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Introduction
Borabicyclo[3.3.1]nonane, more commonly referred to as 9-BBN, is an organoborane compound widely used in organic synthesis, particularly for hydroboration reactions.
The dimeric form of 9-BBN, a stable compound at room temperature, has become an essential reagent due to its favorable properties such as selective reactivity, stability in air-free conditions, and ease of handling.
This monograph delves into the chemistry, structure, applications, and broader impact of the 9-BBN dimer in modern chemistry.
Chemical Structure and Nomenclature
The dimeric structure of 9-BBN arises from the interaction of two borabicyclo[3.3.1]nonane units.
The name is derived from the parent bicyclic structure, indicating a nine-membered system bridged by a boron atom.
The compound belongs to the class of bicycloalkyl boranes and is often referred to by its abbreviated name, 9-BBN dimer, in chemical literature.
Molecular and Electronic Structure
The 9-BBN dimer exhibits a symmetrical dimeric arrangement formed via a B–B bond.
The dimerization results in a planar boron-boron interface, stabilizing the structure.
Each boron center remains tricoordinate and electron-deficient, making the compound highly reactive toward unsaturated carbon-carbon bonds.
Computational studies reveal a stable HOMO-LUMO gap, indicating its relative stability under ambient conditions.
Physicochemical Properties
Appearance: White to off-white crystalline powder
Melting Point: ~200 °C (decomposes)
Boiling Point: Not distillable; decomposes upon heating
Density: ~0.9 g/cm³
Solubility: Soluble in ethers, THF, and hydrocarbons
Stability: Stable under inert atmosphere; reacts with air/moisture
Synthesis and Dimerization Process
9-BBN is synthesized by reacting 1,5-cyclooctadiene with borane (BH3).
The initial reaction yields the monomeric 9-BBN, which readily undergoes dimerization under mild conditions, especially in non-coordinating solvents.
The dimerization is exothermic and facilitated by the electron-deficient nature of boron.
Reaction Scheme:2 C8H15BH → C16H28B2
Mechanism of Dimer Formation
The dimer forms through a head-to-tail association of two 9-BBN monomers.
The boron atom of one unit forms a bond with the vacant p-orbital of the other, leading to the stable dimer.
The mechanism is stereoelectronically controlled and avoids the formation of higher aggregates.
Spectroscopic Characterization
1H NMR: Multiplets due to cyclooctyl protons
11B NMR: Single sharp resonance for tricoordinate boron
13C NMR: Signals corresponding to the cyclic framework
IR Spectrum: B–H stretching around 2500 cm⁻¹
Mass Spectrum: Molecular ion peak at m/z = 250.91 (dimer)
Thermal Stability and Decomposition
The dimer is thermally stable up to ~200 °C but decomposes upon further heating.
Decomposition leads to loss of hydrogen and boron-containing fragments.
It should be stored under inert gas and at low temperatures to prevent degradation.
Solubility and Handling Conditions
The dimer is soluble in tetrahydrofuran (THF), diethyl ether, and hydrocarbon solvents like toluene.
It must be handled under an inert atmosphere (argon or nitrogen) and is typically used as a solution in THF.
Reactivity Profile of 9-BBN Dimer
The compound exhibits high selectivity in hydroboration reactions, reacting preferentially with terminal alkenes and alkynes.
Its bulkiness suppresses over-addition, making it ideal for mono-hydroboration.
It also undergoes transmetallation and cross-coupling under appropriate conditions.
Applications in Organic Synthesis
9-BBN dimer is extensively employed in:
Hydroboration of alkenes and alkynes
Conversion to alcohols via oxidation
Selective reduction of carbonyl compounds
Formation of C–C bonds in Suzuki-type couplings
Comparison with Monomeric 9-BBN
While both forms are reactive, the dimer is more stable and easier to handle.
The monomer is often generated in situ from the dimer for immediate reactions.
The dimer exhibits slightly lower reactivity due to steric hindrance but improved selectivity.
Use in Asymmetric Synthesis and Catalysis
Modified 9-BBN dimers with chiral auxiliaries have been developed for enantioselective hydroboration.
These chiral boranes have been used in the synthesis of optically active alcohols and amines.
Use in Pharmaceutical Intermediates
9-BBN dimer has found utility in the synthesis of intermediates for antiviral, anti-inflammatory, and anticancer drugs.
Its predictable hydroboration profile ensures minimal side reactions and high yields.
Use in Polymer Chemistry
It is used to functionalize unsaturated polymers, introducing boron functionalities that can be further transformed.
This enables the preparation of boron-containing specialty polymers and cross-linkable materials.
Role in Material Science and Functionalization
The boron units derived from 9-BBN dimer are used in modifying surfaces and creating boron-doped materials with unique electronic and optical properties.
Boron Chemistry Context
9-BBN dimer exemplifies the utility of electron-deficient boranes in organic and inorganic synthesis.
It is a cornerstone in the study of boron-carbon and boron-hydrogen chemistry.
Industrial Applications and Scale-Up
Used in manufacturing fine chemicals and pharmaceuticals.
Commercially available in large quantities, its production is amenable to batch and flow chemistry techniques.
Purification and Storage Requirements
Purification involves recrystallization under inert atmosphere.
It should be stored in tightly sealed containers under dry argon at 0–5 °C.
Compatibility with Solvents and Reagents
Compatible with non-protic solvents and non-oxidizing agents. Incompatible with water, alcohols, acids, and oxidizers.
Computational Studies and Theoretical Models
Quantum chemical studies confirm its dimeric stability and predict favorable interaction energies. DFT calculations are used to predict reactivity pathways.
Crystallographic Data and 3D Structure
Single-crystal X-ray diffraction reveals a symmetrical dimer with a B–B bond length of ~1.75 Å.
The bicyclo[3.3.1] framework is rigid and influences its stereoselectivity.
Safety, Handling, and Toxicology
Hazards: Flammable, corrosive to eyes and skin
Toxicity: Low acute toxicity; reacts violently with moisture
Handling: Use gloves, goggles, and inert gas techniques
Environmental Fate and Degradation
Degrades in the presence of water or air to boric acid and organic residues.
Not environmentally persistent but requires controlled disposal.
Waste Disposal Guidelines
Neutralize with alcohol under inert atmosphere before disposal.
Follow local hazardous waste regulations for boron-containing organics.
Regulatory Status and Guidelines
Listed under TSCA and REACH regulations.
No significant restrictions under current guidelines but must be handled as a reactive reagent.
Alternatives and Related Boron Hydrides
Catecholborane
Pinacolborane
Triethylborane
Compared to these, 9-BBN offers better selectivity and stability.
SAFETY INFORMATION ABOUT 9-BORABICYCLO(3,3,1)NONANE,DIMER
First aid measures:
Description of first aid measures:
General advice:
Consult a physician.
Show this safety data sheet to the doctor in attendance.
Move out of dangerous area:
If inhaled:
If breathed in, move person into fresh air.
If not breathing, give artificial respiration.
Consult a physician.
In case of skin contact:
Take off contaminated clothing and shoes immediately.
Wash off with soap and plenty of water.
Consult a physician.
In case of eye contact:
Rinse thoroughly with plenty of water for at least 15 minutes and consult a physician.
Continue rinsing eyes during transport to hospital.
If swallowed:
Do NOT induce vomiting.
Never give anything by mouth to an unconscious person.
Rinse mouth with water.
Consult a physician.
Firefighting measures:
Extinguishing media:
Suitable extinguishing media:
Use water spray, alcohol-resistant foam, dry chemical or carbon dioxide.
Special hazards arising from the substance or mixture
Carbon oxides, Nitrogen oxides (NOx), Hydrogen chloride gas
Advice for firefighters:
Wear self-contained breathing apparatus for firefighting if necessary.
Accidental release measures:
Personal precautions, protective equipment and emergency procedures
Use personal protective equipment.
Avoid breathing vapours, mist or gas.
Evacuate personnel to safe areas.
Environmental precautions:
Prevent further leakage or spillage if safe to do so.
Do not let product enter drains.
Discharge into the environment must be avoided.
Methods and materials for containment and cleaning up:
Soak up with inert absorbent material and dispose of as hazardous waste.
Keep in suitable, closed containers for disposal.
Handling and storage:
Precautions for safe handling:
Avoid inhalation of vapour or mist.
Conditions for safe storage, including any incompatibilities:
Keep container tightly closed in a dry and well-ventilated place.
Containers which are opened must be carefully resealed and kept upright to prevent leakage.
Storage class (TRGS 510): 8A: Combustible, corrosive hazardous materials
Exposure controls/personal protection:
Control parameters:
Components with workplace control parameters
Contains no substances with occupational exposure limit values.
Exposure controls:
Appropriate engineering controls:
Handle in accordance with good industrial hygiene and safety practice.
Wash hands before breaks and at the end of workday.
Personal protective equipment:
Eye/face protection:
Tightly fitting safety goggles.
Faceshield (8-inch minimum).
Use equipment for eye protection tested and approved under appropriate government standards such as NIOSH (US) or EN 166(EU).
Skin protection:
Handle with gloves.
Gloves must be inspected prior to use.
Use proper glove
removal technique (without touching glove's outer surface) to avoid skin contact with this product.
Dispose of contaminated gloves after use in accordance with applicable laws and good laboratory practices.
Wash and dry hands.
Full contact:
Material: Nitrile rubber
Minimum layer thickness: 0.11 mm
Break through time: 480 min
Material tested:Dermatril (KCL 740 / Aldrich Z677272, Size M)
Splash contact
Material: Nitrile rubber
Minimum layer thickness: 0.11 mm
Break through time: 480 min
Material tested:Dermatril (KCL 740 / Aldrich Z677272, Size M)
It should not be construed as offering an approval for any specific use scenario.
Body Protection:
Complete suit protecting against chemicals, The type of protective equipment must be selected according to the concentration and amount of the dangerous substance at the specific workplace.
Respiratory protection:
Where risk assessment shows air-purifying respirators are appropriate use a fullface respirator with multi-purpose combination (US) or type ABEK (EN 14387) respirator cartridges as a backup to engineering controls.
If the respirator is the sole means of protection, use a full-face supplied air respirator.
Use respirators and components tested and approved under appropriate government standards such as NIOSH (US) or CEN (EU).
Control of environmental exposure
Prevent further leakage or spillage if safe to do so.
Do not let product enter drains.
Discharge into the environment must be avoided.
Stability and reactivity:
Chemical stability:
Stable under recommended storage conditions.
Incompatible materials:
Strong oxidizing agents:
Hazardous decomposition products:
Hazardous decomposition products formed under fire conditions.
Carbon oxides, Nitrogen oxides (NOx), Hydrogen chloride gas.
Disposal considerations:
Waste treatment methods:
Product:
Offer surplus and non-recyclable solutions to a licensed disposal company.
Contact a licensed professional waste disposal service to dispose of this material.
Contaminated packaging:
Dispose of as unused product
