Organogelators derived from [3.3]metacyclophane skeleton with a urea unit

Article abstract of DOI:10.1246/cl.2012.485

Dithia[3.3]metacyclophanes with a urea unit having long alkyl chains have been prepared. It has been found out that some of them give stable organogels in some solvents, and their gelating ability depends on structural properties of both aromatic componen

Full text of DOI:10.1246/cl.2012.485

doi:10.1246/cl.2012.485
Published on the web April 21, 2012
485
Organogelators Derived from [3.3]Metacyclophane Skeleton with a Urea Unit
Akihiko Tsuge,* Ryuichiro Matsushita, Katsuhiko Sakura, Tetsuji Moriguchi, and Koji Araki
Department of Applied Chemistry, Kyushu Institute of Technology,
1-1 Sensui-cho, Tobata-ku, Kitakyushu, Fukuoka 804-8550
(Received February 7, 2012; CL-120100; E-mail: tsuge@che.kyutech.ac.jp)
Dithia[3.3]metacyclophanes with a urea unit having long
alkyl chains have been prepared. It has been found out that some
of them give stable organogels in some solvents, and their
gelating ability depends on structural properties of both aromatic
components in the dithia[3.3]metacyclophane.
HSH2C
CH2SH
OCnH2n+1
NO2
S
OCnH2n+1
1a; n = 0
S
During the past several years, a great deal of research has
focused on low-molecular-weight organogelators (LMOGs)1
because they are not only of fundamental scientific interest
but also have promising technological applications in various
areas. Large structural diversity has been proposed to fulfill the
requirements of the organogelators capable of gelling organic
solvents in which gelators consisting of the cyclic component
have also been investigated. As those cyclic gelators, macro-
cyclic compositions such as calixarene,2 resorcinarene,3 cucur-
bit[7]uril,4 crown ether,5 cyclodextrin,6 cyclophane,7 and de-
hydrobenzoannulene-based macrocycles8 have been employed.
On the other hand, there has been only one example on an
organogelator based on the small-sized cyclic compound
reported by Abe9 to the best of our knowledge; however, in
this report [2.2]paracyclophane has been employed as the base to
construct the imidazole dimer photochromic organogel.
We have been interested in small-sized cyclophanes,
especially [n.n]metacyclophanes in terms of their specific ³
systems due to the strong transannular ³-electronic interactions
between aromatic components in close proximity.10 These
[n.n]metacyclophanes can also be characterized by their con-
formational properties like the conversion between the syn- and
the anti-conformers. From the view points of creating novel
organogelators the [n.n]metacyclophane skeleton could be a
unique candidate because of two components in the cyclophane
compounds arranged in the layered structure. In order to realize
the organogelators consisting of the metacyclophane skeleton we
focused on the dithia[3.3]metacyclophanes because we have
established the synthetic strategy to obtain their derivatives.
Thus, we have designed dithia[3.3]metacyclophanes which
carry the appropriate long alkyl chain connected by the urea unit
to the aromatic ring.
1b; n = 3
1c; n = 6
1d; n = 8
1e; n = 10
1f; n = 12
3a; n = 0 3d; n = 8
3b; n = 3 3e; n = 10
3c; n = 6 3f; n = 12
OCnH2n+1
S
BrH2C
CH2Br
S
NH2
NO2
4a; n = 0 4d; n = 8
4b; n = 3 4e; n = 10
4c; n = 6 4f; n = 12
2
OCnH2n+1
S
S
H2N
OCmH2m+1
H
N
H
N
R
C
O
6a; m = 6
6b; m = 8
6c; m = 10
6d; m = 12
5a; n = 3, R=Br
5b; n = 6, R=Br
5c; n = 12, R=Br
5d; n = 12, R=H
OCnH2n+1
S
H
N
H
N
S
OCmH2m+1
C
O
7a; n = 0, m = 12 7d; n = 8, m =10
7b; n = 3, m = 12 7e; n = 10, m = 8
7c; n = 6, m = 12 7f; n = 12, m = 6
5-Alkoxy-1,3-bis(sulfanylmethyl)benzenes 1a-1f were ob-
tained by reduction of the corresponding diethyl 5-alkoxyiso-
phthalates with hydrogen gas in the presence of 10% Pd/C,
followed by chlorination using thionyl chloride and subsequent
reaction with thiourea in yields of 20-35%. Bromination of 1,3-
dimethyl-5-nitrobenzene with NBS gave the bis(bromomethyl)
compound 2 in a yield of 55%. Cyclization of 1a-1f and 2 using
K2CO3 as a base under highly dilute conditions11 afforded the
corresponding dithia[3.3]metacyclophanes 3a-3f in yields of
34-57%.
H3C
H3C
H3C
H
N
H
N
OC12H25
OC6H13
C
O
H3C
9
8
The aminocyclophanes 4a-4f were prepared by reductions
of 3a-3f in yields of 73-90% (Scheme 1).
Scheme 1. Chemical structures of [3.3]metacyclophanes and
referential compounds.
Chem. Lett. 2012, 41, 485-487
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

486
Table 1. Gelation properties of cyclophanes 5a-5d in various
solventsa
Cyclophane
5a
5b
5c
5d
Hexane
I
I
I
I
Cyclohexane
Chloroform
Benzene
I
I
G (0.6)
G (0.2)
S
S
S
S
S
S
S
S
S
S
S
S
Figure 2. X-ray crystal structure of 5a.
Toluene
Table 2. Gelation properties of cyclophanes 7a-7f in various
solventsa
aI: insoluble, S: soluble, G: gel. The values given in
parentheses are the minimum concentration (mg mL¹1) to
achieve gelation.
Cyclophane
7a
7b
7c
7d
7e
7f
Hexane
Cyclohexane
Ethanol
Chloroform
Benzene
Toluene
I
P
P
S
PG
PG
P
I
I
I
I
G (1.1) G (1.1) G (2.1) G (1.5) G (0.6)
P
S
S
S
G (2.0)
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
aI: insoluble, S: soluble, P: precipitate, G: gel, PG: partial gel.
The values given in parentheses are the minimum concen-
tration (mg mL¹1) to achieve gelation.
2 µm
Figure 1. Optical and SEM images of gel from 5d in
cyclohexane.
The treatments of 4b, 4c, and 4f with 4-bromophenyl
isocyanate or phenyl isocyanate afforded the desired cyclo-
phanes 5a-5d (Scheme 1).12 In the reaction of 4a and 6d
utilizing triphosgene the cyclophane 7a was obtained in a yield
of 34%. Similar reactions between 4b-4f and 6a-6d were
carried out to prepare the cyclophanes 7b-7f (Scheme 1).12
The gelation behaviors of the cyclophanes 5a-5d were
studied in a variety of solvents and some results are summarized
in Table 1.
1 µm
Figure 3. Optical and SEM images of gel from 7f in
cyclohexane.
In the series of the cyclophanes 5a-5d the C12 alkyl chain
was found necessary to gelate the solvents. Interestingly 5d can
form gels at a lower concentration than 5c, which could be
attributed to the bromine atom reducing the hydrogen bondings
of the urea unit. Figure 1 shows the optical image and the SEM
analysis of 5d. Elongated fibers containing slender ones were
7b-7f. As an example the gel formed from 7f was shown in
Figure 3. 7f exhibits a network of slender fibers.
Quite interestingly, 7c is the only cyclophane that can gelate
ethanol, even though the cyclophane 7f likewise has the C6 and
C12 alkyl chains. The alcohols such as methanol, buthanol,
hexanol, and isopropanol were found to be also gelated by 7c.
The cyclophanes 7d and 7e having the C8 and C10 alkyl chains
exhibit a similar trend for gelation. The minimum concentration
to achieve gelation for cyclohexane was observed for the
cyclophane 7f. Considering the cyclophane 7c with the C6 and
C12 alkyl chains, the long C12 alkyl chain on the opposite side
of the urea unit should be closely related to the gelation. In order
to know the contribution of the cyclophane skeleton to gelation
properties we have synthesized the partial units of most
powerful gelater 7f. The 1:1 mixture of 8 and 9 has no ability
to gelate any solvents examined, meaning that the cyclophane by
which these two long alkyl chain units are connected is essential
for the gelator.
¹1
observed. The C=O stretching at 1654 cm was seen for 5d,
indicating formation of the hydrogen-bondings. The other
cyclophanes were either too soluble or too insoluble in the
solvents examined.
Although we have already clarified the structure of the
dithia[3.3]metacyclophane system, the X-ray analysis13 of the
cyclophane 5a was carried out in order to know the basic
cyclophane structure with the urea group.
As expected the syn conformation has been confirmed
meaning the urea group and the long alkyl chain are located in
close proximity in the solid state (Figure 2).
The gelation properties of the cyclophanes 7a-7f carrying
two kinds of alkyl chains were also examined (Table 2).
In contrast to 5d the cyclophane 7a shows no clear gelation
in spite of the C12 alkyl chain. This fact demonstrates the
position of a long alkyl spacer could play a critical role for
gelation behaviors in this cyclophane system. Formation of
organogel was observed in cyclohexane for other cyclophanes
In conclusion we have achieved organogelators based on
small-sized cyclophanes. This first development should disclose
a new aspect of the organogelator because cyclophanes have a
unique ³-electron system as well as a dynamic structure. Further
research according to this concept is in currently underway in
our laboratory.
Chem. Lett. 2012, 41, 485-487
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

487
References and Notes
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Chem. Lett. 2012, 41, 485-487
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/