Hydrogen-bonded capsules

The future prospects for the capsule project emerge from these considerations. Further increasing the size of the capsule and building chemical functionalities into the inner cavity would allow a closer emulation the functions of enzymes, especially those that require cofactors in order to catalyze chemical transformations. Another important aspect is to design capsules that can combine stereospecificity and catalysis - that is accelerate stereoselective transformations. Capsules that reversibly dimerize in water would probably contribute a lot more to our understanding of non-covalent forces and solvent effects in this most biorelevant medium. So far, water solubility and assembly have not been achieved with hydrogen-bonded capsules. 

Hydrogen bonded capsule consisting of calixarene dimers will be briefly presented in Section 8.3.2. 

Hydrogen-bonded capsules and other higher architectures... 

Figure 10.45 Multi-component, hydrogen-bonded capsules (a) C-methyl[4]resorcarene 4,4 -bipyri-dine capsule with two included p-nitrobenzene guests,49 (b) rigidified [4]resorcarene tetracarboxylic acid held together by 2-aminopyrimidine with two included p-nitrobenzene guests.50...

Scheme 3.8 A Diels-Alder reaction is accelerated in a tennis-ball like hydrogen bonded capsule (35)2 (R1 = R2 = 4-n-heptylphenyl,...

The existence for such twistomers was first described by Chapman and Sherman for a self-assembled hydrogen-bonded capsule formed by the hydroxy... 

Chiral discrimination of guests by inclusion in hydrogen-bonded capsules of tetraureas is a highly interesting and challenging problem. The analysis of such systems by NMR spectroscopy is complicated by two facts ... 

As already mentioned, cavitand 126 forms a hydrogen-bonded capsule in the presence of a base. A single crystal X-ray structure could be obtained for the inclusion complex with pyrazine.285 Figure 38 shows its overall D4 symmetry (not regarding the guest) due to the twisting of the two cavitands. As mentioned before, this chiral arrangement is also found at low temperature in solution. 

Custelcean R, Bock A, Moyer BA (2010) Selectivity principles in anion separation by crystallization of hydrogen-bonding capsules. J Am Chem Soc 132 7177-7185... 

Hydrogen-bonded capsules in polar, protic solvents, A. Shivanyuk and J. Rebek, Jr, Chem. Commun., 2001, 2374. 

Scheme 4-12 Formation of tetrameric hydrogen-bonded capsule 52.

Figure 3.54 Examples of multi-component, hydrogen bonded capsules (see text for further details).

Figure 3.55 A hexameric hydrogen bonded capsule (3.34)j, with one individual component highlighted (for clarity, isobutyl groups have been omitted).

Figure 4.4 Supramolecular hydrogen bonded capsule developed by Rebek et ai (left) a dimeric structure and (right) various tetrameric structures.

Homodimeric hydrogen-bonded capsules have been successfully employed to bind and separate fullerenes (Section 3.1.2). A heterodimeric capsule 32 forms through charge-assisted hydrogen bonding between a carboxylic acid and an amine-appended CTV and encapsulates a number of small neutral guests (Scheme 15). ... 

---