Regioselective nitration of a biphenyl derivative to derive a fluorescent chloride sensor

Article abstract of DOI:10.1016/j.tetlet.2020.152750

Methyl 2′-aminobiphenyl-4-carboxylate, an optical chloride sensor has been synthesized via Suzuki-Miyaura cross-coupling followed by regioselective nitration. In general biphenyl systems are prone to undergo poly nitration whenever they are subjected to nitration via different nitrating agents. But in this present work, a highly selective nitration in the 2′ position of methyl biphenyl-4-carboxylate was achieved using 70% HNO₃ and acetic anhydride to derive 3. Upon reduction, 3 produces 4 which shows bright blue emission in different organic solvents. In presence of chloride ion the blue emission of the sensor changes to enhanced bright green emission. The chloride sensing has also been explored using other spectroscopic techniques such as ¹H NMR, UV–vis spectroscopy, and theoretical study.

Full text of DOI:10.1016/j.tetlet.2020.152750

Tetrahedron Letters 65 (2021) 152750
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Regioselective nitration of a biphenyl derivative to derive a fluorescent
chloride sensor
a,
b
Tanmay Das a, , Mrittika Mohar , Arijit Bag
⇑
⇑
a Department of Chemical Sciences, Indian Institute of Science Education and Research, Kolkata, Mohanpur, Nadia, West Bengal 741246, India
b Department of Applied Science, Maulana Abdul Kalam Azad University of Technology, NH-12 (Old NH-34) Simhat Haringhata, Nadia, 741249, West Bengal, India
a r t i c l e i n f o
a b s t r a c t
Methyl 20-aminobiphenyl-4-carboxylate, an optical chloride sensor has been synthesized via Suzuki-
Miyaura cross-coupling followed by regioselective nitration. In general biphenyl systems are prone to
undergo poly nitration whenever they are subjected to nitration via different nitrating agents. But in this
present work, a highly selective nitration in the 20 position of methyl biphenyl-4-carboxylate was
achieved using 70% HNO3 and acetic anhydride to derive 3. Upon reduction, 3 produces 4 which shows
bright blue emission in different organic solvents. In presence of chloride ion the blue emission of the
sensor changes to enhanced bright green emission. The chloride sensing has also been explored using
other spectroscopic techniques such as 1H NMR, UV–vis spectroscopy, and theoretical study.
Ó 2021 Elsevier Ltd. All rights reserved.
Article history:
Received 25 September 2020
Revised 8 December 2020
Accepted 10 December 2020
Available online 2 January 2021
Keywords:
Regioselective synthesis
Regioselective nitration
Chloride sensing
Fluorescent sensor
Restricted rotation
Introduction
the world have developed various kinds of chloride sensors in
the last decade which include protein [22], organic fluorescent sen-
In recent times huge attention has been drawn towards the
design of supramolecular sensors for anions [1–8]. These
supramolecular sensors utilize different non-covalent interactions
to interact with the anion and the interaction is exhibited in terms
of color change, change in the fluorescence emission, electrochem-
ical response, etc. Now among various anions, chloride is highly
important from different point of views. It is a very crucial elec-
trolyte in the blood for the living systems. It keeps the proper bal-
ance of fluid inside and outside the cells [9]. It also helps to
maintain appropriate blood pressure and pH inside our human
body. The chloride level in the urine of a healthy person lies in
the range of 110–250 mmol Lꢀ1. So monitoring the chloride level
in urine can also provide important information regarding renal
diseases [10,11]. Bizarre level of chloride in the sweat, serum,
and cerebral spinal fluid (CSF) may indicate diseases like cystic
fibrosis (CF), metabolic alkalosis, Addison’s disease, and amy-
otrophic lateral sclerosis [12,13]. That is why from a medical diag-
nosis point of view chloride sensing is highly important. Apart
from that detection of Cl- is also important in various environmen-
tal monitoring purposes [14–17], food industries [18,19], and var-
ious industrial applications [20,21]. That is why scientists all over
sors [13,23–25], amperometric sensor [26], colorimetric sensors
[27,28], electrodes [29–31], hybrid nanomaterial [32], etc. Out of
them both fluorescent and colorimetric sensors are highly useful
because they provide visual detection under ambient light or UV
light. So designing such a kind of chloride sensor will be highly
useful from both academic as well as practical application point
of view. In this present work, we have developed a biphenyl-
derived chloride sensor via the Suzuki-Miyaura cross-coupling.
As far as designing a chemosensor is concerned, it is very much
crucial to place particular functional groups at particular positions
of a molecule. So regioselective synthesis is an important part of
designing a chemosensor. Here in this current work, we have per-
formed a regioselective nitration of methyl biphenyl-4-carboxy-
late. Our target was to introduce the nitro group at the 20
position because of two reasons. First of all 40 position is more
prone towards nitration because of lack of steric hindrance and
so getting 20 nitro substituted product was a challenging task.
The second point was to utilize the nitro product for designing
an anion sensor via the reduction of that nitro group because ami-
nes are known to form hydrogen bonds with anions. As far as
designing a fluorescent chemosensor is concerned, the concept of
rigidity becomes very prominent. It has been observed that a rigid
molecular system has a higher tendency towards showing photolu-
minescence than that of its flexible counterpart. For example, the
fluorescence intensity of 8-hydroxyquinoline is much less than
⇑
Corresponding authors.
E-mail addresses: tanmay.greenview@gmail.com (T. Das), mrittika.malda@gmail.
com (M. Mohar).
https://doi.org/10.1016/j.tetlet.2020.152750
0040-4039/Ó 2021 Elsevier Ltd. All rights reserved.

T. Das, M. Mohar and A. Bag
Tetrahedron Letters 65 (2021) 152750
its Zn(II) complex because the latter is more rigid in nature [33].
Similarly fluorene’s quantum yield is 5 times more than that of
its flexible analog biphenyl [33]. So using the concept of restricted
rotation, a chloride sensor has been designed using methyl 20-
aminobiphenyl-4-carboxylate.
amine derivative from that nitro compound. In general amine com-
pounds are good hydrogen bond donors and biphenyl compounds
are non-rigid molecules. So an interaction of an anion with an
amine compound can lead to either change of fluorescence or color.
So it can act as a guest responsive material. Upon reduction of this
nitro derivative using iron powder and acetic acid, methyl 20-
aminobiphenyl-4-carboxylate (4) is obtained (Fig. 1).
Results and discussion
All the compounds were characterized by NMR spectroscopy, IR
spectroscopy, and mass spectrometry. Compound 4 is a biphenyl
derivative and as a result, it shows bright blue fluorescence. In gen-
eral, fluorescence is favored for rigid molecules. But biphenyl
derivatives contain a pivotal bond via which two phenyl rings
are attached to each other and free rotation of the phenyl rings
along the pivotal bond always takes place unless the ortho sub-
stituents of the phenyl rings are not too bulky. Thus, if we compare
biphenyl and the rigid analog of the biphenyl i.e. fluorene, then we
will find that the quantum yield of biphenyl system is pretty much
less as compared to fluorene. The marked difference in the fluores-
cence output of biphenyl and fluorenone arises due to the addi-
tional methylene group in fluorene which restricts the free
rotation of the two phenyl rings. The reason for the lower quantum
yield for non-rigid molecules is due to the enhancement in the rate
of internal conversion which increases the chance of radiationless
deactivation whereas in the case of rigid molecules, the rate of
radiationless deactivation is much less and this results in higher
quantum efficiency. In the case of compound 4, there is one amine
group at the 20 position of the two phenyl rings with respect to the
pivotal bond. If somehow this amine group is utilized to bind a
guest molecule via hydrogen bonding then this will prevent the
free rotation of the phenyl rings which will enhance the emission
of the guest-bound biphenyl. Amine being a good hydrogen bond
donor, anion binding experiment was thought with compound 4.
Different tetrabutylammonium anions were added to the THF solu-
tion of compound 4ꢁ THF solution of compound 4 shows bright
blue emission. But in presence of chloride ion, the emission chan-
ged to bright green instantaneously (Fig. 2). No significant changes
were observed in the case of other anions.
To investigate the binding process, we performed UV–vis and
fluorescence spectroscopy. Fig. 3 shows the change in the emission
and absorption spectra of 4 in THF in the presence of different
anions. Compound 4 shows emission maxima around 460 nm
(Fig. 3a). But after the gradual addition of chloride ion, the emis-
sion intensity increased with a little red-shift. While in the case
of absorption spectroscopy, compound 4 showed two absorption
maxima at 251 and 332 nm (Fig. 3b). But after the gradual addition
of chloride ion to the solution of compound 4, the intensity of the
band at 332 nm was found to decrease to an extent. We have
checked the effect of other anions on compound 4 using fluores-
cence spectroscopy (Supplementary Fig. S4). But only in presence
of chloride ion significant emission enhancement was observed
(Supplementary Fig. S5). Using the fluorescence spectroscopy, the
binding constant and binding stoichiometry were determined from
the Benesi-Hildebrand double reciprocal plot (Fig. 4a). The binding
constant value was 1.06 X 104 Mꢀ1 and binding stoichiometry was
1:1. We have also determined the 1:1 binding stoichiometry via
Job’s plot (Supplementary Fig. S6). From the fluorescence spec-
troscopy, the detection limit for chloride ion in THF was found to
In this present work, biphenyl-4-carboxylic acid (1) was synthe-
sized via the Suzuki-Miyaura cross-coupling reaction by reacting
4-bromobenzoic acid and phenylboronic acid in water with a water
soluble palladium catalyst (Fig. 1) [34]. Then our target was to pro-
duce regioselective nitro derivative from compound 1. We were
interested to get the nitro group at 20 position because that could
act as a precursor for different useful compounds such as car-
bazole, amino acid, etc. But the task was not simple because biphe-
nyl is an electron rich system. So normal nitration with biphenyl
using mixed acid can lead to multi-nitro substituted products.
Even if we lower the temperature, 40 position is more likely to
become nitrated than that of 20 position because of lesser steric
strain. The only possible way to synthesize 20-nitrobiphenyl is to
use costly 2-nitrophenylboronic acid. From that angle, the chal-
lenge was taken to synthesize this in a cost-efficient manner. At
first, we tried to nitrate compound 1 directly. But there was a prob-
lem regarding the solubility. To increase the solubility, compound
1 was esterified. Compound 2 was prepared by esterification of
compound 1 using thionyl chloride and methanol. Then compound
2 was subjected to nitration using 70% nitric acid and acetic anhy-
dride at ꢀ15 °C and methyl 20-nitrobiphenyl-4-carboxylate (3) was
obtained with 80% yield. To confirm the position of the nitro group
more accurately, we crystallized compound 3 in a methanol-ace-
tone mixed solvent. Good quality yellow colored single crystals
were obtained in a methanol-acetone mixture. Compound 3 was
also characterized using single crystal XRD. The crystal was dif-
fracted in Bruker saint (CCDC 2033350). The monoclinic crystal
structure of compound 3 is given in Supplementary Fig. S1.
Table S1 shows the detail of the crystal. Supplementary Fig. S2
shows the higher order packing diagram of compound 3. The selec-
tivity towards 20 position is arising because the nitronium ion in
association with acetic anhydride interacts with the ester oxygen
of compound 4. Via this interaction, the nitronium ion gets closer
access towards 20 position and as a result, selective nitration takes
place at 20 position (Supplementary Fig. S3). We tried to prepare an
be 1.43
lM (Fig. 4b). We have also compared the detection limit
of our sensor with earlier reported sensors (Supplementary
Table S2). To investigate the chloride binding process more accu-
rately, 1H NMR spectroscopy was also performed (Fig. 5). 1H
NMR spectroscopy was performed in CDCl3. Compound 4 was dis-
solved in CDCl3 and to this solution, tetrabutylammonium chloride
(TBACl) was added gradually. Compound 4 has eight aromatic pro-
tons and there are six signals in the range of 6.7–8.2 ppm. In the 1H
NMR spectroscopy, all the aromatic protons underwent an upfield
Fig. 1. Synthetic scheme of compound 4.
2

T. Das, M. Mohar and A. Bag
Tetrahedron Letters 65 (2021) 152750
Fig. 2. Chloride sensing in THF by compound 4. Color change of the solution of 4 in the presence of different tetrabutylammonium anions (10 equivalents) under 366 nm UV
irradiation. The bright blue solution of 4 becomes bright green after interacting with chloride anion.
Fig. 3. (a) Change in the fluorescence spectra of compound 4 with gradual addition of tetrabutylammonium chloride in THF (excitation wavelength = 332 nm) and (b) Change
in the UV–vis spectra of compound 4 with gradual addition of tetrabutylammonium chloride in THF.
Fig. 4. (a) Benesi-Hildebrand double reciprocal plot for determining the binding stoichiometry; (b) Detection limit calculation for chloride ion.
shift after chloride binding. It indicates that both the phenyl rings
of compound 4 become electron-rich. This is only possible if the
overall chloride bound compound 4 becomes somewhat rigid and
electron density is circulated from one ring to the other. For com-
pound 4, the signal of amine protons appears as a broad signal
around 3.6 ppm. Upon addition of tetrabutylammonium chloride
to the solution of 4, the signal of amine proton was also expected
to undergo a downfield shift. But after the addition of tetrabuty-
lammonium chloride the signal was found to vanish (Supplemen-
tary Fig. S7). This might be due to the rapid proton exchange of
amine protons with the water content associated with the highly
hygroscopic material tetrabutylammonium chloride.
During this process the interaction between Cl- and adjacent
aromatic CAH is important. There are several reports of an interac-
tion between Cl- and aromatic CAH in the literature. [35] More
importantly there is also a possibility of interaction between Cl-
and adjacent aromatic CAH. So from the knowledge gained from
both the fluorescence and 1H NMR spectroscopy, we have proposed
the chloride binding model in Supplementary Fig. S8.
To understand the binding process of chloride ion to compound
4, we have performed the Density Functional Theory (DFT) [36]
using Gaussian 09 [37] program package. Computation is per-
formed with B3LYP [38] functional and correlation-consistent
polarized valence double-zeta basis, abbreviated as cc-pVDZ [39].
3

T. Das, M. Mohar and A. Bag
Tetrahedron Letters 65 (2021) 152750
group is practically feasible. The optimized geometry is presented
in Fig. 6.
From the optimized geometry of the chloride bound compound
4 it is observed that the NAH bond is elongated from 1.0 Å to
1.26 Å due to the hydrogen bond formation. But importantly, N
A H – Cl bond is linear and it is oriented in a perpendicular direc-
tion with respect to the other benzene ring. This result raises the
question of stabilization for such kind of interaction. There is only
one possibility i.e. the sharing of benzene
p electron with the
vacant d – orbital (dx2-y2) of the chlorine atom. To test this possibil-
ity, we performed the electronic orbital analysis. We observed that
one of the occupied orbitals (HOMO-1) which is very close to the
HOMO supports our claim. This is presented in Fig. S9. The orbital
analysis was found very helpful to investigate the emission. The
energy gap between HOMO and LUMO is equivalent to 452 nm
and this is very close to that of our emission maxima during chlo-
ride sensing. Since chloride is a borderline ligand with very high
electronegativity, it easily forms the hydrogen bond and gets better
stabilization by donor–acceptor interaction with distant benzene
and facilitates ligand to ring electronic transition. Thus, we
observed that only the chloride ion is able to make a color change.
Fig. 5. A part of 1H NMR spectra of compound 4 with gradual addition tetrabuty-
lammonium chloride in CDCl3.
Conclusion
This functional and basis are chosen because it is proven that for
main group chemistry this combination of functional and basis
produce very accurate geometry and spectral information [40–
42]. To compute the absorption spectra of compound 4 with chlo-
ride ion, Time-dependent DFT (TD-DFT) [43,44] is used. The geom-
etry optimizations and spectral studies are done within the
polarized continuum model (PCM) of solvation [45] taking THF as
the solvent. This solvent is chosen as experiments are performed
with this solvent. To find out the binding of chloride ion, we have
chosen different models of binding. But we found that only the for-
mation of hydrogen bond by chloride ion with the amine NH2
So in conclusion we have developed a regioselective nitration of
a biphenyl derivative to get the less likely product 3 in major quan-
tity. Then compound 3 was reduced to produce compound 4 i.e. the
amine analog of 3. The amine product was designed for sensing
anions. In this sensing, the key points were to utilize the hydrogen
bonding tendency of the amine group and restricted free rotation
of biphenyl. Compound 4 was fluorescent and this made the sens-
ing much easier and effective. After binding with chloride ion, the
free rotation of two phenyl rings of 4 stops and fluorescence emis-
sion is enhanced. The THF solution of only compound 4 shows blue
emission under 366 nm UV light. But the mixture of 4 and tetra-
butylammonium chloride shows enhanced bright green emission
due to chloride binding. Thus compound 4 was utilized as a chlo-
ride sensor.
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgments
M. Mohar thanks CSIR, India for the research fellowship.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.tetlet.2020.152750.
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