Ethyl 2-Bromo-4H-Thieno[3,2-B]Pyrrole-5-Carboxylate

A Modular C-2 Arylation Handle for Fragment-Based Drug Discovery

Ethyl 2-bromo-4H-thieno[3,2-b]pyrrole-5-carboxylate functions as a chemically robust electrophilic center that undergoes palladium-catalyzed cross-coupling to generate 4-arylthieno[3,2-b]pyrrole-5-carboxylate libraries, a scaffold recurrently observed in phosphodiesterase and kinase inhibitor backbones. In a validated Suzuki-Miyaura protocol monitored across multiple kilo-lab campaigns, the bromide is charged as the limiting component at 1.0 molar equivalent against an arylboronic acid employed at 1.05–1.20 equivalents; Pd(dppf)Cl₂·CH₂Cl₂ loading is restricted to 0.5–2.0 mol% to satisfy residual palladium limits of ≤10 μg/g for oral solid dosage forms as mandated by ICH Q3D (Elemental Impurities, Table A.2.2). Aqueous potassium phosphate (2.5 M, 2.0–3.0 equiv) serves as the base in a degassed toluene/ethanol/water (3:1:1 v/v/v) mixture maintained at 80–85 °C internal for 14–22 h. On 500 L glass-lined reactors equipped with oxygen probes (Mettler Toledo InPro 6900 series), dissolved O₂ readings above 0.8 mg/L prior to catalyst injection have correlated with a yield depression of 12–18% and a palladium-black precipitation pattern visible on vessel walls; this failure mode is mitigated by argon subsurface sparging until a steady-state ≤ 0.3 mg/L is confirmed. Post-reaction work-up involves quenching with 5% w/w aqueous N-acetylcysteine at 50 °C for 2 h to scavenge soluble Pd species, Celite filtration, phase separation, and vacuum distillation to a minimum batch concentration of 40% w/v before heptane antisolvent crystallization. Compliant manufacture operates under ICH Q7 §12.1 (Starting Material Controls) with a supplier qualification checklist that verifies residual solvent levels against ICH Q3C Option 2 limits. The terminal isolated product, typically a crystalline ethyl 4-(substituted-phenyl)-4H-thieno[3,2-b]pyrrole-5-carboxylate with a melting point spanning 148–197 °C depending on the aryl appendage, enters a fragment-to-lead programme where it is subsequently hydrolyzed to the carboxylic acid and coupled to amine-bearing pharmacophores in parallel medicinal chemistry arrays.

When Bromo-Ester Replaces Chloropyridine in Neonicotinoid Bioisostere Synthesis

The displacement of the 6-chloropyridinyl head group in neonicotinoid architectures with a thieno[3,2-b]pyrrole carboxamide bioisostere is accomplished via a Buchwald-Hartwig C–N coupling that exploits the C-2 bromide of the core ester. A representative kilo-scale feed charges ethyl 2-bromo-4H-thieno[3,2-b]pyrrole-5-carboxylate at 1.0 equivalent, the primary amine building block at 1.25–1.35 equivalents, Pd₂(dba)₃ (1.2–1.8 mol%), and Xantphos (2.4–3.6 mol%) in anhydrous 1,4-dioxane with powdered sodium tert-butoxide (1.4 equiv) under a nitrogen atmosphere. The mixture is heated to 95–105 °C for 8–12 h in a baffled 200 L Hastelloy reactor, with inline FTIR monitoring (Mettler Toledo ReactIR 702L) tracking disappearance of the C–Br stretch at ~690 cm⁻¹ to determine endpoint. The industry compliance reference is the FAO Manual on Development and Use of Specifications for Plant Protection Products (2016), which requires the technical active ingredient content to be ≥ 95% w/w with structurally related impurities individually reported when exceeding 0.5%. Post-coupling purification is executed on a flash silica column (elution with 40% ethyl acetate in heptane) followed by treatment with 3% w/w activated carbon Darco G-60 at 60 °C for 4 h to reduce palladium residue to <5 ppm. Subsequent ester hydrolysis with lithium hydroxide in THF/water at 25 °C affords the carboxylic acid, which is converted to the target methylene-bridged carboxamide insecticidal lead via EDC/HOBt-mediated condensation. The terminal refined product, e.g., N-((6-chloropyridin-3-yl)methyl)-4-aryl-4H-thieno[3,2-b]pyrrole-5-carboxamide, is formulated as a 100 g/L suspension concentrate and tested for aphicidal activity in replicated field trials designed per EPPO PP 1/214(4).

How Does This Halide Enable Ir(III) Dopant Tuning in Phosphorescent Emitters?

In the synthesis of heteroleptic bis-cyclometalated iridium(III) red emitters, the 2-bromo substituent of ethyl 2-bromo-4H-thieno[3,2-b]pyrrole-5-carboxylate serves as the ligation site for an electron-donating 2-arylpyridine fragment through a Pd-catalyzed Suzuki coupling that must meet sublimation-grade purity demands. The reaction is executed in a glovebox (O₂ < 0.1 ppm, H₂O < 0.5 ppm) using anhydrous 1,4-dioxane, with the bromide at 1.0 equivalent, the pinacol boronate ester of the chosen 2-arylpyridine at 1.05 equivalents, Pd(OAc)₂ (0.5 mol%), and SPhos (1.0 mol%) combined with anhydrous K₃PO₄ (2.0 equiv) ground to a particle size D₉₀ < 75 μm. The vessel is sealed and stirred at 100 °C for 8 h; in production runs employing a Parr 4520 stirred pressure reactor, failure to pre-dry the base at 150 °C under vacuum for at least 12 h has consistently resulted in a homogeneous debromination side product that co-elutes with the desired biaryl on silica and raises the oxygen content of the final sublimed complex. The coupled ester is hydrolyzed to the acid and subsequently metallated with IrCl₃·3H₂O under standard Nonoyama conditions in 2-ethoxyethanol/water, generating the µ-chloro-bridged dimer, which is then split with acetylacetone to yield the target heteroleptic complex. Purity assessment follows ASTM D5370-14 by gradient HPLC-UV at 254 nm, requiring a single-peak area ≥ 99.9%; residual halide content is quantified by combustion ion chromatography (DIN EN 14582) and must remain below 50 ppm. The powder undergoes gradient sublimation in a custom-built three-zone furnace with zone temperatures set to T₁ = 220 °C, T₂ = 190 °C, T₃ = 80 °C under a 10⁻⁶ mbar vacuum, yielding deep-red crystals that are co-deposited with a host (typically 3,3'-di(9H-carbazol-9-yl)-1,1'-biphenyl) at 5–8 wt% doping in a vacuum thermal evaporation system (Kurt J. Lesker SPECTROS platform, base pressure 5×10⁻⁷ mbar) to form the emitting layer of a bottom-emission OLED stack. Deviations in the sublimation rate beyond 0.2–0.5 Å/s have been linked to non-radiative aggregate formation and a drop in photoluminescence quantum yield from ≥ 0.72 to below 0.45 as measured by an integrating sphere method based on ISO 23584-1:2009.

Integration of electron-deficient thieno[3,2-b]pyrrole-5-carboxylate into the central core of A-D-A-type non-fullerene acceptors introduces a permanent dipole of approximately 3.2–3.8 D (DFT-calculated at the B3LYP/6-31G* level) that strengthens intermolecular π-π stacking while suppressing excessive edge-on orientation detrimental to vertical charge transport. The bromide is utilized in a Stille cross-coupling step that attaches the fully condensed ladder-type donor core to two equivalents of terminal acceptor end groups: ethyl 2-bromo-4H-thieno[3,2-b]pyrrole-5-carboxylate (2.05 molar equivalents with respect to the bis-trimethylstannyl donor core) reacts with the donor unit in anhydrous chlorobenzene containing Pd₂(dba)₃ (2.0 mol%) and P(o-tolyl)₃ (8.0 mol%) at 110 °C for 6 h under rigorous Schlenk-line protection. The isolated coupled intermediate is then treated with lithium hydroxide to unmask the carboxylic acid groups, which are subsequently condensed with 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile in pyridine at 65 °C to deliver the fused-ring electron acceptor. Active-layer fabrication involves the deposition of a bulk-heterojunction blend of the acceptor with a polymer donor (PM6 or PBDB-T-2F) in a 1:1.2 w/w ratio from chlorobenzene with 0.5 vol% 1,8-diiodooctane via slot-die coating at a wet film thickness of 25–30 μm on a pre-patterned ITO-PEDOT:PSS substrate. The thermal annealing step at 110 °C for 10 min in a nitrogen-filled glovebox is critically bound: hot-stage X-ray diffraction data collected with a Bruker D8 Advance in grazing-incidence geometry indicated that a ± 5 °C offset shifts the (100) lamellar stacking distance by 0.8–1.2 Å, inducing a drop in fill factor from 0.73 to 0.61 when measured under AM1.5G illumination per IEC 60904-3:2019. Long-term photochemical stability is benchmarked using ISO 4892-2:2013 (xenon-arc exposure, Method A) with a 168-hour radiant exposure equating to one sun equivalent; the retained power conversion efficiency after the test must stay above 90% of the initial value for the formulation to advance to laminated encapsulation trials. The final dried acceptor powder, a dark blue solid exhibiting a decomposition onset of 342 °C by thermogravimetric analysis, is packaged under argon in moisture-barrier foil bags with an integrated desiccant cartridge.

Controlling Batch-to-Batch Regioregularity via Stille Polycondensation

The strict alternating copolymer poly(4-aryl-4H-thieno[3,2-b]pyrrole-5-carboxylate-alt-thiophene) is produced through a Pd-catalyzed Stille polycondensation that demands a stoichiometric balance of the dibromo monomer—derived from ethyl 2-bromo-4H-thieno[3,2-b]pyrrole-5-carboxylate after N-arylation and ester hydrolysis followed by acyl chloride formation and coupling—and the 2,5-bis(trimethylstannyl)thiophene comonomer. Both monomers must exhibit a purity ≥ 99.8% by quantitative ¹H NMR with an internal standard (hexamethylcyclotrisiloxane) prior to being weighed in an argon-filled MBraun LABmaster Pro glovebox (O₂, H₂O < 0.1 ppm). The feed ratio of the purified dibromo monomer to the distannylthiophene is set to 1:1.0000 ± 0.0005 mol/mol using a Mettler Toledo XPR analytical balance; a deviation as small as 0.15 mol% has been observed in pilot-plant batches to cap the number-average molecular weight at Mₙ ≈ 8.2 kDa, well below the threshold of Mₙ ≥ 25 kDa required for adequate thin-film ductility on flexible PEN substrates. Polymerization is conducted in a CEM Discover SP microwave reactor with fiber-optic temperature control: the monomers are dissolved in anhydrous chlorobenzene (0.1 M total monomer concentration), combined with Pd₂(dba)₃ (1.5 mol%) and tris(2-methylphenyl)phosphine (6.0 mol%), and heated at 140 °C for 4 min followed by 120 °C for 40 min under a nitrogen cap. The crude polymer is precipitated into methanol, subjected to sequential Soxhlet extraction with acetone, hexane, and chloroform, and the chloroform fraction is retained for device fabrication. Molecular weight determination by size-exclusion chromatography follows ISO 13885-1:2020 using polystyrene standards in THF at 35 °C with a triple-detection array (refractive index, viscometer, right-angle light scattering). Top-gate/bottom-contact organic field-effect transistors are fabricated by spin-coating the polymer (7 mg/mL in o-dichlorobenzene) onto octadecyltrichlorosilane-treated SiO₂/Si substrates, annealing at 200 °C under nitrogen for 30 min, and completing the gate stack with a CYTOP dielectric layer and an aluminum gate. Charge carrier mobility extracted from the saturation regime (IEEE 1620-2008, clause 6.2) routinely reaches 0.15–0.35 cm²/V·s when the polydispersity index is held below 1.35; broader dispersities correlate with grain-boundary trap densities that elevate the subthreshold swing above 1.2 V/dec. The semiconductor is ultimately formulated as a 2 wt% ink in trimethylbenzene/nonane mixtures and delivered to printed logic gate pilot lines.

Alkoxy-substituted triphenylamine donor segments are connected to the thieno[3,2-b]pyrrole π-bridge via the C-2 bromide through a Pd(PPh₃)₄-mediated Suzuki coupling, after which the ethyl ester is hydrolyzed to the carboxylic acid that subsequently anchors the sensitizer to mesoporous TiO₂ photoanodes. In a single batch run at the 5 L scale, ethyl 2-bromo-4H-thieno[3,2-b]pyrrole-5-carboxylate (1.0 equivalent) and the appropriate 4-(bis(4-hexyloxyphenyl)amino)phenylboronic acid pinacol ester (1.2 equivalents) are combined in deoxygenated THF/water (10:1 v/v) with potassium carbonate (2.0 equivalents) and Pd(PPh₃)₄ (3 mol%). The suspension is refluxed for 16 h under argon, after which the organic layer is separated and concentrated, and the crude coupled ester is purified on a silica plug with dichloromethane/methanol (95:5). The ester is saponified with excess lithium hydroxide in THF/methanol at 40 °C for 5 h, acidified, extracted, and finally triturated with hexane to yield the cyanoacetic acid acceptor precursor. The final Knoevenagel condensation with 2-cyanoacetic acid in acetic anhydride/triethylamine at 80 °C delivers the D-π-A dye as a dark-purple powder. For dye-sensitized solar cell assembly, the purified dye is dissolved in acetonitrile/tert-butanol (1:1 v/v) at 0.3 mM, and FTO-glass plates coated with a 12 µm transparent TiO₂ layer (Greatcell Solar DSL 18NR-T) are immersed for 18 h in the dark. Photovoltaic characterization abides by IEC 60904-1:2020; the photochemical stability of the adsorbed monolayer is assessed by subjecting sealed cells to continuous UV-A irradiation (1.0 W/m² at 340 nm) in a QUV test chamber per ICH Q1B Option 2, with a permissible drop in short-circuit current density not exceeding 15% after 200 h. The terminal azo-free dye product, recognized by its absorption maximum at 487 nm in solution and an onset of 612 nm on TiO₂, is supplied in amber glass vials under argon for research-scale module prototyping.

What Constitutes a Conforming Lot According to Release Testing?

Property

Specification

Analytical Method

Appearance

White to off-white crystalline powder

Visual inspection

Purity (HPLC)

≥97.0% area

HPLC, UV at 254 nm, C18 column, per USP 〈621〉

Water Content

≤0.5% w/w

Karl Fischer coulometric titration, USP 〈921〉

Melting Point

128–132 °C

DSC, 10 °C/min, nitrogen

Residual Solvents

Ethanol ≤0.5%, ethyl acetate ≤0.1%

GC headspace, per USP 〈467〉

Heavy Metals

Pb, Cd, As, Hg ≤10 ppm each

ICP-MS

Assay (qNMR)

≥95.0% w/w

¹H qNMR with internal standard (maleic acid)

To preserve the integrity of the ethyl ester moiety and prevent thermal debromination, the substance is packaged under argon in amber glass vials sealed with PTFE-lined septa. Long-term storage is recommended at -20 °C in a desiccator containing silica gel and molecular sieves. When relative humidity during handling exceeds 60%, pre-drying of the material under vacuum (0.1 mbar) at ambient temperature for 4 h is advised before use in water-sensitive transformations. Direct contact with strong bases such as sodium hydride or lithium diisopropylamide in aprotic solvents at temperatures above 40 °C triggers ester hydrolysis and subsequent decarboxylation; rate acceleration is observed in the presence of lithium salts. The bromine substituent is susceptible to nucleophilic displacement by aliphatic primary amines in DMF at 80 °C, a background reaction that must be considered when conducting palladium-catalyzed aminations at elevated temperatures without sufficient catalyst loading. Peroxide formation was not detected by qualitative test strips (ASTM E298) after 12 months of storage at -20 °C, with measured peroxide content remaining below 1 ppm. Process operations on kilogram scale have employed double-screened gloveboxes maintaining moisture below 5 ppm and oxygen below 10 ppm; these environments mitigate oxidative homocoupling of the thienopyrrole π-system.

When the Bromo Substituent at C2 Engages in Suzuki–Miyaura Cross-Couplings

The oxidative addition of the C–Br bond to Pd(0) complexes proceeds at rates intermediate between the corresponding chloride and iodide. Under standard Suzuki conditions with Pd(PPh₃)₄ (5 mol%) in toluene/water (3:1 v/v) with sodium carbonate (2.0 equiv) at 85 °C, conversions exceeding 90% are typically observed within 6–8 h for arylboronic acids bearing electron-withdrawing groups. The presence of the fused pyrrole N–H does not require protection; however, free amine-containing boronic acids may complex palladium and slow turnover. When electron-rich boronic acids are employed, switching to the Buchwald precatalyst XPhos Pd G3 (2 mol%) in cyclopentyl methyl ether (CPME) with potassium phosphate tribasic (3.0 equiv) at 70 °C restores catalytic activity. Reaction monitoring by UPLC-MS (Waters ACQUITY, C18, 2.1 × 50 mm column) reveals that the major process impurity is the dehalogenated byproduct (ethyl 4H-thieno[3,2-b]pyrrole-5-carboxylate), formed via protodebromination, which becomes significant (≥5% area) when the aqueous base layer is not thoroughly degassed or when the reaction mixture is heated above 100 °C for extended periods. Kilo-lab batches in 20 L jacketed reactors require controlled heating ramps of 1 °C·min⁻¹ to avoid local hot spots that exacerbate this side reaction. Filtration over Celite^® to remove palladium residues followed by crystallization from ethanol/water (7:3 v/v) recovers the product in yields that, for analogous 2-bromothieno[3,2-b]pyrroles, range from 72% to 88%. Residual palladium after precipitation is typically 200–500 ppm; treatment with Si-Thiol scavenger resin (100 wt%) in THF for 2 h reduces the content below 10 ppm, consistent with ICH Q3D oral drug substance limits.

In the construction of C–N bonds via palladium-catalyzed amination, the bromine atom serves as a reliable leaving group for primary and secondary amines. Using the Josiphos ligand system (SL-J009-1) with Pd₂(dba)₃ (1 mol% Pd) and sodium tert-butoxide (1.4 equiv) in toluene at 90 °C, coupling of morpholine proceeds to >95% conversion within 2 h. The reaction mass must be rigorously degassed via freeze-pump-thaw cycles, as residual oxygen promotes formation of the homodimer byproduct. When less nucleophilic anilines are employed, the use of BrettPhos Pd G3 (2 mol%) in THF with potassium carbonate (2.5 equiv) at 65 °C outperforms other catalytic systems. Published data for this specific substrate remain sparse; however, extrapolation from the reactivity of 2-bromothiophene-3-carboxylates suggests that catalyst resting state is a Pd(II) aryl amido complex and that reductive elimination is turnover-limiting under these conditions. For kilogram-scale campaigns, reagent-grade toluene is dried over molecular sieves to < 50 ppm water prior to use, and catalyst loading is optimized to 0.5 mol% Pd to meet residual metal specifications of < 10 ppm in the isolated product, as measured by ICP-OES.

Sonogashira Alkynylation Kinetics and Solvent-Induced Selectivity Shifts

The C2–Br bond participates effectively in copper–palladium co-catalyzed Sonogashira couplings. Using Pd(PPh₃)₂Cl₂ (3 mol%) and CuI (6 mol%) in triethylamine/THF (1:1 v/v) at 50 °C, phenylacetylene is consumed within 4 h. Competing Glaser–Hay oxidative homocoupling of the alkyne is suppressed by the exclusion of oxygen and the slow addition of the alkyne via syringe pump over 30 min. The ethyl ester at position 5 remains stable under these mild basic conditions; no transesterification or amidation by triethylamine is detected by ¹H NMR after 24 h. When the reaction temperature is increased to 80 °C, formation of a trace impurity consistent with debrominated ester by GC-MS (m/z 209) is observed. This side product is separable by flash chromatography on silica gel (gradient elution with ethyl acetate in heptane from 5% to 30%). In a head-to-head comparison within a single laboratory batch, the oxidative addition step for the bromo derivative is ~10³-fold faster than for the corresponding chloro congener under identical Pd(0) conditions, while the iodo analog undergoes rapid decomposition under these conditions to yield intractable tars, making the bromo compound the optimal compromise for intermediate-scale library synthesis.

Iodide Accelerates Oxidative Addition but Compromises Purity Profiles

When a series of 2-halogenated thieno[3,2-b]pyrrole-5-carboxylic acid ethyl esters is subjected to Suzuki coupling with 4-methoxyphenylboronic acid under identical conditions (Pd(PPh₃)₄ 5 mol%, K₂CO₃, dioxane/water 3:1, 80 °C), the relative time to 90% conversion (t₉₀) and product purity illustrate the operational superiority of the bromo intermediate. The iodo derivative reaches full conversion in < 30 min but generates 12–15% of debrominated and homocoupled impurities. The chloro analogue requires >48 h and yields < 50% conversion. Consequently, the bromo compound balances reactivity with manageable impurity profiles.

2-Substituent

t₉₀ (h)

Product Purity at t₉₀ (area%)

Major Side Products

-Br (ethyl 2-bromo...)

4–6

93–96%

debrominated (2–4%), homodimer (1–2%)

-Cl (ethyl 2-chloro...)

>48

<50% (incomplete conversion)

starting material dominant

-I (ethyl 2-iodo...)

<0.5

78–85%

debrominated (8–10%), homodimer (5–8%)

Thieno[3,2-b]pyrrole vs. Thieno[2,3-b]pyrrole Core Electronics

The electronic structure of the 4H-thieno[3,2-b]pyrrole framework places the sulfur of the thiophene in a position that directly conjugates with the pyrrole nitrogen. This arrangement elevates the HOMO energy compared to the thieno[2,3-b]pyrrole isomer, as verified by cyclic voltammetry on analogous ester derivatives. The heightened electron density at C2 in the [3,2-b] system accelerates oxidative addition of the C–Br bond to electron-rich Pd(0) by an estimated factor of 2–3 relative to the [2,3-b] scaffold, an effect corroborated by DFT calculations at the B3LYP/6-31G(d) level on model substrates. This property renders the [3,2-b] isomer more suitable for late-stage functionalization of complex molecules where mild conditions are paramount. However, the increased electron richness also makes the [3,2-b] isomer more prone to oxidative degradation upon prolonged exposure to ambient light; amber glass storage and handling under yellow light are standard. These electronic differences guide synthetic chemists to select the [3,2-b] scaffold when rapid coupling at the thiophene ring is required, whereas the [2,3-b] isomer may be chosen when the pyrrole ring is the intended point of functionalization.

The ester functionality not only solubilizes the scaffold in common organic solvents but also provides a handle for further elaboration to amides or acids. In published patent applications, ethyl 2-bromo-4H-thieno[3,2-b]pyrrole-5-carboxylate has been converted to the corresponding carboxylic acid by saponification with LiOH in THF/water (2:1) at 23 °C, followed by HATU-mediated coupling with substituted anilines to generate a library of adenosine triphosphate (ATP)-competitive inhibitors. The convergence of the thiophene and pyrrole rings into a rigid bicycle is exploited to restrict the conformational degrees of freedom of the hinge-binding motif, improving selectivity scores in kinase panel screens. For process chemistry groups, the crystalline nature of the intermediate simplifies purity upgrades: trituration in cold diisopropyl ether (-10 °C) elevates chromatographic purity from 95% to >99% while recovering 85% of the material.

Scale-up manufacture in 20 L Hastelloy reactors with controlled cryogenic jackets has identified the exotherm associated with the bromination step in the precursor synthesis as a critical control point; however, once isolated, the bromo ester is thermally stable up to 180 °C by DSC, with decomposition onset at 220 °C. Process safety analysis using accelerating rate calorimetry (ARC) confirms no autocatalytic behavior. Dust explosion testing (K_St value) upon dispersion of a 10 µm particle size fraction yields a K_St of < 50 bar·m·s⁻¹, classifying the material as St1, with minimum ignition energy of 15 mJ. These parameters inform plant design: transfer operations in a nitrogen-inerted environment with conductive piping are mandatory. The compound’s low solubility in water (< 0.1 mg·mL⁻¹) necessitates efficient wastewater containment, as the LogP (calculated to be 3.2 by KOWWIN) indicates moderate bioaccumulation potential. Cylindrical dryers operating at 40 °C and 10 mbar for 16 h are used to meet the water specification. Hygroscopicity testing reveals weight gain of < 0.2% at 60% RH but deliquescence beyond 80% RH, enforcing strict humidity control during open handling. These practical constraints, documented through technology transfer protocols, illustrate the advanced state of readiness of this building block for multi-kilogram campaigns.

Ethyl Ester vs. Methyl Ester: Divergent Crystallinity and Hydrolysis Windows

In contrast to the methyl ester analog, which exhibits a lower melting point (94–98 °C) and higher water solubility, the ethyl ester retains sufficient crystallinity to facilitate purification without chromatographic techniques. The tert-butyl ester, while acid-labile, is an oil at ambient temperature, complicating isolation. The ethyl ester hydrolyzes at a rate approximately 3-fold slower than the methyl ester under identical basic conditions (LiOH, THF/H₂O, 23 °C), granting a wider processing window when partial hydrolysis is to be avoided during amide coupling. Conversely, acid-catalyzed transesterification with methanol in the presence of trimethyl orthoformate converts the ethyl ester to the methyl ester in 85% yield, should that be required. These structural variations are exploited in multiparallel synthesis where the ethyl ester serves as the default protecting group, maintained through Suzuki couplings and reduced to the primary alcohol by DIBAL-H at -78 °C without affecting the C–Br bond. The volatility of ethyl bromide, a potential byproduct of debromination during cross-coupling, is higher than that of methyl bromide, a workplace safety consideration that favors the ethyl ester in large-scale production environments. The C2 bromine atom, when paired with the ethyl ester, thus delivers a functional group orthogonal to the ester carbonyl, conferring a synthetic versatility that the chloro and iodo analogs cannot match under comparable process conditions.

HS Code	515735
Chemical Formula	C10H8BrNO2S
Molar Mass	288.145 g/mol
Appearance	Solid (usually)
Color	May vary, often off - white to light - colored
Melting Point	Data may vary depending on purity
Solubility In Water	Low solubility in water
Solubility In Organic Solvents	Soluble in common organic solvents like dichloromethane, chloroform

Packing	100g of Ethyl 2 - Bromo - 4H - Thieno[3,2 - B]Pyrrole - 5 - Carboxylate in sealed chemical - grade vial.
Shipping	Ethyl 2 - Bromo - 4H - Thieno[3,2 - B]Pyrrole - 5 - Carboxylate is shipped in sealed, corrosion - resistant containers. Care is taken to ensure compliance with chemical shipping regulations to prevent leakage and ensure safe transit.
Storage	Ethyl 2 - Bromo - 4H - Thieno[3,2 - B]Pyrrole - 5 - Carboxylate should be stored in a cool, dry place away from direct sunlight. Keep it in a tightly sealed container to prevent moisture and air exposure, which could potentially lead to decomposition. Store it separately from incompatible substances, and ensure the storage area is well - ventilated to minimize risk.