TBHP/Cu(OAc)2 mediated oxidation of pyrazolines: A convenient method for the preparation of pyrazoles

Article abstract of DOI:10.1080/00397911.2021.1885716

An efficient and simple oxidative protocol has been developed for the preparation of pyrazoles from pyrazolines mediated by TBHP/Cu(OAc)₂ at room temperature. The present protocol has been successfully applied for the preparation of various pyrazole compounds from heterocyclic pyrazolines.

Full text of DOI:10.1080/00397911.2021.1885716

Synthetic Communications
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Chemistry
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TBHP/Cu(OAc)2 mediated oxidation of pyrazolines:
A convenient method for the preparation of
pyrazoles
Sai Teja Kolla, Ramya Somanaboina & China Raju Bhimapaka
To cite this article: Sai Teja Kolla, Ramya Somanaboina & China Raju Bhimapaka (2021) TBHP/
Cu(OAc)2 mediated oxidation of pyrazolines: A convenient method for the preparation of pyrazoles,
Synthetic Communications, 51:9, 1425-1432, DOI: 10.1080/00397911.2021.1885716
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SYNTHETIC COMMUNICATIONSV
2021, VOL. 51, NO. 9, 1425–1432
https://doi.org/10.1080/00397911.2021.1885716
TBHP/Cu(OAc)2 mediated oxidation of pyrazolines: A
convenient method for the preparation of pyrazoles
Sai Teja Kollaa,b, Ramya Somanaboinaa,b, and China Raju Bhimapakaa,b
aDepartment of Organic Synthesis & Process Chemistry, CSIR-Indian Institute of Chemical Technology,
b
Hyderabad, India; Academy of Scientific and Innovative Research (AcSIR), Ghaziabad, India
ABSTRACT
ARTICLE HISTORY
Received 29 December 2020
An efficient and simple oxidative protocol has been developed for
the preparation of pyrazoles from pyrazolines mediated by TBHP/
Cu(OAc)2 at room temperature. The present protocol has been suc-
cessfully applied for the preparation of various pyrazole compounds
from heterocyclic pyrazolines.
KEYWORDS
Pyrazolines; pyrazoles;
Cu(OAc)2; TBHP
GRAPHICAL ABSTRACT
Introduction
Pyrazolines[1,2] and pyrazoles[3,4] are important heterocyclic compounds plays an
important role in the pharmaceutical and agrochemical industries. Pyrazole scaffold is
present in drug molecules such as celecoxib, lonazolac and Viagra. Ruxolitinib, Ipazilide
and Fezolamine are furthermore important pyrazole drug molecules are well docu-
mented in the literature.[5] Further, pyrazoles act as ligands in coordination chemistry;
serve as optical brighteners and UV stabilizers.[6] The most common method for the
preparation of pyrazoles involves the condensation of hydrazine derivatives with 1,3-
dicarbonyl compounds.[7–9] 1,3-Dipolar addition reactions of diazo compounds with
olefins or alkynes and transition-metal catalyzed cross-coupling reactions are other
approaches are available for the preparation of pyrazoles.[10,11] Recently, Togo et al.
reported the 3-arylpyrazoles by the reaction of arenes with b-bromopropionyl chloride
and hydrazines.[12]
Conversely, pyrazolines are important heterocycles have shown a wide range of
pharmacological properties such as anticancer, antibacterial, antidepressant activity and
anti-WN virus activities.[13–16] Due to the attractive medicinal properties of the pyrazo-
lines various efficient approaches have been developed for the preparation of these
CONTACT China Raju Bhimapaka
chinaraju@iict.res.in
Department of Organic Synthesis & Process Chemistry,
CSIR-Indian Institute of Chemical Technology, Hyderabad 500007, India.
Supplemental data for this article can be accessed on the publisher’s website.
ß 2021 Taylor & Francis Group, LLC

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S. T. KOLLA ET AL.
compounds. Matthias Beller et al. reported regioselective synthesis of pyrazolines from
phenylhydrazines with 3-butynol in the presence of Zn(OTf)2.[17] Ohno et al. reported
poly substituted pyrazolines by three-component, one-pot reaction between aldehydes,
hydrazines and alkynes promoted by gold catalyst.[18] Wang et al. achieved pyrazolines
by three-component annulations reaction of aldehydes, hydrazines and alkenes catalyzed
by Cu(OTf)2.[19] Heng-shan Wang et al. reported series of pyrazolines starting from
propargyl alcohols and hydrazines using t-BuOK as a catalyst.[20]
The oxidation of pyrazolines to pyrazoles is one of the quite simple method due to
its simplicity and versatility. To the best of our knowledge, there is only one oxidative
method which is available in the literature for the preparation of 3-methyl-1-phenyl-
1H-pyrazole from 3-methyl-1-phenyl-4,5-dihydro-1H-pyrazole has been reported by
Kost et al. with sulfur at 180–200 ꢀC.[21] Bismuth(III) nitrate pentahydrate
(Bi(NO3)3.5H2O) has been used as a mild, efficient and inexpensive oxidant for the
aromatization of several 1,3,5-trisubstituted 2-pyrazolines to pyrazoles in acetic acid
under microwave irradiation.[22] The oxidation of pyrazolines has been achieved by Liu
et al. by the treatment of catalytic amount of HIO3 or I2O5 in water.[23] Yu et al.
reported 1,3,5-trisubstituted pyrazolines were oxidized to the corresponding pyrazoles in
high yields by molecular oxygen in the presence of catalytic amount of N-hydroxyph-
thalimide (NHPI) and Co(OAc)2 in acetonitrile at room temperature.[24] A simple and
high yielding method for the synthesis of tri-substituted pyrazoles via Fe (III) catalyzed
aerobic oxidative aromatization of pyrazolines has been reported by Balakrishna and
coworkers[25] Off late, Hayashi et al. has reported the catalytic oxidation of pyrazolines
by Pd/C under aerobic condition.[26]
However, most of these reagents occupied with several disadvantages such as time
consuming, unavailability of the reagents and toxicity due to the presence of certain
toxic elements with unsatisfactory yields of the products. Thus, in view of these draw-
backs, there is a need to search for better reagents which gives high yields, environmen-
tally safe and commercially available reagents for the conversion of pyrazolines to
pyrazoles. As part of our ongoing research, we have reported the synthesis of pyrazoles,
triazolyl pyrazoles and chromenopyrazoles.[27–29] Herein, we report the oxidation of
pyrazolines to pyrazoles mediated by TBHP/Cu(OAc)2.[30] Further, we have prepared
heterocyclic pyrazolines and converted them into pyrazoles conveniently and the results
are depicted below.
Results and discussion
In an initial experiments, the 3-methyl-1-phenyl-4,5-dihydro-1H-pyrazole 3a has been
chosen as a substrate to establish the reaction conditions. The required substrate 3a was
prepared by the reaction of but-3-en-2-one (1a, 1.0 equiv.) with phenylhydrazine hydro-
chloride (2a, 1.0 equiv.) in the presence of glacial AcOH at room temperature (Scheme
1, 50–60 ꢀC).[31] Thus obtained pyrazoline (3a, 1.0 equiv.) was subjected for oxidation
with catalysts such as NBS, DDQ, CAN and Oxone (1.0 equiv., Scheme 1). The pyrazole
compound 4a procures 50% yield at room temperature and 90% yield under reflux con-
ditions with NBS (Table 1, entries 2–5). The pyrazole compound 4a was obtained in
80% yield with Oxone but it is a time consuming process. The compound 4a was

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Scheme 1. Preparation of 3-methyl-1-phenyl-1H-pyrazole 4a.
Table 1. Optimization of reaction conditions for the preparation of 4a.
Entry
Catalyst (equiv.)
Peroxide (equiv.)
Solvent
Time (h)
Yield (%)a,b
01
02
03
04
05
06
07
08
09
10
11
12
13
14
15
16
17
18
19
AcOH (1.0)
NBS (1.0)
DDQ (1.0)
CAN (1.0)
Oxone (1.0)
CuI (1.0)
CuCl2 (1.0)
Cu(OAc)2 (1.0)
Cu(OAc)2 (1.0)
Cu(OAc)2 (1.0)
Cu(OAc)2 (0.3)
–
–
–
–
–
–
–
–
–
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
DCM
48
24
48
48
24
24
24
24
24
48
48
24
18
24
0.1
02
04
09
1.5
15
50
20
60
80
20
82
Trace
10
60
40
60
80
60
95
92
90
90
92
–
–
–
EtOH
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
TBHP (1.0)
TBHP (5.0)
TBHP (1.0)
TBHP (5.0)
TBHP (3.0)
TBHP (3.0)
TBHP (2.0)
TBHP (5.0)
–
Cu(OAc)2 (0.3)
Cu(OAc)2 (0.3)
Cu(OAc)2 (0.3)
Cu(OAc)2 (0.15)
Cu(OAc)2 (0.1)
Cu(OAc)2 (0.1)
aRoom temperature.
bIsolated yields.
The bold values indicating the better condition.
characterized by spectral data and compared with literature (see Supporting
Information).[21]
Next, the reaction 3a has been tested with copper catalysts such as CuI, CuCl2 and
Cu(OAc)2 (1.0 equiv.) in solvents such as DCM, CH3CN and C2H5OH (Table 1, entry
6–11). The pyrazole 4a was obtained in very good yields with CuCl2 and Cu(OAc)2 in
acetonitrile. The product yield was reduced drastically (40%), when the reaction con-
ducted with 30 mol% equivalent of copper catalyst and the reaction takes long time
(Table 1, entry 11). The reaction also conducted with peroxide tert-butyl hydroperoxide
(TBHP, 70 wt% in water, 1.0 & 5.0 equiv.) in acetonitrile at room temperature (Table 1,
entries 12–13). The reaction required 5.0 equivalents of TBHP to complete the reaction
which gives 4a. Then, we have planned the reaction with the combination of Cu(OAc)2
and TBHP. Accordingly, pyrazoline 3a (1.0 equiv.) was stirred in acetonitrile with cata-
lytic amount of Cu(OAc)2 (30 mol%) in the presence of TBHP (tert-butyl hydroperoxide

1428
S. T. KOLLA ET AL.
Table 2. Synthesis of pyrazoles 4a–4q.
Entry
Compound
R
R1
R2
Yield (%)a
1
2
3
4
5
6
7
8
4a
4b
4c
4d
4e
4f
4g
4h
4i
4j
4k
4l
4m
4n
4o
4p
4q
4r
H
H
H
H
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
Ph
H
CH3
OCH3
Cl
Br
H
CH3
OCH3
Cl
Br
H
CH3
OCH3
Cl
Br
m-CH3
H
91
78
87
92
80
80
83
95
92
85
86
95
94
96
95
71
68
–
H
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
9
10
11
12
13
14
15
16
17
18b
Ph
Ph
Ph
Ph
Ph
p-Cl Ph
p-OMe Ph
H
aIsolated yields of pyrazoles 4a–q.
bNo reaction.
(–70 wt% in water, 1.0 & 5.0 equiv.) at room temperature. Interestingly, the combin-
ation of these catalysts worked well and produced pyrazole 4a obtained 95% yield in
10 min (Table 1, entries 14–15). Further, we have reduced the mole ratio of reagents
and the couple of reactions have been conducted with Cu(OAc)2 (10–30 mol%) and
TBHP (2.0–5.0 equiv, Table 1, entries 16–19) provided the compound 4a but this modi-
fications took longer hours (1.5–9 h).
Hence, Cu(OAc)2 (30 mol%) in the presence of TBHP (5.0 equiv.) with acetonitrile at
room temperature was found to be better condition to get the compound 4a with very
good yield. This, interesting result prompted us to explore the methodology with several
of pyrazolines to pyrazoles and the results are depicted in Table 2. The required pyrazo-
lines 3b–q were prepared from (E)-pent-3-en-2-one 1a, (E)-4-phenylbut-3-en-2-one 1b
and chalcones 1c with phenylhydrazine hydrochlorides 2a–e in the presence of glacial
acetic acid at 50–60 ꢀC in methanol. The pyrazolines 3b–q are known compounds and
compared with literature.[17,32–34] Thus, obtained pyrazolines 3b–q were subjected for
oxidation with Cu(OAc)2 (30 mol%) in presence of TBHP (5.0 equiv.) in acetonitrile at
room temperature. These reactions were undergone facilely and provided the pyrazoles
4a–q (Table 2). These pyrazoles 4a–q are known compounds and compared with litera-
[17,21,32,35–39]
ture.
Having achieved the preparation of pyrazoles 4a–q, the method has been applied to
another set of pyrazoline compounds to show the diversity of present protocol. The

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Table 3. Oxidation of pyrazolines 3s–3za to pyrazoles 4s–4za.
aIsolated yields of pyrazoles 4s–za.
pyrazolines such as furan (3s), thiophene (3t), pyrazole (3u), pyridyl (3v), 2-chloropyr-
idyl (3w–y), quinoline (3z), coumarin (3za) have been prepared from corresponding
carboxaldehydes with phenylhydrazine hydrochlorides 2a–c as per the established
experimental procedure (see Supporting Information). The pyrazoles 3s–t, 3v–y are
known compounds and compared with literature.[40–43] The pyrazolines 3u, 3z and 3za
are unknown and characterized by spectral data (see Supporting Information). Thus
obtained pyrazoline compounds 3s–za were subjected for the oxidation under the opti-
mized conditions which are provided series of corresponding pyrazoles 4s–za. The pyra-
zoles 4s–t, 4v are known compounds and compared with literature.[41,43–45] The
compounds 4u, 4w–za are unknown and characterized by spectral data and incorpo-
rated in Supporting Information (Table 3).
Conclusion
In conclusion, an efficient and inexpensive method has been developed for the oxidative
aromatization of pyrazolines to pyrazoles in the presence of catalytic amount of
Cu(OAc)2 with TBHP (70 wt% in water) at room temperature. The procedure offers
several advantages such as simple experimental procedures, utilization of commercially
available catalyst with good yields which make this an attractive alternate approach for
the preparation of pyrazoles.

1430
S. T. KOLLA ET AL.
Experimental section
Procedure for the preparation of 3-methyl-1-phenyl-1H-pyrazole (4a)
Cu(OAc)2 (10 mol%) was added to a stirred solution of 4,5-dihydro-1H-pyrazole (3a,
1.0 mmol) in acetonitrile at room temperature and followed by TBHP (tert-butyl hydro-
peroxide (70 wt% in water, 5.0 mmol). The reaction mixture was stirred at the same
temperature and the reaction was monitored by TLC. After completion of the reaction
(10 min), the solvent was removed under reduced pressure and subjected for the column
chromatography purification using silica gel (60:120, ethyl acetate/hexane 2:98) afforded
1H-pyrazole (4a) as colorless solid. The pyrazoles 4b–za were prepared from corre-
sponding pyrazolines 3b–za under optimized reaction conditions.
1
Supporting Information (copies of H, 13C NMR and HRMS spectra of all the pre-
pared compounds) associated with this article can be found through the
“Supplementary Content” section of this article’s webpage.
Acknowledgment
We thank Director, CSIR-IICT for providing the research facilities.
Funding
B. China Raju acknowledges SERB (DST), India for the financial support [EEQ/2017/000314].
IICT manuscript communication no. IICT/Pubs./2020/280.
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