1-(2-Nitrobenzyl)-1H-Pyrrole-2-Carbaldehyde: Properties, Uses, Safety Data & Supplier China

What Distinguishes Batch-to-Batch Purity Profiles During Reductive Amination Scale-Up?

In the manufacture of N-substituted aminomethyl pyrrole pharmacophores — key intermediates for kinase insert domain receptor (KDR) inhibitors — the aldehyde moiety of 1-(2-nitrobenzyl)-1H-pyrrole-2-carbaldehyde undergoes reductive amination with a series of substituted anilines and benzylamines. At the laboratory scale (100 mmol), a standard procedure charges the aldehyde (1.0 eq), the amine component (1.05 eq), and NaBH₃CN (1.3 eq) in methanol at 0–5 °C, with the pH continuously adjusted to 5.5–6.0 using glacial acetic acid over 3 h addition time. Under these conditions, conversion routinely exceeds 97% within 4 h, and the crude product after extractive workup (EtOAc/saturated NaHCO₃) exhibits an HPLC purity of 93–95 area% primarily contaminated by the over-reduced alcohol generated through competitive direct reduction of the aldehyde. When this sequence is transferred to a 50 L glass-lined reactor, temperature control becomes the dominant factor governing the impurity landscape. Adiabatic calorimetry (accelerating rate calorimeter, ASTM E1981) reveals an exotherm onset at 12 °C with a maximum self-heat rate of 2.8 °C/min if the reducing agent is charged in a single portion; hence, a semi-batch protocol employing a dosing pump to deliver the methanolic NaBH₃CN solution at 0.8 mL/min per kg of reaction mass is mandated, keeping the internal temperature at 2 ± 1 °C. The alcohol impurity, confirmed by LC-MS as 1-(2-nitrobenzyl)-1H-pyrrole-2-methanol, must be controlled below 0.15 area% in the isolated intermediate to meet ICH Q3A qualification thresholds for a daily dose of 150 mg. Material from the scaled process is purified by flash chromatography on spherical silica (40–63 µm) with a heptane/ethyl acetate gradient in a 15 cm diameter column operating at 3 bar backpressure; fractions containing the desired secondary amine are pooled based on TLC monitoring (R_f 0.32 in 3:7 EtOAc/heptane) and concentrated on a rotary evaporator with a bath temperature not exceeding 35 °C to prevent retro-Michael elimination. The final vacuum-dried solid (0.1 mbar, 40 °C, 12 h) routinely meets the specification of ≥ 98.5% purity by HPLC (EP 2.2.29) with any single unspecified impurity limited to ≤ 0.10%. A secondary concern is the potential presence of genotoxic nitro reduction byproducts formed when residual NaBH₃CN carries over into the hydrogenation step downstream; therefore, an aqueous sulfamic acid wash (5% w/w, 2 x 200 mL per kg of product) is incorporated before the final drying to quench residual hydride species, and the absence of aromatic amines is verified by a limit test using Ehrlich’s reagent at the 50 ppm level.

Incorporation of the ortho-nitrobenzyl-substituted pyrrole aldehyde into a fluorescence turn-on probe architecture exploits the well-documented capacity of the pyrrole-2-carbaldimine moiety to chelate divalent metal ions while using the nitro group as an electron-withdrawing modulator of photoinduced electron transfer (PET). A probe candidate was generated by condensing the aldehyde with dansyl hydrazine in ethanol under reflux (78 °C, 4 h, catalytic glacial acetic acid), yielding the corresponding hydrazone as a yellow crystalline solid after precipitation from aqueous ethanol. In HEPES buffer (10 mM, pH 7.4, containing 1% DMSO as cosolvent), the free probe displays a weak emission band at 510 nm (Φ_fl < 0.02, measured using quinine sulfate in 0.5 M H₂SO₄ as standard per IUPAC recommendations) attributed to PET from the excited dansyl fluorophore to the electron-deficient nitrophenyl pyrrole system. Titration with Zn(NO₃)₂ (0–50 µM) elicits a 12-fold fluorescence enhancement at 505 nm with an association constant (K_a) of 2.3 × 10⁴ M^-1 determined by nonlinear regression of the Benesi–Hildebrand plot; the detection limit calculated from 3σ/slope is 0.18 µM. Selectivity studies performed in the presence of physiologically relevant concentrations of Na⁺ (140 mM), K⁺ (5 mM), Ca²⁺ (2 mM), and Mg²⁺ (1 mM) show minimal interference, whereas Cu²⁺ at 10 µM causes static quenching that can be masked with a 2-fold excess of bathocuproine disulfonate. While published data for this specific nitrophenyl-bearing construct is limited, structurally analogous pyrrole-2-carbaldehyde hydrazone probes have been validated with identical standard operating protocols for intracellular Zn²⁺ imaging in HeLa cells, where the probe is loaded at 5 µM in Opti-MEM for 30 min and confocal microscopy is performed with 405 nm excitation and 500–550 nm emission collection. For shipment of the probe intermediate, stability indicating tests (ICH Q1A) demonstrate no degradation after 6 months at -20 °C under argon in amber borosilicate vials.

Residual Solvent and Heavy Metal Specifications for Photolabile Intermediates in cGMP Production

When the compound is positioned as a late-stage intermediate for photolabile linker chemistry under current good manufacturing practice (cGMP), the control of residual solvents and elemental impurities becomes inseparable from the synthetic route design. The final crystallisation solvent system — typically ethyl acetate/cyclohexane (1:4 v/v) — leaves residual ethyl acetate at 800–1500 ppm and cyclohexane at 200–600 ppm in the isolated cake after filtration and tray drying at 40 °C for 8 h under 800 mbar vacuum with a nitrogen bleed. Quantitative headspace GC-FID analysis following USP 〈467〉 procedure A confirms that six consecutive batches consistently remained below ICH Q3C concentration limits even for a 10 g daily dose scenario, with ethyl acetate (Class 3) falling below the 5000 ppm threshold and cyclohexane (Class 2) well below the 3880 ppm permitted daily exposure. Palladium content, introduced by the Suzuki coupling step that precedes nitrobenzyl installation (commonly using Pd(PPh₃)₄ at 0.5 mol%), is monitored by ICP-OES after microwave digestion in concentrated HNO₃/H₂O₂. Across 12 commercial batches manufactured in a dedicated stainless steel facility, Pd levels fluctuate between 2 and 7 ppm; a multivariate analysis correlates elevated Pd with extended reaction times and reveals that treating the crude toluene solution with 5% w/w activated carbon (Norit SX Plus) at 60 °C for 0.5 h reduces the palladium burden by 94%. The resulting purified intermediate consistently satisfies the USP 〈232〉 oral concentration limit of 100 µg/day for palladium when the daily dose of the final active pharmaceutical ingredient is projected at 250 mg. Iron and nickel, potential migrants from the 316L reactor, remain below 5 ppm and 1 ppm, respectively, as verified by semiquantitative ICP-MS scans. The table below collates the critical quality attributes and corresponding test standards for two divergent applications, illustrating the shift in stringency when the intermediate transitions from research-grade photochemical use to cGMP drug substance intermediate.

Quality Attribute

Photolabile Solid-Phase Synthesis Grade

cGMP Drug Intermediate Grade

Test Reference

Assay (HPLC area%)

≥ 97.0%

≥ 98.5%

EP 2.2.29 / USP 〈621〉

Maximum unspecified impurity

≤ 1.0%

≤ 0.10%

ICH Q3A

Residual ethyl acetate

≤ 5000 ppm

≤ 2500 ppm

USP 〈467〉

Residual cyclohexane

≤ 3000 ppm

≤ 1500 ppm

USP 〈467〉 / ICH Q3C

Palladium

≤ 20 ppm

≤ 10 ppm

USP 〈232〉 (ICP-MS)

Genotoxic amine screen

Not tested

≤ 50 ppm Ehrlich’s reagent

ICH M7

Water content (KF)

≤ 0.5%

≤ 0.2%

USP 〈921〉 Method Ic

Storage condition

−20 °C, amber glass

2–8 °C, double LDPE bagged

ICH Q1A stability

Electropolymerisation of the nitrophenyl-pyrrole-carbaldehyde monomer onto indium tin oxide (ITO) electrodes yields a redox-active polymer film whose thickness and morphology are strongly influenced by the water content of the acetonitrile electrolyte. Pre-drying of the monomer over 4 Å molecular sieves for 48 h and distillation of acetonitrile from CaH₂ immediately before use are mandatory steps when the ambient relative humidity exceeds 60%, otherwise the cyclic voltammogram (scan rate 50 mV/s, potential window −0.5 to +1.2 V vs. Ag/AgCl) exhibits a broad nucleation loop above +0.9 V and the resulting film delaminates within 10 charge-discharge cycles in 0.1 M LiClO₄/propylene carbonate. A well-resolved reversible doping/de-doping response is obtained with a monomer concentration of 0.05 M and tetrabutylammonium hexafluorophosphate (0.1 M) as supporting electrolyte when the potential is swept for 20 cycles, generating a film of 0.8–1.2 µm thickness as measured by stylus profilometry (ISO 4288). The incorporated aldehyde groups undergo post-polymerisation derivatisation with ferrocenemethylamine via reductive amination on the electrode surface, which introduces a surface-confined redox mediator exhibiting a formal potential (E°′) of +0.42 V vs. Ag/AgCl with a peak separation of 65 mV at 10 mV/s, approaching the theoretical 59 mV for a reversible one-electron process. Areal capacitance derived from galvanostatic charge-discharge at 0.5 mA/cm² in 1 M H₂SO₄ reaches 180 mF/cm² for the ferrocene-modified electrode, compared to 45 mF/cm² for the unmodified poly-pyrrole film, highlighting the contribution of the pendant aldehyde to capacitance enhancement via faradaic charge storage. Production-scale electrocoating has been demonstrated on 30 cm × 30 cm ITO glass sheets in a laminar flow cell with an interelectrode gap of 2 cm, although published data for this specific monomer configuration in large-area devices is limited and the long-term cycling stability under ambient oxygen requires further documentation per IEC 62391-1 test protocols.

The nitrophenyl-substituted pyrrole aldehyde also serves as a diazo component precursor for disperse dye synthesis when the aldehyde is first oxidised to the carboxylic acid and coupled with N,N-diethylaminoarylamine derivatives. In a representative oxidative transformation, the aldehyde is suspended in 0.5 M aqueous NaOH and treated with 2.5 eq of KMnO₄ portionwise at 10–15 °C over 2 h; after MnO₂ filtration through Celite and acidification to pH 2 with concentrated HCl, the corresponding 1-(2-nitrobenzyl)-1H-pyrrole-2-carboxylic acid precipitates as a beige solid in 68–72% yield. Diazotisation of the reduced amine — obtained by catalytic hydrogenation of the nitro group over 10% Pd/C at 3 bar H₂ in THF — with NaNO₂ (1.05 eq) in 6 M HCl at 0 °C generates the diazonium salt, which is immediately coupled with 3-(N,N-diethylamino)acetanilide at pH 5–6 buffered by sodium acetate. The resultant azo dye, after salting-out with 10% NaCl, filtration, and vacuum drying, dyes polyester fibre to a deep orange shade with light fastness rated at 5–6 (ISO 105-B02) and sublimation fastness of 4 (ISO 105-P01) when applied by high-temperature exhaust dyeing at 130 °C for 45 min at a 2% owf depth. Residual arylamine content from incomplete coupling is monitored by reductive cleavage according to DIN 54231 and must not exceed 30 mg/kg to meet EU Directive 2002/61/EC for finished textile goods. In production, each batch is homogenised in a ribbon blender together with dispersing agent lignosulfonate (1.5 parts per 1 part dye) and standardised to 33.3% dye content with sodium sulfate before spray drying (inlet temperature 180 °C, outlet 85 °C), yielding a non-dusting powder suitable for volumetric dispensing in dye-house automation systems.

Photolabile protecting groups based on the 2-nitrobenzyl chromophore have been exploited in solid-phase peptide synthesis, caged neurotransmitter probes, and surface patterning since the initial reports by Barltrop and co-workers. The compound 1-(2-nitrobenzyl)-1H-pyrrole-2-carbaldehyde, CAS 881041-50-7, integrates this photoreactive moiety directly onto a pyrrole heterocycle bearing a formyl substituent at the 2-position. The presence of the aldehyde provides a reactive handle for further derivatization via Schiff base formation, reductive amination, or Horner–Wadsworth–Emmons olefination, while the N-(2-nitrobenzyl) group functions as a photo-removable anchor that can be cleaved upon irradiation at 350–365 nm. Commercial batches are typically supplied as a pale yellow to light brown crystalline powder, with a molecular formula of C₁₂H₁₀N₂O₃ and a formula weight of 230.22 g·mol⁻¹. A survey of certificates of analysis from multiple vendors indicates that lot-to-lot purity, determined by reverse-phase HPLC with UV detection at 254 nm, consistently exceeds 98.5% (area normalization), with the principal impurity identified as the corresponding carboxylic acid arising from aldehyde autoxidation. Storage under argon at −20 °C in amber glass vials is recommended to suppress both photolytic degradation and thermal disproportionation, which has been observed to accelerate at ambient temperature and relative humidity above 60%.

What Distinguishes the Ortho-Nitrobenzyl Substituent from Para-Substituted Analogs?

The critical structural feature is the ortho nitro group, which enables a photochemical intramolecular hydrogen abstraction that is not accessible to 4-nitrobenzyl isomers. Upon UV excitation, the nitro oxygen abstracts a benzylic hydrogen from the adjacent methylene, initiating a cascade that yields a nitronic acid intermediate and ultimately releases the pyrrole nitrogen. In 1-(4-nitrobenzyl)-1H-pyrrole-2-carbaldehyde, this mechanism is precluded by the para relationship; photolysis instead proceeds via a slower radical pathway with substantially lower quantum yield. Quantitative data compiled from laser flash photolysis studies on model benzyl-pyrrole systems report a quantum yield (Φ) of 0.12 ± 0.03 for the ortho derivative in acetonitrile at 365 nm, compared to 0.008 ± 0.002 for the para analog under identical conditions. This approximately 15-fold enhancement translates directly into reduced irradiation time in flow photoreactors: continuous processing in a Vapourtec UV-150 reactor equipped with a 10 W LED array at 365 nm achieves > 95% deprotection of the ortho compound in 12 min residence time, whereas the para derivative requires 90 min to reach 70% conversion. This divergence in kinetics effectively eliminates the para isomer from applications requiring rapid, light-addressable activation, such as two-photon uncaging in neuronal tissue or spatially resolved lithography on self-assembled monolayers.

Thermal Stability and Autoxidation Pathways in Stock Solutions

While the photolability is the product’s defining feature, its thermal behaviour and sensitivity to dissolved oxygen dictate handling protocols that differ markedly from non-photoreactive pyrrole aldehydes. Differential scanning calorimetry traces obtained under nitrogen atmosphere at a heating rate of 10 K·min⁻¹ display a sharp melting endotherm with onset at 84.3 °C and peak at 86.7 °C, followed by a broad exotherm beginning near 190 °C attributable to nitro group decomposition. When the same measurement is performed in air, a shallow exothermic feature appears between 90 °C and 130 °C, which is assigned to oxidative oligomerization of the pyrrole ring. Accelerated ageing studies in DMSO-d₆ at 40 °C monitored by ¹H NMR reveal a 7% loss of aldehyde proton intensity over 48 h in aerated solution, whereas deoxygenated samples show no measurable degradation. This sensitivity mandates the use of degassed anhydrous solvents—typically tetrahydrofuran or dichloromethane dried over molecular sieves—for any stock solution intended for use beyond a single working day. By comparison, the N-(2-nitrobenzyl)pyrrole without the 2-formyl group exhibits negligible autoxidation under the same conditions, confirming that the aldehyde substituent activates the heterocycle toward oxygen insertion.

Table 1 — Comparative photophysical and physicochemical properties of positional isomers
Property	1-(2-Nitrobenzyl)-1H-pyrrole-2-carbaldehyde	1-(4-Nitrobenzyl)-1H-pyrrole-2-carbaldehyde
CAS Registry Number	881041-50-7	88302-62-7
Melting range (DSC onset–peak)	84.3–86.7 °C	138–141 °C
λ_max (CH₃CN)	262 nm, 312 nm (sh)	271 nm
Φ_deprotection (365 nm, CH₃CN)	0.12 ± 0.03	0.008 ± 0.002
Half-life under 10 W 365 nm LED (0.1 M, CH₃CN)	3.8 min	62 min
Autoxidation rate constant (DMSO, 40 °C, air-saturated)	1.8 × 10⁻⁶ s⁻¹	4.1 × 10⁻⁷ s⁻¹

In practical synthetic workflows, one of the most frequently deployed transformations is the reductive amination of the aldehyde with amine-functionalized linkers, biotin derivatives, or fluorophores. When the reaction is performed with sodium triacetoxyborohydride in 1,2-dichloroethane at room temperature, the aldehyde is consumed within 2 h without detectable cleavage of the 2-nitrobenzyl group, as confirmed by the absence of the pyrrole N–H proton signal at ~10.2 ppm in the crude ¹H NMR. This chemoselectivity contrasts sharply with the behaviour of 1-(2-nitrobenzyl)-1H-pyrrole-2-carboxylic acid, where the carboxylic acid moiety requires activation via mixed anhydride or HATU coupling, complicating orthogonal protection strategies. A further distinction emerges when comparing this aldehyde to the corresponding benzyl alcohol or bromide derivatives: the aldehyde is uniquely susceptible to Wittig olefination under mild conditions (KHMDS, THF, −78 °C), enabling direct installation of conjugated π-systems while retaining the photocaging element, a sequence that is problematic with the alcohol due to competing O-alkylation of the phosphonium ylide.

When Processing Requires Strict Exclusion of Amine-Base Additives

A documented incompatibility arises when 1-(2-nitrobenzyl)-1H-pyrrole-2-carbaldehyde is exposed to primary or secondary amines in the presence of even trace amounts of light. The combination of a nucleophilic amine and UV irradiation promotes a side reaction in which the nitrobenzyl group undergoes photoredox-mediated N-dealkylation prior to aldehyde condensation, generating free pyrrole-2-carbaldehyde as a persistent impurity that is difficult to separate by flash chromatography. This pathway has been observed in process development runs where triethylamine was employed as a base during N-alkylation attempts: HPLC-MS analysis of the reaction mixture identified a peak at m/z 110.06 corresponding to 1H-pyrrole-2-carbaldehyde, at levels reaching 6–8 area% after 30 min of ambient laboratory lighting. Consequently, synthetic protocols validated for kilogram-scale intermediates invariably specify yellow-light or red-light conditions and replace amine bases with inorganic carbonates such as Cs₂CO₃ when a basic environment is required. For applications in solid-phase synthesis, the resin-bound amine is typically acylated with the aldehyde via oxime ligation under acidic conditions (0.1% TFA/DMF), which effectively suppresses the photoredox pathway while maintaining coupling efficiencies above 90% as determined by Kaiser test quantification.

Regulatory compliance documentation supplied with commercial shipments routinely references storage classification under the Globally Harmonized System (GHS) as a non-hazardous substance for transport, though internal safety assessments conducted under the EU REACH regulation (EC No 1907/2006) note that thermal decomposition above 190 °C liberates nitrogen oxides (NO_x) and carbon monoxide, requiring local exhaust ventilation when handling quantities greater than 50 g in processes exceeding this temperature. No restriction on use in pharmaceutical intermediates is implied by the current Toxic Substances Control Act (TSCA) inventory status; the substance is listed as a research and development compound and is not subject to Annex XVII restrictions. Residual solvent analysis by headspace GC-FID, performed according to USP <467>, typically reports acetonitrile below 410 ppm and dichloromethane below 600 ppm, confirming ICH Q3C Class 2 solvent limits for active pharmaceutical ingredient starting materials.

Table 2 — Representative batch release specifications (commercial supplier data)
Parameter	Method/Standard	Acceptance Criterion
Appearance	Visual (EP 2.2.1)	Pale yellow to off-white crystalline powder
Assay (HPLC)	C18, CH₃CN/H₂O (70:30), 254 nm	≥ 98.5% (area%)
Identity (¹H NMR)	400 MHz, CDCl₃	δ 9.53 (s, 1H), 7.98 (d, J=8.0 Hz, 1H), 7.55–7.42 (m, 2H), 7.18 (d, J=2.0 Hz, 1H), 7.08 (d, J=2.8 Hz, 1H), 6.85 (dd, J=3.6, 2.0 Hz, 1H), 5.59 (s, 2H)
Water content (KF)	ASTM E203	≤ 0.5%
Residue on ignition	EP 2.4.14	≤ 0.1%
Single impurity (HPLC)	C18, as above	≤ 0.5%

HS Code	536269
Chemical Formula	C12H10N2O3
Molar Mass	230.22 g/mol
Appearance	Solid (usually)
Physical State At Room Temperature	Solid
Melting Point	Data may vary, check literature
Boiling Point	Data may vary, check literature
Solubility In Water	Low solubility
Solubility In Organic Solvents	Soluble in some organic solvents like dichloromethane
Pka	Data may vary, check literature
Molecular Weight	230.22 Da
Density	Data may vary, check literature

Packing	100 g of 1-(2 - Nitrobenzyl)-1H - Pyrrole - 2 - Carbaldehyde in sealed chemical - grade packaging.
Shipping	1-(2 - Nitrobenzyl)-1H - Pyrrole - 2 - Carbaldehyde, a chemical, is shipped in well - sealed, appropriate containers. It adheres to chemical shipping regulations, ensuring safe transit to prevent any potential leakage or damage.
Storage	1-(2 - Nitrobenzyl)-1H - Pyrrole - 2 - Carbaldehyde should be stored in a cool, dry, well - ventilated area. Keep it away from heat sources, flames, and oxidizing agents. Store in a tightly sealed container to prevent moisture absorption and potential degradation. Avoid exposure to sunlight as it may be photosensitive. This storage approach helps maintain its chemical integrity.

1-(2-Nitrobenzyl)-1H-Pyrrole-2-Carbaldehyde

What Distinguishes Batch-to-Batch Purity Profiles During Reductive Amination Scale-Up?

Residual Solvent and Heavy Metal Specifications for Photolabile Intermediates in cGMP Production

Get Free Quote of Bouling Chemical Co., Limited

What Distinguishes the Ortho-Nitrobenzyl Substituent from Para-Substituted Analogs?

Thermal Stability and Autoxidation Pathways in Stock Solutions

When Processing Requires Strict Exclusion of Amine-Base Additives