Knoevenagel condensation in aqueous media promoted by 2,2′-bipyridinium dihydrogen phosphate as a green efficient catalyst

Article abstract of DOI:10.1007/s11164-021-04445-3

A 2,2′-Bipyridine-based ionic compound named 2,2′-bipyridinium dihydrogen phosphate was synthesized by addition of phosphoric acid to a solution of 2,2′-Bipyridine in dichloromethane. After the characterization using FT-IR, mass, ¹H, ¹³C and ³¹P NMR techniques, it was used as a Bronsted dicationic acidic catalyst for the promotion of the synthesis of 2-arylidene malononitrile and 5-arylidene barbituric acid derivatives via Knoevenagel condensation reaction in water. Some of the advantages of this method are the utilization of an easy preparable, cost-effective and eco-friendly organic salt as a catalyst within high rates and yields of the reactions, simple and quick work-up and acceptable reusability of the catalyst.

Full text of DOI:10.1007/s11164-021-04445-3

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-021-04445-3
Knoevenagel condensation in aqueous media promoted
by 2,2′‑bipyridinium dihydrogen phosphate as a green
efcient catalyst
Shila Darvishzad1 · Nader Daneshvar2 · Farhad Shirini1,2 · Hassan Tajik1,2
Received: 7 December 2020 / Accepted: 25 March 2021
© The Author(s), under exclusive licence to Springer Nature B.V. 2021
Abstract
A 2,2′-Bipyridine-based ionic compound named 2,2′-bipyridinium dihydrogen
phosphate was synthesized by addition of phosphoric acid to a solution of 2,2′-Bipy-
1
ridine in dichloromethane. After the characterization using FT-IR, mass, H, 13C
and 31P NMR techniques, it was used as a Bronsted dicationic acidic catalyst for
the promotion of the synthesis of 2-arylidene malononitrile and 5-arylidene barbi-
turic acid derivatives via Knoevenagel condensation reaction in water. Some of the
advantages of this method are the utilization of an easy preparable, cost-efective
and eco-friendly organic salt as a catalyst within high rates and yields of the reac-
tions, simple and quick work-up and acceptable reusability of the catalyst.
Keywords Knoevenagel condensation · Dicationic Brönsted acidic ionic liquid ·
2,2-bipyridinium dihydrogen phosphate · 2-arylidene malononitrile derivatives ·
5-arylidene barbituric acid derivatives · Aqueous media
Introduction
Ionic liquids are known for their unique properties such as recyclability, low vapor
pressure, tunable viscosity, good thermal and chemical stability and the ability to
dissolve many organic and inorganic substances [1–3].
Brönsted acidic ionic liquids (BAILs) are a category of ionic liquids that con-
tain protic acidic sites in their structures, so in addition to the properties of general
*
Farhad Shirini
shirini@guilan.ac.ir
Nader Daneshvar
ndxdaneshvar@gmail.com
1
2
Department of Chemistry, College of Sciences, University of Guilan, 41335-19141, Rasht, Iran
Department of Chemistry, College of Science, University of Guilan, University Campus 2,
Rasht, Iran
Vol.:(0123456789)
1 3

S. Darvishzad et al.
ionic liquids, they also have acidic properties. Because that the number of possi-
ble combinations and nature of cations and anions in ionic liquids determines their
physical and chemical properties [4, 5], halides, tetrafuoroborates, hexafuorophos-
phates, nitrates, phosphates, sulfates and sulfonates are common anions used in
the preparation of acidic ionic liquids [6]. Due to the non-toxicity and less pollu-
tion of phosphoric acid in comparison with other conventional mineral acids, the
use of phosphorous anion in the preparation of ionic liquids is more eco-friendly.
In this way, some of the BAILs containing phosphate anion have been synthesized
and their structures, properties and applications have been studied. Some of these
compounds are choline dihydrogen phosphate (CDHP) as a bufer ionic liquid [7],
l-butyl-3-methylimidazolium dihydrogen phosphate ([Bmim][H2PO4]) as a catalyst
for the Mannich reaction [8] and Esterifcations [9], diammonium hydrogen phos-
phate (DHP) as a catalyst for the synthesis of 2-aminothiazole and 2-iminothiazoli-
dine derivatives [10], a series of alkylammonium salts ([Et3NH][H2PO4], [Et2NH2]
[H2PO4], [EtNH3][H2PO4]) as catalyst and medium for Fischer esterifcation reac-
tion [11] and [Et3NH][H2PO4] and [Me3NH][H2PO4] in the cracking of dialkoxy-
propanes [12].
The Knoevenagel condensation is an important reaction that occurs between an
aldehyde (or ketone) and a compound that consists of an active methylene moiety
leading to a carbon–carbon double bond formation [13]. In this regard, the Kno-
evenagel condensation reaction between malononitrile and aldehydes is a suitable
way for the preparation of alkenes containing electron-withdrawing groups caused
them to be used as various chemical intermediates in a variety of organic reactions
[14–18].
Barbituric acid contains an active methylene group and because of it can be
entered in condensation reactions with aldehydes or ketones. This cyclic amide hav-
ing a pKa=4.01 in water. It is partially soluble in solvents such as water and metha-
nol [19]. The reaction of barbituric acid with carbonyl compounds was studied since
1864 [20]. The products of this reaction are useful as intermediates in the synthesis
of heterocyclic compounds [21], benzyl barbituric derivatives [22], oxadiazafavines
[23], and unsymmetrical disulfdes [24].
Due to its importance and applications, diferent catalysts and conditions have
been employed for the Knoevenagel reaction which of the [(DABCO)2C3H5OH].2Cl
[25], Bis-Su [26], [C4(MIm)2].2Cl [27], [C4(MIm)2].2HSO4 [27], γ-Fe2O3@
SiO2@[Bis-APTES]Cl2 NPs [28], Taurine [29, 30], [H2-pip][H2PO4]2 [31],
H2-Bsim[ClO4]2 [32] and H2-Bim[HSO4]2 [32] are the most important ones.
Experimental
Materials
All chemicals were obtained from Fluka, Merck and Aldrich Chemical Companies.
All yields refer to isolated pure products. Products were characterized by their phys-
ical constants, comparison with authentic samples and IR and NMR spectroscopy.
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Knoevenagel condensation in aqueous media promoted by…
The purity determination of the substrate and reaction monitoring was accompanied
by thin-layer chromatography (TLC) on silica-gel polygram SILG/UV 254 plates.
Instrumentation
The FT-IR spectra were recorded on a Perkin- Elmer Spectrum BX series spec-
1
trophotometer as neat flms on KBr pellets. The H NMR (500 MHz), 13C NMR
(125 MHz) and 31P NMR (202 MHz) spectra were run on a Bruker AVANCE spec-
trometer in DMSO using TMS as an internal standard (δ in ppm). Melting points
were recorded on a Büchi B-545 apparatus in open capillary tubes.
Preparation of 2,2′‑bipyridinium dihydrogen phosphate ([H2‑Bpy][H2PO4]2)
In a 100 mL round-bottomed fask in an ice bath, 2,2′-bipyridine (10.0 mmol,
1.562 g) was dissolved in 30.0 mL dichloromethane. Then an excess amount of
phosphoric acid 85% (w/w) (2.0 mL~28 mmol) was added drop-wise to it. Dur-
ing the addition of H3PO4, precipitation of the ionic product is observable. After
completion of the addition, the mixture was stirred for an additional 24 h. Then,
the solvent was decanted. The obtained white solid was washed with diethyl ether
(3×30 mL) and ethanol (2×20 mL). Finally, the solid was completely dried using
a rotary system and the desired [2,2′-bipyridine]-1, 1′-diium dihydrogen phosphate
was obtained in 93% yield (Scheme 1).
Spectral and physical data of [H2‑Bpy][H2PO4]2
M.P: 250 °C Dec; FT-IR (KBr, cm−1) υmax: 3103, 2707, 2330, 1708, 1604, 1583,
1
1528, 1455, 1435, 1356, 1224, 953, 807, 751, 608, 545; H NMR (500 MHz,
DMSO) δ (ppm): 7.42 (m, 2H), 7.92 (t, J=8.5 Hz, 2H), 8.38 (d, J=7.5 Hz, 2H),
8.70 (s, 2H), 10.53 (S, 6H); 13C NMR (125 MHz, DMSO) δ (ppm): 121.1, 124.9,
138.1, 149.7, 155.6; 31P NMR (202 MHz, DMSO) δ (ppm): 2.26.
General procedure for the synthesis of 2‑arylidene malononitriles in the presence
of [H2‑Bpy][H2PO4]2
In a 25 mL round-bottomed fask a mixture of an aromatic aldehyde (1.0 mmol),
malononitrile (1.1 mmol) and [H2-Bpy][H2PO4]2 (10 mg) in water (5 mL) was
Scheme 1 Preparation of [H2-Bpy][H2PO4]2
1 3

S. Darvishzad et al.
stirred in an oil-bath at 70 °C. The progress and completion of the reaction were
monitored by thin-layer chromatography (TLC) using n-hexane: ethyl acetate (7:3)
as an eluent. After completion of the reaction, the obtained residue was separated
by fltration and washed several times with water. Finally, the product was dried at
room temperature and characterized by its spectral data and melting points while
compared with the authentic samples.
General procedure for the synthesis of 5‑arylidene barbituric acids
in the presence of [H2‑Bpy][H2PO4]2
In a 25 mL round-bottomed fask a mixture of aldehyde (1.0 mmol), barbituric acid
(1.0 mmol) and [H2-Bpy][H2PO4]2 (10 mg) was stirred in water (5 mL) and heated
in an oil-bath at 70 °C. The progress and completion of the reaction were monitored
by TLC [n-hexane: ethyl acetate: EtOH (7:3:1)]. After completion, the product was
separated by fltration and washed several times with water. Finally, the product was
dried at 50 °C and characterized by its spectral data, melting point and comparison
with authentic samples.
Results and discussion
Based on the obtained structural information and our previous work with bpy
(SO3H)2(HSO4)2 [34], it is predicted that [H2-Bpy][H2PO4]2 can be used as an ef-
cient catalyst to promote the reactions needing acidic catalysts to speed-up, so we
were interested to investigate the performance of this catalyst in the acceleration of
the Knoevenagel condensation reaction for the preparation of 2-arylidene malononi-
trile and 5-arylidene barbituric acid derivatives.
Characterization of the catalyst
After preparation of the [H2-Bpy][H2PO4]2, various spectroscopic techniques were
used to characterize the structure of the compound. Here are some elucidations in
detail:
Comparison of the solubility and melting points
Investigations on the solubility of 2,2′-bipyridine and [H2-Bpy][H2PO4]2 revealed
that while 2,2′-bipyridine quickly dissolves in dichloromethane and ethanol the ionic
product is completely insoluble in these solvents. Moreover, [H2-Bpy][H2PO4]2 dis-
solves in water very fast while 2,2′-bipyridine is slightly soluble in it.
A comparison between the melting points showed that [H2-Bpy][H2PO4]2 be
decomposed at temperatures higher than 250 °C, while the precursor melts around
70–73 °C.
The above macroscopic diferences prove that the nature of 2,2′-bipyridine is
changed in the designed ionic structure.
1 3

Knoevenagel condensation in aqueous media promoted by…
FT‑IR analysis
In the FT-IR spectrum of 2,2′-bipyridinium dihydrogen phosphate, the broaden-
ing in the area of 2100–3100 cm−1 is attributed to acidic N−H and O−H stretch-
ing vibrations. P=O stretching vibration is observed at 1224 cm−1 and the two
peaks at 953, and 807 cm−1 are assigned to asymmetric and symmetric stretching
vibrations of P−O.
NMR spectroscopy
In the 1H NMR spectrum of [H2-Bpy][H2PO4]2, four peaks related to eight hydro-
gens of the 2,2′-bipyridine ring are observed in the aromatic area. Single peak
with the integral of six is related to six acidic hydrogens of the compound (two
NH+ and two H2PO‾4) which are appeared in the same chemical shift.
The 13C NMR spectrum of [H2-Bpy][H2PO4]2 as a symmetrical compound,
shows 5 peaks in the aromatic area related to fve types of carbons.
In the 31P NMR spectra of [H2-Bpy][H2PO4]2 as a symmetrical compound,
one peak at 2.26 ppm is observed which is related to the phosphorus of dihydro-
gen phosphate anion. This single peak proves the presence of H2PO4 anion in the
structure of the catalyst.
Mass spectroscopy
In the mass spectrum of [H2-Bpy][H2PO4]2, the molecular ion peak (M+)
appeared at m/e = 352.0, which is corresponding to the mass of the catalyst. Also,
other peaks related to the other fragmentations such as bipyridine at m/e = 156.2
and pyridine m/e = 78.0 can be observed.
Optimizations and derivations
To evaluate the efectiveness of various factors in the Knoevenagel reactions, the
infuence of diferent solvents, temperatures and the amounts of the catalyst was
investigated in the preparation of 4-chlorobenzaldehyde derivatives of both of the
target reactions (Tables 1 and 2).
The obtained data showed that the best results can be achieved in the presence
of 10 mg of [H2-Bpy][H2PO4]2 in the water at 70 °C (Scheme 2).
After the optimization studies, a variety of 2-arylidene malononitrile and
5-arylidene barbituric acid derivatives were prepared efciently from the reaction
of malononitrile or barbituric acid with diferent types of aldehydes in the pres-
ence of [H2-Bpy][H2PO4]2 (Tables 3 and 4).
1 3

S. Darvishzad et al.
Table 1 Optimization of the reaction conditions for the synthesis of 4-chlorobenzylidene malononitrile
Entry
Catalyst (mg)
Solvent
Temperature (°C)
Time (min.)
Yield (%) (isolated)
1
2
3
4
5
6
7
8
–
5
5
H2O
H2O
H2O
–
H2O
H2O
H2O
H2O
Refux
RT
Refux
100
RT
50
70
Refux
70
Refux
Refux
Refux
Refux
60
60
20
60
16
13
10
10
10
60
60
60
60
Trace*
Trace*
72
Trace*
84
90
96
95
96
Trace*
Trace*
Trace*
Trace*
10
10
10
10
10
15
10
10
10
10
9
H2O
10
11
12
13
CH3OH
CHCl3
CH3CN
n-hexane
*Reaction was not completed
Table 2 Optimization of the reaction conditions for the synthesis of 4-chlorobenzylidene barbituric acid
Entry
Catalyst (mg)
Solvent
Temperature (°C)
Time (min.)
Yield (%) (isolated)
1
2
3
4
5
6
7
8
–
4
4
6
H2O
H2O
H2O
H2O
H2O
–
H2O
H2O
H2O
H2O
H2O
H2O
CH3OH
CH3OH
Refux
RT
Refux
RT
Refux
100
RT
50
60
70
Refux
70
60
60
60
60
23
60
31
17
13
10
9
Trace*
Trace*
Trace*
Trace*
87
Trace*
90
90
92
95
93
95
6
10
10
10
10
10
10
12
10
10
9
10
11
12
13
14
8
60
60
Refux
Refux
Trace*
Trace*
*Reaction was not completed
1 3

Knoevenagel condensation in aqueous media promoted by…
Scheme 2 Synthesis of 2-arylidenemalononitrile and 5-arylidene barbituric acid derivatives using the
[H2-Bpy][H2PO4]2
Table 3 Synthesis of 2-arylidenemalononitrile derivatives
Entry
Aldehyde
Symbol
Time (min.)
Yield (%)a
M.p. (°C)
Observed
Reported [Ref.]
1
2
3
4
5
6
7
8
C6H5CHO
4-FC6H4CHO
1a
1b
1c
1d
1e
1f
1 g
1 h
1i
15
8
13
10
9
10
12
8
15
12
9
89
94
90
96
97
92
93
94
88
97
89
92
95
90
88
82–83
123–125
90–93
82–84 [25]
123–125 [26]
90–92 [28]
2-ClC6H4CHO
4-ClC6H4CHO
4-BrC6H4CHO
2-NO2C6H4CHO
3-NO2C6H4CHO
4-NO2C6H4CHO
2-OHC6H4CHO
4-OHC6H4CHO
2-CH3OC6H4CHO
4-CH3OC6H4CHO
4-CH3C6H4CHO
Terephthalaldehyde
Cinnamaldehyde
160–162
157–159
139–141
102–104
160–162
99–100
184–186
80–82
114–115
132–134
294–296
125–126
160–162 [28]
156–158 [29]
138–140 [29]
103–105 [29]
161–163 [25]
99–101 [29]
183–185 [28]
81–82 [26]
113–115 [25]
133–135 [28]
295–297 [29]
125–127 [29]
9
10
11
12
13
14
15
1j
1 k
1 l
1 m
1n
1o
10
8
10
12
aIsolated yields
1 3

S. Darvishzad et al.
Table 4 Synthesis of 5-arylidenebarbituric acid derivatives
Entry
Aldehyde
Symbol
Time (min.)
Yield (%)a
M.p. (°C)
Observed
Reported [Ref.]
1
2
3
4
5
6
7
8
C6H5CHO
4-FC6H4CHO
2a
2b
2c
2d
2e
2f
2 g
2 h
2i
13
10
14
12
10
10
14
15
10
9
8
8
7
9
17
15
14
91
93
89
88
95
85
87
91
90
96
89
90
94
90
87
79
92
254–256
>300
253–255 [32]
>300 [30]
2-ClC6H4CHO
3-ClC6H4CHO
4-ClC6H4CHO
4-BrC6H4CHO
2-NO2C6H4CHO
3-NO2C6H4CHO
4-NO2C6H4CHO
4-OHC6H4CHO
2-OCH3C6H4CHO
3-OCH3C6H4CHO
4-OCH3C6H4CHO
4-CH3C6H4CHO
Terephthalaldehyde
Cinnamaldehyde
Furfural
248–250
248–250
298–300
290–292
271–273
231–233
259–261
>300
269–271
228–230
>300
278–280
>300
249–251 [32]
275–247 [31]
298–300 [33]
290–291 [33]
271–273 [30]
230–232 [30]
259–260 [32]
>300 [31]
270–272 [31]
230–232 [32]
295–297 [31]
278–280 [31]
>300 [31]
9
10
11
12
13
14
15
16
17
2j
2 k
2 l
2 m
2n
2o
2p
2q
262–264
257–259
260–261[32]
258–260 [31]
aIsolated yields
Scheme 3 The plausible mechanism for the formation of 2-arylidene malononitrile and 5-arylidene bar-
bituric acid derivatives in the presence of [H2-Bpy][H2PO4]2
1 3

Knoevenagel condensation in aqueous media promoted by…
The results showed that all the products can be obtained in high yields dur-
ing short reaction times, without the considerable influence of the electron-
donating or -withdrawing substituents located on the aromatic ring of the
aldehyde.
Reusability of the catalyst
To show the reusability potential of the catalyst, the synthesis of compounds
1d and 2e was studied again. In these investigations and after the completion
of the reactions, the products were isolated by filtration and the same reac-
tions repeated in the filtrated solution without the addition of extra amounts
of the catalyst. The results show that this catalyst can be reused at least four
times in both of the mentioned reactions without significant loss of its cata-
lytic activity.
Mechanistic study
The proposed mechanism for the synthesis of 2-arylidene malononitrile in the pres-
ence of [H2-Bpy][H2PO4]2 is shown in Scheme 3. Based on this mechanism, at frst
aldehyde and malononitrile are activated by the acidic protons of the catalyst. After
that, the activated aldehyde is attacked by the activated nitrile compound to obtain
intermediate (Ia). After the elimination of a molecule of water, the corresponding
2-arylidene malononitrile is achieved. This scheme also presents a suggested mecha-
nism for the catalytic role of [H2-Bpy][H2PO4]2 in the 5-arylidene barbituric acid
synthetic process. As can be seen, the catalyst enolized barbituric acid attacks the
aldehyde which is also activated by the acidic proton of the catalyst. After the elimi-
nation of a water molecule from (Ib), the desired 5-arylidene barbituric acid deriva-
tive is achieved.
Comparison of the catalytic ability
To demonstrate the efciency of the proposed method, and also carry out a cor-
rect comparison and investigation of the produced catalyst function, the results
related to the preparation of the Knoevenagel condensation products have been
compared with the same results obtained using other catalysts. In all cases, the
introduced catalyst show advantages from the view-point of lesser amounts
of the used catalyst, lower temperature and the lack of use of organic solvents
(Table 5).
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S. Darvishzad et al.
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Knoevenagel condensation in aqueous media promoted by…
Conclusion
In this work, we described an efcient and clean synthetic protocol for the prepara-
tion of arylidene malononitrile and arylidene barbituric acid derivatives catalyzed
by [H2-Bpy][H2PO4]2 as a new Brönsted acidic ionic liquid via Knoevenagel con-
densation reactions. This method ofers important benefts such as the use of non-
toxic catalyst and solvent, high rates and yields of the reactions, ease of work-up
and separation of the products and high thermal stability and good reusability of the
catalyst.
Supplementary Information The online version contains supplementary material available at https://doi.
org/10.1007/s11164-021-04445-3.
Acknowledgements We are thankful to the Research Council of the University of Guilan for the partial
support of this research.
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