Introduction

2,5-Dihydropyrrol-3-yl boron reagents combine a nitrogen-containing five-membered ring with a transformable C–B bond. Through Suzuki–Miyaura coupling, aryl or heteroaryl groups can be introduced at C3, followed by reduction of the double bond to give 3-substituted pyrrolidines. Liashuk et al. established a multigram-scale synthesis of these building blocks: N-Boc-3-pyrrolidinone was converted through triflation and palladium-catalyzed borylation into a 2,5-dihydropyrrol-3-yl pinacol boronic ester, which was further converted into the corresponding potassium organotrifluoroborate. The synthesis was performed on a scale of up to approximately 60 g per batch, affording either pure trifluoroborate or technical-grade pinacol boronic ester.[1]

Pinacol boronic esters (Bpin) and potassium organotrifluoroborates (R–BF₃K) retain the same 2,5-dihydropyrrol-3-yl carbon skeleton but differ in the coordination structure of the boron center, physical properties, and pre-reaction activation processes. Bpin represents the material form that can proceed directly from borylation into downstream coupling, whereas BF₃K provides a crystalline salt form suitable for isolation as a pure product, storage, and commercialization.[1,3]

1. The 2,5-Dihydropyrrol-3-yl C–B Bond and Structural Diversification at C3

The 2,5-dihydropyrrole ring contains a C=C bond that can be further reduced. When the boryl group is located on an alkenyl carbon, the C–B bond can be converted into a C–C bond through Suzuki–Miyaura coupling, allowing different aryl and heteroaryl groups to be attached at the C3 position of the five-membered ring.[1]

The core boron building block discussed in this article can be represented as N-Boc-2,5-dihydropyrrol-3-Bpin. Its boryl group is located at the C3 position of the five-membered ring, making it a direct precursor for subsequent C3 diversification through Suzuki–Miyaura coupling (Figure 1).

Figure 1. Structure of N-Boc-2,5-dihydropyrrol-3-Bpin (CAS 212127-83-8).

The C3 position is connected to Bpin and can be further converted into a C3–C bond through Suzuki–Miyaura coupling.

The basic structural transformation is:

N-Boc-3-pyrrolidinone

→ N-Boc-2,5-dihydropyrrol-3-yl boron reagent

→ 3-Aryl/heteroaryl-2,5-dihydropyrrole

→ 3-Aryl/heteroarylpyrrolidine

This route first establishes a common C3–B connection point and then generates different C3-substituted structures by changing the coupling electrophile. The C–B bond serves as a handle for late-stage structural diversification, eliminating the need to reconstruct the pyrrolidine scaffold for each individual C3 substituent.[1]

2. Conversion of the Carbonyl Position into a C(sp²)–Bpin Connection Point

2.1 Enol Triflation of N-Boc-3-pyrrolidinone

The carbonyl group of the starting material N-Boc-3-pyrrolidinone is located at the target C3 position. Liashuk et al. used potassium tert-butoxide (t-BuOK) as the base in the triflation step, and optimization showed that this base was suitable for multigram-scale reactions and one-pot operation.[1]

This step involves carbonyl enolization followed by O-triflation, with the net structural transformation:

C=O → enolate → C=C–OTf

Here, OTf denotes the triflate group. The original carbonyl carbon becomes incorporated into the newly formed alkenyl structure, converting the C3 carbonyl functionality into an alkenyl triflate capable of participating in palladium-catalyzed borylation.

This transformation serves two functions: the carbonyl group defines the site of functionalization, while the triflate group provides the reactive site for subsequent C–B bond formation.

2.2 Palladium-Catalyzed Borylation of the Alkenyl Triflate

The alkenyl triflate is subsequently subjected to palladium-catalyzed borylation with bis(pinacolato)diboron (B₂pin₂). Formation of alkenyl Bpin compounds from alkenyl halides or alkenyl triflates and B₂pin₂ is a classical type of Miyaura borylation.[2]

The net functional-group transformation is:

C(sp²)–OTf → C(sp²)–Bpin

A typical catalytic cycle involves oxidative addition of the alkenyl electrophile to low-valent palladium, transfer of the boryl group to the palladium center, and reductive elimination to form the C–B bond.[2]

These two sequential transformations produce a clearly defined functional-group relay at C3 of N-Boc-3-pyrrolidinone:

C3 carbonyl → C3 alkenyl OTf → C3 alkenyl Bpin

The resulting C(sp²)–Bpin no longer preserves the original carbonyl chemistry; instead, it converts the C3 position into a connection point for subsequent C–C coupling.

3. Boron-Center Structures and Material Properties of Bpin and BF₃K

Pinacol boronic esters (Bpin) and potassium organotrifluoroborates (R–BF₃K) contain the same transferable organic group R but differ in the coordination number, charge state, and heteroatom coordination environment of the boron center. Bpin is a neutral three-coordinate boronic ester, whereas R–BF₃K is a four-coordinate organotrifluoroborate. These differences in boron-center structure further affect Lewis acidity, solubility, solid-state properties, and isolation methods.[3]

The three-coordinate boron atom in Bpin possesses an empty p orbital, and the lone pairs on the two pinacol oxygen atoms can donate electron density to the boron center, thereby reducing its electron deficiency. Once R–BF₃K forms a four-coordinate tetrahedral borate, it no longer exhibits the characteristic empty-p-orbital Lewis acidity of a neutral three-coordinate boron center; the three B–F bonds and ionic structure impart physicochemical properties distinct from those of neutral Bpin.[3]

4. Bpin as a Material Form for Direct Sequential Coupling

After Bpin is formed by palladium-catalyzed borylation, if the next step is a Suzuki–Miyaura coupling, the Bpin material can be used directly once it meets the quality requirements of the downstream reaction, without first being converted into another independently isolated boron derivative. Liashuk et al. identified this mode of use as a principal process advantage of the Bpin form and distinguished it from the storage function of BF₃K. [1]

4.1 Transmetalation Pathways of Bpin

The key steps of Suzuki–Miyaura coupling include:

Oxidative addition of the electrophile → transmetalation of the organoboron component → C–C bond-forming reductive elimination

The participation of Bpin in transmetalation is strongly condition-dependent. In their experimental systems, Thomas, Zahrt, Delaney, and Denmark conducted structural, kinetic, and computational studies of multiple boronic esters and demonstrated that these boronic esters could form transmetalation precursors and directly transfer their organic groups to palladium without prior complete hydrolysis to the corresponding boronic acids.[5]

In aqueous, basic systems, Bpin may simultaneously undergo equilibria involving hydrolysis and boronate formation. The actual transmetalation pathway is jointly controlled by the boronic ester structure, base, water content, solvent, and palladium ligand system. [3,5]

The main advantage of Bpin as a material form for sequential coupling arises from process continuity:

Palladium-catalyzed borylation → Bpin → Suzuki–Miyaura coupling

This pathway reduces the additional operations required to convert Bpin into a storage-oriented derivative and subsequently reactivate it. The process choice is therefore based on continuity of the reaction sequence and does not imply a fixed intrinsic reactivity ranking between Bpin and BF₃K.

5. BF₃K for Isolation as a Pure Product and for Storage

Organoboronic acids or boronic esters can be converted into potassium organotrifluoroborates through treatment with fluoride. KHF₂ is widely used as a fluoride source for this conversion. During conversion of Bpin into R–BF₃K, the pinacol ligand is replaced by fluoride to generate a four-coordinate trifluoroborate.[3]

The structural transformation can be represented as:

R–Bpin → R–BF₃K

This conversion simultaneously changes the charge state, polarity, solvent partitioning behavior, and solid-state properties of the target compound. R–BF₃K compounds can commonly be isolated by precipitation, crystallization, or selective dissolution; pinacol generated during conversion from Bpin can be removed through the corresponding workup procedures.[3]

The multigram-scale route established by Liashuk et al. produced two clearly defined material outputs: technical-grade 2,5-dihydropyrrol-3-yl Bpin and pure trifluoroborate. The authors used the trifluoroborate form for storage and commercialization.[1]

The two boron forms correspond to different process requirements:

Bpin: retain the borylation product form for direct transition into downstream coupling

BF₃K: convert into a crystalline salt to obtain an independently isolable and storable building block

This functional division allows the same C–B building block to switch between continuous synthesis and independent material supply without changing the transferable 2,5-dihydropyrrol-3-yl skeleton.

6. Hydrolysis of BF₃K and Formation of the Active Transmetalation Species

R–BF₃K is a four-coordinate organotrifluoroborate. In aqueous Suzuki–Miyaura coupling systems, R–BF₃K can undergo hydrolysis, during which the fluorine ligands at boron are progressively replaced by hydroxyl groups, ultimately generating the corresponding boronic acid R–B(OH)₂. Mechanistic studies by Lennox and Lloyd-Jones showed that, under the Suzuki–Miyaura conditions they investigated, the organotrifluoroborate itself was not the principal transmetalating species; rather, the boronic acid produced by complete hydrolysis was the major species involved in transferring the organic group to palladium.[3,4]

The simplified process is:

R–BF₃K → hydrolysis → R–B(OH)₂ → transmetalation → Pd–R

Kinetic studies by Lennox and Lloyd-Jones on the hydrolysis of potassium organotrifluoroborates showed that the hydrolysis rate depends on the structure of the organic group, pH, solvent composition, and the phase behavior and mixing state of the reaction system. Some R–BF₃K compounds required acid catalysis for effective hydrolysis under the Suzuki–Miyaura conditions studied, and hydrolysis rates could differ substantially among different structures.[4]

6.1 Hydrolysis Rate and Low-Concentration Delivery of Boronic Acid

The hydrolysis rate of R–BF₃K and the rate at which the catalytic cycle consumes boronic acid together determine the concentration of R–B(OH)₂ in the reaction system. When these rates are appropriately matched, the generated boronic acid can continuously enter the catalytic cycle without excessive accumulation, thereby reducing competing reactions such as oxidative homocoupling and protodeboronation.[3,4]

This can be represented as:

Continuous hydrolysis of R–BF₃K

→ R–B(OH)₂ maintained at a relatively low concentration

→ Continuous transmetalation

→ Suppression of oxidative homocoupling and protodeboronation

This effect depends on kinetic matching between hydrolysis and catalytic turnover. If hydrolysis of R–BF₃K is too slow, the rate of boronic acid generation may limit transmetalation; if hydrolysis is rapid and causes boronic acid to accumulate, the ability of low-concentration delivery to suppress competing reactions is correspondingly diminished.[4]

R–BF₃K therefore undergoes the following reaction sequence:

Four-coordinate organotrifluoroborate

→ Hydrolysis to form the boronic acid

→ Transmetalation

→ C–C bond formation

7. Bpin/BF₃K Form Selection and the Multigram-Scale Synthetic Workflow

The route established by Liashuk et al. enables the preparation of up to approximately 60 g of 2,5-dihydropyrrol-3-yl boron building blocks per batch using conventional laboratory equipment and commercially available reagents, producing two product forms: pure organotrifluoroborate and technical-grade pinacol boronic ester.[1] The term “technical grade” in the original study describes the material state of the Bpin product and does not represent a universally defined purity grade.

The two product forms correspond to different downstream workflows:

N-Boc-3-pyrrolidinone

→ Alkenyl triflate

→ 2,5-Dihydropyrrol-3-Bpin

→ Suzuki–Miyaura coupling

→ 3-Substituted 2,5-dihydropyrrole

or:

2,5-Dihydropyrrol-3-Bpin

→ 2,5-Dihydropyrrol-3-BF₃K

→ Isolation as a pure product

→ Storage or commercialization

The two forms retain the same transferable 2,5-dihydropyrrol-3-yl skeleton, with their differences concentrated in the chemical form of the boron center and the resulting material properties. Thus, the choice of boron-reagent form is primarily determined by downstream process requirements, while the C3–B bond continues to serve its synthetic role in subsequent Suzuki–Miyaura coupling to form a C3–C bond.

8. Representative Chemicals Related to Form Control of 2,5-Dihydropyrrol-3-yl Boron Reagents and Suzuki Coupling

Table 1. Core Scaffolds, Key Intermediates, and Boron-Containing Building Blocks

Table 2. Reagents for Triflation, Palladium-Catalyzed Borylation, and Boron-Form Conversion

Table 3. Reagents for Suzuki Coupling and Downstream Transformations

Table 4. Bromopyridine Electrophiles for Suzuki Coupling

Note: The products listed above are representative Aladdin products related to scientific research. Specific applications should be determined according to product specifications, batch COAs, and the intended reaction or evaluation system. Additional product specifications, grades, and COA information can be searched on the Aladdin website using the product name, CAS number, or catalog number.

References

[1] Liashuk O. S.; Demchuk O. P.; Hryshchuk O. V.; Grygorenko O. O. 2,5-Dihydro-1H-pyrrol-3-yl Boronic Derivatives: Multigram Synthesis and Coupling Reactions. Organic Process Research & Development, 2024, 28(4): 1061–1069. DOI: 10.1021/acs.oprd.3c00454.

[2] Takagi J.; Takahashi K.; Ishiyama T.; Miyaura N. Palladium-Catalyzed Cross-Coupling Reaction of Bis(pinacolato)diboron with 1-Alkenyl Halides or Triflates: Convenient Synthesis of Unsymmetrical 1,3-Dienes via the Borylation-Coupling Sequence. Journal of the American Chemical Society, 2002, 124(27): 8001–8006. DOI: 10.1021/ja0202255.

[3] Lennox A. J. J.; Lloyd-Jones G. C. Selection of Boron Reagents for Suzuki–Miyaura Coupling. Chemical Society Reviews, 2014, 43: 412–443. DOI: 10.1039/C3CS60197H.

[4] Lennox A. J. J.; Lloyd-Jones G. C. Organotrifluoroborate Hydrolysis: Boronic Acid Release Mechanism and an Acid–Base Paradox in Cross-Coupling. Journal of the American Chemical Society, 2012, 134(17): 7431–7441. DOI: 10.1021/ja300236k.

[5] Thomas A. A.; Zahrt A. F.; Delaney C. P.; Denmark S. E. Elucidating the Role of the Boronic Esters in the Suzuki–Miyaura Reaction: Structural, Kinetic, and Computational Investigations. Journal of the American Chemical Society, 2018, 140(12): 4401–4416. DOI: 10.1021/jacs.8b00400.

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