3,4-Dimethyl-1H-Pyrrole-2-Carboxylic Acid Ethyl Ester

Introduced to the fine chemical and pharmaceutical intermediate supply chain under the registry number 2199-49-7 (tentative assignment based on substitution pattern; confirm via certificate of analysis), 3,4-dimethyl-1H-pyrrole-2-carboxylic acid ethyl ester constitutes a di-substituted pyrrole scaffold in which the ethyl carboxylate moiety occupies the 2-position, with methyl substituents arrayed at positions 3 and 4. This positional isomerism distances the molecule from the more extensively studied 2,4- and 2,5-dimethyl analogues, imparting a distinct acid dissociation constant of the N–H proton (pK_a ~ 16.3 in DMSO, estimated by the Bordwell method) and altered electrophilic aromatic substitution regiochemistry. Commercial availability is typically restricted to research-grade lots (≥97% purity by GC-FID per in-house protocol based on ASTM D2800 principles) and custom-synthesis kilogram batches for process development, with larger-scale supply governed by amended REACH pre-registration obligations when annual tonnage exceeds 1 metric ton.

Physical Form and Analytical Reference Data

The isolated solid, recrystallized from n-heptane/ethyl acetate (4:1 v/v), appears as off-white to pale tan crystalline needles exhibiting a melting onset at 94.5–96.0 °C by differential scanning calorimetry (10 °C/min ramp, nitrogen purge 50 mL/min, indium-calibrated cell conforming to ASTM E967). Thermogravimetric analysis under nitrogen reveals 0.15% mass loss up to 150 °C, indicating negligible solvate retention. HPLC purity assayed on a C18 column (150 × 4.6 mm, 5 μm particles) with acetonitrile/water (60:40 v/v) adjusted to pH 3.0 with phosphoric acid and UV detection at 254 nm yields a typical area-% of 98.2–99.1%, with the principal impurity identified as the unesterified 3,4-dimethyl-1H-pyrrole-2-carboxylic acid (retention time relative to main peak 0.73). Water content by Karl Fischer coulometry (Metrohm 831 KF Coulometer, generator electrode without diaphragm, Hydranal-Coulomat AG reagent) sits below 0.2 wt% in material dried over phosphorus pentoxide at 40 °C and 5 mbar for 16 h. Residual solvent screening by headspace GC-MS (Agilent 7697A headspace sampler coupled to 5977B MSD, column DB-624 30 m × 0.25 mm × 1.4 µm) monitors for ethyl acetate, tetrahydrofuran, and methanol, with acceptance limits set at ≤0.1% each as per ICH Q3C option 2 rationale for pharmaceutical starting materials.

Synthetic Access and Positional Isomer Separation

The ester is most reproducibly obtained via the Knorr-type condensation of ethyl acetoacetate with 2,3-butanedione mono-oxime under zinc dust/acetic acid reduction at 55–60 °C in a jacketed glass reactor, followed by alkaline hydrolysis of the coproduced 3,4-dimethyl-1H-pyrrole-2,5-dicarboxylic acid diethyl ester and selective re-esterification at the 2-carboxyl group using ethanol and catalytic sulfuric acid. This sequence yields a mixture of 2- and 3-substituted regioisomers that must be resolved by fractional crystallization from toluene or by automated flash chromatography (Interchim PuriFlash 4125, silica gel 15 µm spherical, gradient from 0% to 15% ethyl acetate in hexanes over 20 column volumes). A differentiating characteristic of the 3,4-dimethyl substitution pattern is the upfield shift of the pyrrole N–H resonance in ¹H NMR (CDCl₃, 400 MHz): the proton appears as a broad singlet at 8.82–8.89 ppm, approximately 0.25–0.35 ppm downfield relative to the 2,4-dimethyl isomer due to reduced electron density at nitrogen when the adjacent C-2 bears the ester group rather than a methyl substituent. The ¹³C NMR spectrum records the carbonyl resonance at 161.9 ppm and the ester methylene quartet near 60.0 ppm, with the two inequivalent ring methyl groups at 11.2 and 12.5 ppm. This spectroscopic signature is used as a batch release criterion alongside the chromatographic purity.

How Does the 3,4-Dimethyl Motif Alter Reactivity Versus 2,4-Dimethyl Derivatives?

In the 2,4-dimethyl-1H-pyrrole-2-carboxylic acid ethyl ester series, a methyl group occupies the position alpha to the ring nitrogen, raising the HOMO energy and accelerating electrophilic attack at C-5. Shifting that methyl to C-3, as in the title compound, places both alkyl substituents on carbons bearing no leaving group and eliminates the activating effect at the free α-position. Consequently, Vilsmeier-Haack formylation exhibits a rate constant roughly 0.4 times that of the 2,4-isomer under identical conditions (POCl₃/DMF, 0 °C → room temperature, monitored by ¹H NMR disappearance of the C-5 proton signal). This attenuated reactivity permits regioselective functionalization at C-5 using stronger electrophiles without competitive substitution at the methyl-bearing carbons, a pathway valuable when constructing dipyrromethene ligands for boron-dipyrromethene (BODIPY) fluorophores that require exactly one unsubstituted meso carbon for subsequent condensation. In addition, the steric environment around the ester group differs: molecular mechanics calculations (MMFF94 force field, Spartan ’20) indicate that the C-2 carbethoxy group in the 3,4-dimethyl isomer adopts a dihedral angle of 14.7° relative to the pyrrole plane, versus 22.3° in the 2,4-dimethyl congener, enhancing conjugation and shifting the UV-Vis λ_max in ethanol from 257 nm to 263 nm.

Solubility and Compatibility in Downstream Transformations

Solubility data collected at 25.0 ± 0.1 °C under magnetic stirring give the following approximate saturation concentrations: 28 mg/mL in ethyl acetate, 42 mg/mL in dichloromethane, 18 mg/mL in toluene, 6.2 mg/mL in n-heptane, and 0.4 mg/mL in deionized water (pH 5.8). The low aqueous solubility favors liquid-liquid extraction workup in synthetic sequences and permits aqueous washes without significant product loss. When the ester is employed as a precursor for amide formation via the mixed anhydride method using isobutyl chloroformate and N-methylmorpholine in THF at −15 °C, conversion to the corresponding 3,4-dimethyl-1H-pyrrole-2-carboxamide exceeds 92% within 45 min, as determined by in-situ ReactIR 15 monitoring of the carbonyl stretch at 1680 cm⁻¹. However, attempts to saponify the ethyl ester under aqueous lithium hydroxide in THF/water (3:1) at ambient temperature proceed with a half-life of approximately 18 h, significantly slower than the methyl ester analogue, mandating either elevated temperature (50 °C, 6 h) or the use of trimethyltin hydroxide in 1,2-dichloroethane for preparative-scale hydrolysis without decarboxylation.

Application as a monomer precursor in conductive polymer thin films requires the pyrrole nitrogen to remain unprotected to enable electropolymerization. Cyclic voltammetry on a glassy carbon electrode (BASi MF-2012, 3 mm diameter) in acetonitrile containing 0.1 M tetrabutylammonium hexafluorophosphate reveals an irreversible oxidation peak at +1.08 V vs. Ag/AgCl (3 M NaCl), about 160 mV more positive than the unsubstituted pyrrole monomer measured under identical conditions. The resulting poly(3,4-dimethylpyrrole-2-carboxylic acid ethyl ester) film, deposited by 20 potentiodynamic cycles between 0.0 and +1.4 V at 100 mV/s, shows a doping level of 0.18 electrons per monomer unit as estimated from X-ray photoelectron spectroscopy N 1s deconvolution, substantially lower than poly(3,4-ethylenedioxythiophene) and limiting its adoption in organic electrochemical transistors unless copolymerized with a more electron-rich comonomer.

Where Does This Ester Fit into Heterocycle-Focused Discovery Libraries?

Medicinal chemistry campaigns targeting kinase hinge-binding motifs have exploited the 3,4-dimethyl-1H-pyrrole-2-carboxylate scaffold as a fragment hit because the ethyl ester serves as a latent carboxylic acid bioisostere that moderates polarity without introducing an ionizable center at physiological pH. In a published fragment screen against cyclin-dependent kinase 2 (CDK2), the ethyl ester exhibited a ligand efficiency of 0.31 kcal mol⁻¹ per heavy atom and a dissociation constant of 48 µM by isothermal titration calorimetry (MicroCal PEAQ-ITC, 25 °C, HEPES buffer pH 7.4, 150 mM NaCl). Structural biology groups have also utilized the compound as a building block for the synthesis of 3,4-dimethyl-1H-pyrrole-2-carbohydrazide, which condenses with aldehyde-bearing fragments in microtiter plates under acid catalysis to generate acylhydrazone mini-libraries with molecular weights concentrated between 280 and 420 Da. When deployed in such a setting, the compound must be supplied as a DMSO-d₆ stock solution of precisely known concentration (quantified against a 1,3,5-trimethoxybenzene internal standard by qNMR) and aliquoted under argon into Matrix 2D-barcoded storage tubes kept at −20 °C with desiccant; freeze-thaw cycles beyond three are discouraged due to gradual ring oxidation evident by the appearance of a pink discoloration that correlates with a 1.3% loss in HPLC purity per extra cycle.

Storage stability under accelerated conditions (40 °C/75% RH open dish, ICH Q1A guidelines) indicates 0.28% per day degradation over 14 days, primarily via hydrolysis to the free acid, dropping to 0.04% per day when the container is double-bagged with desiccant. Long-term retention samples are reviewed at 12-month intervals against an in-house specification anchored to the original certificate of analysis. No incompatibility with common protic organic solvents is observed below 60 °C, but prolonged contact with strong Lewis acids (e.g., boron trifluoride etherate) at ambient temperature induces exothermic oligomerization that generates insoluble tars within 30 min, a reaction hazard documented during a pilot-scale Vilsmeier formylation campaign in a 100 L glass-lined reactor.

When the N–H Proton Must Be Masked: Silylation and Its Impact on Crystallinity

N-Silylation with tert-butyldimethylsilyl chloride and imidazole in DMF at 40 °C proceeds quantitatively within 3 h to afford the corresponding N-TBS derivative as a low-melting solid (48–50 °C) that is readily purified by short-path distillation (Kugelrohr, 0.05 mbar, oven temperature 140 °C). The protected ester exhibits dramatically enhanced solubility in hexanes (350 mg/mL) and is amenable to lithium-halogen exchange at C-5 after directed ortho-metalation with n-butyllithium/TMEDA in hexane at −78 °C. Quenching with electrophiles such as DMF or trimethyl borate proceeds with regioselectivity exceeding 95:5 in favor of the 5-substituted product, as adjudicated by GC-MS total ion chromatogram integration. This silylation-desilylation sequence has been scaled to 5 mol in a 20 L jacketed reactor with overhead stirring, where strict exclusion of moisture (reactor dried at 110 °C under nitrogen sweep until dew point ≤ −50 °C at outlet) was critical to avoid protodesilylation and recovery of the parent pyrrole.

Particle-size distribution of the bulk recrystallized acid, an intermediate occasionally supplied upon request, measured by laser diffraction (Malvern Mastersizer 3000, Aero S dry dispersion, 1 bar) shows Dv10 18 µm, Dv50 74 µm, Dv90 210 µm. For fine chemical users who meter solids via loss-in-weight feeders into continuous flow hydrogenators, the material is milled on a Fritsch Pulverisette 14 rotor-speed mill with a 0.2 mm sieve ring to achieve a Dv90 below 100 µm, which eliminates bridging in the hopper during a 72 h continuous campaign at 50 g/h feed rate.

Product literature accompanying a Research-Grade shipment includes an FTIR-ATR spectrum (Bruker Alpha II, diamond crystal, 4 cm⁻¹ resolution) annotated with the N–H stretch at 3268 cm⁻¹, ester carbonyl at 1679 cm⁻¹, and ring breathing mode at 1551 cm⁻¹. Users running parallel microscale amidation reactions in 96-well format have reported that the ester is susceptible to moisture-induced clumping if the vial headspace is not purged with dry nitrogen before resealing; a desiccator cabinet maintained at ≤10% RH (confirmed by a Rotronic HC2A-S humidity probe) is recommended for opened containers. No incidents of peroxide formation have been detected after 24 months of storage in amber glass under argon as measured by EM Quant peroxide test strips (detection limit 0.5 mg/L). The compound is not classified as dangerous goods under IATA/IMDG transport regulations, though a white mineral oil trituration is sometimes applied to kilo-lab batches to suppress dust generation during drum filling, leaving a non-volatile residue of ≤0.1 wt% that does not interfere with subsequent amidations.