Structurally, boronic acid compound is a trivalent boron-containing organic compound with alkyl substituent and hydroxyl groups to fill the valence on the boron atom. Due to the electron deficiency, the sp2-hybridized boron atom possesses an empty p-orbital. This low-energy orbital is orthogonal to three substituents. Unlike carboxylic acids and their carbon analogues, boronic acids do not exist in nature.
Chemical use: Boronic acid compounds are synthetically derived from primary sources of boron, such as boric acid that is made by acidification of boronic acid with carbon dioxide. Their unique properties and reactivity as mild organic Lewis acids, coupled with their stability and ease of handling, make boronic acid compounds attractive synthetic intermediates. Moreover, boronic acid compounds can be considered as environmentally friendly compounds due to their low toxicity and their degradation into boric acid, so they can be used in a plenty of chemistry reactions, such as Suzuki coupling reaction, Chan–Lam coupling, Liebeskind–Srogl coupling, C–H coupling reactions, protonolysis and so on. For example, the borylation of arenes can lead to the formation of synthetically versatile products from unactivated arene reagents.
Figure 1. Iridium-catalyzed borylation of arenes
Biological use: Boronic compound is one of the most useful materials for biological applications because of its ready inter-convertility, Lewic acidity, and unique behavior upon neutron bombardment. Based on these properties, boronic compounds can be used as enzyme inhibitors, sensors, lectin mimics that can be termed as boronolectins, boron neutron capture therapy agents, transmembrane transporters, and they also can be used for bioconjugations and immobilization.
Figure 2. Boronic acid-based thrombin inhibitors
Medicinal use: The use of boronic compounds as enzyme inhibitors largely reflects the usefulness of boron. For example, the bortezomib, which is a boronic acid-based inhibitor, has been approved by FDA for treatment of multiple myeloma.
1. Ishiyama T, Takagi J, Ishida K, et al. Mild iridium-catalyzed borylation of arenes. High turnover numbers, room temperature reactions, and isolation of a potential intermediate[J]. Journal of the American Chemical Society, 2002, 124(3): 390-391.