Organic cations in constrained systems

Institut fur Chemie, Humboldt-Universitat zu Berlin, D-12489 Berlin, Germany 

Molecular Encapsulation Organic Reactions in Constrained Systems Edited by Udo H. Brinker and Jean-Luc Mieusset 2010 John Wiley Sons, Ltd 

The host molecules considered here possess a more or less rigid structure surrounding a cavity that can form complexes with organic cations having various shapes. The strength of binding depends on several parameters such as  

Charged groups such as sulfanyl substituents may serve as secondary binding sites operating through salt bridges. 

Because ion pairs are formed in low polarity aprotic solvents, the anion of organic cations also contributes to the complexation. It has been found that the stability of complexes between aromatic receptors and ammonium and iminium ions is decreased in the order picrate trifluoroacetate r Br Cr tosylate acetate. Thus, the lower the cation-anion attraction, the stronger is the cation-rr interaction.  

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Organic Cations in Constrained Systems 333 Table 12,1 Association constants (M" ) of host-guest complexes in CDCI3 solution... 

Saturation index calculations made as part of a species distribution problem allow an assessment to be made of the effect of organic acids on the likely state of heterogeneous equilibria in an aqueous system (see Drever 1988, for discussion and definitions). By comparing saturation indices for minerals in systematically different waters we can predict the likely behavior of these minerals in the presence of organic acids. The predictions about mineral stability vary with the precise constraints that are placed on the calculations, in particular whether the cations are constrained to be in equilibrium with a mineral phase or set as a total concentration, the temperature, the partial pressure of CO2, and the anionic composition of the water. Conclusions that differ from those presented here may be possible, nevertheless, some consistent trends emerge that are related to observations made in the preceding section about speciation. 

The hemispherands, spherands, calixarenes, and related derivatives. A number of hosts for which the pre-organization criterion is half met (the hemispherands) (Cram et al., 1982) or fully met (the spherands) (Cram, Kaneda, Helgeson Lein, 1979) have been synthesized. An example of each of these is given by (251) and (252), respectively. In (251), the three methoxyl groups are conformationally constrained whereas the remaining ether donors are not fixed but can either point in or out of the ring. This system binds well to alkali metal ions such as sodium and potassium as well as to alkylammonium ions. The crystal structure of the 1 1 adduct with the f-butyl ammonium cation indicates that two linear +N-H - 0... 

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