Anionic capsule

In addition to calixarenes, Sherman has reported the ability of a resorcin[4]arene, a bowl-shaped molecule with eight hydroxyl groups at the upper rim, to form a homodimer in the solid state.39 Deprotonation of two hydroxyl groups of a re-sorcin[4]arene using l,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as a base produced a dianion that self-assembled to form an anionic capsule. The dimer hosted a molecule of pyrazine. Four FI+ DBU ions also interacted with the periphery of the dimer via four N+ H O hydrogen bonds (Fig. 19). 

Murayama and Aoki have also revealed the ability of a resorcin[4]arene to assemble with hydroxylated solvent molecules to form an assembly with a structure that conforms to a tetragonal prism. Specifically, two tetraethylresorcin[4]arenes assembled with eight water molecules to give an anionic capsule in the solid state.55 The capsule accommodated a tetraethylammonium cation, which interacted with the inner walls of the host via cation-Tt forces. Similar assemblies involving derivatized resorcin[4]arenes have also been described.56 In particular, the resorcin[4]arene of Aoki et al.56 was also shown to assemble with 10 water molecules to give a capsule that hosted both a triethylammonium cation and a water molecule.57 The encapsulated guests assembled via a N+-H O-hydrogen bond. Shivanyuk et al. have also described the ability of two hydroxylated resorcin[4]arenes to assemble with 16 water molecules to form a capsule that accommodated either four acetonitrile molecules or a quinuclidinium ion.58... 

Complete resolntion of isomers of a chiral anionic capsule 577 has been performed in [119] with the chiral A -methylnicotinium cation as a guest. The latter is an ideal probe for determining the diastereomeric excess of the resolved capsule, as it readily undergoes further exchange reaction... 

Figure 4d represents in situ encapsulation processes (17,18), an example of which is presented in more detail in Figure 6 (18). The first step is to disperse a water-immiscible Hquid or soHd core material in an aqueous phase that contains urea, melamine, water-soluble urea—formaldehyde condensate, or water-soluble urea—melamine condensate. In many cases, the aqueous phase also contains a system modifier that enhances deposition of the aminoplast capsule sheU (18). This is an anionic polymer or copolymer (Fig. 6). SheU formation occurs once formaldehyde is added and the aqueous phase acidified, eg, pH 2—4.5. The system is heated for several hours at 40—60°C.

Ong et al. [134] found that several hydrophilic anionic, non ionic, or cationic surfactants can alleviate the deleterious effect of magnesium stearate over-mixing on dissolution from capsules when added with the lubricant in a ratio as low as 1 5 (w/w). These successful surfactants were sodium A-lauroyl sarcosinate, sodium stearoyl-2-lactylate, sodium stearate, polox-amer 188, cetylpyridinium chloride, and sodium lauryl sulfate. The lipophilic surfactant glyceryl monostearate did not alleviate the magnesium stearate mixing effect. A reduction in thier particle size was shown to enhance effectiveness, particularly in the case of surfactants with low solubility and slow dissolution rate. 

It is possible to extract or remove ionic species, both anions and cations, from soil using ion exchange resins. Both anion and cation exchange resins have been used as well as combinations of the two. Resins can be added to the soil and mixed, or they can be contained in a bag (Procedure 11.11), on a strip, or in capsules buried in soil. Mixing resins with soil allows for more intimate contact with soil and with the soil solution. However, one is faced with separation of the resin from soil at the end of some extraction time. Resins in bags, on strips, or as capsules can easily be removed from soil. However, the resins do not have as intimate contact with soil in this procedure. Good relationships between all these methods and standard extraction methods have been obtained and all approaches have found utility in determining the amounts of various ions in soil. 

Similarly, analogous Rh(III) hydrolytic cations can be isolated by crystallisation with the super-anion, however these complexes show quite different hydrogen bonding networks. A second type of Russian Doll super-anion is found in the complex [Rh4(OH)6(H2O)12](20.21+8H). 33H2O [46]. The dehydrated Russian Doll Na([18]crown-6)(19)2 7 21 is skewed by comparison with the capsule structure of 20. The two trans aquo ligands are not present in the dehydrated super-anion, and the axial Na coordination positions are occupied by oxygens... 

The drug dissolution profiles from capsules have been documented to change with time due to changes in the gelatin capsule shell properties, interaction between gelatin and an encapsulated ingredient such as anionic compounds (e.g., substituted benzoic and sulfonic acid dyes), and compounds with keto groups. The... 

Atwood, J. L., Szumna, A., Anion-sealed single-molecule capsules. Chem. Commun. 2003, 940-941. 

An example of this design was recently reported by Atwood and coworkers, who showed that an extended, deep-cavity resorcinarene derivative can completely encapsulate a NMe4+ cation with a Cl" anion positioned by hydrogen bonding at the capsule entrance (see schematic receptor 12). [20]... 

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