Synthesis and molecular recognition of novel multiimidazole cyclophanes

Article abstract of DOI:10.1002/jhet.188

(Chemical Equation Presented) Cyclophanes based on 2,2′-biimidazole and 2,2′-bibenzimidazole were synthesized as receptors. UV spectroscopic titration in chloroform at 25°C showed 1:1 complexes between the cyclophanes and the guests, and the binding constants (K) and Gibbs free energy changes (-ΔG₀) were calculated according to the modified Benesi-Hildebrand equation.

Full text of DOI:10.1002/jhet.188

November 2009
Synthesis and Molecular Recognition of Novel Multiimidazole
Cyclophanes
1137
Xiao-Wei Xu, Xin-Long Wang, Ai-Ming Wu, Zhi-Ming Zheng, Mei-Gui Yi,
and Rong Xiao*
School of Chemical Engineering, Sichuan University, Chengdu, Sichuan 610065, China
*
E-mail: xu02115101@gmail.com
Received January 5, 2009
DOI 10.1002/jhet.188
Published online 2 November 2009 in Wiley InterScience (www.interscience.wiley.com).
0
0
Cyclophanes based on 2,2 -biimidazole and 2,2 -bibenzimidazole were synthesized as receptors. UV
ꢀ
spectroscopic titration in chloroform at 25 C showed 1:1 complexes between the cyclophanes and the
guests, and the binding constants (K) and Gibbs free energy changes (ꢁDG
0
) were calculated according
to the modified Benesi-Hildebrand equation.
J. Heterocyclic Chem., 46, 1137 (2009).
INTRODUCTION
out by Shi and Thummel [6]. Although much progress
has been achieved [7], the construction of cyclophanes
are of only one or two imidazole rings and has not yet
overtaken the natural high efficiency and selectivity. We
recently synthesized novel multiimidazole cyclophanes
linked by alkyl groups [8]. However, these receptors only
show poor recognition ability for amino acid esters. Non-
covalent interactions, especially, hydrogen bonding and
p-p stacking interaction may contribute to the attractive
forces between hosts and guests. Because of the greater
It is known that molecular recognition is a fundamen-
tal characteristic of biochemical system. The design and
synthesis of receptors is of prime importance in supra-
molecular chemistry, due to its biological, medical, and
environmental relevances [1,2]. During the last two dec-
ades, considerable attention have been addressed to de-
velop systems capable of recognizing, self-assembling,
catalyzing, and sensing, but the design of effective hosts
for guests are still particularly challenging because of
the characteristics of guests [3].
0
chemical stability and bidentate chelating sites [9], 2,2 -
0
biimidazole (Biim) and 2,2 -bibenzimidazole (BiBim) are
In biochemical host-guest processes, noncovalent inter-
actions, such as, hydrogen bonding and coordination
bonding play a central role in the creation of a variety of
molecular architectures for molecular recognition, molec-
ular self-assembly, and supramolecular catalysis. Imidaz-
ole can serve as proton donor-acceptor, general acid-
base, nucleophile, and selective binding group [4]. Com-
pounds with imidazole ring systems have many pharma-
ceutical activities and play important roles in biochemical
processes [5]. Imidazole-based cyclophanes have
received increasing attention in host-guest chemistry and
biomimetic chemistry. Pioneering work has been carried
chosen as the component. We report, herein, the facile
synthesis and characterization of novel multiimidazole
cyclophanes which are comprised of four imidazole rings
linked by ether chains (Scheme 1), and found that these
artificial receptors exhibit excellent recognition ability to-
ward amino acid esters.
RESULTS AND DISCUSSION
Design and synthesis of cyclophanes 5a,b and
6a,b. Cyclophanes 5a,b and 6a,b with different cavity
and structure were designed and synthesized.
V
C
2009 HeteroCorporation

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138
X.-W. Xu, X.-L. Wang, A.-M. Wu, Z.-M. Zheng, M.-G. Yi, and R. Xiao
Vol 46
Scheme 1
0
Cyclophane 5a contains four imidazole rings that could
function as binding sites and two oxygen atoms on the
macrocyclic skeleton that might serve as two hydrogen
bonding recognition sites. Cyclophane 5b also has four
imidazole rings but with bigger cavity. For comparison,
cyclophanes 6a,b with more aromatic BiBim subunits
linked to polyether dichlorides were also synthesized.
The multiimidazole compounds are generally consid-
ered to be difficult to prepare because of low solubility
and several reactive sites. The reaction conditions have
a significant influence on the selectivity. The N-alkyla-
tion of Biim and BiBim may have many possible substi-
bridged cyclization, 1,1 -disubstitution. By optimizing
conditions, these competitive reactions can be mini-
mized. Temperature and basicity are important in
obtaining products. The temperature of the reaction
affects on the yields of [1 þ 1] and [2 þ 2] cyclization.
Excessive heating may lead [1 þ 1] condensation. If ex-
cessive base are used, the mixture of quaternization,
0
0
N,N -bridged cyclization and 1,1 -disubstitution are
formed. Under equivalent basicity to Biim and BiBim,
1-substitution of Biim and BiBim are produced. The
bridged Biim and BiBim intermediates were synthesized
by the N-alkylation of Biim and BiBim, respectively,
with the corresponding polyether dichlorides in the pres-
0
tutions at different sites, such as, quaternization or N,N -
ence of a slight excess of NaH or KOH in dry DMF at
ꢀ
7
0 C. The cyclization reactions of the bridged
Figure 1. Spectrum of 5a with varying Val-OMe concentration at
ꢁ3
ꢀ
ꢀ
ꢁ5
2
5 C ꢂ 0.1 C. The concentration of 5a is 6.8 ꢃ 10 mol dm . The
ꢁ3 ꢁ5 ꢁ5
concentration of Val-OMe (mol dm ) are 0, 10 ꢃ 10 , 20 ꢃ 10
,
0 0 0
Figure 2. Typical plots of [G] [H] /DA versus [G] for the host-guest
0 ꢃ 10ꢁ , 40 ꢃ 10 , 50 ꢃ 10 , 60 ꢃ 10 , and 70 ꢃ 10 reading
5
ꢁ5
ꢁ5
ꢁ5
ꢁ5
complexation of Val-OMe and 5a at 25 C ꢂ 0.1 C. [Color figure can
be viewed in the online issue, which is available at www.interscience.
wiley.com.]
ꢀ
ꢀ
3
from a to h. [Color figure can be viewed in the online issue, which is
available at www.interscience.wiley.com.]
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

November 2009
Synthesis and Molecular Recognition of Novel Multiimidazole Cyclophanes
1139
Table 1
Binding constants (K) and Gibbs free energy of complexation (2DG
0
) for the 1:1 complexes between cylcophanes and guests in CHCl
3
at 25°C.
a
3
K (dm mol
ꢁ1
ꢁ1
Entry
Host
Guest
)
ꢁDG
0
(kJ mol
)
1
2
3
4
5
6
7
8
9
5a
5a
5a
5a
5a
5b
5b
5b
5b
5b
6a
6a
6a
6a
6a
6b
6b
6b
6b
6b
Ala-OMe
Val-OMe
Leu-OMe
Pro-OMe
Gly-OMe
Ala-OMe
Val-OMe
Leu-OMe
Pro-OMe
Gly-OMe
Ala-OMe
Val-OMe
Leu-OMe
Pro-OMe
Gly-OMe
Ala-OMe
Val-OMe
Leu-OMe
Pro-OMe
Gly-OMe
377
2656
707
1330
160
14.71
19.55
16.27
17.83
12.58
15.03
17.02
15.31
19.64
16.20
16.90
20.08
18.13
15.61
16.36
17.08
17.02
16.33
16.32
17.89
429
960
480
2750
687
10
11
12
13
14
15
16
17
18
19
20
913
3295
1502
544
734
980
960
725
723
1358
a
Ala-OMe, alanine methyl ester; Leu-OMe, leucine methyl ester; Pro-OMe, proline methyl ester; Val-OMe, valine methyl ester; Gly-OMe, glycine
methyl ester.
ꢀ
intermediates were carried out in DMF at 95 C. The
slow addition of the intermediates made the cyclization
facile and efficient. The structures proposed for these
novel multiimidazole cyclophanes were confirmed by
tem formed can be calculated according to the modified
Hildebrand-Benesi equation [11], eq. (2).
½
Gꢄ ½Hꢄ =DA ¼ 1=KDe þ ½Gꢄ =De
(2)
0
0
0
1
elemental analysis, MS, and H NMR.
Molecular recognition of hosts in UV spectroscopic
titration. Among the various methods to characterize
host-guest interactions, the UV-vis titration method is
widely used for its high sensitivity to host-guest binding
where [H]
represents the total concentration of host,
0
[G] denotes the total concentration of guest amino acid
0
derivatives, De is the difference between the molar
extinction coefficient for the free and complexing cyclo-
phane, DA denotes the changes in the absorption of the
host on adding amino acid derivatives. For all guest
[
1,10]. In this article, the binding constants (K) of inclu-
sion complexes of aforementioned receptors with amino
acid esters were determined on the basis of the differen-
tial UV spectrometry in chloroform. In UV spectro-
scopic titration experiments, addition of varying concen-
tration of amino acid derivatives resulted in a gradual
increase of the characteristic absorptions of the host
molecules. Typical UV spectral changes upon the addi-
tion of valine methyl ester (Val-OMe) to host 5a are
shown in Figure 1.
molecules examined, plots of calculated [G] [H] /DA
0 0
values as a function of porting the 1:1 complex forma-
tion. Typical plots are shown for the complexation of
compound 5a with Val-OMe in Figure 2.
The association constants (K) and the free-energy
change (ꢁDG
) calculated from the slope and the inter-
0
cept are shown in Table 1. Inspection of Table 1 shows
that these receptors can recognize the differences
between the cyclophane size and shape of amino acid
derivatives. Our study clearly demonstrates that the dif-
ferent cavity size strongly influences the recognition
ability of the cyclophane receptor. The multiimidazole
cyclophane 5a shows selective binding to amino acid
derivatives in chloroform. For example, the association
constants (K) of 5a and 5b for Val-OMe are 2656 and
960, respectively. The similar structure of 5a and 6a
gives us the similar association constants (K) for Val-
OMe, which are 2656 and 3295, respectively. Thus, the
With the assumption of a 1:1 stoichiometry, the com-
plexation of amino acid derivatives (G) with cyclophane
(
H) is expressed by eq. (1):
H þ G ꢀ GH
(1)
Under the conditions used, the concentration of the
receptors (6.8 ꢃ 10ꢁ mol dm ) is much lower than
5
ꢁ3
that of amino acid derivatives, that is, [H] <<[G] .
0
0
Therefore, the stability constant of supramolecular sys-
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

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X.-W. Xu, X.-L. Wang, A.-M. Wu, Z.-M. Zheng, M.-G. Yi, and R. Xiao
Vol 46
cavity size, steric effects, and structural rigidity of the
receptors may play an important role in the selective
recognition.
Figure 3 and Table 1 show the selective recognition
ability of the receptor 6a with a-animo acid esters,
affording the K of 544–3295 and the DG of 15.61–20.08
0
ꢁ
1
KJ mol . The selectivity is highly sensitive to the chain
length and shape of the substituted group in amino acids.
Indeed, the receptor 6a exhibits stronger binding for
amino acid esters containing an acyclic group, inferring
the hydrogen bonding interaction between the receptor
and the amino acid is the principal attractive interaction
involved. The acyclic Val-OMe is included most effec-
tively by 6a, giving the highest and the strongest binding.
According to CPK model, when Val-OMe is embedded
into the cavity, the supramolecular complex should be
formed (Fig. 4). It has been proposed that the dimension
of the cavity of cyclophane 6a is more suitable for Val-
OMe than other amino acid esters.
Figure 4. Proposed recognition mechanism of Val-OMe by host 5a.
[
Color figure can be viewed in the online issue, which is available at
www.interscience.wiley.com.]
Different structure strongly influences the recognition
ability of the receptor for amino acid esters. The similar
structures 5b and 6b give us the association constants
EXPERIMENTAL
Physical measurements. Melting points were taken on a X-
(
K) for Gly-OMe, which are 687 and 1358, respectively.
1
From this result, we think that the p-p stacking interac-
tion between the receptor and the aromatic side chain of
amino acid play an important role in recognition.
Because 5b and 6b have bigger cavity, they cannot be
propitious to the formation of stable supramolecular sys-
tem. They give us low selective ability.
6 micro-melting point apparatus and were uncorrected.
H
NMR spectra were recorded on a Bruker AVANCE/400 MHz
instrument, and chemical shifts in ppm were reported with
TMS as the internal standard. Mass spectra (ms) were
measured on a VG Autospec 3000 mass spectrometer. Ele-
mental analyses were performed on a Carlo Erba 1106 in-
strument. UV-vis spectra were obtained with TU-1810
spectrophotometer.
Reagents and general techniques. Anhydrous DMF was
purified according to the standard method. Biim [12], BiBim
CONCLUSION
In conclusion, a facile synthesis led to cyclophanes
based on biimidazole and bibenzimidazole units as mo-
lecular recognition motifs for amino acid esters. These
receptors exhibit excellent recognition ability toward
amino acid derivatives. The cavity size, steric effects,
and structural rigidity, hydrogen bond, and p-p stacking
interaction between the aromatic groups may be respon-
sible for the recognition of amino acid derivatives.
[
13], and polyether dichlorides [14] were prepared according
to literature procedure. Amino acid methyl ester hydrochloride
used was prepared by adding dropwise SOCl
sion of the free amino acid in absolute methanol at 0 C. Free
amino acid esters were obtained by neutralization with
into a suspen-
ꢀ
2
NH
3
2
ꢅnH O before use. All other chemicals and reagents were
obtained commercially and used without further purification.
General procedure for preparation of 3a, b and 4a, b. In
a 25-mL flask, Biim or BiBim (5 mmol), DMF (10 mL) and
NaH or KOH (6 mmol) were stirred at room temperature. Pol-
yether dichlorides (5 mmol) was added quickly. The mixture
ꢀ
was stirred for 12 h at 70 C. The solid was filtered and
washed with a less absolute ethanol. The combined solvent
was removed in vacuo and the residue was purified by column
chromatography on silica gel (dichloromethane/ethanol 19:1 or
petroleum ether/ethyl acetate 4:1) to give the pure products
3
a,b and 4a,b.
-(5-Chloro-3-oxa-pentyl)-2,2 -biimidazole, 3a. Red viscous
0
1
1
liquid, yield 63.1% H NMR (CDCl ) d: 3.539–3.564 (t, 2H,
3
CH
2
2
Cl), 3.681–3.709 (t, 2H, ClCH
ACH
2
2
ACH O), 3.912–3.937 (t,
H, NCH
2
0
2
O), 4.875–4.900 (t, 2H, NCH
0
2
), 7.114 (d, 2H,
BiIm 5,5 -H), 7.144 (d, 2H, BiIm 4,4 -H) ppm; ms (m/z):
þ
2
5
1
41.08 (MþH ). Anal. Calcd for C H ClN O: C 49.90, H
10
13
4
Figure 3. Recognition ability of the cyclophane 6a for a-animo acid
esters. [Color figure can be viewed in the online issue, which is avail-
able at www.interscience.wiley.com.]
.44, N 23.28, Cl 14.73; found: C 50.08, H 5.47, N 23.48, Cl
4.85.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

November 2009
Synthesis and Molecular Recognition of Novel Multiimidazole Cyclophanes
1141
0
1
-(8-Chloro-3,6-dioxa-octyl)-2,2 -biimidazole, 3b. Red vis-
1
acid esters in chloroform was added in portions via microsyr-
inge to the cell. The concentration of guest increased along
with each addition, as far as the concentration of guest reached
about 15-fold of the concentration of host. Different absorption
spectra were obtained directly using the instrument according
to its normal procedure. The absorption of the guest was can-
cous liquid, yield 42.1% H NMR (CDCl ) d: 3.575–3.590 (t,
3
2
H, CH
2
Cl), 3.598–3.612 (t, 2H, ClCH
2
2
ACH O), 3.678–3.850
(
m, 4H, OCH ACH O), 3.869–3.914 (t, 2H, NCH ACH O),
2
2
2
2
0
4
.876–4.900 (t, 2H, NCH
0
2
), 7.114 (d, 2H, BiIm 5,5 -H), 7.154
þ
(
d, 2H, BiIm 4,4 -H) ppm; ms (m/z): 285.10 (MþH ). Anal.
0
celled by using the guest solutions of [G] concentration for
4 2
Calcd for C12H17ClN O : C 50.62, H 6.02, N 19.68, Cl 12.45;
each titration as the reference solution. The whole volume of
guest solution added to the cell did not exceed 100 lL to dis-
pel the effect of volume change. For example, when the con-
found: C 50.83, H 6.15, N 19.80, Cl 12.54.
0
1
-(5-Chloro-3-oxa-pentyl)-2,2 -bibenzimidazole,
1
4a. Pale
yellow solid, yield 61.6% H NMR (CDCl ) d: 3.498–3.508 (t,
3
centration of host 5a was 6.8 ꢃ 10ꢁ mol dm , its maximum
5
ꢁ3
2
H, CH Cl), 3.724–3.733 (t, 2H, ClCH ACH O), 4.149–4.158
2
2
2
absorption wavelength was at 242 nm, and the absorbance A
was 0.422. When the guest Val-OMe was portion-wise added
0
(
t, 2H, NCH
2
ACH
2
O), 5.294–5.302 (t, 2H, NCH
2
), 7.261–
þ
7
.850 (m,8H, ArAH) ppm; ms (m/z): 343.12 (MþH ). Anal.
ꢁ
ꢁ5
5
ꢁ5
ꢁ5
to the cell to make its concentration of 10 ꢃ 10 , 20 ꢃ 10
,
4
Calcd for C18H19ClN O: C 63.06, H 5.59, N 16.34, Cl 10.34;
ꢁ
5
ꢁ5
ꢁ5
3
mol dm , respectively, the maximal absorption increased
0 ꢃ 10 , 40 ꢃ 10 , 50 ꢃ 10 , 60 ꢃ 10 , 70 ꢃ 10
ꢁ
found: C 63.16, H 5.61, N 16.56, Cl 10.50.
0
3
1
-(8-Chloro-3,6-dioxa-octyl)-2,2 -bibenzimidazole, 4b. Pale
1
orderly and gave the corresponding DA (AꢁA ) values 0.016,
yellow solid, yield 40.9% H NMR (CDCl
H, CH Cl), 3.641–3.669 (t, 2H, ClCH ACH
m, 4H, OCH ACH O),4.116–4.125 (t, 2H, NCH
.186–5.194 (t, 2H, NCH ), 7.186–7.781 (m,8H, ArAH) ppm;
3
) d: 3.534–3.586 (t,
O), 3.925–3.950
ACH O),
0
0
.029, 0.036, 0.040, 0.044, 0.049, 0.052. According to eq. (2),
2
2
2
2
plots of calculated [G] [H] /DA values as a function of [G]
0
(
2
2
2
2
0
0
values gave an excellent linear relationship (Fig. 2). From
5
2
þ
Figure 2, we can obtain the association constant K ¼ 2655
ms (m/z): 285.10 (MþH ). Anal. Calcd for C H ClN O : C
1
2
17
4 2
3
ꢁ1
.
dm mol
5
1
0.62, H 6.02, N 19.68, Cl 12.45; found: C 50.83, H 6.15, N
9.80, Cl 12.54.
General procedure for the synthesis of cyclophanes 5a, b
REFERENCES AND NOTES
ꢀ
and 6a, b. To a stirred and warmed (95 C) solution of NaH or
KOH (2.5 mmol) and DMF (5 mL), compounds 3a, b and 4a,
b (2.1 mmol) in DMF was added dropwise over 5 h. The mix-
ture was stirred at this temperature for 20 h. The solid was fil-
tered and washed with a less absolute ethanol. The combined
solvent was removed in vacuo and the residue was purified by
column chromatography on silica gel (dichloromethane/ethanol
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Xie, R.-G. Tetrahedron Asymmetry 2003, 14, 3651.
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14:1 or petroleum ether/ethyl acetate 1:1) to give the pure
products 5a, b and 6a, b.
Cyclophane 5a. Off-white solid, yield 45.6%, mp 186–
ꢀ
1
88 C. H NMR (CDCl
1
3
) d: 3.770–3.794 (t, 8H, OCH
2
),
.950–3.971 (t, 8H, NCH ), 7.192 (s, 4H, BiIm 5,5 -H), 7.323
0
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2
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0
þ
(
s, 4H, BiIm 4,4 -H) ppm; ms (m/z): 409.5 (MþH ). Anal.
Calcd for C20 : C 58.81, H 5.92, N 27.43; found: C
8.63, H 5.95, N 27.23.
Cyclophane 5b. Off-white solid, yield 24.1%, mp 172–
24 8 2
H N O
5
1
[5] (a) Lombardino, J.G.; Wiseman, E. H. J Med Chem 1974,
17, 1182; (b) Sundberg, R. J.; Martin, R. B. Chem Rev 1974, 74, 471.
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ꢀ
1
74 C. H NMR (CDCl ) d 3.610–3.632 (t, 8H, OCH A-
3
2
CH O), 3.683 (s, 8H, NCH ACH O), 3.735–3.757 (t, 8H,
2
2
2
0
0
NCH
2
), 7.030 (s, 4H, BiIm 5,5 -H), 7.149 (s, 4H, BiIm 4,4 -H)
þ
ppm; ms (m/z): 497.2 (MþH ). Anal. Calcd for C24
32 8 4
H N O :
[
7] (a) Jered, C. G.; Richard, S. S.; Jody, M. T.; Chrys, W.;
C 58.05, H 6.50, N 22.57; found: C 57.87, 6.46, 22.28.
Cyclophane 6a. Off-white solid, yield 42.3%, mp 161–
Claire, A. T.; Wiley, J. Y. Organometallics 2001, 20, 1276; (b) Semih,
D.; Jered, C. G.; Matthew, J. P.; Claire, A. T.; Wiley, J. Y. Tetrahe-
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Dirk, W.; Helmar, G. Tetrahedron 2006, 62, 731.
ꢀ
1
63 C. H NMR (CDCl
1
3
) d: 3.797–3.821 (t, 8H, OCH
2
),
4
.189–4.212 (t, 8H, NCH
2
), 7.405–7.948 (m, 16H, ArAH)
þ
ppm; ms (m/z): 609.1 (MþH ). Anal. Calcd for C36
C 71.04, H 5.30, N 18.41; found: C 70.87, H 5.28, N 18.23.
Cyclophane 6b. Off-white solid, yield 22.3%, mp 124–
126 C. H NMR (CDCl
CH O), 3.559–3.644 (m, 8H, NCH
32 8 2
H N O :
[
[
8] Xiao, R.; Su, X.-Y.; Xie, R.-G. J Chem Res 2007, 278.
9] Tadokoro, M.; Nakasuji, K. Coord Chem Rev 2000, 198,
ꢀ
1
205.
3
) d: 3.424–3.474 (m, 8H, OCH
2
A-
[
10] Yuan, Y.; Gao, G.; Jiang, Z.-L.; You, J.-S.; Zhou, Z.-Y.;
Yuan, D.-Q.; Xie, R.-G. Tetrahedron 2002, 58, 8993.
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2703; (b) Cramer, F.; Saenger, W.; Spatz, H. C. J Am Chem Soc
977, 99, 6392.
2
2
ACH O), 4.053–4.080 (t,
2
8
6
5
H, NCH
2
), 7.259–7.717 (m, 16H, ArAH) ppm; ms (m/z):
[
þ
97.0 (MþH ). Anal. Calcd for C H N O : C 68.95, H
4
0 40 8 2
.79, N 16.08; found: C 70.13, H 5.77, N 15.96.
UV titration. UV spectra were recorded on a TU-1810 UV-
1
[
12] Xiao, J.-C.; Jean’ne, M. S. J Org Chem 2005, 70, 3072.
ꢀ
ꢀ
vis spectrophotometer at 25 C ꢂ 0.1 C with a 1-cm quartz
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Journal of Heterocyclic Chemistry
DOI 10.1002/jhet