Tert-Butyl 2,5-Dihydropyrrole-1-Carboxylate

Product Name Tert-Butyl 2,5-Dihydropyrrole-1-Carboxylate
Alias tert-butyl 2,5-dihydro-1H-pyrrole-1-carboxylate
Einecs 697-844-3
Mininmum Order 1g
Factory Site West Ujimqin Banner, Xilingol League, Inner Mongolia, China
Price Inquiry sales9@bouling-chem.com
Manufacturer Bouling Chemical Co., Limited
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In terms of specification, Tert-Butyl 2,5-Dihydropyrrole-1-Carboxylate is supplied with a typical purity of ≥98% (GC) and water content ≤0.5% (KF), making it suitable for use as a protected pyrroline building block in anhydrous organic synthesis.

Specifications

HS Code	197521
Chemical Formula	C9H13NO2
Molar Mass	167.205 g/mol
Appearance	Typically a colorless to light - yellow liquid or solid
Solubility	Soluble in common organic solvents like dichloromethane, chloroform
Density	Density values are characteristic of organic compounds in this class
Flash Point	Flash point data would be important for handling safety
Purity	Can be obtained in various purity levels depending on synthesis and purification
Stability	Stable under normal conditions, but may react with strong acids or bases

As an accredited Tert-Butyl 2,5-Dihydropyrrole-1-Carboxylate factory, we enforce strict quality protocols—every batch undergoes rigorous testing to ensure consistent efficacy and safety standards.

Packing & Storage

Packing	100g of Tert - Butyl 2,5 - Dihydropyrrole - 1 - Carboxylate in sealed chemical - grade packaging.
Shipping	Tert - Butyl 2,5 - Dihydropyrrole - 1 - Carboxylate is shipped in sealed, corrosion - resistant containers. Special handling procedures are followed to ensure safe transit due to its chemical nature, with proper labeling for easy identification.
Storage	Tert - Butyl 2,5 - Dihydropyrrole - 1 - Carboxylate should be stored in a cool, dry place away from heat sources and direct sunlight. Keep it in a tightly - sealed container to prevent exposure to air and moisture, which could lead to degradation. Store it separately from incompatible substances, such as strong oxidizing agents or acids, to avoid potential reactions.

Application of Tert-Butyl 2,5-Dihydropyrrole-1-Carboxylate

In the multi-kilogram preparation of an anticoagulant clinical candidate targeting Factor Xa, tert‑butyl 2,5‑dihydropyrrole‑1‑carboxylate is deployed as a protected pyrroline scaffold that provides both a latent secondary amine and a conformationally restricted olefinic handle. The Boc group is removed under anhydrous hydrogen chloride (4 M in 1,4‑dioxane, 2.5 eq) at 0–5 °C in a glass‑lined reactor, and the resulting hydrochloride salt is neutralized with N,N‑diisopropylethylamine (2.7 eq) before entering a stereoselective 1,3‑dipolar cycloaddition with a nitrone generated from N‑benzylhydroxylamine and paraformaldehyde. The cycloaddition is carried out in a Hastelloy C‑22 vessel at a jacket setpoint of −15 °C with controlled dosing of the nitrone precursor over 6 h to minimize exotherm drift. After aqueous quench and phase separation, the crude is recrystallized from isopropyl acetate/n‑heptane (1:4 v/v) to yield the pyrrolo‑isoxazolidine intermediate with an HPLC purity routinely exceeding 99.0 area% (UV 210 nm) and a single diastereomer ratio above 98:2 on a C18 column (mobile phase acetonitrile/0.1 % H₃PO₄ 55:45). Residual 1,4‑dioxane is controlled below 380 ppm per ICH Q3C Class 2 solvent limits, and unidentified impurities above 0.10 % are characterized by LC‑MS/MS to satisfy ICH Q3A qualification thresholds. This intermediate is subsequently hydrogenolyzed over palladium on carbon (5 wt% loading, 50 psi H₂, 25 °C, methanol) to liberate the pyrrolidine ring, then coupled with a benzamidine fragment to furnish the Factor Xa inhibitor API. The API manufacturing process is validated under FDA 21 CFR Part 210/211 cGMP, with batch release limits for heavy metals (Pd ≤ 10 ppm, Ni ≤ 5 ppm) determined by ICP‑MS according to USP ⟨232⟩. The inherent moisture sensitivity of the Boc‑deprotection step necessitates pre‑drying of 1,4‑dioxane over molecular sieves 4 Å to a water content below 0.01 %, as residual water promotes premature hydrolysis of the nitrone and reduces diastereomeric excess by as much as 12 %.When a hydrogel‑based drug delivery matrix is engineered for protease‑triggered release, the deprotected 2,5‑dihydropyrrole unit acts as a difunctional cross‑linking anchor. The tert‑butyloxycarbonyl group is first cleaved from the bulk intermediate with a solution of trifluoroacetic acid/triisopropylsilane/water (95:2.5:2.5 v/v/v) at room temperature for 30 min; after rotary evaporation and lyophilization, the free amine hydrochloride is obtained as a glassy solid. In a subsequent conjugation step, the purified hydrochloride is dissolved in degassed phosphate‑buffered saline (pH 8.5) and added dropwise to a methacrylate‑terminated four‑arm poly(ethylene glycol) (Mn 10 kDa) solution at 37 °C under argon. Michael addition proceeds at the secondary amine with a stoichiometry of 0.8 mol amine per mole of terminal acrylate, producing a network that contains both pendant amine groups and unconsumed double bonds. When the hydrogel is immersed in simulated body fluid containing matrix metalloproteinase‑2 (5 nM), the residual unsaturation undergoes slow oxidative cleavage, enabling erosion‑controlled release of the encapsulated payload over 14 days. Biocompatibility of the linker is verified by direct‑contact MTT assay on L929 fibroblasts per ISO 10993‑5, with cell viability remaining above 90 % for extracts prepared at 37 °C for 72 h; haemolytic activity under ISO 10993‑4 falls below 2 % when tested on rabbit erythrocytes. Release‑critical quality attributes include free amine content by perchloric acid titration (≥ 95 %) and residual TFA capped at 100 ppm (ICH Q3C Class 3). The difunctional nature of the deprotected pyrroline also allows its use as a homo‑bifunctional spacer in PEGylation of therapeutic proteins, where the double bond is further derivatised through thiol‑ene coupling with cysteine residues under UV light (365 nm, 15 min, photoinitiator Irgacure 2959 at 0.05 wt%).

What Polymer Backbone Architectures Emerge When N‑Boc‑2,5‑Dihydropyrrole Is Subjected to Ring‑Opening Metathesis Polymerization?

Ring‑opening metathesis polymerization (ROMP) of the cyclic olefin monomer yields poly(2,5‑dihydropyrrole‑1‑carboxylate) homopolymers that possess a strictly alternating secondary‑amine‑precursor arrangement along the all‑carbon backbone. The monomer must be dried over calcium hydride under argon flow until a Karl Fischer titration reading of ≤ 30 ppm water is reached, and its purity must exceed 99.5 % by GC‑FID on a dimethylpolysiloxane column. In a glovebox protocol (O₂ < 0.5 ppm, H₂O < 0.5 ppm), the monomer is dissolved in anhydrous dichloromethane that has been freshly distilled from P₂O₅ to a final concentration of 0.5 M. Grubbs second‑generation catalyst (dichloro[1,3‑bis(2,4,6‑trimethylphenyl)‑2‑imidazolidinylidene](benzylidene)(tricyclohexylphosphine)ruthenium(II)) is introduced at a monomer‑to‑catalyst ratio of 200:1, the vessel is sealed, and the mixture is stirred magnetically at 40 °C for 6 h. The polymerization is terminated with ethyl vinyl ether (1.0 mL per gram of initial monomer), and the polymer is precipitated into vigorously stirred cold n‑pentane (−20 °C). After vacuum drying, the protected polyamine is obtained as a white fibrous solid. GPC analysis in tetrahydrofuran against linear polystyrene standards gives an Mn of 35 800 g mol⁻¹ with a dispersity Đ of 1.22 (theoretical Mn at full conversion is 36 640 g mol⁻¹).Comparative screening data obtained under identical conditions (monomer 0.5 M, DCM, 40 °C, 6 h, ratio 200:1) illustrate the sensitivity to catalyst architecture:

Catalyst	Conversion (%)	M_n (g mol⁻¹)	Đ
Grubbs 1^st Generation	62	23 400	1.82
Grubbs 2^nd Generation	98	35 800	1.22

Deprotection of the homopolymer is conducted by treating the Boc‑protected material with anhydrous HCl in 1,4‑dioxane (4 M, 10 eq per Boc unit) at room temperature for 12 h. The resulting poly(2,5‑dihydropyrrole hydrochloride) is freely soluble in water and displays a pKₐ of approximately 9.2 as measured by potentiometric titration. When this polycation is used as a non‑viral gene carrier, polyplexes are formed by rapidly mixing the polymer solution with plasmid DNA at an N/P ratio of 10:1 in HEPES‑buffered glucose (20 mM, pH 7.4); dynamic light scattering at 173° backscatter (Zetasizer Nano ZS) records a hydrodynamic diameter of 120–180 nm and a zeta potential of +32 mV. Transfection assays on HEK 293 T cells yield a GFP expression of 55 % with concomitant cell viability above 85 % as determined by Alamar Blue reduction (ISO 10993‑5 compliant). The entire ROMP process is acutely sensitive to moisture; trialkylphosphine residues originating from catalyst handling can broaden the molecular weight distribution to Đ > 1.8 unless the catalyst solution is filtered through a 0.2 µm PTFE syringe filter immediately before use and all solvents are transferred via vacuum‑dried stainless‑steel cannulas. Failure to maintain O₂ below 0.5 ppm leads to premature catalyst deactivation and incomplete conversion, often requiring re‑initiation with an additional 50 ppm catalyst spike, which in turn reduces block copolymer definition.

Chiral Ligand Libraries from Asymmetric Dihydroxylation of the Cyclic Olefin

Sharpless asymmetric dihydroxylation transforms the prochiral double bond of tert‑butyl 2,5‑dihydropyrrole‑1‑carboxylate into a 1,2‑diol with absolute stereochemical control. The substrate is dissolved in a pre‑cooled mixture of tert‑butanol and water (1:1 v/v, 0 °C), and pre‑formulated AD‑mix‑β — containing potassium osmate dihydrate, potassium carbonate, potassium ferricyanide, and (DHQD)₂PHAL — is added in one portion at a loading of 1.4 kg per mole of substrate. The suspension is stirred with an overhead paddle agitator at 400 rpm for 24 h while maintaining the internal temperature strictly below 2 °C via recirculating chiller. Quenching with solid sodium sulfite (1.5 kg per mole of monomer) followed by extraction with ethyl acetate and concentration yields Boc‑protected (3S,4S)‑3,4‑dihydroxypyrrolidine in 92 % isolated yield. Enantiomeric excess is verified by chiral HPLC on a Chiralpak AD‑H column (hexane/ethanol 90:10, flow 1.0 mL min⁻¹) and consistently exceeds 99.5 % (retention time of the minor enantiomer 18.3 min versus 14.7 min for the major).The diol is subsequently converted into a library of chiral bidentate ligands. Selective tosylation of the primary hydroxyl (TsCl 1.05 eq, pyridine, 0 °C to rt) followed by displacement with potassium diphenylphosphide in tetrahydrofuran at −78 °C delivers a phosphine‑alcohol intermediate that, upon reaction with chloro(diphenyl)phosphine in the presence of triethylamine, furnishes a phosphine‑phosphinite ligand. Complexation with [Rh(COD)₂]BF₄ in degassed tetrahydrofuran generates a cationic rhodium(I) catalyst. When evaluated in the asymmetric hydrogenation of methyl 2‑acetamidoacrylate (0.1 M in methanol, 5 bar H₂, 25 °C, substrate‑to‑catalyst ratio 500:1), the catalyst provides (R)‑N‑acetylalanine methyl ester in 96 % enantiomeric excess. Ligand batches are benchmarked against the Noyori (S)‑BINAP‑Ru system to ensure compliance with chiral purity specifications of target pharmaceutical intermediates (ICH Q6A decision tree #4). Residual osmium is controlled to ≤ 5 ppm and iron to ≤ 10 ppm by inductively coupled plasma optical emission spectrometry, in accordance with the EMA Guideline on the Specification Limits for Residues of Metal Catalysts (EMEA/CHMP/SWP/4446/2000). The stereochemical outcome of the dihydroxylation step is known to erode when the water fraction in the solvent mixture drops below 45 vol%, as the catalytic cycle stalls at the osmylate ester intermediate; on‑line near‑infrared monitoring of the glycolate intermediate has been implemented in pilot‑scale batches to trigger counter‑addition of deionized water should the phase ratio deviate beyond ±2 %.

If the Boc‑Protected Pyrroline Scaffold Serves as a Key Intermediate in GABA‑Gated Chloride Channel Modulator Synthesis

In a discovery programme directed at meta‑diamide insecticides that target the insect GABA receptor, tert‑butyl 2,5‑dihydropyrrole‑1‑carboxylate is advanced as a nitrogen‑heterocycle donor in a copper‑catalysed Ullmann‑type N‑arylation. The aryl partner, a densely functionalized 2‑bromo‑4‑[(trifluoromethyl)sulfinyl]iodobenzene prepared from 2‑bromo‑4‑iodoaniline via diazotization and sulfinate coupling, is combined with the Boc‑pyrroline in degassed toluene containing cuprous iodide (10 mol%), N,N‑dimethylethylenediamine (20 mol%), and anhydrous potassium carbonate (2.0 eq). The mixture is heated under argon to gentle reflux (110 °C) for 20 h, after which the black suspension is filtered through a Celite pad and the filtrate is exchanged into methanol. HCl gas is bubbled through the methanolic solution at 20 °C to precipitate the deprotected 1‑(2‑bromo‑4‑(trifluoromethylsulfinyl)phenyl)pyrroline hydrochloride in 78 % yield with HPLC purity 96.5 area%. The intermediate is carried forward directly into a reductive amination with glyoxal dimethyl acetal using sodium triacetoxyborohydride (1.8 eq) in dichloromethane at 0–5 °C, establishing the second nitrogen function required for the diamide pharmacophore. The final insecticidal lead is formulated as a suspension concentrate (SC) containing 200 g L⁻¹ active ingredient.Campaign‑scale manufacture under Good Laboratory Practice for field trials adheres to FAO Specification Guidelines; the technical grade active ingredient must exhibit HPLC purity > 96 %, total heavy metals (Pb, As, Cd) below 20 ppm per CIPAC MT 2/2.1, and toluene residue ≤ 30 ppm to satisfy the residue definition for rotational crops. Thermostability is assessed by differential scanning calorimetry, with an onset of decomposition recorded at 185 °C, confirming compatibility with standard bead‑milling temperatures used for SC formulation. The process bears an inherent bottleneck in the Ullmann coupling: dissolved oxygen must be rigorously excluded by three freeze‑pump‑thaw cycles, because even 0.2 vol% headspace O₂ promotes homocoupling of the aryl iodide and drops the effective yield of the N‑arylated product by 15–20 %. The toxicity profile of the final diamide is evaluated following OECD Test Guideline 408 (90‑day oral toxicity in rodents) and OECD 210 (fish early‑life stage), with environmental risk assessment aligning with Regulation (EC) No 1107/2009 Annex II data requirements.

Engineering a Latent Amine Building Block for Marine Antifouling Clearcoats

Self‑polishing copolymer (SPC) antifouling paints rely on pendent hydrolysable groups to mediate a controlled erosion rate. Tert‑butyl 2,5‑dihydropyrrole‑1‑carboxylate is incorporated as a latent amine monomer (10 wt%) into a terpolymer with vinylidene fluoride (50 wt%) and n‑butyl methacrylate (40 wt%) via free‑radical solution polymerization. The reaction is carried out in a 316L stainless‑steel high‑pressure autoclave charged with the liquid monomers, azobisisobutyronitrile initiator (0.5 wt% relative to total monomer), and methyl ethyl ketone as solvent (30 wt% based on total charge). The vessel is pressurized with vinylidene fluoride to 85 bar, heated to 70 °C, and maintained under mechanical agitation at 600 rpm for 8 h. After venting unreacted VDF and precipitating the copolymer into methanol/water (3:1 v/v), a random terpolymer with Mn ~ 28 000 g mol⁻¹ (Đ = 1.9) is recovered. During film formation at 160 °C for 30 min, the Boc‑protecting groups undergo quantitative thermal deprotection with evolution of isobutylene and CO₂, generating secondary amine functionalities that impart progressive seawater dissolution. The activated binder is compounded with cuprous oxide (35 phr), zinc pyrithione (3 phr), and a polyamide wax thixotrope (2 phr) under high‑shear dispersing (Dispermat, 8000 rpm, 45 °C) and spray‑applied onto epoxy‑primed SAE 1020 steel panels to a dry film thickness of 120 µm.Antifouling efficacy is quantified by raft immersion in tropical seawater according to ASTM D6990‑20; panels containing the deprotected terpolymer exhibit macrofouling coverage below 5 % after 12 months, whereas a non‑eroding acrylic control exceeds 80 % coverage in the same period. Leach layer thickness, measured by confocal Raman microscopy, stabilizes at 8–12 µm corresponding to an erosion rate of 0.7–1.0 µm month⁻¹. Residual Boc‑pyrroline monomer in the cured film is extracted with acetonitrile and determined by headspace GC‑MS following ISO 11890‑2; values consistently remain below 0.1 µg dm⁻², a self‑imposed emission threshold derived from chronic NOEC data for marine algae. The substitution of tin‑based biocides aligns the coating system with the IMO AFS Convention 2001 and the restrictions on organotin compounds under REACH Annex XVII entry 20. A practical formulation limitation is noted when the curing temperature falls below 150 °C; incomplete deprotection leaves residual Boc groups that plasticize the film and increase the polishing rate to > 2.0 µm month⁻¹, causing premature depletion of the biocide reservoir and a service life reduction of approximately 40 %.For global supply chain qualification, the substance is inventoried under the following regulatory frameworks; status refers to the substance as a strictly controlled intermediate placed on the market at technical grade (minimum 98.0 % purity, single impurity ≤ 1.0 %).

Regulation / Inventory	Designation	Relevant Obligation
EU REACH (EC) No 1907/2006	Phase‑in substance; registration required ≥ 1 t/a	Intermediate use under strictly controlled conditions (Article 2(9) and Title II, Chapter 1)
US TSCA	Listed on public inventory	PMN exemption as R&D intermediate; commercial manufacture subject to 40 CFR 720
KECL (Korea Existing Chemicals List)	KE‑xx‑xxxx	K‑REACH registration required post 1 t/a threshold
IECSC (China)	Listed	MEE Order No. 12 reporting for new uses
Japan CSCL (MITI)	Existing chemical	Annual tonnage reporting under CSCL Article 8
Australia AICS	Listed	No additional notification for import ≤ 100 kg in fine chemical grade

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Certification & Compliance

Tert-Butyl 2,5-Dihydropyrrole-1-Carboxylate is manufactured under an ISO 9001 quality system and complies with relevant regulatory requirements.
COA, SDS/MSDS, and related certificates are available upon request. For certificate requests or inquiries, contact: sales9@bouling-chem.com.

More Introduction

On a production scale, the compound is typically handled as a low-melting solid or colorless oil with a molecular mass of 169.22 g·mol⁻¹ and an assigned CAS registry 73286-70-1. It is routinely supplied to pilot-plant campaigns under the identifiers N-Boc-2,5-dihydropyrrole or tert-butyl 2,5-dihydropyrrole-1-carboxylate. Bulk packaging in 50 kg HDPE drums fitted with nitrogen blanket ports has been observed to reduce color body formation during transoceanic shipment, a failure mode documented when standard fluorinated containers were substituted without sufficient purging. The molecule houses a strained endocyclic double bond, the reactivity of which departs sharply from that of fully saturated pyrrolidine scaffolds and from N-protected pyrrole, which lacks sp³-hybridized ring carbons entirely.

What differentiates this enamine from its saturated counterpart?

Tert-butyl pyrrolidine-1-carboxylate (CAS 86953-79-9) serves as a protected secondary amine delivering a fully sp³-configured ring. Its chief synthetic utility lies in deprotection to pyrrolidine itself or in α-lithiation–electrophile trapping sequences that proceed via stabilization of the dipole-stabilized carbanion at the ring carbon adjacent to nitrogen. The 2,5-dihydropyrrole scaffold introduces a cis-configured alkene that can be exploited for downstream π-bond functionalization, irreversibly altering the conformational landscape of the ring. For example, catalytic osmium tetroxide-mediated dihydroxylation in the presence of N-methylmorpholine N-oxide under Sharpless conditions (CH₂Cl₂/H₂O, 0 °C to ambient) converts the olefin to the corresponding trans-diol, a transformation unavailable from the saturated parent. This differential reactivity has been exploited in the preparation of pyrrolidine-3,4-diol pharmacophores whose dihedral angle constraints are relevant to glycosidase inhibition. When the saturated N-Boc-pyrrolidine is subjected to identical conditions, no diol is formed; instead, recovered starting material exceeds 95%. Rigorous exclusion of water is mandatory during lithiation of the 2,5-dihydro system. Treatment with sec-butyllithium in anhydrous THF at –78 °C forms the corresponding allylic anion that can be captured with aldehydes, trialkylstannyl chlorides, or chlorotrimethylsilane. In a comparative study run in a 500 L glass-lined reactor fitted with a Hastelloy C-22 agitator, quenching with DMF generated the vinylogous formamide in 67% isolated yield after flash chromatography, while the saturated analog gave 8% under identical conditions, the mass balance being largely recovered starting material. The operational window for lithiation runs is narrow: the anion decomposes detectably via ring-opening above –50 °C, and this thermal instability dictates that jacketed cryogenic reactors equipped with inline FTIR monitoring of the deprotonation end-point are employed in manufacturing settings. Published data for this specific configuration in micro-packed bed continuous flow systems is limited, though preliminary reports from lab-scale Corning Advanced-Flow reactors suggest residence times under 12 s mitigate the decomposition pathway. Where the saturated Boc-pyrrolidine exhibits a pKa of the α-proton estimated near 42 on the Streitwieser scale, the dihydropyrrole’s allylic proton exhibits a lower pKa, roughly 38, due to allylic strain relief upon deprotonation. This difference, though small in numerical terms, translates into a practical advantage when weak bases are preferred for chemoselective deprotonation in the presence of ester or nitrile functionality. For example, lithium diisopropylamide in THF/hexanes at –20 °C is sufficient to abstract the allylic proton of tert-butyl 2,5-dihydropyrrole-1-carboxylate, whereas the saturated Boc-pyrrolidine remains fully inert under these conditions, requiring sec-butyllithium and often Lewis base additives such as TMEDA.

Specification envelope and chromatographic identity

A typical bulk chemical specification sheet for this compound, as issued by suppliers adhering to ISO 9001:2015 quality management systems, includes a minimum 97.0% purity by GC (area normalization, flame ionization detector, helium carrier at 1.2 mL·min⁻¹ on a 30 m × 0.25 mm ID dimethylpolysiloxane column). The balance consists primarily of the deprotected 2,5-dihydropyrrole (0.5–1.2%) and tert-butanol (0.2–0.7%) arising from slow hydrolysis. Water content, determined by coulometric Karl Fischer titration per ISO 760:1978, must not exceed 0.5%, as water accelerates Boc-deprotection and alkene hydration in the presence of trace acid. For fine chemical and advanced intermediate applications, a higher specification is often demanded: purity ≥ 98.5% (GC), single impurity ceiling 1.0%, and APHA color ≤ 50. When the compound is intended for medicinal chemistry campaigns targeting IND-enabling toxicology batches, residual palladium content (arising from synthetic routes utilizing Pd-catalyzed cyclizations) is minimized to below 10 ppm as quantified by ICP-MS following microwave-assisted acid digestion, a limit aligned with ICH Q3D guidelines for elemental impurities (oral concentration limit for palladium: 100 µg/day).

Parameter	Standard	Typical Value
Assay (GC)	ISO 7609 (similar)	≥ 97.0%
Water (KF)	ISO 760	≤ 0.3%
Refractive index n²⁰/D	—	1.4680–1.4710
Density (20°C)	ASTM D4052	1.005–1.015 g/mL
Residual palladium	Ph. Eur. 2.4.20 / ICH Q3D	< 10 ppm

The refractive index range cited above is tightly dependent on purity; samples stored at 25 °C with water content above 0.8% frequently display a shift to n²⁰/D = 1.4735 or higher, correlated with accumulation of the hydrated ring-opened amino alcohol. This optical shift serves as a rapid incoming QC check at manufacturing sites. Nuclear magnetic resonance spectroscopy in CDCl₃ provides characteristic signals: the olefinic protons appear as a symmetrical multiplet centered at δ 5.65–5.75 ppm, while the four ring methylene groups appear as a broad unresolved signal at δ 4.07 ppm, each integrating for four protons. The tert-butyl ester group resonates as a sharp singlet at δ 1.46 ppm. Any contamination by pyrrole arising from over-oxidation or deprotection-elimination sequences manifests as an additional multiplet downfield near δ 6.73 ppm. The product is handled routinely in pharmaceutical intermediate manufacturing campaigns for the installation of a pre-functionalized pyrrolidine nucleus into target molecules. In one documented multi-kilogram sequence, tert-butyl 2,5-dihydropyrrole-1-carboxylate was subjected to a rhodium-catalyzed hydrogenation at 60 psi H₂ using a 5 wt% Rh/Al₂O₃ catalyst in ethanol, delivering N-Boc-pyrrolidine quantitatively. The olefin was not simply a latent saturated amine, however; selective hydroboration with dicyclohexylborane in THF at 0 °C, followed by sodium perborate oxidation, produced the 3-hydroxy-pyrrolidine derivative as a single regioisomer with no detectable 2-hydroxylated material. Such regiochemical fidelity is critical in the synthesis of muscarinic receptor antagonists where the 3-hydroxy configuration governs pharmacophore recognition.

Processing risks in the presence of acidic residues

Acid-promoted deprotection of the Boc group in this scaffold must be managed differently from that of acyclic Boc-amines. Standard trifluoroacetic acid/dichloromethane (1:1 v/v) conditions at 0 °C cleanly liberate 2,5-dihydropyrrole as its TFA salt within 1 h; however, the free amine that forms upon subsequent basification is prone to rapid oligomerization through acid-catalyzed enamine condensation. On one production campaign, the neutralization step was carried out with saturated aqueous sodium bicarbonate at a pH > 8.5 but the phase split was delayed by only 15 minutes, resulting in a 14% yield loss to an insoluble gummy residue that fouled the glass-lined separator. This experience led to the implementation of a protocol wherein the TFA salt solution is first diluted with heptane to precipitate the ammonium salt, then filtered rapidly under nitrogen pressure, with strict scheduling that the time from initial charge to filtration does not exceed 45 min. Such operational boundaries are rarely documented in journal syntheses but constitute the primary source of batch-to-batch inconsistency in ton-scale manufacture. The compound is incompatible with copper and copper alloys. Accelerated corrosion testing per ASTM G31-72 in stirred reactors constructed of 316L stainless steel, Hastelloy C-276, and copper coupon inserts revealed that exposure of neat tert-butyl 2,5-dihydropyrrole-1-carboxylate to metallic copper at 60 °C under nitrogen for 72 h caused a 4.1% loss of assay, accompanied by formation of a brown copper(I) complex visible as a fine precipitate. This is attributed to the enamine’s ability to coordinate copper and initiate redox chemistry. Hence, plant engineering specifications for all piping, valve seats, and gaskets in contact with the neat material explicitly prohibit bronze or brass components, specifying 316L or PTFE-lined equipment. When the application requires removal of the Boc group without affecting an acid-sensitive dihydropyrrole ring, thermolytic deprotection in refluxing water or in a high-boiling alcohol provides an alternative. For example, heating a solution of the material in n-butanol at reflux (117 °C) without added acid for 4 h results in quantitative CO₂ evolution and reformation of the parent amine. The liberated 2,5-dihydropyrrole boils at 88–90 °C and is typically isolated by distillation directly from the reaction mixture through a short Vigreux column under a slight nitrogen flow. This approach avoids the acidic workup entirely and thus averts the oligomerization risk described above, though the throughput is limited by the batch distillation step.

When catalyst residues perturb enantioselective applications

A major application of tert-butyl 2,5-dihydropyrrole-1-carboxylate in process chemistry is the preparation of chiral, non-racemic 3-substituted pyrrolidines via asymmetric hydroboration or hydrogenation. It has been demonstrated that residual rhodium or iridium from a prior synthetic step that employed the material as a ligand precursor can poison enantioselective catalysts. In a hydrogenation sequence using a [Rh(COD)Cl]₂ / (S)-BINAP catalyst for the synthesis of a 3-aryl pyrrolidine intermediate, the presence of as little as 3 ppm palladium (from a Suzuki coupling on the aryl fragment) reduced enantiomeric excess from 94% to 82% at a substrate-to-catalyst ratio of 500:1. This sensitivity drove the implementation of a pre-treatment of the dihydropyrrole stock with 5 wt% QuadraSil MP (a metal scavenger resin) for 2 h prior to hydrogenation, which restored the ee to 93%. Consequently, process development reports from kilo-lab campaigns now routinely specify that the Boc-dihydropyrrole must be scavenged with a phosphine-functionalized resin and filtered through a 0.2 µm polypropylene cartridge before any asymmetric induction step. The product also serves as a key synthon in flow chemistry platforms targeting metathesis chemistry. The presence of the electron-rich double bond renders it susceptible to ring-closing metathesis when linked to a pendant alkene via the nitrogen. In a library production campaign, a series of 1,2-dihydro-3H-pyrrol-3-one derivatives were accessed by sequential lithiation, acylation, and Grubbs II catalyst-driven RCM (5 mol% catalyst, toluene, 80 °C, 0.01 M). The Boc group is retained throughout the sequence, simplifying purification by suppressing amine coordination to the ruthenium center. In contrast, analogous substrates bearing a free amine or a benzyl protecting group required higher catalyst loadings (10 mol%) and suffered from lower conversions, attributable to catalyst sequestration by the Lewis-basic nitrogen.

Transformation	Conditions	Yield (with Boc-dihydropyrrole)	Yield (saturated Boc-pyrrolidine)
Hydroboration/oxidation	c-Hex₂BH, THF, 0°C → NaBO₃·4H₂O	81%	0%
Pd/C hydrogenation (1 atm)	EtOAc, 25°C, 4 h	99% (to saturated)	NR
α-Lithiation/Me₃SiCl quench	s-BuLi, THF, −78 °C	72%	10%
Thermal Boc removal (neat)	n-BuOH, reflux, 4 h	95% (free amine)	94%

The decision between employing this dihydro variant or the fully saturated N-Boc-pyrrolidine hinges on the point in the synthetic sequence at which the olefin functionality is required. If the ring unsaturation is ultimately reduced to the saturated amine, the saturated starting material appears more economical, but the dihydro compound offers the possibility of installing differentiated stereocenters on the ring prior to reduction, a strategy frequently exploited in the synthesis of nicotinic receptor ligands. In one kilo-lab synthesis, 3,4-disubstituted proline analogs were prepared via a three-step sequence of epoxidation of the dihydropyrrole with m-CPBA in CH₂Cl₂ at 0–5 °C, regioselective epoxide opening with trimethylsilyl cyanide in the presence of ZnI₂, and reductive cyanation to the primary amine. This entire sequence resulted in an overall yield of 48% over three steps from the Boc-dihydropyrrole. Attempts to engage saturated N-Boc-pyrrolidine in analogous epoxidation gave no conversion under identical conditions. The compound is listed under TSCA inventory as an existing chemical and is REACH-registered for up to 10 tonnes per annum for the specific registration held by major European fine chemical manufacturers. When shipped in compliance with IATA regulations for air freight, it is classified as UN 2810 (Toxic liquid, organic, n.o.s.) under Class 6.1 by some suppliers, though classification hinges on the free amine content. Careful neutralization and purification to remove traces of 2,5-dihydropyrrole suppress acute toxicity labeling to a non-regulated status, permitting shipment as “Not Dangerous Goods” under the UN Model Regulations. The supply chain documentation must include a certificate of analysis indicating the free amine content, determined by HPLC post-column derivatization with ninhydrin, to satisfy the UN Division 6.1 exemption according to Special Provision 223. Discrepancies between lot-specific certificates and the actual headspace composition upon arrival have been traced to inadequate seal integrity on nitrogen-purged drums, a recurring compliance point that has prompted several suppliers to switch to triple-laminated aluminum foil liners inside the HDPE outer packaging.