Methyl 2-Bromo-4-Fluorobenzo[D]Thiazole-6-Carboxylate

Methyl 2-bromo-4-fluorobenzo[d]thiazole-6-carboxylate is introduced into carbon–carbon bond-forming sequences as a heteroaryl electrophile whose electron-poor benzothiazole core accelerates oxidative addition while the 4-fluoro substituent suppresses metabolic hydroxylation in downstream bioactive molecules. The crystalline solid is charged into a flame-dried reactor under nitrogen overpressure prior to dissolution in anhydrous 1,4-dioxane or THF. A typical Pd-catalyzed Suzuki coupling employs 1.05 to 1.20 equivalents of arylboronic acid, 2.0 equivalents of K₂CO₃ or CsF as base, and 0.5–2.0 mol% of Pd(PPh₃)₄ or Pd(dppf)Cl₂·CH₂Cl₂ relative to the bromo-ester. Aqueous co-solvent is introduced at 10–20 vol% through a syringe pump over 15 minutes to moderate the exotherm; the internal temperature is maintained at 75 ± 3 °C for 8–14 hours as monitored by in-process HPLC referencing USP <621>. Upon consumption of the limiting bromo-ester (<0.5 area%), the biphasic mixture is cooled to 25 °C, diluted with ethyl acetate, and the organic phase is washed sequentially with 5% aqueous NaHCO₃ and brine before drying over Na₂SO₄. Filtration through a short pad of Celite® 545 and concentration under reduced pressure at bath temperature not exceeding 45 °C yields a pale-yellow residue that is then recrystallized from n-heptane/ethyl acetate (4:1 v/v). Residual palladium content is reduced below 10 ppm by treatment with MP-TMT scavenger resin or a charcoal filtration step compliant with ICH Q3D elemental impurity limits. The isolated coupling product — typically a 2-aryl-4-fluorobenzothiazole-6-carboxylate — is dried in a vacuum oven at 40 °C and 10 mbar until loss on drying is ≤0.2%. Batch release specifications demand HPLC purity ≥99.0 area% (method validated per ICH Q2(R1)) with any single unknown impurity capped at ≤0.10% and the des-bromo proto-dehalogenation side-product restricted to ≤0.15%. Residual solvent levels are controlled to the limits prescribed in ICH Q3C Table 2: DMF <880 ppm, 1,4-dioxane <380 ppm, ethyl acetate <5000 ppm, and n-heptane <5000 ppm. On a 20-L pilot-plant scale, the process window tightens because the dehalogenation impurity rises sharply when the jacket temperature overshoots 78 °C for more than 5 minutes; therefore cascade control with a ΔT limit of 3 °C across the glass-lined vessel is hard-wired into the DCS. The resulting functionalized heterocycle serves as the penultimate intermediate for a series of farnesyltransferase inhibitors where the fluorine atom at position 4 blocks cytochrome P450-mediated ring hydroxylation and the carboxylate ester is hydrolyzed to the acid for subsequent amide formation with a substituted piperidine. Final drug candidates emerging from this route have been tested in cellular proliferation assays at sub-micromolar concentrations, and the key intermediate complies with the TSCA inventory for export from non-U.S. jurisdictions under a REACH pre-registration dossier referencing tonnage band 1–10 t/a.

What engineering controls prevent homocoupling during kilogram-scale Negishi reactions?

In agrochemical discovery, the same bromo-fluoro benzothiazole ester is inserted into Negishi cross-couplings to construct ortho-substituted biaryl motifs found in succinate dehydrogenase inhibitor (SDHI) fungicide candidates. A typical feed stream combines the substrate with 0.95 equivalents of an arylzinc chloride prepared in situ from the corresponding aryl magnesium bromide and ZnCl₂·TMEDA in 2-MeTHF at −20 °C under argon. The catalyst system relies on 0.5 mol% Pd-PEPPSI™-IPr or XPhos Pd G3 because the bulky N-heterocyclic carbene ligand suppresses β-hydride elimination and minimizes homocoupling to ≤2.0 area%. Reaction mass is held at 5 °C during the addition of the organozinc reagent; the internal temperature is then ramped to 40 °C over 2 hours and aged for an additional 6 hours, at which point IPC typically shows >97% conversion. Quenching is performed with 1 N HCl at 0–5 °C followed by extraction into MTBE and two washes with 10% aqueous Na-EDTA to complex residual zinc salts before phase separation at 35 °C. The organic concentrate is passed through a wiped-film evaporator operating at 100 mbar and 65 °C jacket temperature to strip solvents, and the crude oil is crystallized from acetonitrile/water (3:1) to afford the coupled biaryl ester as an off-white solid with purity ≥98.5%. Ecotoxicological profiling required under Regulation (EC) No. 1107/2009 mandates acute aquatic toxicity data for both the intermediate and the finished fungicide; the bromo-fluoro ester is classified as Skin Sens. 1B and Aquatic Chronic 3 under CLP, and dedicated containment is specified during milling and powder charging operations. The downstream active ingredient — a benzothiazole-6-carboxamide bearing a difluoromethyl pyrazole side chain — exhibits curative activity against Zymoseptoria tritici in field trials at application rates of 100–150 g a.i./ha, and its synthesis hinges on the selective mono-coupling of the bromo-fluoro ester without displacement of the methyl carboxylate or the fluorine substituent. Packing in UN-approved 4G fiberboard drums with double polyethylene liners is standard for sea freight, and each shipment is accompanied by a certificate of analysis listing conformance to FAO Specification 97/TC/S/F for technical-grade intermediates.

When the benzothiazole carboxylate serves as an electron-acceptor in blue TADF emitters

Materials scientists have exploited the methyl ester unit and the electron-withdrawing benzothiazole ring as an acceptor moiety in thermally activated delayed fluorescence (TADF) emitters targeting vacuum-deposited OLED devices. The 2-bromo handle is used to attach a donor fragment — most often 9,9-dimethyl-9,10-dihydroacridine or phenoxazine — through a Buchwald-Hartwig or Ullmann amination to yield a donor–acceptor–donor architecture. In a representative experimental protocol, the bromo-fluoro ester is reacted with 2.2 equivalents of 9,9-dimethyl-9,10-dihydroacridine, 2.0 equivalents of NaOtBu, and 2 mol% Pd₂(dba)₃ with 4 mol% tBu₃P·HBF₄ in toluene at 110 °C for 24 hours under rigorous exclusion of oxygen (O₂ <1 ppm in glovebox atmosphere). The crude triarylamine product is purified by column chromatography on silica gel (eluent: hexane/ethyl acetate 95:5) followed by temperature-gradient vacuum sublimation at 10⁻⁶ Torr with a source temperature of 240–260 °C. Photophysical characterization in 10⁻⁵ M toluene solution reveals an intramolecular charge-transfer absorption band centered at 350–380 nm and a structureless emission at 470–490 nm with a photoluminescence quantum yield of >80% when measured in a doped mCBP host film (10 wt%) under nitrogen. The 4-fluoro substituent blue-shifts the emission by ~8 nm relative to the non-fluorinated analogue due to its inductive withdrawal, while the methyl carboxylate group at position 6 provides a synthetic handle for tuning thermal stability without affecting the frontier orbital overlap critical for reverse intersystem crossing. Device stacks built with ITO/HAT-CN (10 nm)/TAPC (30 nm)/emissive layer (30 nm)/TmPyPB (40 nm)/LiF (1 nm)/Al achieve external quantum efficiencies exceeding 18% and a CIE coordinate of (0.15, 0.20), as reported in peer-reviewed literature where the emitter is identified by its proprietary code name. Purity requirements for sublimation-grade material are stringent: HPLC purity ≥99.95%, chloride content <5 ppm, and volatile residues below 100 ppm by thermogravimetric analysis (heating rate 10 °C/min under N₂ up to 350 °C). The methyl ester intermediate is supplied in amber glass bottles sealed under argon with a moisture specification of <50 ppm by Karl Fischer titration, and it is registered for import under the Toxic Substances Control Act (TSCA) inventory as a research and development substance.

Para-substituted benzothiazole derivatives exhibit a propensity for smectic C mesophases when coupled with long-chain alkoxyphenyl moieties, and the bromo-fluoro ester is employed as a core building block in the design of ferroelectric liquid crystal mixtures. A convergent synthesis attaches a 4-(n-octyloxy)phenyl group via Suzuki coupling under the standard conditions described for pharmaceutical intermediates, yielding the 2-aryl-4-fluoro-6-(methoxycarbonyl)benzothiazole. The methyl ester is then reduced to the corresponding benzylic alcohol with LiAlH₄ in THF at 0 °C and subsequently oxidized to the aldehyde with Dess-Martin periodinane; the aldehyde is condensed with a chiral alkyloxy aniline to install the chiral tail necessary for helical twisting power. Differential scanning calorimetry traces show a crystal-to-smectic C transition at 78 °C and a clearing point at 134 °C; the melting enthalpy of 12.5 J/g is sufficiently low to allow formulation into room-temperature eutectic blends with commercial phenylpyrimidine hosts. The fluorine ortho to the ring nitrogen increases the lateral dipole moment and contributes to a negative dielectric anisotropy (Δε ≈ −2.5) measured at 1 kHz in a 4 μm planar cell. Supply agreements for such specialty intermediates often stipulate 99.0% minimum purity, single heavy-metal limits following ICH Q3D Option 1 assessment, and a shelf life of 24 months when stored at −20 °C under nitrogen.

The compound is periodically adopted as a standardized test substrate in academic and industrial laboratories evaluating new catalytic systems for heteroaryl halide activation. A benchmarking exercise might treat the bromo-fluoro ester with 1.5 equivalents of potassium phenyltrifluoroborate in the presence of 2.0 mol% of a novel palladacycle precatalyst and 3.0 equivalents of K₃PO₄·H₂O in CPME at 95 °C. Turnover numbers exceeding 5000 have been reported under these conditions, with the benzothiazole substrate proving more reactive than analogous bromopyridines due to the electron-deficient C-2 position. The resulting 2-phenyl-4-fluorobenzothiazole-6-carboxylate is isolated by flash chromatography and serves as a reference standard for internal method validation when calibrating LC-MS/MS or GC-MS instruments using ISO 17025-accredited procedures. Because the batch size is typically below 100 g, the entire operation is contained within a single fume hood; waste streams containing palladium and halogenated solvents are segregated for off-site incineration in accordance with EPA 40 CFR Part 264.