A selective chromogenic molecular sensor for acetate anions in a mixed acetonitrile-water medium

Article abstract of DOI:10.1039/b805291c

Quinonehydrazone compound 2, as a new chromogenic anion sensor, can selectively detect AcO^- over F^- and other anions in mixed acetonitrile-water media. The deprotonation of the N-H proton of the sensor is responsible for the drastic color change. An acidic C-H group in the receptor, probably acting as an accessorial binding site, is essential to the selectivity and affinity for sensing the acetate anions. The Royal Society of Chemistry 2008.

Full text of DOI:10.1039/b805291c

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
A selective chromogenic molecular sensor for acetate anions in a mixed
acetonitrile–water medium†
Shuzhen Hu,a,b Yong Guo,a Jian Xua and Shijun Shao*a
Received 28th March 2008, Accepted 28th April 2008
First published as an Advance Article on the web 13th May 2008
DOI: 10.1039/b805291c
Quinonehydrazone compound 2, as a new chromogenic anion sensor, can selectively detect AcO− over
F− and other anions in mixed acetonitrile–water media. The deprotonation of the N–H proton of the
sensor is responsible for the drastic color change. An acidic C–H group in the receptor, probably acting
as an accessorial binding site, is essential to the selectivity and affinity for sensing the acetate anions.
effort has been spent in searching for this.7,8 On the other hand,
Introduction
charge-neutral organic chromogenic anion sensors applicable in
It is well known that anions play an important role in a wide
highly competitive solvents such as alcohols or water are rare.
range of chemical and biological processes, therefore the search
Recently, Gunnlaugsson and co-workers reported a chromogenic
sensor for AcO− in a 1 : 1 EtOH–H2O aqueous solution.8 Duan
and co-workers have reported a chromo- and fluorogenic hybrid
chemosensor for F−, this chemosensor contains a quinonehydra-
zone group and allows the detection of F− in water with the
naked eye.9 Herein, we report a new chromogenic anion sensor
2 containing a quinonehydrazone group that can be used in a
mixed acetonitrile–water medium and shows selectivity for AcO−
over other anions such as F− and H2PO4−, which have similar
basicity to AcO−.
for chemosensors for recognizing and sensing anions has attracted
growing attention. Most chemosensors developed so far are
chromogenic and/or fluorescent sensors, which efficiently change
their photophysical properties (kmax shift of the UV–vis spectrum,
change in the quantum yield or emission wavelength etc.) in the
presence of anions.1 Color changes, which can be detected by
the naked eye, are widely used to signal events since they are
low cost and do not require any spectroscopic instrumentation.
The nature of the interaction between anions and charge-neutral
organic receptors is primarily based on hydrogen-bonding. In this
sense, the receptors must provide one or more hydrogen-bonding
donor groups, X–H, where X is typically a nitrogen or oxygen
atom. The recognition studies are preferably carried out in aprotic
media (e.g., DMSO, acetonitrile, CHCl3 etc.), to avoid competition
of the solvent (e.g., water or alcohols) as a hydrogen-bonding
donor.1b
Results and discussion
Synthesis of receptors
The compounds 1 and 2 were prepared by the reaction of N-
butylisatin with the corresponding hydrazine in acetic acid, while
compound 3 was obtained from the reaction of N-butylisatin with
hydrazine hydrate followed by reaction with 2-chloro-1,3-dinitro-
5-(trifluoromethyl)benzene. The N-butylisatin was prepared by a
method similar to that described in the literature.10
Carboxylate anions are important constituents of biological and
synthetic organic molecules such as amino acids and proteins.2
A number of chromogenic and/or fluorescent sensors for AcO−
have been reported in recent years.3 Unfortunately, these AcO−
chemosensors usually also display responses to other basic anions
such as H2PO4 and F−, especially the latter. For many charge-
−
neutral chromogenic and/or fluorescent anion sensors reported
so far, it has been confirmed that not hydrogen-bonding but
deprotonation of the receptor was the cause of the response.
This issue has been explicitly discussed, for instance firstly by
Gale and co-workers,4 then Gunnlaugsson and co-workers1a,5
and Fabbrizzi’s groups1b,6 etc., in a variety of systems. Due to
the strong ability of F− to deprotonate the receptor in aprotic
solvents, it is generally difficult to achieve selectivity for AcO−
over F− using charge-neutral chromogenic sensors, yet much
aKey Laboratory for Natural Medicine of Gansu Province, Lanzhou Institute
of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000,
P. R. China. E-mail: shaoguo@lzb.ac.cn
bGraduate School of the Chinese Academy of Sciences, Beijing 100039,
P. R. China
† Electronic supplementary information (ESI) available: Titrations of 1, 2
and 3 with various anions in various solutions; MS and 1H NMR spectra
of 1, 2 and 3; 13C NMR spectra of 1 and 3; and the IR spectrum of 2. See
DOI: 10.1039/b805291c
UV–vis studies in acetonitrile
The interaction of the receptors 1, 2 and 3 with anions was firstly
investigated in acetonitrile by UV–vis spectroscopy. Receptor 2
This journal is
The Royal Society of Chemistry 2008
Org. Biomol. Chem., 2008, 6, 2071–2075 | 2071
©

View Article Online
(2.5 × 10−5 M) shows an intense absorption band centered at
been observed during the UV–vis titration, and basic anions
deprotonated the receptor directly. So the process can be described
by the proton-dissociation equilibrium:
378 nm and a less strong, broad, shoulder peak at about 410 nm.
Upon addition of anions such as F−, Cl−, Br−, I−, HSO4
,
−
ClO4−, AcO− and H2PO4− (all as their tetrabutylammonium salts),
LH + X− ꢀ L− + HX
(3)
only F−, AcO− and H2PO4 remarkably changed the absorption
−
spectrum of 2, whereas the other anions tested induced negligible
responses (Fig. 1a). During the titration of F− with a solution of
receptor 2, the intensity of the absorptions at 378 nm and 410 nm
decreased and a new absorption band centered at 552 nm appeared
(Fig. 1b), which was responsible for simultaneously changing the
solution of 2 from yellow to purple. The strong absorption band
at 552 nm can be assigned to the deprotonated form ([L]−) of the
“azophenol tautomer” in the presence of the interrelated anion
(i.e., F−, AcO− or H2PO4−) with delocalisation of the negative
charge from the nitrogen atom to the oxygen atom.11 This was
confirmed by the Brønsted acid–base reaction of 2 with a strong
base [Me4N]OH (see ESI, Fig. S1).† Typically, the deprotonation
of the receptor induced by anions can be described by the following
two equilibria developed by Fabbrizzi’s group:6
The equilibrium constant (or proton-dissociation constant) K for
F− was calculated to be 3.73 × 104 969 M−1 through nonlinear
least squares fitting according to a 1 : 1 stoichiometry. AcO−
−
and H2PO4 can induce similar absorption spectrum and color
changes, and the equilibrium constant is 1.88 × 104 485 M−1 for
AcO− and 2.40 × 103 56 M−1 for H2PO4−, respectively (see ESI,
Fig. S2).† The affinity of receptor 2 toward anions in acetonitrile
solution is: F− > AcO− > H2PO4 ꢀ Cl−, Br−, I−, HSO4 and
ClO4−, owing to the basicity of the anions.12 Similar results were
also obtained upon addition of the representative anions to a
solution of receptor 1 or 3 (see ESI, Fig. S3).†
−
−
UV–vis studies in acetonitrile–water
LH + X− ꢀ [LH · · · X]−
(1)
(2)
When we checked the reversibility of these responses, an excep-
tional result occurred in the interaction of receptor 2 with AcO−.
The absorption spectrum and color changes were not fully reversed
upon addition of a small amount of a competitive hydrogen-
bonding solvent such as water. To explore this further, a UV–vis
spectral titration in 90 : 10 (v/v) acetonitrile–water was carried out.
Under these conditions, free receptor 2 (6.25 × 10−6M) showed
an intense absorption band centered at 377 nm with a shoulder
at about 410 nm. Upon the addition of anions, 2 showed high
[LH · · · X]− + X− ꢀ L− + [HX2]−
Probably due to the strong acidity of the N–H proton in the
receptor 2, a genuine H-bond complex, [LH · · · X]−, has not
selectivity for AcO− over other anions including F− and H2PO4
−
(Fig. 2). The absorption band at 377 nm decreased in intensity
and a new strong band at 551 nm appeared, and the color of
the solution changed from yellow to purple immediately. The
presence of other anions such as F− and H2PO4− did not interfere
in the changes to the absorption bands induced by AcO− (see
ESI, Fig. S4).† The equilibrium constant for AcO− was calculated
to be 8.62 × 103
97 M−1 according to a 1 : 1 stoichiometry.
The results imply that receptor 2 can selectively sense AcO− in a
competitive mixed acetonitrile–water medium. The response to
AcO− can be fully reversed upon addition of a large amount
of water (up to 30 vol%, the quantity of water depending upon
the concentrations of receptor and anion added) to the purple
acetonitrile–water solution containing receptor 2 and AcO−, as
the color of the solution turned back to yellow. However, under
the same conditions, neither receptor 1 nor receptor 3 has any
response to these representative anions, which means that they
cannot serve as anion sensors in such media.
Possible binding mode and 1H NMR studies
It seemed that the selectivity of receptor 2 for AcO− could be
ascribed to the basicity of the anions in aqueous solution (the
pKb values of AcO−, F− and H2PO4 in water are 9.24, 10.83
−
and 11.88, respectively13). Nevertheless, why can receptor 2 sense
AcO− in a competitive mixed acetonitrile–water medium but 1
and 3 cannot? Theoretically, the acidity of the N–H group of
receptor 3 is stronger than that of receptor 2. So the selectivity
may not be dependent simply upon the basicity of the anions and
the acidity of the N–H group. The possibility of the participation
Fig. 1 Changes in the UV–vis absorption spectrum of 2 (2.5 × 10−5 M)
in acetonitrile solution: (a) upon addition of 100 equiv. of various anions;
(b) upon addition of F− from 0 to 100 equiv.
2072 | Org. Biomol. Chem., 2008, 6, 2071–2075
This journal is
The Royal Society of Chemistry 2008
©

View Article Online
of 2 (2.5 × 10−2 M) was titrated with AcO−, and the shifts of the
aromatic C–H protons of the 2,4-dinitrophenyl motif, Ha, Hb and
Hc, were monitored (Fig. 3a). Upon gradual addition of AcO−,
the signal of the acidic N–H proton at 12.10 ppm disappeared,
and the electron density in the 2,4-dinitrophenyl motif increased
with the through-bond propagation and caused a shielding effect,
leading to progressive upfield shifts of the Ha and Hb signals.
On the other hand, a large downfield shift of Hc from 7.93 ppm
(J = 7.2 Hz) to 8.58 ppm (Dd = 0.65 ppm) was observed. The
result suggests that Hc was probably involved in the interaction
of the anion with the receptor, and the polarization of the C–Hc
bond induced by a through-space effect would cause a deshielding
effect. Though the large downfield shift of Hc observed is not
enough evidence to account for the C–H interaction, considering
the significant selectivity of receptor 2 toward AcO−, we tentatively
ascribed the selectivity to the existence of the C–H group. There
is increasing evidence that C–H groups within the charge-neutral
receptors can also interact with the anion.14 Recently, Hay and
Bryantsev15 reported that the aryl C–H group could be a strong
hydrogen-bonding donor when electron-withdrawing substituents
are present, exhibiting hydrogen-bonding strengths comparable
to those obtained with O–H and N–H groups. Furthermore, to
mimic the mixed acetonitrile–water environment, a 1H NMR
Fig. 2 (a) Changes in the UV–vis absorption spectrum of 2 (6.25 ×
10−6 M) in an acetonitrile–water (90 : 10, v/v) solution upon addition
of 200 equiv. of anions. (b) UV–vis titration of receptor 2 (6.25 ×
10−6 M) in an acetonitrile–water (90 : 10, v/v) solution upon addition
of AcO− (0–520 equiv.). (c) Color changes of 2 (6.25 × 10−6 M) in an
acetonitrile–water (90 : 10, v/v) solution induced by addition of 200 equiv.
of anions (from left to right: H2PO4−, AcO−, I−, Br−, Cl−, F−, and anion
free).
of the C–Hc group of receptor 2 in the interaction with anions
cannot be ruled out. This interaction can perhaps help to stabilize
the deprotonation. Whereas receptor 3, lacking such extra binding
from C–Hc, interacts with anions through the N–H group only, and
such an interaction may easily be destroyed by water. In the case
of receptor 1 only one electron-withdrawing group, –NO2, exists
and the acidity of the corresponding C–H proton is probably not
strong enough to bind the anions tightly and ensure the efficient
deprotonation of the receptor in a competitive mixed acetonitrile–
water medium.
To further prove the interaction of receptor 2 with AcO−, a
1H NMR titration was conducted. Unfortunately, because of
the limited solubility of receptor 2 in acetonitrile-d3 solution,
experiments were carried out in CDCl3 solution. A CDCl3 solution
Fig. 3 (a) Titration of receptor 2 (2.5 × 10−2 M) in CDCl3 solution with
AcO− (0, 0.25, 1.2, 2.4 equiv.). The dashed lines track the shift of the Hc
signal. (b) Titration of receptor 2 (2.0 × 10−2 M) in a CDCl3–EtOD (1 :
1, v/v) solution with AcO− (0, 0.5, 1.0, 3.0, 6.5 equiv.). The dashed lines
track the shift of the Hc signal.
This journal is
The Royal Society of Chemistry 2008
Org. Biomol. Chem., 2008, 6, 2071–2075 | 2073
©

View Article Online
spectroscopy titration of 2 with AcO− was carried out in a 1 :
1 (v/v) CDCl3–EtOD solution, as shown in Fig. 3b. The signal
of the N–H proton disappeared in such a protic solvent system.
Upon addition of AcO−the spectra changes were similar to those
found in the CDCl3 solution, with the smaller shifts due to the
hydrogen-bonding effect of the EtOH solvent, which weakens the
interaction of 2 with the anion.
under reduced pressure. The residue was dissolved in CHCl3
and washed well with water. The organic layer was dried with
anhydrous Na2SO4 and evaporation of the solvent afforded N-
butylisatin as a deep-red oil. Recrystallization in acetone–water
afforded N-butylisatin as red needles. TLC indicated that it was a
single compound, which was used directly in next step.
Synthesis of receptor 1
UV–vis studies in CHCl3–EtOH
N-Butylisatin (0.406 g, 2 mmol) and 4-nitrophenylhydrazine
(0.306 g, 2 mmol) were dissolved in acetic acid and heated to
reflux for 2 h. After cooling to room temperature, the mixture was
poured into ice–water and the precipitate was collected and dried.
Recrystallization from ethanol afforded 1 as a yellow solid. Yield
86%.
Corresponding UV–vis spectral titrations in a 1 : 1 (v/v) CHCl3–
EtOH solution were performed, as shown in Fig. 4. In this solution
system, receptor 2 showed high selectivity for AcO−, similar to that
observed in the 90 : 10 (v/v) acetonitrile–water solution. Only
AcO− induced remarkable absorption and color changes, with a
decrease of the absorption band at 374 nm and the appearance of a
new strong absorption band at 556 nm. The equilibrium constant
was calculated to be 8.76 × 102 5 M−1. F−, Cl−, and H2PO4− can
induce slight changes in the absorption spectrum but no obvious
color changes can be observed. Other anions such as Br−, I−,
MS (ESI): m/z 339.4 (M + H+)
1H NMR (400 MHz, CDCl3), d (ppm): 12.929 (s, 1H, N–H),
8.169–8.146 (d, 2H), 7.607–7.589 (d, 1H), 7.316–7.279 (t, 3H),
7.112–7.073 (t, 1H), 6.874–6.855 (d, 1H), 3.753–3.716 (t, 2H),
1.760–1.638 (m, 2H), 1.429–1.336 (m, 2H), 0.967–0.931 (t, 3H).
13C NMR (400 MHz, CDCl3), d (ppm): 161.957, 147.754,
142.297, 141.432, 130.911, 129.625, 125.761, 122.836, 120.281,
119.860, 113.356, 109.054, 39.335, 29.684, 20.142, 13.632.
−
HSO4−, ClO4 display almost no response. However, under the
same conditions, receptors 1 and 3 remained silent to the anions.
Synthesis of receptor 2
N-Butylisatin (0.406 g, 2 mmol) and 2,4-dinitrophenylhydrazine
(0.4 g, 2 mmol) were dissolved in acetic acid and heated to reflux
for 2 h. After cooling to room temperature, the mixture was poured
into ice–water and the precipitate was collected and dried. Further
purification by silica column chromatography (CH2Cl2–petroleum
ether (b.p. 60–90 ◦C), 3 : 1, v/v) afforded 2 as an orange-red solid.
Yield 78%.
MS (ESI): m/z 382.5 (M − H+)
1H NMR (400 MHz, CDCl3), d (ppm): 12.105 (s, 1H, N–H),
9.174–9.177 (d, 1H), 8.448–8.452 (d, 2H), 7.926–7.945 (d, 1H),
7.463–7.501 (t, 1H), 7.190–7.228 (t, 1H), 6.949–6.969 (d, 1H),
3.772–3.808 (t, 2H), 1.646–1.720 (m, 2H), 1.378–1.434 (m, 2H),
0.937–0.974 (t, 3H).
Fig. 4 Changes in the UV–vis absorption spectrum of receptor 2 (2.5 ×
10−5 M) in a CHCl3–EtOH (1 : 1, v/v) solution upon the addition of
100 equiv. of anions.
IR (KBr): 3441, 3329, 3087, 2957, 2931, 2861, 1733, 1589, 1499,
1335, 1184, 1093, 1026, 830, 738 cm−1.
Conclusions
Synthesis of receptor 3
In conclusion, we have developed a new chromogenic anion
molecular sensor, which can selectively detect AcO− over F− and
other anions in a competitive mixed acetonitrile–water medium.16
An acidic C–H group in the receptor, probably acting as an
accessorial binding site, was essential to the selectivity and affinity
for sensing the acetate anions.
N-Butylisatin (0.406 g, 2 mmol) and hydrazine hydrate (1 mL,
large excess) were dissolved in ethanol and heated to reflux for 12 h.
After cooling to room temperature, the mixture was poured into
ice–water and the precipitate was collected, washed well with water
and dried. The precipitate was dissolved in ethanol, 2-chloro-1,3-
dinitro-5-(trifluoromethyl)benzene (0.5 g, 1.8 mmol) was added
and the mixture was heated to reflux for 12 h. After cooling to
room temperature, the solvent was removed and purification by
silica column chromatography (CH2Cl2–petroleum ether (b.p. 60–
90 ◦C), 3 : 1, v/v) afforded 3 as a red solid. Yield 30%.
Experimental
Synthesis of N-butylisatin
Sodium (0.46 g, 0.02 mol) was dissolved in 30 mL anhydrous
ethanol and stirred for 30 minutes. Isatin (2.94 g, 0.02 mol) was
added, the mixture was stirred for 2 h at room temperature, then
the solvent was removed under reduced pressure. A large excess of
1-iodobutane (25 mL) was added and the mixture was heated
to reflux for 16 h, then the excess 1-iodobutane was removed
MS (ESI): m/z 450.5 (M − H+)
1H NMR (400 MHz, CDCl3), d (ppm): 14.463 (s, 1H, N–H),
8.336 (s, 2H), 7.506–7.487 (d,1H), 7.391–7.352 (t, 1H), 7.123–7.084
(t, 1H), 6.891–6.872 (d, 1H), 3.789–3.752 (t, 2H), 1.731–1.673 (m,
2H), 1.431–1.374 (m, 2H), 0.973–0.936 (t, 3H).
2074 | Org. Biomol. Chem., 2008, 6, 2071–2075
This journal is
The Royal Society of Chemistry 2008
©

View Article Online
13C NMR (400 MHz, DMSO-d6), d (ppm): 160.255, 142.956,
138.239, 136.105, 135.115, 134.389, 133.032, 131.997, 127.262,
123.177, 120.320, 118.715, 114.174, 111.174, 110.339, 29.121,
19.555, 13.569.
Quesada, New J. Chem., 2006, 30, 1019; (g) C. Caltagirone, G. W. Bates,
P. A. Gale and M. E. Light, Chem. Commun., 2008, 61.
5 T. Gunnlaugsson, P. E. Kruger, P. Jensen, F. M. Pfeffer and G. M.
Hussey, Tetrahedron Lett., 2003, 44, 8909.
6 (a) M. Boiocchi, L. D. Boca, D. Esteban-Go`mez, L. Fabbrizzi, M.
Licchelli and E. Monzani, J. Am. Chem. Soc., 2004, 126, 16507; (b) D.
Esteban-Go`mez, L. Fabbrizzi and M. Licchelli, J. Org. Chem., 2005,
70, 5717; (c) D. Esteban-Go`mez, L. Fabbrizzi, M. Licchelli and E.
Monzani, Org. Biomol. Chem., 2005, 3, 1495.
7 B. Garc´ıa-Acosta, X. Albiach-Mart´ı, E. Garc´ıa, L. Gil, R. Mart´ınez-
Ma´n˜ez, K. Rurack, F. Sanceno´n and J. Soto, Chem. Commun., 2004,
774.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (grant nos. 20372067 and 20672121).
8 T. Gunnlaugsson, P. E. Kruger, P. Jensen, J. Tierney, H. D. Ali and
G. M. Hussey, J. Org. Chem., 2005, 70, 10875.
9 Z. Lin, S. Ou, C. Duan, B. Zhang and Z. Bai, Chem. Commun., 2006,
624.
References and notes
1 (a) T. Gunnlaugsson, M. Glynn, G. M. Tocci (ne´e Hussey), P. E. Kruger
and F. M. Pfeffer, Coord. Chem. Rev., 2006, 250, 3094; (b) V. Amendola,
D. Esteban-Go`mez, L. Fabbrizzi and M. Licchelli, Acc. Chem. Res.,
2006, 39, 343; (c) R. Mart´ınez-Ma´n˜ez and F. Sanceno´n, J. Fluoresc.,
2005, 15, 267; (d) T. Gunnlaugsson, H. D. P. Ali, M. Glynn, P. E.
Kruger, G. M. Hussey, F. M. Pfeffer, C. M. G. dos Santos and J. Tierney,
J. Fluoresc., 2005, 15, 287; (e) R. Mart´ınez-Ma´n˜ez and F. Sanceno´n,
Chem. Rev., 2003, 103, 4419; (f) P. D. Beer and P. A. Gale, Angew.
Chem., Int. Ed., 2001, 40, 486.
10 E. J. Moriconi and J. J. Murray, J. Org. Chem., 1964, 29, 3577.
11 The hydrazone of isatin exists mainly as the hydrazone tautomer form,
which is greatly favored in the presence of electronegative groups such as
p-nitro in the phenylazo ring. In our case, the charge-neutral azophenol
tautomer of the receptors can be ruled out. For related examples, see:
(a) D. B. Pendergrass, Jr, I. C. Paul and D. Y. Curtin, J. Am. Chem.
Soc., 1972, 94, 8730; (b) G. R. Desiraju, H. R. Luss and D. L. Smith,
J. Am. Chem. Soc., 1978, 100, 6375; (c) F. A. Snavely and S. Un, J. Org.
Chem., 1981, 46, 2764, and references cited therein.
2 (a) R. J. Fitzmaurice, G. M. Kyne, D. Douheret and J. D. Kilburn,
J. Chem. Soc., Perkin Trans. 1, 2002, 841; (b) S. J. Brooks, P. A. Gale
and M. E. Light, Chem. Commun., 2006, 4344.
12 Fluoride is a special case: see ref. 1b.
13 (a) W. P. Jencks in, J. March, Advanced Organic Chemistry, Wiley, New
York, 3rd edn, 1985; (b) F. G. Bordwell, Acc. Chem. Res., 1988, 21, 456.
14 (a) F. M. Pfeffer, P. E. Kruger and T. Gunnlaugsson, Org. Biomol.
Chem., 2007, 12, 1894; (b) C. R. Bondy, P. A. Gale and S. J. Loeb,
J. Am. Chem. Soc., 2004, 126, 5030; (c) H. Maeda and Y. Kusunose,
Chem.–Eur. J., 2005, 11, 5661; (d) R. Miao, Q. Zheng, C. Chen and Z.
Huang, Tetrahedron Lett., 2005, 46, 2155; (e) J. Y. Kwon, Y. J. Jang,
S. K. Kim, K. Lee, J. S. Kim and J. Yoon, J. Org. Chem., 2004, 69, 5155;
(f) S. In, S. J. Cho, K. H. Lee and J. Kang, Org. Lett., 2005, 7, 3993.
15 (a) V. S. Bryantsev and B. P. Hay, Org. Lett., 2005, 7, 5031; (b) V. S.
Bryantsev and B. P. Hay, J. Am. Chem. Soc., 2005, 127, 8282.
16 We have to point out that the selectivity of receptor 2 for AcO− over
3 (a) N. J. Singh, E. J. Jun, K. Chellappan, D. Thangadurai, R. P.
Chandran, I.-C. Hwang, J. Yoon and K. S. Kim, Org. Lett., 2007,
9, 485; (b) Y. Wu, X. Peng, J. Fan, S. Gao, M. Tian, J. Zhao and S.
Sun, J. Org. Chem., 2007, 72, 62; (c) V. Thiagarajan, P. Ramamurthy,
D. Thirumalai and V. T. Ramakrishnan, Org. Lett., 2005, 7, 657;
(d) R. Nishiyabu and P. Anzenbacher, Jr, J. Am. Chem. Soc., 2005,
127, 8270; (e) D. H. Lee, K. H. Lee and J. Hong, Org. Lett., 2001, 3,
5.
4 (a) S. Camiolo, P. A. Gale, M. B. Hursthouse, M. E. Light and A. J. Shi,
Chem. Commun., 2002, 758; (b) P. A. Gale, K. Navakhun, S. Camiolo,
M. E. Light and M. B. Hursthouse, J. Am. Chem. Soc., 2002, 124,
11228; (c) S. Camiolo, P. A. Gale, M. B. Hursthouse and M. E. Light,
Org. Biomol. Chem., 2003, 1, 741; (d) P. A. Gale, Acc. Chem. Res., 2006,
39, 465; (e) L. S. Evans, P. A. Gale, M. E. Light and R. Quesada, Chem.
Commun., 2006, 965; (f) L. S. Evans, P. A. Gale, M. E. Light and R.
F− could only be achieved in dilute solution (ranging from ∼10−6
M
to ∼10−4 M); under higher concentrations (ranging from ∼10−3 M to
∼10−2 M, suitable for 1H NMR spectroscopy study), F− and H2PO4
−
can produce similar responses and thus interfere with the selectivity.
This journal is
The Royal Society of Chemistry 2008
Org. Biomol. Chem., 2008, 6, 2071–2075 | 2075
©