1,3-Alternate calix arene-based

Synthesis of the closely related acyclic (19) and macrocyclic (20) polyradicals has recently been reported (Figure 5.1).1231 The -conjugated carbanions (e.g., the calix[4]-arene-based tetraanion and the related calix[3]arene-based trianion) were synthesized and studied.1241 Oxidation of these tetra- and tri-anions gave the corresponding tetra- and tri-radicals, respectively. It has been shown in closely related systems that it is not the shape or overall geometric symmetry of the molecules, but rather it is the juxtaposition of the carbenic centers within the jt-cross-conjugated structure, that is most important in determining the spin multiplicity of the alternant hydrocarbon molecule.1251... 

Copper-mediated coupling reactions of / -iodocalixarenes with phthalimide followed by hydrazinolysis should be mentioned as an alternative and independent strategy to obtain p-aminocalixarenes . The carbazole-substituted derivatives 50 (Figure 8) were obtained similarly by Ulhnan coupling . CMPO derivatives (51c), urea compounds (51b), available also via the isocyanates (51a), may be mentioned additionally. Mono- (52) and diimides with acidic functions pointing towards the cavity, and the calix[6]arene-based acetylcholine esterase mimic (53) are more sophisticated examples. 

Fig. 17. A molecular Pasteur pipette . On addition of acid, the tertiary amine group of the upper N04 crown compartment pushes the Ag(I) ion down to the bis(ethoxyethoxy) chelating compartment, from which the metal is released to the solution. On addition of base, Ag(I) is sucked back to the N04 compartment. The reversible process is intramolecular and the metal center travels up and down across the aromatic portion of the 1, 3-alternate calix[4]arene 10...

Another family of calixarene-based ditopic receptors are the l,3-calix[4]arene-Z w(crown ethers), where the calixarene is locked into the 1,3-alternate conformation, and substrate binding normally occurs at the crown ether loops. These may form mononuclear or dinuclear complexes with alkali inetal cations, or may bind neutral guest molecules. The unsymmetrical 1.3-alternate calix[4]ar-ene-l,3-crown-6 2,4-(l,2-phenylene)-crown-6 forms a 1 1 complex with Cs. The crystal structure shows that the cation is bound only within the adapted phenylene-crown-6 loop, and a molecule of acetonitrile is bound within the unmodified crown-ether loop. ... 

Novel calix[4]arene-based anion sensor 334 with two coumarin units attached via amido functions acting also as binding sites was described. This compound may be considered as a potential fluorescent chemosensor for F. Reference calixarene 335 was also synthesized and its 1,3-alternate conformation was deduced from the H NMR spectrum [192] (Scheme 107). 

Calixarenes are potential platforms on which specific binding arms can be grafted. The extractive properties of these molecules for metallic ions depend on the cavity size, the conformation, and the nature of the ligating groups. Different calix[4] arene-crown-6 derivatives in the 1,3-alternate conformation have been studied for Cs recovery from both basic and acidic solutions (257-262). Calixarene-based picol-inamide ligands have been proposed as candidates for separating actinides from lanthanides (263, 264). 

The covalent connection of two tetra-urea calix[4]arenes via their narrow rims leads to molecules which are bifunctional with respect to their ability to form dimeric capsules. This was used by Rebek and Castellano to form self-assembled polycaps , including also alternating structures based on the heterodimerization of aryl- and tosylureas [42,63]. The idea was later extended to switchable polymers [64]. We were recently able to show that the covalent connection of two tetra-urea calix[4]arenes via a rigid spacer between urea functions also leads to molecules 24, which can form polymeric assemblies (Scheme 5.22) [57,65]. 

Mono-O-alkylation of calix[4]arenes has been achieved using weak bases (K2CO3 in acetonitrile, or CsF in DMF) , and monoethers of calrx[5]arenes were synthesized under similar conditions . 1.1 Equivalents of K2CO3 in acetone have been used also to prepare the monomethyl (1.1 mol Mel, 70°C, 2 bar) or monobenzyl ether (1.1 mol BnCl, reflux) of cahx[6]arene 2c ° in yields of ca 80%. Excess of Mel (15.5 mol), KH (1.9 equivalents) in TFIF (RT, sonification) is an alternative in the former case. The mono-0-alkylated 2e with a covalently attached Ceo moiety may be mentioned as an interesting example among the calrx[8]arenes . 

A rather sophisticated ditopic ligand, based on a calix[4]arene, suitable for acidity controlled metal translocation has been recently described by Shinkai [23]. The calix-derivative 10 exhibits an 1, 3-alternate conformation and possesses two distinct coordinating compartments an N04-crown-like moiety on one side, and a bis(ethoxyethoxy) chelating subunit on the opposite... 

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