4,4'-Bis(Bpin)biphenyl: Properties, Synthesis & Applications

4,4'-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)biphenyl is a difunctional arylboronate ester that serves as a versatile, bench-stable building block for Suzuki–Miyaura cross-coupling chemistry and the construction of π-conjugated materials.

CAS Number

179613-19-5

Molecular Formula

C₂₄H₃₂B₂O₄

Molecular Weight

422.13 g/mol

Appearance

White to off-white crystalline solid

Functional Groups

Two pinacol boronate ester groups at the 4,4'-positions of a biphenyl core

Key Application

Difunctional monomer for Suzuki–Miyaura polymerization and small-molecule synthesis

Storage

Store in a cool, dry place away from moisture and light

Chemical Identity and Physical Properties

4,4'-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)biphenyl (CAS 179613-19-5) is a symmetric diboronate ester built on a rigid biphenyl scaffold. Each terminal aryl ring bears a pinacol boronate ester group at the para position, conferring both high symmetry and excellent crystallinity. The compound has a molecular formula of C₂₄H₃₂B₂O₄ and a molecular weight of approximately 422.13 g/mol.

As a pinacol-protected boronate, this reagent is significantly more stable toward air, moisture, and silica gel chromatography than the corresponding free boronic acid, 4,4'-biphenylenediboronic acid. It is typically isolated as a white to off-white crystalline solid with good thermal stability, allowing routine handling on the benchtop for short periods. Long-term storage is recommended in a sealed container under inert atmosphere in a cool, dry location.

The extended π-conjugated biphenyl backbone renders the molecule of interest not only as a synthetic intermediate but also as a functional chromophore in optoelectronic device research.

Synthesis and Preparation

The most widely applied route to 4,4'-bis(Bpin)biphenyl is the iridium- or palladium-catalyzed C–H borylation or, more classically, the reaction of 4,4'-biphenyldiboronic acid with an excess of pinacol in the presence of a dehydrating agent. Alternatively, the compound is accessible via lithiation of 4,4'-dibromobiphenyl followed by quenching with 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (iPrOBpin), and subsequent aqueous workup.

A particularly practical laboratory preparation involves the palladium-catalyzed Miyaura borylation of 4,4'-dibromobiphenyl with bis(pinacolato)diboron (B₂pin₂) under mild conditions (e.g., KOAc base, PdCl₂(dppf) catalyst, DMSO or dioxane solvent, 80–100 °C). This approach offers excellent functional-group tolerance and scalability. The crude product can be purified by recrystallization from common organic solvents such as ethanol or dichloromethane/hexane mixtures.

Purity of the final material is typically assessed by ¹H NMR, ¹¹B NMR (a single resonance near δ 30 ppm is characteristic of an sp²-hybridized pinacol boronate ester), and HPLC.

Applications in Cross-Coupling and Polymer Chemistry

The primary utility of 4,4'-bis(Bpin)biphenyl lies in its role as a difunctional coupling partner in Suzuki–Miyaura reactions. Because it carries two reactive boronate ester groups, it can undergo step-growth AB₂ or AA/BB-type polymerization with dihalogenated comonomers (e.g., dibromo-fluorenes, dibromo-thiophenes, or dibromo-carbazoles) to produce high-molecular-weight conjugated polymers for organic photovoltaics (OPV), organic light-emitting diodes (OLEDs), and organic field-effect transistors (OFETs).

In small-molecule synthesis, the reagent serves as a biphenylene linker to construct extended π-systems, macrocycles, covalent organic frameworks (COFs), and metal–organic frameworks (MOFs). Its symmetric structure simplifies reaction design and minimizes regiochemical complexity.

The compound is also employed in the synthesis of hole-transport materials for perovskite solar cells and in the preparation of fluorescent probes, where the biphenyl core contributes to blue-shifted emission profiles relative to longer acene systems.

Handling, Storage, and Safety Considerations

4,4'-Bis(Bpin)biphenyl is considered a low-to-moderate hazard compound under standard laboratory conditions. Organoboron pinacol esters are generally less acutely toxic than their corresponding boronic acids or boron trihalides, but standard laboratory precautions should be observed: use of nitrile gloves, safety glasses, and a laboratory coat is recommended. Operations generating fine dust should be performed in a fume hood.

The compound should be stored away from strong oxidizers, strong acids, and strong bases, which can hydrolyze the boronate ester bonds and liberate the free boronic acid. Moisture ingress should be minimized; bottles should be backfilled with argon or nitrogen upon opening and resealed promptly.

Recommended storage temperature is room temperature or below (2–8 °C for extended storage), in a tightly closed container. Always consult the current Safety Data Sheet (SDS) provided by Aladdin Scientific before use.

Frequently asked questions

What is the difference between 4,4'-bis(Bpin)biphenyl and 4,4'-biphenyldiboronic acid?

The pinacol ester form (Bpin) is the boronate-protected derivative of the free boronic acid. The Bpin compound offers significantly greater air and moisture stability, improved solubility in common organic solvents, and easier purification by column chromatography. Both forms are reactive in Suzuki–Miyaura coupling, as the pinacol ester is readily transmetalated under standard aqueous base/palladium conditions.

Can this compound be used directly in Suzuki–Miyaura polymerization to make conjugated polymers?

Yes. As a symmetric AA-type bis-boronate monomer, 4,4'-bis(Bpin)biphenyl can be paired with a complementary BB-type dihalide comonomer (e.g., 2,7-dibromofluorene) under Pd catalysis to give alternating conjugated copolymers via step-growth polymerization. High monomer purity and precise stoichiometry are critical for achieving high molecular weights.

How should I activate the pinacol boronate ester for Suzuki coupling?

No pre-activation is required. Under typical Suzuki–Miyaura conditions—aqueous base (e.g., K₂CO₃, Cs₂CO₃, or KOAc), a Pd(0) or Pd(II) precatalyst, and an organic co-solvent (THF, DMF, dioxane, or toluene) at 60–100 °C—the pinacol ester undergoes in situ hydrolysis and transmetalation to the palladium center. Adding a phase-transfer catalyst or increasing base concentration can accelerate the reaction.

What analytical techniques are used to confirm purity and identity?

Identity is typically confirmed by ¹H NMR (aromatic protons and pinacol methyl signals), ¹¹B NMR (single resonance ~30 ppm for sp²-B in pinacol boronate), and mass spectrometry. Purity is assessed by HPLC (reverse-phase or normal-phase) or GC where applicable. Melting point determination also provides a useful purity indicator for this crystalline solid.

References

1. Miyaura, N.; Suzuki, A. Palladium-Catalyzed Cross-Coupling Reactions of Organoboron Compounds. Chemical Reviews 1995, 95 (7), 2457–2483. doi:10.1021/cr00039a007
2. Ishiyama, T.; Murata, M.; Miyaura, N. Palladium(0)-Catalyzed Cross-Coupling Reaction of Alkoxydiboron with Haloarenes: A Direct Procedure for Arylboronic Esters. Journal of Organic Chemistry 1995, 60 (23), 7508–7510. doi:10.1021/jo00128a024
3. Schluter, A. D.; Wegner, G. Palladium Catalyzed Polycondensation of Aryl Monomers: A Route to Ladder and Other Rigid-Rod Polymers. Acta Polymer 1993, 44, 59–69.

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