Boronic Acids and Esters

Introduction

Boronic Acids (substance 1 in Fig 1) contain a sp2-hybridized boron atom connected with two hydroxyl groups, and the structure of boronic acid crystals is planar triangle. Boronic Acids are weak acid, but the acid property does not from the proton itself, because boron is an electron-deficient atom, which can react with hydroxyl ions of water molecules to release protons and produce substance 2 in Fig 1. In the light of this electron-deficient property, they can react with electron donating moieties like diols, amines to form boronate esters (substance 3 in Fig 1). Besides, the obtained boronate esters can be hydrolyzed to boronic acids in acidic water and to borate in alkaline water.

Boronic Acids and Esters Fig.1. Boronic acid equilibrium in the presence of a generic 1,2- diol (Polym. Chem. 2016, 7, 5484-5495.)

Almost all the boronic acids and esters are soluble in water, thus its storage should be avoided in humid environments. The common industrial production of boronic acids is the reaction between borax and sulfuric acid (Na2B4O7+H2SO4+5H2O→4H3BO3+Na2SO4). Boronic acids and esters have a very wide range of applications including glassmaking, medical engineering, adhesion agent, chemical industry and so on.

Applications

Glassmaking: Boronic acids play a dual role of flux and network forming in glass and glass fiber fabrication. Generally speaking, boronic acids can reduce the melting temperature to facilitate wire drawing. Besides, they can also reduce viscosity, control thermal expansion, prevent impermeability, improve chemical stability, and improve resistance to mechanical and thermal shocks.

Biological recognition: There are many receptors in living organisms comprising catechol, saccharide, pyridyl and anion which can play a role as biological receptor to react with boronic acids donators (Fig.2). Therefore boronic acids derivatives can be used as sensors to detect above related substances.

Boronic Acids and Esters Fig.2. Reactivity of boronic acid towards various analytes (J. Chem. Biol. 2013, 6, 161-174.)

Catalyst: For example, boronic acids can also take part in catalyst fabrication, which promote the establish of boronate enolate in aqueous environment, finally advance the reaction of asymmetric aldol (Fig.3).

Boronic Acids and Esters Fig.3. Catalysts of boronic acids and esters (Acc. Chem. Res. 2009, 42, 756-768.)

Advanced materials: Boronic ester can be used in the fabrication of Vitrimer, a kind of high-performance material with reversible exchengeable chemical bonds (Fig.4). The metathesis of boronic esters can be acted as the reversible chemical bonds to endow the permanent crosslinking and machinability, as well as excellent chemical resistance and dimensional stability.

Boronic Acids and Esters Fig.4. Fabrication of boronic ester Vitrimer (Science, 2017, 356, 62-65.)

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