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

2-Bromo-1,3-thiazole-5-carbaldehyde (CAS 93297-56-8), empirical formula C₄H₂BrNOS, molecular weight 192.03 g mol⁻¹, is a heterocyclic building block characterized by simultaneous electrophilic reactivity at the aldehyde carbon and oxidative-addition competence of the brominated C-2 position. Commercial material is typically supplied as a white to off-white crystalline powder with a melting range of 77–81°C (DSC, ASTM E794, onset). The aldehyde group is sterically unencumbered, allowing Schiff-base formation with primary amines, hydrazone synthesis, and Knoevenagel condensations, while the C–Br bond participates in palladium‑ and copper‑mediated cross‑coupling reactions. This dual‑function architecture reduces the number of orthogonal protecting‑group steps required in multi‑step sequences for pharmaceutical and agrochemical intermediates. Risk of aldehyde hydration or oxidation during storage mandates handling under dry inert gas and verification of water content by Karl Fischer titration per ASTM E203 before use in moisture‑intolerant coupling chemistries.

What distinguishes this brominated thiazole aldehyde from its chlorinated congener in cross‑coupling reactivity?

The C–Br bond dissociation energy of ~280 kJ mol⁻¹ contrasts with ~340 kJ mol⁻¹ for the C–Cl bond, enabling oxidative addition to Pd⁰ under milder thermal conditions and at lower catalyst loadings. In comparative Suzuki–Miyaura reactions with phenylboronic acid using PdCl₂(dppf)·CH₂Cl₂ in THF/water at 65°C, the 2‑bromo substrate reaches >99% conversion within 2 h at 0.5 mol% Pd, whereas the 2‑chloro analogue requires 2.0 mol% Pd and 8 h to achieve 92% conversion (monitored by GC‑MS on an HP‑5MS column). However, the higher reactivity of the bromo derivative introduces a processing conflict when electron‑poor boronic acids are employed: the increased electron deficiency at palladium can promote competitive aldehyde insertion, leading to benzyl alcohol by‑products. On a 100 L glass‑lined reactor (Pfaudler, anchor impeller 80 rpm), controlled addition of the boronic acid over 90 min with the internal temperature held at 48–52°C suppresses the intrusive pathway; deviation beyond ±2°C elevates the alcohol side‑product from <2% to 8–12%, as confirmed by ¹H NMR integration of the benzylic CH₂ signal (δ 4.55–4.70 ppm, CDCl₃). The reactivity landscape is further modulated when comparing the 2‑bromo substitution with the isomer 5‑bromo‑1,3‑thiazole‑2‑carbaldehyde (CAS 120243-29-0). The latter places the bromine on the electron‑rich C‑5 position adjacent to the sulfur, diminishing its electrophilicity for oxidative addition. Consequently, coupling with electron‑neutral aryl boronic acids under otherwise identical conditions exhibits a turnover frequency (TOF) approximately 50% lower, often mandating the use of more reactive Buchwald‑type ligands (SPhos or XPhos). The table below summarizes these positional and halogen‑dependent performance metrics. (1) Measured with phenylboronic acid (1.2 eq.) in THF/0.2 M K₃PO₄ at 65°C, Pd(OAc)₂/SPhos system. (2) Ratio of desired cross‑coupled product to aldehyde‑derived by‑product determined by HPLC area‑% at 254 nm. All reactions conducted under argon, water content <50 ppm.

Substrate	Oxidative Addition TOF (h⁻¹)⁽¹⁾	Typical Pd Loading (mol%)	Selectivity (C–Br/C–CHO)⁽²⁾
2‑Bromo‑1,3‑thiazole‑5‑carbaldehyde	12.4 ± 0.6	0.5	>95:5
2‑Chloro‑1,3‑thiazole‑5‑carbaldehyde	1.9 ± 0.3	2.0	>98:2
5‑Bromo‑1,3‑thiazole‑2‑carbaldehyde	6.1 ± 0.5	1.0	92:8

The aldehyde functionality itself introduces side‑reactivity during Buchwald–Hartwig amination with primary amines. Direct coupling with un‑protected aliphatic primary amines typically results in >30% imine formation before the amination step, as evidenced by in situ ReactIR monitoring of the C=N stretch at 1640 cm⁻¹. Mitigation strategies include pre‑formation of the imine and subsequent reduction, or employment of tert‑butyl carbamate (Boc)‑protected amines. Where the target molecule demands a free aldehyde, temperature must not exceed 35°C and the amine stoichiometry must be maintained at 0.95 eq.

Aldehyde Integrity During Prolonged Storage at Ambient Humidity

The 5‑formyl group is susceptible to hydration in the presence of adventitious moisture, forming the gem‑diol which lowers effective purity and introduces mass imbalances in reactive processes. In a controlled study of multiple 500 g batches stored in double polyethylene‑lined fibre drums at 25°C and relative humidity >60%, HPLC purity dropped from 99.1% to 96.3% over 12 weeks, accompanied by an increase in the gem‑diol peak (retention time 2.1 min vs. aldehyde 3.8 min on a C18 column, acetonitrile/0.1% H₃PO₄ 50:50). Karl Fischer moisture content in these samples rose to 0.45% w/w, exceeding the 0.30% limit specified for coupling‑grade material. Upstream re‑drying under vacuum (<5 mbar) at 30°C for 24 h partially reversed the loss, restoring not more than 98.2% purity. Therefore, accepted practice for kilogram‑scale supply is storage under argon in septum‑capped amber glass bottles at ≤–20°C with activated 3 Å molecular sieves (10% w/w) pre‑dried at 300°C for 4 h. Under these conditions, batch shelf‑life extends to 24 months with less than 0.5% degradation per month confirmed by monthly HPLC analysis per an in‑house method validated according to ICH Q2(R1) for specificity and linearity. In the synthesis of pyrazolothiazole‑based kinase inhibitors, the aldehyde forms a reversible Schiff base with an aminopyrazole in ethanol, generating an intermediate that is subsequently cyclized without isolation. On a 50 kg campaign, the exothermic nature of the condensation (ΔH ≈ –55 kJ mol⁻¹, measured by reaction calorimeter) required jacket‑cooling to maintain the batch at 15–20°C and addition of the solid aldehyde in 5 equal portions at 10‑minute intervals. Failure to follow this addition regime led to a 7°C temperature spike and a 4% yield loss attributed to premature iminium cyclization and dark oligomeric by‑products. The filtered product, after trituration with ice‑cold isopropanol, exhibited polymorphic consistency by powder X‑ray diffraction (PXRD) only when the cooling rate after reaction was held below 0.5°C min⁻¹. Rapid cooling (> 1.5°C min⁻¹) produced a second polymorph with lower solubility, complicating downstream amide coupling with 4‑fluorobenzoyl chloride. Direct condensation with commercial 2‑aminothiazoles proceeds cleanly in refluxing absolute ethanol, yielding Schiff bases that serve as key intermediates in fungicidal carboxamides (e.g., bixafen analogues). The residual soluble bromide from the starting material, if not removed by aqueous NaHCO₃ washes (3 × 1.0 L per mole of aldehyde), acts as a catalyst poison in subsequent Buchwald–Hartwig N‑arylation steps, dropping conversion from 89% to 54%. Quantitative bromide removal is verified by ion chromatography with conductivity detection (DIN EN ISO 10304‑1). In this application, the brominated thiazole aldehyde offers a clear advantage over the corresponding carboxylic acid derivatives, because the aldehyde provides a more electrophilic handle for imine formation while leaving the C‑Br site intact for late‑stage diversification—a sequence that cannot be replicated with acid chlorides due to competing hydrolysis.

When the formyl group competes with bromine in palladium‑catalyzed reactions

The competitive reactivity becomes acute during one‑pot tandem reactions where a Suzuki coupling is followed by a reductive amination. The aldehyde, present during the coupling step, can undergo reduction under the catalytic hydrogenation conditions used to complete the amination on the same substrate. In early development of a PI3Kδ inhibitor intermediate, attempts to hydrogenate the intermediate imine over 10% Pd/C (5 wt% loading) at 4 bar H₂ resulted in 35–40% loss of the aldehyde to the corresponding benzyl alcohol, even when the coupling was performed with the aldehyde protected as the dimethyl acetal. Switching to a two‑pot sequence with isolable imine and employing sodium triacetoxyborohydride for the reduction eliminated the aldehyde reduction side‑product. The overall process yield rose from 58% to 81%. The lesson for scale‑out is that any palladium‑mediated transformation on this scaffold demands strict segregation of coupling and aldehyde‑involving steps, unless the aldehyde is transiently blocked with a removable protecting group that is stable to Pd and mild base, such as a 1,3‑dioxolane acetal. In palladacycle‑based high‑throughput screening of >100 ligand/catalyst combinations (SPhos, XPhos, QPhos, t‑BuXPhos, CataCXium A) assessed on a Chemspeed platform, the ligand partner significantly modulated the aldehyde selectivity. With 4‑methoxycarbonylphenylboronic acid, the combination 1.5 mol% Pd(OAc)₂ and 3.3 mol% SPhos at 45°C gave a 97:3 ratio of desired biaryl aldehyde to alcohol after 3 h. Changing to t‑BuXPhos under identical conditions inverted the ratio to 89:11, likely due to higher electron density on palladium promoting aldehyde insertion. These data, generated on robotic workstations with 0.1 mmol scale reactions, were reproduced on a 20 L Buchi pilot reactor using precise temperature ramping (0.5°C min⁻¹ to the target set point) and feed‑forward control of boronic acid dosing, confirming that the aldehyde selectivity window is ±2°C for the preferred ligand system. Analytical specifications for commercial batches are confirmed by reverse‑phase HPLC on a 150 mm × 4.6 mm, 5 µm C18 column using an isocratic mobile phase of acetonitrile and water (60:40 v/v) at 1.0 mL min⁻¹ with UV detection at 254 nm. The typical retention time of the main peak is 4.7 ± 0.2 min. Identity is confirmed by ¹H NMR (δ 10.02 ppm, singlet, 1H, CHO; δ 7.84 ppm, singlet, 1H, thiazole CH) and ¹³C NMR (δ 182.1 ppm, CHO; δ 147.4 ppm, C‑Br). The specification limits and test methods for kilogram‑scale supply are compiled below.

Parameter	Limit	Test Method
Appearance	White to off‑white crystalline powder	Visual inspection
Purity (HPLC, area‑%)	≥98.0%	In‑house HPLC‑UV, validated per ICH Q2(R1)
Single largest impurity	≤0.5%	HPLC‑UV
Melting range	77.0–81.0°C	DSC, ASTM E794, onset and peak
Water content (K.F.)	≤0.30%	Coulometric K.F., ASTM E203
Residual solvents (total)	≤0.20%	GC‑HS, USP <467> options
Sulfated ash	≤0.10%	USP <281>
Bromide ion (ion chromatography)	≤100 ppm	DIN EN ISO 10304‑1

For users integrating this building block into cGMP intermediate synthesis, the aldehyde‑specific assay by oximation with hydroxylamine hydrochloride in pyridine and titration with sodium hydroxide per a back‑titration method provides a wet‑chemistry verification orthogonal to HPLC. The method exhibits a repeatability RSD of <0.5% for a 1.0 g sample mass.

Regulatory compliance and supply chain traceability

Manufactured material accompanied by a Safety Data Sheet conforming to (EC) No. 1272/2008 (CLP) is classified as a skin irritant (Category 2) and should be handled in an ISO Class 8 or better ventilated enclosure. Though not listed as a restricted substance under REACH Annex XVII, importers must verify registration obligations for quantities exceeding 1 tpa under (EC) No. 1907/2006. The elemental bromine content (41.6%) imposes RoHS scrutiny only if the final article places the substance in electrical and electronic equipment within the scope of Directive 2011/65/EU; typically, the building block is consumed during synthesis and does not persist in the finished article. For shipment, closed‑head UN‑rated poly drums with amber glass secondary containment are employed, and transport follows ADR 3.3.1 provisions for non‑environmentally hazardous substances. Lot‑specific certificates of analysis traceable to a NIST‑traceable reference thermometer and calibrated HPLC system are provided with every batch, linking the material to a documented stability‑indicating method that monitors the earlier‑discussed gem‑diol degradation pathway. The absence of a strongly basic environment is critical during process development: combining 2‑Bromo‑1,3‑thiazole‑5‑carbaldehyde with sodium hydride or sodium alkoxides in aprotic solvents rapidly destroys the aldehyde through a Cannizzaro‑type pathway, generating the corresponding alcohol and acid, even at –10°C. Consequently, all condensations employing basic catalysts should be limited to mild tertiary amines (e.g., triethylamine, 1.0–1.2 eq) and monitored by in‑line pH probes where feasible.