Correlation between the Selectivity and the Structure of an Asymmetric Catalyst Built on a Chirally Amplified Supramolecular Helical Scaffold

Article abstract of DOI:10.1021/jacs.6b01306

For the first time, supramolecular helical rods composed of an achiral metal complex and a complementary enantiopure monomer provided a good level of enantioinduction in asymmetric catalysis. Mixtures containing an achiral ligand monomer (BTA^PPh2, 2 mol %) and an enantiopure ligand-free comonomer (ester BTA, 2.5 mol %), both possessing a complementary benzene-1,3,5-tricarboxamide (BTA) central unit, were investigated in combination with [Rh(cod)₂]BAr_F (1 mol %) in the asymmetric hydrogenation of dimethyl itaconate. Notably, efficient chirality transfer occurs within the hydrogen-bonded coassemblies formed by BTA Ile and the intrinsically achiral catalytic rhodium catalyst, providing the hydrogenation product with up to 85% ee. The effect of the relative content of BTA Ile as compared to the ligand was investigated. The amount of chiral comonomer can be decreased down to one-fourth of that of the ligand without deteriorating the enantioselectivity of the reaction, while the enantioselectivity decreases for mixtures containing high amounts of BTA Ile. The nonlinear relationship between the amount of chiral comonomer and the enantioselectivity indicates that chirality amplification effects are at work in this catalytic system. Also, right-handed helical rods are formed upon co-assembly of the achiral rhodium complex of BTA^PPh2 and the enantiopure comonomer BTA Ile as confirmed by various spectroscopic and scattering techniques. Remarkably, the major enantiomer and the selectivity of the catalytic reaction are related to the handedness and the net helicity of the coassemblies, respectively. Further development of this class of catalysts built on chirally amplified helical scaffolds should contribute to the design of asymmetric catalysts operating with low amounts of chiral entities.

Full text of DOI:10.1021/jacs.6b01306

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Article
Correlation between the selectivity and the structure of an asymmetric
catalyst built on a chirally amplified supramolecular helical scaffold
Alaric Desmarchelier, Xavier Caumes, Matthieu Raynal, Anton
Vidal-Ferran, Piet W. N. M. van Leeuwen, and Laurent Bouteiller
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b01306 • Publication Date (Web): 21 Mar 2016
Downloaded from http://pubs.acs.org on March 22, 2016
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Journal of the American Chemical Society
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Correlation between the selectivity and the structure of an asymmetric
catalyst built on a chirally amplified supramolecular helical scaffold
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†
†
,†
‡,&
Alaric Desmarchelier, Xavier Caumes, Matthieu Raynal,* Anton VidalꢀFerran, Piet W. N. M. van
||
†
Leeuwen, and Laurent Bouteiller
†
Sorbonne Universités, UPMC Univ Paris 06, CNRS, Institut Parisien de Chimie Moléculaire, Equipe Chimie des Po-
lymères, 4 Place Jussieu, F-75005 Paris, France
‡
Institute of Chemical Research of Catalonia (ICIQ), Avgda. Països Catalans 16, 43007 Tarragona, Spain.
&
Catalan Institution for Research and Advanced Studies (ICREA), Passeig Lluís Companys 23, 08010 Barcelona, Spain
§
LPCNO, INSA, 135 Avenue de Rangueil, F-31077 Toulouse, France
ABSTRACT: For the first time, supramolecular helical rods composed of an achiral metal complex and a complementary enan-
tiopure monomer provided good level of enantioinduction in asymmetric catalysis. Mixtures containing an achiral ligand
PPh2
monomer (BTA
, 2 mol%) and an enantiopure ligand-free co-monomer (ester BTA, 2.5 mol%), both possessing a comple-
mentary benzene-1,3,5-tricarboxamide (BTA) central unit, were investigated in combination with [Rh(cod) ]BAr (1 mol%) in
2
F
the asymmetric hydrogenation of dimethyl itaconate. Notably, efficient chirality transfer occurs within the hydrogen-bonded
co-assemblies formed by BTA Ile and the intrinsically achiral catalytic rhodium catalyst, providing the hydrogenation product
with up to 85% ee. The effect of the relative content of BTA Ile compared to the ligand was investigated. The amount of chiral
co-monomer can be decreased down to one fourth of that of the ligand without deteriorating the enantioselectivity of the
reaction while the enantioselectivity decreases for mixtures containing high amounts of BTA Ile. The non-linear relationship
between the amount of chiral co-monomer and the enantioselectivity indicates that chirality amplification effects are at work
PPh2
in this catalytic system. The rhodium complex of BTA
and BTA Ile co-assemble under the form of right-handed helical
rods as confirmed by various spectroscopic and scattering techniques. Remarkably, the major enantiomer and the selectivity
of the catalytic reaction are related to the handedness and the net helicity of the co-assemblies, respectively. Further devel-
opment of this class of catalysts built on chirally-amplified helical scaffolds should contribute to the design of asymmetric
catalysts operating with low amounts of chiral entities.
principle. Following the pioneering results of Green and
Introduction
9a,12
co-workers on poly(isocyanates),
this phenomenon has
Chirality amplification, the phenomenon by which a small
asymmetric bias is translated into a large chiral prefer-
ence, is a central topic of investigation mainly aiming at
been observed experimentally in a great number of exam-
ples which in turn have been consistently modeled at the
theoretical level. Such an approach is particularly ap-
pealing for the construction of asymmetric catalysts since
it might enable a good control of the asymmetric envi-
ronment around the metal center with a limited amount
of chiral inducers.
13
1
elucidating the origin of homochirality. It also opens new
avenues in the preparation of enantio-enriched mole-
2
cules and materials with promising perspectives of appli-
3
cations including the development of sensors, stimuli-
4
5
responsive gels, liquid crystal displays, stationary phas-
Surprisingly, only very recently chirality amplification in
helical polymers was applied for the first time in the con-
6
7
es and catalysts.
14
Chirality amplification effects are particularly strong in
struction of efficient asymmetric catalysts. Suginome
8
7a,7c
dynamic molecular helices, such as those formed by
and
co-workers
demonstrated
that
covalent
9
10
covalent and supramolecular polymers, and folda-
mers
poly(quinoxaline-2,3-diyl)s terpolymers containing achiral
phosphine monomers (0.4%), achiral phosphine-free
8b,11
since a minimal energetic bias at the monomeric
15
level is cooperatively transferred along the polymeric
main chain. In particular, when a few enantiopure mon-
omers (called "the sergeants") impose their handedness to
a large number of achiral ones (called "the soldiers"), the
macromolecular helicity obeys the sergeants-and-soldiers
monomers (82%) and only 18% of enantiopure mono-
meric units formed purely one-handed helical structures
that promoted palladium-catalyzed asymmetric hydrosi-
lylation and Suzuki-Miyaura cross-coupling reactions
16
with remarkably high enantioselectivity.
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Scheme 1. Possible strategies [I]-[III] for the construction of supramolecular helices supporting catalytic metal centers (the arrows illustrate the chirality
transfers).
Helical supramolecular polymers also showed interesting
to the structure of the co-assemblies, and notably to their
helicity (as measured by CD spectroscopy), which may
facilitate further development of this unique class of cata-
lysts.
features for catalysts development: i) they are stimuli-
17
responsive, ii) their supramolecular structure may en-
hance the activity and selectivity of catalytic centers ar-
18
ranged on their scaffold, and iii) their composition can
be tuned by simply mixing different types of complemen-
Results and discussion
17b,19
tary monomers.
Despite these promising achieve-
The magnitude of chirality transfer and chirality amplifi-
cation effects displayed by supramolecular assemblies
between achiral and enantiopure monomers depend,
ments, little is known about how the chirality of a supra-
molecular polymer can be transferred to intrinsically-
20
10
achiral metal centers located at its periphery, which can
amongst other factors, on the nature of the monomers.
in turn be used as catalytic metals for asymmetric reac-
Ester BTAs (Chart 1) were chosen as chiral co-monomers
since they are derived from an important accessible chiral
pool and can be prepared straightforwardly in two syn-
19,21
tions.
One can envisage three strategies for the con-
struction of chiral supramolecular helices supporting
catalytic metal centers: the use of enantiopure monomers
only ([I], Scheme 1), the use of a mixture of an achiral
ligand monomer with a enantiopure ligand-free monomer
in a nearly stoichiometric ratio ([II], chirality transfer)
and the use of an enantiopure monomer as the minor
24
thetic steps (see the Supporting Information). When
studied individually, ester BTAs exhibit unusual self-
association properties in cyclohexane compared to classi-
23
cally-investigated alkyl BTAs since the nature of the
substituent at the stereogenic carbon and the concentra-
tion determine the nature of the dominant species in
solution, i.e. stacks or dimers. Moreover, preliminary
results indicated that co-assemblies formed between an
achiral alkyl BTA and an ester BTA, used as the enanti-
opure co-monomer, display strong chirality amplification
component ([III], chirality amplification). While strategy
19,21
[
[
I] has already been successfully described,
strategies
II] and [III] remain to be implemented in an efficient
way. The challenge in these cases is to obtain a good level
of enantioinduction since any achiral metal catalysts that
will not be located in a chiral environment will signifi-
24c
effects.
22
cantly decrease the selectivity of the reaction. However,
Catalytic experiments. Mixtures composed of an achiral
supramolecular co-polymers used as efficient scaffolds for
asymmetric catalysis should possess unique features: i)
they can be prepared by simply mixing the different mon-
omers alleviating the synthetic efforts that are required
for the preparation of covalent co-polymers and ii) chiral-
ity amplification effects that operate in these polymers
can be used to decrease the amount of chiral inducers
required to promote asymmetric catalysis.
PPh2
BTA ligand (BTA
, 2 mol%), the metal precursor
1 mol%) and an enantiopure co-
(
[Rh(cod) ]BAr ,
2
F
monomer (ester BTA) were tested in the rhodium-
catalyzed hydrogenation of dimethyl itaconate. Catalytic
components were mixed in CH Cl in order to ensure the
2
2
PPh2
formation of the Rh complex between BTA
and
[
Rh(cod) ]BAr then CH Cl was removed under vacuum
2
F
2
2
and replaced by the solvent selected for catalysis. In the
Here, we demonstrate that supramolecular helical rods
formed between an achiral rhodium complex of a ben-
zene-1,3,5-tricarboxamide (BTA) ligand and an enanti-
following, the molar ratio between the ester BTA and
PPh2
BTA
initially present in the catalytic mixtures will be
23
0
designated as R esterBTA.
opure BTA co-monomer can indeed be used as efficient
scaffolds for the asymmetric hydrogenation of dimethyl
itaconate (up to 85% ee). Chirality amplification effects
are at work in this catalytic system, which allows decreas-
ing the amount of chiral co-monomer down to one fourth
of that of the ligand monomer without deteriorating the
enantioselectivity of the catalytic reaction. Moreover, we
show that the selectivity of the catalytic reaction is related
Chirality transfer. The ease of preparation of the catalytic
mixtures allowed us to test ten ester BTAs (R esterBTA=1.25)
0
as potential enantiopure co-monomers for the catalytic
reaction (Table 1). In hexane/CH Cl (10:1), the hydrogena-
2
2
tion product of dimethyl itaconate was obtained with
significant ee (enantiomeric excess) for all ester BTAs.
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Chart 1. Structures of the BTA ligands and co-monomers used in the asymmetric hydrogenation reaction.
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Table 1. Asymmetric hydrogenation of dimethyl ita-
conate with mixtures of BTA ligand, [Rh(cod) ]BAr
and ester BTA: screening of various co-monomers.
monomers BTA Nle and BTA (R)-Nle furnish opposite
enantiomers with similar ee (74% and 72% ee respectively,
entry 7 and 8). Control experiments (entries 11-12) con-
firmed that the selectivity of the reaction stems from the
2
F
[a]
formation of hydrogen-bonded co-assemblies between
PPh2
the rhodium complex of BTA
BTA
and ester BTAs. Indeed,
Me
PPh2
, a BTA ligand in which the amide groups have
19
been N-methylated (Chart 1), shows no selectivity in
presence of BTA Ile (entry 11). BTA Ile alone provided no
Entry
BTA ligand
Co-monomer
ee (%)
ee either, demonstrating that ester BTAs do not influence
the stereochemical outcome of the catalytic reaction by
fortuitous coordination to the Rh center (entry 12).
PPh2
1
BTA
BTA Phe
BTA (R)-Ala
BTA Met
BTA Leu
BTA Phg
BTA (R)-Abu
BTA (R)-Nle
BTA Nle
40 (S)
-46 (R)
53 (S)
PPh2
2
3
4
5
6
7
8
9
BTA
Table 2. Screening of the catalytic conditions with
PPh2
BTA
0
[a]
BTA Ile as the co-monomer (R BTAIle=1.25).
PPh2
BTA
54 (S)
62 (S)
-66 (R)
-72 (R)
74 (S)
85 (S)
PPh2
BTA
[dimethyl
itaconate] (M)
Entry
solvent
ee (%)
PPh2
BTA
PPh2
[b]
BTA
1
hexane/CH
2
Cl
2
10:1
5:1
0.4
0.4
0.4
0.4
0.4
0.2
0.8
0.4
85±7 (S)
85 (S)
52 (S)
51 (S)
0
PPh2
BTA
2
3
4
5
6
7
8
hexane/CH
hexane/CH
2
Cl
2
2
PPh2
BTA
BTA Val
2
Cl
2:1
PPh2
[b]
10
BTA
BTA Ile
85±7 (S)
Me
BTA
PPh2
toluene
11
BTA Ile
0
0
-
12
BTA Ile
CH
2
Cl
2
[c]
3
PPh2
1
BTA
BTA Ile
nd
hexane/CH
hexane/CH
hexane/CH
2
2
2
Cl
2
10:1
10:1
10:1
85 (S)
55 (S)
[
a]
0
R
0
PPh2
esterBTA=1.25, n BTA
=4.0 µmole, [dimethyl itaconate]=0.4 M. Full
Cl
2
2
conversion except for entry 13. Each experiment was performed in tripli-
cate (except for control experiments 11-13) and the indicated ee corre-
sponds to the average value (standard deviation ≤ 5%). Positive value of
[c]
74 (S)
Cl
25
the ee corresponds to the (S) enantiomer. See Chart 1 for the structures
[
a]
PPh2
BTA
2 F
(2 mol%), [Rh(cod) ]BAr (1 mol%), BTA Ile (2.5 mol%), room
of the BTAs. See the SI for more details on the preparation of the catalytic
[
b]
[c]
temperature unless otherwise stated. Full conversion. Each experiment
was performed in duplicate and the indicated ee corresponds to the
average value (standard deviation ≤ 5%). See Table S.2 for additional
mixtures. Based on repeatability tests (16 runs, Table S.1). The cata-
lytic mixture is filtered and catalysis is performed with the supernatant:
no hydrogenation product was obtained. nd = not determined.
[
b]
screening experiments. Based on repeatability tests (16 runs, Table S.1).
c] Catalytic experiment performed at -20 °C.
[
The selectivity of the catalytic reaction strongly depends
on the nature of ester BTA (40% ee<selectivity<85% ee)
thus showing the importance of having a library of enan-
tiopure co-monomers at one's disposition for catalytic
screening. BTA Ile (Entry 10) and BTA Val (Entry 9) pro-
vided the (S) enantiomer of the hydrogenation product
with the best selectivity (85% ee), a selectivity similar to
We performed additional catalytic experiments with mix-
PPh2
0
tures composed of BTA
and BTA Ile (R BTAIle=1.25,
Tables 2 and S.2). For catalytic experiments conducted in
a mixture of hexane/CH Cl 10:1, the results were repeata-
2
2
ble with a mean value of 85±7% ee based on 16 runs (Entry
, Table 2 and Table S.1). The amount of CH Cl in this
1
1
9
2
2
the one obtained with chiral monomers ([I], Scheme 1).
solvent mixture can be increased to 20% without altering
the selectivity (entry 2). However, the selectivity signifi-
cantly decreases in a more polar hexane/CH Cl 2:1 mix-
By inverting the chirality of the co-monomer, we were
able to access both enantiomers of the hydrogenation
product with the same BTA ligand. For example, co-
2
2
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ture and in toluene and no selectivity is obtained in
CH Cl (entries 3-5). This is related to the disruption of
the hydrogen-bonded assemblies in these solvents. In
pure hexane, the selectivity appears to vary significantly
from one run to another presumably because catalyst
aggregates of variable size form under these conditions
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5
5
5
5
5
5
6
2
2
1
9
(
Table S.2 and vide infra). Changing the nature of the
alkane solvent does not significantly influence the selec-
26
tivity outcome of the reaction (Table S.2). While a lower
concentration in substrate does not change the selectivity
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(
entry 6), the ee significantly drops at higher substrate
concentration perhaps as a result of competitive interac-
tions between the substrate and the hydrogen-bonded
assemblies (entry 7). Finally, lowering the temperature
(
-20 °C) leads to a decrease in the selectivity (entry 8 and
Table S.2).
These screening experiments reveal that both the nature
of the enantiopure co-monomer and the experimental
conditions are crucial factors to obtain optimal selectivity
for these catalytic systems. Taken all together, these re-
sults indicate that the supramolecular chirality displayed
by co-assemblies formed between an achiral ligand mon-
Figure 1. Enantioselectivity of the hydrogenation reaction as a function
0
0
0
0
PPh2
of R BTAIle. R BTAIle=n BTA Ile/n BTA
. Inset: zoom on the region for
0
values of R BTAIle<1.5
.
structure the catalyst evolves as a function of the compo-
28
sition of the co-assemblies (vide infra).
Nature of the catalytically active co-assemblies. In order to
rationalize the catalytic results, it is important to gain
information into the assembly process and on the solubil-
0
omer and an enantiopure co-monomer (R esterBTA=1.25) can
be efficiently transferred to the peripheral rhodium cen-
ters which in turn promote the hydrogenation reaction
with very good enantioselectivity ([II], Scheme 1).
ity properties of the resulting co-assemblies. Firstly, the
PPh2
catalytic mixture (BTA
+BTA Ile+[Rh(cod) ]BAr ) is
2 F
Chirality amplification. Encouraged by our initial screen-
prepared in CH Cl , a solvent in which no selectivity is
2
2
0
PPh2
ing performed with R esterBTA=1.25, we probed the influence
observed suggesting that co-assembly between BTA
of the amou
coordinated to Rh and BTA Ile does not occur to a signif-
icant extent (Table 2). In fact, co-assembly likely occurs
upon evaporation of CH Cl and the rhodium complex
2 2
remains insoluble as we observed no significant dissolu-
tion of the resulting yellow solid upon addition of hex-
ane/CH Cl 10:1, the solvent mixture used in the catalytic
2 2
experiments. However, this second solvent does have an
influence on the size of the solid aggregates as we ob-
nt of enantiopure co-monomer on the selectivity of the
catalytic reaction. BTA Ile, one of the two most efficient
ester BTAs, was selected and was mixed in variable
PPh2
amounts (0.1-8 mol%) to fixed quantities of BTA
(2
0
PPh2
mol%, n BTA
=4.0 µmole) and [Rh(cod) ]BArF (1
mol%). The enantioselectivity of the hydrogenation
2
27
0
reaction exhibits a strong dependence on R
(Figure 1,
BTAIle
0
0
0
PPh2
R
=n BTAIle/n BTA
). In fact, 48% ee is observed
for the mixture containing the lowest amount of BTA Ile
served well-dispersed aggregates in hexane/CH
Cl mix-
2 2
BTAIle
tures but not in pure hexane (Table S.2) We also found
.
0
(
R
BTAIle=0.05 i.e. 0.1 mol% loading in BTA Ile); then the ee
that the supernatant obtained after filtration of this yel-
low solid is not catalytically active (Table 1, entry 13). If we
consider that the solubility of the co-assemblies does not
evolve during the catalytic reaction, then this result sug-
gests that the reaction is actually catalyzed by insoluble
increases up to plateau at 78-85% ee for
a
0
0
0
.25≤R BTAIle≤1.25 and then decreases for R BTAIle >1.25. The
non-linear increase of the selectivity of the catalytic reac-
0
tion (see inset in Figure 1) as a function of R
clearly
BTAIle
reveals that chirality amplification effects are at work in
this catalytic system ([III], Scheme 1). As a result of these
chirality amplification effects, the amount of enantiopure
co-monomer can be decreased down to one fourth of that
of the ligand without deteriorating the enantioselectivity
of the catalytic reaction. Further characterization is re-
quired to relate the chirality amplification effects dis-
played by this catalyst to the net helicity of the co-
assemblies, i.e. to the bias between left and right-handed
helical fragments. Also, these effects alone cannot explain
co-assemblies formed between the rhodium complex of
PPh2
BTA
and BTA Ile after evaporation of CH Cl .
2 2
Characterization of the assemblies. With the objective of
correlating the selectivity of the catalytic reaction with
the structure of the BTA assemblies, we precisely probed
PPh2
the homo- and co-assembly properties of BTA
and
BTA Ile, in solution and in the solid-state, by means of
Fourier Transform-Infrared (FT-IR) spectroscopy, UV
absorption, Circular Dichroism (CD) and Small Angle
Neutron Scattering (SANS) analyses. These analyses were
the entire selectivity outcome of the catalytic reaction as
performed both in presence and absence of Rh coordinat-
0
PPh2
the enantioselectivity decreases at higher R
values
BTAIle
ed to BTA
.
0
(
R
≥1.5). This suggests that the
PPh2
BTAIle
Homo-assemblies. When studied separately BTA
and
BTA Ile display different association properties in cyclo-
2
9
PPh2
hexane. BTA
forms long and rigid stacks (Figure S.1)
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60
PPh2
Figure 2. Quantification of the amount of BTA Ile present in the co-assemblies. Left: FT-IR spectrum (zoom on the NH region) of the mixture of BTA
0
PPh2 0
and BTA Ile (R BTAIle=1.00, [BTA
] =8.0 mM) in cyclohexane. Simulated spectra for the extreme cases for which all or no BTA Ile is present in the co-
assemblies. RBTAIle in stacks (here 0.44) is deduced from the amount of BTA Ile in dimers. Right: RBTAIle in stacks as a function of R BTAIle (with or without
). For the co-assemblies in presence of Rh, the amount of BTA Ile in the insoluble stacks is deduced from the amount of BTA
Ile that remains as dimers in the cyclohexane phase. The black line (y=x) helps to visualize mixtures for which BTA Ile is fully incorporated into stacks.
0
PPh2
Rh coordinated to BTA
0
PPh2
R
BTAIle in stacks=nBTA Ile in stacks/n BTA
.
-
1
as shown by: i) FT-IR bands at 3245 cm (NH bonded to
the composition and structure of the co-assemblies. Mix-
-
1
-1
PPh2
amide CO), 1634 cm and 1549 cm (amide CO bonded to
tures of BTA
and BTA Ile, with fixed amount of
and variable amount of BTA Ile (0.05≤
≤4.0), proved to be fully soluble in cyclohexane. For
BTAIle
0
-
1
H
PPh2
NH, amide I and II bands respectively) and, ii) the q
BTA
0
dependence of the SANS intensity at low q values charac-
teristic of isolated cylindrical rigid rods (r=12.6 Å,
L>200 Å). In striking contrast, BTA Ile only forms dimers
in which the amide NH are linked to the ester carbonyl
instead of the amide carbonyl. BTA Ile does not form
stacks across the whole range of concentration investigat-
ed (0.05-50 mM). The dimers of BTA Ile possess the same
spectroscopic and scattering signature as those previously
R
mixtures with R BTAIle≤0.18, FT-IR analyses show that all
BTA monomers are in stacks inferring that all BTA Ile
monomers are incorporated into the co-assemblies (case
ii above). In contrast, for others mixtures, FT-IR analyses
indicate both the presence of stacks and dimers suggest-
PPh2
ing that BTA Ile only partly co-assembles with BTA
(case iii). In that case, the quantity of BTA Ile that is
incorporated into the stacks is deduced from the amount
of BTA Ile that remains as dimers as measured by FT-IR
(Figures 2a and S.5) and SANS analyses (Figure S.6). Ac-
cordingly, co-assemblies are observed for all mixtures
24c
characterized for BTA Nle (for a proposed molecular
arrangement and analyses see Figure S.2). The assembly
of BTA Ile into dimers in cyclohexane is in sharp contrast
with its ability to form stacks in the solid-state (Figure
S.3). The positive Cotton effect observed above 200 nm in
CD analyses infers the preferential formation of right-
(Table S.3) and the ratio of BTA Ile that actually co-
PPh2
assembles with BTA
is defined as RBTAIle in
3
0
0
PPh2
handed helical stacks of BTA Ile as previously found in
stacks=nBTA Ile in stacks/n BTA
.
3
1
the X-ray structure of a related ester BTA.
The same approach can be applied to mixtures of
PPh2
Importantly, the rhodium complex formed by reacting
BTA
, BTA Ile and [Rh(cod) ]BAr in order to deter-
2 F
PPh2
BTA
with [Rh(cod) ]BAr , in a 2:1 ratio, is soluble in
2 F
mine the composition of the co-assemblies between the
PPh2
CH Cl but not in cyclohexane. UV and FT-IR analyses of
rhodium complex of BTA
and BTA Ile. Here, all mix-
2
2
the thus obtained solid indicate that this rhodium com-
tures form heterogeneous suspensions in cyclohexane and
characterization of the soluble part indicates that it only
contains BTA Ile (in the form of dimers, Figure S.8), i.e.
all Rh and all ligand is in the solid. For all mixtures, the
amount of BTA Ile in the soluble phase is significantly
lower than the quantity of BTA Ile initially introduced in
the mixtures, which indicates (and allows us to quantify)
the presence of BTA Ile in the solid (Table S.4). It is im-
portant to note that the presence of BTA Ile in the solid
demonstrates its co-assembly with the Rh complex of
since BTA Ile is fully soluble in cyclohexane.
Thus, in contrast to co-assemblies without Rh, the co-
assemblies between BTA Ile and the rhodium complex of
are insoluble and can be isolated from non-
incorporated dimers of BTA Ile by simple centrifugation.
plex assembles into stacks (Figure S.4b,c). As expected,
PPh2
given that BTA
is achiral, these stacks show no helical
preference (Figure S.4a).
Composition and structure of the co-assemblies. Upon
mixing two complementary monomers, one can envisage:
i) the formation of their homo-assemblies exclusively
(
narcissistic self-sorting), ii) the formation of co-
assemblies exclusively (social self-sorting) and iii) an
intermediate situation in which homo- and co-assemblies
are concomitantly present. Determining whether the co-
assembly process follows one of the above hypotheses (i-
iii) is challenging particularly when the species have simi-
lar analytical signatures. In our case, stacks and dimers
can be easily differentiated by means of spectroscopic and
scattering analyses which allow us to precisely determine
PPh2
BTA
PPh2
BTA
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Page 6 of 11
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1
2
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2
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0
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0
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0
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0
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
H
PPh2
H
PPh2
Figure 3. a) CD spectra of the co-assemblies formed between the rhodium complex of BTA
(8 μmole in BTA
for all mixtures) and BTA Ile. For
the CD spectra of all the mixtures see Figure S.9. b) Plots of the anisotropy factor (g) of co-assemblies and of the enantioselectivity of the hydrogenation
reaction as a function of RBTAIle in stacks. The lines are drawn to guide the eye. g values measured at λ=249.0 nm.
0
RBTAIle in stacks can be plotted as a function of R BTAIle
the solid state, Figure S.3) to the achiral ligand mono-
mers.
(
Figure 2b). For both types of co-assemblies (with and
without Rh), the same trend is observed. BTA Ile is quan-
Subsequently, the helicity of the co-assemblies between
the Rh complex of BTA
PPh2
titatively incorporated into the co-assemblies for mixtures
and BTA Ile was measured for
0
with R BTAIle<0.25. For higher BTA Ile contents, the incor-
all the mixtures and expressed by the Kuhn anisotropy
factor (g=ꢀε/ε at λ=249.0 nm). As expected for helical
assemblies displaying chirality amplification properties,
the helicity does not increase linearly as a function of the
amount of enantiopure monomer in the co-assemblies
(i.e. as a function of RBTAIle in stacks, Figure 3b). Maximum
g values are reached for RBTAIle in stacks ≥0.25, which co-
incides with the minimum content of chiral co-monomer
poration of BTA Ile in the co-assemblies levels off (plat-
eau value at R
in stacks≈0.3-0.5) and then increases
BTAIle
0
again for R BTAIle≥1.0. This non-uniform trend can be ex-
plained by potentially different thermodynamic stabilities
displayed by the co-assemblies depending on their con-
tent in BTA Ile. Interestingly, the ratio of BTA Ile present
in both types of co-assemblies (with or without Rh) are
virtually the same. It suggests that the coordination of the
0
(R BTAIle=0.25) required to obtain the highest enantioselec-
tivity for the hydrogenation reaction.
+
PPh2
[Rh(cod)] fragment to BTA
does not significantly
modify the composition of the co-assemblies.
Rationalization of the selectivity outcome. The selectivi-
ty observed in the catalytic hydrogenation of dimethyl
itaconate (major enantiomer and enantioselectivity) can
be related to the structure of the co-assemblies. Firstly,
the handedness of the stacks dictates the nature of the
major enantiomer. Indeed, the (R) hydrogenation product
was obtained with our previously investigated chiral BTA
The structure of the co-assemblies between BTA Ile and
PPh2
the Rh complex of BTA
was then probed in the solid
state by UV and FT-IR analyses (Figure S.9). Despite their
different composition (Figure 2a), similar spectra with
bands that are characteristic of the stack form were ob-
served for all mixtures. Accordingly, the different selectiv-
ities displayed by the co-assemblies for the catalytic reac-
tion cannot be related to a change in the association pat-
tern of the central BTA units.
19
ligand that formed left-handed helical stacks whereas
the (S) hydrogenation product is furnished by the rhodi-
um catalyst supported by the right-handed helical co-
assemblies of BTA
H
PPh2
Chirality of the co-assemblies. The chirality of the co-
and BTA Ile. Secondly, the net
assemblies, with and without Rh, was then probed by CD
helicity of the co-assemblies explains the first two regimes
of the selectivity outcome of the catalytic reaction: stacks
containing less than one BTA Ile monomer for four
PPh2
spectroscopy. For mixtures of BTA
and BTA Ile (solu-
ble co-assemblies), CD spectra must be interpreted care-
fully since dimers of BTA Ile may contribute to the over-
H
PPh2
BTA
monomers (regime [I], Figure 3b), do not (all)
all CD signals. Importantly, for mixtures with
have the same handedness, so the catalytic selectivity is
not optimal. For stacks containing between ca. one fourth
and one half of BTA Ile relatively to BTA
[II]), all the stacks have the same handedness (although
the composition is evolving), so the catalytic selectivity is
roughly constant. For regime [I] and [II], there is a direct
correlation between the net helicity of the co-assemblies
formed between the Rh complex of BTA
and the selectivity outcome of the catalytic reaction. It is
further corroborated by the fact that dimethyl itaconate,
0
R
BTAIle≤0.25, a Cotton effect is observed, the shape of
H
PPh2
which is similar to that commonly found in CD spectra of
BTA helical stacks exhibiting a preferred handedness in
(regime
3
2
23,30
the solid state and in solution
(Figure S.7). A similar
positive Cotton effect is observed in the CD spectra of the
mixtures with Rh (insoluble co-assemblies, Figures 3a and
3
3
H
PPh2
Figure S.9) indicating that in both cases right-handed
and BTA Ile
3
0
helical stacks are formed. Clearly, BTA Ile imposes its
preferred handedness (as observed for its helical stacks in
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H
R*
1
2
3
4
5
6
7
8
9
N
H
N
R*
H
N
O
H
R*
O
O
n
H
R
N
N
Ph2
P
H
O
Z
O
[
Rh]
N
O
R
Ph2P
H
Z
N
R
H
O
H
N
N
fixed planar
chirality
O
10
11
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59
60
R
(
Z rings)
O
(1-n)
transfer of chirality
structure [A]
Figure 4. Proposed supramolecular structures [A] and [B] for the catalysts and postulated mechanism for the chirality transfer.
the substrate of the catalytic reaction, has no significant
influence on the nature of the co-assemblies (Figures 2b
and 3a), at least at the concentration used in our opti-
mized conditions (Table 2).
Conclusions
The above results clearly reveal the potential of supramo-
lecular helical co-assemblies as efficient scaffolds for
asymmetric reactions. Achiral rhodium complexes of BTA
ligands, when combined with suitable enantiopure com-
plementary co-monomers, are located within the chiral
environment displayed by the co-assemblies and promote
the hydrogenation of dimethyl itaconate with good level
of enantioinduction. The selectivity of the catalytic reac-
tion is correlated to the handedness, the helicity and the
According to our experimental results, the decrease in
selectivity observed for mixtures containing higher con-
tent of enantiopure co-monomer cannot be explained by
a different structure or a lower net helicity of the stacks.
To explain this decrease in selectivity (regime [III], Figure
3
b), we propose that the binding mode of the PPh units
2
to the Rh center evolves with the composition of the co-
assembly (Figure 4). In structure [A], two neighboring
diphenylphosphino (PPh ) units maintained by the right-
handed helical stacks are well positioned to chelate one
binding mode of the PPh units to the catalytic rhodium
2
center. The handedness and the helicity are dictated by
the enantiopure co-monomer while the preferred binding
mode of the phosphine ligands is related to their relative
position in the helical stacks. Importantly, the helicity of
co-assemblies is not proportional to the amount of enan-
tiopure co-monomer. Benefiting from the chirality ampli-
fication properties displayed by the co-assemblies, opti-
mal selectivity for the catalytic system can be obtained
with the amount of chiral co-monomer being one fourth
of that of the achiral ligand. Performing an asymmetric
reaction with little or no help of chiral entities raises
many fundamental questions with potentially important
2
3
0
3
4
Rh center. We hypothesize that within this structure,
chirality transfer occurs from the right-handed helices of
the polymeric scaffold to the connecting rings (labeled as
rings Z in structures [A] and [B]) facing one another with
their Si,Si (or Re,Re) faces and from there to the periph-
eral PPh units which may adopt the well-known C quad-
rant configuration (regimes [I] and [II]). In contrast, in-
creasing the amount of enantiopure co-monomers in the
stacks reduces the probability of having two consecutive
BTA ligands, which prevents the coordination of Rh by
3
5
2
2
2c
applications. To date, only very rare but fascinating
examples of asymmetric auto-catalysts are able to pro-
mote asymmetric reactions by chirality amplification of a
two PPh units belonging to the same stack. Accordingly,
2
in the speculative structure [B], Rh is acting as a bridge
between two right-handed helical stacks and although
these stacks exhibit the same handedness they cannot
efficiently transfer their helical chirality via a planar chiral
rearrangement of the rings Z to the environment of the
Rh catalytic center (regime [III]).
2d,2h,36
minute chiral imbalance.
knowledge, the present asymmetric metal catalyst also
constitutes the first example in which sub-
To the best of our
a
stoichiometric amount of a chiral inducer relative to the
achiral ligand can be used without eroding the stereose-
lectivity of the catalytic reaction. Helped by our proposed
rationale of the selectivity outcome of the catalytic reac-
tion, we are currently selecting combinations of achiral
and chiral monomers based on their chiroptical proper-
ties with the aim of further reducing the amount of chiral
inducer needed to get optimal selectivity.
Based on structures [A] and [B], a consistent rationale of
the selectivity outcome of the catalytic reaction is ob-
tained. In this specific catalytic system, CD experiments
may be used to predict: i) the major enantiomer produced
by the catalytic reaction and ii) the minimum amount of
chiral co-monomer required to get the optimal selectivity.
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 8 of 11
mM in CH Cl ). The components are then mixed together in
2
2
EXPERIMENTAL SECTION
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
glass vials equipped with small magnetic stirring bars, in the
PPh2
Materials Preparation. All amino acids were purchased
from Sigma-Aldrich or Alfa Aesar (99% ee) and used as re-
ceived. Benzene-1,3,5-tricarbonyl chloride was purchased
from Alfa Aesar, dimethyl itaconate, 1-dodecanol p-
following order: BTA Ile (desired volume) and BTA
(200
µL, 8 µmole). The vials are left open and stirred overnight in
a fume-hood. The resulting solids were put under vacuum
-
3
(10 mbar) for 3 hours before the addition of 1.0 mL of cyclo-
TsOH.H O, carbonyldiimidazole and trimesic acid were
hexane and sonicated for a few seconds to obtain a solution
2
PPh2
acquired from Sigma Aldrich, and were used directly. The
([BTA
]=8.0 mM and 0.42<[BTA Ile]<32.0 mM). The solu-
PPh2 19 Me
PPh2 19
synthesis and characterization of BTA
,
BTA
,
tions were briefly heated to reflux and cooled to r.t. before
3
7
24c
24c
24c
[
Rh(cod) ]BAr , BTA Met, BTA Phe, BTA Nle and
analyses. For CD analyses: Solutions with a fixed concentra-
2
F
24c
PPh2
tion in BTA
BTA (R)-Nle have been described previously. The experi-
mental details for the synthesis and characterization of the
ester BTAs are provided in the Supporting Information. Ra-
cemic ester BTAs, (rac)-BTAs, were prepared for the purpose
of determining the optical purity of (R)-BTAs and BTAs (en-
antiomeric and diastereomeric excesses, see the Supporting
Information for analytical details).
(1.0 mM) and variable concentrations in BTA
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Ile (0.05-4.0 mM) were prepared in the same way than solu-
tions for FT-IR analyses. For SANS analyses: Solutions with a
PPh2
-3
fixed concentration in BTA
(3.5 g.dm , 5.1 mM) and vari-
able concentrations in BTA Ile (2.2 and 24.5 mM) were pre-
pared in C D in the same way than solutions for FT-IR and
6
12
CD analyses.
Preparation of the Rh complex of BTA
PPh2
Catalytic experiments. Preparation of the catalytic system:
Mother solutions of each component were prepared sepa-
rately in dry CH Cl without any precaution to exclude air or
and BTA Ile
mixtures for spectroscopic analyses: Rh containing mix-
2
2
tures were prepared similarly to those used in the catalytic
0
moisture. This solvent readily dissolves each compound
listed below. The following mother solutions are required:
ester BTA: 50.0 mM; BTA ligand: 40.0 mM; [Rh(cod) ]BAr :
experiments (vide supra). Solutions at different R
values
BTAIle
were made using mother solutions of [Rh(cod) ]BAr (40.0
2
F
PPh2
2
F
mM in CH Cl ), BTA
2
2
(40.0 mM in CH Cl ) and BTA Ile
2 2
20.0 mM; dimethyl itaconate: 1.0 M. For catalytic reactions
(40.0 mM in CH Cl ). The components are then mixed to-
2 2
performed with variable amounts of BTA Ile, a 10.0 mM
mother solution of BTA Ile was used. The components are
then mixed together in glass vials suitable for a 24-well pres-
surized reactor (CAT-24 reactor provided by HEL®) equipped
with small magnetic stirring bars, in the following order:
ester BTA (desired volume), BTA ligand (100 µL, 4 µmole),
gether in an Eppendorf tube® in the following order: BTA Ile
PPh2
(desired volume), BTA
(200 µL,
8
µmole) and
0
[Rh(cod) ]BAr (100 µL, 4 µmole). In one case (R BTAIle=1.22)
2
F
dimethyl itaconate (400 µmole) was also added to the cata-
lytic mixtures. The resulting solutions were left to evaporate
overnight in a fume hood. The resulting solids were put
-3
[
Rh(cod) ]BArF (100 µL, 2 µmole) and dimethyl itaconate
under vacuum (10 mbar) for 3 hours before the addition of
2
(
200 µL, 200 µmole). The vials are left open and stirred over-
1.0 mL of cyclohexane. The Eppendorf tubes® were sonicated
for 1 min before centrifugation using a Gilson GmCLab® at
6000 rpm for 30 min. The resulting solids and supernatants
were then separated. The solids were dried under vacuum
3
-1
night in a well-ventilated fume-hood (900 m .h flow) to
evaporate the CH Cl . The next day, the vials are dried under
2
2
-
3
a 10 mbar vacuum for 1 hour to obtain a dry, gum-like solid
(the dried reaction mixture). Catalysis: The aforementioned
dried reaction system is taken up (desired volume) in the
desired solvent, sonicated for a few seconds to obtain a
bright yellow suspension, briefly heated to solvent reflux,
and then transferred to the CAT-24 reactor, which is sealed
and set on a magnetic stirrer at room temperature and at
-
3
(10 mbar) for 3 hours.
FT-IR measurements: FT-IR measurements were per-
formed on a Nicolet iS10 spectrometer. Solution spectra were
measured in KBr or CaF cells of 0.5 mm or 1.0 mm path
2
length and are corrected for air, solvent and cell absorption.
FT-IR spectra of the solids were recorded after evaporation of
-
1
1000 rpm for one hour. The reactor is then purged 3 times
a CHCl solution (8.85 g.L ) of the sample over KBr pellets.
3
with hydrogen gas (3-5 bar) before pressurizing it again at 3
Circular dichroism (CD): CD measurements were per-
formed on a Jasco J-1500 spectrometer equipped with a Pelti-
er thermostated cell holder and Xe laser (lamp XBO 150W/4).
Data was recorded at 20°C with the following parameters: 20
bar of H . The hydrogenation is performed at room tempera-
2
ture, 1000 rpm stirring, over 16 hours without compensating
H consumption. Determination of the conversion and the
2
1
-1
enantiomeric excess: The conversion was determined by H
nm.min sweep rate, 0.05 nm data pitch and 1.0 nm band-
NMR after evaporation of the catalytic solutions under vacu-
um. Ee was measured by chiral GC (Betadex 225, capillary
width and between 350 and 180 nm. The obtained signals
were processed as follow: solvent and cell contribution was
subtracted and the signals were smoothed (Savitzky-Golay
method). Spectra were corrected for solvent and cell contri-
bution. Kuhn anisotropy factors (g) are dimensionless and
expressed as follows: g=θ/(32980×Abs), where θ is the meas-
ured ellipticity (mdeg) and Abs the absorbance measured at
the same wavelength. For CD measurements in solution, a 1
30.0mx250µmx0.25µm, flow = 1.5mL/min, PHe = 17.6 psi,
isotherm at 70°C (10 min) then 2°C/min until 95°C, tr(R)=27.5
min, tr(S)=27.8 min). For examples of GC spectra (one race-
mate and the result of the catalytic reaction performed with
0
PPh2
BTA Ile (R BTAIle=1.25), BTA
and [Rh(cod) ]BAr )) see the
2 F
Supporting Information. Assignment of enantiomers was
25
made according to published data. In Table 1, Table 2, Table
S.1 and Table S.2, enantiomeric excesses in favor of the (R)
enantiomer are set as negative values (those of the (S) enan-
tiomer are positive).
mm quartz cell (cyclohexane phase of catalytic mixtures) or a
PPh2
0.1 mm dismountable quartz cell (mixtures of BTA
and
BTA Ile) was used. Molar CD values are reported in
-
1
-1
L.mol .cm and are expressed as follows: ꢀε = θ/(32980 × l
c) where θ is the measured ellipticity (mdeg), l is the optical
H
PPh2
Preparation of BTA
Rh) for spectroscopic analyses: For FT-IR analyses: Solu-
and BTA Ile mixtures (without
×
PPh2
path length in cm and c is the total concentration ([BTA
+ [BTA Ile]) in mol.L . For all samples, LD contribution was
]
0
-
1
tions at different R
values were made using mother
BTAIle
PPh2
solutions of BTA
(40.0 mM in CH Cl ) and BTA Ile (40.0
2 2
negligible (ΔLD < 0.005 dOD). For CD measurements of
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solids, a CHCl solution (8.85g.L ) of the sample was spin
S.; Kagan, H. B. Angew. Chem. Int. Ed. 2009, 48, 456-494. (h)
Soai, K.; Kawasaki, T.; Matsumoto, A. Acc. Chem. Res. 2014, 47,
3643-3654. (i) Dijken, D. J.; Beierle, J. M.; Stuart, M. C. A.;
Szymanski, W.; Browne, W. R.; Feringa, B. L. Angew. Chem. Int.
Ed. 2014, 53, 5073-5077. (j) Kawasaki, T.; Araki, Y.; Hatase, K.;
Suzuki, K.; Matsumoto, A.; Yokoi, T.; Kubota, Y.; Tatsumi, T.;
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coated over a quartz plate using a Laurell WS-650-23 Spin
Coater (3000 rpm). For all the samples, no linear dichroism
effects were present and the shape of the CD signal was in-
dependent of the orientation of the quartz slide.
UV spectroscopy: UV absorption spectra were extracted
from CD analyses on each of the above samples and obtained
after correction for air, solvent and cell absorption.
Small-angle neutron scattering (SANS) analyses: SANS
measurements were made at the LLB (Saclay, France) on the
Pace instrument, at two distance-wavelength combinations
_
Soai, K. Chem. Commun. 2015, 51, 8742 8744. (k) Bukhryakov, K.
V.; Almahdali, S.; Rodionov, V. O. Langmuir 2015, 31, 2931-2935.
(3) (a) Yashima, E.; Matsushima, T.; Okamoto, Y. J. Am. Chem.
Soc. 1995, 117, 11596-11597. (b) Yashima, E.; Matsushima, T.;
Okamoto, Y. J. Am. Chem. Soc. 1997, 119, 6345-6359.
(4) (a) Aparicio, F.; Matesanz, E.; Sanchez, L. Chem. Commun.
0
1
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9
0
-
3
-1
to cover the 4 × 10 to 0.24 Å q-range, where the scattering
vector q is defined as usual, assuming elastic scattering, as
q=(4π/λ)sin(θ/2), where θ is the angle between incident and
scattered beam. Data were corrected for the empty cell signal
and the solute and solvent incoherent background. A light
water standard was used to normalize the scattered intensi-
2
012, 48, 5757-5759. (b) Duan, P. F.; Cao, H.; Zhang, L.; Liu, M.
H. Soft Matter 2014, 10, 5428-5448.
5) Eelkema, R.; Feringa, B. L. Org. Biomol. Chem. 2006, 4, 3729-
(
3745.
(6) Shimomura, K.; Ikai, T.; Kanoh, S.; Yashima, E.; Maeda, K.
Nat. Chem. 2014, 6, 429-434.
(7) (a) Yamamoto, T.; Adachi, T.; Suginome, M. ACS Macro Lett.
2013, 2, 790-793. (b) Ke, Y.-Z.; Nagata, Y.; Yamada, T.; Suginome,
M. Angew. Chem. Int. Ed. 2015, 54, 9333-9337. (c) Nagata, Y.;
Nishikawa, T.; Suginome, M. J. Am. Chem. Soc. 2015, 137, 4070-
-
1
ties to cm units.
ASSOCIATED CONTENT
Experimental procedures, including synthesis and character-
ization of BTAs and their precursors, additional catalytic
experiments (Tables S.1 and S..2), and spectroscopic and
scattering analyses (Figures S1-S9; Tables S3-S5). This materi-
al is available free of charge via the Internet at
http://pubs.acs.org.
4
(
3
1
073.
8) (a) Rowan, A. E.; Nolte, R. J. M. Angew. Chem. Int. Ed. 1998,
7, 63-68. (b) Pijper, D.; Feringa, B. L. Soft Matter 2008, 4, 1349-
372.
9)(a) Green, M. M.; Park, J. W.; Sato, T.; Teramoto, A.; Lifson,
(
S.; Selinger, R. L. B.; Selinger, J. V. Angew. Chem. Int. Ed. 1999,
38, 3139-3154. (b) Maeda, K.; Yashima, E. Top. Curr. Chem. 2006,
265, 47-88. (c) Yashima, E.; Maeda, K.; Iida, H.; Furusho, Y.;
Nagai, K. Chem. Rev. 2009, 109, 6102-6211.
AUTHOR INFORMATION
Corresponding Author
(
10) (a) Palmans, A. R. A.; Meijer, E. W. Angew. Chem. Int. Ed.
007, 46, 8948-8968. (b) Liu, M. H.; Zhang, L.; Wang, T. Y.
Chem. Rev. 2015, 115, 7304-7397.
11) Lockman, J. W.; Paul, N. M.; Parquette, J. R. Prog. Polym. Sci.
005, 30, 423-452.
*
E-mail: matthieu.raynal@upmc.fr
2
Notes
The authors declare no competing financial interest.
(
2
ACKNOWLEDGMENT
(12) (a) Green, M. M.; Reidy, M. P.; Johnson, R. J.; Darling, G.;
Oleary, D. J.; Willson, G. J. Am. Chem. Soc. 1989, 111, 6452-6454.
(b) Green, M. M.; Garetz, B. A.; Munoz, B.; Chang, H. P.; Hoke,
S.; Cooks, R. G. J. Am. Chem. Soc. 1995, 117, 4181-4182.
This work was supported by the French Agence Nationale de
la Recherche (project ANR-13-BS07-0021 SupraCatal). A. V.-F.
would like to thank MINECO (CTQ2014-60256-P) for finan-
cial support. Jacques Jestin (LLB, Saclay) is acknowledged for
assistance with SANS experiment, Nicolas Vanthuyne (iSm2,
Marseille) for chiral HPLC analyses, Fabrice Mathevet
(UPMC, Paris) for his help in the preparation of spin coated
samples, Bruno Giordano Alvarenga (UPMC, Paris) and Lu-
dovic Dubreucq (UPMC, Paris) for their help in the synthesis
and characterization of some precursors and BTA monomers
and Marta Serrano Torne (ICIQ, Tarragona) for her help in
the preparation of the catalytic experiments.
(13) (a) Green, M. M.; Peterson, N. C.; Sato, T.; Teramoto, A.;
Cook, R.; Lifson, S. Science 1995, 268, 1860-1866. (b) van Gestel,
J.; van der Schoot, P.; Michels, M. A. J. Macromolecules 2003, 36,
6
668-6673. (c) van Gestel, J. Macromolecules 2004, 37, 3894-
3898. (d) van Gestel, J.; van der Schoot, P.; Michels, M. A. J. J.
Chem. Phys. 2004, 120, 8253-8261. (e) Markvoort, A. J.; ten
Eikelder, H. M. M.; Hilbers, P. A. J.; de Greef, T. F. A.; Meijer, E.
W. Nat. Commun. 2011, 2, 509-517. (f) ten Eikelder, H. M. M.;
Markvoort, A. J.; de Greef, T. F. A.; Hilbers, P. A. J. J. Phys. Chem.
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(
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showed the same trend than the one
0
observed for BTA Ile: 55% ee (R BTANle=0.25), 74% ee
(R BTANle=1.25) and 35% ee (R BTANle=4.0)
0
0
0
1
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0
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(
(
2 2
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5a
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26) In contrast to this observation, the conformation of
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(
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