3-Amino-5-Nitro-2,1-Benzoisothiazole (CAS 84387-89-3), a heterocyclic building block with the molecular formula C7H5N3O2S and a formula weight of 195.20 g mol⁻¹, is supplied as a yellow to orange crystalline powder. The compound exists predominantly in the amino tautomeric form, as confirmed by 15N CP/MAS NMR data showing a single sp²-hybridized endocyclic nitrogen environment. Commercial production employs a nitration pathway on 3-amino-2,1-benzisothiazole under mixed-acid conditions (HNO₃/H₂SO₄, ≤5 °C), followed by drowning, neutralization with aqueous ammonia, and recrystallization from isopropanol/water (3:1 v/v). The product exhibits a melting point with decomposition of 195–198 °C (capillary, ASTM E324) and limited solubility in cold water but dissolves readily in DMF, DMSO, and hot ethanol. Its principal value lies in the simultaneous presence of a primary aromatic amine and a nitro group on the electron-deficient benzisothiazole scaffold, enabling selective derivatization through diazotization, reduction, or nucleophilic aromatic substitution without mutual interference when proper protecting-group strategies are observed.
Molecular Architecture and Tautomeric Behavior
The fused 2,1-benzisothiazole system allocates the sulfur atom at position 1 adjacent to a nitrogen at position 2, creating a 10π-electron heteroaromatic ring with a computed aromatic stabilization energy of 89 kJ mol⁻¹ (B3LYP/6-311++G**). The 3-amino group can, under strongly alkaline conditions (pH > 12), partially deprotonate to an imino anion, shifting the UV-Vis absorption maximum from 385 nm to 422 nm. This tautomeric equilibrium has been monitored by stopped-flow spectrophotometry, yielding a forward rate constant of 4.2 × 10³ s⁻¹ at 298 K in 0.1 M NaOH. The nitro substituent at position 5 exerts a powerful electron-withdrawing effect (σm = 0.71), which deactivates the carbocyclic ring toward electrophilic attack and imparts a dipole moment of 7.8 D (measured in dioxane at 25 °C). Single-crystal X-ray diffraction data (Cambridge Structural Database deposition) reveal planar molecules stacked with an interplanar spacing of 3.45 Å, promoting charge-transfer interactions that contribute to its intense color as a precursor in disperse dye manufacture.
What Distinguishes the 3-Amino Substituent from Analogous 3-Hydroxy Derivatives?
Replacing the 3-amino group with a hydroxyl function—as in 3-hydroxy-5-nitro-2,1-benzisothiazole—alters the dominant reaction manifold from diazo coupling to O-alkylation and ester formation. The amino derivative undergoes clean diazotization with NaNO₂ in concentrated HCl at 0–5 °C to generate a stable diazonium salt that couples with electron-rich aromatics (e.g., N,N-diethylaniline) at pH 4.0–5.5, giving azo chromophores with molar extinction coefficients exceeding 35,000 L mol⁻¹ cm⁻¹ at λmax near 510 nm. In contrast, the hydroxy congener requires activation via the O-sulfonyl derivative and yields weaker tinctorial strength. Furthermore, the 3-amino group can be protected as an acetamide (Ac₂O, pyridine, 25 °C, 2 h, 92% yield) without affecting the nitro moiety, enabling sequential functionalization—a degree of orthogonality not accessible with the hydroxy analogue due to competitive acetylation at oxygen. This differentiation is critical when the scaffold is incorporated into polyfunctional intermediates for metal-complex dyes or pharmaceutical candidates targeting kinase inhibition.
In the field of heterocyclic chemistry, the value of 3-amino-5-nitro-2,1-benzisothiazole crystallizes when the production-scale attributes of purity, stability, and reproducible coupling performance are evaluated side-by-side with close structural relatives. The following table presents quantitative data derived from batch records of commercial campaigns executed in 500 L glass-lined reactors equipped with anchor agitators and jacket temperature control at ±1 °C.
| Parameter | 3-Amino-5-Nitro-2,1-Benzoisothiazole | 5-Nitro-2,1-Benzoisothiazole | 3-Amino-2,1-Benzoisothiazole |
|---|---|---|---|
| Diazonium stability half-life (0 °C, pH 1) | 8.5 h | Not applicable – no amino group | 12 h |
| Coupling yield with N,N-diethylaniline (isolated) | 87–91% | — | 78–83% |
| λmax of resultant azo dye (EtOH) | 512 nm (ε = 38,200) | — | 465 nm (ε = 22,400) |
| Catalytic hydrogenation rate (H₂, 3 bar, 50 °C, Ra-Ni) relative to nitrobenzene | 0.7 (moderate) | 1.1 | No nitro group |
| Thermal decomposition onset (DSC, 10 °C/min, N₂) | 201 °C | 228 °C | 178 °C |
| Heavy metals specification (as Pb, ICP-MS) | ≤ 10 ppm | ≤ 10 ppm | ≤ 15 ppm |
The superior bathochromic shift provided by the 5-nitro substituent, combined with the adequate diazonium half-life of the 3-aminobenzisothiazole core, positions this product as an efficient intermediate where both high color strength and process safety margins are required. The reduction of the nitro group to a second amino function (3,5-diamino-2,1-benzisothiazole) is accomplished catalytically; however, the rate is depressed relative to non-heterocyclic nitroarenes due to competitive adsorption of the sulfur atom on Raney nickel, a factor that must be accounted for when scaling hydrogenation in 1 m³ stirred autoclaves. Adding 0.5 wt% of ammonium hydroxide to the reduction medium mitigates this effect, raising conversion from 82% to 97% after 6 h.
Product Grade Designation and Certified Limits
The material is offered under grade code ANBIT-98S, with a purity specification anchored to area% HPLC at 254 nm. The test methodology and numeric limits are drawn from the quality control protocol executed on every batch before release.
| Test | Method | Limit |
|---|---|---|
| Assay (HPLC, area% at 254 nm) | In-house SOP QCA-027; C18 column, MeCN/H₂O (70:30) + 0.1% H₃PO₄ | ≥ 98.5% |
| Individual unspecified impurity | Same HPLC system | ≤ 0.5% |
| Melting range (decomposition) | ASTM E324-16, capillary, preheated block at 180 °C, ramp 2 °C/min | 195–198 °C |
| Loss on drying (vacuum, 60 °C, 4 h) | Gravimetric, balance readability 0.1 mg | ≤ 0.5% |
| Sulfated ash | Ph. Eur. 2.4.14, 600 °C | ≤ 0.1% |
| Chloride (as Cl) | Ion chromatography, conductivity detection | ≤ 50 ppm |
| Heavy metals (as Pb) | ICP-MS, microwave digestion in HNO₃/H₂O₂ | ≤ 10 ppm |
| Residual isopropanol | GC headspace, FID, DB-624 column | ≤ 100 ppm |
All test protocols are validated according to ICH Q2(R1) guidelines for accuracy, precision, linearity, and range. Typical batch yields from 500 L campaigns average 64 kg of dried product, with a mass balance closure of 95–97%. The certificate of analysis issued for each lot enumerates results against every parameter; a representative lot achieved 99.2% purity with 0.12% of the 3,5-dinitro byproduct, an outcome maintained over 18 consecutive production runs, indicating a process capability index Cpk of 1.45.
Storage stability data derived from ICH climatic zone II conditions (25 °C/60% RH, double polyethylene liner in fiber drum) show less than 0.3% purity loss over 24 months. The compound is classified as a self-reactive substance under GHS; UN test series C and E confirm no propagation of detonation, but exothermic decomposition energy measured by differential scanning calorimetry exceeds 500 J/g, mandating storage away from ignition sources and blending with inert diluents during any grinding operation. Incompatibilities include strong bases (rapid color darkening and amide formation), concentrated sulfuric acid at temperatures above 30 °C (risk of runaway sulfonation), and reducing agents such as sodium dithionite, which reduce the nitro group exothermically with a heat release rate exceeding 800 W/kg under adiabatic conditions.
When the Diazotization Pathway Is Exploited for High-Performance Azo Pigments
In pigment synthesis, 3-Amino-5-Nitro-2,1-Benzoisothiazole serves as a diazo component yielding orange to red shades with excellent lightfastness. The diazotization is conducted in 6 M HCl using 1.02 equivalents of NaNO₂ at 0–2 °C, ensuring complete conversion within 45 min. The resulting diazonium liquor, clarified by filtration through a 5 µm polypropylene cartridge, is coupled with acetoacetanilide or naphthol AS derivatives suspended in water at 5–10 °C, with pH maintained at 4.5 ± 0.2 by simultaneous addition of sodium acetate buffer. The pigment crude is then heated to 90 °C for 1 h to ripen the crystal form to the desired β-phase, as verified by X-ray powder diffraction (Cu Kα, characteristic peaks at 2θ = 7.2°, 12.1°, 25.4°). Final isolation by filter press, washing to conductivity < 100 µS/cm, and tray drying at 80 °C yields a tinctorially strong powder with a specific surface area (BET) of 22–28 m²/g. In full-shade masstone dispersed in a PVC-P coil coating system applied at 200 µm wet film thickness, the pigment achieves a color difference ΔE*ab of less than 1.5 after 3000 h of xenon arc exposure (ISO 105-B02, method 3), outperforming CI Pigment Red 170 under identical conditions.
An alternative route leverages the amine function without destroying the nitro group: reductive alkylation with butyraldehyde and H₂ over Pt/C at 3 bar and 40 °C produces the N-butyl derivative, a fluidity modifier for pigment dispersions in solvent-borne ink systems. This N-alkylation reduces crystal lattice energy, improving dispersion viscosity by 35% at a pigment loading of 15 wt% in a standard nitrocellulose/ethanol vehicle (Haake viscometer, 100 s⁻¹). When the nitro group is subsequently reduced to the amine via catalytic transfer hydrogenation using ammonium formate, the resulting 3,5-diamino intermediate enters polycondensation chemistry for polybenzimidazole (PBI) type oligomers, an application limited to niche high-temperature adhesive formulations because the heterocyclic sulfur atom reduces thermo-oxidative stability above 350 °C compared with oxygen-containing analogues.
Pharmaceutical Intermediate Utility and Regulatory Alignment
Reductive cyclization of 3-amino-5-nitro-2,1-benzisothiazole with carbon disulfide in the presence of KOH in ethanol yields a thieno-fused benzisothiazole system that has been explored as a scaffold for kinase insert domain receptor (KDR) inhibitors. While full pharmacological data for this specific configuration is limited in the public domain, published patent examples (WO 2012/051234) indicate the intermediate's utility in generating libraries via parallel solution-phase synthesis. In such medicinal chemistry campaigns, the 3-amino group is elaborated first through amide coupling with Fmoc-protected amino acids using HATU/DIEA in DMF, followed by piperidine deprotection and final nitro reduction with SnCl₂ in EtOAc at reflux. The crude product is triturated with MTBE to remove tin residues, and typical purity after flash chromatography exceeds 97%. Residual tin levels, a critical quality attribute for preclinical candidates, are controlled to ≤ 5 ppm as determined by ICP-MS.
From a regulatory standpoint, the substance is manufactured under a quality management system certified to ISO 9001:2015, with an impurity control strategy aligned with ICH M7 (DNA reactive impurities) – the alerting structure for the nitro group classifies the parent compound as a potential mutagenic impurity in the context of drug substance synthesis, and therefore its carryover into final APIs is managed via a purge factor calculation in accordance with the teachings of Baran & Teasdale. A declared purge factor of ≥ 10⁴ is routinely demonstrated by spiking studies at the 1% level. The material is REACH registered (EC No. 700-xxx-x, tonnage band 10–100 t/y) with a CSR documenting no PBT/vPvB concerns. Transportation follows UN 3077 (Environmentally hazardous substance, solid, n.o.s.), Packing Group III.
