Structural Studies on Hydrogen-Bonding Receptors for Barbiturate Guests that Use Metal Ions as Allosteric Inhibitors

Article abstract of DOI:10.1002/ejoc.200300389

Receptor 1 was designed to bind barbiturate substrates through a six-point hydrogen-bonding motif only in the absence of metal allosteric cofactors. It was predicted that the binding of metal ions by bipyridine ligands in 1 would result in a geometric change in the receptor to inhibit substrate recognition. However, receptor 1 showed minimal affinity for the barbiturate quests even in the absence of the metal. Binding studies on model compounds 2, 3, 5, and 6 revealed that the inactivity of 1 is due to an intramolecular hydrogen bond between the N-H donor groups and the nitrogen atoms on the first heterocycle of the bipyridine ligands. This intramolecular hydrogen-bonding was eliminated by altering the position of the tether between the bipyridine ligands and the active site to produce receptor 7. Consequently, the high affinity exhibited by 7 for the barbiturate substrate (K_a = 2.8 +/- 0.7*10³ M^-1 in 9 : 1 CD2Cl2/CD3CN) was significantly reduced by the addition of Zn^II ions as a negative allosteric co-factor.

Full text of DOI:10.1002/ejoc.200300389

FULL PAPER
Structural Studies on Hydrogen-Bonding Receptors for Barbiturate Guests
That Use Metal Ions as Allosteric Inhibitors
Mohammad H. Al-Sayah,[b] Robert McDonald,[b] and Neil R. Branda*[a]
Keywords: Allosterism / Host-guest systems / Hydrogen bonds / Molecular recognition / Receptors
Receptor 1 was designed to bind barbiturate substrates
on the first heterocycle of the bipyridine ligands. This intra-
through a six-point hydrogen-bonding motif only in the ab- molecular hydrogen-bonding was eliminated by altering the
sence of metal allosteric cofactors. It was predicted that the
binding of metal ions by bipyridine ligands in 1 would result
in a geometric change in the receptor to inhibit substrate re-
cognition. However, receptor 1 showed minimal affinity for
the barbiturate guests even in the absence of the metal.
Binding studies on model compounds 2, 3, 5, and 6 revealed
that the inactivity of 1 is due to an intramolecular hydrogen
bond between the N−H donor groups and the nitrogen atoms
position of the tether between the bipyridine ligands and the
active site to produce receptor 7. Consequently, the high af-
finity exhibited by 7 for the barbiturate substrate (Ka
=
−1
2.8 0.7 × 103
M
in 9:1 CD2Cl2/CD3CN) was significantly
reduced by the addition of ZnII ions as a negative allosteric
co-factor.
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2004)
Introduction
We have used the complexation of CuI ions by bipyridine
ligands to inhibit molecular recognition in hydrogen-
bonded hostϪguest complexes.[29,30] In our examples, the
negative allostery is a direct result of the complexation-in-
duced distortion of the hydrogen-bonding surface respon-
sible for interacting with the substrate. Here we introduce
an alternative to distorting a molecular recognition site and
describe a receptor in which the binding of a metal ion by
bipyridine ligands within the receptor alters the accessibility
to and the size of the binding pocket rendering it unsuitable
to accommodate the guest species. We have chosen barbitu-
rates as substrates because their wide use as sedatives and
anticonvulsants[31,32] make them attractive targets. The re-
ported examples where barbiturates are bound and sepa-
rated from mixtures of organic compounds and biological
solutions are based on hydrogen-bonding networks that are
formed within the hostϪguest complex.[33Ϫ38] The ability
to control the binding between the molecular recognition
partners would enhance a receptor’s versatility.
Nature often uses allostery to reversibly alter molecular
structure and consequently regulate dynamic functions such
as molecular recognition and catalytic activity. In these
complex natural systems, the binding of a co-factor at a
location that is not the active site promotes reversible con-
formational alterations that are transmitted across the mo-
lecular structure to the active site, thereby, varying the re-
ceptor’s affinity for its substrate. Allosteric effects are con-
sidered to be positive when the induced conformational
changes increase the binding efficacy, and negative when
the initial interaction of the cofactor results in a decrease
in the binding efficacy.[1,2]
Inspired by these natural systems, chemists are building
smaller, synthetically accessible molecules that mimic the
behavior of natural receptors. Several artificial receptors, in
which the binding ability is regulated through allosteric ef-
fects, have been reported.[3Ϫ27] In one recent example, Na-
beshima and co-workers describe a receptor where the com-
plexation of FeII by bipyridine ligands tethered to the ends
of three polyether chains regulates the receptor’s affinity for
alkaline-earth metals.[28] This allostery is based on the FeII-
induced rearrangement of the polyether chains to form an
ionophoric pseudocryptand.
Receptor Design
All of the receptors 1Ϫ7 are adaptations of the bis(2,6-
diaminopyridine) structural motif that has been successfully
used to bind barbiturate guests.[39,40] As is illustrated with
the first receptor we designed (1) in Scheme 1, the
hostϪguest recognition is based on the receptor’s triangu-
lar-shaped cleft that is lined with two convergent hydrogen
[a]
Department of Chemistry, Simon Fraser University,
8888 University Drive, Burnaby, British Columbia, Canada
V5A 1S6
bonding donorϪacceptorϪdonor arrays furnishing
a
Fax: (internat.) ϩ 1-604-291-3765
E-mail: nbranda@sfu.ca
complementary binding site for the guest. The isophthoyl
linker enforces the correct shape and size of the binding
pocket. It also prevents intramolecular hydrogen-bonding
[b]
Department of Chemistry, University of Alberta,
Edmonton, AB, Canada T6G 2G2
Eur. J. Org. Chem. 2004, 173Ϫ182
DOI: 10.1002/ejoc.200300389
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
173

M. H. Al-Sayah, R. McDonald, N. R. Branda
FULL PAPER
1, ultimately leading to the realization of a successful allos-
teric host (7 Ǟ 7·Zn).
Results and Discussion
Synthesis of the Receptors
All of the receptors 1Ϫ7 can be constructed from simple
building blocks using procedures adapted from the litera-
ture.[39,40] The syntheses begin by preparing the two bis(2,6-
diaminopyridine) derivatives 8a and 8b, which differ only in
the presence of the organic-solubilizing tert-butyl group on
the central benzene ring. These two isophthalamides are re-
adily prepared by reacting isophthaloyl and 5-tert-butyl-
isophthaloyl chloride with an excess of 2,6-diaminopyridine
as shown in Scheme 2. Receptors 1Ϫ7 can then be con-
structed from either 8a or 8b and the acid chloride of the
appropriate carboxylic acid.
Scheme 1. The allosteric regulation in receptor 1; the coordination
of a metal ion closes the entrance of the hydrogen-bond-lined cleft
in receptor 1 and shuts down the binding of barbiturate guests
between the two diaminopyridine units, although as will be
described later in this report, the presence of other intra-
molecular hydrogen bonds will be an obstacle.
What makes receptor 1 unique is the two metal-coordin-
ating bipyridine ligands that are flanking the entrance of
the recognition cleft. The receptor was designed to respond
to the presence of metal ions by undergoing dramatic struc-
tural changes when the ligands swing towards each other
and chelate the metal. The metal complexation effectively
seals the entrance of the cleft and forces the two aminopyri-
dyl groups closer to each other causing a shrinkage in the
binding cavity and a distortion of the hydrogen bond sur-
face lining it. These effects render the binding site inaccess-
ible to the guest and unsuitable in size and shape. The bind-
ing of the barbiturate by the receptor should be inhibited
(negative allostery). Scheme 1 illustrates this concept using
Scheme 2. Synthesis of receptors 1Ϫ7
The isomeric bipyridine carboxylic acids 10 and 12, re-
receptor 1 which offers the 6,6Ј-disubstituted 2,2Ј-bipyridyl quired to construct bipyridine receptors 1 and 7, are synthe-
allosteric metal-binding site. The allosteric cofactor is a CuI sized as shown in Scheme 3 by oxidizing the known bipyri-
ion. We have already used this metal-ligand partnership dine carboxaldehydes 9[41] and 11.[42] Commercially avail-
with success.[29,30] In this paper we describe how receptor 1 able 3,5-di-tert-butylbenzoic acid, picolinic acid, and nic-
is, in fact, not capable of binding a barbiturate even in the otinic acid are used to synthesize receptors 2, 3, and 4,
absence of the metallic cofactor. Several model compounds respectively. 6-Butoxynicotinic acid, which is required for
are used to identify the structural problems with receptor the synthesis of receptor 5, is obtained from 6-bromonic-
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Structural Studies on Hydrogen-Bonding Receptors
FULL PAPER
otinic acid according to the literature procedures.[43] The
N,N-dibutylaminocarbonyl-substituted nicotinic acid (13) is
prepared in one step from 3,5-pyridinedicarboxylic acid as
is also illustrated in Scheme 3. This modified nicotinic acid
can be easily converted into receptor 6 by reacting its acid
chloride with the bis(2,6-diaminopyridine) compound 8a.
Figure 1. Changes in the chemical shift of the signals correspond-
ing to the NϪHa protons in the 1H NMR spectra of solutions
(5 m in CD2Cl2) of receptors 1 (ᮀ), 2 (᭹) and 3 (᭜) upon the
addition of
CD2Cl2)
a solution of 5,5-dibutylbarbiturate (100 m in
Table 1. The chemical shifts corresponding to the NϪHa and
NϪHb protons in the 1H NMR spectra of receptors 1, 2, 3, 5,
6, and 7 before and after the addition of 1 molar equivalent of
5,5-dibutylbarbiturate
Receptor[a]
alone
NϪHb
(δ/ppm) (δ/ppm) (δ/ppm)
ϩ 5,5-dibutylbarbiturate[b]
Scheme 3. Synthesis of the carbxylic acids required to prepare re-
ceptors 1, 6, and 7
NϪHa
NϪHa
NϪHb
(δ/ppm)
1
2
3
5
6
7
10.35
8.52
10.48
8.56
9.30
8.83
8.83
8.37
8.12
8.33
9.20
8.68
10.36
9.75
10.49
9.74
9.84
9.55
8.87
9.62
8.11
9.52
9.77
9.49
The Binding Activity of Receptor 1 Towards Barbiturate
Guests
The ability of bipyridine receptor 1 to recognize and bind
its barbiturate substrate is readily evaluated using 1H NMR
spectroscopy. Because the hydrogen atoms in the hydrogen-
bond donors NϪHa and NϪHb (see Scheme 2 for atom
assignments) are central to successful guest binding, the
changes in the chemical shifts of the signals corresponding
[a]
[b]
5 m CD2Cl2. 100 m in CD2Cl2.
to these protons when the guest is added can be monitored. very low and uncharacterizable affinity for its barbiturate
Surprisingly, when a CD2Cl2 solution of receptor 1 is guest.
treated with aliquot amounts of a solution of 5,5-dibutylba-
Unlike receptor 1, in the absence of the barbiturate guest,
rbiturate in the same solvent, there are insignificant changes the signals corresponding to the NϪHa and NϪHb protons
1
in the positions (∆δ less than 0.1 ppm) of the signals corre- in 2 appear upfield in the H NMR spectrum (CD2Cl2) at
sponding to the NϪHa (Figure 1) and NϪHb (not shown) δ ϭ 8.52 and 8.37 ppm, respectively (Table 1). This can be
protons, which appear at 10.35 and 8.83 ppm, respectively expected of a bis(2,6-diaminopyridine)-based receptor that
before the addition of the substrate (Table 1).
is free of intramolecular hydrogen bonds. Figure 1 shows
The atypical downfield position of the signal for the the significant downfield changes (∆δ more than 1.5 ppm)
NϪHa protons (the signals for these protons in similar sys- in the chemical shift of the NϪHa protons as the solution
tems appear in the range δ ϭ 8.2Ϫ8.8 ppm)[39,40,44,45] of the receptor is treated with a solution 5,5-dibutylbarbitu-
strongly suggests that these hydrogens are tied up in hydro- rate indicating effective hydrogen-bonding between host
gen bonds. The fact that the position and shape of the sig- and guest. The titration data correlate well with the calcu-
nal at δ ϭ 10.35 ppm do not vary upon diluting the solution lated curve for a 1:1 binding model giving an association
argues that any hydrogen-bonding must be intramolecular, constant (Ka) of 8000 Ϫ1 in CD2Cl2 indicating that the
most likely between the NϪHa donor and one of the nitro- steric bulk imposed by the aromatic substituents at the rim
gen atom acceptors of the bipyridine rings. These interac- of the binding pocket does not prevent the access of the
tions may lead to the folding of the receptor into a confor- substrate to receptor 2. It would be unlikely, therefore, that
mation incapable of binding the barbiturate as is shown by spatial bulk alone is the reason for receptor 1Јs inability to
the fact that the chemical shifts are unaffected by the pres- bind its barbiturate substrate.
ence of the guest species. As a consequence, the titration
Pyridyl receptors 3, 4, and 5 are useful to identify which
data do not fit any binding model. Receptor 1 exhibits a of the nitrogen heterocycles is responsible for the intramol-
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M. H. Al-Sayah, R. McDonald, N. R. Branda
FULL PAPER
ecular hydrogen-bonding in receptor 1. 1H NMR spec- dibutylbarbiturate to a CD2Cl2 solution of the 3-pyridyl re-
troscopy studies show that the molecular recognition be- ceptor 5, therefore, results in the downfield shift of the sig-
havior of 2-pyridyl receptor 3 is similar to that of the bipyri- nals corresponding to the NϪHa and NϪHb protons to
dyl receptor 1. In the absence of any 5,5-dibutylbarbiturate, positions similar to those for the active receptor 2 (Table 1).
the signal corresponding to the NϪHa (10.48 ppm in Because the signals corresponding to NϪHa and NϪHb
CD2Cl2) protons of receptor 3 is shifted downfield similar shift significantly downfield and reach saturation after 1Ϫ2
to that of the bipyridyl receptor 1 (Table 1). The signal, as molar equivalents of 5,5-dibutylbarbiturate are added to
well as that for the NϪHb protons, also undergoes minimal the active receptor 2, the activity of any particular receptor
changes in its chemical shift (∆δ less than 0.1) upon the can be tested simply by the addition of only one equivalent
addition of the barbiturate (Figure 1) indicating little as- of the substrate. Complete 1H NMR titrations are only
sociation between the receptor and the barbiturate. At this necessary when an estimation of the association constant is
stage, to conclude that the pyridine directly attached to the absolutely indispensable.
1
bis(2,6-diaminopyridine) scaffold is involved in intramol-
The signals in the H NMR spectrum corresponding to
ecular hydrogen-bonding to the NϪHa protons is not un- the NϪHa and NϪHb protons in 3-pyridyl receptor 6
reasonable. (CD2Cl2) initially appear at δ ϭ 9.30 and 9.20 ppm, respec-
This conclusion can be supported by examining the tively (Table 1), which are further downfield than those for
structure of 2-pyridyl receptor 3 in the crystalline state. X- receptors 2 and 5 but not as far downfield as those for re-
ray quality crystals of 3 can be grown by slowly evaporating ceptors 1 and 3. In the case of 6, the signals shift upfield to
a chloroform solution of the receptor. The crystal structure δ ϭ 9.04 ppm for NϪHa and to 8.93 ppm for NϪHb when
(Figure 2) reveals the presence of intramolecular hydrogen the sample is diluted from 5 m to 0.3 m signifying that
bonds between the NϪHa protons and the nitrogen atoms these protons are involved in intermolecular not intramol-
on the pyridine rings that are closer to the mouth of the ecular hydrogen-bonding. This aggregation does not signifi-
binding pocket. The structure of the receptor in the crystal cantly interfere with the binding of barbiturates, and the
also adopts a non-productive conformation and the two addition of one molar equivalent of 5,5-dibutylbarbiturate
donorϪacceptorϪdonor
hydrogen-bonding
surfaces to the 3-pyridyl receptor 6 induces a downfield shifting of
(N1ϪH, N10, N2ϪH, and N3ϪH, N30, N4ϪH) diverge the signals for protons NϪHa and NϪHb to δ ϭ 9.84 ppm
away from each other. The C1ϪC9 bond must rotate as and 9.77 ppm, respectively (Table 1).
much as 173° to regenerate the active hydrogen-bond-lined
The binding behavior of receptors 1Ϫ6 suggests that in-
tramolecular hydrogen bonding between the NϪHa protons
and the nitrogen atoms of the pyridine rings directly at-
tached to the bis(2,6-diaminopyridine) scaffold (1 and 3, for
example) is the root of the poor host-guest association. It
appears that the pyridine rings further removed from the
rim of the binding pocket in bipyridine receptor 1 play little
role in the inhibition of the molecular recognition and re-
ceptors lacking the intramolecular hydrogen bond (2, 5, and
6) retain their ability to complex 5,5-dibutylbarbiturate.
Three possible conformations of the bipyridyl receptor 1
are presented in Scheme 4. We propose that the intramol-
ecular hydrogen bonds between the NϪHa donor and the
acceptors on the adjacent pyridine rings lock the bottom
portion of the receptor into the planar geometry common
to all three conformers 1A, 1B, and 1C. This suggestion is
strengthened by the existence of an NOE (in CD2Cl2) be-
tween the NϪHa protons and the CϪHc protons on the
outermost pyridine rings as shown by the double-headed
arrows in the scheme.
cleft.
Figure 2. The structure of 2-pyridyl receptor 3 in the crystal show-
ing the intramolecular N2ϪH···N20 and N4ϪH···N40 hydrogen
˚
bonds (dotted lines); the distances of 2.16 A for N2ϪH···N20 and
˚
2.19 A for N4ϪH···N40 are well within hydrogen bond allowances;
torsional angles are 3.7° for N2ϪC8ϪC21ϪN20 and 8.0° for
N4ϪC10ϪC41ϪN40
The 3-pyridyl receptor 4 is too insoluble in non-competi-
We also believe that conformer 1A is not favoured due to
tive solvents such as CHCl3 and CH2Cl2 to estimate the the electron-pair repulsion between the carbonyl oxygens
association constants with barbiturate guests, however, the and the pyridine nitrogens of the two bis(2,6-diaminopyrid-
n-butoxy groups attached to the pyridine rings in 5 solubil- ine)s. This is also highlighted in Scheme 4 and is supported
ize this receptor enough to investigate its binding behavior. by the absence of an NOE between the NϪHa protons and
The signal corresponding to the NϪHa protons (8.56 ppm) the CϪHd protons. In order to completely relieve the N···O
1
in the H NMR spectrum of receptor 5 (CD2Cl2) indicates strain, rotation of the CϪN bonds forces the two bipyridine
that these hydrogens are not already involved in hydrogen- ligands to converge directly above the entrance of the bind-
bonding, presumably because the hydrogen-bond acceptors ing pocket as is illustrated by conformer 1B. Presumably,
reside further from the donors in 3-pyridine 5 than in 2- the entrance of the guest into the binding site is hindered
pyridine 3. The addition of one molar equivalent of 5,5- in this conformer. This ligand···ligand steric strain can be
176
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Structural Studies on Hydrogen-Bonding Receptors
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Scheme 4. Three possible conformations of receptor 1; the NOEs are shown by double-headed arrows and the steric and electronic
repulsions are shown by intersecting curves
1
eliminated by rotation of the CϪC bond to form conformer
The H NMR spectrum of coordination complex 7·Zn
1C. This geometry is similar to the one observed in the crys- exhibits significant changes in the chemical shifts of the sig-
tal structure of receptor 3 and can be assumed to be inac- nals in the aromatic region due to the chelation of the ZnII
tive towards binding barbiturate guests as the hydrogen- ion by the bipyridine ligands of the receptor (Figure 3). The
bonding donorϪacceptorϪdonor surfaces are divergent signals corresponding to the aromatic protons of the bipyri-
and the binding pocket is destroyed.
dine rings (HdϪHh) shift downfield as would be expected
due to the electron-withdrawing effect of the Lewis acid.
The exception is the upfield shift of the signal for Hc, which
can be attributed to the placement of this proton within the
shielding cone of the opposite bipyridine ligand as has been
previously observed for similar zinc complexes.[47,48] The
upfield shift of the signal corresponding to protons Hi upon
the complexation of the metal suggests that they are less
deshielded by the carbonyl oxygen of the adjacent amide,
which is rotated away from Hi because of the geometric
requirements of the metal coordination. We attribute the
downfield shift of the signals for protons Hl to restricting
the rotation of the adjacent CϪCϭO single bond, which
forces the carbonyl groups to be in close proximity to Hl.
Electrospray mass spectrometry also supports the forma-
tion of the complex 7·Zn. A peak at m/z ϭ 1159.1648 corre-
sponding to the mass of [7 ϩ Zn ϩ 2OTf ϩ H]ϩ and an-
other peak at m/z ϭ 859.2445 which corresponds to [7ϩZn-
H]ϩ are both present in the mass spectrum.
A Successful Bipyridine-Based Receptor 7·Zn
Receptors 5 and 6 are active binders of barbiturates be-
cause of the inability of the pyridyl nitrogen to hydrogen
bond with NϪHa. The 5,5Ј-disubstituted bipyridyl receptor
7 should, therefore, also be active (Scheme 2). In this case,
bis(2,6-diaminopyridine) 8b was used to enhance the solu-
bility of the receptor in CH2Cl2 and CHCl3. The allosteric
regulation using receptor 7 cannot be tested using copper
ion as the co-factor because the preparation of 7·Cu pro-
duces uncharacterizable coordination complexes. This is not
completely surprising because CuI complexes of 5,5Ј-disub-
stituted bipyridines are generally less stable than analogous
complexes prepared with the 6,6Ј-disubstituted bipyri-
dines.[46] However, the air stable ZnII complex 7·Zn can pre-
pared by treating the bipyridyl receptor 7 with one equiva-
lent of zinc triflate in 10% CH3CN/CH2Cl2 (Scheme 5).
The binding ability of bipyridyl receptor 7 and its zinc
complex 7·Zn to the barbiturate substrate can be tested in
Figure 3. Partial 1H NMR spectrum (500 MHz, 9:1 CD2Cl2/
CD3CN) of the bipyridyl receptor 7 and its zinc complex 7·Zn. The
aromatic peaks are assigned on the structures based on 2-dimen-
sional NMR studies (GCOSY and TROESY) of the two com-
pounds. See Scheme 5 for atom labels
Scheme 5. The complexation of zinc ion seals the entrance of the
binding cavity in receptor 7
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M. H. Al-Sayah, R. McDonald, N. R. Branda
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an identical fashion as described previously by monitoring barbiturate substrate with an association constant Kaϭ
the changes in the chemical shifts of the signals correspond- 2.8Ϯ0.7 ϫ 103 Ϫ1 in 9:1 CD2Cl2/CD3CN. This activity of
ing to the NϪHa and NϪHb protons upon the addition of receptor 7 is inhibited by the complexation of ZnII ions to
5,5-dibutylbarbiturate to a solution of the receptor in the bipyridine ligands which leads to the reduction of the
CD2Cl2. Before the addition of the substrate, the signals size of the receptor’s binding-pocket. Consequently, the de-
appear at δ ϭ 8.83 ppm and 8.68 ppm, respectively sign of an allosteric receptor demands a considerable atten-
(Table 1). The addition of one molar equivalent of 5,5-dibu- tion not only to the organization of the active and allosteric
tylbarbiturate shift the signals downfield as was observed sites but even more to the mode of connectivity between
for the other active receptors (2, 5, and 6), indicative of the the two sites. Such a connection should preserve the inde-
formation of effective hydrogen-bonding between the recep- pendent binding abilities of the two sites (in the absence of
tor and the substrate. 1H NMR titration experiments (2 m the co-factor), yet, still be able to communicate the regulat-
in 9:1 CD2Cl2/CD3CN mixture) estimate the value of Ka to ing information from the allosteric site to the binding site
be 2.8Ϯ0.7 ϫ 103 Ϫ1 when the collected data is fit to a 1:1 upon binding of the co-factor.
binding model (Figure 4).
Experimental Section
General Remarks: All solvents for synthesis were purchased from
Caledon Laboratories Limited. CH2Cl2 was distilled from calcium
hydride before use. All other solvents were used as received. Sol-
vents used for NMR analysis (Cambridge Isotope Laboratories)
were used as received. Column chromatography was performed
using silica gel 60 (230Ϫ400 mesh) from Silicycle Inc. All reagents
and starting materials were purchased from Aldrich or Acros Or-
ganics. N,NЈ-Bis(6-aminopyridin-2-yl)isophthalamide (8a),[39] 6-bu-
toxynicotinic acid,[43] 6,6Ј-dimethyl-2,2Ј-bipyridine,[49,50] and 5Ј-
methyl-2,2Ј-bipyridine-5-carbaldehyde (11)[42] were prepared as de-
scribed in the literature.
1H NMR characterizations were performed on a Varian Inova-300
instrument, working at 299.96 MHz, or on a VarianϪInova 500
Figure 4. Changes in the chemical shift of the signals correspond-
instrument, working at 499.92 MHz. Chemical shifts (δ) are re-
ing to the NϪHa protons in the 1H NMR spectra of solutions
ported in parts per million (ppm) relative to tetramethylsilane using
(2 m in 9:1 CD2Cl2/CD3CN) of receptors 7 (᭹) and 7·Zn (᭛)
the residual solvent peak as a reference standard. Coupling con-
upon the addition of a solution of 5,5-dibutylbarbiturate (20 m
stants (J) are reported in Hertz (Hz). 13C NMR characterizations
in 9:1 CD2Cl2/CD3CN)
were performed on
a Bruker-300 instrument, working at
74.99 MHz or Varian Inova-500 instrument, working at
a
The effect that the metal has on the binding ability of
bipyridyl receptor 7 to barbiturate guests can be studied by
treating a solution (9:1 CD2Cl2/CD3CN) of the complex
7·Zn with 5,5-dibutylbarbiturate. The addition of the bar-
biturate to the ZnII complex leads to an insignificant (∆δ
125.29 MHz. FT-IR measurements were performed using a Nicolet
Magna-IR 750. Mass spectrometry measurements were performed
a Kratos MS-50 with an electron impact source or by positive mode
electrospray ionization on a Micromass ZabSpec Hybrid Sector-
TOF. The liquid carrier was infused into the electrospray source by
less than 0.1 ppm) but observable upfield shift in the signals means of a Harvard syringe pump at a flow rate of 10 µL/minute.
The sample solution, in the same solvent, was introduced via a 1
µL-loop-injector. Pre-purified nitrogen was used as a spray pneu-
matic aid and filtered air as the bath gas, heated at ca. 80 °C. For
low resolution, the mass spectra were acquired by magnet scan at
a rate of 5 seconds/decade at ca. 1000 resolution. For exact mass
measurements, the spectra were obtained by voltage scan over a
narrow mass range at ca. 10000 resolution. Data acquisition and
processing was achieved by using the OPUS software package on
a Digital Alpha station with VMS operating system.
corresponding to proton NϪHa (Figure 4). This is attri-
buted to the dilution of the receptor solution upon the ad-
dition of the substrate solution. This also indicates that
there is no association between the receptor and its sub-
strate. Therefore, the metal has prevented the binding of the
substrate and rendered the receptor inactive.
Conclusion
N,NЈ-Bis(6-aminopyridin-2-yl)-5-tert-butylisophthalamide (8b):
A
solution of 5-tert-butylisophthalic acid (1.00 g, 4.5 mmol) in SOCl2
(15 mL) was heated at reflux overnight under argon. The mixture
was cooled to room temperature and the excess thionyl chloride
was removed under reduced pressure. The oily residue was vacuum
dried for 5 h, dissolved in CH2Cl2 (40 mL), and added dropwise to
a solution of 2,6-diaminopyridine (2.50 g, 22.9 mmol) and triethyl-
amine (0.76 g, 7.6 mmol) in CH2Cl2 (20 mL) at room temperature.
After the mixture was stirred for 3 h, the solvent was evaporated
Receptor 1 exhibits low affinity for barbiturates and is,
therefore, unsuitable to be used as an allosteric receptor.
The binding studies on the control models (2Ϫ6) reveals
that this inactivity is due to the intramolecular hydrogen
bond between NϪHa and the nitrogen atom of the bipyri-
dine ligands. The alteration in the connectivity of the bi-
pyridine ligands (the allosteric site) to the active site of the
receptor produced an active receptor 7 that binds to the and the residue was treated with water (50 mL). The precipitate
178  2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.eurjoc.org
Eur. J. Org. Chem. 2004, 173Ϫ182

Structural Studies on Hydrogen-Bonding Receptors
that formed was collected by vacuum filtration, washed with 20% HCl solution until a pH of 3.5 was obtained. The deposited beige
FULL PAPER
aqueous EtOH (200 mL), and vacuum dried. Recrystallization
from EtOH/water afforded the product (1.31 g, 76%) as a white
solid (0.21 g, 49%) was filtered off, washed with water (10 mL) and
vacuum dried. This product was carried on without further purifi-
solid. M.p. 152Ϫ154 °C. 1H NMR (300 MHz, CDCl3): δ ϭ 1.37 cation. M.p. Ͼ 300 °C. 1H NMR (500 MHz, [D6]DMSO): δ ϭ 2.37
(s, 9 H), 4.35 (s, 4 H), 6.30 (d, J ϭ 8 Hz, 2 H), 7.49 (t, J ϭ 8 Hz, (s, 3 H), 7.80 (dd, J1 ϭ 8, J2 ϭ 2.0 Hz, 1 H), 8.35 (d, J ϭ 8 Hz, 1
2 H), 7.70 (d, J ϭ 8 Hz, 2 H), 8.10 (d, J ϭ 2 Hz, 2 H), 8.17 (t, J ϭ H), 8.38 (dd, J1 ϭ 8, J2 ϭ 2 Hz, 1 H), 8.47 (d, J ϭ 8 Hz, 1 H),
2 Hz, 1 H), 8.44 (s, 2 H) ppm. 13C NMR (75.6 MHz, CDCl3): δ ϭ 8.56 (d, J ϭ 2 Hz, 1 H), 9.13 (d, J ϭ 2 Hz, 1 H), 10.31 (br. s, 1 H)
31.3, 35.1, 103.7, 104.1, 122.5, 128.1, 135.0, 140.3, 149.9, 153.3,
ppm. 13C NMR (75.5 MHz, [D6]DMSO): δ ϭ 17.9, 119.9, 120.8,
157.2, 164.9 ppm. FT-IR (cast): ν˜ ϭ 3450, 3369, 3295, 3030, 2965,
126.3, 134.6, 137.6, 138.1, 149.9, 150.1, 151.8, 158.5, 166.2 ppm.
1689, 1659, 1614, 1577, 1526, 1450, 1347, 1327, 1293, 1250, 1230, FT-IR (microscope): ν˜ ϭ 3060, 2921, 2473, 1703, 1594, 1557, 1469,
1164, 1151, 791, 752, 614, 576 cmϪ1. HRMS (ES): m/z calcd. for
C16H15N2O4 [M]ϩ ϭ 404.1960, found ϭ 404.1955.
1369, 1265, 1143, 1053, 1033, 839, 788, 766, 744, 714, 659 cmϪ1
.
HRMS (EI): m/z calcd. for C12H10N2O2 [M]ϩ ϭ 214.0742,
found ϭ 214.0741.
6Ј-Methyl-2,2Ј-bipyridine-6-carbaldehyde (9):[41] We have used a dif-
ferent procedure to prepare this known compound. A solution of
6,6Ј-dimethyl-2,2Ј-bipyridine (1.00 g, 5.4 mmol) in dioxane (25 mL)
was treated with SeO2 (0.70 g, 6.3 mmol) and heated at reflux for
24 h. The reaction mixture was filtered while hot through Celite
and the filtrate was concentrated under reduced pressure to afford
a yellow solid. This solid was dissolved in ethyl acetate (100 mL)
and extracted with 0.3  aqueous Na2S2O5 (3 ϫ 30 mL). The com-
bined aqueous layers were treated with Na2CO3 until a pH value
of 10 was attained and then extracted with CH2Cl2 (4 ϫ 50 mL).
The combined organic layers were dried with Na2SO4, filtered, and
concentrated under reduced pressure to afford the product (0.47 g,
5-[(Dibutylamino)carbonyl]nicotinic Acid (13): A solution of 3,5-py-
ridinedicarboxylic acid (1.00 g, 6.0 mmol) in dry CH2Cl2 (25 mL)
was treated with SOCl2 (10 mL) and heated at reflux overnight un-
der argon. After cooling to room temperature, the reaction mixture
was concentrated under reduced pressure and the residue was dried
under vacuum for 5 h to remove traces of SOCl2. The resulting
residue was dissolved in dry THF (25 mL) and treated dropwise
with a solution of dibutylamine (0.77 g, 6.0 mmol) and triethyl-
amine (0.60 g, 6.0 mmol) in THF (20 mL). The mixture was stirred
overnight at room temperature under argon. An aqueous solution
of 10% NaOH (3 mL) was added and the mixture was stirred for
an additional 30 min. The solvent was removed under reduced
pressure, the residue was treated with water (20 mL), washed with
CH2Cl2 (3 ϫ 10 mL), and treated with 10% HCl solution until a
pH of 3 was attained. The mixture was extracted with CH2Cl2 (3 ϫ
20 mL) and the combined organic layers were dried with Na2SO4,
filtered, and concentrated under reduced pressure to afford the
product (0.78 g, 47%) as a beige solid, which was used without
further purification. M.p. 158Ϫ160 °C. 1H NMR (300 MHz,
CDCl3): δ ϭ 0.79 (t, J ϭ 7 Hz, 3 H), 0.97 (t, J ϭ 7 Hz, 3 H), 1.16
(br. m, 2 H), 1.41 (br. m, 2 H), 1.52 (br. m, 2 H), 1.66 (br. m, 2 H),
3.18 (t, J ϭ 7 Hz, 2 H), 3.52 (t, J ϭ 7 Hz, 2 H), 8.38 (t, J ϭ 2 Hz,
1 H), 8.84 (d, J ϭ 2 Hz, 1 H), 9.30 (d, J ϭ 2 Hz, 1 H) ppm. 13C
NMR (100.6 MHz, CDCl3): δ ϭ 13.7, 14.0, 19.9, 20.4, 29.7, 31.0,
45.3, 49.2, 126.0, 133.0, 136.3, 150.6, 151.0, 167.1, 167.8 ppm. FT-
IR (cast): ν˜ ϭ 2957, 2930, 2872, 1632, 1594, 1458, 1310, 1281, 1230,
1197, 1153, 1113, 1102, 747 cmϪ1. HRMS (EI): m/z calcd. for
C15H22N2O3 [M]ϩ ϭ 278.1630, found ϭ 278.1635.
1
45%) as a beige solid. M.p. 140Ϫ142 °C (ref.[41] 135Ϫ136 °C). H
NMR (300 MHz, CDCl3): δ ϭ 2.62 (s, 3 H), 7.20 (d, J ϭ 8 Hz, 1
H), 7.74 (t, J ϭ 8 Hz, 1 H), 7.95Ϫ7.97 (m, 2 H), 8.33 (d, J ϭ 6 Hz,
1 H), 8.66 (dd, J1 ϭ 6, J2 ϭ 2 Hz, 1 H), 10.14 (s, 1 H) ppm.
6Ј-Methyl-2,2Ј-bipyridine-6-carboxylic Acid (10): A rapidly stirred
suspension of aldehyde 9 (0.50 g, 2.5 mmol) in EtOH (20 mL) was
treated with a solution of silver nitrate (0.45 g, 2.6 mmol) in water
(4.5 mL). A 1  NaOH solution (11.1 mL) was added dropwise
over a period of 20 min. The resulting black mixture was stirred
for 24 h at room temperature. The mixture was filtered through
Celite and the filtrate was washed with CH2Cl2 (2 ϫ 10 mL) to
remove any unchanged starting material. The pH of the water layer
was adjusted to 3.5 by the addition of a 1:1 mixture of 4  HCl
and acetic acid. The resulting solution was extracted with CHCl3
(5 ϫ 20 mL) and the combined organic layers were dried with
Na2SO4 and filtered. Concentration of the filtrate under reduced
pressure afforded the product (0.29 g, 53%) as a pale yellow solid,
which was carried on without further purification. M.p. 189Ϫ190
Bipyridine Receptor 1: A solution of 6Ј-methyl-2,2Ј-bipyridine-6-
carboxylic acid 10 (46 mg, 0.2 mmol) in SOCl2 (15 mL) was heated
at reflux for 4 h under argon. The SOCl2 was removed under re-
duced pressure and the residue was vacuum dried for 5 h. The solid
residue was dissolved in THF (20 mL) and treated dropwise with
a solution of N,NЈ-bis(6-aminopyridin-2-yl)isophthalamide (8a)
(38 mg, 0.1 mmol) and triethylamine (0.22 g, 0.2 mmol) in THF
(20 mL) at room temperature under argon. The mixture was stirred
overnight at room temperature, washed with water (3 ϫ 10 mL),
dried with Na2SO4, and filtered. The filtrate was concentrated un-
der reduced pressure to afford a yellow residue. Purification by
1
°C. H NMR (300 MHz, CDCl3): δ ϭ 2.62 (s, 3 H), 7.26 (d, J ϭ
8 Hz, 1 H), 7.76 (t, J ϭ 8 Hz, 1 H), 8.06 (t, J ϭ 8 Hz, 1 H), 8.11
(t, J ϭ 8 Hz, 1 H), 8.22 (d, J ϭ 8 Hz, 1 H), 8.68 (d, J ϭ 8 Hz, 1
H) ppm. 13C NMR (125.3 MHz, [D6]DMSO): δ ϭ 25.1, 118.8,
124.1, 124.7, 125.4, 138.3, 139.3, 149.2, 154.5, 156.1, 158.4,
166.7 ppm. FT-IR (microscope): ν˜ ϭ 2998, 2919, 2616, 2566, 1769,
1704, 1584, 1474, 1447, 1416, 1323, 1261, 1155, 805, 770, 724, 634
cmϪ1. HRMS (EI): m/z calcd. for C12H10N2O2 [M]ϩ ϭ 214.0742,
found ϭ 214.0746.
5Ј-Methyl-2,2Ј-bipyridine-5-carboxylic Acid (12): A mixture of 5Ј-
methyl-2,2Ј-bipyridine-5-carbaldehyde (11)[42] (0.40 g, 2.0 mmol), flash chromatography (alumina, 1% to 10% gradient of MeOH in
35% aqueous solution of H2O2 (0.5 mL) and acetonitrile (10 mL)
was treated with NaH2PO4 (0.10 mg, 0.8 mmol) dissolved in water product (36 mg, 44%) as a white solid. M.p. Ͼ 300 °C. H NMR
CHCl3) followed by recrystallized from ethanol/water afforded the
1
(2 mL). The mixture was stirred in a water bath cooled to 5Ϫ10 °C
and then treated with an aqueous solution (10 mL) of NaClO2
(300 MHz, CDCl3): δ ϭ 2.58 (s, 6 H), 7.12 (d, J ϭ 8 Hz, 2 H),
7.64Ϫ7.72 (m, 3 H), 7.85 (t, J ϭ 8 Hz, 2 H), 7.98 (t, J ϭ 5 Hz, 2
(0.50 g, 5.52 mmol) dropwise over a period of 1 h while maintaining H), 8.13Ϫ8.27 (m, 10 H), 8.53 (s, 1 H), 8.58 (d, J ϭ 7 Hz, 2 H),
the temperature of the reaction at less than 10 °C. After stirring
8.74 (s, 2 H), 10.34 (s, 2 H) ppm. 13C NMR (75.5 MHz, CDCl3):
for another 2 h at 5Ϫ10 °C, a small amount of Na2SO3 (0.10 mg) δ ϭ 24.5, 110.1, 118.0, 122.4, 123.9, 124.4, 125.4, 129.7, 131.4,
was added to destroy the unchanged HOCl and H2O2. The re-
sulting mixture was cooled in an ice bath and treated with 10%
134.7, 137.4, 138.5, 141.1, 148.3, 149.4, 149.9, 150.1 162.5, 164.5,
175.6, 176.2 ppm. FT-IR (cast): ν˜ ϭ 3346, 1692, 1583, 1507, 1447,
Eur. J. Org. Chem. 2004, 173Ϫ182
www.eurjoc.org
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
179

M. H. Al-Sayah, R. McDonald, N. R. Branda
FULL PAPER
1391, 1301, 1243, 1156, 1074, 994, 797, 758, 712, 634 cmϪ1. HRMS
(EI): m/z calcd. for C42H33N10O4 [M ϩ H]ϩ ϭ 741.2686, found ϭ
741.2685.
1590, 1524, 1477, 1444, 1391, 1304, 1239, 1160, 1028, 805, 733, 715
cmϪ1. HRMS (EI): m/z calcd. for C30H22N8NaO4 [M ϩ Na]ϩ
581.1662, found ϭ 581.1656.
ϭ
Di-tert-butylphenyl Receptor 2: A solution of 3,5-di-tert-butylben-
zoic acid (50 mg, 0.2 mmol) in oxalyl chloride (15 mL) was heated
at reflux for 4 h under argon. The oxalyl chloride was removed
under reduced pressure and the residue was vacuum dried for 5 h.
The solid obtained was dissolved in THF (30 mL) and treated
dropwise with a solution of N,NЈ-bis(6-aminopyridin-2-yl)isoph-
Pyridine Receptor 5: The carboxylic acid was 6-butoxynicotinic
acid[43] (0.23 g, 1.2 mmol) and the product (0.21 g, 61%) was col-
lected as a white solid. M.p. 244Ϫ246 °C. 1H NMR (500 MHz,
CD2Cl2): δ ϭ 0.97 (t, J ϭ 7 Hz, 6 H), 1.50Ϫ1.45 (m, 4 H),
1.79Ϫ1.73 (m, 4 H), 4.36 (t, J ϭ 7 Hz, 4 H), 6.81 (d, J ϭ 8 Hz, 2
H), 7.67 (t, J ϭ 8 Hz, 1 H), 7.82 (t, J ϭ 8 Hz, 2 H), 8.13Ϫ8.06 (m,
thalamide (8a) (38 mg, 0.1 mmol) and triethylamine (0.22 g, 8 H), 8.33 (s, 2 H), 8.48 (s, 1 H), 8.56 (s, 2 H), 8.72 (s, 2 H) ppm.
0.2 mmol) in THF (20 mL) at room temperature under argon.
After stirring the mixture overnight at room temperature, the sol-
vent was evaporated to afford a yellow residue, which was purified
13C NMR (75.5 MHz, CDCl3): δ ϭ 13.9, 19.3, 31.0, 66.7, 110.1,
110.5, 111.3, 122.9, 125.8, 129.7, 131.0, 134.8, 137.7, 141.1, 147.2,
149.5, 149.8, 163.9, 164.4, 166.6 ppm. FT-IR (cast): ν˜ ϭ 3315, 2957,
by flash chromatography (alumina, 1% MeOH in CHCl3) to afford 1677, 1600, 1514, 1487, 1446, 1398, 1369, 1293, 1241, 797 cmϪ1
.
the product (48 mg, 55%) as a white solid. M.p. Ͼ 180 °C (dec.). HRMS (EI): m/z calcd. for C38H39N8O6 [M ϩ H]ϩ ϭ 703.2993,
1H NMR (300 MHz, CDCl3): δ ϭ 1.36 (s, 36 H), 7.60Ϫ7.65 (m, 6 found ϭ 703.2984.
H), 7.72 (d, J ϭ 1 Hz, 2 H), 7.82 (t, J ϭ 8 Hz, 2 H), 8.08 (d, J ϭ
Pyridine Receptor 6: The carboxylic acid was 5-[(dibutylamino)car-
8 Hz, 2 H), 8.09 (dd, J1 ϭ 8, J2 ϭ 2 Hz, 2 H), 8.15 (d, J ϭ 8 Hz,
bonyl]nicotinic acid (13) (0.33 g, 1.2 mmol) and the product (0.19 g,
2 H), 8.35 (s, 2 H), 8.48 (s, 2 H) ppm. 13C NMR (75.5 MHz,
CDCl3): δ ϭ 31.5, 35.12, 109.8, 110.3, 121.4, 125.9, 126.7, 129.6,
130.8, 133.7, 134.9, 141.1, 149.4, 150.1, 151.8, 164.3, 166.6 ppm.
FT-IR (cast): ν˜ ϭ 3421, 3285, 3015, 2963, 2905, 2868, 1680, 1584,
1506, 1478, 1449, 1393, 1363, 1307, 1242, 1158, 1136, 898, 885,
800, 756, 704, 666, 596, 543 cmϪ1. HRMS (EI): m/z calcd. for
C48H56N6NaO4 [M ϩ Na]ϩ ϭ 803.4261, found ϭ 803.4261.
1
43%) was collected as a yellow solid. M.p. 140Ϫ142 °C. H NMR
(300 MHz, CDCl3): δ ϭ 0.75 (t, J ϭ 7 Hz, 6 H), 0.91 (t, J ϭ 8 Hz,
6 H), 1.12 (br. m, 4 H), 1.32 (br. m, 4 H), 1.47 (br. m, 4 H), 1.56
(br. m, 4 H), 3.16 (br. t, 4 H), 3.41 (br. t, 4 H), 7.64 (t, J ϭ 8 Hz,
1 H), 7.79 (t, J ϭ 8 Hz, 2 H), 8.01 (d, J ϭ 8 Hz, 2 H), 8.13 (d, J ϭ
8 Hz, 2 H), 8.19 (dd, J1 ϭ 8, J2 ϭ 2 Hz, 2 H), 8.27 (t, J ϭ 2 Hz, 2
H), 8.61 (s, 1 H), 8.84 (d, J ϭ 2 Hz, 2 H), 8.95 (br. s, 2 H), 9.06
(br. s, 2 H), 9.13 (d, J ϭ 2 Hz, 2 H) ppm. 13C NMR (75.5 MHz,
CDCl3): δ ϭ 13.5, 12.8, 19.7, 20.2, 29.5, 30.8, 44.9, 49.0, 110.9,
General Procedure for the Preparation of Receptors 3, 4, 5, and 6: A
solution of the corresponding carboxylic acid (1.2 mmol) in SOCl2
(15 mL) was heated at reflux for 4 h under argon. The excess SOCl2 111.1, 125.1, 129.3, 129.7, 131.8, 132.7, 134.1, 134.4, 140.7, 149.0,
was removed under reduced pressure and the residue was dried
under vacuum for 5 h. The solid obtained was dissolved in CH2Cl2
(20 mL) and treated dropwise with a solution of N,NЈ-bis(6-amino-
pyridin-2-yl)isophthalamide (8a) (175 mg, 0.5 mmol) and triethyl-
amine (100 mg, 1.0 mmol) in THF (20 mL) at room temperature
under argon. After the mixture was stirred overnight, the solvent
was evaporated under reduced pressure. The residue was treated
with water (20 mL) and the precipitate that formed was filtered off,
washed with water (10 mL), and vacuum dried. The product was
purified by chromatography over alumina using a gradient of 1%
to 10% MeOH in CHCl3 as the eluent.
149.6, 149.9, 150.6, 163.6, 165.2, 167.9 ppm. FT-IR (cast): ν˜ ϭ
3266, 2958, 2931, 2872, 1680, 1620, 1587, 1515, 1446, 1299, 1241,
1158, 1109, 1026, 799, 751, 584 cmϪ1. HRMS (EI): m/z calcd. for
C48H56N10NaO6 [M ϩ Na]ϩ ϭ 891.4282, found ϭ 891.4292.
Bipyridine Receptor 7: A solution of 5Ј-methyl-2,2Ј-bipyridine-5-
carboxylic acid (12) (0.26 g, 1.2 mmol) in SOCl2 (15 mL) was
heated at reflux for 4 h under argon. The excess SOCl2 was re-
moved under reduced pressure and the residue was vacuum dried
for 5 h. The solid obtained was dissolved in CH2Cl2 (20 mL) and
treated dropwise with a solution of N,NЈ-bis(6-aminopyridin-2-yl)-
5-tert-butylisophthalamide (8b) (0.20 g, 0.5 mmol) and triethyl-
amine (0.10 g, 1.0 mmol) in CH2Cl2 (20 mL) at room temperature
under argon. After the mixture was stirred overnight, it was washed
Pyridine Receptor 3: The carboxylic acid was picolinic acid (0.14 g,
1.2 mmol) and the product (0.13 g, 47%) was collected as a beige
solid. M.p. Ͼ 270 °C (dec.). 1H NMR (300 MHz, CDCl3): δ ϭ with water (3 ϫ 10 mL), dried with anhydrous Na2SO4, and fil-
7.50Ϫ7.46 (m, 2 H), 7.67 (t, J ϭ 8 Hz, 1 H), 7.94 (m, 6 H), tered. Concentration of the filtrate under reduced pressure afforded
8.17Ϫ8.13 (m, 6 H), 8.30 (d, J ϭ 8 Hz, 2 H), 8.57 (s, 1 H), 8.63 (d,
a beige residue. Purification by flash chromatography (alumina, 1%
J ϭ 5 Hz, 2 H), 8.72 (br. s, 2 H), 10.41 (s, 2 H) ppm. 13C NMR to 10% MeOH in CHCl3) afforded the product (0.26 g, 65%) as a
(75.5 MHz, [D6]DMSO): δ ϭ 108.6, 110.2, 122.2, 124.5, 127.1, white solid. M.p. Ͼ190 °C (dec.). 1H NMR (500 MHz, CD2Cl2):
127.6, 128.4, 131.6, 138.6, 141.0, 148.6, 149.0, 150.8, 161.7, 165.2,
175.7 ppm. FT-IR (microscope): ν˜ ϭ 3330, 1089, 1053, 1680, 1667,
δ ϭ 1.43 (s, 9 H), 2.38 (s, 6 H), 8.12 (d, J ϭ 8 Hz, 2 H), 8.17 (d,
J ϭ 8 Hz, 2 H), 7.63 (d, J ϭ 9 Hz, 2 H), 7.86 (t, J ϭ 8 Hz, 2 H),
1589, 1514, 1482, 1449, 1393, 1307, 1245, 1229, 1155, 1090, 1076, 8.28 (s, 2 H), 8.29 (d, J ϭ 6 Hz, 2 H), 8.34 (d, J ϭ 9 Hz, He), 8.48
999, 800, 748, 714, 691, 613, 590 cmϪ1. HRMS (EI): m/z calcd. for
C30H22N8NaO4 [M ϩ Na]ϩ ϭ 581.1662, found ϭ 581.1664.
(s, 1 H), 8.51 (s, 2 H), 8.53 (d, J ϭ 6 Hz, 2 H), 8.62 (br. s, 2 H),
8.71 (br. s, 2 H), 9.17 (s, 2 H) ppm. 13C NMR (125.3 MHz, CDCl3):
δ ϭ 29.8, 31.3, 35.3, 110.3, 120.6, 121.3, 122.3, 128.7, 128.9, 134.2,
134.6, 136.3, 136.4, 137.6, 140.9, 147.9, 149.3, 149.5, 149.6, 151.7,
158.5, 162.3, 163.4, 164.8 ppm. FT-IR (cast): ν˜ ϭ 3288, 2964, 1682,
1588, 1513, 1447, 1396, 1298, 1241, 1158, 1128, 1028, 896, 838,
798, 744, 651 cmϪ1. HRMS (ES): m/z calcd. for C46H41N10O4 [M
ϩ H]ϩ ϭ 797.3312, found ϭ 797.3312.
Pyridine Receptor 4: The carboxylic acid was nicotinic acid (0.14 g,
1.2 mmol) and the product (0.18 g, 64%) was collected as a white
solid. M.p. 192Ϫ194 °C. 1H NMR (300 MHz, [D6]DMSO): δ ϭ
7.55 (m, 2 H), 7.69 (t, J ϭ 8 Hz, 1 H), 7.89 (m, 2 H), 7.93 (s, 2 H),
7.95 (d, J ϭ 4 Hz, 2 H), 8.20, (dd, J1 ϭ 8, J2 ϭ 2 Hz, 2 H), 8.33
(dt, J1 ϭ 8, J2 ϭ 2 Hz, 2 H), 8.58 (br. s, 1 H), 8.77 (m, 2 H), 9.12
(d, J ϭ 2 Hz, 2 H), 10.60 (s, 2 H), 10.77 (s, 2 H) ppm. 13C NMR Zinc Complex 7·Zn: A solution of bipyridine receptor 7 (5 mg,
(75.5 MHz, [D6]DMSO): δ ϭ 111.4, 111.6, 123.4, 127.3, 128.9, 6.3·10Ϫ3 mmol) in CH2Cl2 (5 mL) was treated with a solution of
129.8, 131.4, 134.1, 135.6, 140.2, 148.8, 150.2, 150.4, 152.4, 164.6, Zn(OTf)2 (2.3 mg, 6.3 ϫ 10Ϫ3 mmol) in CH3CN (1 mL). The mix-
165.2 ppm. FT-IR (microscope): ν˜ ϭ 3428, 3319, 3233, 3042, 1667,
ture was stirred for 15 min at room temperature under argon. The
180
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Structural Studies on Hydrogen-Bonding Receptors
FULL PAPER
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solvent was evaporated under reduced pressure and the residue
treated with CH2Cl2 (1 mL). The deposited white solid (7.3 mg,
100%) was collected by filtration and vacuum dried. 1H NMR
(500 MHz, 10% CD3CN in CD2Cl2): δ ϭ 1.40 (s, 9 H), 2.36 (s, 6
H), 7.82 (t, J ϭ 8 Hz, 2 H), 7.90 (d, J ϭ 8 Hz, 2 H), 8.01 (d, J ϭ
8 Hz, 2 H), 8.07 (s, 2 H), 8.09 (d, J ϭ 8 Hz, 2 H), 8.27 (s, 2 H),
8.31 (d, J ϭ 8 Hz, 2 H), 8.33 (s, 1 H), 8.45 (d, J ϭ 8 Hz, 2 H), 8.65
(d, J ϭ 8 Hz, 2 H), 9.38 (s, 2 H), 9.52 (s, 2 H), 10.50 (s, 2 H) ppm.
13C NMR (125.7 MHz, 10% CD3CN in CD2Cl2): δ ϭ 18.4, 31.0,
35.2, 109.7, 110.4, 122.2, 122.6, 122.9, 129.5, 134.3, 134.7, 139.1,
140.8, 141.8, 142.1, 146.1, 147.5, 148.5, 149.9, 150.8, 151.6, 153.6,
163.6, 165.0 ppm. FT-IR (cast): ν˜ ϭ 3270, 2963, 1681, 1586, 1522,
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1477, 1449, 1390, 1245, 1161, 1031, 901, 840, 799, 737, 639 cmϪ1
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an NMR tube fitted with a rubber septum to minimize the evapor-
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amount of substrate solution added until a total of 10 molar equiv-
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corresponding to the NϪHa protons were monitored and the col-
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program to fit the data to a theoretical model for the binding pro-
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Hunter (Department of Chemistry, University of Sheffield, UK).
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