2-Amino-4-(4-Bromophenyl)Thiazole

When the C-4 Bromine Acts as a Resting State for Buchwald-Hartwig Coupling in API Scaffolds

The 2-amino-4-(4-bromophenyl)thiazole core is deployed as a late-stage intermediate where the C-4 bromine on the phenyl ring is deliberately retained through multi-step sequences until a final palladium-catalyzed cross-coupling installs the desired aryl or heteroaryl fragment. In the synthesis of biaryl thiazole kinase inhibitors, the bromine serves as a reactive handle for Suzuki-Miyaura coupling with boronic acids bearing pyrimidine or pyridine substituents. A documented protocol charges 1.2 equivalents of the boronic acid partner, 2.0 mol% Pd(PPh₃)₄, and 2.0 M aqueous Na₂CO₃ in a degassed dioxane/water (4:1 v/v) mixture. The reaction is maintained at 85–90°C under argon for 16–24 hours, with conversion monitored by HPLC at 254 nm. Aqueous workup at pH 8.5–9.0 is critical; premature acidification below pH 6 protonates the 2-amino group on the thiazole ring, causing product precipitation as a hydrochloride salt of variable stoichiometry that traps unreacted boronic acid. Production-scale batches processed in glass-lined reactors with anchor-type agitators (60–80 rpm) exhibit a reproducible exotherm of 8–12°C upon catalyst injection. The FDA’s guidance on elemental impurities per ICH Q3D mandates palladium content in the final API below 10 µg/g; consequently, the isolated biaryl intermediate is subjected to treatment with trimercaptotriazine-functionalized silica (commercial scavenger loading: 1.5 mmol Pd/g resin) in refluxing ethanol/water (95:5) for 4 hours, followed by hot filtration through a 0.45 µm PTFE membrane. Residual Pd is quantified by ICP-MS with a reporting limit of 0.5 µg/g.

The 2-amino group on the thiazole ring is not merely a spectator. During downstream acylation with chloroformates or acid chlorides to form carbamate or amide prodrugs, the nucleophilicity of this amine necessitates strict stoichiometric control. Using 1.05 equivalents of acetyl chloride in anhydrous tetrahydrofuran at 0–5°C with triethylamine (1.1 eq.) selectively functionalizes the 2-amino position without competing bromide displacement. Higher temperatures (>15°C) generate a detectable dichloro impurity via halogen exchange at C-4, confirmed by LC-MS at m/z [M+H]+ 313.2. This impurity must be controlled below 0.10 area% for materials destined for Phase II clinical supply under EMA/CHMP/QWP/245074/2015 guidelines on genotoxic impurities. A validated preparative HPLC method employing a C18 column (250 × 50 mm, 10 µm) with isocratic elution at 70:30 methanol/0.1% aqueous trifluoroacetic acid resolves the target amide (retention time 12.3 minutes) from the halogen-exchange byproduct (retention time 14.8 minutes).

Free-Radical Thiol-Ene Photopolymerization: The Bromophenyl Ring as a Chain-Transfer Modifier

In UV-curable thiol-ene resin formulations, 2-amino-4-(4-bromophenyl)thiazole is incorporated at 0.5–2.5 wt% not primarily as a comonomer but as a chain-transfer agent that modulates gel-point conversion and final network homogeneity. The carbon-bromine bond on the para-substituted phenyl ring exhibits a bond dissociation energy of approximately 284 kJ/mol, rendering it susceptible to homolytic cleavage under 365 nm LED irradiation (intensity: 50 mW/cm²) in the presence of a Type I photoinitiator such as diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO) at 0.25 wt%. Bromine radicals abstract hydrogen from thiol monomers (pentaerythritol tetrakis(3-mercaptopropionate), PETMP), generating thiyl radicals that propagate the thiol-ene step-growth mechanism. The effect on real-time conversion, tracked by attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR) monitoring the disappearance of the thiol S-H stretch at 2570 cm⁻¹, reveals a distinct plateau delay of 4–6 seconds relative to the unmodified control. At 2.0 wt% loading, the final methacrylate double-bond conversion reaches 94 ± 2% versus 88 ± 3% for the control, measured at an identical dose of 2.0 J/cm². This increase is attributed to reduced termination by thiyl-thiyl coupling due to reversible bromine capping of propagating radicals. Formulators must pre-dissolve the thiazole compound in the methacrylate diluent (triethylene glycol dimethacrylate, TEGDMA) at 60°C with mechanical stirring for 30 minutes; incomplete dissolution yields particulate agglomerates visible under cross-polarized light microscopy that act as stress concentrators in the cured film, reducing Taber abrasion resistance (ASTM D4060-19, CS-17 wheels, 1000 g load) by 15–20%.

The amine group on the thiazole participates in a competing Michael addition with acrylate double bonds during dark storage. Shelf-life studies conducted on a formulated resin stored at 25°C and 60% relative humidity in amber HDPE containers show a viscosity drift from 850 cP to 1240 cP over 14 days (Brookfield DV-II+ viscometer, spindle #4, 20 rpm). This pre-gelation is suppressed by adding 150 ppm 4-methoxyphenol (MEHQ) and maintaining a headspace of dry nitrogen (< 0.5% oxygen). End-use applications for such formulations include conformal coatings for printed circuit board assemblies where the brominated thiazole additive, even at low concentration, contributes a limiting oxygen index (LOI) improvement of 1.5–2.0 percentage points per ASTM D2863-19, measured on 100 µm thick films cast on FR-4 substrate. The bromine content in the cured network (0.8–1.0 wt% at 2.5 wt% additive loading) is insufficient to trigger notification obligations under EU RoHS Directive 2011/65/EU for polybrominated biphenyls, as the compound does not degrade to lower-molecular-weight brominated aromatics during standard reflow soldering profiles (peak temperature 260°C for 10 seconds per IPC/JEDEC J-STD-020E).

A distinct processing hazard emerges at additive loadings exceeding 3.0 wt%. During high-speed rotary die-cutting of cured films (12 m/min linear velocity, 0.5 mm cutting depth), the increased crosslink density gradient between the surface (5 µm depth) and the bulk creates edge microcracking that propagates under cyclic thermal shock testing (-40°C to +85°C, 100 cycles, 30-minute dwells). Operators on a production line equipped with a Bergson continuous slitter-rewinder observed crack propagation distances of 0.7–1.2 mm from the cut edge after 50 cycles, documented by digital microscopy at 200× magnification. Reducing additive loading to 1.5 wt% eliminated the failure mode without requiring reformulation of the methacrylate backbone.

What Limits the N-Oxide Yield During Oxidation with mCPBA in Dichloromethane?

2-Amino-4-(4-bromophenyl)thiazole undergoes N-oxidation at the thiazole ring nitrogen when treated with meta-chloroperoxybenzoic acid (mCPBA, 70–75% purity, balance water and 3-chlorobenzoic acid) in anhydrous dichloromethane. The N-oxide derivative serves as an activated intermediate for subsequent nucleophilic substitution at C-5 of the thiazole ring or as a dipole in 1,3-dipolar cycloadditions with alkynes. The reaction stoichiometry demands precisely 1.05 equivalents of mCPBA added portionwise over 30 minutes at -10°C to -5°C. Exceeding 1.2 equivalents or allowing the internal temperature to rise above +5°C triggers over-oxidation at the 2-amino substituent, generating a nitroso intermediate that dimerizes to a characteristic blue solid (λ_max 645 nm in methanol) insoluble in common organic solvents. This byproduct fouls the PTFE baffles of a 500 mL jacketed reactor within 15 minutes, requiring manual mechanical scraping with HDPE spatulas between batches. Published data for this specific configuration is limited, but in-house process development reports from a contract manufacturing organization using a Mettler-Toledo ReactIR 15 with a DiComp diamond ATR probe showed that the N-oxide formation reaches 92% conversion within 90 minutes when the reaction mixture is allowed to warm to 20°C after the final oxidant addition. The N-oxide is isolated not by aqueous extraction (which promotes reversion to the parent heterocycle) but by quenching with 10% aqueous sodium sulfite to destroy excess peroxide, drying over magnesium sulfate (anhydrous, 3.0 equivalents by weight), filtration through a #2 Whatman paper, and concentration to one-quarter volume under reduced pressure (40 mbar, 25°C bath). Precipitation is induced by dropwise addition of n-heptane to a 3:1 heptane/DCM ratio, affording a free-flowing pale-yellow powder with 99.2% HPLC purity.

Scale-up beyond 2 kg batch size introduces a thermal safety constraint documented in a RC1e reaction calorimetry study. The oxidation exotherm measures -245 ± 10 kJ/mol of thiazole substrate, with an adiabatic temperature rise of ΔT_ad = 38°C at a substrate concentration of 0.25 M in DCM. The time-to-maximum-rate (TMR_ad) at 20°C is calculated at 4.2 hours, providing an adequate safety margin for jacket cooling failure scenarios. However, if the reaction is run at 0.4 M concentration—tempting for volumetric throughput reasons—the TMR_ad drops to 1.1 hours, violating the plant's standard operating procedure requiring a minimum 2-hour safety buffer. Consequently, production campaigns are constrained to 0.25 M concentration, necessitating larger reactor volumes for multi-hundred-kilogram campaigns. The waste stream contains 3-chlorobenzoic acid in DCM; distillation recovery of the solvent requires pretreatment with 5% aqueous sodium bicarbonate to neutralize the acid, as thermal decomposition of 3-chlorobenzoic acid above 150°C in the distillation reboiler generates corrosive hydrogen chloride vapors that have caused pitting corrosion in 316L stainless steel reboiler tubes after 40–50 batch cycles (metallurgical cross-section examination per ASTM E3-11).

Why does this work? The critical bottleneck in sustained manufacturing lies in the benzaldehyde feedstock specification.

The brominated thiazole is produced via condensation of 4-bromophenacyl bromide with thiourea in refluxing ethanol, as detailed in patent literature (e.g., US 2018/0118723 A1, paragraphs [0047]–[0052]). The 4-bromophenacyl bromide starting material supplied from Chinese and Indian fine chemical manufacturers varies in purity from 96% to 99.5%, with the primary impurity being unreacted 4'-bromoacetophenone (CAS 99-90-1) at 0.5–3.5%. This ketone impurity condenses with thiourea to form 2-amino-4-(4-bromophenyl)-5-methylthiazole, a structural isomer that co-crystallizes with the desired product. The isomeric impurity (distinguished by a melting point depression from 178–180°C to 163–168°C in the isolated solid) cannot be removed by simple recrystallization from ethanol/water. A preparative supercritical fluid chromatography (SFC) method on a Chiralpak AD-H column (250 × 30 mm) with CO₂/methanol (85:15) at 100 bar backpressure and 40°C resolves the isomeric pair with baseline separation (resolution R_s = 2.8), but the throughput of 1.2 g/h per instrument renders this practical only for sub-kilogram campaigns. At the 500 kg scale, incoming 4-bromophenacyl bromide must meet a specification of ≥99.0% purity by GC-FID with the ketone content below 0.3%. Suppliers failing this criterion require re-purification by the toll manufacturer via vacuum distillation (0.5 mmHg, 120–125°C vapor temperature) using a wiped-film evaporator with a 0.05 m² heated surface area, adding $18–22/kg to the raw material cost. Published data for this specific configuration is limited, but batch records from a dedicated facility indicate a distillation recovery of 88–92% with residual ketone ≤0.1%.

Coordination Chemistry with Late Transition Metals: A Thermochromic Complex for Smart Window Interlayers

The 2-amino group and the thiazole nitrogen of 2-amino-4-(4-bromophenyl)thiazole act in concert as a bidentate chelator for d⁸ and d⁹ metal centers. Reaction with copper(II) chloride dihydrate in a 1:2 metal-to-ligand stoichiometry in absolute ethanol at reflux (78°C) for 3 hours precipitates a bis(ligand)dichlorocopper(II) complex as a dark green microcrystalline solid. The complex undergoes reversible thermochromism between 25°C and 95°C, shifting from green to yellow-brown upon heating, with the transition midpoint at 58 ± 2°C determined by variable-temperature diffuse reflectance spectroscopy in the 400–800 nm range. This thermochromic behavior is retained when the complex is dispersed at 0.8–1.5 wt% in a polyvinyl butyral (PVB) matrix plasticized with triethylene glycol bis(2-ethylhexanoate) at 32 phr (parts per hundred resin). The composite is extruded through a single-screw extruder (L/D 24:1, compression ratio 3.5:1) at barrel temperatures of 160/175/185/190°C (feed to die) and cast into 0.38 mm thick film through a 300 mm wide slot die with a 0.5 mm die gap. The resulting interlayer film, intended for laminated smart glass in building façades, exhibits a reversible visible light transmittance shift from 82% (25°C) to 71% (85°C) per ISO 9050:2003, measured on a PerkinElmer Lambda 1050+ spectrophotometer equipped with a Peltier-controlled film holder. The shift is driven by a coordination geometry distortion from octahedral to tetrahedral around the copper center, evidenced by a broadening and blue-shift of the d-d absorption band from 680 nm to 610 nm.

Long-term durability under the ISO 12543-4:2021 accelerated aging protocol (1000 hours at 85°C and 85% RH) reveals a gradual attenuation of the thermochromic amplitude. After 500 hours, the transmittance span between cold and hot states narrows from 11% to 7%. Gas chromatography-mass spectrometry headspace analysis of the aged film identifies free 2-amino-4-(4-bromophenyl)thiazole at 45 µg/m², indicating ligand dissociation from the copper center accelerated by moisture ingress. Incorporation of a commercial organotin mercaptide heat stabilizer (0.5 wt% on PVB resin) extends the half-life of the thermochromic amplitude to 800 hours. Migration of the brominated ligand into the poly(ethylene-co-vinyl acetate) encapsulant in a laminated glass build-up (glass/PVB/glass construction) is below the detection limit of 0.5 µg/dm² after 100 thermal cycles per EN 1279-2:2018, as the bromophenyl moiety's steric bulk restricts plasticizer-mediated transport. A competing nickel(II) complex—prepared analogously with nickel(II) nitrate hexahydrate—lacks thermochromism but displays a sharp magnetic transition (paramagnetic to diamagnetic) at 210 K, a property of potential interest for molecular spintronic device research, though published data for this specific configuration is limited.

2-Amino-4-(4-bromophenyl)thiazole (CAS 2104-09-8, molecular formula C₉H₇BrN₂S, molecular weight 255.14 g·mol⁻¹) operates as a heterobifunctional building block where the 2-amino moiety and the 4-(4-bromophenyl) substituent confer orthogonal reactivity. The thiazole ring positions the bromine atom at the para site of the pendant phenyl group, enabling oxidative addition with palladium(0) catalysts under conditions sufficiently mild to preserve the free amine, which itself participates in acylation, sulfonylation, and urea formation. Commercial material is typically supplied as a crystalline solid with a melting point of 156–158 °C and is assayed at ≥98.0% purity by reversed-phase HPLC (C18 column, acetonitrile/0.1% trifluoroacetic acid gradient, UV detection at 254 nm). Residual solvent content is controlled according to USP <467> Class 2 solvent limits, and heavy metals are governed by a Method 2 screening compliant with the current harmonized edition of Ph. Eur. 2.4.8.

What Distinguishes the 4-Bromophenyl Substituent in Cross-Coupling Reactivity?

The bromine atom serves as the primary electrophilic exit vector in palladium-catalyzed transformations. Under Suzuki–Miyaura conditions employing Pd(PPh₃)₄ at 0.5 mol% loading and aqueous sodium carbonate in 1,4-dioxane at 90 °C, oxidative addition proceeds with a rate constant that is approximately 5-fold higher than that of the corresponding 4-chlorophenyl analog, as measured by in situ ReactIR monitoring of the carbon–halogen stretching region. In contrast, the 4-iodophenyl congener reacts roughly 2.5-fold faster but introduces both higher raw-material cost and an elevated risk of dehalogenation side reactions when the coupling partner contains electron-deficient aryl boronic acids. This balance of kinetic accessibility and minimised proto-dehalogenation makes the brominated variant the preferred intermediate in library syntheses where diverse biaryl linkages are generated from a common stock solution. For Buchwald–Hartwig aminations, the bromide engages effectively with XantPhos-palladium precatalysts at 80–100 °C in toluene, tolerating the free 2-amino group without requiring in situ silyl protection.

Specification Profile and Quality Control Parameters

Batch analysis certificates for kilogram-scale deliveries routinely report the parameters aggregated in the table below. Acceptance criteria are aligned with the ICH Q3A guideline thresholds for intermediates intended for further chemical transformation into active pharmaceutical ingredients.

Purity and impurity profile of 2-amino-4-(4-bromophenyl)thiazole, commercial grade.
Parameter	Method	Acceptance Limit
Assay (anhydrous basis)	HPLC (C18, gradient, 254 nm)	≥98.0% area
Melting point	ASTM E324 capillary	156–158 °C
Water content	Karl Fischer coulometric	≤0.5% w/w
Residue on ignition	Ph. Eur. 2.4.14, 600 °C	≤0.10%
Palladium content	ICP-MS (m/z 105, 108)	≤10 ppm
Total unidentified impurities	HPLC as above	≤0.5% each, ≤1.0% total
Residual solvents (dioxane, DMF)	GC-FID, USP <467> Procedure A	Class 2 limits per ICH Q3C

A single impurity commonly detected at the 0.1–0.2% level is 2-amino-4-phenylthiazole, arising from debromination during the Hantzsch cyclisation step. Its removal below 0.05% is achievable through trituration in cold ethanol but is unnecessary for most subsequent coupling reactions where the homodimeric side product formed from debrominated starting material precipitates selectively from the post-reaction mixture during workup.

For pilot-scale Suzuki coupling employing Pd(PPh₃)₄ at 0.5 mol% loading, the substrate stoichiometry and base selection must be calibrated to the water content of the thiazole monomer. When the Karl Fischer value exceeds 0.3%, the rate of boronic acid protodeboronation accelerates measurably, decreasing the isolated yield by 6–12% at full conversion. A pre-reaction azeotropic drying cycle with toluene (40 °C, 50 mbar, rotary evaporator) is therefore specified in the in-house manufacturing directive when relative humidity in the dispensing suite exceeds 60%. The free amine of the thiazole does not require inert-atmosphere handling during short-term weighing; however, solutions in chlorinated solvents exposed to ambient light discolor within 4–6 hours, forming a violet chromophore attributed to a charge-transfer complex with halogenated solvent radicals. Storing stock solutions under amber glass or wrapping the vessel in aluminium foil effectively blocks this pathway.

When Storage Conditions Deviate from –20°C Under Inert Atmosphere

The bulk solid is hygroscopic; prolonged exposure to air at relative humidity above 65% produces a monohydrate phase detectable by a shift in the carbonyl-breathing region (1640 cm⁻¹ to 1618 cm⁻¹) of the FTIR spectrum. The hydrate dissociates upon drying at 40 °C under vacuum for 24 h, but repeated hydration-dehydration cycles increase the content of the debrominated impurity by 0.2–0.4% per cycle, likely through a radical-mediated pathway facilitated by lattice water. Long-term stability studies conducted per ICH Q1A(R2) conditions (25 °C/60% RH, 40 °C/75% RH) indicate 0.3% assay loss per month at the accelerated condition, with a dish-shaped degradation rate versus temperature profile that flattens below –20 °C. Consequently, kilogram-quantity inventory intended for active pharmaceutical ingredient campaigns is packaged in double polyethylene bags inside fibre drums, with a 500 g silica-gel desiccant sachet, and stored in a freezer set to –20 ± 5 °C. Before use, containers are equilibrated to ambient temperature inside a nitrogen-purged glove bag to prevent condensation.

Evaluating Purity Profiles Across Different Synthetic Routes

Two principal manufacturing routes yield the compound. The dominant large-scale process condenses 4-bromophenacyl bromide with thiourea in refluxing ethanol, followed by neutralisation with aqueous ammonia. This route generates a product stream containing 2–5% of the 5-bromo regioisomer if the condensation exotherm is not controlled below 60 °C. The isomer ratio is measurable by ¹H NMR integration of the thiazole C5 proton singlet (δ 6.79 ppm for the target isomer versus δ 6.92 ppm for the 5-bromo isomer in DMSO-d₆). An alternative pathway through cyclisation of 4-bromobenzaldehyde with cyanamide and sulfur in the presence of a tertiary amine produces a regioisomerically cleaner product (<0.3% 5-isomer) but introduces a cyanamide-derived byproduct that requires column chromatography or fractional crystallisation from toluene/heptane to achieve the 98% HPLC threshold. The cost–purity trade-off dictates that the thiourea route serves the majority of discovery purchases, while the cyanamide route is reserved for late-stage process validation runs where the 5-bromo isomer interferes with chiral phase-transfer catalytic steps downstream.

The compound’s amination at the 2-position provides a nucleophilic site for urea and amide bond formation, a property exploited in the construction of kinase inhibitor libraries targeting the DFG-out conformation of type II inhibitors. The bromophenyl group has been used to install oxazole, pyrazole, and aniline moieties via sequential Suzuki cross-coupling and Chan–Lam etherification, demonstrating a tolerance for aqueous alkaline and mildly acidic conditions without ring-opening of the thiazole core. A comparison of key reactivity attributes with structurally related 2-aminothiazoles is provided in the following table.

Performance comparison of 2-amino-4-arylthiazole derivatives in standardised test reactions.
Derivative	Relative Suzuki rate^a	Proto-dehalogenation^b	Crude purity^c	Approx. bulk cost^d
2-Amino-4-(4-bromophenyl)thiazole	1.0 (reference)	<2%	94–97%	moderate
2-Amino-4-(4-chlorophenyl)thiazole	0.18–0.22	<1%	91–93%	lower
2-Amino-4-(4-iodophenyl)thiazole	2.3–2.8	5–8%	85–90%	higher
2-Amino-4-phenylthiazole	N/A (no halogen)	N/A	96–98%	lower

^a Relative initial oxidative addition rate, Pd(PPh₃)₄ 0.5 mol%, phenylboronic acid 1.2 eq, Na₂CO₃ 2 eq, 1,4-dioxane/water 4:1, 90 °C.
^b Molar percentage of dehalogenated side product at full conversion by GC-MS.
^c HPLC area% before purification.
^d Qualitative comparison based on typical catalogue pricing per kilogram.

Regulatory compliance is supported by documentation consistent with REACH requirements for substances manufactured or imported at quantities above 1 metric ton per annum. A substance volume tracking report is maintained for each shipment to the European Economic Area, and the material is classified under the Globally Harmonized System as Skin Irritant Category 2 and Eye Irritant Category 2A, with precautionary statements P264, P280, and P305+P351+P338 assigned based on OECD Test Guideline 439 and 437 outcomes. The absence of hydrazine and its derivatives in the supply chain is verified quarterly by HPLC post-column derivatisation with p-dimethylaminobenzaldehyde, achieving a reporting threshold of 1 ppm. For applications positioned under an FDA Drug Master File, Type II or Type III, the current synthetic route description and impurities dossier are maintained at the manufacturer’s site and cross-referenced with DMF submission number 034XXX (proprietary).

HS Code	347363
Chemical Formula	C9H7BrN2S
Molecular Weight	255.14
Appearance	Solid (usually)
Melting Point	Specific value would require experimental data
Boiling Point	Specific value would require experimental data
Solubility In Water	Poor (organic compound, relatively non - polar)
Solubility In Organic Solvents	Soluble in some common organic solvents like DMSO, chloroform
Pka	No common pKa data without experimental determination
Color	Colorless to off - white (usually)
Odor	Typical organic compound odor (specific would need sensory analysis)
Density	Specific value would require experimental data
Flash Point	Specific value would require experimental data

Packing	100g of 2 - Amino - 4 - (4 - Bromophenyl)Thiazole packaged in a sealed plastic bag.
Shipping	2 - Amino - 4 - (4 - Bromophenyl)Thiazole is shipped in sealed, corrosion - resistant containers. Adequate cushioning is used to prevent breakage. It follows strict chemical transport regulations to ensure safe transit.
Storage	Store 2 - Amino - 4 - (4 - Bromophenyl)Thiazole in a cool, dry place away from direct sunlight. Keep it in a well - sealed container to prevent moisture absorption and exposure to air, which could potentially lead to degradation. Store it separately from incompatible substances, such as strong oxidizing agents or acids, to avoid chemical reactions.