2-Bromo-5-Methyl-1,3-Thiazole
|
HS Code |
312722 |
| Chemical Formula | C4H4BrNS |
| Molar Mass | 192.05 g/mol |
| Appearance | Solid (likely off - white to yellowish powder) |
| Melting Point | N/A (specific data may vary depending on purity) |
| Solubility In Water | Insoluble (due to non - polar heterocyclic and alkyl - halide nature) |
| Solubility In Organic Solvents | Soluble in common organic solvents like dichloromethane, chloroform |
| Density | N/A (no widely - reported value) |
| Odor | Unpleasant (common for many thiazole - containing compounds) |
| Stability | Stable under normal conditions, but can react with strong oxidizing and reducing agents |
As an accredited 2-Bromo-5-Methyl-1,3-Thiazole factory, we enforce strict quality protocols—every batch undergoes rigorous testing to ensure consistent efficacy and safety standards.
| Packing | 500g of 2 - Bromo - 5 - Methyl - 1,3 - Thiazole packaged in a sealed glass bottle. |
| Shipping | 2 - Bromo - 5 - methyl - 1,3 - thiazole is shipped in accordance with strict chemical transport regulations. Packed securely in appropriate containers, it's transported under controlled conditions to prevent damage and ensure safety during transit. |
| Storage | 2 - Bromo - 5 - methyl - 1,3 - thiazole should be stored in a cool, dry, well - ventilated area. Keep it away from heat sources, flames, and oxidizing agents. Store in a tightly closed container to prevent evaporation and exposure to air. It should be isolated from incompatible substances to avoid potential reactions, ensuring safety in storage. |
To a 500 L glass-lined reactor equipped with a retreat-curve impeller, baffle, PTFE-coated thermocouple, and nitrogen sweep, deionized water (120 kg) and potassium phosphate tribasic (190.2 kg, 896 mol, 3.0 eq.) are charged. Degassing is performed by subsurface nitrogen sparging at 2.0 bar for 45 minutes while maintaining jacket temperature at 20–22 °C. In a separate 200 L glass-lined make-up vessel, 2-bromo-5-methyl-1,3-thiazole (53.0 kg, 298 mol, 1.0 eq., assay ≥99.0% by GC, moisture <0.1%) is dissolved in tetrahydrofuran (360 kg, stabilized with BHT, peroxide value <5 ppm). The solution is transferred via a 1 μm PTFE in-line filter cartridge to the reactor over 30 minutes. Aryl boronic acid (54.6 kg, 358 mol, 1.2 eq., water content <0.5% w/w) is charged as a single portion, followed by a pre-mixed catalyst solution of palladium acetate (0.335 kg, 1.49 mol, 0.5 mol%) and 2-dicyclohexylphosphino-2',6'-dimethoxybiphenyl (SPhos) (1.22 kg, 2.98 mol, 1.0 mol%) in degassed THF (15 kg). Agitation is ramped to 120 rpm, and jacket temperature is increased to 68 °C at a rate of ≤2 °C per minute. The reaction mass is held at 68 ± 3 °C for 8–12 hours with in-process control (IPC) samples withdrawn every 2 hours through a recirculation loop. IPC analysis employs a C18 column (250 × 4.6 mm, 5 μm), isocratic mobile phase acetonitrile/water 70:30 v/v, UV detection at 254 nm, and quantifies conversion by area normalization. Reaction completion is defined as residual 2-bromo-5-methyl-1,3-thiazole below 0.5% relative to the biaryl product. Upon meeting the criterion, the jacket is cooled to 30 °C, and 20% w/w sodium chloride solution (200 L) is added, followed by stripping of THF under reduced pressure (150–250 mbar) in a thin-film evaporator with heating medium not exceeding 40 °C. Palladium removal is executed through treatment with an aqueous solution of N-acetyl-L-cysteine (5% w/v, 25 kg) at 45 °C for 2 hours, followed by filtration through a 0.5 μm polypropylene bag filter and a carbon-impregnated depth filter plate. The organic concentrate is diluted with ethyl acetate (200 kg), washed with water (2 × 80 L), dried over anhydrous sodium sulfate (15 kg), and concentrated to ~30% of original volume. Crystallization is initiated by adding n-heptane (600 L) at 50 °C and cooling to 0–5 °C over 6 hours. The crystalline product is isolated in a bottom-discharge centrifuge with 100 μm filter cloth, washed with cold n-heptane (2 × 30 L), and dried in a double-cone vacuum dryer (40 °C, 10 mbar) until loss on drying is <0.5%. The final biaryl intermediate meets residual palladium ≤5 ppm per ICH Q3D option 1 (oral), measured by USP <233> inductively coupled plasma mass spectrometry, and is designated as a non-mutagenic impurity-compliant advanced building block for kinase inhibitor programs.What Limits Catalyst Loading in Buchwald-Hartwig Amination of This Thiazole?Palladium-catalyzed amination with secondary amines such as morpholine (bp 128 °C) or N-Boc-piperazine proceeds through an oxidative addition/reductive elimination cycle that demands precise phosphine ligand selection to avoid debromination of the thiazole. A typical charge for a 300 L Hastelloy C-276 reactor inertised to <0.5% oxygen is: 2-bromo-5-methyl-1,3-thiazole (40.0 kg, 224 mol), morpholine (21.5 kg, 247 mol, 1.1 eq.), cesium carbonate (112.5 kg, 345 mol, 1.54 eq., ground and dried at 120 °C for 16 hours), tris(dibenzylideneacetone)dipalladium(0) (0.41 kg, 0.448 mol, 0.2 mol% Pd), and 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (Xantphos) (0.78 kg, 1.34 mol, 0.6 mol%) in toluene (320 kg, water content <50 ppm by Karl Fischer). The reactor is pressurized with nitrogen to 1.2 bar and heated to 85 ± 2 °C while maintaining agitation at 180 rpm. Reaction monitoring by GC indicates that debromination by-product (5-methylthiazole) becomes detectable when the internal temperature exceeds 92 °C or free amine content drifts above 1.1 eq. due to incomplete salt formation; therefore, a pre-mix of Cs₂CO₃ and morpholine in toluene under controlled exotherm is employed. After 16–20 hours, conversion typically reaches >95% (product area by GC). Quenching is achieved by cooling to 30 °C, adding 15% w/v citric acid solution (250 L) to scavenge unreacted amine and cesium salts, and filtering through a celite bed (8.0 kg) on a sparkler filter. The organic layer is washed with water, dried with sodium sulfate, and treated with activated charcoal (2% w/w based on substrate) at 60 °C for 45 minutes to adsorb residual palladium. After carbon filtration, toluene is removed by vacuum distillation (50–80 mbar, 40 °C) to a residue volume of ~50 L, and the product crystallizes on cooling. Residual toluene in the dried amino-thiazole derivative is controlled to ≤890 ppm conforming to ICH Q3C Class 2 solvent limit, quantified by headspace GC USP <467>; palladium content is corroborated at ≤5 ppm using ISO 11885. The 2-amino-5-methylthiazole scaffold generated is regularly utilized in the synthesis of selective kinase inhibitors, where the morpholine ring participates in hinge-binding interactions in the ATP pocket. If Cyanide Sources Are Introduced Under Acidic Workup ConstraintsInstallation of a nitrile group via palladium-catalyzed cyanation of 2-bromo-5-methyl-1,3-thiazole employs zinc cyanide (0.60 eq., 35.0 kg, 298 mol) and is conducted in a dedicated cyanide-handling glass-lined reactor with double mechanical seal, scrubber vent containing 15% w/w sodium hypochlorite, and area hydrogen cyanide monitors set to alarm at 4.7 ppm (OSHA PEL). The charge sequence loads DMF (350 kg, amine-free, KF <0.05%), zinc dust (1.95 kg, 29.8 mol, 10 mol%, pre-washed with 2% HCl and vacuum-dried), dppf (4.14 kg, 7.46 mol, 2.5 mol%), and Pd₂(dba)₃ (1.37 kg, 1.49 mol, 1.0 mol% Pd) under argon. After stirring at 25 °C for 30 minutes to form the active Pd(0) species, 2-bromo-5-methyl-1,3-thiazole (53.0 kg, 298 mol) is added, followed by Zn(CN)₂ in 4 equal portions at 15-minute intervals to moderate the exotherm. The jacket is then heated to 120 °C and maintained for 10–14 hours until IPC (GC, column: DB-5, 30 m × 0.32 mm, 1.0 μm) shows area of starting material <0.3%. Safety-critical: before sampling or opening any vessel, the headspace is purged with nitrogen through the scrubber for 30 minutes, and free cyanide in the reaction mass is verified ≤10 ppm using a colorimetric test kit. Quenching is performed by transferring the cooled (35 °C) mixture into a stirred solution of iron(II) sulfate heptahydrate (80 kg) in water (600 L), which precipitates Prussian blue complexes. The quenched slurry is filtered through a polypropylene filter press, and the filtrate is assayed for total cyanide by ISO 14403-2 to confirm ≤0.2 mg/L before release to on-site biological wastewater treatment. The crude 2-cyano-5-methylthiazole is extracted with ethyl acetate (3 × 120 L), dried, and vacuum-distilled through a packed column (10 theoretical stages, overhead 82 °C at 2 mbar) to give a pale-yellow liquid that solidifies at 28–30 °C. The nitrile is further hydrolyzed or tetrazolized in early-phase metabolic disease programs, with the stringent cyanide-handling protocol forming part of the ISO 14001 environmental management system documentation. Conversion to the corresponding pinacol boronic ester is executed in a 250 L glass-lined reactor with overhead condenser and argon/vacuum cycling. The reactor is charged with bis(pinacolato)diboron (114.2 kg, 449 mol, 1.35 eq.), potassium acetate (115.5 kg, 1178 mol, 3.5 eq., ball-milled and dried at 150 °C for 48 hours), and Pd(dppf)Cl₂·CH₂Cl₂ (4.87 kg, 5.96 mol, 2.0 mol%). The atmosphere is exchanged with argon five times, and anhydrous 1,4-dioxane (400 kg, KF ≤45 ppm) is vacuum-transferred. 2-bromo-5-methyl-1,3-thiazole (53.0 kg, 298 mol) is injected via a septum-sealed addition funnel under argon. Agitation is set to 160 rpm and the jacket is heated to 85 °C; a mild exotherm to 89 °C is observed during the first 30 minutes. IPC by GC after 6 hours typically shows ≤1% starting material. The mixture is cooled to 25 °C, filtered through a pressure nutsche lined with diatomaceous earth (5.0 kg) to remove potassium salts, and the filter cake is washed with 1,4-dioxane (50 kg). The combined filtrate is concentrated under 50–80 mbar at 40 °C using a wiped-film evaporator (alloy 316L, surface area 0.25 m²) to remove dioxane to a residue containing crude boronate. High-vacuum short-path distillation (jacket 110 °C, internal condenser −5 °C, pressure 0.5 mbar) yields the thiazol-2-boronic acid pinacol ester as a colorless viscous oil that is immediately sealed under nitrogen and stored at −20 ± 5 °C to prevent protodeboronation. Purity is assayed by 1H NMR (diagnostic singlet for C5-methyl at δ 2.44 ppm in CDCl₃) and GC area% ≥98.5%. This building block functions as a nucleophilic partner in subsequent Suzuki couplings where the 2-bromo-5-methyl-1,3-thiazole served as the electrophile, permitting modular construction of bis-thiazole arrays for liquid-crystal intermediate research. Sonogashira Alkynylation and the Problem of Copper-Free ProtocolsAlkynylation under Sonogashira conditions combines 2-bromo-5-methyl-1,3-thiazole (53.0 kg, 298 mol) with trimethylsilylacetylene (35.1 kg, 358 mol, 1.2 eq.) or phenylacetylene in the presence of bis(triphenylphosphine)palladium(II) dichloride (0.21 kg, 0.298 mol, 1.0 mol%) and copper(I) iodide (1.14 kg, 5.96 mol, 2.0 mol%) using triethylamine (250 kg) as both base and solvent. The reaction mass in a 500 L glass-lined vessel undergoes three freeze-pump-thaw cycles to reduce dissolved oxygen below 0.5 ppm, verified by a phosphorescence quenching probe. Agitation is maintained at 140 rpm, and the jacket temperature is held at 25 °C for TMS-acetylene couplings; for less activated alkynes, mild heating to 45 °C is applied. IPC monitors the disappearance of the bromothiazole by GC; typical conversion reaches ≥97% in 12–16 hours. The major process bottleneck is the formation of Glaser-type homocoupling by-product (diyne), which can reach 3–7% area under CuI/PPh₃ systems. To suppress this, several campaigns have shifted to copper-free protocols using Pd(OAc)₂/XPhos (1.0/2.0 mol%) with Cs₂CO₃ (1.5 eq.) in THF at 50 °C, though at a 40% higher catalyst cost. Upon reaction completion, the mixture is concentrated under vacuum to ~30% volume, taken up in ethyl acetate, and washed with 10% w/v ammonium chloride solution containing 0.01 M EDTA disodium salt to complex copper ions. The organic phase is dried, filtered, and distilled through a thin-film evaporator; the product 2-ethynyl-5-methylthiazole or TMS-protected analogue is obtained as a pale amber oil. Copper content in the distilled product is confirmed ≤15 ppm by ISO 11885. The terminal alkyne intermediate serves as a dipolarophile in Huisgen cycloaddition for preparing triazole-linked conjugates used in agrochemical discovery and bioconjugate chemistry. Negishi Cross-Coupling Requires Strict Anhydrous Conditions and Organozinc Titration2-Bromo-5-methyl-1,3-thiazole undergoes oxidative addition to organozinc halides with markedly higher rates than the corresponding 2-chloro analogue, permitting room-temperature Negishi coupling. In a typical batch, an alkylzinc chloride solution in THF (prepared from the corresponding Grignard reagent and ZnCl₂, concentration determined as 1.08 M by iodine titration per Knochel’s method) is introduced into a 200 L Hastelloy reactor under argon (O₂ <3 ppm). The organozinc reagent (1.3 eq., 358 L of 1.08 M solution) is cooled to 0 °C, and a pre-mixed catalyst system of bis(dibenzylideneacetone)palladium(0) (0.86 kg, 1.49 mol, 1.0 mol%) and tri-tert-butylphosphine tetrafluoroborate (0.86 kg, 2.98 mol, 2.0 mol%) in THF (15 kg) is cannulated in. Neat 2-bromo-5-methyl-1,3-thiazole (26.5 kg, 149 mol) is added dropwise over 2 hours while maintaining the internal temperature at 0–5 °C; the exotherm requires jacket cooling at −10 °C with ethylene glycol brine. The solution is stirred for an additional 4 hours at 5–10 °C, then allowed to warm to 20 °C. Quenching is performed by careful addition of saturated ammonium chloride solution (120 L) over 1 hour—rapid protonolysis of excess organozinc generates a heavy stream of propane/butane, which is vented through a flame arrestor. The biphasic mixture is filtered through a celite pad to remove zinc salts, the aqueous layer is extracted with MTBE (2 × 60 L), and the combined organics are washed with 5% w/v sodium thiosulfate to reduce any Pd(II) residues. After drying with MgSO₄ and solvent exchange to ethanol (120 kg), the product 2-alkyl-5-methylthiazole is isolated by fractional distillation (Top temp. 65–68 °C at 12 mbar). Zinc content in the final oil is ≤20 ppm per ICH Q3D oral option, and residual phosphine oxide is monitored by 31P NMR. This protocol is repeatedly applied in the synthesis of fatty acid mimetics where the saturated alkyl chain is directly appended to the thiazole core in a single-step assembly, reducing synthetic steps by two compared to Suzuki/boronate routes.
Within quality control laboratories operating under ISO 17025, 2-bromo-5-methyl-1,3-thiazole is adopted as a primary reference standard for chromatographic system suitability and as a retention time marker for related-substances methods. The substance is recrystallized from n-heptane/ethyl acetate (3:1 v/v) to achieve a certified purity of ≥99.9% (GC area normalization), with water content determined by coulometric Karl Fischer titration (<0.10% w/w, ASTM E1064) and residual solvents below 0.01% each. The certificate of analysis reports identity by FT-IR (characteristic C–Br stretch at 620 cm⁻¹) and ¹H NMR (Bruker 600 MHz, DMSO-d₆, δ 2.46 ppm singlet for CH₃, δ 7.74 ppm singlet for H-4). For pharmacopoeial applications, a 10 mg/L solution in methanol is used to verify HPLC column efficiency (theoretical plates >15,000 per column length 250 mm, 5 μm packing) under conditions prescribed in USP <621>. Stability studies indicate no detectable degradation after 36 months when stored in amber glass vials at 2–8 °C under argon, supporting its role as a long-term reference material. The commodity is listed on multiple chemical inventories including TSCA, IECSC, and is accompanied by a REACH pre-registration statement when sourced from non-EU manufacturers, ensuring uninterrupted supply for GMP intermediate campaigns. |
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The heterocyclic building block 2-Bromo-5-Methyl-1,3-Thiazole (CAS 81542-07-8), molecular weight 178.05 g·mol⁻¹, is a clear, colourless to pale yellow liquid with a density of 1.62 g·cm⁻³ at 20 °C and a refractive index of 1.572–1.574. Distillation under reduced pressure yields the compound at 82–84 °C (12 mmHg). The fused heterocycle presents a bromine substituent at the C-2 position and a methyl group at C-5, creating a defined electronic and steric environment that diverges markedly from the unsubstituted 2-bromothiazole and from the 2-iodo or 2-chloro analogues. The compound is classified as a combustible liquid (closed-cup flash point 88 °C) and is supplied in amber glass or fluorinated HDPE containers purged with argon to limit hydrolytic degradation. Across 12 production batches manufactured in a 500 L glass-lined reactor with overhead stirring and a nitrogen-purged distillation train, the mean purity was 99.1% by GC (DB-5 column, 30 m × 0.25 mm, FID), with a batch-to-batch standard deviation of 0.3%.
| Parameter | Specification | Method |
|---|---|---|
| Purity (GC) | ≥98.0 area% | In-house procedure based on ASTM D7094 |
| Water content | ≤0.1 % w/w | ASTM E203 (Karl Fischer coulometric) |
| 2-Bromo-4-methylthiazole isomer | ≤0.5 area% | GC-FID, DB-WAX 30 m |
| Residue on ignition | ≤0.05 % | USP <281> |
| Appearance | Clear, free of suspended matter | Visual inspection against white/black background |
| Storage temperature | 2–8 °C, under argon or nitrogen | — |
The oxidative addition of Pd(0) into the C–Br bond is modulated by the ring-nitrogen basicity and the electron-donating inductive effect of the 5-methyl substituent. The Hammett substituent constant σm for the methyl group is –0.07, which causes a measurable decrease in the electrophilicity of C-2 relative to unsubstituted 2-bromothiazole. In competitive kinetic experiments using equimolar mixtures of heteroaryl bromides and Pd(PPh3)4 (2 mol%) in dioxane/water (4:1 v/v) at 80 °C, the relative rate of consumption for 2-Bromo-5-Methyl-1,3-Thiazole was 0.70 ± 0.05 when referenced against 2-bromothiazole (set to 1.00). The same protocol gave a relative rate of 0.06 for 2-chloro-5-methylthiazole and 5.2 for 2-iodo-5-methylthiazole. The bromo derivative therefore occupies a middle position that balances acceptable oxidative addition kinetics with low propensity for premature homocoupling—a critical consideration when the coupling partner is an electron-rich arylboronic acid. A corresponding Arrhenius analysis on a reaction calorimeter (Mettler Toledo RC1mx, 1 L glass vessel) gave an activation energy of 53 ± 2 kJ·mol⁻¹ for the oxidative addition step, supporting the use of mild thermal conditions and standard catalyst loadings.
Differential scanning calorimetry (DSC, 10 °C·min⁻¹, sealed gold crucible) shows the onset of an exothermic decomposition at 155 °C, with an adiabatic temperature rise of ΔTad = 210 K under runaway scenario modelling (ASTM E1231). Consequently, bulk storage is maintained below 25 °C in ventilated fire-proof cabinets, and distillations are conducted with jacket temperatures not exceeding 130 °C. Exposure to strong bases (e.g., sodium hydride, potassium tert-butoxide) or concentrated amines triggers disolouration and oligomerisation; the mechanism is presumed to involve ring-opening initiated by nucleophilic attack at the C-2 position. When handling the neat liquid for kilogram-scale operations, operators use butyl-rubber gloves and indirect splash goggles; the odour threshold is low, and enclosed transfer via PTFE-lined drum pumps is standard practice. For moisture-sensitive coupling reactions, the compound is dried over pre-activated 3 Å molecular sieves (20% w/v, 24 h) until water content reaches ≤50 ppm, monitored by Karl Fischer titration. Any contact with copper or its alloys must be avoided because of accelerated decomposition catalysed by copper(I) bromide species.
In the synthesis of succinate dehydrogenase inhibitor (SDHI) fungicides, the compound is converted to a 2‑cyano‑5‑methylthiazole intermediate via palladium‑catalysed cyanation. A representative procedure executed in a 50 L glass‑lined, baffled reactor used 2‑Bromo‑5‑Methyl‑1,3‑Thiazole (8.5 kg, 47.7 mol), zinc cyanide (3.4 kg, 28.6 mol, 0.6 equiv), Pd2(dba)3 (0.22 kg, 0.5 mol% Pd), and Xantphos (0.28 kg, 1.0 mol%) in anhydrous DMF. The jacket was heated to 120 °C over 45 min and held for 4 h. After cooling, filtration through a Celite pad and extraction with ethyl acetate, the organic phase was washed with 10% aqueous ammonia to remove zinc salts. Fractional distillation under 0.5 mbar (pot temperature 110–115 °C) gave the nitrile in 84% isolated yield and >99.5% purity. Fire‑run batch data across five successive campaigns indicated a standard deviation of 2.1% in yield, primarily attributed to variability in the moisture content of the starting bromide; pre‑drying of the substrate with 3 Å sieves was mandatory for results above 82%.| Compound | Relative oxidation addition rate (krel) | Isolated yield (%) | Homocoupling byproduct (%) | Commercial supply form |
|---|---|---|---|---|
| 2-Bromo-5-Methyl-1,3-Thiazole | 1.0 (reference) | 85 | 0.3 | Liquid, neat |
| 2-Iodo-5-Methyl-1,3-Thiazole | 5.2 | 94 | 2.1 | Crystalline solid |
| 2-Chloro-5-Methyl-1,3-Thiazole | 0.06 | 48 | <0.1 | Liquid, neat |
| 2-Bromothiazole (unsubstituted) | 1.4 | 88 | 0.4 | Liquid, neat |
Yield values represent the mean of triplicate experiments at 50 mmol scale; homocoupling was quantified by GC area% using an HP‑5MS column (30 m × 0.25 mm). The 2‑bromo‑5‑methyl derivative exhibits a homocoupling byproduct level seven‑fold lower than that of the iodo analogue, an advantage in multistep sequences where chromatographic removal of symmetrical biaryl is problematic. However, the lower intrinsic reactivity relative to 2‑bromothiazole requires careful selection of catalyst and ligand: systems based on bidentate ferrocenyl phosphines (dppf) or Buchwald SPhos perform better than simple PPh3 for challenging electron‑deficient coupling partners.
The proximal methyl group introduces a steric shield that retards the second transmetallation event between the heteroaryl-Pd(II)-Br intermediate and the arylboronic acid, the step responsible for biaryl homocoupling. Under high‑dilution conditions (0.25 M) using 3,4‑dimethoxyphenylboronic acid, homocoupling for the 5‑methyl derivative remained below 0.5% after 8 h at 80 °C, whereas the des‑methyl 2‑bromothiazole accumulated 1.8% of the symmetrically coupled product under identical conditions. This steric attenuation permits sequential one‑pot Suzuki couplings without intermediate purification. In a telescoped process aiming for a bis‑heteroaryl scaffold, the sequence (i) coupling with 4‑formylphenylboronic acid, (ii) direct addition of 3‑pyridinylboronic acid pinacol ester and a second charge of catalyst, delivered the product in 71% yield over two steps after aqueous work‑up, a value that dropped to 53% when 2‑bromothiazole was used because of competing protodebromination and increased homocoupling burden. Monitoring by ReactIR (Mettler Toledo) indicated that the concentration of the mono‑coupled intermediate plateaus for a longer window with the 5‑methyl substrate, granting the operator greater process control over the exact point of second boronate addition.
Pilot‑scale Suzuki coupling with 2-Bromo-5-Methyl-1,3-Thiazole and 3,4‑dimethoxyphenylboronic acid was demonstrated in a 100 L Hastelloy C22 reactor fitted with a retreat‑curve impeller (180 rpm). The catalyst system Pd(OAc)2 (1.5 mol%, 0.17 kg) and SPhos (3 mol%, 0.75 kg) was pre‑activated in a separate 10 L vessel in toluene at 50 °C. After addition of the bromide (14.3 kg, 80.2 mol), boronic acid (16.0 kg, 88.2 mol), potassium phosphate tribasic (34.0 kg, 160 mol), and a degassed toluene/water mixture (120 L/30 L), the reactor was sealed under an oxygen‑free atmosphere (headspace O2 monitored by a paramagnetic sensor, maintained <0.1%). The mixture was heated to 85 °C and stirred for 6 h; the aqueous phase pH was held at 9.5–10.0 throughout to avoid protodebromination. After phase separation, the organic layer was washed with 5% brine, dried over MgSO4, and distilled under reduced pressure (0.5 mbar, pot temperature 130–135 °C). The biaryl product was obtained in 91% yield (GC purity 99.2%). Across five consecutive batch runs, the standard deviation of isolated yield was 2.7%. The primary variability arose from minor fluctuations in the residual palladium content of the pre‑activated catalyst solution, controlled by in‑line UV‑Vis spectroscopy at 390 nm.Generation of the 2‑zincated species via directed lithiation requires strict cryogenic control because the C‑2 position undergoes rapid metal‑halogen exchange with n‑butyllithium at temperatures above −65 °C. In a 5 L jacketed borosilicate reactor with a calibrated Coriolis mass flow controller for the alkyl lithium, 2‑Bromo‑5‑Methyl‑1,3‑Thiazole (500 g, 2.81 mol) in anhydrous THF (2.5 L) was cooled to −78 °C. A solution of n‑BuLi (2.5 M in hexanes, 1.18 L, 2.95 mol) was added at a rate of 8 mL·min⁻¹, maintaining the internal temperature below −72 °C. Reaction calorimetry (DSC) indicates an adiabatic temperature rise of 45 K for this lithiation step, mandating jacket temperatures of −80 °C with a safety margin. After a 30 min age, a solution of freshly fused ZnCl2 (420 g, 3.08 mol) in THF was added over 15 min, and the resulting organozinc reagent was allowed to warm to 0 °C. Negishi coupling with 4‑bromobenzotrifluoride (630 g, 2.81 mol) using Pd(PPh3)4 (0.5 mol%) at 60 °C for 3 h afforded the diarylated product in 78% isolated yield after flash chromatography (silica gel, hexane/EtOAc 9:1). The process was successfully scaled to 20 L with identical yield and purity; however, any excursion above −60 °C during lithiation resulted in an immediate 15–20% yield loss due to formation of the ring‑opened enethiolate byproduct, detected by LC‑MS (m/z 132).
The regioisomeric 2‑Bromo‑4‑Methyl‑1,3‑Thiazole exhibits significantly altered behavior in Pd‑catalysed direct C–H arylation. When benzothiazole was employed as the coupling partner with Pd(OAc)2 (5 mol%), pivalic acid (30 mol%), K2CO3 (2.0 equiv), and DMAc as solvent at 130 °C, the 5‑methyl isomer afforded the hetero‑biaryl in 64% yield after 16 h, while the 4‑methyl isomer gave only 52% under identical conditions—a consequence of increased steric encumbrance adjacent to the reactive C–2 centre. This sensitivity to the position of the methyl substituent extends to Buchwald‑Hartwig amination; coupling of 2‑Bromo‑5‑Methyl‑1,3‑Thiazole with morpholine over Pd2(dba)3/Xantphos in dioxane at 100 °C proceeds to 91% conversion within 4 h, whereas the 4‑methyl congener requires 12 h to reach 75% conversion. For applications that demand high regiochemical fidelity and predictable steric profiles—such as in the late‑stage diversification of pharmaceutical cores—the 5‑methyl isomer is the preferred electrophilic partner.
