Accessing highly electron-rich calix[n]arene (n?=?4 and 8) derivatives from acid-catalyzed condensation of 1,3,5-tripropoxybenzene

Article abstract of DOI:10.1016/j.tetlet.2019.151215

Synthesizing novel electron-rich calix[n]arene derivatives to alter the electronic properties of calixarene-based materials has been one of the long-standing interests. Herein, two new electron-rich calix[n]arenes (n = 4 and 8) with different sizes were s

Full text of DOI:10.1016/j.tetlet.2019.151215

Tetrahedron Letters xxx (xxxx) xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Accessing highly electron-rich calix[n]arene (n = 4 and 8) derivatives
from acid-catalyzed condensation of 1,3,5-tripropoxybenzene
⇑
Denan Wang , Saber Mirzaei, Sergey V. Lindeman, Rajendra Rathore
Department of Chemistry, Marquette University, Milwaukee, WI 53201-1881, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Synthesizing novel electron-rich calix[n]arene derivatives to alter the electronic properties of calixarene-
based materials has been one of the long-standing interests. Herein, two new electron-rich calix[n]arenes
Received 22 August 2019
Revised 25 September 2019
Accepted 26 September 2019
Available online xxxx
(
n = 4 and 8) with different sizes were synthesized by acid-catalyzed condensation reaction of 1,3,5-
1
13
tripropoxybenzene and paraformaldehyde. Both derivatives were fully characterized with H and
C
NMR, mass spectrometry and X-ray crystallography. Furthermore, their electrochemical properties and
oxidized product (cation radicals) have also been investigated.
Ó 2019 Elsevier Ltd. All rights reserved.
Keywords:
Calix[n]arene
Acid-catalyzed synthesis
Cation radical
Electrochemical property
Introduction
among them must be small (i.e., in the range of 1–3 kcal/mol)
[
6]. Higher alkyl ethers of p-tert-butylcalix[4]arene (e.g., propyl
Calixarenes have been widely used as a synthetic framework in
many aspects including functional materials, supramolecular
chemistry and molecular sensors [1]. Base-catalyzed condensation
of 4-tert-butylphenol/formaldehyde allows the preparation of calix
ethers, 2) are conformationally rigid and they do not interconvert
due to prevention of flipping the aromatic rings (Fig. 1A). These dif-
ferent rigid conformers (i.e., 2paco, 2cone, 21,3-alt, and 21,2-alt) of pro-
pyl ethers of p-tert-butylcalix[4]arene can be synthesized and
isolated, albeit via multistep syntheses [2a,b].
[
n]arenes with varied ring sizes (where n = 4, 5, 6, 7 and 8) [2].
Thermodynamically stable calix[4]arene can be easily isolated
and it remains the most celebrated and explored member of this
macrocyclic arene family [3]. However, high yield synthesis, isola-
tion and purification of higher homologues remained a challenge
for synthetic chemists [2a,b]. Beyond the difficulty of synthesis
and purification, modulation of the electronic properties is another
challenge for calixarene materials. In this regard, the development
of new electron-rich calixarenes can potentially improve the bind-
ing affinity towards electron acceptors in host-guest chemistry;
increase the stability of its oxidized product (cation radical species)
and promote the development and implementation as molecular
sensors [4]. Therefore, our group has been interested in synthesiz-
ing new types of calixarene derivatives with highly electron-rich
property.
Recently, Ogoshi and coworkers reported that an electron-rich
calix[4]arene derivative 3 can be readily accessed via a simple
acid-catalyzed condensation of 1,3,5-trimethoxybenzene and
paraformaldehyde in excellent yield (Fig. 1B) [7]. Interestingly,
however, this condensation reaction produces exclusively only
one conformation (3paco) as confirmed by NMR spectroscopy and
X-ray crystallography [7]. Our group further studied the electronic
properties of this particular calixarene and the application of its
cation radical as a NO sensor. The results revealed that the cation
radical of 3paco have an unprecedented binding affinity for nitric
oxide (k > 2 Â 10 ) [8]. As the energy differences amongst the four
conformers of 3 are not prohibitively large, their synthesis should
be feasible as rigid conformers by employing 1,3,5-tripropoxyben-
zene as the reactant instead of 1,3,5-trimethoxybenzene (Fig. 1B).
On the other hand, the larger steric of propoxy groups have possi-
bility to produce larger sizes of highly electron rich calix[n]arene
(n > 4) derivatives.
1
2
It has been shown that methyl ether of p-tert-butylcalix[4]
arene (1) is conformationally mobile at ambient temperatures.
Modest cooling (À30 °C) of a solution of methyl ether of p-tert-
butylcalix[4]arene (1) in CH
2
Cl
2
shows the presence of four distinct
Accordingly, herein we report that the reaction of readily-avail-
conformations by NMR spectroscopy [5]. The presence of all four
conformers in the same solution suggests that energy differences
able 1,3,5-tripropoxybenzene with paraformaldehyde in CH
2
Cl
2
in
3
-
the presence of an acid as the catalyst (such as BF or CH
3
ÁOEt
2
3
SO
H) produces rigid conformers of calix[4]arene (41,3-alt) together
with decent amounts (>45% yield) of calix[8]arene (5). We will also
⇑
Corresponding author.
https://doi.org/10.1016/j.tetlet.2019.151215
040-4039/Ó 2019 Elsevier Ltd. All rights reserved.
0
Please cite this article as: D. Wang, S. Mirzaei, S. V. Lindeman et al., Accessing highly electron-rich calix[n]arene (n = 4 and 8) derivatives from acid-cat-
alyzed condensation of 1,3,5-tripropoxybenzene, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2019.151215

2
D. Wang et al. / Tetrahedron Letters xxx (xxxx) xxx
Scheme 1. Synthesis of highly electron rich calix[n]arene (n = 4 or 8) derivatives.
effects of several factors including the influence of different cata-
lyst, the amount of catalyst, reaction solvent, temperature and
reaction time.
First, in order to investigate the influence of catalyst, both Lewis
(
entries 1–3) and Brønsted acids (entries 4–8) have been screened.
As listed in Table 1, BF O showed the best results with the yield
ÁEt
2% of calix[4]arene and 45% of calix[8]arene among the investi-
gated Lewis acids. The total yield is less than 10% when FeCl or
AlCl was applied as catalyst. Entries 4 and 5 illustrated that the
weak Brønsted acids (CH CO H and H PO ) have very poor or no
3
2
1
3
3
3
2
3
4
Fig. 1. A) Four interconvertible conformations of 4-tert-butylcalix[4]arene methyl
ethers (1) and their rigidification in the corresponding propyl ethers (2). B) The
catalytic effect toward this transformation. To the contrary of weak
Brønsted acids, the stronger ones (entries 6–8) showed generally
better catalytic effects. Among them, methanesulfonic acid
preparation of electron-rich calix[4]arene derivative 3paco
.
(
3
MeSO H, entry 8) showed the best result with 21% of calix[4]arene
and 41% of calix[8]arene. Although, the resulting product is domi-
nated by calix[8]arene, methanesulfonic acid has the best catalytic
effect to produce the smaller macrocycle molecule (calix[4]arene).
This acid showed promising results for synthesis of several differ-
ent macrocyclic arenes (e.g., cyclotetraveratrylene and pillarenes)
show that the composition of 41,3-alt and higher macrocycles calix
8]arene (5) can, in part, be controlled by choice of acid/amount
[
of acid catalyst. The structures of these, hitherto unknown,
highly-electron-rich calixarene derivative were established by
NMR spectroscopy, mass spectroscopy and X-ray crystallography
and further evaluated by DFT calculations. Also, we have studied
their electrochemical and spectral properties of one-electron oxi-
dized products (cation radical). The details of these findings are
described herein.
[
10]. In order to selectively make calix[4]arene, methanesulfonic
acid was applied as a catalyst in the subsequent condition opti-
mization reactions.
Entries 1–4 in Table 2 revealed that the amount of catalyst at
2
0% and 50% have comparable product composition and are
slightly better than 5% or 100% of catalyst. For solvent screening
entries 5–7), dichloromethane and chloroform indicated better
(
Results and discussion
results, the reaction did not happen in the solvents such as THF
and toluene; while produced insoluble by-products (may be higher
oligomer or polymer molecules) in more polar solvent acetonitrile.
Entries 8–9 showed that the reaction works better at room
Synthesis
In order to synthesize highly electron-rich calixarene deriva-
tives, we chose 1,3,5-tripropoxybenzene with three more bulky
and more electron-rich propoxy groups as a starting material.
Firstly, the starting material (1,3,5-tripropoxybenzene) was syn-
thesized according to literature with the alkylation of phlorogluci-
nol (Scheme 1) [9]. The calixarene derivative was then prepared by
the acid catalyzed condensation reaction of 1,3,5-tripropoxyben-
zene with paraformaldehyde according to the reported method
by Ogoshi and coworkers [7]. Interestingly, we found that this
reaction resulted in a more complicated product in comparison
to the methoxy substituted analog which leads to the one domi-
nant product (3paco). After the purification with column chro-
matography, two new compounds, calix[4]arene (41,3-alter) and
calix[8]arene (5) were obtained with the yield 12% and 45%,
respectively. In order to alter the product distribution and synthe-
size calix[4]arene more efficiently, we systematically studied the
Table 1
The catalyst screen in the synthesis of calix[n]arene (n = 4 or 8).
Entry
Cat.
%Yield of OPr-calix[n]arene
n = 4
n = 8
1
2
3
4
5
6
7
8
9
BF
FeCl
AlCl
CH COOH
PO
SO
CO
3
ÁEt
2
O
12
2
2
0
1
4
7
21
0
45
3
6
0
3
12
20
41
0
3
3
3
H
H
CF
3
2
4
4
3
2
H
MeSO
none
3
H
Please cite this article as: D. Wang, S. Mirzaei, S. V. Lindeman et al., Accessing highly electron-rich calix[n]arene (n = 4 and 8) derivatives from acid-cat-
alyzed condensation of 1,3,5-tripropoxybenzene, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2019.151215

D. Wang et al. / Tetrahedron Letters xxx (xxxx) xxx
3
Table 2
The condition optimization in the synthesis of calix[n]arene (n = 4 or 8).
Entry
Amount of catalyst
Solvent
Temp
% Yield of OPr-calix[n]arene
n = 4
n = 8
1
2
3
4
5
6
7
8
9
5%
CH
CH
CH
CH
CHCl
MeCN
THF
2
2
2
2
Cl
Cl
Cl
Cl
2
2
2
2
20
20
20
20
20
20
20
0
10
21
20
12
7
0
0
0
29
41
43
34
21
4
20%
50%
100%
20%
20%
20%
20%
20%
20%
20%
3
0
0
CH
2
CH
2
CH
2
CH
2
Cl
2
Cl
2
Cl
2
Cl
2
35
20
20
2
5
8
38
31
a
a
10
1
b
b
1
10
Note: areaction time 2 h; breaction time 6 h.
temperature. However, decreasing the temperature (entry 8) halts
the reaction and the starting materials remain untouched. To the
contrary of low temperatures, a large amount of white insoluble
by-product is formed at high temperature, which greatly reduces
the reaction yield. The reaction time was also screened, and 4 h
has given the better yield (entries 10–11). In summary, both Lewis
(
BF
3
ÁEt
2 3
O) and Brønsted (MeSO H) acids produce calix[n]arenes
(n = 4 and 8) with high total yield. Additionally, methanesulfonic
acid as catalyst can slightly alter the product distribution and
increase the yield of calix[4]arene to 21%.
Both calix[n]arenes (n = 4 and 8) can be recrystallized from a
mixed solvent of dichloromethane and acetonitrile to obtain high
quality single crystals for X-ray diffraction experiments. The
results of X-ray diffraction revealed that this novel calix[4]arene
hold with 1,3-alter conformer noted as 41,3-alter. This result is dif-
ferent from the structure of 3paco, which contain methoxy instead
of propoxy groups. To better comprehend this change, we utilized
the DFT level calculation to simulate the energy profile of four pos-
sible conformations (4paco, 4cone, 41,2-alter and 41,3-alter at Fig. S4). It
was found that 41,3-alter has the lowest energy and is around
6
kcal/mol lower than the second stable 4paco. This is in agreement
to our experimental results from both NMR and solid-state struc-
ture (X-ray).
Crystal structures
In the structure of 41,3-alter, there are two sets of facial interacted
aromatic rings, and the dihedral angles are 47.7° and 56.4°, respec-
tively (Fig. 2). Such a large dihedral angle is mainly due to the steric
effect of propoxy groups. The distance between two sets of facial
aromatic rings is ꢀ6.42 Å, in contrast to ꢀ5.60 Å in 21,3-alter; there-
fore, only weak p-p interaction can be observed in the structure of
4. This orientation also shortens the distance between inner oxy-
gen atoms on the opposite rings with ꢀ 3.85 Å in average. Fig. 2C
Fig. 2. Side view (A), top view (B), and packing (C) of the crystal structures of
1,3-alter). Hydrogens and solvent molecules were omitted for clarity; oxygen atoms
are red; phenyl rings are gold; alkyl groups are bright gray and bridge carbon are
dark green. (For interpretation of the references to color in this figure legend, the
reader is referred to the web version of this article.)
shows the packing of crystal structures which indicates weak
(4
1
intermolecular CH. . .
p
interactions. Variable temperature
H
NMR experiment were performed on both calix[4]arene (4) and
calix[8]arene (5) in the temperature range from À40 to 55 °C
(Figs. S1 and S2). In the case of calix[4]arene (4), the aromatic
region only display one sharp signal (6.14 ppm) in the whole tem-
perature range and the bridged-methylene proton showed at
ture (À20 and À40 °C) and two of propoxy signal merged at room
temperature. Which indicate that the molecule is interconversion
of conformers at room temperature and can be frozen at À20 °C
or below. In the solid state, the octameric 1,3,5-tripropoxybenzene
forms a molecule resembling through the bonding of methylene
groups as bridge, which looks like a garland structure (Fig. 3A).
The bridging methylene groups are alternately oriented up and
down relative to its mean plane (Fig. 3B). All internal (positioned
3
.90 ppm as a sharp peak, while the peaks raised from two kinds
of propoxy group slightly shifted to lower field and became broad
at lower temperature. The VT-NMR result revealed that 41,3-alt is
not conformationally flexible in the range from À40 to 55 °C. The
variable temperature NMR characterization data of 5 revealed only
one sharp signal in the aromatic region, however, there are three
kinds of propoxy group signal were observed at a lower tempera-
Please cite this article as: D. Wang, S. Mirzaei, S. V. Lindeman et al., Accessing highly electron-rich calix[n]arene (n = 4 and 8) derivatives from acid-cat-
alyzed condensation of 1,3,5-tripropoxybenzene, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2019.151215

4
D. Wang et al. / Tetrahedron Letters xxx (xxxx) xxx
Fig. 4. Cyclic voltammograms (CVs, solid lines, blue color for 41,3-alter, red for 5) and
overlaid square waves (SW, dashed lines) of 2 mM 41,3-alter (top) and 5 (bottom) in
À1
2
CH Cl
2
(0.1 M nBu
4
NPF
6
) at a scan rate of 100 mVs . (For interpretation of the
references to color in this figure legend, the reader is referred to the web version of
this article.)
for the effects of the facially-interacted aromatic rings on the sta-
bilization of the cation-radical species.
Cation radical generation
We further generated the cation radical of 41,3-alter in solution
+Å
by using THEO SbCl
6
as one electron oxidant (THEO = 1,2,3,4,5,
Fig. 3. Top view (A), and side view (B) of the crystal structures of new calix[8]arene
6,7,8-octahydro-9,10-dimethoxy-1,4,5,8-dime-thano-anthracene,
(5). Hydrogens and solvent molecules were omitted for clarity; oxygen atoms are
+
À1
À1
E
[
red = 0.67 V vs. Fc/Fc , kmax = 518 nm,
12]. The Fig. 5 illustrates the electronic spectra obtained upon
an sub-stoichiometric addition of a solution of neutral 41,3-alter in
e
max = 7300 cm mol
)
red; phenyl rings are gold; alkyl groups are bright gray and bridge carbon are dark
green; and all alkyl groups are omitted in the side view mode. (For interpretation of
the references to color in this figure legend, the reader is referred to the web version
of this article.)
+Å
CH
2 2 6
Cl to a solution of THEO SbCl as oxidant. After the addition
+Å
of 1 equiv of neutral 41,3-alter, the oxidant THEO SbCl
6
was com-
+Å
pletely converted to 41,3-alter which was determined by a quantita-
tive deconvolution of the component spectra of various species
present at each titration point (Fig. 5). It was found that a broad
between methylene bridges) propoxy groups are oriented inwards
+Å
(
also alternating up and down) and ordered.
near-infrared absorption peak on 4
,3-alter
spectrum, started from
1
8
00 nm and all the way up to 3000 nm. This charge transfer band
is mainly derived from two pairs of facial interacted aromatic rings.
Electrochemical property
+Å
-1
À1
The extinction coefficient of 41,3-alter (1250 cm mol ) is slightly
+Å
À1
À1
lower than 3paco (1700 cm mol ) due to the larger distance
between two pairs of facial aromatic rings as well as more elec-
tron-rich nature in the structure [8].
The electrochemical properties of 41,3-alter and 5 were investi-
gated by cyclic voltammograms (CV) and square waves voltammo-
grams (SWV) and the results are summarized in Fig. 4. The CV of
4
1,3-alter shows two reversible oxidation peaks at 0.42 and 0.68 V
+
vs. Fc/Fc . The first oxidation potential is lower than the analog
Conclusion
3paco by 0.11 V [8]. This difference can be attributed to two factors:
1
) propoxy group is slightly more electron donating than methoxy
In conclusion, we report the preparation of two novel highly
group (Fig. S7); and each molecule contain twelve propoxy groups
which can significantly reduce the oxidation potential; 2) the
structure distinction in the stable conformation. The structure of
electron-rich calix[n]arenes (n = 4 and 8) by the acid-catalyzed
condensation
paraformaldehyde. After screening the reaction condition, we
found both BF O and methanesulfonic acids produce calix[n]
ÁEt
reaction
of
1,3,5-tripropoxybenzene
and
41,3-alter contains two sets of facial interacted aromatic rings, which
3
2
can stabilize the generated cation radicals by one-electron oxida-
tion. However, the structure of 3paco has only a pair of parallel aro-
matic rings with an oxidation potential of 0.53 V. Similar
stabilization effect was observed in 2paco and 21,3-alter system. The
redox potential of 21,3-alter is lower than 2paco by 0.12 V [11]. For
the calix[8]arene (5) molecule, the CV shows three sets of reversi-
ble couples (0.54, 0.65 and 0.76 V). No facial interacted aromatic
structure was observed in 5. Indeed, the first oxidation potential
is higher than 41,3-alter by 0.12 V, which provides more evidence
arenes (n = 4 and 8) with high total yield. Additionally, methane-
sulfonic acid as catalyst can slightly alter the product distribution
and increase the yield of calix[4]arene. Both calixarene derivatives
are fully characterized by NMR spectra, mass spectroscopy and
X-ray diffraction. In addition, the electrochemical properties and
cation radical of 41,3-alter and 5 were characterized. Studies on the
application of such electron-rich calixarene derivatives as sensors
and the interaction with metal cations and small molecules (e.g.,
NO) will be reported in due course.
Please cite this article as: D. Wang, S. Mirzaei, S. V. Lindeman et al., Accessing highly electron-rich calix[n]arene (n = 4 and 8) derivatives from acid-cat-
alyzed condensation of 1,3,5-tripropoxybenzene, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2019.151215

D. Wang et al. / Tetrahedron Letters xxx (xxxx) xxx
5
+
Å
Fig. 5. Left: Spectral changes observed upon the reduction of 0.074 mmol THEO in CH
2
Cl
2
(3 mL) by adding 2.13 mmol solution of 41,3-alter in CH
2
Cl
2
. Right: Plot of the mole
+
Å
+Å
fractions of THEO (red) and 41,3-alter (black) against added equivalents of neutral 41,3-alter. Symbols represent experimental points, whereas the solid lines show best-fit to
+Å
+Å
experimental points using
version of this article.)
D
G = Eox (41,3-alter À Ered (THEO ) = À246 mV. (For interpretation of the references to color in this figure legend, the reader is referred to the web
(
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We thank the NSF (CHE-1508677) for financial support. This
work used the high-performance computing cluster at the Extreme
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(
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Please cite this article as: D. Wang, S. Mirzaei, S. V. Lindeman et al., Accessing highly electron-rich calix[n]arene (n = 4 and 8) derivatives from acid-cat-
alyzed condensation of 1,3,5-tripropoxybenzene, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2019.151215