Calix arene capsules

Supramolecular self-assembly can be as simple as acetic acid dimerization through mutually complementary hydrogen bonds or as complex as supramolecule formation from multiple components as demonstrated by numerous examples of resorcin[4]arene and calix[4]arene capsules. One of the more interesting directions to have been taken has been the design and synthesis of self-complementary... 

This tetrakis-urea was expected to self-assemble in apolar media by hydrogen bond formation between urea moieties of two different units. The analogous behavior of calix[4]arene tetrakis-ureas is well documented [75], but, in contrast to the corresponding calix[4]arene capsules, those deriving from 31 would feature hydrogen bond donor sites within the cavity (Fig. 5). 

The cavity of these calixarenes is usually too small to accommodate fullerenes, and is confirmed by solution, unless they have extended cavity walls as in calix[4]-naphthalenes. Nevertheless, some calix[4]arenes were shown to form stable crystalline complexes with C60 with the fullerene eno- to the calixarene cavity, and they include p-Ph-calix[4]arene, which has a toluene molecule in the cavity, the overall structure dominated by fullerene-fullerene and eso-calixarene fullerene interactions p-Br-calix[4]arene propyl ether, the structure showing very close interfullerene contacts in a columnar structure (Fig. 2d), which most likely results in opposing induced dipoles from the unidirectionally aligned calix-arenes " and /7-I-calix[4]arene benzyl ether, where the fullerenes are ordered without appreciable interfullerene interactions. There is also a calix[4]resorcinarene, R = CH2CH2Ph, 3, which has a molecular capsule derived from head-to-head hydrogen bonding of two resorcinar-enes and propan-2-01 molecules, with the fullerenes also... 

Figure 5 Formation of a calix[4]arene capsule takes place in benzene but not in CD2CI2. (Reproduced from Ref. 8. American Chemical Society, 1997.)...

Figure 14 illustrates the presence of exchange crosspeaks in the NOESY spectra of a self-assembled tetraurea calix[4]arene capsule 5. For this capsule, the correlation time is long and NOE cross-peaks are of the same... 

Two synthetic strategies with similar efficiencies shown in Scheme 2.172 have been used in [197] for the synthesis of a tris-pyridyl calix[6] arene capsule 276. According to H NMR data, its cage framework has a Cjv-symmetric flattened cone-conformation and is able to coencapsulate Na cation and solvate ethanol molecule, giving a heteroguest 1 1 1 cage complex. At the same time, 276 does not form similar complexes with an encapsulated K+ cation in the absence of ethanol, and vice versa. The X-rayed crystal of the protonated form of 276 contains three cage molecules, two of which... 

Hydrogen-bonded bis-calix[4]arene capsules 364 and 365 have been prepared in [42] by... 

The use of RTF allowed encapsulation a guest organic molecule by dimeric para-phosphonic acid-containing calix[5]arene capsule 382 [50]. This hydrogen-bonded capsule binds carboplatin... 

Cho YL, Rudkevich DM, Shivanyuk A, Rissanen K, Rebek J Jr (2000) Hydrogen-bonding effects in calix[4]arene capsules. Chem Eur J 6(20) 3788-3796... 

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. 

Fig. 9 A Calix[4]arene tetraurea dimeric capsules with a particular head-to-tail hydrogen bond network. B Achiral tetraurea units form achiral homodimeric capsules, while C two different achiral units forms chiral heterodimeric capsules...

Fig. 11 Heterodimeric diastereoisomeric capsules arising from dimerization of achiral sulfonyl tetraurea and chiral tetraurea calix[4]arenes...

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... 

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

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

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 structure of the capsule, proposed initially on the basis of these NMR-data, was completely confirmed by the first single crystal X-ray structure (Figure 5.2) [34]. The two calix[4]arenes are connected via their wide rims by a seam of intermolecular hydrogen bonds alternatingly involving the urea residues of both calixarenes. The directionality of this hydrogen bonded belt is the reason for the inequivalence of the two aromatic protons of a given phenolic unit mentioned above. 

---