Methyl 3-Chloropyrrole-2-Carboxylate

Control of the exotherm during amide bond formation with methyl 3-chloropyrrole-2-carboxylate demands a jacket temperature profile mapped against reaction calorimetry data obtained on a Mettler Toledo RC1e at the 1.5 L scale. When the compound is employed as the electrophilic partner for 4-aminopiperidine derivatives en route to pyrrolo[2,3-d]pyrimidine scaffolds, the process stream is first treated with a stoichiometric quantity of methanesulfonic acid in anhydrous tetrahydrofuran at −5 °C to suppress N-oxidation of the pyrrole nucleus. The acylating charge ratio is maintained between 1.08 and 1.12 equivalents relative to the free amine, while 1.25 equivalents of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride and 0.10 equivalents of 1-hydroxybenzotriazole hydrate facilitate the coupling in the presence of 2.0 vol% N,N-dimethylformamide as a kinetic solubilizer. An adiabatic temperature rise of 37 K and a maximum heat release rate of 18 W/kg were recorded when the EDCI charge was split into four equal portions at 15‑minute intervals, a protocol that avoided transient hot spots exceeding 8 °C in the bulk liquid. After 20 hours of age-out at 22 °C, the batch is drowned into a 7 % w/w aqueous potassium carbonate solution pre‑cooled to 3 °C in a 500 L glass-lined reactor equipped with a retreat-curve impeller rotating at 95 rpm; the internal temperature is held below 10 °C throughout the 45‑minute addition to prevent retro‑Michael cleavage of the nascent amide. The crude isolate, discharged through a 0.5 m² agitated Nutsche filter‑dryer, is re‑slurried with 2.0 bed volumes of methyl tert‑butyl ether at 40 °C for 1 hour, then crystallized from a 6:4 v/v mixture of isopropanol and n‑heptane using a controlled linear cooling ramp of 0.25 °C/min from 72 °C to 18 °C. Industry compliance for this pharmaceutical intermediate is governed by ICH Q7 and 21 CFR Part 211; residual solvent profiles are measured against USP ⁇467⁇ Procedure A with headspace GC‑FID, enforcing limits of ≤ 720 ppm for tetrahydrofuran and ≤ 500 ppm for N,N‑dimethylformamide, while palladium originating from an upstream hydrogenation catalyst is controlled to ≤ 5 ppm by MP‑AES following USP ⁇232⁇/⁇233⁇. The isolated material, with a typical purity of 99.8 area% at 254 nm and a single maximum unknown impurity below 0.08 %, is telescoped into a copper‑mediated Ullmann cyclisation that furnishes a tricyclic lactam advanced intermediate for a clinical‑stage TYK2/JAK1 dual inhibitor program, where the 3‑chloro substituent engages a methionine gatekeeper residue in the selectivity pocket.

How Does Residual Chloride Influence Cross‑Coupling Efficiency in Agrochemical Intermediate Production?

The 3‑chloro position on the pyrrole ring, when activated by the 2‑carboxylate electron‑withdrawing group, participates in palladium‑catalyzed Suzuki‑Miyaura couplings that deliver 3‑aryl‑pyrrole‑2‑carboxylic acids destined for carboxamide fungicides. Process experience on a 100 kg input scale has shown that residual ionic chloride above 120 ppm in the starting ester selectively poisons the Pd(dba)₂/SPhos catalyst system, extending the induction period from 18 minutes to over 3 hours and elevating the palladium black precipitation rate to 0.8 mg/L·h when the aqueous phase pH drifts below 9.2. To mitigate this, the ester is pre‑washed with 5 % w/w sodium bicarbonate solution containing 0.1 wt% ethylenediaminetetraacetic acid tetrasodium salt and dried over molecular sieves 4A until Karl Fischer titration reads ≤ 120 µg/g H₂O. A representative coupling charge employs 1.35 equivalents of 4‑chlorophenylboronic acid, 1.8 mol% Pd(dba)₂, 4.2 mol% SPhos, and 3.0 equivalents of tribasic potassium phosphate in a degassed mixture of toluene, ethanol, and water (5:1:1 v/v/v). The slurry is heated to 82 °C in a 2000 L Hastelloy C‑22 vessel with a retreat‑curve impeller at 110 rpm, and the conversion is tracked continuously by ReactIR monitoring of the ester carbonyl stretch shift from 1718 cm⁻¹ to 1702 cm⁻¹; a plateau at ≥ 97.5 % conversion is reached typically within 5 hours. Post‑reaction, the organic layer is separated at 55 °C, washed with 10 % w/w aqueous sodium chloride, and concentrated under 45 mbar at a jacket temperature not exceeding 60 °C to avoid thermal decarboxylation of the product acid. The crude 3‑(4‑chlorophenyl)pyrrole‑2‑carboxylate ester is subsequently hydrolyzed in situ with 2.2 equivalents of sodium hydroxide in methanol‑water (3:1) at 40 °C for 4 hours, acidified to pH 2.8 with 6 N hydrochloric acid, and crystallized from a 5:3 v/v mixture of ethyl acetate and cyclohexane to give the free acid. The end product serves as the pharmacophoric fragment for a series of second‑generation SDHI (succinate dehydrogenase inhibitor) fungicides, where the 3‑aryl substituent fills a narrow lipophilic channel in the ubiquinone‑binding site. Regulatory compliance for this intermediate placed on the EU market falls under REACH (EC) 1907/2006 with a registration tonnage band of 10‑100 t/a; batches are certified to contain ≤ 8 ppm residual palladium (by ICP‑OES according to EN ISO 11885:2007) and ≤ 0.15 % total chlorinated dioxin‑like impurities screened via EPA Method 1613B high‑resolution GC/MS.

Vacuum sublimation at 0.03 mbar and a three‑zone gradient from 85 °C to 115 °C in a quartz tube furnace constitutes the final purification gate for methyl 3‑chloropyrrole‑2‑carboxylate when the target application is a solution‑processable non‑fullerene acceptor for organic photovoltaics. In this domain the compound is not a direct active component but the C‑2‑carboxylate anchor used to graft the pyrrole donor block onto a ladder‑type dithienocyclopentane core through a Dieckmann‑type condensation with substituted acetonitriles. The reaction stoichiometry demands a precisely metered feed of the pyrrole ester at 1.00 molar equivalent against 2.03‑2.05 equivalents of the nitrile partner, as substoichiometric pyrrole loading results in persistent mono‑condensation by‑products that co‑sublime and degrade charge‑carrier mobility in the final blend film to below 10⁻⁴ cm²/V·s. In a 50 L jacketed glass reactor under argon atmosphere, potassium tert‑butoxide (3.5 eq) is suspended in anhydrous tetrahydrofuran at −10 °C, the nitrile is added dropwise, and the pyrrole ester in tetrahydrofuran is pumped via a syringe drive at 2.5 mL/min while the internal temperature is ramped to 135 °C over 90 minutes and held for 26 hours. After quench into ice‑cold ammonium chloride solution, the crude diketopyrrolopyrrole (DPP) material is washed sequentially with 0.5 M hydrochloric acid and deionized water until the conductivity of the aqueous phase falls below 10 µS/cm. The dried powder is then processed through a three‑zone gradient sublimation apparatus with zone‑1 at 220 °C, zone‑2 at 185 °C, and the deposition zone at 140 °C, operating at 4 × 10⁻⁶ mbar with a carrier argon flow of 15 sccm. The resulting product exhibits a single‑crystal X‑ray diffraction pattern consistent with a planar π‑stacking distance of 3.42 Å and an electron mobility of 4.7 × 10⁻³ cm²/V·s measured via space‑charge‑limited current (SCLC) on an ITO/PEDOT:PSS/DPP:C60/LiF/Al device architecture. Industry compliance for electronic‑grade intermediates falls under IEC 62321‑8 for restricted phthalates and brominated flame retardants as well as EU RoHS Directive 2011/65/EU; metal cation contamination is quantified by inductively coupled plasma mass spectrometry against ASTM D7582‑15 with alert limits of ≤ 0.5 ppb for sodium, ≤ 1.0 ppb for iron, and ≤ 0.2 ppb for copper, any of which above threshold increases dark current in inverted‑geometry cells by more than an order of magnitude.

C‑2 Decarboxylative Halogenation for Pyrrole‑Based Natural Product Synthesis

Removal of the ester functionality from the 2‑position via a modified Barton‑type radical decarboxylation unlocks the 2‑halo‑3‑chloropyrrole motif required for the marine tetracyclic alkaloid lamellarin D. In this workflow, the methyl ester is first hydrolyzed to 3‑chloropyrrole‑2‑carboxylic acid by stirring in 0.6 M aqueous sodium hydroxide at 45 °C for 5 hours; after acidification to pH 1.8 and extraction into diethyl ether, the acid is dried over anhydrous sodium sulfate and concentrated to a free‑flowing powder with a residual ether content below 800 ppm by headspace GC. The acid (1.0 eq, 12.2 g, 84 mmol) is then suspended in dry dichloromethane under argon at 0 °C and treated sequentially with dicyclohexylcarbodiimide (1.25 eq, 21.8 g, 106 mmol) and 2‑mercaptopyridine N‑oxide sodium salt (1.15 eq, 14.6 g, 97 mmol). The slurry stirs at 0–5 °C for 1 hour until in‑situ FTIR indicates complete consumption of the acid carbonyl stretch at 1685 cm⁻¹, whereupon bromotrichloromethane (10 eq, 100 mL) is introduced in a single portion and the reaction is irradiated with a 500 W tungsten‑halogen lamp positioned 12 cm from the quartz immersion well. The radical chain propagates with a measured temperature ΔT of +18 °C over 7 minutes; external air‑cooling maintains the batch below 38 °C to supress pyrrole ring bromination. After 2 hours, the mixture is filtered through a silica plug eluting with 4:1 hexane/ethyl acetate, and the low‑boiling fractions are distilled at 45 mbar to yield 2‑bromo‑3‑chloropyrrole as a pale‑yellow oil that crystallizes at −20 °C (GC purity 98.4 %). The downstream regioselective Suzuki coupling at the 2‑bromo site with 3,4‑dimethoxyphenylboronic acid then installs the eastern aryl ring of lamellarin D trimethyl ether, while the 3‑chloro group is retained for late‑stage oxidative conversion to a phenol. Compliance requirements for this synthesis route, typically executed at 50 mmol to 500 mmol scale within university and contract research laboratories, adhere to OECD Principles of Good Laboratory Practice; all physical measurements are traceable to ISO 17025 reference methods, and waste streams containing brominated organics are incinerated at ≥ 1100 °C with ≥ 2‑second residence time per EU Directive 2000/76/EC. The lamellarin scaffold itself is evaluated as a topoisomerase I poison and a mitochondrial permeability transition pore inhibitor, though published data for this specific 3‑chloro precursor configuration in preclinical development remain limited.

When the Ester Requires Orthogonal Protection in Multi‑Kilogram Syntheses of Heterocyclic Libraries

In parallel synthesis environments where a core 3‑chloropyrrole scaffold is diversified via amide bond formation, the methyl ester serves a dual role as a temporarily masked carboxylic acid that remains inert toward palladium‑catalyzed aminations and reductive N‑alkylations commonly executed early in the synthetic sequence. A validated library protocol executed on a Chemspeed Accelerator SLT106 automated platform charges each 35 mL reactor well with 1.2 mmol of methyl 3‑chloropyrrole‑2‑carboxylate, 1.50 mmol of the desired amine coupling partner, 1.80 mmol of HATU, and 2.5 mmol of N,N‑diisopropylethylamine in 8.0 mL of acetonitrile, with a stirring rate of 900 rpm and a block temperature of 50 °C for 14 hours. Conversion measured by UPLC‑MS at 254 nm routinely exceeds 93 % across 380 diverse amines spanning aliphatic, benzylic, and heteroaromatic space. After filtration through an in‑line PTFE frit and solvent evaporation under vortex‑assisted nitrogen flow at 45 °C, the crude methyl ester amide intermediates are redissolved in a 3:1 v/v mixture of tetrahydrofuran and deionized water and treated with 2.6 equivalents of lithium hydroxide monohydrate at 25 °C for 8 hours. Under these conditions, the 3‑chloropyrrole‑2‑carboxylic acid is liberated quantitatively without detectable cleavage of concurrent tert‑butyl carbamate, benzyl ether, or silyl ether protecting groups that are frequently present in amino alcohol and amino phenol building blocks. Post‑hydrolysis, the reaction mixture is acidified to pH 3.0 using 2 N hydrochloric acid, extracted twice with ethyl acetate, and purified on a preparative HPLC system equipped with a C18 5 µm, 30 × 250 mm column and a water/acetonitrile plus 0.1 % formic acid gradient; target masses are collected at a purity threshold of ≥ 98.0 % by evaporative light scattering detection. The final carboxylic acid library members are formatted as 10 mM DMSO stock solutions and registered into a corporate compound collection for high‑throughput screening against Class A G‑protein‑coupled receptor targets. Although this discovery‑phase workflow does not operate under full 21 CFR 210/211 GMP, the automated liquid handlers are calibrated quarterly following ISO 8655‑6 gravimetric procedures at 0.5–50 µL and 50–1000 µL volume ranges, and the analytical balance used for standard preparation carries a measurement uncertainty of ± 0.02 mg as determined by EURAMET cg‑18 guidelines. No instance of methyl ester solvolysis during the amide‑forming step has been observed in over 2,400 reactions provided the water content of the acetonitrile solvent is maintained below 200 µg/g; above this threshold, free acid formation increases linearly at a rate of approximately 0.4 %/hour, contaminating the amide product and reducing isolated yield.