Boron (B) is a group 13 (IIIA) metalloid element. Its chemical properties are more similar to carbon and silicon than the other group 13 elements, although boron is more electron deficient than carbon and silicon. Many boron compounds are found in nature, mainly in vegetables, fruits, and nuts. Although there are many boron compounds, boronic acids are the most frequently studied in organic chemistry. Boronic acids are used as building blocks and intermediates in synthetic chemistry due to their versatile reactivity, stability, and low toxicity. Boronic acids can be used in metal catalyzed reactions such as the Suzuki–Miyaura cross-coupling reaction, acid catalysis, asymmetric synthesis of amino acids, and hydroboration.
Boronic acids are considered Lewis acids with a pKa value of 4-10. The pKa values can vary depending on the substituents on the boronic acids and derivatives. In general, aryl boronic acids are more acidic than alkyl boronic acids. Most boronic acids are stable to air and moderately stable to most reaction conditions. Aliphatic boronic acids are more easily oxidized than aryl- and alkenyl- boronic acids. These compounds are also prone to deboronation with bases, acids, nucleophiles, and oxidants due to the presence of empty p-orbital on boron. Boronic esters are usually preferred because boronic acids and boronic esters can be routinely converted to each other and boronic esters are easier to purify and characterize. Acyclic boronic esters are susceptible to hydrolysis to produce boronic acids, whereas bulky cyclic boronic esters are more robust.
One of the most noticeable applications of organoboron compounds is the metal-catalyzed Suzuki-Miyaura cross-coupling reaction. It is a complex reaction for the formation of new carbon-carbon bonds, involving steps such as oxidative addition, ligand substitution, transmetalation, and reductive elimination. Boronic esters containing a trivalent ligand such as N-methyliminodiacetic acid (MIDA) have been used to improve the stability towards air and other reaction conditions of the boron partner in the Suzuki–Miyaura reaction. Since MIDA boronic esters are easily cleaved under mildly basic conditions, they are used as boronic acid protecting groups and allow the development of iterative cross-coupling strategies. Boronic acids and esters have also been extensively developed and used in drug design and discovery as protecting and masking groups for drug reservoirs or as potential nanocarriers for drug transport across cell membranes. Moreover, peptide boronic acids become as prime candidates for the development as proteasome inhibitors due to the interaction of covalent boron with nucleophilic residues.
Amerigo Scientific offers boronic acids and their derivatives for use in coupling reactions to form carbon–carbon bonds, such as Suzuki–Miyaura cross-coupling reaction, Stille coupling, Sonogashira coupling, Chan–Lam coupling, and Liebeskind–Srogl coupling.
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