Hydrogen Bond-Induced Rigid Oligoanthranilamide Ribbons That Are Planar and Straight

Article abstract of DOI:10.1021/ol036108b

(Matrix presented) A new series of oligoanthranilamides has been synthesized starting from suitably modified p-phenylenediamine and p-phthalic acid derivatives. Due to strong intramolecular three-center hydrogen bonds, the new oligomers self-assemble into highly stable straight and planar molecular ribbons, which have been characterized by X-ray crystallography and ¹H NMR, IR, and UV-vis spectroscopy.

Full text of DOI:10.1021/ol036108b

ORGANIC
LETTERS
2004
Vol. 6, No. 2
229-232
Hydrogen Bond-Induced Rigid
Oligoanthranilamide Ribbons That Are
Planar and Straight
Zong-Quan Wu, Xi-Kui Jiang, Shi-Zheng Zhu, and Zhan-Ting Li*
Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
354 Fenglin Lu, Shanghai 200032, China
ztli@mail.sioc.ac.cn
Received October 28, 2003
ABSTRACT
A new series of oligoanthranilamides has been synthesized starting from suitably modified p-phenylenediamine and p-phthalic acid derivatives.
Due to strong intramolecular three-center hydrogen bonds, the new oligomers self-assemble into highly stable straight and planar molecular
ribbons, which have been characterized by X-ray crystallography and 1H NMR, IR, and UV−vis spectroscopy.
Linear planar oligo-aromatic molecules with controllable
length not only bear great promise as materials for electronic
devices1 but also are important building blocks for as-
sembling discrete supramolecular systems.2 A large number
of conjugated arylene, arylene ethynylene, and vinylene
oligomers with extended skeletons have been prepared.
Nevertheless, the coplanarity of most of these covalently
bonded systems has not been really achieved since the
relative rotation of their adjacent subunits cannot be ef-
ficiently prevented.3 Following the increasing use of non-
covalent interactions in constructing various non-natural
folding systems,4,5 we had recently initiated a new project
for developing new noncovalent approaches to construct rigid
and planar molecular ribbons, which could be further
functionalized and used as new generation of molecular
scaffolds for further supramolecular assembly or recognition
chemistry. In this paper, we describe a new series of
oligoanthranilamides that self-assemble into highly stable
planar and straight molecular ribbons with controlled strength
in both solution and solid state as a result of strong
intramolecularly three-centered hydrogen bonding.6-8
Compounds 1-5, which possess one to seven aromatic
units, respectively, have been designed and synthesized.
(4) (a) Bassani, D. M.; Lehn, J.-M.; Baum, G.; Fenske, D. Angew. Chem.,
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10.1021/ol036108b CCC: $27.50 © 2004 American Chemical Society
Published on Web 12/17/2003

Instead of using 2,5-dialkoxy-1,4-phenylenediamine, which
would lead to the formation of five-membered ring hydrogen
bonds,4h,6c we herein chose to use 2,5-diaminoterephthalate
as one of the two key intermediates to construct the ribbon
skeletons, since stable six-membered-ring hydrogen bonds
would also be expected to be generated.9
Scheme 1
incorporated with four longer hexyloxyl chains, was also
prepared, as shown in Scheme 2. The synthesis began with
6, which was first monoprotected with (Boc)2O to produce
11. Acylation of 11 with 12 in hot chloroform with NEt3 as
base afforded 13, which was then treated with TFA to give
14. Diol 1512 was then alkylated with n-hexyl bromide in
the presence of potassium carbonate, and hydrolysis of the
product with potassium hydroxide, followed by refluxing in
thionyl chloride, produced diacyl chloride 18. The latter
The synthetic routes for 1-4a are presented in Scheme 1.
Treatment of diamine 610 with acetic anhydride in the
presence of NEt3 in refluxed THF afforded 1 in 70% yield,
whereas the reaction of 6 with acyl chloride 7 in hot
chloroform with NEt3 as base produced 2 in 52% yield.
Under conditions analogous to that for 2, 811 reacted with 9
to afford 3 in 62% yield. For the preparation of 4a, precursor
10 was first produced in 70% yield by treating 7 with excess
of 6 in refluxing chloroform; subsequent reaction of 10 with
8 afforded 5-mer 4a in 62% yield.
Scheme 2
Since compound 4a is of low solubility in common
nonpolar solvents such as chloroform, 5-mer 4b, which was
(6) For three-center hydrogen bonding in folding molecules, see: (a)
Hamuro, Y.; Geib, S. J.; Hamilton, A. D. J. Am. Chem. Soc. 1996, 118,
7529. (b) Hamuro, Y.; Geib, S. J.; Hamilton, A. D. J. Am. Chem. Soc.
1997, 119, 10587. (c) Parra, R. D.; Zeng, H.; Zhu, J.; Zheng, C.; Zeng, X.
C.; Gong, B. Chem. Eur. J. 2001, 7, 4352. (d) Gong, B.; Zeng, H.; Zhu, J.;
Yuan, L.; Han, Y.; Cheng, S.; Furukawa, M.; Parra, R. D.; Kovalevsky, A.
Y.; Mills, J. L.; Skrzypczak-Jankun, E.; Martinovic, S.; Smith, R. D.; Zheng,
C.; Szyperski, T.; Zeng, X. C. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 11583.
(7) For an example of hydrogen-bond-mediated polyarenes, see: Delnoye,
D. A. P.; Sijbesma, R. P.; Vekemans, J. A. J. M.; Meijer, E. W. J. Am.
Chem. Soc. 1996, 118, 8717.
(8) For reviews of covalent bond-mediated molecular ribbons, see: (a)
Kohnke, F. H.; Stoddart, J. F. Pure Appl. Chem. 1989, 61, 1581. (b) Girreser,
U.; Giuffrida, D.; Kohnke, F. H.; Mathias, J. P.; Philp, D.; Stoddart, J. F.
Pure Appl. Chem. 1993, 65, 119. (c) Schro¨der, A.; Mekelburger, H.-B.;
Vo¨gtle, F. Top. Curr. Chem. 1994, 172, 179.
(9) Etter, M. C. Acc. Chem. Res. 1990, 23, 120.
(10) Sinnreich, J. Synthesis 1980, 578.
(11) Marvel, K. J. Org. Chem. 1953, 18, 707.
230
Org. Lett., Vol. 6, No. 2, 2004

Scheme 3
reacted with 14 in refluxing chloroform to produce 4b in
65% yield. As expected, compound 4b is highly soluble in
nonpolar solvents such as chloroform and dichloromethane.
The 7-mer compound 5 was then prepared, as shown in
Scheme 3. Diacid 17 first reacted with benzyl bromide in
DMF with sodium hydride as base to produce 19 in 63%
yield. Acid 19 was then converted to 20 with hot thionyl
chloride. The reaction of 20 with amine 216a also in hot
chloroform afforded 22 in 70% yield. Subsequent deprotec-
tion of the carboxylic acid group by Pd-catalyzed hydrogena-
tion gave 23 in 75% yield. DCC-mediated coupling reaction
of 23 with 6 in dichloromethane with HOBt as catalyst was
then carried out to produce 24 in 56% yield. Finally, 24
reacted with 18 in hot chloroform with NEt3 as base to afford
5 in 85% yield.
Single crystals of 1-3 were grown by slow evaporation
of the chloroform solution at room temperature. Figure 1a
shows the crystal structure of 1. As expected, two six-
membered ring hydrogen bonds (NH‚‚‚O distance ) 1.81
Å) between the adjacent amide NH and ester CdO groups
are formed, which lead to a perfectly planar conformation.
Evidence for the straight and planar features of the linear
molecules are provided by the X-ray structures of 3-mer 2
and 3. As shown in Figure 1b,c, both 2 and 3 possess four
six-membered-ring hydrogen bonds (NH‚‚‚O distances )
1.81 and 1.82 Å). All three benzene units and the amide
groups in both compounds share one plane due to the
presence of these strong three-center intramolecular hydrogen
bonds (see the packing diagrams of 2 and 3 in the Supporting
Figure 1. Crystal structures of 1 (a), 2 (b), and 3 (c). All the
molecules adopt perfectly flat conformations, and both 2 and 3
possess straight π-extended skeletons, due to the strong six-
membered ring hydrogen bonds.
Information), whereas all the side methyl and ethyl groups
point away from the central aromatic skeleton. Since the
longer oligomers 4 and 5 also possess the identical repeated
structural feature, it is reasonable to consider that these longer
oligomers should also adopt similar planar and straight
conformations. A CPK model of planar 7-mer 5, with a
length of 4.30 nm, is shown in Figure 2.
1
The amide NH signals of the H NMR spectra of all
compounds 1-5 in chloroform-d appeared at downfield
positions (Table 1). The results provide strong evidence to
support that strong six-membered ring hydrogen bonds are
(12) Itami, K.; Palmgren, A.; Thorarensen, A.; Ba¨ckvall, J.-E. J. Org.
Chem. 1998, 63, 6466.
Org. Lett., Vol. 6, No. 2, 2004
231

Table 2. Stretching Frequencies (ν, cm-1) of the NH Groups
of Compounds 1-5 at Ambient Temperature
CHCl3
KBr
CHCl3
KBr
1
3
3304
3263
3304
3264
2
3225 3225
4a 3227 2931 3228 2932
4b 3225 2980
3225 2928
5
3250 3235 2940 3250 3234 2935
Figure 2. CPK model of 7-mer 5, produced with AccuModel 1.1
(the hexyl group were replaced with methyl group for clarity).
that 1-5 should adopt intramolecularly hydrogen-bonded
planar conformations.
UV-vis experiments in chloroform reveal that the long-
wavelength absorption band (1, 369 nm; 2, 370 nm; 3, 354
nm; 4b, 399 nm; 5, 404 nm) shifted bathochromically with
the increase of the number of the repeating benzene units.
These transitions may also be explained by considering that
the longer molecules have stronger amide bond delocalization
and increased π-conjugation as a result of the planarity of
the straightly arranged repeating aromatic units.5,7 This is in
also formed in solution for the molecules. Moreover, 2D-
NOESY H NMR experiments for 4b (10 mM) in chloro-
1
form-d revealed NOE connections of moderate strength
between the NH signals and the adjacent OCH2 and
OCH2CH2 signals. Further support for the intramolecularly
hydrogen-bonded π-extended structures in solution comes
from variable-temperature investigations of the amide NH
resonances in chloroform-d (Table 1), which revealed rather
small chemical shift changes with the temperature (e4.5 ×
10-3 ppm/K). This is consistent with their involvement in
the formation of intramolecular hydrogen bonds in solution.13
Dilution experiments (from 40 to 2 mM) in chloroform-d
revealed no great change of the NH signals (<0.03 ppm)
for 1-5, indicating the absence of intermolecular hydrogen
bonding. This result also excludes the possibility of important
intermolecular π-π stacking.14 In addition, when the solvent
was changed from chloroform-d to DMSO-d6 (10 mM, 25
°C), the NH signal of 2 displayed a small change (0.17 ppm).
In sharp contrast, a very large change (2.47 ppm) was
observed for the NH of N-phenyl benzamide when the
solvent was changed from chloroform to DMSO.6c This result
also strongly suggests that stable intramolecular hydrogen
bonds are formed in the oligomers.
The NH stretching frequencies (ν) of compounds 1-5
were measured in CHCl3 (3 mM) and with the KBr disk
method by IR spectroscopy (Table 2), which provides
additional evidence to support the formation of intramolecu-
lar hydrogen bonds in both solution and solid state. For all
molecules, intramolecularly hydrogen bonded N-H stretch
peaks were observed (<3310 cm-1), but no typical peaks
were displayed in the free N-H stretch region (3400-3500
cm-1).15 In addition, the NH stretching frequencies in
chloroform are independent of concentration changes. These
observations, similar to the above NMR results, also indicate
1
good agreement with the above X-ray, H NMR, and IR
results.
In summary, we report the first general approach to
constructing highly stable straight and planar molecular
ribbons with controlled length by utilizing highly stable
intramolecular hydrogen bonds. In the past decade, various
secondary interactions had been applied for building a large
number of folding structures, the present work demonstrates
that straight molecular architectures with controllable length
can also be generated from readily available precursors with
the help of strong secondary interactions. In principle, longer
oligomers of the same skeleton could be readily prepared
by simply increasing the number of the repeating aromatic
units. By introducing additional functional groups, such as
pyridine or acetylene, to the ends of the new skeletons, new
generation of extremely long and rigid molecules have been
prepared for assembling new giant supramolecular squares
or rectangles, which will be reported in due course.
Acknowledgment. We are grateful to the Ministry of
Science and Technology (No. G2000078101), the National
Natural Science Foundation (No. 20172069, No. 90206005),
the Chinese Academy of Sciences, and the State Laboratory
of Bioorganic and Natural Products Chemistry of China for
financial support of this work.
Supporting Information Available: The experimental
procedures and characterizations for 1-3, 4a,b, and 5, the
packing diagrams of 2 and 3 in the solid state, and the X-ray
crystallographic data for 1-3. This material is available free
of charge via the Internet at http://pubs.acs.org.
Table 1. 1H NMR Chemical Shifts (ppm) of the Amide
Protons of Compounds 1-5 in CDCl3
OL036108B
30 °C
50 °C
∆δ (ppm)/T (× 103)
3.00
4.50
4.50
(13) Kesseler, H. Angew. Chem., Int. Ed. Engl. 1982, 21, 512.
(14) (a) Shetty, A. S.; Zhang, J. S.; Moore, J. S. J. Am. Chem. Soc. 1996,
118, 1019. (b) Hunter, C. A.; Lawson, K. R.; Perkins, J.; Urch, C. J. J.
Chem. Soc., Perkin Trans. 2 2001, 651. (c) Tobe, Y.; Utsumi, N.; Kawabata,
K.; Nagano, A.; Adachi, K.; Araki, S.; Sonoda, M.; Hirose, K.; Naemura,
K. J. Am. Chem. Soc. 2002, 124, 5350.
(15) (a) Legon, A. C. Chem. Soc. ReV. 1990, 19, 197. (b) Nowick, J. S.;
Powell, N. A.; Martinez, E. J.; Smith, E. M.; Noronha, G. J. Org. Chem.
1992, 57, 3763. (c) Dado, G. P.; Gellman, S. H. J. Am. Chem. Soc. 1994,
116, 1054.
1
2
3
10.89
12.12
12.33
10.83
12.03
12.22
4a a 12.34 12.18
4b 12.01 11.88
11.92 11.79
4.50 4.50
5
12.35 12.14 11.93 12.27 12.08 11.86 4.00 3.00 3.50
a Values recorded at 25 °C.
232
Org. Lett., Vol. 6, No. 2, 2004