Boron and Boronate Compounds

Jul 1, 2021 | RESEARCH

Boronate Acid Compounds

Boronates are esters and salts of boronic acid. Boronic acid is a compound related to boric acid in which one of the three hydroxyl groups has been replaced by either an alkyl or an aryl group. The boronic acids are derived from boron, an element that is found in the earth’s crust as boric oxide (B(OH)O). Boronic acids can be produced by reacting a borate salt with either acetic or propionic acid to form the corresponding borinic acid.

boron - boronate compound

BORONATE COMPOUNDS

Different compounds known as boronates are borinic esters and boric acid. The compounds can be obtained from borate esters[18] by condensation with alcohols and diols. Phenylboronic acid can be selfcondensed to the cyclic trimer called triphenyl anhydride or triphenylboroxin.

Boric acids are boronates that have a borate anion as their conjugate base. Boronic esters contain boric acid with the carboxylic acid replaced by another organic group, such as alkyl or phenyl groups. The boronic esters can be hydrolyzed to boronic acids.

Different boronates are used in the production of polymers, detergents, pesticides, and more. They also have many other uses as well such as use as a catalyst for organic reactions.

A borate salt can be hydrolyzed to form boric acid or acylated with an acid to form boronic acid.

Boric oxide is boronate salt that has a borate anion as its conjugate base. Boronic esters contain boric acid with the carboxylic acid replaced by another organic group, such as alkyl or phenyl groups. The boronic esters can be hydrolyzed to boronic acids.

The use of boronate compounds has grown over the period of time and advancement in boronic acid-related chemistry. In the global world today, boronates are used in a wide variety of products like toothpaste, cosmetics, detergents, plastics, and insecticides. 

USES IN CHEMISTRY

Boronate esters are used as catalysts for organic reactions, such as hydroboration-oxidation, borylation, and pinacol coupling reactions to name just a few examples. These are explained as:

The hydroboration-oxidation reaction is the reaction in which boron-containing substrates are brominated, then oxidized to borate esters.

The borylation reaction is the process of adding boron trihalides (for example BCl) or boric acid derivatives such as orthoboric acid and metaboric acid to alkenes, alkynes, or other unsaturated compounds. boron-containing substrates are brominated, then oxidized to borate esters

The pinacol coupling reaction is the process of adding a boronic acid or its related compound with pinacols and catalysts in sequence to form long chains.

Diethanolamine complexed heterocyclic boronic acids (DABO boronates) are air-stable reagents that can be used directly in Suzuki-Miyaura reactions, which are the reactions in which boronic acids are used to form boron-carbon bonds and it is an organic reaction, classified as a cross-coupling reaction, where the coupling partners are a boronic acid and an organohalide and the catalyst is a palladium(0) complex. In simple words, it is the reaction where boronic acid esters are in the presence of water or a protic co-solvent.

Boronates, which are also classified as organoborate compounds, have been used in chemical catalysis for many years because they can easily form electron deficiency with boron-oxygen bonds with an atom in a broad range of organic substrates. Boronic esters have proved to act as protective groups in carbohydrate chemistry: processes for acylation, silylation and alkylation of glycoside-derived boronates

Acylation is the process in which boronates are converted to a boronic acid ester with an acyl group and then the boronic acid is treated with the base.

Silylation is the process in which the  boronate ester reacts with a silyl halide, in which boronic acid is an electron-deficient boron atom.

Alkylation is the process of adding an alkyl group to boric acids (B(OH)) and borate salts are formed through the reaction of alcohols or amines.

PURIFICATION OF BORONATES

Recrystallization in benzene, dichloroethane, and EtOAc can be good to moderate yielding. Derivatization follows treating the impure product with the base, separating the resultant salt (obtained from base) by solvent extraction, and treating the salt with acid to afford the pure boronic acids.

Recrystallization in benzene is the process in which boronic acid is dissolved in benzene and the solution is heated to evaporate. The resultant crystals are filtered, washed with anhydrous ether, and dried under a vacuum.

Derivatization is defined as the process in which boronic acid is treated with a base to produce an alkali borate and the salt is extracted as such from the mixture of boronate esters. The resultant salts are subsequently hydrolyzed or acylated, yielding pure boronic acids.

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