Bridged calixarenes moieties

In contrast in the bridged calixarene (see Figure 7.15), the methyleneoxy units between the 1,10-phenanthroline and the calixarene act as flexible joints and this leads to a different geometry in which the 1,10-phenanthroline bridge is tilted towards one chain of the calixarene rim. The calixarene macrocycle is too small to accommodate the carboxylate or to let an alkene approach the carbenoid. This results in an orientation in which both, the alkene and the carboxylate residue of the carbene moiety, have to be squeezed in the gap between the 1,10-phenanthroline and part of calixarene rim. By this sterical shielding of the calixarene, the carboxylate and the residues of the alkene are oriented next to each other, and thus the predominant formation of the endo-product is observed. " ... 

Other components of the design [138—142] include the choice of the crown size or even the use of open-chain metalloclefts, not necessarily polyether armed. The issue of special importance for extraction and transport applications is the selection of substituents ensuring a necessary lipo-philicity-solubility balance. For example, use of a cyclohexano moiety as shown above, instead of the more common o-phenylene bridge between nitrogens, enhances solubility in the membrane phase. The modification of the polyether chain with binaphthyl or calixarene substituents provides high membrane transport rates due to increased ionophore lipophilicity [138,142]. Some representative examples (initial fluxes, in 10 mol cm h , through o-nitrophenyloctyl ether-impregnated Accurel membrane 1 M source urea [138]) are as follows. 

Solvent-dependent anion selectivity was observed for a receptor consisting of two calixarene units bridged by a ferrocenyl moiety [179a]. 

Another type of modified calixarene has been prepared by attaching the ferrocene moieties to the bridge connecting the benzene rings in the calix[4]arenes [190] or both to the bridge and the aromatic rings, 95 [191, 192]. 

Yet another approach for differentiation of substituents at the lower rim involves directional bridging. The bridge can be placed at any position, however, in practice, the most convenient location is the lower rim oxygen atoms (Fig. 2.16e). For example such directional linkages composed of carboxamide [72] or ester [73] moieties were introduced by a cyclization reaction to give inherently chiral calixarene 60 possessing C2 symmetry. 

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