Sulfated polyborate: a new and eco-friendly catalyst for one-pot multi-component synthesis of 3,4-dihydropyrimidin-2(1H)-ones/thiones via Biginelli reaction

Article abstract of DOI:10.1039/c6nj03120j

In this report, an efficient high-yield method for the synthesis of 3,4-dihydropyrimidin-2(1H)-ones/thiones via a three-component Biginelli reaction with various aldehydes, β-ketoesters/β-diketones and urea/thiourea is described. The reaction was catalyze

Full text of DOI:10.1039/c6nj03120j

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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
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Sulfated polyborate: a new and eco-friendly catalyst for one-pot
multi-component synthesis of 3,4-dihydropyrimidin-2(1H)-ones/
thiones via Biginelli reaction
Received 00th January 20xx,
Accepted 00th January 20xx
Chetan K. Khatri, Deelip S. Rekunge and Ganesh U. Chaturbhuj*
DOI: 10.1039/x0xx00000x
This report describes an efficient method for the synthesis of three-component Biginelli reaction of various aldehydes, β-
ketoesters/β-diketone and urea/thiourea catalyzed by sulfated polyborate with high yields under a solvent free condition
at 100 °C. The catalyst has been prepared and used as a Bronsted as well as Lewis acid catalyst for the reaction. The
catalyst was prepared by dehydration of boric acid followed by sulfonation and characterized by different analytical
techniques. The key advantages of the present method are high yields, short reaction time, solvent free condition, easy
workup, recyclability of catalyst and ability to tolerate a variety of functional groups which gives economical as well as
ecological rewards.
www.rsc.org/
The most simple and straightforward reaction reported by
Biginelli involves the one-pot three component condensation
of an aldehyde, ethyl acetoacetate, and urea.6,7 However, this
Introduction
More than a century ago Italian chemist Pietro Biginelli has
reported the synthesis of 3,4-dihydropyrimidin-2(1H)-ones
(DHPM). However, in last few years, its attractiveness and
demand have grown in the field of catalysis. Much effort has
covered for new synthetic methodologies, due to their
pharmacological properties such as anti-viral, anti-tumour,
anti-bacterial, anti-inflammatory, anti-hypertensive and most
importantly, as calcium channel modulators.1-4 Monastrol is a
3,4-dihydropyrimidin-2(1H)-thione, which blocks mitosis by
specifically inhibiting the motor activity of the mitotic kinesis
Eg5 and it is a lead molecule for the development of new
anticancer drugs. In addition, (R)-SQ32926 acts as an
antihypertensive agent with potent oral activity, whereas
promising anticancer activity is also reported with Mon-97 (Fig.
1).5
procedure suffered from shortcomings such as the harsh
reaction conditions, long reaction time and frequently low
yields. This has rekindled development of new methodologies
for the Biginelli reaction, which involves the use of a number
of Bronsted and Lewis acid catalysts such as sulfated silica
tungstic acid,8 SiO2-polyphosphoric acid (SiO2-PPA),9
Al(HSO4)3,10 Al2O3-MeSO3H,11 polyvinyl sulfonic acid,12 sulfated
tungstate,13 Wells-Dawson hetero-poly acid,14 boric acid,15
silica-HBF4,16 H2SO4-Al2O3,17 Al2O3-SO3H,18 p-toluene sulfonic
acid,19 sulfated zirconia,20 cellulose sulfuric acid,21 silica sulfuric
acid,22 PPF-SO3H,23 montmorillonite KSF,24 H2SO4-silica,25
SnCl2.2H2O,26
graphite,27
Bi(NO3)3,28
L-proline,29
molybdenum(VI) dichloride dioxide,30 1-methyl-3-(3-tri
methoxysilylpropyl) imidazolium hydrogen sulfate immobilized
on magnetic Fe3O4 nanoparticles,31 organosilane sulfonated
graphene oxide nanocatalyst,32 ZnO nanoparticles,33
OH
NO2
OH
phosphomolybdic
acid
immobilized
on
imidazole
functionalized Fe3O4-SiO2 core-shell nanosphere34 and so on.
3,4-dihydropyrimidin-2(1H)-ones/thiones have also been
synthesized under microwave and ultrasound irradiations
using different catalysts.35-38 However, some of the newer
literature methods suffer from drawbacks of unsatisfactory
yields, longer reaction time, extractive product isolation
procedures with toxic organic solvents, use of expensive,
metal-based, toxic/corrosive catalysts, which limits their use
due to environmental issues.
O
O
O
CONH2
O
EtO
NH
iPrO
N
Ph
NH
N
S
N
N
S
H
H
Monastrol
Mon-97
(R)-SQ 32926
Fig. 1 Pharmacologically active 3,4-dihydropyrimidin-2(1H)-ones/thiones
A literature search revealed that boric acid catalyzes many
important organic transformations at a temperature above
100 °C.39 Boric acid dehydrates above 100°C and converts to its
polymeric forms, which presumably is the active species
Department of Pharmaceutical Sciences and Technology, Institute of Chemical
Technology, Mumbai, India. E-mail address: gu.chaturbhuj@gmail.com; Tel: +91 22 3361
2212
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ascertained on JASCO P-2000 polarimeter. Chemicals and
solvents used were of LR grade and purchased from SD fine,
AvraSynthesis and Spectrochem and used without purification.
The purity determination of the starting materials and the
reaction monitoring was accomplished by thin-layer
chromatography (TLC) on Merck silica gel G F254 plates. All the
products are known compounds and were characterized by
melting points in comparison with the literature values, FTIR
and 1H NMR spectroscopy for structural identification.
Scheme 1 Schematic representation of sulfated polyborate catalyzed synthesis of 3,4-
dihydropyrimidin-2(1H)-ones/thiones
Results and discussion
catalyzing the reaction.39,40 Dehydrative polymerization of
boric acid liberates of water molecules which may hamper the
progress of the reaction.
Preparation and characterization of the catalyst
Boric acid was heated in a petri dish at 200 °C for 4h to convert
it to the polyboric acid; resultant glassy solid was ground into
fine powder. Polyboric acid powder (5 g) was suspended in
chloroform (20 ml) in 250 ml round bottom flask,
chlorosulfonic acid (4.23 ml) was added drop wise over 30
minutes at room temperature. The mixture was stirred for 120
minutes at the same temperature. The reaction was quenched
by adding ethanol (10 ml). Residual HCl gas was eliminated by
nitrogen flush, the solid was filtered and washed several times
with chloroform. Finally solid sulfated polyborate was dried at
100 °C in hot air oven till constant weight.
This inspired us to develop a polymeric boric acid catalyst with
mild Bronsted acidity. To accomplish this boric acid was
dehydrated at 200 °C to convert it into its polymeric Lewis acid
form and then sulfonated to introduce the mild Bronsted acid
character. Boron being an electron deficient element and
electron withdrawing effect of adjacent sulfate enhances its
Lewis acidity; hence it has both Lewis as well as Bronsted acid
characters (Scheme 1).
The development of novel synthetic procedures with an
objective of green chemistry and green technology, the use of
recyclable catalysts for organic synthesis to maximize catalyst
efficiency and minimize waste production been currently in
demand. To achieve these objectives, herein this report
describes sulfated polyborate as an efficient and eco-friendly
catalyst for the 3,4-dihydropyrimidin-2(1H)-ones/thiones
synthesis under a solvent free condition with high yields and
non-extractive product isolation. The catalyst was prepared
from boric acid which is a readily available, non-toxic, and
inexpensive starting material. To the best of our knowledge,
this is the first report on the use of sulfated polyborate for the
synthesis of 3,4-dihydropyrimidin-2(1H)-ones/thiones (Scheme
1). This catalyst is environmentally benign due to its mild
acidity and non-toxic nature.
The catalyst was characterized by techniques such as
potentiometric analysis, Fourier transform infrared
spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron
microscopy (SEM) energy dispersive X-ray spectroscopy
(EDAX).
Accurately weighed catalyst (0.1g) was dissolved in a mixture
of water and glycerine (2:1) and titrated against standard 0.1N
NaOH solution. The concentration of H+ was found as 1
mmol/g at first equivalence point which indicates the presence
of SO3H group and the total concentration of H+ were found as
19.5 mmol/g at second equivalence point due to H+ of
associated B-O-H and SO3H groups. First equivalence point was
absent in boric acid as well as its polymeric form prior to
sulfonation (Fig. 2).
FTIR spectrum of the catalyst was recorded to ascertain
incorporation of the sulfonic acid group in polyboric acid.
Sulfated polyborate exhibited characteristic absorption bands
at 3446 and 1629 cm-1 that could be assigned to the -OH
stretching and bending of the SO3H groups, which can be
regarded as ‘‘Bronsted active sites’’ of the catalyst. The band
at 3221 cm-1 corresponds to O-H stretching of B-O-H and at
1469 cm-1 for B-O stretching. Bands at 1294 cm-1 for O=S=O
asym. stretching, 1068 cm-1 for sym. stretching, 1004 cm-1 for
S=O stretching of the SO3H group. Band at 580 cm-1 represents
O-B-O stretching of catalyst (Fig. 3).41,42
Experimental
Materials and methods
Melting points of all the compounds were recorded by
AnalabThermoCal melting point apparatus in open capillary
tube and are uncorrected. The FTIR spectra (KBr) were
recorded on Shimadzu FTIRAffinity-1 Fourier Transform
Infrared spectrophotometer. 1H NMR spectra were recorded
on MR400 Agilent Technology NMR spectrometer using
tetramethylsilane (TMS) as an internal standard and DMSO-
d6/CDCl3 as
a solvent. X-ray diffractograms (XRD) were
Powder XRD pattern of sulfated polyborate shown significant
peaks positioned at 2θ = 28.1° which confirms the presence of
B–O bonds in the crystalline structure of the catalyst (Fig. 4).43
The surface morphology of sulfated polyborate catalyst in the SEM
image clearly indicated that the catalyst is crystalline with various
shapes and sizes in the range of 4-10 µm (Fig. 5).
recorded on Rigakuminiflex X-ray Diffractometer. The SEM-
EDAX characterization was performed on a JEOL JSM-638DLA
scanning electron microscope equipped with energy dispersive
X-ray spectrometer. The potentiometric analysis was
performed on Elico LI 120 pH meter. The chirality has been
2 | J. Name., 2012, 00, 1-3
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250
200
150
100
50
Boric acid
Polyboric acid
Sulfated polyborate
0
0
5
10
15
Volume of NaOH
20
25
30
Fig. 4 XRD pattern of the catalyst
Fig. 2 Potentiometric titration curves of boric acid, polyboric acid and sulfated
polyborate catalyst
Fig.5 SEM image of the catalyst
Fig. 3 FTIR spectrum of the catalyst
not advantageous (Table 1, entries 5 and 6), hence 5 wt %
catalyst loading was chosen for further study.
The average elemental distribution mapping of the catalyst by EDAX
showed boron: oxygen: sulfur signal ratio of 30.32: 68.73: 0.96 wt %
over different areas.
Temperature played an important role in the synthesis of 3,4-
dihydropyrimidin-2(1H)-ones (Table 1, entry 7 and 8). The
temperature effect was examined at ambient, 70 °C and 100 °C
under the solvent free condition with sulfated polyborate as a
catalyst. The reaction does not proceed at room temperature
while proceeds with low product yield at 70 °C. Further
increasing temperature to 100 °C resulted in increased product
yield in shorter reaction time (Table 1, entries 5). Therefore,
this was the optimum temperature for performing the
reaction.
Application of sulfated polyborate in Biginelli reaction
We structured our study to investigate the suitability of
sulfated polyborate as a catalyst for Biginelli reaction at
different reaction conditions. For the preliminary
experiment benzaldehyde (2 mmol),
a representative
substrate, ethyl acetoacetate (2 mmol) and urea (2.4 mmol)
were used to afford Ethyl 6-methyl-2-oxo-4-phenyl-1,2,3,4-
tetrahydropyrimidine-5-carboxylate. (Table 1 and 2) The
product has a chiral center and the present method produces
racemate which is confirmed by polarimetry.
The effect of various solvents on time and yield of the reaction
was ascertained (Table 2, entries 1-7). None of the solvents
has the advantage of time and yield over solvent free
condition. Hence, the solvent free condition was regarded as
best for the cost and environmental acceptability.
Effect of the catalyst loading on time and yields of the reaction
was assessed (Table 1, entries 1-6). The reaction does not
proceed in the absence of a catalyst at room temperature
(Table1, entry 1) while proceeds at 100 °C with low product
yield (Table 1, entry 2). An increase of the catalyst loading
increased the product yield with a reduction in reaction time
(Table 1, entries 3-6). The catalyst loading beyond 5 wt % was
In all the experiments the products were isolated by aqueous
quenching followed by filtration and washing the solids with
water.
The reusability of the catalyst in the model reaction under
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Table 1 Effect of catalyst loading for the Biginelli reaction
Table 3 Efficiency of sulfated polyborate in comparison with literature reported
catalysts for the Biginelli reaction
Entry
Catalyst
(wt %)
Temperature
(°C)
Time
(min)
Yielda
(%)
Entry Catalyst
Reaction
Condition
Time Yield Ref.
(%)
1
2
3
4
5
6
7
8
-
rt
360
120
60
NR*
28
1
2
3
Sulfated
polyborate
Solvent
free/100 °C
20
min
15
94
94
85
This
work
8
-
100
100
100
100
100
rt
1
81
Sulfated silica Solvent
2.5
5
30
89
tungsten acid
SiO2-PPA
free/70 °C
min
Acetonitrile/ref 30
lux
9
20
94
min
10
5
20
94
NRa
4
5
Al(HSO4)3
Solvent
free/120 °C
Ethanol/60 °C
35
min
1h
79
98
10
11
60
5
70
60
76
Al2O3-
MeSO3H
PSA
Sulfated
tungstate
aIsolated Yield
*NR= No Reaction
6
7
Water/90 °C
Solvent
free/80 °C
1h
1h
94
92
12
13
Table 2 The effect of various solvents for the Biginelli reaction
8
9
Wells-Dawson Solvent
1.5h
2h
83
97
14
15
Entry Solvent
Temperature
(°C)
Time
(min)
20
Yielda
(%)
94
heteropoly
acid
free/80 °C
Boric acid
Glacial acetic
acid/ 100 °C
Ethanol/rt
Hexane/reflux
Solvent
1
2
3
4
5
6
7
Solvent free
Ethanol
100
reflux
reflux
reflux
reflux
reflux
100
60
69
10
11
12
Silica-HBF4
H2SO4-Al2O3
Al2O3-SO3H
2h
2h
2.1h
94
95
88
16
17
18
Acetonitrile
Tetrahydrofuran
Water
60
62
60
58
60
Traces
60
62
free/120 °C
Ethanol/reflux
Solvent
Toluene
DMF
60
60
13
14
p-TSA
Sulfated
zirconia
2.5h
4h
91
94
19
20
aIsolated Yield
free/80 °C
15
16
Cellulose
sulfuric acid
Silica sulfuric
acid
Water/100 °C
5h
6h
80
91
21
22
solvent free condition at 100 °C was evaluated. In this study,
after completion of each reaction cycle, water was added and
the product was filtered off. The filtrate was evaporated under
vacuum rotary evaporator to recover catalyst. Recovered
catalyst was recycled for four times with no significant loss in
catalytic activity (Fig. 6).
Ethanol/reflux
17
18
PPF-SO3H
Montmorilio
nite KSF
Ethanol/reflux
Solvent
free/130 °C
8h
48h
81
82
23
24
In comparison with literature reported catalysts used for
Biginelli reaction of benzaldehyde, ethyl acetoacetate, and
urea; the present method has an advantage in many cases
with respect to reaction condition, workup procedure, time
and yields (Table 3).
To investigate the substrate scope, optimized reaction
condition was applied to substituted aromatic/aliphatic/α,β-
unsaturated aldehydes, β-ketoesters/β-diketone and urea/
thiourea variants.49 All the substrate variants reacted well and
afforded higher yields of the corresponding 3,4-
dihydropyrimidin-2(1H)-ones/thiones within short reaction
time (Table 4). Several electron releasing or electron
withdrawing substituents at ortho and para positions of
aromatic aldehydes have been examined. The nature of
substitution has no significant effect on reaction time and
yields. However, for 4-N,N-dimethyl, and 2-methoxy
substrates, the reaction time was longer with similar product
yield presumably due to electron releasing effects (Table 4,
entry 9, 10).
100
80
60
40
20
0
3,4-dihydropyrimidin-2(1H)-thiones were also synthesized
using thiourea instead of urea under optimized reaction
conditions. Reaction with thiourea took little longer reaction
time with similar product yields (Table 4, entry 12-16).
1
2
No. of cycles
3
4
5
Fig. 6 Reusability of the catalyst
This protocol was extendable to aliphatic aldehydes (Table 4,
4 | J. Name., 2012, 00, 1-3
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Table 4 Sulfated polyborate catalyzed Biginelli reaction under solvent free condition
Entry
Aldehydes
(R)
β-Ketoesters
R2
X
Time
(min)
Yielda(%)
M.P. °C
R1
Obs.
Lit.
1
C6H5
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
OCH2CH3
O
O
O
O
O
O
O
O
O
O
O
S
20
20
30
20
20
20
20
20
45
30
20
30
45
30
45
45
30
20
20
45
20
30
94
91
86
89
91
86
89
88
90
90
90
92
90
91
87
88
83
92
91
85
94
90
204-205
199-200
232-234
211-212
214-215
199-200
210-211
184-185
232-233
259-260
214-215
207-208
155-156
181-182
188-189
216-217
154-155
189-190
230-231
221-223
212-213
176-177
205-20634
200-20134
234-23521
213-21434
215-21634
203-20434
209-21034
182-18422
230-23232
262-26334
215-21634
208-21034
152-15334
179-18034
189-19044
218-22021
155-15645
NA*
2
4-CH3O-C6H4
4-HO-C6H4
4-Br-C6H4
4-Cl-C6H4
4-CH3-C6H4
4-NO2-C6H4
4-F-C6H4
4-N(CH3)2-C6H4
2-CH3O-C6H4
2-Cl-C6H4
H-C6H4
OCH2CH3
OCH2CH3
OCH2CH3
OCH2CH3
OCH2CH3
OCH2CH3
OCH2CH3
OCH2CH3
OCH2CH3
OCH2CH3
OCH2CH3
OCH2CH3
OCH2CH3
OCH2CH3
OCH2CH3
OCH2CH3
OCH2CH3
OCH2CH3
OCH2CH3
OCH3
3
4
5
6
7
8
9
10
11
12
13
4-CH3O-C6H4
4-Cl-C6H4
2-CH3O-C6H4
2-Cl-C6H4
C3H7
S
14
S
15
S
16
S
17
O
O
O
O
O
O
18
c-C3H5
19
c-C6H11
232-23446
223-22434
216-21847
168-17048
20
C6H5CH=CH
C6H5
21
22
C6H5
R1,R2=CH2C(CH3)2CH2
aIsolated Yield
*NA= Not Available in literature
entry 17-19) and cinnamaldehyde (Table 4, entry 20). On The Authors are grateful to University Grant Commission for their
other hand, variations in β-ketoesters and a β-diketone were financial support.
also adaptable (Table 4, entry 21-22).
This protocol tolerates a variety of substituents on aromatic
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New Journal of Chemistry
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DOI: 10.1039/C6NJ03120J
Graphical abstract:
Rapid, high yielding, solvent free reaction with easy workup procedure using recyclable catalyst with no loss in catalytic activity.