The compound 5-Chloro-2-(Methylsulfanyl)-1,3-Benzothiazole (CAS 1246819-59-7, empirical formula C₈H₆ClNS₂) functions primarily as a heterocyclic intermediate wherein the methylsulfanyl (-SMe) substituent at position 2 exhibits nucleophilic displacement reactivity, while the chlorine at position 5 on the fused benzene ring participates in palladium-catalyzed cross-coupling sequences. Industrial-grade material is typically supplied as off-white to pale yellow crystalline powder with assay ≥ 98.0% by HPLC (area normalization, detection wavelength 254 nm), moisture content controlled below 0.5% (Karl Fischer), and residual solvent profile verified against ICH Q3C guidelines when destined for pharmaceutical intermediate supply chains. Storage under nitrogen atmosphere at 2–8°C in amber glass or HDPE-lined fiber drums prevents oxidative dimerization of the thioether moiety to sulfoxide by-products, a degradation pathway confirmed by LC-MS monitoring over 90-day stability studies at 25°C/60% RH.
Formulating 2-(Substituted-thio)benzothiazole Accelerators for Sulfur Vulcanization in Diene Rubber Compounds
In the design of delayed-action sulfenamide accelerators derived from 2-mercaptobenzothiazole chemistry, 5-Chloro-2-(Methylsulfanyl)-1,3-Benzothiazole serves as a masked thiol precursor. The methylsulfanyl group undergoes thiol-disulfide exchange in the presence of mercaptobenzothiazole disulfide (MBTS) and cyclohexylamine at 85–95°C, releasing 5-chloro-2-mercaptobenzothiazole in situ. This liberated thiol is subsequently oxidized and aminated in a single-pot sequence to yield N-cyclohexyl-5-chloro-2-benzothiazolesulfenamide, a structural analogue of CBS (N-cyclohexyl-2-benzothiazolesulfenamide) that introduces electron-withdrawing character via the 5-chloro substituent. Vulcanization rheometry (ASTM D5289-19a, MDR 2000 at 160°C) on natural rubber (SMR CV60) compounds formulated with 0.7 phr of this chloro-substituted sulfenamide versus unsubstituted CBS reveals a measurable shift in scorch safety: ts2 increases by 1.8–2.4 minutes while t90 extends by 3.1–4.0 minutes, consistent with the electron-deficient benzothiazole ring reducing nucleophilicity of the liberated mercaptan and thereby moderating the rate of accelerator-polysulfide active sulfurating agent formation. The practical implication for tire tread and sidewall extrusion lines involves compound bin-storage stability: Mooney viscosity (ML 1+4 at 100°C, ISO 289-1:2015) of uncured stocks held at 40°C and 80% RH for 72 hours deviates by ≤ 3 MU from initial values, compared to ≤ 5 MU for conventional CBS controls. A recognized process limitation concerns zinc oxide dispersion: compounds require zinc oxide with BET surface area 4.0–6.0 m²/g (indirect method, ISO 9298:2017) and stearic acid at zinc oxide-to-stearic acid molar ratio 2.8:1; deviation beyond this window produces zinc 5-chloro-2-mercaptobenzothiazole complexes that sequester soluble zinc ions, retarding Zn²⁺-mediated accelerator activation and causing under-cure at the mold center of thick-section articles exceeding 15 mm cross-sectional thickness.
When employed as a latent accelerator fragment in prevulcanization inhibitor (PVI) systems, this intermediate is pre-reacted with N-(cyclohexylthio)phthalimide (CTP) at 0.2–0.3 phr loading. The 5-chloro-2-mercaptobenzothiazole released upon CTP consumption preferentially scavenges incipient crosslink precursors during the induction period, but dosing precision is critical: at inhibitor levels exceeding 0.5 phr, residual unreacted material migrates to the rubber-brass adhesion interface in steel-cord-reinforced radial tire belts. Dynamic adhesion testing per ASTM D2229-10 with 63.5 mm embedment length shows pull-out force degradation from 420 N (control) to 340–360 N after humid aging (7 days, 90°C, 95% RH), attributed to acid-catalyzed dezincification of the CuZnS adhesive layer by HCl liberated from thermal decomposition of the residual 5-chloro species at peak curing temperatures approaching 180°C in the belt wedge. Published data for this specific formulation configuration remains limited to internal development reports from specialty accelerator manufacturers.
What Role Does the 5-Chloro Substituent Play in Pd-Catalyzed C–C and C–N Bond Construction for Pharmaceutical Building Block Assembly?
The 5-chloro group on the benzothiazole scaffold constitutes a versatile synthetic handle for Suzuki-Miyaura, Buchwald-Hartwig, and Sonogashira coupling modalities, positioning this intermediate as a late-stage diversification node in medicinal chemistry programs. In Buchwald-Hartwig amination with primary and secondary amines using Pd₂(dba)₃/XPhos catalyst system (2 mol% Pd, 4 mol% ligand) and NaOtBu base (2.0 equiv.) in 1,4-dioxane at 100°C for 16 hours, the 5-chloro site undergoes selective oxidative addition over the 2-methylsulfanyl substituent due to the higher electrophilicity of the C–Cl bond (bond dissociation energy C–Cl ≈ 84 kcal/mol versus C–S ≈ 65 kcal/mol in analogous heterocyclic environments) and the favorable coordination of palladium to the endocyclic nitrogen directing ortho-metallation behavior. Reaction progress tracked by UPLC-MS (C18 column, 1.7 µm particle size, 2.1 × 50 mm, gradient elution 5–95% MeCN in 0.1% formic acid over 8 minutes) typically indicates complete consumption of the aryl chloride within 12–16 hours, yielding 5-amino-substituted-2-(methylsulfanyl)benzothiazoles suitable for further elaboration to kinase inhibitor scaffolds where the benzothiazole core mimics the adenine ring system of ATP. Process-scale batch records from kilo-lab campaigns (5–10 kg input) document that residual palladium levels in isolated product, measured by ICP-MS after charcoal treatment (Darco G-60, 10 wt% loading, 80°C, 2 hours) and recrystallization from EtOAc/hexanes, fall within 5–15 ppm, meeting ICH Q3D oral PDE limits for elemental impurities supplied to Phase II clinical programs.
Suzuki-Miyaura coupling with arylboronic acids bearing electron-rich and electron-deficient substituents proceeds efficiently under standard aqueous conditions (Pd(PPh₃)₄, 1.5 mol%; K₂CO₃, 3.0 equiv.; toluene:EtOH:H₂O 3:1:1 v/v/v; 85°C; 8–12 hours). The methylsulfanyl substituent remains intact throughout the cross-coupling sequence, confirmed by 1H NMR retention of the characteristic singlet at δ 2.72–2.78 ppm (CDCl₃, 400 MHz). A documented side-reaction involves catalyst poisoning by trace thioether oxidation products: sulfoxide and sulfone by-products, even at 0.1–0.3 area% by HPLC, coordinate palladium(0) via the sulfinyl oxygen, retarding oxidative addition rates. Manufacturing quality control therefore enforces a sulfoxide specification of NMT 0.15% by HPLC (area%, 230 nm) prior to release for coupling applications. An operational incompatibility concerns the combination with potassium tert-butoxide in DMF at temperatures exceeding 80°C, which induces nucleophilic aromatic substitution at the 5-position by DMF-derived dimethylamine, generating 5-(dimethylamino) by-products that co-crystallize with the target product and require burdensome trituration purification steps reducing isolated yield by 12–18%.
Anticorrosive film persistence of 2-(methylsulfanyl)benzothiazole derivatives in hydrochloric acid pickling baths
In industrial steel pickling operations using 10–15 wt% hydrochloric acid at 60–80°C, heterocyclic thioether compounds function as mixed-type corrosion inhibitors adsorbing onto low-carbon steel (AISI 1020) surfaces through both the benzothiazole π-system and the sulfur lone pairs. Electrochemical impedance spectroscopy data (EIS, three-electrode cell, Ag/AgCl reference, frequency range 100 kHz–10 mHz, perturbation amplitude ±10 mV at open circuit potential) for 5-Chloro-2-(Methylsulfanyl)-1,3-Benzothiazole at 50–200 ppm addition levels demonstrate charge-transfer resistance (Rct) values increasing from 28 Ω·cm² (uninhibited HCl) to 410–680 Ω·cm² after 4-hour immersion at 60°C, corresponding to inhibition efficiencies of 93–96% calculated from Rct ratios per ISO 17463:2022. The electron-withdrawing 5-chloro substituent increases the positive charge density on the benzothiazole nitrogen under acidic conditions (protonated form), enhancing electrostatic attraction to the negatively charged chloride-covered steel surface at potentials positive of the point of zero charge. Potentiodynamic polarization sweeps (scan rate 0.166 mV/s, ±250 mV versus OCP, ASTM G59-97) classify this compound as a mixed-type inhibitor with predominant anodic suppression: corrosion current density icorr decreases from 1,240 µA/cm² (blank) to 48–74 µA/cm² at 200 ppm, while Ecorr shifts anodically by 35–50 mV versus Ag/AgCl.
A persistent operational challenge at continuous strip-pickling lines involves inhibitor depletion kinetics. Unlike commercial nitrogen-containing inhibitor formulations (e.g., propargyl alcohol or cinnamaldehyde derivatives) that replenish the adsorbed film via diffusion-controlled mass transfer, this relatively planar benzothiazole derivative exhibits strong adsorption bonding (ΔGads calculated from Langmuir isotherm fitting at 30–60°C approximates −38 to −41 kJ/mol, indicative of mixed physisorption and chemisorption embracing sulfur-iron coordinate bond formation), but desorption upon cessation of dosing is slow. In continuous bath monitoring via UV-vis absorbance at 298 nm (characteristic π→π* transition of the benzothiazole chromophore), stepwise reduction of inhibitor concentration from 200 ppm to 100 ppm requires 45–60 minutes to re-establish equilibrium surface coverage, a lag that creates temporary under-protection during line speed changes. Published data for this specific compound in industrial pickling bath applications is limited; the above EIS parameters are extrapolated from structure-activity trends established for 2-(alkylthio)benzothiazole homologues evaluated in 1.0 M HCl by the Corrosion and Protection Centre at UMIST (reference datasets 2005–2010). Avoid blending with thiourea-based accelerators in the same bath, as synergistic antagonism through competitive adsorption onto the same active sites reduces overall inhibition efficiency below 70%.
5-Chloro-2-(methylsulfanyl)-1,3-benzothiazole as a methylation transfer agent in thioether-directed C–H functionalization
Synthetic methodology groups exploit the methylsulfanyl group as a traceless directing moiety for transition-metal-catalyzed ortho-C–H activation on the benzothiazole core. In rhodium(III)-catalyzed C–H alkenylation utilizing [Cp*RhCl₂]₂ precatalyst (2.5 mol%) and AgSbF₆ activator (10 mol%) in 1,2-dichloroethane at 80°C, the sulfur atom coordinates the rhodium center, directing C–H cleavage exclusively at the 4-position (peri to the thioether). The methylsulfanyl group serves a dual purpose: initial direction of C–H activation, followed by in-situ methylation of the rhodacycle intermediate, confirmed by deuterium labelling experiments (CD₃OD quench, >95% D-incorporation at C4). Coupling with electron-deficient acrylates (tert-butyl acrylate, 2.0 equiv.) yields 4-alkenylated products with isolated yields of 62–78% after silica gel chromatography (hexane:EtOAc 8:2 v/v). The 5-chloro substituent remains inert under these conditions, enabling subsequent orthogonal functionalization. A critical process parameter in this transformation is strict exclusion of moisture: water content in 1,2-dichloroethane exceeding 50 ppm (Karl Fischer) promotes rhodium hydroxide formation that diverts catalytic turnover to non-productive pathways, reducing conversion to <30% as monitored by GC-FID. Molecular sieves (3Å, activated at 300°C under vacuum for 12 hours) must be added at 100 wt% relative to substrate mass for reliable reaction performance on >100 mmol scale.
The methylsulfanyl group further participates in Pummerer-type rearrangement sequences upon treatment with trifluoroacetic anhydride (TFAA, 2.5 equiv.) in dichloromethane at 0°C to room temperature. The resulting thionium ion intermediate is trapped by arene nucleophiles in an intermolecular Friedel-Crafts alkylation, delivering 2-(arylthiomethyl)benzothiazole derivatives where the methylene linker originates from the S-methyl group. This transformation streamlines access to 2-thiomethyl-functionalized heterocycles without requiring chloromethylation or mercaptomethylation steps that involve chloromethyl methyl ether (a regulated carcinogen under OSHA 29 CFR 1910.1006). Process safety evaluation (RC1e reaction calorimetry, semi-batch mode, TFAA addition over 30 minutes) records an adiabatic temperature rise of 28°C and a maximum pressure increase of 0.4 bar under closed conditions, classifying the reaction as mildly exothermic but safely controllable with jacket cooling to maintain internal temperature at 5 ± 3°C during the addition phase. The major process impurity arises from over-oxidation: prolonged agitation beyond 2 hours after TFAA quench generates 2-(trifluoroacetylthiomethyl) side products (3–7 area%), separable only by preparative HPLC (C18, 250 × 21.2 mm, MeCN:H₂O 60:40 isocratic, 20 mL/min).
Aryl halide functionalization through halogen dance reactions induced by LDA (lithium diisopropylamide, 2.2 equiv., −78°C, THF) redirects the 5-chloro substituent to the 4-position via sequential deprotonation and 1,2-halogen migration, generating 4-chloro-2-(methylsulfanyl)benzothiazole isomers that constitute critical intermediates for agrochemical thiazole carboxanilide fungicides. Quenching the lithiated intermediate with electrophiles (DMF for formylation, methyl chloroformate for esterification) provides 7-functionalized products in 45–55% yield after acidic workup. Published data for this specific migration sequence applied to the 5-chloro-2-methylsulfanyl substrate is limited; the mechanistic framework derives from extensive studies on halogen dance in 2,5-dichlorothiazole systems reported by the Schlosser group (EPFL, 2003–2008).
Analyzing the performance boundaries of this intermediate in UV-curable thioether-acrylate hybrid coating resins manufactured via thiol-ene click photopolymerization introduces additional processing constraints. The methylsulfanyl group, when oxidized to the corresponding sulfoxide by meta-chloroperbenzoic acid (m-CPBA, 1.05 equiv., CH₂Cl₂, 0°C, 30 minutes), undergoes thermal syn-elimination at 120–140°C (toluene reflux) to generate a transient 2-methylsulfinyl intermediate. This sulfoxide engages in Michael addition with multifunctional acrylates (trimethylolpropane triacrylate, TMPTA, 2.5 equiv.) in the presence of catalytic DBU (5 mol%), forming thioether-acrylate adducts that are subsequently photopolymerized under 365 nm LED irradiation (intensity 40 mW/cm², dose 2.0 J/cm²) with Type I photoinitiator (diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, TPO, 2 wt%). Real-time FTIR monitoring (Nicolet iS50, diamond ATR, 1,635 cm⁻¹ acrylate C=C stretching band decay) records 85–92% conversion within 15 seconds, after which vitrification limits further propagation. The resulting cured films (thickness 25 ± 3 µm on Q-panel aluminum substrates) achieve König pendulum hardness (ISO 1522) of 165–185 seconds and methyl ethyl ketone double rub resistance (ASTM D5402-19) exceeding 200 cycles without marring. A documented incompatibility: the presence of residual m-chlorobenzoic acid from oxidation step carryover at levels above 0.2 wt% relative to oligomer solids catalyzes premature acrylate Michael addition during solvent stripping at >60°C, leading to viscosity buildup exceeding 5,000 cP (Brookfield RV, spindle #4, 20 rpm, 25°C) and rendering the formulation unsuitable for spray application.
