Other Host Molecules - Building Three-Dimensional Cavities

Other Host Molecules - Building Three-Dimensional Cavities 

As seen in the examples shown above, the attachment of an additional part to a two-dimensional cychc host can be an effective way to improve the recognition abihty of the host. Therefore, well-designed three-dimensional hosts show superior molecular recognition properties, hi this section we introduce some three-dimensionally designed hosts. 

Cyclophanes are cyclic hosts made by Unking aromatic rings. Several examples of cyclophanes are depicted in Fig. 2.21. While the cyclophane in (a) is a simple two-dimensional cyclophane, the cyclophane in (b) has four alkyl chains attached. Using a similar molecular design process, a cyclophane with eight alkyl chains can be synthesized and is called an octopus-type cyclophane. The alkyl chains self-assemble in an aqueous phase and form a three-dimensional cavity. Cyclophanes with rigid steroidal walls are called steroid 

Adding legs or walls to the two-dimensional cyclic cavity leads to the formation of three-dimensional cavities. Further addition of a cap to the cavity creates an enclosed cavity space. Such spaces are called molecular capsules, and the trapped guest is shielded from the outer environment. If unstable species are trapped in the molecular capsule, their lifetimes can be extended and their properties are easily measured. An example of an unstable species that is stabilized inside a molecular capsule is shown in Fig. 2.22. Photoirradiation of the benzocyclobutendiol in the molecular capsule at -196 °C converts it to benzyne via benzocyclopropenone. Although benzyne is usually quite unstable, benzyne trapped in the molecular capsule can be characterized with H-NMR and i C-NMR at -75°C. 

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Other Host Molecules - Building Three-Dimensional Cavities Cyclophanes are cyclic hosts made from aromatic rings that mainly recognize hydrophobic guest molecules. Three-dimensional cavities can be constructed by attaching tails, walls and caps to the cyclic hosts. 

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