Self-assembly of 2-(guanidiniocarbonyl)-pyrrole-4-carboxylate in dimethyl sulfoxide: An entropy driven oligomerization

Article abstract of DOI:10.1016/S0040-4020(01)00156-9

The self-assembly of 2-(guanidiniocarbonyl)-pyrrole-4-carboxylate (2), which is a heteroditopic zwitterion with self-complementary binding groups, was studied in DMSO. Through intermolecular ion pairing between the carboxylate of one and the guanidinium group of another molecule, 2 forms linear chain like oligomers in solution as can be seen from the corresponding shift changes in the NMR spectrum. Concentration dependent NMR studies at different temperatures allowed us to calculate the corresponding binding constants and thermodynamic parameters for this association process. These data show that the oligomerization is endothermic and therefore an entropy-driven process. The release of ordered solvent molecules into the bulk solution during complexation is hence responsible for the oligomerization.

Full text of DOI:10.1016/S0040-4020(01)00156-9

TETRAHEDRON
Pergamon
Tetrahedron 57 "2001) 3063±3067
Self-assembly of 2-ꢀguanidiniocarbonyl)-pyrrole-4-carboxylate in
dimethyl sulfoxide: an entropy driven oligomerization
p
Carsten Schmuck
Institut f uÈ r Organische Chemie, Universit aÈ t zu K oÈ ln, Greinstrasse 4, 50939 K oÈ ln, Germany
Received 20 December 2000; accepted 31 January 2001
AbstractÐThe self-assembly of 2-"guanidiniocarbonyl)-pyrrole-4-carboxylate "2), which is a heteroditopic zwitterion with self-
complementary binding groups, was studied in DMSO. Through intermolecular ion pairing between the carboxylate of one and the
guanidinium group of another molecule, 2 forms linear chain like oligomers in solution as can be seen from the corresponding shift changes
in the NMR spectrum. Concentration dependent NMR studies at different temperatures allowed us to calculate the corresponding binding
constants and thermodynamic parameters for this association process. These data show that the oligomerization is endothermic and therefore
an entropy-driven process. The release of ordered solvent molecules into the bulk solution during complexation is hence responsible for the
oligomerization. q 2001 Published by Elsevier Science Ltd.
1
.Introduction
self-association properties of the regioisomeric zwitterion 2
are reported. In contrast to zwitterion 1, the two self-comple-
mentary binding groups in 2 are not arranged in a U-shaped
pattern as necessary for dimerization but rather in a linear
way. Therefore, zwitterion 2 cannot form discrete dimers in a
head-to-tail orientation due to geometric reasons and hence
quite different association properties are expected.
1
±3
Molecular recognition and especially self-assembly can
lead to the formation of highly complex and fascinating
structures both in the solid state
Over the past few years, intense research has been directed
towards gaining an understanding of the concepts and prin-
ciples that govern these processes. Arti®cial self-assembling
systems have been devised, which may prove useful one day
in the design of novel materials and nanostructures.
especially for polar solvents the design of self-assembling
systems is still a challenging task, due to the limited strength
4
±9
10±13
and in solution.
1
4,15
But
2.Results and discussion
2.1. Synthesis
1
6±18
of non-covalent interactions in such solvents.
As the
polarity of the surrounding solvent increases, the strength
of hydrogen bonds and electrostatic interactions, mainly
used for molecular recognition, decreases rapidly, due to
the competitive solvation of donor and acceptor sites by
the solvent. However, it would be very desirable to have
access to self-assembling systems that also function in more
polar "i.e. more `natural') solvents such as DMSO or
The synthesis of 2 is outlined in Scheme 1. Vilsmeier±
Haack formylation of 2-"methoxycarbonyl)-pyrrole "3)
3
3
34
gave a 2:1 mixture of the corresponding 5 and 4 substituted
aldehydes 4a and b, which were separated by chromato-
3
graphy. After oxidation of 4b with permanganate accord-
5
3
4
ing to a literature procedure, the acid 5 was treated with
guanidinium chloride in sodium methoxide to give the
desired zwitterion 2.
1
9±27
water.
Recently, we introduced a novel class of carboxylate±
guanidiniocarbonyl pyrrole zwitterions, which show strong
self-association in polar solution. For example, the
heteroditopic zwitterion 1 forms discrete dimers held
2.2. Self-association properties
2
8,29
1
In contrast to zwitterion 1, the H NMR spectrum of 2 does
not show any signs of intermolecular association at sub-
millimolar concentrations in [D ]DMSO. Whereas the
together by ion pairing in combination with multiple hydro-
The stability constant for the dimer 1 in
6
3
0±32
gen bonds.
spectrum of 1 clearly showed a strong interaction between
the carboxylate group and the guanidiniocarbonyl pyrrole
2
12
21
DMSO could be estimated as K<10 M . Herein, the
2
moiety, the spectrum of 2 at this concentration "0.1 mM at
8
3
6
303 K) is consistent with a non-interacting species:
a
Keywords: supramolecular chemistry; molecular recognition; guanidinium
cations; zwitterions; oligomerization.
broad signal at dˆ8.2 for the 4 guanidinium NH protons,
2
p
a broad signal for the amide NH at dˆ11.0, and a singlet at
dˆ12.7 for the pyrrole NH. However, these shifts are
Tel.: 1221-4470-3094; fax: 149-221-470-5102;
e-mail: carsten.schmuck@uni-koeln.de
0040±4020/01/$ -see front matter q 2001 Published by Elsevier Science Ltd.
PII: S0040-4020"01)00156-9

3064
C. Schmuck / Tetrahedron 57 (2001) 3063±3067
O
O
O
O
O
O
N
H
H
H
O
HN
NH2
N
NH2
N
NH2
NH2
N
=
N
H
O
NH2
NH2
H
O
1
2
2
O
H
H
MeO
a
MeO
N
H
N
+ MeO
O
O
H
O
N
O
H
3
4a
Figure 2. Formation of linear aggregates through self-assembly of
zwitterion 2.
4b
O
O
OH
O
b
c
H
isothermic binding curve "Fig. 3), clearly showing that
concentration dependent intermolecular association takes
place.
4b
MeO
2
H N
N
N
H
N
O
NH2
O
H
39
5
2
As the complexation is fast on the NMR time scale, the
observed chemical shift dobs is the weighted average of
the shifts for the complexed "doligo) and the uncomplexed
Scheme 1. Synthesis of compound 2 "a) POCl
3
, DMF, CH
2
2
Cl , chromato-
graphy on silica gel "hexane/ethyl acetate 60:40) gave pure 4b in 24% yield
besides 60% of 4a; "b) KMnO ; acetone/water, 67%; "c) guanidinium
chloride "5 equiv.), NaOMe "5 equiv.), MeOH, re¯ux overnight, 47%.
4
40,41
molecule "dfree):
dobs ˆ doligop 1 dfreeꢀ1 2 p†
ꢀ1†
concentration dependent "Fig. 1). In a 100 mM solution in
[
D6]DMSO, the signal for the guanidinium NH protons has
2
shifted to dˆ8.5 and the signal for the amide NH to dˆ12.0,
The parameter p is the fraction of complexed molecules at
each concentration. From this value, the number n of aggre-
gated monomers can be estimated according to Eq. "2):
respectively. Both signals also become broader.
4
0
These results are consistent with the formation of linear
aggregates through intermolecular aggregation between
the carboxylate group of one molecule and the guanidinium
group of another "Fig. 2). As already mentioned above,
zwitterion 2 cannot form discrete dimers due to geometric
reasons. The rather linear arrangement of the self-
complementary binding groups allows only a linear aggre-
n ˆ 1=ꢀ1 2 p†
ꢀ2†
If one takes the chemical shift of the highest diluted solution
"0.1 mM) dfreeˆ8.2 as that of the free, uncomplexed
molecule and the extrapolated value from the binding
isotherm for high concentrations as that of the complexed
molecule within the oligomer "doligoˆ8.7), one can estimate
that at a concentration of 100 mM, where the observed
chemical shift is dobsˆ8.5, an average of 2.5 molecules of
2 are aggregated. This means that in the concentration range
studied, dimerization is the main important aggregation
process for zwitterion 2 which has to be considered.
However, in these dimers only one of the two binding
groups of each molecule is used in contrast to the discrete
dimers formed by zwitterion 1. At concentrations above
100 mM, higher oligomers such as trimers or tetramers
may also form in signi®cant amounts. Therefore using
3
7,38
gation leading to the formation of chain-like oligomers.
To determine the binding constant for this association
process, we studied the concentration dependence of the
H NMR spectrum of 2 in [D6]DMSO in the concentration
1
range 1±100 mM following the shift changes of the
guanidinium NHs. The signal for the amide NH becomes
increasingly broad with increasing concentration "Fig. 1) so
that, despite the fact that it shows a larger shift change in
total, this signal is less suitable for an accurate quantitative
analysis of the oligomerization process. A plot of the
observed chemical shift versus concentration gives an
1
Figure 1. Parts of the H NMR spectrum "300 MHz, 303 K) of 2 in [D
the amide NH "concentrations from bottom to top: 1, 2.5, 5, 10, 25, 50, 75 and 100 mM).
6
]DMSO showing the complexation induced shift changes of the guanidinium NHs and

C. Schmuck / Tetrahedron 57 (2001) 3063±3067
3065
H
H
O
H
N
O
R
H
H
H
N
H
N
H
H
O
N
O
N
H
N
N
H
N
H
O
O
R
Scheme 2. Tridentate "left) versus bidentate "right) binding mode in
complexes between guanidiniocarbonyl pyrroles and carboxylates.
agreement with those of other literature data, which
have given association constants for anion binding
by simple guanidinium cations in DMSO ranging from
1
3
21 30,46±49
1
0 to 10 M .
2.3. Temperature dependance
Figure 3. Complexation induced shift changes of the guanidinium NHs in 2
in the concentration range 1±100 mM at various temperatures "from bottom
to top 303, 323, 343 and 363 K). The solid lines represent the curve ®ts
We have also studied the association process of zwitterion 2
at different temperatures from 303 to 363 K using the NMR
dilution experiment described above following the
complexation induced shift change of the guanidinium
NH. The obtained binding curves are shown in Fig. 3.
Non-linear regression analysis using the dimerization
model "Eq. "3)) gave satisfactory curve ®ts in all cases.
6
according to Eq. "3). "300 MHz, [D ]DMSO).
only NMR data from the concentration range #100 mM,
one can calculate the binding constant Kass for the dimeriza-
4
2
tion according to the method of Bangerter and Chan. The
observed chemical shift depends then on the total concen-
tration C and the association constant K in the following
Interestingly, the association constant increases with
increasing temperature from 22.2 M
21
at 303 K to
21
ass
70.5 M at 363 K. From a van't Hoff plot of the calculated
binding constants, the thermodynamic parameters for the
oligomerization process of zwitterion 2 in DMSO can be
obtained "Fig. 4).
4
3
way "Eq. "3)):
p
1 4K C 2 1 1 8K C
1
ass
ass
dobs ˆ dfree
1
ꢀdoligo 2 dfree†
4
K C
ass
ꢀ
3†
These data show that the association of 2, at least in the
temperature range studied, is endothermic "DHˆ
2
1
1
7.6 kJ mol ) and therefore driven by entropy "DSˆ
2
3.4 J K mol ). Similar endothermic binding events
1
21
Using a non-linear curve ®tting procedure for the data in
Fig. 3, the binding constant Kass for the aggregation of
8
have been reported previously for the binding of anions
such as sulfate to ditopic guanidinium cations in polar
2
1
zwitterion 2 is calculated to be 22.2 M at 303 K. This
binding constant is lower by ca. 3 orders of magnitude
than the association constants that we found previously for
carboxylate binding by the isomeric zwitterion 1 and similar
4
6,50
solvents
or for the formation of `softball' dimers in
5
1
chloroform. The formation of salt bridges and hydrogen
bonds between a donor and acceptor requires ®rst a
desolvation of the corresponding binding sites. In highly
4
4
guanidiniocarbonyl pyrroles. This dramatic decrease in
binding strength going from 1 to 2 suggests that the binding
mode is quite different for these two guanidinium cations.
We have shown before that in the case of 1, the carboxylate
group is bound in a tridentate fashion by one of the
guanidinium NHs, the amide NH and the pyrrole NH
2
8
"
Scheme 2). However, in the case of 2, we do not see
any signi®cant complexation induced down®eld shift of
the pyrrole NH in the NMR spectrum even at high concen-
trations. Also the observed maximum shift changes for both
the amide and the guanidinium NHs are much less
pronounced than in 1, in which, for example, the amide
NH shifts nearly 4 ppm to dˆ15.7, respectively. From
these results one can conclude that in the zwitterion 2, the
carboxylate group is bound only in a bidentate way by the
two H atoms of the guanidinium moiety as also shown in
Scheme 2. Such a bidentate binding mode is energetically
4
5
less stable than the tridentate one. Hence, the binding
mode in the linear oligomers of 2 resembles that of a simple
guanidinium cation and not a guanidiniocarbonyl pyrrole.
Figure 4. van't Hoff plot for the oligomerization of 2 in the temperature
range 303±363 K "values above the line are aggregation constants K in
ass
2
1
21
M , values in parentheses are temperatures in K).
And indeed the binding constant of 22.2 M is in good

3066
C. Schmuck / Tetrahedron 57 (2001) 3063±3067
polar solvents, such as DMSO "eˆ49, mˆ3.96 D), the
energy necessary for this desolvation can obviously be
larger than the energy gain resulting from the following
noncovalent interaction between donor and acceptor,
making the overall process endothermic. However, the
release of ordered solvent molecules from the binding
sites increases the entropy of the system, so that the binding
"d, pyrrole C3), 124.8 "s, pyrrole C2), 127.6 "s, pyrrole C4),
128.6 "d, pyrrole C5), 161.3 "s, ester C), 185.6 "s, CHO).
1
4b. H NMR "300 MHz, [D ]DMSO) dˆ3.90 "s, 3H,
6
methyl), 6.91 "m, 2H, pyrrole CH), 9.65 "s, 1H, CHO),
C NMR "75.5 MHz,
1
3
9.96 "s, 1H, pyrrole NH).
[D ]DMSO) dˆ52.2 "q, methyl), 115.7 "d, pyrrole C4),
6
5
2
event is still energetically favorable.
119.7 "d, pyrrole C3), 128.1 "s, pyrrole C2), 134.5 "s,
pyrrole C5), 160.8 "s, ester C), 180.3 "s, 1C, CHO).
4
ꢀ
.1.2. 2-ꢀGuanidiniocarbonyl)-1H-pyrrole-4-carboxylate
3.Conclusions
34
2). The ester 5 "2.00 g, 12 mmol) and guanidinium hydro-
chloride "5.70 g, 60 mmol) were added to a solution of
sodium methoxide, prepared from sodium "1.29 g,
In summary, it was shown here by concentration and
temperature dependent NMR studies that zwitterion 2
forms linear aggregates in an endothermic and therefore
entropy driven process. This underlines the importance of
solvation effects for effective host±guest binding in polar
solvents. It is therefore necessary for the future design of
stable supramolecular arrangements to not only concentrate
on enthalpic contributions but also on the role the solvent
plays for the overall binding.
56 mmol) in methanol "50 mL). The reaction mixture was
re¯uxed for 12 h and then the solvent was evaporated. The
oily residue was dissolved in water "50 mL). Upon acidi®-
cation with concentrated hydrochloric acid, the crude
product precipitated, which was ®ltered off and washed
with water and a small portion of ice cold methanol to afford
1
a white solid "1.10 g, 47%). H NMR "300 MHz,
[
D ]DMSO): dˆ7.61 "d, 1H, pyrrole CH), 7.79 "s, 1H,
6
pyrrole CH), 8.21 "brs, 4H, guanidinium NH ), 11.02 "brs,
2
1
3
1
H, amide NH), 12.03 "s, 1H, pyrrole NH). C NMR
"75.5 MHz, [D
]DMSO): dˆ115.38 "CH), 116.24 "CH),
27.45, 131.25, 155.23, 159.61, 161.18 "all quat. C).
O "picrate salt, 443.08): calcd: C, 35.02;
H, 2.93; N, 22.12; found: C, 35.25; H, 3.15; N, 22.36.
4.Experimental
6
1
4
.1. General remarks
C H N O10´1H
13 11 7 2
Solvents were dried and distilled under argon before use. All
other reagents were used as obtained from either Aldrich or
Fluka. All experiments were run in oven dried glass ware
under argon unless otherwise stated. Products were dried in
4
.2. NMR experiments
Solutions of zwitterion 2 with varying concentrations were
obtained by diluting aliquots of a 100 mM stock solution
with [D6]DMSO to a total volume of 600 ml. The chemical
shifts were recorded "Bruker AM300 spectrometer) for each
sample at increasing temperatures starting from 303 K in
intervals of 20 degrees. After changing the temperature,
an equilibration time of 5 min was allowed for each sample
before recording the spectrum.
2
3
high vacuum "10 mbar) over phosphorus pentoxide at
1
13
room temperature overnight. H and C NMR spectra
were recorded on a Bruker AM300 spectrometer. Shifts
are reported relative to the deuterated solvents. Elemental
analysis was carried out with an Elementar Vario EL.
4
.1.1. Methyl 4-ꢀformyl)-1H-pyrrole-2-carboxylate ꢀ4b).
Phosphoryl chloride "60.7 g, 396 mmol) was added within
5 min to ice cold DMF "28.9 g, 396 mmol). The reaction
1
mixture was allowed to warm to room temperature, diluted
with CH Cl "200 mL) and cooled again to 08C. A solution
Acknowledgements
2
2
3
of ester 3 "45.0 g, 360 mmol) in CH Cl "200 mL) was
4
2
2
This work was ®nancially supported by the Deutsche
Forschungsgemeinschaft "SCHM 1501/1-1; 1501/1-2).
The author thanks Professor Albrecht Berkessel, K oÈ ln, for
his generous support.
added dropwise within 1 h, while the temperature was
kept at 08C. After the addition was complete, the reaction
mixture was re¯uxed for 30 min under vigorous stirring,
cooled to 108C and hydrolyzed with a solution of sodium
acetate "162.0 g) in water "500 mL). The phases were sepa-
rated and the aqueous phase was extracted three times with
diethyl ether "200 mL). The combined organic extracts were
washed with a saturated sodium carbonate solution until no
References
further CO evolved and dried with solid sodium carbonate.
2
1. Philp, D.; Stoddart, J. F. Angew. Chem., Int. Ed. Engl. 1996,
35, 1154±1196.
The solvent was removed under reduced pressure and the
oily, orange residue was puri®ed by chromatography "silica
gel, hexane/ethyl acetate 60:40) to yield 4a "33.0 g,
2. Lehn, J.-M. Supramolecular Chemistry: Concepts and
Perspectives; VCH: Weinheim, 1995.
216 mmol, 60%) and 4b "13.5 g, 88 mmol, 24%) both as
slightly yellow solids.
3. Whitesides, G. M.; Mathias, J. P.; Seto, C. T. Science 1991,
254, 1312±1319.
4
5. Weber, E. "Ed.). Design of Organic Solids. Top. Curr. Chem.
. Bishop, R. Synlett 1999, 1351±1359.
1
4
methyl), 7.29 "m, 1H, pyrrole CH), 7.56 "m, 1H, pyrrole
a. H NMR "300 MHz, [D ]DMSO) dˆ3.87 "s, 3H,
6
1998, 198.
6. Desiraju, G. R. Angew. Chem., Int. Ed. Engl. 1995, 34, 2328±
2345.
1
3
CH), 9.82 "s, 1H, CHO), 10.06 "s, 1H, pyrrole NH).
NMR "75.5 MHz, [D ]DMSO) dˆ52.0 "q, methyl), 114.2
C
6

C. Schmuck / Tetrahedron 57 (2001) 3063±3067
3067
7
8
9
. Desiraju, G. R. Crystal Engineering: The Design of Organic
Solids; Elsevier: New York, 1989.
of Anions; Bianchi, A., Bowman-James, K., Garc Âõ a-Espa nÄ a,
E., Eds.; Wiley-VCH: New York, 1997; pp 79±146.
32. Atwood, J. L.; Steed, J. W. Structural and topological aspects
of anion coordination. In Supramolecular Chemistry of
Anions; Bianchi, A., Bowman-James, K., Garc Âõ a-Espa nÄ a, E.,
Eds.; Wiley-VCH: New York, 1997; pp 147±216.
33. Silverstein, R. M.; Ryskiewicz, E. E.; Willard, C. In Organic
Synthesis; Wiley: New York, 1963; Collect. Vol. IV, pp 831±
833.
. Wright, J. D. Molecular Crystals; Cambridge University
Press: Cambridge, 1987.
. Chin, D. N.; Palmore, T. R.; Whitesides, G. M. J. Am. Chem.
Soc. 1999, 121, 2115±2132.
1
0. For a sampling self-assembled structures in solution: Kang, J.;
Rebek, Jr., J. Nature 1997, 385, 50±52.
1
1. Kang, J.; Rebek, Jr., J. Nature 1996, 382, 239±241.
2. Meissner, R. S.; deMendoza, J.; Rebek, Jr., J. Science 1995,
1
34. Barker, P.; Gendler, P.; Rapoport, H. J. Org. Chem. 1978, 43,
4849±4853 "and references therein).
2
70, 1485±1488.
1
1
1
1
3. For a comprising review article see: Conn, M. M.; Rebek, Jr.,
J. Chem. Rev. 1997, 97, 1647±1668.
4. Terfort, A.; Bowden, N.; Whitesides, G. M. Nature 1997, 386,
35. Whereas it is claimed in Ref. 33 that a Vilsmeier±Haack
formylation of methoxy carbonyl pyrrole 3 only yields the
5-substituted aldehyde 4a, we always obtained mixtures of
4a and its regioisomer 4b.
1
62±164.
5. Bowden, N.; Terfort, A.; Carbeck, J.; Whitesides, G. M.
Science 1997, 276, 233±235.
36. Dixon, R. D.; Geib, S. J.; Hamilton, A. D. J. Am. Chem. Soc.
1992, 114, 365±366.
6. For solvent effects on the strengths of H-bonds in complexes,
see: Horvath, P.; Gergely, A.; Noszal, B. J. Chem. Soc., Perkin
Trans. 1 1996, 1419±1422.
37. For a recent example for the self-organization of other hetero-
ditopic molecules into oligomers in rather unpolar solvents see
e.g.: Ashton, P. R.; Parson, I. W.; Raymo, F. M.; Stoddart, J. F.;
White, A. J. P.; Williams, D. J.; Wolf, R. Angew. Chem., Int.
Ed. Engl. 1998, 37, 1913±1916.
1
7. Kelly, R.; Kim, M. H. J. Am. Chem. Soc. 1994, 116, 7072±
7
080.
1
8. Ariga, K.; Anslyn, E. V. J. Org. Chem. 1992, 57, 417±419.
9. Some general examples, but not self-assembling structures, of
arti®cial receptors that function in more polar solvents, can be
found in: Hossain, A.; Schneider, H.-J. J. Am. Chem. Soc.
38. Yamaguchi, N.; Nagvekar, D. S.; Gibson, H. W. Angew.
Chem., Int. Ed. Engl. 1998, 37, 2361±2364.
1
39. Wilcox, C. S. In Frontiers in Supramolecular Chemistry and
Photochemistry; Schneider, H. J., D uÈ rr, H., Eds.; VCH:
Weinheim, 1990; pp 123±144.
1
998, 120, 11208±11209.
2
2
0. Breslow, R.; Yang, Z.; Ching, R.; Trojandt, G.; Odobel, F.
J. Am. Chem. Soc. 1998, 120, 3536±3537.
40. Connors, K. A. Binding Constants; Wiley: New York, 1987
"Chapters 2 and 5).
1. Peczuh, M. W.; Hamilton, A. D.; Sanchez-Qesada, J.; de
Mendoza, J.; Haak, T.; Giralt, E. J. Am. Chem. Soc. 1997,
41. Davis, Jr., J. C.; Deb, K. K. Adv. Magn. Reson. 1970, 4, 201±
270.
1
19, 9327±9328.
2. Berger, M.; Schmidtchen, F. P. J. Am. Chem. Soc. 1996, 118,
947±8948.
42. Bangerter, B. W.; Chan, S. I. J. Am. Chem. Soc. 1969, 91,
3910±3921.
2
2
2
2
2
2
8
43. For a general discussion of chain-association equilibria studies
by NMR see: LaPlanche, L. A.; Thompson, H. B.; Rogers,
M. T. J. Phys. Chem. 1965, 69, 1482±1488.
3. Fan, E.; Van Arman, S. A.; Kincaid, S.; Hamilton, A. D. J. Am.
Chem. Soc. 1993, 115, 369±370.
4. Schieûl, P.; Schmidtchen, F. P. Tetrahedron Lett. 1993, 2449±
44. Schmuck, C. Chem. Eur. J. 2000, 6, 709±718.
45. Schmuck, C.; Lex, J. Org. Lett. 1999, 1, 1779±1781.
46. Berger, M.; Schmidtchen, F. P. J. Am. Chem. Soc. 1999, 121,
9986±9993.
2
452.
5. Furuta, H.; Magda, D.; Sessler, J. L. J. Am. Chem. Soc. 1991,
13, 978±985.
1
6. Kelly-Rowley, A. M.; Cabell, L. A.; Anslyn, E. V. J. Am.
Chem. Soc. 1991, 113, 9687±9688.
7. Constant, J. F.; Fahy, J.; Lhomme, J. Tetrahedron Lett. 1987,
47. Berger, M.; Schmidtchen, F. P. Angew. Chem., Int. Ed. Engl.
1998, 37, 2694±2696.
48. Berger, M.; Schmidtchen, F. P. J. Am. Chem. Soc. 1996, 118,
8947±8948.
1
777±1780.
2
2
3
8. Schmuck, C. Eur. J. Org. Chem. 1999, 2397±2403.
9. Schmuck, C. J. Org. Chem. 2000, 65, 2432±2437.
0. Review articles on the binding of carboxylates by arti®cial
49. Peschke, W.; Schmidtchen, F. P. Tetrahedron Lett. 1995, 36,
5155±5158.
50. Linton, B.; Hamilton, A. D. Tetrahedron 1999, 55, 6027±
6038.
receptors
Schmidtchen, F. P.; Berger, M. Chem. Rev. 1997, 97, 1609±
646.
including
guanidinium-based
systems:
51. Meissner, R.; Garcias, X.; Mecozzi, S.; Rebek, Jr., J. Am.
Chem. Soc. 1997, 119, 77±85.
1
3
1. Schmidtchen, F. P. In Arti®cial Anion Hosts: Concepts for
Structure and Guest Binding in Supramolecular Chemistry
52. Williams, D. H.; Westwell, M. S. Chem. Soc. Rev. 1998, 27,
57±64 "and references therein).