Capsules molecular hydrogen-bonded

Atwood JL, Barbour LJ, Jerga A (2002) Organization of the interior of molecular capsules by hydrogen bonding. Proc Natl Acad Sci USA 99 4837... 

The design and synthesis of supramolecular architectures with parallel control over shape and dimensions is a challenging task in current organic chemistry [13, 14], The information stored at a molecular level plays a key role in the process of self-assembly. Recent examples of nanoscopic supramolecular complexes from outside the dendrimer held include hydrogen-bonded rosettes [15,16], polymers [17], sandwiches [18, 19] and other complexes [20-22], helicates [23], grids [24], mushrooms [25], capsules [26] and spheres [27]. 

An interesting example of less rigid cages is also provided by molecular self-assembling capsules obtained in the Rebek group [56]. 81 was found to form dimers held together by hydrogen bonds between the donors at its ends and the acceptors in the middle. 

McKinlay, R. M., Thallapally, P. K., Atwood, J. L., Hexameric C-alkylpyrogallol[4]arene molecular capsules sustained by metal-ion coordination and hydrogen bonds. Chem. Commun. 2006, 2956-2958. 

Kobayashi, K., Shirasaka, T., Yamaguchi, K., Sakamoto, S., Horn, E., Furukawa, N., Molecular capsule constructed by multiple hydrogen bonds self-assembly of cavitand tetracarboxylic acid with 2-aminopyrimidine. Chem. Commun. 2000, 41-42. 

Atwood, J. L., Barbour, L. J., Jerga, A., Hydrogen-bonded molecular capsules are stable in polar media. Chem. Commun. 2001,2376-2377. 

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. 

A molecular capsule can be composed of more than two units also a sphere can be obtained assembling together a generic number n of slices. Rebek and collaborators, in their extensive exploration of the properties and features of self-assembled capsules [36], developed a L-shaped molecule characterized by the presence at one end of the glycoluril hydrogen bond system, and at the other end, separated by an aromatic ring, a sulfamide moiety (Fig. 14). 

Other types of molecular capsules with included anions have been described, but they do not involve directed hydrogen-bonding interactions between functional groups on the inside of the capsule and the guests [78, 79]. 

A self-assembling molecular capsule with inwardly directed hydrogen bond acceptors and donors composed of two identical resorcarene derivatives with phenylalanine residues on each aromatic subunit, l-40, was recently reported by Kuberski and Szumna [87]. 

The encapsulation of chemical space on the scale of simple molecules has been a topic of considerable interest for more than two decades. For the formation of capsules two independent strategies have emerged covalent synthesis and self-assembly. The groups of Cram [8], Collet [9], and Sherman [10] have synthesized capsules capable of encapsulating up to three small molecular guests [11]. Rebek and co-workers pioneered the use of self-assembly to produce a variety of assemblies held together by hydrogen bonds [12-15], Multi-component systems have... 

It is clear that, in addition to infinite frameworks, observations made above can be applied to discrete assemblies. Specifically, the basic principle learned from the infinite frameworks, the projection of hydrogen bond donors into a cavity can organize the guests at a remarkable level, can be applied to the organization of guests within a molecular capsule. 

It is appropriate to draw an analogy between 10a, the complex with eight acetone guests, four of which are hydrogen bonded to the framework of the host, per cavity, and 11, the capsule which binds six Et20 molecules on the interior. We have not yet discovered a capsule analog of 10b, but it will surely be possible to obtain such intricate order with a molecular capsule, probably even with that of 11. 

Some other capsules have been described in which both complimentary functions, carboxylic acids and nitrogen-containing aromatic rings, are respectively attached to the rims of two bowl-shaped molecules. Reinhoudt has prepared selfassembling, multi-hydrogen bonding molecular capsules in which a calix[4]arene substituted with four carboxylic functions at the upper rim interacts with a complimentary calix[4]arene with four pyridines attached to the lower rim. These capsules have been identified by H-NiMR, IR and VPO measurements but encapsulation properties have not been reported [106]. 

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