1H-Pyrrole-2-Carboxylic Acid, 4-Bromo-, Ethyl Ester

Chemical and Physical Specifications for 4-Bromo-1H-Pyrrole-2-Carboxylic Acid Ethyl Ester

The compound 1H-pyrrole-2-carboxylic acid, 4-bromo-, ethyl ester (CAS 433267-55-1) is supplied as a white to off-white crystalline powder with a molecular formula of C₇H₈BrNO₂ and a molecular weight of 218.05 g·mol⁻¹. Routine release by the manufacturer includes reversed-phase HPLC assay using a C18 column (5 μm, 250 × 4.6 mm) with acetonitrile/0.1% trifluoroacetic acid gradient, with system suitability evaluated per Ph. Eur. 2.2.46. The acceptance criterion for principal peak area is ≥98.5%, with any single unspecified impurity limited to ≤0.5%. Lot-to-lot variability in the positional isomer 1H-pyrrole-2-carboxylic acid, 5-bromo-, ethyl ester is maintained at <0.2% by control of the bromination temperature profile below −10 °C during production. Melting point, determined by differential scanning calorimetry at a scan rate of 10 K·min⁻¹ under nitrogen, is consistently observed in the range 89–92 °C. Residual solvent analysis via headspace GC-FID conforms to ICH Q3C options, with ethanol typically below 500 ppm and methyl tert-butyl ether below 300 ppm. The product is packaged in amber glass vials under argon with a septum overcap to maintain headspace oxygen below 100 ppm prior to first opening.

Representative certificate of analysis parameters for a production-scale batch

Parameter	Method	Specification	Typical Value
Assay (HPLC, area%)	Ph. Eur. 2.2.46	≥98.5%	99.3%
5-Bromo isomer	HPLC, relative retention	≤0.5%	0.08%
Water (Karl Fischer)	Ph. Eur. 2.5.12	≤0.5%	0.12%
Residue on ignition	Ph. Eur. 2.4.16	≤0.1%	0.04%
Melting point (DSC onset)	Internal STM-024	87–93 °C	90.5 °C

In large-scale pharmaceutical intermediate manufacturing, the ethyl ester derivative is preferred over the corresponding methyl ester for downstream processes requiring selective hydrolysis of a more hindered ester adjacent to the brominated pyrrole ring. The compound exhibits moderate solubility in dimethylformamide (>100 mg·mL⁻¹), tetrahydrofuran (~75 mg·mL⁻¹), and ethyl acetate (~45 mg·mL⁻¹), with limited solubility in hexane, characteristics that govern solvent selection for palladium-catalyzed transformations where high substrate concentration reduces total reactor volume on a pilot scale. Under REACH regulations, the substance is registered as a full phase-in substance manufactured in the EU at tonnages below 1 t/a, assigned to use descriptor sector SU 8/9 (manufacture of fine chemicals/laboratory reagents).

What Limits Oxidative Addition of the 4-Bromo Pyrrole in Palladium Catalysis?

In the context of Suzuki–Miyaura coupling, the 4-bromo substituent occupies a position that is electronically deactivated relative to the 2- or 5-positions of the pyrrole ring, resulting in oxidative addition rates that are substantially lower than those of typical aryl bromides. Kinetic profiling under standardized conditions—using phenylboronic acid (1.2 equiv), Pd(PPh₃)₄ (2 mol%), and 2 M aqueous potassium carbonate in dioxane at 80 °C—shows that the 4-bromo ethyl ester reaches full conversion in 6–8 h, compared to 2–3 h for the corresponding 4-iodo analogue and >24 h for the 4-chloro compound, which fails to exceed 10% conversion under identical conditions. The sluggish kinetics are attributable to the electron-rich nature of the pyrrole ring in its neutral form; addition of 1.05 equiv of sodium hydride prior to catalyst loading to generate the pyrrolide anion enhances the oxidative addition rate by approximately a factor of 5, reducing reaction time to ~1.5 h at 60 °C, but introduces a narrow processing window because the anion is susceptible to ring-opening if localized temperature excursions exceed 10 °C above setpoint. When bulkier phosphine ligands such as SPhos or XPhos are employed at 1 mol% Pd₂(dba)₃ with 2.4 mol% ligand, conversion profiles shift markedly. SPhos reduces the required temperature to 65 °C while maintaining >95% conversion in 4 h. However, the ligand cost differential imposes a procurement trade-off evaluated on a per-kilogram product basis. Batch records from a CDMO running the coupling at 50 kg input scale highlight that residual palladium in the crude product typically rises to 120–180 ppm after a single aqueous workup when using Pd(PPh₃)₄, necessitating treatment with a trimercaptotriazine-functionalized silica scavenger (5 wt% relative to substrate) to bring residual metal below the 10 ppm threshold required for further cGMP steps. A side reaction observed in production is untargeted homocoupling of the pyrrole boronic acid intermediate formed via competing transmetallation with product, which consumes the boronic acid coupling partner. This is suppressed by maintaining a precise stoichiometric ratio of arylboronic acid to substrate at 1.05 ± 0.02 equivalents and by dosing the boronic acid as a solution over 90 min using a peristaltic pump. Deviation outside this band results in up to 15% homocoupling byproduct, which co-elutes closely with the desired product during silica gel flash chromatography employing hexane/ethyl acetate 8:1.

Hydrolysis Rate Constants for Ethyl versus Methyl Ester Analogues

Conversion to the free carboxylic acid is a routine activation step prior to amide bond formation in medicinal chemistry programs. The hydrolysis rate of 1H-pyrrole-2-carboxylic acid, 4-bromo-, ethyl ester under alkaline conditions is deliberately slower than that of its methyl ester congener, a feature exploited when simultaneous presence of other methyl esters in a molecular scaffold requires chemoselectivity. In a standardized assay with 1.2 equiv LiOH monohydrate in THF:water 3:1 at 0 °C, the ethyl ester displays a pseudo-first-order rate constant of 0.15 h⁻¹ versus 0.47 h⁻¹ for the methyl ester. This translates to a half-life of 4.6 h for the ethyl and 1.5 h for the methyl, enabling straightforward kinetic resolution by monitoring at 214 nm with inline HPLC. The reaction is quenched with 0.5 M citric acid to pH 3–4, and the free acid is extracted into ethyl acetate with loss to the aqueous phase typically below 2% at that pH. Process chemists note that exceeding 2.0 equiv of base or raising temperature above 25 °C dramatically accelerates a deleterious ring-opening pathway that forms 3-bromo-2-aminofuran derivatives, indicated by an absorption band at 1720 cm⁻¹ in IR monitoring. When this byproduct exceeds 3% peak area by HPLC, the batch is typically discarded due to difficulty of removal by crystallization. The ethyl ester thus provides a safety margin over the methyl in scale-up that is documented in multiple process development reports across contract research organizations. Directly following hydrolysis, the derived 4-bromo-1H-pyrrole-2-carboxylic acid is of insufficient purity for peptide coupling without recrystallization from ethanol/water. Typical isolated yield of the acid after drying at 40 °C under 10 mbar vacuum for 16 h is 81–85%, with purity exceeding 99.0%. In amide coupling using HATU (1.2 equiv) and N,N-diisopropylethylamine (3.0 equiv) in DMF, the free acid reacts with primary amines quantitatively within 2 h, whereas secondary amines require heating to 50 °C for 12 h, a differential that further underscores the steric environment imposed by the bromine at the 4-position. When the 4-bromo substitution pattern is critical for target engagement in kinase active sites, the ethyl ester is frequently present in the initial fragment screening hits. Structure–activity relationship studies have demonstrated that replacement of the 4-bromo with 4-chloro leads to a 10- to 20-fold reduction in binding affinity in a subset of tyrosine kinase profiles, attributable to the weaker halogen bond donor character of chlorine. Conversely, substitution with 4-iodo introduces metabolic lability at the C–I bond, with microsomal stability studies (human liver microsomes, 1 mg·mL⁻¹ protein, 60 min incubation) showing >50% parent loss compared to <12% for the bromo analogue, making the 4-bromo compound the optimal vector for early lead optimization.

Comparative reactivity of 4-halo pyrrole-2-carboxylic acid ethyl esters in palladium-catalyzed C–C bond formation

4-Substituent	Coupling partner	Catalyst system	Time (h)	Conversion (%)
Br	PhB(OH)2	Pd(PPh3)4, K2CO3, dioxane/H2O, 80 °C	6	97
I	PhB(OH)2	Pd(PPh3)4, K2CO3, dioxane/H2O, 80 °C	2	99
Cl	PhB(OH)2	Pd(PPh3)4, K2CO3, dioxane/H2O, 80 °C	24	<10
Br	Ethynylbenzene	PdCl2(PPh3)2, CuI, Et3N, 50 °C	12	91
Br	Vinylboronic acid pinacol ester	Pd(dppf)Cl2, Na2CO3, DME/H2O, 70 °C	8	83

Storage outside the original argon-purged container consistently increases the carbonyl degradation pathway. Long-term stability studies in the manufacturer’s quality control archive, conducted per ICH Q1A(R2) at 25 °C/60% RH over 36 months, indicate that the product retains >98% purity only when septum-sealed under argon. Once opened and exposed to ambient atmosphere (50–70% RH), hygroscopicity drives hydrolysis to the free acid at a rate of approximately 0.8% per week. Laboratories using the compound in parallel synthesis arrays where multiple vial openings are required are recommended to portion the material under anhydrous conditions into smaller single-use vials and to store the bulk at −20 °C. Incompatibilities documented in process safety reports include strong exotherms upon contact with lithium aluminum hydride and with chlorinating agents such as thionyl chloride, which can induce uncontrollable gas evolution in the absence of adequate venting. The brominated pyrrole ring is photolabile; exposure to UV light at 254 nm initiates dehalogenation in solution, reducing assay by ~5% over 48 h, an effect minimized by the use of amber glass and light-protected staging areas.