Anthracene-labeled pyridinium-based symmetrical chiral chemosensor for enantioselective recognition of l-tartrate

Article abstract of DOI:10.1016/j.tetlet.2014.01.016

A new anthracene-based chiral chemosensor 1 has been designed and synthesized. l-Valine has been used as the chiral source in the design. The chemosensor 1 has been established as an efficient enantioselective sensor for l-tartrate. While in the presence of l-tartrate the fluorescent sensor 1 in DMSO exhibits considerable increase in emission, the isomeric tartrate brings relatively small change. The enantiomeric fluorescence difference ratio (ef) has been determined to be 29.38.

Full text of DOI:10.1016/j.tetlet.2014.01.016

Tetrahedron Letters 55 (2014) 1342–1346
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Anthracene-labeled pyridinium-based symmetrical chiral
chemosensor for enantioselective recognition of L-tartrate
⇑
Kumaresh Ghosh , Tanmay Sarkar
Department of Chemistry, University of Kalyani, Kalyani, Nadia 741235, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A new anthracene-based chiral chemosensor 1 has been designed and synthesized. L-Valine has been
Received 21 October 2013
Revised 3 January 2014
Accepted 7 January 2014
Available online 10 January 2014
used as the chiral source in the design. The chemosensor 1 has been established as an efficient
enantioselective sensor for -tartrate. While in the presence of -tartrate the fluorescent sensor 1 in DMSO
exhibits considerable increase in emission, the isomeric tartrate brings relatively small change. The
enantiomeric fluorescence difference ratio (ef) has been determined to be 29.38.
Ó 2014 Elsevier Ltd. All rights reserved.
L
L
Keywords:
Anthracene-labeled symmetric chiral sensor
Enantioselective recognition
Sensing of L-tartrate
Fluorometric discrimination
Pyridinium amide
L-Valine as chiral source
Synthetic fluorescent chemosensor that discriminates the
synthesis and chiral recognition properties of L-valine coupled
enantiomers of a particular chiral guest by exhibiting different
fluorescence behaviors draws attention in the area of molecular
recognition.1 Now-a-days fluorescence technique is widely used
over the other different analytical methods, as fluorescence-based
enantioselective sensors can provide high sensitivity and real-
time measurement.2 In the past several years, there has been an
anthracene labeled pyridinium-based symmetrical sensor 1. The
chiral chemosensor 1 shows good fluorometric discrimination
between
L
- and
D-tartrates in DMSO.
O
-
PF6
H
N
N
O
N
H
H
O
interest in the enantioselective recognition of
a-hydroxycarboxy-
lic acids1a,3 due to their presence as the structural unit of many
natural products and drug molecules. They can also serve as the
multifunctional precursors to a great variety of organic com-
pounds.4 Among the different hydroxyl carboxylic acids, tartaric
acid is a common natural product present in wines and other
grape-derived beverages. Large accumulation of tartaric acid
causes human fatality with death. Therefore, recognition of this
molecule especially its anionic form is important. Scrutiny of the
literature reveals that hydrogen bonding receptors for both
neutral5 and anionic6 forms of tartaric acid or its derivatives are
known. Very few reports are available for chiral recognition of
the anionic form of tartaric acid in the literature. Recently, we
O
H
H
N
N
N
O
-
PF6
H
O
1
Scheme 1 describes the synthesis of sensor 1. Initially, N-Boc-
valine acid 2 derived from N-Boc- -valine ester, was coupled with
3-aminopyridine to afford the compound 3. Then quaternization
L-
L
of the pyridine ring nitrogen in
3
using 9,10-bis(chloroꢀ-
methyl)anthracene followed by exchange of Clꢀ ions with PF6
ions introduced the chemosensor 1. All the compounds were char-
acterized by 1H NMR, 13C, FTIR, and mass analyses.
have reported
molecule that fluorometrically discriminates
-isomer in DMSO.7 The inspiring results have prompted us fur-
L-valine derived benzimidazole based urea
L-tartrate from its
The solution phase binding interaction of 1 with the tetrabutyl-
D
ammonium salts of
D-/L-tartaric and R-/S-mandelic acids was
ther to undertake a new design which is comprised of pyridinium
amide motifs. In this regard, we herein report the design,
investigated in DMSO by UV–vis and fluorescence techniques.
The chemosensor 1 in DMSO showed an intense emission at
432 nm when excited at 380 nm. However, upon progressive
addition of the tetrabutylammonium salts of D-/L-tartaric and R-/
⇑
Corresponding author. Fax: +91 33 2582 8282.
E-mail address: ghosh_k2003@yahoo.co.in (K. Ghosh).
S-mandelic acids to the solution of 1 (c = 1.12 ꢁ 10ꢀ4 M) in DMSO,
0040-4039/$ - see front matter Ó 2014 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2014.01.016

K. Ghosh, T. Sarkar / Tetrahedron Letters 55 (2014) 1342–1346
1343
O
H
H
N
H
(iii)
O
N
(ii)
O
H
O
(i)
1
H
O
N
CO2H
N
O
CO2Me
N
O
H
H
3
2
Scheme 1. Reagents and conditions: (i) LiOH, MeOH–H2O, stirring, 4 h; (ii) 3-aminopyridine, DCC, DMAP, dry CH2Cl2, stirring, 19 h; (iii) (a) 9,10-bis(chloromethyl)anthracene,
CH3CN, reflux, 72 h; (b) NH4PF6, MeOH/H2O.
the intensity of monomer emission at 432 nm suffered change to
different extents. During fluorometric titration with -tartrate,
the emission of 1 at 432 nm increased significantly and it was dis-
tinguishable from its -isomer. In relation to this, the fluorescence
ratio of 1 at 432 nm for all anions except L-tartrate was found to be
negligible in magnitude.
sion intensity in the same region is initially decreased and then in-
creased (Fig. 2b). Such initial decrease in emission followed by a
recovery reflects the different binding modes due to which, the
photoinduced electron transfer (PET) occurring in-between the
binding site and the excited state of anthracene is regulated in dif-
ferent ways. We believe that the initial decrease in emission is due
to the hydrogen bonding interaction of the individual arm of 1.
During the titration, in the presence of excess concentration of
L
D
Figure 1 shows the change in fluorescence ratio of 1 in the pres-
ence of 20 equiv amounts of tetrabutylammonium salts of -/ -tar-
D
L
taric and R-/S-mandelic acids in DMSO. As can be seen from
Figure 1, although the enantiomers of mandelate are hardly dis-
criminated, receptor 1 shows sharp fluorometric discrimination
D-tartrate, a conformational change takes place due to which the
bridging of the two binding arms occurs and the emission is regu-
lated in the increasing mode. This is supported by the observation
noted in emission titration using R-/S-mandelates. Under similar
conditions, enantiomers of mandelate quenched the emission to
smaller extents (Supporting information).
between
Figure 2a and b show the change in emission of 1 upon increas-
ing the addition of tetrabutylammonium salts of -/ -tartaric acids
D- and L-tartrates.
D
L
in DMSO, respectively. From Figure 2a, it is clear that upon gradual
Time-resolved fluorescence decay profile of 1 in DMSO with
L-
addition of
L
-tartrate (c = 2.2 ꢁ 10ꢀ3 M) to the receptor solution in
and -tartrates shows different behaviors. The decay curve for 1
D
DMSO, the emission intensity at 432 nm is considerably enhanced.
monitored at 440 nm (kexc = 380 nm) was fitted to a three expo-
nential decay with a major (1.24 ns, 68.66%) and minor compo-
nents (76.4 ps, 18.50%; 4.87 ps, 12.84%). While in the presence of
In contrast, upon addition of
D
-tartrate (c = 2.2 ꢁ 10ꢀ3 M) the emis-
L-tartrate the decay curve of 1 fits to three exponential decay, un-
der similar conditions in the presence of -tartrate it fits to two
D
exponential decay (Supporting information). In the decay profile
of 1, the major component for anthracene (lifetime 1.24 ns) shows
significant increase in lifetime in the presence of D- and L-tartrates
and contributes to the total fluorescence with different pre-expo-
nential factors. This indicated the different interaction behaviors
of 1 toward L- and D-tartrates in the excited state.
However, in the emission titration spectra of 1 with all the
guests small inflection at ꢂ525 nm was noticed. This is presumably
attributed to the guest chelation induced formation of intermolec-
ular anthracene excimer8 which disappears upon strong interac-
tion with
L-tartrate.
The selective recognition effect on the guest of the
D
-/L-isomers
of tartrate was understood from the enantiomeric fluorescence dif-
ference ratio, ef½ef ¼ ðIL ꢀ I0Þ=ðID ꢀ I0Þꢃ. I0 represents the fluores-
cence emission intensity in the absence of the chiral substrate. IL
Figure 1. Change in fluorescence ratio of 1 (c = 1.12 ꢁ 10ꢀ4 M) at 432 nm upon
addition of 20 equiv amounts of anions.
240
220
200
900
800
(b)
(a)
250
200
150
100
50
700
180
160
140
120
100
600
800
500
400
300
200
600
100
0
5
10
[G]/[H]
15
20
0
5
10
[G]/[H]
15
20
400
200
0
0
400
450
500
550
600
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
Figure 2. Fluorescence titration spectra of 1 (c = 1.12 ꢁ 10ꢀ4 M) in DMSO upon addition of tetrabutylammonium salts of (a)
L-tartaric (Inset: change in emission at 432 nm
with [G]/[H]) and (b)
D
-tartaric acids (concentration of guests was 2.2 ꢁ 10ꢀ3 M) (kexc = 380 nm).

1344
K. Ghosh, T. Sarkar / Tetrahedron Letters 55 (2014) 1342–1346
and ID are the fluorescence intensities in the presence of
L- and
D
-
discrimination between L- and D-tartrates in the ground state.
tartrates, respectively. The value of ‘ef’ is determined to be 29.38
for the chemosensor 1. This large value of ‘ef’ signifies that chemo-
Figure 5 reveals the DFT optimized geometries with corresponding
hydrogen bonding features.
sensor 1 exhibits a good enantioselective response toward
L-tar-
trate. The steric fit of the -isomer into the syn conformation of 1
L
presumably indicates strong hydrogen bonding interaction. The
equilibrium anti conformation 1X9 will go to the syn conformation
1Y in the presence of tartrate (dicarboxylates) due to the formation
of greater number of hydrogen bonds (Fig. 3).
To be confirmed with the suggested mode of interaction in
Figure 3, 1H NMR studies of 1 in the presence of equiv amounts
of L- and D-tartrates were done in d6-DMSO (Fig. 4). The amide
proton Ha and carbamate proton Hf in 1 underwent downfield
chemical shift during complexation. The pyridinium ring protons
Hb and Hc moved much to the downfield directions and thereby
indicated their involvement in complexation. Careful study reveals
that the chemical shift of the indicated protons is slightly more in
the presence of
L
-tartrate than observed with
D
-tartrate.
- and L-tar-
DFT calculations of the complexes of 1 with both
D
trates in the gas phase were further carried out to realize the
hydrogen bonding interaction in the ground state.10 It has been no-
ticed that the complex of L-tartrate is slightly stable than its iso-
Figure 5. DFT optimized geometries of the complexes of 1 with (a)
(hydrogen bond distances: a = 1.80 Å, b = 1.58 Å, c = 2.06 Å, d = 1.95 Å, e = 1.58 Å,
f = 1.73 Å and (b) -tartrate (hydrogen bond distances: a = 1.74 Å, b = 1.71 Å,
L-tartrate
meric complex by ꢀ0.30 kcal/mol. This small energy difference
D
between the complexes corroborates the small efficiency of 1 in
c = 2.07 Å, d = 2.03 Å, e = 1.60 Å, f = 2.95 Å, g = 1.84 Å).
O
-
H
PF6
N
O
N
N
H
H
O
O
H
N
H
H
N
-
O
N
H
HO
HO
COO-
COO-
PF6
O
1X
H
D- tartrate
Weak interaction
(more steric)
O
-
PF6
H
N
N
O
N
H
H
O
O
H
H
N
Strong interaction
(less steric)
N
N
O
-
PF6
H
O
COO-
H
H
OH
1Y
OH
COO-
L- tartrate
Figure 3. Probable conformations of 1 and their preferential interactions.
-
PF 6
-
PF 6
N+
c
N+
d
b
e
NH
HN
a
g
O
O
H
h
H
HN
NH
O
f
O
O
O
1
Figure 4. 1H NMR titration of (i) 1 (c = 2.9 ꢁ 10ꢀ3 M) with (ii)
D-tartrate and (iii) L-tartrate.

K. Ghosh, T. Sarkar / Tetrahedron Letters 55 (2014) 1342–1346
1345
In the interaction process, the stoichiometry of the complexes
of 1 with both - and -tartrates was determined to be 1:1 as
confirmed by Job plot.11 Figure 6a, for example, shows the Job plot
values12 for the complexes of 1 with R- and S-mandelate were
determined to be (2.79 0.39) ꢁ 103 Mꢀ1 and (2.07 0.22) ꢁ 103
Mꢀ1, respectively, (Supporting information).
D
L
for 1 with
L
-tartrate. Non linear fitting of the emission titration
The selective recognition effect of a particular chiral isomer was
understood from the change in emission of 1 while it remains with
the mirror image isomer in the solution. As can be seen from
data gave the binding constant (Ka)11 value of (6.31 0.05) ꢁ 103
Mꢀ1 for
L-tartrate (Fig. 6b). However, non linear fitting of the
emission titration data obtained from the progressive addition of
Figure 7,
D
-tartrate-induced change in emission of 1 was further
-tartrate
D
-tartrate upto 5 equiv amounts introduced Ka value of (8.18
perturbed to the considerable extent upon addition of L
(Fig. 7a), the reverse one was noticed to be insignificant (Fig. 7b).
0.55) ꢁ 102 Mꢀ1, which is less compared to the case with
L-tartrate.
The emission titration data beyond the addition of 5 equiv
The UV–vis study of 1 in the presence of tetrabutylammonium
amounts of
D-tartrate did not fit well in non linear equation to give
salts of D-/L-tartaric and R-/S-mandelic acids in DMSO showed mar-
any reliable binding constant value. The binding constant (Ka)
ginal change in absorbance for anthracene (Supporting information).
-5
(a) 1.6x10
(b) 900
K= (6.31 0.056)x103 M-1
R = 0.991
800
1.2x10-5
8.0x10-6
700
600
500
400
300
200
100
4.0x10-6
0.0
2.0x10-4 4.0x10-4 6.0x10-4 8.0x10-4 1.0x10-3
0.0
0.0
0.2
0.4
0.6
0.8
XG
[G]
Figure 6. (a) Fluorescence Job plots for 1 with
L
-tartrate in DMSO ([H] = [G] = 5.00 ꢁ 10ꢀ4 M); (b) Binding constant curve for 1 with
L-tartrate.
1 +20 equiv. L- tartrate = A
A + 20 equiv. L- tartrate
800
600
400
200
0
800
600
400
200
0
A + 20 equiv. D- tartrate
1 +20 equiv.
D- tartrate = A
Receptor 1
Receptor 1
400
450
500
550
600
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
Figure 7. Fluorescent response of receptor 1 (c = 1.14 ꢁ 10ꢀ4 M) to (a)
D
-tartrate (c = 2.2 ꢁ 10ꢀ3 M) in the presence of
L
-tartrate (c = 2.2 ꢁ 10ꢀ3 M) and (b)
L-tartrate
(c = 2.2 ꢁ 10ꢀ3 M) in the presence of
D
-tartrate (c = 2.2 ꢁ 10ꢀ3 M) in DMSO.
3
(a)
(b)
3
2
1
0
2
1
0
300
350
400
450
500
300
350
400
450
500
Wavelength (nm)
Wavelength (nm)
Figure 8. Change in absorbance of 1 (c = 1.12 ꢁ 10ꢀ4 M) upon gradual addition of (a)
L-tartrate, (b) D-tartrate in DMSO.

1346
K. Ghosh, T. Sarkar / Tetrahedron Letters 55 (2014) 1342–1346
(c) Ogoshi, H.; Mizutani, T. Acc. Chem. Res. 1998, 31, 81–89; (d) Schmidtchen, F.
P. Top. Curr. Chem. 2005, 255, 1–30; (e) Pu, L. Acc. Chem. Res. 2012, 45, 150–163;
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The change in absorbance in the region ꢂ300 nm (attributed to the
pyridinium binding site) was considerable for tartrates (Fig. 8a and
b). But no characteristic distinguishable spectral feature was
observed in UV.
2. (a) Pu, L. Chem. Rev. 1998, 98, 2405–2494; (b) Gunnlaugsson, T.; Glynn, M.;
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The stoichiometries of the tartrate complexes with 1 were also
1:1 as determined from Job plot11 by the UV-method (Supporting
information). The binding constant values (Ka) for the isomers of
tartrate with 1 were determined to be (9.56 2.3) ꢁ 103 Mꢀ1 and
3. Enantioselective recognition of a-hydroxyl carboxylic acids: (a) Chi, L.; Zhao, J.;
James, T. D. J. Org. Chem. 2008, 73, 4684–4687; (b) Xu, M.-H.; Lin, J.; Hu, Q.-S.;
Pu, L. J. Am. Chem. Soc. 2002, 124, 14239; (c) Altava, B.; Burguete, M. I.; Carbo,
N.; Escorihuela, J.; Luis, S. V. Tetrahedron: Asymmetry 2010, 21, 982–989; (d) Lin,
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cited therein.
(8.55 1.9) ꢁ 103 Mꢀ1 for
D- and L-tartrates, respectively. This small
difference in Ka values in the ground state is in agreement with the
DFT results, shown in Figure 5. Similarly, analysis of the absorption
data provided the binding constants (Ka) (1.47 0.19) ꢁ 103 Mꢀ1 for
R-mandelate and (1.08 0.24) ꢁ 103 Mꢀ1 for S-mandelate.
In conclusion, chemosensor 1 is successfully capable of discrim-
4. (a) Yang, D.; Li, X.; Fan, Y.-F.; Zhang, D.-W. J. Am. Chem. Soc. 2005, 127, 7996–
7997; (b) Duke, R. M.; Gunnlaugsson, T. Tetrahedron Lett. 2007, 48, 8043–8047;
(c) Coppola, G. M.; Schuster, H. F.
VCH: Weinheim, Germany, 1997.
a-Hydroxyl Acids in Enantioselective Synthesis;
inating L-tartrate from D-tartrate fluorimetrically with an ‘ef’ value
of 29.38. The mandelates being smaller in size than tartrate are un-
able to bridge the two pyridinium motifs in 1 and thereby induce
small change in emission without showing any measurable dis-
tinctive feature. The bridging of the two binding arms in 1 by iso-
meric tartrates induces differential hydrogen bonding perturbation
5. (a) Han, F.; Chi, L.; Liang, X.; Ji, S.; Liu, S.; Zhou, F.; Wu, Y.; Han, K.; Zhao, J.;
James, T. D. J. Org. Chem. 2009, 74, 1333–1336; (b) Zhao, J.; Fyles, T. M.; James, T.
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Lithgow, A.; Crego, M.; Caballero, C.; Moran, J. R. Tetrahedron Lett. 1998, 39,
7401–7404.
due to which substantial change in emission takes place and L-tar-
trate is selectively distinguished from its mirror image isomer.
However, the use of pyridinium motif in devising such simple
receptor for chiral recognition of tartrate is a first time approach
after a recent report on lactate recognition13 by pyridinium-based
receptor from our laboratory.
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D.; Puccioni, S. Chem. Commun. 2012, 48, 10428–10430; (b) Qin, H.; He, Y.; Hu,
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James, T. D. J. Am. Chem. Soc. 2004, 126, 16179–16186.
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6132.
Acknowledgments
10. DFT calculation was done by using Gaussian 09 (Revision A.02): Frisch, M. J.;
Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.;
Montgomery, J. A.; Vreven, T., Jr.; Kudin, K. N.; Burant, J. C.; Millam, J. M.;
Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega,
N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.;
Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts,
R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J.
W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.;
Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick,
D.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul,
A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong,
M. W.; Gonzalez, C.; Pople, J. A. DFT Calculation was done by Using Gaussian 09
(Revision A.02); Gaussian, Inc.: Wallingford CT, 2004.
We thank the DST New Delhi, India for financial support
[project SR/S1/OC-76/2010(G); Date: 23.08.2011]. T.S. thanks the
CSIR, New Delhi, India for a fellowship.
Supplementary data
Supplementary data (figures showing the change in fluores-
cence and UV–vis titrations of receptor 1 with the anions, binding
constant curves, Job plots, fluorescence decay spectra, experimen-
tal procedure, 1H, 13C NMR, and mass spectra) associated with this
article can be found, in the online version, at http://dx.doi.org/
10.1016/j.tetlet.2014.01.016.
11. Job, P. P. Ann. Chim. 1928, 9, 113–203.
12. Valeur, B.; Pouget, J.; Bourson, J.; Kaschke, M.; Eensting, N. P. J. Phys. Chem.
1992, 96, 6545–6549.
References and notes
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Garten, S.; Biedermann, P. U.; Topiol, S.; Agranat, I. Chirality 2010, 22, 662–674;