7-Bromo-1,3-Benzothiazole

In late-stage functionalisation workflows, 7-bromo-1,3-benzothiazole serves as a halogen-bearing electrophile for palladium-catalysed cross-coupling. A common protocol employs Suzuki-Miyaura conditions: the bromoarene (1.0 eq), an arylboronic acid (1.05–1.3 eq), and tetrakis(triphenylphosphine)palladium(0) (0.5–2.0 mol%) are charged to a glass-lined reactor, followed by degassed 1,4-dioxane/water (4:1 v/v) and anhydrous potassium carbonate (2.0–2.5 eq). The mixture is held at 80–90 °C under nitrogen for 6–16 h, with conversion monitored by HPLC at 254 nm. After cooling, the organic layer is washed with brine, dried over MgSO₄, and concentrated. Flash chromatography on silica gel (eluent: ethyl acetate/heptane 1:9 to 1:4) delivers the biaryl product in 70–88% isolated yield. Residual palladium is controlled to <10 ppm via treatment with a metal scavenger (e.g., SiliaMetS® Thiol), compatible with ICH Q3D Option 1 limits for parenteral administration. The intermediate purity specification is set at ≥98.5% (HPLC, area%), with single unknown impurity ≤0.5%, and is accompanied by a certificate of analysis compliant with ISO 9001:2015. In medicinal chemistry campaigns, the resulting biaryl scaffold is elaborated into ATP-competitive kinase inhibitors. For instance, introduction of a 2-amino substituent followed by acylation yields a structural analogue of the benzothiazole-containing clinical candidate BLU-945; isolated enzyme assays against EGFR L858R/T790M mutant show IC₅₀ values below 10 nM under phosphate-buffered conditions (pH 7.4, 10 µM ATP, DELFIA® platform). The brominated precursor thus enables rapid SAR exploration at the solvent-exposed region of the hinge-binding motif.

A typical agrochemical discovery program targets the replacement of the chlorine atom in commercial succinate dehydrogenase inhibitor (SDHI) fungicides with a more lipophilic bromobenzothiazole fragment. The 7-bromo-1,3-benzothiazole is first subjected to a lithium-bromine exchange using n-butyllithium (1.05 eq, hexane solution) in anhydrous THF at -78 °C under argon; after 30 min, dimethylformamide (1.5 eq) is introduced, the cooling bath is removed, and the mixture is allowed to reach 0 °C over 2 h. Quenching with saturated NH₄Cl, extraction with MTBE, and vacuum distillation yields 7-formyl-1,3-benzothiazole in 65–78% yield. The aldehyde is then condensed with 2-aminobenzamide in refluxing ethanol in the presence of a catalytic amount of p-toluenesulfonic acid, furnishing a Schiff base that is oxidised with iodine in DMSO at 90 °C to give the 2-(benzothiazol-7-yl)benzimidazole core. This heterocyclic framework is subsequently treated with thionyl chloride in toluene at 75 °C to install a 2-chloromethyl handle, which is displaced with 4-fluorophenol (1.2 eq, K₂CO₃, DMF, 60 °C) to produce the final fungicidal lead compound. Bioassay results against Zymoseptoria tritici in microtiter plate format (EPPO Standard PP 1/26) indicate mycelial growth inhibition of ≥90% at 50 ppm. For field trial translation, the active ingredient is formulated as a 250 g/L suspension concentrate (SC) using an Atlox™ 4913/Agnique™ BL 3625 emulsifier blend and wet-milled to a particle size D₉₀ <5 µm (Malvern Mastersizer 3000). The formulated product meets the FAO/WHO Joint Meeting on Pesticide Specifications (JMPS) definition for suspension concentrate stability: persistent foam ≤10 mL after 1 min (CIPAC MT 47.2), pourability residue ≤5%, and wet sieve retention on a 75 µm test sieve ≤1%. Analytical control of the technical material requires a purity of ≥96% (GC-FID), with dibromo analogue ≤1.0% and des-bromo impurity ≤0.5%, in accordance with SANCO/3030/99 rev.5 guidelines.

When the Thermal Activation of Delayed Fluorescence Emitters Demands a Brominated Acceptor Core

The electron-deficient nature of the benzothiazole ring, combined with the polarisable bromine substituent, makes 7-bromo-1,3-benzothiazole a valuable precursor for thermally activated delayed fluorescence (TADF) materials used in highly efficient organic light-emitting diodes. In a representative synthetic sequence, the bromine atom at position 7 undergoes Buchwald-Hartwig amination with 9,9-dimethylacridan (1.2 eq) in the presence of tris(dibenzylideneacetone)dipalladium(0) (3 mol%) and 2-dicyclohexylphosphino-2′,4′,6′-triisopropylbiphenyl (XPhos, 9 mol%) in anhydrous toluene at 110 °C for 20 h. The crude product is purified by column chromatography (hexane/CH₂Cl₂ 4:1) and subsequently subjected to gradient train sublimation under high vacuum (<1×10⁻⁶ mbar) in a two-zone furnace, with the source temperature ramped from 140 °C to 190 °C at a rate of 2 °C/h. This process delivers material with a purity exceeding 99.9% (HPLC, 270 nm) and a halide content verified <5 ppm by ion chromatography, critical for suppressing exciton quenching in the emissive layer. The sublimed powder is co-evaporated with a host matrix (e.g., mCBP) in a Kurt J. Lesker SPECTROS™ deposition system at 2×10⁻⁷ Torr base pressure, forming a 30 nm thick emissive layer. Photophysical characterisation in a doped film (10 wt% emitter in PMMA) reveals an energy gap ΔE_ST of 0.08 eV determined from the onset of fluorescence and phosphorescence spectra at 77 K, and a transient photoluminescence decay exhibiting prompt and delayed components with lifetimes of 25 ns and 8.4 µs, respectively. The resulting bottom-emission OLED device (ITO/HAT-CN/NPB/TCTA/mCBP:emitter/TPBi/LiF/Al) exhibits a maximum external quantum efficiency (EQE) of 21.4% with Commission Internationale de l’Eclairage coordinates (0.15, 0.20), measured in an integrating sphere according to a protocol aligned with IEC 62341-2-1. Compliance with Directive 2011/65/EU (RoHS 2) regarding hazardous substances is verified by XRF screening, while the absence of restricted polybrominated diphenyl ethers is confirmed by GC-MS per IEC 62321-6:2015.

Fluorogenic probes for intracellular glutathione (GSH) are constructed by exploiting the nucleophilic aromatic substitution of the 7-bromo group with thiolate anions. The bromo-thiazole scaffold is first N-alkylated with a pyridinium salt (ethylene-bridged indolium iodide) in acetonitrile under reflux for 18 h, producing a styryl-type cyanine precursor with strong absorption at 540 nm. To the crude dye dissolved in degassed DMSO, a solution of sodium sulfide nonahydrate (2.2 eq) and sodium methanethiolate (0.4 eq) in methanol is added dropwise at 25 °C; the reaction progress is tracked by the disappearance of the aryl bromide stretch at 1085 cm⁻¹ in FTIR. After 4 h, the mixture is poured into ice-cold diethyl ether precipitating the disulfide-linked probe, which is collected and washed with acetone to obtain a dark purple solid in 58% yield. The probe exhibits a fluorescence turn-on ratio of 65-fold at 605 nm upon addition of 2 mM GSH in HEPES buffer (pH 7.4, 37 °C), as concentration-time curves fit a first-order kinetic model with a rate constant of 0.18 min⁻¹. Selectivity was profiled against cysteine, homocysteine, and dithiothreitol; the response to GSH was 12-fold higher than to cysteine, consistent with the thiol-disulfide exchange mechanism reported in peer-reviewed fluorogenic assays. For live-cell imaging, HeLa cells are incubated with 2 µM probe in serum-free DMEM for 20 min at 37 °C followed by PBS wash; confocal microscopy (Zeiss LSM 880, λ_ex 561 nm, λ_em 600–650 nm) reveals punctate cytoplasmic staining with an average intensity 4.8-fold above the unstained control. The brominated intermediate must be stored under dry conditions at -20 °C under argon to avoid hydrolytic degradation, with a retest date assigned after 24 months based on accelerated stability studies at 40 °C/75% RH per ASTM F1980-21. While no harmonised pharmacopoeial monograph exists for research probes, the material is routinely tested for bacterial endotoxins (LAL assay, <0.25 EU/mg) and screened for mutagenic impurities under ICH M7 (TTC 1.5 µg/day) when supplied to pharmaceutical researchers transitioning from hit-to-lead programs.

Constructing P,N-Ligands with a Benzothiazole Backbone for Asymmetric Allylic Alkylation

In homogeneous catalysis, the planar benzothiazole ring provides a rigid, π-accepting scaffold that, when decorated with a chiral phosphine vector, yields highly enantioselective ligands for palladium-mediated transformations. The synthetic route begins with the regioselective metallation of 7-bromo-1,3-benzothiazole using turbo-Grignard iPrMgCl·LiCl (1.1 eq) in THF at -20 °C for 45 min, achieving complete magnesium-halogen exchange confined to the C-2 position as confirmed by deuterium quench ¹H NMR analysis. The resulting heteroaryl Grignard solution is cannulated into a chilled (-40 °C) solution of chlorodiphenylphosphine (1.3 eq) in THF and stirred for 2 h while slowly warming to ambient temperature. After aqueous workup under nitrogen, the crude 2-(diphenylphosphino)-7-bromobenzothiazole is crystallised from hot ethanol/water (3:1) to furnish colourless needles in 81% yield with >99.5% purity by ³¹P NMR. The bromide handle at position 7 is subsequently utilised in a second Suzuki coupling with pinacolboronate ester of (S)-BINOL to generate a C₁-symmetric phosphine-phosphoramidite hybrid ligand. In a standard Pd(0)-catalysed asymmetric allylic alkylation of 1,3-diphenylallyl acetate with dimethyl malonate (BSA/KOAc, CH₂Cl₂, 25 °C), a catalyst loading of 1.5 mol% [Pd(C₃H₅)Cl]₂ to 3.0 mol% ligand provides the (R)-allylation product in 93% enantiomeric excess (chiral HPLC, Chiralpak AD-H, hexane/i-PrOH 90:10, 0.8 mL/min, retention times of 12.3 and 14.1 min for the two enantiomers). Kinetic profiling under these conditions reveals a substrate-dependent turnover frequency reaching 850 h⁻¹ at 10 min conversion, followed by a gradual decline attributed to precipitation of the Pd(0) species. The brominated ligand precursor is stored and manipulated in a nitrogen-atmosphere glovebox (H₂O and O₂ <0.1 ppm) and is supplied with a certificate of analysis referencing ³¹P NMR and inductively coupled plasma mass spectrometry (ICP-MS) data: Fe <10 ppm, Ni <5 ppm, Pd <2 ppm. While no formal regulatory standard governs research-grade ligand intermediates, the material is pro-actively tested against the methodology outlined in the EP monograph 2.2.58 for melting point and water content (Karl Fischer, <200 ppm), ensuring reproducible catalytic performance across different batches used in kilo-lab campaigns running under cGMP-like conditions.