Boronic acid | EPFL Graph Search

A boronic acid is an organic compound related to boric acid () in which one of the three hydroxyl groups () is replaced by an alkyl or aryl group (represented by R in the general formula ). As a compound containing a carbon–boron bond, members of this class thus belong to the larger class of organoboranes. Boronic acids act as Lewis acids. Their unique feature is 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 they can 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.

Multicomponent Assembly of Boronic Acid-Based Macrocycles, Cages, and Polymers

Burçak Içli

This work describes the synthesis and characterization of boronic acid-based supramolecular structures. Different reversible interactions such as boronate ester formation, imine condensation and dative B–N bond formation are used to assemble macrocycles, cages and polymers. Multicomponent condensation reactions of different diamines with tetraols and formylphenylboronic acids give macrocycles via formation of imine bonds and boronate esters. The [4+2+2] condensation products are generally preferred but larger macrocycles are observed for some combinations of staring materials. The larger macrocycles are in dynamic equilibrium with the smaller [4+2+2] products. Utilization of a triamine instead of the diamine results in the formation of molecular cages. The syntheses of these cages require the formation of 18 covalent bonds between 11 building blocks, and, interestingly, can be performed in a ball mill. Multicomponent reactions of diboronic acids, catechol derivatives, and different pyridyl ligands are described. The condensation of diboronic acids with catechols gives dioxaboroles. Upon crystallization, the ester aggregates with the N-donor ligands via dative B−N bonds. Depending on the nature of the pyridyl ligand, molecularly defined macrocycles, cages or polymeric structures are obtained. One-dimensional polymers are formed with 4,4-bipyridine and 1,2-di(4-pyridyl)ethylene, whereas a two-dimensional network is obtained with the tetradentate ligand tetra(4-pyridylphenyl)ethylene. A tripyridyl linker results in the formation trigonal prismatic cages. The size of the cages can be varied by changing the diboronic acid building block. The cages are able to encapsulate polyaromatic molecules such as triphenylene or coronene. The results highlight the potential of dative B−N bonds in structural supramolecular chemistry and crystal engineering.

EPFL2012

Metal-based Chemosensors for Carbohydrates and Biologically Important Ligands

Vera Janowski

Chemosensors based on indicator displacement were developed for the optical detection of different analytes in aqueous solution. The sensing of carbohydrates, including glucose, in aqueous solution at pH 7.4 was achieved by a fluorescent indicator displacement assay (IDA) based on a palladium complex. The assay is easy to perform as it just requires mixing of a commercially available dye and a simple Pd complex. The sensitivity of the assay is similar to what is typically found for boronic acid-based systems, but its selectivity profile towards different carbohydrates is quite distinct. Furthermore the same complex was employed for the successful differentiation of various diols. A palladium-based sensor is a novelty as almost all carbohydrate sensors exclusively utilize boronic acids as binding site. Various palladium complexes were evaluated and the usability for carbohydrate sensing was found to be strongly dependent on the ligands. Ruthenium-arene complexes are known to have potent anticancer properties. There is an interest in a quick and simple method to give information about the stability towards ligand exchange of these complexes in aqueous solutions. Indicator displacement assays, which can give information about the binding strength of different ligands to a ruthenium-arene complex, were investigated in water at pH 7.4. Various biomolecules (different amino acids, maltol etc.) could be differentiated by a colorimetric (naked eye) IDA. Additionally, a correlation between the binding strength of the bidentate ligand and the amount of indicator displaced was found. Matrix metalloproteinase (MMPs) are endoproteinases and their dysfunction can result in various health problems (e.g. arthritis). In order to provide a quick and simple tool for the evaluation of new MMP inhibitors, colorimetric IDAs were developed. For calibration of the system, MMP inhibitors were synthesized and their IC50 values for different MMP isoforms determined. Although the new MMP inhibitors have very similar structures a differentiation by an indicator displacement array was achieved. The IC50 values were, with a couple exceptions, rather high and similar. None of the new MMP inhibitors exhibited specific inhibition of one of the studied MMP isoforms.

EPFL2012

Half-clathrochelate Complexes as Versatile Links in Supramolecular Chemistry

Erica Giraldi

Clathrochelate complexes have been widely investigated with different size and shape and used as building blocks for the obtainment of discrete supramolecular architectures. These clathrochelate complexes are very stable and kinetically inert complexes. A new link, described as a 'half-clathrochelate complex' with one boronate ester-cap and one 'open' side can be used as novel structural motifs in supramolecular chemistry. Since the metal ion is not completely encapsulated, these complexes are more labile than standard clathrochelate complexes with two boronate ester caps. The half-clathrochelate complexes are obtained by metal-templated condensation reactions of 2-pyridylaldoximes with boronic acids. For applications in supramolecular chemistry, half-clathrochelate complexes are very intriguing compounds for several reasons: the central metal ion can be varied to obtain diamagnetic or paramagnetic complexes and substituents at the pyridyl group of the oxime ligand can be used to fuse half-clathrochelate complexes into defined supramolecular assemblies such as dinuclear triple-stranded helicates or tetrahedral cages.

EPFL2021

Metal-based Chemosensors for Carbohydrates and Biologically Important Ligands

Vera Janowski

EPFL2012