1H-Pyrrole-2-Carboxylic Acid, 3-Methyl-

Pharmaceutical synthesis campaigns utilizing 3-methyl-1H-pyrrole-2-carboxylic acid (CAS 42894-13-9) as a carboxylate building block frequently encounter a narrow processing window defined by the lability of the pyrrole ring toward electrophilic substitution and the thermal sensitivity of the free acid. The compound is charged as a pre-dried powder with moisture content verified below 0.15% by Karl Fischer titration (Metrohm 870 KF Titrino plus) after vacuum desiccation over P₂O₅ for 24 h at 40°C. Activation for amide bond formation employs EDCI·HCl (1.1 equiv), HOBt monohydrate (1.1 equiv), and N-methylmorpholine (2.5 equiv) in anhydrous DMF under a dry nitrogen blanket within a jacketed 10 L borosilicate glass reactor equipped with a retreat-curve impeller rotating at 200 rpm. Coupling to primary aliphatic amines proceeds at 0–5°C for 2 h, then ambient temperature for 16 h, yielding the corresponding carboxamide. Process HPLC purity monitoring uses an Agilent ZORBAX Eclipse Plus C18 column (4.6 × 150 mm, 5 µm) with acetonitrile/water (0.1% TFA) gradient at 1.0 mL/min and 254 nm detection; target area% ≥ 98.0%. Residual DMF is controlled below 880 ppm per ICH Q3C (Option 2) limits, verified by headspace GC-MS (Agilent 7697A/5977B). Heavy metals by USP 〈231〉 are sustained below 10 ppm. A critical incompatibility arises with strong mineral bases: contact with NaH or t-BuOK at temperatures exceeding 60°C triggers rapid decarboxylation, generating 3-methylpyrrole as the dominant by-product, confirmed by inline ReactIR monitoring of the disappearing carboxylate stretch at 1550 cm⁻¹ and the appearance of pyrrole ring breathing modes. The free acid is therefore never exposed to basic conditions above ambient temperature during downstream derivatization sequences. Several investigational kinase inhibitor scaffolds incorporating a 3-methylpyrrole-2-carbonyl motif are accessed via subsequent regioselective bromination at the 5-position using NBS in DMF at −10°C, followed by Suzuki-Miyaura cross-coupling with arylboronic acids using Pd(PPh₃)₄ (2 mol%) and Na₂CO₃ (2 M aq., degassed) in dioxane at 85°C for 8 h. The halogenated intermediate is isolated with a Buchner filtration train lined with PTFE filter cloth (10 µm) and dried in a vacuum tray dryer (45°C, −0.95 bar). Exposure to ambient humidity above 55% RH during handling leads to surface hydration that elevates the free acid content and retards coupling kinetics; therefore all dispensing is conducted within a glove bag maintained at ≤30% RH with a dynamic nitrogen purge.

Low-Temperature Vilsmeier-Haack Sequence for Pyrrole Nitrile Formation: Thermal Boundaries and Quench Protocol

Conversion of 3-methylpyrrole-2-carboxylic acid into the corresponding 5-cyano analog, a key intermediate for 2-aryl-pyrrole insecticidal scaffolds, proceeds via sequential Vilsmeier-Haack formylation, oximation, and dehydration. The formylation step is acutely exothermic. Phosphorus oxychloride (1.05 equiv) is added dropwise to dry DMF (3.0 equiv) at 0°C with jacket cooling, forming the Vilsmeier reagent; the acid is then charged portionwise while maintaining the internal temperature at 0–5°C. Deviation above +5°C during reagent formation causes premature chlorination by-products that persist through subsequent steps. The mixture is warmed to 55°C ± 2°C over 30 min and held for 4 h; temperatures exceeding 57°C result in tar formation and a yield drop below 55%. The reaction mass is quenched onto crushed ice with vigorous stirring, adjusting pH to 7.0 with 30% NaOH at a rate that prevents the bulk temperature from rising above 25°C. Extraction with ethyl acetate (three 1.5 L portions), drying over anhydrous MgSO₄, and concentration in a Büchi R-300 rotary evaporator with bath temperature clamped at 38°C and vacuum gradually reduced to 50 mbar give the crude 5-formyl derivative. The oximation employs hydroxylamine hydrochloride (1.2 equiv) and sodium acetate (1.5 equiv) in ethanol/water (4:1 v/v) at reflux for 3 h; oxime precipitation is completed by chilling to −5°C. Dehydration to the nitrile is executed with acetic anhydride (2.0 equiv) at reflux (120°C) for 2 h, then poured onto 200 g of ice. The crude 3-methyl-5-cyanopyrrole-2-carboxylic acid is recrystallized from toluene/hexane (1:3) and dried under vacuum at 50°C. Purity by HPLC (area%) is required ≥ 98.5% for use in subsequent 2-aryl coupling. This nitrile intermediate is reactive toward Grignard reagents and must be stored under argon with molecular sieve 4Å in sealed amber glass vials to prevent moisture uptake and photolytic degradation. Within the EU, the compound falls under REACH Regulation (EC) No 1907/2006; as a non-isolated intermediate used on-site for captive synthesis, it may qualify for the limited registration exemption under Article 2(7)(b), provided strictly controlled conditions are documented. Residual POCl₃ and DMF are monitored below 500 ppm and 880 ppm respectively by GC-MS for compliance with internal occupational exposure limits.

When 3-methylpyrrole-2-carboxylic acid is introduced at concentrations of 50 to 200 ppm into 1 M HCl containing AISI 1018 cold-rolled carbon steel coupons, weight loss immersion tests according to ASTM G31-72 (standard practice for laboratory immersion corrosion testing of metals) reveal mixed-type inhibition behavior with a pronounced effect on the anodic dissolution branch. Coupons of dimensions 50 × 25 × 2 mm are wet-ground to 600-grit SiC finish, degreased in acetone, and dried before immersion for 24 h at 25 ± 0.5°C in a 500 mL glass cell open to air. Inhibition efficiency is calculated as η% = (1 − (w_i/w₀)) × 100, where w₀ and w_i denote weight loss in uninhibited and inhibited solutions. The concentration-dependent data appear in Table 1. Electrochemical validation employed a Gamry Interface 1010E potentiostat with a conventional three-electrode flat cell: saturated calomel reference, platinum mesh counter, and the AISI 1018 working electrode masked to 1 cm². Potentiodynamic polarization scans from −250 mV to +250 mV vs. open circuit potential at 0.5 mV/s showed a reduction in corrosion current density from 1.18 mA/cm² (blank) to 0.11 mA/cm² at 200 ppm inhibitor. The Tafel slopes indicate a mixed inhibitor without altering the hydrogen evolution mechanism. Addition of potassium iodide at 50 ppm synergistically boosts inhibition efficiency to above 93% at only 100 ppm of the pyrrole acid through co-adsorption, shifting the pitting potential in cyclic polarization by +180 mV. Above 50°C, efficiency drops sharply—the Langmuir adsorption isotherm is no longer followed—consistent with physisorption dominant on the steel surface. XPS survey scans of inhibited coupons detect N 1s peaks at 399.8 eV, confirming organic film deposition. For metalworking fluid integration, the acid must be pre-neutralized with triethanolamine to a pH of 8.0–8.5 to ensure solubility in semi-synthetic concentrates and to avoid proton-driven desorption at low sump pH. Immersion testing of copper alloys (C 11000) in the same inhibited HCl reveals increased weight loss relative to blank, indicating that the compound promotes dezincification of brass and must not be applied in multi-metal systems containing yellow metals without specific inhibitor packages. No chronic aquatic toxicity data are published; therefore, the inhibitor is handled as Category 3 chronic under CLP (EC) No 1272/2008 until screening data are available, and disposal of spent baths must comply with local industrial effluent limits for heterocyclic nitrogen compounds.

Under azeotropic reflux with cyclohexane entrainer and p-toluenesulfonic acid monohydrate (2 mol%) catalysis, 3-methylpyrrole-2-carboxylic acid is quantitatively converted to ethyl 3-methylpyrrole-2-carboxylate, a compound evaluated for nutty, caramelic taste modification in experimental flavor formulations. A 500 mL three-neck flask fitted with a Dean-Stark trap, water-cooled condenser, and PTFE-coated magnetic follower is charged with the acid (0.5 mol), absolute ethanol (2.5 mol, molar ratio 1:5), and cyclohexane (80 mL). The mixture is heated to vigorous reflux; the trap is drained periodically until water collection ceases (9–12 h). TLC monitoring on silica gel 60 F₂₅₄ (hexane:ethyl acetate 3:1, visualization by UV 254 and KMnO₄ dip) indicates complete consumption of the acid. Cooling, sequential washing with saturated NaHCO₃ (2 × 50 mL) and brine, drying over Na₂SO₄, and filtration through a 0.45 µm PTFE syringe filter precede solvent removal under reduced pressure. The crude ester is purified via vacuum fractional distillation using a 30 cm Vigreux column with a reflux ratio of 8:1. The fraction boiling at 95–97°C at 2 mmHg is collected; GC-FID analysis (Agilent DB-5 column, 30 m × 0.25 mm × 0.25 µm, oven program from 80°C to 280°C at 15°C/min) shows area% purity ≥ 99.0%. Olfactory evaluation of a 1% (w/w) solution in ethanol reveals a primary nutty, subtly roasted character with a faint maple undertone. The ester is not listed in the Union list of flavouring substances per Regulation (EC) No 1334/2008; any incorporation into food flavourings therefore requires a separate EFSA safety evaluation and Commission authorisation. For fragrance applications, conformity to IFRA standards is not yet established, and the material is handled as a research chemical under Cosmetic Regulation (EC) No 1223/2009 provisions for experimental components. Air contact during handling darkens the ester through pyrrole ring oxidation; therefore all analytical reference samples are stored under argon in amber ampoules at −20°C, and opened containers are blanketed with nitrogen after each use.

When pH Drifts Above 9.2 During Azo Coupling, Chromaticity and Tinctorial Strength Deteriorate Irreversibly

3-Methylpyrrole-2-carboxylic acid functions as an electron-rich coupling component for para-substituted aryldiazonium salts in the synthesis of disperse and acid azo dyes producing yellow to orange shades on cotton and polyester. A representative laboratory-scale synthesis couples diazotized 4-nitroaniline with the sodium salt of the pyrrole acid. 4-Nitroaniline (0.02 mol) is suspended in 6 M HCl (15 mL) and cooled to 0–5°C in an ice-salt bath; a solution of NaNO₂ (0.022 mol) in deionized water (5 mL) is added dropwise over 20 min while maintaining the temperature strictly below 5°C. The clear diazonium solution is added in a fine stream to a precooled (0–5°C) aqueous solution of the sodium 3-methylpyrrole-2-carboxylate (0.02 mol) in 10% NaOH, with sodium carbonate dosing to clamp the coupling pH at 8.5–9.0. When addition is complete, the suspension is stirred for 2 h at 0–5°C, during which precipitation of the dye is completed by salting out with 15% (w/v) NaCl. Filtration through a sintered glass funnel (porosity 3) and vacuum-drying at 40°C afford the crude dye. Dyeing on scoured cotton knit fabric is performed at 2% owf in a Mathis Labomat BFA-12 using a liquor ratio of 20:1, with 20 g/L Na₂SO₄ and 10 g/L Na₂CO₃ at 60°C for 60 min. After soaping with 1 g/L Sandozol NE at 95°C for 15 min, color fastness properties are evaluated against ISO 105-B02 (xenon arc, Blue Wool references 6 and 7) and ISO 105-C06 test C2S (wash at 60°C with ECE phosphate-based detergent and sodium perborate). Table 2 presents representative fastness ratings for a dye of approximately 85% tinctorial strength compared to C.I. Disperse Yellow 3. Any drift in coupling pH above 9.2 results in a noticeable hypsochromic shift and a drop in lightfastness of one full grey scale grade due to partial decomposition of the diazonium intermediate and formation of tarry by-products that dull the shade. The synthesis effluent is treated with 500 mg/L FeCl₃ and 200 mg/L Ca(OH)₂ for coagulation, achieving a COD reduction below 0.5 kg/t of fabric, consistent with the EU BAT-associated emission level (AEL) for textile finishing effluent (0.5 kg COD/t in BAT conclusion TXT BREF, August 2022). Residual aromatic amines are monitored by LC-MS/MS to below 30 ppm per Directive 2002/61/EC stipulated method for azo colorants. The dye is not registered in the European Union list of authorised food contact colorants and is intended only for textile industrial use under REACH.

Brightener Performance in Nickel Electroplating Baths Evaluated by Hull Cell and Cyclic Voltammetric Stripping

In conventional Watts nickel baths, 3-methylpyrrole-2-carboxylic acid serves as a Class I brightener and levelling supplement at concentrations of 10–50 mg/L, exerting its strongest effect on high-current-density zones. A standard bath composition is nickel sulfate hexahydrate (300 g/L), nickel chloride hexahydrate (45 g/L), and boric acid (40 g/L), operated at pH 4.2 ± 0.1 and a temperature of 55 ± 1°C. The acid is added as a 5% stock solution in deionized water pre-adjusted to pH 4.0 with NaOH. A Hull cell test ( 267 mL panel, current 2 A, duration 5 min, magnetic air agitation) reveals a fully bright, mirror-like deposit from 1.0 to 8.0 A/dm² when the additive concentration is maintained at 20 mg/L. Below 10 mg/L, low-current-density clouding appears; above 50 mg/L, pitting on the high-current edge emerges and the cathodic current efficiency declines by 2–4 percentage points. Additive consumption is tracked by cyclic voltammetric stripping (CVS) on a Metrohm 850 Professional IC with a 3-mm glassy carbon rotating disk electrode at 1500 rpm, scanning from 0.1 V to −1.6 V vs. Ag/AgCl at 100 mV/s; the stripping peak area is linear with concentration in the range 5–60 mg/L. The brightener acts through specific adsorption on the nickel surface, inhibiting lateral growth and promoting fine-grained deposits. Internal deposit stress, determined by the bent cathode method (ASTM B849-02 using a steel strip cathode 100 × 25 × 0.2 mm), remains below 40 MPa, suitable for functional engineering coatings. Neutral salt spray resistance (ASTM B117, 48 h on 12 µm nickel over steel, single-layer, unpassivated) shows no white corrosion products; red rust appears only after 72 h. Hexavalent chromium passivation is not required, supporting compliance with EU End-of-Life Vehicle Directive 2000/53/EC (Annex II exemption expirations). For bath make-up and replenishment, separate addition lines for the brightener and for amine-based carriers are mandatory because direct blending of the pyrrole acid with sulfamate or allyl amine-based carriers at alkaline pH leads to irreversible adduct formation that precipitates as a dark sludge, as confirmed by filtration rate decay in pilot-scale 800 L working baths. Bath effluent is treated by precipitation with calcium hydroxide at pH 11, reducing nickel concentration below 0.5 mg/L to meet the EU industrial wastewater discharge standard under Directive 2010/75/EU.