Phenylboronic acid

Phenylboronic acid (PBA) [98-80-6] or benzeneboronic acid is an organoboronic acid acid substituted with phenyl group. It is a white powder with the melting point of 215-216 °C.^[1] Phenylboronic acid is soluble in most polar organic solvents. It is skin, eye and respiratory irritant which is harmful if swallowed (rat, oral, LD₅₀ = 740 mg/kg).^[2]

Preparation of Phenylboronic acid:

Benzeneboronic acid was first reported by Michaelis and Becker in 1880 who prepared benzeneboronyl dichloride by heating boron trichloride and diphenyl mercury at 180° to 200°C in a sealed tube. Benzeneboronyl dichloride was found to hydrolyze easily, giving benzeneboronic acid.^[3] One of the most common preparations is based on reaction of phenylmagnesium bromide with trimethyl borate to form the ester, which is then hydrolyzed to the product.^[4] Another synthetic approach, which gives lower yields, involves the reaction of phenyllithium with a borate ester.^[5]

Application:

Phenylboronic acid participates in numerous cross coupling reactions. One of most famous example is the Suzuki–Miyaura reaction between boronic acid and organohalide in the presence of Pd(0) catalyst and base.^[6] Heck-type cross coupling of phenylboronic acid and alkenes and alkynes has been demonstrated.^[7] The thermal dehydration of boronic acids gives boroxines, the trimeric anhydrides of phenylboronic acid.³ The boronic acid-diol interaction has found many applications. Five and six membered cyclic arylboronate esters are formed upon binding between arylboronic acids and cis-1,2-diols or 1,3-diols respectively. Boronic acid-based sensors for sugars have been developed, particulary for the monitoring of glucose levels in blood of diabetic patients, and the development of glucose-responsive materials that automatically release insulin when glucose levels are too low.^[8]

Product categorization (Chemical groups):

Main category:

• Boronic acids derivatives

Second level:

• Aromatic compounds

Third level:

• Benzene derivatives

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