Crystal Structure of N-Isopropylanthranilic Acid Trimer - J

X-ray Structure Analysis Online 2014, VOL. 30
2014 © The Japan Society for Analytical Chemistry
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X-ray Structure Analysis Online
Crystal Structure of N-Isopropylanthranilic Acid Trimer
Akihiro YOKOYAMA,*† Natsumi KAWANO,* Yuka WADA,** Michihiro NISHIKAWA,*
Kazuto TAKAISHI,* and Tsutomu YOKOZAWA**
*Department of Materials and Life Science, Faculty of Science and Technology, Seikei University,
3-3-1 Kichijoji-kitamachi, Musashino, Tokyo 180-8633, Japan
**Department of Material and Life Chemistry, Kanagawa University,
Rokkakubashi, Kanagawa-ku, Yokohama 221-8686, Japan
The crystal structure of N-isopropylanthranilic acid trimer was characterized by a single-crystal X-ray diffraction method
to study the conformation of oligo(o-benzamide). The compound crystallized in a triclinic space group, P1, with a =
7.720(4)Å, b = 9.006(4)Å, c = 20.131(9)Å, a = 92.459(7)˚, b = 95.299(7)˚, g = 91.099(7)˚, V = 1391.9(11)Å3, and Z = 2.
In the crystal structure, one of the amide linkages adopts a cis conformation, while the other adopts a trans conformation.
(Received March 24, 2014; Accepted April 26, 2014; Published on web July 10, 2014)
Helical molecules have attracted attention in recent years, and
many types of helical polymers and oligomers have been
reported in the literature.1,2 In the course of our study to develop
a method for the controlled synthesis of N-substituted
polybenzamides,3 we demonstrated that N-substituted oligo- and
poly(p-benzamide)s adopt a helical conformation with three
monomer units per turn, arising from the cis conformation of
N-substituted aromatic amide linkages and the syn arrangement
of the three consecutive benzene units connected by two amide
linkages.4 However, a circular dichroism (CD) study of random
copolymers of N-substituted poly(p-benzamide)s indicates that
the stability of the helical structure is very low, and that the
polymers should exist in equilibrium between the helical
conformation and a random conformation.5
The structures and methods of synthesis of N-substituted
oligo- and poly(o-benzamide) have rarely been reported. A
study by Ollis’s group reported the structure of oligo(obenzamide), as studied by an X-ray structural analysis of the
N,N¢-dimethylated anthranilic acid trimer.6 In the crystal
structure of the trimer, the amide linkage at the N-terminal side
adopts a cis conformation, but that at the C-terminal adopts the
trans conformation. The N-substituted aromatic amide linkages
generally prefer the cis conformation.7 Hence, if a bulky
substituent is introduced at the amide nitrogen atom of oligo(obenzamide), all of the amide linkage may adopt the cis
conformation in order to avoid the repulsion between the
Fig. 1
Chemical structure of 1.
† To whom correspondence should be addressed.
E-mail: [email protected]
N-substituent and the benzene unit connected to the carbonyl
group at the same amide linkage. This leads to a helical
structure in the oligo(o-benzamide). To determine the effect of
the bulky N-substituent on the conformation of amide linkages,
we designed the anthranilic acid trimer 1 bearing N-isopropyl
groups as an N-substituent (Fig. 1); the structure of 1 was
studied by X-ray structure analysis.
The target compound 1 was synthesized in a stepwise manner.
As in the case of the reaction we reported previously,8 the dimer
methyl ester was synthesized by the reaction of methyl
N-isopropylanthranilate with N-isopropylisatoic anhydride in
Table 1
Crystal and experimental data
Chemical formula: C31H37N3O4
Formula weight = 515.65
T = 123 K
Crystal system: triclinic
Space group: P1
a = 7.720(4)Å
a = 92.459(7)˚
b = 9.006(4)Å
b = 95.299(7)˚
c = 20.131(9)Å
g = 91.099(7)˚
Z=2
V = 1391.9(11)Å3
Dx = 1.230 g/cm3
Radiation: Mo Ka (l = 0.7107 Å)
F(0 0 0) = 552
m(Mo Ka) = 0.082 mm–1
Crystal size = 0.40 ¥ 0.40 ¥ 0.40 mm3
No. of reflections collected = 13096
No. of independent reflections = 5940
q range for data collection: 3.05 to 27.48˚
Data/Restraints/Parameters = 5940/0/343
Goodness-of-fit on F2 = 1.147
R indices [I > 2s(I)]: R1 = 0.0539, wR2 = 0.1334
R indices (all data): R1 = 0.0903, wR2 = 0.1462
(D/s)max = 0.002
(Dr)max = 0.26 eÅ–3
(Dr)min = –0.22 eÅ–3
Measurement: Rigaku Saturn 70 CCD
Program system: WinGX
Structure determination: SIR92
CCDC deposition number: 988994
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X-ray Structure Analysis Online 2014, VOL. 30
Fig. 3 Packing view of 1 in the unit cell along the b axis. The
dashed line indicates p-p stacking.
Fig. 2 ORTEP drawing of 1 with atom labeling. The thermal
ellipsoids are shown at the 50% probability level. Hydrogen atoms
are omitted for clarity.
Table 2 Selected bond lengths (Å), bond angles (˚), and
torsion angles (˚)
Bond lengths
Bond angles
An ORTEP drawing of 1 is shown in Fig. 2. Selected bond
lengths, bond angles, and torsion angles are given in Table 2.
The torsion angle of C8–N1–C12–C13 [13.3(3)˚] reveals that the
amide linkage at the C-terminal side is almost planar, and
adopts a cis conformation. On the other hand, the torsion angle
of C18–N2–C22–C23 is 177.85(18)˚, indicating that the amide
linkage at the N-terminal side is also planar, but adopts the trans
conformation. These results are contradictory with respect to
the N,N¢-dimethylated trimer.6 The crystal structure of 1 does
not involve intramolecular any p-p interaction. However, the
packing diagram of 1 shows an intermolecular offset stacking
between the benzene rings at the N-terminal side of two
molecules of 1 (Fig. 3); the shortest C–C distance between two
benzene rings is 3.53 Å. It is possible that the p-p interaction
may disturb the cis preference of the amide linkage at the
N-terminal side. Synthesis and structure analyses of longer
oligoamides are in progress, in order to determine the effect of
crystal packing at the terminal unit.
Acknowledgments
This work was partially supported by a Grant from Seikei
University.
Torsion angles
References
the presence of lithium bis(trimethylsilyl)amide as a base. The
same reaction between the dimer methyl ester and
N-isopropylisatoic anhydride was carried out to obtain the target
compound 1. Single-crystals suitable for X-ray analysis were
obtained by the slow diffusion of hexane into a THF solution of
the trimer 1.
X-ray diffraction data with 13096 reflections (of which 5940
were unique) were collected at 123 K on a Rigaku Saturn 70
CCD diffractometer with Mo Ka radiation. Data were corrected
for both Lorentz and polarization effects. The crystal and
refinement data are presented in Table 1. The structure was
solved by direct methods and expanded using Fourier
techniques.
The non-hydrogen atoms were refined
anisotropically.
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