Lewis acid-catalyzed synthesis of dodecamethoxycalix[4]arene from 1,3,5-trimethoxybenzene and its conformational behavior and host-guest property

Article abstract of DOI:10.1016/j.tet.2009.10.059

Lewis acid-catalyzed condensation of 1,2- and 1,3-dimethoxybenzenes with paraformaldehyde afforded an ortho-bridged cyclic trimer (1) and a meta-bridged cyclic tetramer (2), respectively. Furthermore, condensation of 1,3,5-trimethoxybenzene with paraformaldehyde in the presence of Lewis acid catalyst successfully rendered the first dodecamethoxy-substituted calix[4]arene (3) with high yield. From X-ray crystallography, it was found that 3 formed the partial cone conformation. The conformational behavior of 3 in the solution was investigated by variable temperature ¹H NMR measurements. The partial cone structure observed in the solid state was retained in the solution at low temperatures. Furthermore, because of the slow conformational exchanges of 3 on the NMR time scale, bimodal conformational exchanges were found. The host-guest property of 3 with the electron accepting guest, tetracyanoethylene (TCNE) was examined by UV-Vis measurements, and the ability to associate with the 3-TCNE complex was three times higher than that of the 2-TCNE complex. The observation is due to the superior electron donating property and slow conformational exchanges of 3 compared with those of analogous 2.

Full text of DOI:10.1016/j.tet.2009.10.059

Tetrahedron 65 (2009) 10644–10649
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Lewis acid-catalyzed synthesis of dodecamethoxycalix[4]arene
from 1,3,5-trimethoxybenzene and its conformational behavior
and host–guest property
*
Tomoki Ogoshi , Keisuke Kitajima, Kenji Umeda, Sachi Hiramitsu, Suguru Kanai,
*
Shuhei Fujinami, Tada-aki Yamagishi, Yoshiaki Nakamoto
Graduate School of Natural Science and Technology, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Lewis acid-catalyzed condensation of 1,2- and 1,3-dimethoxybenzenes with paraformaldehyde afforded
an ortho-bridged cyclic trimer (1) and a meta-bridged cyclic tetramer (2), respectively. Furthermore,
condensation of 1,3,5-trimethoxybenzene with paraformaldehyde in the presence of Lewis acid catalyst
successfully rendered the first dodecamethoxy-substituted calix[4]arene (3) with high yield. From X-ray
crystallography, it was found that 3 formed the partial cone conformation. The conformational behavior
of 3 in the solution was investigated by variable temperature 1H NMR measurements. The partial cone
structure observed in the solid state was retained in the solution at low temperatures. Furthermore,
because of the slow conformational exchanges of 3 on the NMR time scale, bimodal conformational
exchanges were found. The host–guest property of 3 with the electron accepting guest, tetracyano-
ethylene (TCNE) was examined by UV–Vis measurements, and the ability to associate with the 3-TCNE
complex was three times higher than that of the 2-TCNE complex. The observation is due to the superior
electron donating property and slow conformational exchanges of 3 compared with those of analogous 2.
Ó 2009 Elsevier Ltd. All rights reserved.
Received 20 August 2009
Received in revised form
15 October 2009
Accepted 15 October 2009
Available online 21 October 2009
Keywords:
Conformational behavior
Lewis acid-catalyzed synthesis
Macrocyclic compound
Host–guest property
1. Introduction
condensation should proceed gradually and then react in-
termediately to form stable macrocycles. The use of BF3 O(C2H5)2 in
ꢀ
Calixarenes have attracted substantial attention in recent years.
This can be attributed to their unique structural and conformational
properties allowing their use in the synthesis of supramolecular
architectures.1 For the synthesis of calixarenes, the classical pro-
cedure reported by Gutsche et al. is the base catalyzed condensa-
tion of phenolic compounds. The other synthetic procedure uses
Lewis acid catalysts. Due to the versatility and facility of Lewis acids
in organic synthesis, the application of Lewis acids for the synthesis
of calixarenes has received considerable attention.2–6 Successful
Lewis acid-catalyzed synthesis of new calixarenes has been dem-
onstrated by Iwanek et al.2 By using the Lewis acid catalyst, novel C4
symmetric calix[4]resorcinarene has been reported by Mocerino
et al.3 Among these studies, we have recently synthesized new
cyclic compounds of pillar[5]arenes by condensation of 1,4-dime-
thoxybenzene with the Lewis acid catalyst.5 Pillar[5]arenes form
a unique symmetrical pillar architecture, that is, able to stably
capture guest molecules. For the synthesis of pillar[5]arene, boron
calixarenes or calixresorcinarenes synthesis has also been exam-
6
ꢀ
ined. Therefore, in this study, by using BF3 O(C2H5)2 as Lewis acid,
we investigated the condensation of various di- and trimethoxy-
benzenes with paraformaldehyde. The Lewis acid-catalyzed con-
densation of 1,2- and 1,3-dimethoxybenzene produced
cyclotriveratrylene (Fig. 1, 1) and octamethoxycalix[4]resorcinarene
(Fig. 1, 2), respectively. Furthermore, by the Lewis acid condensa-
tion of 1,3,5-trimethoxybenzene, the novel meta-bridged
macrocyclic tetramer of 4,6,10,12,16,18,22,24,25,26,27,28-dodeca-
methoxycalix[4]arene (Fig. 1, 3) was obtained. Due to the bulky
dodeca-methoxy substituents on the upper and lower rims, 3
should show unique conformational characteristics and host–guest
properties compared to typical calixarenes and calixresorcinarenes.
ꢀ
trifluoride diethyl etherate [BF3 O(C2H5)2] was an appropriate
ꢀ
Lewis acid. Due to the weak acidity of BF3 O(C2H5)2, the
* Corresponding authors. Tel.: þ81 76 234 4775; fax: þ81 76 234 4800.
E-mail address: ogoshi@t.kanazawa-u.ac.jp (T. Ogoshi).
Figure 1. Macrocycles prepared from Lewis acid-catalyzed condensation of methoxy-
benzenes with paraformaldehyde.
0040-4020/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2009.10.059

T. Ogoshi et al. / Tetrahedron 65 (2009) 10644–10649
10645
In this research, we revealed conformational behavior and the
2.2. Lewis acid-catalyzed condensation of
trimethoxybenzenes
host–guest property of 3 compared to those of 2, which had
a similar structure except for the lack of tetra-methoxy substituents
on the lower rim.
Lewis acid-catalyzed condensation of trimethoxybenzenes with
paraformaldehyde was investigated using GPC measurements
ꢀ
(Fig. 2c–e). BF3 O(C2H5)2 was used as a Lewis acid. In 1,2,3-trime-
2. Results and discussion
thoxybenzene, a broad peak at the elution volume of 17–20 mL was
observed (Fig. 2c), strongly suggesting the formation of polymer. In
the case of 1,2,4-trimethoxybenzene, multiple peaks at an elution
volume of 20–23 mL were found (Fig. 2d), indicating the prepara-
tion of the mixture of oligomers. On the other hand, by the con-
densation of 1,3,5-trimethoxybenzene, a sharp peak at 21.8 mL was
mainly seen (Fig. 2e). The observation indicates that a particular
oligomer is selectively obtained by using 1,3,5-trimethoxybenzene.
The product was successfully re-crystallized from acetone. From
X-ray crystallography (Fig. 3), it was found that the crystalline was
the cyclic tetramer of 1,3,5-trimethoxybenzene (3). 1,3,5-Trime-
thoxybenzene units were connected by methylene bridges at meta-
positions. The structure of 3 was the partial cone conformation
(three benzene rings ‘up’ and one ‘down’). From these observations,
it was confirmed that cyclic tetramer (3) was obtained by con-
densation of 1,3,5-trimethoxybenzene and paraformaldehyde
using Lewis acid catalyst. On the other hand, the reaction of 1,2,3-
and 1,2,4-trimethoxybenzenes with Lewis acid catalyst produced
2.1. Lewis acid-catalyzed condensation
of dimethoxybenzenes
GPC elution traces after Lewis acid-catalyzed condensation of
dimethoxybenzenes with paraformaldehyde are shown in
Figure 2a and b. As a Lewis acid, we employed boron trifluoride
ꢀ
diethyl etherate [BF3 O(C2H5)2]. In both 1,2- and 1,3-dimethoxy-
benzenes, the sharp peaks were observed at 22.5 and 22.0 mL, re-
spectively, indicating that particular oligomers were selectively
obtained. By washing with acetone, ortho-bridged cyclic trimer,
cyclotriveratrylene (1) and meta-bridged cyclic tetramer, octame-
thoxycalix[4]resorcinarene (2) were successfully isolated. These
macrocycles were characterized completely by 1H NMR and FAB-
Mass measurements. While these macrocycles are known com-
pounds, this procedure is the facile and improved method to obtain
high yield of macrocycles 1 (54.8%) and 2 (84.8%) compared with
the other synthetic methods.7
Figure 2. GPC traces of the obtained products from (a) 1,2-dimethoxybenzene, (b) 1,3-dimethoxybenzene, (c) 1,2,3-trimethoxybenzene, (d) 1,2,4-trimethoxybenzene and (e) 1,3,5-
trimethoxybenzene.

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T. Ogoshi et al. / Tetrahedron 65 (2009) 10644–10649
polymer and oligomers, respectively. The difference in reactivity
was depended upon the substituent position of the methoxy
groups. Since the 1,3,5-trimethoxybenzne has 3 equiv reactive po-
sitions on the aromatic ring, the reaction proceeds cleanly. Two
isomers of 1,2,3- and 1,2,4-trimethoxybenzenes do not have equal
reaction sites: therefore, these two isomers should produce
a complex mixture of polymer and oligomers.
To examine the mechanism for the formation of the macrocyclic
3, the reactions with the other Lewis acids were carried out. By using
the other Lewis acids such as aluminum (III) chloride, iron (III)
chloride, and tin (IV) chloride, polymers formed and 3 was not
obtained selectively (Supplementary data). Since the acidity of these
ꢀ
Lewis acid is stronger than that of BF3 O(C2H5)2, the reactions took
place effectively and thus formation of the polymers should occur.
ꢀ
When the weak BF3 O(C2H5)2 catalyst was used for the reaction, the
condensation should proceed gradually and then the intermediate
should form thermally stable macrocyclic tetramer of 3.
2.3. Conformational behavior of 3
Figure 3. The crystal structure of 3.
To examine the conformational behavior of the 3 in the solution,
measurements of the variable temperature 1H NMR were carried
out (Fig. 4a).
Figure 4. (a) Variable temperature 1H NMR spectra of 3 in CDCl3. (b) The arrangement of the benzene proton peaks at low temperature. (c) First exchange process of 3. The peak at
5.23 ppm in Figure 4 (a) at ꢁ20 ꢂC should correspond to toluene formed the complex with 3 or noise from the device.

T. Ogoshi et al. / Tetrahedron 65 (2009) 10644–10649
10647
In CDCl3 at 20 ꢂC, proton resonances from the 3 were largely
broadened. At low temperatures, these peaks became sharp and
split. The result indicates that conformational exchanges of the 3
were slow on the NMR time scale at low temperatures. In the 1H
methoxy groups on the lower rim of 3 extremely lowered the
conformational exchanges of 3 considerably.
2.4. Host–guest property of 3
NMR spectrum of 3 in CDCl3 at ꢁ40 ꢂC, three singlets [
¼5.92 (2H),
d
6.29 (1H), 6.35 (1H)] were found, indicating that the partial cone
conformation adopted in the solid state should be retained in the
solution at the temperature. (The assignment of these peaks is
shown in Fig. 4b). At 12 ꢂC, the first coalescence of both the singlets
Since 3 is composed of an electron donor of 1,3,5-trimethoxy-
benzene, the formation of the host–guest complex with a strong
electron acceptor of tetracyanoethylene (TCNE) was examined. We
measured UV–Vis absorption spectra of TCNE upon the addition of
3 (Fig. 6).
[
d
¼6.29 (proton a) and 6.35 (proton b)] was observed. The co-
alescence behavior derived from the equilibrium between the two
equivalent species of the partial cone conformation was attributed
to the flipping of face-to-face ‘flat’ and ‘down’ benzene rings (first
exchange process, Fig. 4c). From the coalescence temperature,
a free-energy barrier of 60.3 kJ/mol was calculated for the exchange
process. As the temperature increased, the second coalescence of
When 3 was added to the TCNE solution, a new peak around
718 nm was observed. Since there were no absorption peaks around
this region in both TCNE and 3, it was assumed that the peak was
derived from the charge–transfer complex between TCNE and 3.
The stoichiometry of the host–guest complex between 3 and TCNE
was determined by Job plots8 (Fig. 7, left). The peak top was ob-
served in molar fraction: Xguest¼0.5, indicating the formation of
a ratio of 1:1 host–guest complex. To determine the association
constant of the complex, titration experiments were conducted by
adding 3 to the TCNE solution. From the Benesi–Hildebrand treat-
ment (Fig. 7, right), the association constant of the 3-TCNE complex
was found to be 60 Mꢁ1. As a control experiment, we measured the
association constant between the 1,3,5-trimethoxybenzene
the singlets [
d
¼6.29 (proton a0) and 5.89 (proton c0)] was found. The
second coalescence behavior corresponded to the conformational
exchange from a partial cone to cone. From the second coalescence
temperature, a free-energy barrier for the conformational inversion
was determined to be 62.9 kJ/mol. In the case of 3 in toluene-d8, the
first and second coalescence temperatures were found at 24 ꢂC and
66 ꢂC, respectively (Fig. 5).
Figure 5. Variable temperature 1H NMR spectra of 3 in toluene-d8.
From the coalescence temperatures, free-energy barriers for the
first and second exchange processes in toluene-d8 were determined
to be 61.2 kJ/mol and 66.1 kJ/mol, respectively, which were higher
than those in CDCl3. These data indicate that conformational in-
version of 3 in toluene-d8 was slower than that in CDCl3. In the case
of 2, which had a similar structure to 3 except for the lack of the
methoxy groups in the lower rim, the proton peaks of 2 in toluene-
d8 did not change even at a low temperature (Supplementary data).
The foundation indicates that conformational exchanges of 2 were
fast on the NMR time scale at low temperatures. From these ob-
servations, it was found that the steric hindrance of the substituted
(monomer) and TCNE. The association constant of the 1,3,5-tri-
methoxybenzene–TCNE complex determined by the Benesi–Hil-
debrand method was 6 Mꢁ1 (Supplementary data), indicating that
the association ability of 3 with TCNE was almost 10 times higher
than that of 1,3,5-trimethoxybenzene. While various charge–
transfer complexes between TCNE and electron donors have been
reported, their general association constants are not so high (under
20 Mꢁ1).9 Accordingly, the complexation ability of 3 with TCNE is
superior to usual donor molecules. The macrocyclic structure of 3
should enhance the association ability with TCNE. In the case of 2,
the association constant of the 2-TCNE complex calculated by the

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T. Ogoshi et al. / Tetrahedron 65 (2009) 10644–10649
temperatures. As the temperature was raised, bimodal conforma-
tional exchanges of 3 were found from variable temperature 1H
NMR measurements. Furthermore, due to the superior electron
donating property and slow conformational exchanges of 3, 3
formed a strong complex with the electron accepting guest of TCNE.
4. Experimental
4.1. Materials
All solvents and reagents were used as supplied.
4.2. Measurements
The 1H NMR spectra were recorded at 270 MHz and 400 MHz
and 13C NMR spectra were recorded at 67.5 MHz and 100 MHz with
a JEOL-JNM EX270 and EL400 spectrometers, respectively. The FTIR
spectra were obtained using a JASCO FT-IR460 plus infrared spec-
trometer. Gel permeation chromatography (GPC) analysis was
carried out on Shodex GPC LF804 by using THF as an eluent at 25 ꢂC
at a flow rate of 1 mL minꢁ1 after calibration with the standard
polystyrene samples. The absorption spectra were recorded with
a JASCO V-630 spectrophotometer at room temperature.
Figure 6. Absorption spectra of TCNE (3.33 mM) in chloroform upon addition of 3. The
concentration of 3 varied from 0 to 18.3 mM.
Figure 7. Job (left) and Benesi–Hildebrand (right) plots for 3 and TCNE.
Benesi–Hildebrand plots was 20 Mꢁ1 (Supplementary data). The
association constant of 3-TCNE complex was three times higher
than that of the 2-TCNE complex. The observation is due to the
superior electron donating property and slow conformational ex-
changes of 3 compared with those of 2. 3 was perceived to be
a useful host for the electron accepting guests.
4.3. Cyclotriveratrylene (1)
Paraformaldehyde (0.31 g, 10 mmol) was added under nitrogen
atmosphere to
a solution of 1,2-trimethoxybenzene (1.38 g,
10 mmol) in 1,2-dichloroethane (20 mL). The reaction flask was
capped and nitrogen bubbled through the solution for 30 min.
Following this, boron trifluoride diethyl etherate [BF3 O(C2H5)2,
ꢀ
1.25 mL, 10 mmol] was added to the solution and the mixture was
stirred at 30 ꢂC for 3 h. The solution was poured into methanol and
the resulting precipitate was collected using a filtration. After
washing with acetone, cyclotriveratrylene (1) was obtained (Yield:
3. Conclusion
We investigated Lewis acid-catalyzed condensation of various
methoxybenzenes with paraformaldehyde. In the case of 1,2- and
1,3-dimethoxybenzenes, the macrocycles 1 and 2 were selectively
obtained. By using 1,3,5-trimethoxybenzene as a monomer, the
novel cyclic tetramer of 3 was synthesized at a high yield. From
these results, it was found that the Lewis acid-catalyzed conden-
sation of methoxybenzenes was a facile and useful procedure at
obtaining macrocycles. Conformational exchanges of the new 3
were extremely slow on the NMR time scale at low temperatures
compared to that of the analogous 2. Due to dodeca-methoxy
substituents of 3, the partial cone conformation observed in the
solid state X-ray structure was retained in the solution at low
54.8%). 1H NMR (CDCl3, 270 MHz, 25 ꢂC, ppm):
d 6.83 (s, 6H, phe-
nyl), 4.80 (d, 3H, methylene), 3.84 (s, 18H, methoxy), 3.58 (d, 3H,
methylene). FAB mass m/z¼450 (Mþ).
4.4. Octamethoxycalix[4]resorcinarene (2)
This compound was prepared from 1,3-dimethoxybenzene
(1.38 g, 10 mmol), paraformaldehyde (0.31 g, 10 mmol) and boron
ꢀ
trifluoride diethyl etherate [BF3 O(C2H5)2, 1.25 mL, 10 mmol] under
the same conditions as those used for the preparation of 1. After

T. Ogoshi et al. / Tetrahedron 65 (2009) 10644–10649
10649
washing with acetone, 2 was obtained (Yield: 84.8%). 1H NMR
(CDCl3, 270 MHz, 25 ꢂC, ppm):
6.41 (s, 4H, phenyl), 6.18 (s, 4H,
with aluminum (III) chloride, iron (III) chloride, and tin (IV) chlo-
ride, variable temperature 1H NMR spectra of 2, absorption spectra
of TCNE with 1,3,5-trimethoxybenzene and 2, Benesi–Hildebrand
plots for determination of association constants of TCNE with 1,3,5-
trimethoxybenzene and 2 are provided. Supplementary data as-
sociated with this article can be found in the online version, at
doi:10.1016/j.tet.2009.10.059.
d
phenyl), 3.77 (s, 24H, methoxy), 3.69 (s, 8H, methylene). FAB mass
m/z¼600 (Mþ).
4.5. 4,6,10,12,16,18,22,24,25,26,27,28-dodecamethoxy-
calix[4]arene (3)
This compound was prepared from 1,3,5-trimethoxybenzene
(1.68 g, 10 mmol), paraformaldehyde (0.31 g, 10 mmol) and boron
trifluoride diethyl etherate [BF3 O(C2H5)2, 1.25 mL, 10 mmol] under
the same conditions as those used for the preparation of 1. The solid
material obtained was re-crystallized from acetone to yield 3 as
a white solid. (Yield: 43.5%). 1H NMR (CDCl3, 400 MHz, 25 ꢂC, ppm):
References and notes
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Acknowledgements
This work was supported by a Grant-in-Aid (KIBAN C-20550120)
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Appendix. Supplementary data
1H and 13C NMR spectra of 3, X-ray crystallographic data in CIF
format, GPC traces of the products from 1,3,5-trimethoxybenzene