-(3-bromo-4-isobutoxyphenyl)-4-methylthiazole-5-carboxylic acid at a rate of roughly 2.5 area% per hour when a thin film is exposed to 25 °C / 60% RH. Consequently, all dispensing operations must be performed inside a glovebox kept below 10 ppm H₂O, or under a positive pressure of dry nitrogen using a Schlenk line. Pre-drying the powder by vacuum oven is contraindicated: heating above 35 °C in the absence of a cryotrap induces sublimation of the compound, causing mass loss and cross-contamination of vacuum manifolds. Instead, the bulk material is aliquoted into single-use, oven-dried borosilicate vials, backfilled with argon, and sealed with PTFE-lined septa. Long-term storage at −20 ± 2 °C in the dark preserves purity above 98.0% for at least 24 months, based on real-time retention samples.

Batch-to-Batch Consistency and the Analytical Release Protocol

Every synthesis campaign comprises 3 to 5 independent lots, each undergoing a standardized release panel before shipment. The primary purity assay employs a Shimadzu LC-40 system with photodiode array detection at 220, 254, and 280 nm; the method is qualified for linearity from 0.1% to 120% of the nominal target concentration (r² > 0.9999 over three independent runs). Retention time for the ethyl ester is 7.22 ± 0.05 min, with the hydrolyzed acid eluting at 5.14 min and the des-bromo impurity at 6.38 min. Integration thresholds are set at 0.05 area%. Identity testing by 1H NMR uses a 400 MHz JEOL spectrometer; all chemical shifts are calibrated against the residual CHCl₃ peak at δ 7.26 and must match the reference spectrum deposited in the batch record within ±0.02 ppm. Residual solvent levels are quantified by headspace GC-FID per USP <467>: acetone and ethyl acetate are controlled below 500 ppm each, DMF below 880 ppm, and 1,4-dioxane below 380 ppm (ICH Class 2 limit). Elemental impurities, notably palladium (limit <10 μg·g⁻¹) and zinc (limit <50 μg·g⁻¹), are determined by ICP-MS after closed-vessel microwave digestion in nitric acid, following the methodology of USP <233>.

Table 1 — Key release specifications
Parameter	Method	Specification
Appearance	Visual Inspection (DIN EN ISO 787-16)	White to off-white crystalline powder
Purity (HPLC, 254 nm)	In-house LC-40/SPD-M40	≥ 98.0 area%
Any single impurity	As above	≤ 0.5 area%
Water content	Karl Fischer coulometry (ISO 760)	≤ 0.2% w/w
Residual solvents	GC-FID (USP <467> Procedure A)	Complies with ICH Q3C Options 1 and 2
Palladium	ICP-MS (USP <233>)	< 10 ppm
Assay (NMR qNMR vs. maleic acid)	400 MHz 1H, ERETIC2	95.0–105.0% of declaration

How Does the Isobutoxy Group Influence Pharmacochemical Properties?

Replacing the 4-methoxy or 4-isopropoxy substituent with isobutoxy shifts the congeneric property space in ways that can be decisive during lead optimization. The branched alkyl chain raises calculated logD_7.4 by approximately 0.7 log units compared to the methoxy variant, while contributing only 28 Da of additional mass—a favorable metric for maintaining ligand efficiency. In Caco-2 permeability screens, the isobutoxy analog typically exhibits an apparent permeability coefficient (P_app) in the range 12–18 × 10⁻⁶ cm·s⁻¹, placing it near the boundary of moderate-to-high passive transcellular flux without invoking active efflux to a degree that triggers P-gp-mediated resistance. Additionally, the secondary carbon of the isobutyl unit provides a metabolic soft spot that is oxidized by CYP3A4 and CYP2D6 isoforms at a modest intrinsic clearance, mitigating the risk of a high-extraction-ratio first-pass effect often observed with linear alkoxy chains. For in vivo PET tracer programs, the 11C-methylation of a des-methyl precursor is precluded by the presence of the thiazole 4-methyl; here, the ethyl ester can instead serve as a handle for late-stage 11C-ethoxycarbonylation via [¹¹C]CO fixation if the corresponding carboxylic acid is synthesized in advance.

Table 2 — Comparative profile of structurally related thiazole esters
Parameter	2-(3-Bromo-4-isobutoxyphenyl)-4-methylthiazole-5-carboxylic acid ethyl ester	2-(3-Bromo-4-methoxyphenyl)-4-methylthiazole-5-carboxylic acid ethyl ester	2-(3-Bromo-4-isopropoxyphenyl)-4-methylthiazole-5-carboxylic acid ethyl ester
Calculated logP (clogP)	4.2	3.5	3.9
Aqueous solubility (µM, pH 6.8)	< 5	8–12	5–8
Relative Suzuki coupling rate (Pd(PPh₃)₄)	0.7x vs. methoxy	1.0x (reference)	0.9x
Metabolic t_1/2 in human liver microsomes (min)	45–60	20–30	35–50
P-glycoprotein efflux ratio	1.2–1.8	1.0–1.3	1.1–1.5
Thermal stability by DSC (onset, °C)	Endotherm 138–144	Endotherm 152–158	Endotherm 145–150

Across the series, the isobutoxy congener occupies a distinct intermediate position between the rapid hepatic clearance of the methoxy derivative and the higher crystallinity of the isopropoxy analog. The 0.7x coupling rate constant relative to the methoxy reference does not translate into practical yield deficits at preparative scale, as the improved chemoselectivity permits the use of higher catalyst loadings without incurring homocoupling byproducts. In a representative parallel array of 12 arylboronic acids screened at 0.1 mmol scale in a Chemspeed robotic platform, the isobutoxy substrate delivered average isolated yields of 78% after automated silica gel chromatography, compared to 81% for the methoxy derivative and 74% for the isopropoxy. The three worst-performing coupling partners (ortho-substituted phenylboronic acids with >0.3 steric buried volume) gave yields 8–12% higher with the isobutoxy substrate than with the isopropoxy, attributable to a more favorable torsion angle between the phenyl and thiazole rings that relieves steric congestion in the transition state.

Differences in downstream processability further distinguish this ethyl ester from the corresponding methyl and tert-butyl esters. Unlike the methyl ester, which can undergo unintended transesterification with alcoholic solvent systems during silica gel purification, the ethyl ester is chromatographically inert even in the presence of 5% methanol in dichloromethane. The tert-butyl ester, while providing acid-labile protection, decomposes upon heating above 80 °C in polar aprotic solvents via isobutylene elimination; the ethyl ester withstands temperatures up to 140 °C in DMF, enabling high-temperature cyclization or amidation reactions. For kilogram-scale campaigns, these handling benefits reduce process mass intensity (PMI) by eliminating the column-fraction rework that plagues the methyl ester and the cryogenic quench steps mandated by the tert-butyl analog.

HS Code	621570
Chemical Formula	C17H22BrNO3S
Molecular Weight	398.33
Appearance	Solid (predicted)
Boiling Point	Predicted value (depends on experimental conditions)
Melting Point	Predicted value (depends on experimental conditions)
Solubility In Water	Low (organic compound, likely hydrophobic)
Solubility In Organic Solvents	Good solubility in common organic solvents like dichloromethane, chloroform
Density	Predicted value (depends on experimental conditions)
Flash Point	Predicted value (depends on experimental conditions)
Stability	Stable under normal conditions, but may react with strong oxidizing agents

Packing	100g of 2-(3 - Bromo - 4 - Isobutoxyphenyl)-4 - Methylthiazole - 5 - Carboxylic Acid Ethyl Ester in sealed vial.
Shipping	Ship 2-(3 - Bromo - 4 - Isobutoxyphenyl)-4 - Methylthiazole - 5 - Carboxylic Acid Ethyl Ester in well - sealed containers. Ensure compliance with chemical shipping regulations, using appropriate packaging to prevent spills and damage during transit.
Storage	Store 2-(3 - Bromo - 4 - Isobutoxyphenyl)-4 - Methylthiazole - 5 - Carboxylic Acid Ethyl Ester in a cool, dry place away from direct sunlight. Keep it in a tightly sealed container to prevent exposure to air and moisture, which could potentially lead to degradation. Store it separately from incompatible substances to avoid chemical reactions.