Oxidative organophotoredox catalysis: A regioselective synthesis of 2-nitro substituted imidazopyridines and 3-substituted indoles, initiated by visible light

Article abstract of DOI:10.1039/c6nj02365g

We have established a mild, metal-free, one pot, visible light-catalyzed procedure for a highly regioselective synthesis of 2-nitro-3-arylimidazo [1,2-a] pyridines via nitroalkene and 2-aminopyridine under an open atmosphere involving a photoredox catalys

Full text of DOI:10.1039/c6nj02365g

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Oxidative organophotoredox catalysis: a
regioselective synthesis of 2-nitro substituted
imidazopyridines and 3-substituted indoles,
initiated by visible light†
Cite this:DOI: 10.1039/c6nj02365g
Snehlata Yadav,a Madhulika Srivastava,a Pratibha Rai,a Bhartendu Pati Tripathi,a
Anu Mishra,a Jaya Singhb and Jagdamba Singh*a
We have established a mild, metal-free, one pot, visible light-catalyzed procedure for a highly regio-
selective synthesis of 2-nitro-3-arylimidazo [1,2-a] pyridines via nitroalkene and 2-aminopyridine under
an open atmosphere involving a photoredox catalyst, Eosin Y, which is an inexpensive organic dye that
has been employed as a photo sensitizer in the conversion. This protocol serves as an example of green
chemistry, due to the fact that molecular oxygen and visible light have been utilized effectively for the
transformation. The procedure involves intramolecular C–N heterocyclization, followed by aerobic
oxidation and C–C bond formation at room temperature. This green protocol has also been successfully
extended to the regioselective synthesis of 3-substituted indoles via indole and nitroalkene by a free
radical pathway.
Received (in Montpellier, France)
28th July 2016,
Accepted 30th September 2016
DOI: 10.1039/c6nj02365g
www.rsc.org/njc
Nitroalkenes are key intermediates in the synthesis of various
products, for example pharmacologically active compounds,8
Introduction
A desperate demand for highly eco-efficient and feasible synthetic fungicides9 and insecticides.10 This wide utility is due to the
protocols can be regarded as the impetus for several novel electron-deficiency of nitroalkenes and the easy conversion of
discoveries, since it provokes innovative reimagining of published the nitro part into different functional groups.11 Our interest in
approaches, which can in turn lead to the genesis of new the synthesis of nitroalkenes was drawn by their important role
chemistry.1 Currently, visible light photoredox catalysts (VLPC) in medicinal chemistry and synthetic chemistry.12 We have put
have emerged as new aids for heterocyclic synthesis2 due to the our best efforts to propose two highly regioselective synthetic
fact that visible light is an eco-friendly and unending resource routes; one leading to the formation of 2-nitroimidazopyridines
that is easy to handle and its use promotes an elevation of green via nitroalkenes and 2-aminopyridine and the other producing
standards for chemical transformations.3 The research company 3-substituted indoles via indoles and nitroalkenes. Several
of Stephenson et al., MacMillan et al. and Yoon et al. described bioactive compounds, natural products and biomolecules having
the use of photo catalysts in effectively promoting chemical fused heterocycles form an important key structural unit in
transformations via reductive or oxidative quenching.4 The agrochemical and pharmaceutical molecules.13 Among them,
visible light photoredox catalyst is particularly attractive because imidazopyridine, an elite class of nonbenzodiazepine,14 is one
it can activate atmospheric oxygen, a green and natural oxidizing of the crucial nuclei having broad applications in material science
agent.5 In contrast, transition metal catalysts suffer from many and bioscience.15 Both 2-arylimidazopyridine and 3-arylimidazo-
drawbacks, like adverse inherent malignancy, high cost and low pyridine moieties exhibit activities like being antimicrobial, anti-
durability.6 On the other hand, some organic dyes are eco-efficient, viral, fungicidal, etc.16 In particular, 3-arylimidazopyridines exhibit
easily handled and find vast applications in visible light-catalyzed many biological activities, some of which are given in Fig. 1.17
reactions.7
Nitroalkenes, being highly electron deficient, act as strong
Michael acceptors in addition reactions with nucleophiles.18
For example, derivatives of 3-substituted indoles, which serve as
valuable ingredients for the construction of bioactive compounds,
can be synthesized by the conjugate addition of indoles (good
Michael donors) to nitroalkenes (good Michael acceptors). Several
bioactive natural products, agrochemicals and drugs can be
a Environmentally Benign Synthesis Lab, Department of Chemistry,
University of Allahabad, Allahabad-211002, India. E-mail: dr.jdsau@gmail.com;
Tel: +919415218507
b Department of Chemistry, LRPG College, Sahibabad, Uttar Pradesh, India.
E-mail: nimesh.singh@in.abb.com
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6nj02365g synthesized via the alkylated indoles.19 For example, Panobinostat20a
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Fig. 1 Some biologically active heterocycles containing 3-aryl substituted imidazopyridines and 3-substituted indoles.
Scheme 1 Visible light catalyzed synthesis of 3-aryl substituted imidazopyridine and 3-substituted indoles.
(LBH-589), a hydroxamic acid,20b is useful in a large number of ultrasound assisted methods,33 Bronsted acid,34 palladium(II)
anti-cancerous drugs and Umifenovir (Arbidol)20c is used in surfactant combined catalysts,35 silanediol,36 N-bromosuccin-
antiviral drugs (Fig. 1).20 There is a variety of synthetic protocols imide,37 hydrogen bond donor catalyst Feist’s acid,38
reported for the construction of both 3-aryl substituted imidazo- Montmorillonite K10,39 catalyst free,40 solvent free methods,41a
pyridines and 3-substituted indoles. While some routes mentioned HY zeolite under solvent free condition41b and Baker yeast cata-
in literature for the synthesis of 3-substituted imidazopyridines lyzed methods.42 Recently, various visible light-catalyzed reactions
are catalysed by transition metals, there are others that are
transition metal free methods. For instance, palladium-catalyzed
direct arylation,21 reaction between aryl chloride and imidazoles
catalyzed by palladium complexes, both with phosphines/NHC22
and without phosphines,23 from alkynyliodonium salts,24 oxime
esters with pyridine,25 catalytic C–H arylation using magnetically
recyclable Pd–Fe3O4 with aryl bromide,26 regioselective arylation
with aryl tosylates and mesylates in the presence of palladium-
phosphine complexes27 and Fridael Craft alkylation via nitro-
styrene and 2-aminopyridine catalysed by Fe(NO3)3Á9H2O.28
Similarly, several synthetic methods have been reported for
3-substituted indoles involving boric acid,29 metal halide
hydrates,30 t-butyl ammonium hydrogen sulfate,31 graphite oxide,32 Scheme 2 Visible light synthesis of 3-arylimidazopyridine.
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Table 1 Screening and control experimentsa
Table 3 Substrate scopea
Yieldb
(%)
Entry Nitroalkene (1) 2-Aminopyridine (2) Product (3)
Entry
Visible light
Eosin Y
Air
Time (h)
Yieldb (%)
1
78
1
2
3
4
5
6
7
8
+
+
À
+
+
+
+
+
+
À
+
+
+
+
+
+
+
+
+
5
8
10
7
9
8
78
Trace
Tracec
Trace
68d
À
+
+
N2
+
56e
2
3
75
69
10
12
Trace
n.d.f
a
Reaction conditions: 1a (1.0 mmol), 2a (1.0 mmol), CH3CN (5 ml),
green LEDs (2.5 W, l = 535 nm) irradiation under an open atmosphere
b
c
d
at r.t. Isolated yield of product. Reaction performed in dark. Reaction
performed in daylight. Reaction performed under compact florescent
light. Reaction was quenched with TEMPO (2.0 equiv.).
e
f
of indoles have been reported, like aerobic thiocynation using
nano photocatalyst,43 C–H arylation via diazonium salt, a-arylation
of a-amino carbonyl compounds44 and C-3 formylation reactions.45
There is a big concern regarding the reactions of nitrostyrenes
and 2-aminopyridines about the selectivity in formation of
3-arylimidazopyridine and 2-arylimidazopyridine. Despite the fact
that a huge number of methods46 have been reported for the syn-
thesis of 2-arylimidazopyridines, there are only a very few reported
methods for the synthesis of 3-arylimidazopyridine scaffolds.
4
5
75
67
Table 2 Optimization of solvent and catalyst amounta
6
76
7
8
9
74
Entrya
Eosin Y (mol%)
Solvent
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
2
2
2
2
2
2
3
1
CH3CN
THF
Toluene
DMF
DMSO
EtOAc
CH3CN
CH3CN
5.0
78
60
31
39
45
71
78
48
7.30
7.30
6.30
5.45
5.30
5.0
79
73
6.30
a
Reaction conditions: 1a (1.0 mmol), 2a (1.0 mmol), solvent (5 ml),
green LEDs (2.5 W, l = 535 nm) irradiation under an open atmosphere
b
at r.t. Isolated yield of product.
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Table 3 (continued)
any one of the above mentioned reaction conditions, a very small
amount of product was formed (Table 1, entries 2, 3 and 4).
The use of either a compact fluorescent lamp or daylight in place
of LEDs was found to be less effective (Table 1, entries 5 and 6).
Green LED light was therefore essential to effective catalysis of the
reaction by Eosin Y. Only a trace amount of product was obtained
under nitrogen atmosphere, thereby indicating that the presence
of oxygen is also essential for the reaction to be efficient (Table 1,
entry 7). Continuous experimentation led to the conclusion that all
of the initial reaction parameters, i.e., visible light, photocatalyst
and air, are necessary for successful synthesis of the desired
product. It is also noteworthy that when 2.0 equiv. of TEMPO,
a well known radical scavenger, was added under the ideal
reaction conditions the desired product scaffold was not formed
(Table 1, entry 8), clearly indicating that the reaction involved a
radical mechanistic pathway.48
Our next endeavour was to optimize the solvent and the
amount of catalyst. It was observed that acetonitrile was the
best among the tested solvents, providing the maximum yield
(Table 2, entry 1). The optimum loading of catalyst for the
reaction was 2 mol% (Table 2, entry 7). On decreasing the catalyst
loading from 2 mol% to 1 mol%, the yield was significantly
decreased (Table 2, entry 8). However, an increase in catalyst
loading did not result in an increase in yield.
Yieldb
(%)
Entry Nitroalkene (1) 2-Aminopyridine (2) Product (3)
10
73
75
11
12
76
Next, we established the scope and functional group affinity
under the optimized reaction conditions. A broad range of
substituted 2-aminopyridines and nitrostyrenes were studied
to observe the electronic effects of the substrates. Nitrostyrenes
having electron-donating and electron-withdrawing functional
groups did not affect the course of reaction and the desired
products were obtained in good yields. However, aliphatic
nitrostyrenes did not result in the target molecule.28 Furthermore,
we observed that 2-aminopyridines with methyl substitution in
different positions reacted smoothly with nitrostyrene and gave
good yields. Thus, the substituent position of 2-aminopyridine
had negligible effect on the chemical yield of the desired product
(Table 3).
a
Reaction conditions: 1 (1.0 mmol), 2 (1.0 mmol), CH3CN (5 ml), green
LEDs (2.5 W, l = 535 nm) irradiation under an open atmosphere at r.t.
b
for 5 h. Isolated yield of product.
Therefore, we have focused our efforts on development of a
protocol that can control the selectivity of groups on imidazo-
pyridine via reaction between 2-aminopyridine and nitroalkene.
In continuation of our previous work47 for the development of
green, metal free, facile and safe protocols, herein, we have
proposed a visible light-catalyzed synthesis of 2-nitroimidazo-
pyridines. In addition, we have extended our protocol by the
effective synthesis of 3-substituted indoles via nitroalkenes
(Scheme 1).
On the basis of our observation and literature survey28,44,47
the plausible mechanism of the reaction is presented in Scheme 4.
In the first step, the endocyclic nitrogen of 2-aminopyridine
(2) undergoes Michael addition with nitrostyrene (1) to give
Michael adduct A. Organophotoredox catalyst Eosin Y (EY) is
excited to its singlet state (1EY*) on absorption of visible light.
The singlet state further converts into its more stable triplet
Result and discussion
For our initial approach, nitrostyrene (1a) and 2-aminopyridine state (3EY*) via intersystem crossing and then undergoes single
(2a) were taken as model substrates; they were irradiated electron transfer (SET) forming radical cation (B). The in situ
together under green LEDs at room temperature in acetonitrile formation of superoxide radical anion (O2)ꢀÀ and its combination
solution and in the presence of 2 mol% Eosin Y under with two hydrogens of B results in the formation of intermediate
open atmosphere. Interestingly, the reaction did not give C, along with the removal of H2O2. Furthermore, another
the expected 2-arylimidazopyridine as the major product. equivalent of Eosin Y undergoes SET and gives radical cation
Instead, 3-arylimidazopyridine (3a) was obtained in 78% yield D. This radical cation D undergoes intramolecular cyclization,
(Scheme 2).
followed by oxidation, which leads to formation of the desired
This remarkable regioselectivity mediated by visible light product 3 by removal of H2O2. Formation of hydrogen peroxide
and Eosin Y inspired us to carry out a batch of control experiments was confirmed using KI/starch indicator.49 It was further proven
to increase the yield of the 3-substituted product and decrease the experimentally that the reaction was not quenched in the
overall reaction time. It was observed that in the absence of presence of DABCO (2 mol%), which indicated that the triplet
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Scheme 3 Plausible photocatalytic pathway for synthesis of 3-aryl substituted imidazopyridine.
Scheme 4 Photocatalytic approach for synthesis of 3-substituted indole.
oxygen is utilized in the reaction mechanism.50 Presence of best solvent for our protocol (Table 4, entry 5). We also observed
triplet oxygen is a characteristic feature of a radical reaction that only a very trace amount of product was formed in the
(Scheme 3).
absence of light (Table 4, entries 9 and 10), which again
We further augmented the scope of our protocol by carrying indicates that light is essential for our protocol. A very trace
out Friedel Craft alkylation of the nitroalkene derivatives with amount of product was obtained in the presence of TEMPO,
indole. We irradiated nitrostyrene (1, 1 mmol) and indole (4, showing that the reaction undergoes radical pathways (Table 4,
1 mmol) in ethanol with visible light, and a regioselective entry 11).
Michael type addition took place, resulting in 3-substituted
indole 5 in excellent yield (Scheme 4).
Inspired by these results, we explored the generality of
our protocol with respect to various substituted derivatives
We performed several experiments to ensure the best reac- of indoles and nitroalkenes (Table 5). Several products
tion conditions (Table 4). It was observed that ethanol was the were obtained with substituents at the 3-position of the
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Table 4 Optimization of reaction conditionsa
Table 5 (continued)
Entry
Reaction conditions
Yieldb (%)
Yieldb
(%)
1
2
DMF
DMSO
Trace
45
Entry Nitroalkene (1)
Indole (4)
Product (5)
3
4
5
CH3CN
THF
EtOH
60
Trace
75
6
7
8
MeOH
70
85
80
Trace
72
6
84
81
78
65
LEDs, EtOH
CFL, EtOH
EtOH
EtOH
LEDs, EtOH
9c
10d
11e
Trace
a
7
8
9
Reaction conditions: 4a (1.0 mmol), 1a (1 mmol), solvent (5 ml), green
LEDs (2.5 W, l = 535 nm) irradiation under an open atmosphere at r.t. for
2 h. b Isolated yield of product. Reaction performed in dark. Reaction
c
d
e
performed in daylight. Reaction was quenched with TEMPO (2.0 equiv.).
Table 5 Substrate Scopea
Yieldb
(%)
Entry Nitroalkene (1)
Indole (4)
Product (5)
10
48
1
85
a
Reaction conditions: 4 (1.0 mmol), 1 (1 mmol), ethanol (5 ml), green
LEDs (2.5 W, l = 535 nm) irradiation under an open atmosphere at r.t.
b
for 2 h. Isolated yield of product.
2
3
82
80
indole nucleus, showing that Michael addition took place
regioselectively.
Further to our extended protocol, when indole (4) reacted
with nitroalkene (1), it gave Michael type adduct (E) through
a free radical pathway. This intermediate E undergoes a
[1, 3] hydrogen shift and gives the desired product 5. The
(Scheme 5).
To conclude, we have developed an eco-friendly, efficient,
metal free, visible light-catalyzed, aerobic, oxidative cyclization
of 2-aminopyridine with nitroalkene, as well as a Friedel Craft
alkylation of indole with nitroalkene for regioselective construction
of the 2-nitroimidazopyridine nucleus. Eosin Y was used as a
photoredox catalyst and atmospheric oxygen as an oxidant. This
protocol tolerates a broad range of functional groups. The
proposed method will undoubtedly prove to be a superior
choice as compared to the previously reported methods due to
its green reagents, feasibility and high efficiency.
4
5
78
82
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Scheme 5 Plausible photocatalytic pathway for synthesis of 3-substituted
indoles.
Acknowledgements
We sincerely thank SAIF, Punjab University, Chandigarh, for
providing micro-analyses and spectra. Authors S. Yadav and
B. P. Tripathi are thankful to UGC, New Delhi for Research
fellowships. Authors M. Srivastava and P. Rai are thankful to
UGC, New Delhi, for the award of Senior Research Fellowship
(SRF). Author A. Mishra is thankful to UGC, New Delhi, for the
award of Junior Research Fellowship (JRF).
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