Calix arene Urea Dimeric Capsules

Calix[4]arene provides a suitable semi-rigid concave scaffold for the construction of dimeric capsules. Both the upper and lower rim can be decorated with a wide variety of functional groups, in particular the urea moiety is known to provide a good combination of hydrogen bond donor and acceptor character implemented in the same functional group and this sequence of atoms have been extensively employed in the constructions of calix[4]arene dimeric molecular capsules (Fig. 9). Characteristic of these dimers is the presence of a cyclic seam of hydrogen bonds that seal the capsule and provides an internal molecular cavity that easily accommodates molecules as big as benzene. 

Equimolar amounts of achiral urea 1 and the optically active calix[4]urea 5 give rise to the formation of a heterodimeric capsule with only one cyclic arrangement of the urea hydrogen bonds (Fig. 11), and in the presence of a chiral racemic substrate like norcamphor two diastereoisomeric complexes were present in a 1.3 to 1 ratio. 

Because of their efficiency in natural systems in giving supramolecular aggregates, aminoacids have been implemented extensively in calix[4]arene scaffolds. Di- and tetrasubstitution with aminoacid derivatives have been investigated observing different behaviors depending on the orientation of the chiral residues attached to the calix[4]arene scaffold. Derivatives compris- 

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Tetra-urea Calix[4]arenes - From Dimeric Capsules to Novel Catenaries and Rotaxanes... 

Fig. 19 Achiral calix[4]arenes with different substituents on the urea residues lead to diastereoisomeric chiral dimeric capsules...

Hydrogen-bonded dimers of tetra-urea calix[4]arenes offer a unique, unprecedented preorganization [28], Before we describe how this can be used for the rational construction of molecules with novel topologies, it is reasonable to discuss the properties of such dimeric capsules and the conditions under which they exist. 

The covalent connection of two tetra-urea calix[4]arenes via their narrow rims leads to molecules which are bifunctional with respect to their ability to form dimeric capsules. This was used by Rebek and Castellano to form self-assembled polycaps , including also alternating structures based on the heterodimerization of aryl- and tosylureas [42,63]. The idea was later extended to switchable polymers [64]. We were recently able to show that the covalent connection of two tetra-urea calix[4]arenes via a rigid spacer between urea functions also leads to molecules 24, which can form polymeric assemblies (Scheme 5.22) [57,65]. 

The first examples of urea-containing dimeric capsules were discovered independently by Rebek and Bohmer [165,166]. These are based on calix[4]arene ethers, in which the lower rim ether units help to fix the calixarenes in a cone conformation via intramolecular interactions. Figure 60 shows the generic structure of such systems. Synthetically, the urea calixarenes are readily prepared. The calixarenes are first nitrated, followed by a reduction of these groups into amines. The urea derivatives are fixed to the upper rim by reaction of the p-amino calixarenes with isocyanates. Many systems were studied using differently substituted ureas which contain either short alkyl chains or simple phenyl derivatives. Studies also involved the changing of the lower rim ether substituents. 

Urea-functionalized calix[4]arene syntones shown in Scheme 3.4, fixed in their cone-conformation, have been synthesized in [4]. These syntones nndergo dimerization and hybridization in apolar solvents, leading to hybrid dimeric capsules with two nonequivalent urea-substituted arene fragments and their homodimeric analogs as weU. 

The geometry of achiral dimeric calix[4]arene endowed with urea functionalities offers further possibilities to obtain homodimeric chiral capsules. In fact, simply employing two different substituents A and B on the urea or on the ether part of the calix[4]arene aromatic rings and considering all the possible combinations depicted in Fig. 19, it was possible to obtain up... 

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