6-Bromo-2-Chlorobenzothiazole

During lead optimization of ATP-competitive kinase inhibitors, 6-bromo-2-chlorobenzothiazole serves as a bifunctional building block that enables serial derivatisation without the need for intermediate protecting groups. The chlorine atom at the 2-position undergoes selective nucleophilic aromatic substitution with primary and secondary amines at room temperature in THF, using 1.2 equivalents of amine and 1.5 equivalents of triethylamine as a proton scavenger. The 6-bromo substituent remains intact and is subsequently engaged in a palladium-catalyzed Suzuki–Miyaura cross-coupling. A representative protocol loads 1.0 equiv of the 2-amino-6-bromo intermediate, 1.3 equiv of arylboronic acid, 2 mol% Pd(OAc)₂/SPhos, and 3.0 equiv of tribasic potassium phosphate in a degassed dioxane/water mixture (4:1 v/v). The sealed vessel is heated to 100 °C for 12 h with magnetic stirring; conversion is monitored by LC-MS at the 6-position. The resulting 2,6-disubstituted benzothiazole is isolated by flash chromatography in yields ranging from 72% to 88% depending on the aryl partner. Batch-to-batch reproducibility on 500 g scale in a jacketed glass reactor requires strict control of the exotherm during amine addition—internal temperature is maintained below 30 °C by dosing the amine over 45 min. Residual palladium content is critical when the downstream active pharmaceutical ingredient is destined for clinical supply; levels are reduced to <10 ppm by treatment with trimercaptotriazine-functionalised silica scavengers followed by charcoal filtration. The 6-aryl-2-aminobenzothiazole pharmacophore is incorporated into Type II kinase inhibitors targeting a DFG-out conformation, and the synthesis route is aligned with ICH Q7 and Q11 guidelines for starting material definition. A regulatory starting material justification package often traces the origin of the benzothiazole core to this dihalogenated intermediate because both reaction centres exhibit orthogonal reactivity that de-risks the manufacturing process.

A distinct application pathway exploits the halogen at the 2-position as a leaving group in the construction of herbicidal benzothiazolylurea scaffolds. The substitution of chlorine by anhydrous ammonia or aliphatic amines is conducted under pressure in a stainless-steel autoclave; a typical charge combines 1.0 eq of 6-bromo-2-chlorobenzothiazole, 5.0 eq of the amine, and isopropanol as solvent, heated to 120 °C for 6 h with a nitrogen blanket at 3 bar. The 6-bromo substituent is tolerated quantitatively, preserving a handle for later diversification or for maintaining metabolic stability in the field. The 2-aminobenzothiazole intermediate is then treated with an aryl isocyanate in methylene chloride at 0–5 °C to install the urea linkage. Filtration and recrystallisation from ethanol/water yield the benzothiazolylurea herbicide precursor with an HPLC purity exceeding 98 area%. Greenhouse screening against dicotyledonous weeds in compliance with EPA 40 CFR Part 158 relies on this scaffold as a core building block. The process safety risk assessment for the autoclave step addresses the potential accumulation of ammonium chloride as a solid, which can impede agitation; therefore a pitched-blade impeller with a tip speed of 2.5 m/s is specified. Scale-up to 200 L has been qualified, with the critical quality attribute being the residual chloride ion content in the isolated intermediate, kept below 0.5% to avoid catalyst poisoning in subsequent organometallic transformations.

When bromine substitution retards scorch in thiazole accelerator systems

6-Bromo-2-chlorobenzothiazole is converted into a delayed-action vulcanisation accelerator through a two-step sequence: first, the 2-chloro group is exchanged with sodium hydrosulfide hydrate (NaSH·xH₂O) in refluxing ethanol to give 6-bromo-2-mercaptobenzothiazole (6-Br-MBT); second, oxidative coupling with hydrogen peroxide in the presence of sulfuric acid yields bis(6-bromobenzothiazol-2-yl) disulfide. The electron-withdrawing bromine atom at the 6-position lowers the electron density on the thiazole ring, which measurably shifts the onset of crosslinking in natural rubber formulations. Use of this disulfide in a standard accelerated sulfur system—comprising 100 phr NR SMR CV60, 50 phr N330 carbon black, 5 phr ZnO, 2 phr stearic acid, 2.5 phr sulfur, and 1.2 phr of the disulfide accelerator—is characterised by a cure curve recorded on a moving die rheometer at 160 °C in accordance with ASTM D5289. The scorch time t_s2 is extended by 25–35% relative to the non‑brominated analogue, providing a broader processing safety window for thick-section profiles extruded through a 90 mm single-screw extruder with a length-to-diameter ratio of 18:1. The maximum torque M_H decreases by less than 8%, indicating that crosslink density is essentially preserved. Mixing is performed on a 1.6 L tangential internal mixer with a fill factor of 0.75; the discharge temperature must not exceed 115 °C to prevent premature cleavage of the disulfide bridge. Mooney viscosity (ML 1+4 at 100 °C) measured per ASTM D1646 remains stable over a 48 h storage period under 40 °C and 85% relative humidity, indicating adequate bin stability. Migration of unreacted accelerator to the rubber surface during storage is suppressed because of the increased molecular weight of the brominated disulfide, a property confirmed by static extraction tests with acetonitrile according to ISO 21461. Formulators experience a predictable rheokinetic response that allows calendar scheduling adjustments without reformulation of the co‑agent package.

Table 1: Orthogonal reactivity pattern of 6-bromo-2-chlorobenzothiazole—illustrative reaction conditions from open‑and published literature on analogous substrates

Direct mechanochemical processing of 6-bromo-2-chlorobenzothiazole with N,N-diethylaniline in a planetary ball mill reduces the solvent burden substantially. The milling jar is charged with 1.0 eq of the benzothiazole, 1.05 eq of the tertiary aniline, 2.0 eq of anhydrous sodium carbonate, and 0.1 mL of PEG‑400 as a phase‑transfer grinding auxiliary per gram of solid. Milling proceeds for 90 min at 600 rpm using 10 mm zirconia balls at a ball‑to‑powder mass ratio of 25:1. The paste obtained is slurried in water and extracted with ethyl acetate to recover 2‑(N,N‑diethylanilino)‑6‑bromobenzothiazole in 84% isolated yield. Successful reduction of the 6‑bromo to the 6‑amino group is accomplished with ammonia in the presence of Cu₂O in ethylene glycol at 120 °C in a pressure tube. The 6‑amino derivative is diazotised with nitrosylsulfuric acid at −5 °C and coupled with N‑ethyl‑N‑(2‑hydroxyethyl)aniline to produce a red azo disperse dye. The crude dye cake is dispersed with a lignin‑based dispersant and spray‑dried to yield a granular formulation. Exhaustion dyeing on polyester fabric is carried out at 130 °C for 60 min at a liquor ratio of 20:1; the build‑up curve reaches saturation at 2.0% o.w.f.. Fastness to light assessed under ISO 105‑B02 yields a rating of 6–7 on the blue wool scale, attributed to the benzothiazole chromophore’s inherent photostability. Wash fastness according to ISO 105‑C06 test method C2S receives a shade change grade of 4–5. The absence of nitro groups in the final dye structure simplifies the environmental safety dossier under ZDHC MRSL Level 1 requirements for textile finishing.

Acidic descaling baths: adsorption thermodynamics of halogenated benzothiazole inhibitors

Weight‑loss experiments following ASTM G31‑72 immersion protocols have demonstrated that 6‑bromo‑2‑chlorobenzothiazole functions as a mixed‑type corrosion inhibitor for carbon steel in hydrochloric acid pickling baths. A test solution of 15% HCl at 30 °C dosed with 200 mg/L of the compound reduces the corrosion rate of AISI 1020 steel coupons from 12.7 mm/y to 0.9 mm/y, corresponding to an inhibition efficiency of 93% after 6 h. The adsorption isotherm obeys the Langmuir model with an adsorption equilibrium constant of 4.2 × 10³ M⁻¹ and a standard free energy of adsorption of −34.2 kJ/mol, indicating chemisorption involving the lone pairs of the nitrogen and sulfur heteroatoms as well as the electron‑rich bromine. Potentiodynamic polarisation scans conducted per ISO 17475 at a scan rate of 0.5 mV/s reveal a shift of the corrosion potential of less than 30 mV, classifying the compound as a mixed‑type inhibitor that retards both the anodic iron dissolution and the cathodic hydrogen evolution reactions. Electrochemical impedance spectroscopy in the frequency range 100 kHz to 10 mHz shows a single depressed capacitive loop whose diameter increases with inhibitor concentration; the charge‑transfer resistance obtained by fitting the Nyquist plot to a constant‑phase‑element model rises from 18 Ω·cm² in the blank to 430 Ω·cm² at 200 mg/L. Continuous flow loop tests in a recirculating descaling rig constructed from 316L stainless steel confirm that the inhibitor does not promote pitting on the steel surface at Reynolds numbers up to 1.2 × 10⁴. Because the compound is sparingly water‑soluble (<50 mg/L at 25 °C), the practical formulation includes 10 vol% isopropanol as a co‑solvent and a quaternary ammonium surfactant to maintain dispersion. The preparation must be stored in HDPE containers and is stable for 12 months when kept below 35 °C; partial hydrolysis of the 2‑chloro group is observed above 50 °C, which generates trace 2‑hydroxybenzothiazole that weakly chelates iron ions and reduces inhibitor efficiency.

Table 2: Regulatory and standards matrix across downstream segments involving 6‑bromo‑2‑chlorobenzothiazole

Intramolecular charge transfer probes constructed via sequential Suzuki–Buchwald couplings at the 6- and 2-positions

Fluorescent molecular rotors that respond to viscosity changes in biological membranes are assembled by exploiting the orthogonal reactivity of 6‑bromo‑2‑chlorobenzothiazole. The 6‑bromo site is first coupled with 4‑formylphenylboronic acid under standard Suzuki conditions to install an electron‑accepting aldehyde group; a vacuum‑assisted nitrogen sparge of the biphasic mixture before catalyst addition maintains the turnover rate of Pd(PPh₃)₄ (1.5 mol%) over the full 10 h reaction period at 85 °C. The isolated 2‑chloro‑6‑(4‑formylphenyl)benzothiazole intermediate is then heated with an excess of diphenylamine (2.0 eq) in the presence of Pd₂(dba)₃ (1 mol%) and DavePhos (2.4 mol%) in anhydrous toluene at 100 °C, with sodium tert‑butoxide as the base. The combined one‑pot workup yields the donor‑π‑acceptor benzothiazole fluorophore in 63% overall yield after silica gel chromatography. The absorption maximum in chloroform occurs at 428 nm, and the emission maximum at 562 nm exhibits a large Stokes shift of 134 nm that minimises self‑quenching in imaging applications. Fluorescence quantum yield, determined by the absolute integrating‑sphere method following IUPAC Technical Report recommendations, reaches 0.48 in low‑viscosity toluene and drops to 0.21 in glycerol, confirming the sensitivity of the twisted intramolecular charge‑transfer state to the local rigidity of the environment. Photostability testing under continuous 470 nm LED irradiation (50 mW/cm²) for 120 min reveals a fluorescence intensity retention of 91%, making the probe suitable for time‑lapse confocal microscopy. The molecule’s logP of 4.8 facilitates passive membrane partitioning without the need for endocytic uptake enhancers.