Francine Furtado Nachtigall

Interaction of Calix[4]arene and Aliphatic Amines: A Combined NMR, Spectrophotometric and Conductimetric Investigation

The constants related to the equilibria between calix[4]arene and aliphatic amines in acetonitrile were measured by conductimetric and spectrophotometric methods, and the structures of the salts studied by NMR spectroscopy. In concentrations about 10-4 mol L-1, free ions are at higher proportion, whereas at 10-2 mol L-1 there is an increase of ion-pairs proportion. The values of the constants found for acid-base reactions allow an evaluation of the value of pKa for calix[4]arene in acetonitrile as 16.6 pKa units. The analysis of data agrees with a proposal of exo-calix ammonium ion.

Endo-cavity Complexes between Calix[6]arene Dianions and Aliphatic Ammonium Cations: Structure of a Hexylammonium Complex by X-ray Crystallography

The interactions of calix[6]arene and p-tert-butylcalix[6]arene with aliphatic amines in acetonitrile solution were studied by spectrophotometric titrations in the UV region and 1H NMR spectrometry. Calix[6]arenes can undergo two deprotonations by aliphatic amines and the extent of the second proton transfer is mainly governed by the size of the α-substituent of the amine. 1H NMR spectra show that the macrocycle adopts a 1,2,3-alternate conformation and that the α-hydrogens of the ammonium ions are shielded by the π-clouds of the aromatic rings. The X-ray crystal structure of the dihexylammonium complex of the p-tert-butylcalix[6]arene dianion, reported here, confirms the 1,2,3-alternate conformation and shows one of the two ammonium moieties encapsulated in the inner cavity of the macrocycle.

Synthesis of N-phtaloyl amino acid p-tert-butylcalix[4]arene esters

1-3-Diesters of p-tert-butylcalix[4]arene were obtained upon reaction with N-phtaloyl amino acid chlorides. Reactions with chiral amino acids were accompanied by complete or partial racemization in the reaction media (Et3N/CH2Cl2).

Chiral lactic acid and ethyl lactate p-tert-butylcalix[4]arene derivatives

Ethyl (S)-(–)-O-tosyllactate reacted with p-tert-butylcalix[4]arene in K2CO3–acetone with inversion of configuration on the asymmetric centre, and only the bis-substitution product on the distal oxygens was obtained. Further attempts to react the bis(lactate) p-tert-butylcalix[4]arene derivative with BrCH2CO2CH3 under the same conditions failed. X-Ray analysis of the lactate derivative shows two independent molecules in the unit cell; one CH3 of the ethyl radical is immersed in the cavity of the other molecule. Both adopt a slightly distorted cone conformation. NMR analysis shows that carbons and hydrogens of the methylene bridges and meta positions are diastereotopic. Ester hydrolysis was carried out without racemization, and the diastereotopic behaviour of the lactic acid derivative was maintained.