Unprecedented catalytic activity of Fe(NO3)3·9H2O: Regioselective synthesis of 2-nitroimidazopyridines via oxidative amination

Article abstract of DOI:10.1021/ol502218u

A unique iron-catalyzed oxidative diamination of nitroalkene with 2-aminopyridine for the synthesis of 2-nitro-3-arylimidazo[1,2-a]pyridines with complete regioselectivity has been achieved under mild and aerobic reaction conditions. This is the first method for the synthesis of 2-nitroimidazo[1,2-a]pyridines. These scaffolds were also synthesized directly from styrenes.

Full text of DOI:10.1021/ol502218u

Letter
pubs.acs.org/OrgLett
Unprecedented Catalytic Activity of Fe(NO ) ·9H O: Regioselective
3
3
2
Synthesis of 2‑Nitroimidazopyridines via Oxidative Amination
Kamarul Monir, Avik Kumar Bagdi, Monoranjan Ghosh, and Alakananda Hajra*
†Department of Chemistry, Visva-Bharati (A Central University), Santiniketan 731235, India
*
S Supporting Information
ABSTRACT: A unique iron-catalyzed oxidative diamination of nitroalkene with 2-aminopyridine for the synthesis of 2-nitro-3-
arylimidazo[1,2-a]pyridines with complete regioselectivity has been achieved under mild and aerobic reaction conditions. This is
the first method for the synthesis of 2-nitroimidazo[1,2-a]pyridines. These scaffolds were also synthesized directly from styrenes.
midazopyridine, an important class of nitrogen containing
heterocycles, shows a wide range of biological activities such
I
as antitumor, antiparasitic, antiviral, antimicrobial, fungicidal,
1
anti-inflammatory, hypnotic, etc. These derivatives are also
GABA and benzodiazepine receptor agonists, β-amyloid
2
formation inhibitors, and cardiotonic agents. In addition, this
motif is the core structure of some marketed drugs such as
necopidem, saripidem, zolimidine, olprinone, zolpidem, and
3
alpidem. Furthermore, a few of them exhibit excited-state
4
intramolecular proton transfer.
Accordingly, a variety of synthetic strategies have been
developed for the construction of imidazo[1,2-a]pyridine
scaffolds. These methods include three-component coupling
Figure 1. Regioselectivity in synthesis of imidazo[1,2-a]pyridine
scaffolds.
5
6
of 2-aminopyridine, aldehyde, and alkyne /isonitrile /nitro-
7
8
alkane, condensation of aminopyridine and α-haloketone, and
9
10
oxidative coupling of 2-aminopyridine with ketone / alkyne /
diketone /chalcone. Additionally intramolecular aminoox-
ygenation and intramolecular C−H amination are also
important strategies for the synthesis of these derivatives.
Furthermore, the coupling between 2-aminopyridine and
development of new methods for the synthesis of this class of
compounds with a variety of substituents at the 2 and 3
positions. Nitroimidazopyridines are important derivatives and
are generally used as a key intermediate to synthesize polyfused
1
1
12
1
3
14
17
imidazopyridine derivatives. Recently nitroalkenes have been
used as a coupling partner with 2-aminopyridines for the
1
5,16
nitroolefins
is an attractive way to synthesize these
15
scaffolds. It is interesting to note that in most of the cases
the 2-aminopyridine has been employed as the starting material
due to its commercial availability and binucleophilic nature. In
this context, the reaction of 2-aminopyridine proceeds through
initial nucleophilic addition in two ways which controls the
regioselectivity: by either the exocyclic amino group (Path a,
Figure 1) or endocyclic pyridinium nitrogen (Path b, Figure 1).
Generally in the presence of metal catalysts the exocyclic N-
atom of the 2-aminopyridine moiety first reacts with the
coupling partner. As a consequence the 3-functionalized
imidazopyridine moieties are obtained in most of the cases.
However, the methodologies for the synthesis of 2-function-
synthesis of 3-nitroimidazopyridines. However, there is no
such method for the synthesis of the 2-nitroimidazopyridines.
Therefore, we became interested in whether we can change the
substituent selectivity on the imidazo[1,2-a]pyridine moiety by
the reaction between 2-aminopyridine and nitroalkene under
suitable reaction conditions.
In recent times much attention has been drawn to the
exploration of the catalytic properties of the iron salts in organic
transformations from the aspect of pure science as well as
18
sustainable chemistry. Iron is the second most abundant
metal in the earth; in addition, it is inexpensive, safe, and
19
environmentally benign. Various types of reactions, such as
oxidation, Michael addition, hydrosilylation, hydrogenation,
8
c,d,10b,11
alized imidazopyridines are limited.
The pharmacological activity of imidazopyridine derivatives
is shown to be dependent on the nature of substituents at
different positions. There is continuous effort toward the
Received: July 28, 2014
Published: August 20, 2014
©
2014 American Chemical Society
4630
dx.doi.org/10.1021/ol502218u | Org. Lett. 2014, 16, 4630−4633

Organic Letters
Letter
cross-coupling reaction, cycloaddition, isomerization, rearrange-
ment, etc. have been efficiently catalyzed by the iron catalysts.
catalyst in 1,2-dichlorobenzene (1,2-DCB) solvent at 80 °C
under aerobic reaction conditions (Table 1, entry 1). The
desired product was obtained only in 10% isolated yield after 6
h, and no improvement of the yield was observed even after 12
h. Inspired by this result, the effects of different solvents such as
1,2-dichloroethane (1,2-DCE), DMSO, DMF, and acetonitrile
were tested, and the best result was achieved in 1,2-DCE at 80
20
On the other hand, the direct C−H amination is a
challenging field of synthetic organic chemistry as there is no
21
need to prefunctionalize the starting materials. Transition
metals such as Pd-, Cu-, Rh-, and Ru-catalysts have been
22
extensively utilized for this purpose. Various terminal oxidants
are required to carry out these transformations among which
molecular oxygen is the most preferable one, as it only
°
C affording a 76% isolated yield (Table 1, entries 2−5). 1,2-
19d
DCE may act as both solvent and oxidant for this reaction.
23
produces water as the byproduct. However, the application of
iron salts for the direct C−H amination employing molecular
oxygen as the terminal oxidant has not been extensively
FeCl , FeBr , and Fe(OTf) were totally ineffective for this
3
3
3
transformation (Table 1, entries 6−8). Other metal salts such
as CuI, Cu(NO ) ·3H O, and AgNO also were not capable of
3
2
2
2
2
4
explored so far.
producing the desired product (Table 1, entries 9−11). The
effect of the catalyst loading was also screened. No improve-
ment in the yield was observed by increasing the catalyst
loading whereas decreasing the catalyst loading decreased the
yield (Table 1, entries 12−13). No substantial increase in the
yield was observed by increasing the temperature, and a lower
yield was obtained at 60 °C even after 10 h (Table 1, entries
In continuation of our research to develop newer method-
ologies for the construction of functionalized imidazo[1,2-
9
a,12,16
a]pyridine derivatives,
herein we are demonstrating a
unique Fe(NO ) ·9H O-catalyzed one-step synthesis of 2-
3
3
2
nitro-3-arylimidazo[1,2-a]pyridine derivatives with rare selec-
tivity via C−H amination in ambient air (Scheme 1).
1
4−15). No product was formed in the absence of any catalyst,
Scheme 1. Fe(NO ) ·9H O-Catalyzed Synthesis of 2-Nitro-
3
3
2
but only the Michael product was obtained through the
addition of endocyclic pyridinium nitrogen. Finally, the
optimized reaction conditions were obtained using 20 mol %
Fe(NO ) ·9H O in 1,2-DCE at 80 °C for 6 h under aerobic
3
-arylimidazo[1,2-a]pyridines
3
3
2
conditions (Table 1, entry 3).
With optimized reaction conditions in hand, we then turned
our attention toward the scope of the reaction, and the results
are shown in Scheme 2. A wide range of substituted
nitroalkenes and aminopyridines were subjected to proof the
general applicability of our present procedure. 2-Amino-
pyridines substituted with a methyl group at different positions
First 2-aminopyridine and (E)-1-(2-nitrovinyl)benzene were
selected as the model substrates to find suitable reaction
conditions by using various metal catalysts and solvents. The
results are summarized in Table 1. Initially the reaction was
a
,b
carried out employing 20 mol % Fe(NO ) ·9H O as the
3
3
2
Scheme 2. Substrate Scopes
a
Table 1. Optimization of the Reaction Conditions
b
entry
catalyst (mol %)
solvent
yield (%)
1
2
3
4
5
6
7
8
9
Fe(NO ) ·9H O (20)
1,2-DCB
DMF
10
3
3
2
Fe(NO ) ·9H O (20)
ND
76
3
3
2
Fe(NO ) ·9H O (20)
1,2-DCE
DMSO
3
3
2
Fe(NO ) ·9H O (20)
<10
<10
ND
ND
ND
ND
ND
3
3
2
Fe(NO ) ·9H O (20)
CH CN
3
3
2
3
FeCl (20)
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
3
FeBr (20)
3
Fe(OTf) (20)
3
CuI (20)
10
11
12
13
14
15
16
Cu(NO ) ·3H O (20)
3
2
2
c
AgNO (20)
ND
2
Fe(NO ) ·9H O (10)
51
74
3
3
2
Fe(NO ) ·9H O (30)
3
3
2
d
Fe(NO ) ·9H O (20)
74
3
3
2
e
Fe(NO ) ·9H O (20)
64
3
3
2
c
−
ND
a
Reaction conditions: 1 mmol of 1a and 1 mmol of 2a in the presence
b
of catalyst in solvent (2 mL) at 80 °C for 6 h. Isolated yields. ND:
Not detected in TLC. Michael adduct was obtained. Reaction
carried out at 100 °C. Reaction carried out at 60 °C for 10 h.
c
d
a
Reaction conditions: 1 mmol of 1 and 1 mmol of 2 in the presence of
e
b
Fe(NO ) ·9H O in 1,2-DCE at 80 °C for 6 h. Isolated yield.
3
3
2
4
631
dx.doi.org/10.1021/ol502218u | Org. Lett. 2014, 16, 4630−4633

Organic Letters
Letter
efficiently react with nitroalkenes to afford the corresponding
products with good yields (3ba, 3ca, 3da). The position of the
substituent on 2-aminopyridines almost had no influence on
the chemical yields. 2-Aminopyridines with a methyl sub-
stituent at the 3, 4, and 6 positions afforded the products with
comparable yields under the present reaction conditions.
Nitroalkenes bearing several functionalities such as −Me,
product was obtained in 76% yield (Scheme 4, eq B). This
result indicates that probably 1,2-dichloroethane acts as the
solvent as well as the oxidant to complete the catalytic cycle for
19d
this reaction.
The reaction was also carried out in the
presence of 1.5 equiv of TEMPO under the same reaction
conditions; no desired product was observed which favors the
formation of imidazo[1,2-a]pyridine through the radical
mechanistic pathway (Scheme 4, eq C).
A plausible mechanism of the reaction is represented in
Scheme 5. The first step is the Michael addition of 2-
−
SMe, −Cl, and even −F are unaffected under the present
reaction conditions (3ab, 3cb, 3ac, 3cc, 3dc, 3ad, 3af, 3ag,
cg). Acid sensitive groups such as −COOMe and oxy-
3
methylene containing nitroalkenes were also successfully
employed to produce the desired products (3ae and 3ah)
with good yields. Moreover, heteroaryl nitroalkene gave the
corrosponding product with good yield (3ai). However,
aliphatic nitroalkenes such as 3-methyl-1-nitrobut-1-ene and
Scheme 5. Probable Mechanism
(
2-nitrovinyl)cyclohexane did not afford the desired products.
To make our present methodology more convenient and
practical, we started the reaction from styrene by the in situ
formation of nitrostyrene employing a recent methodology by
2
0c
Maiti et al. Comparable yields were obtained in this one-pot
procedure also (Scheme 3). Furthermore, 2-nitro-3-phenyl-
imidazopyridine (3aa) was easily converted to a 2-amino
derivative (4aa) by simple reduction (see Supporting
Information).
aminopyridine to the nitroalkene by the endocyclic nitrogen
which forms the Michael adduct (A). Subsequently oxidation
occurred by Fe(NO ) through SET which was followed by the
3
3
Scheme 3. One-Pot Synthesis of Imidazopyridines from
Styrenes
intramolecular cyclization to form the intermediate F. The
intermediate F afforded the desired product (3aa) through
deprotonation.
It is noteworthy to mention that the isolated Michael adduct
(
(
A) did not undergo cyclization in the presence of CuBr
15
Scheme 6). Other Fe-salts except Fe(NO ) ·9H O are not
3
3
2
also effective for this oxidative amination. This indicates that
Fe(NO ) ·9H O plays a unique role for this cyclization.
3
3
2
Scheme 6. Specific of Role of Fe(NO ) in Oxidative
Cyclization
3
3
To prove the mechanistic pathway, we studied the reaction
under different reaction conditions. Initially the reaction was
carried out without any catalyst under optimized reaction
conditions, and no desired product was obtained; only the
Michael product (A) was isolated even after 18 h in 48% yield
(
Scheme 4, eq A). The same Michael adduct was also obtained
Scheme 4. Additional Experiments
In summary, we have developed an Fe(NO ) ·9H O
3
3
2
catalyzed oxidative amination of nitroalkenes with 2-amino-
pyridines for the regioselective synthesis of 2-nitro-3-aryl-
imidazo[1,2-a]pyridine derivatives. To the best of our knowl-
edge this is the first report on the synthesis of 2-nitroimidazo-
[1,2-a]pyridines. The reaction proceeds through regioselective
Michael addition following a unique Fe(NO ) ·9H O catalyzed
3
3
2
oxidative intramolecular amination. These derivatives were
directly synthesized from styrenes also employing this strategy.
Easily available basic chemicals as the starting materials, a less
expensive metal catalyst, aerobic reaction conditions, tolerance
of a wide range of functional groups, and operational simplicity
are the notable advantages of this present protocol. These
advantages render this protocol facile and suitable to create a
diversified library of 2-nitro-3-arylimidazo[1,2-a]pyridine de-
rivatives.
in the presence of Fe(NO ) ·9H O within 2 h under the
3
3
2
optimized reaction conditions. Now to demonstrate the
oxidative coupling we applied the optimized reaction conditions
to the Michael product, and the corresponding imidazo[1,2-
a]pyridine was obtained in quantitative yield. When the
reaction between 1a and 2a was carried out with 20 mol %
Fe(NO ) ·9H O under an argon atmosphere, the desired
3
3
2
4
632
dx.doi.org/10.1021/ol502218u | Org. Lett. 2014, 16, 4630−4633

Organic Letters
Letter
(
14) Wang, H.; Wang, Y.; Peng, C.; Zhang, J.; Zhu, Q. J. Am. Chem.
ASSOCIATED CONTENT
Supporting Information
■
Soc. 2010, 132, 13217.
15) (a) Yan, R.-L.; Yan, H.; Ma, C.; Ren, Z.-Y.; Gao, X.-A.; Huang,
*
S
(
Additional data, spectral data of all compounds, and scanned
spectra of new compounds. This material is available free of
charge via the Internet at http://pubs.acs.org.
G.-S.; Liang, Y.-M. J. Org. Chem. 2012, 77, 2024. (b) Yan, H.; Yan, R.;
Yang, S.; Gao, X.; Wang, Y.; Huang, G.; Liang, Y. Chem.Asian J.
2012, 7, 2028. (c) Xie, M.-S.; Chu, Z.-L.; Niu, H.-Y.; Qu, G.-R.; Guo,
H.-M. J. Org. Chem. 2014, 79, 1093.
AUTHOR INFORMATION
Corresponding Author
(16) Santra, S.; Bagdi, A. K.; Majee, A.; Hajra, A. Adv. Synth. Catal.
■
*
2
(
013, 355, 1065.
17) (a) Desbois, N.; Chezal, J.-M.; Fauvelle, F.; Debouzy, J.-C.;
E-mail: alakananda.hajra@visva-bharati.ac.in.
Lartigue, C.; Gueiffier, A.; Blache, Y.; Moreau, E.; Madelmont, J.-C.;
Chavignon, O.; Teulade, J.-C. Heterocycles 2005, 65, 1121.
Notes
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b) Rahimizadeh, M.; Pordel, M.; Bakavoli, M.; Eshghi, H. Dyes
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The authors declare no competing financial interest.
(
ACKNOWLEDGMENTS
A.H. acknowledges the financial support from CSIR, New Delhi
Grant No. 02(0168)/13/EMR-II). K.M. and A.K.B. thank
■
(
Organic Chemistry; Plietker, B., Ed.; Wiley-VCH: Weinheim, Germany,
2008.
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