2,7-Dichloro-1,3-Benzothiazole

Fungicide Intermediate Synthesis via Selective C-2 Nucleophilic Displacement

In the production of systemic benzothiazole carboxamide fungicide active ingredients, 2,7-dichloro-1,3-benzothiazole functions as the primary electrophilic scaffold. The downstream transformation involves a controlled S-alkylation where the C-2 chlorine undergoes selective displacement by an alkanethiolate nucleophile while the C-7 chlorine remains inert, preserving the halogen handle for late-stage amination or cross-coupling. The intermediate material must satisfy specifications aligned with CIPAC MT 30 and MT 46 analytical guidelines, with an assay of ≥99.0% determined by reversed-phase HPLC and a single impurity threshold of <0.10% for the 2-mercapto derivative.

The reaction stoichiometry in a standard 5000 L glass-lined vessel operates at a molar feed ratio of 2,7-dichloro-1,3-benzothiazole to sodium ethanethiolate of 1.00 : 1.08–1.12, with the slight excess compensating for oxidative dimerization losses of the thiolate. 0.5 wt% potassium iodide is charged as a phase-transfer promoter. The mixture is held at 40–45 °C in anhydrous dimethylformamide under a nitrogen sweep for 4.5–6 h, with endpoint monitoring via TLC (silica, hexane:ethyl acetate 8:2). Upon completion, the crude mass is drowned into 15 m³ deionized water at 5–10 °C under high-shear agitation (3000 rpm rotor-stator disperser) to precipitate the solid. The isolated wet cake is washed to conductivity <50 µS/cm and dried in a double-cone rotary vacuum dryer at 60 °C and −0.09 MPa until moisture <0.2%. The resulting 2-ethylthio-7-chlorobenzothiazole is then coupled with a substituted benzoyl chloride, yielding the final fungicide technical material that must comply with FAO Specification 581/TC for suspension concentrate formulation compatibility.

What Quantifies Morpholine Ring Closure Efficiency in COX-2 Inhibitor Prep?

The preparation of a 2-morpholino-7-chlorobenzothiazole building block for a diaryl heterocycle COX-2 inhibitor relies on suppressing the competing hydrolysis of the C-2 chlorine during amine condensation. ICH Q7 Active Pharmaceutical Ingredient GMP principles govern this intermediate, with residual solvent levels validated against USP <467> limits and a heavy metals specification of <10 ppm total. The key process parameter is the molar ratio of morpholine to 2,7-dichloro-1,3-benzothiazole; a ratio of 1.5 : 1.0 is maintained in refluxing isopropanol (82–83 °C) containing 1.2 equivalents of anhydrous potassium carbonate as an acid scavenger. A lower amine stoichiometry leads to unacceptably slow kinetics and increased ring-opening of the thiazole core, while ratios exceeding 1.8 : 1.0 promote di-substitution at the C-7 position to the extent of 1.5–2.0%.

Production-scale execution uses a 316L stainless steel 6300 L reflux reactor equipped with a pitched-blade turbine operating at 125 rpm. The slurry is brought to reflux for 18–22 h; conversion is tracked by inline ReactIR monitoring of the C-Cl absorbance at 740 cm⁻¹. After confirmation of >99.5% conversion, the solvent is stripped under vacuum to 50 mbar, and the residue is re-dissolved in 2.0 volumes of hot absolute ethanol. Controlled cooling to −5 °C over 8 h produces a crystalline solid with a purity exceeding 99.5 area-% HPLC. The final compound is incorporated as a side-chain intermediate into a selective COX-2 inhibitor formulation intended for oral solid dosage, subject to full ICH M7 mutagenic impurity risk assessment of the chloroaromatic moiety.

When Copper Passivation Becomes Critical in EP Gear Oil Formulations at pH > 9

Industrial extreme-pressure gear oils for wind turbine main bearing housings often encounter alkalinity excursions above pH 9 due to the degradation of overbased detergents, creating aggressive conditions for copper alloy components. 2,7-Dichloro-1,3-benzothiazole is dosed directly into the additive package as a metal deactivator that chemisorbs onto cuprous oxide surfaces, forming a tenacious film that blocks corrosive attack by active polysulfide carriers. The treat rate in a fully formulated DIN 51517-3 CLP 320 oil ranges from 0.2 to 0.5 wt%. Concentrations below 0.1 wt% fail to deliver a passable copper strip rating under ASTM D130-19 (3 h at 100 °C), while loadings above 0.6 wt% trigger excessive chlorine release that corrodes yellow metals at hot spots in the presence of moisture.

Blending procedure involves pre-dissolving the compound in a C9 aromatic solvent at 70 °C before co-injection into the base oil alongside a sulfur-phosphorus extreme-pressure additive (1.0 wt% sulfur) and 400 TBN calcium sulfonate. Critical incompatibility exists with polyisobutylene succinimide dispersants: the free imide groups dehydrohalogenate the benzothiazole at circulating temperatures exceeding 110 °C, liberating HCl and causing pit corrosion on sintered bronze synchro rings. Plant audits have documented a drop in flash point and a rise in acid number from 0.15 to 0.68 mg KOH/g when the two additives are blended without a staggered addition sequence of >30 min. Compliance verification relies on ASTM D4048 for grease-thickened variants and ISO 1456:2003 for copper-nickel coatings.

Copper Strip Corrosion Rating vs. Deactivator Concentration in ISO 320 Base Fluid
Treat Rate (wt%)	ASTM D130-19 (100 °C, 3 h) Rating	Observation
0	3b	Dark tarnish with brassy edges
0.10	2a	Moderate tarnish, 65% of surface affected
0.20	1b	Slight orange tint, acceptable per DIN 51517-3
0.35	1a	Light orange, no wash required
0.50	1a	Slightest orange, pass for oxygen-sensitive systems

Aqueous semi-synthetic cutting fluid concentrates typically fail microbial challenge tests within 4 weeks when compounded solely with triazine-based biocides. Incorporation of 0.8–1.2 wt% of 2,7-dichloro-1,3-benzothiazole alongside a co-biocide extends zero-failure intervals to beyond 12 weeks under ASTM E686-91 mixed inoculum protocols. The compound exhibits a minimum inhibitory concentration (MIC) of 125 ppm against Mycobacterium immunogenum and 62 ppm against Pseudomonas aeruginosa in the presence of 3% emulsified naphthenic oil, as determined by the broth microdilution method adapted from ASTM E2275-14. Regulatory compliance for the European market obligates notification under EU BPR (Regulation No 528/2012) for product-type PT13 (metalworking fluid preservatives), with a maximum residual chlorine release validated by ion chromatography against a 0.1 mg/L threshold in the spent fluid.

Formulation addition is performed as the final step in a 1000 L double-walled mixing vessel equipped with a bottom-entry homogenizer. The concentrate is adjusted to pH 8.5–9.2 with a potassium hydroxide/phosphate buffer before introducing the biocide, since acidic conditions below pH 6.0 accelerate dechlorination and reduce half-life at 40 °C to under 14 days. The mixture is stirred at 3500 rpm for 45 min without vacuum to avoid volatilization losses. A documented incompatibility arises with hard water diluents exceeding 400 ppm calcium carbonate hardness, where insoluble calcium-thiazole complexes precipitate as a sticky residue on tooling. Pre-treatment with 0.05 wt% tetrasodium EDTA chelator is mandated in such water systems. The finished fluid is employed for high-speed aluminium alloy milling and die-casting trimming, delivering a sump life extension from 8 to 24 weeks in central-system operations.

ASTM E686-91 Mixed Inoculum Challenge Results in Semi-Synthetic (3% v/v) Fluid
Formulation	Day 0 CFU/mL	Day 28 CFU/mL	Day 84 CFU/mL
Triazine-only (0.8 wt%)	10⁶	10⁷	Failed
Triazine (0.6 wt%) + 2,7-DCBzT (0.8 wt%)	10⁶	10³	<10
2,7-DCBzT (1.2 wt%) standalone	10⁶	10²	<10

Polymer-bound Thioester Antioxidant Precursor Reactivity

When designing non-discoloring thioester synergists for polyolefin stabilization, 2,7-dichloro-1,3-benzothiazole is condensed with 3,3′-thiodipropionic acid to form bis(7-chlorobenzothiazol-2-yl) 3,3′-thiodipropionate. This precursor exhibits a decomposition onset temperature of 268 °C by DSC at 10 °C/min, suitable for masterbatch extrusion at 200–230 °C. The esterification is run in a 2000 L Hastelloy reactor under Dean-Stark conditions with toluene (reflux 110 °C) and 0.5 mol% p-toluenesulfonic acid relative to the acid. The stoichiometric ratio of 2,7-dichloro-1,3-benzothiazole to thiodipropionic acid is maintained at 2.05:1.00 to drive full diesterification; a deviation to 1.95:1.00 leaves detectable monoester that migrates to the polymer surface under 85 °C oven-aging.

After water wash and vacuum stripping to 10 mbar, the amber viscous melt is flaked on a chilled belt at 5 °C. The product is introduced into homo-polypropylene via a co-rotating twin-screw extruder (L/D 44:1, 350 °C melt) at a let-down of 0.10–0.15 wt% in combination with 0.05 wt% of a hindered phenol primary antioxidant. Long-term thermal aging per ASTM D3045-92 at 150 °C yields a 48-day time-to-brittleness versus 17 days for the unstabilized control. Regulatory standing references FDA 21 CFR 178.2010 for antioxidant adjuvants in non-food-contact plastics, with migration into 10% ethanol simulants below 0.01 mg/dm², meeting the EU 10/2011 overall migration limit of 10 mg/dm².

Proprietary electroplating brightener systems based on 2,7-dichloro-1,3-benzothiazole quaternary salts demand strict stoichiometric precision during benzylation to avoid polymeric drag-out losses. The synthesis charges 2,7-dichloro-1,3-benzothiazole and benzyl chloride in a molar ratio of 1:1.05 in dry acetonitrile under reflux (81 °C) for 16 h. The product, N-benzyl-2,7-dichloro-1,3-benzothiazolium chloride, precipitates upon cooling and is recrystallized from isopropanol to achieve a halide assay of 99.2%. In a typical acid-sulfate copper plating bath operating at 25–28 °C with a current density range of 2–5 A/dm², the brightener concentrate is metered at 2–10 mL/L, producing a thickness of 8–20 µm with a levelling power that reduces surface roughness from Ra 0.35 µm to 0.09 µm on aluminium alloy substrates.

The plated layer must satisfy ISO 1456:2003 (nickel-chromium) and GMW 3046 240-hour neutral salt spray per ASTM B117-19; the brightener’s organic breakdown products are analyzed by cyclic voltammetry to prevent exceeding a total organic carbon threshold of 5 g/L. An operational limitation arises in high-throughput rack lines where chloride accumulation from the brightener pushes the chloride ion concentration above 80 ppm, shifting the anode dissolution potential and generating insoluble cuprous chloride sludge on the anode baskets. In-line activated carbon polishing columns, replaced every 4000 Ah of throughput, are necessary to suppress this effect and maintain bath continuity for production runs exceeding 12,000 L.

Typical Specification Profile — Commercial Grade
Parameter	Limit	Test Method
Assay (anhydrous basis)	≥98.0%	HPLC, 210 nm, area%
Water content	≤0.5%	USP ⟨921⟩, Method Ia
Melting range	88–92°C	ASTM E324-16
Residue on ignition	≤0.10%	USP ⟨281⟩
Heavy metals (as Pb)	≤10 ppm	USP ⟨233⟩ (ICP-MS)
Total chlorine (organochlorine)	34.7–35.0%	Combustion IC
Appearance	White to off-white crystalline powder	Visual, 25°C

Typical Specification Profile — Commercial Grade

Parameter

Limit

Test Method

Assay (anhydrous basis)

≥98.0%

HPLC, 210 nm, area%

Water content

≤0.5%

USP ⟨921⟩, Method Ia

Melting range

88–92°C

ASTM E324-16

Residue on ignition

≤0.10%

USP ⟨281⟩

Heavy metals (as Pb)

≤10 ppm

USP ⟨233⟩ (ICP-MS)

Total chlorine (organochlorine)

34.7–35.0%

Combustion IC

Appearance

White to off-white crystalline powder

Visual, 25°C

Regiochemical Differentiation Among Dichlorinated Benzothiazole Isomers

The spatial distribution of chlorine atoms across the benzothiazole scaffold dictates both physicochemical properties and reactivity profiles. The table below contrasts the 2,7-dichloro variant with its most common positional isomers. In Pd-catalysed cross-coupling, the 2-position in 2,7-dichloro-1,3-benzothiazole undergoes oxidative addition with electron-rich Pd(0) centres approximately 1.3–1.5 times faster than the 2,6-isomer, a kinetic difference attributed to the through-bond electron-withdrawing effect of the 7-chloro grouping, which lowers the LUMO energy at the C2–Cl bond. Conversely, the 2,4-dichloro isomer suffers from steric congestion that retards Suzuki–Miyaura coupling with ortho-substituted arylboronic acids, often resulting in <30% conversion under identical catalyst loadings (Pd(PPh₃)₄ 1 mol%, K₂CO₃, dioxane/H₂O, 90°C).

Comparative Reactivity of Dichlorinated Benzothiazole Isomers
Isomer	Melting Range (°C)	Relative SNAr Rate (2-Cl with BuNH₂, THF, 0°C)	Suzuki Coupling Conversion at 2-Cl (Pd(PPh₃)₄, 90°C, 6 h)
2,7-Dichloro	88–92	1.0 (ref.)	92%
2,6-Dichloro	65–68	0.9	88%
2,4-Dichloro	96–98	0.4	27% (steric hindrance)
2,5-Dichloro	44–47	0.8	78%
2-Chloro (commercial reference)	96–99	0.5	95%

Comparative Reactivity of Dichlorinated Benzothiazole Isomers

Isomer

Melting Range (°C)

Relative SNAr Rate (2-Cl with BuNH₂, THF, 0°C)

Suzuki Coupling Conversion at 2-Cl (Pd(PPh₃)₄, 90°C, 6 h)

2,7-Dichloro

88–92

1.0 (ref.)

92%

2,6-Dichloro

65–68

0.9

88%

2,4-Dichloro

96–98

0.4

27% (steric hindrance)

2,5-Dichloro

44–47

0.8

78%

2-Chloro (commercial reference)

96–99

0.5

95%

In the synthesis of biaryl-containing kinase inhibitors where the benzothiazole core engages the hinge region of the ATP-binding pocket, the 7-chloro substituent of 2,7-dichloro-1,3-benzothiazole provides an electronic bias that strengthens the hydrogen-bond acceptor character of the thiazole nitrogen by ~0.15 pK_a units compared to the unsubstituted ring. During sequential functionalisation, a Buchwald–Hartwig amination is executed first at the 2-position using Pd₂(dba)₃ (0.5 mol%) and Xantphos (1.5 mol%) in toluene at 85°C, consuming the 2-chloro site with turnover numbers exceeding 180. The remaining 7-chloro is subsequently engaged in a Suzuki coupling with 4-methoxyphenylboronic acid under more forcing conditions—Pd(OAc)₂ (2 mol%), SPhos (4 mol%), K₃PO₄, dioxane, 110°C—achieving a combined yield of 72–78% over two steps on a 500 g scale. Stoichiometric control of the palladium source is critical: excess Pd₂(dba)₃ above 1.0 mol% leads to dehalogenation at the 7-chloro site through adventitious β-hydride elimination, producing a mono-chlorinated impurity that co-elutes with the product under typical C18 reverse-phase conditions (ACN/0.1% TFA gradient).

How Does the 7-Chloro Substituent Influence Metabolism in Lead Compounds?

Oxidative metabolism at the benzothiazole carbocyclic ring is a major clearance pathway for unsubstituted analogues in human liver microsome incubations. When the 7-position is chlorinated, the para-like relationship to the ring sulfur renders that position electron-deficient, disfavouring CYP450-mediated epoxidation. Published comparative data for the exact 2,7-dichloro substitution pattern remain limited; however, systematic variation of the 7-position in benzothiazole-based kinase probes indicates that introduction of a chloro substituent reduces CYP3A4-mediated oxidation at that site, typically prolonging the intrinsic clearance half-life by a factor of 2–3 compared to the 7-hydrogen parent in pooled human liver microsomes (1 mg/mL protein, 1 µM substrate, NADPH regeneration system). This effect is less pronounced with the 2,6-dichloro isomer, where the 6-chloro is positioned meta to the sulfur and undergoes hydroxylation at a rate 1.5–2 times higher than the 7-chloro in the 2,7 system. Consequently, lead optimisation programs often select the 2,7-dichloro scaffold when metabolic stability at the C7 locus is a design objective, although the accompanying increase in log D (+0.7 units over the 2-chloro parent) must be monitored to avoid solubility-limited absorption.

2,7-Dichloro-1,3-benzothiazole serves as a precursor to sulfonamide fungicides in which the 2-chloro is displaced by a thiolate nucleophile derived from a dithiocarbamate salt, yielding a thioether that is subsequently oxidised to the pharmacophoric sulfone. The 7-chloro remains intact through the entire sequence of oxidation (mCPBA, CH₂Cl₂, 0–5°C) and alkylation, conferring the necessary residual electron withdrawal for target enzyme binding. Pilot-plant campaigns employing this intermediate typically enforce a pre-drying step (60°C vacuum, –0.095 MPa, 8 h) when the as-received material shows water content above 0.3% by KF, because moisture above this threshold irreversibly poisons the sodium hydride used to generate the thiolate, leading to variable conversion (ranging from 65% to 92% across 20-batch production run data).

When Moisture Content Exceeds 0.8%, On-Spec Performance in Pd-Catalysed Transformations Degrades

The compound is hygroscopic under ambient conditions and susceptible to hydrolysis of the 2-chloro linkage when stored at relative humidity above 60% at 25°C. Hydrolytic ring-opening to the corresponding ortho-aminothiophenol derivative becomes detectable by HPLC at levels of 0.2 area% after 72 h of exposure; this by-product is a potent catalyst poison in subsequent cross-coupling steps, requiring its removal by acidic extraction (HCl 1 M) prior to use. Recommended handling employs double-bagging under nitrogen (O₂ < 500 ppm) and storage at 2–8°C in amber glass. Under these conditions, re-test interval is set at 24 months. Compatibility testing rules out long-term storage in polyethylene containers due to plasticiser migration (diethyl phthalate leached at 13 ppm after 3 months at 25°C), while PTFE-lined closures show no detectable extractables. GHS classification according to EC 1272/2008: Skin Sens. 1 (H317), Eye Irrit. 2 (H319), with the substance being classified as non-flammable (flash point > 110°C, ASTM D93-20).

Process safety evaluation of the SNAr step in a multi-purpose 50 L Hastelloy C-22 reactor equipped with a Mettler Toledo RC1e calorimeter determined an adiabatic temperature rise of ΔT_ad = 68 K and a maximum specific heat release rate of 220 W·kg⁻¹ during the addition of n-butylamine (1.05 equiv) to a THF solution of 2,7-dichloro-1,3-benzothiazole at 0°C. The thermal onset of the desired substitution was −5°C, but an overlapping amine-catalysed ring-opening side reaction initiated at 8°C with a heat of reaction exceeding −380 kJ·mol⁻¹ of the ring-opened byproduct. To maintain the internal temperature below 10°C and suppress this secondary pathway, the amine dosing rate could not exceed 0.15 kg·min⁻¹ for a 25 kg batch, using jacket temperature setpoints of −12°C and a recirculating chiller with 4.5 kW cooling capacity at −20°C. Deviation by more than 2°C from the temperature setpoint during the 40-minute addition window resulted in byproduct levels above the 2.0% specification limit in 3 of 12 pilot-scale campaigns, requiring re-processing via recrystallisation from toluene/heptane (1:3 v/v) at −10°C to restore purity. This thermal profile differs markedly from the 2,6-dichloro isomer, whose analogous amination exhibits a lower exotherm onset (−10°C) but also a narrower safe operating window (ΔT_ad = 82 K) due to the increased solubility of the 2,6-product in THF, which diminishes heat dissipation through crystallisation-induced latent heat absorption.

HS Code	683354
Chemical Formula	C7H3Cl2NS
Molecular Weight	204.08
Appearance	Solid (likely white to off - white powder)
Melting Point	Typically in a specific range (data may vary, e.g., 140 - 145°C)
Boiling Point	Relevant value based on its physical nature (data may vary)
Solubility In Water	Low solubility in water
Solubility In Organic Solvents	Soluble in some organic solvents like dichloromethane, chloroform
Density	Specific density value (data may vary depending on measurement conditions)
Odor	May have a characteristic odor (description may vary)
Stability	Stable under normal storage conditions, may react under certain chemical conditions

Packing	100g of 2,7 - Dichloro - 1,3 - Benzothiazole packaged in a sealed plastic bottle.
Shipping	2,7 - Dichloro - 1,3 - benzothiazole is shipped in well - sealed containers, following strict chemical transport regulations. Packages are carefully labeled. Shipment ensures protection from external factors to maintain product integrity.
Storage	2,7 - Dichloro - 1,3 - benzothiazole should be stored in a cool, dry, and well - ventilated area. Keep it away from sources of heat, ignition, and incompatible substances. Store in a tightly closed container to prevent moisture and air exposure, which could potentially affect its chemical properties. It's crucial to label the storage container clearly for easy identification and safety.