An organogel system can control the stereochemical course of anthracene photodimerization

Article abstract of DOI:10.1039/b820565e

A novel photoresponsive organogel with a binary gelator containing 2-anthracenecarboxylic acid shows a high degree of stereochemical control, resulting in head-to-head photocyclodimers exclusively together with significant enantiomeric excess induced by t

Full text of DOI:10.1039/b820565e

View Article Online / Journal Homepage / Table of Contents for this issue
Chemical Communications
www.rsc.org/chemcomm
Number 16 | 28 April 2009 | Pages 2053–2224
ISSN 1359-7345
COMMUNICATION
Norifumi Fujita, Seiji Shinkai et al.
FEATURE ARTICLE
An organogel system can control the Toshifumi Dohi and Yasuyuki Kita
stereochemical course of anthracene Hypervalent iodine reagents as a new
1359-7345(2009)16;1-0
photodimerization
entrance to organocatalysts

COMMUNICATION
www.rsc.org/chemcomm | ChemComm
An organogel system can control the stereochemical course of anthracene
photodimerizationwz
Arnab Dawn,y Norifumi Fujita,*z Shuichi Haraguchi, Kazuki Sada and Seiji Shinkai*y
Received (in Cambridge, UK) 18th November 2008, Accepted 29th January 2009
First published as an Advance Article on the web 9th February 2009
DOI: 10.1039/b820565e
A novel photoresponsive organogel with a binary gelator
containing 2-anthracenecarboxylic acid shows a high degree
of stereochemical control, resulting in head-to-head photocyclo-
dimers exclusively together with significant enantiomeric excess
induced by the chiral counterpart of the gelator.
Chirality that is induced, monitored, controlled, or applied via
the principles of a supramolecular approach is a modern
interdisciplinary field of research that deals with the transfer
of asymmetry information within molecular systems via
noncovalent interactions from a chiral entity to a prochiral
substrate.1 Primarily, they include electrostatic, hydrogen-
bonding,2 van der Waals and p–p interactions,3 in addition
Scheme 1 Schematic presentation of [4 + 4] photocyclodimerization
to steric effects,4 acting individually or cooperatively to
of 2-anthracenecarboxylic acid in the free state and in the cyclohexane
produce the chiral structures. In this context, stereochemical
gel of binary system 1.
control in a chiral photochemical process such as photo-
cycloaddition is one of the most intriguing issues from the
mechanistic and synthetic points of view.5 Substrate preorien-
outside the binding site. A solid state approach with a similar
system, in which stereodifferentiation and chiral information
are provided by the rigid solid environment, gives more h–h
photoproducts though the conversion is very low.10 On the
other hand, reaction media like a Langmuir–Blodgett assem-
bly, micelles, liquid crystals and polymer aggregates have not
been employed extensively or the rate of success is very low.
Even in this series, however, one phase is really missing, that is
the gel state which is capable of affording novel supra-
molecular architectures created by low molecular-weight
gelator (LMG) assemblies. Photo-induced trans-to-cis
isomerization of azobenzene in cholesterol based gelator has
been developed.11a,b Chiral silver nanoparticles and chiral
nanoparticulate films have been prepared through the in situ
reduction of an organogel formed by a silver(I)-coordinated
organogelator.11c Photoresponsive LMG-containing stilbene,11d
9-anthracenecarboxylate,11e and LMG based on monomeric
and dimeric derivatives of anthracene have also been
studied.11f However, the product analysis and the stereo-
chemical control over the photoproducts have not been
explored properly, probably because of their complexity. It
should be an encouraging step to explore the stereochemical
control of anthracene derivatives using the gel matrix.
Recently, LMG chemistry using cholesterols12a–c and sub-
stituted gallic acids12d–f as the backbone of a gelator has been
developed. Keeping the above mentioned goal in mind, we
have designed a binary gelator containing 2-anthracene-
carboxylic acid (2Ac) attached noncovalently with a gelator
component containing the gallic acid backbone coupled with
D-alanine by means of electrostatic and hydrogen-bonding
interactions. The reasons behind the selection of such a system
are three-fold: (i) the noncovalently attached anthracene
tation prior to photoirradiation, governed by supramolecular
interactions, has recently been exploited to achieve high
regio-6 and enantioselectivities.7 Among different stereo-
chemical processes, anthracene photodimerization is one of
the oldest known photochemical reactions.8 However, the
stereochemistry of photocyclodimerization of unsymmetri-
cally substituted anthracenes has been less studied until
recently, probably due to the complex nature of their photo-
products which include four [4 + 4] cyclodimers (Scheme 1),
anti- and syn- head-to-tail (h–t, A and B) and anti- and
syn- head-to-head (h–h, C and D) among which B and C are
chiral. Among the different hosts or template molecules, the
inherently chiral cavity of g-cyclodextrin has been utilized to
study the stereoselectivity and enantioselectivity of the photo-
dimers obtained from 2-anthracenecarboxylate.9 Those
approaches could improve the low selectivity for h–h photo-
dimers by manipulating the electrostatic interaction between
host and guest, controlling external stimuli such as tempera-
ture, solvent, pressure and also by a steric effect operating
Department of Chemistry and Biochemistry, Graduate School of
Engineering, Kyushu University, Fukuoka 819-0395, Japan
w This article is dedicated to Prof. Roeland J. M. Nolte on the
occasion of his 65th birthday.
z Electronic supplementary information (ESI) available: Experimental
procedures for the preparation of 1 and the characterization data. See
DOI: 10.1039/b820565e
y Present address: Faculty of Engineering, Sojo University, Kumamoto
860-0082, Japan. E-mail: shinkai_center@mail.cstm.kyushu-u.ac.jp;
Fax: +81 92 805 3814.
z Present address: Department of Chemistry and Biotechnology,
School of Engineering, The University of Tokyo, Tokyo 113-8656,
Japan. E-mail: fujita@macro.t.u-tokyo.ac.jp; Fax: +81 3 5841 7310.
ꢀc
This journal is The Royal Society of Chemistry 2009
2100 | Chem. Commun., 2009, 2100–2102

moiety should have sufficient room for cyclodimerization
reaction in the closely packed gel state and also this photo-
responsive component may induce the gel–sol phase transition
by photo-stimuli, (ii) intermolecular hydrogen-bonding
through two amide groups and possible p–p stacking of the
anthracene molecules are expected to provide an appropriate
platform for stereochemical selection of the photoproducts in
the 1D arrangement in the gel state, and (iii) there is scope
for the chiral alanine moiety to induce enantioselectivity in
the anthracene photoproducts via perturbation of the pre-
orientation in the ground state.
(50 1C, sol state), gave a substantial amount (76%) of h–h
dimers but with lesser chiral induction (2% ee). In the solid
state, however, a photodimerization process did not take
place. In THF, a nongelating medium, the stereoselectivity
was reversed to give h–t dimers as major photoproducts. It is
thus clear that a high degree of stereoselectivity is inherent in
the gel system. One should notice that the condition employed
for the photoreaction here is close to the ambient condition
even though the system shows a very high degree of stereo-
selectivity and also some chiral induction. These easily
performable reaction conditions definitely enhance the merit
of the system.
In a typical synthetic procedure, first the D-alanine was
condensed with gallic acid based amine, to which was then
attached the 2Ac moiety (equimolar) from THF solution to
give the binary organogelator 1 (Scheme 1). The product was
used in all the studies described here. The gelation ability of 1
was tested in different organic solvents (hexane, cyclohexane,
toluene, p-xylene, benzene, THF, etc.). Among these it was
found to gelate cyclohexane reversibly with a low critical
gelator concentration (2.5 ꢁ 10ꢂ3 mol dmꢂ3). High trans-
parency of the system is a really advantageous condition
that is indispensable for applying the system to photochemical
purposes. For the photoreaction, a gel sample with medium
gelator concentration (9.9 ꢁ 10ꢂ3 mol dmꢂ3) has been
chosen because the higher the gelator concentration, the
more rigid will be the molecular packing in the gel state
but at the same time the efficiency of the photoirradiation
would be less.
The presence of excess chirality in the photoproducts has
encouraged us to investigate the circular dichroism (CD) of the
sample in the ground state. The CD spectrum (Fig. S3z) of the
cyclohexane gel of 1 clearly shows the CD signal corresponding
to the wavelength of the 2Ac absorption band. It means that
the 2Ac molecules are arranged in a chiral environment
induced by the chiral gelator. This is supported by the fact
that the photoproduct distribution and ee are determined by
the orientation in the ground state. The CD signal in the sol
state (at 50 1C) is negligibly weak as is evident from the lower
ee obtained in the sol state. It is worth noticing that in the gel
state, even the noncovalent interaction can convey the chiral
message to the achiral counterpart. This process is facilitated
by the orientational rigidity in the gel matrix.
Fluorescence spectra (Fig. S4z) of the gel sample at room
temperature show a decrease in the fluorescence intensity
together with a significant red shift of lmax compared to that
in the sol state. The red shift of the fluorescence maxima can be
attributed to the p–p stacking of anthracene molecules in the
gel state.13 The decrease in the intensity probably results from
the concentration quenching of the fluorescence in the closely
packed gel state compared to the more isotropic sol state.14
The investigation of the morphology (Fig. S5z) of the gel
sample gives a clear view of a one-dimensional fibrillar
arrangement. XRD analysis of the xerogel prepared from 1
in cyclohexane shows two peaks (Fig. S6z) at 2y = 4.301
(20.5 A) and 9.601 (9.2 A) which are assigned to the different
spacings in the proposed molecular packing (Fig. S7z) based
on energy minimization.
The gel sample prepared under the inert conditions was
photoirradiated at a wavelength of 366 nm with a medium
pressure mercury lamp through optical filters for 2 h at 10 1C.
Photocyclization is not reversible at this wavelength. With the
progress of photoirradiation, the intensity of UV-absorption
band (1La) (Fig. S1z) of 2Ac monomer gradually decreased
with a simultaneous change in the physical state from gel to
sol. It is worth mentioning that the gel melting temperature
(35 1C) for this system is well above the photoirradiation
temperature. One can propose, therefore, that the observed
phase transition should be ascribed to a photo-induced
process. After the photoreaction, the photoproducts and
unreacted 2Ac were separated and isolated from the gelator
component, which was then subjected to HPLC analysis. The
chromatogram (Fig. S2z) showed no signal for the h–t photo-
dimers which are normally the major products in isotropic
conditions. In this process only h–h photodimers were
produced with some moderate (10%) enantiomeric excess
(ee) (Table 1). This system, even at higher temperature
The goal of this work is to control the stereochemical
selectivity of photodimers by taking advantage of the probable
one-dimensional supramolecular assembly in the gel phase. In
this regard, the gel structure or in other words orientation of
the 2Ac molecules in the gel matrix should be the key factor.
The HPLC analysis of the photoproducts provides direct
Table 1 Distribution and enantiomeric excess (ee) of the products obtained from [4 + 4] photocyclodimerization of 2Ac in sample 1
Relative yield (%)a % eea
A
B
C
D
A + B
C + D
B
C
Sample
Temp./1C
Irradiation time/min
Conversion (%)
Gel of 1 in cyclohexane
Sol of 1 in cyclohexane
1 in solid state
10
50
10
10
120
30
120
30
51
61
0
0
0
48
36
—
20
52
40
—
14
0
24
—
66
100
76
—
—
1
—
0.5
ꢂ10
ꢂ2
—
ꢂ0.7
10
—
40
14
—
26
1 in THF (nongelating)
80
34
a
The absolute configurations of B and C were not determined. The first eluted enantiomer is given a positive sign. Errors in relative yields are
ꢃ0.5% and in % ee are ꢃ1% for major products and ꢃ3% for minor products.
ꢀc
This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 2100–2102 | 2101

evidence for the gel structure. Formation of only h–h photo-
dimers from the gel state definitely indicates that 2Ac
molecules are placed only in a h–h fashion (Fig. S7z). The
arrangement is so compact and directional that the non-
covalent interaction becomes enough to transfer the chirality
to the ground state packing of 2Ac molecules and this
preorientation in the ground state actually determines the ee
of the photoproducts. On the other hand, a photo-induced
structural change (gel to sol transition) that is fairly difficult in
the gel system can easily be attained in the present system.
Attachment of 2Ac molecules in the binary gelator via
hydrogen-bonding is evident from FT-IR spectra of the
xerogel (Fig. S8z) where the CQO stretching frequency
(at 1682 cmꢂ1) of 2Ac shifts to lower energy in the gel state.
In addition, unlike the previously reported system,11e the
ammonium counterpart is capable of forming intermolecular
hydrogen-bonding (via two amide groups) to align itself in the
gel structure, even though this does not seem to be enough
to resist the structural change during the photoreaction.
This situation means that 2Ac molecules definitely have an
active participation in constructing the gel structure via their
p–p stacking interaction which is more favourable in a h–h
orientation. The p–p stacking together with the intermolecular
hydrogen-bonding via two amide linkages cooperatively
construct the one-dimensional arrangement of the gelator
molecules (Fig. S7z) which is facilitated by the nonpolar
environment of the cyclohexane.
Notes and references
1 G. A. Hembury, V. V. Borokov and Y. Inoue, Chem. Rev., 2008,
108, 1.
2 (a) B. Huang and J. R. Parquette, J. Am. Chem. Soc., 2001, 123,
2689; (b) L. J. Prins, F. D. Jong, P. Timmerman and
D. N. Reinhoudt, Nature, 2000, 408, 181; (c) J. H. K. Ky Hirschberg,
L. Brunsveld, A. Ramzi, J. A. J. M. Vekemans, R. P. Sijbesma and
E. W. Meijer, Nature, 2000, 407, 167; (d) R. S. Johnson,
T. Yamazaki, A. Kovalenko and H. Fenniri, J. Am. Chem. Soc.,
2007, 129, 5735; (e) T. Moriuchi, M. Nishiyama, K. Yoshida,
T. Ishikawa and T. Hirao, Org. Lett., 2001, 3, 1459.
3 (a) H. von Berlepsch, C. Bottcher, A. Ouart, C. Burger, S. Dahne
and S. Kirstein, J. Phys. Chem. B, 2000, 104, 5255; (b) D. Liu,
D. A. Williamson, M. L. Kennedy, T. D. Williams, M. M. Morton
and D. R. Benson, J. Am. Chem. Soc., 1999, 121, 11798.
4 C. W. Wu, T. J. Sanborn, R. N. Zuckermann and A. E. Barron,
J. Am. Chem. Soc., 2001, 123, 2958.
5 (a) H. Rau, Chem. Rev., 1983, 83, 535; (b) Y. Inoue, Chem. Rev.,
1992, 92, 741; (c) A. G. Griesbeck and U. J. Meierhenrich, Angew.
Chem., Int. Ed., 2002, 41, 3147.
6 (a) D. M. Bassani, V. Darcos, S. Mahony and J. P. Desvergne,
J. Am. Chem. Soc., 2000, 122, 8795; (b) N. D. McClenaghan,
C. Absalon and D. M. Bassani, J. Am. Chem. Soc., 2003, 125,
13004.
7 (a) T. Bach, H. Bergmann, B. Grosch and K. Harms, J. Am. Chem.
Soc., 2002, 124, 7982; (b) B. Grosch, C. N. Orlebar, E. Herdtweck,
M. Kaneda, T. Wada, Y. Inoue and T. Bach, Chem.–Eur. J., 2004,
10, 2179.
8 (a) H. D. Becker, Chem. Rev., 1993, 93, 145, and references therein;
(b) H. D. Roth, Angew. Chem., Int. Ed. Engl., 1989, 28, 1193.
9 (a) A. Nakamura and Y. Inoue, J. Am. Chem. Soc., 2005, 127,
5338; (b) C. Yang, A. Nakamura, T. Wada and Y. Inoue, Org.
Lett., 2006, 8, 3005; (c) C. Yang, A. Nakamura, G. Fukuhara,
Y. Origane, T. Mori, T. Wada and Y. Inoue, J. Org. Chem., 2006,
71, 3126; (d) C. Yang, T. Mori, Y. Origane, Y. H. Ko,
N. Selvapalam, K. Kim and Y. Inoue, J. Am. Chem. Soc., 2008,
130, 8574; (e) H. Ikeda, T. Nihei and A. Ueno, J. Org. Chem., 2005,
70, 1237.
In conclusion, we have demonstrated that a binary gelator
containing 2Ac noncovalently linked with 3,4,5-tris(n-dodecyloxy)-
benzoylamide substituted D-alanine can act as an efficient
gelator of a nonpolar solvent like cyclohexane. The photo-
cyclodimerization of the 2Ac in the gel matrix gives only h–h
photodimers which are the minor products under the normal
isotropic condition. It is also worth noticing that a low but still
significant ee (10%) in the photodimer could be induced from
the chiral component of the gelator. As a consequence of the
photoreaction, a significant gel to sol phase transition can
easily be induced. Thus, the present system is the first example
in which the stereochemical control of the anthracene photo-
dimerization has been investigated in the gel phase and as a
result a very high degree of stereoselectivity is achieved under
simple conditions. This study is believed to open a new door
towards stereochemical control in a supramolecular assembly
created by organogel systems.
10 C. Yang, M. Nishijima, A. Nakamura, T. Mori, T. Wada and
Y. Inoue, Tetrahedron Lett., 2007, 48, 4357.
11 (a) K. Murata, M. Aoki, T. Nishi, A. Ikeda and S. Shinkai,
J. Chem. Soc., Chem. Commun., 1991, 1715; (b) K. Murata,
M. Aoki, T. Suzuki, T. Harada, H. Kawabata, T. Komori,
F. Ohseto, K. Ueda and S. Shinkai, J. Am. Chem. Soc., 1994,
116, 6664; (c) Y. Li and M. Liu, Chem. Commun., 2008, 5571;
(d) J. Eastoe, M. Sanchez-Dominguez, P. Wyatt and
R. K. Heenan, Chem. Commun., 2004, 2608; (e) M. Ayabe,
T. Kishida, N. Fujita, K. Sada and S. Shinkai, Org. Biomol.
Chem., 2003, 1, 2744; (f) C. Wang, D. Zhang, J. Xiang and
D. Zhu, Langmuir, 2007, 23, 9195.
12 (a) K. Sugiyasu, N. Fujita and S. Shinkai, Angew. Chem., Int. Ed.,
2004, 43, 1229; (b) S.-i. Kawano, N. Fujita and S. Shinkai, J. Am.
Chem. Soc., 2004, 126, 8592; (c) S.-i. Kawano, N. Fujita and
S. Shinkai, Chem.–Eur. J., 2005, 11, 4735; (d) P. Mukhopadhyay,
Y. Iwashita, M. Shirakawa, S.-i. Kawano, N. Fujita and
S. Shinkai, Angew. Chem., Int. Ed., 2006, 45, 1592;
(e) T. Kitahara, M. Shirakawa, S.-i. Kawano, U. Beginn,
N. Fujita and S. Shinkai, J. Am. Chem. Soc., 2005, 127, 14980;
(f) M. Shirakawa, N. Fujita, T. Tani, K. Kaneko and S. Shinkai,
Chem. Commun., 2005, 4149.
A.D. warmly thanks Mr T. Kitahara, Mr T. Hara and
Mr H. Sato for their kind help in different stages of experi-
ment. Financial support was provided by Grants-in-Aid
(Nos. 19022031, 20045014, and 20685010) and the Global
COE program ‘Science for future molecular systems’ from
MEXT of Japan and a JSPS fellowship for A.D.
13 A. Ajayaghosh and S. J. George, J. Am. Chem. Soc., 2001, 123,
5148.
14 J. M. Jean and K. B. Hall, Proc. Natl. Acad. Sci. U. S. A., 2001, 98, 37.
ꢀc
This journal is The Royal Society of Chemistry 2009
2102 | Chem. Commun., 2009, 2100–2102