Synthesis and conformational properties of tetranitroazacalix[4]arenes

Article abstract of DOI:10.1016/j.tetlet.2008.11.095

Tetranitroazacalix[4]arenes have been synthesized by the nucleophilic aromatic substitution of 1,5-difluoro-2,4-dinitrobenzene with 1,3-diaminobenzenes. An X-ray crystal structure analysis revealed that the azacalixarenes adopt a non-symmetrical 1,3-alter

Full text of DOI:10.1016/j.tetlet.2008.11.095

Tetrahedron Letters 50 (2009) 620–623
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Synthesis and conformational properties of tetranitroazacalix[4]arenes
*
Hisatoshi Konishi , Shun Hashimoto, Terunobu Sakakibara, Shingo Matsubara,
Yusuke Yasukawa, Osamu Morikawa, Kazuhiro Kobayashi
Department of Chemistry and Biotechnology, Graduate School of Engineering, Tottori University 4-101 Koyama-minami, Tottori, Tottori 680-8552, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Tetranitroazacalix[4]arenes have been synthesized by the nucleophilic aromatic substitution of 1,5-
difluoro-2,4-dinitrobenzene with 1,3-diaminobenzenes. An X-ray crystal structure analysis revealed that
the azacalixarenes adopt a non-symmetrical 1,3-alternate conformation, and the dinitrobenzene rings
strongly conjugate with the bridging nitrogen atoms. In the 1H NMR spectrum (CDCl3, 30 °C), the tetra-
isopropyl derivative 3b displays a pair of diastereotopic methyl signals of the isopropyl groups in agree-
ment with the frozen 1,3-alternate conformation on the NMR time scale. The free energy of activation
Received 14 September 2008
Revised 8 November 2008
Accepted 13 November 2008
Available online 30 November 2008
Keywords:
Azacalixarene
Nucleophilic aromatic substitution
Inversion barrier
(
D
G–298) for the macrocyclic inversion was determined to be 87.5 kJ molÀ1 by temperature-dependent
NMR spectroscopy.
Ó 2008 Elsevier Ltd. All rights reserved.
Temperature-dependent NMR
Calixarenes, or [1n]metacyclophanes, are some of the most
widely used molecular scaffolds for designing sophisticated func-
tional molecules. This is due to their easy availability, interesting
conformational properties, and versatile introduction of functional
groups.1 Recently, much attention has focused on the synthesis of
hetero atom-bridged calixarenes2 because of their fine tunable
molecular structures. Among them, the oxacalix[4]arenes are read-
ily prepared by the nucleophilic aromatic substitution of activated
1,3-dihalobenzenes with 1,3-dihydroxybenzenes.3 The high selec-
tivity for the formation of cyclic tetramers observed during their
synthesis without using high dilution conditions is a consequence
of the thermodynamic product control.4 Thus, the C–O bond for-
mation is reversible and the cyclic tetramer is the most stable
product. An analogous thermodynamically controlled synthesis of
the thiacalixarenes has also been reported.5
On the other hand, several types of nitrogen atom-bridged
calixarenes, that is, azacalixarenes, have been prepared by Pd-cat-
alyzed aryl amination reactions.6 These reactions require a long
reaction time at high temperature and produce a mixture of cyclic
oligomers of various ring sizes.7 Meanwhile, some azacalixarenes
containing 1,3,5-triazine units have been synthesized by the reac-
tion of cyanuric chloride with 1,3-diaminobenzenes in the absence
of metal catalysts.8 We have developed the facile synthesis of the
azacalix[4]arenes (Scheme 1). Our synthetic route involves the aro-
matic nucleophilic substitution of 1,5-difluoro-2,4-dinitrobenzene
1. This synthetic approach provided azacalix[4]arenes consisting
of two dinitrobenzene moieties at the distal positions. The strong
conjugation between the dinitrobenzenes and the bridging nitro-
gen atoms was revealed by X-ray crystallography. In order to
investigate the energy barrier of the macrocyclic ring inversion
by temperature-dependent NMR spectroscopy, the azacalix[4]-
arene bearing isopropyl substituents at the 4,6-positions of the
aromatic rings was synthesized.
The reaction of 1 with 1,3-diaminobenzene 2a was conducted in
DMF in the presence of K2CO3 at 100 °C for 2 h. Recrystallization of
the precipitated crude product from DMSO produced the tetranit-
roazacalix[4]arene 3a in 49% yield.9 On the other hand, under
analogous conditions, the reaction of 1 with 1,5-diamino-2,4-diiso-
propylbenzene 2b produced the cyclic tetramer 3b in rather low
yield (12%) accompanied by the significant formation of lower lin-
ear oligomers.10 Moreover, the GPC separation of the crude reac-
tion mixture from 2b provided no evidence for the formation of
larger macrocyclic compounds. These results appear to be due to
the sterically encumbering isopropyl substituents at the ortho
positions of the amino groups.
The 1H NMR spectroscopic analysis demonstrates the highly
symmetrical structure of 3. In DMSO-d6 at 50 °C, the signals of
the aromatic protons in 3a appear as an AB2C-spin system and
two singlets. Its intra-annular protons (Hin) of the dinitrobenzene
moieties resonate at 5.48 ppm. The 1H NMR spectrum (400 MHz,
CDCl3, 30 °C) of the tetraisopropyl derivative 3b shows the aro-
matic protons as four singlets, and the Hin protons resonate at
5.30 ppm. The Hin signals of 3a and 3b are shifted to a higher field
when compared to the corresponding aromatic protons of 2. These
high field shifts are attributed to the influence of the ring current
effect by the two adjacent benzene rings. Thus, it can be presumed
that the preferred conformations of 3 are more likely to be the
* Corresponding author. Tel.: +81 857 31 5262; fax: +81 857 28 6437.
E-mail address: konis@chem.tottori-u.ac.jp (H. Konishi).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.11.095

H. Konishi et al. / Tetrahedron Letters 50 (2009) 620–623
621
Hex
O2N
HN
NO2
NH
R
R
R
R
R
O2N
F
NO2
F
K2CO3/ DMF
Hin
H2N
NH2
R
HN
NH
2a R = H
2b R = i-Pr
1
O2N
NO2
3a R = H
3b R = i-Pr
Scheme 1. Preparation of azacalix[4]arene 3 by nucleophilic aromatic substitution.
1,3-alternate ones. Interestingly, the resonance of the isopropyl
groups in 3b appears as a pair of doublets (1.07 and 1.18 ppm)
and a septet (2.97 ppm). Furthermore, in the 13C NMR spectrum
(125 MHz, CDCl3, 30 °C) of this compound, two methyl carbon sig-
nals (22.0 and 22.5 ppm) are observed. These observations indicate
that the two methyl groups in the isopropyl group are dia-
stereotopic.
calculations support the fact that the diastereotopicity of the gem-
inal methyl groups is not due to the restricted rotation of the iso-
propyl groups, but due to the slow macrocyclic ring inversion. The
simulation spectra are shown in Figure 1 (right).11 Based on these
data, the Eyring plot of ln(k/T) versus 1/T was constructed (Fig. 2).
Based on the slope and intercept of this straight line (r2 = 0.999),
D
H– = 78.0 kJ molÀ1 and
D
S– = À31.8 J KÀ1 molÀ1 were deter-
Moreover, the temperature-dependent 1H NMR analysis dem-
onstrated the spectral changes of the signals in the isopropyl
methyl protons in the range of 363–423 K, which are shown in
Fig. 1 (left). The exchange of the magnetic environment of two
methyl groups can be interpreted as a result of the ring inversion
process. This interpretation is supported by molecular mechanics
calculations. The energy-minimized structure of 3b optimized by
the MM3 force field adopt 1,3-alternate conformation, and the iso-
propyl substituents are bisected by the attached aromatic rings.
Furthermore, the computed rotational barrier about the Ar-C bond
of the isopropyl group is 25.7 kJ molÀ1, indicating its free rotation
at ambient temperature on the NMR time scale. Therefore, these
mined. Thus, the free energy of activation for the inversion
(D
G–298) was estimated to be 87.5 kJ molÀ1. This value is consider-
ably higher than that of the azacalix[4]arene 4 (D
G–301 58.5 kJ
molÀ1),7j bearing N-benzyl moieties and no nitro groups
(Fig. 3).
The solid state structures of 3a and 3b were determined by a
single-crystal X-ray crystallographic analysis.12 Their ORTEP draw-
ings are shown in Figure 4. In both molecules, the four nitrogen
atoms at the bridging positions are located nearly in the mean
plane defined by these atoms with a maximum deviation of
0.022 Å for 3a and 0.045 Å for 3b. The benzene rings of 3a and
the diisopropylbenzene of 3b are almost perpendicular to these
mean planes, whereas the dinitrobenzene rings are oriented out-
ward. The dihedral angles between the opposite dinitrobenzene
rings are 127.2° for 3a and 103.4° for 3b. The difference in these
dihedral angles may be ascribed to the steric repulsion of the iso-
propyl groups and nitro groups. Overall, the calix[4]arenes 3 adopt
a non-symmetrical 1,3-alternate conformation.
Each of the nitro groups is essentially coplanar with the at-
tached benzene ring, and all the nitrogen atoms in the bridging
Figure 1. Left: temperature dependent 1H NMR spectra of the isopropyl methyl
signals of 3b at 400 MHz in DMSO-d6. Right: line-shape simulations obtained with
the indicated rate constants.
Figure 2. The Eyring plot for the ring inversion of azacalix[4]arene 3b in DMSO-d6.

622
H. Konishi et al. / Tetrahedron Letters 50 (2009) 620–623
compound 3a, the bond length between the nitrogen atom and
t
-Bu
i-Pr
HN
i-Pr
NH
the dinitrobenzene carbon (1.29 Å) is significantly shorter than
that between the nitrogen atom and the benzene carbon (1.41 Å).
A similar situation exists in compound 3b, in which the corre-
sponding bond lengths are 1.35 and 1.44 Å. Obviously, the
shortening of the C–N bond lengths as compared to that of the
azacalix[4]arene 57j arises from the conjugation of the dinitroben-
zene rings with the bridging nitrogen atoms.
In the solid state, there are intramolecular hydrogen bonding
interactions between one of the oxygen atoms of nitro group at
the ortho position and the N–H proton (OÁÁÁH–N hydrogen bond-
ing), which are shown in Figure 4. The OÁÁÁH–N distances ranging
from 1.86 to 2.12 Å are much shorter than the overall mean
OÁÁÁH–N hydrogen bond length (2.30 Å), which was retrieved from
the Cambridge Structural Database.13 Thus, the 1,3-alternate con-
formation is considered to be stabilized by the hydrogen bonding
interactions. In the 1H NMR spectra, the low field chemical shifts
(9.62 ppm for 3a, 9.64 ppm for 3b) of the N–H protons as compared
to the azacalix[4]arene 5 (5.58 ppm)7j are observed, which indicate
the presence of the OÁÁÁH–N hydrogen bondings in solution.
The 1,3-alternate conformation of the azacalix[4]arene bearing
four methoxy groups at the intra-annular positions is inflexible
Bn
Bn
Bn
Bn
O2N
N
N
N
N
NO2
NO2
t-Bu
t-Bu
O2N
HN
i-Pr
NH
i-Pr
t
-Bu
3b
4
Figure 3. Azacalix[4]arenes possessing diastereotopic protons used for tempera-
ture-dependent NMR experiments.
positions adopt the sp2 configuration. These structural features
demonstrate that the dinitrobenzene rings conjugate with the
bridging nitrogen atoms. This is further corroborated by compari-
son of the bond lengths between the nitrogen atom and its
connecting aromatic carbons, which are shown in Figure 5. In
Figure 4. X-ray crystal structure of azacalix[4]arene (a) 3a and (b) 3b with thermal ellipsoids drawn at the 50% probability level. Atom coloring: O, red; N, blue; C and H,
white. The dotted lines show the intramolecular hydrogen bondings between the N-H proton and one of the oxygen atoms of nitro group. Solvent molecules are omitted for
clarity.
t-Bu
1.41 Å
1.29 Å
1.44 Å
1.35 Å
i-Pr
HN
i-Pr
NH
1.41 Å
HN
HN
NH
NH
O2N
O2N
NO2
NO2
NH
NH
NO2
NO2
t-Bu
t-Bu
O2N
HN
O2N
HN
HN
i-Pr
NH
i-Pr
t-Bu
3a
3b
Figure 5. The averaged C-N bond lengths of azacalix[4]arenes.
5

H. Konishi et al. / Tetrahedron Letters 50 (2009) 620–623
623
in solution.14 This is because its small annulus prevents the pas-
sage of the methoxy groups. On the other hand, in the present case,
the nitro groups at the extra-annular position play an important
role in producing a more rigid macrocyclic framework. There are
three reasons which may explain the effect of the nitro group on
the conformational inflexibility. The first is the conjugation be-
tween the dinitrobenzene rings and the bridging nitrogen atoms,
by which the bridging CN bonds are considerably shortened. The
second is the intramolecular hydrogen bondings between the N–
H proton and one of the oxygen atoms of nitro group at the ortho
position. This interaction is expected to reduce the mobility of
the 1,3-dinitrobenzene rings. The third is the steric hindrance be-
tween the nitro groups and the neighboring isopropyl substituents.
The bulky alkyl groups may destabilize the transition state of the
ring inversion, thus increasing the macrocyclic inversion barrier.
For all these reasons, the conformational flexibility of 3b is signif-
icantly diminished when compared to 4.
In summary, we have found that the tetranitroazacalix[4]arenes
can be synthesized by facile nucleophilic aromatic substitution,
and that the dinitrobenzene units strongly affect the conforma-
tional properties of the azacalix[4]arenes both in the solid state
and in solution. Further investigations are planned to provide addi-
tional information with regard to the effect of the nitro groups on
the conformational properties of the heteroatom-bridged calix-
arenes.
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9. 14,16,54,56-Tetranitro-2,4,6,8-tetraaza-1,3,5,7(1,3)-tetrabenzenacyclooctaphane
(3a): A mixture of 1 (5.0 mmol, 1.02 g), 2a (5.0 mmol, 0.54 g), and K2CO3
(10 mmol, 1.38 g) in DMF (25 ml) was stirred at 100 °C for 2 h under Ar. To this
solution were added water and methanol, and the crude product that
precipitated was collected by suction and washed with methanol. The
insoluble material was recrystallized from DMSO to produce the pure
azacalix[4]arene 3a (0.66 g, 49%). Mp 270 °C (dec.), 400 MHz 1H NMR
(DMSO-d6, 50 °C) d 5.48 (s, 2H), 7.14 (m, 4H), 7.15 (m, 2H), 7.47 (t, 2H,
J = 8.0 Hz), 9.03 (s, 2H), 9.62 (s, 4H, NH), 125 MHz 13C NMR (DMSO-d6, 50 °C) d
96.0, 124.7, 125.3, 125.7, 127.2, 131.0, 138.7, 147.1.
10. 14,16,54,56-Tetraisopropyl-34,36,74,76-tetranitro-2,4,6,8-tetraaza-1,3,5,7(1,3)-
tetrabenzenacyclooctaphane (3b): The reaction of
analogously conducted as above for 4 h. Recrystallization of the
crude product from toluene/ethyl acetate produced the
azacalix[4]arene 3b (42 mg, 12%). Mp 320 °C (dec.), 400 MHz 1H
NMR (CDCl3, 30 °C) 1.07 (d, 12H, J = 7.0 Hz, –CH(CH3)2), 1.18 (d,
1
and 2b was
d
Supplementary data
12H, J = 7.0 Hz, –CH(CH3)2), 2.97 (sept, 4H, J = 7.0 Hz, –CH(CH3)2), 5.30
(s, 2H, Hin), 6.95 (s, 2H, H), 7.36 (s, 2H, H), 9.34 (s, 2H, Hout), 9.64 (s,
4H, NH), 125 MHz 13C NMR (DMSO-d6, 130 °C)
d 22.0 (CH3), 22.5
Supplementary data (Experimental procedures for the prepara-
tion of compound 2b) associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2008.11.095.
(CH3), 27.5 (CH), 94.5, 124.2, 124.4, 127.7, 128.3, 133.7, 146.0, 147.9.
11. Budzelaar, P. H. M. gNMR Ver. 5.1. http://home.cc.umanitoba.ca/~budzelaa/.
12. X-ray crystal structure analysis: The X-ray data were collected at 173 K using a
Rigaku R-AXIS RAPID-S imaging plate area detector with graphite
monochromated Mo K
a (k = 0.7107 Å) radiation using the x scan mode. The
References and notes
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ꢀ
acetone: C36H40N8O8Á(C3H6O)2, M = 828.92, triclinic, space group P1 (No. 2),
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c
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