Biphenyl Bis(amino alcohol) Oxalamide Gelators: Complex Gelation Involving Coupled Equilibria, Central-to-Axial Chirality Transfer, Diastereoisomer Interconversion, and Self-Sorting

Article abstract of DOI:10.1002/ejoc.201501261

Chiral gelators 3 and 4, with two valinol- or leucinol-oxalamido arms attached to the 2,2′-positions of the proatropisomeric biphenyl group, were prepared, and their gels were studied. Compound (R,R)-3 in the solution and gel states forms a mixture of major [(R,aR,R)-3] and minor [(R,aS,R)-3] diastereomers due to central-to-axial chirality transfer. ¹H NMR studies of its toluene gel provide evidence of diastereomer interconversion and self-sorting, which results in exclusive incorporation of (R,aR,R)-3 into the gel network. Gels formed in the 10^-3 M concentration range show an irregular T_g/concentration dependence, which is in contrast to those formed in the 10^-2 M concentration range. The peculiar properties of the former gels may be explained by kinetic effects due to the presence of coupled equilibria comprising diastereomer interconversion and (R,aR,R)-3 self-assembly where the rate of gelation becomes dependent on the rate of formation of the gelling (R,aR,R)-3 from the nongelling (R,aS,R)-3.

Full text of DOI:10.1002/ejoc.201501261

DOI: 10.1002/ejoc.201501261
Full Paper
Diastereoselective Self-Sorting
Biphenyl Bis(amino alcohol) Oxalamide Gelators: Complex
Gelation Involving Coupled Equilibria, Central-to-Axial Chirality
Transfer, Diastereoisomer Interconversion, and Self-Sorting
Tomislav Portada,[a] Krešimir Molčanov,[b] Nataša Šijaković Vujičić,[a] and Mladen Žinić*[c]
Abstract: Chiral gelators 3 and 4, with two valinol- or leucinol- 10–3
M
concentration range show an irregular Tg/concentration
dependence, which is in contrast to those formed in the 10–2
M
oxalamido arms attached to the 2,2′-positions of the proatrop-
isomeric biphenyl group, were prepared, and their gels were
studied. Compound (R,R)-3 in the solution and gel states forms
a mixture of major [(R,aR,R)-3] and minor [(R,aS,R)-3] dia-
stereomers due to central-to-axial chirality transfer. 1H NMR
studies of its toluene gel provide evidence of diastereomer in-
terconversion and self-sorting, which results in exclusive incor-
poration of (R,aR,R)-3 into the gel network. Gels formed in the
concentration range. The peculiar properties of the former gels
may be explained by kinetic effects due to the presence of
coupled equilibria comprising diastereomer interconversion
and (R,aR,R)-3 self-assembly where the rate of gelation becomes
dependent on the rate of formation of the gelling (R,aR,R)-3
from the nongelling (R,aS,R)-3.
Introduction
aggregates following a multistep hierarchical pathway.[2d,2e] The
majority of chiral gelators studied to date have central chirality;
axially chiral gelators are very rare.[5,6] Recently, we reported the
design and properties of chiral gelators combining central and
axial chirality in the same molecule. Such gelators contain a
conformationally flexible proatropisomeric 2,2′-dioxybiphenyl
unit and amino acid chiral centres located in the 2,2′-biphenyl
arms (Figure 1, I).[7] We have shown that in contrast to related
molecular systems, here an efficient central-to-axial chirality
transfer occurs through the intermolecular interactions in the
gelator assemblies.[8] Such intermolecularly induced axial chiral-
ity, being the consequence of a specific gelator self-assembly
motif, may be denoted as transient since it disappears and reap-
pears during the thermoreversible sol-to-gel transitions. This
may be of interest for the design of dynamic chirality-controlled
supramolecular systems and devices. To further explore the po-
tential of such systems, we have prepared a series of new gelat-
The supramolecular chemistry of low-molecular-weight organo-
and hydrogelators is a highly active field of research. Gels
formed by the self-assembly of small organic molecules are of
wide interest as dynamic soft materials with numerous possible
applications, especially in biomedicine and nanotechnology.
Such gels are used as cell scaffolds or supports for functional
and responsive biomaterials, biosensors, and nanowires.[1] On
the other hand, they also represent an excellent experimental
system that enables in depth studies of self-assembly, a general
phenomenon of the highest importance in biology and con-
temporary materials science.[2] Chiral information encoded in
the structure of gelators has been found to have a profound
influence on both their self-assembly and gelation properties.[3]
For example, the racemic forms are generally weaker gelators
than the corresponding pure enantiomers, although exceptions
are known.[4] Importantly, it has been shown that in gels, as well
as in other kinds of supramolecular polymers, efficient chirality
transfer from the molecular to the supramolecular level may
occur through the self-assembly of chiral monomers into helical
[a] Department of Organic chemistry and Biochemistry,
Ruđer Bošković Institute,
Bijenička 54, 10002 Zagreb, Croatia
www.irb.hr
[b] Laboratory for Chemical and Biological Crystallography,
Ruđer Bošković Institute,
Bijenička 54, 10002 Zagreb, Croatia
[c] Croatian Academy of Sciences and Arts,
Zrinski trg 11, 10000 Zagreb, Croatia
E-mail: zinic@irb.hr
Supporting information and ORCID(s) from the author(s) for this article
are available on the WWW under http://dx.doi.org/10.1002/
ejoc.201501261.
Figure 1. Structures of proatropisomeric 2,2′-biphenyl-containing di(amino
acid oxalamide) I and di(amino alcohol oxalamide) II gelators.
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ors of type II, which differ from I by their constitution, the rigid- of rac- and meso-3, which were separated by column chroma-
ity of their 2,2′-biphenyl arms, and the shorter distance from tography. Detailed synthetic procedures and structural charac-
the chiral centre to the proatropisomeric biphenyl unit.
terization data are provided in the Exp. Sect. provided as Sup-
porting Information.
In this paper, we report the synthesis and gelling properties
of selected type II gelators, and characterization the of gel and
solid-state assemblies formed by these gelators by FTIR and
NMR spectroscopy, circular dichroism (CD), differential scanning
calorimetry (DSC), X-ray crystallography, and transmission elec-
tron microscopy (TEM). These studies reveal their dramatically
different properties compared to type I gelators.[7] We provide
the evidence that in lipophilic solvents as well as in gels, cen-
trally chiral gelator (R,R)-3 forms a mixture of major [(R,aR,R)-3]
and minor [(R,aS,R)-3] diastereomers due to induced axial (aR
and aS) chirality in the biphenyl unit, and that by diastereo-
selective self-sorting only the major diastereomer is self-assem-
bled into the gel network. We also present evidence that at
Gelling Properties and TEM Studies
Optically active valinol and leucinol oxalamides (R,R)-3 and
(R,R)-4, respectively, showed moderate gelation ability towards
toluene, o-xylene, m-xylene, tetraline, and decalin, while with
the other tested solvents of various polarities the gelation
failed. The two gelators showed different gelation efficiencies
towards very similar solvents such as o-, m-, and p-xylene. o-
Xylene is the best at forming gels; the m and p isomers form
gels much less efficiently, or crystallization was observed. In
highly polar solvents such as MeOH, EtOH, acetone, and DMSO,
the compounds are soluble. They are also soluble in solvents of
low or medium polarity such as dichloromethane, 2-octanol,
THF, and dioxane. The gelators are insoluble in water, cyclohex-
ane, and diethyl ether.
The gelation testing was carried out by consecutive addi-
tions of small measured volumes of the tested solvent, followed
by heating and cooling of the sample until a weak gel was
formed. We observed that some dilute samples showed rather
slow gelation rates that needed 10–24 h of ageing at room
temperature.
lower (10–3
M
) concentrations of (R,R)-3 in toluene, long-lived
nonequilibrium aggregate systems are formed that need pro-
longed ageing (up to 216 h at +4 °C) to transform into gels. In
contrast, (R,R)-3 at 10–2
M
concentrations forms stable toluene
gels that form instantaneously by the cooling of hot samples.
The formation of nonequilibrium systems in the 10–3
concen-
M
tration range is explained by kinetic effects under coupled equi-
libria conditions comprising diastereomer interconversion and
(R,aR,R)-3 network-assembly/disassembly where the overall
gelation rate becomes determined by slow formation of the
gelling (R,aR,R)-3 from the nongelling (R,aS,R)-3 diastereomer.
It is well documented that optically pure gelators are more
efficient that the corresponding racemates, which tend to crys-
tallize; however, exceptions to this rule are known.[10] Com-
pound rac-3 showed only very slow gelation of small volumes
of toluene and o-xylene, and almost instantaneous gelation of
tetraline. Compound meso-3, as has been reported for other
types of gelators with internal symmetry, was found to lack any
gelation ability, and in most cases tended to crystallize.
TEM investigation of the (R,R)-3 toluene gel and the unstable
Results and Discussion
Synthesis
Hydrogenation of 2,2′-dinitrobiphenyl in ethanol using a Pd/C
catalyst gave 2,2′-diaminobiphenyl in quantitative yield
(Scheme 1).[9] Reaction of diaminobiphenyl with ethyl chlorofor- partial gel of rac-3 revealed clear differences between the two
mate in dichloromethane in the presence of triethylamine gave systems (Figure 2, a and b). In the first gel, very long fibres
1 (85 % yield). Diester 1 was treated with selected amino alco- showing P helicity and with diameters in the 10 nm range were
hols in dichloromethane to give biphenyldioxalamide deriva- observed. This shows that molecular chirality is successfully
tives 3 and 4. The reaction of 1 with rac-valinol gave a mixture transferred into supramolecular chirality of the toluene gel ag-
Scheme 1. Synthesis of biphenyldioxalamide derivatives 3 and 4, yields are given in parentheses.
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gregates. In contrast, the image of the unstable rac-3 toluene
gel (Figure 2, b) shows the presence of short fibre bundles and
crystals.
Figure 2. TEM images of (a) (R,R)-3, and (b) rac-3 toluene gels. In (a), very tiny
(d = 10 nm) and long fibres with P helicity can be observed. In (b), short fibre
bundles are present together with crystals.
Figure 4. Hydrogen-bonded chains in the crystal packing of rac-3 extend in
the direction [101]. Symmetry operators i: –x, y, 3/2 – z; ii: 1/2 – x, 1/2 – y,
2 – z.
CD Investigation
X-ray Molecular and Crystal Structure of rac-3
As shown in Table 1, both, (R,R)-3 and (R,R)-4 are capable of
Crystals of rac-3 suitable for X-ray analysis were obtained from
[D8]toluene. The molecule of rac-3 is C2-symmetric, with the
midpoint of the C-1–C-1i (symmetry operator i: –x, y, 3/2 – z)
bond located on the crystallographic twofold axis. Therefore,
the two stereogenic centres (atoms C-9 and C-9i) are related by
a rotation of 180° (Figure 3, a). The axially chiral conformation
of the molecule is stabilized by a pair of intramolecular
hydrogen bonds between the N-2 amine nitrogen and the
hydroxyl group (Figure 3, b and Table S1, Supporting Informa-
tion).
gelling decalin at minimal gelation concentrations (mgc) of
1.8 × 10–2 and 1.54 × 10–2
M, respectively. The electronic ab-
sorption and circular dichroism (CD) spectra of enantiomeric
gelators (R,R)-3 and (S,S)-3 were recorded in decalin at
4.07 × 10–3
M concentrations, which is below the minimal gela-
tion concentration (mgc) (Figure 5, a and b). The CD spectra of
the enantiomers have a mirror image relationship, with bands
at 291, 261, and 231 nm. In the electronic absorption spectra,
the biphenyl absorptions appear at 271 nm, the shoulder at
around 240 nm, and a weak band at 212 nm (Figure 5, a). The
absorption at 240 nm could be ascribed to the biphenyl A band
appearing between 240 and 250 nm.[11] It is well documented
that the sign of the Cotton effect corresponding to the biphenyl
A band is related to the twist of the biphenyl system: a positive
Cotton effect corresponds to an M-twist, and a negative Cotton
effect to a P-twist of the biphenyl system.[12,13] Also, a recent
experimental and theoretical study of chiral 2,2′-(2-butoxy)bi-
phenyls showed the appearance of a positive-negative-positive
sequence of Cotton effects at around 280, 240, and 210 nm,
respectively, for the P (aS) biphenyl atropisomer, and the oppo-
site sequence for the M (aR) isomer.[14] In the CD spectra of
(R,R)-3 and (S,S)-3, the corresponding biphenyl bands are
slightly redshifted due to 2,2′-carboxamide instead of 2,2′-but-
oxy biphenyl substitution and partial aggregation (vide infra).
Nevertheless, the appearance of a negative-positive-negative
sequence of Cotton effects in the CD spectrum of (R,R)-3, and
the opposite sequence in the spectrum of (S,S)-3, suggests an
Figure 3. View of the (R,aR,R) enantiomer of rac-3 showing: (a) C2 symmetry
(position of the crystallographic twofold axis is indicated by a black ellipse),
and (b) conformation of the molecule stabilized by a pair of symmetry-related
intramolecular hydrogen bonds. Symmetry operator i: –x, y, 3/2 – z.
The molecules link to their inversion-related neighbours by a M-twist of the biphenyl in the former and a P-twist in the latter,
pair of inversion-related hydrogen bonds between the hydroxyl corresponding to their (R,aR,R) and (S,aS,S) configurations, re-
group and the O-2 carbonyl oxygen, thus forming hydrogen- spectively. As described in the next section, NMR spectroscopic
bonded chains running in the direction [101] (Figure 4, Table S1 studies of (R,R)-3 in [D8]toluene and CDCl3, at room temperature
in Supporting Information). The chains consist of alternating showed that an equlibrium mixture of (R,aR,R)-3 and (R,aS,R)-3
(R,aR,R) and (S,aS,S) enantiomers.
diastereomers in a ratio close to 5:1 is present in those solvents.
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Table 1. Gelation efficiency of (R,R)-, rac-, and meso-3 and (R,R)-4 towards
selected solvents expressed as the maximum solvent volume (Vmax [mL])
gelled by 10 mg of gelator.[a]
in 3 induce axial (aR) chirality in the biphenyl fragment, and
the (S) centres induce (aS) chirality.
The concentration-dependent CD spectra of (R,R)-3 in deca-
lin are shown in Figure 5 (c). At the highest concentration of
Solvent
(R,R)-3
rac-3
meso-3
(R,R)-4
the gel sample (2.3 × 10–2
M), a strong negative Cotton effect
CH2Cl2
sol.
ins.
sol.
sol.
sol.
ins.
sol.
sol.
sol.
ins.
sol.
ins.
sol.
ins.
sol.
sol.
Cyclohexane
Acetone
MeOH
appears at 303 nm; its intensity decreases with a decrease of
the concentration, followed by a blueshift from 303 nm to
295 nm at 1.1 × 10–3
M
concentration, the latter corresponding
to a weak gel (Figure 5, c, red curve), and to 291 nm at
5.7 × 10–3
(sol, green curve). Further decrease of the concen-
EtOH
sol.
Water
DMSO
ins.
sol.
1.8
2.0
0.7
cryst.
1.4
1.1*
sol.
ins.
ins.
sol.
ins.
sol.
ins.
sol.
0.5**
2.3
1.1**
0.7**
0.7**
1.3*
sol.
M
Toluene
o-Xylene
m-Xylene
p-Xylene
Tetraline
Decalin
Octan-2-ol
Diethyl ether
iPr2O
1.4**
1.6**
cryst.
cryst.
1.0
cryst.
sol.
ins.
cryst.
cryst.
cryst.
cryst.
cryst.
cryst.
cryst.
ins.
tration does not affect the position of the maximum at 291 nm.
At the same time, the positive Cotton effect at 261 nm and the
negative one at 231 nm have their lowest intensities at the
highest concentration in the gel state, and both increase in
intensity for lower gelator concentrations. The CD spectra in the
gel state are dominated by the presence of highly aggregated
gelator molecules, which is reflected in the appearance of a
strong redshifted CD band at 303 nm. A circular differential
scattering effect may also contribute to the intensity of the lat-
ter band under high aggregation conditions. Such an effect is
known to appear in the CD spectra of macromolecules with
sizes larger than λ/20 nm, and it has also been observed in
the CD spectra of gels formed by other types of oxalamide
gelators.[15,7] In such cases, the negative differential scattering
band may diminish the intensity of the positive Cotton band at
261 nm, and enhance the intensity of the negative 291 nm
band. The observation that decreasing the gelator concentra-
ins.
Ins.
THF
Dioxane
sol.
sol.
sol.
sol.
sol.
sol.
sol.
sol.
[a] Gelation tests were carried out in test tubes. Abbrevations: sol.: soluble;
ins.: insoluble; cryst.: crystallization; *: crystallization by ageing; **: very slow
gelation that needs 10–24 h at room temp.
tion to 5.7 × 10–3
M to give a much less aggregated sol phase
(green curve in Figure 5, c) results in a strong decrease in inten-
sity and a blueshift of the 303 nm band, and an increase in
intensity of the 261 nm band reinforces the above conclusion.
For this concentration and also lower gelator concentrations,
the expected bisignate CD spectra originating from coupled bi-
phenyl excitons were observed.
The changes in the temperature-dependent CD spectra of
(R,R)-3 (Figure 5, d) taken at a sample concentration below the
mgc (c = 2.62 × 10–3 ; mgc = 1.8 × 10–2
M M), and hence contain-
ing less of the aggregated gelator, should also reflect the varia-
tions in the (R,aR,R)-3/(R,aS,R)-3 equilibrium at different temper-
atures. Increasing the temperature from 20 to 100 °C induces a
steady decrease in intensity of both the negative 291 and the
positive 263 nm maxima, together with their slight blueshifts
to 289.8 and 256 nm, respectively. The observed changes could
be explained by the combined effects of disaggregation and
decrease of the (R,aR,R)-3/(R,aS,R)-3 ratio due to the intercon-
version of the more stable former diastereomer into the less
stable latter diastereomer with the opposite (aS)-biphenyl con-
figuration. The fact that even at 100 °C, where the degree of
aggregation should be much smaller, a strong CD spectrum
could still be observed reinforces the conclusion about its origin
from the intrinsic chirality of the dissolved diastereomers.
Figure 5. (a) Electronic absorption and (b) mirror image CD spectra of (R,R)-3
and (S,S)-3, dissolved in decalin (c = 4.07 × 10–3
M
): (R,aR,R)-3 (black curve)
and (S,aS,S)-3 (red curve); (c) (R,R)-3 concentration-dependent CD spectra in
decalin: c = 2.3 × 10–2 , black; 1.1 × 10–2 , red; 5.7 × 10–3
, green;
2.5 × 10–3 , blue; 1.1 × 10–3 , cyan; 3.7 × 10–3
, magenta; and (d) (R,R)-3
temperature-dependent CD spectra in decalin (c = 4.17 × 10–3
; temp. range
M
M
M
M
M
M
M
20 °C, black, to 100 °C, dark blue) showing decrease of signal intensities as
indicated by the arrows, followed by slight blueshifts of the maxima.
Hence, also in the lipophilic decalin, the observed CD spectrum
of (R,R)-3 may in fact correspond to a mixture of the predomina-
ting (R,aR,R)-3 and the less abundant (R,aS,R)-3 diastereomer
[and similarly for (S,S)-3, a mixture of the corresponding (S,aS,S)
and (S,aR,S) diastereomers], possibly in a ratio higher than that
observed in [D8]toluene. The CD-determined axial chirality of
the biphenyls induced by the (R) and (S) centres of 3 is consist-
FTIR and NMR Spectroscopic Studies
ent with the (aR)- and (aS)-biphenyl configurations of the An FTIR spectroscopic study of the (R,R)-3 toluene gel revealed
enantiomers found in the crystal structure of rac-3 (Figure 3). the presence of bands at 3425, 3323, and 3274 cm–1, corre-
Hence, in solution as well as in the solid state, the (R) centres sponding to hydrogen-bonded and free OH and NH vibrations.
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In the amide I region, bands were observed at 1682 and
1650 cm–1, corresponding to free and hydrogen-bonded amide
groups, respectively. The temperature-dependent FTIR spectra
(Figure S1, Supporting Information) show that increasing the
sample temperature results in a decrease in intensity of the
3323, 3274, and 1650 cm–1 bands, and an increase in intensity
of the 3425 and 1682 cm–1 bands, as a result of the breakage
of hydrogen bonds. In the amide I region, a new strong band
at 1691 cm–1 appears at the highest sample temperature. These
observations indicate that both OH and amide groups partici-
pate in hydrogen bonding in the aggregates.
1H NMR spectra of (R,R)-3 were recorded in [D8]toluene as a
gelling solvent, and in CDCl3 and [D6]DMSO representing non-
gelling solvents. The spectra in the lipophilic [D8]toluene [vis-
cous solution; concentration of (R,R)-3 slightly below mgc] and
CDCl3 are shown in Figure 6 (a and b, respectively). The [D8]tolu-
ene spectrum shows two singlets corresponding to the bi-
phenyl NH (δ = 9.53 and 9.68 ppm; integral ratio 5:1), and two
doublets corresponding to the C*-NH protons (δ = 8.56 and
7.69 ppm; integral ratio 5:1). In the CDCl3 spectrum, the corre-
sponding singlets appear at δ = 8.91 and 9.19 ppm (integral
ratio 3.8:1), and doublets at δ = 7.87 and 7.51 ppm (integral
ratio: 3.8:1). In both spectra, also the two doublets of the bi-
phenyl C-3 hydrogens appear in the range δ = 8.0–9.0 ppm
with intensity ratios corresponding to those measured for the
NH signals. In the spectrum recorded in [D6]DMSO (Figure 6, c),
also two biphenyl NH singlets (δ = 9.66 and 9.58 ppm), two C*-
NH doublets (δ = 8.39 and 8.32 ppm), and two biphenyl C-3-H
doublets (δ = 8.0–8.1 ppm) were observed, with intensity ratios
of 1.15:1. These observations are consistent with a diastereo-
meric equilibrium containing 83 and 80 % of the predominating
diastereomer in [D8]toluene and CDCl3, respectively, while in
[D6]DMSO practically a 1:1 mixture of diastereomers was ob-
served. The results clearly show that in both lipophilic solvents
favouring formation of hydrogen bonds, the gelling [D8]toluene
and the nongelling CDCl3, one diastereomer predominates,
whereas in the hydrogen-bond-competitive [D6]DMSO solution,
practically a 1:1 mixture of diastereomers is formed.
Figure 6. 1H NMR amide-NH proton regions of (R,R)-3 in a) [D8]toluene;
b) CDCl3; and c) [D6]DMSO.
stereomer. The results of an NH/D exchange experiment with a
sample of (R,R)-3 in CDCl3 solution support the above conclu-
sion (Figure S3, Supporting Information). Both biphenyl NH sin-
glets, as well as the lower intensity C*-NH doublet, disappeared
within 24 h after the addition of D2O. This indicates that these
NH are free or only slightly involved in hydrogen bonding. In
contrast, the H/D exchange of the higher intensity C*-NH pro-
tons was found to be much slower, and the signal was still
observable after 7 d. Since (R,R)-3 should be much less aggre-
gated in the CDCl3 solution than in the toluene gel sample, the
observed very slow NH/D exchange rate of the C*-NH protons
of the predominating diastereomer can be best explained by
their involvement in strong intramolecular hydrogen bonding.
These results also suggest that the structure of the predomina-
ting diastereomer closely resembles that of the intramolecularly
hydrogen bonded (R,aR,R)-3 enantiomer found in the crystal
structure of rac-3 (Figure 3), and that the less abundant dia-
stereomer [i.e., (R,aS,R)-3] lacks the intramolecular C*-NH···HO
bonds. As additional support for the above conclusion, in the
intramolecularly hydrogen bonded structure the biphenyl NH
are located close to the biphenyl and oxalamide carbonyl π-
systems (Figure S4, Supporting Information), and should be
more shielded (observed δ = 9.53 ppm) than the biphenyl NH
of the less abundant diastereomer (observed δ = 9.68 ppm,
Figure 6, a).
The X-ray molecular and crystal structures of rac-3 (Figures 3
and 4) show the presence of two intramolecular NH···OH
hydrogen bonds and four intermolecular OH···O=C< hydrogen
bonds, respectively. Since (R,R)-3 is capable of forming a gel
1
with toluene, the H NMR spectra of the gel sample were ana-
lysed to find out whether intra- and intermolecular hydrogen-
bonding motifs similar to those observed in the crystal struc-
ture could also stabilize the gel aggregates. The NH-tempera-
ture coefficients (ΔδNH/ΔT [ppm/K]) calculated from the tem-
perature-dependent spectra (Figure S2, Supporting Informa-
tion) show that with a temperature increase, the biphenyl NH
of the major and minor diastereomer, and the C*-NH of the
minor diastereomer are only weakly shifted upfield (ΔδNH/ΔT =
3.3 × 10–3; 2.7 × 10–3 and 2.1 × 10–3 ppm/K, respectively) com-
pared to the strong upfield shift of the doublet due to the C*-
NH of the major diastereoizomer (ΔδNH/ΔT = 11.1 × 10–3 ppm/
K). This confirms the involvement of the C*-NH of the major
To find out whether both, or exclusively one of the two dia-
stereomers (R,aR,R)- and (R,aS,R)-3 is assembled in the [D8]tolu-
1
ene gel aggregates, a variable-temperature H NMR investiga-
tion was carried out in the presence of tetrachloroethane as
an internal standard. When the temperature was increased, the
intensity of all the gelator signals increased due to disassembly
of NMR-silent gel network into smaller NMR-observable aggre-
gates.[16,17] In the 1H NMR spectrum of the [D8]toluene gel sam-
ple (c = 1.48 × 10–2
M) recorded at room temperature, signals of
diastereomer in the intramolecular hydrogen bonding, and the the dissolved major and minor diastereomer could be observed
lack of such hydrogen bonding in the less abundant dia- (Figure S5, Supporting Information). The variations of the rela-
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tive concentrations of the dissolved major (R,aR,R)-3diss and mi- organization similar to that in the gel fibres.[17] The NOE interac-
nor (R,aS,R)diss, diastereomers, as well as that of the assembled tions observed for the major diastereomer and identified as in-
diastereomer, that occur with an increase in temperature were tramolecular (H/H distances in the molecular model below 5 Å;
calculated based on the intensity changes of the biphenyl NH, red dotted arrows) or intermolecular (H/H distances exceeding
C*-NH and biphenyl C-3-H signals relative to the intensity of 5 Å; blue arrows) are shown in Figure 8. The intramolecular
the internal standard (Figure 7, a).
NOEs correspond nicely to the distances between the respec-
tive protons measured from the intramolecularly hydrogen-
bonded crystal-structure conformation (Figure 3, a). The intra-
molecular NOEs found in the [D6]DMSO solution sample were
similar, except for additional NH/HO and iPr-CH/HO interactions
(black arrows), which were not identified in the NOESY spec-
trum of the gel due to the uncertain position of the OH signal.
The latter observations suggest that the conformation of the
amino alcohol/oxalamide arm is similar in the gel aggregates
and in the DMSO solution. This could be explained by taking
into account the conformations of the amino acid and the
amino alcohol/oxalamide fragments found earlier in the X-ray
structures of various bis(amino acid or amino alcohol) oxal-
amide gelators; those conformations as well as the crystal struc-
ture conformation of 3 (Figure 3, a) are characterized by the
syn-periplanar positions of the C*-H and the oxalamide carbonyl
group (Figure 8, b, partial Newman projection of the C*–NH
bond).[19] Such a conformation is apparently the most stable
one due to the lowest steric hindrance between the C*-H (with
H corresponding to the small group, S) and the oxalamide carb-
onyl group, compared to larger hindrance in the other possible
conformations with the M or L group close to the carbonyl
group. Consequently, the two larger CH*-substituents (iPr as L,
and hydroxymethylene as M in the case of 3) are located above
and below the oxalamide plane, resulting in a rigid conforma-
tion with little rotational freedom around the C*H–NH bond.
Figure 7. (a) Variation of relative concentrations of (R,aR,R)-3diss (■), (R,aS,R)-
3
diss (●), and (R,aR,R)-3ass (▲) in the temperature-dependent 1H NMR spectra
of (R)-3 [D8]toluene gel; (b) variation of (R,aR,R)-3diss/(R,aS,R)-3diss ratio with
temperature increase.
The concentrations of the assembled gelator were calculated
from the known total concentration of (R,R)-3 and the NMR-
determined concentrations of the dissolved diastereomers. The
concentration of (R,aR,R)-3diss shows a sharp increase between
35 and 50 °C, corresponding to the gel melting interval (inde-
pendently determined Tg = 54 °C). The concentration of minor
diastereomer (R,aS,R)-3diss shows a slow and steady increase be-
tween 25–90 °C (Figure 7, a). The concentration curve of the
assembled gelator shows a sharp decrease in the same temper-
ature range (Figure 7, a, green curve), and is clearly related to
the (R,aR,R)-3diss concentration curve. This result shows that ex-
clusively the (R,aR,R) diastereomer is assembled into the gel
network the due to a diastereoselective self-sorting proc-
ess.[3a,18] The variation of the (R,aR,R)-3diss/(R,aS,R)diss-3 diastere-
omeric ratio (Figure 7, b) shows that it changes from approxi-
mately 2.5:1 in the sol at 90 °C to 8.5:1 in the gel state at 20–
40 °C.
Figure 8. (a) NOE effects observed in (R,R)-3 [D8]toluene gel and [D6]DMSO
solution samples. Intramolecular NOEs observed in both [D8]toluene gel and
[D6]DMSO solution samples (red dotted arrows) and NOEs observed only in
the [D6]DMSO solution sample (black dotted arrows). Proton–proton distan-
ces in Å units measured from the extended molecular model (SYBYL package)
with syn-periplanar C*-H/carbonyl position are given above the arrows. The
blue arrows connect protons at distances exceeding 5 Å, thus indicating in-
termolecular NOEs observed only in the [D8]toluene gel sample. (b) Partial
Newman projection of valinol-oxalamide fragment showing syn-periplanar
arrangement of C*-H (small group, S)/oxalamide carbonyl, and positions of
CH2OH (medium group, M) and iPr (large group, L).
NOESY experiments with (R,R)-3 [D8]toluene gel and
[D6]DMSO solution samples were run to get additional informa-
tion on the gelator conformations in both systems, and to iden-
tify intermolecular NOEs, which could provide information on
the primary organization of gelator molecules in the aggregates
(Figure S6, Supporting Information). As was recently shown, in
Hence, similar conformations of the amino alcohol/oxal-
the gel phase smaller aggregates dissolved in the entrapped amide arms in 3 can be expected to appear in the gel aggre-
solvent can be observed by NMR spectroscopy, and these have gates, in solution, and in solid-state aggregates.
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NOE interactions of the biphenyl C-3-H with the iPr methyl bly. In contrast, gels formed in the 10–2
M range formed imme-
and methyne protons were observed in the toluene gel spec- diately by cooling to room temperature, and showed the com-
trum, despite the fact that their intramolecular distance exceeds mon Tg increase with increasing gelator concentration. Only
6 Å. This points towards an intermolecular origin for these slight variations of the Tg values within an ageing time range
NOEs. To observe such NOEs, the iPr and biphenyl groups of 24–216 h were observed (Table 2), showing that the gels
should be located in close proximity in the aggregates, similar were at thermodynamic equilibrium.
to the packing arrangement of the enantiomers in the crystal
structure of rac-3 (Figure 3, b). The intermolecular distances be-
Table 2. Tg [°C] values determined after specified ageing intervals [h] of the
(R,R)-3 toluene gel samples[a] in the 10–2
M concentration range.
tween the biphenyl C-3-H and the iPr-CH and the methyl pro-
tons in the crystal structure of rac-3 are 2.3 and 3.5 Å, respec-
tively. These distances would allow the observation of inter-
molecular NOEs. This fact suggests that a similar arrangement
of (R,aR,R)-3 with a consecutive up and down orientation of
biphenyl groups could be expected in the gel assemblies.
The observed central (R) and (S) to axial (aR) and (aS) chirality
transfer, respectively, appears to be the consequence of intra-
molecular hydrogen bonding. The intramolecular C*-NH···OH
bonding in the case of the (R) centres is possible for the axially
chiral (aR) conformation of the biphenyl system without violat-
ing the syn-periplanar arrangement of the C*-H and the neigh-
bouring oxalamide carbonyl group (Figure 9, a). However, such
hydrogen bonding is not possible for the (R) centres and the
(aS) configuration of the biphenyl system (Figure 9, b). In a
molecular model of the latter conformation, the C*-NH and
CH2OH groups of neighbouring arms are over 7 Å apart, and
may come within the hydrogen-bonding distance by rotation
around the C*H–NH bond, and by violation of the energetically
most stable C*-H/>C=O syn-periplanar arrangement.
c [10–2
M]
Ageing interval [h]
72
24
48
144
216
3.12
2.35
1.56
65
61
56
62
59
53
63
61
54
63
62
54
64
60
54
[a] Gels were prepared in NMR tubes.
The determination of the Tg values of the gels formed in the
lower concentration range was carefully repeated in a way that
for each specified concentration five gel samples were prepared
by the heating/cooling procedure, and each sample of the spec-
ified concentration was aged from 24 up to 216 h at +4 °C
before the Tg was determined. In this way, each sample was
subjected to a single heating/cooling step before the determi-
nation of the Tg. For the gel samples with 2.4, 3.6, 4.8, 6.0, and
7.2 × 10–3
M concentrations aged for 24, 48, 72, and 144 h (Fig-
ure 10, a–d) a highly irregular variation of the Tg values with
both increasing gelator concentration and ageing time was ob-
served. For the 24 h aged sample (Figure 10, a) the Tg values
increased from 33 to 47 °C for concentrations of 2.4, 3.6, 4.8,
and 6.0 × 10–3
M, while for the highest concentration of
Figure 9. Conformations of (a) intramolecularly hydrogen bonded (R,aR,R)-3
and (b) non-hydrogen bonded (R,aS,R)-3 generated by molecular modelling
(SYBYL of TRIPOS Inc. software).
Concentration-Dependent Formation of Equilibrium and
Nonequilibrium Gel States
The properties of (R,R)-3 toluene gels were assessed by con-
structing phase diagrams showing the dependence of gel melt-
ing temperatures (Tg [°C], determined by tube inversion
method) on gelator concentration. In the preliminary experi-
ments, highly peculiar behaviour of gels prepared with (R,R)-3
concentrations in the 10–3
M range was observed. In some
cases, gels were formed at lower concentrations, and failed to
form at higher concentrations, and Tg values showed highly ir-
regular dependence on the gelator concentrations. Also, when
the heating/cooling procedure was repeated with the same
sample, the Tg values obtained were found to differ considera-
Figure 10. Dependence of (R)-3 toluene gel melting temperatures (Tg [°C]) on
gelator concentration (c = 2.4, 3.6, 4.8, 6.0, and 7.2 × 10–3
M) for gel samples
aged for (a) 24, (b) 48, (c) 72, (d) 144, and (e) 216 h.
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7.2 × 10–3
M
, only a weak gel with partial fluidity was obtained 0 °C (4.3 and 2.1 kJ/mol). This indicates a low level of aggrega-
(Tg denoted as 25 °C). Surprisingly, all of the 48 h aged samples tion that becomes higher for the sample aged for 2 d at +4 °C
gave partially fluid weak gels (Figure 10, b). The samples aged (15.9 kJ/mol). The highest enthalpy change of 23.9 kJ/mol (sum
for 72 and 144 h (Figure 10, c and d) showed irregular melting- of two transitions) was measured for the sample aged for 8 d
temperature changes with increasing concentrations. Only the at room temp., which indicates that a higher extent of aggrega-
216 h aged samples (Figure 10, e) showed the expected Tg in- tion results from ageing at a higher temperature. In contrast,
crease with increasing gelator concentration, revealing that for the 7.2 × 10–2
M gel, a much stronger single endothermic
ageing between 144 and 216 h is needed for each sample of transition was observed for the sample aged for just 30 min at
specified concentration to reach thermodynamic equilibrium. 0 °C (ΔH = 14.4 kJ/mol; Table S2, Supporting Information). This
The variation of the Tg values did not change when the ageing gel also gave a much higher gel melting temperature (Tg
=
time at +4 °C was lengthened for any of the tested concentra- 69.1 °C). These two values are only slightly different for the
tions; the Tg values of more aged samples were often found to samples aged for 2 and 14 d at +4 °C, which is consistent with
be lower than those of less aged samples (Figure S7, Support- a gel state at thermodynamic equilibrium (Figure 11, b and
ing Information). These results imply that the low self-assembly Table S2, Supporting Information).
rate may not be the major factor causing the irregular Tg/ageing
time relationship, since in that case, the Tg values should in-
crease with prolonged ageing, due to a higher degree of aggre-
gation. The results presented in Figure 10 (a–e) clearly show
The gel formation/melting diagrams for the (R,R)-3 toluene
gel determined by 1H NMR spectroscopy (Figure 7, a and b)
clearly show that the (R,aR,R)diss-3/(R,aS,S)diss-3 diastereomeric
ratio varies from 2.5:1 in the sol state at 90 °C to 19:1 [sum
that in the 10–3
M concentration range, nonequilibrium gel
of the network-assembled and dissolved gelator; (R,aR,R)-3ass
+
states are formed that need up to 216 h of ageing at +4 °C to
reach thermodynamic equilibrium.
(R,aR,R)-3diss] in the gel state at 20 °C (Figure 7, a). Hence, in
the gelation process starting from hot sol, at least two coupled
DSC measurements support the above conclusions. For the equilibria are involved, that of the nongelling (R,aS,R)-3 to gel-
gel formed at a concentration of 7.2 × 10–3
M
, only weak transi- ling (R,aR,R)-3 diastereomer interconversion, and that of the
tions were seen. One or two endothermic transitions, and in (R,aR,R)-3 assembly/disassembly equilibrium. The formation of
some cases also exothermic transitions appearing at different nonequilibrium gel states at the lower concentration could be
temperatures could be observed. The transitions observed indi- explained by kinetic effects as a result of the slow nongelling-
cate the formation of different kinds of assemblies. The thermo- to-gelling diastereomer interconversion step becoming the
grams obtained are different for each ageing time, and the gel rate-determining step in the gelation.
We have also found that the formation of gels from 10–3
M
melting temperatures vary irregularly with increasing ageing
times (Figure 11, a and Table S2, Supporting Information).
samples could be greatly accelerated by using ultrasound. The
application of sonication is reported to increase gelling ability
in some cases through local temperature increases, which in-
duce disassembly of the nonproductive gelator aggregates and
enable productive assembly into fibrous aggregates that can
entangle and form gels.[20] The observed effect of sonication in
our case could be explained both by an increased interconver-
sion rate of the nongelling (R,aS,R)-3 into the gelling (R,aR,R)-3
diastereomer as a result of temperature increase, and by sonica-
tion-induced disassembly of nonproductive (R,aR,R)-3 or
(R,aS,R)-3 aggregates indicated by appearance of two transi-
tions in DSC experiments. In conclusion, all the experimental
results taken together suggest that the peculiar behaviour of
gels formed in the 10–3
M concentration range may be best
explained by the involvement of multiple coupled equilibria, as
shown in Figure 12.
Figure 11. DCS heating diagrams of (R,R)-3 toluene gels at: a) 7.2 × 10–3
concentration, ageing time: 1 min at 0 °C (black), 2 d at +4 °C (red), 4 d at
+4 °C (blue); 8 d at +4 °C (gray); and b) 7.2 × 10–2
concentration, ageing
M
M
time: 30 min at 0 °C (black), 1 d at +4 °C (red), and 14 d at +4 °C (blue); for the
corresponding cooling diagram, see Figure S8 in the Supporting Information.
Figure 12. Multiple equilibria involved in gelation: dissolved (R,aR,R)-3diss
/
(R,aS,R)-3diss diastereomer interconversion, coupled with (R,aR,R)-3diss produc-
tive aggregation (self-assembly) into gel network [(R,aR,R)-3Pagg], and dia-
stereomer nonproductive (NPagg) aggregations in the sol phase.
The sums of the enthalpy changes determined for both tran-
sitions are very low for the samples with short ageing times at
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Conclusions
the Croatian Science Foundation (HrZZ) (project number IP-11-
2013-7387).
Chiral 2,2′-biphenyl-substituted amino alcohol oxalamide gelat-
ors 3 and 4 were prepared and studied as rare examples of
proatropisomeric chiral gelators, having both central and axial
chirality (Figure 1). The results show that type I and type II
gelators have dramatically different properties. With type I ge-
lators, central-to-axial chirality transfer was found to occur only
upon self-aggregation.[7] In contrast, the crystal structure of rac-
3, together with the results of the concentration- and tempera-
ture-dependent CD and 1H NMR studies, revealed that in gelled
toluene and nongelled chloroform and DMSO, a mixture of
(R,aR,R)-3 and (R,aS,R)-3 diastereomers is formed due to central-
to-axial chirality transfer. Crystallographic and spectroscopic (1H
NMR, CD) evidence supports an intramolecularly C*–NH···OH
hydrogen-bonded structure for the major (R,aR,R)-3 dia-
stereomer, and a structure for the minor (R,aS,R) diastereomer
lacking such hydrogen bonding (Figure 9). The temperature-
dependent NMR study of the (R,R)-3 solution samples (CDCl3
and [D6]DMSO) and of the [D8]toluene gel revealed the occur-
rence of thermally induced diastereomer interconversion and
diastereomer self-sorting processes, the latter resulting in the
self-assembly into the gel network exclusively of the major
(R,aR,R)-3 diastereomer.
Keywords: Supramolecular chemistry · Self-assembly ·
Chirality transfer · Sol-gel processes · Gels
[1] For some recent reviews on low-molecular-weight gelators, see: a) K.
Hanabusa, M. Suzuki, Polym. J. 2014, 46, 776–782; b) Z. F. Sun, Q. Y.
Huang, T. He, Z. Y. Li, Y. Zhang, L. Z. Yi, ChemPhysChem 2014, 15, 2421–
2430; c) X. D. Yu, L. M. Chen, M. M. Zhang, T. Yi, Chem. Soc. Rev. 2014,
43, 5346–5371; d) Q. i. Zhenhui, C. A. Schalley, Acc. Chem. Res. 2014, 47,
2222–2233; e) P. F. Duan, H. Cao, L. Zhang, M. H. Liu, Soft Matter 2014,
10, 5428–5448; f) C. Tomasini, N. Castellucci, Chem. Soc. Rev. 2013, 42,
156–172; g) J. W. Steed, Chem. Commun. 2011, 47, 1379–1383; h) J. W.
Steed, Chem. Soc. Rev. 2010, 39, 3686–3699; i) A. Ayaghosh, V. K. Praveen,
C. Vijayakumar, Chem. Soc. Rev. 2008, 37, 109–122; j) P. Dastidar, Chem.
Soc. Rev. 2008, 37, 2699–2715; k) M. Sada, N. Takeuchi, M. Fujita, M.
Numata, S. Shinkai, Chem. Soc. Rev. 2007, 36, 415–435; l) M. George, R. G.
Weiss, Acc. Chem. Res. 2006, 39, 489–497; m) J. van Esch, B. L. Feringa,
Angew. Chem. Int. Ed. 2000, 39, 2263–2266; Angew. Chem. 2000, 112,
2351–2354.
[2] a) K. K. Diehn, O. H. Hyuntaek, R. Hashemipour, R. G. Weiss, S. R. Ragha-
van, Soft Matter 2014, 10, 2632–2640; b) A. R. Hirst, S. Roy, M. Arora, A. K.
Das, N. Hodson, P. Murray, S. Marshall, N. Javid, J. Sefcik, J. Boekhoven,
J. H. van Esch, S. Santabarbara, N. T. Hunt, R. V. Ulijn, Nature Chemistry
2010, 2, 1089–1094; c) A. R. A. Palmans, E. W. Meijer, Angew. Chem. Int.
Ed. 2007, 46, 8948–8968; Angew. Chem. 2007, 119, 9106–9126; d) J. J. D.
de Jong, T. D. Tiemersma Wegman, J. H. van Esch, B. L. Feringa, J. Am.
Chem. Soc. 2005, 127, 13804–13805; e) A. Aggeli, I. A. Nyrkova, M. Bell,
R. Harding, L. Carrick, T. C. B. McLeish, A. N. Semenov, N. Boden, Proc.
Natl. Acad. Sci. USA 2001, 98, 11857–11862; f) P. Jonkheijm, P.
van der Schoot, A. P. H. J. Schenning, E. W. Meijer, Science 2006, 313, 80–
83.
The experimental results described for the (R,R)-3 toluene
gel show that coupled equilibria comprising network-assembly/
dissolution of (R,aR,R)-3 and diastereomer interconversion are
involved, giving gels with dramatically different properties in
different (10–2 and 10–3
M) concentration ranges. At lower con-
[3] a) D. K. Smith, Chem. Soc. Rev. 2009, 38, 684–694; b) A. Brizard, R. Oda,
I. Huc, Top. Curr. Chem. 2005, 256, 167–218.
centrations, gelation is very slow, and needs 9 d of ageing at
+4 °C to reach equilibrium (Figure 10). The formation such long-
lived nonequilibrium states could be explained by kinetic ef-
fects, due to a slow nongelling-to-gelling diastereomer inter-
conversion step becoming the rate-determining step at certain
concentrations. It was also found that gelation could be greatly
accelerated by sonication. This, together with the DSC results,
which indicate the formation of two kinds of aggregates, sug-
gest the formation of nonproductive assemblies of gel-forming
(R,aR,R)-3 or nongelling (R,aS,R)-3 diastereomers. The described
gelation of toluene by (R,R)-3 represents a highly complex and
unique gelation system comprising central-to-axial chirality
transfer, diastereomer interconversion, diastereoselective self-
sorting, and multiple coupled equilibria. It gives equilibrated
and long-lived nonequilibrium gel systems, depending on the
gelator concentration. A detailed understanding of such dy-
namic systems of higher complexity could be of the utmost
interest for the future design of soft materials and systems for
advanced applications.
[4] a) M. Liu, L. Zhang, T. Wang, Chem. Rev. 2015, 115, 7304–7397; b) Z. Shen,
T. Wang, M. Liu, Langmuir 2014, 30, 10772–10778; c) X. Luo, B. Liu, Y.
Liang, Chem. Commun. 2001, 1556–1557; d) J. Makarević, M. Jokić, Z.
Raza, Z. Štefanić, B. Kojić-Prodić, M. Žinić, Chem. Eur. J. 2003, 9, 5567–
5580; e) V. Čaplar, M. Žinić, J.-L. Pozzo, F. Fages, G. Mieden-Gundert, F.
Vögtle, Eur. J. Org. Chem. 2004, 19, 4048–4059.
[5] C. Wang, D. Zhang, D. Zhu, Langmuir 2007, 23, 1478–1482.
[6] B. Escuder, M. LLusar, J. F. Miravet, J. Org. Chem. 2006, 71, 7747–7752.
[7] J. Makarević, Z. Štefanić, L. Horvat, M. Žinić, Chem. Commun. 2012, 48,
7407–7409.
[8] a) S. Superchi, R. Bisaccia, D. Casarini, A. Laurita, C. Rosini, J. Am. Chem.
Soc. 2006, 128, 6893–6902; b) E. Manna, A. Montero, M. A. Maestro, B.
Herradon, Helv. Chim. Acta 2002, 85, 3624–3638; c) B. Herradon, A. Mon-
tero, E. Manna, M. A. Maestro, CrystEngComm 2004, 6, 512–521.
[9] R. E. Moore, A. Furst, J. Org. Chem. 1958, 23, 1504–1506.
[10] a) X. Luo, B. Liu, Y. Liang, Chem. Commun. 2001, 1556–1557; b) J. Makar-
ević, M. Jokić, Z. Raza, Z. Štefanić, B. Kojić-Prodić, M. Žinić, Chem. Eur. J.
2003, 9, 5567–5580; c) V. Čaplar, M. Žinić, J.-L. Pozzo, F. Fages, G. Mieden-
Gundert, F. Vögtle, Eur. J. Org. Chem. 2004, 4048–4059.
[11] H. Suzuki, Electronic Absorption Spectra and Geometry of Organic Mol-
ecules, Academic Press, New York, 1967, p. 262 and 272.
[12] K. Mislow, E. Bunnenberg, R. Records, K. Wellman, C. Djerassi, J. Am.
Chem. Soc. 1963, 85, 1342–1349.
[13] J.-P. Mazaleyrat, K. Wright, A. Gaucher, N. Toulemonde, M. Wakselman, S.
Oancea, C. Peggion, F. Formaggio, V. Setnicka, T. A. Keiderling, C. Toniolo,
J. Am. Chem. Soc. 2004, 126, 12874–12879.
[14] T. Mori, Y. Inoue, S. Grimme, J. Phys. Chem. A 2007, 111, 4222–4234.
[15] C. Bustamante, I. Tinolo Jr., M. F. Maestreo, Proc. Natl. Acad. Sci. USA 1983,
80, 3568–3572.
Supporting Information (see footnote on the first page of this
article): Equipment, procedures, and experimental conditions; tem-
perature-dependent FTIR spectra, DSC spectra, NOESY spectra of
gel samples, NMR spectra of synthesized compounds.
[16] a) J. Makarević, M. Jokić, B. Perić, V. Tomišić, B. Kojić-Prodić, M. Žinić,
Chem. Eur. J. 2001, 7, 3328–3341; b) B. Escuder, M. Llusar, J. F. Miravet, J.
Org. Chem. 2006, 71, 7747–7752; c) A. R. Hirst, I. A. Coates, T. R. Bouche-
teau, J. F. Miravet, B. Escuder, V. Castelletto, I. W. Hamley, D. K. Smith, J.
Am. Chem. Soc. 2008, 130, 9113–9121.
Acknowledgments
This work was supported by the Croatian Ministry of Science,
Education and Sports (project number 098-0982904-2912) and
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[17]
[20] a) D. Bardelang, Soft Matter 2009, 5, 1969–1971; b) J. Wu, T. Yi, T. Shu,
M. Yu, Z. Zhou, M. Xu, Y. Zhou, H. Zhang, J. Han, F. Li, C. Huang, Angew.
Chem. Int. Ed. 2008, 47, 1063–1067; Angew. Chem. 2008, 120, 1079–1083.
S. Bouguet-Bounnet, M. Yemloul, D. Canet, J. Am. Chem. Soc. 2012, 134,
10621–10627.
[18] a) M. M. Safont-Sempere, G. Fernandez, F. Wurthner, Chem. Rev. 2011,
111, 5784–5814; b) K. Osowska, O. Š. Miljanić, Synlett 2011, 1643–1648.
[19] L. Frkanec, M. Žinić, Chem. Commun. 2010, 46, 522–537.
Received: October 1, 2015
Published Online: January 22, 2016
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