Special Topic Crown Ethers

We have mentioned more than once the remarkable solvating powers of ethers. Ethers are Lewis bases and can donate electrons to Lewis acids, thus stabihzing them. One example of this stabilization is the requirement for an ether, usually diethyl ether or THF, in the formation of a Grignard reagent. Ethers are also polar compounds and therefore are able to stabilize other polar molecules through non-covalent dipole-dipole interactions. 

This stabilization was carried to extremes in work that ultimately led to the 1987 Nobel Prize in Chemistry for Charles J. Pedersen (1904—1989) of du Pont, Donald J. Cram (1919-2001) of UCLA, and Jean-Marie Lehn (b. 1939) of Universite Louis Pasteur in Strasbourg for opening the field of host-guest chemistry. Pedersen discovered that certain cyclic polyethers (the hosts) had a remarkable affinity for metal cations (the guests). Molecules were constructed whose molecular shapes created different-sized cavities into which different metal ions fit well. Because of their vaguely crown-shaped structures, these molecules came to be called crown ethers. Rgure 6.60 shows two of them. 

FIGURE 6.60 Two crown ethers. The first number in the name shows the number of atoms in the ring, and the final number shows the number of heteroatoms in the ring. In these molecules the heteroatoms are all oxygen, but others are possible. 

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All chiral crown ethers incorporating one carbohydrate subunit possess two diastereo-topic faces of the macrocyclic ring. They are able to form diastereoisomeric complexes with primary alkylammonium cations. Since nonbonding interactions are responsible of the chiral recognition of optically active species, it would be desirable to form monofacial ligands in which the inclusion of a chiral molecule (or chiral ion) could proceed from the sterically hindered side only. This special molecular architecture may be followed by the fusion of the cryptand framework and the chiral unit. 

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