Rigid-cone calix arenes

Arduini, A., Fabbi, M., Mantovani, M., etal., Calix[4]arenes blocked in a rigid cone conformation by selective functionalization at the lower rim. J. Org. Chem. 1995, 60, 1454-1457. 

Phosphorylation of calix(4)arene and t-butyl-calix(4)arene produced the di-substituted product (113 R = H, Bu ). The dominant conformer formed is a stereochemically-rigid cone where the angles of inclination of the benzene ring,... 

Calixarene derivatives possessing one and two crown units on the lower rim were readily prepared via a new regioselective 1,2-dealkylation technique of tetraalkylated calixarene using TiBr4- Conformationally rigid cone-formed biscrown-type calix[4]arene 17 (Structure 10) showed selective extractability in the order of Rb K Na > Cs. This selectivity is believed to be due to its sandwich-type complexation [49]. 

Table V displays the sodium ion selectivity for three kinds of bridged type cal-ix[4]arene derivatives. Since the capping of the lower rim of calix[4]arene with a bridge of different length methylene groups results in the fixation of a rigid cone conformation and the change of the cavity size, the Na" " selectivity is expected to be improved.

Thus, it has been shown that calix[4]aryl esters exhibit remarkably high selectivity toward Na [11-14]. This is attributable to the inner size of the ionophoric cavity composed of four 0CH2C=0 groups, which is comparable to the ion size of Na, and to the cone conformation that is firmly constructed on the rigid ca-lix[4]arene platform (Scheme 2). 

Arduini. A. Giorgi. G. Pochini. A. Secchi. A. Ugozzoli, F. Interaction of the aromatic cavity of rigid calix[4]arene cone conformers with acid CHj and CH2 containing guests in apolar solvents. Tetrahedron 2001. 57. 2411-2417. 

Calixarenes, shown schematically in Fig. 24, are prepared by the condensation of p-tert-butylphenol with paraformaldehyde. In the cone conformation, calix[4]arene has the shape of a chalice or vase. Figure 25 exhibits this geometry, and the inclusion of a toluene molecule is apparent. Closer observation of Fig. 25 shows that an Na+ ion is also held in the base of the chalice. Thus, this cal-ixarene can include both neutral molecules and ions at the same time. The similarity to the cyclodextrins is clear, but the calixarenes are not as rigid. 

The crown ether bridge in calix[4]arene 19 freezes the calixarene stmcture in a rigid flattened-cone conformation also pushing the two metal ions apart. The bimetallic complex 19—Zu2 cleaves HPNP eight times less effectively than the flexible 2—Zu2 [17], thus confirming that a certain degree of flexibility is beneficial and is an important requisite in a supramolecular catalyst. 

Following a complementary approach for reducing the conformational flexibility of calix[4]arenes, but preserving their cone structure, we introduced rigid bridges containing aromatic or other n donor groips at the upper rim of the calix. ... 

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