Boronic acid

  A boronic acid is an alkyl or aryl substituted boric acid containing a carbon to boron chemical bond belonging to the larger class of organoboranes. Boronic acids act as Lewis acids. Their unique feature are that they are capable of forming reversible covalent complexes with sugars, amino acids, hydroxamic acids, etc. (molecules with vicinal, (1,2) or occasionally (1,3) substituted Lewis base donors (alcohol, amine, carboxylate). The pKa of a boronic acid is ~9, but upon complexion in aqueous solutions, they form tetrahedral boronate complexes with pKa ~7. They are occasionally used in the area of molecular recognition to bind to saccharides for fluorescent detection or selective transport of saccharides across membranes.

Boronic acids are used extensively in organic chemistry as chemical building blocks and intermediates predominantly in the Suzuki coupling. A key concept in its chemistry is transmetallation of its organic residue to a transition metal.

The compound bortezomib with a boronic acid group is a drug used in Chemotherapy. The boron atom in this molecule is a key substructure because through it certain proteasomes are blocked that would otherwise degrade proteins

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Boronic acids

Many air-stable boronic acids are commercially available. They are characterised by high melting points.

Borinic acids and esters

Borinic acids and borinic esters have the general structure R₂BOR.

Boronic esters

When hydrogen is replaced by any organic residue the resulting compound is called a boronic ester or boronate ester. The compounds can be obtained from boric esters ^[2] by condensation with alcohols and diols. Phenylboronic acid can be selfcondensed to the cyclic trimer called triphenyl anhydride or triphenylboroxin ^[3]

Compounds with 6-membered cyclic structures containing the C-O-B-O-C linkage are called dioxaborolanes and those with 5-membered rings dioxaborinanes.

Boronate or borate salts

Boronate salts or borate salts (not encouraged) are ate complexes and have the general structure R₄B^-M⁺ for example potassium tetraphenylborate.

Boronic acids in organic chemistry

Suzuki coupling reaction

Boronic acids are used in organic chemistry in the Suzuki reaction. In this reaction the boron atom exchanges its aryl group with an alkoxy group from palladium.

Chan-Lam coupling

In the Chan-Lam coupling the alkyl, alkenyl or aryl boronic acid reacts with a N-H or O-H containing compound with Cu(II) such as copper(II) acetate and oxygen and a base such as pyridine ^{[5] [6]} forming a new carbon-nitrogen bond or carbon-oxygen bond for example in this reaction of 2-pyridone with trans-1-hexenylboronic acid:

The reaction mechanism sequence is deprotonation of the amine, coordination of the amine to the copper(II), transmetallation (transferring the alkyl boron group to copper and the copper acetate group to boron), oxidation of Cu(II) to Cu(III) by oxygen and finally reductive elimination of Cu(III) to Cu(I) with formation of the product. Direct reductive elimination of Cu(II) to Cu(0) also takes place but is very slow. In catalytic systems oxygen also regenerates the Cu(II) catalyst.

Conjugate addition

The boronic acid organic residue is a nucleophile in conjugate addition also in conjunction with a metal. In one study the pinacol ester of allylboronic acid is reacted with dibenzylidene acetone in a such a conjugate addition ^[7]:

The catalyst system in this reaction is tris(dibenzylideneacetone)dipalladium(0) / tricyclohexylphosphine.

Another conjugate addition is that of gramine with phenylboronic acid catalyzed by cyclooctadiene rhodium chloride dimer ^[8]:

Oxidation

Boronic esters are oxidized to the corresponding alcohols with base and hydrogen peroxide (for an example see: carbenoid)

Homologization

• In boronic ester homologization an alkyl group shifts from boron in a boronate to carbon ^[9]:

In this reaction dichloromethyllithium converts the boronic ester into a boronate. A lewis acid then induces a rearrangement of the alkyl group with displacement of the chlorine group. Finally an organometallic reagent such as a Grignard reagent displaces the second chlorine atom effectively leading to insertion of a RCH₂ group into the C-B bond. Another reaction featuring an boronate alkyl migration is the Petasis reaction.

Electrophilic allyl shifts

Allyl boronic esters engage in electrophilic allyl shifts very much like silicon pendant in the Sakurai reaction. In one study a diallylation reagent combines both ^[10][11]:

Hydrolysis

Hydrolysis of boronic esters back to the boronic acid and the alcohol can be accomplished in certain systems with thionyl chloride and pyridine ^[12].

See also

• Boron bonded to three oxygen atoms: boric acid and borates

References