Ag NPs decked GO composite as a competent and reusable catalyst for ‘ON WATER’ chemoselective synthesis of pyrano[2,3-c:6,5-c′]dipyrazol]-2-ones

Article abstract of DOI:10.1016/j.tetlet.2017.02.014

We have synthesized and characterized Ag NPs decked GO composite and studied its role as reusable catalyst for the ‘ON WATER’ chemoselective synthesis of pyranodipyrazolones via the reaction of different carbonyl compounds with 3-methyl-1-phenyl-1H-pyrazol-5(4H)-one. This method illustrates significant selectivity for pyranodipyrazolones over arylmethylene bispyrazolols and arylmethylenepyrazolones. Synergistic effect of heterogenic nature of water with reactants and Ag NPs/GO had profuse outcome on reaction as indicated by high TOF (18.03 × 10^?5mol g^?1min^?1). Furthermore, catalyst was recycled for 7-times without significant loss of activity.

Full text of DOI:10.1016/j.tetlet.2017.02.014

PII:
S0040-4039(17)30178-8
DOI:
http://dx.doi.org/10.1016/j.tetlet.2017.02.014
TETL 48625
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
4 November 2016
4 February 2017
6 February 2017
Please cite this article as: Dandia, A., Gupta, S.L., indora, A., Saini, P., Parewa, V., Rathore, K.S., Ag NPs decked
GO composite as a competent and reusable catalyst for ‘ON WATER’ chemoselective synthesis of pyrano[2,3-c:
6,5-c']dipyrazol]-2-ones, Tetrahedron Letters (2017), doi: http://dx.doi.org/10.1016/j.tetlet.2017.02.014
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Ag NPs decked GO composite as a
competent and reusable catalyst for ‘ON
WATER’ chemoselective synthesis of
pyrano[2,3-c:6,5-c']dipyrazol]-2-ones
Leave this area blank for abstract info.
Anshu Dandia, *a Shyam L. Gupta,a Aayushi indora,a Pratibha Saini,a Vijay Parewa*a, Kuldeep S. Rathoreb
ON WATER SYNTHESIS
N
N
Ph
N
N
Ph
OHHO
Ph
N
N
Ph
Ph
N
N
A
X
ONLY
PRODUCT
g
O
O
O
N
N
O
2
P
Annulated and
N
Spiro Systems
H
s
/
+
,
T
R
G
O
X
No column chromatography
Aqueous mediated synthesis
Excellent chemoselectivity
One pot
O
Ph
N
N
Waste-free (Only side product: H2O )
Reusable catalyst

1
Tetrahedron Letters
Ag NPs decked GO composite as a competent and reusable catalyst for ‘ON
WATER’ chemoselective synthesis of pyrano[2,3-c:6,5-c']dipyrazol]-2-ones
Anshu Dandia*a, Shyam L. Guptaa, Aayushi indora a, Pratibha Sainia, Vijay Parewa*a, Kuldeep S. Rathoreb
a Centre of Advanced Studies, Department of Chemistry,
University of Rajasthan, Jaipur-302004, India
bSemiconductor and Polymer Science Laboratory. Department of Physics, University of Rajasthan, Jaipur
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
We have synthesized and characterized Ag NPs decked GO composite and studied its role as
reusable catalyst for the ‘ON WATER’ chemoselective synthesis of pyranodipyrazolones via the
reaction of different carbonyl compounds with 3-methyl-1-phenyl-1H-pyrazol-5(4H)-one. This
method illustrates significant selectivity for pyranodipyrazolones over arylmethylene
bispyrazolols and arylmethylenepyrazolones. Synergistic effect of heterogenic nature of water
with reactants and Ag NPs/GO had profuse outcome on reaction as indicated by high TOF
(18.03x 10-5 mol g-1 min-1). Furthermore, catalyst was recycled for 7-times without significant
loss of activity
Available online
Keywords:
Ag NPs/GO composite
On water synthesis
Chemoselectivity
Pyranodipyrazolones
2009 Elsevier Ltd. All rights reserved.
Pyranopyrazoles play essential role as biologically active
compounds and represent an interesting template for medicinal
chemistry. Many of these compounds are known for their
antimicrobial1, insecticidal2, anti-inflammatory3 and anticancer
activities4. Furthermore, pyranopyrazoles show molluscicidal
activity5 and are identified as screening kit for Chk1 kinase
inhibitor6. They also find applications as pharmaceutical
ingredients and biodegradable agrochemicals7. Some pyrano[2,3-
c]pyrazoles have been evaluated for their bovine brain adenosine
A1, A2A receptor binding affinity. Also, these pyranopyrazoles are
of interest because of their structural similarity to a wide variety
of flavones and flavanones that exhibit interesting biological
activity8. Despite their pharmaceutical, industrial, and synthetic
importance, only a few methods for their preparation have been
reported so far9.
numerous areas due to their various distinctive properties11,12,13
.
But, the practical applications of Ag NPs in catalytic science are
often hindered by their self-aggregation14-15. Hence, development
of dispersed & stable Ag NPs substrates has been become a topic
of immense concern. Moreover, among diverse carbon
nanomaterials, graphene oxide can be considered as
a
heterogeneous support for various other nanoparticles,16 as they
exhibit highly accessible oxygen-containing functionalities or
binding sites with intrinsically high surface area.
Besides this, water has attracted considerable interest in the
context of sustainable chemistry, as it can be used as an efficient
reaction medium for many nanocatalytic routes.17 The unique
structure and physicochemical properties of water lead to
particular interactions which might greatly facilitates the reaction
course18. Thus, attainment of sustainable synthetic strategy
inherently involves implementation of developed catalytic
processes in water to achieve greener syntheses.
A cautious study dictates that most of the methodologies suffer
some drawbacks e.g: reaction under high temperatures, use of
expensive and environmentally harmful catalysts, harsh reaction
conditions, formation of a mixture of products, multi step
procedure, longer reaction times and involvement of volatile
organic solvents in high quantity which limits the broad use of
these methodologies. Moreover, the major drawback of almost all
existing methods is the sinking turn over number (TON) or turn
over frequency (TOF),as the catalysts are destroyed in the work-
up process and cannot be recovered or reused, which is vital from
the industrial point of view. As a result, the development of new
synthetic methods for this purpose remains an attractive goal.
In the above regards and as a consequence of our interest in
development of new nanocatalytic processes and sustainable
synthesis of biologically important scaffolds19, we herein report,
‘on-water’ chemoselective synthesis of pyranodipyrazolones via
the reaction of various carbonyl compounds with 3-methyl-1-
phenyl-1H-pyrazol-5(4H)-one using Ag NPs decorated reduced
graphene oxide (Ag NPs/GO) composite which is a facile, leach
free and easily recyclable heterogeneous catalyst at room
temperature (Scheme 1).
An exponentially growing research field of nanocatalysis in
modern science10 has met the challenges of energy and
sustainability. In this context, Ag NPs are being widely used in

2
Tetrahedron Letters
N
N
N
Ph
N
Ph
OHHO
5
A
g
Ph
N
N
Ph
N
N
N
Ph
X
ONLY
PRODUCT
P
O
O
O
s
N
/
T
R
G
O
N
,
+
O
c
o
2
H
m
p
X
o
s
i
Diverse carbonyl
compounds
2
t
e
3a-q
O
Ph
Fig. 2. XRD patterns of Ag NPs/GO composite
N
N
Particle size
~ 28 nm
4
Scheme 1: Chemoselective synthesis of pyranodipyrazolones
The product distribution of the reaction of various carbonyl
compounds with 3-methyl-1-phenyl-1H-pyrazol-5(4H)-one has
been reported to strongly depend on the reaction conditions. The
reaction
gave
a
mixture
of
compounds20,
arylmethylenepyrazolones (4), arylmethylene bispyrazolols (5)
and pyranodipyrazolones (3). Sometimes 4 and 5 have been
isolated as the main product instead of pyranodipyrazolones21.
This phenomenon triggered our enthusiasm towards the exclusive
formation of the pyranodipyrazolone derivatives.
Fig. 3. TEM image of Ag NPs/GO composite
GO, Ag NPs/GO and Ag NPs/rGO nanocomposites were
fabricated by simple one-pot chemical route19f and well
characterized by XRD, TEM, SEM and EDX analyses. XRD
patterns of GO, Ag NPs/rGO and Ag NPs/GO composite are
given in Fig.1 (a), (b) and Fig.2 respectively. For GO sheets, a
sharp characteristic peak is observed due to the (001) reflection
of GO. In the XRD pattern of Ag NPs/rGO and Ag NPs/GO
composite, the diffraction peaks for crystallographic planes of Ag
NPs phase with rGO and Ag NPs phase with GO are observed.
The average particle size of Ag NPs was ~20 nm on the basis of
Scherrer formula. In TEM analysis (Fig.3), GO sheet showed
modification of plenty of Ag NPs (~ 28 nm) after blending the
GO with silver colloids. Ag NPs were adsorbed on both sides of
GO. In SEM analyses, Ag NPs were completely distributed on
GO indicating strong interactions between GO and Ag NPs (Fig.
S1a; see ESI). According to EDX analysis, the Ag NPs/GO
composite contains C, O, and Ag. The signals of C and O
originated due to GO and that of Ag resulted from the decorated
Ag NPs. The % amount of various elements is shown in
respective figure (Fig. S2; see ESI).
To appraise the catalytic activity of prepared nanocomposite for
the “on water” synthesis of pyrano[2,3-c:6,5-c']dipyrazol]-2-
ones, reaction of 4-bromo benzaldehyde 1 and 3-methyl-1-
phenyl-1H-pyrazol-5(4H)-one 2 served as model reaction at room
temperature. A controlled experiment was investigated in the
absence of catalyst. But the result indicates the need of a catalyst.
Thus in order to achieve suitable conditions, we chose various
catalysts and the results are summarized in Table 1. As compared
to the blank reaction, analogous results were found in CAN (ceric
ammonium nitrate), PTSA (p-toluenesulfonic acid), Sulfamic
acid and L-Proline. Moreover, silver based bulk catalyst could
only afford the desired compound in low yields. When the
reaction was carried out in the presence of catalytic amount of Ag
NPs, the desired product 3f was isolated in moderate yield.
Further GO and rGO sheets also gave good yields (Table 1, entry
9-10). However, the best result in terms of turnover frequency
(TOF: number of moles of product produced per gram of catalyst
used per min22, 19f) attained by Ag NPs/GO composite catalyst (10
wt% loading, TOF = 18.03 x 10-5 mol g-1 min-1). It was found
that 10 wt% of Ag NPs/GO composite is sufficient to push the
reaction forward. Any excess amount did not show further
increase in yield. Additionally, we have also checked the
catalytic activity of AgNPs/rGO composite on this reaction,
lower catalytic activity was observed as compared to AgNPs/GO.
This may be explained due to presence of reduced or lower
catalytic sites in AgNPs/rGO composite as compared to
AgNPs/GO.
(a)
(b)
Fig. 1. XRD patterns of graphene oxide (a) and Ag NPs/rGO (b)

3
Table 1. Comparison of catalytic activity of Ag NPs/GO composite with other catalytic systems in different conditions for
synthesis of 4-(4-bromophenyl)-3,5-dimethyl-1,7-diphenyl-4,7-dihydro-1H-pyrano[2,3-c:6,5-c']dipyrazole (3f)a.
S. No.
Catalyst
Solvent
Time(h
)
Yield (%)b
TON (x10-5 mol g-1)
TOF (x10-5 mol g-1 min-1)c
1.
2.
-
CAN
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
4
4
60
63
65
62
62
68
67
76
72
68
96
-
-
1065
1099
1048
1048
1150
1133
1286
1218
1150
1622
4.44
4.58
4.36
4.36
9.58
9.44
10.71
10.15
9.58
18.03
3.
PTSA
4
4.
Sulfamic acid
L-Proline
AgCl
4
5.
4
6.
2
7
AgNO3
Ag NPs
GO
2
8.
2
9.
2
10.
11.
rGO
2
Ag NPs/GO
1.5
12.
13.
14.
15.
16.
17.
18.
19.
20
Ag NPs/GOd
Ag NPs/GOe
Ag NPs/GO
Ag NPs/GO
Ag NPs/GO
Ag NPs/GO
Ag NPs/GO
Ag NPs/GO
Ag NPs/GO
H2O
H2O
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
96
40
1021
1432
1218
980
877
-
11.35
15.91
13.53
10.88
9.75
-
H2O/ MeOH(1:1)
MeOH
72
58
CH3CN
THF
52
Trace
Trace
54
DCM
-
-
EtOH
912
-
10.12
-
Toluene
Trace
21
22
Ag NPs/GO
Ag NPs/rGO
Neat
H2O
1.5
1.5
32
78
541
6.00
1320
14.66
[a] Reactions were performed using 1:2 ratio of 4-bromo benzaldehyde 1 and 3-methyl-1-phenyl-1H-pyrazol-5(4H)-one 2 at room temperature with 10 wt% of
the catalyst. [b] Isolated yield. [c] TOF is defined as the number of moles of product produced per gram of catalyst used per min. [d] with 15 wt% of the catalyst
[e] with 5 wt% of the catalyst
The effect of various solvents on the model reaction was also
conducted. When organic solvents such as DCM, THF and
toluene were used, trace amount of desired product was detected.
While polar protic solvents accelerate the reaction. Additionally,
we carried out the reaction in heterogeneous as well as
homogeneous aqueous systems. Methanol was used to make the
reaction homogeneous in water. Under analogous conditions,
higher conversion of reactants into the product was observed in
the heterogeneous compared to the homogeneous conditions.
This result indicates that using only water as the solvent in the
reaction afforded excellent product yield. Hence, apart from the
catalytic role of Ag NPs/GO composite which activates the
carbonyl groups of the reactants, heterogeneity also plays crucial
role in rate accelerations. Furthermore, it can also be expected

4
Tetrahedron
that, due to the good dispersion of this nanocomposite in water
spectrometric studies. For instance, IR spectrum of the product 3f
showed important absorption band at 1598 cm-1 due to the
presence of C=N group in the cyclic ring system while the band
at 1152 cm-1 was due to the presence of cyclic ether (pyran)
as compared to other solvents, most of the catalytic sites of Ag
NPs/GO composite were exposed to the reaction environment.
The subsequent solvent-free synthesis takes virtually many hours
and gave poor result, which shows that the rate improvement is
not the individual effect of an increase in the concentration of
reactants (Table 1, entry-21). The on-water idea assists easy
isolation of solid product from the reaction mixture.
1
functionality. In H NMR singlet at δ 2.31 was assigned to the
methyl protons of pyrazolone unit while singlet at δ 4.94 was
attributed to the methane (CH) proton and the region between δ
7.18-7.71 was due to the 14 aromatic protons. In 13C NMR study,
peak at δ 150.9 confirmed the presence of cyclic ether carbon
associated with pyrazolone unit while peak at δ 32.5 can be
attributed to the methane CH carbon. The structure of product 3f
was further established by mass spectrometry which showed a
molecular ion peak [M+ H2O]+ at m/z 516.
The general efficiency of this protocol was then studied for the
synthesis of a variety of pyranodipyrazolone derivatives and the
results are summarized in Fig.4. A series of aromatic aldehydes
bearing either electron-donating or electron-withdrawing groups
on aromatic ring were investigated. The substituting group on the
phenyl ring did not seem to affect the yield of product or the rate
of the reaction significantly. To demonstrate the versatility of this
technique for generating variety of pyranodipyrazolone
derivatives, various isatins, heterocyclic aldehydes and
acenapthaquinone were also tested. These carbonyl compounds
react smoothly with 3-methyl-1-phenyl-1H-pyrazol-5(4H)-one 2
to afford the desired products in excellent yields. Further, ICP-
AES analysis showed that there was no metallic leaching in the
final product.
In IR spectrum, the lack of the peak of carbonyl group and –
OH ruled out the formation of 1-phenyl-3- methyl-4-
arylmethylenepyrazol-5-ones (4) and 4,4’-(arylmethylene)-bis-
(3-methyl-1-phenyl-1H-pyrazol-5-ols (5) respectively instead of
pyrano[2,3-c:6,5-c']dipyrazol]-2-one derivatives. This was also
confirmed by the absence of characteristic peak of vinylic proton
and –OH in 1H NMR spectrum for 4 and 5 correspondingly.
A plausible mechanism for the formation of the product is
presented in Scheme 2. It involves electrophilic activation by
coordinating with the Lewis acid sites of Ag NPs/GO composite.
Initially, increase in the reactivity of carbonyl group of 4-bromo
benzaldehyde 1f occurs which then reacts with one unit of
pyrazolone 2 through Knoevenagel condensation to form
Knovengeal adduct 4. Further Ag NPs/GO composite activates
the adduct 4 by coordinating with oxygen and facilitates the
Michael addition of second pyrazolone unit and forms hydroxyl
derivate 5', which upon subsequent cyclization gives the desired
product 3f by loss of water molecule and finally, releasing the
catalyst for the next cycle. Hence, it is assumed that the higher
and well dispersed Lewis acid sites of Ag NPs/GO composite
enhances the reactivity of reactant and intermediate towards the
Knovengeal -Michael addition sequence followed by ensuing
cyclization. Due to this rationale, the present protocol results in
pyranodipyrazolones selectively and exclusively instead of 4 and
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
O
O
N
N
N
N
O
N
N
O
N
N
N
N
N
N
N
N
N
N
Cl
Me
OMe
3a; 90 min.; 89%
3d; 90 min.; 90%
3c; 90 min.; 92%
Ph Ph
3b; 90 min.; 91%
Ph
Ph
Ph
Ph
N
Ph
Ph
O
O
N
N
O
N
N
N
N
O
N
N
N
N
N
N
N
N
N
MeO
OMe
NO2
F
Br
OMe
3e; 90 min.; 90%
Ph Ph
3h; 90 min.; 94%
Ph Ph
3g; 90 min.; 93%
Ph Ph
3f; 90 min.; 96%
Ph Ph
O
O
N
N
N
N
N
N
O
O
N
N
N
N
N
N
5.
N
N
Ph
N
N
N
N
O
O
2
S
NH
3j; 90 min.; 89%
Ph Ph
OPh
Ph
N
3l; 90 min.; 88%
3k; 90 min.; 90%
HO
3i; 90 min.; 90%
N
CHO
O
O
N
O
H
Ph Ph
O
Ph
Ph
Ph
Ph
N
N
N
O
N
N
Ph
O
O
N
N
N
N
N
N
N
Br
N
4'
N
N
N
N
II
Br
1f
O
O
H3C
O
NH
NH
3m; 90 min.; 93%
Ph Ph
O
Ag NPs/GO
composite
Knoevenagel
condensation
NH
3n; 90 min.; 91%
Br
Br
Cl
NH
H2O
3p; 90 min.; 88%
3o; 90 min.; 89%
For next
cycle
O
N
N
N
N
O
Ph
N
N
Br
O
NH
O2N
4
+
Ph
N
N
Ph
N
N
3q; 90 min.; 84%
Michael
addition
O
N
N
Fig. 4. Products of chemoselective synthesis of pyrano[2,3-c:6,5-
Ph
HO
c']dipyrazol]-2-ones
Ph
N
O
-
H
2
In the reaction of carbonyl compounds with 3-methyl-1-
phenyl-1H-pyrazol-5(4H)-one, the formation of the required
pyranodipyrazolones was accompanied by the occurrence of
arylmethylene bispyrazolols and arylmethylenepyrazolones as
side products and sometimes these products have been isolated as
the main products. The present protocol resulted in
pyranodipyrazolones selectively and exclusively. The structure of
N
O
Ph
N
N
Ph
N
N
Br
3f
O
OH
O
N
N
Ph
H
Br
5'
Br
Ph
Scheme 2: Plausible mechanism
1
the products was established by IR, H, 13C NMR and mass
N
N
OH
OH
Br
N
Ph
N
5

5
5.
6.
(a) Foloppe, N.; Fisher, L.M.; Howes, R.; Potter, A.; Robertson,
Recycling experiments were also being performed for the model
reaction. Upon completion of the reaction, the resulting solid
precipitate was filtered and dried along with the Ag NPs/GO
composite. Then, the solid precipitate was dissolved in ethanol
and the catalyst was recovered by filtration. The catalyst was
washed with acetone and reused in subsequent 7 reaction cycles
without any significant loss in its catalytic activity (Fig. 5). Up to
99% ,catalyst recovery has been observed during the process.
TEM and SEM image of reused nanocomposite was also
recorded which shows that the structural integrity remains
unaltered after the reaction and thus proves the efficiency of
nanocomposite as a recyclable catalyst (Fig. S1b, S3b; see ESI)..
A.G.S.; Surgenor, A.E. Bioorg. Med. Chem. 2006, 14, 4792.; (b)
Kimata, A.; Nakagawa, H.; Ohyama, R.; Fukuuchi, T.; Ohta, S.;
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(a) Sosnovskikh, V.Y.; Barabanov, M.A.; Usachev, B.I.;
Irgashev, R.A.; Moshkin, V.S. Russ. Chem. Bull. 2005, 54, 2846.;
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Rodinovskaya, L.A.; Gromova, A.V.; Shestopalov, A.M.;
Nesterov, V.N. Russ. Chem. Bull. 2003, 52, 2207.
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8.
(a) Harborne, J.B.; Marby, T.J.; Marby, H. The Flavanoids;
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(a) Dekker, M. Metal Nanoparticles. Synthesis Characterization
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Fig. 5 Recyclability of Ag NPs/GO composite
Hence, we conclude that a highly efficient protocol has been
developed for the “on water” chemoselective synthesis of
structurally complex and diverse pyrano[2,3-c:6,5-c']dipyrazol]-
2-one derivatives catalyzed effectively by Ag NPs/rGO
composite. Moreover, this method can be considered as an ideal
tool for green synthesis because it minimizes the generation of
waste along with the formation of multiple bonds in a single step.
The process has high atom economy and is ecologically benign,
since only two molecules of water are lost. Three rings of the
fused-ring framework were constructed during the reaction and in
addition, catalyst could be easily recovered and recycled at least
7 times without significant loss of catalytic activity.
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Financial assistance from the CSIR [02(0257)/16/EMR-II], DST
[ECR/2015/000479] and UGC [30-91/2015(BSR)], New Delhi
are gratefully acknowledged. We are thankful to the National
Facility for Drug Discovery Complex, Saurashtra University,
Rajkot, IIT Delhi, Delhi and Therachem Research Medilab,
Jaipur for spectral analyses.
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Tetrahedron
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Supporting data (detailed experimental procedures and spectral
data of compounds) associated with this article can be found in
electronic supplementary information (ESI).
Click here to remove instruction text...
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7
Highlights
 On water chemo-selective organic synthesis.
 Synergistic effect of Ag NPs/GO composite and water.
 pyrano[2,3-c:6,5-c']dipyrazol]-2-one formed selectively
 Easy recovery and high recyclability of catalyst.