Iron promoted C3-H nitration of 2H-indazole: direct access to 3-nitro-2H-indazoles

Article abstract of DOI:10.1039/c8ob00931g

An efficient C3-H functionalization of indazole has been demonstrated. Notably, this method involves chelation-free radical C-H nitration on 2H-indazole. The radical mechanism was confirmed by control experiments and quantum chemical calculations. The synthetic utility has been proven by the synthesis of bio-relevant benzimidazoindazoles via reductive cyclization.

Full text of DOI:10.1039/c8ob00931g

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Iron promoted C3–H nitration of 2H-indazole:
direct access to 3-nitro-2H-indazoles†
Cite this: DOI: 10.1039/c8ob00931g
Received 20th April 2018,
Accepted 29th June 2018
Arumugavel Murugan, Koteswar Rao Gorantla, Bhabani S. Mallik * and
Duddu S. Sharada
*
DOI: 10.1039/c8ob00931g
rsc.li/obc
An efficient C3–H functionalization of indazole has been demon- them, C–H functionalization of heteroarenes via a radical
strated. Notably, this method involves chelation-free radical C–H pathway is one of the most appealing and straightforward
nitration on 2H-indazole. The radical mechanism was confirmed strategies.7
by control experiments and quantum chemical calculations. The
Heteroaromatic nitro compounds are of great significance
synthetic utility has been proven by the synthesis of bio-relevant due to their importance as key precursors in organic synthesis8
benzimidazoindazoles via reductive cyclization.
and their potential for further transformations. In recent
years, several chemists have achieved a transition-metal-cata-
lyzed radical C–H nitration on arenes/heteroarenes by using
various nitro sources,9 such as AgNO2,9a–d tBuONO,9e,f
Cu(NO3)2
and Fe(NO3)3·9H2O.11d Among all metal nitrates,
Introduction
9h,i
Fe(NO3)3·9H2O as a non-toxic, inexpensive and green reagent
is well known for various radical nitration strategies such as
nitration on olefins,10a–c allenes10d and imines.10e However, its
application in aromatic nitration is rare, except for a recent
nitration on 8-aminoquinoline.11d Hence, there is a broad
scope for the development of strategies for radical nitration of
arenes and heteroarenes (Scheme 1).11
As an important class of heteroarenes, indazole motifs are
embedded in pharmaceuticals with a broad range of biological
activities,12,13 including antitumor,14 antimicrobial,15 anti-
inflammatory,16 and HIV-protease inhibition.17 Hence, there
have been extensive efforts directed towards the synthesis of
indazole derivatives. Due to the recent emergence of radical
C–H functionalization as a new paradigm in contemporary chem-
istry and as a part of our research program on C–H functionali-
zation and indazole chemistry,18 we were interested in develop-
ing radical C–H functionalization as a new approach for func-
Direct C–H functionalization has become a reliable and robust
method for various transformations,1 i.e. the carbon–hydrogen
bond to the carbon–carbon and carbon–heteroatom bonds to
construct complex molecules due to its high step- and atom-
economy.2 Recently, direct C–H functionalization via a radical
pathway has become a rapidly expanding area of research and
has emerged as a promising and sustainable approach towards
molecular construction often complementary to traditional
methods.3 Despite these developments selective C–H
functionalization via a radical pathway is still in its infancy
and controlling the reactivity and chemo/regioselectivity of
radical species for the selective C–H functionalization requires
efforts to find more mild and suitable methods which poses
many opportunities and challenges to address.4
Owing to the ubiquitous nature of heteroarenes in agro-
chemicals and pharmaceuticals,5 the direct C–H functionali-
zation of heteroarenes is a highly attractive strategy providing a
concise route for complex molecules as well as for late-stage
functionalization (LSF) of bioactive molecular scaffolds and
hence offers tremendous opportunities for chemists.6 Among
Department of Chemistry, Indian Institute of Technology Hyderabad, Kandi-502285,
Sangareddy, Telangana, India. E-mail: sharada@iith.ac.in; Fax: (+040) 2301 6032;
Tel: (+040) 2301 7058
†Electronic supplementary information (ESI) available. CCDC 1825398. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c8ob00931g
Scheme 1 Radical C–H nitration of heteroarenes.
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tionalized indazoles. Recently, Wu et al. have demonstrated (Table S2, ESI†). Interestingly, we have observed the formation
the regioselective C–H functionalization of 8-aminoquinoline of the product with very good yield in the case of DCE as a
through theoretical data calculations.7e Accordingly, we solvent (Table 1, entry 10). Notably, when we performed the
planned to support our findings on radical C–H functionali- reaction in open air, we observed the formation of the expected
zation through quantum chemical calculations. Herein, we are product with moderate yield (Table 1, entry 19). Subsequently,
delighted to disclose for the first time
a radical C–H we screened the reaction with different equivalents of nitro
functionalization of 2H-indazole through Fe(NO3)3·9H2O pro- sources and oxidants (Tables S1 & S3, ESI†). From these experi-
moted C3–H nitration.
ments we concluded that 2 equiv. of nitro source and 1 equiv.
of oxidant are necessary for the complete conversion of the
starting materials.
With the optimized conditions in hand, we next examined
the scope of this methodology with different substituted 2H-
Results and discussion
To achieve our goal, we initiated our preliminary experiments indazole systems (Table 2). The methylenedioxy substitution
with 2H-indazole (1a) as a model substrate and Fe(NO3)3·9H2O gave good yield when compared to halo substitution at the 5th,
as a nitro source in MeCN as a solvent at 80 °C under an 6th and 7th positions of 2H-indazoles 2c–g.
oxygen atmosphere (Table 1, entry 1). As expected, a C3-nitro
Next, we examined the scope of substrates with various sub-
functionalized product (2a) was obtained albeit in very poor stitutions (halogen, alkyl, alkoxy) on the amine partner of 2H-
yield. Inspired by this result, we screened various nitro sources indazoles resulting in desired products with moderate to good
(Table 1, entries 2–8). However, we did not observe any satis- yields. Having the benzyl group in place of aryl at the 2nd posi-
factory yields.
tion of 2H-indazoles 2w & x did not show any improvement in
Next, we turned to the use of the oxidant (2,2,6,6-tetra- the yield.
methylpiperidin-1-yl)oxidanyl (TEMPO) in the reaction.
Unfortunately, amine partner bearing electron withdrawing
Surprisingly, we observed the expected product in very good groups, such as nitro, nitrile and ester groups did not afford
yield in the presence of TEMPO (Table 1, entry 9). the desired products 2aa–ac. In addition, 2H-indazoles with
Furthermore, our attempts of replacing TEMPO with other oxi- alkyl substitution at the 2nd position of 2ad & ae could not
dants went in vain (Table S1, ESI†).
afford the nitration product. Furthermore, our attempts to
Furthermore we also screened the reaction with different carry out nitration on other heteroarenes (indole, imidazole,
solvents (Table 1, entries 10–18) and varying temperatures 1H-indazole) went in vain.
Based on the experiments performed for the optimization
of reaction conditions shown in Table 1, we observed that
other nitro sources failed to give any nitro product, hence in
order to prove the role of Fe(NO3)3·9H2O, we planned to
Table 1 Optimization of reaction conditions for the synthesis of 2aa
examine the reaction with metal free nitro sources, however, in
the presence of Fe/Cu as a promotor which resulted in good
yield (Scheme 2b), thus indicating the dual role of
Fe(NO3)3·9H2O. Furthermore, the reaction in the absence of
any promotor with metal-free nitrate (TBN) did not afford the
Entry Nitro source
Oxidant Solvent
Time (h) Yieldb (%)
desired product (Scheme 2c). Hence, the promotor is necessary
for the nitration of indazole at the C3 position. To prove the
radical pathway, we performed HR-MS analysis of the crude
reaction mixture, which showed 2,2,6,6-tetramethylpiperidin-1-
ol. To know the role of oxygen, we have conducted a couple of
control experiments. We obtained the desired product in 10%
and 15% yield when the reaction was carried out in the
absence of O2 and TEMPO respectively (Scheme 2e & d).
However, these experiments are not conclusive. When we per-
formed the reaction in the presence of an excess amount of
TEMPO (2 equiv.) under an argon atmosphere, 80% yield of
the desired product was obtained (Scheme 2f). This indicates
that the presence of oxygen reduces the amount of TEMPO sig-
nificantly (optimized conditions i.e. 1 equiv. of TEMPO under
an oxygen atmosphere). From these experiments, we can con-
clude that ‘O2’ might involve either in the recycling of TEMPO
(TEMPOH to TEMPO) or in the direct oxidation of intermedi-
ate B to intermediate C. Finally, to confirm the C3-functionali-
zation of 2H-indazole, we performed the reaction with C3 sub-
1
2
3
4
5
6
7
8
Fe(NO3)3·9H2O
Ni(NO3)2·6H2O
CAN
—
—
—
—
—
—
—
—
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
5
5
5
5
5
5
5
5
5
5
5
12
12
15
Trace
Trace
n.d.c
n.d.c
n.d.c
n.d.c
10
tBuONO
AgNO3
AgNO2
NaNO2
Cu(NO3)2·3H2O
Fe(NO3)3·9H2O
Fe(NO3)3·9H2O
Fe(NO3)3·9H2O
Fe(NO3)3·9H2O
Fe(NO3)3·9H2O
Fe(NO3)3·9H2O
Fe(NO3)3·9H2O
Fe(NO3)3·9H2O
Fe(NO3)3·9H2O
Fe(NO3)3·9H2O
Fe(NO3)3·9H2O
9
TEMPO MeCN
TEMPO DCE
TEMPO CHCl3
TEMPO DMF
TEMPO DMSO
TEMPO Dioxane 12
TEMPO Toluene 12
TEMPO H2O
TEMPO EtOH
TEMPO MeOH
TEMPO DCE
65
85
10
11
12
13
14
15
16
17
18
19
55
nrd
n.d.c
n.d.c
n.d.c
n.d.c
n.d.c
n.d.c
60.d
12
12
12
5
a Reaction conditions: 1a (1 mmol), nitro source (2 mmol), oxidant
(1 mmol), solvent (1 mL), oxygen balloon, 80 °C, 5–12 h. b Isolated
yield of chromatographically pure products. c Starting materials recov-
ered. d Reaction carried out in open air.
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Table 2 Substrate scope for the C3-nitration of 2H-indazolea,b
a Conditions: 1a (1 mmol), Fe(NO3)3·9H2O (2 mmol), TEMPO (1 mmol), DCE (2 mL), oxygen balloon, 80 °C, 5–8 h. b Isolated yield of chromatographi-
cally pure products.
stituted 2H-indazole under standard conditions (Scheme 2g), ity of a specific atom. Among various atoms, the largest pz
which did not afford any nitro substituted 2H-indazole. The orbital occupancy at the C3 carbon atom implicates that
X-ray crystallography analysis of compound 2t further supports C3 may be the most likely electrophilic reactive site. Thus the
the nitration at the C3 position.
theoretical calculations support the only C3–H nitration on
To further support our finding, we carried out quantum 2H-indazole.
chemical calculations7e to investigate the charge distribution
Based on the control experiments, literature reports7e,9a,11d
on indazole. The theoretical data of atoms (C3, C4, C6) on the and quantum chemical calculations, we have proposed a
Fe coordinated indazole intermediate A are shown in Table 2. plausible mechanism for the synthesis of 3-nitro-2-phenyl-2H-
The charges of the C3, C4 and C6 as calculated were found to indazole as depicted in Scheme 3. Initially, coordination of
be −0.025, −0.027 and −0.037 respectively (Table 3).
2-phenyl-2H-indazole with Fe(NO3)3·9H2O led to the formation
•
The charges include the contribution from all valance elec- of intermediate A. Meanwhile, the nitro radical (NO2 ) gener-
trons and based on the literature precedence7e the pz orbital ated from Fe(NO3)3·9H2O under thermal conditions19 would
occupancy would be a more effective way to assess the reactiv- react at the electrophilic reactive site of the intermediate A in
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Table 4 Synthesis of benzimidazoindazolesa,b
a Reaction conditions: 2a (0.1 mmol), P(OEt)3 (1 mL), 100 °C, 1 h.
b Isolated yield of chromatographically pure products.
Conclusions
In conclusion we have developed a novel protocol for the
radical C–H nitration of 2H-indazoles. This method offers che-
lation-free C–H nitration on 2H-indazole by using the in-
expensive and nontoxic Fe(NO3)3·9H2O under mild conditions.
Moreover, the mechanistic pathway was inferred on the basis
of control experiments and quantum chemical calculations.
The synthetic utility of nitroindazoles was demonstrated by the
synthesis of bio-relevant benzimidazoindazoles. This unique
radical C–H nitration of 2H-indazoles should open new
avenues for the C–H functionalization of 2H-indazoles and
studies in this direction are currently underway in our
laboratory.
Scheme 2 Control experiments.
Table 3 Charge distribution and pz orbital occupancy of the C3, C4
and C6 atoms in structure A
Atom
Hirshfeld
pz orbital occupancy
C3
C4
C6
−0.025
−0.037
−0.027
0.997
0.990
0.980
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We gratefully acknowledge the Department of Science and
Technology-Science and Engineering Research Board
(DST-SERB-EMR/2016/000952) New Delhi, India and the
Indian Institute of Technology Hyderabad (IITH) for financial
support. AVM and GKR thank the UGC, New Delhi, India for
the award of a research fellowship.
Scheme 3 Plausible mechanism.
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