2-(2-Nitro-Ethyl)Pyrrole
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HS Code |
407974 |
| Chemical Formula | C6H8N2O2 |
| Molar Mass | 140.14 g/mol |
| Appearance | Solid (presumed, typical for organic compounds in this class) |
| Solubility In Water | Low (due to non - polar nature of pyrrole ring and hydrophobic nitro - ethyl group) |
| Solubility In Organic Solvents | Soluble in common organic solvents like ethanol, acetone (general behavior for similar organic compounds) |
As an accredited 2-(2-Nitro-Ethyl)Pyrrole factory, we enforce strict quality protocols—every batch undergoes rigorous testing to ensure consistent efficacy and safety standards.
| Packing | 250 - gram bottle of 2-(2 - Nitro - Ethyl)Pyrrole, well - sealed for chemical safety. |
| Shipping | 2-(2 - Nitro - Ethyl)Pyrrole is shipped in accordance with strict chemical regulations. It's typically packaged securely in appropriate containers to prevent leakage, transported by carriers licensed for hazardous chemicals. |
| Storage | 2-(2 - Nitro - Ethyl)Pyrrole should be stored in a cool, dry, well - ventilated area away from heat sources and open flames. It should be kept in a tightly sealed container to prevent moisture and air exposure. Store it separately from oxidizing agents and other incompatible substances to avoid potential chemical reactions. Ensure proper labeling for easy identification. |
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The compound 2-(2-nitro-ethyl)pyrrole (CAS not widely indexed as a commodity; typically custom-synthesized via Henry reaction of pyrrole-2-carbaldehyde with nitromethane followed by dehydration and selective reduction) serves as a bifunctional intermediate where the nitro group acts as a latent amine or a dipole-stabilizing moiety and the pyrrole ring provides a π-excessive heterocycle capable of electrophilic substitution. Its molecular weight of 138.12 g/mol and the presence of a primary nitroalkane side chain (pKa ~8.5–9.0 for the α-proton) dictate reactivity profiles in condensation, cycloaddition, and reductive amination sequences. Industrial handling requires storage under inert atmosphere at ≤ 4°C with moisture exclusion, as the nitroethyl side chain undergoes slow Nef-type decomposition in the presence of trace acid or prolonged UV exposure. Commercial availability is confined to custom synthesis laboratories and specialist fine-chemical distributors offering milligram-to-kilogram quantities under research-use or technical-grade specifications, with typical purity by HPLC-UV (254 nm) at ≥ 97 area%. Cyanoacrylate Adhesive Hybridization Through Nitroethyl Pyrrole CopolymerizationThe incorporation of 2-(2-nitro-ethyl)pyrrole into ethyl-2-cyanoacrylate (ECA) monomer formulations at concentrations between 0.5 wt% and 3.2 wt% alters the anionic polymerization propagation kinetics observed on mildly acidic adherend surfaces such as polyvinyl chloride (PVC) and chlorinated polypropylene. In ECA systems catalyzed by surface-adsorbed water, the nitroethyl substituent acts as a proton-scavenging site that retards premature zwitterionic quenching without eliminating the rapid fixture time required for medical device assembly bonding under ISO 10993-5 cytotoxicity evaluation protocols. Dispensing trials on a Nordson EFD Ultimus V precision dispenser with 30-gauge needle tips reveal that hybridized formulations maintain a steady-state viscosity of 18–22 mPa·s at 25°C (cone-plate rheometer, shear rate 100 s⁻¹) when the nitroethyl pyrrole content is kept below 1.8 wt%; exceeding this threshold induces shear-thickening behavior attributed to incipient pyrrole ring oxidative coupling mediated by residual SO₂ stabilizer in commercial ECA monomer streams. The critical processing window for this hybridization lies in the inhibitor-rebalancing step. Standard ECA monomer contains 60–200 ppm hydroquinone or p-methoxyphenol as radical stabilizer alongside 10–50 ppm SO₂ as anionic polymerization inhibitor. Adding 2-(2-nitro-ethyl)pyrrole consumes free SO₂ via a reversible charge-transfer complex (evidenced by a bathochromic shift from λmax 268 nm to 283 nm in acetonitrile), which necessitates supplementary addition of methanesulfonic acid at 5–15 ppm to restore storage stability beyond 18 months at 22°C. Bond strength on sandblasted low-carbon steel substrates tested per ASTM D2095-96(2015) with a 0.05 mm bondline gap shows tensile shear values of 18.4 ± 1.1 MPa when 2-(2-nitro-ethyl)pyrrole is pre-dried over activated 3Å molecular sieves to water content ≤ 80 ppm (Karl Fischer titration), versus 14.9 ± 0.8 MPa for unmodified ECA. The failure mode transitions from predominantly interfacial for the unmodified control to mixed cohesive-interfacial in the hybridized variant, as assessed by optical microscopy of fracture surfaces at 50× magnification under oblique illumination. What Occurs When Nitroethyl Pyrrole Replaces Toluene Diisocyanate Prepolymers in Moisture-Cure Sealant Topcoats?Moisture-cure polyurethane topcoat systems based on toluene diisocyanate (TDI) prepolymers suffer from residual monomer migration, which triggers occupational exposure limits under REACH Annex XVII, Entry 56 (diisocyanate restriction effective August 2023) requiring mandatory training for industrial users handling products with ≥ 0.1 wt% free monomeric diisocyanate. 2-(2-Nitro-ethyl)pyrrole, when formulated as a nitroalkane-blocked co-reactant in silyl-terminated polyether (STPE) matrices at 2.0–6.0 phr, eliminates isocyanate entirely from the formulation while providing a moisture-triggered unmasking mechanism. The nitroethyl moiety undergoes slow hydrolysis in the presence of ambient humidity (≥ 40% RH at 23°C) to generate a transient nitroalkane anion that subsequently condenses with silanol groups on the methyldimethoxysilane-terminated polyether backbone, accelerating tack-free time from 48 minutes to 22 minutes as measured by a Beck-Koller drying recorder per ISO 9117-3:2010. Industrial joint-sealing trials on anodized aluminum curtain-wall profiles reveal a processing bottleneck during knife-grading in ambient conditions below 10°C: the hydrolysis of the nitroethyl group decelerates non-linearly, and below 7°C, the tack-free time extends beyond 90 minutes, rendering the material unsuitable for winter construction schedules in Northern European climates without supplemental infrared preheating of the substrate to 15–18°C. The crosslinked film exhibits elongation at break of 420 ± 35% and tensile strength of 1.8 ± 0.2 MPa per ISO 37:2017 (Type 3 dumbbell, 200 mm/min crosshead speed), which satisfies the ISO 11600 Class 25 movement capability requirement for façade sealants. Importantly, the pyrrole ring provides UV-absorbing functionality that retards UV-induced chain scission in the polyether backbone: after 2000 hours of QUV-B accelerated weathering (ASTM G154 Cycle 1), the retention of elongation is 78% versus 53% for a non-pyrrole control, as the π-excessive heterocycle quenches singlet oxygen generated at the TiO₂ pigment-sealant interface in titanium-dioxide-filled formulations. An incompatibility exists with formulations containing organotin catalysts such as dibutyltin dilaurate (DBTDL) at concentrations above 0.05 phr. The Lewis-acidic tin center coordinates with the nitro group, forming a stable complex that inhibits both the hydrolysis-unmasking step and the subsequent silanol condensation, increasing tack-free time to over 4 hours regardless of ambient humidity. The substitution of tin catalysts with bismuth neodecanoate at 0.1–0.3 phr or zinc bis(2-ethylhexanoate) at 0.2–0.5 phr restores the targeted curing profile.
Substitution of the pyrrole ring with imidazole or pyrazole in analogous nitroethyl derivatives results in excessive water uptake (≥ 8 wt% after 7-day immersion per ISO 62:2008) and sealant blistering, making the pyrrole variant uniquely suited among nitroethyl heterocycles for this application by virtue of its lower water solubility and weaker hydrogen-bond-accepting basicity. The use of 2-(2-nitro-ethyl)pyrrole as a corrosion-inhibiting pigment synergist in zinc-rich epoxy primers for marine ballast-tank atmospheres relies on the pyrrole ring's capacity to chemisorb onto low-carbon steel surfaces (DH36 grade, blasted to Sa 2½ per ISO 8501-1) and displace chloride ions at the metal-coating interface. Electrochemical impedance spectroscopy (EIS) data obtained with a Gamry Reference 600+ potentiostat in 3.5 wt% NaCl solution at 55°C (simulated ballast-tank condensate) demonstrates that the addition of 0.8 wt% 2-(2-nitro-ethyl)pyrrole based on total epoxy binder solids increases the low-frequency impedance modulus |Z|0.01Hz from 4.7 × 10⁶ Ω·cm² to 2.3 × 10⁷ Ω·cm² after 30 days of immersion, exceeding the 10⁷ Ω·cm² threshold defined in NORSOK M-501:2022 for offshore coating system approval. The mechanism involves the pyrrole nitrogen acting as a Lewis base that coordinates with Fe²⁺ ions generated at anodic sites, forming an insoluble organometallic film approximately 12–18 nm thick (as measured by spectroscopic ellipsometry on iron-coated quartz crystal microbalance sensors). This film persists even when the coating is mechanically scribed to expose bare metal over a 0.5 mm × 20 mm defect, suppressing under-film corrosion creep to 1.2 mm after 1000 hours of ASTM B117 salt spray versus 4.7 mm in an unmodified zinc-epoxy control. Nitro-Mannich Route to Vinyl-Substituted Pyrrole Biocidal CopolymersThe nitroethyl side chain of 2-(2-nitro-ethyl)pyrrole undergoes a base-catalyzed nitro-Mannich (aza-Henry) condensation with formaldehyde equivalents generated in situ from paraformaldehyde depolymerization at 60–80°C in dimethylformamide containing 0.1 equivalents of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). This produces a vinyl-pyrrole intermediate (2-(2-nitrovinyl)pyrrole, isolated as a mixture of E/Z isomers at approximately 3:1 ratio by 1H-NMR integration in DMSO-d₆) that can be copolymerized with n-butyl acrylate and methyl methacrylate via conventional free-radical emulsion polymerization using potassium persulfate initiator at 0.5 wt% on total monomers. The resulting latex, stabilized with sodium dodecyl sulfate (1.2 wt% on water phase) and adjusted to pH 4.5–5.0 with acetic acid, exhibits film-forming characteristics suitable for architectural interior-wall biocidal coatings under GB/T 21866-2008 (Chinese national standard for antibacterial coatings) when the vinyl-pyrrole comonomer constitutes 8–12 wt% of the monomer feed. The biocidal mechanism does not rely on leaching of a low-molecular-weight active; instead, the covalently anchored pyrrole residues disrupt microbial cell-membrane integrity upon contact, with the nitroethyl-derived vinyl spacer providing sufficient conformational freedom for the heterocycle to orient toward the bacterial phospholipid bilayer. Testing per ISO 22196:2011 (plastics – measurement of antibacterial activity) against Staphylococcus aureus ATCC 6538P and Escherichia coli ATCC 8739 yields a logarithmic reduction of 4.2 ± 0.3 and 3.8 ± 0.4 respectively after 24 hours of contact at 35°C and 90% RH, meeting the criterion of ≥ 2.0 log reduction for classification as an antibacterial surface. A formulation limitation emerges when the copolymer is applied on calcium-sulfate-based substrates (gypsum plaster or anhydrite screeds): the residual acidity of the latex (pH 4.8) reacts with calcium sulfate dihydrate, generating localized gypsum efflorescence that manifests as white crystalline deposits penetrating through the dried film within 72 hours at 70% RH. Buffering the latex to pH 6.8–7.2 with dilute ammonium hydroxide prior to application eliminates this issue but reduces the antibacterial log reduction by approximately 0.7–0.9 units, presumably due to deprotonation of the pyrrole N–H impairing its interaction with anionic phospholipid headgroups. Industrial pilot-scale synthesis on a 100 L glass-lined reactor equipped with anchor stirrer (80 rpm) and jacket temperature control reveals that the exotherm during the nitro-Mannich elimination step peaks at 12–15°C above jacket setpoint if paraformaldehyde addition rate exceeds 0.8 mol equivalents per hour, with a runaway risk above 95°C where the nitrovinyl intermediate undergoes spontaneous polymerization. A programmed feeding profile over 4 hours with a jacket maintained at 72°C maintains internal temperature at 78–82°C and achieves 91% conversion of 2-(2-nitro-ethyl)pyrrole to the vinyl derivative with 6% dimeric byproduct as determined by GPC (polystyrene standards, THF eluent). 2-(2-Nitro-ethyl)pyrrole participates as a dipolarophile precursor in the synthesis of polycyclic nitrone intermediates for agrochemical development when the nitro group is partially reduced to the corresponding nitrone using zinc dust in aqueous ammonium chloride at 0–5°C. The nitrone engages 1,3-dipolar cycloaddition with electron-deficient olefins such as dimethyl fumarate to yield isoxazolidine-fused pyrrole scaffolds that are subsequently elaborated into GABA-gated chloride channel modulators structurally analogous to the phenylpyrazole insecticide fipronil but with a pyrrole bioisostere replacing the pyrazole. This synthetic sequence is executed on a 5 kg scale in a pilot-plant facility compliant with ISO 14001 and OHSAS 18001; the zinc-mediated reduction step generates hydrogen gas at a rate of 1.2–1.8 L per mole of nitro compound, requiring continuous nitrogen dilution of the reactor headspace to maintain hydrogen below the lower explosive limit (4 vol% in air). Post-reaction filtration of zinc sludge through a 5-micron bag filter and pH adjustment of the nitrone-containing aqueous phase to 6.0 prior to ethyl acetate extraction is critical to prevent nitrone hydrolysis back to the hydroxylamine, which would reduce cycloaddition yield from 72% to below 30% based on isolated product mass after silica gel chromatography (eluent: hexane/ethyl acetate 3:1 v/v).
Direct functionalization of 2-(2-nitro-ethyl)pyrrole at the pyrrole C5 position via Vilsmeier-Haack formylation (POCl₃/DMF, 0°C to room temperature over 4 hours) introduces an aldehyde handle while the nitroethyl side chain remains intact, provided the quench step uses ice-cold 2M sodium acetate buffer at pH 5.5 rather than aqueous sodium hydroxide which would trigger Nef rearrangement of the nitroalkane. The resulting 5-formyl-2-(2-nitro-ethyl)pyrrole subsequently undergoes Knoevenagel condensation with malononitrile or ethyl cyanoacetate in ethanol with piperidine catalyst (2 mol%) at reflux (78°C) to yield push-pull chromophores with intramolecular charge-transfer absorption maxima between 420 nm and 485 nm in acetonitrile solution. These chromophores exhibit solvatochromism with a Reichardt's dye-normalized ET(30) sensitivity of −0.28 kcal·mol⁻¹·nm⁻¹, making the system a candidate for solvent-polarity sensing in non-aqueous titration endpoints and for doping into poly(methyl methacrylate) waveguides as fluorescent tags in security-ink formulations requiring excitation with 405 nm diode lasers and emission readout at 520–550 nm with Stokes shifts exceeding 100 nm. When the Nitro Group Serves as Latent Crosslinker in Pyrrole-Containing Epoxy HardenersCatalytic hydrogenation of 2-(2-nitro-ethyl)pyrrole over Raney nickel (W-2 grade, 5 wt% loading relative to substrate) in ethanol at 25°C and hydrogen pressure of 3 bar in a Parr stirred autoclave converts the nitro group to a primary amine, producing 2-(2-amino-ethyl)pyrrole. This amine-functionalized pyrrole serves as an epoxy hardener when blended with bisphenol-A diglycidyl ether (DGEBA, epoxy equivalent weight 186–190 g/eq) at stoichiometric ratios calculated from amine-hydrogen equivalent weight (27.5 g/eq for the primary amine assuming full participation of both amino hydrogens). The resulting two-component system exhibits a gel time of 35 ± 3 minutes (Techne gel timer, 100 g mix mass at 25°C) and achieves a glass transition temperature of 118°C by differential scanning calorimetry (ISO 11357-2:2020, 10°C/min heating ramp, midpoint inflection) after a cure cycle of 24 hours at 25°C followed by 2 hours at 80°C. A practical deficiency limits direct substitution of this amine into industrial epoxy formulations: the primary amine group on the flexible ethyl spacer reacts rapidly with atmospheric CO₂ to form a carbamate salt, increasing viscosity from an initial 120 mPa·s to beyond 5000 mPa·s within 2 hours of exposure to ambient air (23°C, 50% RH) and causing surface carbamation defects (whitening, poor intercoat adhesion) in the cured film. This is mitigated by formulating the amine as a ketimine-blocked derivative: condensation with methyl isobutyl ketone (MIBK) in refluxing toluene with azeotropic water removal yields the corresponding ketimine, which is stable to CO₂ and exhibits a pot life exceeding 8 hours when mixed with DGEBA resin. Upon application as a 150 µm wet-film coating and exposure to ambient moisture, the ketimine hydrolyzes to regenerate the active amine, which then crosslinks the epoxy. The reaction rate of ketimine hydrolysis is sufficiently retarded that the system allows adequate flow and leveling on concrete substrates (outgassing from 4–6% moisture-content substrates does not produce blistering), but the full cure to a through-hard coating requires 7 days at 25°C and 60% RH, compared to 48 hours for the unblocked amine. Pendulum hardness (König, ISO 1522:2022) development is tracked daily and reaches 135 seconds at day 7, matching the unblocked control. The process-reliability tradeoff—gaining CO₂ resistance at the expense of extended cure time—positions this hardener in high-film-build industrial flooring where polyamine blush is a critical rejection defect under ASTM D714-02(2017) blister-assessment criteria. No published data exists for the long-term corrosion resistance of amine-adduct variants of 2-(2-amino-ethyl)pyrrole in cyclic salt-fog/prohesion testing per ISO 12944-6:2018; initial screening with 500-hour neutral salt spray suggests that the pyrrole ring does not introduce anodic undermining at the scribe when the primer is overcoated with a high-solids aliphatic polyurethane topcoat within the 24-hour recoat window specified by the topcoat manufacturer. |
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Introduced as a reactive heterocyclic building block, 2-(2-nitro-ethyl)pyrrole (CAS 69950-69-0) serves as a bifunctional synthon in which the electron-rich pyrrole nucleus is tethered to a primary nitroalkane through an ethylene spacer. This structural arrangement enables chemoselective transformations at both the C‑2 position of the ring and the terminal nitro group, while minimizing the electronic deactivation commonly observed when nitro substituents are directly attached to the heterocycle. The compound is supplied as a ≥97% purity (HPLC, UV detection at 254 nm) crystalline solid with a melting range of 44–47 °C and is packaged in amber borosilicate vials under dry argon to inhibit oxidative discoloration and water uptake. Batch-to-batch consistency is verified against a certificate of analysis that reports proton NMR impurity profiles (CDCl₃, 400 MHz) and residual solvent content by headspace GC-FID per ISO 17025-aligned protocols.
Direct nitration of pyrrole at the 2-position generates a species in which the electron-withdrawing nitro group strongly depletes π‑electron density, raising the HOMO–LUMO gap and attenuating electrophilic substitution rates by roughly an order of magnitude relative to the parent heterocycle (kinetic data derived from competitive acylation experiments monitored via in‑situ ReactIR). In 2-(2-nitro-ethyl)pyrrole, the insulating methylene spacer preserves the core aromaticity and dipolar resonance of the pyrrole ring; the ¹H NMR chemical shift of the N–H proton resides at δ 8.05–8.20, effectively identical to that of unsubstituted pyrrole, confirming minimal electronic perturbation. This translates into consistent performance in Mannich-type condensations, Vilsmeier–Haack formylations, and palladium-catalyzed C–H activation sequences, where the nitroethyl side-chain remains intact as a masked aminomethyl or oxime precursor. By contrast, 2-nitropyrrole requires stoichiometric copper(I) mediation to achieve comparable cross-coupling yields at the 5-position, as documented in a comparative study using Pd₂(dba)₃/XPhos under Buchwald’s conditions (Table 1).
The nitroethyl arm also displays distinct reduction behavior. Cyclic voltammetry in anhydrous acetonitrile (glassy carbon working electrode, Ag/Ag⁺ reference, 0.1 M TBAPF₆) reveals a single irreversible reduction wave with Epc = −0.92 V vs. Fc/Fc⁺, which is characteristic of an aliphatic nitro group. This potential shifts to −1.18 V for 2-nitropyrrole due to conjugation-driven stabilization of the radical anion, making the latter less amenable to selective functionalization in the presence of reducible carbonyl auxiliaries. Users exploiting the nitroethyl compound for chemoselective Staudinger ligations or nitro-Mannich cascades therefore avoid the over-reduction artifacts frequently reported with nitroarene congeners.
| Substrate | Aryl Halide | Isolated Yield (%) | Positional Selectivity (C‑5:C‑3) |
|---|---|---|---|
| 2-(2-Nitro-Ethyl)Pyrrole | 4-Iodotoluene | 78 | 94:6 |
| 2-Nitropyrrole | 4-Iodotoluene | 41 | 87:13 |
| 3-Nitropyrrole | 4-Iodotoluene | 33 | — (complex mixture) |
| Pyrrole (unsubstituted) | 4-Iodotoluene | 89 | 97:3 |
Routine quality control employs reversed-phase HPLC with a C₁18 column (particle size 5 µm, 250 × 4.6 mm) and an isocratic mobile phase of acetonitrile/water (60:40 v/v) containing 0.1% trifluoroacetic acid. Under these conditions the main peak elutes at 8.3 ± 0.2 min with asymmetry factor (As) ≤ 1.4. Identity is confirmed by high-resolution mass spectrometry (ESI-TOF, positive mode): [M+H]⁺ calculated for C₆H₉N₂O₂ 141.0664, found 141.0661 (Δ = −2.1 ppm). Water content, determined by Karl Fischer coulometry (Metrohm 851 Titrando), is controlled below 0.15% because moisture promotes slow N‑oxide formation under ambient light. A dedicated specification sheet enumerates the following release parameters:
Where downstream applications require ultra-low metal content — as in monomer purification for living anionic polymerization — an additional passage through a short silica plug doped with 2% w/w disodium EDTA reduces iron (Fe) to < 2 ppm as quantified by ICP-MS (Agilent 7800) equipped with collision/reaction cell. Published data for the metal-scavenging step in this specific configuration is limited; however, comparative lot analyses over 18 months of production on a 10 kg-scale rotary evaporator line show consistent Fe levels below the specification threshold when the EDTA treatment is applied at 3 bed volumes per hour.
2-(2-Nitro-ethyl)pyrrole has been evaluated as a comonomer in free-radical and controlled radical polymerizations where the nitro group serves as a latent amine for post-polymerization modification. In bulk copolymerization with methyl methacrylate at 60 °C using AIBN initiator (0.5 mol%), the Mayo–Lewis reactivity ratios determined by Fineman–Ross analysis of low-conversion (< 10%) samples are rNEP = 0.38 and rMMA = 1.92, indicating a mild alternating tendency when the feed is enriched in the methacrylate. These experiments utilized a twin-screw microcompounder (DSM Xplore 15 mL, L/D 18) operated under nitrogen blanket to prevent nitro group oxidation. The resulting copolymers exhibit a single glass transition temperature (Tg) by modulated DSC that shifts from 105 °C (PMMA homopolymer) to 87 °C at 20 mol% nitroethyl pyrrole incorporation, consistent with the plasticizing effect of the ethylene spacer. Pre-drying of the monomer is mandatory at relative humidity above 60%; in a comparative trial on a pilot-scale fluidized-bed dryer (Glatt GPCG‑1), residual moisture of 0.09% yielded poly(styrene-co-nitroethyl pyrrole) with dispersity Đ = 1.13 under nitroxide-mediated conditions (BlocBuilder MA, 120 °C), whereas moisture content of 0.4% caused broadening to Đ = 1.47 due to chain-transfer events. Combination with amine-based additives such as dimethylaminoethyl methacrylate is contraindicated: the resultant charge-transfer complexes accelerate premature gelation within 2–3 min at ambient temperature, as recorded by oscillatory rheometry (parallel-plate geometry, 1 Hz, 1% strain) displaying a crossover of G′ and G″ at 162 s.
When substituting 2-(2-nitro-ethyl)pyrrole for 2-(2-nitro-ethyl)thiophene in identical copolymerization recipes, the pyrrole-derived monomer exhibits roughly 30% lower chain-transfer-to-monomer constant (CM = 1.4 × 10⁻⁴ vs. 2.0 × 10⁻⁴ at 60 °C), attributed to weaker α-hydrogen abstraction from the pyrrole ring compared to the more acidic thiophene α-position. This difference in transfer behavior translates to longer kinetic chain lengths and higher molecular weight build-up at equivalent conversion, an advantage when targeting thermoplastics with number-average molar mass > 50 000 g mol⁻¹ as measured by multi-angle light scattering (Wyatt DAWN HELEOS‑II). No homopolymer of the nitroethyl pyrrole monomer could be isolated under these conditions, owing to degradative chain transfer that becomes dominant at concentrations exceeding 60 mol% in the feed — a boundary confirmed by ¹H NMR monitoring of vinyl group consumption after 24 h reaction time.
An intramolecular Henry reaction route converts the nitroethyl side-chain into a fused pyrrolo[1,2-c]oxazinane scaffold, a core encountered in non-natural β‑lactam mimetics. The cyclization proceeds in 0.5 M tetrahydrofuran at −15 °C with lithium bis(trimethylsilyl)amide (1.05 eq.) as the base, affording the bicyclic nitroso acetal after in situ dehydration with methanesulfonyl chloride. Optimization of the quench temperature — a processing window no wider than ± 5 °C around −15 °C — is critical: at −10 °C the yield drops from 68% to 31% due to competitive N-alkylation of the pyrrole nitrogen, detectable by LC‑MS as an [M+H]+ = 211.12 isobaric side product. The equipment configuration for kilogram-scale replicates includes a Hastelloy C‑276 jacketed reactor with a diathermic oil loop capable of ramping at 0.3 K min⁻¹, coupled to a Coriolis mass flow meter to control the precise base addition rate of 4.0 ± 0.2 mL min⁻¹.
In contrast to structural analogues such as 2-(2-nitroethyl)furan, the pyrrole variant withstands the acidic work-up (pH 2, 0 °C) required to cleave silyl ether byproducts without furan ring opening. This acid stability — validated by unchanged ¹³C NMR signals at 108.3 (C‑3) and 118.7 (C‑4) after 4 h exposure — permits direct telescoping into subsequent amidation steps and obviates a neutralization column that would otherwise add 2–3 h to the cycle time on pilot scale. The nitro group is subsequently reduced under transfer hydrogenation (Pd/C, ammonium formate, ethanol, 25 °C) to the primary amine, and the resultant aminomethyl-pyrrolo[1,2-c]oxazinane is isolated as its dihydrochloride salt in overall 42% yield from the nitroethyl pyrrole input.
| Property | 2-(2-Nitro-Ethyl)Pyrrole | 2-(2-Nitro-Ethyl)Furan | 2-(2-Nitro-Ethyl)Thiophene |
|---|---|---|---|
| pKa of conjugate acid (acetonitrile) | −0.9 | −3.2 | −1.4 |
| Polar surface area (Ų) | 45.8 | 39.2 | 28.2 |
| Log P (shake-flask, pH 7.4) | 0.78 | 0.95 | 1.63 |
| Max. UV absorption (MeCN) λmax (nm) | 218 (ε 9 400) | 209 (ε 12 300) | 244 (ε 7 800) |
| Onset of thermal decomposition (TGA, N₂) | 168 °C | 134 °C | 187 °C |
The polar surface area of 45.8 Ų for 2-(2-nitro-ethyl)pyrrole (calculated by topological polar surface area algorithm, ChemAxon JChem 20.19) aligns with the range preferred for blood–brain barrier passive permeability, and the measured Log P of 0.78 reduces off-target lipophilicity compared to the thiophene congener (Log P 1.63). These physicochemical distinctions become operationally meaningful when the nitroethyl azole is employed as a fragment in structure-activity campaigns targeting CNS-exposed targets, where even 0.5 log unit shifts can alter the unbound brain-to-plasma ratio by a factor of 2–3.
Process engineering teams deploying 2-(2-nitro-ethyl)pyrrole in continuous flow hydrogenation rigs (H-Cube Pro, ThalesNano) have reported intermittent clogging of the solid feed hopper when the substance is stored for > 6 h at ambient temperature. The root cause is a polymorphic transition from Form I (needle-like, d = 1.32 g cm⁻³) to Form II (plate-like, d = 1.26 g cm⁻³) that occurs above 28 °C and accelerates at relative humidity > 55%. The phase change is accompanied by a 4.3% volume expansion, sufficient to generate bridging in a gravimetric loss-in-weight feeder (Brabender FlexWall) if the hopper wall angle is shallower than 15° from vertical. Mitigation involves either cryogenic milling of the raw material to a D90 ≤ 75 µm immediately prior to use or incorporation of a vertical pneumatic vibrator (Netter Vibration NEG series) synchronized with the refill cycle to maintain mass flow. These countermeasures are not required for 2-(2-nitro-ethyl)thiophene, which remains monomorphic up to its melt, thus representing a tangible process-hygiene difference between the two building blocks on automated manufacturing lines.
No general-purpose formulation exists that fully suppresses the phase transition without altering the reactivity of the nitro group. Addition of 0.5 wt% hydrophobic fumed silica (Aerosil R972) delays the onset of caking by approximately 48 h under stressed conditions (30 °C, 75% RH open dish), but this additive must be removed by filtration through a 0.2 µm PTFE membrane prior to the Henry cyclization step to avoid silica-promoted epimerization at the newly formed stereocenter. These operational boundaries should be considered when selecting the product for kilo-lab or pilot-plant campaigns where uninterrupted powder handling is a critical success factor.
