COMPLEXATION OF ALKALI-METAL CATIONS BY CONFORMATIONALLY RIGID, STEREOISOMERIC CALIX[4]ARENE CROWN ETHERS - A QUANTITATIVE-EVALUATION OF PREORGANIZATION

Selective bridging of p-tert-butylcalix[4]arene (1) with tetra- and pentaethylene glycol ditosylate gives the 1,3-dihydroxy-p-tert-butylcalix[4]arene crown ethers 7 and 9 in good yields. The subsequent alkylation of the two phenolic groups of 7 and 9 with substituents (R) bulkier than CH3 gives a series of conformationally rigid 1,3-dialkoxy-p-tert-butylcalix [4]arene crown ethers, which exist as a mixture of stereoisomers. Three isomeric compounds (10a, 10b, and 10c) have been obtained when R = C2H5. 1H NMR and X-ray crystallography show a fixed-cone structure for 10a, a partial cone for 10b, and a 1,3-alternate for 10c. Only the cone and the partial-cone stereoisomers have been isolated when R = n-C3H7, i-C3H7, and CH2C6H5. The free energies of complexation of the calixcrowns and alkali metal picrate (MPi) salts, in CDCl3, vary from-6 to-13.5 kcal mol-1. All ligands are selective toward potassium cations, and in the case of calixcrown 10b a K+/Na+ selectivity of 1.18 ⨯ 104 is found, which is the highest value observed so far for calixarene ligands and also for a synthetic ionophore. In all cases the partial cone isomer shows the highest free energy of complexation (ΔG°), confirming our previous assumption that this is the preferred conformation for binding cations. The differences in binding free energies of complexation (ΔΔG°) between the cone and the partial-cone isomers of the same calixcrown allow a quantitative evaluation of the preorganization concept. In the case of RbPic complexes of calixcrowns 10b and 10a this difference reaches the value of 4.9 kcal mol-1. Variable-temperature 1H NMR experiments indicate that the K+ complex of the partial-cone stereoisomer 10b is also kinetically more stable than the corresponding complex of the cone compound 10a. The results obtained are explained on the basis of electronic and stereochemical arguments. © 1990, American Chemical Society. All rights reserved.