1-N-Boc-Pyrrole-2-Boronic Acid, Pinacol Ester

Chemical Identity and Model Designation

The compound 1-N-Boc-pyrrole-2-boronic acid, pinacol ester corresponds to the systematic IUPAC designation tert-butyl 2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-1H-pyrrole-1-carboxylate. The commercial product line is typically assigned a catalogue identifier suffixed with a batch-specific lot code; for instance, a common high-purity research-grade variant carries the model number BPC-2247 (as registered by multiple custom synthesis laboratories). The CAS registry number 850567-52-3 unambiguously identifies the substance across regulatory filings and inventory systems. Molecular formula: C₁₅H₂₄BNO₄. Formula weight: 293.17 g·mol⁻¹. The molecule consists of a pyrrole heterocycle protected at nitrogen with a tert-butoxycarbonyl (Boc) group and substituted at the 2-position with a pinacol boronate ester, conferring both latent nucleophilicity and crystallinity. --- A single-sentence summary of storage practice is appropriate here. The solid is thermally stable at ambient temperature but is recommended for long-term storage at 2–8 °C under inert atmosphere to suppress hydrolytic deboronation; desiccated conditions (moisture content < 50 ppm H₂O in headspace) are maintained in original fluoropolymer-lined closures to preserve anhydride integrity over 24-month retest intervals.

What Differentiates This Boronate Ester from Unprotected Pyrrole-2-Boronic Acid?

The immediate structural distinction—the combination of an acid-labile Boc protecting group and a cyclic pinacol ester—resolves two persistent limitations encountered with the parent pyrrole-2-boronic acid. Free pyrrole-2-boronic acid undergoes rapid protodeboronation under mildly acidic or even neutral aqueous conditions; the C–B bond cleavage is catalysed by the electron-rich pyrrole ring, with measured half-lives of less than 2 h in pH 7.4 phosphate buffer at 37 °C (as determined by ¹¹B NMR kinetic studies reported in peer-reviewed organometallic literature). The pinacol ester retards this pathway by steric shielding of the boron centre and by reducing Lewis acidity. The Boc group further deactivates the pyrrole toward electrophilic deboronation while permitting orthogonal deprotection with trifluoroacetic acid (TFA) in dichloromethane or HCl in dioxane, liberating the NH-pyrrole for subsequent functionalisation. In contrast, the 1-N-tosylpyrrole-2-boronic acid pinacol ester—a common alternative—requires harsher alkaline or reductive conditions to remove the sulfonamide, conditions frequently incompatible with base-sensitive substrates installed during multistep sequences. Thus, the Boc variant occupies a strategic niche for convergent synthesis routes where acid sensitivity elsewhere in the molecule is manageable but strong bases or nucleophiles are contraindicated.

Validated Specification Profile and Analytical Certificates

Routine release testing follows a compendial-style monograph harmonised with ICH Q6A guidelines. A representative certificate of analysis for a research-grade lot is tabulated below.

Typical release specifications for 1-N-Boc-pyrrole-2-boronic acid, pinacol ester (Lot analysis representative; inter-lot variability ± 0.3% area for HPLC purity)

Parameter	Method	Specification
Appearance	Visual (USP 〈695〉)	White to off-white crystalline powder
Assay (anhydrous, solvent-free basis)	HPLC, area % at 254 nm	≥ 98.0% (typically 99.2–99.7%)
Water content	Karl Fischer coulometry (USP 〈921〉)	≤ 0.5% w/w
Residual solvents	Headspace GC-FID (USP 〈467〉)	Ethyl acetate ≤ 5000 ppm; n-heptane ≤ 500 ppm
Melting range	Differential scanning calorimetry, onset, 10 °C/min	94–98 °C
Heavy metals	ICP-OES (USP 〈233〉)	Pd ≤ 20 ppm; Fe ≤ 10 ppm

The HPLC method uses a C18 column (150 × 4.6 mm, 5 µm) with a gradient of acetonitrile and 0.1% phosphoric acid. The high-wavelength detection at 254 nm exploits the pyrrole chromophore, while the boronate ester itself is UV-transparent above 230 nm. Trace palladium, a common residual from the Miyaura borylation step used in manufacture, is controlled to prevent interference in subsequent catalytic cycles; levels above 50 ppm Pd have been documented to suppress turnover in certain Buchwald–Hartwig aminations run on the deprotected intermediate. --- The solid tends to entrain ethyl acetate from crystallisation. On a pilot-plant campaign conducted in a 50-L glass-lined reactor with retreat-curve impeller agitation at 120 rpm, vacuum drying at 40 °C (≤ 10 mbar) for 16 h reduced residual ethyl acetate from 12,000 ppm to 2,100 ppm, meeting the ICH class 3 limit (5000 ppm). This drying protocol is supplied as a processing recommendation for kilogram-scale operations where oven-tray surface area-to-mass ratios deviate from laboratory lyophilisation conditions.

When Suzuki–Miyaura Coupling Demands Orthogonal Protection

In convergent fragment couplings toward pharmaceutical intermediates, the 1-N-Boc-pyrrole-2-boronic acid pinacol ester is typically deployed under standard aqueous-basic Suzuki–Miyaura conditions: 1.5–2.0 eq of boronate relative to aryl halide, 2 mol% Pd(PPh₃)₄ or Pd(dppf)Cl₂, and 2 M aqueous Na₂CO₃ in a mixed solvent system of 1,4-dioxane/water (4:1 v/v). The reaction temperature is typically maintained at 80–90 °C for 6–18 h. Under these conditions, the Boc group remains intact with >95% retention, provided the aqueous phase pH does not drop below 9 during the course of the reaction. This contrasts with 1-N-Boc-indole-2-boronic acid pinacol ester, where the indole nitrogen is more susceptible to thermal Boc loss owing to enhanced N–H acidity in the transition state; comparative kinetic profiling by ¹H NMR in D₂O/dioxane-d₈ at 80 °C revealed a Boc deprotection half-life of 4.2 h for the indole analogue vs. 11.5 h for the pyrrole derivative. Process-scale experience on a 200-g input batch in a 5-L jacketed vessel noted an exothermic induction period of approximately 4–6 min following palladium injection; a controlled ramp of internal temperature from 22 °C to 78 °C over 30 min prevented overshoot that otherwise accelerated protodeboronation. Post-reaction workup involved extraction with methyl tert-butyl ether, washing with brine, and concentration to an oil that crystallised upon seeding. The isolated yield of the Boc-protected biaryl was 84% after flash chromatography, with < 2% of the des-bromo homocoupling dimer. Published data for microwave-assisted variants are limited, but preliminary reports indicate that a sealed-vessel protocol at 120 °C for 30 min using Pd(OAc)₂/SPhos can reduce reaction time to 0.5 h with 78% yield, albeit with 5–7% Boc cleavage.

Compatibility Boundaries and Operational Pitfalls

Incompatibilities are pronounced with primary and secondary amines at elevated temperature; the boronate ester undergoes transesterification with amine-based additives or solvent traces such as ethanolamine, forming a B–N adduct that precipitates from solution and retards cross-coupling. In a documented production deviation, replacement of anhydrous K₃PO₄ with K₂CO₃ ground in a mortar contaminated with hexamethylenetetramine residues caused a 60% reduction in catalytic turnover frequency, traced to amine–boron adduct formation. Therefore, dedicated, amine-free preparation vessels and high-purity bases are specified. Pre-drying of dioxane over activated 4 Å molecular sieves to a water content of < 50 ppm is necessary when the coupling partner is an electron-deficient aryl chloride, as water competes for oxidative addition sites on palladium(0), shifting the selectivity toward homocoupling. The solid exhibits a tendency to clump when relative humidity exceeds 60% during dispensing; this clumping, if not controlled, alters gravimetric accuracy for milligram-scale reactions and introduces variable water content into the reaction mixture. A nitrogen-purged glovebox with a dew point of −40 °C is therefore the recommended dispensing environment for sub-1 g quantities.

Comparative Performance in Matrix Screening

A non-exhaustive cross-comparison of pyrrole-derived boronate reagents is given, focusing on attributes pertinent to early-stage route scouting.

Comparative characteristics of selected pyrrole-2-boronate reagents under standard Suzuki conditions (aryl bromide, 2 eq boronate, 2 M Na₂CO₃, dioxane, 80 °C)

Reagent	Protecting Group Stability Range (pH, temp)	Observed Major Side Product	Isolated Yield (±5%)
1-N-Boc-pyrrole-2-boronic acid, pinacol ester	pH 9–13, ≤ 90 °C	Protodeboronation (3–7%)	82–88%
1-N-Boc-pyrrole-2-boronic acid	pH 10–13, ≤ 70 °C	Protodeboronation (15–22%)	56–65%
1-N-Tosyl-pyrrole-2-boronic acid, pinacol ester	pH 9–14, ≤ 100 °C	Sulfone reduction (< 2%)	85–90%
1-N-methyl-pyrrole-2-boronic acid, pinacol ester	pH 9–13, ≤ 80 °C	N-demethylation (trace)	78–83%

The tosyl-protected variant yields marginally higher cross-coupling efficiency under basic conditions, but the deprotection step—commonly requiring sodium amalgam or magnesium in methanol—introduces a problematic exotherm and generates sulfinate waste streams that complicate downstream purification in cGMP settings. The Boc derivative, while slightly less robust toward aqueous base at extended reaction times, can be deprotected using anhydrous HCl in dioxane at 0–25 °C within 2 h without trace metals contamination. This makes it the preferred choice for active pharmaceutical ingredient (API) intermediates where heavy metal scavenger steps must be minimised.

Pre-formulation and Solubility Data for Automated Parallel Synthesis Platforms

For high-throughput experimentation (HTE) using Chemspeed or Symyx robotic platforms, stock solutions are routinely prepared in anhydrous 1,4-dioxane or toluene at concentrations of 0.2–0.5 M. Solubility in 1,4-dioxane at 23 °C was measured as 0.48 M by gravimetric filtration; in toluene, solubility drops to 0.12 M. The solution exhibits no visible precipitation after 48 h under nitrogen, but ¹¹B NMR monitoring indicates gradual solvolysis (< 3% over 72 h). For extended robotic campaigns exceeding 24 h, fresh stock preparation is advised. When the platform requires a 1,2-dimethoxyethane (DME) solvent due to ligand constraints, solubility is 0.31 M, but storage at −20 °C under argon is mandatory to forestall peroxide-induced deboronation; DME peroxides can react with the boronate to generate radicals that cleave the C–B bond, a pathway confirmed by EPR spin-trap studies in model systems. --- Differential scanning calorimetry reveals a sharp endothermic melt at 94.5 °C (onset) accompanied by a decomposition exotherm above 200 °C (ΔH_dec = −485 J·g⁻¹). Accelerating rate calorimetry (ARC) on a 2-g sample in a titanium bomb did not detect self-sustaining decomposition below 180 °C, classifying the compound as thermally stable under standard storage and handling regimes. Nonetheless, process safety evaluations recommend against exposure to strong Lewis acids (e.g., BF₃·Et₂O) in bulk, as the exothermic transesterification can initiate a runaway above 150 °C.

Regulatory Starting Material Status and Supply Chain Integrity

In filings for drug master files (DMF), the substance is commonly declared as a regulatory starting material when at least two steps separate it from the final API, provided that the borylation chemistry is established as a robust, non-critical transformation. Audited vendors maintain ISO 9001:2015 certification and supply a certificate of analysis including residual palladium quantification by ICP-MS to a reporting limit of 1 ppm. A change in the pinacol source—from synthetic pinacol to bio-derived pinacol containing 0.3% mesityl oxide—was flagged during a vendor qualification campaign, as the α,β-unsaturated ketone impurity poisoned the palladium catalyst in a subsequent Heck coupling. Consequently, a dedicated GC-MS impurity screen for mesityl oxide (≤ 100 ppm) has been appended to the in-house monograph. No further structural elaboration is needed beyond these application-linked profiles; the technical niche occupied by this boronate ester is defined by the intersection of acid-labile protecting group chemistry and air-stable, crystalline handling.