1H-Pyrrole-2-Carboxylic Acid, 5-Bromo-, Methyl Ester

What Regioselective Handling Protocols Apply During Brominated Pyrrole Ester Cross-Couplings?

In the manufacture of a key intermediate for orally administered tropomyosin receptor kinase (TRK) inhibitors—such as those targeting NTRK fusion-positive solid tumours under IND-stage development—5-bromo-1H-pyrrole-2-carboxylic acid methyl ester undergoes a palladium-catalysed Suzuki-Miyaura coupling with (3-methoxy-4-pyridin-4-ylphenyl)boronic acid pinacol ester. The electron-withdrawing methyl ester at C2 deactivates the pyrrole ring sufficiently to retard oxidative addition at the C5–Br bond relative to unsubstituted pyrrole, yet competitive transesterification of the methyl ester with the boronate cannot be disregarded in aqueous-alcoholic media. Process R&D has established that using 1.08 equivalents of the boronate, Pd(OAc)₂ at 0.5 mol%, XPhos at 1.0 mol%, and powdered K₃PO₄ (2.5 eq) in a de-gassed dioxane/water mixture (4:1 v/v) at 85 °C for 6 hours suppresses ester hydrolysis below 0.3%. The reaction mass is charged to a 500 L Hastelloy C-22 reactor fitted with a retreat-curve impeller and a double mechanical seal, heated via external half-pipe jacket circulation. After Celite-pad filtration and solvent swap to ethyl acetate, the crude intermediate is crystallised from n-heptane/ethyl acetate (5:1) using controlled cooling from 60 °C to -5 °C at 0.2 °C/min, yielding a white powder with HPLC purity ≥99.0 area% (C18, 220 nm) and Pd content ≤7 ppm by ICP-MS, compliant with ICH Q3D Guideline for Elemental Impurities. The terminal active pharmaceutical ingredient is a chiral pyrrolopyrimidine TRK inhibitor candidate, formulated as a sulfate salt for capsule filling under 21 CFR 211 conditions. A recurrent batch-failure mode occurs when residual water in the reactor exceeds 0.05% Karl Fischer: the liberated pyrrole-2-carboxylic acid chelates Pd(II), precipitating inactive palladium carboxylates that halt conversion at 60–70%.

In the late-stage construction of the oxazolidinone antibiotic tedizolid phosphate (CAS 856867-55-5), 5-bromo-1H-pyrrole-2-carboxylic acid methyl ester functions as a masked precursor to the C-ring heterocyclic appendage via amide coupling with the des-tetrazolyl oxazolidinone amine core. The methyl ester is deliberately retained throughout the coupling to prevent the free carboxyl group from participating in a Curtius-type rearrangement side reaction when the amine is activated. Manufacturing under full cGMP (ICH Q7 and 21 CFR 211) mandates incoming material specifications that include bromide content by potentiometric titration against silver nitrate (98.5–101.5% on anhydrous basis) and total organic volatile impurities screened as per USP ⟨467⟩ with a limit of 500 ppm for dichloromethane. The supply chain must supply the ester in double polyethylene-lined fibre drums under nitrogen blanket, stored at 2–8 °C to prevent methyl ester migration to the N–H position, a known solid-state rearrangement that generates the 5-bromo-1-methyl isomer. During the amide coupling executed in a 1000 L glass-lined reactor equipped with a temperature probe accurate to ±0.3 °C, the oxazolidinone amine (1.00 equivalent), HOBt·H₂O (1.30 eq), and EDC·HCl (1.30 eq) are dissolved in anhydrous DMF, cooled to 0–5 °C, and a solution of the pyrrole ester (0.99 eq to prevent dimerisation on the pyrrole nitrogen) in DMF is metered in via a peristaltic pump over 45 min. After 2 h at 0–5 °C and 16 h at 22 °C, the batch is quenched into purified water (5 volumes) and the precipitated product isolated by centrifuge filtration, washed, and recrystallised from isopropanol/water (3:2) to yield the penultimate intermediate as a monohydrate. This intermediate is subsequently phosphorylated with POCl₃ and hydrolysed to tedizolid phosphate monosodium salt, an antibacterial active approved under NDA 205435 and compliant with EMA/CHMP/4445/2015. Process robustness trials demonstrated that water ingress above 0.1% KF in the DMF feed causes premature ester saponification; the resulting acid reacts with EDC to form an unreactive N-acylurea, decreasing yield by 12–18% and generating an impurity that co-elutes with the product during preparative HPLC purification.

Catalyst Deactivation Pathways in Continuous-Flow Hydrogenation of 5-Bromo Pyrrole-2-Carboxylate Esters

When the targeted downstream intermediate is the fully debrominated methyl 1H-pyrrole-2-carboxylate—required as a building block for halogen-exchange-based synthesis of aphicidal 2-aryl-5-trifluoromethylpyrroles—catalytic hydrodebromination becomes the preferred route over stoichiometric metal reductions to avoid heavy metal waste streams. The substrate is, however, a potent catalyst poison: the pyrrole π-system binds strongly to palladium surfaces, and bromide ions generated during the reaction accelerate Ostwald ripening of Pd(0) crystallites. To sustain process viability, the hydrogenation is conducted in a ThalesNano H-Cube Pro continuous-flow reactor equipped with a proprietary CatCart® column packed with 5% Pd/C (type THS02231, particle size 50–70 µm). The raw methyl ester is dissolved in methanol at 0.50 M and fed at 1.0 mL/min under a hydrogen pressure of 50 bar at 60 °C; these conditions represent the operational addition ratio wherein residence time is precisely 2.3 min. Under these parameters, an initial conversion of 99.8% declines to 95% after 120 hours of cumulative runtime, at which point the differential pressure across the catalyst bed reaches 12 bar and the cartridge is replaced. Post-hydrogenation, the methanolic stream passes through an in-line activated carbon cartridge (30×4.6 mm) to scavenge dissolved Pd(II) and residual bromide, then is concentrated under reduced pressure (45 °C, 50 mbar) and purified by short-path distillation using a Büchi KDL 5 unit with an evaporator temperature of 80 °C and pressure 0.1 mbar, yielding a colourless, mobile oil with assay 99.5% by quantitative ¹H NMR. The terminal pesticide actives are members of the N-phenylpyrazole and fluorinated pyrrole acaricide classes subject to registration data requirements under EPA 40 CFR Part 158 and must comply with CIPAC test methods for physico-chemical properties. Quality management of the intermediate manufacturing adheres to ISO 9001:2015. Regeneration of spent catalyst cartridges involves air calcination at 400 °C for 4 hours under a flow of 100 mL/min synthetic air, yet activity recovery rarely exceeds 82% of the fresh cartridge value, establishing an intrinsic lifetime limitation.

Polymer-grade 5-bromo-1H-pyrrole-2-carboxylic acid methyl ester is incorporated as a heterocyclic electron-rich donor monomer in alternating donor-acceptor copolymers designed for hole-transport layers (HTL) in phosphorescent organic light-emitting diodes. The pendant methyl ester group simultaneously solubilises the growing polymer chain in aromatic solvents and fine-tunes the HOMO energy level to −5.32 eV as determined by photoelectron spectroscopy in air (PESA, Riken Keiki AC-3) according to the procedure described in ASTM E1490-11. Stoichiometric control is the dominant process variable: the molar ratio of the pyrrole diester monomer to 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene must be maintained at 1.000:1.000 with a tolerance of ±0.002. Deviations beyond 0.5 mol% shift the number-average molecular weight (M_n) below the critical chain entanglement threshold of ~12 kDa, resulting in pinhole-riddled films after spin-coating. The standard Suzuki polycondensation recipe is executed in a 5 L jacketed borosilicate glass reactor (Büchi Polyclave) outfitted with an IKA Eurostar 60 overhead stirrer set at 200 rpm. Toluene (1.2 L) and aqueous sodium carbonate (2.0 M, 0.4 L) serve as the biphasic medium, tetrakis(triphenylphosphine)palladium(0) is introduced at 1.5 mol% relative to total monomers, and Aliquat 336 (5 drops) is added as a phase-transfer catalyst. The stirring is continued at 95 °C for 48 hours under argon, with an additional portion of catalyst (0.5 mol%) injected after 24 hours to compensate for palladium black precipitation. Upon completion, the toluene layer is washed with an aqueous solution of ethylenediaminetetraacetic acid disodium salt (0.1 M, 3 × 200 mL) to extract palladium residues, dried over anhydrous magnesium sulfate, and concentrated to 50 mL. The crude copolymer is precipitated into methanol (1.5 L), collected, and sequentially Soxhlet-extracted with methanol, acetone, and hexane for 24 hours each to remove oligomers (M_n < 3 kDa) and unreacted monomer. The fraction soluble in chloroform yields a bright yellow fibrous solid upon re-precipitation. Gel-permeation chromatography against polystyrene standards in THF at 40 °C (ISO 13885-1) reports M_n = 28–45 kDa and a polydispersity index of 1.8–2.4. The terminal product is an ink-jet printable HTL formulation that, after thermal curing at 180 °C, forms a 40 nm thick layer exhibiting a hole mobility of 2.1 × 10⁻⁴ cm²/V·s measured by the space-charge-limited current method. A process-critical quality gate is residual bromine content in the finished polymer: X-ray fluorescence screening according to IEC 62321-8 must return a value ≤50 ppm. Even 200 ppb of mobile bromide ions migrate under the 3–5 V DC bias of an OLED pixel and initiate electrolytic dark-spot corrosion, reducing device half-life at 1000 cd/m² from 10,000 hours to under 700 hours in accelerated shelf-life tests conducted per ISO 13406-2. All materials supplied for this application chain require a REACH registration dossier covering annual tonnage bands up to 1–10 tonnes and a signed SVHC declaration.

When the Methyl Ester Is Retained as a Latent Carboxyl Protecting Group in Fmoc-Solid Phase Peptide Synthesis

Synthesis of antifungal cyclic heptapeptides belonging to the keramamide class requires the unnatural amino acid (2S)-2-amino-3-(5-bromo-1H-pyrrol-2-yl)propanoic acid as a turn-inducing residue. The methyl ester of 5-bromo-1H-pyrrole-2-carboxylic acid serves as a completely orthogonal carboxyl protecting group during the elongation of the linear precursor on an Fmoc-Wang resin. In contrast to allyl or benzyl esters, the methyl ester withstands the repetitive 20% piperidine/DMF Fmoc-deprotection cycles ( 2 × 5 min at 75 °C under microwave heating in a CEM Liberty Blue synthesizer) without detectable transesterification at the resin-bound peptide ester linkage. The methyl ester is cleaved selectively only after full chain assembly, immediately prior to resin-bound head-to-tail cyclization. Hydrolysis is achieved by treating the preswollen peptidyl-resin with a degassed solution of 0.2 M lithium hydroxide in THF/deionised water (3:1) for 90 min at 30 °C; the free carboxyl group is then activated in situ using HATU (3.95 equivalents relative to resin substitution) and DIEA (8.0 eq) in N-methylpyrrolidone under a pseudo-dilution condition of 0.1 mM peptide concentration to favour intramolecular ring closure. The coupling efficiency of the ester-monomer itself during chain assembly requires a 4.0-molar excess over the resin loading (0.5 mmol/g), with a double-coupling protocol of 60 min each at 75 °C. Process-scale batches use disposable 30 mL polypropylene syringe reactors fitted with a polyethylene frit (20 µm) and agitated on a rotating incubator at 25 rpm. Following TFA/triisopropylsilane/water (95:2.5:2.5) cleavage at 38 °C for 2.5 h and precipitation in cold diethyl ether, the crude cyclic peptide is purified on a preparative RP-HPLC system equipped with a C18 column (250 mm × 50 mm, 10 µm, 100 Å) using a linear gradient of 20–50% acetonitrile in water with 0.1% trifluoroacetic acid over 40 min at a flow rate of 80 mL/min. The terminal active product is API-grade cyclic peptide KNP-1 analogue sulfate, targeting invasive aspergillosis through inhibition of 1,3-β-D-glucan synthase at an IC₅₀ of 8.6 nM. Good manufacturing practice compliance is realised under ICH Q7 and European Pharmacopoeia general monograph 01/2021:2034 for synthetic peptides. Complete removal of the methyl ester is verified by ¹³C NMR disappearance of the methoxy signal at 51.8 ppm; residual methyl ester content above 0.2% directly parallels a cyclodimer impurity that reaches 4.7 area%, necessitating a second HPLC pass and 22% yield loss.

Electron-Deficient Monomer Matching in n-Type Organic Field-Effect Transistor (OFET) Semiconductors

Solution-processed n-channel OFETs based on donor-acceptor copolymers exploit the dual electron-withdrawing effect of the 5-bromo substituent and the carbomethoxy group on the pyrrole ring to depress the LUMO energy level to −3.62 eV, as estimated by density functional theory at the B3LYP/6-31G(d) level and validated by cyclic voltammetry using Bu₄NPF₆ (0.1 M in acetonitrile) with a scan rate of 50 mV/s and a ferrocene internal standard (ASTM E1490-11). This monomer is copolymerised with naphthalene-1,4,5,8-tetracarboxylic diimide via direct heteroarylation polymerisation, thus avoiding pre-functionalised organometallic monomers that contribute to metal-rich impurities in the active channel. In a nitrogen-filled MBraun UNIlab glovebox (H₂O and O₂ < 0.1 ppm), a 100 mL Schlenk tube is charged with the diimide (1.000 mmol), the pyrrole ester (1.000 mmol, ±0.5 mg), tris(dibenzylideneacetone)dipalladium(0) (2 mol%), tricyclohexylphosphine tetrafluoroborate (4 mol%), potassium carbonate (3.0 eq, ground in situ), and pivalic acid (0.3 eq) in anhydrous N,N-dimethylacetamide (5 mL). The reaction is heated at 120 °C for 24 h with magnetic stirring at 300 rpm. After precipitation into methanol and sequential Soxhlet extraction as described previously, a dark blue solid is recovered. The addition ratio defined as 1.000:1.000 is the single largest factor dictating charge carrier mobility: a 1 mol% excess of the bromopyrrole ester caps the polymer chains with electron-deficient end groups that trap electrons, reducing the saturated field-effect mobility from 0.06 cm²/V·s to 0.008 cm²/V·s. The semiconductor ink is prepared at 5 mg/mL in chlorobenzene, heated to 80 °C for 2 h, and spin-coated at 1500 rpm onto an n-octadecyltrichlorosilane-treated SiO₂/Si wafer (gate dielectric thickness 300 nm). Thermal annealing at 200 °C for 30 min inside an inert-atmosphere hotplate improves π-stacking order, evidenced by a (100) diffraction peak with d-spacing 24.8 Å by grazing-incidence X-ray diffraction. Gold source-drain electrodes (50 nm) are thermally evaporated through a shadow mask defining a channel length of 50 µm and width of 1000 µm. Mobility values extracted from transfer characteristics in the saturation regime per ASTM D6900-20 typically range 0.02–0.08 cm²/V·s, with threshold voltages of 5–15 V and on/off current ratios exceeding 10⁴. The final product is a flexible printed RFID antenna and tag transponder that meets the communication protocol requirements of ISO/IEC 18000-6C. Device reproducibility is compromised when residual palladium from the polymerisation exceeds 100 ppm, as determined by microwave-digestion ICP-MS. Palladium aggregates act as deep charge traps, inducing a gate leakage current above 1 nA that violates the pass/fail criterion set in IEC 62860-1 for printed electronic circuits.