Helical structures of N-alkylated poly(p-benzarnide)s

Article abstract of DOI:10.1021/ja0455291

Poly(p-benzamide)s 1 bearing a chiral side chain on the nitrogen atom were synthesized by chain-growth polycondensation methodology. The polyamides exhibited well-defined molecular weights with narrow polydispersities. Solutions of the polyamides in sever

Full text of DOI:10.1021/ja0455291

Published on Web 05/17/2005
Helical Structures of N-Alkylated Poly(p-benzamide)s
Aya Tanatani,†,‡ Akihiro Yokoyama,§ Isao Azumaya,| Yoshinori Takakura,§
Chikashi Mitsui,§ Motoo Shiro, Masanobu Uchiyama,†,@ Atsuya Muranaka,#
Nagao Kobayashi,*,# and Tsutomu Yokozawa*,†,§
Contribution from the Synthesis and Control, PRESTO, Japan Science and Technology Agency
(JST), Japan, Department of Applied Chemistry, Kanagawa UniVersity, Rokkakubashi,
Kanagawa-ku, Yokohama 221-8686, Japan, Faculty of Pharmaceutical Sciences at Kagawa
Campus, Tokushima Bunri UniVersity, 1314-1 Shido, Sanuki, Kagawa 769-2193, Japan, Rigaku
Corporation, X-ray Research Laboratory, Matsubaracho 3-9-12, Akishima, Tokyo 196-8666,
Japan, Graduate School of Pharmaceutical Sciences, The UniVersity of Tokyo, 7-3-1 Hongo,
Bunkyo-ku, Tokyo 113-0033, Japan, and Department of Chemistry, Graduate School of Science,
Tohoku UniVersity, Sendai 980-8578, Japan
Received July 26, 2004; E-mail: yokozt01@kanagawa-u.ac.jp; nagaok@mail.tains.tohoku.ac.jp
Abstract: Poly(p-benzamide)s 1 bearing a chiral side chain on the nitrogen atom were synthesized by
chain-growth polycondensation methodology. The polyamides exhibited well-defined molecular weights
with narrow polydispersities. Solutions of the polyamides in several organic solvents (CH3CN, CHCl3, and
CH3OH) showed dispersion type CD signals characteristic of coupled-oscillator and much larger as compared
with the corresponding monomer. The CD signals were dependent on the temperature and molecular weight
of the polyamides but independent of the solvent, as far as examined. An exciton model analysis of the
absorption and CD spectra provided a clear-cut picture for the secondary structure of these polyamides in
solution that the N-alkylated poly(p-benzamide)s possess a right-handed helical conformation ((P)-helix).
In the solid states, the results of X-ray crystallographic analysis of 4-(methylamino)benzoic acid oligomers
substantiated that they have a helical conformation with three monomer units per turn.
lenes6 are helical polymers consisting of a stiff backbone, within
Introduction
which helix-helix or helix-coil interconversion readily occurs
The helix is nature’s most attractive and important confor-
mational motif, as seen in the structure and function of DNA
and proteins.1 Great efforts have been made to understand the
properties of helical structure and to construct helical polymers2
and oligomers3 possessing useful functions, such as molecular
recognition of chiral guests and asymmetric catalytic ability in
organic synthesis. In contrast to stable helical polymers with a
rigid backbone, polyisocyanates,4 polysilanes,5 and polyacety-
due to the small energetic barriers to helical reversal. These
helical polymers are classified as dynamic helical polymers and
are characterized by a dependence of the helical conformation
in solution upon polymer chain length, temperature,7 and so
forth.
Aromatic polyamides composed of N-unsubstituted amide
bonds, such as poly(p-phenylene terephthalamide) and poly(p-
benzamide), possess extended rodlike structures.8 The amide
linkages of these polymers are in trans conformation9 and form
hydrogen bonds between polymer chains to give a sheet
structure. If the nitrogen of the amide bonds is alkylated, the
polymers are thought to take coiled structures,10 although no
detailed investigation to elucidate their structures has been
reported. On the other hand, the conformation of N-alkylated
† PRESTO, JST.
‡ Current address: Institute of Molecular and Cellular Biosciences, The
University of Tokyo, Tokyo, Japan.
§ Kanagawa University.
| Tokushima Bunri University.
Rigaku Corporation.
@ The University of Tokyo.
# Tohoku University.
(1) (a) Watson, J. D.; Crick, F. H. C. Nature 1953, 171, 737-738. (b) Branden,
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Cornelissen, J. J. L. M.; Rowan, A. E.; Nolte, R. J. M.; Sommerdiik, N. A.
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(9) In this paper, the terms cis and trans amide conformation are used to show
the relative positions of aromatic groups connected to the amide group.
9
10.1021/ja0455291 CCC: $30.25 © 2005 American Chemical Society
J. AM. CHEM. SOC. 2005, 127, 8553-8561
8553

A R T I C L E S
Tanatani et al.
Scheme 1. Synthesis of Monomer 2
aromatic amides of low molecular weight has been investigated
in detail, and it was shown that N-alkylated aromatic amides
favor the cis conformation in the crystal and in solution.11 Fur-
ther, X-ray analysis of N,N′-dimethyl-N,N′-diphenylbenzene-
dicarboxamides demonstrated that, when three benzene units
are connected by two N-alkylated amide bonds, the amide bonds
adopt the cis conformation, and the three benzene units are not
coplanar.12 In this case, the terminal benzene rings can be
arranged in two ways: on opposite sides (anti conformation)
or on the same side (syn conformation) of the central benzene
ring.
These results, as well as the finding that the condensation of
4-(methylamino)benzoic acid gave exclusively the cyclic tri-
mer,13 led us to postulate that N-alkylated poly(p-benzamide)s
may form a helical conformation in solution, despite their high
flexibility as compared to the known helical polymers and lack
of apparent force to fix the backbone. Further, we have
developed a chain-growth polycondensation methodology to
synthesize N-alkylated poly(p-benzamide)s with well-controlled
molecular weight and narrow polydispersity (Mw/Mn e 1.1).14
Because the method provides N-alkylated poly(p-benzamide)s
having desired length and narrow polydispersity without dif-
ficulty or cumbersome fractionation, it will be helpful to gain
an understanding of the chain length-dependent chiroptical
properties of this important class of aromatic polyamides
(aramides). In this paper, we show that N-alkylated poly(p-
benzamide)s 1 bearing a chiral side chain on the nitrogen atom
take a right-handed helical structure in solution.15 The temper-
ature-16 and chain length-dependent17 circular dichroism (CD)
signals indicate that the polyamides have dynamic helical
properties. The helical structure of the N-alkylated aromatic
polyamides is clearly and unambiguously supported by the
exciton analysis of both electronic absorption and CD spectra.
The X-ray crystallographic analysis of the oligomers of 4-
(methylamino)benzoic acid demonstrates that they have a helical
conformation with three monomer units per turn in the crystal.
Scheme 2. Chain-Growth Polycondensation for Polyamides 1
group as a chiral side chain because poly(p-benzamide)s having
a tri(ethylene glycol) monomethyl ether unit on the nitrogen
atom show higher solubility than those with an octyl or dodecyl
unit.18 To synthesize the polyamides with our chain-growth poly-
condensation methodology, we first prepared the monomer 2
as shown in Scheme 1. Introduction of the chiral side chain
into the 4-aminobenzoate unit was achieved by Fukuyama’s
method19 using sulfonamide and chiral alcohol. Thus, phenyl
4-aminobenzoate 314b was converted to sulfonamide 4 by treat-
ment with 2-nitrobenzenesulfonyl chloride. Alkylation of 4 was
carried out under the Mitsunobu conditions with 5, which was
prepared as described in the literature.20 Removal of the 2-nitro-
benzenesulfonyl group from 6 was performed by treatment with
PhSH and Cs2CO3 to give the monomer 2 in high yield.
Chain-growth polycondensation of 2 was carried out in the
presence of the initiator 7 using a combination of N-octyl-N-
triethylsilylaniline, CsF, and 18-crown-6 as a base14a (Scheme
2). Polyamides 1a-f with varying lengths (Mn ) 1700-14 300)
and narrow polydispersities (Mw/Mn ) 1.05-1.17) were ob-
tained by changing the feed ratio of 7 to 2 (Table 1).
Results and Discussion
1. Synthesis of Polyamides. We designed poly(p-benza-
mide)s 1 possessing an N-(S)-2-(methoxyethoxyethoxy)propyl
2. UV and CD Investigations of Chiral Polyamides. The
polyamide 1f showed broad electronic absorption in the 240-
(10) (a) Preston, J.; Krigbaum, W. R.; Asrar, J. In Cyclopolymerization and
Polymer with Chain-Ring Structures; Butler, G. B., Kresta, J. E., Eds.;
ACS Symposium Series 195; American Chemical Society: Washington,
DC, 1981; pp 351-362. (b) Burch, R. R.; Manring, L. E. Macromolecules
1991, 24, 1731-1735.
(15) Helical structures of aromatic polyamides without N-substituents. See: (a)
Kondo, F.; Kakimi, S.; Kimura, H.; Takeishi, M. Polym. Int. 1998, 46,
339-344. (b) Agata, Y.; Kobayashi, M.; Kimura, H.; Takeishi, M. Polymer
2002, 43, 4829-4833.
(11) (a) Itai, A.; Toriumi, Y.; Tomioka, N.; Kagechika, H.; Azumaya, I.; Shudo,
K. Tetrahedron Lett. 1989, 30, 6177-6180. (b) Nishimura, T.; Maeda, K.;
Yashima, E. Chirality 2004, 16, S12-S22.
(16) (a) Shinohara, K.; Yasuda, S.; Kato, G.; Fujita, M.; Shigekawa, H. J. Am.
Chem. Soc. 2001, 123, 3619-3620. (b) Morino, K.; Maeda, K.; Okamoto,
Y.; Yashima, E.; Sato, T. Chem.s Eur. J. 2002, 8, 5112-5120. (c) Tabei,
J.; Nomura, R.; Masuda, T. Macromolecules 2003, 36, 573-577.
(17) (a) Gu, H.; Nakamura, Y.; Sato, T.; Teramoto, A.; Green, M. M.; Andreola,
C.; Peterson, N. C.; Lifson, S. Macromolecules 1995, 28, 1016-1024. (b)
Takei, F.; Yanai, K.; Onitsuka, K.; Takahashi, S. Chem.sEur. J. 2000, 6,
983-993.
(12) (a) Yamaguchi, K.; Matsumura, G.; Kagechika, H.; Azumaya, I.; Ito, Y.;
Itai, A.; Shudo, K. J. Am. Chem. Soc. 1991, 113, 5474-5475. (b) Azumaya,
I.; Kagechika, H.; Yamaguchi, K.; Shudo, K. Tetrahedron 1995, 51, 5277-
5290.
(13) (a) Azumaya, I.; Okamoto, T.; Imabeppu, F.; Takayanagi, H. Tetrahedron
2003, 59, 2325-2331. (b) Azumaya, I.; Okamoto, T.; Imabeppu, F.;
Takayanagi, H. Heterocycles 2003, 60, 1419-1424.
(14) (a) Yokozawa, T.; Asai, T.; Sugi, R.; Ishigooka, S.; Hiraoka, S. J. Am.
Chem. Soc. 2000, 122, 8313-8314. (b) Yokozawa, T.; Ogawa, M.; Sekino,
A.; Sugi, R.; Yokoyama, A. J. Am. Chem. Soc. 2002, 124, 15158-15159.
(c) Yokozawa, T.; Yokoyama, A. Polym. J. 2004, 36, 65-83.
(18) Unpublished results.
(19) (a) Fukuyama, T.; Jow, C-.K.; Cheung, M. Tetrahedron Lett. 1995, 36,
6373-6374. (b) Fukuyama, T.; Cheung, M.; Jow, C.-K.; Hidai, Y.; Kan,
T. Tetrahedron Lett. 1997, 38, 5831-5834.
(20) Prince, R. B.; Brunsveld, L.; Meijer, E. W.; Moore, J. S. Angew. Chem.,
Int. Ed. 2000, 39, 228-230.
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Helical Structures of N-Alkylated Poly(p-benzamide)s
A R T I C L E S
Table 1. Molecular Weights and Polydispersities of Polyamides 1
a
polymer
Mn
Mw/Mn
1a
1b
1c
1d
1e
1f
1700b
3600
1.17
1.08
1.06
1.05
1.05
1.08
5200b
6900
8000
14300
a Measured by MALLS. b Calculated values obtained from the Mn(NMR)
- Mn(MALLS) correlation line by using the Mn(NMR) values.
Figure 2. Dependence of CD spectra (308 nm) on molecular weight of
polyamides 1 measured in CHCl3 at 0 °C (blue line), 15 °C (green line),
30 °C (gray line), and 45 °C (red line). The concentration of each polymer
solution was adjusted so that the absorbance at 300 nm was 1 ([1a-f] )
47-57 mg/L).
results in CHCl3 are illustrated in Figure 2. Some of the salient
features of this molecular weight dependence are that the CD
intensity increases rapidly with increasing molecular weight
when the molecular weight is small but reaches saturation when
it is large. This kind of molecular weight dependence of the
CD spectra is one of the criteria for judging the helical nature
of the polymer, as pointed out by Green et al.17a
Polyamides 8, prepared from phenyl (R)-(3,7-dimethyloctyl-
amino)benzoate (9) as a monomer instead of 2, did not show
any significant Cotton effect at any chain length (for details of
the experiments, see Supporting Information). Considering the
solvent independence of the induced Cotton effect of 1, the lack
of CD spectra is not due to the hydrophobic side chain of 8 but
might be due to the fact that the chiral carbon is placed one
methylene unit further away from the amide backbone as
compared to that of 1 and thereby has a diminished directing
influence on the helicity of the polymer chain.23
Figure 1. (a) CD and (b) UV spectra of polyamide 1f in (A) CH3CN and
(B) CHCl3 at 0 °C (blue line), 15 °C (green line), 30 °C (gray line), and 45
°C (red line) and monomer 2 at 15 °C (pink line). Concentration: [1f] )
60 mg/L in CH3CN and 47 mg/L in CHCl3 and [2] ) 13 mg/L in CH3CN
and 16 mg/L in CHCl3.
360 nm region, which consisted apparently of a peak at ca. 260-
280 nm and a shoulder at ca. 300 nm in acetonitrile (CH3CN),
chloroform (CHCl3) (Figure 1), and methanol (CH3OH) (Figure
S2 in Supporting Information). In the CD spectra, an intense
(ca. 20 000 < [θ] < 50 000 at 0 °C) plus-to-minus pattern was
detected in the same wavelength region, viewing from longer
wavelength, and the positions of the peak and trough roughly
corresponded to the two absorption peaks. Monomeric chro-
mophore 2 showed much weaker (ca. [θ] < 1000) integral type
CD signals that were similar in shape to absorption spectra aside
from its sign21 so that the intense dispersion type CD spectra
are not due to the intrinsic chirality of the monomer units but
3. X-ray Crystallographic Analysis of Oligoamides. The
helical structure of N-alkylated aromatic polyamides was fur-
ther confirmed by X-ray crystallographic analysis of phenyl
esters of the trimer (10), tetramer (11), and pentamer (12) of
4-(methylamino)benzoic acid, as well as the N-Cbz (Cbz:
benzyloxycarbonyl) and carboxylic acid-terminated tetramer
(13), as model compounds of the polyamides. The oligoamides
10-13 were synthesized by the sequential and stepwise depro-
tection-condensation procedure.24 The crystallographic data are
summarized in Table 2.25
to an excess of a single-handed screw-sense structure of 1.22
A
similar CD pattern in the three solvents suggests that there is
no significant influence of the solvent polarity on the conforma-
tion of the polyamides 1. CD signals of polyamides 1 were
highly temperature dependent, decreasing with increasing tem-
perature. This observation suggests that polyamides 1 possess
a flexible helical structure.
Polyamides 1 with various molecular weights exhibited
similarly shaped CD spectra, but the CD intensity showed a
remarkable molecular weight dependence at fixed temperatures
in the range of 0-45 °C and in all solvents examined. The
(23) Nakako, H.; Mayahara, Y.; Nomura, R.; Tabata, M.; Masuda, T. Macro-
molecules 2000, 33, 3978-3982.
(21) (a) Schipper, P. E. J. Am. Chem. Soc. 1979, 101, 6826-6829. (b) Kobayashi,
N.; Higashi, R.; Titeca, B. C.; Lamote, F.; Ceulemans, A. J. Am. Chem.
Soc. 1999, 121, 12018-12028.
(22) Berova, N.; Nakanishi, K.; Woody, R. W. Circular Dichroism: Principles
and Applications, 2nd ed.; Wiley-VCH: New York, 2000.
(24) See Supporting Information for details.
(25) The space groups to which crystals 10-13 belong are all achiral. Therefore,
a unit cell of each crystal has equal numbers of the two enantiomers (right
turn and left turn) of the helices.
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A R T I C L E S
Tanatani et al.
Table 2. Crystallographic Data
10
11
12
13
recryst solvents
formula
formula wt
crystal system
a (Å)
toluene
C30H27N3O4
493.56
monoclinic
9.921(3)
18.683(3)
14.218(3)
CCl4
ethyl acetate
C46H41N5O6‚C4H8O2
847.97
toluene
a
C38H34N4O5‚2(CCl4)
934.36
monoclinic
14.883(6)
13.509(4)
C40H36N4O7‚H2O
702.76
triclinic
10.704(3)
11.993(3)
13.714(3)
87.72(2)
87.36(2)
85.13(2)
1751.2(8)
P1h (#2)
2
triclinic
11.819(2)
13.490(2)
15.6589(9)
106.26(1)
106.353(10)
100.402(5)
2206.7(5)
P1h (#2)
b (Å)
c (Å)
20.809(9)
R (deg)
â (deg)
101.40(2)
92.07(4)
γ (deg)
V (Å3)
2583.3(10)
P21/n (#14)
4
4180(2)
P21/c (#14)
4
space group
Z value
2
T (K)
296
6.90 (Cu KR)
145.3
5355
4885
93
113
0.87 (Mo KR)
60.1
26443
12722
93
µ (cm-1
)
5.87 (Mo KR)
60.1
43830
11959
11953
0.051
0.94 (Mo KR)
60.1
21316
9693
9693
2θmax (deg)
obs refl
ind refl
refl used
Rint
4612
0.034
12722
0.057
0.039
R
0.073
0.062
0.095
0.076
Rw
0.103
0.063
0.125
0.109
GOF
1.82
0.97
1.01
1.01
a C4H8O2 is ethyl acetate.
Figure 3. Top view (upper) and side view (lower) of (a) schematic representations and the crystal structures of (b) trimer 10, (c) tetramer 11, (d) pentamer
12, and (e) N-Cbz and carboxylic acid-terminated tetramer 13. Solvent molecules, terminal O-phenyl groups, and hydrogen atoms are omitted for clarity.
The crystal structure of 10 is shown in Figure 3b with
schematic representations of the oligoamides in Figure 3a. The
two amide bonds are in the cis conformation, and the terminal
benzene rings are orientated at the same side of the plane of
the central benzene ring (syn conformation), as expected. The
top view of Figure 3b demonstrates that the three benzene rings
are arranged in a cyclic manner. These structural properties are
similar to those of N,N′-dimethylisophthalic dianilide.12 Figure
3c,d shows the crystal structures of 11 and 12, respectively.
The structural properties of 11 and 12 are similar to those of
10: all of the amide bonds are in the cis conformation, the
consecutive three benzene rings are in the syn conformation,
and the main chains are arranged in a cyclic manner (Figure
3c,d, top view). In addition, Figure 3 shows the growth of the
helical motif from 10 to 11 and 12: when one or two
4-(methylamino)benzoic acid units are added to the ester
terminal of 10, the additional units accumulate regularly on each
unit of 10. All of the properties mentioned previously clearly
indicate that, in the crystal structures, 11 and 12 adopt a helical
conformation with three monomer units per turn.
To investigate the effect of the terminal structure of the
oligoamides on their conformation, the crystal structure of 13,
which has N-Cbz and carboxylic acid terminals, was determined
by X-ray analysis (Figure 3e). A comparison of the structure
of 13 with that of 11, which has the same number of monomer
Figure 4. Intramolecular edge-to-face aromatic interactions (red dotted
lines) in the crystal of (a) 11 and (b) 12.
units but different terminal structures, shows that the backbone
arrangements are almost the same, and the conformation of 13
is also helical. The major difference between 11 and 13 lies in
the tilt angles between the aromatic rings and the amide bonds.
As shown in Figure 4, intramolecular edge-to-face aromatic
interactions,26 in which the ring center-hydrogen atom distance
is between 3.07 and 3.45 Å, are observed in the crystal structures
of 11 and 12. However, we can find neither intramolecular
aromatic interaction in 10 and 13 nor effective intermolecular
interactions in 10-13. These results indicate that the helical
structures of 10-13 are enforced mainly by the inherent
structural propensities of aromatic amide bonds, such as
(26) Jennings, W. B.; Farrell, B. M.; Malone, J. F. Acc. Chem. Res. 2001, 34,
885-894.
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Helical Structures of N-Alkylated Poly(p-benzamide)s
A R T I C L E S
crystal structures of the oligo(N-methyl-p-benzamide)s showed
a partial turn of helix, the geometry of the polymers in solution
can be deduced, as a first step, to be a helical conformation
with three monomeric units per turn.
The key feature of the present system for analyzing spectra
is that the polymer consists of a distinctive light-absorbing
component, a 4-(alkylamino)benzoyl unit. This type of chromo-
phore that has both electron-releasing and -withdrawing groups
at para positions of benzene can be considered as a spectro-
scopically well-defined subunit that is only slightly perturbed
by the rest of the system upon polymerization.28 Thus, the
electronic wave functions in the chromophore are assumed to
have no electron exchange with the rest of the system. In such
a case, Kasha’s molecular exciton model has been applied to
account for the electronic absorption and the CD spectral
features of chiral oligomers and polymers.27 The molecular
exciton model is a state interaction perturbation theory that deals
with the excited state resonance interaction among chromophores
in weakly coupled electronic systems. Since the monomer unit
considered here corresponds to an intrinsically achiral chromo-
phore, the CD signals must be induced predominantly by the
Kuhn-Kirkwood coupled-oscillator mechanism.29 Nakanishi
and Harada have successfully applied this theory to determine
the absolute configurations of many natural organic molecules
by measuring the CD spectra of their dibenzoate derivatives
(dibenzoate exciton chirality method).28
5.1. TDDFT Calculations of a Model Monomer. Before
applying the exciton theory to our system, we need to assign
the absorption bands of the monomeric unit unambiguously so
that the absorption spectrum of a model compound, 4-(methy-
lamino)benzoic acid, was calculated using the time-dependent
DFT method at the B3LYP/6-31G* level.30 As shown in Figure
6, an intense absorption band was estimated to lie at 266 nm,
reproducing well with the experimental absorption band of
monomer 2 at ca. 300 nm (Figure 1). This band is associated
predominantly with the HOMO f LUMO transition (ca. 89%)
and polarized parallel to the long axis of the molecule. The two
frontier orbitals indicate that the transition is of the π-π* type
of a benzene chromophore with intramolecular CT character.28
Thus, electrons rearrange from the electron-releasing amino side
to the electron-withdrawing carbonyl side during the transition
since the amino nitrogen and the carbonyl carbon contribute
significantly to the benzene HOMO and LUMO levels, respec-
tively. Essentially identical spectra were predicted by the
semiempirical ZINDO/S calculations.31 As a result, we can
assume that the CD signals observed between 240 and 340 nm
in the polymeric system (Figure 1) arise from excitonic
interactions between the transitions along the long axis of each
4-aminobenzoyl chromophore.
Figure 5. Room-temperature electronic absorption spectra of 4-(methy-
lamino)benzoic acid oligomer phenyl esters in CHCl3. The absorption
intensity is normalized at the strongest peak. Concentration and observed
absorbance at 300 nm: 1-mer (concentration ) 7.6 mg/L, Abs300 ) 0.93),
2-mer (34 mg/L, 0.98), 3-mer (20 mg/L, 0.97), 4-mer (22 mg/L, 0.89), 5-mer
(23 mg/L, 0.96), 6-mer (17 mg/L, 0.89), 7-mer (20 mg/L, 0.86), and 8-mer
(33 mg/L, 0.89).
regularity of CorthosCipsosNsC(dO) and CorthosCipsosC(d
O)sN torsion angles, as well as the cis conformation of the
amide bonds and the syn arrangement of the benzene rings.
Considering the structural similarity of the polymers 1 and
oligomers 10-13, the conformation of N-alkylated poly(p-
benzamide)s should be controlled by the same inherent factors,
in accordance with the previously mentioned results that the
polyamides 1 possess helical conformations independently of
the solvent polarity.
4. UV Spectra of Oligo(benzamide)s. To reach a better
understanding of the relations between the molecular structure
and the spectral features, we measured electronic absorption
spectra of a series of 4-(methylamino)benzoic acid oligomer
phenyl esters up to an octamer (Figure 5). The single absorption
band seen at ca. 300 nm for the monomer split into two distinct
peaks (280 and 320 nm) in the dimer. These bands can be
attributed to the transitions from the ground state to the higher
and lower excited state as a result of the strong exciton
interactions of the monomer units (details of the exciton analysis
are discussed in section 5.2).27 Interestingly, the spectral features
of the other higher oligomers appeared to be similar to one
another. The absorption peaks of these oligomers were observed
in the range of 260-280 nm with a shoulder at ca. 300 nm,
depending on the number of the repeating unit; as the number
of chromophoric unit increased, the absorption peak of the
oligomers blue-shifted systematically, while the shoulder band
shifted to the red. More importantly, the absorption spectra of
these oligomers were essentially identical to those of the
polymers, implying that even small oligomers (trimer to
octamer) are long enough to show a helical turn in solution.
5. Exciton CD Analysis. As mentioned in the second section,
monomer 2 was almost CD silent, while polyamides 1 showed
intense bisignate CD signals depending on temperature (0-45
°C), regardless of the solvent polarity. This experimental
evidence substantiates that polymers 1 adopt a well-defined
helical geometry under the experimental conditions. Since the
5.2. Exciton Energy Level Diagram. To relate the spectro-
scopic properties observed in the benzamide system to their
(28) Harada, N.; Nakanishi, K. Circular Dichroic Spectroscopy: Exciton
Coupling in Organic Stereochemistry; University Science Books: Mill
Valley, CA, 1983.
(29) (a) Kuhn, W. Trans. Faraday Soc. 1930, 26, 293-308. (b) Kirkwood, J.
G. J. Chem. Phys. 1937, 5, 479-491. (c) Tinoco, I. AdV. Chem. Phys.
1962, 4, 113-160. (d) Muranaka, A.; Okuda, M.; Kobayashi, N.; Somers,
K.; Ceulemans, A. J. Am. Chem. Soc. 2004, 126, 4596-4604.
(30) (a) Furche, F.; Ahlrichs, R.; Wachsmann, C.; Weber, E.; Sobanski, A.;
Vogtle, F.; Grimme, S. J. Am. Chem. Soc. 2000, 122, 1717-1724. (b)
Dreuw, A.; Head-Gordon, M. J. Am. Chem. Soc. 2004, 126, 4007-4016.
(c) Stephens, P. J.; McCann, D. M.; Devlin, F. J.; Cheeseman, J. R.; Frisch,
M. J. J. Am. Chem. Soc. 2004, 126, 7514-7521.
(27) (a) Kasha, M.; Rawls, H. R.; El-Bayoumi, M. A. Pure Appl. Chem. 1965,
11, 371-392. (b) Kasha, M. In Spectroscopy of the Excited State; Bartolo,
B., Eds.; Plenum: New York, 1976; pp 337-363.
(31) Ridley, J. E.; Zerner, M. C. Theor. Chim. Acta 1976, 42, 223-236.
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A R T I C L E S
Tanatani et al.
Figure 6. (a) Stick absorption spectrum of 4-(methylamino)benzoic acid
as a model monomer calculated by the TDDFT method (B3LYP/6-31G*
level). The arrow in the inset indicates the transition electric dipole moment
(µ) corresponding to the lowest-energy transition. (b) Frontier molecular
orbitals.
Figure 8. (a) Schematic exciton band energy diagrams for a cyclic trimer
and a helical polymer with three light-absorbing units per turn. The pink
and red arrows indicate the transition electric dipole moments of each
4-aminobenzoyl chromophore and the net transition electric moments,
respectively. The dotted bands correspond to optically forbidden exciton
bands for the polymer. (b) Optically allowed exciton states of the helical
N-alkylated p-benzamide polymer. The geometry of the model oligomer
was built using molecular mechanics calculations.
The lowest exciton state is optically forbidden due to a
cancellation of the net transition dipole moment. The higher
energy state, which has one nodal plane, is a doubly degenerate
state with optically allowed nature. It is therefore possible to
assign the absorption bands of the trimer in Figure 5. As would
be expected, the low-energy exciton state in a helical trimer
gains a net transition moment (µz) perpendicular to the plane
since each transition moment in the trimer is not aligned in a
plane in contrast to an ideal planar trimer. As a consequence,
the absorption shoulder observed at around 300 nm can be
attributed to the partially allowed transition to the lowest exciton
state, the polarization of which is parallel to the helix axis. The
absorption band with the center at ca. 260-270 nm should be
a transition to the doubly degenerate higher-energy exciton state,
which is polarized perpendicular to the helix axis.
The exciton band structure of a model helical polymer with
three chromophoric units per turn is presented in the right-hand
side of Figure 8a. In analogy with a cyclic trimer, the possible
optical transitions by electric dipole radiation for larger polymers
are severely limited by symmetry, although a quasi-continuous
exciton band is generated.27 In the present system, one of the
optically allowed exciton states corresponds to the deepest
exciton state, in which all electric transition moments couple
one another, similar to the in-phase coupling of the cis-dimer
and cyclic trimer. The other allowed state is doubly degenerate
and has one nodal plane perpendicular to the helix axis, similar
to the out-of-phase coupling of the dimer and trimer. Figure 8b
shows each coupling mode corresponding to the optically
allowed exciton states of the polymer. As a consequence, it is
Figure 7. Schematic exciton band energy diagrams for cis- and trans-
dimers of 4-(alkylamino)benzoic acid. The pink and red arrows indicate
the transition electric dipole moments of each 4-(alkylamino)benzoyl
chromophore and the net transition electric moments, respectively.
molecular geometry, first a qualitative exciton energy level
diagram of the dimer is considered. Within the point-dipole
approximation, the exciton interaction can be represented as a
coupling of electric transition dipole moments (µ) of each
chromophore.27 Figure 7 shows the schematic energy diagram
and the vector model for a cis-dimer whose conformation has
been confirmed experimentally by the previous studies of
N-methylbenzanilide.11a It is readily seen that the in-phase
arrangement of the transition dipoles leads to an electrostatic
attraction, producing a lower exciton state, whereas the out-of-
phase arrangement of the transition dipoles causes repulsion,
producing a higher exciton state. The magnitude of the net
transition moment (µnet) of the higher state is about 1.73 times
larger than that of the lower state, so that the absorption intensity
corresponding to a transition from the ground state to the higher
exciton state becomes more intense than that to the lower exciton
state, as is indeed observed in Figure 5. If the dimer adopts a
trans conformation, the transition to the higher exciton state
becomes optically forbidden because of the cancellation of the
net moment. As a result, the experimental spectral data observed
for the dimer are reasonably explained within the simple vector
model for the cis-dimer.
We then consider a cyclic planar trimer as a model system
for the helical polymer. As shown in Figure 8, the formation of
a cyclic trimer results in three exciton states by its symmetry.27
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Helical Structures of N-Alkylated Poly(p-benzamide)s
A R T I C L E S
Figure 9. Overviews of the direction of the net electric (µ, red) and magnetic (m, blue) transition dipole moments. The theoretically predicted CD signals
are shown at the bottom. Note that the x’’- and z’’-axes are tilted slightly as compared with the x- and z-axes. The blue circle represents a circulation of
charge arising from the coupling of two transition electric moments.
possible to assign that the absorption shoulder and the positive
CD band of poly(N-alkyl-p-benzamide)s 1 at ca. 300 nm arise
from the transition to the lowest exciton state and that the intense
absorption and the negative CD at ca. 260 nm are associated
with the transition to the doubly degenerate exciton state.
polymer with three light-absorbing units per turn are presented
in Figure 9. It should be noted that the coupling of the transitions
along the long axis induces the net transition magnetic dipole
moment (m). The lower energy exciton state can be represented
as head-to-tail couplings of neighboring transition moments,
giving rise to a transition magnetic moment (m) parallel or
antiparallel to the z-axis because of a circulation of the transition
electric moments (µ). On the other hand, the doubly degenerate
higher exciton state has a magnetic moment (m) directed along
the x′- or y-axis. By considering the scalar product of µ and m,
a theoretical CD pattern can be predicted for our system. For
example, in the case of the (P)-helix, m and µ become opposite
in sign along the x′ and y-axes, which generate CD at shorter
wavelengths so that the CD sign becomes negative, while they
are the same sign along the z-axis so that a positive CD signal
is observed at longer wavelengths. Note that the positive and
negative CD signals have the same intensity because of the sum
rule.28 As would be expected, the CD pattern of the (M)-helix
is the mirror image of that of the (P)-helix. Since the CD patterns
of polymer 1 we studied exhibited a plus-to-minus CD pattern,
in ascending energy, with the almost identical intensity, it can
be concluded that N-alkylated poly(p-benzamide) 1 preferen-
tially takes a right-handed helical geometry ((P)-helix) in
chloroform, methanol, and acetonitrile. TDDFT (B3LYP/6-
31G*) calculations of the crystal structures of the oligo(N-
methyl-p-benzamide) phenyl esters support this assignment,
although the number of the monomer units, the crystal packing,
and the terminal phenyl groups may contribute to the CD spectra
to some extent (see Supporting Information). Hence, the
calculated CD spectrum of the (P)-trimer shows a plus-to-minus
CD pattern, while that of (M)-tetramer shows a minus-to-plus
CD pattern, in agreement with the assignment based on the
molecular exciton model.
5.3. Assignment of the Helical Structure of Polymer 1.
To determine the helicity of the present polymeric system from
the observed CD pattern, a qualitative coupled-oscillator analysis
is performed.29 In principle, the CD intensity (rotational strength
R) of a transition in a collection of randomly oriented chiral
molecules is expressed by the Rosenfeld equation;32
R ) Im(µ‚m)
(1)
where µ is the transition electric dipole moment and m describes
the transition magnetic dipole moment, which is related to the
net circulation of electrons. The direction of m is such that when
viewed from its tip, the rotation of charge is anticlockwise. Im
denotes the imaginary part of µ‚m. The equation implies that
the sign of the CD is simply determined by the relative
arrangement of µ and m. In the case of the present polymers,
the two coupling modes that give rise to the lower energy
positive CD and the higher energy negative CD bands were
associated with the net electric transition moments that lie,
respectively, parallel and perpendicular to the helix axis. Since
the coupling between the transition electric moment (µ) and
the magnetic moment (m) is described as a scalar product from
the Rosenfeld equation, we need to consider only these transition
moments along the same axis for determining the origin of the
optical activity.
The overviews of the direction of the net transition moments
for a right-handed ((P)-helix) and left-handed ((M)-helix) helical
(32) Rodger, A.; Norde´n, B. Circular Dichroism and Linear Dichroism; Oxford
University Press: Oxford, 1997.
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A R T I C L E S
Tanatani et al.
Hz, 1 H), 7.65 (t, J ) 7.9 Hz, 1 H), 7.50 (s, 1 H), 7.42 (t, J ) 7.0 Hz,
2 H), 7.37 (d, J ) 6.9 Hz, 2 H), 7.27 (t, J ) 7.0 Hz, 1 H), 7.17 (d, J
) 8.6 Hz, 2 H).
Conclusion
In conclusion, we have demonstrated that the polyamides 1,
synthesized by chain-growth polycondensation of 2, take a
helical structure in solution, based upon the following: (1)
significant amplification of the CD signal of the polymer as
compared with that of the monomer, (2) temperature dependence
of the CD signal, with the higher values at lower temperature,
and (3) chain length (or molecular weight) dependence of the
CD spectra. The helical structure in the solid state was also
confirmed by X-ray crystallographic analysis of the N-methyl
aromatic oligoamides 10-13, which show a helical conforma-
tion with three monomer units per turn, despite their highly
flexible backbone. The helical structure of N-alkylated poly(p-
benzamide)s probably stems from the inherent structural pro-
pensities of cis conformation of N-alkylated amide bonds and
syn arrangement of the benzene rings.
It has been further demonstrated that the present exciton
model analysis provides a clear-cut picture for qualitative
understanding of the secondary structure of N-alkylated poly-
(benzamide)s in solution. The positive and negative CD signals
observed at 300 and 260 nm, respectively, in the polymeric
system were successfully assigned to the exciton bands arising
from the couplings of the long axis polarized transition of each
4-(alkylamino)benzoyl chromophore. Taking into account the
Rosenfeld equation, we have elucidated that polymer 1 prefer-
entially adopts a right-handed helical structure ((P)-helix) under
the experimental conditions. It is important to note that the
helicity of the polymer could be derived, without a great deal
of calculation, by inspection of the CD spectra and applying
the molecular exciton model, despite its crude nature.
Synthesis of 6. Diethyl azodicarboxylate (DEAD, 40% in toluene,
5.75 g, 13 mmol) was added to a solution of 4 (4.78 g, 12 mmol), 520
(1.50 g, 8.1 mmol), and PPh3 (3.47 g, 13 mmol) in dry THF (40 mL)
at 0 °C under an Ar atmosphere. The reaction mixture was stirred at
room temperature for 15 h and evaporated. The residue was purified
by silica gel column chromatography (AcOEt/hexane ) 1:1) to give 6
1
as a colorless oil (3.16 g, 67%). H NMR (600 MHz, CDCl3) δ 8.14
(d, J ) 8.6 Hz, 2 H), 7.66 (t, J ) 7.9 Hz, 1 H), 7.62 (d, J ) 7.9 Hz,
1 H), 7.59 (d, J ) 7.9 Hz, 1 H), 7.50 (t, J ) 7.9 Hz, 1 H), 7.47 (d, J
) 8.6 Hz, 2 H), 7.44 (t, J ) 7.6 Hz, 2 H), 7.29 (t, J ) 7.6 Hz, 1 H),
7.20 (d, J ) 7.6 Hz, 2 H), 3.93-3.87 (m, 2 H), 3.64-3.58 (m, 4 H),
3.53-3.49 (m, 4 H), 3.46-3.42 (m, 1 H), 3.37 (s, 3 H), 1.21 (d, J )
6.5 Hz, 3 H).
Synthesis of 2. A solution of benzenethiol (0.754 g, 6.8 mmol) in
CH3CN (20 mL) and Cs2CO3 (5.62 g, 17 mmol) was added to a solution
of 6 (3.16 g, 5.7 mmol) in CH3CN (60 mL). The reaction mixture was
heated at 70 °C for 2 h and allowed to cool to room temperature. The
reaction mixture was diluted with water and extracted with CH2Cl2.
The organic layer was washed with brine, dried over anhydrous Na2-
SO4, and evaporated. The residue was purified by silica gel column
chromatography (AcOEt/hexane ) 1:1) to give 2 as a yellow oil (1.88
1
g, 89%). H NMR (600 MHz, CDCl3) δ 8.01 (d, J ) 8.6 Hz, 2 H),
7.40 (t, J ) 7.6 Hz, 2 H), 7.23 (t, J ) 7.6 Hz, 1 H), 7.19 (d, J ) 7.6
Hz, 2 H), 6.63 (d, J ) 8.6 Hz, 2 H), 5.01 (br s, 1 H), 3.81-3.75 (m,
2 H), 3.69-3.66 (m, 4 H), 3.61-3.58 (m, 3 H), 3.40 (s, 3 H), 3.33-
3.29 (m, 1 H), 3.14-3.10 (m, 1 H), 1.25 (d, J ) 6.2 Hz, 3 H). HRFAB-
MS [M + H] Calcd for C21H28NO5: 374.1967; Found: 374.1969.
Typical Procedure of Polymerization: Synthesis of Polyamide
1f. CsF (0.083 g, 0.55 mmol) was placed in a round-bottomed flask
and dried by being heated at 250 °C under reduced pressure for 20
min. A solution of 7 (0.0024 g, 0.0099 mmol), 2 (0.185 g, 0.49 mmol),
and N-octyl-N-triethylsilylaniline (0.151 g, 0.47 mmol) in 0.7 mL of
dry THF was added to the flask under a N2 atmosphere. A solution of
18-crown-6 (0.272 g, 1.03 mmol) in 0.3 mL of dry THF was added to
the mixture at room temperature, and the whole was stirred overnight
at room temperature. The reaction mixture was quenched with saturated
aqueous ammonium chloride (4 mL) and extracted with CH2Cl2. The
organic layer was washed with water, dried over anhydrous Na2SO4,
and evaporated. Removal of N-octylaniline and isolation of the polymer
formed were carried out by preparative HPLC (eluent: chloroform)
with the use of polystyrene gel columns to afford the polyamide 1f
(0.0825 g).
X-ray Crystallography. The structural determination of compound
10 was made on a Rigaku AFC5R diffractometer with graphite-
monochromated Cu KR radiation at 296 K to a maximum 2θ value of
145.3°. The structure was solved by direct methods (SIR92) and
expanded using Fourier techniques (DIRDIF94). The non-hydrogen
atoms were refined anisotropically. The structural determinations of
compounds 11-13 were made on a Rigaku RAXIS-RAPID Imaging
Plate diffractometer with graphite-monochromated Mo KR radiation
Experimental Procedures
Syntheses of 10-13 are described in the Supporting Information.
Column chromatography was performed on a silica gel (Silica gel 60,
spherical, 40-100 µm, Kanto or Kieselgel 60, 230-400 mesh, Merck)
with a specified solvent. Commercially available (Kanto) tetrahydro-
furan (THF, stabilizer-free) was used as a dry solvent. 1H NMR spectra
were obtained on an ECA-600 spectrometer with tetramethylsilane as
the internal standard (0.00 ppm). Mass spectra were recorded on a JEOL
JMA-HX110 spectrometer. The Mn and Mw/Mn values of polymers were
measured on a Shodex GPC-101 equipped with Shodex UV-41, Shodex
RI-71S, and DAWN EOS multiangle laser light scattering (MALLS,
Wyatt Technology Corp) detectors and two Shodex KF-804L columns
(bead size ) 7 µm, pore size ) 200 Å). THF was used as the eluent
(temperature ) 40 °C, flow rate ) 2 mL/min). Calibration was carried
out using polystyrene standards. Isolation of polyamides was carried
out with a Japan Analytical Industry LC-908 Recycling Preparative
HPLC (eluent: chloroform) with the use of two TOSOH TSK-gel
columns (2× G2000HHR). UV-vis spectra were measured on a JASCO
V-550 spectrometer. CD spectra were measured on a JASCO J-820
using a 10 mm quartz cell. The concentration of each solution for CD
and UV experiments was adjusted so that the absorbance at 300 nm
was 1.
(33) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M.
A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.;
Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa,
J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.;
Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo,
J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.;
Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.;
Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels,
A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;
Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson,
B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Rev.
B.04; Gaussian, Inc.: Pittsburgh, PA, 2003.
Synthesis of 4. 2-Nitrobenzenesulfonyl chloride (0.239 g, 1.1 mmol,
1.1 equiv) was added to a solution of 314h (0.205 g, 0.96 mmol) in
pyridine (1 mL) at 0 °C. The reaction mixture was stirred at room
temperature for 2 h and poured into 1 M hydrochloric acid (20 mL).
The aqueous layer was extracted with CH2Cl2 twice. The combined
organic extracts were washed with brine, dried over anhydrous Na2-
SO4, and evaporated. The residue was purified by silica gel column
chromatography (AcOEt/hexane ) 1/3) to give 4 as a white solid (0.367
1
g, 96%). H NMR (600 MHz, CDCl3) δ 8.13 (d, J ) 8.6 Hz, 2 H),
7.98 (d, J ) 7.9 Hz, 1 H), 7.89 (d, J ) 7.9 Hz, 1 H), 7.74 (t, J ) 7.9
9
8560 J. AM. CHEM. SOC. VOL. 127, NO. 23, 2005

Helical Structures of N-Alkylated Poly(p-benzamide)s
A R T I C L E S
at 93 K for 11 and 13 or 113 K for 12 to a maximum 2θ value of
60.1°. The structures were solved by direct methods (SIR97) and
expanded using Fourier techniques (DIRDIF94). The non-hydrogen
atoms were refined anisotropically.
the COE project, Giant Molecules and Complex Systems, 2005,
from the Ministry of Education, Culture, Sports, Science and
Technology, Japan.
Calculation. All calculations were carried with a Gaussian 03 (G03)
program package33 using the hybrid density functional method based
on Becke’s three-parameter exchange function and the Lee-Yang-
Parr nonlocal correlation functional (B3LYP).34 We used 6-31G* basis
set for all atoms. Geometry optimization and time-dependent DFT
calculations were performed at the same level.
Supporting Information Available: Synthesis of 8-13 and
other phenyl esters of 4-(methylamino)benzoic acid oligomers,
Mn and Mw/Mn of 8, correlation plot of the values of Mn of 1
determined by 1H NMR to those determined by GPC and
MALLS, UV and CD spectra measured in methanol, dependence
of CD intensities on molecular weight of polyamide 1f measured
in CH3CN and CH3OH at various temperatures, crystallographic
information (ORTEP drawings and CIF files) of oligoamides
10-13, and calculated UV and CD spectra of the phenyl esters
of the oligo(N-methyl-p-benzamide). This material is available
free of charge via the Internet at http://pubs.acs.org.
Acknowledgment. We thank Yuko Inagaki for the UV
measurements. This work was partially supported by a Grant-
in-Aid (15750108 and 17350063) for Scientific Research and
(34) (a) Becke, A. D. Phys. ReV. 1988, A38, 3098-3100. (b) Becke, A. D. J.
Chem. Phys. 1993, 98, 1372-1377. (c) Becke, A. D. J. Chem. Phys. 1993,
98, 5648-5652. (d) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. 1988, B37,
785-788.
JA0455291
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