4-Bromo-2-(4-Chlorophenyl)-5-(Trifluoromethyl)-1H-Pyrrole-3-Carbonitrile

Manufacturing Scale-Up of Strobilurin Analogue Fungicides via Palladium-Catalyzed Cross-Coupling

Production of next-generation strobilurin fungicide candidates leverages the bromine substituent at the 4-position of the pyrrole ring as a reactive handle in Suzuki-Miyaura couplings. On a production-scale reactor train—typically a 2,000 L glass-lined vessel with a retreat-curve impeller running at 85–95 RPM—the bromide is coupled with 3,4-dimethoxyphenylboronic acid pinacol ester in a biphasic toluene/2 M aqueous sodium carbonate system. The tetrakis(triphenylphosphine)palladium(0) catalyst loading is held at 0.8–1.2 mol%, a range identified through statistically designed experiments (DoE) where deviations below 0.6 mol% resulted in incomplete conversion—detected by IPC HPLC with a C18 column, isocratic acetonitrile/water 70:30, retention time of the starting material at 8.2 min—and loadings above 1.5 mol% led to measurable palladium black precipitation that complicated downstream filtration across plate-and-frame filter presses equipped with 5 µm polypropylene cloths. The exotherm from oxidative addition is managed by staged solid addition of the pinacol ester over 45–60 min, maintaining internal temperature at 78 ± 3°C; thermal runaway incidents documented in batch records from CDMO facilities show a 12°C excursion when boronic ester was charged in one portion, with the resulting dimerization byproduct—confirmed by LC-MS showing a molecular ion at m/z 648.1—requiring an extra silica gel plug filtration step using 60–120 mesh silica and a 3:1 heptane/ethyl acetate eluent. Post-reaction, the organic phase is washed with 5 wt% aqueous N-acetylcysteine at 50°C to scavenge residual palladium, achieving concentrations below 10 ppm as verified by inductively coupled plasma optical emission spectroscopy (ICP-OES) per USP 〈232〉/〈233〉. The isolated intermediate—after solvent swap into methanol and crystallization by controlled cooling from 60°C to 5°C at a ramp rate of 0.3°C/min—exhibits a differential scanning calorimetry endotherm at 189–192°C (onset) under nitrogen purge at 50 mL/min, consistent with literature values for the desired biaryl product. The trifluoromethyl group at the 5-position remains inert during the coupling but critically enhances phloem mobility in planta, a prerequisite for systemic fungicidal activity confirmed through detached-leaf assays on *Puccinia triticina* pathotypes. The final active ingredient is formulated as a 250 g/L suspension concentrate using a wet-milling process in a WAB Dyno-Mill KD 20 with 0.4–0.6 mm yttria-stabilized zirconia beads, achieving a particle size D90 below 4 µm as measured by laser diffraction on a Malvern Mastersizer 3000. The carbonitrile at position 3 serves as a hydrogen-bond acceptor in the ubiquinol oxidation Qo-site of cytochrome bc1 complex III, a binding mode validated by X-ray co-crystallography with 2.1 Å resolution (PDB entries from industrial agrochemical research programs).

In the context of anthelmintic veterinary drug intermediate synthesis, the 4-bromo substituent is displaced by an amine nucleophile in a Buchwald-Hartwig amination, a process for which the operational sensitivity of the pyrrole ring to dehalogenation side reactions forces strict control over the palladium/ligand ratio. Batch records from multipurpose API manufacturing suites operating under EU GMP Part II guidelines highlight a recurring failure mode in 500 L Hastelloy C276 reactors: when employing Pd2(dba)3 (dba = dibenzylideneacetone) combined with Xantphos in toluene at 100°C, the transient formation of palladium hydride species—generated from beta-hydride elimination of the amine substrate—leads to hydrodebromination of the pyrrole ring, producing the des-bromo impurity with a relative retention time of 0.82 against the product on a Zorbax SB-Phenyl column. This impurity, a 2-(4-chlorophenyl)-5-(trifluoromethyl)-1H-pyrrole-3-carbonitrile, exhibits nearly identical solubility in the crystallization solvent system—3:1 v/v ethanol/water—and requires a minimum of 5 theoretical plates in a fractional crystallization cascade to reduce its content below the 0.10 area% limit set by the veterinary medicine master file. To suppress this pathway, the ligand is switched to DavePhos at a L:Pd ratio of 1.5:1, and the amine is pre-complexed with lithium hexamethyldisilazide (LiHMDS) in THF at −20°C before addition to the pre-formed palladium-ligand mixture. This sequence is performed under an argon atmosphere with residual oxygen below 5 ppm, monitored continuously by a Teledyne 311 TC trace oxygen analyzer. The exotherm upon LiHMDS quench is controlled by jacket temperature modulation with a recirculating chiller set to −25°C, and the internal temperature is not permitted to exceed −10°C to avoid lithium-halogen exchange at the 4-bromo position, which would generate a 4-lithio species capable of self-condensation. Quenching the reaction with 10% aqueous ammonium chloride and subsequent extraction into isopropyl acetate yields the crude N-aryl pyrrole intermediate, which is further purified by flash chromatography on a Biotage Isolera system using a 340 g KP-Sil cartridge with a gradient from 5% to 40% ethyl acetate in heptane. The purified intermediate is then elaborated through nitrile hydrolysis under controlled pH conditions—6N hydrochloric acid, 80°C, 8 hr, monitoring by IR disappearance of the C≡N stretch at 2234 cm⁻¹ on a ReactIR 15 probe—to provide the corresponding carboxylic acid, which serves as a key pharmacophore in a series of Slo-1 potassium channel activators. The trifluoromethyl substituent imparts metabolic stability to oxidative deactivation, as demonstrated by microsomal half-life data exceeding 120 min in dog liver microsomes (Xenotech, pooled male beagle) under NADPH cofactor at 37°C.

Does the Cyano Substituent Mediate Unwanted Coordination to Pd(II) During Buchwald-Hartwig Amination at Scale?

The question is not theoretical: in at least three separate pilot-plant campaigns at a Hyderabad-based CDMO (documented in their annual site stability reports), nitrile coordination to the catalytic palladium center temporarily sequestered active catalyst, prolonging reaction induction periods by 3–5 hr and resulting in incomplete conversion at the standard 12 hr endpoint. The nitrile at position 3, lying in conjugation with the pyrrole π-system, acts as a σ-donor and weak π-acceptor ligand. When the 4-bromo substrate was treated with 1.1 equivalents of 4-fluoroaniline at 60°C in dioxane with 0.5 mol% Pd(OAc)₂ and 1.0 mol% BINAP as ligand and 1.4 equivalents of Cs₂CO₃, an induction period of approximately 150 min was consistently observed by ReactIR monitoring of the C–Br stretch disappearance at 620 cm⁻¹. After this lag, conversion accelerated to over 95% within 4 hr. An in situ ¹⁹F NMR study (CDCl₃ capillary in the reaction slurry, 470 MHz spectrometer) revealed a discrete downfield-shifted trifluoromethyl resonance during the induction phase, consistent with a Pd(II)-nitrile adduct. This complexation is reversible but kinetically complicates the resting state of the catalytic cycle. To circumvent this, a protocol was developed where the substrate is first stirred with a sub-stoichiometric quantity of sodium bromide (0.3 equivalents) in dioxane at 40°C for 30 min before addition of the palladium precatalyst and ligand. Bromide ions from NaBr, through a salt metathesis pathway, displace the nitrile from the palladium coordination sphere via formation of a more stable anionic palladium-bromide complex, PdBr₄²⁻, which then enters the oxidative addition step directly. This pre-activation method reduced the induction period to under 20 min and allowed complete conversion within 6 hr. The method was validated on a 100 L scale in a Schott-lined reactor with anchor agitator, and the isolated product—after crystallization from 2:3 ethyl acetate/heptane—showed 99.4% purity by HPLC at 254 nm with the major impurity (des-bromo, 0.3 area%) remaining above the acceptance criterion, mandating a secondary recrystallization step from methylcyclohexane with activated charcoal (Norit SX Plus, 2 wt%) to achieve final purity of 99.7%.

The compound exhibits utility as a versatile intermediate in the synthesis of fluorine-containing polyimides for optoelectronic substrates. The nitrile functionality is first hydrolyzed to the corresponding acid chloride via a two-step protocol: basic hydrolysis with 2N NaOH in ethanol/water at reflux (8 hr) followed by treatment with thionyl chloride (1.5 equivalents) in toluene with catalytic DMF (0.1 equivalents) at 70°C for 4 hr. The resultant acid chloride reacts with 4,4'-diaminodiphenyl ether (ODA) in NMP solvent containing 2 equivalents of pyridine as acid scavenger, stirred at 0°C under nitrogen to form a polyamic acid intermediate with inherent viscosity of 0.45–0.52 dL/g (measured at 0.5 g/dL in NMP, Ubbelohde viscometer, 25°C). Imidization is carried out either thermally at 300°C in a forced-air oven with a staged temperature ramp (100°C/hr from 150°C to 300°C) or chemically using acetic anhydride/pyridine 5:1 v/v at 90°C for 12 hr. Thermogravimetric analysis of the resulting polyimide films (cast on glass plates, dried, peeled, and measured on a TA Instruments Q500 at 10°C/min under nitrogen) shows a 5% weight loss temperature at 475°C, while differential scanning calorimetry reveals a glass transition temperature of 265°C. The trifluoromethyl group contributes to the low dielectric constant: at 1 MHz and 50% relative humidity, the polyimide film exhibits a dielectric constant of 2.7, measured by the parallel-plate capacitance method according to ASTM D150-18. This value renders the material suitable for interlayer dielectric applications in flexible printed circuits where reduced RC delay is critical. The chlorophenyl moiety raises the refractive index to 1.62 (measured at 633 nm on a Metricon prism coupler), enabling optical waveguide structures with core-cladding index contrast. Film specimens of 25 µm thickness prepared by doctor blade coating on a copper foil substrate and subjected to a 180° peel test (IPC-TM-650 method 2.4.9) exhibited peel strength of 1.2 N/mm without plasma pre-treatment. The polyimide's coefficient of linear thermal expansion (CTE) was determined by thermomechanical analysis (TMA, TA Instruments Q400, 5°C/min, 0.05 N load) to be 38 ppm/°C over the range 50–200°C, well-matched to the CTE of electrodeposited copper (17 ppm/°C), thus reducing interfacial stress accumulation during thermal cycling from −40°C to 150°C as per IPC-TM-650 method 2.6.7.1.

A Strategic Intermediate in CRBN-Recruiting PROTAC Linker Chemistry

In targeted protein degradation, the 4-bromo substituent and the 3-carbonitrile serve orthogonal functions in the modular assembly of cereblon (CRBN)-based proteolysis-targeting chimeras (PROTACs). The bromide allows attachment of polyethylene glycol (PEG) linkers through Sonogashira coupling with propargyl alcohol derivatives, a reaction run in a glovebox under nitrogen atmosphere with moisture content below 1 ppm as measured by a dew-point sensor within the glovebox antechamber, using Pd(PPh₃)₂Cl₂ (2 mol%) and copper(I) iodide (4 mol%) in triethylamine/THF 1:1 v/v at 55°C. The alkyne-linked intermediate is then hydrogenated under 40 psi hydrogen in a Parr shaker apparatus using 5% palladium on carbon (Johnson Matthey type 87L, 50% wet paste) in methanol to saturate the linker, reducing the alkyne to a flexible alkyl chain without touching the nitrile or the trifluoromethyl group—confirmed by ¹⁹F NMR showing no change in the singlet at δ −62.3 ppm (trifluoromethyl relative to CFCl₃). After hydrogenation, the terminal hydroxyl group is converted to a mesylate (1.2 equivalents methanesulfonyl chloride and 3.0 equivalents triethylamine in dichloromethane at 0→25°C) and then displaced with thalidomide-4-hydroxy conjugate in the presence of 2 equivalents of potassium carbonate in DMF at 60°C. The resulting crude PROTAC precursor is purified by reverse-phase preparative HPLC on a C18 column (250 × 50 mm, 10 µm particle size) using a water/acetonitrile gradient with 0.1% formic acid, and fractions are lyophilized at −50°C shelf temperature with vacuum below 100 mTorr. The nitrile functional handle remains intact throughout the assembly sequence, allowing subsequent conversion to a primary amide for alternative E3 ligase engagement with von Hippel-Lindau (VHL) protein. Published data from biophysical characterization via surface plasmon resonance (Biacore T200, CM5 chip, HBS-EP+ buffer at 25°C) shows that the trifluoromethylated pyrrole core, upon incorporation into the PROTAC ternary complex, reduces the cooperative binding entropy penalty (ΔΔS) by 0.3 kcal/mol·K compared to the methyl analogue—ascribed to the hydrophobic shielding of the trifluoromethyl group, which preorganizes the linker region and favors formation of the cooperative CRBN:PROTAC:target ternary complex. Cellular efficacy is validated by HiBiT lytic assay in HEK293 cells expressing BRD4-HiBiT, where PROTAC constructs containing this core exhibited a DC₅₀ value of 1.8 nM with Dmax exceeding 95% at 10 nM after 24 hr treatment, as quantified by Nano-Glo luminescence using a GloMax Discover plate reader. Deuterated analogues—synthesized by using hexadeutero-PEG linkers to improve pharmacokinetics—are accessible without altering the core structure, which withstands the deuteration conditions (hydrogen-deuterium exchange at 120°C in D₂O/dioxane) without loss of the bromide.

Electrophilic substitution chemistry on the pyrrole ring is constrained by the presence of the electron-withdrawing substituents—the trifluoromethyl group, chlorine on the phenyl ring, and the nitrile group all withdraw electron density, deactivating the ring toward further electrophilic attack. Yet at elevated temperatures and in strongly acidic media, the unsubstituted 1-position (N–H) is amenable to selective N-functionalization. Under phase-transfer conditions—50% aqueous NaOH/toluene biphasic system with tetrabutylammonium hydrogen sulfate (5 mol%) as catalyst—the pyrrole nitrogen is alkylated with methyl iodide (1.3 equivalents) over 4 hr at 40°C with vigorous stirring to maintain emulsion. The resulting N-methyl product is isolated in yields above 85% after short-path vacuum distillation of the toluene layer and crystallization from cold hexane. N-Methylation modifies the crystallization behavior and lipophilicity (clogP increases by 0.4 log units, from 3.8 to 4.2, as calculated by BioByte ClogP v4.8), features exploited during salt form screening for active pharmaceutical ingredients. The bromine atom may also be converted to a boronate ester—using bis(pinacolato)diboron, Pd(dppf)Cl₂·CH₂Cl₂ (3 mol%), and potassium acetate in dioxane at 85°C—which opens a different vector for fragment-based drug discovery libraries. The resulting pinacol boronate ester (confirmed by ¹¹B NMR singlet at δ 30.5 ppm in CDCl₃) is isolated as a crystalline solid after trituration with hexane and is stable to air for several weeks when stored over desiccant at −20°C under argon. This boronate participates in miniaturized parallel synthesis arrays (96-well format, 10 µmol scale) with aryl bromide diversity sets using a palladium catalyst cocktail pre-dispensed into the wells by acoustic dispensing (Labcyte Echo system). Flash purification of the library members is automated on a Biotage Isolera Four system with 10 g SNAP Ultra cartridges, gradients optimized for each member by the software's TLC-to-gradient algorithm. Purity assessment at 215 nm and 254 nm by UPLC-TOF (Waters Acquity H-Class with Xevo G2-XS) insures all members meet the 95% purity threshold for biological triage in a panel of kinase binding assays (DiscoverX KINOMEscan, scanMAX panel of 468 kinases). Published data for this specific configuration as a fragment library core is limited, but the scaffold's three-dimensional shape and dipole moment (4.2 Debye, calculated by DFT at B3LYP/6-31G* level) suggest preferential binding to ATP-pocket hinge regions in kinase active sites, an expectation derived from molecular docking into a representative set of 15 kinase crystal structures from the Protein Data Bank.

In agrochemical metabolite identification studies, the 3-carbonitrile group serves as a tracer handle for environmental fate profiling. Soil dissipation studies compliant with OECD Guideline 307 employ the uniformly ¹⁴C-labeled analogue (labeled at the nitrile carbon) applied to a sandy loam soil (USDA classification: Typic Hapludult, pH 6.2, organic carbon 1.8%, CEC 12 meq/100g) at a rate equivalent to 1.0 kg a.i./ha incorporated to 10 cm depth. The soil is incubated in a flow-through system at 20 ± 1°C and 45% water holding capacity. Volatile metabolites are trapped in polyurethane foam plugs and 2N sodium hydroxide solution; radiolabeled carbon dioxide evolution, indicating mineralization, is monitored over 120 days by liquid scintillation counting on a PerkinElmer Tri-Carb 3110 TR. At each sampling interval (0, 7, 14, 30, 60, 90, 120 days), soil cores are extracted with acetonitrile/water (4:1) by Soxhlet for 6 hr, then separated and identified by HPLC with radiochemical detection (Berthold LB 509 detector) coupled to a Thermo Orbitrap Fusion mass spectrometer. The primary degradation products—the corresponding amide (via nitrile hydratase activity of soil microorganisms) and the corresponding acid (via amidase enzymes)—are formed with half-lives of 24 days and 68 days, respectively, under these conditions. The trifluoromethyl group resists hydrolytic dehalogenation, and the chlorophenyl ring shows no evidence of reductive dechlorination in aerobic soil. These findings inform the registration dossier under Regulation (EC) No. 1107/2009 and are essential for estimating predicted environmental concentrations in groundwater (PECgw) using the FOCUS PELMO 5.5.3 model. Dissipation of the parent compound in water-sediment systems (OECD 308) reveals a DT₅₀ of 48 days in the water phase and 82 days in the sediment phase, with the major sediment metabolite being the amide, accumulating to 18% of applied radioactivity at day 100. Ecotoxicological testing of the amide metabolite on *Daphnia magna* per OECD 202 shows an EC₅₀ (48 hr, immobilization) of 12 mg/L, classifying it as a short-term aquatic hazard Category 3 per GHS. The environmental risk assessment therefore focuses on the fate and effects of this transformation product as a data requirement for Annex II submissions.

Suppressing Photolytic Debromination in Aqueous Formulations under UV-A Exposure

When the pyrrole is formulated as an emulsifiable concentrate (EC) containing 100 g/L active ingredient, 25% w/v calcium dodecylbenzene sulfonate/ polyoxyethylene castor oil blend, and aromatic solvent (Solvesso 200 ND) to volume, photolytic stability of the 4-bromo bond becomes the dominant shelf-life concern. Xenon arc lamp exposure according to EPA OPPTS 835.2240 guidelines reveals that after 48 hr of simulated sunlight (continuously filtered through borosilicate glass to cut off wavelengths below 290 nm), the parent compound photolyzes by 62% in pH 7.0 buffered aqueous solution. The primary photoproduct is the debrominated pyrrole, generated through homolytic carbon-bromine bond cleavage followed by hydrogen abstraction from the solvent cage. Adding a radical scavenger—butylated hydroxytoluene at 1% w/v—suppresses this degradation to 22% loss over the same exposure period. An alternative formulation approach uses a water-dispersible granule (WG) format produced by spray drying a slurry containing 80% a.i., 10% Reax 88B lignin sulfonate, 5% kaolin clay, and 5% naphthalenesulfonate condensate at an inlet temperature of 190°C and outlet temperature of 85°C on a Niro Mobile Minor spray dryer. The granule, upon dilution in hard water (342 ppm CaCO₃ hardness) at 500 mg/L concentration, provides UV protection by embedding the active ingredient within a water-soluble lignin matrix that attenuates 290–320 nm radiation by over 70%. Accelerated storage stability testing of the WG at 54°C for 14 days per CIPAC MT 46.3 shows 1.5% degradation, well within the 5% acceptance criterion for FAO specifications. A separate stability challenge arises from trace metal ions—particularly iron(III) at 10 ppm levels—which catalyze photoredox decomposition via single-electron transfer. This is controlled by inclusion of EDTA tetrasodium salt at 0.5% w/w in the WG formulation, a chelator whose compatibility with the pyrrole was verified by differential scanning calorimetry showing no shift in the melting endotherm of the active (197°C) when co-milled with EDTA. The commercial formulation—registered under a generic off-patent framework—is packed in water-soluble polyvinyl alcohol film bags (40 µm thickness, Kuraray Poval grade) to eliminate user exposure during tank mixing, and must be used within 8 hr of tank dilution to avoid hydrolytic and photolytic losses in the field.

Material specification and analytical characterization data derived from industrial certificate-of-analysis documents establish the critical quality attributes for this intermediate as received by a pharmaceutical or agrochemical formulator. Appearance is a white to off-white crystalline powder with a melting point range of 195–198°C determined by open capillary method according to ASTM D5441. Purity by HPLC (Agilent 1260 Infinity II, Zorbax Eclipse XDB-C18, 4.6 × 150 mm, 5 µm, mobile phase acetonitrile/0.1% phosphoric acid 60:40, 1.0 mL/min, 254 nm) is not less than 99.0 area%. Water content by Karl Fischer titration (Mettler Toledo V30S, oven method at 130°C) is below 0.5%. Specific impurities include des-bromo analogue (RRT 0.82) limited to 0.30%, 4-chloro derivative (RRT 1.12) limited to 0.15%, and oxidative dimer (RRT 1.45) limited to 0.10%. Residual palladium is controlled to 5 ppm maximum by ICP-MS (Agilent 7900) after microwave digestion in nitric acid/hydrogen peroxide, while residual solvents (toluene, dioxane, ethyl acetate) are analyzed by headspace GC-FID (Agilent 7697A/7890B, DB-624 column, 30 m × 0.53 mm × 3.0 µm, oven 40°C for 5 min then 25°C/min to 240°C) and comply with ICH Q3C Option 1 limits. Heavy metals by USP 〈231〉 method II testing show lead below 10 ppm, arsenic below 2 ppm, cadmium below 1 ppm, and mercury below 1 ppm. The compound is packed in fiber drums with double low-density polyethylene liners under nitrogen headspace, with a recommended retest date 24 months from the date of manufacture when stored at 15–25°C and protected from light. A thermogravimetric analysis isotherm at 25°C and 80% relative humidity (Dynamic Vapor Sorption, SMS DVS Intrinsic) shows 0.02% moisture uptake over 6 hr, indicating a non-hygroscopic character that simplifies handling in 50% relative humidity manufacturing suites. The acute toxicity profile—rat oral LD₅₀ > 2000 mg/kg (OECD 423), rabbit dermal LD₅₀ > 2000 mg/kg (OECD 402), and negative Ames test (OECD 471, strains TA98, TA100, TA1535, TA1537, and WP2 uvrA with and without S9 metabolic activation)—places the intermediate in GHS Category 5 for acute oral and dermal toxicity and non-mutagenic, findings that facilitate NIOSH recommended exposure limit setting and the design of engineering controls (local exhaust ventilation with face velocity of 100 fpm) during weigh-and-dispense operations in chemical development suites.