Bismuth nitrate as a source of nitro radical in ipso-nitration of carboxylic acids

Article abstract of DOI:10.1016/j.poly.2019.04.005

Aromatic nitro compounds are extensively used in synthetic chemistry. We disclose a new approach to obtain nitroarenes regioselectively starting from carboxylic acids under acid-free reaction conditions.

Full text of DOI:10.1016/j.poly.2019.04.005

Accepted Manuscript
Bismuth Nitrate as a Source of Nitro Radical in Ipso-Nitration of Carboxylic
Acids
Soumitra Agasti, Siddhartha Maiti, Soham Maity, M. Anniyappan, M.B.
Talawar, Debabrata Maiti
PII:
S0277-5387(19)30244-X
https://doi.org/10.1016/j.poly.2019.04.005
POLY 13865
DOI:
Reference:
To appear in:
Polyhedron
Received Date:
Revised Date:
Accepted Date:
14 January 2019
18 March 2019
4 April 2019
Please cite this article as: S. Agasti, S. Maiti, S. Maity, M. Anniyappan, M.B. Talawar, D. Maiti, Bismuth Nitrate
as a Source of Nitro Radical in Ipso-Nitration of Carboxylic Acids, Polyhedron (2019), doi: https://doi.org/10.1016/
j.poly.2019.04.005
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Bismuth Nitrate as a Source of Nitro Radical in Ipso-Nitration of
Carboxylic Acids
+
¶
+
§
,§
Soumitra Agasti, Siddhartha Maiti, Soham Maity, M. Anniyappan, M. B. Talawar,* and
+
,
Debabrata Maiti *
+Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai, 400076,
India *Email: dmaiti@chem.iitb.ac.in
¶Department of Biosciences and Bioengineering, Indian Institute of Technology Bombay, Powai,
Mumbai, 400076, India
§High Energy Materials Research Laboratory Pune (HEMRL), Ministry of Defense Research
and Development Organisation, *Email: talawar.mb@hemrl.drdo.in
TABLE OF CONTENTS
ABSTRACT
Aromatic nitro compounds are extensively used in synthetic chemistry. We disclose a new
approach to obtain nitroarenes regioselectively starting from carboxylic acids under acid-free
reaction conditions.
INTRODUCTION
Nitroarenes are extensively used in synthetic chemistry for the preparation of
1
2
3
4
5
pharmaceuticals, explosives, dyes, materials etc. Consequently, selective incorporation of
nitro group onto an arene has drawn significant attention in recent years.6-11 Conventional synthesis
of nitroarenes primarily relied upon mixed acid reagents (HNO
3
/H
2
SO ) for decades and was
4
restricted by harsh condition, limited functional group tolerance, poor substrate scope and
formation of undesired mixture of regioisomers or oxidized by-products.6

Scheme 1.Previous Approaches of ipso-Nitration
An alternative approach made use of ipso-functionalization strategy to circumvent these
1
2
problems, wherein nitro group was introduced in a regioselective manner under user-friendly
1
3
conditions. Arylboronic acids were selected as a fitting candidate for this purpose due to its
successful application in a number of C–C and C–X bond formations and cross coupling
reactions.14-18
Nitrodeboronation pathway dominated the regioselective synthesis of aromatic nitro compounds
(
Scheme 1).19-24 On the other hand, starting from aryl chlorides, triflates and nonaflates, Pd-
catalyzed nitration reactions expanded the scope of nitroarene synthesis (Scheme 1).25-27 Recently,

chelation assisted ortho C–H functionalization has also been employed to introduce nitro group in
a regioselective manner.28, 29
Scheme 2. Our Previous Work on Unsaturated Systems and ipso-Functionalization Strategy with
Nitro Radical
We have previously reported a number of efficient nitration reactions through radical pathway
(
Scheme 2).30-34 During the course of our study, we noticed that bismuth nitrate can be utilized as
.35-37 In this context, a decarboxylative
nitration,38-40 if developed would be highly interesting since carboxylic acids are widely
a source of nitro radical with inorganic oxidant K
2
S
2
O
8

available.41, 42 Furthermore, carboxylic acids can be prepared easily and handled without much
care.43-52
Notably, transition metal catalyzed radical protodecarboxylations are well known and
intermediacy of an aryl radical has been suggested.53-57 Intrigued by the prospect of trapping this
aryl radical intermediate with an in-situ generated nitro radical, we set out to explore an ipso-
nitration starting from arylcarboxylic acids (Scheme 3).
Scheme 3.ipso-Nitration of Carboxylic Acids
RESULTS AND DISCUSSION
Preliminary investigations in different solvents indicated that nitroarenes indeed could be
generated though the initial yield was only 5%. Different nitrate/nitrite salts such as
Fe(NO
3
)
3
.9H
2
O, Pb(NO
3
)
3
, AgNO etc. were investigated as potential nitrating agents but only
2
trace amount of nitroarenes was detected. Interestingly, use of catalytic Ag-salts as an additive
had no effect on the reaction outcome. It was rather surprising in view of its unique ability to
trigger decarboxylation at an elevated temperature.53-55, 58, 59 Use of different bases also did not
alter the outcome to a significant extent.60 After extensive optimization with several reaction
parameters, we found that 2 equiv. of Bi(NO
3
)
3
and 3.5 equiv. of K
2
S
2
8
O could generate
nitrobenzene in 18% isolated yield in acetonitrile (MeCN) at 130 °C. Such an elevated reaction
temperature was accounted for the high activation energy of the decarboxylation step. After

thorough optimization study we could not find a better solvent than acetonitrile to reduce the
excess pressure in the reaction setup. Note that 70% of unreacted benzoic acid (starting material)
was isolated from this reaction.
Scheme 4. Nitration of Aromatic Carboxylic Acidsa
a
Reaction conditions: ArCOOH (0.5 mmol), Bi(NO
3
)
3 2
.5H O (2 equiv.), K
2
S
2
O (3.5 equiv.), MeCN (3 mL),
8
1
30 °C, 24 h, air. Note that unreacted ArCOOH was isolated from each of these reaction.60 bTrace amount
c
of α-isomer was formed. Starting from anthracene-9-carboxylic acid.
Next, we decided to investigate the scope of the reaction with other aromatic carboxylic acids
(
Scheme 4). 1-Naphthoic acid exhibited increased formation of nitro product up to 43% isolated

yield under the standard condition (2b). The 4-fluoronaphthoic acid gave desired product in 41%
yield (2c). Similarly, 2-napthoic acid and its bromo derivative underwent decarboxylative
nitration under the present reaction conditions (2d and 2e). However, anthracene-9-carboxylic
acid gave dinitro product in 32% isolated yield (2f).
Encouraged by these results we applied the standard condition on heteroaromatic carboxylic
acids based on pyridines and quinolines (Scheme 5). Notably, incorporation of electrophilic nitro
group on electron deficient heteroarenes remained an unsolved problem so far. Nicotinic acid
indicated that nitration could be performed on such moiety (4a, 22%). 2-Picolinic acid gave the
desired product in an improved 60% isolated yield. However, presence of bromo and chloro group
at different positions substantially decreased the yield (4c and 4d). Quinoline-2-carboxylic acid
also underwent successful nitration. Reaction with related 4-methoxy-quinoline-2-carboxylic was
sluggish and nitroarene product was obtained in 11% yield.
Scheme 5. Nitration of Heteroaromatic Carboxylic Acidsa

aReaction conditions: ArCOOH (0.5 mmol), Bi(NO
)
.5H
O (2 equiv.), K
S
3
3
2
2
2
8
O (3.5 equiv.),
MeCN (3 mL), 130 °C, 24 h, air. Note that unreacted arylcarboxylic acid was isolated from each
of these reaction.60 bMixture of 2-nitroquinoline and 3-nitroquinoline (yield determined by GC).
We hypothesized that the reaction proceeded through a mechanism involving radical
intermediates. To verify our hypothesis, a series of experiments with radical scavengers were
performed. Radical quenchers like TEMPO, HOBT and 2, 4, 6-tri tert-butylphenol was added in
the reaction mixture under standard reaction conditions.
Table 1. Reaction with Radical Scavengers

Entry
radical scavenger
yield (%)b
1
2
3
4
-
TEMPO
45
18
5
HOBT
2, 4, 6-tri tert-butylphenol
18
a
Reaction conditions: ArCOOH (0.25 mmol), Bi(NO
3
)
3 2
.5H O (2 equiv.), K
2
S
O
2 8
(3.5 equiv.), MeCN (2
b
mL), 130 °C, 24 h, air, 0.5 mmol scavenger. Yields determined by GC.
In presence of TEMPO, desired 1-nitronaphthalene was formed only in 18% yield (Table 1).
HOBT and 2, 4, 6-tri tert-butylphenol also quenched the reaction significantly and gave 5% and
8
% yields, respectively. However, 45% yield of 1-nitronaphthalene has been detected without any
scavenger. Significant decrease of product formation in presence of radical scavenger indicates in
favor of aryl radical formation, which was quenched in the reaction medium.
Based on these experimental observations, a radical based pathway is described in Scheme 6.
Under heating condition, homolytic cleavage of perdisulfate anion will produce sulphate radical
7
anion. This sulphate radical anion will capture hydrogen radical from starting material and as a
result intermediate I will form along with hydrogen sulphate.16, 61 In situ removal of CO
intermediate I will generate aryl radical species.15, 62
from
2
Under elevated temperature, bismuth
nitrate will produce nitro radical and this nitro radical will be intercepted by aryl radical species to
give desired nitro arene.35

Scheme 6. An Outline of the Possible Mechanistic Pathway
CONCLUSION
In conclusion, we have discovered that carboxylic acids can be converted to nitroarenes by an ipso-
nitration reaction with Bi(NO
3
)
3
/K
2
S
2
8
O . Both aromatic and hetero aromatic carboxylic acids were
converted to the nitroarenes under an acid-free condition. Although low to moderate yields are
obtained at present, such a cost-effective method is expected to trigger systematic development of
decarboxylative nitration reactions. From initial understanding, a radical pathway seemed to be
operative. Further improvement of the decarboxylative nitration methodology, synthetic
application and mechanistic details are the subject of our present research.
EXPERIMENTAL SECTION
General Procedure A for Decarboxylative Nitration of Aryl Carboxylic acid:

An oven dried resealable screw cap standard reaction tube containing a magnetic stir bar was
charged with potassium persulfate (1.75 mmol, 472.5 mg), bismuth nitrate (1.0 mmol, 486mg).
Then aryl carboxylic (0.5 mmol) was introduced in this mixture followed by acetonitrile (3 mL)
was added in it. The tube was placed in a preheated oil bath at 130 °C and the reaction mixture
was stirred vigorously for 24 h in air atmosphere. The reaction mixture was cooled to room
temperature, diluted with 2 mL ethyl acetate and filtered through celite, eluting with additional 10
mL of ethyl acetate. The filtrate was concentrated and the resulting residue was purified by column
chromatography.
Nitrobenzene: (Scheme 4, entry 2a) (liquid) Isolated yield=18%. Recovered Starting
Material (R.S.M) 68%: 1H NMR (400 MHz, Chloroform-d) δ: 7.52 (tt, J = 7.7, 7.7, 1.4, 1.4 Hz,
2
1
H), 7.68 (td, J = 7.5, 7.4, 1.2 Hz, 1H), 8.16 – 8.21 (m, 2H). 13C NMR (101 MHz, CDCl3) δ:
23.49, 129.39, 134.72, 148.18. GCMS (m/z): 123.0[M]+
1
-nitronaphthalene :(Scheme 4, entry 2b) (solid) Isolated yield=43% (R.S.M-39%): 1H NMR
(
(
400 MHz, Chloroform-d) δ: 7.53 (t, J = 8.0, 8.0 Hz, 1H), 7.61 (ddd, J = 8.1, 6.9, 1.2 Hz, 1H), 7.71
ddd, J = 8.6, 6.9, 1.4 Hz, 1H), 7.95 (ddt, J = 8.2, 1.3, 0.6, 0.6 Hz, 1H), 8.11 (dd, J = 8.2, 1.1 Hz,
1
H), 8.22 (dd, J = 7.6, 1.2 Hz, 1H), 8.56 (dq, J = 8.8, 0.9, 0.9, 0.9 Hz, 1H). 13C NMR (101 MHz,
CDCl3) δ: 123.26, 124.19, 124.30, 125.27, 127.52, 128.78, 129.62, 134.49, 134.85, 146.72.
GCMS (m/z): 173.1[M]+
1
1
2
-fluoro-4-nitronaphthalene: (Scheme 4, entry 2c) (solid) Isolated yield=41% (R.S.M-36%):
H NMR (400 MHz, Chloroform-d) δ: 7.22 (td, J = 8.9, 8.9, 2.6 Hz, 1H), 7.71 (dddd, J = 8.2, 6.9,
.7, 1.1 Hz, 1H), 7.81 (dddd, J = 8.6, 6.9, 2.7, 1.4 Hz, 1H), 8.23 (dt, J = 8.4, 1.6, 1.6 Hz, 1H), 8.31

–
1
2
1
8
8.35 (m, 1H), 8.70 (ddt, J = 9.0, 2.1, 1.0, 1.0 Hz, 1H). 13C NMR (101 MHz, CDCl3) δ: 108.39,
08.62, 121.48, 123.80, 125.81, 125.92, 127.96, 130.79, 160.86, 163.48. GCMS (m/z): 191.1[M]+
-bromo-6-nitronaphthalene: (Scheme 4, entry 2d) (solid) Isolated yield=35% (R.S.M-37%):
H NMR (400 MHz, Chloroform-d) δ: 7.72 – 7.76 (m, 1H), 7.90 (t, J = 9.6, 9.6 Hz, 2H), 8.10 –
.16 (m, 1H), 8.28 (dd, J = 9.0, 2.3 Hz, 1H), 8.77 – 8.80 (m, 1H). 13C NMR (101 MHz, CDCl3)
δ: 120.71, 124.65, 124.82, 128.85, 130.44, 130.62, 131.60, 131.79, 136.87, 145.86. GCMS (m/z):
2
52.0[M]+
2
-nitronaphthalene: (Scheme 4, entry 2e) (solid) Isolated yield=30% (R.S.M-33%):1H NMR
(
(
400 MHz, Chloroform-d) δ: 7.67 (dddd, J = 21.7, 8.2, 6.9, 1.3 Hz, 2H), 7.92 – 7.99 (m, 2H), 8.03
ddt, J = 8.0, 1.3, 0.7, 0.7 Hz, 1H), 8.24 (dd, J = 9.0, 2.3 Hz, 1H), 8.80 (dd, J = 2.3, 0.7 Hz, 1H).
1
1
9
3C NMR (101 MHz, CDCl3) δ: 119.46, 124.84, 128.11, 128.19, 129.74, 129.93, 130.19, 132.13,
36.02, 145.68. GCMS (m/z): 173.0[M]+
,10-dinitroanthracene : (Scheme 4, entry 2f) (solid) Isolated yield=32% (R.S.M-35%): 1H
NMR (400 MHz, Chloroform-d) δ: 7.81 (dd, J = 5.8, 3.3 Hz, 4H), 8.29 – 8.36 (m, 4H). 13C NMR
101 MHz, CDCl3) δ: 127.44, 133.73, 134.33, 183.36. GCMS (m/z): 268.2[M]+
-nitropyridine : (Schem 5, entry 4a) Isolated yield=22% (R.S.M-56%):1H NMR (400 MHz,
Chloroform-d) δ: 7.55 (ddd, J = 8.4, 4.8, 0.8 Hz, 1H), 8.51 (ddd, J = 8.4, 2.7, 1.5 Hz, 1H), 8.93
dd, J = 4.8, 1.5 Hz, 1H), 9.47 (d, J = 2.8 Hz, 1H). 13C NMR (101 MHz, CDCl3) δ: 124.03,
(
3
(
1
31.27, 145.38, 146.26, 155.08. GCMS (m/z): 124.0[M]+
2
-nitropyridine :(Scheme 5, entry 4b) Isolated yield=60% (R.S.M-23%): 1H NMR (400
MHz, Chloroform-d) δ: 7.70 (ddd, J = 7.5, 4.7, 1.0 Hz, 1H), 8.07 (ddd, J = 8.1, 7.5, 1.8 Hz, 1H),
.28 (dt, J = 8.2, 0.9, 0.9 Hz, 1H), 8.68 (ddd, J = 4.6, 1.9, 0.8 Hz, 1H). 13C NMR (101 MHz,
CDCl3) δ: 118.22, 129.36, 140.12, 149.22. GCMS (m/z): 124.1[M]+
8

5
-bromo-2-nitropyridine :(Scheme 5, entry 4c) Isolated yield=32% (R.S.M-39%): 1H NMR
400 MHz, Chloroform-d) δ: 8.18 (t, J = 1.3, 1.3 Hz, 2H), 8.71 (q, J = 1.4, 1.4, 1.4 Hz, 1H). 13C
NMR (101 MHz, CDCl3) δ: 119.56, 127.27, 142.60, 150.36. GCMS (m/z): 202.9[M]+
-chloro-6-nitropyridine :(Scheme 5, entry 4d) Isolated yield=30% (R.S.M-44%): 1H NMR
400 MHz, Chloroform-d) δ: 7.48 (dd, J = 8.0, 0.9 Hz, 1H), 7.83 (t, J = 7.8, 7.8 Hz, 1H), 8.12 (dd,
(
2
(
J = 7.6, 0.9 Hz, 1H). 13C NMR (101 MHz, CDCl3) δ: 121.37, 127.60, 140.19, 150.04, 150.39.
GCMS (m/z): 158.5[M]+
4
-methoxy-2-nitroquinoline :(Scheme 5, entry 4f) Isolated yield=11% (R.S.M-63%): 1H
NMR (400 MHz, Chloroform-d) δ: 4.15 (s, 3H), 7.63 (ddd, J = 8.3, 6.9, 1.2 Hz, 1H), 7.68 (s, 1H),
.79 (ddd, J = 8.5, 6.9, 1.5 Hz, 1H), 8.07 (dt, J = 8.5, 0.9, 0.9 Hz, 1H), 8.23 – 8.27 (m, 1H). 13C
NMR (101 MHz, CDCl3) δ: 56.50, 97.85, 122.27, 122.79, 128.11, 129.68, 131.07, 147.50, 164.31,
72.30. GCMS (m/z): 204.1[M]+
7
1
ASSOCIATED CONTENTS
Keywords
Nitration; nitro radical; decarboxylation
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This activity is supported by DRDO, India. Financial support received from CSIR-India (senior
research fellowships to S.A. and S.M.) is gratefully acknowledged.
Supporting Information

1
13
Figures giving H and C NMR of all the compounds and details of the optimization study is
available in the supporting information. Supplementary data to this article can be found online at
https://doi.org/10.xxxx/j.poly.2019.xxx
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