2,4-Dibromothiazole

When 2,4-Dibromothiazole Serves as a C-2 Selective Electrophile in Cross-Coupling Cascades

In the synthesis of kinase inhibitor scaffolds requiring differentiated C-2 and C-4 aryl decorations, the inherent electronic bias of 2,4-dibromothiazole dictates a sequential oxidative addition pathway. Under standard Suzuki-Miyaura conditions employing Pd(PPh₃)₄ at a loading of 0.5–1.0 mol%, the C-2 bromide undergoes oxidative addition preferentially due to lower electron density at the 2-position relative to the 4-position, a kinetic profile confirmed by Hammett σₚ correlation studies and computational DFT electrostatic potential mapping at the B3LYP/6-31G(d) level. The first coupling at C-2 with arylboronic acids proceeds in dioxane/water (4:1 v/v) containing 2.0 equiv of K₂CO₃ at 80°C for 4–6 hours, yielding the 2-aryl-4-bromothiazole intermediate with ≥85% regioselectivity. A subsequent Pd₂(dba)₃·CHCl₃/XPhos system (0.8 mol%) then addresses the C-4 position using more electron-rich coupling partners, a telescoped sequence that circumvents intermediate isolation on pilot scale when executed under nitrogen inertization and inline FTIR monitoring at 1550–1650 cm⁻¹ for C-Br bond consumption verification. This specific application is governed by ICH Q11 (Development and Manufacture of Drug Substances) for starting material designation and control strategy definition; the overall process yield typically exceeds 72% over two steps when the aqueous phase is back-extracted with ethyl acetate in a continuous centrifugal extractor (CINC V-02 or equivalent) operating at 3500 rpm. End-product types include Type I inhibitors targeting the DFG-in conformation of VEGFR-2 and selective dual inhibitors of MEK1/2, with batches manufactured under cGMP requiring residual palladium levels below 10 ppm as determined by ICP-MS per USP 〈233〉. The 2,4-dibromothiazole addition in the charge vessel must account for its tendency to sublime slowly under vacuum; weighing operations are conducted at atmospheric pressure in a humidity-controlled suite (RH ≤30%) because moisture accelerates dehalogenation side reactions at elevated temperatures. Cross-contamination risk in multi-purpose facilities is mitigated by dedicated glass-lined reactors (Pfaudler AE-series) with PTFE-lined manway gaskets and a validated cleaning protocol involving a 1.0 N NaOH reflux cycle followed by a pyridine/water azeotrope rinse to remove adsorbed brominated residues. Process analytical technology (PAT) implementations on production campaigns frequently deploy ReactIR 15 probes for endpoint determination, with the C-2 coupling deemed complete when the dibromothiazole starting material peak at ∼3100 cm⁻¹ (aromatic C-H stretch) drops below the detection threshold relative to the internal standard peak of the pre-catalyst ligand.

Regiochemical Fidelity Demands in Multi-Brominated Building Blocks: A Medicinal Chemistry Perspective

During the construction of adenosine A_2A receptor antagonists featuring a central thiazole pharmacophore, the differential leaving-group aptitude of bromine at the 2- versus 4-position becomes the critical process parameter differentiating the successful batch from a failed campaign. Published data confirm that 2,4-dibromothiazole undergoes palladium-catalyzed amination (Buchwald-Hartwig) with primary amines selectively at C-4 when employing BrettPhos Pd G3 precatalyst at 1.2 mol% in THF with LiHMDS (1.5 equiv) at ambient temperature (20–25°C), a counter-intuitive selectivity pattern attributable to the LUMO coefficient distribution in the π-system rather than simple steric arguments. The amination product, 4-amino-2-bromothiazole, is then isolated by aqueous workup with 10% citric acid and crystallized from heptane/EtOAc to ≥99.0% purity (HPLC area%, 254 nm). This intermediate subsequently participates in a Negishi coupling at C-2, where the organozinc reagent is generated in situ from the corresponding aryl iodide and ZnCl₂ in DMA at 65°C, catalysed by Pd-PEPPSI-IPent at 0.3 mol%. The synthetic sequence is subject to ICH Q3A impurity thresholds, with the key debrominated by-product (4-amino-thiazole) controlled to ≤0.15% in the final API via a trituration step in MTBE/heptane (1:3). On manufacturing scale, the amination step exhibits a pronounced exotherm (ΔH_rxn ≈ −85 kJ/mol), requiring a controlled addition rate of the LiHMDS solution not exceeding 0.8 equiv/h into a jacketed reactor maintained at 15°C internal temperature with a safety margin of 20°C below the thermal runaway onset as determined by RC1e adiabatic calorimetry. The end-product class encompasses central nervous system-penetrant A_2A antagonists and PET tracer precursors for neuroimaging studies requiring ¹⁸F-labelled thiazole cores. Single-use reactor liners are frequently employed in late-stage clinical manufacturing to eliminate the cleaning validation burden for brominated heterocycles, with extractable data for low-density polyethylene film documented per USP 〈661.2〉.

How Does Halogen Bonding from Thiazole Bromine Atoms Modulate Ligand Geometry in MOF-74 Analogs?

Metal-organic frameworks incorporating 2,4-dibromothiazole as a pillaring ligand precursor exploit the structure-directing influence of Br···O halogen bonding interactions during solvothermal synthesis. The compound is first converted to 2,4-bis(4-carboxyphenyl)thiazole via a two-step Pd-mediated coupling with 4-methoxycarbonylphenylboronic acid followed by saponification with NaOH in MeOH/THF/H₂O, yielding the tetratopic ligand in 58–65% overall yield after recrystallization from DMF. When this ligand is combined with Zn(NO₃)₂·6H₂O in DMF/H₂O/EtOH (3:1:1) at 100°C for 24 h in a sealed Parr acid-digestion vessel, the resulting Zn-MOF crystallizes in the monoclinic space group C2/c with a BET surface area of 1,120 m²/g (N₂ adsorption at 77 K, Micromeritics ASAP 2460, outgassed at 120°C for 12 h under 10⁻⁶ torr vacuum). Critically, the bromine atoms remain integral in the framework pores as determined by single-crystal XRD (Mo Kα radiation, λ = 0.71073 Å, Bruker D8 Venture) and their presence enhances the heat of adsorption for CO₂ to 31.2 kJ/mol at zero coverage compared to 26.8 kJ/mol for the dehalogenated analog, as calculated from CO₂ isotherms measured at 273 K and 298 K using the virial equation. This adsorbent material finds application in post-combustion flue gas CO₂ capture systems where the binary CO₂/N₂ selectivity (determined by Ideal Adsorbed Solution Theory at 1 bar and 298 K for a 15:85 CO₂/N₂ mixture) reaches 86. Production scale-up for kilogram quantities of the MOF is conducted in a 20 L Buchi Polyclave reactor under autogenous pressure, with the ligand:metal molar ratio maintained strictly at 1:2.2 and the solvent volume not exceeding 60% of the total reactor capacity to accommodate pressure buildup from DMF decomposition. The powdered MOF is activated by solvent exchange (DMF to MeOH to dichloromethane over 3 days) followed by thermal evacuation at 150°C under dynamic vacuum (≤10⁻⁴ mbar) in a Büchi B-585 glass oven, with the activation endpoint verified when the residual solvent mass loss rate drops below 0.01 wt%/h. The bromine content of the framework raises specific regulatory considerations under REACH Annex XVII regarding persistent, bioaccumulative substances, requiring a documented mass balance demonstrating that bromine leaching into aqueous media at pH values from 4 to 9 remains below the method detection limit of 0.05 mg/L (ICP-OES, PerkinElmer Avio 200). The end product type is an engineered CO₂-philic adsorbent for pressure swing adsorption (PSA) units targeting 95% CO₂ purity from dilute streams.

Nematicidal Oxadiazole Hybrids via C-2 Click Chemistry Conjugation

Discovery-phase synthesis of non-fumigant nematicides for root-knot nematode (Meloidogyne incognita) control leverages copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) at the C-2 position of 2,4-dibromothiazole after initial bromide substitution by sodium azide. The 2-azido-4-bromothiazole intermediate is prepared in DMSO at 25°C (8 h, 92% yield), and its thermal stability was evaluated by differential scanning calorimetry (DSC) giving an onset decomposition temperature of 138°C — a datum that dictates the maximum processing temperature for all subsequent reactions and the upper limit for short-path distillation during solvent recovery. CuAAC conjugation with a panel of terminal alkynes derived from 1,3,4-oxadiazole scaffolds proceeds in THF/H₂O (1:1) with CuSO₄·5H₂O (5 mol%) and sodium ascorbate (10 mol%) at 40°C for 12–16 h, generating the 1,2,3-triazole-linked hybrid in 75–88% chromatographic yield after flash purification (Biotage Isolera One, SNAP KP-Sil 100 g cartridge, heptane/EtOAc gradient). Regulatory toxicology screening of the resulting library falls under OECD 222 (earthworm reproduction test) and OECD 213 (avian acute oral toxicity) for non-target organism risk assessment, data that are mandatory in the EU Annex III dossier for low-risk plant protection products. In vivo glasshouse efficacy assays against M. incognita on tomato (Solanum lycopersicum cv. Moneymaker) at a treatment rate of 2.5 kg AI/ha as a 10% granular formulation (attapulgite carrier, granulated in a Fuji Paudal EXD-60 extruder with 0.8 mm screen) demonstrate root galling reduction of ≥70% relative to untreated controls at 28 days post-inoculation. Formulation development must account for the hydrolytic lability of the C-4 bromide in alkaline clay soils: incubation studies in a Luvisol (pH 7.8, 60% WHC, 20°C) revealed 12.3% degradation of the C-4 Br to the corresponding hydroxy-thiazole after 30 days, necessitating an acidifying co-formulant (citric acid monohydrate at 3 wt% of the granule) to maintain the micro-environment within a pH window of 5.5–6.5. The end-product category encompasses soil-applied nematicidal granules and seed-treatment flowable concentrates registered under the FRAC/Nematode Resistance Action Committee codes for integrated pest management programs in Solanaceous and Cucurbitaceous cropping systems.

A Brønsted Acidity Test for Atropisomeric Biaryl Formation in Palladacycle-Catalyzed Couplings

Atroposelective synthesis of axially chiral biaryls incorporating a 2,4-dibromothiazole moiety positions this substrate as a benchmark electrophile for evaluating newly developed chiral phosphine ligand systems. The C-2 aryl coupling of 2,4-dibromothiazole with 1-naphthylboronic acid pinacol ester in the presence of a (R)-BINAP-palladium catalyst (2 mol% Pd(OAc)₂, 2.4 mol% (R)-BINAP) under phase-transfer conditions (toluene/aqueous K₃PO₄, 3.0 equiv, tetrabutylammonium bromide 5 mol%) at 70°C generates the atropisomeric axis after the second coupling at C-4 with a sterically encumbered ortho-substituted phenylboronic acid. The degree of atropostability of the resulting 2,4-diarylthiazole scaffold is evaluated by variable-temperature HPLC on a CHIRALPAK IA-3 column with the enantiomeric enrichment ratio assessed over a temperature range of 25–80°C in hexane/iPrOH (90:10, 0.8 mL/min flow), with the rotational barrier ΔG^‡_rot calculated from the Eyring plot of the first-order rate constant for interconversion. Published data indicate that the steric contribution of the bromine substituent at the C-4 position elevates ΔG^‡_rot by 3.5–5.0 kcal/mol relative to the C-4 chloro analog, a finding of demonstrated value in designing configurationally stable biaryls with an enantiomerization half-life exceeding 6 months at physiological temperature (37°C, phosphate buffered saline pH 7.4). This physicochemical property is relevant to the development of atropisomeric drug candidates under the ICH M7(R2) guideline for the assessment and control of DNA reactive (mutagenic) impurities, where the configurational stability of an atropisomer determines whether it is treated as a single impurity or as an interconverting pair for the purposes of the threshold of toxicological concern (TTC) assessment. Preparative-scale resolution of the racemate is achieved by supercritical fluid chromatography (SFC, Waters Prep-100 with a CHIRALPAK IG column, 30 × 250 mm, 5 µm particle size) using CO₂/MeOH (70:30, 100 g/min, backpressure 150 bar, 35°C) with a productivity of 1.2 g/h racemate processed per gram of stationary phase. The end products are enantioenriched atropisomeric phosphine ligands for asymmetric catalysis and chirally pure pharmaceutical intermediates where the atropisomer is the active pharmaceutical species and the opposite atropisomer is controlled as an impurity at ≤0.10% (HPLC area%) per ICH Q3A qualified threshold.

What Process Intensification Challenges Emerge When the C-2 Bromine is Replaced by Cyano in Continuous Flow?

The Rosenmund-von Braun cyanation of 2,4-dibromothiazole to yield 4-bromothiazole-2-carbonitrile, a versatile intermediate for agrochemical amide coupling and heterocycle annulation, presents a heat-transfer and residence-time distribution challenge that is addressed through continuous flow processing rather than batch chemistry. In the optimized flow protocol, a DMF solution of 2,4-dibromothiazole (0.5 M) and CuCN (1.05 equiv) is pumped through a heated tubular reactor (PFA tubing, 1.0 mm ID, 15 m length, residence volume 11.8 mL) at a flow rate of 0.2 mL/min corresponding to a residence time of 59 min at 150°C under a back-pressure regulator setpoint of 5 bar to prevent DMF boiling. The reaction mixture exiting the reactor is quenched immediately into an aqueous FeCl₃/HCl solution (10% FeCl₃·6H₂O w/w, 2.0 M HCl) to decomplex the copper-cyanide species; insufficient quenching time (< 5 min) in this step results in cyano-copper complex entrainment into the organic extract, which poisons palladium catalysts in downstream reactions. The organic phase is isolated and the product, 4-bromothiazole-2-carbonitrile, is obtained after solvent distillation and recrystallization from toluene/heptane (1:2) as a pale yellow crystalline solid (mp 78–80°C) with an isolated yield of 81% at 100 g scale processing per day on a Vapourtec R-Series system with the V-3 pump head. Process safety testing on the crude reaction mixture by differential accelerating rate calorimetry (ARC, Netzsch ARC 254) detected an exothermic onset at 185°C with a maximum self-heat rate of 2.3°C/min, providing a thermal runaway time-to-maximum-rate (TMR_ad) of >24 h at the operating temperature of 150°C, well within the Stoessel criticality class 2 envelope for adequate margin. The regulatory framework for this nitrile intermediate, when used in the manufacture of an active substance for crop protection, includes compliance with the European Union's adopted classification for nitriles under Regulation (EC) No 1272/2008 (CLP) regarding acute toxicity and specific target organ toxicity following repeated exposure (STOT RE). Wastewater from the iron chloride quench step must be treated for copper removal (target ≤0.5 mg/L for industrial discharge) by precipitation with Ca(OH)₂ to pH 9.0 followed by clarification in a lamellar settler. The end products are 2-cyano-substituted thiazole building blocks employed in the assembly of Factor Xa inhibitors and selective S1P₁ receptor agonists.

Marine antifouling coating systems incorporating 2,4-dibromothiazole exploit the compound's electrophilic bromine atoms as latent leaving groups that generate biocidal isothiazolinone derivatives upon hydrolytic activation in seawater. The parent compound is not directly dispersed in the coating matrix at a loading rate of 4–8 wt% dry film weight; rather, it is covalently tethered to a silyl acrylate binder backbone via nucleophilic displacement of the C-4 bromide with 3-mercaptopropyltrimethoxysilane, followed by free-radical copolymerization with tributylsilyl methacrylate and methyl methacrylate in xylene initiated by AIBN (1.2 mol%) at 80°C under nitrogen. The resulting self-polishing copolymer (SPC) exhibits a controlled erosion rate of 3.5–4.5 µm/month (measured by static immersion of drawdown films on PVC panels in natural seawater at 25 ± 2°C at the Yerseke field testing station, The Netherlands, per ASTM D6990-20) over an 18-month period. This erosion rate is correlated to the hydrolysis of the silyl ester side-chains, which gradually exposes the immobilized thiazole moiety to the seawater/matrix interface where slow hydrolysis releases the bioactive species at a rate that maintains the surface concentration above the minimum inhibitory concentration for Amphibalanus amphitrite cyprid settlement (EC₅₀ of 0.8 µg/mL). Regulatory compliance for this formulation as a biocidal product under the EU Biocidal Products Regulation (BPR, (EU) No 528/2012) requires an efficacy assessment according to the ECHA Guidance on the BPR: Volume II Efficacy, Assessment and Evaluation (Parts B+C) including raft and panel trials lasting a minimum of 6 months in two geographically distinct marine environments (Atlantic and Mediterranean). Additionally, an environmental risk assessment is mandated under OECD Series on Emission Scenario Documents No 25 for leather processing, providing a worst-case release estimation of the active moiety into harbour sediment with a predicted environmental concentration (PEC) that must fall below the predicted no-effect concentration (PNEC) for benthic organisms derived from a chronic sediment-spiked toxicity test on Corophium volutator (OECD 218). The final antifouling paint is manufactured in high-speed dispersers (Hockmeyer HSD, tip speed 18–22 m/s) with the brominated thiazole-functionalized SPC dissolved in xylene together with cuprous oxide (35–40 wt% dry film) as a co-biocide, an iron oxide pigment (Bayferrox 130, 3–5 wt%) for colour, and a polyamide wax thixotrope (Crayvallac Super, 1.5 wt%) to achieve a sag resistance of ≥400 µm wet film thickness (ASTM D4400-18, Leneta sag bar). The end-use application is a self-polishing antifouling topcoat applied over an epoxy anticorrosive primer (two-coat system, total dry film thickness 300–350 µm) for ship hulls and offshore structures with a design dry-docking interval of 60 months.