One-pot synthesis of 3-hydroxy-2-oxindole-pyridine hybrids via Hantzsch ester formation, oxidative aromatization and sp3 C–H functionalization using FeWO4 nanoparticles as recyclable heterogeneous catalyst

Article abstract of DOI:10.1016/j.catcom.2017.09.027

Synthesis of poly-substituted 3-hydroxy-2-oxindole-pyridine hybrids is reported via sp³ C–H bond functionalization as key steps using FeWO₄ nanoparticles as reusable heterogeneous catalyst. Formation of Hantzsch ester (DHP) followed by aromatization, and sp³ C–H bond functionalization was achieved using FeWO₄ nanoparticles (20 mol%) at 80 °C. Temperature dependent reactivity was observed for mono aldol (at 80 °C) and bis aldol (at 120 °C) products. The catalyst was regenerated and reused up to 6 cycles without losing catalytic activity. The FeWO₄ nanoparticles were also used for oxidative aromatization of different DHP derivatives and for the sp3 C–H functionalization of 2-methyl pyridine.

Full text of DOI:10.1016/j.catcom.2017.09.027

Accepted Manuscript
One-pot synthesis of 3-hydroxy-2-oxindole-pyridine hybrids via
Hantzsch ester formation, oxidative aromatization and sp3 C-
H functionalization using FeWO4 nanoparticles as recyclable
heterogeneous catalyst
Banoth Paplal, Sakkani Nagaraju, Kota Sathish, Dhurke
Kashinath
PII:
S1566-7367(17)30408-9
doi:10.1016/j.catcom.2017.09.027
CATCOM 5209
DOI:
Reference:
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
9 August 2017
19 September 2017
27 September 2017
Please cite this article as: Banoth Paplal, Sakkani Nagaraju, Kota Sathish, Dhurke
Kashinath , One-pot synthesis of 3-hydroxy-2-oxindole-pyridine hybrids via Hantzsch
ester formation, oxidative aromatization and sp3 C-H functionalization using FeWO4
nanoparticles as recyclable heterogeneous catalyst. The address for the corresponding
author was captured as affiliation for all authors. Please check if appropriate.
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ACCEPTED MANUSCRIPT
One-pot synthesis of 3-hydroxy-2-oxindole-pyridine hybrids via Hantzsch ester
3
formation, oxidative aromatization and sp C-H functionalization using FeWO
4
nanoparticles as recyclable heterogeneous catalyst
Banoth Paplal, Sakkani Nagaraju, Kota Sathish and Dhurke Kashinath*
a
Department of Chemistry,National Institute of Technology,Warangal-506004, India
Article history:
Received
Receivedin revisedform
Accepted
Available online

ACCEPTED MANUSCRIPT
2
Abstract
3
Synthesis of poly-substituted3-hydroxy-2-oxindole-pyridine hybrids is reportedvia sp C–H bondfunctionalization as keysteps using FeWO
4
nanoparticles as
3
reusable heterogeneous catalyst. FormationofHantzschester (DHP) followedby aromatization, andsp C–H bondfunctionalizationwas achieved using FeWO
4
o
o
o
nanoparticles (20 mol %) at 80 C. Temperature dependent reactivitywas observed for monoaldol (at 80 C) andbis aldol (at 120 C)products.The catalyst was
regeneratedandreusedup to 6 cycles without losingcatalytic activity. TheFeWO
derivatives and for the sp3 C-H functionalization of 2-methyl pyridine.
4
nanoparticles were also usedfor oxidative aromatization of different DHP
Keywords:
Hantzsch ester
FeWO
4
nanoparticles
3
-hydroxy-2-oxindoles
3
sp C-H functionalization
Hybridmolecules

ACCEPTED MANUSCRIPT
Introduction
Pyridine is an important component present in many marketed drugs and fused compounds like pyrimidine, quino line,
imidazopyridine and other biologically active compounds [1]. The monocyclic poly-substituted pyridines are known to show anti-
hypoxic, anti-ischemic, antihypertensive, immune modulators, antibacterial, anti-inflammatory (Nicoboxil, Clonixin), MET inhibitor
6
activities [2] and required as a dietary supplement in the formofvitamin-B (pyridoxine)[3]. Along with these, the pyridine derivative
are also useful in the agriculture industry as acaricidal, insecticidal, and herbicidal agents [4]. Most of the time, these derivative are
prepared via Hantzsch dihydropyridine ester formation followed by oxidative aromatization reactions [5].
Oxindole moiety is used extensively forthe functionalization under catalytic, conventional conditions and for the synthesis of large
amount of complex and hybrid molecules with improved drug-like properties in different therapeutic areas [6],[7] Particularly, 3-
Hydroxy-2-oxindoles are present in many biologically active synthetic and natural compounds such as convolutamydines [8],
donaxaridine [9], dioxabrassinine [10], welwitindolinones [11], maremycins [12] and show anticancer,
-------------------

Corresponding author: Tel. +91-870- 246-2677;
FAX No. +91-870-246-9547; e-mail: kashinath@nitw.ac.in; kashinath.dhurke@gmail.com
antioxidant, anti-HIV [13], neuroprotective [14], BK channel opener [15], and proteasomeinhibition [16] properties.
Fig. 1. Examples forbiologicallyimportant poly-substitutedpyridines and3-hydroxy-oxindole derivatives
Activation of C-H bond is well known using cooperative catalysis and heteroatom directed concepts in presence of different
transition metalcatalysts underhomogeneousandheterogeneous conditions [17],[18]. Similarly, direct functionalization ofunactivated
3
bonds(benzylic sp C-H functionalization ofazaarenessuch as 2-methylpyridine and 2-methylquinoline) is achieved by the attack of
electrophiles (aldehydes,imines unsaturated systemor enones) [19] in presence of either strong bases [20] or organometallic complex
3
(
based on Yttrium, Zirconium,Scandinavium,Hafniumetc) [21]. Along with these,the sp C-H bond functionalization of azaarenes is
3
also reported by using Brønsted acid (TFA) [22], Lewis acid [Yb(OTf) ] [23] silica-supported dodecatungstophosphoricacid [24],
cyclodextrin [25] etc., to give 3-hydroxy-2-oxindoles. All the above methodsare havingtheirown limitation such as long reaction time,
use of high temperature, non-reusability of the catalyst etc.,
Use of nanomaterials as heterogeneous catalysts for organic transformations is gaining attention in recent years because of their
reactivity,recyclability and handling. Many metal based nanomaterials (Bi, Cu, Zn and Pd) have been reported for multicomponent,
3+2-cycloaddition and crosscoupling reactions etc., [26], [27], [28]. Particularly,iron-based nanoparticles are ofspecialfocusand been
used forthe Hantzschreaction,Mannich reaction,aromatization reactions etc., because oftheirmild reactivity [29], [30]. However,best
3
of our knowledge there are no reports on aldol reaction between poly-substituted pyridines and isatins via sp C–H bond
functionalization.
1
. Experimental
a) Preparation andCharacterization oforiginal and recycledFeWO
FeWO nanoparticles were prepared by usingreported method [31]and characterized by powderXRDdata as shown in Figure 2. After
4
nanoparticlesby powderXRD
4
3
conformation ofthe FeWO nanoparticlesto testthe catalyst activity we selected sp C-H functionalization ofpoly-substituted pyridine
4
with isatin as a model reaction as described in below sections. The catalyst was recovered and reused up to sixcycles for the same
reaction without loss in the catalytic activity which is supported bythe yields ofthe productsand powder XRD data of cycle-6 (Figure
2
).

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4
4
Fig. 2. Powder XRD patterns of FeWO nanoparticles.
b) General procedure forthe preparation ofpyridine-3-hydroxy –oxindole hybrids (One-pot approach): To a mixture of ethyl aceto
acetate or methyl acetoacetate (1 eqv), formaldehyde (1.1 eqv) and NH OAc (1.5 eqv) in acetic acid (3 mL) was added FeWO (20
4
4
o
mol%) at roomtemperature and the mixture was heated at 80 C for 2 h (monitoring by TLC) to give poly-substituted pyridine (3), to
this solutionisatin (1eqv)was added andheating continued at same temperature for 3 h (monitoring by TLC). After that the reaction
mixture was cooled to roomtemperature neutralized with sodiumbicarbonate andextracted with EtOAc (2X10 mL). The organic layers
were washed with brine,dried using sodiumsulphate.Evaporationofthe solvent gave the crude product which was purified by silica
gel column chromatography. Elution of the column with petroleumether-EtOAc gave the desired product.
2
. Results anddiscussion
Considering the importance ofpoly-substituted pyridines, 3-hydroxy-2-oxindole derivatives and in continuation of our interest in
greenermethods [32], [33] herein, we report the first application FeWO nanoparticles as a recyclable heterogeneous catalyst for the
4
3
synthesis of poly-substituted pyridine-3-hydroxy-2-oxindole hybrids via sp C–H bond functionalization of 2,6-dimethylpyridines.
Towards achieving the objective, we started with the preparation of 2,6-dimethylpyridine-3,5-dicarboxylate (3). Thus, ethyl
acetoacetate (EAA) 1, formaldehyde and NH
dihydropyridine-3,5-dicarboxylate (Hantzsch ester)2 in good yield [34]. Then the resultingHantzsch ester was aromatized using Pd/C
35] to give the 2,6-dimethylpyridine-3,5-dicarboxylate (3)in quantitative yield.Then compound 3 was treated with 1-benzylindoline-
4 2 6
OAc were reacted in presence of Bi WO to give diethyl 2,6-dimethyl-1,4-
[
2
o
,3-dione (4a) in acetonitrile (3 mL) at 100 C for 30 h to give the mono aldol product [i.e. dimethyl 2-((1-benzyl-3-hydroxy-2-
oxoindolin-3-yl)methyl)-6-methyl pyridine-3,5-dicarboxylate (5a)] in 30% yield (Scheme-1). After confirmation of the product using
1
13
H-, C-NMR, and mass spectra,optimization studies were conducted using different solvents like DMF, DMSO, MeOH, EtOH. But
the yield of the desired product (5a) was encouraging (Table-1; Entries 1-4).
Scheme 1. Preparation of 2,6-dimethylpyridine-3,5-dicarboxylate (3) and its reaction with N-Benzyl isatin (4a) under catalyst-
free conditions.
Later the intention ofimproving overallreaction conditions andwith the background ofusing heterogeneous catalysts (Bi
ZnOnanoparticles)fororganic transformationsin our laboratory[26],[28] above reaction was performed using Bi WO (20 mol%) and
ZnOnanoparticles (20mol%) in water (entries 5 and 6; Table 1). However,the results are not encouraging. In continuation with this,
FeWO nanoparticles were prepared using literature method,characterized using powder XRD data [31] and used as a catalyst for the
2 6
WO and
2
6
4
present study. Fromthe studies on amounts of the catalyst and solvents for the reaction (entries 7-18; Table 1), it was found that a
combination of FeWO (20 mol%) and AcOH (3 mL) is good for the success of the reaction giving desired product in 3 h with 95%
4
o
yield at 80 C (entry 11; Table 1). To make the generalization, the reactivity of FeCl
3
and Fe powder was also tested and results are
given in Table 1 (entries 16 and 17). After optimizing the reaction condition, differently substituted isatins were reacted with 2,6-
dimethylpyridine-3,5-dicarboxylate (3)to give functionalized pyridine-oxindole hybrids(5a–5v)with good to excellent yields (Scheme
2
).

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Table 1
Optimization of reaction conditions (different catalysts,solvents using 3 and 4a as starting materials)
S. No. Catalyst
Solvent (3ml)
Reaction
Reaction
Isolated
o
a
Time (h) Temp ( C) yields (%)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
No catalyst
No catalyst
No catalyst
No catalyst
DMF
DMSO
MeOH
EtOH
Water
Water
Water
ACN
30
30
30
30
15
15
10
4
100
100
80
35
40
30
30
60
55
70
45
55
60
95
90
90
85
30
70
65
75
80
Bi
2
WO
6
100
100
100
80
ZnO
FeWO
FeWO
4
4
FeWO
FeWO
4
4
DMF
4
80
0
1
2
3
4
5
6
7
8
DMSO
AcOH
AcOH
AcOH
AcOH
HCOOH
AcOH
AcOH
AcOH
4
80
FeWO
4
3
80
FeWO
4
3
90
FeWO
4
3
100
70
FeWO
FeWO
FeCl
4
5
4
15
5
80
3
100
100
80
Fe powder
--
5
15
a
All the reactions were performed at 0.132 moles concentration
After successfully optimization of reaction condition for the poly-substituted pyridine-oxindoles starting from 2,6-
dimethylpyridine-3,5-dicarboxylate (3),the focuswas shifted towardsthe one-potsynthesis ofthe title compounds, Thus, the reaction
of ethylacetoacetate(1),formaldehyde,and NH OAc in presence of FeWO nanoparticles (20 mol%) in AcOH (3 mL) gave Hantzsch
4 4
ester (1,4-dihydropyridine) which was oxidized/aromatized without purification in the same reaction vessel in good yield. This
aromatized product was treated with substituted isatin (1 eq) in the same pot to give the activated/condensed product in 85% yield
(Scheme 2). Aftersuccessfully demonstratingthe one-pot strategyforthe title compound,different functionalized isatins (substitution
on aromatic ring and nitrogen) were prepared and reacted with EAA or methyl acetoacetate (MAA) and NH OAc under optimized
4
conditions to give a library offunctionalized pyridine-oxindole hybrids (5a–5v) with moderate to good yields as shown in Scheme 2.
Among the isatins used,nitro substitutedand N-allylisatins gavemoderate yield (75%),halogen substituted, N-alkylated, alkynylated
and unsubstituted(on nitrogen)isatinsgave goodyields (up to 90%).A notable change in the reactivity was observed at higherreaction
o
temperatures (>80 C) where the formation of more impurities thus decrease in the overall yields of the final product was observed.
o
Also,when the temperature was raised to 120 C, formation ofbis-activatedproductswas observed with low yields (25-30) because of
1
13
the decompositionofthe products formed during thereaction. Allthe productssynthesized were characterized using H-, C-NMR, and
mass spectral data (please see electronic supporting information). Along with above, gram scale (2 g) was performed in one-pot
methods and desired product was obtained with 75% overall yields (in 3 steps).
3
Towards increasing the structural complexity, the bis sp C-H functionalization was attempted. Thus, 2,6-dimethylpyridine-3,5-
o
3
dicarboxylate (3)was treated with isatins usingoptimized reactionsconditionat 120 C. It is interesting to observe that the bis sp C-H
o
functionalization under [N-alkyl isatin (2 Equiv), acetic acid (3ml), 120 C, 1.5-2 h] in moderate yields (20-25%) because of
decomposition of product at higher temperatures (Schemes 2; 6a-6f).

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6
3
Scheme 2. Mono and bis sp C-H functionalization products (3-hydroxy-2-oxindole-pyridine derivatives)
Since the reaction working well with FeWO catalyst for the activated 2-methyl pyridines, to test the reactivity of FeWO
inactivated pyridines,same sp C–Hbond functionalization reaction on2-methylpyridine was tested. In this case, though the reaction
time is more (5 h), the yields products 7a-7h (see Scheme 2 in ESI) are comparable with the literature methods.
4
4
for
3
It is noteworthy to mention, the above one-pot sequence (C-H functionalization) is proceeding smoothly via in situ formation of
pyridine-3,5-di carboxylic esters (type 3) in good yield. This type of transformation is known by step-wise method [5]. Also, poly-
substituted pyridine-3,5-dicarboxylic esters are important for biological activity. Considering this, an attempt to generalize the
aromatization step in presence of FeWO
withdrawing groups) aldehydes (1 eqv.) were treated with ethyl/methyl acetoacetate (1 eqv.) and ammonium acetate (1.5 eqv.) in
optimized conditions (i. e. FeWO (20 mol%) and acetic acid (3 ml) to give fully substituted pyridines derivatives with good to
excellent yields (8a-8l) in 2 h. The advantages of our method are in situ DHP preparation, recyclability of the catalyst.
4
catalyst was taken up. Thus, different aldehydes (with electron donating and electron
4

ACCEPTED MANUSCRIPT
Scheme 2. Synthesis of fully substituted pyridines with optimized condition.
From the mechanism,we assume that the FeWO
group ofpyridine which may be helping to isomerize the sp C-H bond.FeWO
4 3
nanoparticlesare involvedin generatingthe nucleophile via acidic proton of -CH
3
4
nanoparticlesare also involved in activating the isatin
moiety by interaction with the oxygenspresent and making carbonylcarbon present at 3rd positionmore electron deficient.On this, the
nucleophilic attack of isomerized carbon of pyridine results into aldol product (Figure 3).
3
Fig. 3. Plausible mechanism for 3-hydroxy-2-oxindole-pyridines via sp C–H bond functionalization (starting from diethyl 2,6-dimethylpyridine-3,5-
4
dicarboxylate) using FeWO nanoparticles.
Later, the catalyst recovery studies were performed. For this, the catalyst was recovered from the reaction mixture by simple
o
filtration, washed with ethyl acetate (3 X 5 mL), heated at 80 C in the oven for activation and reused for the reaction up to 6 cycles
without losing activity giving the desired product in 80% yield (See Table-1 in ESI). The powder XRD data of recycled catalyst
th
indicate there is no change in the morphology of the catalyst after 6 cycle of the reaction (Figure 1).
3
. Conclusions
In conclusion,we have developed a simple and efficient method for synthesis of poly-substituted 3-hydroxy-2-oxindole-pyridine
3
hybrids via Hantzsch ester formation, oxidative aromatization, and sp C–H bond functionalization (one-pot synthesis) FeWO
4
4
nanoparticles as recyclable heterogeneous catalyst. This is the first example where FeWO nanoparticles have been used for the
3
multicomponent reaction,oxidation and sp C–Hbond functionalization.Also,this is the first repot forthe bis-functionalization of 2,3-
dimethylpyridines.
Acknowledgements
BP thanks MHRD for fellowship. SN thanks, UGC-New Delhi for the fellowship. KS thanks DST for the INSPIRE fellowship. DK
thanks, DST (SERB), New Delhi for the financial support (SB/FT/CS-136/2012 and SB/EMEQ-103/2014).
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Graphical abstract
One-pot synthesis of 3-hydroxy-2-oxindole-
Leave this area blank for abstract info
pyridine hybrids via Hantzsch ester formation,
3
oxidative
aromatization
and
sp
C-H
functionalization using FeWO4 nanoparticles
as recyclable heterogeneous catalyst
Banoth Paplal, Sakkani Nagaraju,Kota Sathish and Dhurke Kashinath*

ACCEPTED MANUSCRIPT
1
0
One-pot synthesis of 3-hydroxy-2-oxindole-pyridine hybrids via Hantzsch ester formation, oxidative
3
aromatization and sp C-H functionalization using FeWO4 nanoparticles as recyclable heterogeneous
catalyst
Banoth Paplal, Sakkani Nagaraju, Kota Sathish and Dhurke Kashinath*
Department of Chemistry, National Institute of Technology, Warangal-506 004, India
Highlights:
1
2
. First example of FeWO4 nanoparticles for organic transformation
. First example for one-pot Hantzsch ester formation, aromatization and sp C–H bond
functionalization
3
3
. Broad substrate scope for 3-hydroxy-2-oxindole hybrids