2-Bromothiazole-5-Carboxaldehyde

In a late-stage functionalisation sequence for a phase IIb Bruton’s tyrosine kinase (BTK) inhibitor candidate, kilogram-scale batches of 2-bromothiazole-5-carboxaldehyde are procured under a full cGMP starting material dossier aligned with ICH Q7 chapters 7 and 11. The aldehyde carbon is the point of structural diversification after the halogen position has been exploited to install a 4-aminopiperidine-substituted phenyl ring via a Suzuki–Miyaura coupling. A typical charge protocol combines 1.00 equivalent of the bromothiazolecarboxaldehyde with 1.12 equivalents of the arylboronic ester pinacol adduct, 2.5 equivalents of anhydrous K₃PO₄, and 0.005 equivalents of Pd(dtbpf)Cl₂ in a deoxygenated THF–water mixture (4:1 v/v, oxygen level ≤ 0.5 ppm measured by a Mettler Toledo InPro 6900 optical probe). The stirred reactor jacket is ramped to 58–62°C over 45 min and held within this window for 7–9 h, with in-process HPLC monitoring (C18 column, UV at 254 nm) requiring ≤ 0.15 area% of the des-bromo by-product before the batch is cooled to 20°C. After vacuum filtration through a Celite pad and aqueous workup at pH 8.5±0.3, the crude 2-arylthiazole-5-carbaldehyde intermediate is isolated as a pale yellow solid typically showing 98.2–99.1% chromatographic purity. This intermediate subsequently undergoes reductive amination with N-Boc-aminopiperidine using NaBH(OAc)₃ (1.4 eq.) in dichloromethane containing 3% acetic acid at 18–22°C, providing the penultimate BTK inhibitor fragment after Boc deprotection with HCl/dioxane.

Residual palladium in the isolated intermediate constitutes a critical quality attribute because the target API specification, set against EMA/CHMP/CVMP/QWP/2467/2019 guidance, caps total Pd at ≤ 5 µg/g. In practice, an activated carbon treatment using Norit SX Plus (10 wt% relative to crude, stirred at 45°C for 2 h in isopropyl acetate) brings Pd content from an initial 120–400 µg/g to 1.7–4.3 µg/g as quantified by ICP-MS (USP 〈233〉). A parallel mutagenic impurity risk assessment according to ICH M7 classifies 2-bromothiazole-5-carboxaldehyde itself as a Class 3 alerting structure (aldehyde cohort of concern with an acceptable intake of 1.5 µg/day); daily dose projections for the drug candidate require that the aldehyde starting material be purged below the threshold toxicological concern limit by the registered regulatory starting material boundary. Spike-and-purge studies on the pilot-plant batch confirm a purge factor exceeding 2.4 × 10⁴ when the aldehyde is completely consumed in the reductive amination step, supporting the selected starting material designation in the Type II DMF.

A residual solvent panel, executed by headspace GC–FID as per USP 〈467〉 Procedure A, routinely quantifies THF (≤720 ppm vs. ICH Class 2 limit of 720 ppm), dichloromethane (≤600 ppm), and isopropyl acetate (≤5000 ppm). For batches shipped to Japanese or Swiss finishing sites, additional compliance with JP 2.46 and Ph. Eur. 2.4.24 is demonstrated by inclusion of a purity factor method in the certificate of analysis. The delivered intermediate is drummed under argon in 25 kg LDPE-lined fibre drums with a retest date of 24 months when stored at 2–8°C.

When fungicidal activity pivots on C-2 aryl substitution breadth

Structure–activity relationship campaigns within the N-phenyl-3-(thiazol-5-yl)pyrazole-4-carboxamide class of succinate dehydrogenase inhibitors (SDHIs) have identified 2-arylthiazole-5-carbaldehyde intermediates as versatile building blocks for generating a library of amide variants. A manufacturing campaign supporting a pre-registration toxicology package for a novel Oomycete-targeting fungicide employed this aldehyde in a telescoped sequence on a 200 L glass-lined reactor train. The initial Suzuki coupling parallels pharmaceutical conditions but is executed with a lower-cost ligand: Pd(OAc)₂ (0.8 mol%) and triphenylphosphine (1.6 mol%) in toluene–ethanol–water (5:2:1), with Na₂CO₃ (3.0 eq.) serving as the base. Tolerance to process water is a decisive economic factor, and the biphasic system achieves full conversion of the bromothiazolecarboxaldehyde within 4–6 h at gentle reflux (78±3°C), monitored by a ReactIR 15 probe for the disappearance of the aldehyde C=O stretch at 1702 cm⁻¹.

The post-coupling aldehyde is reduced in the same solvent matrix by adding 1.05 equivalents of NaBH₄ in 0.5 M aqueous NaOH at 0–5°C, delivering 2-(4-chlorophenyl)thiazol-5-ylmethanol with isolated yields across six consecutive batches of 82–86%. Subsequent activation with methanesulfonyl chloride (1.2 eq., Et₃N 1.5 eq.) in THF at –10°C gives the mesylate, which is displaced in situ with 4-fluoroaniline to furnish the penultimate amine intermediate. The final amidation with 3-difluoromethyl-1-methyl-1H-pyrazole-4-carbonyl chloride, run at 10–15°C in dichloromethane containing a supported DMAP scavenger resin, produces the active ingredient as a white crystalline powder with a differential scanning calorimetry onset melting point of 187.3°C (PerkinElmer DSC 8500, 10°C/min under N₂).

Regulatory compliance for the agrochemical intermediate centres on FAO Specification 581/TC for the technical active constituent, which imposes a chlorinated persistent organic pollutant exclusion and mandates that any polychlorinated biphenyl congeners generated through intramolecular coupling remain below 0.5 mg/kg. The intermediate batch records therefore document a charcoal treatment with a specific Zeta potential measurement (Malvern Zetasizer Nano ZS, target ζ ≥ –35 mV for optimal PCB adsorption) and a confirmatory GC–ECD analysis against Aroclor 1254 standard. Methanol, the only ICH Class 2 solvent used in the final crystallisation, is controlled at ≤3000 ppm in the technical material.

In kilogram-scale production of a phase III kinase inhibitor candidate, the aldehyde function of 2-bromothiazole-5-carboxaldehyde serves as a late-stage diversification handle following successful Suzuki–Miyaura coupling with a boronic ester. The boronate partner is pre-lithiated using n-BuLi at –78°C in anhydrous 2-methyltetrahydrofuran before transmetallation with triisopropyl borate; quenching the 1.00 equivalent boronate into the aqueous Pd-catalysed coupling mixture at 55°C initiates a rapid oxidative addition of the C–Br bond. A statistically designed experimental matrix (JMP, central composite face-centred design) varied Pd-PEPPSI-IPr loading (0.2–0.8 mol%), THF/water ratio, and K₂CO₃ equivalents, with the optimal centre point returning 97.3% in situ conversion within 3.5 h and a des-halo impurity ceiling of 0.07 area%.

The purified 2-arylthiazole-5-carbaldehyde then enters a Knoevenagel condensation with 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile in a pyridine–acetic acid catalytic system at 60°C under Dean–Stark water removal. The product, an intensely purple solid, is intended as the terminal electron-accepting unit in an A–D–A non-fullerene acceptor for organic photovoltaic OPV blends. Ultraviolet photoelectron spectroscopy (Kratos Axis Supra) of spin-coated films from a chloroform–chlorobenzene (97:3) solution gives a highest occupied molecular orbital level of −5.61 eV and an optical bandgap derived from the absorption onset of 1.42 eV, placing the compound within the energy matching window for donor polymer PM6. Published data for fully analogous thiazole-core acceptors are sparse; however, single-junction devices fabricated with a PM6 donor and the present acceptor in an inverted architecture (ITO/ZnO/active/MoO₃/Ag) have yielded power conversion efficiencies of 15.1 ± 0.4% (calibrated under AM 1.5G, 100 mW·cm⁻², with an aperture mask of 0.0342 cm²) – values which should be regarded as indicative because batch-dependent carrier mobility measured by space-charge-limited current (SCLC) models still varies ±12% across six device runs.

Electronic-grade purity far exceeds typical pharmaceutical requirements. Metal ion contamination, particularly sodium and iron, is depressed to ≤50 ppb each by double recrystallisation from electronic-grade toluene and passage through a basic alumina plug. The material is submitted to glow-discharge mass spectrometry (VG9000) for a full-elements screen before acceptance, with the specification aligning loosely with SEMI C27-0920 guideline limits for organic semiconductor precursors.

Does the Thiazole Ring Enable Spin-Crossover in Iron(II) Complexes Beyond Pyridine-Triazole Motifs?

Coordination chemistry applications exploit the bidentate chelation potential of an imine functionalised by condensing 2-bromothiazole-5-carboxaldehyde with 2-aminoethanol. The resultant Schiff base, formed under Dean–Stark reflux in ethanol with a catalytic amount of glacial acetic acid (0.5 mol%), affords a neutral N,S,O-donor ligand after demetallation of the intermediate sodium phenolate. In air-sensitive Schlenk-line synthesis, Fe(BF₄)₂·6H₂O (1.0 eq.) in methanol–water (9:1) is treated with two equivalents of the deprotonated ligand at –10°C, yielding a low-spin Fe(II) complex characterised by a room-temperature magnetic moment of 0.9 μB as determined by Evans NMR method (Bruker AVANCE III 500 MHz, D₂O capillary insert).

Variable-temperature single-crystal X-ray diffraction of the perchlorate salt reveals an abrupt spin transition at 108 K, with the Fe–N(thiazole) bond length shortening from 2.18 Å (high-spin) to 1.97 Å (low-spin). Although the transition temperature is well below ambient, the cooperativity parameter ΔT₈₀ of 6 K suggests an unusually sharp first-order transition, and attempts to engineer higher operational temperatures by ring substitution at the bromine site are under investigation. Published data for this specific Fe(II) system remains limited to single-crystal-to-single-crystal cycling experiments.

The bidentate ligand precursor, 2-bromothiazole-5-carboxaldehyde, participates in this scheme solely as the aldehyde reactant. Manufacturing-scale preparation of the Schiff base ligand for kilogram-scale metallation batches is carried out in single-neutron 100 L reactors with a Hastelloy C-276 wetted surface, as trace chloride corrosion from the BF₄ counterion must be avoided. The final complex is isolated as a hydrate and stored under argon at –20°C to prevent ligand oxidation.

Head-to-head kinetic isotope effect experiments on the thiazole aldehyde substrate itself have been patched into existing mechanistic probes for photoredox-mediated formylation reactions. When 2-bromothiazole-5-carboxaldehyde is employed as a surrogate electrophile in a cyanoalkylation cascade, parallel reactions run with the 13C-labelled aldehyde (Carbon-13 enrichment at the formyl carbon, 99 atom %) allow a determination of the primary KIE by quantitative 13C NMR integration. A recent study integrated with an Anton-Paar Multiwave Pro microwave reactor delivering 100 W at 60°C in a sealed quartz vessel (dioxane, 0.05 M substrate, 2 mol% [Ir(dF(CF₃)ppy)₂(dtbbpy)]PF₆) gave a KIE of 1.08±0.03, consistent with rate-limiting reductive debromination rather than aldehyde attack—a mechanistic nuance that guides the choice of this building block in radical cascade library synthesis. Published data for this exact protocol remains confined to a single proof-of-concept communication with the explicit caveat that scale-up beyond 5 mmol results in a bromide-promoted dimerisation band observable as a 12% GPC impurity at a retention volume of 16.8 mL.

In thermoplastic polyurethane (TPU) film extrusion trials on a Leistritz ZSE 27 MAXX twin-screw extruder (L/D = 48, 27 mm screw diameter, 10-barrel zones), 2-bromothiazole-5-carboxaldehyde was grafted as a pendent aldehyde onto a maleated SEBS backbone via a melt-phase condensation with a diamine linker. The masterbatch formulation consisted of 92.5 parts Kraton FG1924X, 5.0 parts 1,12-diaminododecane, and 2.5 parts aldehyde, fed at 8 kg·h⁻¹ with zone temperatures profiling from 170°C (barrel 2) to 210°C (barrel 9). The incorporation efficiency, tracked by the imine C=N IR peak at 1648 cm⁻¹ vs. the residual aldehyde physisorbed peak, reached 83% at a screw speed of 400 rpm; higher speeds generated shear heating that pushed melt temperature above 230°C and accelerated crosslinking via Schiff base oligomerisation, evidenced by a torque spike of 18% and gel particle formation.

The bromine substituent remains chemically inert under these processing conditions—thermogravimetric analysis coupled with mass spectrometry (TGA–MS, Netzsch STA 449 F3 Jupiter, 10°C·min⁻¹ to 600°C under helium) confirms no HBr off-gassing before 310°C. The resulting film, compression-moulded at 200°C and 5 MPa for 3 min, exhibits a tensile strength at break of 22.1 MPa (ISO 37:2017, type 3 dumbbell, 500 mm·min⁻¹) and a Shore A hardness of 87 after 3 s. The aldehyde termini permit post-film immobilisation of amine-functionalised antimicrobial peptides via a mild reductive amination in phosphate-buffered saline (pH 7.4, NaCNBH₃ 10 mM, 4°C, 16 h), quantified by fluorescamine assay as 8.2±0.7 nmol·cm⁻². Such surface engineering is envisaged for catheter-grade TPU but is not yet covered by a dedicated regulatory framework; leachable aldehyde must be monitored and falls under the inventory mass-based threshold for food-contact substances in EU 10/2011 (overall migration limit 10 mg·dm⁻²).