Autoamplification of molecular chirality through the induction of supramolecular chirality

Article abstract of DOI:10.1002/anie.201311160

The novel concept for the autoamplification of molecular chirality, wherein the amplification proceeds through the induction of supramolecular chirality, is presented. A solution of prochiral, ring-open diarylethenes is doped with a small amount of their chiral, ring-closed counterpart. The molecules co-assemble into helical fibers through hydrogen bonding and the handedness of the fibers is biased by the chiral, ring-closed diarylethene. Photochemical ring closure of the open diarylethene yields the ring-closed product, which is enriched in the template enantiomer. There and back again: Doping a mixture of rapidly interconverting prochiral, open diarylethenes (left) with their enantiopure, closed counterparts, led to formation of gel fibers of preferred helicity (center). This supramolecular chirality was transferred to the molecular level by photochemical ring closing, thus yielding a chiral product (right) which is enriched in the enantiomer originally used as a template.

Full text of DOI:10.1002/anie.201311160

Angewandte
Chemie
DOI: 10.1002/anie.201311160
Chiral Amplification
Autoamplification of Molecular Chirality through the Induction of
Supramolecular Chirality**
´
Derk Jan van Dijken, John M. Beierle, Marc C. A. Stuart, Wiktor Szymanski, Wesley R. Browne,
and Ben L. Feringa*
Abstract: The novel concept for the autoamplification of
molecular chirality, wherein the amplification proceeds
through the induction of supramolecular chirality, is presented.
A solution of prochiral, ring-open diarylethenes is doped with
a small amount of their chiral, ring-closed counterpart. The
molecules co-assemble into helical fibers through hydrogen
bonding and the handedness of the fibers is biased by the
chiral, ring-closed diarylethene. Photochemical ring closure of
the open diarylethene yields the ring-closed product, which is
enriched in the template enantiomer.
opposed to the Soai system which shows autoamplification of
chirality based on chiral catalysts in which the product is
identical to the chiral ligand in the catalyst. Moreover, the
achiral molecule is not a precursor for the chiral molecule and
there is no conversion of one into the other. The key question
is: can supramolecular chirality result in the induction of
chirality at the molecular level for the constituent molecular
component of the chiral supramolecular aggregate?
Here we show that autoamplification of molecular
chirality by induction of supramolecular chirality can be
achieved. A minor amount of a chiral diarylethene in the
closed form induces handedness in a supramolecular aggre-
gate (chiral gel) of the corresponding achiral diarylethene in
the open form. Subsequent photochemical ring closure results
in the formation of extra chiral diarylethene in the closed
form. In other words, we demonstrate that a small amount of
a single enantiomer of a chiral molecule can induce its own
formation through the intermediacy of a chiral, supramolec-
ular assembly of the same molecules.
Autoamplification of chirality, a process which allows the
emergence of homochirality from a pool of near-racemic
compounds,[1] is an intriguing concept with far-reaching
implications for asymmetric synthesis[2] and the origin of
life.[3] In the seminal work by Soai et al.[4] on asymmetric
autocatalysis it is shown that autoamplification of molecular
chirality can be achieved by the product of a reaction acting as
a chiral ligand for a metal, thus forming a complex which
catalyzes the enantioselective formation of this product.
Blackmond and Brown and co-workers provided a mechanis-
tic model for this chiral amplification, one involving a catalyst
which is able to accelerate its own formation, while simulta-
neously suppressing other catalytic species which would lead
to the formation of the other enantiomer.[3,5]
The system presented here is based on amide-modified
diarylethene molecular photoswitches (Scheme 1).[8] Diaryl-
ethenes in their open form (1open) exist in two, rapidly
The concept of chiral amplification has been extended to
noncovalent macromolecular systems[6] and the emergence
and induction of supramolecular chirality in dynamic macro-
molecular aggregates was pioneered by Meijer and co-work-
ers.[7] It was shown that doping of achiral, disc-shaped
molecules with small quantities of chiral analogues results
in the formation of long, chiral, columnar stacks held together
by hydrogen bonding.[7] In the columnar stacks, the chiral
dopant is distinct from the majority of the achiral entity, as
Scheme 1. Helical conformations of the diarylethene 1open and the
formation of the two enantiomers of 1closed upon photochemical ring
closing. § =single enantiomer.
[*] D. J. van Dijken, Dr. J. M. Beierle, Dr. M. C. A. Stuart,
´
Dr. W. Szymanski, Prof. Dr. W. R. Browne, Prof. Dr. B. L. Feringa
exchanging, chiral conformations (P and M helicity) in
solution.[9] During ring closing, which results from irradiation
with UV light, two stereogenic centers are generated, thus
giving rise to two enantiomers (Scheme 1): (R,R)-1closed§ from
Center for Systems Chemistry, Stratingh Institute for Chemistry
University of Groningen
Nijenborgh 4, 9747 AG, Groningen (The Netherlands)
E-mail: b.l.feringa@rug.nl
(P)-1open and (S,S)-1closed from (M)-1open (§ denotes that the
§
[**] We thank Dr. P. de Mendoza and Dr. J. van Herpt for insightful
comments and useful discussions. The European Research Council
(Advanced Investigator grant 227897; B.L.F.), the Ministry of
Education, Culture and Science of the Netherlands (Gravity
program 024.001.035, B.L.F. and W.R.B.), and the US National
Science Foundation (NSF International Postdoctoral Fellowship
OISE-0853019, J.M.B.) are acknowledged for financial support.
compound is a single enantiomer). Functionalization of
diarylethenes with amide moieties allows the photoswitches
to assemble through hydrogen bonding and this results in low-
molecular-weight gelators (LMWGs).[10,11]
For the transfer of chirality, we rely on the sergeant-
soldier principle as introduced by Green and co-workers.[12]
According to this mechanism of chiral amplification, a minor
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201311160.
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amount of the chiral compound (sergeant), used as a dopant,
dictates the overall chirality of a system which is made up of
mainly achiral material (soldiers). In recent years, this
principle has been exploited in the field of supramolecular
chemistry.[13,14]
chirality at the molecular level and completing the cycle
from molecular chirality, through induction of supramolec-
ular chirality, and finally back to molecular chirality, thus
leading to an enriched mixture in the templated enantiomer
((S,S)-1closed§ in Figure 1). If chiral dopant is not added to the
system (Figure 1, non-templated pathway), the gel consists of
an equal amount of P- and M-helical fibers and, upon
irradiation, a statistical 1:1 mixture of the S,S- and R,R enan-
tiomer of 1closed (rac-1closed) is formed.
We envisioned a system in which the chiral information is
exclusively transferred from the ring-closed diarylethene
core, rather than from auxiliary, peripheral stereogenic
centers adjacent to the amide group.[16,17] n-Hexyl and
c-hexyl substituents were chosen to prevent any additional
influence from non-innocent groups, such as phenyl moieties.
The sergeant 1closed§ (R = n-hexyl or c-hexyl) was designed to
In our earlier study of LMWGs of type 1open (Scheme 1)
we showed how 1open, bearing chiral side chains [R = (R)-
phenylethylamine], can undergo ring closure in the gel state
to give 1closed with 96% diastereomeric excess.[15] It was also
shown that these chiral compounds can co-aggregate with
compounds of the same type, which have achiral R groups
(R = c-hexyl), and induce an enantiomeric excess of up to
94% after subjecting the mixture in the gel to ring-closing
conditions.[16] Here we demonstrate how enantiomerically
§
pure 1closed can template its own formation in the gel state,
that is, how the chiral product of the reaction governs its own
formation. The distinctive feature of this system, as compared
to earlier studies,[8,14,16] is the fact that the soldier is a precursor
of the sergeant, and can be converted into it by photochemical
cyclization after the amplification of supramolecular chirality
is established. One can consider this an autocatalytic effect in
a kinetically trapped state.
We show how the use of the prochiral diarylethene
molecular photoswitches 1open, which co-aggregate with
a small fraction of their enantiomerically pure closed counter-
parts 1closed§, results in the formation of chiral supramolecular
gels (Figure 1, templated pathway). The S,S molecular chir-
ality present in the dopant [(S,S)-1closed§] is translated into
a preferred helicity in the chiral supramolecular gel (M
helicity in Figure 1) consisting of 1open (soldier). In the gel
state, the prochiral switches 1open can be photochemically ring-
closed to 1closed by UV irradiation, thereby locking the
be one of the enantiomers of the ring-closed soldier 1open
.
Predicting the outcome of chiral amplification in supramolec-
ular systems is a daunting task[17] and small structural changes
in the chiral dopant can result in formation of the other
enantiomer[16] or even loss of amplification.[14] The fact that
the soldier and sergeant in our system are the same molecule
may have outstanding implications for autoamplification of
chirality. The soldiers 1aopen and 1bopen and enantiopure
sergeants 1aclosed§ and 1bclosed§ were synthesized to investigate
§
the potential for 1closed to act as a template in the gel state
(Scheme 2).
§
Scheme 2. Synthesis of the sergeants 1aclosed and 1bclosed§. DIC=N,N’-
diisopropylcarbodiimide, DMAP=4-(N,N-dimethylamino)pyridine.
The soldiers 1aopen and 1bopen are readily prepared by our
reported procedure.[18] The chiral sergeant, however, under-
goes fast ring opening to the achiral (open) form when
exposed to visible light[19] and the synthesis proved to be
highly challenging. Using a variety of procedures, including
classical resolution, HPLC using a chiral stationary phase, or
asymmetric synthesis, did not result in isolation of enantio-
pure material. We envisioned functionalization of the pre-
cursor 2 with a chiral auxiliary in a manner that not only
resulted in separation of the closed isomers, but also allowed
facile synthesis of the desired enantiomers of the amides
1closed§ (Scheme 2).
Figure 1. Concept for autoamplification of molecular chirality through
the induction of supramolecular chirality. For the templated pathway,
§
a small fraction of chiral dopant (S,S)-1closed is added to a mixture of
prochiral photoswitches [(M)-1open and (P)-1open] prior to gelation. The
formed fibers that result in gel formation are templated by the chiral
§
dopant (S,S)-1closed and one helicity (M) is dominant. Upon ring
closing of the photoswitches with UV light, the chirality of the fibers
results predominantly in the formation of the templated enantiomer
(S,S)-1closed. If no chiral dopant is added to the mixture of prochiral
photoswitches (non-templated pathway), the gel is made up of an
equal mixture of P and M conformers in the fibers. Subsequent ring
closure results in a statistical 1:1 mixture of products.
2
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Table 1: Sergeant-soldier experiments of 1a in toluene.
Towards this goal, the chiral dithioester 3open was prepared
by coupling 2 with Ac-(R)-Cys-OEt. Compound 3open was
then photocyclized with UV light (l = 312 nm) to form a 1:1
mixture of diastereomers (3closed). Separation of the diaste-
reomers, by HPLC using a chiral stationary phase, was
followed by reaction of each of the enantiomerically pure
Entry
Total concentration
ee [%]
Dee
§
1open +1closed (% w/v)
at no induction[a]
observed
[%]
1
2
3
4
5
6
7
8
1.5
1.5
1.5
1.5
1.5
1.5
0.8
0.3
0.2
0.8
1.5
1.0
0
3.2
9.5
0
25
50
44
48
84
47
29
31
0
22
40
32
31
46
38
13
16
25
35
38
§
3closed with an excess of n-hexyl and c-hexyl amine at room
12.5
16.9
37.8
9.0
15.7
14.5
À19.5
À19.6
9.0
temperature (shielded from light) to provide the enantiopure
amides 1aclosed§ and 1bclosed§, respectively. These mild reaction
conditions resulted in full conversions for the last step and no
ring opening was observed.
The photochromic properties of 1open were studied using
UV/Vis spectroscopy (for 1bopen, Figure 2a). Upon irradiation
with UV light (l = 312 nm), the band at l = 262 nm decreases
9
10[b]
11[b]
12[c]
À45
À54
47
For experimental details see the Supporting Information. [a] The fraction
of enantiopure sergeant added is equal to the ee value [%] where there is
no indiction. [b] The opposite enantiomer of the sergeant was used as
a dopant and the other enantiomer of the product was obtained
§
accordingly. [c] The c-hexyl-appended soldier 1bopen and sergeant 1bclosed
§
were used instead of n-hexyl 1aopen and 1aclosed
.
neous gel upon cooling. The gel was irradiated with UV light
(l = 312 nm) and the products were analyzed by HPLC, using
a chiral stationary phase, after dissolution of the gel in EtOH.
If induction of the enantiopure sergeant on the ring-closed
soldier does not take place, that is, the ring closure of the
soldier molecules proceeds in a racemic fashion, then the
amount of added enantiopure sergeant is directly reflected in
the observed ee value.[20] The difference between the
observed ee value and the ee value if no asymmetric induction
occurs, reflects the ee value that was induced by the chiral
sergeant (Dee) and is referred to henceforth as “induction”.
It was found that the sergeant was able to induce its own
formation with 40% (Table 1, entry 3) induction (observed
ee = 50%, Dee = 40%), when compared to the system in the
absence of the sergeant (Table 1, entry 1), after correction for
the amount of sergeant initially added. This establishes, to the
best of our knowledge, for the first time the autoamplification
of molecular chirality by induction of supramolecular chir-
ality. Decreasing the amount of sergeant to 3.2% led to
a lower induction of 22% (Table 1, entry 2). Increasing the
amount of sergeant (up to 37.8%), in contrast, did not
significantly increase the asymmetric induction (Table 1,
entries 4–6). Cryo-transmission electron microscopy (cryo-
TEM) revealed a fibrous network of very high density in both
the gels containing the open and closed forms (Figure 3). We
therefore envisioned that decreasing the total concentration
of the gelator might have an effect on the outcome of the
experiment as the gel would become less dense, but lowering
the concentration from 1.5% w/v to 0.8% did not increase the
induction (Table 1, entry 7). Further decrease in the concen-
tration below a certain threshold (0.3% w/v) resulted in
a drop in induction to approximately 15% (Table 1, entries 8
and 9). When the opposite enantiomer of the sergeant,
1aclosed§, was used as the dopant, the other enantiomer of the
product was obtained (Table 1, entries 10 and 11), thus
providing evidence that the sergeant is responsible for
templating its own formation and that one of the enantiomers
§
Figure 2. Photochromic properties of 1bopen, rac-1bclosed, and 1bclosed
a) UV/Vis absorption spectra of 1bopen (solid line, 5.0ꢀ10À4 m in
.
EtOH) and rac-1bclosed (dashed line, 5.0ꢀ10À4 m in EtOH). Rac-1bclosed
was obtained by irradiation (l=312 nm) of 1bopen. b) CD spectra of
the soldier 1bopen (dashed line, 3.7ꢀ10À5 m), ring-closed soldier
rac-1bclosed (dotted line, 3.7ꢀ10À5 m), and enantiopure sergeant 1bclosed
(solid line, 3.7ꢀ10À5 m) in EtOH.
§
and new bands for rac-1bclosed appear at l = 347 and 528 nm.
CD spectroscopy (Figure 2b) confirmed that 1open closes in
a racemic fashion and has no influence on the outcome of the
sergeant-soldier asymmetric induction experiment. As
expected, no Cotton effect was observed for a solution of
neither the open (1bopen) nor the closed soldier (rac-1bclosed).
The enantiopure 1bclosed§, in contrast, shows a positive Cotton
effect with a maximum at l = 528 nm and a minimum at l =
347 nm.
With the enantiopure sergeants, 1closed§, and soldiers, 1open
in hand we set out to probe the concept presented in Figure 1.
A suspension of the soldier 1aopen in toluene was doped with
a small amount of the enantiomerically pure 1aclosed (for
experimental details, see Table 1 and the Supporting Infor-
mation). The suspension was heated and formed a homoge-
,
§
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polar solvents resulted in poor gel formation or racemic
mixtures.
This observation supports the hypothesis that hydrogen
bonding in the supramolecular gel state helps to preorganize
the achiral 1open soldier with a preferred helical conformation.
In other words, one of the dynamic (and interconverting)
helical conformers (Scheme 1) is selected in the supramolec-
ular aggregate, which is governed by the chirality of the
§
dopant 1closed
.
The gel-to-solution temperature (Tgel) of gelated 1bopen
and rac-1bclosed, using dropping-ball experiments[22] (see the
Supporting Information for experimental details), were also
determined. The gel of 1bopen has a Tgel of 828C and the gel of
rac-1bclosed has a Tgel of 748C, thus suggesting a lower stability
of the aggregates of the closed form.
Figure 3. Cryo-TEM images of self-assembled gel morphologies. a) Gel
fibers of the soldier 1bopen (1.5% w/v in toluene). b) Gel fibers of the
soldier rac-1bclosed (1.5% w/v in toluene) after irradiation of 1bopen with
UV light (l=312 nm).
The data indicate that upon irradiation of the gel, the
closed form goes into solution preferentially over the open
form, a process that would limit the amount of sergeant
present in the gel fibers and be a possible origin of a modest
asymmetric induction. Indeed, it has been shown by the
groups of Sijbesma and Meijer that the presence of chiral
monomers in solution has little or no effect on the chirality of
assemblies if the monomers are not part of the assembly.[23]
In summary, we present herein, to the best of our
knowledge, the first example of autoamplification of molec-
ular chirality by induction of supramolecular chirality. We
have shown how diarylethene photochemical switches can be
used in autocatalytic induction. A small amount of a chiral
sergeant (1closed§) can be added to its precursor 1open without
inhibiting gel formation. The chirality of the gel is controlled
by the chiral sergeant, thereby favoring the formation of
either a P- or M-helical gel. The chiral information is
subsequently locked at the molecular level as the prochiral
soldiers undergo ring closure by irradiation of the gel with
UV light to form one of the enantiomers in excess. This
completes the cycle from molecular chirality by supramolec-
ular chirality back to molecular chirality of the same
molecules and places this system among the very few
examples where a chiral product of a reaction can template
its own asymmetric synthesis. This process is reminiscent of
so-called autocatalytic reactions, however here the auto-
amplification takes place in a kinetically trapped state.
is not simply favored for other reasons. The system gives
comparable results for 1b, which is functionalized with
c-hexyl instead of n-hexyl side chains (Table 1, entry 12),
thus demonstrating that the nature of the alkyl moieties has
a minor influence. In each case, doping the gels with the
enantiopure sergeant 1closed§ resulted in an increase in ee value
§
(after correcting for the initial amount of sergeant 1closed
added) compared to cases where no dopant was added.
As the solvent is an integral part of organogels,[21] a range
of other solvents were investigated to assess their effect on the
asymmetric autoinduction (Table 2). The use of toluene or
Table 2: Sergeant–soldier experiments of 1a in various solvents.[a]
Entry
Solvent
ee [%]
Dee
[%]
at no induction[b]
observed
1[c]
2[c]
3
toluene
c-hexane
benzene
decalin
1-phenyloctane
dibutyl ether
acetonitrile
hexadecane
9.5
11.0
9.6
10.0
12.5
6.0
50
44
27
24
rac
rac
–
35Æ6
33
17
14
–
4[c]
5
6
–
–
–
7[d]
8[d]
11.3
11.9
–
[a] The total gelator concentration was 1.5% w/v. For experimental
details see the Supporting Information. [b] The fraction of enantiopure
sergeant added is equal to the ee value [%] where there is no indiction.
[c] The error indicated is the standard deviation (n=7, see also the
Supporting Information). At a concentration of 0.8% w/v, Dee was the
same. [d] Solid precipitated from the solution upon cooling.
Received: December 23, 2013
Published online: && &&, &&&&
Keywords: chirality · gels · hydrogen bonds · self-assembly ·
.
supramolecular chemistry
cyclohexane gave the best results in these asymmetric auto-
amplification experiments (Table 2, entries 1 and 2). Benzene
and decalin facilitate asymmetric induction, but lower Dee
values (ca. 15%) were observed (Table 2, entries 3 and 4). In
1-phenyloctane, the product was formed as a racemate
(Table 2, entry 5). Although a gel can still be formed in the
more polar dibutylether, the product was obtained as a race-
mate (Table 2, entry 6). The use of acetonitrile and hexa-
decane resulted in a precipitate, rather than a gel (Table 2,
entries 7 and 8). In conclusion, hydrophobic solvents were
required for the observation of an enhancement in ee value
(with toluene being the preferred solvent), as the use of more
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Angew. Chem. Int. Ed. 2014, 53, 1 – 6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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These are not the final page numbers!

.
Angewandte
Communications
Communications
Chiral Amplification
D. J. van Dijken, J. M. Beierle,
´
M. C. A. Stuart, W. Szymanski,
W. R. Browne,
B. L. Feringa*
&&&& — &&&&
Autoamplification of Molecular Chirality
through the Induction of Supramolecular
Chirality
There and back again: Doping a mixture
of rapidly interconverting prochiral, open
diarylethenes (left) with their enantio-
pure, closed counterparts, led to forma-
tion of gel fibers of preferred helicity
(center). This supramolecular chirality
was transferred to the molecular level by
photochemical ring closing, thus yielding
a chiral product (right) which is enriched
in the enantiomer originally used as
a template.
6
www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
These are not the final page numbers!