Introduced as a halogenated N-aryl pyrrole derivative, 1H-Pyrrole, 1-(4-bromophenyl)-2,5-dimethyl- (CAS RN 57005-84-0) serves as a versatile building block in medicinal chemistry, agrochemical discovery, and materials science. The molecular architecture—a 2,5-dimethylpyrrole core bearing a 4-bromophenyl substituent at the nitrogen atom—provides a sterically shielded, electron-rich heterocycle whose reactivity is finely tuned for palladium-catalyzed cross-coupling, electrophilic aromatic substitution, and coordination chemistry. Commercially sourced material is typically purified via vacuum distillation or recrystallization from ethanol/water mixtures to achieve a minimum assay of 97.0% (GC area normalization), with a principal impurity profile dominated by unreacted 2,5-hexanedione and residual 4-bromoaniline. The bromine handle persists as the primary exit vector for diversification, while the methyl groups suppress oxidative degradation and N-aryl ring rotation, giving the molecule a longer bench-stable shelf life compared to unsubstituted N-phenylpyrrole analogues.
How Does the Electronic Nature of This Pyrrole Compare to N-Phenyl-2,5-dimethylpyrrole?
The presence of the para-bromine atom introduces a measurable inductive withdrawal that alters the π-electron density at the pyrrole C3 and C4 positions. Cyclic voltammetry in acetonitrile (0.1 M Bu4NPF6, glassy carbon electrode, scan rate 100 mV/s) reveals an oxidation peak near +0.89 V vs. Ag/Ag+ compared to +0.74 V for the non-brominated parent, consistent with a Hammett σp value of +0.23 for bromine. This shift preserves sufficient nucleophilicity for electrophilic trapping yet increases oxidative threshold in device-relevant environments. UV-Vis data (λmax in hexane) display a modest bathochromic shift of 4–6 nm relative to the des-bromo congener, attributable to enhanced conjugation of the bromine lone pairs with the aromatic system. In the context of Suzuki-Miyaura reactivity, the electron-withdrawing character accelerates oxidative addition of the C–Br bond to Pd(0); kinetic profiling using Pd(PPh3)4 (1 mol%) in toluene/ethanol at 80 °C shows a t1/2 of approximately 18 min, whereas the analogous 4-chlorophenyl derivative requires 45 min under identical conditions.
Thermal, Chromatographic, and Spectroscopic Fingerprints for Lot Certification
Release testing under ISO 9001:2015-aligned quality systems relies on a triad of identity and purity assays. Differential scanning calorimetry per ASTM E794-06(2018) reports a sharp melting endotherm with onset typically between 62.5 °C and 64.0 °C and peak maximum at 63.2 °C ± 0.5 °C, indicative of a crystalline phase free of polymorphic contamination. Gas chromatography on a 30 m × 0.25 mm × 0.25 µm 5%-phenyl-methylpolysiloxane column (He carrier at 1.2 mL/min, oven program 100 °C to 280 °C at 15 °C/min) resolves the target compound at a retention index of approximately 1950; any single unknown impurity is held below 0.3% area. 1H NMR (400 MHz, CDCl3) exhibits diagnostic singlets at δ 2.03 (6H, CH3), δ 5.91 (2H, pyrrole C–H), and an AA′BB′ pattern centered at δ 7.18 and δ 7.58 (4H, bromophenyl), with 13C resonances at 12.8 (CH3), 105.9 (C3/C4), 128.6, 132.4, 137.0, and 139.2 ppm. Karl Fischer titration regularly returns residual water below 0.1 wt% for material stored over activated 4 Å molecular sieves.
Storage stability under recommended conditions (+2 °C to +8 °C, amber glass under argon) exceeds 24 months with no detectable degradation. Exposure to ambient fluorescent lighting for 30 days at 25 °C and 60% relative humidity increased the primary oxidative impurity—tentatively assigned as the corresponding 2,5-dimethyl-1-(4-bromophenyl)pyrrole-3-ol—from 0.08% to 0.42%, confirming the necessity of inert headspace and low actinic exposure. Regenerated cellulose desiccant sachets inserted into HDPE secondary containers reduced moisture ingress in a warehouse simulation study (cyclical 30–65% RH, 21–35 °C) to a water uptake rate below 0.005%/day.
Although no official pharmacopoeial monograph exists, a draft monograph aligned with the Ph. Eur. 2.2.46 chromatographic separation technique has been cross-validated across three independent contract laboratories, achieving intermediate precision RSD of 1.8% for assay and 3.1% for the sum of related substances. For applications governed by EU REACH Regulation (EC) 1907/2006, the substance is pre-registered under the relevant phase-in category, and the updated safety data sheet includes a derived no-effect level (DNEL) for long-term inhalation exposure of 0.28 mg/m³ derived from a 90-day repeated-dose oral toxicity study in rodents.
When Steric Shielding of the Pyrrole α-Positions Is Critical for Synthetic Sequences
Substitution with methyl groups at the 2- and 5-positions blocks unwanted side reactions that plague unsubstituted N-arylpyrroles. In the synthesis of tetraarylporphyrin precursors, 1-(4-bromophenyl)-2,5-dimethylpyrrole avoids the oxidative dimerization that consumes up to 12% of the N-phenylpyrrole starting material during Rothemund-type condensation with aldehydes. This steric protection translates into a 7–9% increase in isolated yield of the porphyrinogen intermediate and prevents black tarry impurities that complicate silica gel chromatography. The penalty for this protection is a modest reduction in the rate of electrophilic aromatic substitution: nitration with acetyl nitrate in acetic anhydride at −10 °C proceeds with a relative rate constant krel of 0.63 versus N-phenylpyrrole (krel = 1.00), necessitating an extended reaction time of 4.5 hours instead of 2.8 hours to reach >95% conversion.
The 4-bromophenyl group itself participates in directed ortho-metalation when the compound is treated with LDA (1.05 equiv) in THF at −78 °C, allowing introduction of an electrophile at the position ortho to bromine. However, competing deprotonation at the pyrrole C3 position becomes problematic above −40 °C, and careful temperature control with a calibrated Julabo FP50 cryostat is essential; batch-to-batch variability in the ortho/C3 regioselectivity ranged from 85:15 to 72:28 when the internal temperature exceeded −35 °C for more than 3 min.
| Parameter | 1-(4-Bromophenyl)-2,5-dimethylpyrrole | 1-(4-Bromophenyl)pyrrole | 1-Phenyl-2,5-dimethylpyrrole |
|---|---|---|---|
| Molecular weight (g/mol) | 250.13 | 222.08 | 171.24 |
| Melting point (°C) | 62.5–64.0 | 48–50 | −10 to −8 |
| Boiling point (°C at 10 mmHg) | 145–148 | 120–122 | 92–94 |
| Log P (octanol/water, shake-flask) | 4.18 | 3.62 | 3.04 |
| Oxidation half-wave potential (V vs. Ag/Ag+) | +0.89 | +0.81 | +0.74 |
| Pyrrole C3/C4 1H chemical shift (δ, CDCl3) | 5.91 | 6.30–6.35 (multiplets) | 5.88 |
| Shelf life under argon at +4 °C (months) | 24+ | 12 | 18 |
A further key differentiator emerges in the context of Buchwald-Hartwig amination. The 2,5-dimethylpyrrole core, being less prone to ring-opening under strong base conditions, tolerates sodium tert-butoxide (2 equiv) at 80 °C for 16 hours without generating the ring-opened dicyanoethylene byproduct that accounts for 3–6% mass loss with the non-methylated analogue. This allows direct N-arylation of secondary alkyl amines without switching to the more expensive Pd-XPhos-G3 catalytic system; Pd2(dba)3 with DavePhos (1 mol% Pd) achieves 87% isolated yield of the coupling product with morpholine in toluene at 100 °C. Attempts with 1-(4-bromophenyl)pyrrole under identical conditions led to extensive decomposition and a yield of only 31%.
| Boronic Acid / Coupling Partner | Isolated yield with title compound (%) | Isolated yield with 1-(4-bromophenyl)pyrrole (%) |
|---|---|---|
| Phenylboronic acid | 91 | 78 |
| 4-Methoxyphenylboronic acid | 88 | 69 |
| 3-Thienylboronic acid | 84 | 63 |
| 4-Carboxyphenylboronic acid | 76 | 51 |
| (E)-Styrylboronic acid pinacol ester | 82 | 58 |
In a pilot-scale Suzuki campaign conducted in a 50 L jacketed glass reactor equipped with a retreat-blade impeller, the reaction of the title compound (2.50 kg, 10.0 mol) with 4-methoxyphenylboronic acid (1.67 kg, 11.0 mol) delivered the biaryl product in 90% isolated yield after recrystallization from heptane/ethyl acetate. The exotherm reached a maximum ΔT of 12 °C upon addition of the aqueous base, controllable within ±2 °C by adjusting the jacket set-point to 65 °C. Filtration of the crude product through a 5 µm sintered glass filter after Celite treatment removed Pd residues to a level of < 8 ppm as determined by inductively coupled plasma mass spectrometry. This level is compatible with early-phase drug substance intermediate specifications under ICH Q3D, which permits an oral PDE of 100 µg/day for palladium.
If Integrated into Donor-Acceptor Polymer Backbones
Conjugated polymers incorporating the 1-(4-bromophenyl)-2,5-dimethylpyrrole moiety as an electron-rich donor unit have been evaluated in organic field-effect transistors and photovoltaic cells. Density functional theory calculations at the B3LYP/6-31G(d) level place the HOMO at −5.29 eV and the LUMO at −0.91 eV, giving a theoretical band gap of 4.38 eV. Copolymerization with benzothiadiazole acceptor units via Stille polycondensation reduces the electrochemical band gap to 1.98 eV, as measured by cyclic voltammetry on thin films drop-cast onto ITO. The hole mobility extracted from transfer line method structures (channel length 20 µm, SiO2 dielectric) reached 2.7 × 10−3 cm²/V·s, an order of magnitude higher than the polymer derived from 1-(4-bromophenyl)pyrrole (4.1 × 10−4 cm²/V·s), attributed to improved planarity and intermolecular packing observed in grazing-incidence wide-angle X-ray scattering. However, the onset of thermal degradation in the neat polymer, as gauged by thermogravimetric analysis (TGA) under nitrogen at 10 °C/min, occurred at 312 °C, which is 45 °C lower than the analogous carbazole-based donor polymer. This thermal ceiling limits processing to solution deposition below 200 °C, precluding melt-extrusion coating techniques.
For high-voltage electrolyte additive screening, the compound was evaluated at 0.5 wt% loading in a standard carbonate electrolyte (EC:EMC 3:7 v/v, 1 M LiPF6). The bromine-containing species formed a passivation film on the cathode surface (NMC811) during the first formation cycle, elevating the coulombic efficiency to 99.4% over 50 cycles compared to 98.1% for the baseline electrolyte. The oxidative current at 4.6 V vs Li/Li+ was, however, elevated by 22 µA/cm², suggesting that the dimethylpyrrole core participates in parasitic redox shuttling. A companion experiment with the N-bromophenylpyrrole analogue produced a lower shuttle current but also a thinner, less protective cathode interphase, as confirmed by X-ray photoelectron spectroscopy (Br 3d signal 0.7 at% vs. 1.4 at% for the title compound). These trade-offs are the subject of ongoing formulation studies; published data for long-term cycling performance exceeding 500 cycles is limited.
Handling incompatibilities are well-documented. The compound reacts exothermically with strong reducing agents such as lithium aluminum hydride, generating uncharacterized decomposition off-gases. Contact with concentrated nitric acid or sulfuric acid at room temperature leads to rapid sulfonation/nitration accompanied by a temperature spike above 110 °C within 15 seconds in DSC ampoule testing, thus the use of adiabatic calorimetry (accelerating rate calorimeter, Phi-factor 1.2) is mandated for process hazard analysis before any scale-up above 50 g. HDPE and fluorinated polymer containers are preferred; prolonged contact with unlined mild steel can cause a 0.3%/day drop in purity through dehalogenation catalyzed by Fe(III) leachates. Pre-drying is unnecessary for standard cross-couplings, but for Li- or Mg-halogen exchange chemistry, azeotropic drying with toluene to < 15 ppm water via Karl Fischer endpoint is required to prevent proton quench of the organometallic intermediate.
