Host-guest complexes solvent influences

Overall, the solvent influence on such host/guest equilibria is obviously a rather complex phenomenon and has to be carefully analysed for each individual solvent-dependent complex formation. A few representative examples of solvent-dependent host/guest complexation reactions are given in the following. 

The stability of the host-guest complex is significantly affected by the nature and composition of the solvent in which the processes occur. This is an important factor when the reaction is carried out in aqueous medium, where hydrophobic interactions mainly contribute to the energy of complex formation due to an increase in its stability. In this regard, the use of water-soluble macrocycles with hydrophobic cavities as components of water-soluble metal complexes in two-phase catalytic systems is of particular interest [38,39]. The catalyst, soluble in the aqueous phase, can be easily separated from the water-insoluble reaction products and reused. It should be emphasized that the activity of conventional catalysts is very low for the reactions involving substrates poorly soluble in water. Due to the formation of water-soluble inclusion host-guest complexes, the macrocyclic receptors not only influence the activity and selectivity of the reaction, but also perform the function of interfacial substrate carrier in aqueous phase. 

In 1994 we published the first chiral dendrimers built from chiral cores and achiral branches [ 1,89], see for instance dendrimer 57 with a core from hydroxy-butanoic acid and diphenyl-acetaldehyde and with twelve nitro-groups at the periphery (Fig. 21). As had already been observed with starburst dendrimers, compound 57 formed stable clathrates with many polar solvent molecules, and it could actually only be isolated and characterized as a complex [2 (57- EtO-Ac (8 H20))]. Because no enantioselective guest-host complex formation could be found, and since compounds of type 57 were poorly soluble, and could thus not be easily handled, we have moved on and developed other systems to investigate how the chirality of the core might be influencing the structure of achiral dendritic elongation units. 

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