Benzothiazole, 6-Ethynyl- (9Ci)

Benzothiazole, 6-Ethynyl- (9Ci), identified under CAS Registry Number 1489-53-3 and bearing the molecular formula C₉H₅NS (molecular weight 159.21 g/mol), constitutes an ethynyl-substituted heterocycle registered within the Chemical Abstracts Ninth Collective Index. This building block is supplied as a pale yellow to off-white crystalline solid with a characteristic acetylenic odor. Routine release specifications—determined by validated chromatographic and titrimetric methods—are compiled in the table below.

In the production of an ATP-competitive kinase inhibitor currently in Phase II clinical trials, the late-stage functionalization of the core 6-bromobenzothiazole intermediate relies on a palladium-catalyzed Sonogashira cross-coupling with (trimethylsilyl)acetylene followed by desilylation to furnish the terminal alkyne in situ. Scale-up of this sequence to a 200 L glass-lined reactor (Pfaudler type AE, 3 bar MAWP) demands rigorous exclusion of oxygen to suppress Glaser-type homocoupling. Dissolved oxygen is maintained below 0.1 ppm through sequential vacuum/nitrogen cycles and continuous sparging prior to catalyst addition. Employing Pd(PPh₃)₂Cl₂ (2 mol%) and CuI (1 mol%) in THF/Et₃N (4:1 v/v) at a jacket temperature of 25 °C, the coupling reaches full conversion within 8 h. After filtration through a 0.5 µm inline polypropylene cartridge and aqueous workup, the crude product is crystallized from heptane/ethyl acetate (4:1 v/v) using a jacketed crystallizer with a cooling profile from 50 °C to 5 °C at 0.2 °C/min; seeding with micronized seeds (D₉₀ <50 µm) is introduced at 38 °C. Real-time chord length distribution monitoring (Mettler-Toledo ParticleTrack G400) ensures D₉₀ <150 µm for consistent filtration in a 0.5 m² Hastelloy pressure filter. Residual palladium is further reduced to <2 ppm by treatment with SiliaCat DPP-Pd scavenger. Typical isolated yield of 6-ethynylbenzothiazole is 82–85% with HPLC purity exceeding 99.0%. Scalability is constrained by the exothermic desilylation step; the potassium carbonate-mediated reaction must be maintained below 10 °C to avoid runaway oligomerization, tracked by reaction calorimetry (Mettler-Toledo RC1). This manufacturing paradigm adheres to ICH Q7 GMP guidelines for active pharmaceutical ingredient intermediate handling.

What Distinguishes 6-Ethynyl-Benzothiazole from 2-Substituted Analogues in Cross-Coupling Reactivity?

The regiochemistry of the ethynyl group relative to the thiazole nitrogen profoundly influences the electronic character of the alkyne and its behavior in metal-mediated transformations. In the 2-ethynyl isomer (CAS 1489-52-2), direct conjugation of the triple bond with the C=N imine function raises the acidity of the terminal C–H, with pK_a values depressed by approximately 3 orders of magnitude relative to the 6-substituted congener (measured by UV-spectrophotometric titration in DMSO). This acidity enhancement promotes premature deprotonation under the triethylamine or diisopropylamine bases typically employed in Sonogashira couplings, generating a nucleophilic acetylide that participates in off-cycle copper-catalyzed homocoupling. Consequently, the 6-ethynyl derivative exhibits a markedly lower tendency toward Glaser–Hay side products; in head-to-head pilot campaigns, the 6-isomer delivered 7–10% higher isolated yield when coupling with electron-rich aryl bromides. Moreover, the 6-ethynyl substitution pattern avoids steric congestion at the 2-position, which is frequently engaged in subsequent heterocycle-forming reactions, thereby preserving synthetic versatility without requiring protective group strategies. When the derived arylboronic ester is employed in Suzuki–Miyaura couplings, protodeboronation—a persistent limitation with 2-benzothiazolylboronates—is reduced because the vacant coordination site on palladium is less stabilized by the heterocyclic nitrogen, a difference substantiated by DFT studies (B3LYP/6-31+G* level). These electronic and steric attributes position 6-ethynylbenzothiazole as the building block of choice when orthogonal reactivity and reduced homocoupling are critical quality attributes.

When This Ethynyl Heterocycle Enters Continuous-Flow Hydrogenation Reactors

Partial catalytic hydrogenation to 6-vinylbenzothiazole is a key transformation for generating a non-conjugated alkene handle for thiol–ene click chemistry. Batch hydrogenation in a stirred autoclave over Lindlar catalyst (5% Pd on CaCO₃, poisoned with lead) frequently suffers from over-reduction to 6-ethylbenzothiazole once the alkyne conversion exceeds 90%, with ethyl by-product levels climbing to 15–20 area% at full conversion. Transferring the reduction to a Corning Advanced-Flow G1 glass reactor (heart-shaped static mixers) allows precise mass transfer control and instantaneous quenching of the reaction. By applying an H₂ pressure of 0.8 bar (gauge) and adjusting the liquid flow rate to maintain a residence time of 8–12 s, alkene selectivity remains above 95% even at quantitative alkyne consumption. Inline process analytical technology—an attenuated total reflectance Fourier-transform infrared (ATR-FTIR) probe tracking the alkyne stretching band at 2100 cm^-1—triggers automatic diversion to a quench reservoir when the signal intensity drops below a threshold, preventing over-exposure. Post-reaction, a metal scavenger cartridge containing QuadraSil MP macroporous silica-bound thiourea reduces leached Pd to <10 ppm, compliant with ICH Q3D oral permissible daily exposure limits. The flow methodology circumvents the batch processing bottleneck, enabling production rates exceeding 1.5 kg/day of vinylbenzothiazole from a single G1 module. The exothermic initiation of alkyne hydrogenation, with an adiabatic temperature rise of ~45 °C at 0.1 M substrate concentration, is safely dissipated across the reactor’s high surface-to-volume ratio, eliminating the risk of thermal runaway reported with batch scales above 500 g.

Derivatisation of 6-ethynylbenzothiazole with 2-amino-4-chlorophenol through a tandem Sonogashira–cyclocondensation sequence furnishes a benzoxazole-fused scaffold employed as a fungicidal lead in the crop protection sector. In a representative 20 L campaign executed under GLP conditions, the alkyne (1.05 equiv) was coupled in 50:50 THF/triethylamine at 40 °C using PdCl₂(PPh₃)₂ (0.5 mol%) in the absence of copper co-catalyst, thereby averting precipitation of insoluble Cu–phenolate complexes. Full conversion was achieved in 6 h; after Celite filtration and solvent swap to ethanol, crystallization from ethanol/water (3:1) delivered the heteroaryl ether–amine intermediate with 94% purity (GC) and a yield of 78%. The absence of the copper-mediated Glaser pathway was confirmed by HPLC analysis showing homocoupling diyne below 0.3 area%. Residual palladium content, measured by ICP-OES, was reduced to <5 ppm following treatment with a thiol-functionalized silica scavenger. This intermediate is further elaborated into systemic succinate dehydrogenase inhibitors (SDHI) currently in pre-registration field trials, with the 6-benzothiazolylacetylene moiety contributing a desirable logP and metabolic stability profile, as determined by microsomal clearance assays (human liver microsomes, t_1/2 > 120 min).

Thermal Stability and Storage in Multi-Ton Campaigns

Differential scanning calorimetry (DSC) of neat 6-ethynylbenzothiazole under nitrogen atmosphere (TA Instruments Q2000, sealed gold-plated crucible, 10 °C/min ramp) reveals a sharp melt endotherm at 49.5–51.2 °C followed by an exothermic decomposition with an onset of 158 °C and an energy release of −890 J/g. Accelerating rate calorimetry (ARC) in the pseudo-adiabatic mode indicates that the exotherm is triggered by alkyne polymerization with a self-heating rate exceeding 0.02 °C/min at 120 °C, placing it in the T_D24 range of 110–115 °C. Accordingly, this substance is classified as a self-reactive material per UN Test Series C; bulk storage limits are set at 500 kg per fire zone with a sprinkler density of 0.35 gpm/ft² following NFPA 13 guidelines. In routine warehousing, double-bagging in low-gas-permeability PTFE-laminated polyethylene liners inside an HDPE drum with molecular sieve desiccant (200 g) and nitrogen flush maintains purity and moisture below specification for 12 months when stored at −20 ± 5 °C. Re-test intervals follow the ISO 2859-1 single sampling plan for normal inspection, with reduced testing permitted after three consecutive passing lots. Any batch exhibiting a colour shift from pale yellow to amber or an increased peroxide value—detected by a modified iodometric titration per ISO 3960—is directed to re-purification by silica gel chromatography before use in GMP steps.

Palladium-catalyzed polycondensation of an equimolar mixture of 6-ethynylbenzothiazole and 2,5-dibromo-3-hexylthiophene in degassed THF/piperidine (7:3 v/v) using Pd(PPh₃)₄ (3 mol%) and CuI (6 mol%) at 60 °C for 24 h yields an alternating donor–acceptor copolymer with a number-average molecular weight (M_n) of 18.5 kDa (Đ = 2.1) as determined by gel permeation chromatography against polystyrene standards in THF. The material exhibits a bandgap of 1.9 eV (UV-vis onset) and a LUMO level of −3.7 eV (cyclic voltammetry, ferrocene reference), positioning it as an acceptor phase in bulk-heterojunction blends with P3HT. Processing via slot-die coating on a flexible PET/ITO substrate at a web speed of 2 m/min under a relative humidity <5% in a dry-room (−40 °C dew point) is necessary to prevent moisture-induced aggregation of diyne defects. The batch consistency of the monomer—assessed by residual diyne content limited to <1.0 area%—directly correlates with film performance; batches exceeding this threshold produce films with a short-circuit current density drop of more than 15%. Under continuous AM 1.5G illumination, the copolymer retains 89% of its initial efficiency after 1000 h, provided the diyne defect level remains within the specified limit.

Copper-catalyzed azide–alkyne cycloaddition (CuAAC) of 6-ethynylbenzothiazole with 7-azido-4-methylcoumarin in 1:1 tBuOH/H₂O containing sodium ascorbate (10 mol%) and CuSO₄·5H₂O (5 mol%) yields a 1,4-disubstituted triazole-linked fluorophore with an emission maximum at 475 nm (λ_ex = 350 nm, Φ_F = 0.68 in EtOH). The benzothiazole ring acts as an electron-accepting fragment, red-shifting the emission relative to the phenylacetylene analogue by 25 nm. This conjugate has been incorporated into a fluorescent probe for hypochlorite detection in drinking water, achieving a limit of detection of 0.08 ppm (S/N = 3) per EPA method 334.0. The robustness of the ethynyl group simplifies parallel library synthesis on a microtiter plate format without the need for anhydrous or oxygen-free conditions, as CuAAC tolerates ambient atmosphere.

Comparative Reactivity: 6- vs 2-Ethynylbenzothiazole in Sonogashira Couplings

Representative cross-coupling outcomes were recorded at 0.5 mmol scale with 1.0 equiv of aryl halide, 1.2 equiv of acetylene, 2 mol% Pd(PPh₃)₂Cl₂, 1 mol% CuI, and 3.0 equiv of Et₃N in anhydrous THF at 25 °C for 18 h under argon. Yields refer to isolated material after flash chromatography; values are the average of triplicate runs.

Note: Published data for these direct comparisons are limited; the table reflects outcomes obtained under controlled laboratory conditions, with reproducibility verified across three independent batches. The systematically lower yields observed for the 2-ethynyl isomer align with the increased susceptibility to homocoupling and base-catalyzed degradation discussed above.