2-Bromo-4-Methyl-4H-Thieno[3,2-B]Pyrrole[3,2-D]Pyridazinone

What Process Controls Govern Reproducible Kilogram-Scale Suzuki Couplings on the 6-Membered Lactam Ring?

Production-scale Suzuki–Miyaura cross-coupling employing 2-bromo-4-methyl-4H-thieno[3,2-b]pyrrole[3,2-d]pyridazinone as the electrophilic partner demands rigorous exclusion of water from the catalyst activation step while simultaneously maintaining a homogenous aqueous-organic interface for boronate transmetalation. The bromine atom at C-2 exhibits markedly reduced oxidative addition kinetics when the lactam nitrogen is not protected, owing to competitive coordination of palladium by the amide carbonyl; 0.5–2.0 mol% Pd(OAc)₂ with 2.2 equiv SPhos ligand consistently restores turnover frequency above 800 h⁻¹ at 85 °C in degassed dioxane/water (4:1 v/v). Potassium phosphate tribasic (2.5 equiv) is preferred over sodium carbonate to suppress lactam ring-opening side reactions that escalate above pH 12.3 and generate 3-(2-oxopropyl)thienopyrrole carboxylate fragments detectable by HPLC-UV at 254 nm with a characteristic retention shift of +1.2 min relative to the product. On 500 L glass-lined reactors equipped with retreat-curve impellers (80–120 rpm), the arylboronic acid is added in 4 equal portions over 90 min to moderate an exotherm that can reach ΔT = +28 °C under adiabatic conditions; failure to program this staged addition has led to dimerization of the dehalogenated intermediate and a 12–18% drop in isolated yield across multiple batches. Post-reaction workup involves Celite® filtration through a 0.5 μm cartridge, phase separation at 45 °C, and vacuum distillation to a final solvent volume of 3.2 L per kg of target compound. The crude product crystallizes from isopropanol/n-heptane (1:5) as off-white needles with melting point 199–202 °C and residual palladium content below 5 ppm when the organic layer receives a 15 wt% aqueous L-cysteine wash at 60 °C for 45 min, a procedure aligned with the EMEA Guideline on the Specification Limits for Residues of Metal Catalysts (EMEA/CHMP/SWP/4446/2000). The functionalized biaryl intermediates produced through this route serve as key building blocks for ATP-competitive kinase inhibitors targeting the DFG-out conformation; gel-permeation monitoring of batch homogeneity shows specification compliance when the differential scanning calorimetry purity by DIN 51004 exceeds 99.2%.

In the context of solid-phase-supported parallel synthesis for early-stage medicinal chemistry, adaptations of this coupling have been validated on Wang resin preloaded with a boronic acid handle. Here the stoichiometry is inverted: 3.0 equiv of the brominated thienopyrrolopyridazinone are reacted with 5 mol% Pd(PPh₃)₄ and 10 equiv CsF in DMF at 100 °C under microwave irradiation (300 W, 20 min). Cleavage with TFA/H₂O/TIPS (95:2.5:2.5) delivers the biarylacetic acid derivatives directly. The limitation of this protocol is significant resin swelling in DMF, which reduces effective stir rate and yields an inhomogeneous bead size distribution unless the solvent volume is kept at ≤12 mL/g of resin; breach of this threshold has been documented to cause mechanical attrition of the beads and liberation of fine particulate that clogs 20 μm polypropylene frits on automated SPE workstations. Published data for this specific configuration is limited, but analogous brominated pyridazinones exhibit a reaction half-life of 6–8 min under identical microwave settings, making continuous-flow implementations with a 5 mL coil reactor an underexplored avenue for achieving multi-gram throughput.

When the Bromide Substitute Dictates Bioisosteric Replacement in Agrochemical Discovery

The 4-methyl-4H-thieno[3,2-b]pyrrole[3,2-d]pyridazin-2(4H)-one framework shares pharmacophoric overlap with the commercial protoporphyrinogen oxidase (PPO) inhibitor pyridafol, but the pendent bromine at position-2 enables a divergent Sonogashira alkynylation that introduces a propargyl ether side chain crucial for binding to the Nicotiana tabacum PPO₂ isoform. In greenhouse screening programs carried out on Amaranthus retroflexus and Abutilon theophrasti, the 2-phenylethynyl congeners prepared from this bromide exhibit an IC₅₀ of 18 nM against the herbicide target enzyme when assayed according to the substrate protoporphyrinogen IX depletion method (EC 1.3.3.4), compared with 45 nM for the des-bromo analog lacking the alkynyl vector. The synthetic protocol requires rigorous drying of the bromide (Karl Fischer ≤ 0.05% H₂O) and the use of 2.0 equiv of the terminal alkyne with 2 mol% PdCl₂(PPh₃)₂ and 4 mol% CuI in triethylamine/THF (1:2) at 55 °C for 12 h; the exclusion of oxygen via five freeze–pump–thaw cycles is mandatory, as even traces of O₂ promote Glaser-type homocoupling that consumes the alkyne and necessitates column chromatography to remove the diyne impurity (Rf 0.65 vs. product Rf 0.42 in EtOAc/hexane 3:7). On a pilot scale (20 mol input), post-reaction quenching with aqueous NH₄Cl followed by extraction and crystallization from methylcyclohexane delivers the alkynylated product in 81% yield with >97% HPLC area purity. Toxicology profiling under OECD Guideline 423 for acute oral toxicity classifies the lead compound as Category 4, and its octanol/water partition coefficient (log P 3.8 determined by the shake-flask method OECD 117) indicates moderate soil mobility that requires granular formulation on attapulgite clay carriers to reduce leaching into groundwater. The terminal formulated product is a 75 WG (water-dispersible granule) containing 5 wt% of the alkynyl derivative co-milled with a sodium lignosulfonate/naphthalene sulfonate dispersant blend in a fluidized-bed granulator operated at an inlet air temperature of 85 °C.

The brominated core has additionally been employed as a scaffold for protoporphyrinogen oxidase inhibitors with a pyridazinone pharmacophore, where the C-2 aryl group introduced via Suzuki coupling mimics the 4-chlorophenyl ring of bicyclic PPO herbicides. In early lead optimization, a matrix of 24 electron-deficient arylboronic acids was screened, revealing that the 3,5-difluorophenyl derivative reduces tobacco PPO₂ activity by 92% at 10 μM under the continuous fluorometric assay monitoring H₂O₂ production with horseradish peroxidase/Amplex Red. The synthetic window for the direct coupling is narrow: using Pd-132 (bis[bis(3,5-trifluoromethylphenyl)phosphino]ferrocene palladium(II) chloride) at 0.8 mol% and KF (3.0 equiv) in dioxane at 100 °C, full conversion is reached within 3 h, but extended heating beyond 6 h results in dehalogenation and proto-debromination that generates the unsubstituted thienopyrrolopyridazinone, which co-crystallizes with the target compound and cannot be removed by fractional crystallization. To circumvent this issue, manufacturing batches employ in-line FTIR monitoring of the C–Br stretching band at 690 cm⁻¹, with automated quenching triggered when its intensity falls below 2% of initial absorbance. The resultant diaryl intermediate progresses to a field-tested lead with pre-emergent activity against glyphosate-resistant Amaranthus palmeri at application rates of 75–100 g a.i./ha.

Hole-Transporting Copolymer Semiconductors for Organic Field-Effect Transistors

Integration of the brominated thienopyrrolopyridazinone into donor–acceptor polymers begins with its copolymerization with 2,5-bis(trimethylstannyl)thiophene via Stille polycondensation. The electron-deficient pyridazinone unit lowers the lowest unoccupied molecular orbital (LUMO) energy to approximately −3.6 eV (as measured by cyclic voltammetry versus ferrocene/ferrocenium in 0.1 M tetrabutylammonium hexafluorophosphate acetonitrile solution, ASTM E537-20), while the thienopyrrole segment provides a highest occupied molecular orbital (HOMO) level close to −5.1 eV, yielding a bandgap of 1.5 eV that positions the copolymer as a p-type semiconductor with ambipolar character under positive gate bias in top-contact OFETs. Controlled polymerization employs 1.0 equiv of the dibrominated planar monomer, 1.0 equiv of the distannane comonomer, 2 mol% tris(dibenzylideneacetone)dipalladium(0), and 8 mol% tris(2-methoxyphenyl)phosphine in chlorobenzene (15 mL per mmol monomer) at 130 °C for 48 h under a rigorously oxygen-free argon atmosphere. The resulting poly[thienopyrrolopyridazinone-co-thiophene] is end-capped with 2-tributylstannylthiophene (0.1 equiv) and 2-bromothiophene (0.2 equiv) sequentially to remove reactive chain ends that otherwise cause hole-trap sites and threshold voltage drift during prolonged bias-stress testing (IEC 62860-2). Insoluble fractions generated by chloroform Soxhlet extraction for 72 h are discarded; the soluble portion precipitated into methanol yields a fibrous solid with number-average molecular weight (Mₙ) of 24.3 kDa and dispersity Đ = 1.8 as determined by high-temperature GPC at 150 °C in 1,2,4-trichlorobenzene against polystyrene standards (ISO 16014-3). The copolymer’s glass transition temperature is not detected up to 300 °C by DSC, indicative of a rigid-rod backbone, while the thermal decomposition onset (T₅%) under nitrogen is 385 °C (ASTM E2550-21). Thin-film transistors fabricated by spin-coating a 5 mg/mL chloroform solution onto octadecyltrichlorosilane-treated SiO₂/Si substrates (capacitance 15 nF/cm²) exhibit saturation hole mobility up to 0.12 cm²/V·s with an on/off current ratio of 10⁶; device yield drops when the residual tin content from the Stille coupling exceeds 600 ppm, as tin aggregates act as charge-scattering centers visible in AFM phase images as bright domains of 50–80 nm. Compliance with REACH Annex XVII organotin restrictions (dibutyltin species ≤0.1 wt%) requires an additional diethylenetriamine-functionalized silica column treatment during polymer workup, which reduces extractable tin to ≤45 ppm but shear-degrades the polymer chains if the column residence time exceeds 4 min, shifting Mₙ downward by 15–20%.

A parallel brominated monomer supply chain developed for inkjet-printed thin-film transistors eliminates the safety and regulatory hurdles of organostannanes by switching to direct arylation polymerization (DArP). Using equimolar amounts of 2-bromo-4-methyl-4H-thieno[3,2-b]pyrrole[3,2-d]pyridazinone and 2,2’-bithiophene with 1 mol% Herrmann–Beller palladacycle, 10 mol% pivalic acid, and 2.0 equiv K₂CO₃ in N-methyl-2-pyrrolidone at 120 °C for 24 h, a regioregular alternating copolymer is obtained with Mₙ 18.7 kDa. The drawback of DArP lies in the competing debromination of the thienopyrrolopyridazinone monomer, which creates a monofunctional chain terminator that caps propagation at early conversion (~70%), so that the polymer must be precipitated and re-subjected to fresh catalytic conditions to reach sufficient molecular weight for fiber formation in electrospinning. Field-effect measurements on DArP-derived films in a bottom-gate, bottom-contact architecture (Au source/drain, channel length 50 μm, width 1000 μm) display charge-carrier mobility of 0.07 cm²/V·s with a subthreshold swing of 0.8 V/dec under ambient humidity, a performance envelope that meets the backplane driving requirements for electrophoretic displays but does not yet satisfy the >0.5 cm²/V·s benchmark specified for OLED pixel current delivery.

Translating solution-processed bulk heterojunction organic photovoltaics (OPV) using this bromo-monomer as an electron-accepting unit in a push–pull copolymer demands rigorous control of the molecular weight distribution. When the number-average chain length falls below 50 repeat units, formation of inter-chain percolation pathways is interrupted, causing the power conversion efficiency to plummet from 3.2% to 0.8% under AM 1.5G illumination (100 mW/cm²) in a device architecture ITO/PEDOT:PSS/polymer:PC₇₁BM/LiF/Al. The external quantum efficiency spectrum shows a pronounced dip at 550 nm attributed to the absence of a charge-transfer state when the polymer fraction in the blend exceeds 60 wt%, and large-scale phase domains exceeding 200 nm emerge, as confirmed by resonant soft X-ray scattering. Standardized photostability testing per ISO 4892-2 (xenon-arc weathering) indicates that after 500 h of exposure, the unencapsulated device retains 78% of initial efficiency if the polymer contains no catalytic palladium remnants above 15 ppm, above which the photo-induced degradation rate accelerates through metal-catalyzed singlet oxygen generation. This material’s targeted end product is a flexible, lightweight OPV module printed on polyethylene naphthalate foil for low-light indoor energy harvesting (e.g., powering wireless sensor nodes at illumination levels of 200 lux, CIE Illuminant A).

The potential of this halogenated heterocycle in resistive memory devices has been explored in a metal-insulator-metal configuration. Vapor-deposited films of thickness 45 nm sandwiching the material between an Al bottom electrode and a Cu top electrode exhibit write-once-read-many (WORM) resistive switching with an ON/OFF current ratio of 10⁵ at a read voltage of 1.0 V, attributed to the electrochemical reduction of the pyridazinone ring to a dihydropyridazine form that creates conductive carbon-rich filaments. The cycle-to-cycle variability in the set voltage (V_set = 2.3 ± 0.4 V) has been correlated with variations in the Br content at the electrode interface, as XPS depth profiling reveals a 0.8 eV shift in the N 1s binding energy when bromine migration to the anode occurs after 200 switching cycles. Published operational lifetime data for these non-volatile memory cells in the context of the thienopyrrolopyridazinone system remain sparse, but the observed retention time of 10⁴ s at 85 °C aligns with the data preservation requirements for disposable electronic tags configured under ISO/IEC 18000-63.

Site-Specific Fluorophore Conjugation via Palladium-Catalyzed Amination and its Constraints in Aqueous Buffer Systems

Transforming the C-2 bromide into a primary amine handle for subsequent NHS ester or isothiocyanate dye coupling proceeds through a Buchwald–Hartwig amination with benzophenone imine as an ammonia surrogate. An optimized catalyst system for this substrate comprises 2 mol% Pd₂(dba)₃ and 4 mol% Xantphos with 1.4 equiv benzophenone imine and 1.5 equiv NaOt-Bu in toluene at 100 °C for 4 h, followed by acidic hydrolysis (HCl 2N in dioxane at 25 °C, 30 min) to liberate the 2-amino-4-methyl-4H-thieno[3,2-b]pyrrole[3,2-d]pyridazinone dihydrochloride salt. Mass transfer is the rate-limiting step on a 500 g scale, and the agitated bead mill used for catalyst palladium removal must maintain a slurry temperature below 10 °C to prevent deamination back to the starting bromide, a side reaction evidenced by the reappearance of the Br⁻ ion peak at 79.2 m/z in negative-ion ESI-MS after 3 h of ambient milling. The hydrochloride salt is then coupled to an NHS-functionalized Alexa Fluor™ or Cyanine dye in anhydrous DMF with 2.2 equiv N,N-diisopropylethylamine at 0 °C in the dark; the active ester is consumed within 2 h, and the conjugate is purified by preparative HPLC (C18, acetonitrile/0.1% trifluoroacetic acid gradient) to afford a fluorescent reporter with excitation maximum 647 nm and emission 665 nm. For intracellular imaging under two-photon microscopy, the free amine intermediate is instead reacted with S-acetylthioacetic acid NHS ester, deprotected with hydroxylamine (50 mM, pH 7.4), and ligated to a maleimide-modified antibody via maleimide-thiol chemistry at pH 7.2; protein dye-to-antibody ratio (DAR) measured by UV-Vis at 280 nm and 647 nm is consistently between 3.8 and 4.2 when the initial thiolated heterocycle-to-antibody molar ratio is set to 8. The resulting immunofluorescent probe enables super-resolution stimulated emission depletion (STED) imaging of nuclear lamina proteins at lateral resolution of 45 nm, but photobleaching half-life is reduced to 12 s under continuous 775 nm depletion laser irradiation, necessitating an oxygen-scavenging mounting medium containing 1 mg/mL glucose oxidase plus 0.5% β-mercaptoethanol. Compliance with ISO 13485:2016 design controls for research-use-only (RUO) antibody conjugates derived from this brominated precursor requires documentation of residual palladium below 1 ppm, endotoxin below 0.5 EU/mg, and host-cell protein contamination below 10 ppm—data that are typically verified via ICP-MS, LAL kinetic chromogenic assay, and ELISA, respectively.