Two-component liquid crystals as chiral reaction media: Highly enantioselective photodimerization of an anthracene derivative driven by the ordered microenvironment

Article abstract of DOI:10.1002/anie.200803242

(Chemical Equation Presented) Tailored to the reaction: Within a liquid-crystalline matrix provided by a chiral amphiphilic amino alcohol, the photodimerization of an anthracenecarboxylic acid proceeds with high probability and with excellent regio-, diastereo-, and enantioselectivities (up to 81% ee).

Full text of DOI:10.1002/anie.200803242

Angewandte
Chemie
DOI: 10.1002/anie.200803242
Chiral Reaction Media
Two-Component Liquid Crystals as Chiral Reaction Media: Highly
Enantioselective Photodimerization of an Anthracene Derivative
Driven by the Ordered Microenvironment**
Yasuhiro Ishida,* Yukiko Kai, Syun-ya Kato, Aya Misawa, Sayaka Amano, Yuki Matsuoka, and
Kazuhiko Saigo*
Inspired by natural systems like enzymes, chemists have long
sought tailored microenvironments that can drive targeted
chemical transformations in a highly selective manner.[1–6] For
this aim, a number of constrained reaction fields have been
explored to date, including crystals,[1] coordination poly-
mers,[2] zeolites,[3] clays,[4] and discrete hosts in homogeneous
solutions.[5] Among these classes of reaction fields, crystals are
regarded as one of the most successful classes, in which
reactive molecules are aligned in a highly defined manner
with infinite periodicity. In particular, crystalline-state reac-
tion fields can provide the ultimate environment for intra- and
intermolecular photochemical reactions, leading to near-
perfect regio- and stereocontrol, and providing access to
realms unexplored in traditional chemistry using homoge-
neous media.[2]
In spite of their prominent advantages, however, crystal-
line-phase reactions have not taken a leading role in current
chemistry, owing mainly to the fatal problem of probability. In
general, crystalline-phase reactions proceed only under
limited conditions, in which crystal packings accidentally
meet topochemically stipulated demands.[2] To overcome such
a limitation, liquid-crystalline phases should be considered as
alternative constrained reaction media, as such phases are
known to realize well-defined molecular alignment and
partial molecular motion at the same time. In fact, liquid-
crystalline reaction media have been extensively investigated
in the last three decades,[6] and recent works have expanded
the scope of applicability of such reaction media to the
fabrication of mesoscaled supramolecular architectures.[7] To
date, however, the application of liquid-crystalline reaction
media in stereocontrolled molecular transformations, espe-
cially enantioselective transformations, has been surprisingly
limited.[8] Herein, we report the first example of an asym-
metric synthesis induced by a chiral liquid crystal (up to
81% ee). Our novel approach employs two-component liquid
crystals, which may be considered as “flexible crystals” or
“ordered solvents”.
Recently, we reported intriguing two-component liquid
crystals, which seemed attractive for the development of
novel liquid-crystalline reaction media; several salts of
amphiphilic carboxylic acids with amino alcohols were
found to exist as thermotropic liquid crystals over wide
thermal ranges.[9] Likewise, the opposite combinations, that is,
salts of amphiphilic amino alcohols with carboxylic acids,
were also anticipated to exhibit liquid-crystalline phases. If
photoreactive carboxylic acids were employed as a compo-
nent of the latter combination, photoreactions in liquid-
crystalline matrices might be easily conducted simply by
photoirradiation of the liquid-crystalline salts (Scheme 1). In
this case, the photoreactive substrates are expected to be
confined in a unique reaction environment, where the relative
orientation of the components is highly defined by multiple
hydrogen-bonding interactions. The structural order of our
system is undoubtedly higher than those of conventional
liquid-crystalline reaction systems, in which substrates are
randomly doped in partially ordered mesogens.[6,8] In partic-
ular, enantiopure amphiphilic amino alcohols are expected to
direct the reaction course in an enantioselective manner with
high efficiency. Taking advantage of the noncovalent inter-
actions between the matrix and substrate, the same amphi-
philic amino alcohol might be able to be applied in the
reactions of various photoreactive carboxylic acids. These
aspects prompted us to develop a chiral, amphiphilic amino
alcohol (1), of which the structure, being reminiscent of
Ephedra alkaloids, is promising for chirality induction.
The amphiphilic amino alcohol 1 was synthesized from
easily accessible building blocks and inexpensive reagents, as
[*] Prof. Y. Ishida, Y. Kai, S.-y. Kato, A. Misawa, Dr. S. Amano,
Prof. K. Saigo
Department of Chemistry and Biotechnology
The University of Tokyo
Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan)
Fax: (+81)3-5802-3348
Prof. Y. Ishida, Dr. Y. Matsuoka
PRESTO
Japan Science and Technology Agency
4-1-8 Honcho, Kawaguchi, Saitama, 332-0012 (Japan)
E-mail: saigo@chiral.t.u-tokyo.ac.jp
[**] We acknowledge Dr. Toshimi Shimizu and Dr. Hiroyuki Minamikawa
(AIST), and Prof. Junji Watanabe and Dr. Masatoshi Tokita (TIT) for
XRD measurements. We would like to thank Prof. Takashi Kato (UT)
for use of the DSC equipment. A part of this work was financially
supported by a Grants-in-Aid for Exploratory Research (No.
16655043) from MEXT (Japan).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200803242.
Scheme 1. Photoreaction in a liquid-crystalline matrix.
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outlined in Scheme 2. A bromobenzene derivative bearing
three alkoxy chains (2)[10] was lithiated with butyllithium and
then coupled with an l-alanine derivative (3)[11] to afford the
ketone 4.[12] The subsequent hydride reduction of 4 by the
classical Meerwein–Ponndorf–Verley method proceeded with
excellent diastereoselectivity to give the alcohol 5 in an
essentially diastereo- and enantiopure form.[13] Then, the
stereochemical configuration at the C1 position of 5 was
inverted by the Mitsunobu reaction to give the corresponding
benzoate 6.[14] Finally, the N- and O-protecting groups were
cleaved to afford the target amino alcohol 1.[15]
Scheme 2. Synthesis of amphiphilic amino alcohol 1. a) 2 (2.3 equiv),
nBuLi (2.3 equiv), diethyl ether/THF, ꢀ78!ꢀ20 8C, 4 h; b) Al(OiPr)3
(0.3 equiv), iPrOH/toluene, 508C, 48 h; c) PhCO2H (2.0 equiv), PPh3
Figure 1. Liquid-crystalline salts 1·8. a) Phase-transition behavior
during a cooling process. Iso=isotropic, Sm=smectic,
Colrec =rectangular columnar. b) A proposed structure deduced from
powder XRD studies.
=
(1.5 equiv), iPrOCON NCO2iPr (1.5 equiv), toluene, RT, 2 h; d) LiAlH4
(4.0 equiv), THF, 08C, 2 h; e) H2 (1 atom), Pd/C (cat.), ethanol, RT,
12 h. Cbz=benzyloxycarbonyl.
However, despite the simplicity of the photochemical process,
the regio- and stereochemical course of the photodimeriza-
tion is complicated when laterally substituted anthracenes
like 8c and 8d are used; owing to the dissymmetric shape of
the reactive unit, the photodimerization gives rise to no less
than four configurational isomers (head-to-head (HH) or
head-to-tail (HT) isomers with syn or anti isomerism, denoted
as synHH, antiHH, synHT, and antiHT), two of which each consist
of a pair of C2-symmetric enantiomers ((R)/(S)-antiHH and
(R)/(S)-synHT).[16,18] Within the framework of classical syn-
thetic methodologies, regio-, stereo-, and enantiocontrol of
this reaction seem to be difficult, and therefore, the environ-
ment-directed control of this reaction has attracted consid-
erable recent attention.[19]
The photodimerization of 2-anthracenecarboxylic acid
(8c) was conducted by irradiation of the salt 1·8c with UV/Vis
light (a 500 W Hg lamp, l > 380 nm). To investigate the
characteristic features of this liquid-crystalline medium, the
photoreaction was carried out in various phases, including
smectic1 (30 and 458C during the first cooling process),
smectic2 (808C during the first cooling process), and isotropic
(1108C during the first heating process) phases. Although the
results of the photoreaction in the crystalline state would
further clarify the characteristics of the liquid-crystalline
medium, the salt 1·8c did not exhibit a definite transition to a
crystalline phase upon cooling to ꢀ208C. Therefore, by using
Salts of the amino alcohol 1 with various carboxylic acids
were prepared by mixing equimolar amounts of the two
components, and the mesomorphic behavior of the resultant
salts was studied by differential scanning calorimetry (DSC),
polarized optical microscopy (POM), and X-ray diffraction
(XRD). As summarized in Figure 1a, 1 forms liquid-crystal-
line salts with a variety of carboxylic acids, including sorbic
acid (8a), cinnamic acid (8b), and anthracenecarboxylic acids
(8c and 8d). During heating and cooling processes above
room temperature, the salts of 8a–c exhibited one or two
kinds of smectic phases, whereas the salt of 8d exhibited a
rectangular columnar phase. A strong XRD reflection
corresponding to a periodicity of 28–31 ꢀ was detected for
all of the liquid crystals, regardless of the packing mode
(smectic or rectangular). Taking into account the unique
periodicity, together with the amphiphilic nature of the 1·8
salts, these liquid-crystalline salts are believed to adopt a
bilayer-like structure, as shown in Figure 1b. As might be
expected, the observed d spacings are shorter than those
anticipated from the extended molecular length of 1, most
likely because of partial interdigitation of the aliphatic tails.
With these liquid-crystalline salts in hand, we attempted
the in situ photoreaction of the carboxylic acid units. We first
focused on the photodimerization of the anthracene deriva-
tives 8c and 8d, a well-established [4+4] cycloaddition.[16,17]
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(1S,2S)-norpseudoephedrine (1’) in place of
1, an analogous crystalline salt (1’·8c) was
prepared and applied in the photoreaction.
After photoirradiation, the reaction mix-
ture was treated with an excess amount of
been reported to be less stable than the HT dimers, owing to
the electrostatic repulsion between the two carboxylate
moieties.[19] As far as we are aware, the HH/HT ratios
achieved herein are among the highest reported for this kind
of bimolecular photoreactions.[6,16–19] These observations
indicate that the high HH selectivities of the dimerizations
are attributable not to an intrinsic preference of the starting
material, but to the special environments provided by the
liquid-crystalline matrices. Assuming a bilayer-like molecular
alignment in the smectic phases, as depicted in Figure 1b, an
intralayer dimerization can reasonably elucidate the nearly
exclusive formation of HH dimers.
trimethylsilyldiazomethane to thoroughly
esterify the carboxyl groups in the system (those in unreacted
8c, the photodimers, and other products), and then subjected
to preparative thin-layer chromatography (TLC) to remove
the amphiphilic amino alcohol 1. The composition of the
1
recovered esters was determined by H NMR spectroscopy
and high-performance liquid chromatography (HPLC), which
allowed us to unambiguously estimate the conversion of 8c
and the regio-, diastereo-, and enantioselectivities of the
photodimerization.
Thus, highly HH-selective photodimerization of 8c was
established by using two kinds of liquid-crystalline reaction
media. Nevertheless, the stereochemical outcome of the
photodimerization, in terms of the synHH/antiHH ratio,
revealed striking differences between the two liquid-crystal-
line phases. Indeed, the smectic1 phase afforded the antiHH
dimer as the main product (synHH/antiHH = 26:72–28:70;
Table 1, entries 1 and 2), whereas the smectic2 phase yielded
As summarized in Table 1, the two smectic phases were
found to realize acceptable reactivity to give a mixture of the
target dimers (9c) in 23–55% yield after photoirradiation for
3 hours (entries 1–3). In the isotropic phase, the photoreac-
tion proceeded so fast that 87% of the starting material were
consumed under conditions differing only in phase and
temperature (entry 4). In contrast, the crystalline phase of
1’·8c did not provide any detectable amount of the photodi-
mer 9c (entry 5), as might be expected from the general
properties of crystalline reaction fields. These observations
indicated the superiority of the liquid-crystalline phases
compared to the crystalline phase in the context of reaction
probability.
Quite interestingly, the regioselectivity (HH/HT ratio) of
the photodimerization changed dramatically depending on
the phase, which clearly demonstrates the peculiar property
of liquid crystals as “constrained” reaction media. In fact,
both of the smectic phases realized excellent regioselectivities
to yield the HH dimers almost exclusively (HH/HT= 97:3–
98:2; Table 1, entries 1–3). These regioselectivities are oppo-
site to the usual tendency (HT> HH), which is governed by
the relative stability of the products; the HH dimers have
almost equal amounts of the two diastereomers (synHH
/
antiHH = 44:53; entry 3). Although the origin of the syn/anti
selectivity is unclear at present, we believe that the packing
mode of the liquid-crystalline matrix has a large influence on
the syn/anti selectivity, which is at least as large as the
influence of the reaction temperature. Indeed, experiments
conducted in the same phase (smectic1) but at different
temperatures (30 and 458C), resulted in virtually the same
syn/anti selectivities (entries 1 and 2).
The liquid-crystalline media were found to offer reaction
environments with excellent chirality-induction ability. Par-
ticularly, the photodimerization performed in the smectic1
phase afforded the antiHH dimer with high enantioselectivity
(up to 81% ee; Table 1, entries 1 and 2). In the case of the
smectic2 phase, the enantioselectivity was also at an appreci-
able level, although somewhat lower (48% ee; entry 3). These
results contrast strikingly with the insufficient enantioselec-
tivity attained in the isotropic phase
(9% ee; entry 4). The outstanding
Table 1: Photodimerization of 2-anthracenecarboxylic acid (8c).
enantioselectivities achieved in the
smectic phases are not likely to be a
simple chirality transfer within the
discrete salt pair; the formation of a
chiral supramolecular architecture
would be essential for efficient
chirality induction. To our knowl-
edge, the reaction conducted in the
smectic1 phase is the first asymmet-
Entry
Medium
T [8C]
State[a]
Yield [%][b]
Product ratio [%][c]
ee [%][d]
ric synthesis induced by a chiral
liquid crystal.[8] Moreover, the pres-
ent system provides a very rare
example of an intermolecular pho-
tochemical reaction in which satis-
factory yield and regio-, diastereo-,
and enantioselectivities were simul-
taneously realized; the antiHH
dimer was produced in 34% overall
yield with 81% ee (entry 2).[19]
antiHT
synHT
antiHH
synHH
1
2
3
4
1
1
1
1
30
45
80
110
30
25
Sm1
Sm1
Sm2
Iso
Cry
Sol
23
48
55
87
<1
88
1
1
1
10
–
1
1
2
12
–
72
70
53
38
–
26
28
44
30
–
+78
+81
+48
+9
–
5
1’
H2O
CH2Cl2
6[19k][e]
43
32
36
22
14
25
7
21
–
–
7[19j]
20
Sol
96
1
[a] Sm=smectic, Iso=isotropic, Cry=crystalline, Sol=solution. [b] Determined by H NMR spectros-
copy. [c] Determined by HPLC. [d] Enantiomeric excess of antiHH-9c determined by chiral HPLC. [e] A
phosphate buffer solution (pH 7.0) was used as the medium.
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We next performed the in situ photodimerization of
another anthracenecarboxylic acid (1-anthracenecarboxylic
acid, 8d)[18,19a–c] in a liquid-crystalline matrix to demonstrate
the general utility of this strategy (Scheme 3). For example,
the salt 1·8d was irradiated with UV/Vis light at 358C, and the
salt took on a rectangular columnar structure. The photo-
dimerization of 8d proceeded with excellent regioselectivity
(HH/HT> 99:1) and notable diastereoselectivity (synHH
/
antiHH = 62:38), as we expected. In this case, the chiral
isomer (antiHH) was not afforded as a major product, and its
enantiomeric excess was unsatisfactory.
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Scheme 3. Photodimerization of 1-anthracenecarboxylic acid (8d).
It is worth noting that both of the substrates 8c and 8d
exhibited a strong propensity to form the thermodynamically
unfavorable HH dimers, despite the lack of similarity in their
molecular shapes. This peculiar regiochemical preference
observed for the two substrates further supports our hypoth-
esis that an intralayer dimerization predominantly proceeds
in a bilayer-like liquid-crystalline matrix.
In summary, the present results unambiguously demon-
strate that liquid-crystalline reaction media can induce
remarkable regio-, diastereo-, and even enantioselectivities
in a photochemical reaction, contrary to popular concep-
tions.[8] In addition, liquid-crystalline reaction media were
found to be tolerant toward variation in substrate shape,
which is possibly one of the most novel features of such
reaction fields compared with traditional solid-state reaction
fields. Further exploration of this strategy would increase the
expediency of chemistry in tailored reaction media, and
would enhance our understanding of chemical events taking
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[15] The diastereomeric purity of the amino alcohol 1 was confirmed
by 1H NMR spectroscopy (300 MHz); the diastereomers of 1
(1R,2S and 1S,2R isomers) were not detected in the spectrum.
An authentic sample of the 1R,2S isomer was independently
synthesized by hydrogenolysis of the benzyloxycarbonyl group
in 5.
Received: July 4, 2008
Published online: September 22, 2008
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Keywords: asymmetric synthesis · chirality · liquid crystals ·
photochemistry · template synthesis
.
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[20] During the review process of our manuscript, a paper describing
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