Copper(ii) ionic liquid catalyzed cyclization-aromatization of hydrazones with dimethyl acetylenedicarboxylate: A green synthesis of fully substituted pyrazoles

Article abstract of DOI:10.1039/c3nj40756j

The Lewis acid room temperature ionic liquid, [n-Bu₄P][CuBr ₃], was found to be an efficient and reusable catalyst for three component synthesis of fully substituted pyrazoles from the reaction of aldehydes, arylhydrazines and dimethyl acetylenedicarboxylate (DMAD). This catalytic system is simple and chemoselective with high yields.

Full text of DOI:10.1039/c3nj40756j

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Copper(II) ionic liquid catalyzed cyclization–aromatization
of hydrazones with dimethyl acetylenedicarboxylate:
a green synthesis of fully substituted pyrazoles†
Cite this: DOI: 10.1039/c3nj40756j
Shirin Safaei, Iraj Mohammadpoor-Baltork,* Ahmad Reza Khosropour,*
Majid Moghadam, Shahram Tangestaninejad and Valiollah Mirkhani
Received (in Montpellier, France)
27th August 2012,
The Lewis acid room temperature ionic liquid, [n-Bu4P][CuBr3], was found to be an efficient and
reusable catalyst for three component synthesis of fully substituted pyrazoles from the reaction of
aldehydes, arylhydrazines and dimethyl acetylenedicarboxylate (DMAD). This catalytic system is simple
and chemoselective with high yields.
Accepted 1st April 2013
DOI: 10.1039/c3nj40756j
www.rsc.org/njc
Recently, multi-component reactions (MCRs) have been reported to stereoselective synthesis of pyrazolidinones.19 Recently, Huang
be efficient, fast and environmentally benign processes compared and co-workers reported copper(I)-catalyzed synthesis of pyrazoles
to the complicated stepwise methods.1 In fact, the importance of in the presence of sodium acetate.20 However, such reagents
MCRs is due to their wide range of applications in pharmaceutical were found to be inefficient for the conversion of hydrazones
chemistry for the production of diversified structural scaffolds and bearing aliphatic and strongly electron donating groups. Also, a
combinatorial libraries for drug discovery.2
stoichiometric amount of base is required for this reaction.
Pyrazole derivatives have been extensively used in the core Therefore, introduction of new and efficient methods for the
structure of biologically active compounds. For example substituted synthesis of pyrazole derivatives, which proceed under mild and
pyrazoles are used as anti-inflammatory,3 anti-cancer,4 anti- environmentally benign conditions, is still in demand.
angiogenic,5 monoamine oxidase inhibitor6 and nitric oxide
Room temperature ionic liquids as green media or catalysts
carrier7 agents. Due to the importance of these heterocycles, for environmentally friendly and economically attractive processes
significant efforts have been devoted to find new synthetic have found many applications in organic synthesis,21 materials
methods for preparation of pyrazole derivatives. Some of these science,22 electrochemistry23 and separation technology.24 Due to
known methods are 1,3-dipolar cycloaddition of hydrazones the wide range of applications of ionic liquids in academic and
with acetylenic compounds,8 condensation of 1,3-diketones9 or industrial fields, great development in the structure and property
chalcones10 with hydrazines, reaction of arylhydrazines with of ionic liquids has been made over the last two decades. In this
3-butynol11 and condensation of hydrazonoyl chlorides with scope, Sun et al developed aerobic oxidation of phenol25 and 2,3,6-
acetylenic compounds.12 Pyrazoles and pyrazolines are also triethylphenol26 to their corresponding quinones in the presence of
prepared by 1,3-dipolar cycloaddition of nitrile imines with [bmim][CuCl3]. Comparison of the results obtained using this
acetylenic compounds13 or cyclic a,b-unsaturated ketones,14 catalytic system with some of those reported in the literature using
[3+2] dipolar cycloaddition of 4-halosydnones with 1-haloalkynes,15 different oxidizing agents or copper(II) salts shows that the reaction
1,3-dipolar cycloaddition of nitrile imines to benzyne,16 iodo- is more selective and environmentally friendly using [bmim][CuCl3].
cyclization of acetylenic hydrazides,17 and cyclization of hydrazone Also the reaction in the presence of catalytic amount of this ionic
dianions with diethyl oxalate.18 1,3-Dipolar cycloaddition of hydra- liquid leads to higher yields and shorter reaction times. In continua-
zones with a-oxo-ketenes has been reported for the three-component tion of our interest towards the development of environmentally-
friendly methods for heterocyclic synthesis,27 herein, we report a
Catalysis Division, Department of Chemistry, University of Isfahan,
Isfahan 81746-73441, Iran. E-mail: imbaltork@sci.ui.ac.ir,
novel and efficient procedure for the synthesis of pyrazoles by simple
cyclization–oxidation of hydrazones with dimethyl acetylenedi-
carboxylate (DMAD) in the presence of copper(II) ionic liquid,
[n-Bu4P][CuBr3], as a reusable catalyst (Scheme 1).
The experimental procedure for these reactions is generally
simple and clean, no toxic organic solvent or corrosive substance
khosropour@chem.ui.ac.ir; Fax: +98 (311)6689732; Tel: +98 (311)7932705
† Electronic supplementary information (ESI) available: Full characterization
data for compounds and crystallographic data for compound 4h. CCDC
867509. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c3nj40756j
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(1.2 mmol) in the presence of [n-Bu4P][CuBr3] (0.25 mmol) at
100 1C was chosen as the optimized reaction conditions.
To survey the scope and generality of this method, a wide
range of aldehydes and arylhydrazines were used under the
optimized reaction conditions (Table 2). The aldehydes bearing
electron-withdrawing or halogen groups in the aromatic ring
reacted smoothly with arylhydrazines (phenylhydrazine, p-chloro
and p-tolylhydrazines) and DMAD in the presence of 25 mol%
[n-Bu4P][CuBr3] to afford the corresponding fully substituted
pyrazoles in good to high yields (Table 2, entries 1–10). The
aliphatic aldehydes such as heptanal and butanal (Table 2,
entries 11 and 12) also reacted efficiently to give the desired
products in good yields. To the best of our knowledge this is the
first report using copper based ionic liquids in the synthesis of
heterocyclic compounds.
Under the same conditions, the reaction of aldehyde or
arylhydrazine bearing strongly electron-donating substituents
like 4-methoxybenzaldehyde or 4-methoxyphenylhydrazine produced
the corresponding pyrazoline and aromatization didn’t occur
even after longer reaction times (Scheme 2). But the compounds
5m and 5n were oxidized to their corresponding pyrazoles using
a stronger oxidant such as H2O2 (Scheme 3).
Scheme 1 Synthesis of pyrazoles via cyclization–aromatization of hydrazones
with DMAD in the presence of [n-Bu4P][CuBr3].
is used and the corresponding products are obtained in high
yields.
In order to achieve the optimized reaction conditions, the
reaction of benzaldehyde, phenylhydrazine and DMAD was
chosen as a model (Table 1). Initially, the model reaction was
carried out in the absence of the catalyst. As shown in Table 1,
the reaction was not successful even after prolonged reaction
times. Then, the reaction was carried out in the presence of
25 mol% CuBr2 and the corresponding product was obtained in
35% yield after 1 h (Table 1, entry 2). In order to liquefy the
reaction mixture for well stirring, we used [n-Bu4P][CuBr3] IL as
a catalyst. The results showed that the reaction was carried out
efficiently and the desired product was obtained in 87% yield
(Table 1, entry 3). Several other Lewis acid ionic liquids such as
[bmim][CuCl3], [bmim][CuBr3], [bmim][FeCl4], [bmim][InCl4]
and [bmim][ZnCl3] were also tested for this reaction (Table 1,
entries 4–8), showing lower yields of the product. It should be
mentioned that in the presence of [bmim][ZnCl3], the corres-
ponding pyrazoline was produced as the major product. Next,
the effects of temperature and different amounts of catalyst
were examined. Lower yields were obtained by decreasing the
temperature and the amount of the catalyst, whereas no
significant improvement was observed when these parameters
were increased (Table 1, entries 9 and 12). Finally, the reaction of
benzaldehyde (1 mmol), phenylhydrazine (1 mmol) and DMAD
The results indicate that the present protocol is potentially
applicable for the chemoselective conversion of aromatic alde-
hydes bearing electron-withdrawing or halogen groups (chloro,
bromo, nitro and cyano) and arylhydrazines (phenylhydrazine,
p-chloro and p-tolylhydrazines) to their corresponding pyrazoles
in the presence of aromatic aldehyde or aryl hydrazine containing
strongly electron-donating substituents (Scheme 4).
The structure of the products was identified by their IR,
1
mass, H and 13C NMR spectra and elemental analysis. Further-
more, the structure of 4h was confirmed by X-ray crystallographic
analysis (CCDC 867509, Fig. 1). The N-bound six-membered phenyl
ring shows positional disorder between the hydrogen and chlorine
atoms at the para-position with a refined site occupancy ratio of
0.648(8)/0.352(8). For the crystal data and structure refinement of
compound 4h see ESI.†
Table 1 Optimization of the reaction conditions for synthesis of pyrazole 4aa
A plausible mechanism for this multi-component reaction is
illustrated in Scheme 5. At first the intermediate hydrazone A
is formed by the condensation of aldehyde 1 with hydrazine 2
(it seems that electron-withdrawing groups on the aromatic
ring of aldehydes increase the electrophilicity of the carbonyl
group towards nucleophilic attack by aryl hydrazine). Then,
complexation of Cu(II) with the triple bond of DMAD 3 and
subsequent transfer of an electron from DMAD to Cu(II) results
in a radical cation intermediate B and Cu(I) species.28 Nucleo-
philic attack of hydrazone A on radical cation B provides C.
One-electron transfer from Cu(I) to C gives D which upon
prototropic shift from nitrogen to carbon gives E which in turn
converts to F by intramolecular nucleophilic attack. The inter-
mediate F is transformed to pyrazoline 5 via a prototropic shift
(in this case, the acidity of hydrogen attached to the carbon of
Entry
Catalyst
Time (h)
Yieldb (%)
1
2
3
4
5
6
No catalyst
CuBr2
10
1
1
1
1
1
1
1
1
1
1
1
—
35
87
65
60
52
40
31
79
87
80
87
[n-Bu4P][CuBr3]
[bmim][CuCl3]
[bmim][CuBr3]
[bmim][FeCl4]
[bmim][InCl4]
[bmim][ZnCl3]
[n-Bu4P][CuBr3]
[n-Bu4P][CuBr3]
[n-Bu4P][CuBr3]
[Bu4P][CuBr3]
7c
8c
9d
10e
11f
12g
a
Benzaldehyde (1 mmol), phenylhydrazine (1 mmol) and DMAD hydrazone is increased due to the presence of electron-withdrawing
b
c
(1.2 mmol), catalyst (25 mol%), at 100 1C. Isolated yield. The corres-
ponding pyrazoline was the major product. 20 mol% of catalyst
was used. 30 mol% of catalyst was used. Reaction was performed
at 90 1C. Reaction was performed at 110 1C.
substituents on the R group and consequently, the conversion of E
to F and finally to 5 is facilitated). Finally, aromatization of 5 under
an ambient atmosphere in the presence of Cu(II) catalyst affords the
d
e
f
g
c
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Table 2 Synthesis of pyrazoles via cyclization–aromatization of hydrazones with DMAD in the presence of [n-Bu4P][CuBr3]a
Entry
Aldehyde (1)
Hydrazine (2)
Product (4)
Time (min)
60
Yieldb (%)
87
1
4a
4b
4c
4d
4e
4f
2
3
4
5
6
7
8
60
90
90
90
90
60
60
66
91
86
92
85
79
86
4g
4h
9
4i
4j
60
60
75
76
10
11
4k
4l
60
60
67
72
12
a
Aldehyde (1 mmol), phenylhydrazine (1 mmol) and DMAD (1.2 mmol), [n-Bu4P][CuBr3] (25 mol%) under solvent-free conditions at 100 1C.
Isolated yield.
b
corresponding pyrazole 4 and releases the Cu(II) species for the species in the reaction. The model reaction was also performed
next catalytic cycle (Scheme 5). To confirm the suggested under an argon atmosphere; the product yield was reduced
mechanism, a radical scavenger such as 1,1-diphenylethylene or considerably. It is also noteworthy that bubbling oxygen into the
2,6-di-tert-butylphenol was added to the reaction mixture. Under reaction mixture accelerated the reaction. These results show that
these conditions, the related hydrazone was observed in the the presence of oxygen is essential. According to the previous
reaction mixture and no further conversion to its corresponding works,25,26 the copper(II) ionic liquid in the presence of oxygen can
pyrazole occurred. This observation proved the presence of radical oxidize the pyrazoline derivatives to pyrazoles.
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Scheme 2 Chemoselective synthesis of pyrazoline.
Scheme 3 Oxidation of pyrazolines 5m and 5n to their corresponding pyrazoles
4m and 4n in the presence of H2O2.
Scheme 5 Proposed mechanism for the cyclization–aromatization reaction of
hydrazone with DMAD.
Reusability and recyclability of a catalyst is of practical
importance in minimizing the amount of waste. In this context,
we investigated the reusability of this ionic liquid in the model
reaction. After completion of the reaction, water (30 ml) was
added and the organic materials were filtered. Then, the water
was evaporated, the catalyst was dried at 80 1C under reduced
pressure for 2 h and reused. The catalyst could be recycled four
times with slight loss of its activity (Fig. 2).
Scheme 4 Chemoselective reaction of 4-cyanobenzaldehyde and 4-methoxy-
benzaldehyde with phenylhydrazine catalyzed by [n-Bu4P][CuBr3].
In conclusion, we have demonstrated an efficient, novel,
eco-friendly and chemoselective method for the synthesis of
fully substituted pyrazoles, using Lewis acidic ionic liquid
[n-Bu4P][CuBr3] as a reusable catalyst. Short reaction times,
simple operation, high yields of products, a green procedure and
avoiding toxic organic solvents and reagents are noteworthy
advantages of the current method.
Fig. 1 The crystal structure of compound 4h.
Experimental
General information
Melting points were determined using a Stuart Scientific SMP2
apparatus and are uncorrected. FT-IR spectra were obtained as
To check the role of Cu(II) in the oxidation process, the
oxidation of dimethyl 1,3-diphenyl-4,5-dihydro-1H-pyrazole-4,5-
dicarboxylate (the corresponding pyrazoline of 4a) was investi-
gated in the absence of catalyst; no desired pyrazole was
obtained under these conditions. This shows that Cu(II) plays
a crucial role in the oxidation step, in addition to activation
of DMAD.
It is noteworthy that in comparison with the previous work
on the synthesis of pyrazoles using Cu(I) as the catalyst and
sodium acetate as the base,20 in the presence of [n-Bu4P][CuBr3]
without any base, the reaction times are shorter (1–1.5 h
compared to 10 h) and the yields are higher (66–92% compared
to 52–88%). It seems that these two synthetic methods proceed
via two different pathways.
Fig. 2 Reusability of the catalyst for synthesis of 4a.
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