Boron Effect on Sugar-Based Organogelators

Article abstract of DOI:10.1002/chem.202001970

The reaction of several alkylglucosides with phenyl boronic acid permitted easy access to a series of alkylglucoside phenyl boronate derivatives. This type of compound has structures similar to those of known benzylidene glucoside organogelators except fo

Full text of DOI:10.1002/chem.202001970

Accepted Article
Accepted Article
Title: Boron effect on sugar-based organogelators
Authors: Andreas D Ludwig, Arnaud Saint Jalmes, Cristelle Mériadec,
Franck Artzner, Olivier Tasseau, Fabienne Berrée, and Loïc
Lemiègre
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10.1002/chem.202001970
Chemistry - A European Journal
FULL PAPER
Boron effect on sugar-based organogelators
Andreas D. Ludwig,[a] Arnaud Saint-Jalmes,[b] Cristelle Mériadec,[b] Franck Artzner,[b] Olivier Tasseau,[a]
Fabienne Berrée*[a] and Loïc Lemiègre*[a]
[a]
[b]
Andreas D. Ludwig, Olivier Tasseau, Dr. Fabienne Berrée and Dr. Loïc Lemiègre
Univ Rennes, Ecole Nationale Supérieure de Chimie de Rennes, CNRS, ISCR - UMR6226, F-35000 Rennes, France.
E-mail: loic.lemiegre@ensc-rennes.fr
Dr. Arnaud Saint-Jalmes, Cristelle Mériadec, Dr. Franck Artzner
Univ Rennes, CNRS, IPR (Institut de Physique de Rennes) - UMR 6251, F-35000 Rennes, France.
Supporting information for this article is given via a link at the end of the document.
Abstract: The reaction of several alkylglucosides with phenyl boronic
acid permitted an easy access to a series of alkylglucoside phenyl
boronate derivatives. This type of compounds has structures similar
to the known benzylidene glucoside organogelators except the
presence of a boronate function in place of the acetal one. Low to very
low concentrations of these amphiphilic molecules produced gelation
of several organic solvents. The rheological properties of the
corresponding soft materials characterized them as elastic solids.
They were further characterized by SEM to obtain more information
on their morphologies and by SAXS to determine the type of self-
assembly involved within the gels. The sensitivity of the boronate
function towards hydrolysis was also investigated. We demonstrated
that a small amount of water (5% v/v) was sufficient to disrupt
organogels leading to the original alkylglucoside and phenyl boronic
acid; an important difference with the stable benzylidene-based
organogelators. Such water sensitive boronated organogelators could
be suitable substances for the preparation of smart soft material for
topic drug delivery.
the esterification of the hydroxyl functions available on the 4,6-
positions of the glucoside.[10] Boronic esters have already been
involved in organogelator structures,[11] but only few examples
have taken advantage of this function for the preparation of sugar-
based organogelators[12]. One important illustration related to our
research work was described by Shimizu et al.[12b-d] who designed
a luminescent organogelator based on a glycolipid naphtyl
boronic ester. We described herein a series of alkylglucoside
phenylboronic esters 1 easily prepared in a single chemical step
(Figure 1). Their chemical structures are similar to known
benzylidene type organogelators 2 that already demonstrated
their ability as good organogelators with typical minimum gelation
concentration between 2 to 10 mg.mL-1.[7e, 7j, 13] The replacement
of the acetal function by a boronic ester group induces two main
differences; 1) the tetrahedric carbon is replaced by a planar
boron atom and 2) the presence of a vacant orbital on boron atom.
As a consequence, the geometry of the molecule, and therefore
its self-assembling properties, are supposed to dramatically
change. Furthermore, the easier hydrolysis of the boronate
function compared with the acetal one might conduce to water
sensitive self-assembly. However, one could imagine that this
slight difference would prevent these less stable boronate
derivatives to provide gelation of organic solvents. Within this new
family of organogelators, we investigated the impact of the alkyl
chain length on the gelation of various solvents. The organogels
Introduction
The preparation of gels with low molecular weight gelators is
possible thanks to weak intermolecular interactions such as van
der Waals interactions, π−π interactions, hydrogen bonding or
electrostatic interactions.[1] The corresponding self-assembly
turns aqueous or organic solutions into hydrogels or organogels
respectively. This important area of research provides soft
materials which find various types of applications in oil
remediation,[2] nanotube formation,[3] drug delivery[4] or sensitive
materials[5]. Typical gelators are based on sugars, lipids, peptides,
aromatics, that usually provide the required weak interactions.
Sugar-based structures are often suitable for the preparation of
hydrogels if the hydrophilicity of the carbohydrate is well balanced
with hydrophobic tails.[6] Usually access to sugar-based
organogelators requires further functionalization of the sugar
moiety with the introduction of aromatic rings, acyl or alkyl chains,
most of the time by creation of C-O linkages.[7] However, sugars
in general, and glucoside derivatives in particular, are known to
react selectively with boronic acids leading to corresponding
boronic esters.[8] Several strategies have been developed for the
detection of sugars or for the formation of molecular self-assembly,
thanks to this easy and efficient reaction.[8a, 9] In regard to the
specific case of glucoside derivatives, the boronic acid promotes
were characterized at
a macroscopic scale by rheology
(mechanical properties, stability), at a microscopic scale by
scanning electron microscopy (SEM) and molecular level was
achieved by small angle X-ray scattering (SAXS) analysis. In
addition, their sensitivity against water was confirmed.
Ph
B
O
O
HO
O
O
HO
Ph
O
O
O
O
1
2
Alk
Alk
OH
OH
Figure 1. Alkylglucoside phenyl boronic esters 1 and known benzylidene
glucoside organogelators 2.
Results and Discussion
The synthesis of boronic esters 1 was simply achieved by mixing
equimolar amounts of alkylglucosides 4 and phenylboronic acid 3
without solvent. Elimination of water by heating under high
1
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10.1002/chem.202001970
Chemistry - A European Journal
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vacuum displaced the equilibrium towards the formation of the
final product (Scheme 1). Complete reactions were observed for
alkylglucosides 4a-h within 15 minutes at 90°C whatever the alkyl
tail equipping the glucoside (n-methyl, n-butyl, n-hexyl, n-heptyl,
n-octyl, n-nonyl, n-decyl and n-dodecyl chains). All boronic esters
1a-h were isolated without purification or after simple washing
with diisopropylether in excellent yields (84-99%) and purities.
cyclohexane providing an organogel at a very low concentration
(2 mg.mL-1) and in only one of the two fatty esters (isopropyl
palmitate).
Table 1. Gelation assays [a]
Entry
Compound
Toluene
CyH
Ethyl
Myristate
Isopropyl
Palmitate
This series of boronic esters with
hydrophilic/lipophilic balance, was further engaged in gelation
assays with various solvents. The a-butyl derivative -1b was
a
broad range of
a
1
2
3
4
5
6
7
8
9
b-D-methyl 1a
I
I
I
I
I
also prepared in the same manner (not shown) in order to
investigate the stereochemical impact of the nature of the
glycosidic bond (a or b).
a-D-n-butyl
b-D-n-butyl
a
-1b
P
nd [b]
GT 10
GO 10
nd [b]
GT 11
GO 10
b
-1b
GT 11
GO 10
GT
I
3
b-D-n-hexyl 1c
b-D-n-heptyl 1d
b-D-n-octyl 1e
b-D-n-nonyl 1f
b-D-n-decyl 1g
b-D-n-dodecyl 1h
Ph
OH
OH
HO
HO
HO
B
Solvent free
O
O
HO
O
GT
9
P
I
GT
GO
9
8
GO
9
+
Ph
B
O
O
Kugelrohr distillation
84-99% yield
O
Alk
OH
Alk
OH
GO 12
GP 12
S
GO 12
GP 11
GT 10
GT 12
3
4a-h
1a (Me), 1b (n-Bu), 1c (n-Hex), 1d (n-Hept),
1e (n-Oct), 1f (n-Non), 1g (n-Dec), 1h (n-Dodec)
GP
I
4
2
GO 12
GT 10
S
Scheme 1. Synthesis of alkylglucoside boronic esters 1a-h.
S
GT
To evaluate the potential of this series of new molecules as
organogelators, we first selected several solvents covering
different solvation behaviors. Toluene represents an example of
aromatic solvent, cyclohexane is a typical cycloalkane, ethyl
myristate and isopropyl palmitate are interesting bio-based
solvents used in cosmetic applications and topical drug
delivery.[4a] Table 1 gathers the results for each molecule/solvent
combination. For each case, we determined if the molecule was
insoluble even after heating (I), soluble after heating (S), behave
as an opaque gel (GO), as a transparent gel (GT) or as a
precipitate upon cooling (P) (Photos of representative gels are
available in the SI). The first member of the series (methyl, 1a)
was not soluble whatever the solvent used (Table 1, entry 1). This
[a] I: Insoluble; S: Soluble; P: Precipitate; GP: Partial gel; GT: Translucent gel;
GO: Opaque gel. Values indicate minimum gelation concentrations (MGC) in
mg.mL-1. [b] Not determined.
Mechanical properties were investigated by rheological
amplitude-sweep. Graphs of G’ and G” against strain % are
shown in Figure 2 (data shown for b-n-butyl derivative, graphs of
other organogels are available in the SI). Experiments were
performed with a frequency of 1 Hz at the MGC described in Table
1.
shortest chain length did not provide
a
sufficient
hydrophilic/lipophilic balance to permit the gelation of the solvents
considered here. Concerning the stereochemistry of the
glycosidic bond, if the a isomer
organogel either in toluene or in cyclohexane, conversely, the b
isomer -1b (entry 3) was able to gel the four solvents giving
a
-1b (entry 2) did not provide
b
translucent organogels with toluene, cyclohexane and fatty esters.
Indeed, stereochemistry is an important feature that dramatically
changes the geometry of molecules and therefore their ability to
self-assemble.[14] For the rest of the study, we only considered b
isomers and evaluated the impact of the length of the alkyl chain
on the gelation properties (entries 3-9). Indeed, the length of the
alkyl chain had a remarkable influence on the gelation properties.
n-Butyl derivative -1b led to organogels with the all four solvents
b
at low to very low concentrations (3 to 11 mg.mL-1) (entry 3). The
addition of two carbon atoms (n-hexyl 1c, entry 4) provided very
similar minimum gelation concentrations (MGC) for the four
solvents. Middle chain length (n-heptyl, 1d and n-octyl, 1e; entries
5-6) led to organogels in all solvents except cyclohexane for which
precipitation or insolubility were obtained. The n-nonyl derivative
1f did not lead to a full gelation except for ethyl myristate (entry
7). Increasing even more the chain length reduced the solubility
in cyclohexane, increased the solubility in toluene but provided
transparent organogels in fatty ester solvents at low
concentrations (10 mg.mL-1) (n-decyl 1g, entry 8). Surprisingly,
the n-dodecyl counterpart 1h (entry 9) had a different behavior in
Figure 2. Storage modulus (G’, plain line) and loss modulus (G”, dashed line)
against strain % of organogels obtained with b-n-butyl
b
-1b in toluene (A),
cyclohexane (B), ethyl myristate (C) and in isopropyl palmitate (D).
Under such conditions, at low strains, all organogels exhibited
storage moduli greater than loss moduli (Table 2). It demonstrates
2
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10.1002/chem.202001970
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that the gels can be considered as elastic solids, but rather soft
ones. Indeed, their elastic moduli reach only values from
hundreds to a few thousands of Pa, and they remain only a few
times larger than the viscous moduli. Interestingly the b-n-butyl
the elastic modulus G'. At amplitudes above 2.5 to 8%, the
elasticity of the organogel vanished, with G' collapsing down to G”.
Under such amplitudes, the organogel structure could not
effectively sustain the deformation and the sample started to
irreversibly flow. Similar conclusions raised from the analysis of
the other organogels (see SI). An experiment at 2% amplitude and
1 Hz frequency showed constant moduli over a period of 2 hours
(Figure 3A). Thus, the constant rheological properties obtained
under such conditions demonstrated the stability of the
organogels over time. In addition, consecutive amplitude-sweep
experiments on a same gel also showed that the low strain regime
was independent of the number of measurements, but that the
deformation threshold (where G’ becomes smaller than G’’) was
dramatically modified after the first run (runs from the black line to
red lines, Figure 3B) and then continuously by the repetition of
tests (from red lines to blue and then green lines, Figure 3B). This
implies that the first self-assembled structures obtained through
the gelation process were irreversibly modified each time that they
were deformed over their deformation threshold (Figure 3B and
SI). Reorganization of the fiber network might occur during the
deformation of the organogels.
derivative
b
-1b provided the largest G’ and G’/G” values after
gelation of toluene (Table 2, entry 1). The n-dodecyl derivative 1h
induced the formation of stiffer gels in cyclohexane (Table 2, entry
17) and n-heptyl 1d and n-decyl 1g counterparts afforded stiffer
gels (larger elastic moduli) in fatty ester solvents (Table 2, entries
9, 15, 16). These observations might result from different
tightness of packing of organogelator fibers within the solvent.
Then interaction of the fibers with the solvent molecules would
vary for each organogelator/solvent couple. Indeed, long chain
fatty ester solvents are reasonably more compatible with long
chain organogels whereas short chain organogelators would
express a more aromatic behavior, more compatible with toluene
for instance.
Table 2. Storage and loss modulus at low strain of organogels.
Entry
Compound
Solvent
Storage
modulus
[Pa] [a]
Loss
modulus
[Pa] [a]
G’/G”
[a]
1
b-D-n-butyl
b-D-n-butyl
b-D-n-butyl
b-D-n-butyl
b
b
b
b
-1b
-1b
-1b
-1b
Tol
CyH
EM
IP
2724
326
213
40
12.8
8.1
2
3
1196
1322
706
279
234
111
90
4.3
4
5.6
5
b-D-n-hexyl 1c
b-D-n-hexyl 1c
b-D-n-hexyl 1c
b-D-n-heptyl 1d
b-D-n-heptyl 1d
b-D-n-heptyl 1d
b-D-n-octyl 1e
b-D-n-octyl 1e
b-D-n-octyl 1e
b-D-n-nonyl 1f
b-D-n-decyl 1g
b-D-n-decyl 1g
b-D-n-dodecyl 1h
b-D-n-dodecyl 1h
Tol
EM
IP
6.4
6
206
2.3
7
449
140
98
3.2
Figure 3. (A) Storage modulus (G’, plain line) and loss modulus (G”, dashed
line) against time (Frequency = 1 Hz and Amplitude = 2%); (B) Ten successive
amplitude-sweep experiments (First experiment in black line then red, blue, and
8
Tol
EM
IP
1181
3023
837
12.1
15.2
8.1
green lines). Data for gel of b-n-butyl -1b in isopropyl palmitate (IP).
b
9
199
103
341
287
104
45
10
11
12
13
14
15
16
17
18
Toluene and cyclohexane-based organogels were freeze-dried
and the corresponding xerogels were analyzed by scanning
electron microscopy (SEM) (Figure 4). The morphology of
xerogels originated from toluene revealed characteristic networks
of fibers entangled one to another in the case of n-butyl, n-hexyl
and n-octyl derivatives (Figures 4A, B, D, E). However, dense
structures corresponding to a bundle of fibers were observed in
the case of n-heptyl counterpart (Figure 4C). This exception would
find its origin in the odd number of carbon atoms within the alkyl
chain compared to the even number of carbon atoms for the other
organogelators. The SEM images of xerogels obtained from
cyclohexane (Figure 4F-H) also revealed fibrillar networks for n-
butyl (Figure 4F) and n-nonyl (Figure 4G) derivatives whereas n-
dodecyl derivative led to a network involving large clusters of
fibers (Figure 4H).
Tol
EM
IP
753
2.2
1075
540
3.7
5.2
EM
EM
IP
126
2.8
2762
1487
429
346
126
35
8.0
11.8
12.3
4.7
CyH
IP
722
154
[a] At strain < 1%, 1 Hz frequency.
At high amplitudes of deformation, a same qualitative behavior is
found for all the organogels: above a deformation threshold
(typically ranging from 1 to 10%), the samples provide a viscous
response to the mechanical solicitation, eventually dominating the
elastic one (G’’ > G’). In the case of b-n-butyl derivative
b
-1b,
above amplitudes of around 2%, some first plastic
rearrangements are induced inside the organogels, decreasing
3
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Figure 5. SAXS profiles of organogel in blue (toluene) and xerogel in red
obtained with b-D-octyl 1e.
The unit cell volume is 3390 Å3 and molecular weight of 1e is 378
g.mol-1. With a density of 1.11, the number of molecules per unit
cell is 6. The molecules may be localized on any position of the
hexagonal lattice. It is simpler to imagine three sets of dimers
centered on the C2 axes (Figure 6A-B). This dimer must exhibit
smaller areas at both ends to fit in the center of the hexagonal
phase and a larger area at its center to accommodate the space
available (Figure 6B). This type of organization can be compared
to self-assemblies involved in cylindrical micelles.[15]
Table 3. Theoretical and observed indexations of the hexagonal phase shown
in Figure 5 (b-D-octyl 1e) (a = 26.8 Å, c = 5.47 Å)
Figure 4. SEM images of xerogels (A: n-butyl/toluene; B: n-hexyl/toluene; C: n-
heptyl/toluene; D: n-octyl/toluene; E: n-octyl/toluene; F: n-butyl/cyclohexane; G:
n-nonyl/cyclohexane; H: n-dodecyl/cyclohexane). Scale bar : 10 µm.
(h, k, l)
qcalc (Å−1
0.2710
0.4693
0.5419
0.7169
0.9387
0.9770
1.1811
1.2417
1.2705
1.3548
1.4080
1.4837
1.5087
1.5796
1.6482
)
qobs (Å−1
0.2709
0.4696
0.5424
0.7172
0.9397
0.9774
1.1811
1.2415
1.2709
1.3544
1.4075
1.4830
1.5085
1.5801
1.6482
)
(1, 0, 0)
(1, 1, 0)
(2, 0, 0)
(2, 1, 0)
(2, 2, 0)
(3, 1, 0)
(3, 2, 0)
(4, 1, 0)
(2, 0, 1)
(5, 0, 0)
(3, 3, 0)
(2, 2, 1)
(5, 1, 0)
(4, 0, 1)
(4, 3, 0)
The fibrillar networks observed by SEM originate from the self-
assembly of the organogelators within the solvent. To better
understand the type of self-assembly involved during gelation, we
embarked on a small angle X-ray scattering (SAXS) study of
some members of the series. Measurements were successfully
achieved for organogels obtained in toluene with b-D-octyl 1e
(Figure 5) and b-D-hexyl 1c (SI). Figure 5 compares SAXS
profiles of the organogel at 20 mg.mL-1 of 1e in toluene (blue) and
the corresponding xerogel (red). Both organogel and xerogel
have the same scattering profiles. Indexations revealed the
existence of a hexagonal phase exhibiting a 3D crystal packing
(Table 3). This hexagonal phase was assigned without any
ambiguity and consequently, tracing qhkl in a hexagonal lattice,
where h, k, and l refer to Miller indices, gave access to repeating
parameters of a = 26.8 Å and c = 5.47 Å.
This in mind,
a compatible dimer structure involves: 1)
disorganized alkyl chains at both ends which fit into the center of
4
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10.1002/chem.202001970
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the hexagonal phase and 2) interdigitated aromatic rings at the
center of the dimer. Several types of packing can be envisaged
based on the interaction between aromatic rings: face to face or
edge to face including T-shaped geometry or herringbone packing.
The later fits the best with the repeating parameter c (5.47 Å)
(Figure 6C). Indeed, edge-to-face stacking (T-shaped geometry)
would have led to a wrong orientation of the two members of the
dimer and face-to-face stacking would have required a larger
repeating parameter (≈ 7 Å). It is also noteworthy that the
herringbone packing was previously observed within the crystal
packing of a similar boronate derivative (methyl-2-deoxy-a-D-
glucopyranoside 4,6-phenylboronate).[16] Thus, Figure 6C
(extended chains are shown for clarity) represents the possible
structure of the dimer, its vertical packing and interdigitation of
aromatic rings in respect of parameter c (5.47 Å). Figure 6d
represents the possible packing of three dimers within the
hexagonal phase, the alkyl chain being disorganized in the center
of each hexagon in respect of parameters a (26.8 Å). A very
similar self-assembly was characterized for the organogel
obtained with b-D-hexyl 1c. However, the scattering profile
exhibited a liquid crystal state instead of the 3D crystal packing.
This hexagonal phase was also assigned without any ambiguity
(see SI). A comparable parameter (a = 28.7 Å) found its origin
from a similar packing model (Figure 6D), the less crystallinity of
the liquid crystal state would explain the slight difference between
the parameters a of 1e and 1c in favor of 1c.
to the sugar moiety. Therefore, this orientation impacts the global
shape of the molecule and is then responsible for the type of
packing observed here and characterized by SAXS.
Ph
B
O
O
HO
O
O
HO
Ph
O
O
O
O
1
2
Me
Me
OH
OH
Figure 7. Representation of the impact of boronate and acetal geometries on
the orientation of the aromatic rings (Minimization obtained at an AM1 level).
As mentioned in the introduction, boronate functions are usually
sensitive to aqueous media whereas benzylidene functions are
stable in the presence of water, at least at neutral pH. Thus, we
evaluated the behavior of the organogels obtained with the n-butyl
derivative 1b selected as an example of the series. The
organogels in toluene, ethyl myristate and isopropyl palmitate
were exposed to water (5% v/v) at room temperature during the
time of the experiment. Until 1h, the gels were partially disrupted
with most of the material being in a gel state, between 1h and 3h,
the gels loosed their structures towards a fluid state with some
pieces of gel still present and after 3h, the gel state of all samples
totally disappeared leading to biphasic or monophasic solutions
depending on the solvent used for the formation of the gels
(Figure 8). We also repeated the same experiments (gel from b-
A
B
=
dimer
butyl -1b in toluene) except that we replaced fresh distilled water
b
by acetate buffer (pH = 4.8) or carbonate buffer (pH = 9.8). Very
similar results were obtained after addition of 5% v/v of these two
buffer solution.
C
D
5.5 Å
Ph
HO
HO
HO
B
OH
OH
O
O
HO
O
O
+
Ph
B
O
O
≈ 27 Å
n-butyl
OH
Herringbone packing
n-butyl
OH
H2O 5% v/v
3 h
Figure 8. Water sensitivity of organogels: disruption of a gel by addition of water
(5% v/v) via hydrolysis of the boronate function (Images shown for isopropyl
palmitate case).
Figure 6. Representation of the hexagonal packing characterized by SAXS for
1e. A: distribution of 5 dimers (6 molecules) within the unit cell; B: self-assembly
of dimers within the hexagonal phase and global shape of the dimer; C: self-
assembly of 4 molecules and herringbone packing; D: self-assembly of 10
molecules within the unit cell.
The 1H NMR analysis of the water phase confirmed the hydrolysis
of the organogelator into its two original components;
phenylboronic acid and n-butyl-b-D-glucoside (Figure 9). It
confirmed the water sensitivity of the boronate function which is
then responsible for the disruption of the organogel.
The sp2 hybridization of a boron atom of a boronate function
induces a planar geometry, whereas the sp3 hybridization of the
carbon atom of an acetal function confers a tetrahedral geometry.
It triggers a dramatic difference between the two organogelators
1 and 2 (Figure 7). Indeed, the particular geometry of the boronate
function offers a different orientation of the aromatic ring in regard
5
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melting point apparatus Stuart SMP10 and are uncorrected. Specific
rotation (in deg cm3 g-1) was measured on a Perkin Elmer-341 polarimeter.
General procedure for the synthesis of boronic esters 1. In a 25 mL round
bottom flask, alkylglucoside (0.3 mmol) and phenylboronic acid (37 mg,
0.3 mmol) were added. The mixture was then stirred in a kugelrohr
distillation apparatus at 90°C; under 0.1 mm Hg for 15 minutes to give the
product pure enough to make gels. Compounds were washed by
diisopropylether when necessary.
Methyl-b-D-glucopyranoside 4,6-phenylboronate 1a.[17] 143 mg (99%);
white solid; mp = 188-190°C. [a]D= -21 (C 0.2, CH2Cl2). 1H NMR (300 MHz,
CDCl3) δ 7.81 (d, J = 7.2 Hz, 2H), 7.46 (t, J = 7.4 Hz, 1H), 7.36 (t, J = 7.4
Hz, 2H), 4.38 (d, J = 7.7 Hz, 1H, H1), 4.32 (dd, J = 10.4, 5.4 Hz, 1H, H6),
4.01 (t, J = 10.3 Hz, 1H, H6), 3.85-3.70 (m, 2H, H3 H4), 3.60 (s, 3H), 3.58-
3.49 (m, 2H, H2 H5), 2.80 (d, J = 1.8 Hz, 1H, OH), 2.55 (d, J = 2.3 Hz, 1H,
OH). 13C NMR (75 MHz, CDCl3) δ 134.3, 131.4, 127.8, 104.4 (C1), 75.1
(C4), 74.7 (C2), 74.3 (C3), 68.7 (C5), 64.2 (C6), 57.7 (OCH3). The carbon α
to the boron was not observed. 11B NMR (96 MHz, CDCl3) δ 28.7. HRMS
(ESI+) (M+Na)+ calculated for C13H17O611BNa 303.10159, found 303.1011.
8.0
7.5
7.0
6.5
6.0
5.5
5.0
4.5
(ppm)
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
Figure 9. 1H NMR spectra in D2O of phenylboronic acid (bottom), n-butyl-b-D-
glucoside (middle) and water phase after disruption of the organogel (
toluene) with 5% v/v of water (top).
b
-1b in
Butyl-b-D-glucopyranoside 4,6-phenylboronate -1b. 134 mg (98%); white
b
solid; mp = 153-155°C; [a]D= -87 (C 1, CH2Cl2). 1H NMR (300 MHz, CDCl3)
δ 7.80 (d, J = 7.1 Hz, 2H), 7.44 (t, J = 7.4 Hz, 1H), 7.35 (t, J = 7.5 Hz, 2H),
4.43 (d, J = 7.7 Hz, 1H, H1), 4.30 (dd, J = 10.4, 5.4 Hz, 1H, H6), 4.00 (t, J
= 10.4 Hz, 1H, H6), 3.92 (dt, J = 9.5, 6.8 Hz, 1H, OCH2), 3.83-3.70 (m, 2H,
H3 H4), 3.63-3.50 (m, 3H, OCH2 H5 H2), 2.95 (s, 1H, OH), 2.67 (d, J = 2.4
Hz, 1H, OH), 1.63 (quint, J = 6.8 Hz, 2H), 1.41 (hex, J = 7.3 Hz, 2H), 0.94
(t, J = 7.3 Hz, 3H).13C NMR (75 MHz, CDCl3) δ 134.1, 131.2, 127.6, 103.2
(C1), 74.9 (C3), 74.5 (C4), 74.1 (C2), 70.3 (OCH2), 68.6 (C5), 64.1 (C6), 31.6,
19.1, 13.8. The carbon α to the boron was not observed. 11B NMR (96 MHz,
CDCl3) δ 26.8. HRMS (ESI+) (M+Na)+ calculated for C16H23O611BNa
345.1485, found 345.1481.
Conclusion
A series of glycolipid phenylboronic esters was easily prepared in
one step from alkylglucosides and phenylboronic acid. These new
amphiphilic structures revealed good gelation abilities for the four
typical organic solvents evaluated in this study (toluene,
cyclohexane, fatty esters). Rheological properties and
morphologies of the organogels were determined for each gel
obtained with all possible solvent/organogelator combinations. It
showed that the alkyl chain length on the glucoside played an
important role on the gelation ability of the different solvents. The
organogelators self-assembled within the solvent to form fibers
observed by SEM. These fibers were the result of the packing of
organogelators in a hexagonal lattice where they behave as
dimers. In addition to the preparation and characterization of
these organogels, we demonstrated that the addition of a small
amount of water (5% v/v) was enough to disrupt the organogels
in few hours (less than 3h). The mechanism of disruption is clearly
related to the hydrolysis of the boronate function. Therefore, the
introduction of a boron atom within the structure of these
organogelators deeply impacts and enriches the properties of the
so formed organogels. For instance, these soft materials
represent useful systems for topical drug delivery with controlled
release upon contact with aqueous media. Such applications and
tuning of the water sensitivity are now under investigation in our
laboratories.
Butyl-a-D-glucopyranoside 4,6-phenylboronate -1b. 115 mg (84%); white
a
solid; mp = 100-102°C. [a]D= +95 (C 1, CH2Cl2). 1H NMR (400 MHz, CDCl3)
δ 7.81 (d, J = 7.1 Hz, 2H), 7.45 (t, J = 7.3 Hz, 1H), 7.34 (t, J = 7.4 Hz, 2H),
4.91 (d, J = 3.9 Hz, 1H, H1), 4.22 (dd, J = 9.2, 4.2 Hz, 1H, H6), 3.94 (t, J =
10.4 Hz, 1H, H6), 3.90-3.85 (m, 2H, H5 H3), 3.79-3.69 (m, 2H, OCH2 H4),
3.66 (dd, J = 9.4, 3.9 Hz, 1H, H2), 3.50 (dt, J = 9.8, 6.5 Hz, 1H, OCH2),
1.61 (quint, J = 6.9 Hz, 2H), 1.43-1.34 (m, 2H), 0.93 (t, J = 7.4 Hz, 3H). 13
C
NMR (101 MHz, CDCl3) δ 134.3, 131.3, 127.7, 98.9 (C1), 75.0 (C4), 73.6
(C3), 72.7 (C2), 68.7 (OCH2), 64.7 (C5), 64.5 (C6), 31.6, 19.5, 13.9. The
carbon α to the boron was not observed. 11B NMR (96 MHz, CDCl3) δ 27.2.
HRMS (ESI+) (M+Na)+ calculated for C16H23O611B Na 345.14854, found
345.1480.
Hexyl-b-D-glucopyranoside 4,6-phenylboronate 1c. 103 mg (98%); white
solid; mp = 173-175°C. [a]D = -40 (C 1, CH2Cl2). 1H NMR (300 MHz, CDCl3)
δ 7.81 (dd, J = 7.1, 1.3 Hz, 2H), 7.45 (tt, J = 7.3, 1.4 Hz, 1H), 7.36 (t, J =
7.3 Hz 2H), 4.44 (d, J = 7.7 Hz, 1H, H1), 4.30 (dd, J = 10.4, 5.4 Hz, 1H, H6),
4.01 (t, J = 10.3 Hz, 1H, H6), 3.91 (dt, J = 9.5, 6.8 Hz, 1H, OCH2), 3.85-
3.73 (m, 2H, H3 H4), 3.61-3.50 (m, 3H, OCH2 H2 H5), 2.85 (d, J = 1.9 Hz,
1H, OH), 2.56 (d, J = 2.4 Hz, 1H, OH), 1.64 (quint, J = 7.5 Hz , 2H), 1.41-
1.27 (m, 6H), 0.90 (t, J = 6.8 Hz, 3H). 13C NMR (75 MHz, CDCl3) δ 134.3,
131.3, 127.8, 103.4 (C1), 75.1 (C2), 74.7 (C4), 74.3 (C3), 70.8 (OCH2),
68.7 (C5), 64.2 (C6), 31.7, 29.7, 25.7, 22.7, 14.2. The carbon α to the
boron was not observed. 11B NMR (96 MHz, CDCl3) δ 27.7. HRMS (ESI+)
(M+Na)+ calculated for C18H27O611BNa 373.1798, found 373.1796.
Experimental Section
General Information. 1H NMR spectra (300 MHz, 400 MHz), 13C NMR (75
MHz, 101 MHz) and 11B (96 MHz) were recorded on Bruker AC 300 and
AC 400 spectrometers. Chemical shifts are given in ppm and coupling
constants J in Hz. Multiplicities are presented as follows: s = singlet, d =
doublet, t = triplet, q = quartet, quint = quintuplet, hex = hexuplet, m =
multiplet, br = broad. The carbon bearing the boron atom was not observed
due to the quadrupolar relaxation mechanism of 11B nucleus. High-
resolution mass spectra (HRMS) were recorded, either on a Bruker MaXis
4G, an Agilent 6510, or a Thermo Fisher Q-Exactive spectrometer (Centre
Régional de Mesures Physiques de l’Ouest, Rennes) using positive ion
Electron-Spray ionization techniques. Melting points were measured on a
Heptyl-b-D-glucopyranoside 4,6-phenylboronate 1d. 103 mg (99%); white
solid; mp = 164-166°C. [a]D= -54 (C 1, CH2Cl2). 1H NMR (300 MHz, CDCl3)
δ 7.84 (d, J = 6.5 Hz, 2H), 7.46 (t, J = 7.4 Hz, 1H), 7.37 (t, J = 7.2 Hz, 2H),
4.45 (d, J = 7.7 Hz, 1H, H1), 4.31 (dd, J = 10.4, 5.3 Hz, 1H, H6), 4.02 (t, J
= 10.4 Hz, 1H, H6), 3.93 (dt, J = 9.5, 6.9 Hz, 1H, OCH2), 3.82 (t, J = 9.1 Hz,
1H, H3), 3.77 (td, J = 9.1, 1.9 Hz, 1H, H5), 3.64-3.50 (m, 3H, H2 H4 OCH2),
2.97 (brs, 1H, OH), 2.69 (brs, 1H, OH), 1.75-1.58 (m, 2H), 1.37 (m, 8H),
0.96 (t, J = 7.3 Hz, 3H). 13C NMR (75 MHz, CDCl3) δ 134.3, 131.3, 127.8,
6
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10.1002/chem.202001970
Chemistry - A European Journal
FULL PAPER
103.4 (C1), 75.0 (C4), 74.7 (C3), 74.2 (C2), 70.8 (OCH2), 68.7 (C5), 64.2
(C6), 31.9, 29.7, 29.2, 26.0, 22.7, 14.2. The carbon α to the boron was not
observed. 11B NMR (96 MHz, CDCl3) δ 27.0. HRMS (ESI) (M+Na)+
calculated for C19H29O611BNa 387.1955, found 387.1950.
Rheology. Organogel samples were presented under disc form (4 cm
diameter). Rheological measurements were performed on an Anton-Paar
MCR301 equipped with an upper plate of 75 mm diameter. The frequency
(ω) was fixed to 1 Hz and the amplitude deformation (γ°) was gradually
increased from 0 to 50% of shearing. Storage modulus G’ and the loss
modulus G” were obtained at 25°C.
Octyl-b-D-glucopyranoside 4,6-phenylboronate 1e. 127 mg (98%); white
solid; mp = 172-173°C. [a]D= -71 (C 1, CH2Cl2). 1H NMR (400 MHz, CDCl3)
δ 7.81 (d, J = 7.1 Hz, 2H), 7.43 (t, J = 7.4 Hz, 1H), 7.34 (t, J = 7.4 Hz, 2H),
4.42 (d, J = 7.7 Hz, 1H, H1), 4.27 (dd, J = 10.4, 5.3 Hz, 1H, H6), 3.98 (t, J
= 10.4 Hz, 1H, H6), 3.95-3.82 (m, 1H, OCH2), 3.78 (t, J = 9.2 Hz, 1H, H3),
3.74 (td, J = 9.1, 1.1 Hz, 1H, H5), 3.64-3.49 (m, 3H, H4 H2 OCH2), 2.85 (brs,
1H, OH), 2.56 (d, J = 2.3 Hz, 1H, OH), 1.64 (quint, J = 6.9 Hz, 2H), 1.40-
1.20 (m, 10H), 0.88 (t, J = 6.7 Hz, 3H). 13C NMR (101 MHz, CDCl3) δ 134.3,
131.3, 127.8, 103.4 (C1), 75.0 (C3), 74.6 (C4), 74.2 (C2), 70.8 (OCH2), 68.7
(C5), 64.2 (C6), 31.9, 29.7, 29.5, 29.3, 26.0, 22.8, 14.2. The carbon α to the
boron was not observed. 11B NMR (96 MHz, CDCl3) δ 26.9. HRMS (ESI)
(M+Na)+ calculated for C20H31O611BNa 401.2111, found 401.2106.
Scanning Electron Microscopy (SEM). Metalisation by Au/Pd. Scanning
Electron Microscopy (SEM) of xerogels were evaluated using the JEOL IT
300 Scanning Electron Microscope. Samples were collected and
deposited on a Teflon plot. Each sample was examined using a voltage of
5 or 10 kV. Images were analyzed by SMileView software.
Small Angle X-Ray Scattering (SAXS). Organogel and xerogel samples
were prepared into capillaries. SAXS experiments were performed using
X-ray patterns collected with a Pilatus 300k (Dectris, Grenoble, France),
mounted on
a microsource X-ray generator GeniX 3D (Xenocs,
Sassenage, France) operating at 30 watts. The monochromatics CuKα
radiation is of λ = 1.541 Å. The diffraction patterns were therefore recorded
for reciprocal spacing q = 4π*sinθ/λ in a range of repetitive distances from
0.015 Å-1 (418 Å) and 1.77 Å-1 (8 Å). Images were transformed to graphics
using the software program Fit2D (ESRF).
Nonyl-b-D-glucoside 4,6 phenyl boronate 1f. 122 mg (94%); white solid;
mp = 170-172°C. [a]D= -76 (C 1, CH2Cl2). 1H NMR (400 MHz, CDCl3) δ
7.84 (d, J = 6.5 Hz, 2H), 7.46 (t, J = 7.5 Hz, 1H), 7.37 (t, J = 7.4 Hz, 2H),
4.45 (d, J = 7.7 Hz, 1H, H1), 4.31 (dd, J = 10.4, 5.4 Hz, 1H, H6), 4.02 (t, J
= 10.4 Hz, 1H, H6), 3.93 (dt, J = 9.5, 6.9 Hz, 1H, OCH2), 3.82 (t, J = 9.1 Hz
1H, H3) 3.78 (td, J = 9.2, 2.2 Hz, 1H, H5), 3.66-3.51 (m, 3H, OCH2 H2 H4),
3.18 (d, J = 2.1 Hz, 1H, OH), 2.92 (d, J = 2.6 Hz, 1H, OH), 1.67 (quint, J =
6.8 Hz, 2H), 1.39-1.25 (m, 12H), 0.91 (t, J = 6.7 Hz, 3H). 13C NMR (101
MHz, CDCl3) δ 134.3, 131.3, 127.8, 103.4 (C1), 75.1 (C3), 74.7 (C4), 74.2
(C2), 70.8 (OCH2), 68.7 (C5), 64.2 (C6), 32.0, 29.7, 29.6, 29.5, 29.4, 26.0,
22.8, 14.2. The carbon α to the boron was not observed. 11B NMR (96 MHz,
CDCl3) δ 28.3. HRMS (ESI) (M+Na)+ calculated for C21H33O611BNa,
415.22624, found 415.2265.
Molecular model. Cartoon representation of the self-assembly of the
organogelators were obtained from the assembly of one molecule
previously minimized. These preliminary minimizations were carried out in
vacuum, in periodic boundaries and standard conditions taking into
account Coulomb and Van der Waals interactions; with semi-empirical and
restricted Hartree-Fock method, AM1[18] coupled with an AMBER[19] force
field; version for Yasara software[20] (YAMBER03). Assembly of several
molecule and final representation were obtained using Samson
software[21]
.
Decyl-b-D-glucoside-4, 6 phenyl boronate 1g. 117 mg (96%); white solid;
mp = 154-156°C. [a]D= -66 (C 1, CH2Cl2). 1H NMR (400 MHz, CDCl3) δ
7.83 (d, J = 7.9 Hz, 2H), 7.46 (t, J = 7.4 Hz, 1H), 7.37 (t, J = 7.4 Hz, 2H),
4.45 (d, J = 7.7 Hz, 1H, H1), 4.30 (dd, J = 10.4, 5.3 Hz, 1H, H6), 4.01 (t, J
= 10.4 Hz, 1H, H6), 3.92 (dt, J = 9.5, 6.9 Hz, 1H, OCH2), 3.82 (t, J = 8.8 Hz,
1H, H3), 3.75 (td, J = 9.2, 2.1 Hz, 1H, H5), 3.65-3.51 (m, 3H, OCH2 H2 H4),
3.31 (d, J = 2.2 Hz, 1H, OH), 3.05 (d, J = 2.7 Hz, 1H, OH), 1.67 (quint, J =
6.9 Hz, 2H), 1.33-1.29 (m, 14H), 0.91 (t, J = 6.8 Hz, 3H). 13C NMR (101
MHz, CDCl3) δ 134.3, 131.3, 127.8, 103.4 (C1), 75.1 (C3), 74.6 (C4), 74.2
(C2), 70.8 (OCH2), 68.7 (C5), 64.2 (C6), 32.0, 29.7, 29.7 (2C), 29.5, 29.4,
26.0, 22.8, 14.2. The carbon α to the boron was not observed. 11B NMR
Acknowledgements
We acknowledge the Ministère de l’Education Nationale, de la
Recherche et de la Technologie. The authors thank F.
Gouttefangeas and L. Joanny from the CMEBA (Centre de
Microscopie Electronique à Balayage et microAnalyse, Rennes)
for assistance in recording SEM images.
(96 MHz, CDCl3)
C22H35O611BNa 429.2424, found 429.2419.
δ
27.5. HRMS (ESI) (M+Na)+ calculated for
Keywords: Organogel • boronate • glycolipid • alkylglucoside •
water sensitive
Dodecyl-b-D-glucoside-4, 6 phenyl boronate 1h. 126 mg (99%); white
solid; mp = 134-136°C. [a]D= -64 (C 1, CH2Cl2). 1H NMR (400 MHz, CDCl3)
δ 7.83 (d, J = 7.4 Hz, 2H), 7.47 (t, J = 7.3 Hz, 1H), 7.37 (t, J = 7.4 Hz, 2H),
4.45 (d, J = 7.7 Hz, 1H, H1), 4.32 (dd, J = 10.4, 5.4 Hz, 1H, H6), 4.02 (t, J
= 10.4 Hz, 1H, H6), 3.93 (dt, J = 9.6, 6.8 Hz, 1H, OCH2), 3.80 (t, J = 9.2 Hz,
1H, H3), 3.76 (t, J = 9.2 Hz, 1H, H5), 3.62-3.54 (m, 3H, H2 H4 OCH2), 2.99
(brs, 1H, OH), 2.72 (brs, 1H, OH), 1.67 (quint, J = 7.1 Hz, 2H), 1.29 (m,
18H), 0.91 (t, J = 6.7 Hz, 3H). 13C NMR (101 MHz, CDCl3) δ 134.3, 131.3,
127.8, 103.4 (C1), 75.1 (C3), 74.7 (C4), 74.3 (C2), 70.8 (OCH2), 68.7 (C5),
64.2 (C6), 32.1, 29.80, 29.78, 29.74 (2C), 29.70, 29.54, 29.50, 26.1, 22.8,
14.3. The carbon α to the boron was not observed. 11B NMR (96 MHz,
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10.1002/chem.202001970
Chemistry - A European Journal
FULL PAPER
Entry for the Table of Contents
B
O
O
O
HO
O
HO
HO
HO
Alk
OH
O
O
Alk
OH
+
OH
B
OH
Synthesis of alkyglucoside phenyl boronic esters provided a series of organogels in several solvents. Similar to benzylidene type
organogelators, the replacement of the acetal group by a boronate function induced singular self-assembling properties and high
sensitivity to water.
Institute and/or researcher Twitter usernames: @LLemiegre
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