3-Ethyl-4-Methyl-5-(2-Hydroxyethyl)Thiazole-3-Ium·Bromide

In the synthesis of flavour esters via cross-benzoin condensation, this thiazolium bromide functions as a nucleophilic carbene precursor that reverses the polarity of the aldehyde carbonyl. Typical catalyst loading ranges from 0.5 mol% to 5.0 mol% relative to the limiting aldehyde, with optimal performance frequently observed at 2.0 mol% when electron‐rich benzaldehyde derivatives are used. The downstream manufacturing process is executed in a jacketed glass‐lined reactor under a dry nitrogen blanket; the bromide is pre‐dried at 40 °C under vacuum (≤5 mbar) for 24 h when ambient relative humidity exceeds 60 % RH, because residual moisture above 2 wt% of the solvent charge hydrolyses the thiazolium ylide intermediate and generates an inactive thiazolone by‐product. A solvent mixture of anhydrous ethanol and deionised water (9:1 v/v) is charged, the aldehydes are added, and the pH is adjusted to 8.5–9.0 with triethylamine, which deprotonates the C‑2 position of the thiazolium ring in situ. The mass is held at 65–70 °C for 6–12 h, then cooled to 5 °C to precipitate the benzoin product. The crude cake is isolated by centrifuge filtration, washed with chilled ethanol, and recrystallised from toluene/hexane (1:3). The final products—typically 4,4′‐dimethoxybenzoin or 4‐hydroxy‐4′‐methoxybenzoin—are employed as key intermediates for violet‐ and raspberry‐toned fragrance formulations and must comply with EU Regulation (EC) No 1334/2008 on flavouring substances and FDA 21 CFR 172.515 (synthetic flavour ingredients used in food). Residual bromide in the finished flavour intermediate is controlled below 10 mg/kg, verified by ion chromatography following a microwave‐assisted digestion. The bromide salt is hygroscopic and requires storage in sealed aluminium‐lined drums with desiccant; exposure to moisture causes rapid deliquescence and loss of catalytic activity. It is incompatible with primary amines, which quaternise the reactive ylide through an irreversible alkylation pathway, and with strong oxidising agents that oxidise bromide to bromine. Published large‐scale campaign data indicate that batch‐to‐batch yield variance remains within ±3 % when strict moisture exclusion and pH control are maintained; excursions outside the pH 8.5–9.0 window lead to a sharp drop in selectivity due to competing Cannizzaro side reactions.

Use of this thiazolium bromide as a masked N‑heterocyclic carbene (NHC) precursor for organometallic catalyst synthesis demands meticulous oxygen and moisture exclusion because the free carbene generated at the C‑2 position is kinetically unstable toward a rapid oxidative desulfurisation that yields a thiazol‐2‑one. In a standard catalyst preparation workflow, the bromide is dissolved in anhydrous tetrahydrofuran (THF, water content <10 ppm by Karl Fischer titration) and treated with a non‐nucleophilic base—typically potassium hexamethyldisilazide (KHMDS, 1.05 eq.) at –78 °C—inside a nitrogen‐filled glovebox where oxygen is maintained below 1 ppm. Deprotonation is complete within 30 min, after which a metal precursor such as Pd(dba)₂ or Ni(cod)₂ is added dropwise at –30 °C to form the corresponding NHC–metal complex. The ligand‐to‐metal ratio is kept at 1.05:1 to ensure full coordination; excess free carbene accelerates decomposition. On a kilogram scale, the same protocol is adapted to a Hastelloy C‑22 reactor equipped with a mechanical seal, a recirculating cryostat capable of –80 °C, and an online oxygen analyser with a detection limit of 0.1 ppm. After complexation, the solution is filtered through a 0.2 µm PTFE membrane under positive nitrogen pressure and immediately used for cross‐coupling reactions, because the palladium complex loses approximately 15 % of its catalytic turnover frequency within 4 h of standing in solution. The final manufactured articles are homogeneous NHC–Pd(II) precatalysts that enable Suzuki–Miyaura coupling of sterically hindered aryl chlorides at catalyst loadings as low as 0.02 mol% Pd, producing biaryl intermediates for active pharmaceutical ingredients (APIs) or liquid‐crystal monomers. The entire synthetic sequence must adhere to ICH Q3D (Guideline for Elemental Impurities) and the residual metal specifications set out in ICH Q3C (Residual Solvents), while the THF solvent is recovered by distillation to meet an NFPA 30 flammable‐liquids design standard for plant safety. A representative laboratory dataset illustrating the influence of base and solvent on carbene generation efficiency is provided below. Operational boundaries: the free carbene is permanently inactivated in the presence of oxygen partial pressures above 5 Pa and water concentrations exceeding 20 ppm; furthermore, even trace quantities of Brønsted acids (pKa < 18) will reprotonate the carbene and halt complex formation.

¹Determined by ¹³C NMR of the in situ generated NHC–chloroform adduct; ranges reflect triplicate runs.

Biphasic nucleophilic aromatic substitution employing this thiazolium bromide as a phase‐transfer catalyst is applied to the efficient fluorination of electron‐deficient aryl chlorides. The catalyst is charged at 1.0–5.0 mol% relative to the haloarene substrate, with 3.0 mol% being the most frequently reported sweet spot in continuous stirred‐tank reactor trials. The downstream process feeds spray‐dried potassium fluoride (KF, 2.5 eq.) and the aryl chloride dissolved in acetonitrile/water (10:1 v/v) into a 316L stainless steel reactor jacketed for 6 bar steam. The thiazolium cation transports fluoride across the interface; the mixture is heated to 80–85 °C under vigorous impeller agitation (Reynolds number > 10⁴) for 8–14 h, monitored by in‐line GC. After conversion exceeds 98 %, the mass is cooled, the organic layer is separated by a disc‐stack centrifuge, and the solvent is stripped under vacuum to yield the crude fluorinated arene. Final purification is achieved by fractional distillation through a packed column (15 theoretical plates). The end products—such as 2,4‐difluoronitrobenzene or 4‐fluorobenzonitrile—serve as building blocks for herbicides (e.g., diflufenican‐type) and fluoroquinolone antibiotics. Compliance requires a full REACH registration dossier under Regulation (EC) No 1907/2006 and an extended safety data sheet (eSDS) covering the bromide salt’s classification as a skin sensitiser (H317). The phase‐transfer performance drops sharply when the catalyst loading exceeds 5.0 mol%, because the amphiphilic thiazolium cation stabilises stubborn macroemulsions that increase the phase disengagement time to over 2 h; demulsifier addition is then necessary, which introduces contamination risk. The salt is also incompatible with oxidizing agents such as hypochlorite that would convert bromide to bromine, and with concentrated sulfuric acid above 90 wt%, which induces rapid Hoffman‐type elimination.

Cyclisation of ortho‐(α,β‐unsaturated carbonyl)‐substituted benzaldehydes via the Stetter reaction represents a highly atom‐economic route to 1,4‐diketone intermediates that are subsequently ring‐closed to pharmacologically relevant tetralone derivatives. The bromide is employed at a loading of 0.5–2.0 mol% relative to the aldehyde, typically 1.0 mol% in validated pilot campaigns. The manufacturing protocol dissolves the substrate in anhydrous N,N‐dimethylformamide (DMF, water <100 ppm) in a glass‐lined vessel, sparges with argon for 45 min, and adds anhydrous potassium carbonate (K₂CO₃, 1.1 eq.) as the proton shuttle. The suspension is heated to 50 °C, and the bromide is introduced as a 0.5 M solution in DMF over 30 min. The carbene is generated catalytically in situ and promotes conjugate addition of the aldehyde to the Michael acceptor, forming a 1,4‐diketo chain. The reaction endpoint is verified by HPLC (area % target > 95 %), after which the mixture is cooled, quenched with 1 M HCl to pH 5.5, and extracted with ethyl acetate. The organic phase is washed with 10 wt% NaCl solution, dried over molecular sieves, and concentrated. The crude diketone is purified by short‐path distillation at 0.5 mbar (jacket 160 °C) to give the key intermediate that is subsequently cyclised in polyphosphoric acid to the tetralone scaffold. The final tetralone‐type product is a precursor to a selective serotonin reuptake inhibitor (SSRI) analogue; all intermediates must meet ICH Q7 cGMP requirements and residual solvent levels must be within the limits of ICH Q3C (DMF not exceeding 880 ppm in the final API precursor). The process is critically sensitive to pH: a bulk pH above 9.0 triggers irreversible aldol condensation of the starting aldehyde, while pH below 7.5 produces insufficient carbene concentration for appreciable turnover. The Stetter cyclisation is therefore run in a buffer of K₂CO₃/KHCO₃ maintaining pH 8.2–8.5. The bromide is incompatible with dimethyl sulfoxide (DMSO) as solvent at temperatures above 40 °C, where gradual oxidation of the ylide by DMSO is observed, and with primary amine bases (e.g., DBU, triethylamine) that will alkylate the thiazolium nitrogen. Published kilogram‐scale production records indicate that a deviation of even ±2 °C in the addition temperature can shift the product distribution by 8–12 % toward the undesired β‐keto ester pathway.

Ranges collected from multi‐batch laboratory and pilot‐scale optimisation reports; yields are corrected for recovered starting material.

Formulation of a non‐aqueous electrolyte for high‐voltage electric double‐layer capacitors (EDLCs) incorporating this thiazolium bromide as a conductive ionic liquid exploits its high thermal stability and wide electrochemical window. In a dry‐room environment with a dew point below –40 °C, the salt is dissolved in propylene carbonate (PC) at a concentration of 1.0–1.5 M, frequently 1.2 M, and stirred for 2 h to ensure complete dissolution before being passed through a 0.2 µm PTFE filter capsule. The electrolyte is vacuum‐injected into spiral‐wound cells comprising activated carbon electrodes (specific surface area > 1500 m²/g, BET method) and a cellulose separator under an absolute pressure of 10 kPa. After a 24 h wet‐standing period, the cells are sealed by crimping in an argon‐filled glovebox. The final products are industrial supercapacitor modules rated for 2.7–3.0 V that are deployed in wind‐turbine pitch control systems and port‐yard rubber‐tyred gantry cranes. The entire device must satisfy IEC 62391-1:2015 (fixed electric double‐layer capacitors for use in electronic equipment) and the transportation safety requirements of UN 38.3. A critical quality attribute is the water content of the electrolyte, which must stay below 20 ppm; at higher moisture levels, evolution of hydrogen at the cathode during charging causes cell bulging and a drop of capacitance retention from 95 % to below 80 % after 5000 cycles. The bromide‐based electrolyte exhibits a voltage stability window limited to 3.2 V on pristine aluminium current collectors; for long‐term durability, carbon‐coated aluminium foils are adopted to mitigate bromide‐induced pitting corrosion. The salt is thermally stable up to 210 °C by TGA (onset of decomposition), but is incompatible with polyolefin separators that contain residual unsaturated groups, which are prone to oxidative crosslinking at the elevated operating potential. Published data regarding field failure modes pinpoint ionic migration of the thiazolium cation into the electrode micropores as a cause of increased equivalent series resistance after prolonged floating at 3.0 V and 65 °C.

Application of this quaternised thiazolium compound as a non‐metallic corrosion inhibitor for carbon steel surfaces in closed‐loop recirculating cooling water systems relies on its ability to adsorb onto the metal oxide layer via electrostatic interaction between the positively charged thiazolium nitrogen and the negatively charged steel surface at typical operating pH 8.0–9.0. The inhibitor is supplied as a 25 wt% aqueous concentrate that is metered into the return header through a positive displacement diaphragm pump to maintain a residual actives concentration of 50–200 mg/L in the circulating water, with 100 mg/L being the accepted setpoint for moderate hardness (150–300 mg/L as CaCO₃). The formulation also contains 2‐phosphonobutane‐1,2,4‐tricarboxylic acid (PBTC, 7.5 wt%) and a low‐molecular‐weight acrylic copolymer (MW ≈ 4500, 5.0 wt%) as a dispersant. System performance is validated by the ASTM D1384-05 (Standard Test Method for Corrosion Test for Engine Coolants in Glassware) adapted for industrial water, with linear polarisation resistance (LPR) probes providing real‐time corrosion rates. Target corrosion rate for AISI 1020 mild steel under these conditions is below 0.025 mm/year; field data from a 500‑TR centrifugal chiller loop over a 12‑month monitoring period demonstrated an average rate of 0.018 mm/year with the thiazolium‐based program, compared to 0.034 mm/year for a tolyltriazole‐only reference. The final service delivered is extended equipment lifetime for tube‐and‐shell heat exchangers, pump impellers, and distribution piping, with compliance required under the US EPA Effluent Limitation Guidelines (40 CFR 423) and the EU Biocidal Products Regulation (BPR, (EU) No 528/2012) where the bromide salt falls under product‐type PT 11 (preservatives for liquid‐cooling and processing systems). A representative set of performance data is tabulated below. The thiazolium bromide is ineffective on copper and brass alloys, where a synergistic dose of benzotriazole (5–10 mg/L) must be co‐applied. It also exhibits poor performance at bulk water pH > 9.5, where a ring‐opening hydrolysis generates a mercapto‐acid derivative that promotes microbiological activity, and it is incompatible with zinc‐based co‐inhibitors, which cause precipitation of a poorly soluble zinc–thiazolium complex.

All coupons were AISI 1020 cold‐rolled steel, exposed at 45 ± 1 °C for 72 h under flow at 1.5 m/s. Post‐experiment mass loss values corrected for cleaning blank.