Synthesis and Complexation Studies of Regioisomers and Conformational Isomers of P-tert-butylcalix[4]arene Bearing Pyridine Or Pyridine N-oxide Pendant Groups At the Lower Rim

Selected title compounds have been prepared in order to provide calix[4]arene ligands having different shapes of the calix[4]arene skeleton (cone, partial cone, 1,2-alternate, or 1,3-alternate) and spacial arrangements (syn-proximal, syn-distal, or anti-distal) of pendant pyridine or pyridine N-oxide binding groups. The structure of 1,2-alternate tetrakis[(2-pyridylmethyl)oxy]calix[4]arene 9, obtained in low yield by direct alkylation of the parent p-tert-butylcalix[4]arene 1 with PicCl . HCl and Cs2CO3, has been elucidated by single crystal X-ray analysis. N-Oxide derivatives have been obtained in good yield by m-CPBA oxidation of the appropriate pyridinocalix[4]arene precursors in dry Et(2)O or THF. The H-1 and C-13 NMR spectral changes upon N-oxide formation are discussed. Extraction studies with alkali metal picrates from an aqueous solution into CH2Cl2 have shown that the ionophoric efficiency is low. The highest phase-transfer values are observed for cone tetrapyridinocalix[4]arene 7 (selectivity follows the order Na+ > K+ > Rb+ > Cs+ > Li+), whereas the corresponding N-oxide 7a has no activity. However, NMR complexation studies with Na+ and K+, while confirming a low complexation rate, indicate that in aprotic solvents tetra-N-oxide derivative 7a is a good complexer having a faster kinetic than its pyridino precursor 7