Calix blocking group

Figure 33. The steric demand of calix[4]arene blocking groups can be varied by attachment of different substituents, furthermore their complexation ability introduces interesting properties to rotaxanes.

The principal idea of this present essay was to show how the unique preorganization of functional groups in self-assembled dimers of tetra-urea calix[4]arenes can be used to prepare novel multi-rotaxanes and -catenanes or topologically even more complex molecules and supramolecular structures. We will conclude by summarizing some related studies in which calixarenes were used in a different way as building blocks for the construction of such structures or assemblies. 

The calix[ ]arene ( indicates the number of aromatic units in the metacyclophane) building block provides a three-dimensional skeleton that is needed to fix different donor ligands in the optimal spatial positions for binding one ion selectively [94,95]. Calix[n]arenes are easily accessible from the base-eatalyzed condensation of p-tert-butylphenol and formaldehyde [96]. Gutsche proposed to define the two faces of a calix[4]arene molecule as the lower (phenolic groups) and the upper rim. The calix[4]arene with free hydroxyl groups is conformationally flexible and the... 

Fig. 3 Self-assembly of three calix[4]arenas (la) functionalired with two melamine groups and six complementary barbiturate building blocks (2) into a (la)326 superstructure. Mixing in of another calixarene building block functionalized with Zn-porphyrin units (lb) generates a small DCL of four superstructures. Addition of a tripyridine guest (3) shifts the equilibrium in favor of the host-guest complex (lb)326 32. (Adapted from Refs. [9] and [10].)...

Stoikov et alP synthesized novel p-tert-hnty thia-calix[4]arenes functionalized with hydrazide groups at the lower rim of the cone and the partial cone in the 1,3-altemate conformations. The affinity of the self-assembly to p-(Al +, Pb +) and c -(Fe +, Co +, Ni +, Cu ", Ag" ", Cd " ") block elements was investigated by... 

In this article we will illustrate our new strategy with a few examples of receptor molecules composed of calix[4]arenes in combination with several other building blocks that exhibit unique complexation properties. Moreover we have found new routes for the selective introduction of functional groups both in calix[4]arenes and resorcinarenes. One of these methods comprises the key step in the synthesis of calix[4]arene-based carcerands in which guest molecules are permanently encapsulated in an asymmetric environment. Finally, the first example of a holand, a molecule with a large rigid cavity of nanosize dimensions, will be described. 

Not only the covalent combination of calixarenes with other building blocks leads to products with precisely defined molecular cavities. Julius Rebek and his group at Scripps were the first to explore the non-covalent assembly of calix[4] arenes to dimeric capsules in which small guest molecules can be complexed [27]. Together with the group of Volker Bohmer they have explored this self-assembly in great detail [28]. This chemistry is described by Aragay and Ballester in Chap. 32. 

Calix[4]arenes with unprotected OH groups in the lower rim undergo through-annulus inversion and therefore cannot be inherently chiral. The substitution of OH groups with alkyl chains (of at least n-Pr size) blocks the inversion and the resulting... 

One of the main advantages of the chemistry of calixarenes is the fact that they can be, with only few limitations, easily derivatised by numerous functional groups. This feature makes calixarenes very attractive starting point in the design of various receptors and building blocks as we can introduce the required substituents into a basic calixarene skeleton almost at will. Especially calix[4]arene became a frequently exploited molecule in supramolecular chemistry and one can find many regioselective transformations of its basic structure in the literature. The combination of well-established chemistry and a tuneable three-dimensi(Mial shape of its molecule cone, partial cone, 1,2-alternate, 1,3-alternate) makes this compound very attractive in the design of more elaborated systems. 

The flexible structure of calix[4]pyrrole possess coupled with their relative ease of synthesis and inherent affinities for anions have made this core structure a key building block in the creation of containers and capsules [76,78]. Examples of calix [4]pyrroles capsules and self-assembled systems are presented in a separate chapter in this book that is authored by the Tarragona group. Thus, only a brief description will be provided here to avoid redundancy. 

Fig. 38.12a where the methyl replacing hydrogen in hydroxyl group is oriented towards the center of the cavity and instead of inclusion complex the co-crystal (ethanol monohydrate clathrate) is formed [25], When the cavity is not blocked like in t-butyldioxocalix[4]arene the ethyl acetate enters the cavity and the inclusion complex is formed (Fig. 38.12b) [26], When the substituent replacing hydrogen in hydroxyl group mimics the para substituent a cavity similar to the one in cone conformation is created and the inclusion is similar to that observed for calix[4] arenes in cone conformation (Fig. 38.12c) [27].

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