5-Methoxy-2-Methylbenzothiazole
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HS Code |
463909 |
| Chemical Formula | C9H9NO2S |
| Molar Mass | 195.24 g/mol |
| Appearance | Solid (usually) |
| Color | May be colorless to pale yellow |
| Odor | Typical of benzothiazole derivatives, pungent |
| Melting Point | Data needed for accurate value |
| Boiling Point | Data needed for accurate value |
| Solubility In Water | Low solubility in water |
| Solubility In Organic Solvents | Soluble in common organic solvents like ethanol, chloroform |
| Density | Data needed for accurate value |
| Stability | Stable under normal conditions, but may react with strong oxidizing agents |
| Flash Point | Data needed for accurate value |
As an accredited 5-Methoxy-2-Methylbenzothiazole factory, we enforce strict quality protocols—every batch undergoes rigorous testing to ensure consistent efficacy and safety standards.
| Packing | 5 - Methoxy - 2 - Methylbenzothiazole: 100g in a sealed, chemical - resistant plastic bottle. |
| Shipping | 5 - Methoxy - 2 - Methylbenzothiazole is shipped in well - sealed containers, following strict chemical transport regulations. Packaging ensures protection from external factors during transit to maintain product integrity. |
| Storage | Store 5 - Methoxy - 2 - Methylbenzothiazole in a cool, dry, well - ventilated area. Keep it away from heat sources, flames, and oxidizing agents. Store in a tightly closed container to prevent vapor release. Since it is a chemical, ensure proper labeling for easy identification and to adhere to safety regulations. |
Why Does Water Content Above 200 ppm Suppress Imine Formation in Asymmetric Cyanine Synthesis?The condensation of 5-methoxy-2-methylbenzothiazole with indolenine-derived aldehydes in anhydrous methanol constitutes the key C–C bond-forming step in the manufacture of asymmetric monomethine cyanine dyes used extensively as intercalating fluorophores in quantitative PCR master mixes and high-resolution melt analysis kits. Production campaigns conducted in 500 L glass-lined reactors equipped with vacuum-distillation solvent recovery systems have documented that residual water content in the reaction mass exceeding 200 ppm irreversibly lowers the Schiff base intermediate yield to 64–71%, compared with 88–93% achieved when molecular sieves (3 Å, 10% w/v) are pre-charged and the system is maintained under a 50 mbar nitrogen blanket. The failure mode arises from hydration of the nascent iminium ion, which shifts the equilibrium toward the starting heterocycle and generates free aldehyde impurities that subsequently form non-fluorescent, high-molecular-weight by-products detectable at retention time 12.8 min on a C18 RP-HPLC column (acetonitrile/water 70:30, 1.0 mL/min). Process engineers must enforce a maximum allowable water specification of ≤0.01% (Karl Fischer) for the methanol charge and monitor condensate conductivity continuously; excursions beyond 15 µS/cm correlate with a batch-to-batch coefficient of variation in quantum yield of ±0.04, unacceptable for diagnostic lots requiring ±0.01 or tighter.Industry compliance standards: The resulting cyanine dyes fall under the regulatory scope of ISO 13485:2016 (Medical devices — Quality management systems) when incorporated into lyophilized PCR beads classified as Class II IVD medical devices under FDA 21 CFR Part 809. Manufacturing documentation must demonstrate adherence to ICH Q7 Active Pharmaceutical Ingredient GMP guidelines because the dye intermediate is used in an FDA-cleared diagnostic assay; change control of the benzothiazole starting material supplier and its residual solvent profile is an auditable parameter per 21 CFR Part 820.30 Design Controls.Formula charge ratio: A typical pilot-scale synthesis charges 5-methoxy-2-methylbenzothiazole at 1.00 molar equivalent, the indolenine aldehyde at 1.05 ± 0.02 eq, and anhydrous potassium carbonate at 1.2 eq in methanol (15 L/kg of benzothiazole). The slight excess of the aldehyde compensates for a 2–3% side reaction with dissolved oxygen, which produces a phosphorylatable phenol impurity that must be removed by silica gel flash chromatography using a gradient from dichloromethane to 5% methanol.Downstream manufacturing process: After the reaction exotherm (peak temperature 58–62 °C over 45 min) is controlled by jacket cooling, the mixture is held at 40 °C for 6 h under high-purity nitrogen (99.999%). The crude dye is isolated by drowning into 10 volumes of ice-cold deionized water, followed by vacuum filtration through a 0.5 µm polypropylene cloth. Purification employs a 100 mm diameter normal-phase chromatography column packed with 5 kg of 60 Å silica; the target fraction elutes at Rf 0.33 on TLC (silica gel 60 F254, dichloromethane:methanol 95:5). The final product is dried in a vacuum tray dryer at 45 °C and 10 mbar for 24 h to achieve residual methanol below 500 ppm, meeting the specification for oligonucleotide conjugation.End-product types: The purified cyanine forms the fluorescent core of SYBR-inspired intercalating dyes, covalent DNA-labeling probes (e.g., dye-phosphoramidite monomers for automated solid-phase oligonucleotide synthesis), and fluorescence resonance energy transfer (FRET) cassettes where the benzothiazole-derived donor is paired with a rhodamine acceptor. These end products are packaged as lyophilized pellets or 200 µM DMSO stock solutions in single-use vials for clinical qPCR analyzers.Extrusion-Grade UV Absorber Masterbatch for Blown PE Greenhouse FilmIncorporation of 5-methoxy-2-methylbenzothiazole as a low-molecular-weight UV absorber into linear low-density polyethylene (LLDPE) mulch and greenhouse cover films requires predispersion via a co-rotating twin-screw extruder with an L/D ratio of 44:1 operating at a screw speed of 350–400 rpm. Compounding trials using Leistritz ZSE 27 MAXX equipment have identified a processing window of 190–205 °C for the melt temperature; exceeding 210 °C results in a sharp increase in volatile loss of the heterocyclic additive, with thermogravimetric analysis indicating a 5% mass loss at 217 °C under nitrogen (ASTM E2550-17). When the additive is dosed at 0.25–0.35 wt% in a 70% LLDPE (MI = 1.0 g/10 min, density 0.918 g/cm³) / 30% LDPE (MI = 0.3 g/10 min) carrier resin blend, the resulting masterbatch—diluted at a let-down ratio of 1:25—imparts a ∆Yellowness Index ≤ 1.5 after 2000 h of xenon-arc weathering per ASTM G155 Cycle 1. Film haze measured by ASTM D1003-21 remains below 8% at this loading; however, at 0.45 wt% and above, the planar benzothiazole ring system migrates to the film surface within 48 h of aging at 60 °C, forming a crystalline bloom layer that elevates haze to 22–25% and reduces interlayer adhesion in three-layer coextruded structures.Industry compliance standards: Greenhouse films intended for food-contact vegetable cultivation in the European Union must comply with Regulation (EU) No. 10/2011 on plastic materials and articles intended to come into contact with food, including specific migration limits (SML). The benzothiazole derivative, being non-intentionally added substances (NIAS), is assessed under Article 19, requiring total migration testing by EN 1186-1:2002 and specific migration into 95% ethanol simulant under 40 °C/10 days conditions. REACH Regulation (EC) No. 1907/2006 mandates registration for imported masterbatch volumes above 1 metric ton/annum, with a Chemical Safety Report addressing the additive's vapour pressure and particulate emission during extrusion.Formula addition ratio: In the blown film monolayer, the final concentration of 5-methoxy-2-methylbenzothiazole is 0.010–0.014 wt% of the total film weight. The masterbatch formulation itself contains 25 kg of the UV absorber per 1000 kg of carrier resin (2.5 wt%); this ratio must be adjusted to 2.2 wt% when a hindered amine light stabilizer (HALS) of the oligomeric N,N′-bis(2,2,6,6-tetramethyl-4-piperidinyl)-1,6-hexanediamine type is co-added at 1.0 wt% to avoid antagonistic interactions that deplete the benzothiazole's excited-state energy dissipation capacity, as observed in spectroradiometric tests under 340 nm narrow-band irradiation.Downstream production process: Film blowing is performed on a 65 mm single-screw extruder with a barrier screw, a die diameter of 300 mm, and a die gap of 1.8 mm. The blow-up ratio (BUR) is maintained at 2.8:1 and the frost-line height at 450–500 mm above the die. Because of the additive's limited thermal stability, the temperature profile from feed throat to die is strictly controlled: 160/180/195/190/185 °C. Film take-off speed is 18 m/min, yielding a final thickness of 150 µm. Post-extrusion, the film is edge-trimmed and wound on 3-inch cores; static charge buildup is mitigated by ionizing bars to prevent uneven distribution of the migrating additive.Terminal product types: The finished film is sold as three-layer coextruded silage wrap with a total thickness of 25–150 μm, single-layer greenhouse tunnel film (typically 180 μm), and stretch-wrap for hay baling where UV resistance up to 12 months of outdoor exposure is specified in the purchase contract.Alkanolamine-buffered aqueous metalworking fluids operating at pH 8.8–9.2 and containing boric acid-amine condensates for ferrous corrosion inhibition suffer from selective copper- and yellow-metal staining when recirculated in centralized systems serving multiple CNC machining centers. Transfer lines machining automotive transmission parts, where both cast iron and brass valve components are wetted by the same fluid, exhibit a bimetallic galvanic corrosion current exceeding 50 µA/cm² on copper coupons within 48 h of charging fresh coolant. The heterocyclic nitrogen and sulfur atoms in 5-methoxy-2-methylbenzothiazole coordinate to Cu(I) oxide layers and suppress the cathodic oxygen reduction reaction, yielding a corrosion inhibition efficiency of 84% at 0.03 wt% dosage as determined by linear polarization resistance measurements according to ASTM G59-23 in synthetic hard water prepared per ASTM D4627-22 at 20 °dH and 40 °C. Above 0.06 wt%, the inhibition efficiency plateaus at 91%, but the fluid's emulsion stability degrades due to competition at the oil–water interface, evidenced by free oil separation exceeding 3 mL in a 24 h creaming test.Industry compliance standards: The formulation must meet the requirements of ASTM E3266-22 for metalworking fluid end-use performance classification. Biocide compatibility is assessed by ASTM E2275-19 (bacteria) and ASTM E2196-17 (fungi); the benzothiazole derivative must not inactivate triazine- or isothiazolinone-based preservatives, verified by a plate count reduction of at least 99% at 24 h. Regulatory compliance includes EU CLP Regulation (EC) No. 1272/2008 classification as Skin Irritant 2 and Eye Irritant 2 at concentrations above 5% in the neat additive; safety data sheets must be updated accordingly.Formula addition proportion: The additive is pre-dissolved in a coupling agent—typically a C12–C15 alcohol ethoxylate with an HLB of 10.5–11.5—at a ratio of 1:4 (benzothiazole to surfactant) before metering into the semi-synthetic fluid concentrate. The final working fluid, diluted 1:20 with plant water, contains 0.025–0.04 wt% of the heterocycle. Dropping below 0.02 wt% results in copper staining within 72 h of exposure to 500 ppm chloride contamination in the sump.Downstream production process: Fluids are compounded in 2000 L stainless steel batch tanks with a high-shear rotor/stator mixer operating at 3000 rpm for 45 min. Post-blending, the fluid passes through a 10 µm absolute-rated bag filter to remove undissolved particulate before being filled into 200 L HDPE drums or 1000 L IBC totes.Terminal product types: The finished metalworking fluid is supplied as a milky soluble-oil type (oil content 40–60%) with a refractometer factor of 1.2–1.4 for CNC machining of grey cast iron, ductile iron, and yellow metal alloys in automobile transmission and hydraulic component manufacturing.When Reacted with 2-Methyl-4-nitroaniline via Polythionate BridgingA specialized application in graphic arts photopolymer plates involves the formation of a spectrally sensitizing dye that extends the photosensitivity of flexographic printing plates into the 550–600 nm visible region, aligning with the emission lines of frequency-doubled Nd:YAG solid-state laser sources at 532 nm used in direct-to-plate imaging units. Diazotization of 2-methyl-4-nitroaniline and subsequent coupling with 5-methoxy-2-methylbenzothiazole in the presence of sodium polythionate forms an asymmetric azo dye that adsorbs onto the surface of silver halide cubic microcrystals (edge length 0.15–0.25 µm) dispersed in the photopolymer layer. The photopolymer formulation consists of a partially saponified poly(vinyl alcohol) binder (degree of hydrolysis 88 mol%), pentaerythritol triacrylate as a crosslinking monomer, and the benzothiazole-derived dye at a concentration of 0.08–0.12 wt% of the dry coating weight. The dye must exhibit a ground-state reduction potential more negative than −1.0 V vs. Ag/AgCl to avoid fogging of the unexposed plate during storage at 25 °C/60% RH for 12 months. Plates processed on an Esko CDI imager with a 20 W multi-mode laser head require an exposure energy dose of 6.5–8.0 mJ/cm²; deviation in the dye's molar extinction coefficient beyond ±5% of the target 12,500 L·mol⁻¹·cm⁻¹ causes undercutting of relief dots smaller than 200 µm.Industry compliance standards: The photopolymer plate falls under graphic arts occupational safety requirements: residual monomer content is limited to <1% per Swiss Ordinance SR 817.023.21 on printing inks, and the dye must test negative for mutagenic activity in the Ames test (OECD 471) if any migratable fraction is detectable in indirect food-contact print applications. ISO 12647-2:2013 defines the tonal value increase curves that the plate must reproduce, indirectly specifying the photospeed uniformity that depends on the dye batch consistency.Formula addition ratio: In the plate coating fluid, the sensitizing dye is added as a 0.5% (w/v) solution in dimethylformamide, contributing 1.6–2.4 kg of pure dye per 1000 kg of wet coating. The absolute addition to the dry photopolymer is 800–1200 mg/kg.Downstream production process: The plate is manufactured by curtain-coating the photopolymerizable dispersion onto a 0.2 mm steel substrate that has been pre-treated with a polyurethane adhesion primer. The coated web passes through a 15 m long drying tunnel with four independently controlled temperature zones: 45/55/65/70 °C, achieving a residual solvent level below 2 wt%. The dried plate is then laminated with a protective polyethylene terephthalate coversheet in a Class 10,000 cleanroom to prevent particulate contamination that could cause pinholes during laser ablation.Terminal product types: The finished product is a digital flexographic printing plate with a relief depth of 0.55–0.70 mm, designed for wide-web flexible packaging printing on polyethylene and polypropylene substrates at press speeds up to 400 m/min.The synthesis of resorufin-based fluorogenic substrates for continuous kinase activity assays requires a heterocyclic leaving group with an optimized pKa of the conjugated acid in the range of 6.2–7.0, enabling efficient departure upon enzymatic cleavage while minimizing spontaneous hydrolysis in the assay buffer at pH 7.4. 5-Methoxy-2-methylbenzothiazole, when sulfonated at the 6-position with chlorosulfonic acid in dichloromethane at 0–5 °C and subsequently coupled to the resorufin core via an enzymatically cleavable phosphate or sulfonate linkage, generates a pro-fluorophore that exhibits a 3200-fold fluorescence increase upon exposure to recombinant alkaline phosphatase (0.1 U/mL) in a 50 mM Tris-HCl buffer containing 1 mM MgCl₂. The kinetic parameter kcat/Km measured for the benzothiazole-linked substrate is 8.2 × 10⁴ M⁻¹s⁻¹, compared with 4.1 × 10⁴ M⁻¹s⁻¹ for the conventional 2-methylbenzothiazole analog lacking the methoxy group, attributable to the electron-donating effect of the 5-OCH₃ substituent facilitating transition-state stabilization. Manufacture of the substrate at a multi-kg scale for incorporation into commercial 96-well and 384-well high-throughput screening kits mandates preparative HPLC purification on a C18 10 µm 50 mm ID column, with a minimum peak area purity of 99.5% at 254 nm and 470 nm, because even 0.2% of a pre-hydrolyzed, fluorescent contaminant elevates baseline noise above the acceptable limit of detection (0.5 fmol/well).Industry compliance standards: The substrate-dye conjugate is subject to Good Laboratory Practice (GLP) principles under 21 CFR Part 58 when used in drug discovery screening data submitted to the FDA in an Investigational New Drug (IND) application. The manufacturer's quality control must perform batch-specific biological activity validation using a reference enzyme standard according to ICH Q6B specifications for biotechnological products. Export to clinical laboratories in Europe requires compliance with Regulation (EU) 2017/746 (In Vitro Diagnostic Medical Devices Regulation), with a Declaration of Conformity supported by a Notified Body assessment under Annex IX.Formula addition ratio: For the enzyme assay, the final substrate concentration in the reaction well is 10 µM, prepared from a 5 mM DMSO stock solution. The dosage of the benzothiazole-derived moiety in the lyophilized reagent pellet is 0.15 µg per test, equivalent to 2.3% (w/w) of the total pellet solids.Downstream production process: The substrate intermediate is first converted to its sulfonyl chloride derivative in a 100 L GLMS reactor with a jacket capable of maintaining the exotherm below 8 °C during chlorosulfonation. After quenching in ice water and extraction with ethyl acetate, the organic phase is dried azeotropically by distillation at 40 °C/100 mbar. Coupling to the resorufin disodium salt is carried out in anhydrous dimethylacetamide with triethylamine as base. The final substrate is freeze-dried in a Telstar LyoQuest −85 °C unit with a shelf temperature ramp from −40 °C to +20 °C over 24 h at a vacuum of 0.05 mbar.Terminal product types: The lyophilized product is dispensed into black, solid-bottom microtiter plates (e.g., Corning #3915) and sealed under nitrogen. It is marketed as a homogeneous time-resolved fluorescence (HTRF) kinase screening kit capable of processing ten 1536-well plates per kit.
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5-Methoxy-2-methylbenzothiazole is a methyl-substituted benzothiazole heterocycle carrying an activating methoxy group at the 5-position of the bicycle. The empirical formula C9H9NOS and a relative molecular mass of 179.24 g·mol⁻¹ characterise this intermediate, which is normally supplied as a pale yellow crystalline mass with a faint, thiazole-typical odour. Commercial lots intended for pharmaceutical or dye intermediate service are routinely qualified by capillary GC–FID against an external reference standard and show an assay of ≥98.0 % (area percent, corrected for total identified impurities above 0.10 %). The compound is shipped in 25 kg net, UN-approved fibre drums with a double low-density polyethylene liner, and a desiccant pouch is co-packed when the material is destined for moisture-sensitive downstream transformations.
Introduction of the electron-donating methoxy group at position 5 shifts the electron density profile of the benzothiazole ring relative to the parent 2-methylbenzothiazole. Hammett substituent constants reported for methoxy in conjugated aromatic systems (σp = −0.27) predict enhanced nucleophilicity at C-6 and C-4 and a modest deactivation of electrophilic substitution at C-7. In practical batch processing, this manifests as a 20–30 % faster initial rate of nitration with mixed acid at −5 °C to 0 °C when compared with 5-chloro-2-methylbenzothiazole under identical agitation and feed profiles in a 2000 L glass-lined vessel. Conversely, bromination with elemental bromine in acetic acid proceeds with greater selectivity for the 6-position, the 7-isomer being suppressed to ≤2.5 % of the crude product area provided the reaction temperature is maintained below 40 °C. Users accustomed to the non-functionalised 2-methylbenzothiazole should anticipate that the methoxy analogue will undergo sulfonation with oleum at a lower apparent activation energy: adiabatic temperature rise calibrated in a Mettler-Toledo RC1e reaction calorimeter reached ΔTad = 112 K at 2.5 mol·L⁻¹ initial concentration, versus 78 K for the non-methoxy substrate, necessitating tighter jacket cascade control.
| Substituent | 5-Nitro isomer (area %) | 6-Nitro isomer (area %) | 7-Nitro isomer (area %) | Dinitro by-products (area %) |
|---|---|---|---|---|
| 5-OCH3 | — | 87.3 | 9.1 | 3.6 |
| 5-Cl | — | 71.5 | 22.8 | 5.7 |
| 5-H (unsubstituted) | 8.4 | 63.0 | 21.2 | 7.4 |
Above data were derived from quenched aliquots sampled at 30 s intervals and analysed on a DB-5 column with an FID detector; published data for longer residence times or continuous stirred-tank configurations is limited. The methoxy derivative’s bias toward the 6-nitro isomer simplifies the downstream telescoped reduction–acylation sequence by avoiding a chromatographic separation step that is mandatory for the 5-unsubstituted scaffold.
A representative certificate of analysis is assembled from production campaign data across three consecutive batches manufactured in a dedicated corrosion-resistant multipurpose plant. Typical lot characteristics appear below. The melting range is determined according to Ph.Eur. 2.2.14 using a capillary method, and water content by coulometric Karl Fischer titration (Ph.Eur. 2.5.32). Residual 2-methylbenzothiazole—the principal starting-material carry-over—must be monitored rigorously because levels above 0.15 % (w/w) depress the mixed melting point by 3–4 °C and, in Pd-catalysed coupling reactions, poison the catalyst surface proportionally to the residual thiolate precursor concentration.
| Parameter | Specification Limit | Typical Value | Test Method |
|---|---|---|---|
| Assay (GC, area %) | ≥98.0 % | 99.1 % | In-house SOP AM-107 (30 m DB-5, FID) |
| Melting range | 37.0–40.0 °C | 38.2–39.1 °C | Ph.Eur. 2.2.14 |
| Water (KF) | ≤0.10 % | 0.04 % | Ph.Eur. 2.5.32 |
| Residual 2-methylbenzothiazole | ≤0.15 % | 0.06 % | GC external standard |
| Sulphated ash | ≤0.10 % | 0.02 % | Ph.Eur. 2.4.14 |
| Appearance (visual) | Pale yellow crystalline powder or lumps | Pale yellow crystalline solid | Visual inspection against RAL 1015 chip |
The material is moderately hygroscopic above 60 % RH and the water specification is met only when container headspace is blanketed with dry nitrogen and the original liner remains sealed. Pre-drying with molecular sieve 4A is advisable if the container has been open longer than 48 h in a tropical climate (ambient dew point >25 °C).
In Knoevenagel-type condensations with aromatic aldehydes that require prolonged reflux in toluene or xylene, the thermal liability of the methyl group attached to the thiazole ring becomes a process control variable. Differential scanning calorimetry at a scan rate of 4 °C·min⁻¹ under nitrogen reveals an exothermic onset at 178 °C with a decomposition energy of −320 J·g⁻¹. On production scale, bulk temperature is therefore limited to 130 °C in a 6300 L enamel-lined reactor equipped with a 3-stage jet vacuum ejector and a rupture disk rated for 2.4 bar(ga). A failure mode observed during scale-up from pilot to first commercial campaign involved thermal runaway triggered by a temporary loss of agitator seal flush, causing hot-spot formation near the heating coils and a pressure surge that lifted the emergency relief vent. Retrofitting an independent high-temperature interlock (set point 140 °C) and replacing the plain jacket with a half-pipe coil design reduced the characteristic cooling time constant from 12 min to 3.5 min, enabling safe operation even at reflux with para-xylene (b.p. 138 °C).
Applications in fluorescent whitening agent synthesis leverage the methoxy group as a donor to extend conjugation across a vinyl-bridged dimeric structure. A typical route condenses 1.0 mol of 5-methoxy-2-methylbenzothiazole with 1.03 mol of terephthalaldehyde in refluxing N,N-dimethylformamide under piperidine catalysis. After a hold time of 6 h, the precipitated bis-styryl product is isolated by centrifugation in a peeler centrifuge (basket diameter 800 mm, G-factor 980), washed with methanol, and dried in a conical paddle dryer at 65 °C and 50 mbar. The methoxy-activated methylene proton is sufficiently acidic (pKa estimated at 12.5–13.0) that enamine formation with secondary amines occurs at 80 °C without pre-treatment with a strong base, a feature that the 6-methoxy isomer does not replicate because steric shielding by the adjacent sulfur atom reduces the rate of condensation by roughly one order of magnitude under the same conditions.
The safety data sheet draws its thermal stability guidance from a programme of accelerated rate calorimetry (ARC) and C80 microcalorimeter experiments. Above 150 °C, the compound evolves gaseous decomposition products that include methyl mercaptan and dimethyl disulfide; the corresponding pressure generation rate reaches 1.2 bar·min⁻¹ at 200 °C in a 10 mL titanium test cell. This behaviour mandates that distillation be performed in a short-path wiped-film evaporator with a jacket temperature not exceeding 120 °C and a residence time below 90 s. A thin-film unit with a 0.16 m² heated surface and rotor clearance of 0.4 mm, operated at 0.5 mbar, routinely reduces a 92 % crude olive-coloured feed to a 99.5 % overhead product with a single pass, yield loss to the residue fraction being held below 3.5 %.
Incompatibility screening against common process auxiliaries indicates that contact with strong oxidising agents—peracetic acid, potassium permanganate in acetone, or concentrated nitric acid above 20 °C—results in violent gas evolution within 15–30 s. Compounds bearing free primary or secondary amine groups form coloured charge-transfer adducts at room temperature; while these adducts can be purified as intermediates, storage of the neat mixture for more than 8 h leads to progressive ring-opening of the thiazole, detected through loss of the characteristic 1470 cm⁻¹ IR band. The compound has been registered under the EU REACH regulation; an inhalation LC50 (rat, 4 h) does not appear in the public dossier, but the occupational exposure limit for airborne thiazole particulates is provisionally set at 0.5 mg·m⁻³ as an 8-hour TWA, and full PPE including nitrile gloves tested to EN 374 and tight-fitting goggles certified to EN 166 is mandated during all open-handling operations.
The isomeric 6-methoxy-2-methylbenzothiazole is commercially available as an off-white powder with a melting range of 105–108 °C, almost 70 °C higher than the 5-methoxy compound. That physical divergence translates into a profound processing difference when the benzothiazole core is pre-functionalised with a bromine atom for Suzuki–Miyaura couplings. The 5-bromo-2-methylbenzothiazole (prepared from the 5-methoxy derivative by nitration–reduction–diazotisation–Sandmeyer) exhibits a coupling rate with phenylboronic acid in a standard Pd(PPh3)4/K2CO3/dioxane–water system that is 2.8 times faster than the 6-bromo isomer measured under identical catalyst loading (0.5 mol %) at 80 °C. The kinetic acceleration is attributed to the reduced steric compression experienced by the metal centre during oxidative addition into the C–Br bond that is para to the methoxy group rather than meta. Production campaigns that switch from the 6-methoxy to the 5-methoxy building block therefore achieve full bromide conversion in 3.5 h versus 10 h, directly enabling the use of a smaller reactor train and lowering the Pd inventory per kilogram of coupling partner.
