Intramolecular hydrogen bonding and anion binding of N-benzamido-N′- benzoylthioureas

Article abstract of DOI:10.1021/jo702159r

(Chemical Equation Presented) N-(p-Dimethylamino)benzoyl-N′- phenylthiourea as an N-acylthiourea is known to be unable to bind anions due to a strong intramolecular hydrogen bond (IHB). We show here that by inserting an amido group in the N′-phenyl side the newly designed N-benzamido-N′- benzoylthioureas, despite this IHB too, bind strongly to anions with binding constants on the order of 10⁶-10⁷ mol^-1 L. Results suggest that potential anion receptors or organocatalysts could be developed on the basis of this framework with a wide structural diversity.

Full text of DOI:10.1021/jo702159r

Intramolecular Hydrogen Bonding and Anion
Binding of N-Benzamido-N′-benzoylthioureas
Wen-Xia Liu and Yun-Bao Jiang*
Department of Chemistry, College of Chemistry and Chemical
Engineering, and The MOE Key Laboratory of Analytical
Sciences, Xiamen UniVersity, Xiamen 361005, China
ybjiang@xmu.edu.cn
ReceiVed October 4, 2007
FIGURE 1. Chemical structure of thiourea-based receptors. Numbering
of -NH’s and aromatic protons is given in 5.
applications of thiourea-based anion receptors.1 In order to
enhance the acidity of thioureido -NH protons and to introduce
additional hydrogen-bonding sites, we have alternatively ex-
plored anion binding performance of N-(p-dimethylaminoben-
zoyl)thiourea (2, Figure 1),3 an N-acylthiourea. Unfortunately,
no response in its absorption and fluorescence spectra toward
anions was observed. This was rationalized to result from a
strong intramolecular hydrogen bond (IHB) between a carbonyl
O atom and a thioureido -NH proton (Figure 1). To prevent
this IHB, we extended to examine N-benzamidothioureas (3 and
4, Figure 1) by inserting an “-NH-” between the carbonyl
group and thiourea moiety.4 Compared with classical N-
phenylthioureas 1, N-benzamidothioureas 3 and 4 bearing an
additional amide group showed a dramatically increased anion
affinity and a more substantial spectral variation upon anion
binding. This is peculiar since the thioureido -NH protons in
3 and 4 are not of higher acidity. It was concluded that this
was due to anion binding induced N-N conformation change
and the resultant intramolecular charge transfer (ICT) in the
anion binding complex. Inspired by and attempting to clarify
the possible contribution of this additional amide group in
N-(p-Dimethylamino)benzoyl-N′-phenylthiourea as an N-
acylthiourea is known to be unable to bind anions due to a
strong intramolecular hydrogen bond (IHB). We show here
that by inserting an amido group in the N′-phenyl side the
newly designed N-benzamido-N′-benzoylthioureas, despite
this IHB too, bind strongly to anions with binding constants
on the order of 106-107 mol-1 L. Results suggest that
potential anion receptors or organocatalysts could be devel-
oped on the basis of this framework with a wide structural
diversity.
Thiourea has been a subject of intensive investigations for
its performance in the construction of anion receptors via double
hydrogen-bonding interaction by thioureido -NH donors.1 This
interest has recently been enhanced because of the promising
progress in the thiourea-based organocatalysts again via hydro-
gen bonding.2 Obviously, the hydrogen-bonding ability of the
thiourea moiety is an important parameter, which in principle
depends on the acidity of thioureido -NH protons and the
number of binding sites. From a structural point of view, a direct
means of tuning this acidity is to introduce a substituent of varied
electron-donating or -withdrawing ability. N-Alkyl and/or N-aryl
substitutions (such as 1, Figure 1) have been the main choices
in this regard and indeed led to great success in design and
(2) For recent examples of thiourea-based organocatalysts, see: (a) Sibi,
M. P.; Itoh, K. J. Am. Chem. Soc. 2007, 129, 8064-8065.(b) Zhang, Z. G.;
Schreiner, P. R. Synlett 2007, 1455-1457. (c) Yamaoka, Y.; Miyabe, H.;
Takemoto, Y. J. Am. Chem. Soc. 2007, 129, 6686-6687. (d) Pan, S. C.;
List, B. Synlett 2007, 318-320. (e) Liu, K.; Cui, H. F.; Nie, J.; Dong, K.
Y.; Li, X. J.; Ma, J. A. Org. Lett. 2007, 9, 923-925. (f) Liu, T. Y.; Li, R.;
Chai, Q.; Long, J.; Li, B. J.; Wu, Y.; Ding, L. S.; Chen, Y. C. Chem.sEur.
J. 2007, 13, 319-327. (g) Pan, S. C.; Zhou, J.; List, B. Angew. Chem., Int.
Ed. 2007, 46, 612-614. (h) Cao, C. L.; Ye, M. C.; Sun, X. L.; Tang, Y.
Org. Lett. 2006, 8, 2901-2904. (i) Song, J.; Wang, Y.; Deng, L. J. Am.
Chem. Soc. 2006, 128, 6048-6049. (j) Lalonde, M. P.; Chen, Y. G.;
Jacobsen, E. N. Angew. Chem., Int. Ed. 2006, 45, 6366-6370. (k) Fuerst,
D. E.; Jacobsen, E. N. J. Am. Chem. Soc. 2005, 127, 8964-8965.
(3) Wu, F. Y.; Li, Z.; Guo, L.; Wang, X.; Lin, M. H.; Zhao, Y. F.; Jiang,
Y. B. Org. Biomol. Chem. 2006, 4, 624-630.
(4) (a) Li, Z.; Liu, Z.; Liao, Q. X.; Wei, Z. B.; Long, L. S.; Jiang, Y. B.
C. R. Chim. 2008, in press. (b) Liu, W. X.; Jiang, Y. B. Org. Biomol. Chem.
2007, 5, 1771-1775. (c) Han, J.; Li, Z.; Liu, W. X.; Yang, R.; Jiang, Y. B.
Acta Chim. Sin. 2006, 64, 1716-1722. (d) Nie, L.; Li, Z.; Han, J.; Zhang,
X.; Yang, R.; Liu, W. X.; Wu, F. Y.; Xie, J. W.; Zhao, Y. F.; Jiang, Y. B.
J. Org. Chem. 2004, 69, 6449-6454. (e) Wu, F. Y.; Li, Z.; Wen, Z. C.;
Zhou, N.; Zhao, Y. F.; Jiang, Y. B. Org. Lett. 2002, 4, 3203-3205.
(1) For recent reviews of thiourea-based anion receptors, see: (a) Gale,
P. A. Acc. Chem. Res. 2006, 39, 465-475. (b) Amendola, V.; Esteban-
Go´mez, D.; Fabbrizzi, L.; Licchelli, M. Acc. Chem. Res. 2006, 39, 343-
353. (c) Special issues on anion sensing and recognition: Coord. Chem.
ReV. 2006, 250, 2917-3244; 2003, 240, 1-226. (d) Mart´ınez-Ma´n˜ez, R.;
Sanceno´n, F. J. Fluoresc. 2005, 15, 267-285. (e) Gunnlaugsson, T.; Ali,
H. D. P.; Kruger, P. E.; Hussey, G. M.; Pfeffer, F. M.; dos Santos, C. M.
G.; Tierney, J. J. Fluoresc. 2005, 15, 287-299.(f) Mart´ınez-Ma´n˜ez, R.;
Sanceno´n, F. Chem. ReV. 2003, 103, 4419-4476. For recent reports, see:
(g) Pfeffer, F. M.; Kruger, P. E.; Gunnlaugsson, T. Org. Biomol. Chem.
2007, 5, 1894-1902. (h) Ros-Lis, J. V.; Mart´ınez-Ma´n˜ez, R.; Sanceno´n,
F.; Soto J.; Rurack, K.; Weiâhoff, H., Eur. J. Org. Chem. 2007, 2449-
2458. (i) Blondeau, P.; Benet-Buchholz, J.; de Mendoza, J. New J. Chem.
2007, 31, 736-740. (j) Yakovenko, A. V.; Boyko, V. I.; Kalchenko, V. I.;
Baldini, L.; Casnati, A.; Sansone, F.; Ungaro, R. J. Org. Chem. 2007, 72,
3223-3231.
10.1021/jo702159r CCC: $40.75 © 2008 American Chemical Society
Published on Web 12/21/2007
1124
J. Org. Chem. 2008, 73, 1124-1127

FIGURE 2. Chemical shifts of -NH protons in DMSO-d6 versus
Hammett constant of substituent X in 5 (a) and Y in 6 (b). For
numbering of -NH protons, see Figure 1 for 5.
FIGURE 3. Absorption spectra of (a) 5c (1.00 × 10-5 mol L-1), (b)
5a (8.76 × 10-6 mol L-1), and (c) 6a (1.30 × 10-5 mol L-1) in MeCN
in the presence of increasing concentration of AcO-.
promoting anion binding of thiourea-based receptors, we envis-
aged to insert an amide group into 2-like N-acylthioureas to
have N-benzamido-N′-benzoylthioureas (5 and 6, Figure 1). By
doing so, the acidity of the thioureido -NH protons and the
number of hydrogen-binding sites in 5 and 6 are also made
higher than those in 3 and 4, favoring not only anion receptor
designing but organocatalyst development as well. Certainly,
an IHB characteristic of N-acylthioureas such as 2 was expected
for 5 and 6 that may prevent anion binding of multiple hydrogen-
bonding nature.3,4 Indeed, the crystal structure of 5c confirmed
the existence of this IHB.5 We report here that 5 and 6, despite
the existence of such a strong CdO‚‚‚HN IHB, do show in their
absorption spectra sensitive response toward model anions
AcO-, F-, and H2PO4- with elevated binding affinity by up to
1 order of magnitude than that of 3 and 4.
1H NMR signals of -NH protons in 5 and 6 in DMSO-d6
were found at lower field than those of 3 and 4. This is likely
due to the IHB in 5 and 6 depicted in Figure 1 and the additional
carbonyl group. Figure 2 shows that the chemical shifts of the
-NH protons in 5 and 6 vary linearly with the Hammett constant
(σ) of substituent X or Y. The slopes of the chemical shift of
-NH1 against σX(Y) are both negative, confirming that the -NH1
proton is indeed involved in an IHB (Figure 1).4,6 It is also noted
that the slope of -0.264 versus σY is more negative than that
of -0.128 versus σX. This means that, with varying X or Y
from electron-donating to -withdrawing, the -NH1‚‚‚OdC IHB
is weakened and substituent Y in 6 exerts a slightly stronger
influence on this IHB. The linear slopes of the chemical shifts
of -NH2 and -NH3 protons in 5 against σX are 0.070 and 0.453,
while those in 6 versus σY are 0.583 and 0.072, respectively
(Figure 2). These slopes are comparable to those of 3 of 0.0977
and 0.4634d and of 4 of 0.523 and 0.125,3 respectively.
Substantially different slopes in the two sets of NMR data of
non-hydrogen-bonded -NH2 and -NH3 protons in 5 and 6
again probe the twisted conformation of the N-N single bond
that stops to some extent the electronic communication of the
substituent effect, a character prevailing in neutral hydrazines7,8
and reported for 34d and 4.3 It could be similarly expected3,4
that no absorption signal change would be observed upon anion
binding to the thiourea moiety in 5 and 6 as it is decoupled
from the N-benzamido chromophore signal reporter. The
intercepts of the aforementioned linear correlations for 5 and 6
are respectively higher than those of 3 and 4, indicating the
expected higher acidity of these non-hydrogen-bonded -NH
protons in 5 and 6.
It was previously reported that, due to such a strong NH‚‚‚
OdC IHB in 2, no response toward an anion in both its
absorption and fluorescence spectra in acetonitrile (MeCN) was
observed.3 The absorption spectra of 5 and 6, however, do show
a sensitive response to anions (Figure 3). Note that the
observation of isosbestic points in the titration traces points to
the formation of well-defined anion binding complexes which
are later shown to be in 1:1 stoichiometry by Job plots.
Molecules in 5 and 6 can be divided into three subgroups on
the basis of their spectral characteristics. The first group includes
5b-d without substituent p-N(CH3)2, whose absorption maxima
are at ca. 260 nm, originating from the benzamide chromophore.
With increasing AcO- concentration, the absorbance at 260 nm
of 5b, for example, decreases while a new band at 335 nm
develops (Figure 3a). The second group includes 5a and 6b-e,
which bear one p-N(CH3)2. When AcO- is introduced, their
absorption spectra, which peaks at 290 and 335 nm (Figure 3b)
due to the charge-transfer chromophore p-(dimethylamino)-
benzamide (DMABA),3,4e undergo relatively minor variations
by a decrease in the absorbance at maximum and an enhance-
ment in absorption between 350 and 400 nm. 6a is in the third
group, which contains two p-N(CH3)2 and shows its absorption
maximum at 347 nm (Figure 3c). This is longer than that with
one DMABA chromophore (335 nm, 5a and 6b-e), which is
presumably assigned to J-aggregates9 of two DMABA chro-
mophores with a J-splitting of 0.26 eV.10 In the presence of
AcO-, the absorption shifts to blue at 335 nm and the
absorbance at 347 nm decreases and a new band at 295 nm
appears (Figure 3c). It is also found that with 5b-d the new
(5) Yusof, M. S. M.; Yamin, B. M.; Shamsuddin, M. Acta Crystallogr.
2003, E59, o810-o811.
(9) (a) Jelley, E. E. Nature 1936, 138, 1009-1010. (b) Kasha, M.; Rawls,
H. R.; El-Bayoumi, M. A. Pure Appl. Chem. 1965, 11, 371-392. (c) Katoh,
T.; Inagaki, Y.; Okazaki, R. J. Am. Chem. Soc. 1998, 120, 3623-3628.
(10) (a) Sasaki, F.; Haraichi, S.; Kobayashi, S. IEEE J. Quantum Electron.
2002, 38, 943-948. (b) Furuki, M.; Tian, M. Q.; Sato, Y.; Pu, L. S.;
Tatsuura, S.; Abe, S. Appl. Phys. Lett. 2001, 79, 708-710. (c) McRae, E.
G.; Kasha, M. J. Chem. Phys. 1958, 28, 721-722.
(6) Wen, Z. C.; Jiang, Y. B. Tetrahedron 2004, 60, 11109-11115.
(7) Nelsen, S. F.; Pladziewicz, J. R. Acc. Chem. Res. 2002, 35, 247-
254.
(8) (a) Lewis, M.; Glaser, R. J. Org. Chem. 2002, 67, 1441-1447. (b)
Dey, S. K.; Lightner, D. A. J. Org. Chem. 2007, 72, 9395-9397.
J. Org. Chem, Vol. 73, No. 3, 2008 1125

TABLE 1. Anion Binding Constants of 5 and 6 in MeCN
Ka, 106 mol-1
L
-
AcO-
F-
H2PO4
5a
5b
5c
5d
6a
6b
6c
6d
6e
7.02 ( 2.23
6.53 ( 1.26
9.72 ( 1.99
13.6 ( 6.3
6.90 ( 1.13
16.7 ( 4.2
4.56 ( 0.75
7.02 ( 2.23
19.6 ( 3.7
2.23 ( 0.59
5.49 ( 0.32
3.62 ( 0.64
4.23 ( 1.12
3.18 ( 0.73
8.93 ( 2.99
4.73 ( 1.03
2.23 ( 0.59
3.67 ( 0.54
0.271 ( 0.032
0.725 ( 0.12
1.17 ( 0.23
0.598 ( 0.047
0.304 ( 0.018
0.962 ( 0.327
0.975 ( 0.497
0.271 ( 0.032
1.16 ( 0.62
1
FIGURE 4. Traces of H NMR titration of 5a in CD3CN of -NH
While substituent dependence of Ka of 5 is similar to that of
3,4d it differs, however, substantially for 6 compared to 4 which
showed an enhanced substituent effect on the anion binding
constant.3 The latter was previously attributed to a positive
feedback of the anion binding induced N-N conformation
change.4d
protons and aromatic -CH protons. [5a] ) 15 mmol L-1, and
concentration ratio of 5a to F- is given in the figure. For numbering
of -NH and aromatic protons, see Figure 1 for 5.
absorption band in the presence of an anion appears at longer
wavelength with decreasing electron-donating X, thereby sug-
gesting the charge-transfer character of this new band as was
shown in the case of 3.4d Spectral variation profiles of 5 and 6
in the presence of F- and H2PO4- are similar to those of AcO-,
It deserves pointing out that both the chemical shifts of
hydrogen-bonded -NH1 and non-hydrogen-bonded -NH2
protons in 6 are sensitively, despite to a varied extent, subject
to Y (Figure 2b). The fact that the anion binding constants of
6 show practically no dependence on Y indicates that the
observed spectral variations in 6 upon anion binding are not
due to deprotonation of -NH2,12 and the positive feedback
toward anion binding resulting from the anion binding induced
N-N conformation change is absent with 6. Since X or Y exerts
essentially no influence on anion affinity of 5 or 6, the aliphatic
counterparts of them are expected to possess similar binding or
hydrogen-bonding capacity. This offers a promising chance for
creating 5-/6-like thiourea-based potential organocatalysts, for
example, the well-known proline derivatives13 of 5 and 6 could
well be the earlybirds of this kind.
whereas Cl-, Br-, I-, HSO4-, NO3-, and ClO4 exert no
-
influence on the absorption spectra (Figure S1, Supporting
Information). It is therefore likewise concluded that the anion
binding mode and its consequence with 5 and 6 are similar to
those with 3 and 4 and that anion binding is of multiple
hydrogen-bonding nature and anion binding induces a confor-
mation change in the N-N single bond that establishes
electronic communication of the anion binding site with the
chromophore in the anion binding complexes.3,4 As an IHB
exists in 5 and 6 that would, in principle, prevent multiple
hydrogen-bonding interaction with anions, breaking of this IHB
is assumed to occur upon anion binding.
Because of the high anion binding constants in MeCN and
the concern of solvent dehydration in hydrogen-bonding-based
organocatalysis, anion binding in H2O-containing MeCN was
examined. It was found that binding of AcO- with 5d, for
example, could efficiently occur in MeCN containing up to 20%
H2O (v/v, Figure S4, Supporting Information), with Ka values
of (1.36 ( 0.63) × 107, (5.76 ( 0.65) × 106, (5.90 ( 0.56) ×
105, (1.97 ( 0.20) × 105, (1.51 ( 0.19) × 105, and (3.30 (
0.94) × 104 mol-1 L, in MeCN containing 0, 1, 5, 10, 15, and
20% H2O, respectively. This implies that 5 and 6 have a higher
capability of competing with H2O for anion binding, a character
significant for anion receptors for practical use and for easy
organocatalytic applications in organic solvents in terms of
solvent dehydration.
Interaction of anions with 5 and 6 was further probed by 1H
NMR titrations in CD3CN. Figure 4 shows, as an example, NMR
traces of 5a titrated by F-. Similar profiles were found with
AcO- and H2PO4-. It is evident in Figure 4 that the signal of
-NH1 at 13.10 ppm disappears upon F- titration, while those
of -NH2 and -NH3 at 9.69 and 9.62 ppm mix and shift
downfield to 10.66 ppm. In the case of AcO- and H2PO4-, they
mix and finally disappear. These observations indeed report the
breaking of CdO‚‚‚HN IHB in 5a upon anion addition and the
hydrogen-bonding interaction of 5a with F-. Accordingly, the
signal of aromatic proton Hb in the N-benzamido moiety shows
a slight downfield shift, whereas those of protons Ha in the
N-benzamido moiety and Hc, Hd, and He in the N′-benzoyl
moiety undergo various degrees of upfield shifts.
Job plots indicate a 1:1 stoichiometry in anion binding to 5
and 6 in MeCN. Anion binding constants of 5 and 6 were
obtained by nonlinear fittings11 of their absorbance variations
against anion concentration (Table 1). Comparing data compiled
in Table 1 with those of 34d and 4,3 it is obvious that the binding
constants of 5 and 6 at 106-107mol-1 L are higher, by up to 1
order of magnitude, than those of 3 and 4. It is interesting to
note that the AcO- binding constants of 5 and 6 depend very
little on X or Y in both pure MeCN and 5% H2O-MeCN in
which the binding constants are 2 orders of magnitude lower
than those in pure MeCN (Table S1, Supporting Information).
In summary, we found that, despite the fact that no anion
binding was previously observed with N-benzoyl-N′-phenylth-
iourea (2) due to a strong IHB, highly efficient anion binding
occurred with the newly designed N-benzamido-N′-benzoylth-
ioureas (5 and 6) bearing this IHB too. The anion affinity is
(12) Absorption and NMR titrations of 5a in MeCN and CD3CN,
respectively, by OH- and F- were carried out up to high anion concentra-
tions. Whereas an additional “abnormal” red shift was observed in the
absorption of 5a at OH- concentration of 40 equiv, a phenomenon ascribed
to deprotonation by the highly basic anion,1b no such red shift was noted
in the case of F- at up to 140 equiv (Figure S2, Supporting Information).
In NMR titration traces, an -NH signal could be seen in the presence of
up to 40 equiv of F-, whereas it disappears when OH- was introduced; in
the aromatic proton portion, variation profiles of proton Hc start to differ at
OH- of 10 equiv (Figure S3, Supporting Information).
(11) Connors, K. A. Binding Constants. The Measurement of Molecular
Complex Stability; John Wiley & Sons: New York, 1987.
(13) Sorensen, E. J.; Sammis, G. M. Science 2004, 305, 1725-1726.
1126 J. Org. Chem., Vol. 73, No. 3, 2008

even higher by up to 1 order of magnitude than that of the
corresponding N-benzamido-N′-phenylthioureas (3 and 4) with-
out such an IHB. Actually, an efficient anion binding could be
easily observed in the presence of water of 10% by volume,
for instance. The anion binding constants of 5 and 6 were found
to be independent of substituent X or Y, the latter differing
strikingly from that of 4, whose anion binding showed an
enhanced Y dependence. Breaking of the IHB that exists in 5
and 6 was therefore revealed to occur upon anion binding. The
N-N single bond in 5 and 6 was shown to be twisted and stops
electronic communication of the thiourea moiety as the anion
binding site with the N-benzamido moiety, and the anion binding
results in a conformation change in the N-N single bond that
couples these two moieties. This N-N conformation change,
however, shows no positive feedback to enhance the Y
dependence of the anion affinity of 6. As the cis-conformation
of the thioureido -NH protons is required in their binding to
anions or other hydrogen-bonding acceptors,14 breaking the IHB
in 5 and 6 shall also be accompanied by a rotation of the IHB
involved benzamide moiety,15 in addition to the aforementioned
N-N conformation change. Whether these conformational
changes contribute to the enhancement of the anion binding
ability of 5 and 6 is not yet clear; it however deserves further
efforts. It is interesting to note that indeed the structural
difference of an additional amide group (-C(O)NH-) between
5/6 and 2 and between 3/4 and 1 appears to play a role in anion
binding promotion in thiourea-based receptors. This component
is thus likely useful in future development of the thiourea-based
anion receptors and organocatalysts. In this connection, since
both substituents X and Y do not exert influence on anion
binding of 5 and 6, more diverse structures can be developed;
in particular, aliphatic counterparts of them could similarly work
well in solvents not necessarily dehydrated for the sake of
hydrogen-bonding interaction.
Experimental Section
General Procedures for Synthesis of N-(Substituted benza-
mido)-N′-(substituted benzoyl)thioureas. Substituted benzoic acid
(3.0 mmol) was added to an excess amount of SOCl2, and the
solution was refluxed for 8 h. After excess SOCl2 was removed
under reduced pressure, benzoyl chloride was obtained. The freshly
prepared chloride was slowly added to a stirred dry MeCN (5.0
mL) solution of ammonium isothiocyanate (4.5 mmol) at 0 °C
followed by refluxing for 8 h. On cooling to room temperature,
solid precipitate was collected and washed with dry MeCN. Filtrate
was reacted under refluxing with substituted N-benzoylhydrazide
(3.0 mmol) in MeCN for 8 h. White or light yellow solid was
filtered and recrystallized three times from THF-EtOH-H2O (6:
3:1, v/v/v) to afford the corresponding crystal product.
Spectral Investigations. UV-vis spectra were recorded using
1
a 1 cm quartz cell. H and 13C NMR spectra were obtained on a
400 MHz NMR spectrometer in DMSO-d6 or CD3CN using TMS
or using residual solvent peak as an internal standard. HRMS data
were recorded on a high-resolution mass spectrometer by injection
of sample solution in methanol. Absorption and NMR titrations
for anion binding were carried out by adding an aliquot of anion
solution into the bulk receptor solution at a given concentration.
Tetrabutylammonium salts of the anions were prepared by neu-
tralization of the corresponding acids with n-Bu4N+OH-.
Acknowledgment. This work has been supported by the
National Natural Science Foundation of China via Grants
20425518 and 20675069.
Supporting Information Available: Characterization data and
1H and 13C NMR spectra of 5 and 6. Absorbance of 5 and 6 against
anion concentration in MeCN and of 5d against [AcO-] in H2O-
MeCN. Substituent dependence of AcO- binding constants of 3,
4, 5, and 6 in MeCN and of 5 and 6 in 5% H2O-MeCN. Absorption
(MeCN) and NMR (CD3CN) spectra of 5a in the presence of F-
and OH-. Anion induced upfield shifts of the NMR signals of
aromatic protons in the N′-phenyl rings of 6a and 6d in CD3CN.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(14) (a) Sessler, J. L.; Gale, P. A.; Cho, W. S. Anion Receptor Chemistry;
RSC Publishing: Cambridge, UK, 2006. (b) Etter, M. C. Acc. Chem. Res.
1990, 23, 120-126.
(15) In the anion binding complex, the CdS and CdO bonds shall be
unparallel. This was also indicated by the observed anion-dependent upfield
shifts in the NMR signals of aromatic protons in the N′-phenyl rings of 6a
and 6d. In the case of 6d with substituent Y ) H, anion induced upfield
shifts become increasingly higher when the anion varies from F- to AcO-
to H2PO4- of decreasing electron density, whereas with 6a bearing a highly
electron-donating Y of p-N(CH3)2, this anion dependence is not that much
(Table S2, Supporting Information).
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J. Org. Chem, Vol. 73, No. 3, 2008 1127