Sugar-integrated gelators of organic fluids: On their versatility as building-blocks and diversity in superstructures

Article abstract of DOI:10.1039/a800825f

Three 1-O-methyl-4, 6-O-benzylidene derivatives of monosaccharides (D-glucose, D-galactose and D-mannose) were synthesised: they acted as versatile gelators of various organic fluids, indicating that saccharides are useful as potential building-blocks for

Full text of DOI:10.1039/a800825f

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Sugar-integrated gelators of organic fluids: on their versatility as
building-blocks and diversity in superstructures
a
a
a
b
c
Kenji Yoza, Yoshiyuki Ono, Kanami Yoshihara, Tetsuyuki Akao, Hideyuki Shinmori, Masayuki
c
c
d
Takeuchi, Seiji Shinkai* † and David N. Reinhoudt
a
Chemotransfiguration Project, Japan Science and Technology Corporation, Aikawa, Kurume, Fukuoka 839, Japan
Biotechnology and Food Research Institute, Fukuoka Industrial Technology Center, Aikawa, Kurume, Fukuoka 839, Japan
Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, Hakozaki, Higashi-ku,
b
c
Fukuoka, 812-81, Japan
d
Chemotransfiguration Project, Faculty of Chemical Technology, University of Twente, 7500 AE Enschede, The Netherlands
Three 1-O-methyl-4, 6-O-benzylidene derivatives of mono-
saccharides ( -glucose, -galactose and -mannose) were
synthesised: they acted as versatile gelators of various
organic fluids, indicating that saccharides are useful as
potential building-blocks for molecular design of chiral
gelators.
in Table 1 denotes that a gel was formed at this stage). Some
solutions gelatinised at a gelator concentration below 1.0 wt%
(SG or super-gelator; Table 1). Solvents in Table 1 are those
which were gelatinised by either 1, 2 or 3.14 Table 1 reveals that,
in general, 2 acts as an excellent gelator of many organic fluids.
In contrast, comparison of the gelation ability for n-heptane and
cyclohexane reveals that 1 is more cohesive and tends to form
a precipitate, whereas 3 is more soluble than the other two
gelators and is frequently unable to coagulate in solution.
What is the origin of the difference in their gelation ability?
The sole structural difference among 1, 2 and 3 is the absolute
configuration of C-2 and C-4. It is reasonable to assume that 1,
2 and 3 show a different ability to form a gel network because
of the different intermolecular hydrogen-bonding interactions.
In the FT-IR spectra, the gel solutions gave two peaks
D
D
D
The development of new gelators of organic fluids has recently
received much attention. They not only gelatinise various
organic fluids but also create novel networks with fibrous
superstructures which can be characterised by SEM pictures of
the xerogels.1
–11
The gelators can be classified into two
categories according to the difference in the driving force for the
molecular aggregation, viz. hydrogen-bond-based gelators and
nonhydrogen-bond-based gelators. Typical examples of the
former group are the aliphatic amide derivatives,1–4 whereas
those of the latter group are cholesterol derivatives.6 The
superstructures of the organic gels of aliphatic amide deriva-
tives show that they satisfy the complementarity of the
intermolecular hydrogen-bonding interactions.5 This obser-
vation stimulated us to use saccharides as a hydrogen-bond-
forming segment in the gelators, because one can easily
introduce a variety of hydrogen-bond-forming, chiral segments
into gelators by selection from a saccharide library. In the
literature examples of saccharide-containing gelators are very
limited in spite of their high potential.8 We synthesised
glucose-based 1, galactose-based 2 and mannose-based 3 and
studied their gelation abilities. We found that the gelation
properties, such as gel stability, superstructure and solvent-
dependence, were profoundly related to the saccharide struc-
ture.
21
(3473–3240 and 3576–3659 cm ) in the nOH region which
–9
could be assigned to hydrogen-bonded OH and free OH groups,
respectively. As shown in Fig. 1, the peak intensity ratio (R) of
hydrogen-bonded OH vs. free OH abruptly increased at the sol-
gel phase-transition concentration. Furthermore, the largest R
value was observed for 2, which also features the greatest
gelation ability. In contrast, the peak arising from hydrogen-
bonded OH groups was hardly observed for 3 up to 0.5 wt%,
indicating that 3 is too soluble to use as a good gelator. These
FT-IR spectral data consistently indicate that the gel network in
the present system is primarily constructed by intermolecular
–9
,12
hydrogen-bonding interactions.
A
computational study
(MOPAC 93 included in CS CHEM3D) indicated that the
Table 1 Organic fluids tested for gelation by 1–3a
Organic fluid
1
2
3
n-Hexane
n-Heptane
n-Octane
Cyclohexane
Methylcyclohexane
Benzene
SG
P
SG
P
SG
G
SG
SG
SG
P
SG
SG
SG
SG
SG
SG
SG
SG
SG
SG
SG
SG
G
SG
SG
SG
SG
SG
P
SG
SG
P
O
O
O
O
HO
O
O
O
O
O
HO
HO
HO
HO
HO
O
O
O
Toluene
p-Xylene
1
2
3
CCl
CS
Et O
4
2
SG
P
G
The synthesis of 1 has already been reported.13 Gelators 2 and
were synthesised in a similar manner by treatment of the
G
SG
S
2
Ph
2
O
3
n-Octanol
S
2
saccharide with benzaldehyde and ZnCl . The products were
Et
Et
3
3
N
SiH
S
P
P
P
G
S
SG
P
identified by 1H NMR and IR spectroscopy and elemental
analyses.
Gp
SG
S
(EtO)
4
Si
The gelation test was carried out as follows: the gelator (1–3:
.0 mg) was mixed with solvent (0.10 ml) in a septum-capped
H
2
O
G
3
test tube and the mixture was heated until the solid dissolved.
The solution was cooled to room temperature and left for 1 h (G
a
G
=
gel; SG
S = solution.
=
supergel; Gp
=
partial gel; P
=
precipitation;
Chem. Commun., 1998
907

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7
6
5
4
3
2
1
0
intramolecular hydrogen-bonding distance (H···O) between
3
-OH and 4-OR of the saccharides is not significantly different
(e.g. 2.5 Å for 1 and 2.38 Å for 2). This suggests that the
differences in gelation ability are not due to the presence of a
special OH group useful for intermolecular hydrogen-bonding,
but rather is due to a structural preference in the molecular
aggregation. We now consider that 2, which has the benzylidene
group and the pyranose ring arranged at almost a right angle, is
able to enjoy both a benzene–benzene p–p stacking interaction
and an OH–OH intermolecular hydrogen bonding interaction,
whereas 1 and 3, which are flatter than 2, cannot enjoy both
interactions simultaneously.
To obtain visual insights into the aggregation mode, we
prepared a dry sample for SEM studies.15 Fig. 2(a) shows a
typical picture obtained from the xerogels of 1 or 2. It is clear
that the gelator forms a three-dimensional network with 50–200
nm frizzled fibrils. On the other hand, the fibrils obtained from
the toluene gel of 3 are more linear (the picture is not shown
here). Very interestingly, the fibrils obtained from the aqueous
gel of 3 show a regular left-handed helical structure [Fig. 2(b)].
Presumably, in an aqueous system the balance between the
hydrogen-bonding interaction and the hydrophobic interaction
is different from that in other, organic media.
(
iii)
i)
R
(
(
ii)
0
.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
[
Gelator] (wt%)
Fig. 1 Plot of the peak intensity ratio (R) of hydrogen-bonded OH groups vs.
free OH groups as a function of [gelator] in toluene at 20 °C. There are two
major peaks for hydrogen-bonded OH groups: the filled points were
obtained from the intensities at nOH 3312–3240 cm2 , whereas the open
points are obtained from those at nOH 3473–3350 cm2 ; (2,5) 1, (8,-) 2
and (.,/) 3. The arrows indicate the sol-gel transition temperatures of (i)
1
In conclusion, the present study has demonstrated for the first
time that saccharides are promising building-blocks for new
gelators with different gelation abilities and different three-
dimensional network structures. We believe that such versatility
of synthesis and diversity of products (including the creation of
the helical structure) cannot be attained in a more simple fashion
than with saccharides as building-blocks.
1
1
, (ii), 2 and (iii) 3.
Notes and References
†
E-mail: seijitcm@mbox.nc.kyushu-u.ac.jp
1
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3
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5
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8
9
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1
1
0 T. Brotin, R. Uterm o¨ hlen, F. Fagles, H. Bouas-Laurent and J.-P.
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1
1
3 M. Svaan and T. Anthonsen, Acta Chem. Scand., 1986, B40, 119.
4 The following solvents were also tested, but 1–3 were soluble in these
solvents and gels were not formed: nitrobenzene, m-cresol, 1,2-di-
chloroethane, CH
onate, acetone, ethyl methyl ketone, DMA, DMF, DMSO, NMP,
MeCN, MeOH, EtOH, BnOH, AcOH, Ac O, PrNH , Et NH, PhNH
pyridine, 2,2,2-trifluoroethanol and glycerol.
2 2 3
Cl , CHCl , THF, dioxane, MeOAc, diethyl mal-
2
2
2
2
,
1
5 For the preparation of dry samples for SEM observations, see ref. 7 and
S. W. Jeong and S. Shinkai, Nanotechnology, 1997, 8, 179.
Fig. 2 SEM pictures of (a) the 1/CCl
4
system and (b) the 3/water system
Received in Cambridge, UK, 30th January 1998; 8/00825F
908
Chem. Commun., 1998