What Is Phenylboronic Acid?
Phenylboronic acid is an organoboron compound.
It has the molecular formula C6H7BO2 and consists of a boron atom bonded to two hydroxy groups and one phenyl group. The phenyl group (C6H5-) is sometimes abbreviated to Ph and written as PhB(OH)2. It is also known as phenylboric acid and benzeneboric acid, and its CAS number is 98-80-6.
It has a molecular weight of 121.93, a melting point of 216°C, and is a white or yellow, odorless crystal or powder at room temperature. It is characterized by its solubility in polar organic solvents. On the other hand, it is virtually insoluble in non-polar solvents such as hexane and carbon tetrachloride. Phenylboronic Acid is a weak Lewis acid with an acid dissociation constant pKa of 8.83.
Uses of Phenylboronic Acid
Phenylboronic acid is used in various organic syntheses, taking advantage of its properties and reactivity as a weak Lewis acid. Examples of typical reactions include the Suzuki-Miyaura coupling reaction for the synthesis of biaryl compounds and palladium-catalyzed direct arylation.
Other compounds are used for their reactivity to make sensors and receptors for hydrocarbons and N-type polymers for all-polymer solar cells. Medical applications include antibiotics, enzyme inhibitors, and neutron capture therapy.
In biochemistry and chemical biology, the substance can also be used in biological reactions such as membrane permeation transport, bio-conjugation reactions, and protein labeling.
Characteristics of Phenylboronic Acid
The boron atoms of phenylboronic acid have empty p orbitals due to sp2 hybridization, and the molecular structure is a planar molecule with C2v molecular symmetry. This planar molecule is hydrogen bonded in bimolecular units with a slight bend across the C-B bond, and the angles formed by the planes of the two PhB (OH) bimolecules are 6.6° and 21.4°, respectively.
These dimeric units form hydrogen bonds with each other to form a rectangular molecular crystal system.
Types of Phenylboronic Acid
Phenylboronic acid is sold primarily as a reagent for research and development. The types of volumes include 1 g, 10 g, 25 g, 50 g, 100 g, and 500 g. It is a reagent product that can be stored at room temperature.
Phenylboronic acid anhydride may be included as an impurity.
Other Information on Phenylboronic Acid
1. Method of Synthesis of Phenylboronic Acid
There are many methods for synthesizing phenylboronic acid. The following is a list of some of the most representative ones.
- Reaction of phenylmagnesium bromide (Grignard reagent) with trimethyl borate to form the ester (PhB(OMe)2) and hydrolysis
- Trapping of electrophilic borates onto phenyl halides or phenyl-metal intermediates synthesized by orthometallation
- Transmetallation of phenylsilanes or phenylstannanes with BBr3 and hydrolysis of the products to obtain phenylborates
- Transition metal-catalyzed bonding of aryl halides and trifluoromethanesulfonates with diboronyl reagents
- Method of synthesis by transition metal-catalyzed C-H activation of aromatics
2. Chemical Reaction of Phenylboronic Acid
Phenylboronic acid is dehydrated to form the trimeric anhydride, boroxyn. This dehydration reaction proceeds thermally, and in some cases, a drying agent is added.
The compound is also useful as a reactant in cross-coupling reactions. A typical example is the Suzuki-Miyaura coupling, in which aryl halides react with phenylboronic acid in the presence of a palladium catalyst and base to produce biaryls.
α-Amino acids can be synthesized without the use of a catalyst by reacting α-keto acids, amines, and phenylboronic acid. There are also reports of Heck’s reactions using phenylboronic acid and alkenes or alkynes.