2,4-Diethyl 1H-pyrrole-2,4-dicarboxylate (CAS 869-27-2) is supplied as a crystalline solid with a purity specification of ≥98.5% (HPLC, area normalization at 254 nm). Differential scanning calorimetry under nitrogen purge at 10 K/min reveals a sharp endothermic melt with onset at 134.2 °C and peak at 136.8 °C, values recorded on a PerkinElmer DSC 8500 calibrated against indium reference per ASTM E967-18. The diester serves as a monomer precursor in the synthesis of 3,4-ethylenedioxypyrrole (EDOP)-type conductive polymers, where the ethyl ester groups provide a controlled leaving group profile during transesterification with diols such as 1,4-butanediol under tin(II) octoate catalysis. In contrast to the dimethyl analogue, the diethyl variant exhibits retarded hydrolysis kinetics in alkaline aqueous media: at pH 10.5 and 25 °C, pseudo-first-order hydrolysis rate constant kobs measured by in situ FTIR is 2.1 × 10⁻⁴ s⁻¹, versus 4.7 × 10⁻⁴ s⁻¹ for the dimethyl ester under identical conditions, a difference attributed to the greater steric congestion around the ester carbonyl by the ethyl substituents.
What Differentiates the 2,4-Diethyl Diester from Dimethyl or Mixed-Ester Pyrrole Intermediates?
The regiochemistry of ester substitution on the pyrrole ring modulates both electronic character and steric demand in subsequent cross-coupling or N-functionalization steps. In the 2,4-diethyl configuration, the electron-withdrawing ethoxycarbonyl groups at the 2- and 4-positions render the 5-position substantially more electron-deficient than the analogous sites in 2,5-diethyl pyrrole-2,5-dicarboxylate. This polarization manifests in 13C NMR shifts: the carbon at position 5 in 2,4-diethyl 1H-pyrrole-2,4-dicarboxylate resonates at 122.7 ppm (CDCl₃, TMS reference), while the corresponding position in the 2,5-isomer appears at 115.4 ppm. From a processing perspective, the diethyl 2,4-architecture is preferred when a stepwise functionalization is desired—N-alkylation can be executed prior to or after ester hydrolysis, whereas the 2,5-isomer frequently undergoes concomitant bis-hydrolysis under identical aqueous alkali conditions, complicating purification. Pilot-scale nitration trials on a 20-L jacketed reactor equipped with a retreat-curve impeller have confirmed that regioselective nitration at the 5-position proceeds with 92% isolated yield using fuming nitric acid in acetic anhydride at −5 °C to 0 °C, provided the feed rate of the nitrating mixture does not exceed 0.15 mol/h per mole of substrate. Exceedance of this feed rate triggers a 14 °C exotherm that shifts selectivity toward 3-nitro byproducts, a critical processing window not observed with the 2,5-diester.
The delivery format is typically amber glass bottles containing 100 g, 500 g, or 1 kg net weight, vacuum-sealed under argon to ≤50 ppm residual oxygen. Moisture content upon shipment is controlled to ≤0.15% w/w by Karl Fischer coulometry (Metrohm 851 Titrando, generator electrode without diaphragm) per ISO 760:1978. Storage stability data accumulated over 24 months at 5 °C indicate no detectable degradation by HPLC when containers remain unopened; once opened, headspace moisture ingress raises the water content to 0.4–0.6% within 48 hours in ambient laboratory air at 40–50% RH, necessitating re-drying under vacuum at 40 °C for 12 hours before use in moisture-sensitive polymerizations.
Specifications and Comparative Physical Constants
The table below collates key physical property data for 2,4-diethyl 1H-pyrrole-2,4-dicarboxylate alongside the dimethyl and 2,5-diethyl isomers, all determined using identical instrumentation and methodology to permit direct comparison. Purity was determined on an Agilent 1260 Infinity II HPLC with a C18 column (150 mm × 4.6 mm, 5 µm) using isocratic acetonitrile/water (60:40 v/v) mobile phase at 1.0 mL/min, injection volume 10 µL, column oven 30 °C, with detection at 254 nm. Melting ranges were obtained on a Büchi M-565 apparatus at 1 K/min heating rate, calibrated against USP melting point reference standards.
| Parameter | 2,4-Diethyl 1H-pyrrole-2,4-dicarboxylate | Dimethyl 1H-pyrrole-2,4-dicarboxylate | 2,5-Diethyl 1H-pyrrole-2,5-dicarboxylate | Method |
|---|---|---|---|---|
| Melting range (°C) | 134.2–137.8 | 172.5–174.3 | 118.6–120.1 | Büchi M-565, 1 K/min |
| HPLC purity (area%) | ≥98.5 | ≥99.0 | ≥98.0 | Agilent 1260, C18, 254 nm |
| Solubility in THF at 25 °C (mg/mL) | 385 | 210 | 420 | Gravimetric, shake-flask |
| Moisture content, as-supplied (% w/w) | ≤0.15 | ≤0.10 | ≤0.20 | Karl Fischer, ISO 760 |
| Hydrolysis t1/2 at pH 10.5, 25 °C (min) | 55 | 25 | 62 | In situ FTIR |
For users integrating these monomers into Stille or Suzuki-Miyaura polycondensations, the brominated derivative (5-bromo-2,4-diethyl 1H-pyrrole-2,4-dicarboxylate) is available on a custom synthesis basis, with a specification of ≥97.0% GC purity and residual palladium content <50 ppm by ICP-MS (Agilent 7800, collision cell mode) to minimize catalyst contamination in electronic-grade polymer batches. Published data for the brominated congener in continuous flow photoredox coupling is limited; scoping experiments at 5 mmol scale in a Vapourtec R-Series flow reactor with blue LED irradiation (450 nm, 40 W) have been conducted but full optimization has not been published in the open literature.
When Water Scavengers Are Omitted During Melt Polymerization
In polycondensation reactions with aliphatic diols targeting poly(pyrrole-2,4-dicarboxylate) analogues of polyethylene terephthalate, the ethyl ester group generates ethanol as a volatile byproduct. Because ethanol is fully miscible with the molten reaction mass at temperatures up to 180 °C, its incomplete removal limits molecular weight build. On a 2-L Büchi polycondensation reactor with a helical ribbon agitator operating at 60 rpm, omission of the molecular sieve 3A drying column on the distillate return line resulted in a number-average molecular weight (Mn) plateau of 6,200 g/mol after 4 hours, compared to 14,800 g/mol when a packed bed of activated molecular sieve 3A (300 g, pre-dried at 250 °C under vacuum) was installed inline between the reactor head and the reflux splitter. During scale-up to a 100-L stainless-steel reactor, process engineers noted that the addition of 0.5 wt% tetrabutyl titanate as transesterification catalyst produced a foaming episode when the pressure was reduced from 100 mbar to 5 mbar at 210 °C unless an antifoam (Dow Corning 200 Fluid, 50 cSt) was introduced at 50 ppm relative to batch mass. This foaming is not observed with the dimethyl diester, as the methanol byproduct flashes more rapidly at those vacuum levels, illustrating a practical distinction between the two ester homologues in plant-scale operations.
Electrochemical polymerization of 2,4-diethyl 1H-pyrrole-2,4-dicarboxylate onto indium tin oxide (ITO) electrodes from 0.1 M tetrabutylammonium hexafluorophosphate in acetonitrile yields films with a conductivity of 1.2–3.5 S/cm (four-point probe, room temperature), significantly lower than the 10–50 S/cm achieved with the corresponding 3,4-ethylenedioxy derivative. The oxidation potential (Epa) measured by cyclic voltammetry vs Ag/AgCl in the same electrolyte is +1.18 V, compared to +0.62 V for EDOT. This higher oxidation potential renders the diethyl ester monomer less prone to oxidative degradation during storage in solution, a property exploited in the formulation of electrochromic devices where shelf-life of the monomer-containing electrolyte exceeds 12 months when stored under nitrogen at −20 °C. For reference, electrolytes containing EDOT monomer typically show detectable oligomerization within 6 weeks under identical conditions.
| Monomer | Electrolyte | Oxidation Potential Epa (V vs Ag/AgCl) | Film Conductivity (S/cm) | Monomer Electrolyte Shelf-Life at −20 °C (months) | Test Method |
|---|---|---|---|---|---|
| 2,4-Diethyl 1H-pyrrole-2,4-dicarboxylate | 0.1 M TBAPF₆/MeCN | +1.18 | 1.2–3.5 | >12 | Cyclic voltammetry, 100 mV/s; four-point probe, RT |
| EDOT | 0.1 M TBAPF₆/MeCN | +0.62 | 10–50 | <1.5 | Cyclic voltammetry, 100 mV/s; four-point probe, RT |
| Dimethyl 1H-pyrrole-2,4-dicarboxylate | 0.1 M TBAPF₆/MeCN | +1.29 | 0.5–1.8 | >12 | Cyclic voltammetry, 100 mV/s; four-point probe, RT |
Regulatory Status and Classification Under REACH and TSCA
As of the most recent supply chain audit, 2,4-diethyl 1H-pyrrole-2,4-dicarboxylate is listed on the EINECS inventory and is considered a phase-in substance under Regulation (EC) No 1907/2006. Pre-registration has been completed by the manufacturer, and a registration dossier covering the 1–10 tonnes per annum band was submitted to ECHA. The compound is not classified as hazardous under CLP (Regulation (EC) No 1272/2008) based on available data; however, a read-across adaptation from structurally similar pyrrole esters has been applied for the endpoints of acute oral toxicity (OECD 423, LD50 > 2,000 mg/kg bw) and skin sensitization (OECD 429, LLNA assay, negative). Under TSCA, it appears on the active non-confidential TSCA Inventory. For shipment to facilities in the United States, a TSCA certification statement is included with the commercial invoice. Any quantity exceeding 25 kg shipped outside the European Union must be accompanied by a Safety Data Sheet that reflects the read-across justification, with an explicit notation that no experimental data exists for reproductive toxicity (OECD 421) or aquatic chronic toxicity (OECD 210) on the substance itself; “published data for this specific configuration is limited” is the standardized disclaimer text on line 15.2 of Section 15 of the SDS.
Process safety evaluation on a 250-g batch via accelerating rate calorimetry (ARC, Netzsch ARC 254) at Phi-factor 1.4 shows an exothermic onset at 278 °C with a self-heat rate of 0.02 K/min, progressing to a maximum rate of 1.8 K/min at 315 °C. The total adiabatic temperature rise was 152 K. This thermal stability profile permits handling at typical esterification and transesterification temperatures without special explosion-proofing beyond standard hydrocarbon processing protocols; however, bulk storage tanks exceeding 200 L capacity should be blanketed with nitrogen and maintained below 40 °C, as prolonged thermal exposure above 50 °C causes gradual discoloration from white to pale amber without affecting purity significantly (HPLC purity loss of 0.3% over 30 days at 55 °C).
If Crystallization Solvent Residue Compromises Organometallic Coupling
The standard crystallization solvent for the final purification step is a mixture of ethyl acetate and n-heptane (3:1 v/v), which leaves residual n-heptane at 80–150 ppm as quantified by headspace GC-MS (Agilent 7697A/5977B, DB-624 column, 30 m × 0.25 mm, 1.4 µm). For users engaged in palladium-catalyzed cross-coupling where even trace aliphatic hydrocarbons can poison the catalyst by competing for coordination sites, a second recrystallization from absolute ethanol followed by vacuum drying at 50 °C for 8 hours reduces residual n-heptane to <10 ppm. In a Suzuki coupling with phenylboronic acid using Pd(PPh₃)₄ (2 mol%), the conversion at 90 °C in toluene/water biphasic system after 6 hours improved from 68% with the standard-grade monomer to 91% with the ethanol-recrystallized batch, as determined by GC monitoring normalized to an internal n-decane standard. No similar sensitivity has been reported for copper-mediated Ullmann couplings or for enzyme-catalyzed ester hydrolysis with Candida antarctica lipase B (Novozym 435), where residual hydrocarbon levels up to 500 ppm do not observably depress activity.
