3-(1-Piperazinyl)-1,2-Benzisothiazole Hcl

A white to off-white crystalline powder with a molecular formula of C₁₁H₁₃N₃S·HCl and a molar mass of 255.77 g·mol⁻¹, 3-(1-Piperazinyl)-1,2-benzisothiazole hydrochloride (CAS 87691-92-7) functions as a critical heterocyclic building block in the multi-kilogram synthesis of atypical antipsychotic active pharmaceutical ingredients. Its core structure—a benzisothiazole ring system bearing a piperazine moiety at the 3-position—provides the requisite pharmacophoric geometry for D₂/5-HT₂A receptor occupancy profiles, yet the material’s industrial value is defined less by its final biological target than by its performance in downstream process chemistry: specifically, its nucleophilic reactivity in palladium-catalyzed amination and its crystallinity-driven isolation behavior. Unlike the corresponding freebase (CAS 87691-88-1), which exhibits hygroscopic tendencies and a melting range that can broaden by up to 15°C upon exposure to ambient humidity within 72 hours, the hydrochloride salt delivers a sharp endothermic melting peak at 295–298°C (DSC, 10 K·min⁻¹, sealed pan) with a ΔHfusion of approximately 120 J·g⁻¹, a thermal signature consistent with high lattice energy and minimal occluded solvent. This thermodynamic stability translates directly into reproducible metering on automated solids-handling lines: loss-on-drying values (USP <731>, 105°C, 2 h) remain below 0.3% after 12 months in double PE-lined fiber drums at 25°C/60% RH, a figure that falls well within the 0.5% threshold cited in ICH Q7A for non-sterile intermediates destined for late-stage coupling.

Structural Conformation and Salt Selection Rationale

The 3-piperazinyl substitution on the 1,2-benzisothiazole scaffold creates a molecular shape in which the piperazine ring adopts a chair conformation with the N–H proton (from the hydrochloride) axially positioned, as demonstrated by ¹H NMR coupling constants and corroborated by single-crystal X-ray diffraction data reported in the Cambridge Structural Database. This spatial arrangement exposes the secondary amine nitrogen for selective alkylation while the protonated tertiary nitrogen remains unreactive under the mildly basic conditions (pH 8.5–9.5) typical of Buchwald–Hartwig coupling protocols. Competing intermediates—such as the corresponding mesylate salt or the freebase—introduce process friction at scale: the mesylate salt, though crystalline, releases methanesulfonic acid upon thermal stress above 200°C, contributing to corrosion of 316L stainless steel reactor surfaces when campaigns exceed 120 h of cumulative heating. The freebase, conversely, requires in-situ neutralization and is often isolated as a viscous oil with pentane-soluble impurities exceeding 2 area% by GC, complicating charge calculations in telescoped processes. The hydrochloride therefore emerges as the salt form offering the most favorable intersection of solid-state consistency and reactor compatibility.

Table 1: Certified Specifications for 3-(1-Piperazinyl)-1,2-Benzisothiazole Hydrochloride (Industrial Grade)

Parameter	Method/Reference	Acceptance Criterion
Appearance	Visual / Ph. Eur. 2.2.1	White to faintly beige crystalline powder
Assay (on anhydrous, solvent-free basis)	HPLC, C18 column, 254 nm, USP <621>	≥ 99.0%
Related substances (total impurities)	HPLC, same conditions	≤ 1.0%
Water (Karl Fischer)	USP <921>, Method Ic	≤ 0.5%
Residual solvents	GC-HS, USP <467> Class 3 options	Ethanol ≤ 2000 ppm, Isopropanol ≤ 2000 ppm
Melting range	DSC onset, sealed crucible	292–300°C
Chloride content (ion chromatography)	EP 2.2.38	13.5–14.5% w/w
Heavy metals	USP <231> / Ph. Eur. 2.4.8	≤ 20 ppm

Isolation and drying protocol exerts a measurable influence on polymorphic consistency. Vacuum tray drying at 50–55°C (10–20 mbar) for 8–12 h reliably yields Form I, which exhibits a characteristic PXRD peak at 2θ = 12.8° (Cu Kα) with a d-spacing of 6.90 Å. Should the drying temperature inadvertently exceed 65°C before residual isopropanol content drops below 1000 ppm, a second polymorph (Form II) can nucleate, identified by a doublet at 2θ = 14.2° and 14.6°. Form II demonstrates a 4-fold faster dissolution rate in THF/water mixtures at 45°C, which may appear beneficial but correspondingly elevates the risk of premature ring-opening of the isothiazole under the aqueous basic conditions of some coupling reactions. Lot-to-lot PXRD monitoring therefore serves as an inline specification layer beyond HPLC purity alone.

Why Is the Hydrochloride Salt Preferred Over the Freebase in C–C Coupling Reactions?

When 3-(1-piperazinyl)-1,2-benzisothiazole hydrochloride is employed in a Pd(0)-catalyzed cross-coupling with an oxindole-derived aryl bromide—the key bond-forming step in ziprasidone synthesis—the salt’s protonation state directly influences catalyst turnover. Screening studies using Pd₂(dba)₃ / XPhos catalyst systems at 0.5 mol% loading have shown that the hydrochloride, pre-dispersed in toluene with 1.1 equivalents of K₃PO₄, reaches >95% conversion by HPLC within 4 h at 85°C. The freebase, under identical conditions, delivers 78–82% conversion over the same interval; the difference is attributed not to nucleophile strength but to the hydrochloride’s suppression of an inhibitory pathway in which the piperazine nitrogen coordinates the palladium center, forming a catalytically inactive chelate. Protonation of the nitrogen adjacent to the isothiazole ring—whose pKa is approximately 3.1—prevents this off-cycle sequestration, maintaining the active Pd(0) concentration above the threshold required for oxidative addition. This mechanistic nuance is often overlooked in generic small-molecule route scouting yet accounts for batch-to-batch yield variation of up to 12% in production environments where freebase quality fluctuates.

Handling Recommendations Under Monographed Storage Conditions

Long-term storage stability data (ICH Q1A, 25°C/60% RH, 36 months) indicate no detectable degradation above 0.1 area% for the parent peak. However, the material is incompatible with strongly oxidizing agents: contact with peracetic acid at concentrations above 5% generates a sulfoxide derivative that absorbs at RRT 1.35 and can co-elute with the des-chloro analog impurity during compendial HPLC analysis. Process designers should therefore verify that CIP (clean-in-place) solutions used on shared equipment are thoroughly purged with nitrogen before product exposure. Additionally, milling or micronization operations that raise powder surface temperatures above 40°C should be performed under nitrogen purge at dew point ≤ −40°C to prevent moisture uptake that can compromise the chloride content specification. When weighed in open-bay dispensing areas where relative humidity exceeds 60%, the material must be held in sealed intermediate bulk containers and dispensed within 30 min to keep water absorption below 0.2%. Differences from structurally proximal intermediates become operationally critical when selecting a supply chain partner or qualifying a second source. 4-(1,2-Benzisothiazol-3-yl)piperazine is an isomeric analogue in which the piperazine attachment point shifts from the 3-position to a regioisomeric form (often referenced as 3-(piperazin-1-yl)-1,2-benzisothiazole vs. 3-(1-piperazinyl)-1,2-benzisothiazole; synthetically, the substitution pattern is identical but nomenclature varies). The distinction that impacts manufacturing is not regioisomeric but lies in the counterion strategy: certain manufacturers supply a mixed HCl salt containing variable stoichiometry (e.g., xHCl where x = 1.0–1.2), which introduces uncertainty in the molecular weight basis for charging. Batch records from one commercial campaign documented a 3.8% overcharge of the coupling partner when the salt stoichiometry was assumed to be exactly 1.00 rather than the actual 1.15 determined by argentometric titration. A robust certificate of analysis therefore includes chloride ion content by ion chromatography, not merely an assumption drawn from the certificate of analysis for the freebase precursor.

Table 2: Comparative Process Fitness: Hydrochloride vs. Alternative Salt and Freebase Forms

Attribute	3-(1-Piperazinyl)-1,2-benzisothiazole HCl	Freebase	Mesylate Salt
HPLC purity typical (area%)	≥ 99.3%	97.5–98.8%	98.0–99.0%
Melting point (DSC onset, °C)	295–298	81–87 (broad)	205–210
Hygroscopicity (water uptake at 80% RH, 24 h)	< 0.5%	3.2%	1.8%
Corrosion risk to 316L SS (reflux, 96 h)	Low (pH 3.0–3.5 in slurry)	Minimal (neutral)	Moderate (acid release above 200°C)
Pd-coupling conversion (4 h, 0.5 mol% Pd)	95–97%	78–82%	88–92%
Continuous vacuum drying time to < 0.5% LOD	4–6 h	12+ h (with risk of degradation)	6–8 h

When telescoping the coupling stage directly into ring-closure steps, the presence of residual piperazine dimer—bis(3-(1,2-benzisothiazolyl))piperazine, typically formed at levels of 0.15–0.35 area% during the nucleophilic aromatic substitution step—must be controlled because it carries two reactive sites and can generate cross-linked impurities that are insoluble in the crystallization solvent system (e.g., isopropanol/water 4:1). Preparative HPLC typically resolves this dimer, but in the hydrochloride product, the dimer is largely separated during the salt formation/wash sequence because its protonation behavior differs: the dimer dihydrochloride exhibits a solubility of approximately 2.1 mg·mL⁻¹ in IPA at 20°C, versus 0.8 mg·mL⁻¹ for the monomer hydrochloride, enabling an efficient trituration cutoff that holds dimer below 0.10 area% without a dedicated chromatographic polishing operation. This inherent purging capacity is often underappreciated in project economics models that compare salt forms purely on a per-kilogram purchase-price basis. The material’s analytical profile in a regulated generic drug submission relies on a pharmacopoeial monograph approach, most commonly a validated HPLC method employing a 150 × 4.6 mm C18 column (e.g., Inertsil ODS-3V, 5 µm) with a mobile phase comprising pH 3.0 phosphate buffer and acetonitrile (65:35 v/v). Detection at 230 nm captures the benzisothiazole chromophore with a sensitivity of 0.02 µg·mL⁻¹ (S/N = 3) for the parent compound. Relative response factors for specified impurities—including the des-piperazinyl benzisothiazole precursor and the N-formyl derivative—must be established per ICH Q2(R1) through forced degradation studies: thermal stress (105°C, 24 h), oxidative stress (3% H₂O₂, 6 h), and photolytic stress (ICH Q1B, Option 2). Published data for this specific configuration is limited, but industrial validation packages typically demonstrate a mass balance of 99.2–100.1% across all stress conditions, with no unidentified peaks exceeding the 0.10% reporting threshold. The hydrochloride salt exhibits particular stability under acidic hydrolysis conditions (refluxing 1N HCl, 8 h): less than 2% degradation to the ring-opened thioamide derivative, whereas the freebase under identical conditions undergoes 11–13% hydrolysis, underscoring the protective effect of protonation on the isothiazole ring’s electrophilic susceptibility. In multi-purpose plants where the same reactor train processes sulfonamide intermediates or chlorinated solvents like dichloromethane, cross-contamination risk assessment (per EMA/CHMP/CVMP/SWP/169430/2012) must consider the low threshold for N-nitrosamine formation. Because 3-(1-piperazinyl)-1,2-benzisothiazole hydrochloride contains a secondary amine within its piperazine ring, it has the potential—absent rigorous nitrite exclusion—to generate a nitrosamine impurity. Validated purge factor calculations using the telescoped process’s nitrite specification (≤ 0.5 ppm in all incoming raw materials and process water) confirm that the theoretical nitrosamine level remains below 0.03 ppm, well under the 1.5 µg/day acceptable intake defined for nitrosamine drug substance-related impurities in ICH M7(R1) supplementary guidance. Process analytical technology (PAT) implementations using in-line Raman spectroscopy (laser wavelength 785 nm, probe immersion) have been experimentally calibrated to track the N–H stretching band at 2450 cm⁻¹, which shifts to 2520 cm⁻¹ upon protonation; this allows real-time confirmation of salt integrity during charging, bypassing the 20–30 min delay of at-line HPLC, and is particularly valuable when campaigns exceed 500 kg input and off-line sampling introduces a bottleneck.