Br?nsted acid-mediated annulations of 1-cyanocyclopropane-1-carboxylates with arylhydrazines: Efficient strategy for the synthesis of 1,3,5-trisubstituted pyrazoles

Article abstract of DOI:10.1039/c6ra14557d

1-Cyanocyclopropane-1-carboxylates are reacted with arylhydrazines to afford 1,3,5-trisubstituted pyrazoles under the influence of a Br?nsted acid. Formally, this transformation can be regarded as an annulation of three-membered rings with α-carbonyl and

Full text of DOI:10.1039/c6ra14557d

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Bronsted acid-mediated annulations of 1-
¨
cyanocyclopropane-1-carboxylates with
Cite this: RSC Adv., 2016, 6, 67724
arylhydrazines: efficient strategy for the synthesis
of 1,3,5-trisubstituted pyrazoles†
Received 5th June 2016
Accepted 12th July 2016
*
Shuwen Xue, Jiaming Liu, Xushun Qing and Cunde Wang
DOI: 10.1039/c6ra14557d
www.rsc.org/advances
1-Cyanocyclopropane-1-carboxylates are reacted with arylhydrazines acceptor cyclopropanes such as 2-arylcyclopropane 1,1-diesters.
to afford 1,3,5-trisubstituted pyrazoles under the influence of 2-Aroyl-3-aryl-1-cyanocyclopropanecarbonates could provide
a Bronsted acid. Formally, this transformation can be regarded as an various 1,3-zwitterionic intermediate through different pathways
¨
annulation of three-membered rings with a-carbonyl and hydrazines. for ring-opening due to involvement in conjugate hybrid from a-
This newly efficient method provides access to a variety of structurally carbonyl group. Based on our studies on the reactivity of 2-aroyl-
diverse pyrazole derivatives. The structures of five typical products 3-aryl-1-cyanocyclopropanecarbonates, we envisioned that the
were confirmed by X-ray crystallography.
reaction of 2-aroyl-3-aryl-1-cyanocyclopropanecarbonates with
arylhydrazines may offer an efficient approach to pyrazole skel-
etons. Although, the reported reactions of substituted cyclopro-
panes with hydrazines afforded pyridazin-3(2H)-ones and indole
derivatives, or ring-opening products.2h,5 To the best of our
knowledge, no example using the annulations of substituted
cyclopropanes with arylhydrazines to afford 1,3,5-trisubstituted
pyrazoles was reported. In this context, the annulation reactions
of 1-cyanocyclopropane-1-carboxylates with arylhydrazines could
provide an easy access to 1,3,5-trisubstituted pyrazoles under
acidic conditions.
As is known to us, a pyrazole unit as a key structural motif
exists in large numbers of pharmaceutically and agrochemically
active compounds.6 Pyrazoles and their derivatives exhibit
antimicrobial, antioxidant, antinammatory, analgesic, anti-
convulsant, anticancer, antiHIV-1 reverse transcriptase and
herbicidal activities7 that are extensively used for the treatment
of metabolic, CNS, and oncological diseases.8 For instances,
a number of compounds containing a pyrazole scaffold have
been successfully commercialized, such as Celebrex, Viagra,
Fipronil, Lonazolac, Rimonabant, and Acomplia (Scheme 1).9
Additionally, pyrazole derivatives are also applied widely in
supramolecular and polymer chemistry, in the food industry,
and in the ne chemical industry such as uorescent bright-
ening agents, while some have liquid crystal properties.10
Pyrazole derivatives were usually synthesized through tradi-
tional approaches including the condensation of 1,3-dicarbonyl
compounds with hydrazines and 1,3-dipolar cycloaddition of
dipolarophiles with appropriate dipoles.11 The importance of the
pyrazole moiety has prompted the development of many prac-
tical and efficient synthetic routes to construct their derivatives.
Recently, the efficient methods have been developed with the
Cyclopropanes and their derivatives have been broadly used as
valuable building blocks for the construction of various carbo-
and heterocyclic systems due to their high p character,
intrinsic ring strain and straightforward synthesis.1 They easily
underwent a variety of ring-opening reactions, and the subse-
quent annulations with dipoles formed ve-, six-, or seven-
membered ring systems.2 Recently we reported the direct
annulations of 2-aroyl-3-aryl-1-cyanocyclopropanecarboxylates
with pyridine derivatives, nitromethane or b-nitrostyrenes to
construct ve-membered cyclic cores in a regioselective
manner,3 which stimulated research into new pathways for the
construction of cyclic units via the ring-opening reactions of 2-
aroyl-3-aryl-1-cyanocyclopropanecarboxylates. Some previous
studies revealed the most importantly synthetic value of
donor–acceptor cyclopropanes has been extensively demon-
strated in the preparation of highly substituted cyclic products
via the vicinal arrangement of donor and acceptor moieties of
cyclopropane that is able to stabilize a 1,3-zwitterionic rela-
tionship for formal [3 + 2] cycloaddition reactions.4
However, the main research efforts have been devoted to use
the skeleton of cyclopropane for the construction of new cyclic
skeletons via the ring-opening reactions of the common donor–
School of Chemistry and Chemical Engineering, Yangzhou University, 180 Siwangting
Street, Yangzhou 225002, P. R. China. E-mail: wangcd@yzu.edu.cn; Fax: +86-514-
8797-5244; Tel: +86-514-8797-5568
† Electronic supplementary information (ESI) available: Reactions conditions and
spectra. CCDC 1427486, 1433734, 1465079, 1457746 and 1473251. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c6ra14557d
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Table 1 Optimization of the reaction conditionsa
Entry
Acid (equiv.)/solvent/T (ꢀC)/ta (h)
Yieldb (%)
1
2
3
4
5
6
7
8
(À)/AcOH/110/12
52
78
82
75
70
68
65
76
80
80
78
84
78
82
80
82
70
65
82
67
72
H2SO4 (1.0)/AcOH/110/12
H2SO4 (1.0)/PhMe/110/12
Sulfamic acid (1.0)/PhMe/110/12
Malonic acid (1.0)/PhMe/110/12
Benzoic acid (1.0)/PhMe/110/12
Cinnamic acid (1.0)/PhMe/110/12
p-TsOH (1.0)/PhMe/110/16
H3PO4 (1.0)/PhMe/110/12
H2SO4 (1.0)/PhMe/120/12
H2SO4 (1.0)/PhMe/100/12
H2SO4 (0.8)/PhMe/110/12
H2SO4 (0.6)/PhMe/110/12
H2SO4 (0.8)/PhMe/110/8
Scheme 1 Pyrazole representative biologically active analogues.
9
aim of increasing the regioselectivity in the diverse synthesis of
substituted pyrazoles. Among them, those involving the forma-
tion of C–N and C–C bonds by multicomponent reactions have
emerged as a promising alternative to conventional method-
ologies.6b,10g,12 Nevertheless, the most common and efficiently
synthetic method for the preparation of substituted pyrazoles
still involves the cyclocondensation of an appropriate hydrazine,
which acts as a double nucleophile, with a three-carbon unit
featuring two electrophilic carbons in a 1,3-relationship, such as
1,3-dicarbonyl, a,b-unsaturated carbonyl compounds, and b-
enaminones or related compounds. In addition, we have
recently published a method for the generation of the specially
1,3-dipolar intermediate from 2-aroyl-3-aryl-1-cyanocyclopropa-
necarboxylates,3,13 which could allow for the construction of the
pyrazole ring via annulation reaction with hydrazines.
10
11
12
13
14
15
16c
17
18
19
20
21d
H2SO4 (0.8)/PhMe/110/16
H2SO4 (0.8)/PhMe/110/12
H2SO4 (0.8)/CH3CN/reux/12
H2SO4 (0.8)/1,4-dioxane/110/12
H2SO4 (0.8)/xylene/110/12
H2SO4 (0.8)/cyclohexane/reux/12
H2SO4 (0.8)/PE (90–120 ꢀC)/110/12
a
Unless otherwise stated, reactions of 1a (1 mmol) and
phenylhydrazine (2 mmol) were carried out in 5 mL of solvent.
b
c
Isolated yield. 1a (1 mmol) and phenylhydrazine (1.5 mmol) were
used. d PE: petroleum ether.
At rst, we treated the 1-cyanocyclopropane-1-carboxylate 1a
with phenylhydrazine (2a, 2 equiv.) in AcOH as the solvent at
110 C for 12 h (Table 1, entry 1). The desired product 3a was
formed in 52% yield. The result indicated that acidic agents
should promote the reaction. Subsequently, while inorganic
acid H2SO4 was used as a promoter, we were pleased to nd out
that the yield was improved to 78% (Table 1, entry 2). Then, we
examined the reaction using toluene as the solvent to replace
AcOH at the above reaction. Aer overnight, the reaction yielded
the required product in 82% yield (Table 1, entry 3). The
similar results (Table 1, entries 5–7). Under the same reaction
conditions, phosphoric acid promoted the title reaction as
much as the reaction activity of sulfuric acid (Table 1, entry 9),
which indicated the detrimental effect of strong acids on the
reaction. The results showed that sulphuric acid afforded the
best result (84% in yield). Further study on the effect of
temperature led to the observation that the reaction at 120 ꢀC or
ꢀ
ꢀ
100 C resulted in a lower yield of 80% and 78%, respectively
(Table 1, entries 10–11). Further study on the effect of the
amount of sulphuric acid led to the observation that the reac-
tion using 0.8 equiv. H2SO4 afforded the best result (Table 1,
entries 12 and 13). When the reaction was carried out in shorter
reaction time (Table 1, entry 14) or in longer reaction time
(Table 1, entry 15), the yield of product 3a was slightly
decreased. Reducing the amount of phenylhydrazine (2a) to 1.5
equiv. did not afford the better result (Table 1, entry 17). Next,
we studied the effect of solvent on reaction, the reaction was
carried out with different solvents such as acetonitrile, 1,4-
¨
preliminary results indicated the Bronsted acid and aprotic
solvent could afford better results. Recently these studies
showed that simple organic acids were used as green and highly
efficient catalysts in the preparation of containing-nitrogen
heterocycles.14 Thus, further many Bronsted acids such as sul-
¨
famic acid, malonic acid, benzoic acid, cinnamic acid, p-tolue-
nesulfonic acid and inorganic acid phosphoric acid were
screened (Table 1, entries 4–9), the desired product 3a was ob-
tained in yield of 75%, 70%, 68%, 65%, 76%, and 81% respec-
tively. Of these acids, sulfamic acid and p-toluenesulfonic acid
gave the moderate yield (ca. 75%), however, the reaction needed
the longer reaction time while p-toluenesulfonic acid was used.
The use of malonic acid, benzoic acid, and cinnamic acid gave
ꢀ
dioxane, xylene, cyclohexane and petroleum ether (90–120 C)
gave the desired product 3a in 65–82% of yields (Table 1, entries
17–21), screening of the solvents revealed that toluene and
xylene were good candidates. Thus, reconsidering easy removal
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Fig. 1 Molecular structure of 1H-pyrazole 3a.
of toluene in the work-up step, we dened the reaction of the 1-
cyanocyclopropane-1-carboxylate 1a with
2 equiv. phenyl-
hydrazine (2a) and 0.8 equiv. sulphuric acid in toluene at 110 ꢀC
for 12 h as the standard conditions (Table 1, entry 10).
The structure of 3a was shown in Fig. 1.15 X-ray crystallo-
graphic analysis determined that product 3a possess three aryls
contiguous substituents at N(1), C(3) and C(5). On the basis of
spectroscopic evidence the structure of compound 3a was
identied as 5-(4-bromophenyl)-3-(4-chlorophenyl)-1-phenyl-
1H-pyrazole (3a).
Fig. 2 Molecular structure of 1H-pyrazoles 3d, 3f, 3g, 3o.
The scope of this transformation was then investigated
under the standard conditions using different 1-cyanocyclo-
propane-1-carboxylates, and substituted arylhydrazines. The
results are summarized in Table 2. The reaction tolerated
different substituents on the aromatic ring of the 1-cyano-
cyclopropane-1-carboxylates and arylhydrazines such as
methyl, methoxy, chloro, iodo, and bromo at ortho-, meta- or
para-positions of phenyl groups. Generally, substrates with
para-position phenyl groups gave the products in higher yields
than those with ortho-, or meta-position phenyl groups. The
electronic properties of the substituents on the benzene ring of
1-cyanocyclopropane-1-carboxylates had a slight effect on the
reaction. The introduction of an electron-withdrawing group
such as Cl, Br or I speeded up the reaction and increased the
yield of product, thus facilitating the synthesis of diversely
substituted 1H-pyrazoles. Substrates with aryl and aroyl groups
bearing electron-donating groups such as methyl, and methoxy
afforded the corresponding pyrazoles in moderate yields (Table
Table 2 Scope with respect to 1-cyanocyclopropane-1-carboxylates
and arylhydrazinesa
Entry
R1
R2
R3
3
Yieldb (%)
1
2
3
4
5
6
7
8
p-Br
p-Cl
p-Cl
p-Br
m-Cl
o-Cl
o-Cl
p-Cl
p-Cl
H
H
H
H
H
H
H
H
H
H
H
H
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
3p
3q
3r
84
87
83
82
76
78
79
80
72
81
85
80
75
73
87
80
81
84
85
73
p-CH3O
H
p-CH3O
p-CH3O
H
p-CH3O
p-CH3O
p-Br
p-Br
H
H
p-CH3O
p-Cl
H
p-Br
9
m-CH3
3-F-4-PhO
p-I
3-F-4-PhO
m-Cl
m-Cl
p-Br
o-Cl
p-CH3
p-Br
10
11
12
13
14
15
16
17
18
19
20
H
p-Cl
p-Cl
p-CH3
H
p-Cl
p-CH3
H
H
p-CH3O
p-Cl
p-CH3O
p-Br
m-Br
3s
3t
a
1-Cyanocyclopropane-1-carboxylates 1a–p (1 mmol), arylhydrazines
2a–c (2 mmol) and sulphuric acid (0.8 equiv.) toluene (5 mL), 110 ꢀC,
12 h. Isolated yield.
b
Scheme 2 Tentative reaction mechanism.
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2, entries 3, 5, 6, 9, 14, and 20). It is worth noting that substrates
with aryl group bearing multi-substituents may also be applied
(entries 10 and 12, Table 2). All corresponding substituted 1H-
pyrazoles were analyzed by their 1H NMR, 13C NMR and MS.
Characteristic 1H chemical shi of 1H-pyrazoles C(4)–H at d ca.
6.75(s), unequivocally indicated the exclusive chemical envi-
ronment of 1H-pyrazole C(4) proton. Products 1H-pyrazoles 3d,
3f, 3g and 3o were further characterized by single X-ray crys-
tallography (Fig. 2).15
On the basis of the above experimental results together with
related reports, the reaction mechanism shown in Scheme 2
was proposed. In terms of pyrazole formations, the condensa-
tion of 1-cyanocyclopropane-1-carboxylates with arylhydrazine
gave rstly the intermediate arylhydrazone [A], then donor–
acceptor cyclopropane was attacked intramolecularly by
a nitrogen of arylhydrazone, the following ring-opening reac-
tion afforded dihydropyrazole salt [B].5 Next, the deprotonation
of the dihydropyrazole salt [B] afforded dihydropyrazole [C].
Subsequent the elimination of ethyl cyanoacetate and aroma-
tization furnished the pyrazole products 3a–t.
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Conclusions
¨
In conclusion, we have developed an efficiently Bronsted acid-
promoted annulation reaction of 1-cyanocyclopropane-1-
carboxylates with arylhydrazines to afford 1,3,5-trisubstituted
pyrazoles in moderate to good yields (72–87%). This reaction
involved the sequential condensation, intra-molecular
addition/ring-opening
reaction
of
2-aroyl-3-aryl-1-
cyanocyclopropanecarboxylates with arylhydrazines to give the
corresponding pyrazoles. The development of this strategy
offers a complementary approach to substituted pyrazole
compounds with advantages that include a variety of cheap and
readily available reactants and a wide range of substrates with
dense or exible substitution patterns.
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Acknowledgements
Financial support of this research by the National Natural
Science Foundation of China (NNSFC 21173181) is gratefully
acknowledged by authors. A Project was Funded by the Priority
Academic Program Development of Jiangsu Higher Education
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67728 | RSC Adv., 2016, 6, 67724–67728
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