Silica chloride catalyzed one-pot synthesis of fully substituted pyrazoles

Article abstract of DOI:10.1016/j.cclet.2011.05.016

Silica chloride catalysted one pot cyclocondensation of aldehydes, ethyl acetoacetate and phenyl hydrazine leading to substituted pyrazoles has been reported.

Full text of DOI:10.1016/j.cclet.2011.05.016

Available online at www.sciencedirect.com
Chinese Chemical Letters 22 (2011) 1187–1190
www.elsevier.com/locate/cclet
Silica chloride catalyzed one-pot synthesis
of fully substituted pyrazoles
*
Dhanaji V. Jawale, Umesh R. Pratap, Jyotirling R. Mali, Ramrao A. Mane
Department of Chemistry, Dr. Babasaheb Ambedkar Marathwada University, Aurangabad (M.S.) 431 004, India
Received 17 February 2011
Available online 20 July 2011
Abstract
Silica chloride catalysted one pot cyclocondensation of aldehydes, ethyl acetoacetate and phenyl hydrazine leading to
substituted pyrazoles has been reported.
# 2011 Ramrao A. Mane. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: One-pot synthesis; Silica chloride; Solvent free; Fully substituted pyrazoles
Pyrazoles are the privileged scaffold and show promising biological activities. Pyrazole is an important core
nucleus of various drugs viz. PNU-32945, Zoniporide and Celecoxib (Fig. 1). These drugs act as HIV-reverse
transcriptase inhibitors [1], sodium hydrogen ion exchanger inhibitors [2] and COX-2 inhibitors [3]. Pyrazoles are also
emerged as potential antihyperglycemic agents [4]. A number of pyrazole compounds specially, 1,3-disubstituted
pyrazoles [5] and 1,3,5-trisubstituted pyrazoles [6,7] have been cited in the literature and are found to elicit anti-
hyperglycemic activity (Fig. 1).
Owing to the important features of pyrazoles, various synthetic methods are reported for the pyrazoles.
Condensation of hydrazonyl halides with b-dicarbonyl compounds and 1,3-dipolar cycloaddition of diazo compounds
with alkynes [8–10] are found to yield pyrazoles. 1,3,5-Trisubstituted pyrazolines have been synthesized by
condensing chalcones with hydrazines. The pyrazolines on aromatization using various oxidants, PhI(OAc)2 [11],
MnO2 [12] and PBr3 [13] are found to give pyrazoles.
The most widely used synthetic protocol for obtaining polysubstituted pyrazoles is by condensing 1,3-dicarbonyl
compounds with hydrazines using acid catalysts like sulphuric acid [14], polystyrensulphonic acid [15] and
hydrochloric acid [16]. To provide an efficient multicomponent one-pot synthesis for polysubstituted pyrazoles L.
Shen et al. have recently reported the cyclocondensation of aldehydes, 1,3-dicarbonyl compounds and phenyl
hydrazines using ytterbium perfluorooctanoate as rare earth condensing catalyst and IBX as an oxidant for obtaining
good to moderate yields of polysubstituted pyrazoles [17,18]. This is a unique report however it has certain limitations
as it needs non readily available and costly rare earth catalyst, ytterbium perfluorobutanoate. It has been revealed that
silica chloride is gaining importance as catalyst in accelerating various organic transformations [19–23] and
cyclocondensations leading of bioactive heterocycles [19–23,27]. Multicomponent reactions are convenient and are
found in use in various organic transformations because of their several advantages [24–26].
* Corresponding author.
E-mail address: manera2011@gmail.com (R.A. Mane).
1001-8417/$ – see front matter # 2011 Ramrao A. Mane. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2011.05.016

1188
D.V. Jawale et al. / Chinese Chemical Letters 22 (2011) 1187–1190
CN
N
N
F C
3
O
N
N
O
NH
NH
N
2
N
N
N
N
N
N
2
O
S
O
H N
2
SAH 57-749
CF
Zoniporide
3
PNU-32945
Celecoxib
Fig. 1. Biologically active substituted pyrazoles.
In continuation of our earlier work on the development of the convenient synthetic protocols for bioactive
molecules [27–30] using some of the green tools and considering the above urgent need here it was thought worthwhile
to develop better synthetic protocol by carrying the cyclocondensation of multicomponents, aromatic aldehydes,
ethylacetoacetate and phenyl hydrazine under neat condition or in organic medium using freshly prepared silica
chloride as catalyst to obtain better yields of the desired polysubstituted pyrazoles.
We have developed one-pot three component convenient synthesis for fully substituted pyrazoles (4a-l). Aromatic
aldehydes (1a-l), phenyl hydrazine (2) and ethyl acetoacetate (3) were allowed to react under solvent free condition at
120 8C in presence of dry silica chloride and obtained high yields of the titled pyrazoles (Scheme 1), detail information
are put in supplementary file.
To investigate the choice of catalyst, the cyclocondensation of 4-bromobenzaldehyde (1b), phenyl hydrazine (2)
and ethyl acetoacetate (3), was carried as a model reaction to afford the pyrazole (4b). Initially the reaction was run in
the absence of catalyst and was observed that after prolonged heating also the cyclocondensation did not run
satisfactorily. Then the above model reaction was performed separately in the presence of various catalysts like acetic
acid, ZnCl2, molecular iodine and silica chloride and the results are summarized in Table 1.
Encouraged by these results an attempt was made to optimize the reaction conditions. The condensation was
performed separately in presence of medium like toluene, acetonitrile, methanol and ethanol. Attempt was made to
carry the condensation in the absence of medium/diluent. It was recorded that when the condensation was run in the
presence of silica chloride using various solvents/diluents like toluene, acetonitrile, methanol and ethanol they were
not found to undergo completion even after prolonged heating. It was recorded that the above model reaction when
carried under solvent free condition at 120 8C, using silica chloride gave high yield of the pyrazole (4b) in short
reaction time (Table 1).
To determine the amount of silica chloride required for the formation of pyrazoles (4b), the reactions were
attempted by varying the amount of silica chloride in the range of 0–35 mol%. It was observed that with increase in
O
OSiCl
O
O
O
CHO
NHNH2
O
O
R
O
1200
C
N
N
R
4(a-l)
1(a-l)
2
3
Scheme 1. Silica chloride catalyzed synthesis of fully substituted synthesis of pyrazoles.
Table 1
Optimization of catalysts.
Catalystsa,b
Reaction time (h)
Yieldc (%)
None
AcOH
ZnCl2
I2
>24
Trace
56
8
5
3
2
58
62
SiO2Cl
80
a
Amount of the catalyst 30 mol%.
Reaction was carried at 120 8C.
Isolated yield of pyrazole (4b).
b
c

D.V. Jawale et al. / Chinese Chemical Letters 22 (2011) 1187–1190
1189
Table 2
Optimization of amount of catalyst.
Catalyst (mol%)a
Reaction time (h)
Yield (%)b
Catalyst (mol%)a
Reaction time (h)
Yield (%)b
0
5
24
10
8
Trace
36
20
25
30
35
3
2
2
2
63
75
80
80
10
15
48
5
55
a
Reaction was carried at 120 8C.
Isolated yield of pyrazoles (4b).
b
concentration of silica chloride the yields of pyrazoles were increased. The optimum concentration of silica chloride
was found to be 30 mol%. With increase in the amount of silica chloride above 30 mol% there was no any substantial
change in the yield and the reaction time (Table 2).
The fascinating scope of this synthetic strategy to afford the fully substituted pyrazoles alleviates the use of toxic
and hazardous organic solvents and non readily available catalysts. This method is useful for both electron donating as
well as electron withdrawing substituent on the aromatic ring of the aldehydes (Table 3).
It was also noted that the heterogeneous catalyst, silica chloride can be recycled and reused in the cyclo-
condensations without any activation for the same reactions and gave better yields of the pyrazoles (4b, Table 4).
The silica chloride has labile chlorine and therefore is known as Lewis acid and also acts as dehydrating agent [26].
In this condensation it might be enhancing electrophilic character of aldehydic carbon and hence would have expedited
the formation of the intermediates, hydrazones. The addition of the hydrazones and enolate, generated from ethyl
acetoacetate might be undergoing rapid cyclization to pyrazolines in presence of silica chloride. The role of silica
chloride as catalyst in the cyclocondensation has presented in Scheme 2.
In conclusion a simple, convenient one-pot synthetic protocol has been developed for the synthesis of fully
substituted pyrazoles. This new synthetic strategy markedly improve the synthetic efficiency, decreases the production
Table 3
Silica chloride catalyzed one-pot synthesis of fully substituted pyrazoles under solvent free condition.
Productsb
R
Yielda (%)
Melting point (8C)
4a
4b
4c
4d
4e
4f
H
76
80
78
79
74
77
75
78
73
74
73
72
143–144
178–179
110–111
176–177
140–141
162–163
178–179
138–139
124–125
175–176
158–159
161–162
4-Br
4-F
4-Cl
3,4-OCH3
3-Cl
4g
4h
4i
4-OH
2-Cl
4-OCH3
4-CH3
4j
4k
4l
3-OH-4-OCH3
3-OH-4-OCH2CH3
a
Isolated yield.
All the products are characterized by 1H NMR and mass spectral analyses.
b
Table 4
Recycling and reusability of silica chloride.
Run
Time (h)a
Yieldb (%)
Ist
2
2
2
2
80
76
73
70
IInd
IIIrd
IVth
a
Reaction was carried at 120 8C.
Isolated yield of pyrazoles (4b).
b

1190
D.V. Jawale et al. / Chinese Chemical Letters 22 (2011) 1187–1190
NHNH
CHO
2
R
R
H
N
N
N
O
R
N
N
O
N
H
R
H
Aromitization
Cl
H
O
Cl
4(a-l)
1
2
O
O
O
-Cl
O
O
OH
SiO
HCl
O
O
2
O
3
SiO
2
SiO
2
SiO
2
Scheme 2. Plausible mechanism for the formation of fully substituted pyrazoles using silica chloride.
of chemical waste without the use of highly toxic reagents and gives fully substituted pyrazoles for making the library
of new pyrazoles.
Acknowledgments
Authors are thankful to Professor D.B. Ingle for his invaluable discussion and guidance and one of the authors, D.V.
Jawale is also thankful to U.G.C, New Delhi, India for the meritorious student research fellowship.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the online version, at doi:10.1016/
j.cclet.2011.05.016.
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