Efficient sonochemical synthesis of novel 3,5-diaryl-4,5-dihydro-1H-pyrazole-1-carboximidamides

Article abstract of DOI:10.1016/j.ultsonch.2009.06.013

An ultrasound-assisted preparation of a series of novel 3,5-diaryl-4,5-dihydro-1H-pyrazole-1-carboximidamides that proceeds via the efficient reaction of chalcones with aminoguanidine hydrochloride under clean conditions is described.

Full text of DOI:10.1016/j.ultsonch.2009.06.013

Ultrasonics Sonochemistry 17 (2010) 34–37
Contents lists available at ScienceDirect
Ultrasonics Sonochemistry
journal homepage: www.elsevier.com/locate/ultsonch
Efficient sonochemical synthesis of novel 3,5-diaryl-4,5-dihydro-1H-pyrazole-
1-carboximidamides
Lucas Pizzuti a,b, Paula L.G. Martins a, Bruna A. Ribeiro a, Frank H. Quina a,c, Ernani Pinto e, Alex F.C. Flores b,
,
*
Dalila Venzke d, Claudio M.P. Pereira a,c,d,
*
a Centro de Capacitação e Pesquisa em Meio Ambiente (CEPEMA-USP), Universidade de São Paulo, 11573-000 Cubatão, SP, Brazil
b Departamento de Química, Universidade Federal de Santa Maria, 97105-900 Santa Maria, RS, Brazil
c Instituto de Química, Universidade de São Paulo, CP 26077, 05513-970 São Paulo, SP, Brazil
d Departamento de Química e Geociências, Universidade Federal de Pelotas, 96010-900 Pelotas, RS, Brazil
e Departamento de Análises Clínicas e Toxicológicas, Faculdade de Ciências Farmacêuticas, Universidade de São Paulo, 05508-900 São Paulo, SP, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
An ultrasound-assisted preparation of a series of novel 3,5-diaryl-4,5-dihydro-1H-pyrazole-1-carboximi-
damides that proceeds via the efficient reaction of chalcones with aminoguanidine hydrochloride under
clean conditions is described.
Received 27 February 2009
Received in revised form 17 June 2009
Accepted 17 June 2009
Ó 2009 Elsevier B.V. All rights reserved.
Available online 22 June 2009
Keywords:
Pyrazoles
Green chemistry
Carboximidamides
Ultrasonic irradiation
1. Introduction
Recently, we have reported the synthesis of many heterocycles
under mild conditions, such as microwaves, sonochemistry and
Pyrazole and derivatives are key substructures in a large variety
of compounds with important biological activities and pharmaco-
logical properties [1–3]. On the other hand, the use of ultrasonic
irradiation to activate organic reactions has recently taken on a
new dimension. It has been used for a great variety of organic reac-
tions, such as oxidations, reductions, cleavage of epoxides, imina-
tion of aldehydes, multicomponent reactions, preparation of
acetylenes, synthesis of ionic liquids, etc. [4–7].
The phenomenon responsible for the beneficial effects of ultra-
sound on chemical reactions is cavitation. During the rarefaction cy-
cle of the wave, the molecules of the liquid are separated, generating
bubbles that subsequently collapse in the compression cycle [8].
These rapid and violent implosions generate short-lived regions with
temperatures of roughly 5000 °C, pressures of about 1000 atm and
heating and cooling rates above 10 billion °C per second [9]. Such
localized hot spots can be thought as microreactors in which the en-
ergy of sound is transformed into a useful chemical form.
aqueous media [9–14]. These observations and our interest in the
clean production of organic compounds triggered our interest in
examining the applicability of ultrasound to synthesize novel pyr-
azole derivatives. We sought methods for conversion of chalcones
to pyrazoles via condensation with hydrazines in the presence of
KOH and ethanol. For the preparation of pyrazole nucleus, an
examination of previously reported methods revealed serious syn-
thetic drawbacks, such as a mixture of products, long reaction
times, high temperatures and use of catalysts [15]. The increasing
environmental consciousness worldwide has triggered the search
for new products and processes that are compatible with an eco-
friendly environment.
2. Method
2.1. Apparatus and analysis
Starting chalcones 1 were prepared according to the literature
and characterized by 1H and 13C NMR [16]. All solvents and
reagents were obtained from Aldrich and used without further
purification. High-resolution mass analyses were carried out on a
time-of-flight instrument (Micro-TOF) equipped with an ESI
source, operated in the positive ion mode. Samples dissolved in
* Corresponding authors. Address: Centro de Capacitação e Pesquisa em Meio
Ambiente (CEPEMA-USP), Universidade de São Paulo, 11573-000 Cubatão, SP,
Brazil. Tel.: +55 13 33629367; fax: +55 13 33629363 (C.M.P. Pereira).
E-mail addresses: alexflores@smail.ufsm.br (A.F.C. Flores), claudio.martin@pq.
cnpq.br, claudiochemistry@gmail.com (C.M.P. Pereira).
1350-4177/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.ultsonch.2009.06.013

L. Pizzuti et al. / Ultrasonics Sonochemistry 17 (2010) 34–37
35
MeOH/H2O 1:1 (ꢀ0.5 mg/ml) were introduced into the source at
5 ml/min via an infusion pump (Cole-Parmer, Vernon Hills, IL,
USA). Nitrogen gas was used for nebulising (2 bar) and drying
(4 l/min, 160 °C). An accurate-mass calibration was obtained using
a solution of sodium trifluoroacetate (10 mg/ml) as internal stan-
dard. NMR spectra were recorded on a Bruker DPX 300 spectrom-
eter (300.13 MHz for 1H and 75.48 MHz for 13C) at 300 K.
The reactions were carried out with a microtip probe connected
to a 500 Watt Sonics Vibracell ultrasonic processor operating at
20 kHz at 25% of the maximum power output. The progress of reac-
tions was monitored on a Thermo Trace GC Ultra chromatograph,
Column I.D., 0.25 mm; Column length, 30 m; Column Head Pres-
sure, 14 psi.
167 °C; 1H NMR (300 MHz; DMSO-d6): d (ppm) 2.92 (dd, 1H,
JAB = 17.6 Hz, JAX = 5.2 Hz, HA), 3.79 (dd, 1H, JAB = 17.6 Hz,
JBX = 12.0 Hz, HB), 3.83 (s, 3H, OCH3), 5.67 (dd, 1H, JBX = 12.0 Hz,
JAX = 5.2 Hz, HX), 6.84–7.74 (m, 9H, aromatic H); 13C NMR
(75 MHz; DMSO-d6): d (ppm) 41.5 (C-4), 55.4 (OCH3), 55.9 (C-5),
111.1, 120.1, 125.3, 126.0, 128.1, 128.4, 129.1, 130.1, 131.7, 149.4
(12C, Ar), 154.2 (C-3), 155.6 (C(NH)NH2).
5-(4-Methoxyphenyl)-3-phenyl-4,5-dihydro-1H-pyrazole-1-
carboximidamide (2d): (0.47 g, 80%) C17H18N4O, 294.35, 147–
149 °C; 1H NMR (300 MHz; DMSO-d6): d (ppm) 3.06 (dd, 1H,
JAB = 17.6 Hz, JAX = 5.8 Hz, HA), 3.71 (s, 3H, OCH3), 3.78 (dd, 1H,
JAB = 17.7 Hz, JBX = 12.1 Hz, HB), 5.43 (dd, 1H, JBX = 12.0 Hz,
JAX = 5.8 Hz, HX), 6.85–7.75 (m, 9H, aromatic H); 13C NMR
(75 MHz; DMSO-d6): d (ppm) 42.7 (C-4), 54.9 (OCH3), 60.2 (C-5),
113.8, 125.9, 126.7, 128.4, 129.0, 131.8, 135.4, 148.2 (12C, Ar),
154.5 (C-3), 158.2 (C(NH)NH2).
2.2. Classical synthesis of 3,5-diaryl-4,5-dihydro-1H-pyrazole-1-
carboximidamides (2)
5-(4-Chlorophenyl)-3-phenyl-4,5-dihydro-1H-pyrazole-1-carb-
oximidamide (2e): (0.58 g, 98%) C16H15ClN4, 298.77, 163–165 °C;
1H NMR (300 MHz; DMSO-d6): d (ppm) 3.04 (dd, 1H, JAB = 17.7 Hz,
JAX = 5.9 Hz, HA), 3.82 (dd, 1H, JAB = 17.7 Hz, JBX = 12.2 Hz, HB), 5.50
(dd, 1H, JBX = 12.1 Hz, JAX = 5.8 Hz, HX), 7.20–7.75 (m, 9H, aromatic
H); 13C NMR (75 MHz; DMSO-d6): d (ppm) 42.4 (C-4), 60.1 (C-5),
126.0, 127.5, 128.3, 128.5, 129.2, 131.3, 131.6, 142.4 (12C, Ar),
148.6 (C-3), 154.5 (C(NH)NH2).
5-(2-Bromophenyl)-3-phenyl-4,5-dihydro-1H-pyrazole-1-carb-
oximidamide (2f): (0.68 g, 99%) C16H15BrN4, 343.22, 168–170 °C;
1H NMR (300 MHz; DMSO-d6): d (ppm) 2.92 (dd, 1H, JAB = 17.7 Hz,
JAX = 5.8 Hz, HA), 3.91 (dd, 1H, JAB = 17.7 Hz, JBX = 12.3 Hz, HB), 5.68
(dd, 1H, JBX = 12.2 Hz, JAX = 5.8 Hz, HX), 7.06–7.74 (m, 9H, aromatic
H); 13C NMR (75 MHz; DMSO-d6): d (ppm) 41.3 (C-4), 60.5 (C-5),
120.8, 126.0, 126.4, 127.8, 128.4, 128.7, 129.1, 131.6, 132.6, 141.8
(12C, Ar), 148.4 (C-3), 154.3 (C(NH)NH2).
A mixture of chalcone 1 (2.0 mmol), aminoguanidine hydro-
chloride (4.0 mmol, 0.44 g) and KOH (4.0 mmol, 0.22 g) in ethanol
(10 ml) was stirred at reflux. After the time indicated (Table 1), the
reaction was allowed to cool, the excess of aminoguanidine hydro-
chloride was removed by filtration in a Büchner funnel and the eth-
anol was evaporated under reduced pressure. The solid residue
was dissolved in water (20 ml) and the product was extracted in
ethyl acetate (3 Â 20 ml). The organic extracts were combined,
dried (MgSO4) and the solvent removed under reduced pressure.
The product 2 obtained was purified by recrystallization from hex-
ane/EtOAc (1:1).
2.3. Ultrasound-promoted synthesis of 3,5-diaryl-4,5-dihydro-1H-
pyrazole-1-carboximidamides (2)
5-(4-Bromophenyl)-3-phenyl-4,5-dihydro-1H-pyrazole-1-carb-
oximidamide (2g): (0.55 g, 80%) C16H15BrN4, 343.22, 136–138 °C;
1H NMR (300 MHz; DMSO-d6): d (ppm) 3.08 (dd, 1H, JAB = 17.8 Hz,
JAX = 5.5 Hz, HA), 3.85 (dd, 1H, JAB = 17.8 Hz, JBX = 12.1 Hz, HB), 5.52
(dd, 1H, JBX = 12.0 Hz, JAX = 5.5 Hz, HX), 7.15–7.77 (m, 9H, aromatic
H); 13C NMR (75 MHz; DMSO-d6): d (ppm) 42.5 (C-4), 60.1 (C-5),
120.0, 126.2, 127.8, 128.5, 129.4, 131.3, 131.4, 142.3 (12C, Ar),
149.7 (C-3), 154.2 (C(NH)NH2).
5-(3,4-Dimethoxyphenyl)-3-phenyl-4,5-dihydro-1H-pyrazole-
1-carboximidamide (2h): (0.64 g, 99%) C18H20N4O2, 324.38, 146–
148 °C; 1H NMR (300 MHz; DMSO-d6): d (ppm) 3.08 (dd, 1H,
JAB = 17.7 Hz, JAX = 5.8 Hz, HA), 3.71 (s, 3H, OCH3), 3.72 (s, 3H,
OCH3), 3.81 (dd, 1H, JAB = 17.7 Hz, JBX = 12.0 Hz, HB), 5.44 (dd, 1H,
JBX = 11.9 Hz, JAX = 5.8 Hz, HX), 6.70–7.77 (m, 8H, aromatic H); 13C
NMR (75 MHz; DMSO-d6): d (ppm) 42.9 (C-4), 55.4 (OCH3), 55.5
(OCH3), 60.6 (C-5), 109.2, 111.9, 117.3, 126.1, 128.5, 129.3, 131.6,
135.4, 147.8, 149.4 (12C, Ar), 148.7 (C-3), 154.4 (C(NH)NH2).
5-(2,4-Dichlorophenyl)-3-phenyl-4,5-dihydro-1H-pyrazole-1-
carboximidamide (2i): (0.58 g, 87%) C16H14Cl2N4, 333.22, 94–96 °C;
1H NMR (300 MHz; DMSO-d6): d (ppm) 2.99 (dd, 1H, JAB = 17.7 Hz,
JAX = 6.1 Hz, HA), 3.91 (dd, 1H, JAB = 17.7 Hz, JBX = 12.3 Hz, HB), 5.69
(dd, 1H, JBX = 12.3 Hz, JAX = 6.0 Hz, HX), 7.08–7.75 (m, 8H, aromatic
H); 13C NMR (75 MHz; DMSO-d6): d (ppm) 40.9 (C-4), 58.1 (C-5),
126.1, 127.4, 127.9, 128.5, 128.8, 129.3, 131.4, 131.6, 139.4 (12C,
Ar), 149.0 (C-3), 154.3 (C(NH)NH2).
A mixture of chalcone 1 (2.0 mmol), aminoguanidine hydro-
chloride (4.0 mmol, 0.44 g) and KOH (4.0 mmol, 0.22 g) in ethanol
(10 ml) was irradiated in an ultrasound probe at 25–30 °C for the
appropriate time (see Table 1). After filtration in a Büchner funnel,
to remove the excess of the aminoguanidine hydrochloride, the
ethanol was evaporated under reduced pressure. The solid residue
was dissolved in water (20 ml) and the product was extracted in
ethyl acetate (3 Â 20 ml). The organic extracts were combined,
dried (MgSO4) and the solvent removed under reduced pressure.
Finally, the 3,5-diaryl-4,5-dihydro-1H-pyrazole-1-carboximida-
mides were obtained in excellent purity.
2.4. Data spectra of products 2a-j
3,5-Diphenyl-4,5-dihydro-1H-pyrazole-1-carboximidamide (2a):
(0.40 g, 75%) C16H16N4, 264.33, 167–169 °C; 1H NMR (300 MHz;
DMSO-d6): d (ppm) 3.05 (dd, 1H, JAB = 17.7 Hz, JAX = 5.6 Hz, HA),
3.84 (dd, 1H, JAB = 17.7 Hz, JBX = 12.1 Hz, HB), 5.53 (dd, 1H,
JBX = 12.0 Hz, JAX = 5.5 Hz, HX), 7.20–7.77 (m, 10H, aromatic H);
13C NMR (75 MHz; DMSO-d6): d (ppm) 42.8 (C-4), 60.6 (C-5),
125.4, 126.1, 126.9, 128.4, 128.5, 129.2, 131.6, 143.1 (12C, Ar),
149.1 (C-3), 154.3 (C(NH)NH2).
5-(2-Methylphenyl)-3-phenyl-4,5-dihydro-1H-pyrazole-1-car-
boximidamide (2b): (0.42 g, 75%) C17H18N4, 278.35, 188–190 °C;
1H NMR (300 MHz; DMSO-d6): d (ppm) 2.38 (s, 3H, CH3), 2.91
(dd, 1H, JAB = 17.6 Hz, JAX = 5.8 Hz, HA), 3.87 (dd, 1H, JAB = 17.6 Hz,
JBX = 12.2 Hz, HB), 5.61 (dd, 1H, JBX = 12.2 Hz, JAX = 5.8 Hz, HX),
6.95–7.75 (m, 9H, aromatic H); 13C NMR (75 MHz; DMSO-d6): d
(ppm) 18.9 (CH3), 41.5 (C-4), 58.0 (C-5), 124.1, 125.9, 125.9,
126.6, 128.5, 129.1, 130.3, 131.8, 133.9, 141.2 (12C, Ar), 148.5 (C-
3), 154.4 (C(NH)NH2).
3-Phenyl-5-(3,4,5-trimethoxyphenyl)-4,5-dihydro-1H-pyrazole-
1-carboximidamide (2j): (0.64 g, 90%) C19H22N4O3, 354.40, 179–
181 °C; 1H NMR (300 MHz; DMSO-d6): d (ppm) 3.09 (dd, 1H,
JAB = 17.7 Hz, JAX = 6.5 Hz, HA), 3.63 (s, 3H, OCH3), 3.73 (s, 6H,
OCH3), 3.81 (dd, 1H, JAB = 17.7 Hz, JBX = 12.2 Hz, HB), 5.41 (dd, 1H,
JBX = 12.0 Hz, JAX = 6.5 Hz, HX), 6.53–7.76 (m, 7H, aromatic H); 13C
NMR (75 MHz; DMSO-d6): d (ppm) 42.8 (C-4), 55.7 (2 OCH3),
59.8 (OCH3), 61.1 (C-5), 102.6, 126.1, 128.5, 129.2, 131.7, 136.3,
139.0, 152.9 (12C, Ar), 149.0 (C-3), 154.8 (C(NH)NH2).
5-(2-Methoxyphenyl)-3-phenyl-4,5-dihydro-1H-pyrazole-1-
carboximidamide (2c): (0.58 g, 98%) C17H18N4O, 294.35, 165–

36
L. Pizzuti et al. / Ultrasonics Sonochemistry 17 (2010) 34–37
Table 1
Preparation of 3,5-diaryl-4,5-dihydro-1H-pyrazole-1-carboximidamides 2.
Chalcone (1)
Product (2)a
MP (°C)
Sonochemical method
Classical method
Time (min)
30
Yieldb (%)
75
Time (h)
3
Yieldb (%)
57
Ph
167À169
O
N
N
Ph
1a
HN
NH2
2a
Ph
Me
O
Me
N
188À190
165À167
147À149
30
30
30
75
98
80
6
6
6
64
60
69
N
Ph
1b
HN
NH2
2b
Ph
N
MeO
O
OMe
N
Ph
1c
HN
NH2
2c
Ph
N
O
Ph
N
OMe
1d
OMe
HN
NH2
2d
Ph
N
O
163À165
168À170
136À138
146À148
30
30
30
30
98
99
80
99
6
6
6
6
66
65
60
66
Ph
N
Cl
1e
Cl
HN
NH2
2e
Ph
N
Br
O
Br
N
Ph
1f
HN
NH2
NH2
NH2
2f
Ph
N
O
Ph
N
Br
1g
Br
HN
2g
Ph
N
O
OMe
OMe
OMe
OMe
Ph
N
1h
HN
2h
Ph
N
Cl
O
Cl
94À96
30
30
87
90
6
3
61
63
Ph
N
Cl
1i
Cl
HN
NH2
2i
Ph
N
O
OMe
OMe
OMe
OMe
Ph
179À181
N
1j
HN
NH2
OMe
OMe
2j
a
Novel compounds.
Yields of isolated products.
b

L. Pizzuti et al. / Ultrasonics Sonochemistry 17 (2010) 34–37
37
for all compounds of less than 5 ppm. More investigations are cur-
rently in progress to explore the reaction.
NH
NH2•HCl
Ph
H2N
N
H
O
N
Ar
NH
N
4. Conclusion
Ph
Ar
KOH, EtOH, )))), 30 min
H2N
−
1a j
In summary, we report a very simple, fast and environmentally
benign synthesis of novel 3,5-diaryl-4,5-dihydro-1H-pyrazole-1-
carboximidamides by simple mixing of the reactants in ethanol un-
der ultrasonic irradiation.
−
2a j
Scheme 1. Synthesis of 3,5-diaryl-4,5-dihydro-1H-pyrazole-1-carboximidamides
2.
Acknowledgements
3. Results and discussion
The authors are grateful to FAPESP, CEPEMA-USP (Centro de
Capacitação e Pesquisa em Meio Ambiente), CNPq (310472/2007-
5, 475575/2008-3), INCT Estudos do Meio Ambiente/CNPq
(573.667/2008-0) and CAPES for financial support.
Strict legislative restrictions on pollution exposure have en-
forced the application of biorenewable solvents in practice [17].
In this endeavor, we report here the synthesis of novel 3,5-dia-
ryl-4,5-dihydro-1H-pyrazole-1-carboximidamides by the reaction
of chalcones with aminoguanidine hydrochloride in KOH. The
starting compounds 1 (chalcone derivatives) were obtained from
acetophenone (0.02 mol) and the appropriate aldehydes
(0.02 mol) by known methods [16].
The mixture of reagents was sonicated by use of an ultrasound
probe for 30 minutes. In our studies, we found that ethanol was the
solvent most appropriate for these reactions, giving the best re-
sults. The scope and generality of this process are illustrated by
the preparation of the series of ten pyrazoles shown in the Table
1 (Scheme 1). We have developed a mild and eco-friendly protocol
for the preparation of novel heterocycles, 3,5-diaryl-4,5-dihydro-
1H-pyrazole-1-carboximidamides under ultrasonic irradiation.
The progress of the reactions was monitored by gas chromatog-
raphy and the pyrazole derivatives were isolated after simple fil-
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All compounds 2 were characterized by 1H NMR and 13C NMR.
The structures 2 were confirmed by mass analyses with errors