Chiral spaces in supramolecular assemblies

The present review is aimed at illustrating to the reader the state of the art on the construction of artificial chiral supramolecular assemblies held together by means of weak intermolecular non-covalent forces in solution except metal-ligand coordination. Chiral supramolecules are self-assembled finite aggregates characterized by the presence of chiral residues or by asymmetric arrangement of components. These assemblies have proven to be powerful tools to explore chirality and its effects in host-guest interactions and represents important steps toward a superior control in asymmetric non-covalent synthesis, which is still a peculiar prerogative of natural systems. The introduction serves as a quick overview of weak (non-covalent) interactions refreshing concepts like cooperativity and multivalency that are crucial to molecular assembly, as well as providing selected examples of supramolecular chiral systems found in nature. Four main strategies have been so far employed for the preparation of chiral supramolecular assemblies: these are discussed on the bases of the nature of the monomeric components employed. Intrinsically chiral scaffolds have been used both in the enantiopure or racemic mixture form to assemble into chiral aggregates. A second approach instead makes use of achiral building blocks to give racemic chiral supramolecules, and a step further consists of the enrichment of these species in one enantiomeric form exploiting the chiral memory effect. The last approach is based on encapsulation of chiral guests, which allows reciprocal sensing of chiral species confined in close proximity within an achiral host.