Molecular engineering. Part 2.1 Influence of side-chain substituents in lariat-type upper-rim calix[4]crowns on their binding properties and the reversal of these

Abstract

Cone-structured upper-rim 1,3-calix[4]crowns have been synthesized from the distal dibromo diol 6 which is itself regioselectively synthesized from p-bromocalix[4]arene hexyl ether by transmetallation, quenching with B(OMe)3 and oxidation. The dibromo substituents in these calix[4]crowns have been transformed into hydrophobic or hydrophilic groups to give lariat-type hosts. Picrate extraction experiments show that alkylammonium ions bind more strongly than alkali-metal cations to hosts having hydrophobic side-arms such as bromo or p-methoxyphenyl, which means that the hydrophobic binding of the calix[4]arene unit and the hydrophilic binding of the crown unit synergistically increase the affinity of the calix[4]crowns for alkylammonium ions. 1H NMR spectra also support the conformational change of the calix[4]crowns from pinched-cone to cone upon complexation. With amide or ester side-arms the affinity tendency is reversed. It is presumed that the hydrophilic side-arms have a high affinity for the charged part of the guest, which leads to a collapse of the hydrophobic cavity.