Expanding cavitand chemistry

• Main Author: Naumann, Christoph
• Format: Others
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/13321

Supramolecular chemistry studies the interactions between hosts, molecules that possess a binding cavity (e. g. cavitands), and guests, any molecules that are attracted by different forces to the cavity of the host. The interaction between host and guest molecules leads to new structures (complexes) by means of self-assembly. The properties of both the host and guest can change once a complex is formed. The study of those (relatively) less complex systems enables us to learn about the interactions between molecules and subsequently helps contribute to the understanding of more complicated natural systems. Cavitands are organic host molecules that are shaped like bowls and manifest enforced cavities capable of binding to neutral guest. [4]Cavitands have been used extensively not only as receptors, but also as components for the preparation of more elaborate hosts-guest systems such as carceplexes, globe-shaped, closed-surface molecules that contain entrapped molecules. In recent years, efforts have been made to use [4] cavitands to create larger cavities. One approach is to link together several [4]cavitands. Another route is to extend the walls via the bridging unit Y. The nature of the interbowl linker Y defines the volume and character of the cavity, as well as the guest(s) that will be recognized. Most common is the methylene linkage, but ethylene, propylene, o-arenes, and benzal bridges have been used as well. This thesis includes the study of a new family of [4] cavitands that are bridged with four carbon, ortho-xylyl linkers and manifest novel conformational properties. Usually, the most popular hosts in supramolecular chemistry are available in an assortment of sizes. In sharp contrast to this, the chemistry of cavitands has only made use of [4]cavitands. This thesis discusses the synthesis and characterization of the first series of [n]cavitands, where n = 4, 5, 6, and 7. Whereas [4]- and [5]cavitands are rigid and planar molecules, the "crown" shaped [6]-, and [7]cavitands were found to be conformationally mobile in solution due to steric strain. Their dynamics was studied using ID NOESY experiments, a very recently introduced technique based on Gaussian shaped selective pulses. The usage, capabilities, and limitations of ID NOESY experiments were explored in detail. [6]Cavitand was derivatized both at the lower rim and at the upper rim of the molecule. The lower rim can be functionalized without attacking the methyl groups on the upper rim, and vice versa, by combining electronic and steric selective effects. [5]Cavitand was used to synthesize the first disulfide carceplex (containing two DMF or DMA molecules as guests), which brings supramolecular chemistry in close contact to naturally occurring effects in peptides. The dynamics of the entrapped DMF molecules in the disulfide-carceplex and other previously described DMF containing carceplexes were studied by ID NOESY experiments in order to elucidate the effect the host induces on the amide activation barrier of guest DMF. The versatility of [5] (and [6]) cavitands is illustrated by linking five (and six) peptide strands to the cavitands via disulfide bonds. === Science, Faculty of === Chemistry, Department of === Graduate