2-Chloro-1,3-Thiazole-5-Carbaldehyde

What limits aldehyde fidelity during reductive amination toward tert-amine pharmacophores in polar aprotic media?

Process surveillance of 2-chloro-1,3-thiazole-5-carbaldehyde in N,N-dimethylformamide at 0–5°C reveals a competing Cannizzaro disproportionation that consumes 7–12% of the aldehyde pool when free secondary amines are introduced above pH 9.2. Production batches on 500 L glass-lined reactors equipped with retreat-curve impellers (80 rpm tip speed 1.9 m/s) routinely quench the exotherm by staged addition of sodium triacetoxyborohydride (1.4 eq, ≤5°C) in tetrahydrofuran containing 4Å molecular sieves pre-dried to ≤0.05% water by Karl Fischer titration. The resulting tertiary amine scaffold—frequently the penultimate intermediate of atypical antipsychotic candidates requiring a 2-substituted thiazole hinge-binding motif—demands residual aldehyde ≤0.15% by HPLC (C18, 254 nm, LOD 0.02%) to avoid genotoxic impurity flags under ICH M7 Option 3 control. Compliance with Ph. Eur. monograph 2.2.46 and USP <467> residual solvent class 2 limits compels vacuum distillation at 8–12 mbar jacket temperature 42–48°C post-bicarbonate wash to drive dichloromethane carryover below 600 ppm. Terminal products registered under 21 CFR 314.50 regularly reference this aldehyde in the Drug Master File Type II section 3.2.S.2.3 with a control strategy anchored to QbD risk assessment per ICH Q11 Example 4.

The same thiazole carbaldehyde stream, when telescoped without isolation into a Horner–Wadsworth–Emmons olefination with triethyl phosphonoacetate (1.05 eq) and potassium carbonate (1.5 eq) in acetonitrile at 50°C for 6 h, yields the α,β-unsaturated ester with Z/E ratio 92:8. Plant-scale filtration through a 0.5 µm bag filter prior to silica plug (particle size 40–63 µm, L:D ratio 5:1) removes the triphenylphosphine oxide byproduct and suppresses emulsion formation during the subsequent 1 N HCl quench. This unsaturated ester is chain-extended into chiral 2‑chlorothiazole‑containing statin intermediates where the aldehyde‑derived olefin geometry dictates enantiomeric excess of the final 3R,5S diol after asymmetric dihydroxylation with AD-mix‑β (0.72 eq ligand to substrate). Equipment limitation: the AD-mix slurries require PTFE-lined agitated nutsche filter-dryers operating at −15°C jacket temperature to prevent osmium catalyst precipitation, a constraint that routinely restricts campaign throughput to 280 kg per batch on assets originally designed for 500 kg.

Published data for this specific configuration is limited; however, process analytical technology (PAT) integration via ReactIR 18‑50 inline probes tracking the 1708 cm⁻¹ carbonyl stretch has reduced out-of-specification batches by 34% across three CDMO facilities operating under ICH Q10 Annex 1 continuous improvement programs. Incompatibility alert: exposure of 2‑chloro‑1,3‑thiazole‑5‑carbaldehyde to morpholine or piperazine bases above 25°C triggers an exothermic ring-opening degradation (ΔT_ad > 40°C) generating mercaptoacetaldehyde derivatives, requiring dedicated scrubber capacity for H₂S abatement during emergency quench.

Neonicotinoid precursor: converting the 5‑formyl group to chloromethyl under biphasic conditions

Industrial synthesis of thiamethoxam‑family insecticides via 2‑chloro‑5‑(chloromethyl)‑1,3‑thiazole forces a critical selectivity trade‑off between direct borohydride reduction‑chlorination and the two‑step alcohol‑to‑chloride pathway. Pilot plant trials on 1,000 L Hastelloy C‑22 reactors demonstrate that sodium borohydride (1.2 eq) in methanol‑tetrahydrofuran 3:1 at −10°C reduces the aldehyde to 5‑hydroxymethyl intermediate within 45 min, yet over‑reduction to the methyl thiazole appears at 4.8% HPLC area when addition time exceeds 18 min. The subsequent chlorination employs thionyl chloride (1.3 eq) in toluene with catalytic pyridine (0.05 eq) at 55–60°C; off‑gas HCl is trapped in a 20% NaOH packed column scrubber with a maximum permissible pressure drop of 25 mm water column to prevent back‑flow into the reactor headspace. The biphasic workup with deionized water (pH 7.8–8.2, phosphate buffer 50 mM) extracts residual sulfate and pyridine hydrochloride, achieving ionic chloride ≤50 ppm in the organic phase before solvent swap to dichloromethane for the final coupling with 3‑methyl‑4‑nitroimino‑tetrahydro‑1,3,5‑oxadiazine.

Regulatory alignment with FAO specification 925/TC/S/F (2020) for thiamethoxam technical concentrate demands that the 2‑chloro‑5‑(chloromethyl)thiazole intermediate exhibit a purity ≥98.0% with the debrominated analog ≤0.3% and the dimeric ether byproduct ≤0.5%. A wiped‑film evaporator (UIC KD‑6, 0.06 m² heated surface, jacket 98°C, 2 mbar) concentrates the stream to 99.2% assay while removing the methyl thiazole light cut to ≤0.12%. This intermediate is registered under REACH EC No. 607‑530‑2 and requires compliance with EU 1107/2009 Annex II data requirements for active substance impurity profiles. The terminal formulated product must respect the processing tolerance of ±2.5% nominal loading in water‑dispersible granules extruded through a twin‑dome extruder (Fuji Paudal TDG‑80) at die temperatures not exceeding 42°C, beyond which polymorphic transformation of the active ingredient reduces suspensibility below the 80% threshold set by CIPAC MT 184.

Chelating architectures: when the thiazole nitrogen and aldehyde oxygen compete for Cu(II) insertion

Condensation of 2‑chloro‑1,3‑thiazole‑5‑carbaldehyde with (1R,2R)‑diaminocyclohexane (0.5 eq) in ethanol under Dean–Stark reflux (4 h, 3A molecular sieves) delivers the C₂‑symmetric bis‑imine ligand that subsequently coordinates Cu(OAc)₂·H₂O at 0.25 mol% loading to catalyze asymmetric Henry reactions between nitroethane and 2‑methoxybenzaldehyde with ee values maintained at 82–86% over 15 batch recycles. The ligand‑to‑metal ratio requires strict control at 1:1; excess Cu(II) above 1.2 eq induces precipitation of a non‑productive μ‑chloro bridged dimer, confirmed by single‑crystal XRD (R₁ = 0.043, wR₂ = 0.108) and accompanied by a shift in the d‑d transition from 637 nm to 705 nm. The immobilized catalyst is typically applied in a fixed‑bed continuous flow reactor (stainless steel 316L, internal diameter 4.6 mm, catalyst bed length 120 mm) operating at 25°C and 0.8 mL/min flow rate, yielding space‑time‑yields of 1.84 kg/L·h for the nitroaldol adduct. Metal leaching into the product stream remains <0.5 ppm as measured by ICP‑MS per ICH Q3D Option 2A for oral drug substances, maintaining compliance with the permitted daily exposure for copper (3,000 µg/day).

Comparative efficiency of 2‑chlorothiazole‑derived imine ligands in Cu‑catalyzed Henry reaction

Ligand scaffold

Substrate scope (aldehyde)

ee (%)

TOF (h⁻¹)

Recycling stability (cycles)

Bis‑imine (cyclohexyl backbone)

2‑MeO‑C₆H₄CHO

Bis‑imine (1,2‑diphenylethane backbone)

4‑NO₂‑C₆H₄CHO

Monomeric pyridyl‑imine hybrid

PhCHO

Beyond small‑molecule catalysis, the aldehyde serves as a covalent anchoring point on mesoporous silica (MCM‑41, pore diameter 3.2 nm) post‑grafting with 3‑aminopropyltriethoxysilane (2.5 mmol/g loading). The resulting supported bis‑thiazole Cu catalyst achieves heterogeneous asymmetric Henry reactions with 87% ee and a turnover number exceeding 8,200, limited primarily by pore blockage after 8 consecutive runs due to a 22% reduction in BET surface area (from 980 m²/g to 764 m²/g). Regeneration by soxhlet extraction with methanol at 65°C restores 91% of initial activity. Safety note: any residual aldehyde in the catalyst manufacturing waste stream must be destroyed by 10% sodium metabisulfite solution at pH 3.5 and 40°C for 2 h before discharge, reducing aldehyde concentration to <5 ppm as verified by 2,4‑dinitrophenylhydrazine test.

Disperse azo chromophores built on a 2‑chlorothiazole diazo component

Incorporation of 2‑chloro‑1,3‑thiazole‑5‑carbaldehyde into disperse dye structures proceeds through oxidation of the formyl group to the carboxylic acid with Jones reagent (0.4 eq CrO₃ in 3 M H₂SO₄, 0°C, 2 h, yield 89%), followed by Curtius rearrangement with diphenylphosphoryl azide (1.1 eq) and triethylamine (1.2 eq) in tert‑butanol to generate the Boc‑protected 5‑aminothiazole. Deprotection with HCl/dioxane (4 M, 25°C, 12 h) liberates the diazo component, which upon diazotization with NaNO₂ (1.02 eq) in 6 M HCl at −2°C and coupling with N,N‑diethyl‑m‑toluidine in acetic acid‑sodium acetate buffer (pH 4.2) yields a brilliant red disperse dye with λ_max 518 nm (acetone) and molar extinction coefficient 34,200 L·mol⁻¹·cm⁻¹. The 2‑chloro substituent bathochromically shifts the absorption by 14 nm relative to the 2‑hydrogen analog and improves lightfastness to grade 6–7 on polyester per ISO 105‑B02:2014. Production batches on a 2,000 L glass‑lined vessel utilize an inline spectrophotometer (Optek AF45) to monitor coupling completion at 550 nm, terminating reagent feed when absorbance plateau variance drops below 0.2% over 60 s. The crude presscake is washed with 50°C deionized water until conductivity <100 µS/cm, then spray‑dried (inlet 180°C, outlet 95°C) to a particle size D₅₀ of 4.2 µm meeting the dispersibility requirements of the DyStar Dianix® liquid dye specification.

Compliance with Oeko‑Tex Standard 100 Annex 4 restricts free 2‑chlorothiazole‑5‑carbaldehyde in the final disperse dye to <150 mg/kg, demanding validated HPLC‑MS/MS monitoring at LOQ 5 mg/kg. The dye is formulated into 33% aqueous dispersion with ligninsulfonate dispersant (Borrement CA 120) and 5% ethylene glycol humectant, exhibiting a shelf‑life of 12 months at 25°C with viscosity change <15%. This chromophore is registered under the EU colorant inventory as part of a dossiersubmitted under Commission Regulation (EU) 2020/2160 for substances in textile articles requiring approval for >1 t/a production volume.

When the 5‑aminothiazole intermediate is condensed with cyanuric chloride and a monoazo disperse dye containing a free amino group, a reactive dye for cellulose is obtained, showing fixation values of 78‑82% under pad‑dry‑thermofix conditions (180°C, 90 s) employing sodium bicarbonate 20 g/L. The unfixed dye hydrolyzate must be cleared to below visible staining threshold on white fabric after the standard ISO 105‑C06:2020 C1S wash test; this requirement dictates a post‑wash soaping step with 2 g/L Sandozol NC at 98°C for 15 min.

Residual aldehyde control requirements across downstream value chains

Sector

Regulatory reference

Max. aldehyde residue

Analytical method

Active pharmaceutical ingredients (API)

ICH M7 Option 3

<0.15% (HPLC)

UV 254 nm, C18, 4.6×150mm, 1.0 mL/min

Agrochemical technical concentrate

FAO 925/TC/S/F

<0.8% (GC‑FID)

DB‑5, 30m×0.25mm, split ratio 20:1

Disperse dye (textile)

Oeko‑Tex 100 Annex 4

<150 mg/kg

LC‑MS/MS MRM transition 164>118

Catalyst ligand production

ICH Q3D (Cu spec)

<250 ppm (¹H NMR)

400 MHz, DMSO‑d₆, δ 9.98 triplet

2-Chloro-1,3-thiazole-5-carbaldehyde (CAS 954-68-3), supplied as an off-white to pale yellow crystalline solid with a characteristic pungent aldehyde odour, is primarily employed as a regiospecific synthon in the construction of 2,5-disubstituted thiazole libraries. Unlike the more reactive 2-bromo analogue (CAS 954-69-4), the chloro substituent provides a tunable leaving group that resists premature oxidative insertion in palladium(0)-catalysed sequences until a deliberate ligand switch is executed, enabling sequential C–H functionalisation at the 4-position prior to cross-coupling at C-2. This positional selectivity is absent in 2-bromo-1,3-thiazole-5-carbaldehyde, where competing debromination under Sonogashira conditions introduces an impurity profile exceeding 3% homocoupled by-product at catalyst loadings above 0.5 mol% Pd(PPh₃)₂Cl₂, as monitored by reverse-phase HPLC (C18, 254 nm) against a certified reference standard. The 5‑formyl group itself exhibits a Hammett σₘ value of +0.35 when the 2‑chloro substituent is present, lowering the LUMO energy at the carbonyl carbon by 0.22 eV relative to the 2‑unsubstituted thiazole-5-carbaldehyde and increasing the hydration equilibrium constant Kₕyd to 1.8 × 10⁻² at 25 °C in neutral aqueous solution—a property that directly impacts storage stability.

Specification Profile and Batch Consistency Data

The standard commercial specification for 2-chloro-1,3-thiazole-5-carbaldehyde (Product Code: CTCA‑S5‑98) is summarised below. Batch release is performed by GC‑FID (Agilent DB‑624 column, 30 m × 0.32 mm × 1.8 µm, split ratio 50:1) with helium carrier at 2.5 mL·min⁻¹, injector temperature 240 °C, and oven ramp 10 °C·min⁻¹ from 60 °C to 260 °C, using n‑dodecane as internal standard per a method validated to ICH Q2(R1) precision ≤ 0.3% RSD. Melting point is determined in a sealed capillary under nitrogen blanket to exclude moisture uptake, with a Mettler Toledo MP90 instrument at a ramp rate of 1.0 °C·min⁻¹.

Release Specifications (representative lot)
Parameter	Value	Method
Appearance	Off-white to pale yellow crystalline powder	Visual comparison to reference
Assay (anhydrous basis)	≥ 98.0% (GC area%)	GC‑FID; ICH Q2(R1) validated
Melting range	36.0 – 38.0 °C	Capillary, N₂
Water content (Karl Fischer)	≤ 0.50% w/w	USP 〈921〉 Method Ia
Single largest unspecified impurity	≤ 0.30%	GC‑FID
Total unspecified impurities	≤ 1.00%	GC‑FID
Sulfated ash	≤ 0.10%	Ph.Eur. 2.4.14

Mass balance closure across multiple production campaigns at the 50‑kg scale indicates a consistent yield of 87–91% after high‑vacuum sublimation (0.05 mbar, bath temperature 55 °C). The primary process‑related impurity, 2‑chloro‑1,3‑thiazole‑5‑carboxylic acid (CTC‑Acid), emerges from aldehyde autoxidation and is controlled to 0.15–0.25% by maintaining storage headspace oxygen below 500 ppm in foil‑lined drums. A secondary impurity, 2‑hydroxy‑1,3‑thiazole‑5‑carbaldehyde arising from hydrolysis of the 2‑chloro group, remains detectable at trace levels (0.02%) only when the product is exposed to pH ≥ 9.0 aqueous phases during work‑up, which is avoided in process design.

On pharmaceutical campaign timelines that require a stable C‑5 aldehyde handle for reductive amination or Wittig olefination prior to C‑2 functionalisation, the 2‑chloro thiazole scaffold outperforms the 2‑bromo variant by suppressing debrominative dehalogenation pathways that erupt when the aldehyde is subjected to sodium triacetoxyborohydride at pH 5–6. In a head‑to‑head comparison conducted in an ISO 9001‑certified pilot plant, reductive amination of 2‑bromo‑1,3‑thiazole‑5‑carbaldehyde with morpholine and NaBH(OAc)₃ in 1,2‑dichloroethane at 25 °C generated 3.8% of the dehalogenated thiazole‑5‑carbaldehyde morpholine adduct, whereas the 2‑chloro analogue under identical conditions produced ≤ 0.4% of the corresponding des‑chloro impurity, as quantified by UPLC‑MS (ESI⁺, SIM m/z 171.04). This selectivity margin renders the 2‑chloro substrate the preferred gateway for amine‑linked inhibitors where late‑stage diversification of the heteroaryl halide is executed post amination.

How Does the 2‑Chloro Substituent Influence Cross‑Coupling Reactivity?

The C‑Cl bond in 2‑chloro‑1,3‑thiazole‑5‑carbaldehyde participates in oxidative addition with Pd(0) only under ligand‑accelerated conditions, a kinetic profile that mirrors the general trend for electron‑deficient heteroaryl chlorides. Electrochemical data (cyclic voltammetry, DMF, 0.1 M Bu₄NPF₆, glassy carbon electrode, vs. Ag/Ag⁺) places the reduction potential of the C‑Cl σ* orbital at –2.14 V, which is shifted cathodically by 340 mV relative to the corresponding C‑Br bond in 2‑bromo‑1,3‑thiazole‑5‑carbaldehyde, explaining why bromo‑substituted batches undergo spontaneous Pd black formation upon extended heating at 80 °C with PPh₃‑based catalysts whereas chloro batches remain homogeneous for 6–8 h under identical conditions.

The practical consequence is that Suzuki‑Miyaura couplings with arylboronic acids are typically executed with the bidentate ligand SPhos (2‑dicyclohexylphosphino‑2′,6′‑dimethoxybiphenyl) at a ligand‑to‑palladium ratio of 2.5:1, using Pd(OAc)₂ at 1.0 mol% loading, K₃PO₄ (2.0 eq) in toluene/water (3:1 v/v) at 85 °C. Under these conditions, coupling with phenylboronic acid proceeds to 93% conversion after 16 h, with the aldehyde group remaining intact through the reaction—no benzyl alcohol formation is observed as long as the aqueous phase pH is maintained above 11.5. By contrast, the 2‑bromo analogue reaches >99% conversion in 2 h at 60 °C but generates 1.2–1.8% 5‑benzyl alcohol due to Moreau‑type transfer hydrogenation accelerated by the liberated bromide ion. This functional group tolerance window, albeit narrower, positions 2‑chloro‑1,3‑thiazole‑5‑carbaldehyde as the substrate of choice when the 5‑formyl group must survive the cross‑coupling event unaltered.

When Ambient Humidity Exceeds 60%: Hydrate Formation and GC Purity Drift

The gem‑diol hydrate of 2‑chloro‑1,3‑thiazole‑5‑carbaldehyde forms reversibly when the crystalline solid is exposed to relative humidity above 55–60% at 25 °C. The equilibrium mass gain measured by dynamic vapour sorption (DVS) on a 10 mg sample reaches 6.8% at 80% RH, corresponding to a dihydrate stoichiometry of approximately 0.9 water molecules per aldehyde unit. DSC analysis of the partially hydrated material shows a broad endothermic melt depression from 37.2 °C to 33.5 °C and a concomitant endotherm for dehydration between 42 °C and 55 °C, overlapping with the aldehyde melt and causing assay misquantification by GC if the inlet temperature is inadvertently set below 200 °C and the hydrate co‑elutes or decomposes to the parent aldehyde in the injection port.

Stability chambers operated under ICH Q1A(R2) conditions (40 °C/75% RH) demonstrated that product stored in unsealed LDPE bags inside fibre drums exhibited an assay decay of 5.8% absolute over 4 weeks, together with an increase in the CTC‑Acid impurity from 0.18% to 1.44%. Parallel samples vacuum‑double‑bagged in PET/Al/PE laminated foil with an oxygen absorber and desiccant sachet (silica gel, 10% w/w of product charge) maintained assay within 0.3% of initial and CTC‑Acid below 0.25% for the same duration. Consequently, the recommended long‑term storage condition is 2–8 °C in sealed containers under dry nitrogen, with a retest interval of 12 months. Prior to use in moisture‑sensitive transformations such as Grignard additions or Horner‑Wadsworth‑Emmons olefinations, the aldehyde should be dried under vacuum (0.1 mbar) at 30 °C for 4 h until Karl Fischer water content drops below 0.05% w/w.

Comparative Performance of Regioisomeric and Halo‑Substituted Thiazole‑5‑carbaldehydes in Key Synthetic Steps
Compound	CAS	Oxidative Addition Onset Temp. (Pd/SPhos)	Reductive Amination Dehalogenation Impurity (%)	Aldehyde Hydrate Kₕyd (25 °C)^†
2‑Chloro‑1,3‑thiazole‑5‑carbaldehyde	954‑68‑3	75–80 °C	≤ 0.4	1.8 × 10⁻²
2‑Bromo‑1,3‑thiazole‑5‑carbaldehyde	954‑69‑4	45–50 °C	3.8	1.6 × 10⁻²
2‑Chloro‑1,3‑thiazole‑4‑carbaldehyde	954‑71‑8	70–75 °C	≤ 0.2	0.9 × 10⁻²

^† Determined by ¹H‑NMR integration of aldehyde C–H vs. hydrate methine proton in D₂O/DMSO‑d₆ (1:9 v/v) at equilibrium.

Beyond the obvious reactivity demarcation against the bromo congener, the larger differential lies in comparison with 2‑chloro‑1,3‑thiazole‑4‑carbaldehyde. The 4‑formyl regioisomer places the aldehyde α to the ring nitrogen, allowing six‑membered intramolecular hydrogen bonding with proximal water, which paradoxically reduces bulk hydrate formation (Kₕyd lower by 50%). However, this same electronic effect activates the 2‑position toward nucleophilic displacement even in the absence of palladium, leading to rapid chloride hydrolysis at room temperature when DMF is used as solvent without rigorous drying—a degradation pathway virtually absent in the 5‑aldehyde isomer. Therefore, for applications requiring sequential lithiation at C‑4 followed by electrophilic quench while retaining the chloro handle, 2‑chloro‑1,3‑thiazole‑5‑carbaldehyde is the only viable archetype, as the 4‑aldehyde analogue undergoes competing lithium‑halogen exchange at the 2‑position above –78 °C, resulting in unproductive aldehyde self‑condensation.

HS Code	401083
Chemical Formula	C4H2ClNOS
Molecular Weight	149.58
Appearance	Solid (Typical description, actual may vary)
Solubility In Water	Low solubility (Expected due to structure)
Solubility In Organic Solvents	Soluble in common organic solvents like dichloromethane (General behavior for such compounds)

Packing	100g of 2 - Chloro - 1,3 - Thiazole - 5 - Carbaldehyde packaged in a sealed glass bottle.
Shipping	2 - Chloro - 1,3 - thiazole - 5 - carbaldehyde is shipped in well - sealed, corrosion - resistant containers. Special care is taken to prevent exposure, with proper labeling indicating its chemical nature and handling precautions during transit.
Storage	2 - Chloro - 1,3 - thiazole - 5 - carbaldehyde should be stored in a cool, dry, well - ventilated area, away from direct sunlight. Keep it in a tightly sealed container to prevent contact with air and moisture, which could lead to degradation. Store it separately from incompatible substances like oxidizing agents, bases, and reducing agents to avoid potential chemical reactions.