Carcerands and Hemicarcerands

Carceplexes and hemicarceplexes, host-guest complexes in which carcerands and hemicarcerands, in particular those including heterocyclic fragments, are hosts 99CRV931. 

It should be stressed that calixarenes, hemispherands and spherands, carcerands and hemicarcerands, and some other molecules discussed elsewhere in this book, belong to the cyclophane group of compounds. 

Scheme 6.19 Preparation of (CH2)n bridged carcerands and hemicarcerands from tetrol 6.99.

Since the preparation of 6.97, an enormous range of carcerand and hemicarcerand species have been prepared based upon both [n]resorcarenes and calix[n]arenes (n = 4,5), all of which exhibit interesting binding behaviour and intra-cavity reactivity, as we will see in the next sections. 

The robust, well-shielded cavity found in hemicarcerands offers tremendous scope for the use of these hosts as micro-reaction vessels in order to protect reactive species from bimolecular decomposition by isolating them from the outside medium. Furthermore, the unique intracavity environment with its fluid-like properties in which guest species are, formally, in a very condensed state at very high pressures, may well result in unique inclusion reactivity. Indeed, the inner volume of carcerands and hemicarcerands has been described as a new phase of matter distinct from solid, liquid and gas. A number of elegant demonstrations have been made of the potential of inclusion reactions, and there is clearly a great deal of scope for their use as molecular reaction vessels. 

Cram [49a] elaborated further on this concept by enclosing space in his carcerands and hemicarcerands (See Scheme 1) to form a new inner phase which he has referred to as a new phase of matter . In contrast to the hollow space found inside clathrates and zeolites for instance, the cages of these molecules are independent of the form and physical state. For example, hemicarcerands and related supramolecular systems (i.e. hemicarceplexes) prevail in the solid, liquid, or gas phase. This characteristic-hollow space, the inside surface — is maintained across all phase transitions. The inner surfaces and spaces of these systems are not manifested as bulk properties. An extensive review on the synthesis of these materials has been published recently [205]. 

Carceroisomerism has also been observed in hemicarceplexes. Paek and coworkers have measured isomerisation energy barriers of carceroisomers in non-centrosymmetric C4v hemicarceplexes, the largest of which was found to be 15.4 Kcal mol-1 for the rotation of NMP inside the cavity [43]. It has also been claimed that the inside of carcerands and hemicarcerands can be considered as a new phase of matter. This suggestion implies effects beyond mere spatial confinement and chemical isolation, for example, a marked change in the physical bulk properties, such as the polarity or polarisability of the host cavity. Nau has obtained evidence that biacetyl included within the cavity of a hemicarcerand may experience an unusual polarisability even higher than that of di-iodomethane by using biacetyl as a solvatochromatic probe for the polarisability of the environment [44]. 

In a range of studies on carcerands and hemicarcerands involving resorcinol (and pyrogallol) structures some remarkable reactions have been effected on trapped molecules (ref. 129). 

Carcerands and Hemicarcerands, p. 189 Concave Reagents, p. i /1 Crown Ethers, p. 326 Cryptophanes, p. 340 Cyclodextrins, p. 398 Cyclodextrins, Applications, p. 405 Cyclophanes Definition and Scope, p. 414 Enzyme Mimics, p. 546 Macrocycle Synthesis, p. 830 Organometallic Anion Receptors, p. 1006 Soft and Smart Materials, p. 1302 X-Ray Crystallography, p. 1586... 

Since their conception. carcerands and hemicarcerands have been used for a number of applications including trapping reactive intermediates and studying templation processes. Examples of these applications. 

Carcerands and Hemicarcerands, p. 189 Catenanes and Other Interlocked Molecules, p. 206 Cavitands, p. 219... 

Carcerands and Hemicarcerands, p. 189 Cryptophanes, p. 340 Glycoluril-Based Hosts, p. 597 Hydrogen Bonding, p. 658 Micelles and Vesicles, p. 861 Molecular Squares, Boxes, and Cubes, p. 909 Platonic and Archimedean Solids, p. 1100 Self-Assembly Definition and Kinetic and Thermodynamic Considerations, p. 1248 Self-Assembly Terminology, p. 1263 Soft and Smart Materials, p. 1302... 

Carcerands and Hemicarcerands, p. 189 Electrochemical Sensors, p. 505 Ion-Selective Electrodes, p. 747... 

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