Flattened Cone Conformation Research Articles

Zinc dimerization of p-tert-butylcalix[4]arene

Abstract Reaction of p-tert-butylcalix[4]arene, 1, with excess ZnEt2 results in complete metallation of the calixarene, or with two equivalents of ZnEt2 selective dimetallation occurs yielding the bis(calix[4]arene) species [{1-H2)Zn}2], 2; distorted four coordinate O-bound zinc centers (Zn-O = 1.896(7) – 1.978(8) Å) fuse the slightly flattened cone conformation calixarene units with methylene chloride included in the divergent cavities.

1,3‐Alternate Calix[4]arenecrown‐5 Conformers: New Synthetic Ionophores with Better K+/Na+ Selectivity than Valinomycin

AbstractNew 25,27‐dialkoxycalix‐[4]arenecrown‐5 conformers 8, 10, and 11 have been synthesized and studied. The compounds 8a and 8b, fixed in 1,3‐alternate structure, have been obtained in 57 and 40% yield, respectively, by reaction of the corresponding 25,27‐dialkoxycalix[4]arenes 7a–b with tetraethylene glycol di‐p‐toluenesulfonate in the presence of Cs2CO3. The cone 10a and 10b and the partial cone 11 conformers were obtained by selective demethylation of the 25,27‐dimethoxycalix[4]arenecrown‐5 (6a) and subsequent dialkylation with NaH/DMF and KOtBu/THF, respectively. In the solid state (X‐ray), compound 6a adopts a flattened cone conformation, which is also found to be most abundant in CD3CN and CD3OD solution. Upon complexation with potassium picrate compound 6a was converted quantitatively into the 1,3‐alternate conformation. All new ligands synthesized were used in the extraction of alkali metal cations from H2O into CHCl3, and as active components in supported liquid membranes and in chemically modified field effect transistors. Results were compared to those obtained with with the natural antibiotic valinomycin 1. All ligands showed high selectivity for potassium. Ligand 8a, fixed in the 1,3‐alternate conformation, is more selective than valinomycin and shows the highest K+/Na+ selectivity known so far.

Cation binding and conformation of octafunctionalized calix[4]resorcinarenes

The preparations of four calix[4]resorcinarene esters and two calix[4]resorcinarene amides are reported. The logarithms of association constants have been determined by the picrate extraction method for a variety of metal cations, including alkali metal, alkaline earth, lanthanum, and silver cations. Strong binding toward silver ion, and selectivity amongst the alkaline earth cations, was observed for the octa-α-(diethyl acetamide) 3c. Flattened cone conformations have been established for both C-undecylcalix[4]resorcinarene octa-α-(methyl acetate), 3a, and octa-α-(diethyl acetamide), 3c, in chloroform solutions at 200 K. Dynamic nuclear magnetic resonance was used to identify rapid conformational interconversions of both 3a and 3c corresponding to pseudorotation between flattened cone conformations. The silver complex of 3c also exhibits pseudorotation; however, the activation barrier was observed to increase from 42 kJ mol−1 in the free ligand to 49 kJ mol−1 in the complex. The NMR data indicate that the metal cation binding occurs in a symmetrical fashion using two pairs of binding sites that are disposed across the macrocyclic ring from one another. Considering these data it is concluded that four carbonyl functions are involved in the binding of a metal cation in a 1:1 complex with the ligand. Key words: resorcinarene, picrate, association constant, dynamic NMR, pseudorotation.