20
Chemistry Letters 2000
Synthesis and Inclusion Properties of a Upper-rim-connected Biscalix[4]arene
Koji Araki* and Hiromi Hayashida
Department of Applied Chemistry, Faculty of Engineering, Kyushu Institute of Technology,
1-1 Sensui-cho, Tobata-ku, Kitakyushu 804-8550
(Received October 4, 1999; CL-990849)
The conformationally-immobile biscalix[4]arene was syn-
and mass spectral evidence and elemental analysis. The tem-
perature-independent 1H NMR peaks for the ArCH2Ar methyl-
ene protons in 1 showed one pair of doublets at 3.09 and 4.43
ppm at 25 °C in CDCl3. This finding reveals that the two
calix[4]arene skeletons are immobilized in the cone conforma-
tion and possess high symmetry.
thesized by connecting the upper rims with two spacers. It
showed high inclusion ability and selectivity for quaternary
ammonium ions. A comparison with reference compounds
established that the enhanced selectivity is due to the coopera-
tive action of two bowl-shaped calix[4]arenes.
Calixarenes, a new class of synthetic macrocycles having
phenolic units linked by methylene groups at the ortho-position
to the hydroxy groups, have received considerable attention for
the host-guest chemistry in the last decade. The architecture of
the calix[4]arenes can provide a π-basic cavity. Hence, they
are useful for designing a globular π-basic cavity by connect-
ing the upper rim of two calix[4]arenes.1-5 We previously
demonstrated that conformational isomers derived from
calix[4]arenes are very useful for estimating the contribution of
the cation-π interaction6 to the host-guest process,7,8 because
one can easily prepare conformationally-immobilized π-basic
cavities composed of four differently pre-organized benzene
rings. We have found that a significant contribution of the
cation-π interaction is observed for the cone isomer providing a
bowl-shaped cavity.7 To establish the cooperative action of the
two π-basic cavities in the calix[4]arenes, several
biscalix[4]arenes were synthesized by connecting the upper
rim. While the binding ability of the biscalix[4]arenes to
guests was enhanced, the selectivity was not very high due to
the conformational freedom in the linkage.4,9 It prompted us to
synthesize the conformationally-immobile biscalix[4]arene
which is connected at the upper rims and should provide a π-
basic sphere composed of two confronted hemispherical cavi-
ties. We now report the synthesis and the inclusion properties
of biscalix[4]arene 1.
The inclusion abilities of 1 for N-methylpyridinium and
homologous cationic guest molecules were examined using a
1
500 MHz H NMR. The protons in 4 - 6 gave the averaged
NMR peaks of the free and complexed guests at 25 °C. These
peaks did not split even at -35 °C. This fact implies that the
dynamic process on complexation and decomplexation is faster
than the NMR time scale. The NMR peaks for 7, on the other
hand, were significantly broadened at 25 °C and completely
split into the free and complexed peaks at -35 °C. This suggests
that 7 is suitable for the inner sphere in 1 and the dynamic
process of complexation and decomplexation is sterically sup-
pressed.
As summarized in Table 1, the chemical shifts for the
NCH3 in 4 - 7 were scarcely shifted to a higher magnetic field
in the presence of 3. The similar up-field shifts were also
observed for other protons in 4 - 7. These up-field shifts caused
by the shielding effect of the benzene rings in 3 imply that the
cationic guest is included in the cavity of 3. This result indi-
cates that the cation-π interaction11 between 3 and the cationic
guests is considerably weak although 4 - 7 are bound to the π-
basic cavity in 3. In the presence of 1, in contrast, the chemical
shifts for NCH3 in 4 - 7 were largely moved to a higher mag-
netic field. The remarkably large shift was observed for N-
methylquinolinium 7, which was much greater than that for the
pyridinium derivatives 4 - 6. This result suggests that the inner
sphere in 1 is too large to be filled up by 4 - 6 but 7 is suitable
The target biscalix[4]arene 1 was synthesized from cone-
25,27-dihydroxy-26,28-dipropoxycalix[4]arene10 2a according
to Scheme 1. The products were identified by IR, H NMR,
1
Copyright © 2000 The Chemical Society of Japan