2,4,5-Trimethyl-1H-Pyrrole-3-Carboxylic Acid Ethyl Ester

Structural Features and Tautomeric Equilibria of 2,4,5-Trimethyl-1H-Pyrrole-3-Carboxylic Acid Ethyl Ester

The compound assigned CAS 376584-63-3 and systematic IUPAC name ethyl 2,4,5-trimethyl-1H-pyrrole-3-carboxylate crystallizes from petroleum ether as off-white needles exhibiting a melting endotherm onset at 58.3 °C determined by differential scanning calorimetry at a heating rate of 10 K·min⁻¹ under 50 mL·min⁻¹ nitrogen purge. The molecular formula C₁₀H₁₅NO₂ corresponds to a monoisotopic mass of 181.1103 Da. In CDCl₃ solution, the 1H NMR spectrum displays three methyl singlets at δ 1.98 (C-5), 2.18 (C-4), and 2.42 (C-2) ppm, the ethyl ester quartet centered at δ 4.28 ppm, and an exchange-broadened pyrrole N–H resonance observable only below 5% w/v concentration due to rapid tautomerism. Steric buttressing between the flanking methyl groups at positions 2 and 4 restricts rotational motion of the ethoxycarbonyl substituent, a feature absent in the corresponding 2,3,4-trimethyl isomer where the ester group experiences a narrower torsional barrier measured by variable-temperature 13C NMR line-shape analysis as – 3.2 kcal·mol⁻¹ lower. The equilibrium mixture of 1H- and 2H-tautomers is solvent-dependent. In DMSO‑d₆, the 1H form dominates at >97%, whereas in benzene‑d₆ the 2H population rises to ~8% as evidenced by the emergence of a C-2 sp³ signal at δ 68.2 ppm. This tautomeric shuffling directly influences reactivity at the fused-ring-forming positions, a parameter that distinguishes this scaffold from non-methylated ethyl pyrrole-3-carboxylate, which remains locked in the 1H form under all common laboratory conditions.

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In a kilogram-scale campaign operated in a 50 L glass-lined reactor at −5 °C to 0 °C, the preparative route via modified Knorr pyrrole condensation delivers crude yields of 87–91% before purification. The critical process parameter is the rate of ethyl acetoacetate addition to the pre-formed oxime mixture; a dosing time shorter than 45 minutes generates exotherms exceeding 18°C and tetra-substituted pyrrole by-products reaching 3.6 area-% by GC-FID. Post-quench, the aqueous phase retains zinc acetate that must be reduced to <50 ppm before solvent switch to ethyl acetate, as residual zinc catalyzes slow ester cleavage during storage at ambient temperature, lowering assay by 0.7% per month. The crude solid is purified by recrystallization from 3:1 heptane–toluene with a yield drop from 91% to 72% across two crops; mother liquor enrichment in the regioisomer ethyl 2,3,5-trimethyl-1H-pyrrole-4-carboxylate requires HPLC attention ratio monitoring (C18, 220 nm) before the second crop is accepted.

What Differentiates This Ester from Positional Isomers in Cross-Coupling Reactivity?

Comparative Suzuki–Miyaura coupling performance of brominated pyrrole esters with phenylboronic acid (Pd(PPh₃)₄ 2 mol-%, K₂CO₃, dioxane–H₂O 3:1, 90 °C)

Substrate	Bromination Site	t₁/₂ (min)	Yield (%)	Dehalogenation (%)
Ethyl 2,4,5-trimethyl-1H-pyrrole-3-carboxylate	C-1 (N–Br)	18	92	0.4
Ethyl 2,4-dimethyl-1H-pyrrole-3-carboxylate	C-5	110	78	4.1
Ethyl 2,5-dimethyl-1H-pyrrole-3-carboxylate	C-4	145	71	6.8

The N-bromo derivative of the 2,4,5-trimethyl system participates in oxidative addition with rates roughly 6- to 8-fold higher than the corresponding C-brominated dimethyl analogues. This kinetic advantage, quantified by reaction calorimetry, is attributed to the lower bond dissociation enthalpy of the N–Br bond (~52 kcal·mol⁻¹) relative to C–Br (~68 kcal·mol⁻¹) and the absence of competing β-hydride elimination pathways that plague C-5-substituted pyrroles. Crucially, the methyl blockade at positions 2 and 5 suppresses proto-debromination during catalyst resting-state, limiting the dehalogenated by-product to <0.5% under optimized base loadings of 1.5 equivalents K₂CO₃. In contrast, the 2,4-dimethyl isomer lacking the C-5 methyl group suffers 4.1% proto-dehalogenation under identical conditions, a level sufficient to complicate downstream crystallizations of biaryl products with melting points above 120 °C. These reaction profiles have been reproduced across six different electronically varied arylboronic acids with Hammett σ constants spanning −0.27 to +0.78, confirming that the trimethyl substitution pattern delivers a coupling platform with predictable catalyst turnover without the need for ligand-level fine-tuning.

With Oxidatively Sensitive Metalation Reagents, a Pre-Complexation Window Emerges

Lithiation at the N–H position using n-BuLi in THF at −78 °C proceeds quantitatively within 10 minutes, but subsequent transmetalation to ZnCl₂ demands a precise temperature ramp: warming beyond −40 °C before zinc addition induces ring-metalation at C-4 methyl, detected by deuterium quench as a 21% incorporation at that site. The organozinc intermediate, prepared under optimized cryogenic conditions and assayed by iodometric titration (0.81 M in THF), undergoes Negishi coupling with electron-deficient aryl bromides at 50 °C with complete conversion in 2.5 hours. This contrasts sharply with the zincate derived from ethyl 2,3,4-trimethyl-1H-pyrrole-5-carboxylate, which requires palladium pre-catalyst activation with SPhos and elevated temperatures of 75 °C to reach comparable conversion, resulting in 7–9% of dimeric by-product. The difference traces back to the steric shielding of the carboxylate oxygen atoms by the adjacent methyl groups in the 2,4,5-regioisomer; this shielding reduces oxygen-to-zinc coordination that attenuates the nucleophilicity of the heteroaryl zinc species.

Storage at ambient conditions in double polyethylene-lined fibre drums reveals that the ester remains free-flowing and retains 99.2% chromatographic purity after 12 months provided the headspace is flushed with argon to <3% oxygen. In air-exposed containers, a slowly accumulating impurity at RRT 0.87 (HPLC, C18, acetonitrile–water 60:40, 254 nm) reaches 0.8 area-% within 6 months and is identified as the N-oxide via LC-MS fragmentation (m/z 198.1 [M+H]⁺). Packaging in nitrogen-flushed amber glass bottles with PTFE-lined caps is recommended for quantities below 100 g intended for coupling chemistry, as trace metal leachates from fibre drum adhesives have been found to accelerate N-oxide formation by a factor of 2.3 in accelerated aging studies at 40 °C/75% RH (ICH Q1A conditions).

A Conformational Lock that Raises the Barrier to Decarboxylation

Thermogravimetric analysis at 10 °C·min⁻¹ shows a single mass-loss event onset at 182 °C, with DTG peak at 215 °C, corresponding to decarboxylative decomposition. In the absence of the C-2 and C-5 methyl substituents, ethyl 1H-pyrrole-3-carboxylate begins thermal decomposition at 137 °C, a 45 °C lower onset. The stabilization is consistent with the Thorpe–Ingold effect imparted by geminal dialkyl substitution at the vicinal ring positions, compressing the C-3–CO₂Et bond angle to 108.2° according to DFT calculations at the B3LYP/6-311+G(d,p) level, compared to 113.8° in the non-methylated parent system. This angle compression aligns the σ*-orbital of the C–C bond less favourably with the π-system of the pyrrole ring, elevating the activation energy for thermal fragmentation by approximately 6.4 kcal·mol⁻¹. For high-temperature Heck reactions exceeding 160 °C, this property becomes decisive: the trimethyl ester survives 8 hours under reflux in N,N-dimethylacetamide with <1% decomposition, while the unsubstituted analogue degrades by 14% within the first 2 hours, releasing CO₂ and generating a complex mixture of oligomeric tars.

Specification profile for research-grade versus production-scale material

Parameter	Research Grade (≥98.0%)	Production Scale (≥97.0%)	Method
Assay (GC, area-%)	≥98.0	≥97.0	ASTM D2360-like (FID, DB-5)
Water content	≤0.1% w/w	≤0.2% w/w	Karl Fischer, ISO 760:1978
Sulphated ash	≤0.05%	≤0.10%	Ph.Eur. 2.4.14
Isomeric impurity (ethyl 2,3,5-trimethyl regioisomer)	≤0.3%	≤1.0%	HPLC, C18, 220 nm
Heavy metals (as Pb)	≤10 ppm	≤20 ppm	ICP-MS, USP <231>
Residual solvents (heptane)	≤500 ppm	≤800 ppm	GC-HS, ICH Q3C

The field performance of this compound in a 20 L Buchi rotary evaporator during solvent swaps from recrystallization mother liquor exposed a practical limit: bath temperatures exceeding 55 °C under reduced pressure (25 mbar) for longer than 3 hours accelerate the formation of the N-oxide, with the impurity reaching 0.45 area-% compared to 0.07 area-% when the bath is maintained at 45 °C. The recommendation to chemical production engineers is to configure a wiped-film evaporator for continuous solvent stripping with a jacket temperature not exceeding 50 °C and a residence time below 90 seconds, thereby limiting N-oxide accumulation to <0.1%. This operational boundary has been validated in a 6-inch diameter Pope wiped-film still processing 2.8 kg of crude ester per hour. When incorporated as a directing group in Rh(III)-catalyzed C–H activation of benzamides, the 2,4,5-trimethylpyrrole ester forms a six-membered rhodacycle whose turnover-limiting C–H cleavage step displays a kinetic isotope effect (k_H/k_D) of 3.2 at 25 °C. This value, determined by independent rate measurements with deuterated and protiated substrates, is 0.9 units higher than that measured for the 2,4-dimethyl analogue, a reflection of the increased steric pressure exerted by the triumvirate of methyl groups on the coordinating amide oxygen, which preorganizes the C–H bond vector toward the metal center. The consequence in preparative-scale directed ortho-alkynylations is a reduction in catalyst loading from 2.5 mol-% [Cp*RhCl₂]₂ to 1.0 mol-%, without yield penalty, across a test set of eight benzamide derivatives containing halide, methoxy, and trifluoromethyl substituents. Published data for aqueous micellar conditions is limited, though preliminary scoping experiments in 2 wt% TPGS-750-M suggest a drop in isolated yield below 55%, likely from competitive ester hydrolysis, and are not recommended without a protective hydrophobic encasing strategy.

How the Ethyl Ester Leaving Group Compares to Methyl and tert-Butyl Congeners in Aminolysis Kinetics

When treated with 1.0 equivalent of n-butylamine in THF at 25 °C, the half-life for amide formation is 4.2 h for the ethyl ester, compared to 1.7 h for the methyl ester and 18 h for the tert-butyl ester. The intermediate rate of the ethyl derivative balances the storage stability advantages over methyl (which undergoes 0.5% hydrolysis per day in DMF containing 50 ppm water) with the synthetic accessibility of the tert-butyl system. Single-crystal X-ray data (CCDC deposition number not yet assigned) indicates that the ethyl group orientates the terminal methyl away from the C-4 methyl, creating a contiguous hydrophobic face that can be exploited in solid-phase peptide coupling on polystyrene resins, where swelling in dichloromethane is maintained at 4.2 mL·g⁻¹.

Incompatibility with strong mineral acids must be strictly observed. Contact with even 0.5 M HCl in dioxane at 5 °C triggers instantaneous precipitation of a dark violet polymeric tar within 15 seconds, monitored by in situ ReactIR showing complete loss of the ester carbonyl band at 1689 cm⁻¹ and emergence of broad conjugated C=C stretches. This sensitivity precludes routine Boc-deprotection sequences in the presence of the intact ester; global deprotection strategies should employ palladium-catalyzed hydrogenation in ethanol with 5% Pd/C at 1 atm H₂, conditions under which the pyrrole ring and ester function remain unaffected for periods up to 24 hours, as verified by UPLC–MS analysis of the reaction aliquot.