Covalent molecular capsules

Kinetically and thermodynamically controlled syntheses of covalent molecular capsules... 

Fabris and coworkers have used the reversible imine chemistry for obtaining covalent molecular capsules 39 and 40 (Fig. 20).132 In one case, enantiopure... 

Molecular capsules composed of cavitands and able to form inclusion complexes have received much attention [29, 77, 78]. They can be formed covalently [78, 79] or reversibly through metal coordination [80-82] or H-bond-... 

A self-threaded rotaxane was also observed as a (by) product, when two homooxacalix[3]arenes were covalently connected to a molecular capsule via three p-xylylenediamide linkers Zhong, Z., Ikeda, A. and Shinkai, S. (1999)... 

Molecular capsules are structurally elaborated receptors that completely surround the hosted molecule(s). Encapsulation based on covalent bonds yields permanent arrangements of molecules-within-molecules. Reversible encapsulation, on the other hand, is based on self-assembling through formation of weak supramolecular bonds and offers possibilities for a dynamic in out exchange of encapsulated molecules. Most of the dimeric capsules developed by Rebek and his group are obtained through reversible self-assembly of resorcinarene subunits. When simultaneously encapsulated in the cylindri-cally shaped inner space of these capsules, two reactant molecules are temporarily isolated from others in solution and display reactivity features different from those in bulk solution. The matter has been extensively reviewed,and will not be discussed here. 

The application of molecular containers as molecular reaction flasks has been a very exciting and rewarding venture in host-guest chemistiy. In recent years, the exploration of reactions and reactivity inside covalent or self-assembled molecular capsules has produced very spectacular and unexpected discoveries. For example, it has been demonstrated that molecular capsules may allow the taming of otherwise fleeting reactive intermediates, alter the regiochemistry of reactions, give rise to new... 

Self-assembled molecular capsules are three-dimensional structures generated through reversible, noncovalent interactions such as hydrogen bonding between complementary subunits. They circumvent many problems of covalent capsules, such as multistep and complicated synthetic methods and lack of versatility. Therefore, recent researches are focused on self-assembled systans [2], Micelles, emulsions, vesicles, and noncovalent molecular capsules are examples of self-assembled NRs [2],... 

Self-assembled molecular capsules come in different shapes and cavity sizes and hold together through a variety of non-covalent forces they are capable of reversible encapsulation. Small molecule guests are entirely surrounded by larger molecular assemblies and the steric barriers imposed by a host retain the encapsulated molecules. 

These molecular capsules self-assemble through a range of forces such as metal-ligand interactions, hydrophobic interactions, hydrogen bonding, and dynamic covalent chemistry (Figure 8.2). Their lifetimes vary from milliseconds to days. 

FIGURE 8.2 Modeled structures of molecular capsules assembled with (left to right) metal-ligand coordination, hydrophobic interaction, hydrogen bond interaction, and dynamic covalent chemistry. Peripheral groups have been removed for viewing clarity. 

De novo design and synthesis of molecular capsules with appropriate curvatures and complementary binding sites is not an easy task. Covalent modifications on the structure of an already known module are laborious and may encounter unwelcome solubility problems or collapses of concave surfaces. 

In conclusion, self-assembled molecular capsules are held together by weak inter-molecular forces such as hydrogen bonds, CH-n interactions, van der Waals forces, strong metal-ligand binding, and even dynamic covalent chemistry. Capsule formation creates an inner cavity with a unique shape that can accommodate guests with complementary shapes and binding sites. The behaviors of molecules in dilute... 

Unusual chemical and photochemical reactivity (or, in contrast, incredible stability) of encapsulated species and the unexpected products of such reactions within the confined cavities of molecular capsules have been observed in many cases. This can be explained by steric restrictions due to relative rigidity of covalent and coordination capsules, by isolation of these guests from external factors such as solvent effects, and by relatively strong supramolecular host-guest interactions. 

Ballester P (2010) Anion binding in covalent and self-assembled molecular capsules. Chem Soc Rev 39 3810-3830... 

In some polysaccharides, the reducing terminal is linked, through a phosphoric diester linkage, to O-1 of a 2,3-di-6 -acylglycerol. This structural feature has been demonstrated for some capsular polysaccharides from E. coli and Neisseria species, - but is probably more common than that. Non-covalent linkage between the lipid part and the cell membrane may explain why extracellular polysaccharides often occur as capsules, and the high (apparent) molecular weight observed for these polysaccharides may be due to micelle formation in aqueous solution. 

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