Guanidine hydrochloride catalyzed efficient one-pot pseudo five-component synthesis of 4,4′-(arylmethylene)bis(1H-pyrazol-5-ols) in water

Article abstract of DOI:10.1080/00397911.2019.1643483

The present methodology describes an efficient, environmentally friendly and simple protocol for the synthesis of some 4,4′-(arylmethylene)bis(1H-pyrazol-5-ol) derivatives through a one-pot pseudo-five-component reaction of hydrazine hydrate/phenyl hydrazine, ethyl acetoacetate, and various aromatic aldehydes catalyzed by guanidine hydrochloride. This condensation reaction was performed by tandem Knoevenagel–Michael reaction in water under refluxing conditions giving the title compounds in 82–92% yields. Atom economy, simple operation, easy work-up, using inexpensive organocatalyst, high yields in short times, clean transformation, and environmentally benign are some of the important features of this new protocol.

Full text of DOI:10.1080/00397911.2019.1643483

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: https://www.tandfonline.com/loi/lsyc20
Guanidine hydrochloride catalyzed efficient one-
pot pseudo five-component synthesis of 4,4′-
(arylmethylene)bis(1H-pyrazol-5-ols) in water
Fatemeh Noruzian, Abolfazl Olyaei, Rahimeh Hajinasiri & Mahdieh
Sadeghpour
To cite this article: Fatemeh Noruzian, Abolfazl Olyaei, Rahimeh Hajinasiri & Mahdieh
Sadeghpour (2019): Guanidine hydrochloride catalyzed efficient one-pot pseudo five-component
synthesis of 4,4′-(arylmethylene)bis(1H-pyrazol-5-ols) in water, Synthetic Communications, DOI:
10.1080/00397911.2019.1643483
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https://doi.org/10.1080/00397911.2019.1643483
Guanidine hydrochloride catalyzed efficient one-pot
pseudo five-component synthesis of 4,40-
(arylmethylene)bis(1H-pyrazol-5-ols) in water
Fatemeh Noruziana, Abolfazl Olyaeib, Rahimeh Hajinasiria, and Mahdieh
Sadeghpourc
b
aChemistry Department, Qaemshahr Branch, Islamic Azad University, Qaemshahr, Iran; Department of
c
Chemistry, Payame Noor University (PNU), Tehran, Iran; Department of Chemistry, Takestan Branch,
Islamic Azad University, Takestan, Iran
ABSTRACT
ARTICLE HISTORY
Received 29 April 2019
The present methodology describes an efficient, environmentally
friendly and simple protocol for the synthesis of some 4,40-
KEYWORDS
(arylmethylene)bis(1H-pyrazol-5-ol) derivatives through
a one-pot
4,40-(Arylmethylene)bis(1H-
pyrazol-5-ols); guanidine
hydrochloride;
pseudo-five-component reaction of hydrazine hydrate/phenyl hydra-
zine, ethyl acetoacetate, and various aromatic aldehydes catalyzed
by guanidine hydrochloride. This condensation reaction was per-
formed by tandem Knoevenagel–Michael reaction in water under
refluxing conditions giving the title compounds in 82–92% yields.
Atom economy, simple operation, easy work-up, using inexpensive
organocatalyst, high yields in short times, clean transformation, and
environmentally benign are some of the important features of this
new protocol.
Knoevenagel–Michael;
water
GRAPHICAL ABSTRACT
Introduction
Heterocycles containing nitrogen atom are of interest because of their biological activ-
ities and medicinal importance including ontological research. Pyrazoles are one of
these heterocyclic compounds. They have been extensively attracted due to their import-
ance as a class of bioactive drug targets in the pharmaceutical industry and the core
structure of numerous biologically active compounds.[1] Pyrazoles, especially 4,40-
(arylmethylene)bis(1H-pyrazol-5-ols) is a very important class of compounds with vari-
ous biological activities such as anti-inflammatory,[2] antipyretic,[3] gastric secretion
CONTACT Abolfazl Olyaei
Olyaei_a@pnu.ac.ir
Department of Chemistry, Payame Noor University (PNU),
Lashkarak Road, Nakhl Street, Tehran PO BOX 19395-4697, Iran.
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lsyc
Supplemental data for this article can be accessed on the publisher’s website.
ß 2019 Payame Noor University

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F. NORUZIAN ET AL.
stimulatory,[4] antidepressant,[5] antifilarial agents,[6] antibacterial,[7] fungicides,[8] pesti-
cides,[9] anticancer,[10] antiviral,[11] antiproliferative,[12] hypoglycemic,[13] and
antioxidant.[14]
One-pot tandem Knoevenagel–Michael reaction is one of the most broadly used
methods for the synthesis of 4,40-(arylmethylene)bis(1H-pyrazol-5-ols) via three-com-
ponent condensation of aldehydes with 2 equiv. of 3-methyl-1H-pyrazol-5(4H)-one
or 3-methyl-1-phenyl-1H-pyrazol-5(4H)-one and pseudo five-component reaction of
hydrazine hydrate/phenyl hydrazine, ethyl acetoacetate and various aromatic alde-
hydes. In recent years, synthesis of 4,40-(arylmethylene)bis(1H-pyrazol-5-ols) has been
reported using MCR in the presence of some catalyst such as Ce(SO4)2 4H2O,[15a]
ceric ammonium nitrate (CAN),[11] PEG-SO3H,[15b] ammonium acetate,[15c] N-meth-
ylimidazolium perchlorate ([MIm]ClO4),[15d] pyridine trifluoroacetate or acetic
[15f],
acid,[15e] Cu-isatin Schiff base supported on c-Fe2O3
Silica-bonded S-sulfonic
acid,[15g] Sulfuric acid ([3-(3-silicapropyl)sulfanyl]propyl)ester,[15h] L-proline,[15i]
Ph3CCl,[15j] sulfonated rice husk ash (RHA-SO3H),[15k] 3-aminopropylated silica gel
(AP-SiO2),[15l] 2-hydroxy ethylammonium propionate,[15m] Ag/TiO2 nano-thin
films,[15n] Mohr’s salt,[15o] HAP@AEPH2-SO3H,[15p] morpholinium glycolate,[15q]
[15r]
nano-NiZr4(PO4)6,
solid state,[15s] and ammonium chloride. However, some of
these methods suffer from moderate yields, long reaction times, harsh reaction
conditions, use of toxic organic solvents, expensive catalysts, many tedious steps or
catalyst preparation.
Organocatalysts have been used widely in many reactions as mono and bifunctional
catalysts because of economic and environmental considerations. Among them, guani-
dines and their corresponding guanidinium salts are becoming powerful tools for activa-
tion of the carbonyl functionality in organic transformations.[16] Moreover, guanidinium
salts, on the other hand, display weak Brønsted acidity and are bidentate, cationic
hydrogen-bond donors, and the possibility of the delocalized guanidinium cation acting
as a metal-free “p-Lewis acid” has even been discussed.[17] In this article, we wish to
introduce guanidine hydrochloride as a highly efficient organocatalyst for the facile and
green synthesis of 4,4ꢀ-(arylmethylene)-bis(3-methyl-1-phenyl-1H-pyrazol-5-ols) in
aqueous media. Using this catalyst, the electrophilic substitution reactions of 3-methyl-
1H-pyrazol-5(4H)-one and 3-methyl-1-phenyl-1H-pyrazol-5(4H)-one with carbonyl
compounds proceeded smoothly to afford the desired products in high yields and rela-
tively short reaction times.
Results and discussion
To find the optimum reaction conditions, a mixture of hydrazine hydrate (2.0 mmol),
ethyl acetoacetate (2.0 mmol), and benzaldehyde (1.0 mmol) was stirred in the absence
of catalyst in water (10 mL) under reflux condition and afforded expected product up to
30% yield within 60 min of reaction time (Table 1). When the reaction was accom-
plished in the presence of guanidine hydrochloride (5% mol), the desired product was
obtained in 78% yield after 60 min (Table 1, entry 2). An increase in the amount of
catalyst from 5 to 10% mol not only decreases the reaction time from 60 to 40 min but
also increased the product yield from 78 to 90% (Table 1, entry 3). Loading of the

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Table 1. Optimization of the synthesis of 4a.
Entry
Solvent
H2O
H2O
H2O
H2O
Catalyst loading (mol%)
Temp. (ꢀC)
Time (min)
Yield (%)
1
2
3
4
5
6
7
8
9
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
70
60
60
40
40
40
60
60
60
60
60
30
78
90
91
91
65
45
41
61
72
5
10
15
20
10
10
10
10
10
H2O
EtOH
CH3CN
THF
10
90
catalyst up to 15 and 20 mol.% did not prove satisfactory in terms of yield or reaction
time (Table 1, entries 4 and 5).
Moreover, we examined the effect of different protic and aprotic solvents such as
ethanol, acetonitrile, and tetrahydrofuran on the model reaction in the presence of
guanidine hydrochloride (10% mol) under reflux conditions within 60 min. As indicated
in Table 1, EtOH, CH3CN, and THF afforded moderate yields of the desired product
(Table 1, entries 6–8). Next, we studied the model reaction under the solvent-free con-
dition at 70 and 90 ꢀC and afforded 61 and 72% yields of the corresponding product
(Table 1, entries 9–10), respectively. Accordingly, the optimization of reaction condi-
tions was established as indicated in entry 3 of Table 1.
We thus used the reaction condition of entry 3 of Table 1 for further exploration pre-
paring compounds 4a–f, having a number of electron-withdrawing and electron-donat-
ing substituents on aromatic aldehydes. The results are summarized in Figure 1. Under
the optimal conditions, products yields (83–92%) were uniformly good and were not
particularly substituent dependent.
After the successful application of guanidine hydrochloride as a organocatalyst in the
synthesis of compounds 4, with the optimal conditions in hand, we decided to use it in
the condensation of phenyl hydrazine instead of hydrazine hydrate with ethyl acetoace-
tate and various aromatic aldehydes in aqueous medium leading to 4,40-(phenylmethyle-
ne)bis(3-methyl-1-phenyl-1H-pyrazol-5-ol) derivatives 5a–f and 4,40,400,4000-(1,4-
Phenylenebis(methanetriyl))tetrakis(3-methyl-1-phenyl-1H-pyrazol-5-ol) (5g) as an
important class of bis(pyrazolyl)methanes. For this purpose, one-pot, two-step domino
reaction of phenyl hydrazine (2b, 2 mmol), ethyl acetoacetate (1, 2 mmol) and aromatic
aldehydes (3, 1 mmol) was investigated. Initially, the reaction of phenyl hydrazine (2b,
2 mmol) and ethyl acetoacetate (1, 2 mmol) in the presence of guanidine hydrochloride
(10 mol.%) in water (10 mL) under reflux condition was studied. The desired 3-methyl-
1-phenyl-1H-pyrazol-5-ol (6b) was obtained in high yield (90%) after 90 min. After
completion of the reaction, various aromatic aldehydes (3, 1 mmol) or terephthaldehyde

4
F. NORUZIAN ET AL.
Figure 1. Synthesis of 4,4’-(arylmethylene)bis(3-methyl-1H-pyrazol-5-ol) derivatives 4a–f.
Scheme 1. One-pot synthesis of 3-methyl-1-phenyl-1H-pyrazol-5-ol (6) and bis(pyrazol-5-ol)s 5a–g
(0.5 mmol) was added to the reaction mixture for the synthesis of a series of com-
pounds 5 (Scheme 1). The reactions proceeded successfully to give bis(pyrazol-5-ol)s
5a–g in 85–92% yields within 30–60 min.
As shown in Figure 2, the aryl aldehydes that possess electron-donating and electron-
withdrawing substituents provided the corresponding products in good to high yields. It
should be noted that the purification of the title compounds 4 and 5 is very easy. After
cooling the reaction mixture to room temperature, the solid resulting from the reaction
was easily separated by simple filtering. Then, the crude product was recrystallized from
ethanol to produce the desired pure product.
In another study, the one-pot pseudo-three-component reaction of 3-methyl-1-phe-
nyl-1H-pyrazol-5-ol (6b, 2 equiv.) and benzaldehyde (1 equiv.) in the presence of orga-
nocatalyst in the water at reflux were examined (Scheme 2). In this process, the same

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Figure 2. Synthesis of 4,4’-(phenylmethylene)bis(3-methyl-1-phenyl-1H-pyrazol-5-ol) derivatives 5a–g.
Scheme 2. One-pot synthesis of bis(pyrazol-5-ol) 5a.
product 5a was obtained after 45 min in 86% yield. However, the other title compounds
5b–g can be obtained by this procedure.
The products 4a–e, 5a–e, and 5g were known compounds, their authenticity was
1
established by H NMR and their melting points compared with that reported in litera-
1
ture. The products 4f and 5f were unknown and established by their H NMR, 13C
NMR, FTIR, CHN, and MS.
A plausible reaction mechanism for the formation of 4 and 5 is proposed in Scheme 3.
On the basis of this mechanism, first, a condensation of phenylhydrazine derivative (2)
with ethyl acetoacetate (1) catalyzed by guanidinium chloride is proposed to give the
adducts 6a–b. Subsequently, the intermediate 8 is likely formed by the condensation of
aromatic aldehyde 3 with 7, which is the tautomer of 6. The next step, Michael addition

6
F. NORUZIAN ET AL.
Scheme 3. Proposed mechanism for the one-pot pseudo-five-component synthesis of bis(pyrazol-5-
ol)s 4 and 5.
of another intermediate of 7 to 8 gives adduct 9 which undergoes tautomerization to
afford the desired products 4 and 5.
Experimental
General procedure for the synthesis of 4,40-(arylmethylene)bis(3-methyl-1H-
pyrazol-5-ol) derivatives 4a–f
Hydrazine hydrate (1.0 mmol), ethylacetoacetate (1.0 mmol), aromatic aldehyde
(0.5 mmol), guanidine hydrochloride (10% mol) and 5 mL of H2O were placed in a
50 mL round bottom flask over a magnetic stirrer and the contents were refluxed for
appropriate times (30–50 min). After completion of the reaction (monitored by TLC),
the reaction mixture was allowed to cool at room temperature and the resulting solid
(crude product) filtered and dried. The crude products were recrystallized from ethanol
to afford the pure products.
General procedure for the synthesis of 4,40-(phenylmethylene)bis(3-methyl-1-
phenyl-1H-pyrazol-5-ol) derivatives 5a–g
Phenyl hydrazine (1.0 mmol), ethylacetoacetate (1.0 mmol), guanidine hydrochloride
(10% mol), and 5 mL of H2O were placed in a 50 mL round bottom flask over a mag-
netic stirrer and the contents were refluxed for 90 min (the progress of the reaction was
monitored by TLC). Then, aromatic aldehyde (0.5 mmol) or terephthaldehyde
(0.25 mmol) was added to the reaction mixture and the contents were refluxed for
appropriate times (30–60 min). After completion of the reaction (monitored by TLC),
the reaction mixture was cooled at room temperature and the resulting solid (crude
product) filtered and dried. The crude products were recrystallized from ethanol to
afford the pure products.

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7
Conclusion
In summary, the present research is an operationally simple, cleaner and highly efficient
method for the green synthesis of one-pot pseudo-five-component synthesis of 4,40-
(arylmethylene)bis(1H-pyrazol-5-ols) in the water at reflux using a catalytic quantity of
guanidine hydrochloride. Compared to the classical methods, a library of pyrazole deriv-
atives was obtained in excellent yields. The current protocol has the advantages of short
reaction time, generality, the use of small amount of the low-cost catalyst, the use of
water instead of toxic solvents, simple environmentally friendly, isolation products, and
being environmentally benign.
1
Full experimental detail for unknown compounds, H, and 13C NMR spectra can be
found through the “Supplementary Content” section of this article’s webpage.
Funding
The authors thank the Research Council of Payame Noor University and Islamic Azad
University, Qaemshahr Branch for financial support.
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