Synthesis and inclusion properties of thiacalix[4]arene tetraamides in cone- And 1,3-alternate conformation

Article abstract of DOI:10.3184/030823407X248630

Novel thiacalix[4]arene tetraamides in cone- and "1,3-alternate conformation were prepared, in which the conetetraamide shows stronger intramolecular hydrogen bonding than 1,3-alternate-tetraamide. The two-phase solvent extraction data indicated that tetr

Full text of DOI:10.3184/030823407X248630

JOURNAL OF CHEMICAL RESEARCH 2007
OCTOBER, 557–560
RESEARCH PAPER 557
Synthesis and inclusion properties of thiacalix[4]arene tetraamides in
cone- and 1,3-alternate conformation
Shofiur Rahman, Nazneen Begum, Carol Pérez-Casas, Akina Yoshizawa and Takehiko Yamato*
Department of Applied Chemistry, Faculty of Science and Engineering, Saga University, Honjo-machi 1, Saga-shi, Saga 840-8502,
Japan
Novel thiacalix[4]arene tetraamides in cone- and 1,3-alternate conformation were prepared, in which the cone-
tetraamide shows stronger intramolecular hydrogen bonding than 1,3-alternate-tetraamide. The two-phase solvent
extraction data indicated that tetraamides show no extractability for alkali metal cations, but small extractability for
dichromate anion (Cr2O72-).
Keywords: thiacalix[4]arene, conformation, ionophores, hydrogen bond, metal complexation, anion recognition, dichromate anion
From both a biological1 and an environmental2 viewpoint the
synthesis of anion receptors is one of the most challenging
targets in supramolecular chemistry. Calixarenes3 and more
recently thiacalixarenes4 have been widely used in molecular
recognition due to their unique three-dimensional structure
with almost unlimited derivatisation abilities. Despite the
importance of the molecular recognition of anions, little
work has been done, while that of cations has been studied
intensively. Selective complexation of anions is more
demanding than that of cations due to many reasons such as
size, charge density, polarisability, solvation energy and pH-
dependent acid-base equilibria.5
Me
Me
Me
Me
HN
Me
NH
C
Me
O O
C
NH
C
NH
C
O
O
S
S
NH
C
NH
C
O
O
O
O
S
S
O
O
O
O
O
O
S
S
S
S
C
O
O
C
HN
NH
A number of examples of substituted calixarenes have been
used for anion complexation.6 Thus, introduction of activated
amides,7 amines,7a urea,8 and thiourea8a into the calixarene
platform, led to the receptors that interact with anions.
Furthermore, such receptors are able to form ditopic systems
capable of ion-pair recognition9 which show significant
relevance to the selective extraction and/or transport of
metals salts across, lipophilic membranes. But receptors with
multiple binding sites have not yet been reported. Resulting
from our interest in the synthesis of ditopic receptors that
function as not only an anion binder but also as cation binder,
we introduced amide functions into the lower rim of the
thiacalix[4]arene. Amide function has been used as an efficient
extractant for both cations and anions due to the high stability
and hydrophobicity.
cone-4
Me
1,3-alternate-4
Me
Fig.1 Structures of cone- and 1,3-alternate-4.
conformation.11 The product structures were supported by
their spectral and analytical data.11b
The 1,3-alternate-tetraamide 1,3-alternate-4 has, at first
glance, two possible binding sites because of the 1,3-alternate
conformation.12 Furthermore, the NH group can bind anions
by hydrogen bonding while the carbonyl and phenolic
oxygens are available for cation binding. In particular, the
presence of the acetate function might provide the possibility
of complexing cations and could work as a controller for the
recognition of cation and/or anions by the amide function
placed at the other site of the thiacalix[4]arene cavity. It is
therefore possible that the two binding sites of 1,3-alternate-4
could participate simultaneously and cooperatively upon ion
binding producing higher forms of molecular behaviour.
Several examples of the formation of intramolecular
hydrogen-bonding among opposing urea groups which can
bind anionic species in calix[4]arenes have been reported.13
Therefore, intramolecular hydrogen bonding may be foreseen
between the NH and CO groups. In order to investigate
the existence of intramolecular hydrogen bonding in 4 the
reference compound 6 was synthesised by condensation
reaction of 514 with p-toluidine in the presence of DCC and
HOBt at room temperature for 15 h in CH2Cl2.
The chemical shifts of the NH protons and the differences
(Dd) in the chemical shifts of 4 from that of the reference
compound 6 in both CDCl3 and DMSO-d6 are shown in
Table 1. Compared with the chemical shift of the NH protons
of 6 (d 8.60 ppm) the corresponding chemical shift at d 9.26
ppm arising from the formation of intramolecular hydrogen
bonding in cone-4 shows a downfield (Dd = –0.66 ppm). The
strong intramolecular hydrogen bonding between NH and
CO groups implies a close contact of the chains OCH2CONH
We now report the synthesis and structural properties
of tetraamides
4 constraining cone- and 1,3-alternate
conformation. Their recognition behaviours were also
1
investigated by H NMR titration experiments in CDCl3 and
solvent extraction experiment.
Results and discussion
Synthesis
cone-Thiacalix[4]arene tetracarboxylic acid cone-3 was
prepared by hydrolysis of cone-5,11,17,23-tetra-tert-butyl-
25,26,27,28-tetrakis[(ethoxycarbonyl)methoxy]-2,8,14,20-
tetrathiacalix[4]arene cone-210 with K2CO3 aq. in a mixture
of DMSO and water, which was prepared by O-alkylation of
1 with ethyl bromoacetate in the presence of NaH according
to the reported procedure.11 Tetrathiacalix[4]arene tetraamide
cone-4 was prepared in 45% yield by condensation reaction of
cone-3 with p-toluidine in the presence of DCC and HOBt at
room temperature for 15 h in CH2Cl2. Similarly, 1,3-alternate-
tetrathiacalix[4]arene tetraamide 1,3-alternate-4 was
prepared in good yield from the corresponding 1,3-alternate-
2 as described above. The conformational assignments
of 4 were not necessary because the tetraacetates 2 were
conformationally immobilised in cone- and 1,3-alternate
* Correspondent. E-mail: yamatot@cc.saga-u.ac.jp
PAPER: 07/4762

558 JOURNAL OF CHEMICAL RESEARCH 2007
OR
OR
OR
OR
tBu
(iv)
(i), (iii)
(76%)
cone-4
(45%)
S
S
S
S
(76%)
cone-2; R= CH2COOEt
cone-3; R= CH2COOH
OH
(85%)
tBu
OH
HO
tBu
OH
OR
(iv)
(53%)
(ii), (iii)
(95%)
OR
OR
1,3-alternate-4
tBu
1
OH
OH
OH
OH
OR
(95%)
1,3-alternate-2; R= CH2COOEt
(85%)
1,3-alternate-3; R= CH2COOH
Scheme 1 Reagents and conditions: (i) BrCH2CO2Et, NaH, THF-DMF, reflux for 36h; (ii) BrCH2CO2Et, Cs2CO3, acetone,
reflux for 36h; (iii) K2CO3, DMSO/water, 120°C for 12h; (iv) p-toluidine, DCC/HOBt, CH2Cl2, room temp. for 15h.
Me
OCH2CONH
Me
OCH2COOH
Me
p-toluidine, DCC/HOBt,
Me
Me
CH2Cl2
room temp. for 15 h.
(65%)
tBu
tBu
6
5
Scheme 2
0.0
-0.03
due to the cone conformation. These downfield shifts are
conspicuous due to several examples of the formation
of intramolecular hydrogen bonding in calix[4]arene
constraining cone conformation are well known. In contrast,
the NH protons of 1,3-alternate-4, constraining 1,3-alternate
conformation, shows an upfield (i.e. Dd = + 0.16 ppm). This
might suggest the steric hindrance of the tert-butyl groups
avoid the formation of intramolecular hydrogen bonding
between the distal positions.
Me
Me
-0.07
-0.05
-0.09
-0.10
-0.30
-0.88
HN
HN
O
C
O
C
+0.12
-0.10
CH
CH
2
2
O
O
1H NMR dilution studies showed no change of the chemical
shift of NH protons due to the concentration-independent
intramolecular hydrogen-bonding in cone-4. Due to NH,
groups can also form intermolecular hydrogen-bonding based
on the solvent, the compound cone-4 was dissolved in the
strongly hydrogen-bonding solvent DMSO-d6. The downfield
shift of NH protons of cone-4 at d 10.14 ppm (i.e. Ddsol.
= –0.88 ppm) also indicates the formation of intermolecular
hydrogen-bonding. By contrast, a smaller downfield shift
S
-0.11
S
-0.11
O
4
2
CH
C
-0.18
-0.02
2
O
cone-4
NH
of NH protons of 1,3-alternate-4 at d 8.74 ppm (i.e. Ddsol.
=
–0.30 ppm) than that of cone-4 was observed, apparently from
the weaker intramolecular hydrogen-bonding between the
distal-amide moieties and then decreased formation of a new
intermolecular hydrogen-bonding attributable to the steric
hindrance of the tert-butyl groups.
Chemical shifts changes of cone- and 1,3-alternate-4
in CDCl3 and DMSO-d6 are shown in Fig. 2. The higher
downfield shift of tert-butyl protons (Ddsol. = –0.18 ppm) in
1,3-alternate-4 than that of cone-4 (Ddsol. = –0.02 ppm) may
be attributed by the conformation change of 1,3-alternate-4
Me
1,3-alternate-4
Fig. 2 Chemical shifts changes of cone- and 1,3-alternate-4 in
CDCl3 and DMSO-d6. Dd = d [in CDCl3] - d [in DMSO-d6].
in DMSO-d6. It was also found that in the cases of cone-4 the
chemical shift of the methylene protons of OCH2CONH shifted
to a lower field (Ddsol. = –0.10 ppm) in DMSO-d6, whereas
Table 1 Chemical shifts (d) of the NH protons of cone- and 1,3-alternate-4a
b
b
c
Compound
CDCl3 dNH (Ddref.NH
)
DMSO-d6 dNH (Ddref.NH
)
Ddsol. NH
cone-4
1,3-alternate-4
reference 6
9.26 (-0.66)
8.44 (+0.16)
8.60
10.14 (-0.25)
8.74 (+1.14)
9.88
–0.88
–0.30
–1.28
aDetermined in CDCl3 or DMSO-d6 by using SiMe4 as a reference and express d in ppm; bDdref. = d [reference 6] – d [thiacalixarene 4];
cDdsol. = d [in CDCl3] – d [in DMSO-d6]
PAPER: 07/4762

JOURNAL OF CHEMICAL RESEARCH 2007 559
an upper field shift (Ddsol. = + 0.12 ppm) was observed in 1,
3-alternate-4, which might be attributable to being to locate
in the area of the ring current effect15 arising from the two
inverted calixarene benzene rings.
Experiment
1
All melting points are uncorrected. H NMR spectra were recorded
at 300 MHz on a Nippon Denshi JEOL FT-300 NMR spectrometer
in deuteriochloroform with Me4Si as an internal reference. IR spectra
were measured as KBr pellets on a Nippon Denshi JIR-AQ2OM
spectrometer. Mass spectra were obtained on a Nippon Denshi JMS-
HX110A Ultrahigh Performance Mass Spectrometer at 75 eV using
a direct-inlet system. Elemental analyses were performed by Yanaco
MT-5.
Binding studies
The recognition properties of receptors 4 were investigated
1
by H NMR titration experiments in CDCl3 toward selected
anions (tetrabutylammonium (TBA) chloride and bromide)
and cations (silver and potassium trifluoromethanesulfonate).
In general, the titration experiments were carried out by the
increasing addition of ion (0.1 mol dm-3) into 5 ¥ 10-6 mol
of the receptor in 0.5 cm3 of CDCl3. Addition of 1 equiv of
KSO3CF3 into either cone- and 1,3-alternate-4 solutions did
not cause significant chemical shift (i.e. greater than 0.01
ppm) even in the presence of an excess of K+ ion. Similar
titration of cone- and 1,3-alternate-4 with 1 equiv or an excess
of AgSO3CF3 did not caused chemical shift.
Disappointingly, in the titration of cone- and 1,3-alternate-
4 with 1 equiv or excess of TBACl or TBABr no significant
shifts were observed indicating that there is little interaction
or no interaction between these molecules and the anionic
species. This implies that the two hydrogen-bonding systems
in 1,3-alternate-4 is not strong enough to form tight complexes
with anion species such as Cl- or Br-.
Materials
cone-(cone-2) and 1,3-alternate-5,11,17,23-tetra-tert-butyl-25,26,
27,28-tetrakis[(ethoxycarbonyl)methoxy]-2,8,14,20-tetrathiacalix
[4]arene (1,3-alternate-2) were prepared in 76 and 95% by
O-alkylation of 1 with ethyl bromoacetate according to the reported
procedure.11
cone-5,11,17,23-Tetra-tert-butyl-25,26,27,28-tetrakis[carboxy-
methoxy]-2,8,14,20-tetrathiacalix[4]arene (cone-3): A solution of
cone-2 (1.07 g, 1.0, mmol), K2CO3 (2.76 g, 20 mmol) in DMSO-
water (6:1, 35 cm3) was heated at 120°C for 12 h and then 2 M HCl
(60 cm3) was added to the cooled mixture in an ice-water bath. The
precipitate was collected by filtration and washed with water, then
dissolved in chloroform.After filtering off the solid residue, the filtrate
was evaporated to dryness to obtain cone-3, which was recrystallised
from water–acetone to obtain a pure sample as a white solid (750 mg,
85%) as colourless prisms, m.p. 333–335°C; IR n (KBr)/cm-1 3480
(OH), 1758 (CO); dH (CDCl3) 1.11(36H, s, tBu), 5.07 (8H, s, OCH2),
7.38 (8H, s, ArH); m/z: 975.1 [M + Na]+. (Found: C, 60.23; H, 5.70.
C48H56O12S4 (953.22) requires C, 60.48; H, 5.92%).
1,3-alternate-5,11,17,23-Tetra-tert-butyl-25,26,27,28-tetrakis
[carboxymethoxy]-2,8,14,20-tetrathiacalix[4]arene (1,3-alternate-3):
A similar procedure to that of cone-2 was carried out to give 1,3-
alternate-3 (85%) as an off-white solid, m.p. 325–326°C; IR n (KBr)/
cm-1 3421 (OH), 1695 (CO); dH (CDCl3) 1.25 (36H, s, tBu), 4.66
(8H, s, OCH2), 7.39 (8H, s, ArH); m/z: 975.1 [M + Na]+. (Found: C,
60.32; H, 5.86. C48H56O12S4 (953.22) requires C, 60.48; H, 5.92%).
cone-5,11,17,23-Tetra-tert-butyl-25,26,27,28-tetrakis[(p-methyl-
phenylamino)carbonylmethoxy]-2,8,14,29-tetrathiacalix[4]arene
(cone-4): To a solution of cone-3 (100 mg, 0.105 mmol), p-toluidine
(108 mg, 1.14 mmol) and 1-hydroxybenzotriazole (HOBt) (26 mg,
0.17 mmol) in CH2Cl2:DMF (4:1, 25 cm3) was added dropwise a
solution of dicyclohexylcarbodiimide (DCC) (190 mg, 0.92 mmol)
in CH2Cl2 (5 cm3) at 0°C. After the mixture was stirred for 20 h at
room temperature, it was condensed under reduced pressure. The
residue was extracted with ethyl acetate (2 ¥ 30 cm3). The combined
extracts were washed with 10% citric acid (2 ¥ 20 cm3), 5% sodium
bicarbonate (20 cm3), water (20 cm3), saturated brine (20 cm3),
dried (MgSO4) and condensed under reduce pressure. The residue
was recrystallised from methanol affording pure compound cone-4
(62 mg, 45%) as colourless prisms, m.p. 245–247°C; IR n (KBr)/cm-1
3721 (NH), 1680 (CO); dH (CDCl3) 1.13 (36H, s, tBu), 2.23 (12H,
s, Ph-CH3), 4.94 (8H, s, OCH2), 6.96 (8H, d, J = 8.4 Hz, Ph-H),
7.38 (8H, s, ArH), 7.42 (8H, d, J = 8.4 Hz Ph-H), 9.26 (4H, t, NH);
dH (DMSO-d6) 1.15 (36H, s, tBu), 2.26 (12H, s, Ph-CH3), 5.04 (8H, s,
OCH2), 7.06 (8H, d, J = 8.4 Hz, Ph-H), 7.49 (8H, s, ArH), 7.51 (8H, d,
J = 8.4 Hz Ph-H), 10.14 (4H, t, NH); m/z: 1308 (M+). (Found: C,
69.64; H, 6.72; N, 4.05. C76H84N4O8S4 (1309.78) requires C, 69.69;
H, 6.46; N, 4.28%).
Solvent extraction
Recently a number of chemically modified calixarenes have
been synthesised that can be used as host for simple anions.16
Dichromate anions in particular are important because of
their high toxicity17 and their presence in soils and waters.18
For this purpose we have preliminary evaluation of the binding
efficiencies of the extractant 4 which has been carried out by
solvent extraction of alkali metal picrates and dichromatefrom
aqueous into dichloromethane. As expected from the titration
experiment described above, both cone- and 1,3-alternate-4
hardly extracted alkali metal cations (Na+, K+) and Ag+ in the
present experimental system. Interestingly, small extractability
for dichromate (Cr2O72-) [extraction% (Na2Cr2O7): 5.0% for
cone-4 and 8.3% for 1,3-alternate-4] and higher extractabil-
ity for 1,3-alternate-4 than that of cone-4 was observed.
Although an explanation for the different extractabilities from
those of metal cations is not clear in the present stage, one
might assume the larger size of dichromate to construct the
intermolecular hydrogen bondings between NH protons of
amide moieties and two oxyanions of dichromate. It was also
found the decreased extractability for K2Cr2O7 (extraction%:
4.1 for cone-4 and 1.2% for 1,3-alternate-4. The larger size of
the K+ than Na+ might affect the present dichromate extraction,
but this is also awaiting confirmation.
1,3-alternate-5,11,17,23-Tetra-tert-butyl-25,26,27,28-tetrakis
[(p-methylphenylamino)carbonylmethoxy]-2,8,14,29-tetra-thiacalix
[4]arene (1,3-alternate-4): A similar procedure to that of cone-4
was carried out to give 1,3-alternate-4 as colourless prisms in 53%
yield. m.p. 230–232°C; IR n (KBr)/cm-1 3393 (NH), 1685 (CO);
dH (CDCl3) 0.70 (36H, s, tBu), 2.32 (12H, s, Ph-CH3), 4.75 (8H, s,
OCH2), 7.14 (8H, d, J = 8.4 Hz, Ph-H), 7.39 (8H, d, J = 8.4 Hz Ph-
H), 7.46 (8H, s, ArH), 8.44 (4H, t, NH); dH (DMSO-d6) 0.88 (36H,
s, tBu), 2.32 (12H, s, Ph-CH3), 4.63 (8H, s, OCH2), 7.19 (8H, d,
J = 8.4 Hz, Ph-H), 7.46 (8H, d, J = 8.7 Hz Ph-H), 7.57 (8H, s,
ArH), 8.74 (4H, t, NH); m/z: 1308.50 (M+). (Found: C, 69.64; H,
6.45; N, 4.27. C76H84N4O8S4 (1309.78) requires C, 69.69; H, 6.46;
N, 4.28%).
4-tert-Butyl-2,6-dimethyl[(4-methylphenylamino)carbonylmethoxy]
benzene (6): To a solution of (4-tert-butyl-2,6-dimethyl)phenoxyacetic
acid 514 (100 mg, 0.43 mmol), p-toluidine (137 mg, 1.28 mmol) and
HOBt (75 mg, 0.17 mmol) in CH2Cl2 (12 cm3) was added dropwise
a solution of DCC (560 mg) in CH2Cl2 (5 cm3) at 0°C. After the
mixture was stirred for 15 h at room temperature, it was condensed
under reduced pressure. The residue was extracted with ethyl acetate
(2 ¥ 30 cm3). The combined extracts were washed with 10% citric
Conclusions
The two novel thiacalix[4] arene amide derivatives in cone-
and 1,3-alternate conformation were prepared, in which the
cone-tetraamide shows stronger intramolecular hydrogen
bonding than 1,3-alternate-tetraamide. Even though there is a
presence of intramolecular hydrogen bonding in cone-4, their
affinity for K+ and anion species is quite small. In the case of
1,3-alternate-4, the complexation of K+ does not induce any
enhancement for anion complexation. The very low affinity of
all thiacalix[4]arene amide derivatives can be ascribe to both
no suitable preorganisation and the two or four intramolecular
hydrogen bonding system. At present, only a dichromate
extraction phenomenon has been observed for cone- and 1,3-
alternate-4; variation of the solvent extraction experiment
conditions and the introduction of the further binding units to
the amide moieties could lead to the new type of dichromate
anion extractants.
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560 JOURNAL OF CHEMICAL RESEARCH 2007
acid (2 ¥ 20 cm3), 5% sodium bicarbonate (20 cm3), water (20 cm3),
saturated brine (20 cm3), dried (Na2SO4) and condensed under reduce
pressure. The residue was recrystallised from methanol gave the title
compound 6 (95 mg, 65%) as colourless prisms; m.p. 204–206°C;
IR n (KBr)/cm-1 3277(NH), 1667 (CO); dH (CDCl3) 1.30 (9H, s,
t-Bu), 2.30 (6H, s, Ph-CH3), 2.35 (3 H, s, Ph-CH3), 4.39 (2H, s,
ArOCH2), 7.05 (2H, s, ArH), 7.18 (2H, d, J = 8.8, Ph-Ha), 7.52 (2H,
d, J = 8.8, Ph-Hb), 8.60 (1H, s, NH); dH (DMSO-d6) 1.24 (9H, s,
tBu), 2.26 (9H, s, Ph-CH3, Ar-CH3), 4.35 (2H, s, ArOCH2), 7.05 (2H,
s, ArH), 7.13 (2H, d, J = 8.3, Ph-Ha), 7.59 (2H, d, J = 8.8, Ph-Hb),
9.88 (1H, s, NH); m/z: 325 (M+). (Found: C, 77.36; H, 8.33; N, 4.27.
C21H27O2N (325.45) requires C, 77.50; H, 8.36; N, 4.3%).
5
6
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7
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Engl., 2001, 40, 486.
1H NMR complexation experiments
8
9
To a CDCl3 solution (4 ¥ 10-3 M) of 4 in the NMR tube was added 1
molar to 2 molar solution of KSO3CF3 or AgSO3CF3. The spectrum
was registered after addition and the temperature of NMR probe kept
constant at 27°C.
Solvent extraction
Picrate and/or dichromate extraction experiments were performed
following Pedersen’s procedure.19 5 cm3 of a 2.5 ¥ 10−4 M aqueous
solution or 1 × 10−4 M aqueous dichromate solution and 5 cm3 of
1 × 10−4 M solution of calixarene in CH2Cl2 were vigorously agitated
in a stoppered glass tube with a mechanical shaker for 2 min then
magnetically stirred in a thermostated water-bath at 25°C for 12 h,
and finally left standing for an additional 30 min. Blank experiments
showed that no metal picrate or dichromate extraction occurred in
the absence of thiacllix[4]arene derivative 4. The percent extraction
(E%) has been calculated as:
10 (a) N. Iki, F. Narumi, T. Suzuki, A. Sugawara and M. Sotaro, Chem. Lett.,
1998, 1065; (b) N. Iki, N. Morohashi, F. Narumi, T. Fujimoto, T. Suzuki
and M. Sotaro, Tetrahedron Lett., 1999, 40, 7337.
11 (a) P. Lhoták, V. Stastny, P. Zlatusková, I. Stibor, V. Michlová,
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(E%) = (A0 − A/A0) × 100,
where A0 and A are the initial and final concentrations of the metal
picrate/dichromate before and after the extraction respectively.
Received 26 July 2007; accepted 28 September 2007
Paper 07/4762
doi: 10.3184/030823407X248630
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PAPER: 07/4762