Supramolecular Cages for the Recognition and Transmission of Chirality

Abstract

When a molecule cannot be superimposed with its mirror image, it is said to be chiral. This property is key in nature, as it is intrinsically present: for example, in nature there are D-sugars and, alternatively, L-amino acids. Chirality therefore plays a very important role in chemistry, as the properties of a compound can vary depending on its form. In the context of a molecule, chirality is most commonly defined by the presence of an asymmetric carbon, i.e. a carbon that has four different substituents. In contrast, when talking about supramolecular chemistry, chirality refers to the distribution of the overall structure in an asymmetric manner, for example, like the secondary or tertiary structure of proteins. In this work, using dynamic covalent chemistry (DCC), a chiral supramolecular cage (due to the presence of an asymmetric carbon) is synthesised. The aim is that a metal coordinates inside it and can recognise the chiral environment and coordinate in a specific way (Λ or Δ) to result in a supramolecular structure with a defined chirality. As a result, chirality will be conveyed from asymmetric carbon to supramolecular chirality. As ligands for the cage synthesis a tripodal tris(ethylamino)amine (Tren) coupled to L-Phenylalanine (Tren-L-Phe) has been used. Tren-L-Phe, which provides the chirality of the asymmetric carbon and acts as an encapsulant. As a second ligand, a bypiridine derivative with carbaldehyde groups as substituents has been used, which allow coupling with the amines of the Tren-L-Phe via an imine bond. Moreover, the bipyridine can coordinate to a metal using the Ione electron pairs of the nitrogen atoms. To determine if the supramolecular cage has formed, whether with a coordinated metal inside or not, mainly 1H NMR and circular dichroism measurements have been used. Using these techniques, it has been observed that by controlling the chirality of the asymmetric carbon, it is possible to control the way in which a metal coordinates inside the cage, confirming that there is chirality induction.