Super Aryl-Extended Calix[4]pyrroles: Synthesis, Binding Studies, and Attempts To Gain Water Solubility

Article abstract of DOI:10.1002/chem.201602987

We describe the synthesis of unprecedented calix[4]pyrrole receptors featuring “super aryl extended” (SAE) cavities. We elaborated the aromatic cavity provided by the αααα-isomer of para-tetraiodo-meso-phenyl calix[4]pyrrole by installing ethynyl-aryl sub

Full text of DOI:10.1002/chem.201602987

DOI: 10.1002/chem.201602987
Full Paper
&
Supramolecular Chemistry
Super Aryl-Extended Calix[4]pyrroles: Synthesis, Binding Studies,
and Attempts To Gain Water Solubility
Luis Escobar,[a] Gemma Aragay,[a] and Pablo Ballester*[a, b]
Abstract: We describe the synthesis of unprecedented cal-
ix[4]pyrrole receptors featuring “super aryl extended” (SAE)
cavities. We elaborated the aromatic cavity provided by the
aaaa-isomer of para-tetraiodo-meso-phenyl calix[4]pyrrole
by installing ethynyl-aryl substituents at its upper rim. We
report the binding properties of the prepared SAE-calix[4]-
pyrrole tetraester towards pyridyl-N-oxides. The binding data
revealed the formation of thermodynamically and kinetically
highly stable 1:1 complexes. The complexation-induced
chemical shifts indicated the formation of hydrogen bonds
and aromatic interactions with the calix-core adopting the
cone conformation. We quantified the additional interactions
established between the four terminal aryl groups and the
para-phenyl substituent of 4-phenyl pyridine N-oxide to be
in the order of 1 kcalmolÀ1. The complex formation rate was
found to be close to the diffusion control suggesting that
the free host adopted a 1,3-alternate conformation. Finally,
we attempted to gain water solubility of SAE-calix[4]pyrroles
using derivatives that display four ionizable or charged
groups at their upper rims.
Introduction
ty. On the other hand, polar interactions provide binding selec-
tivity. In this sense, synthetic receptors deprived of polar
groups in their hydrophobic cavities only obey the selectivity
principle of size and shape complementarity.
Molecular recognition of polar substrates in water using syn-
thetic receptors represents a challenging and exacting endeav-
or,[1–3] aiming to mimic natural protein performance. Encoun-
tered difficulties include: 1) the synthesis of receptors that are
water soluble and do not aggregate at the concentrations re-
quired for conducting NMR binding studies; 2) the incorpora-
tion to the receptors’ scaffolds of converging polar groups
able to establish hydrogen-bonding interactions with those of
the targeted substrates; and 3) burying the receptor’s polar
groups in the interior of hydrophobic cavities of sensible di-
mensions with the aim to reduce water solvation and induce
guest inclusion. Following these guidelines, it is also expected
that liberation of solvation water molecules will provide addi-
tional enthalpic and/or entropic gains to the thermodynamic
stability of the formed complex.[4] In water, the driving force
for the binding of polar substrates consists of a combination
of hydrophobic, dispersive, and polar interactions, which are
not obvious to dissect owing to strong interdependences. On
the one hand, hydrophobic and dispersive interactions are
mainly responsible for the achievement of high levels of affini-
Davis and co-workers achieved remarkable levels of affinity
and selectivity in the binding of carbohydrates in water using
“temple” receptors.[2,5] These synthetic receptors were rational-
ly designed following the guidelines discussed above. The ob-
tained results proved that it is indeed possible to synthesize
receptors that resemble the biological counterparts (i.e., lec-
tins). However, the number of examples of synthetic receptors
where hydrogen-bonding sites converge in a sizeable three-di-
mensional hydrophobic cavity to achieve the selective binding
in water of polar molecules is scarce.[6,7] The extensive use of
aromatic panels in the construction of hydrophobic cavities
makes the adequate positioning of polar groups that can be
displayed towards the included polar guest more difficult.
We[8,9] and others[10] have demonstrated that the water-solu-
ble tetra-a isomers of aryl-extended calix[4]pyrroles are privi-
leged receptors for the recognition of N-oxides in this solvent.
Aryl-extended calix[4]pyrroles were introduced originally by
Sessler[11] and Floriani.[12] In the solid state, the tetra-a isomers
formed deep inclusion complexes with electron-rich neutral
compounds (i.e., EtOH, AcOH, acetone, and N-oxides) in which
the calix[4]pyrrole core adopted the cone conformation. In this
conformation, the aryl-extended calix[4]pyrroles displayed a siz-
able aromatic cavity open at one end and closed at the oppo-
site end by four converging pyrrole units. The included polar
guests established four simultaneous hydrogen-bonding inter-
actions between one of its electron-rich oxygen atoms and the
four pyrrole NHs.[13] The burying of the polar NHs in a deep ar-
omatic cavity isolates them from solvation with bulk water.
The reduced solvation of the polar groups is crucial in main-
[a] L. Escobar, Dr. G. Aragay, Prof. Dr. P. Ballester
Institute of Chemical Research of Catalonia (ICIQ)
The Barcelona Institute of Science and Technology
Avgda. Pa¨ısos Catalans 16, 43007 Tarragona (Spain)
E-mail: pballester@iciq.es
[b] Prof. Dr. P. Ballester
ICREA
Passeig Lluꢀs Companys 23, 08018 Barcelona (Spain)
Supporting information and the ORCID identification number(s) for the au-
thor(s) of this article can be found under http://dx.doi.org/10.1002/
chem.201602987.
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taining their hydrogen-bonding properties for polar substrates
in water.
The elaboration of the aromatic cavity in synthetic receptors
is typically associated with modifications of their binding selec-
tivity and affinity. In this work, we report the enlargement of
the aromatic cavity defined by meso-phenyl substituents in the
tetra-a isomers of aryl-extended calix[4]pyrroles affording un-
precedented calix[4]pyrroles that we termed “super-aryl-ex-
tended” (SAE) (Figure 1). The newly prepared receptors were
obtained by Sonogashira reaction of the parent aryl-extended
aaaa-tetrakis(4-iodo-phenyl)-tetramethyl-calix[4]pyrrole with
4-ethynyl-susbtituted phenyl derivatives (Scheme 1). We com-
pare the binding properties of an aryl-extended calix[4]pyrrole
with its SAE-counterpart in CHCl3 solution. To explore the
water solubility of the SAE-calix[4]pyrroles, we appended ioniz-
able groups at their upper rim. We also show that the incorpo-
ration of four ionizable groups is not enough to gain access to
water solubility.
Scheme 1. a) Synthetic scheme for the preparation of SAE-calix[4]pyrroles 3,
4, and 5; b) line-drawing structure of N-oxides 6 and 7 used as guests and
4-ethynyl substituted calix[4]pyrrole 8[14] employed as reference model in
the ITC binding studies.
Both SAE-calix[4]pyrroles were characterized by a complete set
1
of high resolution spectra (1D and 2D H NMR, 13C NMR and
HR-MS).
In the particular case of tetraester 3a, single crystals suitable
for X-ray diffraction were formed from slow diffusion of aceto-
nitrile vapors into a methylene chloride solution of 3a. In the
solid state, 3a adopted the cone conformation with one mole-
cule of acetonitrile deep included in its cavity (Figure 2). The
nitrogen atom of the bound acetonitrile was involved in four
hydrogen bonds with the pyrrole NHs. The average N···N dis-
tance was 3.24 ꢁ. Remarkably, the aromatic cavity of 3a col-
lapsed in its upper section owing to edge-to-face stacking ge-
ometry (CH–p interactions) adopted by adjacent terminal
phenyl rings of the ethynyl substituents (Figure 2c).
The tetramethyl ester 3a was hydrolyzed with LiOH, fol-
lowed by acidification to provide the tetraacid 4 in 68% yield.
In turn, the tetrapyridyl receptor 3b was reacted with excess
of CH3I to afford the N-methyl tetraquaternized derivative 5
(Scheme 1).
Figure 1. Molecular structures of aaaa-isomers of aryl-extended (left) and
super-aryl-extended (right) calix[4]pyrroles in cone conformation.
Results and Discussion
Synthesis
aaaa-Tetrakis(4-iodo-phenyl)-tetramethyl-calix[4]pyrrole 1 was
prepared by acid-catalyzed condensation of freshly distilled
pyrrole with commercially available 4-iodo-phenyl-methyl
ketone using a literature procedure.[14] Methyl 4-(ethynyl-phe-
noxy)acetate 2a and N-(4-ethynylphenyl)-4-pyridinecarboxa-
mide 2b were prepared in 54 and 50% yield, respectively,
using conditions reported for similar substrates.[15,16] The syn-
thetic procedures are described in detail in the Supporting In-
formation.
Single crystals of the tetraacid 4 suitable for XRD analysis
were obtained by slow diffusion of acetonitrile into an acetone
solution of the compound. The solid state was very similar to
that observed for the parent tetraester 3a. The packing of the
crystal revealed that tetraacid 4 formed dimeric aggregates
with capsular-like topology (Figure 3a). The dimeric capsules
were stabilized by eight intermolecular hydrogen bonds
formed between the carboxylic groups installed at the upper
rim of the monomers. DOSY experiments performed using
[D8]THF as solvent (see the Supporting Information) ruled out
the existence of dimeric aggregates of the tetraacid 4 in solu-
tion.
The super aryl-extended calix[4]pyrroles 3a,b were synthe-
sized by coupling the tetraiodo calix[4]pyrrole 1 with the corre-
sponding 4-ethynyl-substituted derivatives, 2a,b, under palla-
dium/copper-catalyzed Sonogashira conditions. Column chro-
matography purification of the crude reaction mixtures al-
lowed the isolation of the pure SAE-calix[4]pyrroles, 3a and
3b, as solids in 81 and 50% yield, respectively (Scheme 1).
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Binding studies of the tetraester SAE-calix[4]pyrrole 3a with
pyridyl N-oxides
We probed the interaction of the tetraester 3a with pyridyl N-
1
oxides by H NMR spectroscopy. Pyridine N-oxides have rele-
vant biological activity.[17,18] They are known to act as a novel
class of antiviral drugs by inhibiting the replication processes.
Previous investigations of our research group have established
that N-oxides bind calix[4]pyrroles through the formation of
four convergent hydrogen bonds between the oxygen atom of
the N-oxide and the pyrrole NHs.[8,9]
1
The H NMR spectrum of 3a (3 mm) in CDCl3 solution dis-
played sharp and well-defined proton signals that are in agree-
ment with a C4v symmetry. The pyrrole NHs (Ha) resonated at
d=7.6 ppm and the b-pyrrole hydrogen atoms (Hb) appeared
at d=5.7 ppm (Figure 4a, see Scheme 1 for proton assign-
ment). All together, these observations did not evidence the
preferred conformation adopted by the calix[4]pyrrole 3a in
solution. The receptor may be locked in the cone conformation
or experiencing an interconversion process between different
conformers (cone and alternate) at a rate that is fast on the
chemical-shift time scale. Molecular dynamic simulations have
shown that octamethyl calix[4]pyrroles adopted mainly the
1,3-alternate conformation in solvents with reduced hydrogen-
bonding acceptor properties (i.e., chloroform).[19] They also re-
vealed that in non-polar solvents the conformational space of
calix[4]pyrroles was complex and the conformational transi-
tions were fast. Based on the above, we considered that 3a
dissolved in CDCl3 was fluttering mainly between 1,3-alternate
conformations.
Figure 2. X-ray structure of the tetraester SAE-calix[4]pyrrole 3a: a) side
view; b) top view and c) selected view of the edge-to-face CH–p interactions
established between adjacent terminal phenyl rings. The structures are
shown in ORTEP view with thermal ellipsoids set at 50% probability. Hydro-
gens are shown as fixed-size spheres of 0.3 ꢁ radius.
We observed that the addition of 0.5 molarequiv of 4-
methyl pyridine-N-oxide 6 to the CDCl3 solution of 3a induced
the appearance of a new set of proton signals (Figure 4b). We
assigned this new set of signals to the protons of bound 3a.
Significantly, the b-pyrrole protons and the pyrrole NHs in the
bound receptor moved downfield, Dd=0.35 and 2.5 ppm, re-
spectively. On the contrary, the aromatic protons of the added
N-oxide appeared upfield-shifted with respect to the free
counterpart. In particular, the chemical-shift change experi-
enced by the proton alpha to the nitrogen atom (Hx) was
1
Figure 3. X-ray structure of the tetraacid SAE-calix[4]pyrrole 4: a) dimeric ag-
gregate assembly; b) top view of one molecule of 4; and c) hydrogen-bond-
ing interaction between the carboxylic acids at the upper rim. The structures
are shown in ORTEP view with thermal ellipsoids set at 50% probability. Hy-
drogens are shown as fixed-size spheres of 0.3 ꢁ radius.
Figure 4. Selected regions of the H NMR (500 MHz, CDCl3, 298 K) spectra
registered during the titration of 3a with incremental amounts of 6: a) 3a;
b) 6+3a (0.5:1 molar ratio); c) 6+3a (1.5:1 molar ratio); and d) 6. Primed
letters correspond to proton signals in the inclusion complex 6ꢀ3a. *Resid-
ual solvent peak.
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larger than À3.5 ppm. When 1 molarequiv of N-oxide 6 was
added, only the proton signals assigned to bound 3a were de-
tected (see the Supporting Information). The addition of more
than 1 molarequiv of 6 did not produce additional changes in
the signals of the receptor but caused the appearance of the
proton signals of the free N-oxide.
The additional p–p and CH–p interactions displayed in the
energy-minimized 7ꢀ3a complex suggested a potential in-
crease in its thermodynamic stability with respect to the 6ꢀ3a
analogue.
We computed the chemical shifts (d) and the nucleus-inde-
pendent chemical shifts (NICS), which are related to complexa-
tion-induced chemical shifts (Dd), for the hydrogen atoms of
the N-oxide in the 7ꢀ3a complex (see the Supporting Informa-
tion). The sensible agreement that exists between experimen-
tal and calculated values strongly supports the inclusion geom-
etry assigned to the 7ꢀ3a complex in solution.
Taken together, these results indicated that: 1) 3a binds 6
forming a 1:1 complex (6ꢀ3a); 2) the chemical exchange be-
tween the components of the complex and the free counter-
parts is slow on the chemical-shift timescale; 3) the binding
constant is larger than 104 mÀ1; and 4) the 6ꢀ3a complex fea-
tures an inclusion binding geometry. That is, the N-oxide 6 is
included in the aromatic cavity displayed by 3a in the cone
conformation where it experiences a strong magnetic shielding
exerted by the meso-aryl substituents. It is worth noting that
Unfortunately, the tetrapyridyl receptor 3b featured a re-
duced solubility in most non-polar and polar organic solvents,
except DMSO. This characteristic precluded the implementa-
tion of titration experiments with this latter receptor.
the para-methyl substituent of
6 also shifted upfield
(À0.82 ppm) in the complex but to a much reduced extent.
The receptor 3a is locked in the cone conformation upon
binding 6 owing to the establishment of four hydrogen bonds
between the oxygen atom of the N-oxide and the pyrrole NHs.
We observed completely analogous behavior in the binding
dynamics and chemical-shift changes during the titration of 3a
with 4-phenyl-pyridine-N-oxide 7. Remarkably, in the 7ꢀ3a in-
clusion complex the signals of the ortho- and meta-protons in
the para-phenyl substituent of 7 also experienced significant
upfield changes (À0.37 and À1.20, respectively).
Thermodynamic characterization of the inclusion complexes
Pair-wise competitive experiment
To quantify the effect of the additional interactions in 7ꢀ3a,
we determined the ratio between the stability constant values
for the 6ꢀ3a and 7ꢀ3a complexes performing a pair-wise
competitive experiment (Figure 6). The analysis of an equimo-
1
lar (3 mm) CDCl3 solution of 3a and 6 using H NMR spectros-
copy evidenced the quantitative formation of the 6ꢀ3a com-
plex. The addition of 0.7 molarequiv of N-oxide 7 to the above
solution induced not only the observation of the proton sig-
nals of free N-oxide 7, but also those corresponding to the
7ꢀ3a complex and free N-oxide 6. This observation demon-
strated that N-oxide 7 was able to partially replace the bound
N-oxide 6 in the 6ꢀ3a complex. The incremental addition of
N-oxide 7 (1.3 and 1.9 molarequiv) showed that the intensity
of the proton signals assigned to the 7ꢀ3a complex grew at
the expense of those of the 6ꢀ3a counterpart. Using the inte-
gral values of selected proton signals from all the species in-
volved in the equilibrium, we determined an averaged ratio of
stability constants of Ka (7ꢀ3a)/Ka (6ꢀ3a)ꢁ4. This value trans-
lated into ꢁ0.8 kcalmolÀ1 of free-energy advantage in favor of
the 7ꢀ3a complex.
We performed simple molecular-modeling studies (PM6) to
derive the putative structures for the inclusion complexes
6ꢀ3a and 7ꢀ3a (Figure 5). The energy-minimized structures
of both complexes showed the guest included deep in the ar-
omatic cavity of the receptor. The oxygen atom of the N-
oxides formed four hydrogen bonds with NH groups of the
pyrrole core that adopted the cone conformation. In addition,
the pyridyl ring of the guest establishes p–p and CH–p interac-
tions with the meso-phenyl substituents that define the lower
section of the aromatic cavity of the receptor. Nonetheless, ad-
ditional p–p and CH–p interactions exist between the para-
phenyl ring of guest 7 and the terminal phenyl groups that
define the upper section of the aromatic cavity of bound 3a.
Clearly, the inclusion of the guests disrupts the edge-to-face
stacking interactions observed for the CH3CNꢀ3a complex in
the solid state.
As mentioned above, the measured increase in thermody-
namic stability for the 7ꢀ3a complex can be assigned to addi-
tional p–p and CH–p interactions established between the
para-phenyl substituent of N-oxide 7 and the terminal phenyl
groups that defined the upper section of the receptor’s cavity.
Nevertheless, the assessed free-energy difference is significant-
ly lower than we expected. We[20] and others have determined
an average gain in free energy of ꢁ1 kcalmolÀ1 for each CH–p
interaction. Likewise, p–p interactions in organic solvents can
contribute to about 1 kcalmolÀ1 in the complex stability.[21]
Most likely, solvation effects do not allow a direct translation of
enthalpy gain into free energy (vide infra, ITC experiments).
Isothermal titration calorimetry (ITC) experiments
Figure 5. Energy-minimized structures (PM6) of the inclusion complexes
a) 6ꢀ3a and b) 7ꢀ3a. Included guest molecules are shown as CPK models.
3a is depicted in stick representation. Substituents at the para position of
the terminal phenyl substituents of the calixpyrrole walls and non-polar hy-
drogens have been omitted for clarity.
The stability constant of the inclusion complexes, 7ꢀ3a and
1
6ꢀ3a, were too high to be measured accurately using H NMR
titrations. Moreover, we were also interested in determining
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contribution to the binding process. Not surprisingly, except
for the formation of the 6ꢀ3a complex, entropy opposed
complex formation. This is the expected behavior for the for-
mation of a 1:1 complex in non-polar solvents. Remarkably,
less negative entropic terms, and even slightly positive, were
measured for the formation of the complexes with SAE calix[4]-
pyrroles, 7ꢀ3a and 6ꢀ3a, respectively. Most likely, these re-
duced entropy values can be attributed to the release of sol-
vent molecules to the bulk solution upon formation of deep-
cavity inclusion complexes with the SAE calix[4]pyrroles. The
solvation/desolvation processes that are required for the for-
mation of each inclusion complex are significant and diverse
because the differences in binding enthalpy do not directly
relate to the modifications in free energies. It is probable that
the deep-inclusion geometry of the complexes is mainly re-
sponsible for this discrepancy. All binding processes revealed
the existence of enthalpy–entropy compensation effects.[22]
Considering the inclusion complex of 4-methyl-pyridine-N-
oxide 6 and tetraester 3a, 6ꢀ3a, as reference to quantify the
effect produced by the introduction of the para-phenyl group
in the N-oxide guest, we assigned, in agreement with the pair-
wise competitive experiment, a value of 1.0 kcalmolÀ1 to the
additional CH–p and p–p interactions present in the 7ꢀ3a
complex. Alternatively, the energy gain provided by these in-
teractions was estimated using the 7ꢀ8 complex as reference,
resulting in an equivalent value of 1.0 kcalmolÀ1 less stable
than the 7ꢀ3a counterpart. The obtained results confirmed
that the elaboration of the aromatic cavity present in the SAE-
3a enhanced its binding properties towards N-oxide 7 in com-
parison to those of the aryl-extended counterpart 8. On the
other hand, the binding process of N-oxide 6 does not show
noticeable changes in free energy when comparing the com-
plexes formed with SAE calix[4]pyrrole 3a and its aryl-extend-
ed counterpart 8 supporting the lack of additional CH–p and
p–p interactions for the para-methyl substituted guest.
1
Figure 6. Selected regions of the H NMR (500 MHz, CDCl3, 298 K) spectra ac-
quired during the competitive pair-wise binding experiment of N-oxides 6
and 7 and receptor 3a: a) 6ꢀ3a (1:1 molar ratio); addition of incremental
amounts of 7; b) 0.7 molarequiv; c) 1.3 molarequiv; and d) 1.9 molarequiv.
Primed letters correspond to proton signals of the N-oxides in the inclusion
complexes.
the enthalpy and entropy contributions to complex formation
to gain some insight in the reduced thermodynamic advant-
age determined for the 7ꢀ3a complex. We performed ITC ex-
periments in chloroform solution to characterize the binding
process of N-oxides 6 and 7 with the SAE-calix[4]pyrrole 3a.
We also studied the binding of the N-oxides to the aryl extend-
ed calix[4]pyrrole 8 (Schem 1). Receptor 8 seemed to be an
ideal model system to quantify the energy gain provided by
the terminal phenyl substituents incorporated at the upper rim
of tetraester 3a when N-oxide 7 is bound.
In all cases, the obtained experimental binding isotherms
were fit to a theoretical 1:1 binding model (see the Supporting
Information for experimental details). The well-fit data allowed
the direct calculation of the stability constant values (Ka) and
the enthalpy (DH) of binding. The free energy (DG) and the en-
tropy of binding (DS) were mathematically derived from the
thermodynamic constants measured experimentally. The deter-
mined thermodynamic constants are summarized in Table 1.
As expected, all the stability constants determined were
larger than 104 mÀ1 and all binding processes were driven
mainly by enthalpy. In general, the entropic term had a reduced
Kinetic characterization of the inclusion complexes
1
We performed 2D H EXSY experiments using CDCl3 solutions
of the SAE-3a and one of the N-oxides, 6 or 7, in a 1:0.5 molar
ratio, respectively (see the Supporting Information for experi-
mental details). In both cases, we observed clean cross peaks
due to chemical exchange between the separate b-pyrrole
proton signals in the free (Hb) and bound (Hb’) receptor 3a.
Using the integral values of the exchange cross-peaks and the
diagonal peaks, we determined the rate constants (k1’ and kÀ1’)
for the two-sites magnetization exchange process with the
help of the EXSYcalc software[23] (see Supporting Information).
The magnetization exchange-rate constants and the rate con-
stants of the binding equilibrium of 3a with the N-oxides, as-
suming a one-step process, are related as indicated by Equa-
tions (1) and (2).
Table 1. Thermodynamic constant values determined for the binding
equilibria of calix[4]pyrrole receptors with N-oxides. All values represent
the average of at least two ITC titration experiments. Errors (standard de-
viation) are determined to be less than 10%.
Complex
Ka/107[a]
DH[b]
TDS[b]
DG[b]
6ꢀ8
1.3
1.0
1.2
3.6
À13.7
À14.2
À9.3
À3.8
À4.6
0.3
À0.7
À9.9
À9.6
À9.6
À10.6
7ꢀ8
In turn, the equilibrium constant Ka for the binding process
is equal to the forward rate constant (k1) divided by the inverse
rate constant (kÀ1). Hence, we determined the energy barrier
6ꢀ3a
7ꢀ3a
À11.3
[a] MÀ1. [b] kcalmolÀ1 at 298 K.
for the dissociation of the complexes as DG° (6ꢀ3a)=
À1
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The synthesis of SAE-calix[4]pyrroles that are water soluble as
discrete molecules will require the introduction of more than
four charged groups in their scaffolds and we are currently
working towards this goal.
Conclusions
In summary, we described the synthesis of two unprecedented
super aryl-extended neutral calix[4]pyrroles, SAE-3a and -3b,
and their ionizable or charged derivatives 4 and 5. The com-
pounds were characterized by a complete set of high-resolu-
tion spectra. The tetraester 3a and the tetraacid 4 were also
characterized by X-ray diffraction analysis of single crystals. In
the solid state, these two receptors adopted the cone confor-
mation and deeply included one hydrogen-bonded molecule
of acetonitrile in their aromatic cavity. They also displayed in-
tramolecular edge-to-face CH–p interactions between the four
terminal phenyl residues that defined the upper section of the
17.59Æ0.01 kcalmolÀ1 and DG°
(7ꢀ3a)=18.42Æ0.03 kcal
À1
molÀ1. The greater kinetic stability determined for the 7ꢀ3a
complex with respect to the 6ꢀ3a counterpart is in line with
the superior thermodynamic stability of the former. This result
suggested that the structure of the transition state (TS) must
be similar in both cases. We hypothesized that, in the TS, the
receptor adopts a conformation that resembles the 1,3-alter-
nate conformer and the N-oxide is mainly bound with just two
hydrogen bonds. Therefore, the TS structure is more similar to
the starting materials than the inclusion complex. Such TS
qualifies as an early TS and, according to Hammond’s postu-
late, it is characteristic of rapid and exothermic reactions.
Indeed, the ITC experiments showed that the binding equili-
bria are highly exothermic. Using the stability constant values
determined in the ITC experiments and the assessed rate con-
stants for complex dissociation, we calculated the rate con-
stant for the formation of the inclusion complexes. The deter-
mined values were large and close to those expected for a dif-
1
aromatic cavity. We performed H NMR and ITC experiments to
characterize the complexation of tetraester 3a with pyridyl N-
oxides 6 and 7. The obtained results revealed the formation of
thermodynamically and kinetically highly stable 1:1 complexes.
The association constant values of the complexes formed in
chloroform solution were in the order of 107 mÀ1. The results of
the binding experiments indicated that the CH–p and p–p in-
teractions established between the para-phenyl ring in N-oxide
7 and the four terminal phenyls that defined the upper section
of the aromatic cavity of calix[4]pyrrole 3a can be estimated to
be of the order of 1 kcalmolÀ1. Thus, the elaboration of the ar-
omatic cavity present in SAE-3a endowed this compound with
superior binding properties towards N-oxide 7 in comparison
to the simple aryl-extended counterpart. We also determined
the kinetic stability of the inclusion complexes of 3a with the
pyridyl N-oxides and their dissociation energy barriers. Finally,
we calculated that the formation of the complexes was close
to diffusion control. This finding suggested that 3a adopts
a 1,3-alternate conformation when dissolved in CDCl3 solution.
We propose that the TS structure for the dissociation of both
complexes is similar and involves 3a in 1,3-alternate conforma-
tion with the N-oxide interacting with the receptor just by es-
tablishing two hydrogen bonds. The SAE-calix[4]pyrrole deriva-
tives 4 and 5 having ionizable carboxylic groups and positively
charged pyridinium groups, respectively, at their upper rims,
turned out to be insoluble in water at the concentration re-
fusion-controlled process (k1 ~107 mÀ1 À1). All together, these
s
results suggested that, to a large extent, the SAE-calix[4]pyrrole
3a adopts a 1,3-alternate conformation when dissolved in
chloroform solution. A change in the receptor conformation is
not required for the TS formation of the inclusion complex.
This finding is in contrast with our previous results for anion
binding with aryl-extended calix[4]pyrroles in acetonitrile solu-
tion. There, we postulated that the receptor mainly adopted
the cone conformation in acetonitrile solution.[24]
Attempts to gain water solubility with SAE-calix[4]pyrroles
We investigated the solubility of tetraacid SAE-4 in basic water.
Unfortunately, all our attempts to dissolve 4 in basic water at
1 mm concentration resulted in the formation of suspensions.
The addition of 10% acetone to the formed suspensions pro-
duced clear solutions. The analysis of the obtained solutions
1
quired for H NMR spectroscopy. We are currently working on
increasing the number of water-solubilizing groups covalently
connected to the SAE-calix[4]pyrrole scaffold. These receptors
are privileged scaffolds to achieve binding of polar substrates
in water by combining hydrogen bonding and other polar in-
teractions with hydrophobic effects, in an attempt to mimic
nature’s molecular receptors.
1
using H NMR spectroscopy did not show any signal. The lack
1
of proton signals in the H NMR spectrum of the solution sug-
gested the formation of high-order aggregates (i.e., micelles)
owing to the large hydrophobic section of 4.[25] Dynamic light
scattering (DLS) measurements confirmed this hypothesis and
revealed the presence of aggregates with a mean hydrody-
namic diameter of 127Æ3 nm in solution (see the Supporting
Information).
Experimental Section
Likewise, the four positive charges located at the upper rim
of the pyridinium derivative 5 did not promote its solubility in
water at the concentrations required for NMR characterization.
Tetramethyl ester SAE-calix[4]pyrrole (3a): Tetraiodo 1 (50 mg,
0.04 mmol, 1 equiv), Pd(PPh3)2Cl2 (14.86 mg, 0.02 mmol, 0.13 equiv),
CuI (6.45 mg, 0.03 mmol, 0.20 equiv), and methyl 4-(ethynylphe-
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noxy)acetate 2a (48.30 mg, 0.25 mmol, 1.50 equiv) were kept
under Ar atmosphere in a round-bottomed flask. Dry THF (5 mL)
and dry iPr2NH (5 mL) were added. The reaction was stirred at
458C for 5 h. The color of the mixture changed from yellow to
black. After that, the reaction was stopped and concentrated. The
crude was redissolved in CH2Cl2 (10 mL) and washed with HCl
0.2 N (10 mL) and brine (10 mL). The organic layer was dried
(Na2SO4), filtered, and concentrated to dryness. The product was
purified by column chromatography on silica gel (4 g, 99:1
CH2Cl2:AcOEt, product Rf =0.3) to afford 3a as a white solid
(40 mg, 0.03 mmol, 81% yield). Rf =0.3 (99:1 CH2Cl2:AcOEt).
1H NMR (300 MHz, CDCl3, 298 K): d=7.69 (br s, 4H); 7.46–7.44 (m,
8H); 7.41–7.39 (m, 8H); 7.10–7.08 (m, 8H); 6.86–6.84 (m, 8H); 5.76
(s, 8H); 4.61 (s, 8H); 3.81 (s, 12H); 1.98 ppm (s, 12H). 13C{1H} NMR
(100 MHz, CDCl3, 298 K): d=169.25; 157.80; 148.14; 136.30; 133.29;
131.01; 127.59; 121.77; 116.85; 114.80; 106.71; 89.08; 88.45; 65.35;
52.47; 44.80; 27.94 ppm. HR-MS (HPLC-ESI-TOF) (C92H76N4NaO12):
Measured m/z=1451.5330 [M+Na]+; Calcd m/z=1451.5352. FT-IR:
n˜ =3395; 1758 (C=O stretching); 1739 (C=O stretching); 1601;
1511; 1434; 1204 (C-O stretching); 1173 cmÀ1 (C-O stretching);
1074; 828; 771; 533. M.p.>2658C (decompose).
m/z=1371.4769 [MÀH]À; Calcd m/z=1371.4761. FT-IR: n˜ =3412;
2922 (OÀH stretching); 2853; 1724 (C=O stretching); 1514; 1429;
1282; 1240 (CÀO stretching); 1176; 832; 771; 524 cmÀ1. M.p.>
1508C (decompose).
Tetramethyl pyridinium SAE-calix[4]pyrrole (5): Tetrapyridine 3b
(10 mg, 0.01 mmol, 1 equiv) was dissolved in dry DMF (1 mL) and
stirred for 5 min. Then, methyl iodide was added (0.04 mL,
0.64 mmol, 25 equiv). The mixture was stirred at 458C under Ar at-
mosphere for 24 h. The reaction was stopped and the solvent was
removed under vacuum. THF (20 mL) was added and the mixture
was heated at 458C for 2.5 h. The product was collected by filtra-
1
tion (13 mg, 0.01 mmol, 95% yield). H NMR (500 MHz, [D6]DMSO,
298 K): d=10.83 (br s, 4H); 9.78 (br s, 4H); 9.19–9.17 (m, 8H);
8.50–8.48 (m, 8H); 7.83–7.81 (m, 8H); 7.61–7.57 (m, 16H); 7.08–
7.06 (m, 8H); 5.87 (br s, 8H); 4.42 (s, 12H); 1.90 ppm (s, 12H).
X-ray crystallography: Crystallographic data for 3a (colorless crys-
tals): C92H76N4O12·(C2H3N)2.3, M=1552.72 gmolÀ1, crystal size: 0.20ꢂ
0.12ꢂ0.04 mm3, triclinic, space group P-1, a=13.9554 (8), b=
14.9965 (9), c=20.9812 (13) ꢁ, a=99.969 (2), b=106.253 (2), g=
96.512 (2)8, V=4090.6 (4) ꢁ3, Z=2, 1calcd =1.261 mgmÀ3
, m=
0.083 mmÀ1, l=0.71073 ꢁ, T=100 (2) K, 2q range: 3.146–54.4908,
reflections collected: 62632, independent: 18173 (Rint =0.0465),
1467 parameters. The structure was solved by direct methods and
refined by full-matrix least-squares on F2; final R indices [I>2s(I)]:
R1=0.0749, wR2=0.1921; maximal residual electron density:
Tetrapyridine SAE-calix[4]pyrrole (3b): Tetraiodo
0.04 mmol, 1 equiv), Pd(PPh3)2Cl2 (14.86 mg,
1
(50 mg,
0.02 mmol,
0.125 equiv), CuI (6.45 mg, 0.03 mmol, 0.2 equiv) and N-(4-ethynyl-
phenyl)-4-pyridinecarboxamide 2b (56.5 mg, 0.25 mmol, 1.5 equiv)
were kept under Ar atmosphere for 5 min. Dry THF (5 mL) and dry
iPr2NH (5 mL) were added. The color of the mixture changed from
yellow to black. The reaction was stirred at 458C for 3 h. After that,
we observed a white solid in suspension. The mixture was filtered
and the solid was resuspended in an aqueous solution of saturated
NaHCO3 (5 mL). Then, the solid was filtered and washed with satu-
rated NaHCO3 (2 mL). The product was purified by column chroma-
tography on silica gel (4 g, THF, product Rf =0.55 99:1 THF:CH3OH)
to afford tetrapyridine 3b as a white solid (32.85 mg, 0.04 mmol,
50% yield). Rf =0.55 (99:1 THF:CH3OH). 1H NMR (500 MHz,
[D6]DMSO, 298 K): d=10.59 (s, 4H); 9.54 (br s, 4H); 8.73–8.72 (m,
8H); 7.79–7.77 (m, 16H); 7.59–7.57 (m, 8H); 7.52–7.50 (m, 8H);
7.01–6.99 (m, 8H); 5.99 (s, 8H); 1.85 ppm (s, 12H). 13C{1H} NMR
(125 MHz with cryoprobe, [D6]DMSO, 298 K): d=164.08; 150.53;
141.67; 138.97; 136.86; 131.96; 131.32; 127.27; 121.51; 120.64;
120.27; 117.51; 105.27; 89.42; 88.73; 66.99; 44.25; 30.67 ppm. HR-
MS (HPLC-ESI-TOF) (C104H77N12O4): Measured m/z=1557.6117
[M+H]+; Calcd m/z=1557.6185. FT-IR: n˜ =3335 (NÀH stretching);
3306 (NÀH stretching); 1676 (C=O stretching); 1655; 1587; 1517
(NÀH bending); 1406; 1323; 1017; 833; 751; 525 cmÀ1. M.p.>
2458C.
0.764 eꢁÀ3
.
Crystallographic data for 4 (colorless crystals): C88H68N4O12·(C2H3N),
M=1414.51 gmolÀ1, crystal size: 0.10ꢂ0.10ꢂ0.01 mm3, triclinic,
space group P-1, a=13.7935 (13), b=14.7307 (11), c=20.7078
(19) ꢁ, a=70.225 (8), b=82.971 (8), g=86.700 (7)8, V=3929.1
(6) ꢁ3, Z=2, 1calcd =1.196 mgmÀ3, m=0.080 mmÀ1, l=0.71073 ꢁ,
T=100 (2) K, 2q range: 4.146–60.8508, reflections collected: 35755,
independent: 19424 (Rint =0.0610), 974 parameters. The structure
was solved by direct methods and refined by full-matrix least-
squares on F2; final R indices [I>2s(I)]: R1=0.1123, wR2=0.2489;
maximal residual electron density: 0.681 eꢁÀ3
.
CCDC 1486376 (3a) and 1486387 (4) contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre.
Computational modeling: The energy-minimized structures
shown in the manuscript were obtained from Fujitsu Scigress Ver-
sion 2.2.0. The structures were optimized using Molecular Mechan-
ics method and PM6[26] force field. We used Turbomole Version
4.1[27] for chemical shift calculations. The structure was optimized
using DFT+Dispersion method and BP86[28]-D3[29]/def2-SVP level of
theory. Chemical shift values and NICS were determined at BP86-
D3/def2-SVP level of theory.
Tetraacid SAE-calix[4]pyrrole (4): Tetraester 3a (49.30 mg,
0.03 mmol, 1 equiv) was dissolved in THF:H2O (1:1, 10 mL) and
LiOH (6.61 mg, 0.28 mmol, 2 equiv) was added. The reaction was
stirred at RT for 5 h. After that, the THF was removed under
vacuum. Water (10 mL) was added to the mixture and it was
washed with CH2Cl2 (10 mL). Then, the aqueous layer was acidified
with HCl 1 N (pH 3) and a white precipitate appeared, which was
extracted with AcOEt (3ꢂ10 mL). The organic layer was washed
with brine (20 mL), dried (Na2SO4), filtered, and evaporated to
Acknowledgements
The authors thank Gobierno de EspaÇa MINECO and FEDER
Funds (project CTQ2014-56295-R), MECD (grant FPU14/01016),
Severo Ochoa Excellence Accreditation 2014–2018 (SEV-2013-
0319), and ICIQ Foundation for funding. We also thank Eduar-
do C. Escudero-Adꢃn for X-ray crystallography data. We ac-
knowledge the NMR and X-Ray Units of ICIQ.
1
afford 4 as a white solid (32.1 mg, 0.02 mmol, 68% yield). H NMR
(500 MHz with cryoprobe, [D6]acetone, 298 K): d=8.91 (br s, 4H);
7.47–7.45 (m, 8H); 7.41–7.39 (m, 8H); 7.05–7.03 (m, 8H); 6.93–6.91
(m, 8H); 5.97 (d, 8H, J=2.50 Hz); 4.77 (s, 8H); 3.30 (br s, 4H);
1.94 ppm (s, 12H). 13C{1H} NMR (125 MHz with cryoprobe,
[D6]acetone, 298 K): d=170.15; 159.25; 150.90; 138.15; 133.76;
131.95; 128.52; 122.53; 116.66; 115.76; 106.57; 89.95; 88.54; 65.39;
45.53; 29.84 ppm. HR-MS (HPLC-ESI-TOF) (C88H67N4O12): Measured
Keywords: cross-coupling · kinetics · molecular recognition ·
noncovalent interactions · supramolecular chemistry
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Received: June 23, 2016
Published online on && &&, 0000
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FULL PAPER
Super aryl-extended calix[4]pyrroles
displaying a deep aromatic cavity are re-
ported. The 1:1 inclusion complexes
formed with pyridyl-N-oxides in chloro-
form solution are thermodynamically
and kinetically highly stable. The incor-
poration of ionizable and charged
groups to gain water solubility is also
investigated.
&
Supramolecular Chemistry
L. Escobar, G. Aragay, P. Ballester*
&& – &&
Super Aryl-Extended Calix[4]pyrroles:
Synthesis, Binding Studies, and
Attempts To Gain Water Solubility
Chem. Eur. J. 2016, 22, 1 – 9
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ