MOLECULAR ENGINEERING .2. INFLUENCE OF SIDE-CHAIN SUBSTITUENTS IN LARIAT-TYPE UPPER-RIM CALIX[4]CROWNS ON THEIR BINDING-PROPERTIES AND THE REVERSAL OF THESE
H. Ihm et al., MOLECULAR ENGINEERING .2. INFLUENCE OF SIDE-CHAIN SUBSTITUENTS IN LARIAT-TYPE UPPER-RIM CALIX[4]CROWNS ON THEIR BINDING-PROPERTIES AND THE REVERSAL OF THESE, Journal of the Chemical Society. Perkin transactions. I, (13), 1997, pp. 1997-2003
Cone-structured upper-rim 1,3-calix[4]crowns have been synthesized fro
m the distal dibromo diol 6 which is itself regioselectively synthesiz
ed from p-bromocalix[4]arene hexyl ether by transmetallation, quenchin
g with B(OMe)(3) and oxidation. The dibromo substituents in these cali
x[4]crowns have been transformed into hydrophobic or hydrophilic group
s to give lariat-type hosts. Picrate extraction experiments show that
alkylammonium ions bind more strongly than alkali-metal cations to hos
ts having hydrophobic side-arms such as bromo or p-methoxyphenyl, whic
h means that the hydrophobic binding of the calix[4]arene unit and the
hydrophilic binding of the crown unit synergistically increase the af
finity of the calix[4]crowns for alkylammonium ions. H-1 NMR spectra a
lso support the conformational change of the calix[4]crowns from pinch
ed-cone to cone upon complexation. With amide or ester side-arms the a
ffinity tendency is reversed. It is presumed that the hydrophilic side
-arms have a high affinity for the charged part of the guest, which le
ads to a collapse of the hydrophobic cavity.