(rectangular). According to the lattice parameters and the
molecular models, the number of acid–base pairs included in the
layer of a cylinder was estimated; four pairs for the hexagonal
CLC structure and three or four pairs for the rectangular CLC
structure.§ However, by introducing a bulky substituent at the
C-2 position of the 2-amino alcohols, formation of a CLC
structure was significantly inhibited, as no liquid crystalline
phase was observed for the salts with (2)-6, (+)-9, and
(2)-10.
Quite interestingly, the cross-linked material could be
applied as “size- and shape-selective molecular sieves” for
2-amino alcohols. As described above, (2)-7 having a quite
similar structure to the initial guest (2)-8 could efficiently
substitute (2)-8 (Fig. 2a). Contrary to this, when substitution of
(2)-8 with a bulky guest, (2)-10, was attempted, only 27%
exchange was occurred under the same conditions (Fig. 2c). In
the case of (+)-9, which possesses a similar size but a different
shape compared with (2)-8, efficient exchange was again
suppressed (28%, Fig. 2b). Thus observed guest selectivity
motivated us to apply the cross-linked material for the
separation of 2-amino alcohols. For example, from an equimo-
lar mixture of (2)-7 and (+)-9 in methanol, (2)-7 was
predominantly captured by the cross-linked solid, where the
ratios of finally incorporated (2)-7 and (+)-9 were 33 and 9%,
respectively (Fig. 2d).
Since optically active 2-amino alcohols can be used as
templates in the present system, the resultant cross-linked
materials have a potential for the enantioseparation of amines.
As a preliminary study, we performed the resolution of racemic
7 through the guest-exchange reaction of the cross-linked
2·(2)-8. Typically, the cross-linked solid was placed in a
solution of racemic 7 (25 mM) until equiriblium was reached
(59 % exchange), then the supernatant and the precipitate was
separated by centrifugation, and the precipitate was extensively
extracted with aqueous HCl. The supernatant and the collected
polymer extract were analyzed by HPLC in order to estimate the
optical purity of 7. The supernatant and the polymer extract
were (+)- and (2)-enriched (2 and 7% ee) respectively, which is
consistent with our expectation that (2)-7, of which the
stereochemistry at C-1 is the same as that of the initial template
(2)-8, should be preferentially incorporated.
When the amino or hydroxy group of the 2-amino alcohol
was methylated, the resultant CLC phase became unstable; the
salts with 11 and 12 showed a CLC phase at a quite narrow
range of temperature compared with the salt with 3. In addition,
the salts with simple primary amines, (2)-13 and 14, did not
display any LC phase at all (Fig. 1). These observations indicate
that hydrogen-bonding interactions between the carboxyl
groups with the amino and hydroxy groups play an essential role
for the stable CLC phase formation.
When polymerizable acid unit 2 was used in the place of 1,
the salts with 2-amino alcohols showed similar phase profiles to
those of 1, and CLC phases were again observed for the salts
with (2)-4, (+)-5, (2)-7, and (+)-8 (Fig. 1).†‡ In the presence
of a radical photoinitiator, the salt of 2 with (+)-8 (2·(+)-8) could
be photopolymerized to give a cross-linked solid. For example,
when a mixture of 2·(+)-8 and 2-hydroxy-2-methylpropiophe-
none (2.5 wt.%), which showed a rectangular phase at a
temperature range of 215 to 60 °C, was irradiated (l > 300 nm,
a high-pressure mercury lamp) at 16 °C for 48 h, a solid mass
insoluble in common organic solvents was obtained in quantita-
tive yield. Consumption of the acryloyl groups of 2 was
confirmed by IR spectroscopy; the absorption attributed to the
a,b-unsaturated carbonyl groups was significantly diminished
after photoirradiation. The XRD analysis clearly showed that
the cross-linked solid maintained the rectangular columnar
structure, where the lattice constants slightly diminished after
polymerization.†§
Taking advantage of the non-covalent nature of the inter-
actions between the matrix unit and the template unit, 8
incorporated in the cross-linked material could be exchanged
with other 2-amino alcohols.† For example, the solid obtained
by the cross-linking of 2·(2)-8 was milled and soaked in a
solution of (2)-7 in methanol (25 mM). Upon standing the
mixture at 4 °C, the original guest (2)-8 was released into the
supernatant, whereas (2)-7 was adsorbed by the solid, which
was monitored by HPLC. When the exchange reaction was
attained at equilibrium (after 440 h), 57% of (2)-7, with respect
to the original guest, was incorporated to the solid (Fig. 2a). The
XRD analysis indicated that the rectangular columnar structure
was retained even after guest exchange, although the intensity
of the peaks became smaller presumably due to partial disorder
of the original alignment.
In conclusion, we found that the salts of trialkoxybenzoic
acids and 2-amino alcohols generally show a CLC phase. When
a polymerizable acid unit, 2, was used, the resultant columnar
structure could be fixed by an in situ cross-linking reaction,
where the ordered structure was essentially maintained. Since
various carboxylic acids and 2-amino alcohols are available,
porous materials with well-controlled size, shape, and func-
tional-groups could be created by careful choice of these
units.
We acknowledge Dr T. Shimizu and Dr H. Minamikawa
(AIST) for XRD measurements. We would also like to thank
Prof. T. Kato, Prof. Y. Ueda and Dr H. Kageyama (The
University of Tokyo) for the use of the DSC equipment. Part of
this work was financially supported by the Ogasawara Science
and Technology Fund.
Notes and references
‡ The salts were prepared by dissolving equimolar amounts of the
carboxylic acid and the amine in chloroform, followed by the evaporation of
the solvent. The 1 : 1 stoichiometry was confirmed by 1H NMR
spectroscopy.
§ The lattice constants determined by XRD patterns.† 1·3 (hexagonal): a =
38.5 Å; 1·(+)-8 (ordered rectangular): a = 44.3, b = 25.2 Å; 2·(+)-5
(ordered hexagonal): a = 42.2 Å; 2·(+)-8 (ordered rectangular): a = 45.7,
b = 26.9 Å; the cross-linked 2·(+)-8 (rectangular): a = 44.6, b = 26.6
Å.
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Fig. 2 The exchange reaction of the amine units of cross-linked 2·(2)-8 in
methanol at 4 °C. The ratio of the adsorbed 2-amino alcohols ((2)-7 [5],
(+)-9 [Ω], and (2)-10 [-]) with respect to the original guest (2)-8. Initial
conditions: (a) [(2)-7]0 = 25 mM; (b) [(+)-9]0 = 25 mM; (c) [(2)-10]0
25 mM; (d) [(2)-7]0 = [(+)-9]0 = 12.5 mM.
=
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CHEM. COMMUN., 2003, 2338–2339
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Ishida, Yasuhiro
