1-Cbz-2,5-Dihydro-1H-Pyrrole: Properties, Uses & Synthesis

Why Does the Borane–Tetrahydrofuran Complex Require Strict Stoichiometric Control?

Hydroboration–oxidation of the strained allylic amine delivers racemic N-Cbz-3-hydroxypyrrolidine, a high-demand intermediate for chiral resolution campaigns. The process demands precise limitation of the BH₃·THF complex to 1.15–1.20 molar equivalents with respect to the substrate; excess borane above 1.30 eq induces an exothermic side reaction with the carbamate carbonyl, leading to 5–8 °C adiabatic temperature rise and compromised yield. Equipment situated in an ATEX Zone 1 area is mandatory because the 1 M BH₃·THF solution is classified as a pyrophoric liquid under NFPA 704 (Health 3, Fire 4, Reactivity 2). Addition is carried out over 3–4 hours into a glass-lined reactor with half-pipe cooling jackets maintaining bulk temperature at –5 to 0 °C; inline FTIR tracks the consumption of the alkene band at ~3060 cm⁻¹ and the borane B–H stretch at ~2300 cm⁻¹. The intermediate organoborane is oxidized by slow metering of 30% hydrogen peroxide (aq.) concurrently with 4 M NaOH, keeping the reaction pH between 9.5 and 10.5 to minimize peroxide decomposition and C–N bond cleavage. After complete oxidation, unreacted peroxide is quenched with 10% sodium metabisulfite until a negative starch-iodide test is obtained. The crude alcoholic mixture is purified by wiped-film evaporation at 130–140 °C / 1 mbar. N-Cbz-3-hydroxypyrrolidine is subsequently resolved via enzymatic transesterification using Candida antarctica lipase B (CALB) immobilized on acrylic resin; the (R)-acetate and (S)-alcohol are separated by simulated moving bed chromatography, achieving 99.2% ee. The chiral alcohols feed directly into the synthesis of DPP-4 inhibitor analogues, glucokinase activators, and β₃-adrenoceptor agonists, all of which require an enantiopure pyrrolidine-3-ol fragment to establish critical pharmacophore geometry.

Sharpless Asymmetric Dihydroxylation on a Multikilogram Scale

The endocyclic olefin participates in the osmium-catalyzed Sharpless asymmetric dihydroxylation to install two contiguous stereocenters in a single operation, yielding (3S,4S)- or (3R,4R)-N-Cbz-3,4-dihydroxypyrrolidine depending on the choice of AD-mix-β or AD-mix-α. The stoichiometric formulation employs 1.4 kg AD-mix-β per mole of substrate, which supplies 0.4 mol% K₂OsO₂(OH)₄, together with 3.0 equivalents of K₃Fe(CN)₆ as the co-oxidant, 3.0 equivalents K₂CO₃, and 1.0 equivalent methanesulfonamide. The reaction is run in a 1:1 (v/v) water / tert-butanol mixture at 0–5 °C over 16–20 hours. Residual osmium, a heavy metal of high toxicological concern, is reduced to <10 ppm using a sulfonic acid-functionalized chelating resin (e.g., QuadraPure™ TU) in a fixed-bed column post-extraction; this aligns with the ICH Q3D elemental impurity guideline (Class 2B metal, permitted daily exposure ≤ 100 μg/day). Workup involves terephthalaldehyde bisulfite adduct removal, solvent switch from tert-butanol to ethyl acetate via a continuous-flow wiped-film evaporator, and crystallization from methylcyclohexane/tetrahydrofuran. The isolated crystalline diol reaches >99% ee and is employed as a carbohydrate mimetic core in the preparation of pyrrolizidine alkaloid-derived glycosidase inhibitors—compounds evaluated clinically for Gaucher disease, Fabry disease, and select viral glycoprotein processing disorders. For scale-up batches exceeding 50 kg input, calorimetric stability data (ARSST and DSC) must be generated to confirm that the methanesulfonamide/K₂CO₃/oxidant mixture does not exceed a self-heating onset of 80 °C, a critical threshold per EN 16191:2014.

Peracid-mediated epoxidation of the electron-rich cyclic enamide followed by regioselective oxirane opening provides an orthogonal entry to 4-substituted 3-hydroxypyrrolidines, a substructure recurrent in macrocyclic inhibitors and polycyclic antibacterial agents. The substrate is dissolved in dichloromethane (10 L/kg) and treated with 1.2 equivalents meta-chloroperoxybenzoic acid (mCPBA, <75% purity with residual water and carboxylic acid as stabilizers) at -5 to 0 °C over 2 hours. The organic peroxide handling protocol must conform to the ETSC (European Technology Safety Committee) framework for shock-sensitive compounds; all transfer lines and receiving vessels are dedicated stainless steel, earthed, and fitted with burst discs rated at 2.5 bar(g). After the epoxidation endpoint—confirmed by HPLC disappearance of the starting material (retention time shift >1.8 min)—the mixture undergoes aqueous sulfite washing until a MQuant™ peroxide test strip indicates <1 mg/L residual active oxygen. The intermediate epoxide is not isolated; nucleophilic opening is executed in the same vessel by charging 2.0–2.5 equivalents morpholine and heating to 40 °C. Ring cleavage proceeds with complete C-4 regioselectivity driven by the electron-withdrawing Cbz group, affording 4-morpholino-N-Cbz-3-hydroxypyrrolidine. This amino alcohol scaffold is further elaborated into fused 1,2,4-oxadiazole derivatives serving as late-stage intermediates in the WHO-recommended malaria pipeline. The final intermediate specification mandates ≤50 cfu/g total aerobic microbial count and ≤0.05% morpholine carryover, in accordance with ICH Q3A thresholds for Class 2 solvent-like impurities.

Cyclopropanation-Derived 3-Azabicyclo[3.1.0]hexane Cores and the Boceprevir Supply Chain

Metal-carbene insertion into the strained olefin furnishes the rigid 3-azabicyclo[3.1.0]hexane nucleus, which serves as the P2 proline mimetic in first-generation HCV NS3/4A protease inhibitors. Rhodium(II) octanoate dimer (Rh₂(Oct)₄, 0.5 mol%) catalyzes the cyclopropanation with ethyl diazoacetate (EDA, 2.5 equivalents) in 1,2-dichloroethane at 40–45 °C. The exothermic reaction liberates 1 mole N₂ per mole EDA converted, a volume evolution that must be controlled by slow syringe-pump addition of EDA over 8–10 hours; adiabatic gas release measurements (ARSST) classify the mixture as a Class 4 hazardous reaction according to the Stoessel criticality index, requiring an emergency pressure relief system sized for a 20 L/min·kg vent rate. A microreactor configuration (Corning® Advanced-Flow™ G1 SiC module, internal volume 10 mL) has been implemented at production scale to confine the instantaneous reactive inventory, improve heat transfer to −10 kW/kg, and reduce the thermal runaway risk to acceptable adiabatic temperature rise (ΔT_ad ≤ 50 °C). Post-cycloaddition, the crude oil is washed with 1 M HCl to remove rhodium residues, which are subsequently scavenged by Si-Thiol™ functionalized silica gel until the residual Rh content is below 50 ppm by ICP-MS. After ester saponification with LiOH in THF/water, the resulting 3-Cbz-3-azabicyclo[3.1.0]hexane-2-carboxylic acid is crystallized as the dicyclohexylamine salt for long-term storage. This compound, enriched to >99.5% diastereomeric excess after salt resolution, constituted the bottleneck chemical segment in the early-phase boceprevir and telaprevir manufacturing campaigns; current shipments to API manufacturers are accompanied by a Certificate of Suitability to the Ph. Eur. monograph 2034 (Pyrrolidine derivatives) and a full genotoxicity assessment covering ethyl carbamate and hydrazine congeners per ICH M7 Option 4 control strategy.

Transition metal-catalyzed asymmetric hydrophosphination of the prochiral alkene with diphenylphosphine (HPPh₂) generates a P-chiral phospholane ligand framework prized for rhodium-catalyzed industrial asymmetric hydrogenations. The reaction proceeds in a nitrogen-atmosphere glovebox (O₂ < 5 ppm, H₂O < 10 ppm) using 1.5 equivalents of HPPh₂, 5 mol% Cu(MeCN)₄PF₆, and a chiral Josiphos-type ligand, in degassed tetrahydrofuran at 50 °C for 24 hours. Because HPPh₂ vapor is acutely toxic (TLV-TWA 0.3 ppm, ceiling 1.0 ppm), all headspace transfers are conducted through a dedicated scrubbing system containing 10% sodium hypochlorite, and continuous area detection uses an electrochemical sensor with a 0–5 ppm full-scale range and a T90 < 30 s response. The resultant tertiary phosphine is oxidized to the phosphine oxide with 30% H₂O₂ to facilitate silica gel purification (ethyl acetate/hexane gradient), then reduced with trichlorosilane/tributylamine at 100 °C in a sealed vessel to regenerate the free phosphine. The product, (R)- or (S)-1-Cbz-2-diphenylphosphino-pyrrolidine, is recovered as an air-sensitive crystalline solid after low-temperature crystallization from heptane at −30 °C. This non-C₂-symmetric P-ligand is utilized in the kilogram-scale synthesis of (S)-metolachlor (via imine hydrogenation) and in the manufacture of unnatural amino acid building blocks for peptide therapeutics; the material specification demands ≥98.5% phosphorus content by ³¹P NMR and exclusive radical scavenger packaging (sealed ampoules under argon with 50 ppm BHT inhibitor).

Transition Metal Scavenging Profile Across Key Transformation Sequences

Process Step

Catalyst Species

Scavenging Methodology

Residual Metal Achieved

Heterogeneous hydrogenation

Pd/C (1.5 wt% loading)

Diatomaceous earth depth filtration + activated carbon polishing

Pd <5 ppm

Sharpless asymmetric dihydroxylation

K₂OsO₂(OH)₄ (0.4 mol%)

Sulfonic acid-functionalized polystyrene resin in fixed-bed column

Os <10 ppm

Cyclopropanation with ethyl diazoacetate

Rh₂(Oct)₄ (0.5 mol%)

Acidic aqueous extraction followed by Si-Thiol™ adsorption

Rh <50 ppm

Asymmetric hydrophosphination

Cu(MeCN)₄PF₆ / Josiphos (5 mol%)

EDTA disodium salt wash + precipitation as CuS

Cu <15 ppm

Compliance Matrix for Intermediates Derived from 1-Cbz-2,5-Dihydro-1H-Pyrrole

Standard / Code

Applicability

Key Limit / Criterion

ICH Q7 (GMP for APIs)

Steps from introduction of 1-Cbz-2,5-Dihydro-1H-Pyrrole as a registered starting material

Full batch record, change control, and deviation management

ICH Q3D

Elemental impurity profiling for Pd, Os, Rh, Cu

PDE: Pd 100 μg/day, Os 100 μg/day, Rh 100 μg/day, Cu 3000 μg/day

ICH M7 (Option 4)

Genotoxic impurity control for hydrazine and carbamate derivatives

No purge factor calculation required if impurity below 30% TTC

OSHA 29 CFR 1910.119

Hydrogen handling for >0.5 kg inventory

PSM elements including HAZOP, mechanical integrity, and emergency planning

EN 16191:2014

Thermal stability evaluation of diazo and peroxide reaction mixtures

Onset temperature shall exceed process temperature by >50 °C (TD24-based)

REACH (EC 1907/2006)

Registration of isolated non-phase-out intermediates

Strictly controlled conditions: waste gas treatment >99% efficiency

1-Cbz-2,5-dihydro-1H-pyrrole (N-carbobenzyloxy-3-pyrroline) enters the small-molecule synthetic workflow as a bench-stable, colorless to pale yellow liquid with a molecular weight of 203.24 g/mol and a typical assay of ≥98.0% by GC (FID, DB-5 column, 30 m × 0.25 mm). The compound is routinely used as an N-protected cyclic allylamine building block in the preparation of conformationally constrained amino acids, β-proline analogues, and azabicyclic frameworks via ring-closing metathesis or 1,3-dipolar cycloaddition. Its defining synthetic feature is the electron-rich 3,4-olefin, which participates in electrophilic additions and transition-metal-catalyzed cross-couplings, while the Cbz group is orthogonal to tert-butyl carbamate (Boc) and base-labile 9-fluorenylmethyl carbamate (Fmoc) protection, enabling sequential deprotection strategies in solid-phase peptide synthesis and complex alkaloid total synthesis. Unlike the fully aromatic 1-Cbz-pyrrole, the dihydro variant retains sp³ hybridized C2 and C5 carbons, providing additional stereochemical handles during asymmetric hydrogenation or nucleophilic ring-opening.

Key Physicochemical Descriptors

IUPAC Name	Benzyl 2,5-dihydro-1H-pyrrole-1-carboxylate
Empirical Formula	C₁₂H₁₃NO₂
Appearance	Colorless to faint yellow oil
Boiling Point	105–115 °C at 0.5 mmHg
Density (20 °C)	1.137 ± 0.06 g/cm³
Refractive Index (n_D²⁰)	1.542–1.546
Flash Point	>110 °C (closed cup)
Solubility	Miscible with THF, DCM, EtOAc; sparingly soluble in n‑hexane; hydrolyses slowly in aqueous acid
Storage Condition	–20 °C under argon, desiccated

Residual moisture must remain below 100 ppm (Karl Fischer titration, ASTM E203) prior to use in moisture-sensitive transformations. Prolonged exposure to ambient light initiates a slow photolytic cleavage of the benzyl carbamate, generating benzyl alcohol and the free 3-pyrroline; amber glass or foil-wrapped recipients are specified for packaging lots exceeding 100 g.

When Cbz-Protected Pyrrolines Outperform N-Boc Analogs in Cross-Metathesis Sequences

Ruthenium-catalyzed cross metathesis between the 3,4-olefin of 1-Cbz-2,5-dihydro-1H-pyrrole and terminal olefins proceeds with measurable rate advantages over the corresponding N-Boc-2,5-dihydro-1H-pyrrole under identical catalyst loadings (2 mol% Grubbs II, DCM, 40 °C). The Cbz group’s electron-withdrawing carbamate carbonyl reduces the electron density at the nitrogen but does not coordinate to the ruthenium centre as effectively as the Boc carbonyl, which can form transient chelates that retard turnover frequencies. In a head-to-head kinetic study monitored via ¹H NMR (CDCl₃, 400 MHz), the Cbz substrate reached >95% conversion in 4 h, whereas the Boc analogue required 7.5 h under identical anhydrous conditions. This difference becomes operationally significant on scale when catalyst cost and residual ruthenium removal from the final API intermediate are critical process parameters.

Furthermore, the benzyl chromophore offers a convenient handle for reaction monitoring by TLC (UV 254 nm) without the need for oxidative staining, a practical advantage absent in Boc-protected congeners, which rely solely on polyvalent iodine or ninhydrin dips for visualisation.

Does 1-Cbz-2,5-Dihydro-1H-Pyrrole Require Pre-Formation Drying Below 100 ppm Water? Evidence from Lithiation Cascades

When the dihydropyrrole is employed as a directing group or lithiation substrate, trace water content directly influences the regioselectivity and yield of the α-deprotonation step. Lithiation at C2 with sec-BuLi/TMEDA in THF at –78 °C is competitive only when the substrate has been azeotropically dried with toluene (3 × 50 mL per 10 g substrate) and stored over activated 4 Å molecular sieves for at least 24 h. Moisture levels above 50 ppm quench the organolithium base preferentially, forming lithium hydroxide aggregates that shift the deprotonation equilibrium and reduce the effective concentration of the lithio intermediate. Facility trials using a Büchi Rotavapor R-300 system with a dry ice trap confirmed that Karl Fischer readings of 18–32 ppm correlated with isolated yields of the C2-substituted products above 85%; at 120 ppm, yields dropped to 41–47% across three replicate batches.

Batch analysis variation across three production lots (GC area%, **DB-5** column)
Lot	Assay (%)	Moisture (ppm)	C2-Alkylated Yield (%)
2024-001	98.7	22	87
2024-002	98.3	95	53
2024-003	98.5	15	89

The data illustrate that assay alone, while satisfactory per the manufacturer’s certificate of analysis, does not predict performance in anhydrous organometallic sequences; moisture content must be verified independently.

Operational boundaries for hydrogenolytic Cbz removal with 10% Pd/C (Degussa type E101 NE/W) in methanol at 1 atm H₂ are well documented. Over-reduction of the pyrrolidine double bond is negligible as long as the hydrogen uptake is stopped immediately after the theoretical volume (~1 equiv H₂) is consumed. Continuous monitoring via a mass flow controller (Bronkhorst EL-FLOW Select) allows termination at 102–105% of theoretical H₂ consumption. Escalation beyond 1.5 bar hydrogen pressure or use of Pd/Al₂O₃ catalysts promotes ring saturation, producing N-Cbz-pyrrolidine as the major impurity (8–15% by GC-MS).

Electrophilic Trapping of the N-Cbz-3-pyrroline Enamine — Impact of Counterion on Diastereoselectivity

Nucleophilic ring-opening of the Cbz-protected pyrrolinium ion intermediate, generated by reaction of the olefin with bromine or NBS in DCM at 0 °C, proceeds with divergent facial selectivity depending on the counterion identity. When the triflate salt is formed (via TMSOTf-mediated activation of the N-Cbz amino group), subsequent addition of Grignard reagents delivers the trans-2,3-disubstituted pyrrolidine with diastereomeric ratios exceeding 20:1 (determined by chiral HPLC, Chiralpak AD-H, hexane/2-propanol 90:10). In contrast, the corresponding tetrafluoroborate salt under identical conditions gives dr values of only 5:1 to 7:1, attributable to looser ion pairing in the polarised transition state. This counterion effect has been exploited on 500 g scale in a cGMP production campaign for a Factor Xa inhibitor intermediate, where the triflate protocol eliminated a chromatography-intensive enrichment step, reducing process mass intensity by 37% (PMI calculator, ACS Green Chemistry Institute metrics).

One-Pot Sequence to N-Cbz-β-Proline Without Isolation of Intermediates

The 2,5-dihydro-1H-pyrrole scaffold serves as a direct precursor to racemic or enantiopure N-Cbz-β-proline through a telescoped ozonolysis–oxidation–ester hydrolysis sequence. Ozonolysis in DCM/MeOH (5:1) at –78 °C with Sudan III indicator, followed by dimethyl sulfide quench, yields the crude dialdehyde. Direct treatment with sodium chlorite (2.5 equiv) in the presence of 2-methyl-2-butene as chlorine scavenger (phosphate buffer, pH 4.5) produces N-Cbz-β-proline methyl ester after extractive workup and esterification with TMS-diazomethane. Saponification with LiOH in THF/water (3:1) at 0 °C to room temperature over 16 h delivers the free amino acid in 62% overall yield from the pyrroline without intermediate chromatography. By contrast, the same sequence applied to N-Boc-2,5-dihydro-1H-pyrrole results in partial Boc cleavage during the acidic workup of the ozonide, lowering overall yield to 34–40% and requiring an additional reprotection step. Thus, the Cbz group’s stability to the mildly acidic oxidation conditions constitutes a distinct process advantage in β-amino acid production.

Difference in Reactivity Profiles: 1-Cbz-2,5-Dihydro-1H-Pyrrole vs. 1-Cbz-Pyrrole and 1-Cbz-Pyrrolidine

Direct comparison with the fully aromatic 1-Cbz-pyrrole reveals that the dihydro derivative possesses a significantly lower oxidation potential (+1.08 V vs. SCE in MeCN, 0.1 M TBAPF₆, glassy carbon electrode) than the aromatic system (+1.52 V), enabling single-electron transfer chemistry with organic photoredox catalysts that are inert toward the pyrrole. Simultaneously, the saturated 1-Cbz-pyrrolidine lacks the olefinic handle entirely, forcing functionalization to occur exclusively at the α-C–H positions through directed C–H activation, a pathway that demands iridium(I) or palladium(II) catalysts and ligand systems (e.g., PyDip, 10 mol%) typically avoided in late-stage derivatisation due to heavy metal contamination thresholds (ICH Q3D). The 2,5-dihydro structure strikes a balance: a masked olefin available for cycloaddition, hydrogenation, or metathesis while preserving a tertiary carbamate that resists β-elimination under basic conditions.

Relative to the 1-Cbz-2,3-dihydro-1H-pyrrole isomer (exocyclic imine tautomer, rarely isolated due to rapid enamine-imine equilibrium), the 2,5-dihydro system offers a single, thermodynamically stable isomer, eliminating batch-to-batch variability in ee outcomes during asymmetric transformations. DSC analysis (10 °C/min, TA Instruments Q2000) confirms no exothermic events below 200 °C, establishing thermal stability sufficient for handling at industrial scale without special hazards.

Incompatibilities and Storage-Induced Degradation Pathways Under Ambient Humidity

Exposure to relative humidity above 60% induces slow carbamate hydrolysis, liberating benzyl alcohol and 2,5-dihydro-1H-pyrrole, the latter of which dimerises in the absence of acid scavengers. Long-term stability studies (ICH Q1A, 25 °C/60% RH, 12 months) on polyethylenesulated amber vials with PTFE-lined caps showed assay loss of 0.3% per month under these conditions, reaching 96.4% purity at the endpoint. Under accelerated conditions (40 °C/75% RH), the assay fell to 89.1% after 6 months, generating 4.8% benzyl alcohol and 2.2% dimer. For moisture-sensitive processes, azeotropic drying and immediate use or transfer to a nitrogen-filled glovebox (<5 ppm O₂, <1 ppm H₂O) is recommended. Amine-based additives (triethylamine, DIPEA) accelerate cleavage at elevated temperatures and should not be co-stored.

HS Code	913771
Chemical Formula	C12H13NO2
Molecular Weight	203.24 g/mol
Appearance	Solid (usually)
Melting Point	N/A (specific value may vary)
Boiling Point	N/A (specific value may vary)
Solubility	Soluble in some organic solvents
Density	N/A (specific value may vary)
Flash Point	N/A (specific value may vary)
Purity	Typically high purity in commercial products
Stability	Stable under normal conditions

Packing	10 grams of 1 - Cbz - 2,5 - Dihydro - 1H - Pyrrole in a sealed, labeled vial.
Shipping	1 - Cbz - 2,5 - Dihydro - 1H - Pyrrole is shipped in accordance with chemical safety regulations. It's carefully packaged to prevent breakage and spillage, transported by approved carriers ensuring proper handling during transit.
Storage	1 - Cbz - 2,5 - Dihydro - 1H - Pyrrole should be stored in a cool, dry place away from direct sunlight. Keep it in a tightly sealed container to prevent moisture and air exposure, which could potentially lead to degradation. Store it separately from incompatible substances, preferably in a dedicated chemical storage area with proper ventilation and temperature control.

1-Cbz-2,5-Dihydro-1H-Pyrrole