Phenylboronic acid or benzeneboronic acid, abbreviated as PhB(OH)₂ where Ph is the

phenyl group In organic chemistry, the phenyl group, or phenyl ring, is a cyclic group of atoms with the formula C6 H5, and is often represented by the symbol Ph. Phenyl group is closely related to benzene and can be viewed as a benzene ring, minus a hydrogen ...

C₆H₅-, is a boronic acid containing a phenyl

substituent A substituent is one or a group of atoms that replaces (one or more) atoms, thereby becoming a moiety in the resultant (new) molecule. (In organic chemistry and biochemistry, the terms ''substituent'' and ''functional group'', as well as ''side ...

and two hydroxyl groups attached to

boron Boron is a chemical element with the symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the ''boron group'' it has th ...

. Phenylboronic acid is a white powder and is commonly used in

organic synthesis Organic synthesis is a special branch of chemical synthesis and is concerned with the intentional construction of organic compounds. Organic molecules are often more complex than inorganic compounds, and their synthesis has developed into one o ...

. Boronic acids are mild

Lewis acid A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any sp ...

s which are generally stable and easy to handle, making them important to organic synthesis.

Properties

Phenylboronic acid is soluble in most polar organic solvents and is poorly soluble in hexanes and

carbon tetrachloride Carbon tetrachloride, also known by many other names (such as tetrachloromethane, also IUPAC nomenclature of inorganic chemistry, recognised by the IUPAC, carbon tet in the cleaning industry, Halon-104 in firefighting, and Refrigerant-10 in HVAC ...

. This planar compound has idealized C_2V

molecular symmetry Molecular symmetry in chemistry describes the symmetry present in molecules and the classification of these molecules according to their symmetry. Molecular symmetry is a fundamental concept in chemistry, as it can be used to predict or explain m ...

. The boron atom is sp²-hybridized and contains an empty p-orbital. The

orthorhombic In crystallography, the orthorhombic crystal system is one of the 7 crystal systems. Orthorhombic lattices result from stretching a cubic lattice along two of its orthogonal pairs by two different factors, resulting in a rectangular prism with a r ...

crystals use

hydrogen bond In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a ...

ing to form units made up of two molecules. These dimeric units are combined to give an extended hydrogen-bonded network. The molecule is planar with a minor bend around the C-B bond of 6.6° and 21.4° for the two PhB(OH)₂ molecules.Hall, D. G. ''Boronic Acids''; WILEY-VCH: Edmonton, Canada, 2005.

Synthesis

Numerous methods exist to synthesize phenylboronic acid. One of the most common synthesis uses

phenylmagnesium bromide Phenylmagnesium bromide, with the simplified formula , is a magnesium-containing organometallic compound. It is commercially available as a solution in diethyl ether or tetrahydrofuran (THF). Phenylmagnesium bromide is a Grignard reagent. It is o ...

and trimethyl borate to form the ester PhB(OMe)₂, which is then

hydrolyzed Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile. Biological hydrolysis ...

to the product. :PhMgBr + B(OMe)₃ → PhB(OMe)₂ + MeOMgBr :PhB(OMe)₂ + H₂O → PhB(OH)₂ + MeOH Other routes to phenylboronic acid involve

electrophilic In chemistry, an electrophile is a chemical species that forms bonds with nucleophiles by accepting an electron pair. Because electrophiles accept electrons, they are Lewis acids. Most electrophiles are positively charged, have an atom that carri ...

borates to trap phenylmetal intermediates from phenyl halides or from directed ortho- metalation. Phenylsilanes and phenylstannanes transmetalate with BBr₃, followed by hydrolysis form phenylboronic acid. Aryl halides or triflates can be coupled with diboronyl reagents using transition metal catalysts. Aromatic C-H functionalization can also be done using transition metal catalysts.

Reactions

The dehydration of boronic acids gives boroxines, the trimeric anhydrides of phenylboronic acid. The dehydration reaction is driven thermally, sometimes with a dehydration agent. Triphenyl boroxine synthesis01

Phenylboronic acid participates in numerous cross coupling reactions where it serves as a source of a phenyl group. One example is the Suzuki reaction where, in the presence of a Pd(0) catalyst and base, phenylboronic acid and vinyl halides are coupled to produce phenyl alkenes. This method was generalized to a route producing

biaryl Biphenyl (also known as diphenyl, phenylbenzene, 1,1′-biphenyl, lemonene or BP) is an organic compound that forms colorless crystals. Particularly in older literature, compounds containing the functional group consisting of biphenyl less one ...

s by coupling phenylboronic acid with aryl halides. C-C bond forming processes commonly use phenylboronic acid as a reagent.

Alpha-amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha ami ...

s can be generated using the uncatalyzed reaction between alpha-ketoacids, amines, and phenylboronic acid. Heck-type cross coupling of phenylboronic acid and alkenes and alkynes has been demonstrated. Aryl azides and nitroaromatics can also be generated from phenylboronic acid. Phenylboronic acid can also be regioselectively halodeboronated using aqueous bromine, chlorine, or

iodine Iodine is a chemical element with the symbol I and atomic number 53. The heaviest of the stable halogens, it exists as a semi-lustrous, non-metallic solid at standard conditions that melts to form a deep violet liquid at , and boils to a vi ...

: :PhB(OH)₂ + Br₂ + H₂O → PhBr + B(OH)₃ + HBr Boronic esters result from the

condensation Condensation is the change of the state of matter from the gas phase into the liquid phase, and is the reverse of vaporization. The word most often refers to the water cycle. It can also be defined as the change in the state of water vapor to ...

of boronic acids with

alcohol Alcohol most commonly refers to: * Alcohol (chemistry), an organic compound in which a hydroxyl group is bound to a carbon atom * Alcohol (drug), an intoxicant found in alcoholic drinks Alcohol may also refer to: Chemicals * Ethanol, one of sev ...

s. This transformation is simply the replacement of the hydroxyl group by alkoxy or aryloxy groups. This reversible reaction is commonly driven to product by the use of Dean-Stark apparatus or a dehydration agent to remove water. :PhB(OH)₂ + 2 ROH PhB(OR)₂ + 2 H₂O As an extension of this reactivity, PhB(OH)₂ can be used as a protecting group for

diol A diol is a chemical compound containing two hydroxyl groups ( groups). An aliphatic diol is also called a glycol. This pairing of functional groups is pervasive, and many subcategories have been identified. The most common industrial diol is e ...

s and diamines. This reactivity is the basis of the use of phenylboronic acid's use as a receptor and sensor for carbohydrates, antimicrobial agents, and enzyme inhibitors,

neutron capture therapy Neutron capture therapy (NCT) is a type of radiotherapy for treating locally invasive malignant tumors such as primary brain tumors, recurrent cancers of the head and neck region, and cutaneous and extracutaneous melanomas. It is a two-step pro ...

for cancer, transmembrane transport, and bioconjugation and labeling of proteins and cell surface.

Properties

Synthesis

Reactions

See also

References

Further reading