Selective molecular hosts for anions

The ideas fundamental to the design of cation com- plexing host molecules can also be applied in the construction of molecular hosts for anions. Following the basic concept the macrotricyclic ammonium hosts la and 1b were prepared. They were shown to bind anions by encapsulation into their molecular cavities in aqueous solution. This inclusion process causes guest discrimination according to integral properties like charge, hydrophobicity and size. Besides this equilibrium selectivity differential rate effects (kinetic selectivity) are observed favouring reactions, which run through delocalized anionic transition states. In order to improve the selectivity, accessability and applicability the concept of linearly connected locular molecular hosts was developed. A selectivity advantage exclusively attributable to this design can be demonstrated even with the simplest member of the series, the ditopic host 2. A better definition of orientational and distance relations than attainable in the anion inclusion hosts can be expected from bicyclic guanidinium host structures (Fig. 3). The binding mode of oxoanionic guests to symmetrical molecular hosts of this type resembles the interaction pattern found in natural receptors. The synthesis of chiral guanidines via three different approaches opens the way to predetermined enantioselective anion recognition. An initial step in this direction is achieved by linear combination of two guanidinium units to produce the ditopic host 29.