Dimerization and anion binding of a fluorescent phospholipid analogue

Article abstract of DOI:10.1021/jo3009758

A diarylacetylene fluorophore featuring spatially separated urea and phosphocholine (PC) groups forms a macrocyclic head-to-tail dimer stabilized by NH_urea???OP_PC hydrogen bonds. At concentrations above ～2 ×10^-5 M in CH ₂Cl₂, the emission intensity of the dimer is quenched by HCO₃^- and H₂PO₄^- but not by Cl^- and NO₃^-. Under more dilute conditions, all four anions are bound unselectively with association constants on the order of 10⁵ M^-1.

Full text of DOI:10.1021/jo3009758

Note
pubs.acs.org/joc
Dimerization and Anion Binding of a Fluorescent Phospholipid
Analogue
Danielle M. Jessen, Ashley N. Wercholuk, Ber Xiong, Andrew L. Sargent,* and William E. Allen*
Department of Chemistry, Science and Technology Building, East Carolina University, Greenville, North Carolina 27858-4353,
United States
S
* Supporting Information
ABSTRACT: A diarylacetylene fluorophore featuring spatially
separated urea and phosphocholine (PC) groups forms a macrocyclic
“head-to-tail” dimer stabilized by NHurea···OPPC hydrogen bonds. At
concentrations above ∼2 × 10−5 M in CH2Cl2, the emission intensity
of the dimer is quenched by HCO3− and H2PO4− but not by Cl− and
−
NO3 . Under more dilute conditions, all four anions are bound
unselectively with association constants on the order of 105 M−1.
oorly regulated movement of anions across cell mem-
branes lies at the root of a number of diseases.1,2 A small
Scheme 1
P
but operationally diverse set of synthetic transporters have been
developed to facilitate diffusion of species like chloride and
bicarbonate through lipid bilayers. Low molecular weight
“shuttles” shield the ions of interest within a nonpolar envelope,
yielding complexes that can traverse the hydrophobic bilayer
core.3−5 Larger, relatively immobile systems allow anions to
pass among electrophilic sites in membrane-spanning single
molecules6,7 or through pores in noncovalent assemblies.8,9
Regardless of the mechanism of transport, all such systems
must recognize their targets near the aqueous interface, a milieu
rich in electrically charged phosphate and ammonium groups.
Here we evaluate the effect(s) of such groups on anion
recognition in organic solution using a urea-based10−12
fluorescent receptor that bears a biologically relevant
phosphocholine (PC) unit.
Synthesis of the lipid analogue is shown in Scheme 1. 4-
Ethynylaniline reacts slowly with hexyl isocyanate at room
temperature to afford 1. This alkyne was expected to impart
organic solubility to subsequent products while setting the
anion-binding site in conjugation with the final diarylacetylene
reporter.13,14 To ensure that any inductive effects of the PC
would not alter the acidity of the urea, a simple 1,3-propanediol
spacer, inspired by the glycerol found in naturally occurring
lipids, was incorporated into 2. Control compound 3
precipitates from a Sonogashira coupling mixture of 1 and 2
in sufficient purity for use in spectroscopic measurements.
Electron-donor (:NHR) and electron-acceptor (CO) groups
are present at the para-positions; so-called “push-pull”
chromophores are of interest for their nonlinear optical
properties and environmental sensitivity.15,16 The PC head-
group was attached to 3 by adapting previously published
procedures.17−19 Yields of 4 were less than 5% when
acetonitrile was used as the reaction solvent instead of N,N-
dimethylformamide.
−
Addition of 30 equiv of a tetraalkylammonium salt of HCO3 ,
H2PO4 , Cl−, or NO3 reduces the integrated fluorescence
intensity F by 20% (for nitrate) to 70% (for dihydrogenphos-
phate), while causing λem to move by 3−5 nm to longer
wavelengths. An isoemissive point appears at 505 nm during
−
−
−
−
titration with HCO3 (Figure 1) and at 470 nm for NO3 ,
indicative of two-state behavior. Normalized outputs (F/F0)
were plotted vs [anion]total and fit to a hyperbolic function20 to
Nonionic 3 emits violet light with a large Stokes shift in
dichloromethane (λex = 324 nm, λem = 414 nm, ΦF = 0.43).
Received: May 12, 2012
Published: July 16, 2012
© 2012 American Chemical Society
6615
dx.doi.org/10.1021/jo3009758 | J. Org. Chem. 2012, 77, 6615−6619

The Journal of Organic Chemistry
Note
Figure 1. Normalized emission spectra of 3 (1.6 × 10−6 M in CH2Cl2)
during titration with Et4NHCO3 (0 → 30 equiv); λex = 320 nm.
Figure 2. Fluorescence response of 3 (squares) and 4 (diamonds)
−
during titration with Bu4N+ NO3 in CH2Cl2. The concentration of
each sensor was 5.0 × 10−6 M.
derive the 1:1 association constants shown in Table 1. The
Kassoc values for 3 increase with anion basicity.21 Chloride
concentration regime prevented study of the self-association of
4 in the dichloromethane solvent used for fluorescence, so
CD3CN was used instead. The spectrum of a 2.0 × 10−3
M
Table 1. Association Constants (M−1) in CH2Cl2
solution of 4 consists of one set of peaks. A very broad singlet
at ∼6.15 ppm and a sharper one at 8.68 ppm are assigned to the
urea NH. At the same concentration, the corresponding
protons in 3 appear at 5.29 and 7.36 ppm, respectively.
Introducing aliquots of water into acetonitrile samples of 4
induces upfield shifts in both NH signals, consistent with a
process that replaces relatively robust H-bonds (within the
putative dimer) with weaker ones (to water). At 1.5% water by
volume, for example, the sharper resonance is ∼0.7 ppm
removed from its starting point (see the Supporting
Information). Density functional theory (DFT) analyses,28−32
performed in a dielectric continuum corresponding to CH2Cl2,
further support the conclusion that assembly of 4·4 involves
hydrogen-bonding at the urea. Its predicted structure is a
“head-to-tail” macrocycle (ΔEdimer = −12.8 kcal/mol) with
multiple NHurea···OPPC interactions (Figure 3).
a
b
b
−
−
−
HCO3
H2PO4
Cl−
NO3
a
b
3
4
5.2 × 104
1.2 × 104
3.7 × 105
9.6 × 103, 7.4 × 103
4.7 × 103
1.0 × 105
c
a
>105
2.9 × 105
a
b
c
Tetraethylammonium salt. Tetrabutylammonium salt. Titration
curve shape is indicative of strong binding; fitting to the equation in ref
20 produced unacceptably large errors.
binding strengths are effectively the same whether Et4N+ or
Bu4N+ countercations are used, and agree well with the
reported equilibrium constant for 1-(4-nitrophenyl)-3-octylurea
in wet chloroform, ∼8 × 103 M−1.7 For zwitterion 4, at low
concentrations appropriate for quantitative fluorescence meas-
urements (i.e., ≤ 5 × 10−6 M in CH2Cl2), the absorption and
emission maxima (329 nm, 415 nm) and quantum yield (0.36)
are similar to those of its precursor. However, the emission
intensity of 4 is strongly suppressed by all four species in Table
1. Figure 2 illustrates the enhancement in Kassoc for nitrate,
which is typically a poor substrate for synthetic H-bond
donors.22−24 Titrations with bicarbonate yielded binding
isotherms with a pronounced “L”-shape, consistent with tight
association, that could not be reliably fit.25 The sensor is
significantly less radiative in a competitive solvent mixture (λex
= 339 nm, λem = 462 nm, ΦF = 0.08 for a 4.1 × 10−6 M solution
in DMSO containing 0.5% water by volume). Under these
conditions, the presence of 50 equiv of a potent quencher,
Figure 3. Calculated structure of 4·4 in a dichloromethane solvent
field. Hexyl groups attached to urea were truncated to methyl prior to
DFT optimization.
H2PO4 , reduces F by just 10%. Kassoc is 6.7 × 103 M−1, more
than 1 order of magnitude below that observed in dichloro-
methane.
−
Electrically charged anion receptors with self-complementary
groups are known to form cyclic dimers26 or linear oligomers,27
depending on the length and flexibility of the intervening
spacer. Electrospray ionization MS of 4 in acetonitrile−water
identified a species at m/z = 1175.64, matching the calculated
mass of 4·4 + H+, with about one-fifth of the abundance of the
monomer at m/z = 588.29. Poor solubility in the NMR
Electronic absorption spectra of 4 were acquired under
several sets of conditions to determine if the absorbance values
for the two bands near 280 and 330 nm would vary with the
extent of dimerization (Figure 4). In dichloromethane, the ratio
A
281/A332 remains constant at 0.68 as a sample is diluted from
5.3 × 10−5 M to 1.7 × 10−5 M, at which point it begins to fall.
6616
dx.doi.org/10.1021/jo3009758 | J. Org. Chem. 2012, 77, 6615−6619

The Journal of Organic Chemistry
Note
−
changes for H2PO4 are too similar to assign a clear
preference.28 Qualitative fluorescence determinations of anion
sensitivity, acquired at relatively high sensor concentrations in
CH2Cl2, are in general agreement with the computational
−
results. Large excesses of Cl− or NO3 do not diminish F for
2.6 × 10−5 M solutions of 4, but rather cause negligible
increases in intensity. Bicarbonate and dihydrogenphosphate
ions quench such samples. In previous work with synthetic
receptors that feature anion recognition sites isolated from
(positive) charges, high affinities were ascribed to entropic
gains that occur upon substrate binding.34,35 Rigidity in the
receptors is relieved and an ensemble of energetically low-lying
configurations is generated. Separation of 4·4 into its
component monomers would indeed free the PC groups
from conformational restriction. Nevertheless, any positive
changes in entropy that may accompany the dissociation event
are apparently not large enough to offset the enthalpic cost, at
least in a nonpolar solvent.
Structures of the sensor-anion assemblies that are present in
the dilute samples used for Kassoc determinations are unknown.
Proton NMR experiments upon acetonitrile solutions may shed
some light on the complexes, since the monomeric state of 4 is
accessible in this solvent at millimolar concentrations.
Continuous variations plots were generated for 4 in the
presence of tetraethylammonium bicarbonate and chloride in
acetonitrile-d3. Maxima for both appear at a sensor mole
fraction of 0.4, corresponding to a binding stoichiometry that is
intermediate between 1:1 and 1:2 (4-to-anion). Ion-pairing and
urea H-bonding are present. When 10 equiv of Et4NCl is added
to a 2.0 × 10−3 M CD3CN solution of 4, the choline methyl
protons at 3.08 ppm experience a downfield shift of 0.07
ppm.36,37 The NH resonances appear at 7.23 and 10.26 ppm
under these conditions. Treating 3 with the same amount of
chloride ion yields δNH of 7.19 and 10.18 ppm, suggesting that
the coordination environments surrounding Cl− are similar for
both sensors. The urea signals are further downfield for 4 with
10 equiv of Et4NHCO3, at 8.56 and 10.97 ppm, in accord with
the greater calculated binding energy for bicarbonate vs
chloride (“Monomer,” Table 2). An NMR-derived association
constant for 4 + Cl− is 2.4 × 103 M−1, a value typical for
uncharged urea-containing receptors in acetonitrile.38,39 Thus,
the presence of a PC headgroup does not guarantee unusually
strong anion binding in polar solvents.
In summary, dimerization of a fluorescent lipid analogue that
bears urea and phosphocholine groups can render it
unresponsive toward weakly basic anions. Under conditions
where a fraction of the analogue is monomeric (i.e., at
micromolar concentrations in CH2Cl2, or when it is dissolved
in CH3CN or DMSO), strong anion binding is observed only
in nonpolar dichloromethane solvent. Future work will focus on
incorporating the sensor into liposomal membranes and
evaluating its ability to promote anion transport. Presumably,
the directionally organized nature of bilayer molecules will
disfavor self-association of the sensor.
Figure 4. UV/vis traces of 4 in CH2Cl2 () and CH3CN (---). [4] =
2.6 × 10−5 M.
At a concentration within the range above (2.4 × 10−5 M) in 1-
octanol, a solvent with a comparable dielectric constant33 but
with hydrogen-bonding potential, A281/A335 = 0.53. The ratio is
lowest for 4 in acetonitrile (0.49 at 2.2 × 10−5 M). Control
compound 3, which is incapable of phosphodiester-mediated
dimerization, has a similar value (A280/A324 = 0.49 in CH2Cl2).
For comparison, an electronic excitation profile of 4 in its
monomeric form was generated using time-dependent DFT.
Calculated transitions occur at 274 and 374 nm with oscillator
strengths of 0.2962 and 1.4819, respectively, for a ratio of 0.20.
These results seem to define a trend in which samples with
higher dimer content have higher absorbance quotients.
However, an unexpectedly large value is observed when 4 is
dissolved in DMSO-0.5% water (A282/A339 = 0.67 at 4.1 × 10−5
M). Therefore, the absorbance ratio is perhaps better viewed as
an approximate gauge of H-bond strength at the anion
recognition site, and not strictly as an indicator for the self-
association of 4.
DFT treatments show that poorly basic anions are not likely
to disrupt the hydrogen bonds of 4·4. Starting from the
structure of Figure 3, the four anions in turn were placed close
+
to a peripheral RN(CH3)3 unit of an intact dimer, and the
resulting complexes were optimized in a CH2Cl2 solvent field.
Calculated ΔE values for ion-pairing (labeled “Closed” in Table
2) are −6 kcal/mol or greater across the series. When the
macrocycle is opened on one side, there is less driving force for
binding of Cl− and NO3 . Here, a set of NHurea···OPPC H-
−
bonds was broken, an anion was docked to the free urea, and
the nearby choline group was rotated into proximity of the
−
guest (“Opened,” Table 2). The best H-bond acceptor, HCO3 ,
benefits from this mode of association, while the energy
EXPERIMENTAL SECTION
Table 2. Calculated Binding Energies (kcal/mol) for Anion
Complexes in a CH2Cl2 Dielectric Continuum
■
1-(4-Ethynylphenyl)-3-hexylurea (1). 4-Ethynylaniline (2.78 g,
23.7 mmol) and hexyl isocyanate (3.08 g, 24.2 mmol) were combined
in 60 mL of acetonitrile to afford a tea-colored solution. The mixture
was stirred at room temperature under N2 for 4 d, during which time a
precipitate appeared. The solution volume was reduced to
approximately 30 mL using a rotary evaporator, and the crude
product was collected by filtration and washed with chilled CH3CN.
−
−
−
HCO3
H2PO4
Cl−
NO3
closed (4·4·anioncholine
opened (4·anionchol+urea·4)
monomer (4·anionurea
)
−7.0
−8.2
−12.0
−6.2
−5.7
−10.2
−7.2
−5.0
−9.1
−7.3
−5.5
−8.8
)
6617
dx.doi.org/10.1021/jo3009758 | J. Org. Chem. 2012, 77, 6615−6619

The Journal of Organic Chemistry
Note
Urea 1 (3.10 g, 53%) was obtained as an off-white powder: mp 129−
131 °C; 1H NMR (CDCl3) δ 0.87 (t, 3H), 1.27 (s, 6H), 1.48 (m, 2H)
3.02 (s, 1H), 3.21 (q, 2H), 5.10 (t, 1H), 6.91 (s, 1H), 7.27 (d, 2H),
7.39 (d, 2H); 13C NMR (CDCl3) δ 14.0, 22.5, 26.6, 30.0, 31.5, 40.5,
76.2, 83.5, 116.6, 119.5, 133.1, 139.4, 155.4; HRMS (ESI-QToF) calcd
for C15H21N2O (M + H)+ 245.1654, found 245.1644.
ASSOCIATED CONTENT
■
S
* Supporting Information
1H/13C NMR spectra for new compounds, representative
fluorescence titration data, binding and Job plots from NMR,
and atomic coordinates of DFT-calculated structures. This
material is available free of charge via the Internet at http://
pubs.acs.org.
3-Hydroxypropyl 4-Bromobenzoate (2). A suspension of 4-
bromobenzoic acid (0.83 g, 4.1 mmol) and methanesulfonic acid (≪1
drop) in 1,3-propanediol (20 mL, 280 mmol) was heated with stirring
to 115 °C for 4 h. The solid disappeared. Upon cooling to room
temperature, the contents were poured into 200 mL of water, and the
resultant mixture was extracted with EtOAc. The organic phases were
combined, washed several times with water and then with saturated
aqueous NaHCO3, and dried over anhydrous Na2SO4. Filtration and
evaporation of the filtrate provided 1.06 g (99%) of 2 as a colorless
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: allenwi@ecu.edu, sargenta@ecu.edu.
Notes
1
The authors declare no competing financial interest.
oil/low-melting white solid: H NMR (CDCl3) δ 2.01 (m, 2H), 2.48
(br s, 1H), 3.78 (t, 2H), 4.48 (t, 2H), 7.57 (d, 2H), 7.89 (d, 2H); 13C
NMR (CDCl3) δ 32.0, 59.3, 62.4, 128.4, 129.2, 131.3, 132.0, 166.4;
HRMS (ESI-QToF) calcd for C10H12BrO3 (M + H)+ 258.9970, found
258.9977.
ACKNOWLEDGMENTS
■
A.L.S. acknowledges the ECU CACS and the NSF (CNS-
0619285). D.M.J. was supported by an ECU Undergraduate
Research and Creative Activity Award for Spring 2011.
Purchase of NMR and mass spectrometers was made possible
by the NSF (CRIF-0077988 and MRI-0521228, respectively).
We thank Ms. Jenna M. Thuman and Amanda L. Morgan for
assistance with acquisition of spectral data.
3-Hydroxypropyl 4-((4-(3-Hexylureido)phenyl)ethynyl)-
benzoate (3). A pressure tube was charged with 1 (0.71 g, 2.9
mmol), 2 (0.75 g, 2.9 mmol), piperidine (1.5 mL, 15 mmol), and 8 mL
of CH3CN. With stirring, a stream of N2 was gently bubbled into the
gold suspension for 5 min, and then tetrakis(triphenylphosphine)-
palladium(0) (0.067 g, 0.058 mmol) was added. The tube was
immediately sealed and lowered into an oil bath that had been
preheated to 85 °C. After 15 h, the reaction mixture was allowed to
cool to room temperature with slow stirring. The cream-colored
precipitate was collected by filtration and washed with chilled CH3CN
to afford 0.86 g (70%) of 3. Analytical samples were recrystallized from
REFERENCES
■
(1) Quinton, P. M. Am. J. Physiol. Cell Physiol. 2010, 299, C1222−
C1233.
1
(2) Cordat, E.; Casey, J. R. Biochem. J. 2009, 417, 423−439.
hot EtOAc: mp 188−190 °C; H NMR (DMSO-d6) δ 0.90 (t, 3H),
(3) Iglesias-Hernan
T.; Perez-Tomas
1558.
́
dez, P.; Moreno, D.; Araujo Javier, A.; Torroba,
1.31 (s, 6H), 1.48 (m, 2H), 1.93 (m, 2H), 3.14 (q, 2H), 3.64 (q, 2H),
4.39 (t, 2H), 4.48 (t, 1H), 6.08 (t, 1H), 7.40, (d, 2H), 7.44 (d, 2H),
7.56 (d, 2H), 7.97 (d, 2H), 8.52 (s, 1H); 13C NMR (DMSO-d6) δ
14.4, 22.6, 26.5, 30.1, 31.5, 32.0, 57.7, 62.7, 87.7, 93.7, 113.9, 117.8,
128.0, 129.8, 131.8, 132.8, 142.2, 155.3, 165.7; FTIR (ATR, solid) ν
3314 cm−1; HRMS (ESI-QToF) calcd for C25H31N2O4 (M + H)+
423.2284, found 423.2301; UV/vis (CH2Cl2) λmax (ε, M−1cm−1) 324
nm (31000), 280 (16000).
́
́
, R.; Quesada, R. Chem. Commun. 2012, 48, 1556−
(4) Andrews, N. J.; Haynes, C. J. E.; Light, M. E.; Moore, S. J.; Tong,
C. C.; Davis, J. T.; Harrell, W. A.; Gale, P. A. Chem. Sci. 2011, 2, 256−
260.
(5) Hussain, S.; Brotherhood, P. R.; Judd, L. W.; Davis, A. P. J. Am.
Chem. Soc. 2011, 133, 1614−1617.
(6) Perez-Velasco, A.; Gorteau, V.; Matile, S. Angew. Chem., Int. Ed.
2008, 47, 921−923.
3-(4-((4-(3-Hexylureido)phenyl)ethynyl)benzoyloxy)propyl
2-(Trimethylammonio)ethyl Phosphate (4). Alcohol 3 (0.42 g, 1.0
mmol) was dissolved in 3 mL of stirring N,N-dimethylformamide in a
pressure tube. A solution of 2-chloro-1,3,2-dioxaphospholane 2-oxide
(0.71 g, 5.0 mmol) in 1 mL of DMF was added, followed immediately
by neat liquid trimethylamine (1 mL, stored at −40 °C). The tube was
sealed, and the contents were stirred at room temperature for 4 h.
During this time, the gold mixture became opaque. The tube was then
heated to 70 °C for an additional 6 h. Upon cooling, the reaction
vessel was cautiously opened, and the liquid portion was transferred
directly onto a silica gel flash chromatography column that had been
saturated with CH2Cl2−CH3OH−H2O (30:60:10, respectively, by
volume). Fractions containing the desired compound (Rf = 0.17) were
combined and evaporated to a viscous yellow oil that solidified upon
standing under vacuum. The yield of 4 was 0.36 g (61%). Analytical
samples were purified by RP-HPLC on a C18 column using a gradient
of 50:50 CH3CN−H2O → 80:20 CH3CN−H2O, then were
(7) McNally, B. A.; O’Neil, E. J.; Nguyen, A.; Smith, B. D. J. Am.
Chem. Soc. 2008, 130, 17274−17275.
(8) Yamnitz, C. R.; Negin, S.; Carasel, I. A.; Winter, R. K.; Gokel, G.
W. Chem. Commun. 2010, 46, 2838−2840.
(9) Hennig, A.; Fischer, L.; Guichard, G.; Matile, S. J. Am. Chem. Soc.
2009, 131, 16889−16895.
(10) Steed, J. W. Chem. Soc. Rev. 2010, 39, 3686−3689.
(11) Li, A.-F.; Wang, J.-H.; Wang, F.; Jiang, Y.-B. Chem. Soc. Rev.
2010, 39, 3729−3745.
(12) Amendola, V.; Fabbrizi, L.; Mosca, L. Chem. Soc. Rev. 2010, 39,
3889−3915.
(13) Jordan, L. M.; Boyle, P. D.; Sargent, A. L.; Allen, W. E. J. Org.
Chem. 2010, 75, 8450−8456.
(14) Carroll, C. N.; Naleway, J. J.; Haley, M. M.; Johnson, D. W.
Chem. Soc. Rev. 2010, 39, 3875−3888.
1
́ ́
(15) Nepras, M.; Almonasy, N.; Bures, F.; Kulhanek, J.; Dvorak, M.;
lyophilized to afford a white fluffy solid: mp 134−148 °C dec; H
Michl, M. Dyes Pigments 2011, 91, 466−473.
(16) Barsu, C.; Cheaib, R.; Chambert, S.; Queneau, Y.; Maury, O.;
Cottet, D.; Wege, H.; Douady, J.; Bretonniere, Y.; Andraud, C. Org.
Biomol. Chem. 2010, 8, 142−150.
NMR (DMSO-d6) δ 0.88 (t, 3H), 1.28 (br s, 6H), 1.43 (m, 2H), 2.04
(m, 2H) 3.08 (m, 2H), 3.13 (s, 9H), 3.59 (m, 2H), 4.00 (m, 2H), 4.24
(br s, 2H), 4.37 (t, 2H), 6.35 (t, 1H), 7.44 (d, 2H), 7.47 (d, 2H), 7.65
(d, 2H), 7.98 (d, 2H), 8.82 (s, 1H); 13C NMR (DMSO-d6) δ 14.4,
22.6, 26.5, 30.1, 31.5, 44.7, 52.7, 62.5, 63.2, 65.6, 87.7, 94.0, 113.6,
117.7, 128.3, 129.1, 130.1, 131.8, 132.8, 142.4, 155.4, 165.6; HRMS
(ESI-QToF) calcd for C30H43N3O7P (M + H)+ 588.2839, found
588.2836; UV/vis (dry CH2Cl2, 2.6 × 10−5 M) λmax (ε, M−1 cm−1) 332
nm (19000), 281 (12000), 268 (13000), (DMSO, 5.0 × 10−5 M) λmax
(ε, M−1 cm−1) 339 nm (12500), 281 (8800), 270 (9200); fluorescence
(CH2Cl2, < 5 × 10−6 M) λem 415 nm, (CH2Cl2, > 2 × 10−5 M) λem
425 nm.
̀
(17) Seuring, J.; Reiss, P.; Koert, U.; Agarwal, S. Chem. Phys. Lipids
2010, 163, 367−372.
(18) Saugar, J. M.; Delgado, J.; Hornillos, V.; Luque-Ortega, J. R.;
Amat-Guerri, F.; Acuna, A. U.; Rivas, L. J. Med. Chem. 2007, 50, 5994−
̃
6003.
(19) Nederberg, F.; Bowden, T.; Hilborn, J. Macromolecules 2004, 37,
954−965.
(20) Causey, C. P.; Allen, W. E. J. Org. Chem. 2002, 67, 5963−5968.
6618
dx.doi.org/10.1021/jo3009758 | J. Org. Chem. 2012, 77, 6615−6619

The Journal of Organic Chemistry
Note
(21) Ros-Lis, J. V.; Martínez-Man
́
ez, R.; Sancenon
́
, F.; Soto, J.;
Rurack, K.; Weißhoff, H. Eur. J. Org. Chem. 2007, 2449−2458.
(22) Busschaert, N.; Wenzel, M.; Light, M. E.; Iglesias-Hernandez, P.;
Perez-Tomas, R.; Gale, P. A. J. Am. Chem. Soc. 2011, 133, 14136−
14148.
̃
́
́
́
(23) Swinburne, A. N.; Paterson, M. J.; Beeby, A.; Steed, J. W.
Chem.Eur. J. 2010, 16, 2714−2718.
(24) Bisson, A. P.; Lynch, V. M.; Monahan, M.-K. C.; Anslyn, E. V.
Angew. Chem., Int. Ed. 1997, 36, 2340−2342.
(25) Acetate ion (as Bu4N+ salt) is also bound strongly by 4. The
data could not be reliably fit to a 1:1 model. See the Supporting
Information.
(26) Rether, C.; Verheggen, E.; Schmuck, C. Chem. Commun. 2011,
47, 9078−9079.
(27) Schmuck, C.; Rehm., T.; Geiger, L.; Schafer, M. J. Org. Chem.
̈
2007, 72, 6162−6170.
(28) Computations employed the Becke−Tsuneda−Hirao gradient-
corrected exchange-correlation (BOP) functional and a double
numerical plus polarization basis set. For 4·4, its large size and the
conformational mobility of its termini made an exhaustive search of
binding geometries unfeasible. As such, errors in energies are likely
greater than 0.1 kcal/mol. See ref 13 for an example of the BOP
functional applied to H-bonded systems.
(29) Becke, A. D. J. Chem. Phys. 1988, 88, 1053−1062.
(30) Delley, B. J. Chem. Phys. 1990, 92, 508−517.
(31) Tsuneda, T.; Hirao, K. Chem. Phys. Lett. 1997, 268, 510−520.
(32) Klamt, A.; Schueuermann, G. J. Chem. Soc., Perkin Trans. 2 1993,
799−805.
(33) Dichloromethane εr = 9.1, 1-octanol εr = 10.3.
́
(34) Valik, M.; Kral, V.; Herdtweck, E.; Schmidtchen, F. P. New J.
Chem. 2007, 31, 703−710.
(35) Jadhav, V. D.; Schmidtchen, F. P. Org. Lett. 2005, 7, 3311−3314.
(36) A modest shift in RN(CH3)3+ from 3.13 to 3.17 ppm takes place
when a 1.0 × 10−2 M sample of 4 in DMSO-d6 is treated with 10 equiv
of Et4NCl.
(37) Substrate binding within the central cavity of 4·4 via aryl
−
CH···anion interactions can be ruled out; excess HCO3 and Cl−
induce changes of <0.05 ppm in the NMR shifts of these hydrogens in
CD3CN. See: McDonald, K. P.; Ramabhadran, R. O.; Lee, S.;
Raghavachari, K.; Flood, A. H. Org. Lett. 2011, 13, 6260−6263. See
also: Sessler, J. L.; Cai, J.; Gong, H.-Y.; Yang, X.; Arambula, J. F.; Hay,
B. P. J. Am. Chem. Soc. 2010, 132, 14058−14060.
(38) Clare, J. P.; Statnikov, A.; Lynch, V.; Sargent, A. L.; Sibert, J. W.
J. Org. Chem. 2009, 74, 6637−6646.
(39) dos Santos, C. M. G.; McCabe, T.; Watson, G. W.; Kruger, P. E.;
Gunnlaugsson, T. J. Org. Chem. 2008, 73, 9235−9244.
6619
dx.doi.org/10.1021/jo3009758 | J. Org. Chem. 2012, 77, 6615−6619