Molecular cavity inclusion

These ligands form extremely stable cation inclusion complexes, called cryptates, In which the cation Is completely surrounded by the ligand and hidden Inside the molecular cavity, and this leads to a considerable Increase of the interionic distance In the ion pairs. It has been shown that such ligands have a marked activating effect on anionic polymerizations (4,5,6). Moreover, the aggregates are destroyed and simple kinetic results have been obtained In the case of propylene sulfide (7,8,9). ethylene oxide (9,10,11) and cycloelloxanes (12) polymerizations. Though the... 

The majority of food flavors are known to be volatile even at ambient temperatures,The molecular entrapment of flavors reduces their equilibrium vapor pressure via fixation in a molecular cavity. The apolar-apolar interaction between the internal wall of the cyclodextrin and the guest molecule results in remarkable heat resistence of volatiles,Signs of this inclusion phenomenon can be seen in heat stability tests. 

Figure 3 Schematic representation of the formation of inclusion complexes based on the recognition of a guest molecule by a host receptor (a), of a clathrate based on the inclusion of guest molecules within cavities generated upon packing of clathrands in the solid state (b), and of a 1-D inclusion molecular network named koilate formed through interconnection of hollow tectons (koilands) by connector molecules (c).

Enantiomeric resolution of solutes that fit within the molecular cavity, which is chiral, results in the formation of an inclusion complex (Ref. 169 and Fig. 4). In general, the enantiomers are separated on the basis of formation constants of the host-guest complexes. The enantiomer that forms the more stable complex has a greater migration time because of this effect. The chiral recognition mechanism for cyclodextrin enantioseparation has been discussed in several works (163-168). 

Cyclodextrins (CDs) are commonly used to improve the stability of flavor via the encapsulation of certain specific ingredients that naturally exist in food materials. The method is often called molecular encapsulation because the flavor ingredients are encapsulated in the molecular cavity of CDs. CDs form inclusion complexes with a variety of molecules including flavors, fats, and colors. Most natnral and artificial flavors are volatile oils or liquids, and the complexation with CDs provides a promising alternative to the conventional encapsulation technologies for flavor protection. 

Another attempt to use the host-guest complexation of simple cyclophanes has been reported by Schneider They take the easily accessible host 7, an analogue of which had been demonstrated by Koga to bind aromatic guest molecules by inclusion into its molecular cavity, and study its rate effects on nucleophilic aliphatic substitutions of ambident anions (NOf, CN, SCN ) on 2-bromomethylnaphthalene 8 and benzylbromide. Similar bimolecular reactions are well known in cyclodextrin chemistry and other artificial host systems . In addition to the rather poor accelerations observed (see Table 3) the product ratio is changed in the case of nitrite favouring attack of the ambident nucleophile via its nitrogen atom. 

An interesting example of the interplay between solid-state clathrands and solution-phase cavitands is provided by cyclotriveratrylene (CTV, 8). In the solid state, the saucer-shaped CTV molecules stack one on top of another in the two most common phases (x and and hence, while the molecules possess shallow molecular cavities, they do not include guests such as solvent molecules, which instead are located in voids between host stacks. However, larger guests such as buckminsterfuller-ene C6o, organometallic sandwich compounds,or carboranes form intracavity inclusion compounds, and the association persists in the solid state, with potential applications, for example, in the selective purification of fullerenes. Thus, CTV is both a cavitand and a clathrand. The cavitand behavior of CTV is highlighted by the chemistry of the double-CTV cryptophanes that form very stable solution complexes with a variety of halocarbon guests. 

It has been successfully demonstrated by the authors that macro cyclic heterocyclophanes 1-3 exhibit enzyme-like or rdceptor-like functions with remarkable substrate specificity in the solution phase Heterocyclophane 1 is of considerable interest, since 1 forms a molecular cavity inclusion crystal which is to be differentiated from the lattice (cavity) inclusion and an interesting guest-dependent polymorphism and plane chirality in the crystalline phase have been observed K... 

Therefore, it is most appropriate to classify 1-CHCl, as a molecular (cavity) inclusion crystal. 

A racemic inclusion crystal is also obtained when the guest molecule is other than chloroform. 1 forms a similar 1 1 molecular cavity inclusion complex with methylene chloride, but in a different space group C2/c (Table III). In marked contrast to the type I crystal, the heterocyclophane in the CH Clo complex takes both "R"- and "S"-confor-mations (Type II, racemic inclusion crystal). A schematic picture of the host-guest packing in the methylene chloride complex is shown in Fig 2. 

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