The heterocyclic scaffold designated 2-Amino-6-methoxybenzothiazole (IUPAC: 6-methoxy-1,3-benzothiazol-2-amine) is assigned CAS Registry Number 1747-60-0 and is represented by the molecular formula C₈H₈N₂OS, yielding a formula weight of 180.23 g/mol. The substance is routinely supplied as a finely divided pale-yellow to light-brown crystalline powder exhibiting a melting point via differential scanning calorimetry (DSC) of 157–161 °C (onset, 10 K/min ramp under nitrogen). Commercial lots intended for pharmaceutical intermediate service are typically controlled to an HPLC area-purity specification of ≥98.5% (254 nm), with any single unspecified impurity capped at ≤0.50 area-% and total impurities below 1.5 area-%. The primary contaminant of consequence is the deschloro or desbromo analog arising from halide-containing ring-closure routes, as well as the regioisomeric 2-amino-5-methoxybenzothiazole, which co-elutes unless resolved by an optimized chromatographic system. This building block has found a durable niche in dye chemistry, materials science, and pharmaceutical research precisely because the 6-methoxy substituent modulates electronic character without introducing the metabolic instability often associated with halogenated or nitro-functionalized congeners.
What Distinguishes This Heterocycle From the Parent 2-Aminobenzothiazole?
The introduction of a methoxy group at the 6-position shifts the electron density landscape of the benzothiazole nucleus in ways that determine both reaction kinetics and product distribution. The Hammett substituent constant for para-OCH₃ (σₚ = −0.27) quantifies the resonance-donating effect that is transmitted to the 2-amino nitrogen, raising its nucleophilicity and lowering the pKa of the conjugate acid by approximately 0.6–0.8 log units relative to the parent 2-aminobenzothiazole (σₚ for H = 0.00). In practice, this translates to a measurable acceleration in acylation and sulfonylation reactions: the pseudo-first-order rate constant for acetylation with acetic anhydride in DMF at 25 °C is roughly 2.2-fold higher for the methoxy derivative. The dipole moment increase conferred by the methoxy oxygen also elevates the crystalline lattice energy, accounting for the 30–35 °C upward shift in melting point versus the unsubstituted species (mp 126–129 °C). By contrast, 2-amino-6-methylbenzothiazole, where the substituent constant σₚ for CH₃ is a more modest −0.17, melts at 120–125 °C and displays intermediate nucleophilicity, making the methoxy variant the most electron-rich member of the series commonly available at scale. Solubility in polar aprotic solvents such as DMF and NMP exceeds 125 g/L at 20 °C, roughly double that of the parent heterocycle, a factor that simplifies homogeneous operation in downstream amidations.
Manufacture of the azo dye C.I. Disperse Red 179 relies on the electron-rich character introduced by the 6-methoxy group to achieve a deep bluish-red shade on polyester fiber. The diazotization step is conducted at −5 to 0 °C by charging 1.02 molar equivalents of sodium nitrite into a slurry of the amine in 30% hydrochloric acid maintained in a jacketed, glass-lined reactor equipped with a retreat-curve impeller. Completion of diazotization is verified with starch-iodide paper, and excess nitrous acid is quenched with sulfamic acid until a negative test persists for 60 seconds. The resulting diazonium solution is immediately transferred to a coupling vessel containing an equimolar amount of N-ethyl-N-(2-hydroxyethyl)aniline dissolved in dilute acetic acid, with the pH maintained between 4.0 and 4.5 by sodium acetate buffer. Under these conditions, coupling proceeds regioselectively at the para position of the tertiary aniline, and the crude product, isolated by filtration, delivers a yield of 85–88% after methanol washing. A critical in-process control parameter is the level of unreacted free amine remaining in the diazonium liquor; residual 2-amino-6-methoxybenzothiazole above 0.10 area-% relative to the diazo species leads to the formation of a bis-azo impurity that detracts from brightness and fastness. The methoxy auxochrome contributes a bathochromic shift of 18–22 nm in λmax (measured in DMF solution) compared with the dye prepared from 2-aminobenzothiazole, while simultaneously improving wash fastness to ISO 105-C06 C2S rating 4–5 and sublimation fastness to ISO 105-P01 rating 4. On modern package-dyeing equipment operating at 130 °C, the dye exhibits 90–95% exhaustion onto polyester over 45 minutes, a performance envelope that underscores the commercial utility of the electron-releasing methoxy motif.Pharmaceutical Intermediacy and the Challenge of Deschloro Impurities
Within early-phase medicinal chemistry programs, 2-amino-6-methoxybenzothiazole has served as a privileged fragment for the construction of ATP-competitive kinase inhibitors, particularly those targeting VEGFR2 and c-Met receptor tyrosine kinases. The presence of the methoxy group enhances hydrogen-bond acceptor capacity while keeping the logP within a range compatible with Lipinski’s rule-of-five; calculated logP (CLOGP) for the free amine is approximately 1.9, versus 2.8 for the 6-methyl analog. However, when the compound is manufactured via the Hugerschoff thiocyanogenation of p-anisidine—a route that sometimes leaves residual halogenated intermediates—low-level persistence of the 2-amino-6-chlorobenzothiazole contaminant can become a regulatory concern. ICH Q3A guidelines for new drug substances stipulate an identification threshold of 0.10% for a maximum daily intake of ≤2 g/day, and many pharmacopoeial monographs now mandate a resolution of not less than 2.0 between the methoxy and chloro peaks when run on a C18 column (150 × 4.6 mm, 5 µm) with an isocratic mobile phase consisting of 40:60 (v/v) 0.01 M KH₂PO₄ buffer (pH 3.0): methanol at a flow rate of 1.0 mL/min and UV detection at 254 nm. Under these conditions, the typical retention time for the target amine is 8.3 ± 0.2 min, and the deschloro impurity elutes at 10.1 ± 0.2 min. Process chemists have demonstrated that a series of activated carbon treatments combined with a hot filtration at 45 °C can reduce the chlorinated species from an initial 1.8 area-% to below 0.05 area-% without recrystallization, preserving the overall yield above 78%.
When Storage Conditions Dictate Long-Term Stability
Pre-formulated laboratory stock of 2-amino-6-methoxybenzothiazole retains specification for 36 months when stored in sealed, double-lined HDPE containers purged with nitrogen and maintained at 2–8 °C. Accelerated stability studies at 40 °C/75% RH for 6 months show an increase in total impurities from 0.6 area-% to 2.3 area-%, predominantly due to oxidative dimerization leading to the corresponding benzothiazole disulfide and, to a lesser extent, the sulfinic acid. Exposure to relative humidity above 60% at 25 °C triggers measurable non-bridging aggregation (confirmed by dynamic vapor sorption), necessitating a desiccated environment for open operations exceeding 4 hours. For in-process samples taken from the production line, the material is not corrosive to mild steel but may cause sensitization on repeated skin contact; the GHS classification includes Skin Sensitizer Category 1 (H317) as a precautionary statement. Engineering controls at the bench scale include local exhaust ventilation maintaining an airborne concentration below 0.1 mg/m³ as an 8-hour time-weighted average, a limit derived from structural analogy to 2-aminobenzothiazole.
A comparative summary of key attributes across the benzothiazole family clarifies the positioning of the 6-methoxy member for different application classes. The table below captures the critical metrics ordinarily evaluated during incoming raw-material qualification for dye synthesis and pharmaceutical projects.| Parameter | 2-Aminobenzothiazole | 2-Amino-6-methylbenzothiazole | 2-Amino-6-methoxybenzothiazole |
|---|---|---|---|
| CAS Registry Number | 136-95-8 | 2536-91-6 | 1747-60-0 |
| Molecular Weight (g/mol) | 150.20 | 164.23 | 180.23 |
| Melting Point Range (°C) | 126–129 | 120–125 | 157–161 |
| Hammett σₚ of Substituent | 0.00 | −0.17 | −0.27 |
| Typical Commercial Purity (HPLC, 254 nm) | ≥98.0% | ≥97.5% | ≥98.5% |
| Key Distinguishing Impurity | 2-Mercaptobenzothiazole (MBT) | 2-Amino-4-methyl isomer | 2-Amino-6-chloro analog |
| Primary Application Domain | Rubber vulcanization accelerators | Optical brightener precursors | Disperse dye & pharma intermediates |
