Supramolecular ribbons from amphiphilic trisamides self-assembly

Article abstract of DOI:10.1021/jo201055t

Two amphiphilic C₃-symmetric OPE-based trisamides have been synthesized and their self-assembling features investigated in solution and on surface. Variable-temperature UV-vis experiments demonstrate the cooperative supramolecular polymerization of these trisamides that self-assemble by the operation of triple C=O???H-N H-bonding arrays between the amide functional groups and π-π stacking between the aromatic units. The helical organization of the aggregates has been demonstrated by circular dichroism at a concentration as low as 1 × 10^-4 M in acetonitrile. In the reported trisamides, the large hydrophobic aromatic core acts as a solvophobic module impeding the interaction between the polar TEG chains and the amide H-bonds. This strategy makes unnecessary the separation of the amide functional groups to the polar tri(ethylene glycol) chains by paraffinic fragments. Achiral trisamide 1 self-assembles into flat ribbon-like structures that experience an amplification of chirality by the addition of a small amount of chiral 2 that generates twisted stripes.

Full text of DOI:10.1021/jo201055t

ARTICLE
pubs.acs.org/joc
Supramolecular Ribbons from Amphiphilic Trisamides Self-Assembly
Fꢀatima García, Julia Buendía, and Luis Sꢀanchez*
Departamento de Química Orgꢀanica, Facultad de Ciencias Químicas, Ciudad Universitaria s/n, 28040 Madrid, Spain
S Supporting Information
b
ABSTRACT: Two amphiphilic C3-symmetric OPE-based tri-
samides have been synthesized and their self-assembling
features investigated in solution and on surface. Variable-
temperature UVꢀvis experiments demonstrate the cooperative
supramolecular polymerization of these trisamides that self-
assemble by the operation of triple CdO HꢀN H-bonding
3 3 3
arrays between the amide functional groups and πꢀπ stacking
between the aromatic units. The helical organization of the aggregates has been demonstrated by circular dichroism at a
concentration as low as 1 ꢁ 10ꢀ4 M in acetonitrile. In the reported trisamides, the large hydrophobic aromatic core acts as a
solvophobic module impeding the interaction between the polar TEG chains and the amide H-bonds. This strategy makes
unnecessary the separation of the amide functional groups to the polar tri(ethylene glycol) chains by paraffinic fragments. Achiral
trisamide 1 self-assembles into flat ribbon-like structures that experience an amplification of chirality by the addition of a small
amount of chiral 2 that generates twisted stripes.
’ INTRODUCTION
cooperative supramolecular polymerization.11 Herein, we investi-
gate the influence of the attachment of tri(ethelene glycol) (TEG)
chains to C3-symmetrical OPE trisamides on the self-assembly
process in apolar and polar solvents. In the reported TEG-
substituted achiral trisamide 1 and chiral trisamide 2, the large
aromatic skeleton acts as hydrophobic shielding factor that allows
their supramolecular polymerization by the synergy of triple
The delicate balance between hydrophobic and hydrophilic
effects controls the organized self-assembly of molecules in
water. A challenging goal in this topic is to emulate the formation
of complex and functional structures by the assembly of bio-
molecules.1 Drug delivery,2 tissue engineering,3 or hydrogels4
exemplify the applicability of the supramolecular structures
formed in aqueous media. In this regard, a combination of
noncovalent forces like H-bonding, πꢀπ stacking, and ion-
pairing is required to control the hydrophobic/hydrophilic
balance.5 However, highly directional H-bonds operate effi-
ciently only in nonpolar environments,6 and therefore, the
H-bonding arrays in polar media need to be protected by the
creation of a solvophobic pocket.7
A number of examples of artificial molecules utilize water-
compatible ethylene oxide chains to achieve highly organized
supramolecular structures in aqueous environments.8 However,
a drawback of ethylene oxide chains is the competence in the
formation of H-bonds with polarizable NꢀH groups of amides or
ureas that diminishes notably the corresponding binding con-
stant. In these examples, only the separation between the
intermolecular H-bonding donors and acceptors moieties to
the polar ethylene oxide (EO) chains by paraffinic segments
allows stabilizing the final supramolecular structures.4c,9
NꢀH OdC amide H-bonding motifs and πꢀπ stacking.
3 3 3
’ RESULTS AND DISCUSSION
Synthesis. Polar trisamides 1 and 2 were readily synthesized
by a multistep synthetic procedure starting from amines 3a,b that
were obtained by a slightly modified methodology to that
previously reported (Scheme 1 and Supporting Information).13
The condensation of these amines with 4-iodobenzoic acid and
subsequent Sonogashira cross-coupling reaction of the resulting
amides with 1,3,5-triethynylbenzene14 yields the target OPEs 1
and 2 in 56 and 72% yield, respectively (Scheme 1).
The chemical structure of the new compounds has been
1
confirmed by H NMR, 13C NMR, and FTIR spectroscopy
and HRMS (ESI-FT) spectrometry. The C3-symmetry of com-
pounds 1 and 2 results in simple 1H NMR spectra with two sets
of resonances at δ ∼7.8, and 7.6, ascribable to an AA0BB0 spin
system typical of para-substituted aromatic compounds, and one
singlet at δ ∼7.7 corresponding to the central aromatic proton.
The close proximity of the stereogenic center at the polar TEG
chains to the amide proton at δ ∼7.0 changes the multiplicity of
this resonance. Thus, in achiral 1 the signal appears as a triplet but
in chiral 2 a double doublet is clearly observed due to the
Our research group is actively working on the formation of
supramolecular structures by utilizing radial oligo(phenylene
ethynylene) (OPEs) platforms.10,11 The different decoration and
connectivity of the central OPE moiety with polar EO and/or
paraffinic chains render a variety of ensembles of modulated
morphology and dimensionality that also differ in the self-assembly
mechanism.12 While the attachment of the peripheral chains as
alkoxy groups induces an isodesmic self-assembly,10 the connection
of these chains by means of amide functional groups results in a
Received: May 24, 2011
Published: June 21, 2011
r
2011 American Chemical Society
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ARTICLE
Scheme 1. Synthesis of Amphiphilic Trisamides 1 and 2
(Figure 1 and Figure S5, Supporting Information). The amide
resonance of amide 1 disappears and the upfield shift of the
aromatic and TEG signals increases upon addition of CD3OD
into a solution of 1 in CDCl3, a bad solvent for πꢀπ stacking,
keeping the total concentration constant which implies an
efficient aggregation (Figure S5, Supporting Information). Inter-
estingly, the addition of small amounts of highly polar solvents
like CD3OD or even D2O into a solution of 1 in CD3CN does
not break the intermolecular amide H-bonds (Figure 1). Under
these conditions, the solvophobic effect of CD3CN induces an
efficient πꢀπ stacking of the hydrophobic aromatic units
and impedes the approach of the more polar solvents (D2O
or CD3OD) to the amide functional groups. Therefore, in
these polar mixtures, the aggregation of amide 1 is driven by
the triple intermolecular array of H-bonds between the three
amide functionalities strongly reinforced by the πꢀπ stacking of
the aromatic moieties.
To determine the self-assembly mechanism of trisamide 1 in
solution, we have performed variable-temperature UVꢀvis ex-
periments by using apolar methylcyclohexane (MCH) as solvent
even though compound 1 is sparingly soluble in this solvent
(Figure 2). In this experiment, the depletion of two well-defined
waves at ∼300 and ∼314 nm, and the appearance of a shoulder at
∼340 nm, is noticeable upon decreasing temperature
(Figure 2).11a The nonsigmoidal shape of the melting curve is
diagnostic of a nucleationꢀelongation or cooperative mechan-
ism. A cooperative mechanism is defined by several parameters:
φSAT is the quantity necessary to equate φn/φSAT to unity, he is
the enthalpy released during elongation, Te is the temperature at
which the nucleation regime changes into elongation, and Ka
indicates the degree of cooperativity. Fitting the experimental
data to eqs 1 and 2, we can conclude that the supramolecular
polymerization of 1 is exothermic (he = ꢀ52.15 kJmolꢀ1) and
cooperative.
diastereotopic character of the methylene group adjacent to the
amide and the stereogenic center.
Self-Assembly in Solution. We have previously shown that
OPE-based trisamides exhibit a C3 geometry with the OPE
moiety almost planar and the amide groups twisted by 21.5°.
This molecular geometry favors the operation of πꢀπ and
H-bonding intermolecular interactions and induces the self-
assembly of this class of molecules into helical columnar agg-
regates.11a The bands at ∼3340, 1639, and 1546 cmꢀ1 observed
in the FTIR spectra of compounds 1 and 2 are ascribable to the
amide A, amide I, and amide II bands and imply the formation of
ꢂ
ꢀ
ꢁꢃ
ꢀhe
φn ¼ φSAT 1 ꢀ exp
2 ðT ꢀ TeÞ
ð1Þ
RTe
ꢀ
ꢁ
2
he
ꢀ1=3
φn ¼ Ka1=3exp ð Ka
ꢀ 1Þ
2 ðT ꢀ TeÞ
ð2Þ
amide CdO HꢀN H-bonds (Figure S1, Supporting
3 3 3
3
RTe
Information).11a,15 The stability of these CdO HꢀN
3 3 3
H-bonds and, therefore, the possible interaction between the
TEG chains and the amide functionalities in amphiphile 1 have
been investigated by concentration-dependent 1H NMR experi-
ments in deuterated solvents of different polarity (Figure S2ꢀS4,
Supporting Information). In all these experiments, the aromatic
protons shield upon increasing concentration, thus suggesting
the supramolecular interaction of 1 by π-stacking. However, the
resonance diagnostic of the amide protons at δ ∼7.0 is only
visible in CDCl3 and CD3CN. In these solvents, the amide triplet
shifts downfield with increasing concentration, which indicates
the intermolecular interaction of 1 by the operation of H-bond-
ing arrays between the amide groups. In CD3OD, the amide
triplet disappears as a consequence of the H/D exchange with the
solvent. However, the protons of the TEG chains are unaffected
in all these experiments, which could be diagnostic of a negligible
interaction of these peripheral chains with the amide groups.
Therefore, the large aromatic surface of the OPE moiety could
act as solvophobic segment preventing the H-bonding of the
TEG chains with the amide groups.
Considering that the lower the Ka value is the higher the
cooperativity is, the calculated value for Ka of 6 ꢁ 10ꢀ5
indicates that the self-assembly process is highly
cooperative.16 This activation step is 3 orders of magnitude
lower than that reported for previously reported OPE-based
trisamides11a and suggests the difficulty of the molecules
reported herein to form organized structures, most probably
due to the coiling effect experienced by the polar TEG chains
in an apolar media like MCH. Similar results have been
obtained by using a mixture of MCH/1,2-dichloroethane
99/1 at 1 ꢁ 10ꢀ5 M. The slightly higher polarity of 1,2-
dichloroethane increases the Ka value to 5 ꢁ 10ꢀ4 (Figure S6,
Supporting Information). The cooperative mechanism fol-
lowed by amphiphilic trisamide 1 contrasts with the isodesmic
self-assembly experienced by referable triangular-shape OPE-
based amphiphiles reported by our group10a and verifies the
relevance of the directional H-bonding interactions to control
the supramolecular polymerization mechanism.
The examples of helical supramolecular structures from
amphiphilic chiral molecules involving H-bonds are still scarce, as
the 1,3,5-benzenetrisamide is the most utilized structural motif to
The significant role exerted by the aromatic skeleton has been
1
demonstrated by H NMR experiments in solvent mixtures
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ARTICLE
Figure 1. Partial concentration-dependent 1H NMR (300 MHz, 298 K, 1 mM) spectra of 1 in CD3CN (top), CD3CN + 1% CD3OD (middle), and
CD3CN + 1% D2O (bottom) showing the aromatic, the amide, and the TEG protons.
Figure 2. T-dependent UVꢀvis spectra of 1 (MCH, 5 ꢁ 10ꢀ6 M, from
363 to 283 K, at intervals of 5 K). Inset: plot of the molar fraction of
aggregates versus temperature fitted within elongation (red line) and
nucleation (yellow line) regimes. Arrows indicate the direction of
change with increasing temperature.
Figure 3. CD spectra (298 K) of chiral trisamide 2 in CH3CN, 1 ꢁ
10ꢀ4 M (black), CH3CN, 1 ꢁ 10ꢀ5 M (red), CH3CN, 1 ꢁ 10ꢀ4 M + 5%
H2O (green), and CH3CN, 1 ꢁ 10ꢀ4 M + 10% H2O (blue).
amount of water (5%) does not eliminate the H-bonds between
the amide functionalities, and the CD response is also observed.
However, the addition of a larger quantity of water (10%)
definitively breaks the H-bonding arrays, and no CD signal is
observed (Figure 3).
The helical organization of chiral trisamide 2 is also confirmed
by utilizing a more apolar mixture of solvents. The low solubility
of 2 in MCH results in a negligible CD response. However, the
CD spectra of 2 in a 99/1 mixture of MCH and 1,2-dichlor-
oethane exhibits a bisignated Cotton effect that disappears upon
addition of a larger amount of 1,2-dichloroethane (Figure S7,
Supporting Information).
achieve helicity.17 In the case of amphiphilic trisamides 1 and 2,
the synergy of πꢀπ stacking between the aromatic units and
the triple H-bonding array between the amide functionalities
should favor their aggregation into helical columnar stacks.11a
The helical arrangement of chiral trisamide 2 has been corro-
borated by circular dichroism (CD) at different experimental
conditions (Figure 3 and Figure S7, Supporting Information).
Compound 2 exhibits dichroic response in polar CH3CN at 1 ꢁ
10ꢀ4 M, a concentration value more than 2 orders of magnitude
lower than that described for amphiphilic 1,3,5-benzene-
trisamides.9a The CD response of trisamide 2 is canceled by
diluting the sample at 1 ꢁ 10ꢀ5 M. This concentration
dependence implies the helical aggregation of 2. As has been
Electron Microscopy Images of Supramolecular Struc-
tures. Finally, we have visualized the supramolecular aggregates
formed by the directed self-assembly of amphiphilic trisamides
1
observed in H NMR experiments, the addition of a small
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Figure 4. SEM images of the ribbon-like structures formed by the cooperative self-assembly of trisamides 1 (a) and the 9/1 mixture of 1 and 2 (b). The
scale in the insets is 1 μm. The samples were prepared by slow diffusion of MCH vapor into dilute THF (1 ꢁ 10ꢀ4 M) solutions of the corresponding
trisamides.
1 and 2 by scanning electron microscopy (SEM). The slow
diffusion of MCH vapors into a 1 ꢁ 10ꢀ4 M solution of 1 in THF
gives rise to a white suspension formed by strongly stacked fibrils
organized into ribbon-like structures (Figure 4a and Figure S8,
Supporting Information). Similarly to some other chiral mol-
ecules,18 trisamide 2 is not able to self-assemble forming orga-
nized supramolecular structures as demonstrated by the corre-
sponding SEM images (Figure S9, Supporting Information).
Remarkably, the coassembly of chiral trisamide 2 and achiral 1 in
a 1/9 ratio exerts a large influence on the morphology of the
supramolecular structures, most probably due to a “sergeants and
soldiers” effect.19,20 The SEM images of the mixture of both
amphiphiles 1 and 2 do not show a dense packing of ribbons, but
only isolated twisted strips with highly uniform thickness of ∼1
μm are visualized (Figure 4b and Figure S10, Supporting
Information). The stereogenic center present in the peripheral
chains of 2 propagates the chiral information on the whole
aggregate formed by the joint contribution of πꢀπ stacking
between the aromatic units and the H-bonding interactions
between the amide functionalities, provoking an amplification
of chirality phenomenon.21
’ EXPERIMENTAL SECTION
2-(2-(2-Methoxyethoxy)ethoxy)ethanamine (3a) and (S)-2-(2-(2-
methoxyethoxy)ethoxy)propan-1-amine (3b) were prepared by a pro-
cedure slightly modified from that previously reported.13 Compound 5
was prepared according to a previously reported synthetic procedure22
and showed spectroscopic properties identical to those reported therein.
N-(2-(2-(2-Methoxyethoxy)ethoxy)ethyl)-4-iodobenza-
mide (4a). To a stirred solution of 4-iodobenzoic acid (1.72 g, 6.92
mmol) in dry dichloromethane (30 mL) at 0 °C under argon atmo-
sphere were slowly added 1-ethyl-3-(3-dimethylaminopropyl) carbodii-
mide (EDC) (1.46 g, 7.61 mmol) and 4,4-dimethylaminopyridine
(DMAP) (0.93 g, 7.61 mmol). The reaction mixture was stirred for
15 min, and then 2-(2-(2-methoxyethoxy)ethoxy)ethanamine (1.24 g,
7.61 mmol) was added slowly. The reaction mixture was stirred over-
night under room temperature. The reaction mixture was washed with
solutions of HCl (1 N), NaOH (3 M), and water. The organic layer was
dried with MgSO4 and filtered, and the solvent was removed under
reduced pressure. After evaporation of the solvent under reduced
pressure, the residue was purified by column chromatography (silica
gel, CHCl3/CH3OH 40:1) to afford 4a as a yellow oil (1.44 g, 53%). 1H
NMR (CDCl3, 300 MHz): δ 7.72 (2H, Hb, d, J = 8.6 Hz); 7.50 (2H, Ha,
d, J = 8.6 Hz); 6.98 (1H, Hc, br); 3.63ꢀ3.47 (12H, Hd+e+f+g+h+i, m); 3.28
(3H, Hj, s). 13C NMR (CDCl3, 75 MHz): δ 166.7, 137.6, 134.0, 128.8,
98.3, 71.9, 70.5, 70.5, 70.2, 69.6, 60.0, 39.8. FTIR (neat): 618, 660, 753,
846, 938, 1007, 1031, 1106, 1196, 1304, 1353, 1478, 1540, 1588, 1645,
2875, 3064, 3337 cmꢀ1. HRMS-ESI: calcd for C14H20INO4 [M + H]+
394.04371, found 394.04219.
N-((S)-2-(2-(2-Methoxyethoxy)ethoxy)propyl)-4-iodoben-
zamide (4b). To a stirred solution of 4-iodobenzoic acid (1.63 g, 6.60
mmol) in dry dichloromethane (30 mL) at 0 °C under argon atmo-
sphere were slowly added 1-ethyl-3-(3-dimethylaminopropyl) carbodii-
mide (EDC) (1.40 g, 7.25 mmol) and 4-4-dimethylaminopyridine
(DMAP) (0.89 g, 7.25 mmol). The reaction mixture was stirred for
15 min, and then (S)-2-(2-(2-methoxyethoxy)ethoxy)propan-1-amine
(3b) (0.94 g, 7.25 mmol) was added slowly. The reaction mixture was
stirred overnight under room temperature. The reaction mixture was
washed with solutions of HCl (1 N), NaOH (3 M), and water. The
organic layer was dried with MgSO4 and filtered, and the solvent was
removed under reduced pressure. After evaporation of the solvent under
reduced pressure, the residue was purified by column chromatography
(silica gel, CHCl3/CH3OH 40:1) to afford 4b as a colorless liquid
’ CONCLUSION
To summarize, we have synthesized two amphiphilic C3-
symmetrical OPE-based trisamides that experience a cooperative
supramolecular polymerization with very low activation step
(6 ꢁ 10ꢀ5) in apolar MCH. The large hydrophobic aromatic
surface impedes the interaction between the H-bonding amide
groups and the polar TEG chains thus favoring a helical
organization of the reported trisamides by the operation of triple
intermolecular amide CdO HꢀN H-bonds. The hierarchical
3 3 3
self-assembly of achiral trisamide 1 forms fibrillar structures that
bundle efficiently into flat ribbons. The addition of a small
amount of chiral trisamide 2 (10%) to 1 transfers the chirality
embedded in the steroegenic centers to the whole supra-
molecular structure which results in the apparition of twisted
strips. Therefore, the utilization of large hydrophobic cores
opens new avenues for the development of self-assembly in
aqueous media.
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(0.88 g, 33%). 1H NMR(CDCl3, 300MHz):δ7.80 (2H, Hb, d, J =8.6 Hz);
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0
3.69ꢀ3.47 (8H, Hg+h+i+j, m); 3.29 (3H, Hk, s); 3.20 (1H, Hd or d , m);
1.22 (3H, Hf, d, J = 6.2 Hz). 13C NMR (CDCl3, 75 MHz): δ 166.7,
137.6, 134.2, 128.9, 98.1, 74.8, 71.8, 70.7, 70.4, 67.9, 58.9, 44.9, 17.6.
FTIR (neat): 651, 751, 844, 931, 1007, 1032, 1103, 1195, 1292, 1380,
1477, 1538, 1588, 1648, 2877, 2973, 3076, 3331 cmꢀ1. HRMS-ESI:
calcd for C15H22INO4 [M + H]+ 408.05936, found 408.05843.
Compound 1. N-(2-(2-(2-Methoxyethoxy)ethoxy)ethyl)-4-iodo-
benzamide (4a) (1.44 g, 3.7 mmol), 1,3,5-triethynylbenzene (0.18 g, 1.2
mmol), copper(I) iodide (0.01 g, 0.07 mmol), and bis(triphenylphosphine)-
palladium(II) chloride (0.04 g, 0.06 mmol) were dissolved in dry
triethylamine (40 mL) and dry THF (10 mL). The mixture was sub-
jected to several vacuum/argon cycles and heated at 70 °C for 40 h. The
solvent was evaporated under reduced pressure, and the residue was
washed with HCl (1 N) and NH4Cl and extracted with chloroform.
After evaporation of the solvent, the residue was purified by column
chromatography (silica gel, CHCl3/CH3OH 50:1) to afford 1 as a
yellow solid (0.65 g, 56%). 1H NMR (CDCl3, 300 MHz): δ 7.84 (6H,
Hc, d, J = 8.7 Hz); 7.71 (3H, Ha, s); 7.61 (6H, Hb, d, J = 8.7 Hz); 6.87
(3H, Hd, br); 3.70ꢀ3.54 (36H, He+f+h+i+j, M); 3.35 (9H, Hk, s). 13C
NMR (CDCl3, 75 MHz): δ 166.7, 134.5, 131.7, 127.3, 125.8, 123.8,
90.0, 89.7, 71.9, 70.5, 70.5, 70.2, 69.8, 59.0, 39.9. FTIR (neat): 679, 765,
855, 936, 1024, 1105, 1196, 1304, 1353, 1456, 1500, 1546, 1607, 1644,
2875, 3066, 3338 cmꢀ1. HRMS-ESI: calcd for C54H64N3O12 [M + H]+
946.44845, found 946.44755.
Compound 2. N-((S)-2-(2-(2-Methoxyethoxy)ethoxy)propyl)-
4-iodobenzamide (4b) (0.88 g, 2.16 mmol), 1,3,5-triethynylbenzene
(0.10 g, 0.68 mmol), copper(I) iodide (0.008 g, 0.04 mmol), and
bis(triphenylphosphine)palladium(II) chloride (0.008 g, 0.04 mmol)
were dissolved in dry triethylamine (5 mL) and dry THF (10 mL). The
mixture was heated at 70 °C for 36 h. The solvent was evaporated under
reduced pressure, and the residue was washed with HCl (1 N) and
NH4Cl and extracted with chloroform. After evaporation of the solvent
under reduced pressure, the residue was purified by column chroma-
tography (silica gel, CHCl3/CH3OH 50:1) to afford 2 as a brown oil
(0.48 g, 72%). 1H NMR (CDCl3, 300 MHz) δ 7.85 (6H, Hc, d, J = 8.5
Hz); 7.70 (3H, Ha, s); 7.60 (6H, Hb, d, J = 8.5 Hz); 7.03 (3H, Hd, dd, J1 =
0
6.4 Hz, J2 = 6.6 Hz); 3.70ꢀ3.33 (30H, He or e +f+h+i+j+k, m); 3.31 (9H, Hl, s);
3.26 (3H, He or e , m); 1.23 (9H, Hg, d, J = 6.2 Hz). 13C NMR (CDCl3,
0
75 MHz): δ 166.7, 134.6, 134.4, 131.7, 127.3, 125.7, 123.9, 90.1, 89.6,
74.8, 71.9, 70.8, 70.4, 68.0, 58.9, 44.9, 17.7. FTIR (neat): 681, 767, 854,
933, 956, 1025, 1196, 1295, 1377, 1456, 1499, 1543, 1607, 1647, 2876,
2924, 2973, 3061 cmꢀ1. HRMS-ESI: calcd for C57H68N3O12 [M ꢀ H]+
986.48085, found 946.48452.
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’ ASSOCIATED CONTENT
S
Supporting Information. Experimental details and sup-
plementary figures. This material is available free of charge via the
b
Internet at http://pubs.acs.org.
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’ AUTHOR INFORMATION
Corresponding Author
*E-mail: lusamar@quim.ucm.es.
(13) De, S.; Ramakrishnan, S. Chem. Asian J. 2011, 6, 149–156.
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’ ACKNOWLEDGMENT
This work has been supported by MICINN (CTQ2008-
00795). The MICINN of Spain is also thanked by F.G. for an
FPU research grant. We thank Prof. N. Martín, Prof. F. Gavilanes,
and Dr. B. Yꢀelamos (UCM, Spain) for their generous help.
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The Journal of Organic Chemistry
ARTICLE
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(20) The scarce dichroic signal of compound 2 prevents performing
a “sergeants and soldiers” experiment in solution. However, the FTIR
spectrum of a 9/1 mixture of achiral trisamide 1 and chiral trisamide 2
exhibits the amide A, amide I, and amide II bands at 3320, 1639, and
1535 cmꢀ1 diganostic of the formation of an array of amide CdO
3 3 3
HꢀN H-bonds (see Figure S1, Supporting Information).
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6276
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