Concave hosts

Fig. 1. The principle of formation of an inclusion compound, (a) concave host (b) convex guest component (c) host—guest compound.

These examples of template effects illustrate the close relationship between host-guest and rotaxane chemistry. Appropriate concave host molecules bind convex guests in such a way that they can be equipped with two stoppers that mechanically prevent dissociation of the host-guest complex. Any macrocyclic receptor may thus be suitable for the formation of rotaxanes. 

The (V-alkyl ammonium resorcinarene salts (NARSs) are easily prepared from the corresponding resorcinarene tetrahenzoxazines under refluxing conditions in the presence of mineral acids [36]. The hydrogen bonds between the ammonium moieties and halides form a strong circular hydrogen bond seam ( H(R )N (R") H H(R )N (R")H )2 leading to concave host molecules that can... 

Abstract Concave molecules with a periphery of complementary hydrogen-bond donors and acceptors dimerize to form non-covalently bound capsules. These host structures feature the ability to bind appropriate guest molecules... 

For clarity and because of space limitations, we will consider only examples of complexes in which the guest interacts with the host from above (or/and below) the cavity, and the structure has been confirmed by X-ray analysis. These complexes serve as models for the interaction of neutral molecules which are nearly concave/ convex complementary in their. spatial interaction. Although complementary in shape, some guest molecules simply fill spatial voids in the host lattice. These clathrate-like structures will not be considered. 

Glycoluril 1 is a rigid, concave molecule, which can be easily functionalized via its four ureido nitrogen atoms. The two carbonyl groups in 1 are potential hydrogen bond acceptor sites. These features make 1 an excellent building block for synthetic host molecules. Glycoluril is the monomeric unit of cucurbituril,... 

When the concave reagents are compared to other reactions in Supramole-cular Chemistry a distinct difference must be noted Most other approaches try to bind the substrate in a host first. Then this complex reacts with a reagent which either is present in solution or attached to the host. For concave reagents and concave catalysts, however, there is no need for binding of the educt. In contrast, the protonation reactions can be interpreted as a reagent (H ) host complex. 

Fig. 1 Corannulene13 17 (top) is a concave aromatic compound whose preparation was reported in 1966.18 Solid state structures of selected hosts,19-21 cavitands,22 and molecular capsules23,24 were reported in the last four decades. (For color version of this figure, the reader is referred to the web version of this book.)...

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