Amplification of chirality in N,N′-1,2-ethanediylbisbenzamides: From planar sheets to twisted ribbons

Article abstract of DOI:10.1039/c0cc02726j

A remarkable sergeants-and-soldiers effect is observed in the coassembly of N,N′-1,2-ethanediylbisbenzamides onto surfaces that experience an amplification of chirality able to transform flat lamellae into twisted ribbons.

Full text of DOI:10.1039/c0cc02726j

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Amplification of chirality in N,N0-1,2-ethanediylbisbenzamides: from
planar sheets to twisted ribbonsw
´ ´
Fatima Aparicio, Felipe Vicente and Luis Sanchez*
Received 22nd July 2010, Accepted 8th September 2010
DOI: 10.1039/c0cc02726j
A remarkable sergeants-and-soldiers effect is observed in the
coassembly of N,N0-1,2-ethanediylbisbenzamides onto surfaces
that experience an amplification of chirality able to transform
flat lamellae into twisted ribbons.
the commercially available 3-chloro-2-chloromethyl-1-propene
(2) and the corresponding alcohol (1-decanol or (S)-3,7-di-
methyloctanol). The subsequent hydroboration of the olefin,
and Mitsunobu’s reaction of the resulting alcohol (4a,b) with
ethyl 4-hydroxybenzoate yields esters 5a,b. Finally, the acidic
hydrolysis of these esters, followed by the formation in situ of
the corresponding acid chloride derivatives by treatment with
oxalyl chloride and the reaction with ethylenediamine yields
the target N,N0-1,2-ethanediylbisbenzamides with yields of
82 and 71%, respectively (Scheme 1). The chemical structure
of all the new reported compounds has been confirmed by
NMR, FT-IR, and MALDI-TOF techniques (see Supporting
Information).
DNA helices and amyloid or cellulose fibrils are examples of
natural helical structures with specific functions. The helical
morphology of such biomolecules expresses the chirality of
their components and demonstrates the direct relationship
between the molecular and the supramolecular chirality.1
Man-made chiral structures are being amply utilized in
asymmetric chemical synthesis and catalysis,2 liquid crystals,3
and conductive materials.4 However,
a very intriguing
The self-assembly of bis-benzamides 1a and 1b has been
investigated by a number of techniques. Firstly, the aggregation
of these amides in solution has been confirmed by concentration-
dependent 1H NMR experiments in deuterated chloroform
(Fig. 2 and S1).11 This solvent is well-known to stabilize
H-bonded arrays although it is a bad solvent to detect p–p
stacking. These studies reveal a clear deshielding of the N–H
protons (resonances in red in Fig. 2 and S1) with increasing
concentration, diagnostic of the intermolecular interaction of
question around chirality is the origin of homochirality,
that is, how Nature utilized minute enantiomeric excesses to
convert racemates into enantiomerically enriched mixtures.
Despite the relatively high number of examples reported on
the amplification of chirality in solution,5 less efforts have been
made to amplify the chirality onto surfaces thus generating
aggregates with high aspect ratios.6
In our laboratory, self-assembly of a variety of non-ionic
amphiphiles is being studied in solution and onto surfaces.7
We report herein on the spectacular chirality amplification
process of very simple N,N0-1,2-ethanediylbisbenzamides8,9
with the transformation of flat sheets from achiral 1a into
twisted ribbons in the sergeants-and-soldiers10 coassembly of
achiral 1a and chiral 1b (Fig. 1).
The synthesis of compounds 1a and 1b starts with the
preparation of the peripheral wedges 4a and 4b by reacting
Scheme 1 Synthesis of N,N0-1,2-ethanediylbisbenzamides 1a and 1b.
Fig. 1 Chemical structure of bisamides 1a and 1b (top) and schematic
illustration of the self-assembly of 1a into sheets and the coassembly of
1a and 1b into twisted ribbons.
1
Fig. 2 Partial H NMR spectra (300 MHz, 298 K) of 1b at different
´
Departamento de Quımica Organica, Facultad de Ciencias Quımicas,
Ciudad Universitaria, s/n. 28040 Madrid, Spain.
E-mail: lusamar@quim.ucm.es; Fax: (+) 34913944103
w Electronic supplementary information (ESI) available: Supplementary
Fig. S1–S8 and experimental section. See DOI: 10.1039/c0cc02726j
´
´
concentrations in CDCl3. The inset depicts the variation of the N–H
resonance with concentration and the fit to eqn (1). The resonances in
red and in blue correspond to the N–H and ethylene bridge protons,
respectively.
c
8356 Chem. Commun., 2010, 46, 8356–8358
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the bis-benzamides by H-bonding. Plotting the variation of the
chemical shift against concentration shows a non-linear
behaviour that can be fitted to eqn (1) that corresponds to
an isodesmic or equal-K aggregation behaviour. c and Ka
are concentration of amide and the binding constant,
respectively.12,13
H-bonding. The further interdigitation of the paraffinic chains
finally gives rise to the bidimensional structures.
The incorporation of stereogenic centres at the peripheral
chains in bis-amide 1b should induce the self-assembly of this
compound into helical supramolecular structures.5,6 The first
approach to demonstrate the helical character of the aggregates
formed by the association of amide 1b was circular dichroism
(CD) in methylcyclohexane (MCH), an apolar solvent in
which the stabilization of the H-bonding and the p–p stacking
pffiffiffiffiffiffiffiffiffiffi
1 þ 1 ꢀ 4cKa
d ¼ Ddmax þ ðdaggr ꢀ Ddmax
Þ
ð1Þ
2cKa
should be maximized. The CD spectrum of 1b at 1 ꢁ 10ꢀ3
M
The calculated Ka values for amides 1a and 1b applying eqn (1)
are rather low (2.88 and 3.65 Mꢀ1 for 1a and 1b, respectively)
suggesting that the presence of only two H-bonding amide
units is not sufficient to create highly stabilized aggregates in
exhibits a weak bisignated Cotton effect with a zero crossing of
CD signal at the frequency of the absorption maximum
(250 nm) and with negative and positive waves at 248 and
253 nm (Fig. S5). This CD response is too weak to perform an
accurate study of the self-assembly mechanism and also to
investigate the possible sergeants-and-soldiers behaviour in
solution.
1
solution. The concentration-dependent H NMR experiments
also demonstrate the very weak interaction between the benzene
units by p–p stacking. However, a small interaction involving
the ethylene spacer linking the two benzamide moieties is
noticeable (resonances in blue in Fig. 2 and S1). This effect
is indicative of the close proximity of the protons of this spacer
in the aggregated species.
Unexpectedly, SEM images of chiral bis-amide 1b do
not show any organized supramolecular structure most
probably due to the low binding constant determined by the
1
concentration-dependent H NMR studies.18 The situation is
The participation of different forces in the aggregation of
amides 1a and 1b changes drastically in the neat state as
demonstrated by the corresponding FTIR spectra (Fig. S2).
The broad wave at 3330 cmꢀ1 is ascribable to bounded N–H
stretches and confirms the contribution of the amide groups in
H-bonded arrays.14 In addition, the amide I and amide II
bands at 1637 and 1545 cmꢀ1, respectively, corroborate the
H-bonded CQO stretching and the N–H bending modes.15
Finally, the stretching bands at n 2925 and 2865 cmꢀ1 suggest
the peripheral CH2 groups are not in a crystalline state.
Nonetheless, the band at around 1464 cmꢀ1 is indicative of
the interdigitation of these paraffinic substituents.16,17
different when achiral 1a soldiers and chiral 1b sergeants are
coassembled in acetonitrile (1 ꢁ 10ꢀ4 M) and deposited onto a
glass substrate. The presence of only 5 mol% of the sergeant
1b is able to transfer the chirality embedded in the peripheral
chains to the remaining 95% of 1a soldiers as revealed by the
corresponding SEM images (Fig. 4a and b and S6). The
transfer of chirality transforms the lamellae observed for 1a
into micrometre long twisted ribbons of high aspect ratio.
Similarly to achiral 1a, the synergy of p–p, H-bonds and
van der Waals interactions forms bidimensional supramolecular
structures. However, the presence of stereogenic centres in the
coassembly of achiral 1a with chiral 1b provokes the chiral
propagation of the H-bonding of the amide functionalities
reinforcing the formation of twisted ribbons.
The morphology of the supramolecular structures formed
from amide 1a has been investigated by utilizing field-emission
scanning electron microscopy (SEM) onto a glass substrate.
Initially, we prepared a suspension of 1a in acetonitrile
(1 ꢁ 10ꢀ3 M) that was subsequently deposited onto the glass
substrate. The SEM images of this suspension exhibit big
flake-like objects (Fig. 3a and S3). The SEM images of a more
diluted suspension of 1a in acetonitrile (1 ꢁ 10ꢀ4 M) show
stratified clusters of microcrystalline lamellae formed upon the
superimposition of thinner sheets (Fig. 3b and S4). These
lamellae are formed by the synergy of p–p stacking between
the phenyl rings, the H-bonding of the amide groups and
the van der Waals interactions between the peripheral wedges.
p-Stacking induces the formation of one-dimensional aggregates
that are able to propagate along the aggregate axis by
The formation of twisted ribbons by increasing the
percentage of chiral sergeant 1b in the coassembly of 1a and 1b
Fig. 4 SEM images (298 K, 1 ꢁ 10ꢀ4 M in acetonitrile, glass
substrate) of twisted ribbons formed by the coassembly of the achiral
soldier 1a and chiral sergeant 1b at different percentages of chiral
component: 95/5 (a and b), 90/10 (c), and 85/15 (d). The yellow ellipses
in (c) and (d) highlight amorphous material.
Fig. 3 SEM images of the microcrystalline flat sheets formed by the
self-assembly of bis-amide 1a in acetonitrile at 1 ꢁ 10ꢀ3 M (a) and at
1 ꢁ 10ꢀ4 M (b). For an enlarged view of the inset in (b), see Fig. S4.
c
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is also observed (Fig. 4c and d, S7 and S8). When a 10% or
15% molar ratio of chiral 1b is added to 1a, twisted ribbons
are again clearly visible in SEM images. In both cases,
unorganized material appears in the SEM images together
with twisted ribbons which implies that only a minute amount
of chiral sergeant (5% molar ratio) is capable to dictate the
chirality to the whole coassembled structures. In all the tested
cases, the chiral twisted ribbons appear spirally entangled to
form pillars of chiral structures (Fig. 4a and d, S6, and S8).
To summarize, we have demonstrated that it is possible to
apply the sergeants-and-soldiers principle, and therefore an
amplification of chirality, to the self-assembly of very simple
N,N0-1,2-ethanediylbisbenzamides despite these compounds
exhibiting a low binding constant and also a negligible
dichroic response in solution. The self-assembly of achiral
bis-amide 1a forms microcrystalline lamellae due to the
synergy of H-bonding, p–p stacking and van der Waals
interactions. The addition of 1b, which acts as chiral sergeant
in the coassembly of both bis-amides, provokes the transcription
of the chirality embedded in the stereogenic centres at the
peripheral chains generating the formation of twisted ribbons
of high aspect ratio. The results presented herein represent an
excellent example of the study of homochirality onto surfaces
and, at the same time, contribute to increasing the knowledge
about the set of rules governing the generation of chiral
objects that hold a great potential for the development of
supramolecular devices.
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c
8358 Chem. Commun., 2010, 46, 8356–8358
This journal is The Royal Society of Chemistry 2010