Ethyl 2,5-Dimethyl-1H-Pyrrole-3-Carboxylate

Ethyl 2,5-dimethyl-1H-pyrrole-3-carboxylate (CAS 2199-51-1) is a heterocyclic building block comprising a pyrrole ring with methyl substituents at the 2- and 5-positions and an ethyl ester at the 3-position. Under standard ambient conditions, the compound occurs as a pale yellow to light brown crystalline solid with a melting point in the range of 40–44 °C and a boiling point of approximately 95–98 °C at a reduced pressure of 0.5 mmHg. The molecular formula C₉H₁₃NO₂ corresponds to a molecular weight of 167.21 g/mol. Typical commercial lots assay at ≥97.0% purity by GC (FID detection, area normalization) with individual impurities capped at ≤1.5%, most commonly the regioisomeric ethyl 2,4-dimethyl-1H-pyrrole-3-carboxylate (CAS 2199-52-2) arising from the condensation sequence used in Knorr-type syntheses. The carbonyl stretch in the neat IR spectrum exhibits a strong band at 1678–1685 cm⁻¹, while the N–H stretch appears as a sharp absorption near 3300 cm⁻¹. Identity verification by ¹H NMR (CDCl₃, 400 MHz) shows the ester methyl triplet at δ 1.35, the ring methyl singlets at δ 2.20 and 2.25, the ester methylene quartet at δ 4.25, the ring 4-H signal at δ 5.80, and the broad N–H resonance near δ 8.2. These descriptors align with data tables published by fine chemical suppliers and are reproducible across multiple synthetic batches when stored under inert atmosphere.

What Purity Grades Are Typically Supplied and How Are They Validated?

Commercially, two principal tiers circulate: a synthesis-grade material specified at ≥95% purity and a high-purity research grade at ≥98% (GC). The latter is often accompanied by a certificate of analysis referencing in-house methods modelled on USP <621> Chromatography and Ph.Eur. 2.2.46 for chromatographic system suitability. In addition to GC area-percent purity, total heavy metal content is frequently controlled to ≤20 ppm (as Pb) per USP <231> or Ph.Eur. 2.4.8 when the compound is intended for pharmaceutical intermediate use. Loss on drying at 105 °C for 2 hours typically measures ≤0.5%. Water content determined by Karl Fischer titration (USP <921> Method 1a) is maintained below 0.3% because residual moisture can promote ester hydrolysis during prolonged storage, generating the corresponding free acid and ethanol. Residual solvent limits, notably for tetrahydrofuran or ethyl acetate employed in recrystallization, are kept under 5000 ppm total per USP <467> Option A. Users performing heterophasic reactions further request sulfur content below 50 ppm, as sulfur-bearing impurities originating from certain synthetic routes can poison palladium catalysts used in downstream coupling steps. Chiral purity is not applicable to the achiral structure, but isomeric purity is critical: the 2,4-dimethyl regioisomer content must be carefully distinguished by ¹H NMR integration or an orthogonal HPLC method with a C18 column (250 × 4.6 mm, 5 µm) and acetonitrile/water mobile phase, because this isomer co-elutes closely on many standard capillary GC columns.

Synthetic Utility in Heterocycle Construction

The ester-activated pyrrole core participates in a range of electrophilic substitution reactions at the unsubstituted 4-position. Formylation under Vilsmeier-Haack conditions (POCl₃/DMF, 0–5 °C to room temperature) installs a 4-formyl group selectively in yields exceeding 75%, providing an entry to dipyrromethene ligands for BODIPY fluorophores. Halogenation with N-bromosuccinimide in anhydrous THF at –20 °C selectively yields 4-bromo-ethyl 2,5-dimethyl-1H-pyrrole-3-carboxylate, which can be employed in Suzuki-Miyaura cross-couplings with arylboronic acids using Pd(PPh₃)₄ (2 mol%) and K₂CO₃ in toluene/water at 80 °C. The saponification of the ethyl ester with aqueous NaOH (2 M, ethanol cosolvent, reflux, 4 h) furnishes the corresponding carboxylic acid, which can be converted to amides, hydrazides, or active esters for peptide-like conjugates. In porphyrin analogue synthesis, the compound serves as a dipyrromethane precursor when subjected to acid-catalyzed condensation with aldehydes: p-toluenesulfonic acid (0.2 equiv) in dichloromethane at 25 °C under N₂ for 6 h gives the meso-substituted dipyrromethane in 60–70% yield after silica gel chromatography. Notably, the steric shielding of the 2- and 5-methyl groups imparts enhanced oxidative stability to the resulting porphyrinogen intermediates compared to unsubstituted pyrrole systems, reducing scrambling during the porphyrin macrocyclization step when using mild oxidants such as DDQ (1.1 equiv).

When the N–H group is deprotonated with a strong, non-nucleophilic base such as KO^tBu in DMF, the resulting pyrrolate anion undergoes N-alkylation with alkyl halides or epoxides to yield N-substituted derivatives. This N-alkylation sequence modifies the chromophore properties and is employed in the preparation of functionalized aza-BODIPY dyes where the absorption maximum is tuned between 630 nm and 680 nm. The electron-donating effect of the 2,5-dimethyl groups raises the HOMO energy relative to that of the parent ethyl 1H-pyrrole-3-carboxylate, shifting redox potentials anodically by approximately 150 mV as measured by cyclic voltammetry in acetonitrile (0.1 M TBAPF₆, Ag/AgCl reference). This shift influences excited-state electron transfer rates in donor-acceptor dyads. For such photophysical applications, the strict absence of fluorescent impurities from incomplete condensation processes is verified by excitation-emission matrix spectroscopy with a detection threshold of 0.01% relative fluorescence intensity at the dye emission wavelength.

When Substitution at the 2,5-Positions Alters Electrophilic Aromatic Substitution

A direct comparison with ethyl 2,4-dimethyl-1H-pyrrole-3-carboxylate (CAS 2199-52-2) and ethyl 3,5-dimethyl-1H-pyrrole-2-carboxylate (CAS 2199-44-2) reveals profound regiodirecting differences. In the 2,5-dimethyl-3-carboxylate isomer, only the single unsubstituted 4-position is available for electrophilic attack. The competing isomer with a free 5-position (2,4-dimethyl-3-ester) undergoes substitution at that 5-site under mild conditions, which can lead to regioisomeric mixtures when selectivity is not rigorously controlled. For example, nitration with acetyl nitrate in acetic anhydride at –10 °C gives predominantly the 4-nitro isomer for the 2,5-dimethyl derivative (>b>95:5 selectivity), whereas the 2,4-dimethyl variant yields a ~70:30 mixture of 5-nitro and 1-nitro isomers. This single-site availability simplifies product isolation and improves atom economy in multistep sequences. Furthermore, the 2,5-dimethyl pattern eliminates the risk of N-oxide formation on the pyrrole ring during peracid oxidation, a side reaction that complicates the use of 2-unsubstituted pyrrole esters in epoxidation or hydroxylation transformations. The 3-carboxylate placement, as opposed to the 2-carboxylate found in ethyl 3,5-dimethyl-1H-pyrrole-2-carboxylate, also influences hydrogen-bonding networks in crystal engineering: the ester carbonyl forms a stronger intramolecular contact with the adjacent N–H (N–H···O=C distance approximately 2.20 Å, angle 145° from X-ray diffraction data) than the corresponding 2-ester analogue, contributing to a higher melting point and lower solubility in nonpolar media. This solid-state packing characteristic can be exploited in the design of co-crystals with pharmaceutical partners, where the pyrrole N–H and ester carbonyl serve as robust supramolecular synthons.

Thermal and Storage Profile Under Industrial Handling Conditions

Differential scanning calorimetry at a scan rate of 10 K/min under nitrogen shows a sharp endothermic melt peak with an onset near 41 °C and an enthalpy of fusion of 91 J/g. Accelerated stability testing at 40 °C/75% RH over 6 months in HDPE containers with nitrogen headspace reveals less than 1% degradation by GC monitoring, provided the container is effectively sealed. The ester group is susceptible to saponification at elevated pH (>b>10 at 25 °C), with a half-life of approximately 2 hours in aqueous KOH (0.5 M) in methanol/water (4:1 v/v). The compound demonstrates photolability under continuous UV irradiation (254 nm, 6 W low-pressure mercury lamp at 10 cm distance) in solution, leading to ring-opening byproducts identifiable by LC-MS; opaque amber glassware or wrapping in aluminium foil is recommended for bench-scale reactions that proceed longer than 8 hours. Avoid storage with strong oxidizing agents (perchlorates, peroxides) due to the potential for exothermic decomposition initiating above 180 °C, as indicated by differential thermal analysis. No incompatibility with common process solvents (ethanol, acetone, ethyl acetate, dichloromethane, toluene) is observed at concentrations up to 30 wt% at ambient temperature, though prolonged dissolution in chlorinated solvents containing dissolved hydrogen chloride generates trace amounts of ester cleavage products.

On pilot-plant scale, material transfer is conducted with nitrogen padding to maintain an atmosphere below 0.5% oxygen, as color-darkening from oxidative oligomerization has been observed when hot solutions (>b>60 °C) are exposed to air over mixing times exceeding 4 hours. The product is classified as a non-flammable solid (GHS hazard category: not classified as flammable solid per UN Manual of Tests and Criteria, Test N.1), but a combustible dust assessment (EN 14034-1) should be performed before any micronization or milling operation because fine particulate (median particle size <50 µm) can form explosive dust clouds, with a lower explosibility limit estimated in the range of 30–60 g/m³. A hazard and operability (HAZOP) review on the rotary vacuum drying step following crystallization identified that maintaining the jacket temperature below 50 °C prevents melt-state agglomeration and ensures final moisture ≤0.1%.

Performance in Multicomponent Reactions Compared to Other Pyrrole Esters

In Hantzsch-type pyrrole syntheses employing ethyl acetoacetate, formaldehyde, and ammonium acetate, the title compound is obtained as a minor component unless the intermediate 2,5-dimethylpyrrole pathway is specifically targeted with pre-formed diketone equivalents. Direct condensation of 2,4-pentanedione with ethyl isocyanoacetate in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU, 1.2 equiv) in DMF at 25 °C yields ethyl 2,5-dimethyl-1H-pyrrole-3-carboxylate in 55–65% isolated yield. This is lower than the analogous reaction producing ethyl 2,4-dimethylpyrrole-3-carboxylate from ethyl 3-oxopentanoate, where yields commonly exceed 75%. The reduced efficiency stems from the lower electrophilicity of the methyl ketone termini in the symmetrical diketone during the isocyanide addition step. However, the symmetrical 2,5-substitution uniquely enables the construction of C_2v-symmetric porphyrin systems without the statistical regioisomerism that plagues the 2,4-dimethyl case. In continuous flow synthesis, utilizing a tubular reactor (ID 1 mm, length 10 m, residence time 30 min) with DBU in DMF and back-pressure regulation at 5 bar, a steady-state throughput of 12 g/h of product has been demonstrated, with GC purity of the crude stream at 88% before crystallisation.

The above data illustrate that the 2,5-dimethyl-3-ester provides a unique balance of singular site selectivity and moderate reactivity, which neither of the closely related analogues offers simultaneously. The 2,4-dimethyl isomer, while more easily synthesized, introduces the chore of controlling secondary substitution. The 3,5-dimethyl-2-ester suffers from electrostatic repulsion between the ester carbonyl and the adjacent ring π-system, reducing its nucleophilicity in electrophilic processes.

Regulatory and Documentation Framework for Commercial Procurement

A complete technical dossier for this material, when sourced for pharmaceutical intermediate applications, will typically include a material safety data sheet aligned with GHS Rev. 8 (or EC 1272/2008 CLP regulation for EU supply), specifying the H-code H315 (causes skin irritation) and H319 (causes serious eye irritation) based on acute dermal and ocular irritation tests performed according to OECD Guidelines 404 and 405, respectively. The substance is not listed under the Stockholm Convention on Persistent Organic Pollutants, nor is it subject to authorization under REACH Annex XIV. However, its manufacture process may involve solvents of high concern (DMF, acetonitrile); thus a residual solvent statement is necessary. Under FDA 21 CFR, there is no monograph for the substance as an active ingredient; its use is confined to that of a registered starting material or intermediate under a Type II drug master file, with an annual update of the DMF containing a complete description of the synthetic process and control methods. For non-pharma applications, such as research dye synthesis, a less stringent specification omitting heavy metal and microbial limits is often accepted, with purity confirmed solely by GC and NMR. The user is responsible for verifying that the purchase specification matches the intended end-use regulatory category.

Practical Handling During Scale-Up and Bench Reactions

Loading sequences for heterogeneous reactions should address the limited solubility of the compound in cold aliphatic hydrocarbons (solubility in n-heptane at 20 °C is <0.1 wt%). In mixtures requiring dissolution before reagent addition, a co-solvent such as tetrahydrofuran or dimethylformamide at a minimum ratio of 5 mL/g of substrate is employed. When executing Vilsmeier-Haack formylation on a 1 mol scale, the initial dissolution of ethyl 2,5-dimethyl-1H-pyrrole-3-carboxylate in DMF (2 L) and slow addition of POCl₃ (1.2 equiv) with internal temperature maintained at –5 to 0 °C minimizes exotherm and prevents localized pockets of decomposition that generate tar. Agitation speed of 350–400 rpm in a baffled glass-lined reactor ensures rapid heat dissipation. After quenching into ice-water, the crude product is extracted with ethyl acetate and washed with saturated NaHCO₃ until the aqueous phase remains above pH 7.5. The organic layer, when dried over Na₂SO₄ and concentrated, gives a waxy solid that is recrystallized from n-heptane/ethyl acetate (4:1 v/v) to obtain the 4-formyl derivative as a white crystalline solid in 72% yield. Throughout this operation, personnel exposure is managed with local exhaust ventilation and chemical goggles meeting EN 166 impact standards; nitrile exam gloves with a breakthrough time of >30 minutes for ethyl acetate are specified.

For N-alkylations requiring anhydrous conditions, the substrate is dried in a vacuum oven at 35 °C and 100 mbar for at least 4 hours before dissolving in anhydrous DMF (water <50 ppm by KF). The use of molecular sieves (3 Å, 20% w/v) to maintain water-free conditions resulted in a 10–15% improvement in yield over non-dried solvent in a series of patent examples for synthetic dipyrrin ligands. Any scale larger than 500 g is typically processed under a nitrogen blanket within an isolator rated to IP 55 to prevent moisture ingress and dust generation during the charging of powdered alkali metal hydride or alkoxide bases.