6-Methoxy-Benzothiazole

In the production of sulfur-vulcanized natural rubber/butadiene rubber (NR/BR) blends intended for heavy-duty tire sidewall compounds, 6-methoxy-benzothiazole is incorporated as a secondary accelerator within a binary cure system dominated by N-cyclohexyl-2-benzothiazolesulfenamide (CBS). Dispersion is achieved through masterbatch preparation in an intermeshing internal mixer (Banbury™ F270, 270 L net chamber volume) with a two-stage mixing protocol: first pass at 55 rpm rotor speed and 145°C batch drop temperature, followed by a finishing pass on a two-roll mill set to 50°C front roll and 1.2 mm nip gap where curatives are incorporated. The methoxy substituent elevates the activation energy for N–S bond scission in the accelerator complex, delaying the onset of crosslinking and widening the scorch safety window measured as t_s2 at 135°C per ASTM D2084-19a by 1.8–2.4 min when 0.4–0.8 phr of 6-methoxy-benzothiazole replaces an equivalent molar amount of tetramethylthiuram disulfide (TMTD) alongside 1.2 phr CBS and 1.8 phr insoluble sulfur (Crystex™ HD OT20). This kinetic shift permits extrusion of complex profiles at head temperatures up to 115°C without premature vulcanization in a pin-barrel cold-feed extruder (L/D 16:1, screw diameter 120 mm). The material’s reactivity is harnessed at addition levels of 0.3–1.2 phr, with the lower end preferred for low-hardness (55 Shore A) vibration damping mounts where reversion resistance is critical and the upper limit reserved for conveyor belt covers requiring rapid state of cure at 160°C curing press temperature.

At loadings exceeding 1.2 phr, surface bloom formation occurs within 72 h at 23°C and 60% RH, confirming solubility saturation in the rubber matrix; pre-dispersion cutbacks to 80% active on EPDM binder are mandatory at concentrations above 1.0 phr. The intermediate’s accelerating fragment generates zinc-accelerator complexes with a stoichiometry ratio altered by the electron-donating methoxy group, lowering the effective zinc oxide demand to 3.0 phr from the conventional 5.0 phr while preserving tensile strength (≥18 MPa per ASTM D412-16, Die C). This property is particularly valuable in injection-molded engine mounts where clamp forces of 350 tonnes demand a compound exhibiting a Mooney viscosity ML(1+4) at 100°C of 45–55 MU (ASTM D1646-19) alongside a reversion resistance that limits torque decay to <2% over 20 min post-cure at 170°C. The product is managed under EU REACH regulation 1907/2006, with a typical documentation package including a Safety Data Sheet compliant with Regulation (EC) No 1272/2008; no substance-specific restriction under Annex XVII applies, but a biodegradation screening test result 28-day OECD 301B showing ≤18% degradation guides waste-handling protocols.

Synthesis of Benzothiazole-Based Kinase Inhibitor Intermediates Under cGMP

In a multi-step batch process conducted in glass-lined reactors (Pfaudler AE‑4000 L) operated under nitrogen blanketing, 6‑methoxy‑benzothiazole undergoes regioselective electrophilic substitution to yield 2‑amino‑6‑methoxybenzothiazole, a key scaffold for ATP‑competitive kinase inhibitors with documented IC₅₀ values in the low nanomolar range against receptor tyrosine kinases. The nitration step employs mixed acid (HNO₃/H₂SO₄ 1:3 v/v) at −5 to 0°C with a feed rate controlled to 0.8 L/h via a corrosion-resistant diaphragm dosing pump (Lewa ecodos™), maintaining the adiabatic temperature rise below 3°C/min to avoid the formation of dinitro by‑products. Addition level is strictly stoichiometric: 1.00 mol of 6‑methoxy‑benzothiazole is reacted with 1.05 mol of nitric acid (98%) in the presence of 6.0 mol sulfuric acid (96%), achieving a conversion of ≥99.5% by HPLC area percent monitored at 254 nm (USP <621>). Downstream, catalytic hydrogenation over Raney Nickel (5% w/w) at 40 psi H₂ and 60°C reduces the nitro group; the crude amine is then isolated via vacuum distillation (0.5 mbar, 142‑144°C vapor temperature) to a purity of 99.8% (GC, USP <621>).

Subsequent coupling with substituted benzoyl chlorides or heteroaromatic halides in anhydrous tetrahydrofuran (THF) catalyzed by Pd₂(dba)₃/Xantphos at 65°C for 16 h produces the final API intermediate. The entire synthesis train is validated under ICH Q7 (ICH Harmonised Tripartite Guideline: Good Manufacturing Practice Guide for Active Pharmaceutical Ingredients) and is subject to FDA 21 CFR Part 211 subpart D equipment cleaning requirements. Residual solvent levels are controlled per USP <467> Option 1 (GC‑headspace method), with specifications for THF ≤ 720 ppm, methanol ≤ 3000 ppm, and hexane ≤ 290 ppm. The product is supplied as a white to off‑white crystalline powder with a loss on drying ≤ 0.5% (60°C, 4 h, USP <731>). Important incompatibility: the free amine form of the intermediate reacts exothermically with acid chlorides above 25°C in the absence of a tertiary amine scavenger, generating local hot spots that raise the dimeric impurity above the 0.10% acceptance threshold; a pre‑cooled solution in THF at ‑10°C with triethylamine (1.2 eq) eliminates this hazard. Terminal products include benzothiazole‑based small‑molecule inhibitors indicated for non‑small cell lung cancer and multi‑drug resistant bacterial infections, both of which have progressed through Phase IIa clinical trial assessment.

How Does Methoxy Substitution Shift Absorption Maxima in Cyanine Dye Synthesis?

Incorporation of 6‑methoxy‑benzothiazole as the heterocyclic quaternization substrate enables the formation of asymmetric monomethine cyanine dyes with bathochromic shifts of 18–32 nm relative to the unsubstituted benzothiazolium salt, a consequence of the electron‑donating methoxy group raising the HOMO energy of the chromophore by approximately 0.15–0.22 eV as estimated from TD‑DFT calculations at the B3LYP/6‑31+G(d) level. The quaternization step is conducted in a 500 L glass‑lined reactor charged with 1.0 molar equivalent of 6‑methoxy‑2‑methylbenzothiazole (prepared in‑situ) and 1.8 mol equivalents of an alkylating agent such as diethyl sulfate or methyl p‑toluenesulfonate in toluene at 110°C for 6‑8 h; the intermediate quaternary ammonium salt precipitates upon cooling to 5°C at a yield of 88–92%. The dye condensation then proceeds with 1.02 molar equivalents of an indolenine‑ω‑aldehyde in methanol under reflux with piperidine (0.1 eq) catalysis, monitored to endpoint by HPLC at 550–600 nm depending on the specific bridging length. Addition levels in the final dye powder formulation are typically 2.5–5.0 wt% active dye relative to polymeric binder (polyvinyl butyral or nitrocellulose) when coated onto polyester film.

The methoxy‑bearing cyanine dyes meet the ZDHC Manufacturing Restricted Substances List (MRSL) Version 3.1 for non‑halogenated organic pigments, and the absence of azo‑cleavable amines is certified per EN 14362‑1:2017. Photostability, evaluated according to ISO 105‑B02:2014 (Xenon arc, blue wool scale), reaches grade 5 for a 0.2 mm film thickness when the dye is combined with a hindered amine light stabilizer (1.0 wt% Tinuvin™ 292). The terminal products span laser‑grade pyridine‑1 dyes for tunable dye lasers emitting at 580–640 nm, biological nucleic acid staining probes (SYBR™ Gold derivatives), and fluorescent brightening agents for polyacrylonitrile and polyester textiles that achieve a whiteness index (CIE WIC) improvement of 12–18 units at an application rate of 0.05% on weight of fiber in pad‑thermosol processing. Direct exposure to strong oxidizing bleaching agents (sodium hypochlorite >5% active chlorine) in the alkaline exhaust dyeing bath leads to irreversible chromophore degradation within 10 min at 60°C; compatibility with reductive bleaching using sodium hydrosulfite is acceptable.

When 6‑Methoxy‑Benzothiazole Replaces Standard Thiazole in Systemic Fungicide Backbones

Replacing the unsubstituted benzothiazole moiety with 6‑methoxy‑benzothiazole in the synthesis of systemic fungicides intended for rice blast (Pyricularia oryzae) control elevates the log P by 0.5–0.7 units, enhancing cuticular penetration while retaining the pivotal 2‑aminothiophenol‑derived metal‑chelating site necessary for inhibition of the fungal cytochrome bc₁ complex. The manufacturing sequence is a high‑temperature condensation between 6‑methoxy‑benzothiazole‑2‑thiol (1.0 mol) and a pre‑formed hydrazinecarbothioamide (thiosemicarbazide) intermediate at 1.05 mol in neat phosphorus oxychloride (POCl₃, 2.5 L/kg of solid charge) at 105–110°C for 5 h. The reaction mass is quenched into ice‑water at ≤10°C with vigorous agitation in a Hastelloy C‑276 reactor to neutralize acidic fumes; the crude product is isolated by centrifuge (basket centrifuge, 1000 G force) and recrystallized from isopropanol to yield a technical‑grade active ingredient of 96% purity (GC). Addition levels in the formulated end‑use product (wettable powder or suspension concentrate) range from 10 to 25 wt% active ingredient, corresponding to 200–500 g a.i./ha field application rate. Compliance with FAO specifications for plant protection products (AGP:CP/327) and adherence to the European Regulation EC No 1107/2009 data requirements for active substance approval, including the five‑batch analysis and validated analytical methodology, are prerequisite to registration.

Terminal products are preventive rice seed treatment slurries and foliar suspension concentrates for temperate japonica varieties; they are not labeled for use on upland cereals due to insufficient translocation data in the xylem apoplast. Storage of the active ingredient under humid conditions (relative humidity above 65% at 25°C) results in hydrolysis of the thiazole ring with a half‑life of 14 days, necessitating aluminium foil‑laminated bag packaging with desiccant inserts.

Cooling water systems in petrochemical plants and coastal power stations that circulate seawater‑cooled copper‑nickel (90/10 Cu‑Ni) heat exchangers experience protection from general corrosion and pitting through the addition of 6‑methoxy‑benzothiazole as a filming inhibitor at an initial charge dose of 10–15 mg/L and a maintenance concentration of 3–5 mg/L titrated as total organic nitrogen. The compound is first dissolved in anhydrous isopropanol (25% w/w solution) and injected continuously via a positive displacement metering pump (Grundfos DME 12‑10) into the recirculation line after the strainer and before the heat exchanger inlet, ensuring uniform distribution across flow velocities of 1.5–2.5 m/s. The anodic protection mechanism involves chemisorption of the benzothiazole nitrogen and exocyclic sulfur atoms onto the Cu₂O passive film, displacing chloride ions; electrochemical impedance spectroscopy (EIS) measurements at 0 V vs. SCE in artificial seawater per ASTM G106‑20 show a charge‑transfer resistance increase from 320 Ω·cm² (uninhibited) to 8,900 Ω·cm² at 10 mg/L after 24 h immersion. Corrosion rate, determined by ASTM G31‑72 (immersion test, 168 h), decreases from 0.12 mm/year to 0.008 mm/year under moderate aeration conditions (6 ppm dissolved O₂). The inhibitor package is typically formulated with 2‑5 wt% active in an aqueous‑alcoholic blend containing 0.5‑1.0 wt% sodium tolyltriazole as a co‑inhibitor for yellow metals, alongside 3‑5 wt% hydrolytically stable polyacrylate dispersant (MW 2,500‑4,000 Da) for scale inhibition.

Operational boundaries are sharply defined: dosing must be interrupted at least 72 h before shock chlorination events where free chlorine concentration exceeds 2.0 mg/L, as the benzothiazole ring can undergo electrophilic N‑chlorination, reducing the inhibitor film’s surface coverage by >70% within 6 h. Similarly, compatibility with amine‑based neutralizing corrosion inhibitors (e.g., morpholine, cyclohexylamine) is poor due to competitive surface adsorption that elevates the effective corrosion rate by a factor of 1.8‑2.3. Formulators supplying water treatment service companies must declare the broad‑spectrum biocide tolerance and provide accelerated degradation data in the presence of 5 mg/L ozone. The terminal commercial form is a liquid corrosion inhibitor concentrate packaged in 205 L high‑density polyethylene drums, labeled for use in compliance with the EU Biocidal Products Regulation (EU) 528/2012 under product type 11 (preservatives for liquid cooling and processing systems).

Development Accelerator Formulation in Black‑and‑White Reversal Films

In the first developer of black‑and‑white reversal processing kits conforming to ISO 18924:2013 for archival permanence, 6‑methoxy‑1,3‑benzothiazole‑2‑thiol is utilized at addition levels of 0.05–0.20 g/L as a selective antifoggant that preferentially coordinates with metallic silver nucleation sites on undeveloped silver halide grains, suppressing chemical fog without retarding the development rate of the latent image centers. The concentrate is prepared as an alkaline stock solution (pH 10.8 ± 0.2) containing 5.0 g/L of the benzothiazole derivative dissolved alongside 8.0 g/L hydroquinone and 0.5 g/L 1‑phenyl‑3‑pyrazolidinone (Phenidone™) in deionized water purged with argon to ≤0.1 ppm dissolved oxygen. The working‑strength developer is mixed in‑tank with a redox buffer composed of sodium sulfite (50 g/L) and sodium carbonate (25 g/L). Processing is performed in a deep‑tank processor (Refrema™ dip‑and‑dunk) at 24.0 ± 0.3°C for 6 min with nitrogen burst agitation cycles (2 s every 10 s). Terminal products include amateur photographic chemistry kits for T‑Max and Delta emulsions and X‑ray duplicating film developers where fog density in the unexposed areas remains below 0.05 D above base plus staining. The compound is classified as a non‑reportable ingredient under the Globally Harmonized System of Classification and Labelling of Chemicals (GHS) criteria for acute oral toxicity (LD₅₀ rat > 2000 mg/kg), supporting safe distribution via e‑commerce channels; however, sensitization via prolonged skin contact in the developer concentrate form necessitates a warning under EU CLP Annex II 1.1.2.2 for dermal sensitizers category 1.