Anion hosts cyclophanes

The first is a cyclophane-based system reported by Murakami et al. (Fig. 9.12a). [31] The sides of the host consist of tetraaza-[3.3.3.3]paracyclophane units, and its octaprotonated cation has been shown to bind anionic guests. The molecule possesses 48 asymmetric units of [(N/3)-(CH2)- (C6H4)/2 ]/2. 

Figure 20. Potentiometric responses to anionic guest 78 by PVC matrix liquid membranes containing cyclophanes (74, 76,77) or acyclic reference compound (75). (a) Membrane potential as a function of pH in the presence ( ) or absence (o) of 1.00 X 10 M guest, (b) Membrane potential as a function of guest concentration at pH 5.0 (0.01 M AcONa-AcOH buffer) and room temperature (ca. 20 °C). Membrane composition DOS (95 mg) and PVC (44 mg) containing 6.2 x 10 mmol host.

Nitrogen-bearing cyclophanes like 351 [16] and 352 [17] bind larger organic anions in water due to superposition of the hydrophobic effect and electrostatic attraction. The phenanthridinium hosts like 351 have been found to form the most stable nucleotide complexes known so far. On the other hand, free tetrapyrrolic porphyrins do not bind anions since their cavity is too small to take advantage of the convergent N-H dipoles for the complex stabilization [18]. However, expanded diprotonated porphyrins like sapphyrin 353 were shown to form stable complexes with phosphate [19a] and halide [19b] anions. 

A range of cyclophanes (compounds containing a bridged aromatic ring - for a full discussion of cyclophane chemistry see Section 6.5) with protonated nitrogen functionalities have been used with great success as anion binding hosts. In particular, the diphenylmethane moiety is commonly used... 

Another attempt to use the host-guest complexation of simple cyclophanes has been reported by Schneider They take the easily accessible host 7, an analogue of which had been demonstrated by Koga to bind aromatic guest molecules by inclusion into its molecular cavity, and study its rate effects on nucleophilic aliphatic substitutions of ambident anions (NOf, CN, SCN ) on 2-bromomethylnaphthalene 8 and benzylbromide. Similar bimolecular reactions are well known in cyclodextrin chemistry and other artificial host systems . In addition to the rather poor accelerations observed (see Table 3) the product ratio is changed in the case of nitrite favouring attack of the ambident nucleophile via its nitrogen atom. 

A cyclophane-based system reported by Murakami was shown to conform to a structure of a cube (Fig. 4c).The sides of the host consist of tetraaza-[3.3.3.3]paracyclo-phane units, and its octaprotonated cation was shown to bind anionic guests. A polyoxovanadate. (V06)(RP03)g] (R=rBu,OSiMe3), reported by Zubieta and Thom, was also shown to adopt an octahedral framework (Fig. 4d). The vanadium atoms of the shell are located at the vertices of an octahedron, while the phosphorus atoms are located at the comers of a cube. A cube with a structure based on deoxyribonucleic acid (DNA) was described by Seeman (Fig. 4e). The directionality and ability of the double helix to form branched junctions were exploited for the edges and vertices, respectively. 

The cyclophanes of Tabushi (29,30) and Murakami (31) are especially suited for hydrophobicly binding aromatic molecules in aqueous solutions (Figure 4). With cationic cyclophane hosts, both hydrophobic and electrostatic forces come into play, and not only aromatic molecules but their anionic derivatives are also bound (32-34). 

Synthetic molecular receptors command widespread interest as mimics of membrane transport agents and enzyme active sites.[1,2] The development of new host systems for the selective complexation of organic and inorganic cations and anions has mushroomed in recent years however, examples of solution phase coordination of neutral organic molecules are few. Most of these examples involve inclusion of an aromatic hydrocarbon into the hydrophobic pocket of a water soluble cyclodextrin or cyclophane receptor.[3-6] A smaller subset of synthetic receptors possess lipophilic exteriors and hydrophilic cavities for the association of polar substrates in nonpolar media. [7-9] We report here the synthesis and characterization of a new system of the latter type designed to bind polar guests in chloroform solution. 

---