Chloroalkane gel formations by tris-urea low molecular weight gelator under various conditions

Article abstract of DOI:10.1021/jo900894q

(Chemical Equation Presented) C₃-symmetrical tris-urea low molecular weight gelator 1 was synthesized in three steps from phloroglucin. The gelation ability of 1 in four chloroalkanes, i.e., CH₂Cl ₂, CHCl₃, 1,1,2-trichloroethane, and 1,1,2,2- tetrachloroethane, was investigated under various conditions. Thermal treatment was effective in gelating 1,1,2-trichloroethane. In the presence of equimolar 1 and CuBr₂, 1,1, 2-trichloroethane and 1,1,2,2-tetrachloroethane formed gels. Mixtures of 1 and CHCl₃ or 1,1,2, 2-tetrachloroethane gave gels after ultrasound irradiation. CH₂Cl₂ changed into a gel in the presence of equimolar 1 and BiCl₃ after sonication. Spherical particles with rough surfaces were found by SEM observation of CHCl₃ gel prepared from ultrasound irradiation of 1 and Y(NO ₃)₃.

Full text of DOI:10.1021/jo900894q

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Chloroalkane Gel Formations by Tris-urea Low Molecular Weight
Gelator under Various Conditions
Masamichi Yamanaka* and Hiromitsu Fujii
Department of Chemistry, Faculty of Science, Shizuoka University, 836 Ohya, Suruga-ku,
Shizuoka 422-8529, Japan
smyaman@ipc.shizuoka.ac.jp
Received April 30, 2009
C3-symmetrical tris-urea low molecular weight gelator 1 was synthesized in three steps from
phloroglucin. The gelation ability of 1 in four chloroalkanes, i.e., CH2Cl2, CHCl3, 1,1,2-trichloro-
ethane, and 1,1,2,2-tetrachloroethane, was investigated under various conditions. Thermal treatment
was effective in gelating 1,1,2-trichloroethane. In the presence of equimolar 1 and CuBr2, 1,1,
2-trichloroethane and 1,1,2,2-tetrachloroethane formed gels. Mixtures of 1 and CHCl3 or 1,1,2,
2-tetrachloroethane gave gels after ultrasound irradiation. CH2Cl2 changed into a gel in the presence
of equimolar 1 and BiCl3 after sonication. Spherical particles with rough surfaces were found by SEM
observation of CHCl3 gel prepared from ultrasound irradiation of 1 and Y(NO3)3.
Introduction
gelators (LMWGs) have been reported.1-9 The ability to use
several stimuli for the sol-gel phase transition based on the
rational design of LMWGs is a remarkable advantage of
supramolecular gels.10-14 These reversible conversions of rheol-
ogy have potential as future materials with wide application in
Regulated assembly of a low molecular weight compound
builds up a supramolecular gel. Many low molecular weight
*To whom correspondence should be addressed. Phone: (þ81) 54-238-
4936. Fax: (þ81) 54-237-3384.
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Published on Web 06/24/2009
DOI: 10.1021/jo900894q
r
2009 American Chemical Society

Yamanaka and Fujii
JOCArticle
material separations, sensors, drug delivery systems, cosmetics,
etc. One general method for supramolecular gel formation is the
thermal dissolution of LMWG into a solvent and cooling the
mixture. Ultrasound irradiation-induced supramolecular gel
formations have also been reported lately.15 Gel formation by
the cooperative work of two chemicals, i.e., two-component gels,
is another topic of LMWG research.16-18 Ideas of metal-ligand
coordination or host-guest chemistry were adopted in these
two-component gels. Generally, an LMWG gelates solvents
under one specific condition. Multifunctional LMWGs, for
instance, an LMWG gelated by different stimuli depending on
the solvents, are hardly known. Recently, we have developed
tris-urea LMWGs that enable reversible sol-gel phase transi-
tions in response to chemical stimuli.19 Our tris-urea gelator has
a highly symmetrical and readily divisible structure; therefore, it
is easy to synthesize various derivatives using similar proce-
dures.20 In the course of this study, we designed C3-symmetrical
tris-urea 1 as a derivative. Synthesized tris-urea 1 acted as a
talented LMWG beyond expectations. In this paper, we would
like to report that a tris-urea LMWG 1 gels four chloroalkanes
in different ways, i.e., by thermal dissolution and cooling or
ultrasound irradiation in the presence or absence of metal salt.
SCHEME 1. Synthesis of Tris-urea 1
Results and Discussion
Tris-urea 1 was synthesized in three steps from phloro-
glucin (Scheme 1). Nucleophilic aromatic substitution of
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by hydrogen in the presence of palladium on carbon. Tris-
amine 3 was reacted with an excess amount of octylisocya-
nate, and the desired tris-urea 1 was obtained as a white solid.
Gelation experiments using 1 were accomplished in four
chloroalkanes: CH2Cl2, CHCl3, 1,1,2-trichloroethane, and
1,1,2,2-tetrachloroethane. Gel formation was evaluated by
the inverted tube test. A mixture remaining at the top of an
inverted test tube was defined as a gel. The term “partial gel”
was used when some part of a mixture remained at the top of
the inverted test tube and some flowed down the tube slowly
or quickly.
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Thermal treatment and cooling of a mixture of 1 and
chloroalkane solvents were performed (Table 1, Figure 1).
Mixtures of tris-urea 1 and CH2Cl2 or CHCl3 in plugged
test tubes were heated up to 70 °C; however, insoluble
suspensions remained unchanged. Homogeneous solu-
tions were obtained by heating mixtures of 1 and 1,1,
2-trichloroethane or 1,1,2,2-tetrachloroethane in plugged
test tubes to 110 °C. A 1,1,2-trichloroethane solution
of 1 was converted into a white semitransparent gel by
cooling, and the critical gelation concentration (CGC) was
determined to be 10 mM. A 1,1,2,2-tetrachloroethane
solution of 1 gave only a partial gel even at higher concen-
trations (up to 25 mM). A scanning electron microscope
(SEM) image of a xerogel prepared by freeze-drying a 1,1,
2-trichloroethane gel with 1 showed intertwining nanofibers
(Figure 2a). Freeze-drying a sample of the gelous mixture
of 1 and 1,1,2,2-tetrachloroethane indicated micrometer-
sized fibrous aggregates (Figure 2b).
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TABLE 1. Gelation Properties of Tris-urea 1 by Thermal Treatment
and Coolinga
1,1,2-trichloro-
ethane
1,1,2,2-tetra-
chloroethane
additiveb
CH2Cl2
CHCl3
none
CuBr2
I
I
I
I
G (10)
G (5)
PG
G (5)
aTris-urea 1 = up to 25 mM. Abbreviations: G, gel; PG, partial gel; I,
insoluble suspension; values in parentheses refer to critical gelation
concentration (mM). bEquimolar amount for tris-urea 1.
FIGURE 1. Photographs of mixtures of chloroalkanes and 1 after
thermal treatment and cooling: (a) CH2Cl2, (b) CHCl3, (c) 1,1,2-
trichloroethane, (d) 1,1,2,2-tetrachloroethane.
It is known that some low molecular weight compounds
form supramolecular gels by adding metal salt,18 and the
effect of metal salts on the gelation of 1 and chloroalkane
solvents was investigated (Table 1, Figure 3). The addition
of CuBr2 was effective for the gelation of 1 in 1,1,2-trichlor-
oethane. A mixture of 1 and CuBr2 in 1,1,2-trichloroe-
thane gave a pale brown semitransparent gel after thermal
dissolution and cooling. The CGC of 1 with 1,1,2-trichlor-
oethane was reduced from 10 to 5 mM by adding 1 equiv
of CuBr2. A SEM image of the 1,1,2-trichloroethane gel
with 1 and CuBr2 showed spherical particles of 1-2 μm
diameter (Figure 2c). These results indicated that gelation
proceeds with different mechanisms in the presence or
absence of CuBr2. Similar phenomena were also observed
in the gelation of 1 and metal salts with ultrasound
irradiation (vide infra). A dramatic alteration in the gela-
tion ability of 1 in 1,1,2,2-tetrachloroethane was observed
by addition of CuBr2. An equimolar mixture of 1 and
CuBr2 in 1,1,2,2-tetrachloroethane afforded a pale brown
semitransparent gel, and the CGC was 5 mM; however, 1
itself only gave a partial gel in 1,1,2,2-tetrachloroethane
without the metal salt. A SEM image of 1,1,2,2-tetrachlor-
oethane gel with 1 and CuBr2 showed fibrous aggregates
that were much thinner than those prepared from 1 with-
out the metal salt (Figure 2d). Tris-urea 1 and CH2Cl2 or
CHCl3 did not form gels by adding CuBr2 or other
metal salts (such as MgCl2, LaCl3, or BiCl3) in thermal
treatment and cooling.
FIGURE 2. SEM images of gels (xerogels) prepared by thermal
treatment and cooling: (a) 1,1,2-trichloroethane gel of 1, (b) 1,1,2,2-
tetrachloroethane partial gel of 1, (c) 1,1,2-trichloroethane gel of
1 and CuBr2, (d) 1,1,2,2-tetrachloroethane gel of 1 and CuBr2.
Gelation of some kinds of LMWGs has been induced
by ultrasound irradiation.15,19,20 Sonication-induced gela-
tion of chloroalkanes by tris-urea 1 is shown in Table 2 and
Figure 4. A mixture of 1 and CHCl3 formed a semitranspar-
ent gel with ultrasound irradiation for several hours, and the
CGC was determined as 15 mM. 1,1,2,2-Tetrachloroethane
gave a gel with high transparency in the presence of 5 mM
of 1 after sonication. Mixtures of 1 and either CH2Cl2 or
1,1,2-trichloroethane afforded heterogeneous suspensions
even after ultrasound irradiation; however, a mixture of 1
and 1,1,2-trichloroethane formed a gel by thermal dissolu-
tion and cooling. The gelation property of tris-urea 1 was
considerably altered according to the gel-forming methods.
Other solvents such as methanol, acetone, and ethyl acetate
were also gelated by sonication in the presence of appro-
priate amounts of 1. CGCs of 1 were determined as 5 mM
for methanol, 10 mM for acetone, and 15 mM for ethyl
acetate (Figure S1, Supporting Information). It seems
that ultrasound irradiation is generally a better procedure
than thermal treatment for gelation using tris-urea 1. A
xerogel of the CHCl3 gel with 1 showed micrometer-sized
fibrous structures on SEM observation (Figure 5a). Thermal
treatment of a mixture of 1,1,2,2-tetrachloroethane and 1
gave a partial gel that was constructed from micrometer-
FIGURE 3. Photographs of mixtures of chloroalkanes, 1, and
CuBr2 after thermal treatment and cooling: (a) CH2Cl2, (b) CHCl3,
(c) 1,1,2-trichloroethane, (d) 1,1,2,2-tetrachloroethane.
FIGURE 4. Photographs of mixtures of chloroalkanes and 1 after
ultrasound irradiation: (a) CH2Cl2, (b) CHCl3, (c) 1,1,2-trichlor-
oethane, (d) 1,1,2,2-tetrachloroethane.
sized fibrous aggregates (Figure 2b). On the other hand,
nano-ordered thin fibers were observed by SEM measure-
ment of the xerogel prepared by freeze-drying the 1,1,2,
2-tetrachloroethane gel with 1 (Figure 5b). SEM images of
the methanol, acetone, and ethyl acetate gels with 1 showed
intertwined nanofibers on SEM observation.
Gelation experiments were performed using chloroalk-
anes in the presence of 1 and metal salts with ultrasound
irradiation (Table 2). A mixture of 1 and CuBr2 gelled CHCl3
5392 J. Org. Chem. Vol. 74, No. 15, 2009

Yamanaka and Fujii
JOCArticle
TABLE 2. Gelation Properties of Tris-urea 1 by Ultrasound Irradiationa
1,1,2-trichloro-
ethane
1,1,2,2-tetrachlo-
roethane
additiveb
CH2Cl2
CHCl3
none
I
I
G (15)
G (5)
PG
I
I
I
I
G (5)
G (5)
G (20)
I
CuBr2
BiCl3
Y(NO3)3
G (15)
PG
G (10)
aTris-urea 1 = up to 25 mM. Abbreviations: G, gel; PG, partial gel; I,
insoluble suspension. Values in parentheses refer to critical gelation
concentration (mM). bEquimolar amount for tris-urea 1.
with sonication; however, this combination did not give
a gel with thermal treatment. The CGC for CHCl3 was
reduced from 15 to 5 mM by addition of CuBr2. Tris-urea
1
and CuBr2 also gelated 1,1,2,2-tetrachloroethane,
although the CGC was unchanged. The 1,1,2,2-tetrachlor-
oethane gel with 1 and CuBr2 prepared by ultrasound
irradiation was freeze-dried for SEM observation. The
xerogel showed intertwining nanofibers that were some-
what different from the SEM image of the same com-
ponent’s gel prepared by thermal dissolution and cooling
(Figures 2d and 5c). The difference in the two 1,1,2,
2-tetrachloroethane gels with 1 and CuBr2 also appeared
in their mechanical stabilities. The gel-sol transition tem-
perature (Tgel) measured by the ball-dropping method21
was used to estimate the mechanical stabilities of gels pre-
pared in different ways. The Tgel of the gel (10 mM each
of 1 and CuBr2) prepared by thermal dissolution and
cooling was 69 °C, while the Tgel of the gel (10 mM each of
1 and CuBr2) prepared by ultrasound irradiation was 57 °C.
The thermal dissolution and cooling method seemed to
give more stable gels than those prepared using ultrasound
irradiation. Mixtures of 1 and CuBr2 in CH2Cl2 or 1,1,
2-trichloroethane did not form gel with sonication. The
addition of BiCl3 was effective for gelation of a mixture of
1 and CH2Cl2 (Figure 6). Ultrasound irradiation of a mixture
of 1 and BiCl3 (each 15 mM) in CH2Cl2 gave an opaque
gel. Adding an equimolar amount of BiCl3 was essential
to form the CH2Cl2 gel with 1: a shortage or excess of
BiCl3 with 1 no longer formed gels but gave suspensions.
An equimolar mixture of 1 and BiCl3 in CHCl3 afforded
only a partial gel even at higher concentrations (<25 mM).
1,1,2-Trichloroethane did not give a gel by mixing with 1
and BiCl3. A mixture of 1 and BiCl3 in 1,1,2,2-tetrachlor-
oethane formed a gel by sonication; however, the CGC
(20 mM) was higher than the CGC with tris-urea 1 alone.
CH2Cl2 formed a partial gel by mixing with equimolar 1
and Y(NO3)3 after sonication. Tris-urea 1 and Y(NO3)3
(each 10 mM) in CHCl3 formed a semitransparent gel
after ultrasound irradiation. An SEM image of the gel
showed 1 to 5 μm-sized spherical particles with rough
surfaces (Figure 5d). A precipitable suspension of Y(NO3)3
in CHCl3 also showed micrometer-sized spherical particles,
although their sizes were smaller and their surfaces were
smoother than the particles of the gel with 1 and Y(NO3)3
(Figure S2, Supporting Information). The explanation seems
to be that micrometer-sized particles were constructed from
Y(NO3)3 in CHCl3 and their surfaces were covered with
self-assembled tris-urea 1. Both 1,1,2-trichloroethane and
1,1,2,2-tetrachloroethane became suspensions by mixing
with 1 and Y(NO3)3 even after sonication.
FIGURE 5. SEM images of gels (xerogels) prepared by ultrasound
irradiation: (a) CHCl3 gel of 1, (b) 1,1,2,2-tetrachloroethane gel of 1,
(c) 1,1,2,2-tetrachloroethane gel of 1 and CuBr2, (d) CHCl3 gel of 1
and Y(NO3)3.
FIGURE 6. Photographs of mixtures of chloroalkanes, 1, and
BiCl3 after ultrasound irradiation: (a) CH2Cl2, (b) CHCl3, (c)
1,1,2-trichloroethane, (d) 1,1,2,2-tetrachloroethane.
Conclusion
We have synthesized tris-urea LMWG 1 from phloro-
glucin. Tris-urea 1 was able to gelate four chloroalkanes
using several procedures. The process of thermal treatment
and cooling was effective for forming 1,1,2-trichloroethane
gel with 1. 1,1,2,2-Tetrachloroethane changed into a gel in
the presence of both 1 and CuBr2 by thermal treatment
and cooling. Ultrasound irradiation induced CHCl3 gel
formation with 1. Equimolar amounts of 1 and BiCl3 could
form CH2Cl2 gel after ultrasound irradiation. Most gels
showed fibrous structures distinctive of gels on SEM
observation; however, some gels (e.g., the CHCl3 gel with
1 and Y(NO3)3) exhibited spherical particles as consti-
tuents. We have demonstrated that tris-urea 1 can act as a
talented LMWG.
Experimental Section
Synthesis of 1,3,5-Tris(4-nitrophenoxy)benzene (2). To a
mixture of phloroglucin (3.00 g, 23.8 mmol), K2CO3 (65.8 g,
476 mmol), and DMF (240 mL) was added 4-fluoronitroben-
zene (10.0 mL, 94.0 mmol). The reaction mixture was stirred
at 100 °C for 2 days. K2CO3 was removed by filtration, and
then the filtrate was concentrated under reduced pressure. The
residue was dissolved in CH2Cl2 and washed with water
and brine successively. The organic layer was dried over
Na2SO4, and the solvent was removed under reduced pre-
ssure. The crude product was purified by column chromatog-
raphy (SiO2, n-hexane/EtOAc 40/1). The desired product (2)
was obtained as a yellow solid (7.76 g, 67%): mp 195-196 °C;
(21) Takahashi, A.; Sakai, M.; Kato, T. Polym. J. 1980, 12, 335–341.
J. Org. Chem. Vol. 74, No. 15, 2009 5393

Yamanaka and Fujii
JOCArticle
1H NMR (600 MHz, CDCl3) δ 6.65 (s, 3H), 7.13 (d, J = 8.9 Hz,
6H), 8.26 (d, J = 8.9 Hz, 6H); 13C NMR (150 MHz, CDCl3) δ
108.0, 118.6, 126.5, 144.0, 158.2, 161.7; HRMS (ESI, M þ Naþ)
calcd for C24H15N3NaO9 512.0706, found 512.0698.
28.7, 28.8, 28.8, 29.7, 31.2, 99.9, 119.0, 120.2, 137.3, 148.6, 155.2,
160.1; HRMS (ESI, M þ Naþ) calcd for C51H72N6NaO6
887.5411, found 887.5382.
Gelation Experiments. A weighed amount of tris-urea 1 (and
metal salt) was placed in test tube, and 0.3 mL of solvent was
added. For the thermal treatment, mixtures of CH2Cl2 or CHCl3
in closed test tubes were heated at 70 °C over 2 h, and mixtures of
1,1,2-trichloroethane and 1,1,2,2-tetrachloroethane in closed
test tubes were heated at 110 °C until dissolved. All heated
mixtures were gradually cooled to room temperature. For
ultrasound irradiation, mixtures in closed test tubes were placed
in an ultrasonic cleaner and sonicated for 5 h. The bath
temperature of the cleaner was maintained at approximately
25 °C. Gels were evaluated at the point of formation, and partial
gels and suspensions were judged after being kept undisturbed at
room temperature for 1 day.
Synthesis of 1,3,5-Tris(4-aminophenoxy)benzene (3). A mix-
ture of 2 (7.00 g, 14.3 mmol), 10% palladium on carbon (700 mg,
10 wt %), and EtOAc (140 mL) was stirred at ambient tempera-
ture under a hydrogen atmosphere for 16 h. The mixture was
filtered and the solvent removed under reduced pressure. The
crude product was purified by column chromatography (SiO2,
EtOAc). The desired product (3) was obtained as a brown solid
(5.43 g, 95%): mp 142-145 °C; 1H NMR (600 MHz, CDCl3) δ
6.15 (s, 3H), 6.64 (d, J = 8.2 Hz, 6H), 6.84 (d, J = 8.2 Hz, 6H);
13C NMR (150 MHz, CDCl3) δ 100.3, 116.5, 121.5, 143.2, 148.1,
161.1; HRMS (ESI, M þ Naþ) calcd for C24H21N3NaO3
422.1481, found 422.1498.
Synthesis of 1,3,5-Tris(4-octylureidophenoxy)benzene (1). To
a mixture of 3 (1.00 g, 2.50 mmol) and ClCH2CH2Cl (50 mL)
was added octyl isocyanate (2.00 mL, 11.3 mmol). The reaction
mixture was refluxed for 1 day. The mixture was cooled
to ambient temperature, and the solid obtained was collected
by filtration. Further washing of the solid with n-hexane
and CH2Cl2 gave pure 1 as a white solid (1.87 g, 86%): mp
165-166 °C; 1H NMR (600 MHz, CDCl3) δ 0.84 (t, J = 6.9 Hz,
9H), 1.26-1.41 (m, 36H), 3.05 (dt, J = 2.5, 6.5 Hz, 6H), 6.06-
6.08 (m, 6H), 6.94 (d, J = 8.9 Hz, 6H), 7.37 (d, J = 8.9 Hz, 6H),
8.42 (s, 3H); 13C NMR (150 MHz, CDCl3) δ 13.9, 22.1, 26.4,
Acknowledgment. This work was partly supported by
Tokuyama Science Foundation. The research was partially
carried out using an instrument at the Center for Instru-
mental Analysis of Shizuoka University.
Supporting Information Available: Figures S1 and S2 and
1H and 13C NMR spectra of synthesized compounds. This
material is available free of charge via the Internet at http://
pubs.acs.org.
5394 J. Org. Chem. Vol. 74, No. 15, 2009