2-Amino-6-Fluorobenzothiazole

In neuroprotective agent discovery programs pursuing analogues of riluzole—a 2-amino-6-(trifluoromethoxy)benzothiazole approved by FDA for amyotrophic lateral sclerosis—2-amino-6-fluorobenzothiazole serves as a critical halogen-substituted bioisostere. Synthesis proceeds via tandem cyclization of 4-fluoroaniline with potassium thiocyanate and bromine in glacial acetic acid at 0–5 °C, then basified to pH 9–10 with 25% aqueous ammonia to precipitate the free amine. The isolated crude is purified by vacuum distillation at 140–145 °C / 2.5 mbar or by recrystallization from toluene/hexane ( 1:3 v/v) to yield an off-white crystalline solid with HPLC purity ≥99.2% (area%, C18 column, detection 254 nm, mobile phase methanol/water 70:30). In downstream kinase inhibitor and mGluR5 antagonist syntheses, the 2-amino group undergoes HATU-mediated amidation with heteroaryl carboxylic acids (1.05 equiv. acid, 1.2 equiv. DIPEA, DMF, 20 °C, 16 h) to afford advanced intermediates which, following flash chromatography (silica 60 Å, ethyl acetate/heptane gradient), enter biological screening. Residual primary aromatic amine content is monitored per Ph.Eur. 2.4.24 (TLC limit ≤0.1%). In vitro cytotoxicity of resultant analogues is assessed by ISO 10993-5 MTT assay using L929 fibroblasts before advancing candidates to 100-L pilot-scale batches in glass-lined reactors (Pfaudler AE 10 type) under GMP Part 211 conditions. Note that 2-amino-6-fluorobenzothiazole itself has not been registered as an API intermediate in any currently marketed small-molecule therapeutic, and its developmental pipeline exposure is confined to Phase I candidates described in WO 2012/098115 and related patent families; thus, published production-scale data for this specific configuration is limited.

What limits lightfastness in monoazo disperse dyes for polyester when a 6-fluoro auxochrome is present?

2-Amino-6-fluorobenzothiazole is diazotized at −2 to 0 °C in a jacketed glass-lined reactor by adding 40% nitrosylsulfuric acid (1.02 molar equivalents relative to amine) into a slurry of the amine in 85% phosphoric acid over 45 min with vigorous anchor agitation (120 rpm, Rushton turbine optional for viscous stages). The clear diazonium solution is checked for excess nitrous acid with starch-iodide paper (negative) and sulfamic acid (0.5% w/w) is introduced to quench residual nitrosating species. Coupling to N-ethyl-N-(2-hydroxyethyl)aniline is performed at 5–8 °C in a second vessel by delivering the diazo stream subsurface into the coupler pre-dissolved in 10% aqueous H₂SO₄ containing 1% leveling agent (polyvinylpyrrolidone K‑30). Molar ratio diazonium:coupler is maintained at 1:1.03 to avoid bis-azo formation. After coupling completes (30 min, confirmed by Ehrlich’s reagent spot test), the suspension is neutralized to pH 4.5 with sodium acetate, stirred for 2 h, then filtered through a polypropylene filter press and washed with 60 °C demineralized water until conductivity < 100 µS/cm. The wet cake is dried in a fluidized-bed dryer at 80 °C inlet air temperature to a final moisture content of < 0.5% (Karl Fischer). Standardization with sodium lignin sulfonate (20–25 wt%) yields a blue-red disperse dye with high tinctorial strength. Exhaustion dyeing trials on polyester knitted fabric ( 2.0% o.w.f.) in a Mathis Labomat IR dyeing machine at 130 °C for 45 min followed by reduction clearing (NaOH 2 g/L, Na₂S₂O₄ 2 g/L, 80 °C, 20 min) deliver lightfastness ratings measured under ISO 105‑B02:2014 that reveal the impact of the fluorine substituent on photodegradation pathways. The table below collates comparative data where the 6-F derivative is benchmarked against the non-fluorinated analogue.

The fluorinated dye exhibits a hypsochromic shift of 8–12 nm in DMF (λ_max 568 nm vs. 578 nm for the non-fluorinated) and a decade-long lower rate of photofading under high-energy visible light, attributed to the electron-withdrawing fluorine reducing the electron density on the azo chromophore and thereby decreasing the rate of oxidative photofission. Process deviations—specifically diazotization temperature exceeding +3 °C—trigger early decomposition of the diazonium salt, producing a tar-like side product that precipitates in the coupling vessel, reducing final yield by 15–25% and shifting the shade dull. To comply with ZDHC Manufacturing Restricted Substances List 2024, the plant effluent is treated with 0.1% activated carbon and 10 ppm alkylphenol ethoxylate below emission thresholds per EU 2018/858.

Without a distinct heading, the next application emerges directly from coordination chemistry practice. When 2-amino-6-fluorobenzothiazole is refluxed in absolute ethanol with salicylaldehyde or its 3,5-dibromo derivative (molar ratio 1:1, 0.5 mol% acetic acid catalyst, 78 °C, 6 h under N₂), the resulting Schiff base precipitates upon cooling as yellow needles. After recrystallization from acetonitrile (yield 82–88%), this ligand binds Zn²⁺ in methanolic solution with a 1:1 stoichiometry determined by Job’s plot, yielding a fluorescence turn-on response at 510 nm with a detection limit of 1.8 × 10⁻⁸ M as established by the IUPAC 3σ/slope method. While no current ASTM standard directly addresses small-molecule fluorescent sensor validation, the complex’s quantum yield (Φ_F = 0.34 relative to quinine sulfate in 0.1 M H₂SO₄) qualifies it for intracellular imaging applications only after confirming no unspecific mitochondrial membrane potential disruption via JC‑1 assay (ISO 19007:2018 nanoparticle cytotoxicity). The ligand precursor has been scaled to 5‑kg batches in a 50‑L glass reactor with anchor stirrer; addition of the catalyst in a single portion without pre-dilution caused localized overheating and exothermic imine formation resulting in 7% yield loss through oxidation by-product formation, a problem mitigated by metered catalyst dosing over 15 min and continuous N₂ sparging at 2 L/min.

When the scaffold is incorporated into carboxamide fungicide leads

In discovery-phase agrochemical programs targeting succinate dehydrogenase (SDH, EC 1.3.5.1), 2-amino-6-fluorobenzothiazole functions as the amine partner in carboxamide linkage generation. A representative procedure: to a chilled (0–5 °C) solution of the amine (1.0 eq.) in anhydrous dichloromethane (10 volumes) containing pyridine (1.5 eq.) is added a substituted benzoyl chloride (1.08 eq.) dropwise under positive argon pressure. The batch is allowed to warm to 22 °C over 3 h and monitored by TLC (silica, cyclohexane:ethyl acetate 2:1). Upon completion, the mixture is quenched with 1 M HCl (5 volumes), the organic phase separated, washed with saturated NaHCO₃ until pH 8.0, dried over anhydrous Na₂SO₄, and concentrated on a rotary evaporator at 40 °C / 25 mbar. The crude amide is triturated with diisopropyl ether to afford a white solid (purity > 95% by qNMR using 1,2,4,5-tetrachloro-3-nitrobenzene as internal standard). These compounds are submitted for in vitro enzyme inhibition assay on isolated Zymoseptoria tritici SDH complex II following FRAC guideline protocols, where the 6‑F substitution on the benzothiazole ring consistently shifts IC₅₀ values by 0.3–0.5 log units compared to the 6‑H analogue, a potency gain attributable to the improved oxidative metabolic stability imparted by the fluorine atom blocking CYP-mediated hydroxylation. Field trial formulations are prepared as emulsifiable concentrates (EC) containing 100 g/L active ingredient, 8% w/v calcium dodecylbenzenesulfonate/ethoxylated castor oil blend (4:1), and Solvesso™ 150 ND to volume, diluted 200‑fold in water and sprayed at 200 L/ha on Triticum aestivum (BBCH 31–39) against septoria leaf blotch. Residue compliance is evaluated under Regulation (EC) No 396/2005 (maximum residue limit pending). An operational constraint: the carboxamide products show exothermic decomposition onset at 212 °C by DSC (heating rate 10 °C/min, N₂ atmosphere), mandating that micronization by air-jet milling maintain product temperature below 40 °C to avoid amorphous phase formation and subsequent recrystallization in the suspension concentrate that clogs sieve filters (50 µm mesh) on boom sprayers.

Performance-modifying intermediate for high-temperature polyamide heat stabilizers

Polyamide 6,6 compounds processed in continuous fiber spinning lines (Barmag SP8 draw texturing machine) occasionally incorporate aromatic amine antioxidants to suppress thermo-oxidative degradation at melt temperatures exceeding 295 °C. In this context, the reaction product of 2-amino-6-fluorobenzothiazole with epoxidized soybean oil (ESO, oxirane oxygen content 6.8–7.2%) at a molar ratio of 2.1:1 (amine:epoxide) catalyzed by 0.5 wt% triethylammonium bromide in toluene reflux (110 °C, 24 h) yields a benzothiazole-grafted triglyceride additive. After solvent stripping and vacuum drying ( < 1 mbar, 70 °C), this viscous liquid is metered into PA 6,6 at 0.3–0.8 phr via a gravimetric side feeder on a co-rotating twin-screw extruder (L/D 40, screw speed 250 rpm, barrel profile 260–285 °C). The fluorine atom in the 6‑position was found, through comparative oven-aging at 140 °C per ISO 188:2023 on injection-molded tensile bars (ISO 527‑2 type 1A), to reduce the oxygen uptake rate measured by isothermal differential scanning calorimetry (190 °C, 50 mL/min O₂) by 12–18% relative to the unsubstituted benzothiazole adduct, likely due to enhanced radical scavenging capacity at the ortho-position to the heterocyclic nitrogen. Extrusion operators must enforce pre-drying of the additive at 60 °C for 4 h when ambient relative humidity exceeds 60% to avoid hydrolytic ring-opening of residual oxirane groups that catalyze PA 6,6 chain scission at processing temperatures. Compatibility with copper iodide heat stabilizers is poor; combining these two additives at combined loading above 0.6 phr leads to metallic copper precipitation at the die lip within 4 h of continuous operation, monitored by die pressure rise exceeding 15 bar on the extruder head transducer. The resulting stabilized fiber finds application in under-the-hood automotive textiles meeting SAE J369 flammability and long-term heat resistance specifications. No commercial masterbatch containing this specific benzothiazole-ESO adduct has been registered under REACH, so downstream users must prepare a PPORD notification if annual volume exceeds 1 tonne.

As a Building Block in Electroluminescent Polymeric Matrices: Avoiding Aggregation-Caused Quenching

Copolymerization of 2-amino-6-fluorobenzothiazole into conjugated polymer backbones via Buchwald-Hartwig coupling with aryl dihalides has been explored to suppress π‑π stacking in the solid state. The monomer is first N-Boc protected with di‑tert‑butyl dicarbonate (1.2 eq.) in THF/water (2:1) with NaHCO₃ (3 eq.), affording a crystalline intermediate (mp 131–133 °C) susceptible to lithiation at the 4‑position with LDA in THF at −78 °C, followed by quenching with trimethyl borate and subsequent pinacol ester formation. This pinacol boronate ester is then engaged in alternating Suzuki-Miyaura polymerization with 2,7‑dibromo‑9,9‑dioctylfluorene using Pd(PPh₃)₄ (2 mol%), aqueous K₂CO₃ (2 M), and Aliquat® 336 phase transfer catalyst in toluene at 90 °C for 48 h. After end-capping with bromobenzene and phenylboronic acid, the polymer is purified by precipitation into methanol and Soxhlet extraction with acetone. The resulting polyfluorene derivative exhibits a photoluminescence quantum yield of 0.68 in thin-film state under 365 nm excitation, with Commission Internationale de l'Éclairage coordinates (x = 0.15, y = 0.21) corresponding to stable blue emission. Device fabrication on pre-patterned ITO‑glass substrates (sheet resistance 15 Ω/sq) with a PEDOT:PSS hole-injection layer (Heraeus Clevios™ P VP AI 4083, spin-coated at 3000 rpm to 40 nm) and a Ca/Al cathode (20 nm/100 nm) yields an OLED with a turn-on voltage of 4.2 V and a luminance of 1200 cd/m² at 8 V. Critically, the fluorine substituent introduces sufficient steric and electronic disorder to prevent the formation of keto-defect sites (fluorenone emission at 530 nm) that plague unsubstituted analogues after 24 h of continuous operation at 50 mA/cm² under ambient conditions. Scale-up of the Suzuki polymerization step beyond 10 g batch size in a 500‑mL Parr reactor required strict control of the interphase mixing: a self-venting pitched-blade turbine at 650 rpm proved necessary to maintain molecular weight dispersity (Đ) below 2.1, as measured by GPC against polystyrene standards in THF, otherwise bimodal distributions resulted in gel particle content exceeding 5% and compromised the quality of spin-coated films. Before adoption in a display pilot line, the polymer lot must pass an ionic impurity test: chloride content below 10 ppm and palladium content below 50 ppm as determined by ICP‑MS after microwave digestion (EPA Method 3052).