4H-Thieno[3,2-B]Pyrrole-5-Carboxylic Acid, 3-Bromo-

At the core of late-stage functionalisation strategies for clinical-stage kinase inhibitors, the 3-bromo-4H-thieno[3,2-b]pyrrole-5-carboxylic acid scaffold functions as a masked heteroaryl electrophile with dual reactive handles. The carboxylic acid moiety is pre-activated as the corresponding acid chloride using 1.1–1.3 eq oxalyl chloride in anhydrous dichloromethane containing 0.5 mol% dimethylformamide at 0–5 °C, then telescoped into amide bond formation with a protected piperazine derivative to install the solvent-exposed basic tail. The 3-bromo substituent is retained through this sequence and subsequently exploited in a Pd-mediated Suzuki–Miyaura cross-coupling with a boronic acid pinacol ester bearing a 2-aminopyrimidine pharmacophore. Optimised conditions on 500 g scale employ 0.8 mol% Pd(OAc)₂ and 1.6 mol% XPhos in degassed 4:1 v/v 1,4-dioxane/water, with 2.5 eq K₃PO₄ as base, heating to 82 °C for 14 h. Reaction calorimetry data from a Mettler-Toledo RC1e indicate an exotherm of −138 kJ/mol during catalyst activation; controlled dosing of the bromide solution over 45 min and jacket temperature ramping at 0.3 K/min prevent thermal runaway above 88 °C, at which palladium black formation accelerates and yield drops below 72%. Residual palladium is reduced to <10 ppm by treatment with 3 wt% Macroporous polystyrene-bound trimercaptotriazine (MP-TMT) in toluene at 55 °C for 6 h, followed by hot filtration through a 0.2 μm inline PTFE cartridge. The final active pharmaceutical ingredient intermediate is precipitated from 2:1 heptane/ethyl acetate and vacuum-dried at 45 °C and 50 mbar to a loss on drying <0.5%, with HPLC purity >99.3 area% at 254 nm. Residual solvent analysis per USP chapter <467> confirms dichloromethane below 600 ppm and 1,4-dioxane below 380 ppm, consistent with ICH Q3C Option 2 limits for a 10 g daily dose product. The bromide intermediate is stored under nitrogen at –20 °C in amber HDPE containers; accelerated stability testing at 40 °C/75% RH open dish for 4 weeks shows 0.2% de-bromination as the only detectable degradation pathway by LC-MS.

The brominated thieno[3,2-b]pyrrole ester derived by coupling the carboxylic acid with N-hydroxysuccinimide opens a pathway for site-specific lysine conjugation in antibody–drug conjugate (ADC) payload design. The NHS ester is generated in situ using 1.05 eq EDC·HCl and 1.2 eq N-hydroxysaccharin in 1:1 N-methylpyrrolidone/ethyl acetate at 20–25 °C under argon, and the activated ester is purified by rapid silica gel filtration to avoid hydrolytic reversion; moisture specification for the eluting solvent is <100 ppm Karl Fischer. Conjugation to a trastuzumab biosimilar in phosphate-buffered saline pH 7.8 at 4 °C for 2 h yields a drug-to-antibody ratio of 3.8–4.2 by hydrophobic interaction chromatography, with unreacted bromopyrrole scavenged by a 10 mM cysteine quench. The bromine atom permits subsequent radio-iodination via copper-free Sonogashira–type exchange with ¹²⁵I-iodide for biodistribution imaging; that step uses 1.2 eq Na¹²⁵I, 0.15 eq CuI, and 0.3 eq N,N′-dimethylethylenediamine in DMF at 80 °C for 25 min, with radiochemical purity >97% after C18 Sep-Pak desalting. Compliance with ICH M7 for mutagenic impurities requires control of the reduced bromine-free impurity to <1.5 μg/day, achieved by crystallisation from 3:1 acetone/water and a 0.45 μm polishing filtration through activated carbon-impregnated cellulose depth media. The cytofluorimetric release criterion for free drug content is <1.8% by SEC-HPLC.

What Process Controls Suppress Dehydrohalogenation in Direct Arylation Polymerisation?

When 3-bromo-4H-thieno[3,2-b]pyrrole-5-carboxylic acid is converted to its 2-ethylhexyl ester and employed as an AB-type monomer in direct heteroarylation polymerisation (DHAP), the labile α-proton on the pyrrole ring presents a competing dehydrobromination pathway that limits chain growth. Optimisation on a 50 g pilot scale using a microfluidic flow reactor with a 0.5 mm ID PFA coil and residence time of 12 min at 110 °C yields poly(4H-thieno[3,2-b]pyrrole-5-carboxylate) with number-average molecular weight (M_n) of 18.4 kg/mol and dispersity 1.32 by GPC against polystyrene standards in THF. The catalytic system — 2 mol% Herrmann–Beller palladacycle, 4 mol% P(o-anisyl)₃, 1.5 eq K₂CO₃, and 0.5 eq pivalic acid in 10:1 NMP/tert-butylbenzene — maintains selectivity for C–H activation at the 2-position of the pyrrole over β-hydride elimination, as monitored by in situ ReactIR at 1604 cm⁻¹ (carbonyl shift upon monomer consumption). Stoichiometric bromine end-groups are capped by adding 0.3 eq 2-(tributylstannyl)thiophene and heating a further 2 h, then residual tin is leached to <5 ppm by stirring with 5 wt% QuadraSil AP functionalised silica in toluene at 60 °C. Purification by repeated precipitation into methanol/water 4:1 followed by Soxhlet extraction with acetone for 24 h removes oligomers below 3 kg/mol. The polymer’s HOMO level determined by photoelectron spectroscopy in air (PESA) is –5.22 eV, placing it as a donor in bulk-heterojunction blends with ITIC-F. Organic photovoltaic devices fabricated on ITO/PEDOT:PSS with a 1:1.2 donor:acceptor weight ratio and 2 vol% 1-chloronaphthalene additive achieve a power conversion efficiency of 8.9% under AM1.5G illumination (calibrated with a KG5-filtered silicon reference cell). Quality assurance for the electronic-grade monomer mandates inductively coupled plasma mass spectrometry (ICP-MS) verification of critical elemental impurities: Pd <2 ppm, Fe <3 ppm, Cu <1 ppm, in accordance with a modified SEMI C43-1119 specification.

When the 3-bromo-4H-thieno[3,2-b]pyrrole-5-carboxylic acid is elaborated into a bidentate ligand through sequential amidation with 2-(aminomethyl)pyridine and N-alkylation, the resulting N,N-coordinating species serves as a modular building block for palladium(II)-pincer complexes evaluated in Heck arylations. The amidation step uses 1.0 eq acid chloride (prepared as described) and 1.05 eq 2-(aminomethyl)pyridine in dichloromethane with 2.0 eq triethylamine at 0 °C for 1 h, achieving 87% isolated yield after flash chromatography. Subsequent N-alkylation of the pyrrole nitrogen with 1.1 eq 2-bromoethyl methyl sulfide in DMF containing 1.5 eq Cs₂CO₃ at 60 °C for 18 h produces the thioether-functionalised pro-ligand, which is metallated with 0.95 eq Pd(COD)Cl₂ in refluxing acetonitrile under argon. The square-planar pincer complex precipitates as a yellow microcrystalline solid upon cooling and is recrystallised from 3:1 acetonitrile/diethyl ether, with 98.7% purity by elemental analysis (calculated C, 41.32; H, 3.05; N, 8.78; S, 10.08; found C, 41.28; H, 3.10; N, 8.72; S, 10.12). Single-crystal X-ray diffraction confirms a Pd–C bond length of 1.978 Å and Pd–N distance of 2.041 Å, consistent with strong σ-donation from the thienopyrrole ring. In catalytic testing for the coupling of 4-bromoacetophenone with styrene in DMF at 140 °C using 0.5 mol% complex and 1.2 eq sodium acetate, turnover frequencies reach 2,400 h⁻¹ at >95% conversion; ICP-MS of the reaction filtrate indicates 0.3 ppm palladium leaching, supporting a predominantly heterogeneous recycling mechanism. The ligand synthesis is conducted in a 20 L glass-lined reactor with anchor agitator running at 120 rpm, and exothermy during the acid chloride formation is controlled by circulating –20 °C brine through the jacket. All operations involving the bromide monomer and its derivatives follow OSHA HCS 2012 classifications: skin sensitisation category 1, eye irritation category 2A, with a recommended occupational exposure limit of 0.01 mg/m³ 8-hour TWA pending local regulatory adoption.

Post-Polymerisation Functionalisation of Donor–Acceptor Copolymers with Pendant Bromothicnopyrrole Esters

Grafting of polyethylene glycol side chains onto a pre-formed alternating copolymer of dithienogermole and 3-bromo-4H-thieno[3,2-b]pyrrole-5-carboxylic acid 2-octyldodecyl ester proceeds via nucleophilic aromatic substitution of the 3-bromo group with amine-terminated mPEG₂₀₀₀ under microwave-assisted conditions. The parent polymer, synthesised by Stille polycondensation using 0.5 mol% Pd₂(dba)₃·CHCl₃ and 1.5 mol% P(o-tolyl)₃ in chlorobenzene at 130 °C for 48 h, exhibits M_n = 22.1 kg/mol and a 5% weight-loss temperature of 342 °C by TGA under nitrogen. Subsequent treatment with 5 eq methoxy-terminated PEG-amine (M_n = 2000) and 6 eq triethylamine in anhydrous DMSO at 150 °C under microwave irradiation (150 W, 30 min) converts >95% of bromine aryl sites to the corresponding secondary aniline linkages, as evidenced by the disappearance of the C–Br stretching mode at 585 cm⁻¹ in Raman spectroscopy. The resulting brush polymer self-assembles in aqueous media into micelles with hydrodynamic diameter 48.5 nm by dynamic light scattering (DLS, 633 nm laser, backscattering angle 173°) and critical micelle concentration 2.6 × 10⁻³ mg/mL by pyrene fluorescence probe method. Cyclic voltammetry drop-cast films on glassy carbon electrode reveal reversible oxidation waves at +0.62 V and +0.89 V vs. Ag/AgCl, with a HOMO shift to –5.31 eV ascribed to the electron-withdrawing character of the pendant aniline groups. Such amphiphilic conjugated polymers are evaluated as interface modifiers in perovskite solar cells: a 0.5 mg/mL solution in chlorobenzene spin-coated at 4000 rpm for 40 s onto a triple-cation perovskite layer boosts the open-circuit voltage by 45 mV and the fill factor by 3.8% absolute relative to the untreated device, stabilised power output tracking at maximum power point for 300 s yields 20.2% PCE. The synthetic protocol demands strict exclusion of oxygen during the Stille step — glovebox O₂ <2 ppm — and monomer purification by sequential recrystallisation from 2:1 hexane/toluene (Karl Fischer H₂O <30 ppm). Residual tin in the polymer after workup by precipitation into ethanol and column chromatography over neutral alumina is quantified by ICP-OES at <8 ppm, aligning with the 100 ppm threshold under RoHS Directive 2011/65/EU Annex II for homogeneous materials used in optoelectronic components.

Field degradation studies on a series of 2-(3-bromo-4H-thieno[3,2-b]pyrrole-5-carboxamido)benzimidazole fungicides, synthesised by amidation of the acid chloride with 2-aminobenzimidazole in acetonitrile using 1.2 eq N-methylmorpholine as base, reveal that the bromine atom contributes critically to both foliar rainfastness and soil photolysis half-life. The lead candidate, 1-{3-bromo-4H-thieno[3,2-b]pyrrole-5-carbonyl}-1H-benzimidazol-2-amine, is formulated as a 200 g/L suspension concentrate using 3 wt% phenolsulfonate dispersant and 0.5 wt% xanthan gum thickener, milled in a horizontal bead mill (WAB Dyno-Mill KD-20) with 0.4–0.6 mm yttria-stabilised zirconia beads at 2500 rpm to a particle size D₉₀ 2.8 μm by laser diffraction. Biological screening against Mycosphaerella graminicola (wheat leaf blotch) in a greenhouse trial at 75 g a.i./ha applied with a track sprayer delivering 200 L/ha shows 87% disease control, non-inferior to epoxiconazole at the same rate. Residue decline kinetics in wheat grain measured by LC-MS/MS QqQ give a DT₅₀ of 6.2 days following a first-order kinetic model; the primary soil metabolite, 4H-thieno[3,2-b]pyrrole-5-carboxylic acid (debrominated), exhibits an ecotoxicological no-observed-effect concentration (NOEC) of 1.2 mg/L for Daphnia magna, triggering classification as Chronic Aquatic 3 (H412) under GHS. Compliance data packages for registration under EU Reg. 1107/2009 require the submission of identity, physical-chemical properties, and five-batch analysis: the active ingredient content must be >97.0% w/w, with bromobenzene impurity limited to <0.05% w/w and any single unknown impurity <0.8% w/w. Storage stability at 54 °C for 14 days confirms 2.3% degradation, well within the 5% threshold of CIPAC MT 46.3. Process safety data for the amidation step indicate that the reaction mass can auto-accelerate above 110 °C; a reactor safety design with a blow-down vessel sized per DIERS methodology and a quench solution of aqueous sodium hydroxide held at 15 °C is implemented for pilot batches of 200 L.

Why Halogen Retention During Sanger-Type Reagent Synthesis Determines Sequencing Byproducts

Conversion of the carboxylic acid group to a 2,4-dinitrophenyl (DNP) active ester for N-terminal peptide analysis demands retention of the 3-bromo substituent because any premature debromination triggers electrophilic substitution on the pyrrole ring and generates isomeric contaminants indistinguishable from target amino acid derivatives. The DNP ester is prepared by dissolving the carboxylic acid and 1.2 eq 1-fluoro-2,4-dinitrobenzene in dry dimethylacetamide containing 2.0 eq N,N-diisopropylethylamine, and stirring at 25 °C under nitrogen for 18 h. The reaction mixture is diluted with 5 volumes of ethyl acetate, washed sequentially with 0.1 N HCl and brine, dried over anhydrous MgSO₄, and concentrated to a yellow oil that solidifies upon trituration with diisopropyl ether to give pale orange crystals, mp 128–130 °C, yield 78%. Coupling to glycine methyl ester hydrochloride in dichloromethane with 1.0 eq of the DNP ester and 1.1 eq N-methylmorpholine at 0–5 °C produces the DNP-glycine adduct with the intact 3-bromo-4H-thieno[3,2-b]pyrrole-5-carboxamide UV chromophore exhibiting λ_max 342 nm (ε = 2.18 × 10⁴ M⁻¹ cm⁻¹ in acetonitrile). When subjected to Edman degradation conditions — 20% trifluoroacetic acid in acetonitrile at 45 °C for 10 min — the DNP adduct releases the free DNP-glycine quantitatively with no detectable des-bromo byproduct (<0.2% by HPLC at 360 nm), confirming the suitability of this Sanger variant for peptide sequencing where halogen-selective MS detection (³⁷Cl/⁷⁹Br isotope pattern) is desired. Stability testing under typical sequencing reaction conditions, conducted in a total of 12 cycles of TFA exposure, shows 1.8% cumulative dehalogenation. Personnel handling the DNP ester intermediate must use local exhaust ventilation at 0.5 m/s face velocity; occupational monitoring for dinitrofluorobenzene vapour, a known respiratory sensitiser, requires levels <0.5 μg/m³ 8-hour TWA, aligned with a proposed DNEL under REACH.