Electrostatically Enhanced Sulfuric Acid: A Strong Br?nsted Acidic Catalyst for Multi-Component Reactions

Article abstract of DOI:10.1007/s10562-019-02776-w

Abstract: A new electrostatically enhanced sulfuric acid as a strong Br?nsted acidic catalyst has been developed for multi-component reactions. A positively charged center in the catalyst electrostatically activates it for acid-catalyzed multi-component reactions and afforded desired products in short reaction time and near room temperature in EtOH as a green solvent. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-019-02776-w

Catalysis Letters
https://doi.org/10.1007/s10562-019-02776-w
Electrostatically Enhanced Sulfuric Acid: A Strong Brønsted Acidic
Catalyst for Multi‑Component Reactions
Mohadese Yaghoobi Anzabi1 · Hossein Yazdani1 · Ayoob Bazgir1
Received: 3 January 2019 / Accepted: 25 March 2019
© Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract
A new electrostatically enhanced sulfuric acid as a strong Brønsted acidic catalyst has been developed for multi-component
reactions. A positively charged center in the catalyst electrostatically activates it for acid-catalyzed multi-component reac-
tions and aforded desired products in short reaction time and near room temperature in EtOH as a green solvent.
Graphical Abstract
Electrostatically enhanced sulfuric acid:
A strong
Bronsted acid ionic liquid catalyst
OSO3H
N
I
reactions
Multi-component
C8H17
Heterocyles
Synthesis
and short reaction time in
temperature
yield
Good
near room
Keywords Electrostatically enhanced sulfuric acid · Brønsted acid catalyst · Multi-component reaction · GBB reaction ·
Biginelli reaction
1 Introduction
heterocyclic compounds from simple precursors with bond-
forming efciency, high atom-economy, compatibility with
green solvents, and under mild conditions [3, 4]. Due to
savings in time, chemicals and efort, multi-component reac-
tions can increase synthetic efciency and provide a positive
environmental impact by reducing chemical use, and they
may be well classifed as sustainable and eco-compatible
methodologies [1–4].
Heterocycles are an essential building block in biomedicals,
natural products, organic materials, pharmaceuticals, and
many functional molecules [1]. As a result, improvement of
efcient and applicable methodologies for the synthesis of
heterocycles under mild and green reaction conditions is an
interesting area in chemistry [1, 2]. The multi-component
reactions (MCRs), which consist of several simultaneous
bond-forming reactions in a one-pot manner, are an appeal-
ing and vital synthetic method for the construction of various
In most MCRs, substrate activation is required to improve
the reaction rate and yield. Substrate activation often occurs
by proton transfer or hydrogen bonding, which both decrease
the energy of the LUMO and make nucleophilic attack facile
[5]. Generally, MCRs were achieved by the use of multi-
ple catalysts or transition-metal catalysts [6, 7]. However, a
metal-free, acid-catalyzed strategy in one-pot fashion using
Bronsted acids as an excellent alternative to transition-metal
catalysts and should be more efective for the synthesis of
complex organic molecules [8–12]. Due to the correlation
between catalyst acidity and activity [13–16], a variety of
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s10562-019-02776-w) contains
supplementary material, which is available to authorized users.
* Ayoob Bazgir
a_bazgir@sbu.ac.ir
1
Department of Chemistry, Shahid Beheshti University G.C,
1983963113 Tehran, Iran
Vol.:(0123456789)
1 3

M. Y. Anzabi et al.
Brønsted acid catalysts such as TSOH.H2O [5], heteropoly
acids [17], phosphoric acids [18], β-cyclodextrin-SO3H
[19], l-proline [20], thioureas [21], and 1,1′-bi-2-naphthol
derivatives [22], with a wide range of acidities have been
reported for multi-component synthesis and the develop-
ment of new Brønsted acid catalysts have been the subject
of moreresearch activities to date [5]. Notably, most of the
organic Brosted acids have limited reactivity due to its mod-
est acidity and can be commonly increased by the presence of
strong electron-withdrawing groups on their structure.In the
past decade, Brönsted acidic ionic liquids as environmentally
benign media have been extensively applied as a catalyst or
dual catalyst-solvent in organic synthesis because of their
extraordinary properties such as low vapor pressure, non-
infammability, reusability, and acidic nature with high ther-
mal and chemical stability [23].Very recently, incorporation
of positively charged center into the structure of the Bronsted
catalyst has been reported as a new strategy for increasing the
acidity and catalytic activity of the Bronsted acid ionic liquid
catalysts. In 2015, Steven R. Kass and co-workers found that
the catalytic ability of a phenol (Fig. 1, I) can be enhanced
by orders of magnitude by the presence of a charged site [14].
The electrostatically enhancement of acidity and catalytic
reactivity of phosphoric acid derivatives (II) and thioure as
(III) was reported by this group in 2016 (Fig. 1) [15, 16].
To explore this new strategy, we herein report synthesis of
electrostatically enhanced sulfuric acid as a strong Brønsted
acidic ionic liquid (Fig. 1) and investigate its catalytic activity
in some multi-component reactions for heterocycles synthesis
under mild and green reaction conditions.
2 Experimental
2.1 General
The melting point of the products was measured using the
9100 electrothermal device and not corrected. The IR spec-
tra of products were recorded by Thermo Nicolet Nexus 470
FT-IRspectrophotometer. H NMR and 13C NMR spectra
1
were obtained on the AVUCE BRUKERDRX-300. MS
spectra were recorded by Agilent Technology 5973 Network
Mass Selective Detector (EI) 70 eV. All required materials
and solvents that used in this study were purchased from
Merck, Fluka and Across companies, and were used without
purifcation.
2.2 Synthesis of N‑octyl 3‑hydroxypyridinium Salt
3-Hydroxy pyridine (10.5 mmol) and 1-iodoactene
(11 mmol) in dry acetonitrile was stirred and refuxed for
20 h under argon conditions. After cooling the mixture to
ambient temperature, the solvent was evaporated, and the
resulted mixture was purifed by silica gel column chroma-
tography using n-hexane and ethyl acetate as the eluents.
The oily brown product was obtained in 34% yield.
2.3 Synthesis of N‑ocetylpyridinium 3‑sulfuric Acid
Salt
N-octyl 3-hydroxy pyridinium salt (1.7 mmol) was mixed
in dry dichloromethane. The mixture was stirred in an ice
bath in the presence of molecular sieve. Then, chloro-
sulfonic acid (3 mmol) was added dropwise (over a half-
hour). After 3 h, the mixture was brought to room tem-
perature, and after evaporation of the solvent, the mixture
was extracted with water and dichloromethane, and then
the organic phase dried using anhydrous sodium sulfate.
After evaporation of the solvent, the oily brown product
was obtained in 40% yield. The catalyst is stable and can be
stored at room temperature in a vacuum desiccator without
loss of activity.
OH
Electrostatically enhanced phenol (I)
N
X
H
I
C8
17
acid
(II)
Electrostatically
phosphoric
enhanced
OH
O
O
OH
O
O
N
P
N
P
N
X
H
H
C8
C8
O
17
17
H
C8
17
O
X
II
X
Electrostatically enhanced thiourea (III)
2.4 A Typical Procedure for the Synthesis
of ethyl 4‑(4‑methoxyphenyl)‑6‑
methyl‑2‑oxo‑1,2,3,4‑tetrahydropyrimidine‑
5‑carboxylate
S
S
N
N
N
Me
N
N
H
Me
N
H
N
Me
H
H
X
X
X
III
electrostatically enhanced
This work:
sulfuric acid
A mixture of 4-methoxy-benzaldehyde (1 mmol), ethyl ace-
toacetate (1 mmol), urea (1 mmol) and catalyst (0.05 mol %)
in ethanol was stirred at 35 °C for 10 min. After cooling the
reaction mixture, the residue was fltered and washed with
ethanol (88% yield).
H
OSO3
N
I
H
C8
17
Fig. 1 Electrostatically enhanced catalysts
1 3

Electrostatically Enhanced Sulfuric Acid: A Strong Brønsted Acidic Catalyst for…
group was observed as a broad signal at δH 6.21 ppm. The
1H-decoupled 13C NMR spectrum of catalyst showed 13 dis-
tinct signals, in agreement with the proposed structure. The
IR spectrum of catalyst in dry toluene exhibits a broad OH
absorption band at 3260 cm−1. Addition of a small amount
of CD3CN (1% v/v) led to a broadening of the OH band
due to the formation of an ArOH··· NCCD3 hydrogen bond
(Fig. 2). These results confrm that the catalyst is very acidic
in toluene [14]. What’s more; the pH of the 0.01 N and
0.001 M solution of the catalyst was measured to be 2.0 and
3.0 using a calibrated glass electrode on a Metrohm 780 pH
meter. This reveals the EESA completely dissociates in the
water like a strong acid. For comparison, the pH of p-TSOH
was measured to be 3.67.
2.5 A Typical Procedure for the Synthesis
of N‑cyclohexyl 2‑ (4‑methoxyphenyl) ‑imidazo
[1,2‑a] pyridine‑3‑amine (10a)
A mixture of 2-aminopyridine (1.8 mmol), 4-methoxy-ben-
zaldehyde (1 mmol) and cyclohexylisocyanide (1 mmol)
and catalyst (0.1 mol %) in ethanol was stirred at 35 °C for
40 min. Then the solvent was evaporated and the mixture
was extracted by ethyl acetate and sodium bicarbonate solu-
tion. The organic phase was dried with anhydrous sodium
sulfate and after evaporation of solvent recrystallized from
ether/n-hexane. The product was obtained in 84% yield.
2.6 A Typical Procedure for the Synthesis of 2‑
(4‑methoxyphenyl) ‑2,3‑dihydroquinazoline‑4
(1H) ‑one (12d)
A positively charged centre in the catalyst electrostati-
cally enhances its acidity and seems to have promising
potential as Brønsted acid catalyst for acid-catalyzed MCRs.
A mixture of 2-amino-benzamide (1 mmol), 4-methoxy-ben-
zaldehyde (1 mmol) and catalyst (0.05 mol %) in ethanol was
stirred at 35 °C for 5 min. After cooling the reaction mixture
to ambient temperature, the residue was fltered and washed
with ethanol (98% yield).
3.2 The Catalytic Activity of the Catalyst
Considering the very important biologically activities of
3,4-dihydropyrimidin-2-(1H)-ones [24], the Biginelli reac-
tion was the frst probe reaction that implemented to evaluate
the catalytic activity of the synthesized catalyst. The initial
investigation was started by the reaction of ethyl acetoacetate
4, benzaldehyde 5a and urea 6 as a model reaction in difer-
ent conditions (Table 1). The reaction was tested employing
various solvents such as H2O, EtOH, MeCN, EtOH–H2O
(1:1), and EtOH–H2O (2:1) and under neat conditions for
30 min in the presence of 0.05 mol % catalyst at 50 °C
(Table 1, Entries 1–6). A superior yield was obtained when
EtOH was used as the solvent (Table 1, Entry 1). Then, the
efect of the amount of catalyst was investigated under the
optimum reaction conditions (Entries 1, 7 and 8). It was
found that when the amount of catalyst increased from 0.03
and 0.05 to 0.1 mol %, the yield of product 7a increased
3 Results and Discussion
3.1 Synthesis of Catalyst
The preparation of electrostatically enhanced sulfuric acid
(EESA) is schematically illustrated in Scheme 1. The com-
mercially available 3-hydroxypyridine 1 was reacted with
1-iodooctane in dry MeCN to aford the N-octylpyridinium
salt 2 [15, 16]. Subsequently, the reaction of N-octylpyri-
dinium 2 with chlorosulfonic acid in dry CH2Cl2 led to the
fnal catalyst EESA 3.
The structures of the catalyst were deduced from their
1
IR, mass, HNMR, 13C NMR spectra and CHNS analysis
1
(Supporting Information). The HNMR spectrum of cat-
alyst consists of a broad signal for the methyl group (δH
0.83 ppm) and one triplet signal for the N-methylene group
(δH 4.51 ppm). Two broad resonances for the other six
methylene groups are observed at δH 1.22 and 1.87 ppm,
respectively. The aromatic hydrogen atoms give rise to char-
acteristic signals in the aromatic region of the spectrum.
Due to the exchange with water of NMR solvent, the OH
OH
H
OSO3
OH
+
H
ClSO3
CH CN
(Reflux)
(Ar)
3
H
17I
n-C8
20 h
CH Cl 0 o
C
N
C8
,
N
N
2
2
I
17
I
H
H
3 h
C8
3
17
1
2
Fig. 2 IR spectrum of catalyst in PhMe (red) and 1% CD3CN/PhMe
(blue)
Scheme 1 Synthesis of EESA
1 3

M. Y. Anzabi et al.
Table 1 Optimization of the reaction conditionsa
pyridine-3-sulfunic acid and EESA 3 to show the rule of
charged center in the fnal catalyst. 3-Hydroxypyridine 1
is a weak catalyst and after 24 h the desired product was
obtained in trace yield (Entry 16). N-octylpyridinium salt
2 is more efective than 1, but this transformation is still
slow and the desired product was obtained in 20% yield
after 24 h (Entry 17). Pyridine-3-sulfunic acid as Bronsted
catalyst aforded the product 7a in 78% after 10 h (Entry
18). P-TSOH and PhOSO3H were tested and aforded the
product in low yield after 10 min (Entries 19 and 20). There-
fore, the synthesized catalyst 3 is much more active and gave
product with good yield in short reaction time (Entry 14).
These results indicate that the charged center in the catalyst
increases the catalyst activity.
Entry Solvent
Time (min) T (oC) Catalyst Yield (%)b
(mol %)
1
2
3
4
5
6
7
8
EtOH
H2O
Neat
CH3CN
EtOH–H2O (1:1) 30
EtOH–H2O (2:1) 30
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
30
30
30
30
50
50
50
50
50
50
50
50
50
r.t.
35
60
35
35
35
35
35
35
35
35
0.05
0.05
0.05
0.05
0.05
0.05
0.03
0.1
88
<20
45
48
30
42
68
85
<20
63
88
89
88
87
81
Trace
20
78
18
By optimizing the reaction condition, the recycling per-
formance of catalyst was investigated using the same model
reaction. After the separation of the product, the fltrate
was evaporated under reduced pressure at 100 °C. Then,
water and n-hexane were added and the organic phase was
decanted. The organic solution containing catalyst was dried
in under reduced pressure. The recovered catalyst was then
charged with starting materials. The process is followed for
four successive cycles (Table 2) and a slight decrease in the
catalytic activity of catalyst might be ascribed to the slight
deactivation of the catalyst.
30
30
30
30
30
30
20
10
9
–
10
11
12
13
14
15
16c
17d
18e
19f
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
5
Next, the scope of the Biginelli reaction was investigated
with respect to the aromatic aldehydes and the 3,4-dihy-
dropyrimidin-2-(1H)-ones 7 were obtained in good isolated
yields and high TOF in short reaction time at near room
temperature (Table 3).
24 h
24 h
10 h
10
20 g EtOH
10
35
aethyl acetoacetate (1 mmol), benzaldehyde (1 mmol), urea
(1.5 mmol), and solvent (4 mL)
bIsolated yield
Table 2 Reusability of catalyst
Run
1
2
3
4
cCatalyst=3-Hydroxypyridine (0.05 mol %)
dCatalyst=N-octylpyridinium salt
eCatalyst=Pyridine-3-sulfunic acid
fCatalyst=p-TSOH
Yield (%) 87 86 84 80
gCatalyst=PhOSO3H
Table 3 The Biginelli reaction for dihydropyrimidinones
Ar
O
O
O
EtOOC
Cat.
(0.05 mol%)
35 o
NH
from 63% and 88%, and 85%, respectively. It was found
that 0.05 mol % catalyst is sufcient to push this reaction
forward. When this reaction was carried out without a cata-
lyst, the yield of the product was trace (Entries 9). Also,
some experiments were performed at room temperature,
35 °C and 60 °C (Entries 10–12). It was found 35 °C was
the optimum reaction temperature (Entry 11). Additionally,
the lower reaction time tested (Entries 13–15). Delightfully,
the reaction in the presence of 0.05 mol % catalyst in EtOH
at 35 °C aforded the product in 87% isolated yield after
10 min (Entry 14). Then, the model reaction was tested in
the presence of 3-hydroxypyridine 1, N-octylpyridinium2,
+
+ ArCHO
EtO
H2N
NH2
C
EtOH,
10 min.
N
H
O
5
4
6
7
Product 7
R
Ar
Yield (%)
TOF (h−1
)
a
b
c
d
e
f
OEt
OEt
OEt
OEt
OMe
OMe
C6H5
87
88
90
82
92
86
10440
10560
10588
9840
11040
10320
4-MeO–C6H4
4-NO2–C6H4
4-Cl–C6H4
4-NO2–C6H4
4-Cl–C6H4
1 3

Electrostatically Enhanced Sulfuric Acid: A Strong Brønsted Acidic Catalyst for…
Table 4 Comparison of
efciency of various sulfuric
acid-based catalysts in the
Biginelli reaction
Entry Catalyst
Conditions
Time (min) Yield (%) TOF (h−1
)
References
1
2
3
4
5
6
Silica sulfuric acid:
Nano–ZrO2sulfuric acid Solvent-free/90 °C
KHSO4
Starch sulfuric acid
Silica sulfuric acid
EESA
EtOH/Refux
360
45
94
92
92
90
92
90
52
12
230
[25]
[26]
[27]
[28]
[29]
This work
Ethylene glycol/100 °C 120
Solvent-free/120 °C
Solvent-free/120 °C
EtOH/35 °C
42
10
10
2571
18
10588
To indicate the feasibility of the present approach, the
reaction was scaled up to a gram scale. Under the optimized
reaction conditions, ethyl acetoacetate 4 (10 mmol) and ben-
zaldehyde 5a (10 mmol) was reacted with urea (15 mmol) in
the presence of EESA (0.05 mol %) at 35 °C and the corre-
sponding product 7a was achieved in 85% yield after 10 min.
Table 4, compares the efciency and capability of EESA
catalyst with other reported sulfuric acid-based catalysts in
the Biginelli reaction of ethyl acetoacetate, 4-Nitro-benzal-
dehyde and urea. Table 2 shows that all the methods based
sulfuric acid catalysts have good efciency in the reactions,
but some of these methods sufer from one or more of the
drawbacks such as high temperatures and require longer
Table 5 The GBB reaction for imidazo[1,2-a]pyridins synthesis
Product
10
Compound 8 Ar
R
Yield
(%)
TOF (h-
1
)
a
b
c
d
4-MeO-C6H4
84
86
80
82
1440
1474
1371
1405
4-(Me)2N-
C6H4
4-Cl-C6H4
4-MeO-C6H4
e
f
4-Me-C6H4
4-NO2-C6H4
89
90
1525
1542
1 3

M. Y. Anzabi et al.
Scheme 2 Synthesis of quina-
zolinones12
O
O
R
R
NH
Cat. (0.05 mol%)
NH2
+ ArCHO
EtOH, 35 oC
5 min.
N
H
Ar
NH2
11
12
Product
Ar
R
Yield (%) TOF (h-1)
12a
12b
12c
12d
12e
C6H5
H
H
H
H
Cl
93
91
87
98
89
22320
21840
20880
23520
21360
4-Cl-C6H4
4-NO2-C6H4
4-OMe-C6H4
4-OMe-C6H4
Table 6 Comparison of
Catalyst
Conditions
Time (min)
Yield (%)
References
efciency of various catalysts in
the quinazolinone synthesis
Glycerol sulfonic acid
Fe3O4–OSO3H
Cellulose sulfonic acid
Boehmits silica sulfuric acid
EESA
Glycerol, 80 °C
EtOH/Refux
MeCN/r.t.
EtOH/80 °C
EtOH/35 °C
60
25
60
50
5
88
96
88
96
93
[32]
[33]
[34]
[35]
This work
reaction times to aforded reasonable yields. The present
catalytic system had high TOF and ofered high catalytic
activity under mild reaction conditions in short reaction
time and low reaction temperature for the synthesis of
dihydropyrimidinones.
time and low reaction temperature for the synthesis of
quinazolinones.
4 Conclusion
Due to the wide application of Groebke-Blackburn-Bien-
aymé (GBB) reaction for the N-fused heterocycles synthesis
[30, 31], the GBB reaction of aminopyridine 8, aldehydes
5 and isocyanides 9 was the second probe reaction that was
tested in this study. To our delight, the one-pot GBB reaction
aforded the imidazo [1,2-a]pyridine derivatives 10 in high
isolated yield and short reaction time (Table 5).
In conclusion, for the frst time, an electrostatically enhanced
sulfuric acid as a new Brønsted acidic ionic liquid catalyst
was synthesized using commercially available starting
materials. A positively charged center in the catalyst elec-
trostatically enhances its acidity and catalytic activity. The
catalyst activity was tested in some acid-catalyzed reactions
for heterocycles synthesis. Impressively, the catalyst showed
excellent catalyst ability with high TOF and desired prod-
ucts were obtained in good isolated yields and short reaction
times at near room temperature.
The observation of the superior potential activity of the
catalyst in the Bronsted acid catalyzed multi-component
reactions such as Biginelli and GBB reactions encouraged
further investigation of the catalyst activity in other Bronsted
acid catalyzed methods for heterocycles synthesis. There-
fore, the synthesis of quinazolinones 12 by the reaction of
2-aminobenzamide 11 and aldehydes in the same reaction
conditions was investigated. As can be seen from Scheme 2,
the desired products 12 were obtained in good isolated yields
after a short reaction time (5 min.).
Acknowledgements We gratefully acknowledge the fnancial support
from the Research Council of Shahid Beheshti University and the Iran
National Science Foundation (INSF).
Finally, the efciency of the EESA catalyst was com-
pared with some reported catalysts in the reaction of
2-aminobenzamide and benzaldehyde (Table 6). As can be
seen in Table 3, the synthesized catalyst has high catalytic
activity under mild reaction conditions in short reaction
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