Supramolecular chirality of hydrogen-bonded rosette assemblies

The control of chirality in synthetic self-assembled systems remains challenging because of their lower stability and their higher susceptibility to racemization when compared to covalent systems. In this review the supramolecular chirality of noncovalent hydrogen-bonded assemblies formed by multiple cooperative hydrogenbonds between calix[4]arene dimelamines and cyanurates or barbiturates derivatives (rosette assemblies) are described. It is shown that the amplification of chirality (a high enantiomeric or diastereomeric excess induced by a small initial amount of chiral bias) of double and tetrarosette assemblies is influenced by bulky substitution on their components and electronic properties of the substituents as well as their proximity to the rosette core. In the absence of chiral centers in their components, the assemblies form as a racemic mixture of both enantiomers (P and M). The synthesis of enantiomerically pure rosette assemblies is conducted via induction of chirality using chiral barbiturates, followed by substitution of the chiral components for achiral cyanurates (chiral memory concept). The addition of an external auxiliary to a racemic mixture of P and M assemblies leading to the formation of one of the two possible diastereomeric assemblies is also described. Moreover, chiral resolution of self-assembled nanostructures on highly oriented pyrolytic graphite (HOPG) surfaces is also discussed