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Crystalline Molecular Complexes and CompoundsStructures and Principles$
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Frank H. Herbstein

Print publication date: 2005

Print ISBN-13: 9780198526605

Published to Oxford Scholarship Online: September 2007

DOI: 10.1093/acprof:oso/9780198526605.001.0001

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The enclosure species – crown ethers, cryptands and related molecules – as hosts

The enclosure species – crown ethers, cryptands and related molecules – as hosts

(p.27) Chapter 3 The enclosure species – crown ethers, cryptands and related molecules – as hosts
Crystalline Molecular Complexes and Compounds

Frank H. Herbstein

Oxford University Press

Crown ethers are prototype examples of host molecules of the ring type, where small ring sizes interact with metallic cations to form widely-studied ion-molecule complexes, while the larger ring sizes interact with neutral molecules and organic cations of various types to form intramolecular inclusion complexes, rotaxanes, and even catenanes. Although formally two-dimensional rings, the actual conformations taken up by the larger crown ethers are more complicated and include the formation of molecular clefts. The overall shapes of three-dimensional cage molecules are approximately ellipsoidal. The cages are closed to lesser or greater extents, thus permitting ingress and egress of guest molecules tailored in size and shape to match the available portals and cavities. Thermodynamic measurements in solution, principally using NMR methods, provide insight into the energetics of these processes. Although considerable ingenuity has been expended in the synthesis of the hosts, these are more easily obtainable than might be imagined at first thought, and this holds great promise for their widespread use in the future. A crowning achievement of this type of host-guest chemistry is the synthesis of stable but reactive cyclobutadiene incarcerated within the cavity of a hemicarcerand, followed by the analogous preparation of o-benzyne.

Keywords:   crown ethers, intramolecular inclusion complexes, rotaxanes, catenanes, cyclobutadiene, hemicarcerand

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