Synthesis of dihydropyrimidinones using large pore zeolites

Article abstract of DOI:10.1007/s10562-011-0639-6

A series of dihydropyrimidin-2(1H)-one (DHPM) belongs to one of the important class of therapeutic and pharmacological active compound, were synthesized through the multicomponent reactions (MCRs) of aldehydes, ethyl acetoacetate and urea, followed by the

Full text of DOI:10.1007/s10562-011-0639-6

Catal Lett (2011) 141:1541–1547
DOI 10.1007/s10562-011-0639-6
Synthesis of Dihydropyrimidinones Using Large Pore Zeolites
Sunil R. Mistry
Kalpana C. Maheria
•
Rikesh S. Joshi
•
Suban K. Sahoo
•
Received: 14 March 2011 / Accepted: 29 May 2011 / Published online: 16 June 2011
Ó Springer Science+Business Media, LLC 2011
Abstract A series of dihydropyrimidin-2(1H)-one (DHPM)
belongs to one of the important class of therapeutic and
pharmacological active compound, were synthesized through
the multicomponent reactions (MCRs) of aldehydes, ethyl
acetoacetate and urea, followed by the heterogeneous cata-
lyzed Biginelli reaction. In the present endeavour, medium
include calcium channel blockers, antihypertensive and
anti-inflammatory agents [1]. Additionally, their particular
structure has been found in natural marine alkaloid Batz-
elladine A and B which are the first low molecular weight
natural products reported in the literature to inhibit the
binding of HIV gp-120 to CD cells, so disclosing a new
4
(
ZSM-5) and large pore zeolites (Y, BEA and MOR) as well
field towards the development of AIDS therapy [2]. The
original procedure for the preparation of DHPMs was
reported by Biginelli [3], involving one-pot condensation
of ethylacetoacetate, benzaldehyde and urea under strongly
acidic conditions. One major drawback of this so called
Biginelli reaction, however is the low to moderate yield
that is often encountered when substituted aromatic
(20–60%) or aliphatic aldehydes (10%) is used [4].
as dealuminated zeolites BEA, were studied as catalysts. An
excellent activity for DHPMs synthesis is achieved by opti-
mizing accessibility of the reactants to the active sites and the
surface polarity of zeolite catalysts. Moreover, the mechanism
of Biginelli reaction was studied by means of GAUSSVIEW
energy calculations of adsorbed acylimine intermediate on
zeolite by using the density functional method (DFT).
Report concerning this catalytic one pot multicompo-
nent reactions (MCRs), which has been carried out both in
solvent and solvent free condition, are available [3–11].
The catalysts consist of Lewis acids such as BF ÁOEt [5],
Keywords Zeolite Á Biginelli reaction Á DHPM Á
DFT studies
3
2
triflates [6, 7] etc. as well as protic acids such as H SO ,
2
4
1
Introduction
HOAc, Conc. HCl [8] as promoters. Many other methods
including microwave irradiations, ionic liquids and ultra-
sonic mediated [9–11] are also reported. However, many of
these methods are associated with expensive and toxic
reagents, stoichiometric amount of catalyst, reaction time,
unsatisfactory yields, incompatibility with other functional
groups and involve difficult product isolation procedures.
Reported solid acid catalysts for the synthesis of DHPMs
are polyoxometalate [12], heteropoly acids [13, 14], silica
sulfuric acid [15] and ammonium chloride [16]. Recently,
some molecular sieves such as zeolite H-Y, H-ZSM-5,
MCM-41, Ersorb-4 (E4) and Heulandite (HEU) type natural
zeolites [17–19] have been also reported as efficient heter-
ogeneous catalysts for the synthesis of DHPMs. According
to these reports, zeolite showed high catalytic activity in
Biginelli reaction. However, all above protocols seems to
Dihydropyrimidin-2(1H)-one (DHPM) and their deriva-
tives have attracted considerable attention because of their
diverse therapeutic and pharmacological profile which
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-011-0639-6) contains supplementary
material, which is available to authorized users.
S. R. Mistry Á S. K. Sahoo Á K. C. Maheria (&)
Department of Applied Chemistry, Sardar Vallabhai National
Institute of Technology, Surat, Gujarat, India
e-mail: kcm@ashd.svnit.ac.in
R. S. Joshi
Sud-Chemie India Pvt. Ltd., Nandesari, India
123

1
542
S. R. Mistry et al.
Scheme 1 Zeolite catalyzed
synthesis of DHPMs
O
R
O
O
O
Zeolite
Reflux
H C
3
O
NH
CH3
+
H C
3
O
R
H
2
H C
3
N
H
O
1
(a-k)
+
O
4
(a-l)
H N
2
NH2
3
aim mainly at enhancing the yields of the reaction. So far,
not much attention has been paid to the role of surface
acidity and pore structure of solid acid catalysts in the
mechanism of the Biginelli reaction.
All the reactions were carried out in a round bottom
flask attached to a condenser and equipped with a magnetic
stirrer under heating in an oil bath. In a typical reaction, to
a solution of urea (1 mol) and ethyl acetoacetate (1 mol) in
toluene, appropriate amount of aldehyde (1 mol) and zeo-
lite (2 wt%) were added. The reaction mixture was refluxed
for 6 and 10 h with zeolite H-BEA and H-MOR, respec-
tively. After completion of the reaction indicated by TLC,
the spent catalysts were collected by filtration and then
washed with ethanol. Crude product was recovered by
evaporating the solvent under reduced pressure. This
product was purified by recrystallization with ethanol to
afford pure 5-ethoxycarbonyl-6-methyl-4-phenyl-3,4-dihy-
dropyrimidin-2-one having melting point 201 °C (reported,
202 °C) in 85% yield with zeolite H-BEA and 60% yield
with H-MOR.
In the present endeavour, we have shown that with
proper choice of topology, acidity and adsorption charac-
teristic of zeolite catalyst, it is possible to obtain series of
DHPMs with excellent yield through Biginelli reaction.
Because bulky molecules or reaction intermediates are
involved in the synthesis of DHPMs, large pore zeolite
BEA, dealuminated BEA, H-Y and MOR topologies were
chosen in our investigation (Scheme 1).
2
Experimental
2
.1 Catalyst Preparation and Characterization
The desire product, 5-ethoxycarbonyl-6-methyl-4-phe-
nyl-3,4-dihydropyrimidin-2-one (4a) was characterized by
comparison of their physical data with those of known
compound [17–19]. The spectral data of representative
compound including novel DHPM (Table 3, entry 4l)
synthesized using 2-chloro-3-formylquinoline [21] as a
new aldehyde functionality are given below.
The zeolites Na-beta (BEA(12); Si/Al = 12), H-Mordenite
H-MOR(11); Si/Al = 11), Na-Y (Si/Al = 2.43), Na-ZSM-
(Si/Al = 15) zeolites were obtained from Sud-Chemie
(
5
India Pvt Ltd., INDIA. The H-form of zeolite were prepared
by ion exchange of the Na-form samples with aqueous
solution of NH NO (1 M), followed by drying and calci-
4
3
nation at 823 K. Zeolite BEA with different Si/Al ratio were
prepared by dealumination of these parent zeolite with
2.2.1 5-Ethoxycarbonyl-6-methyl-4-phenyl-3,
4-dihydropyrimidin-2-one (4a)
HNO according to the method reported in the literature [20].
3
1
The phase purity and crystallinity of the zeolites were
analyzed by XRD (D8 Advanced Brucker AXS, Germany)
with Cu Ka radiation and nickel filter. Surface area mea-
surement (BET method) was carried out on Micromeritics
Gemini at -196 °C using nitrogen adsorption isotherms.
Acidity of zeolites were determined on Micromeritics
Chemisorb 2720, by a temperature programmed desorption
mp 202–204 °C; H-NMR (DMSO-d ): 1.1 (t, J = 7.2 Hz,
6
3H, CH CH O), 2.24 (s, 3H, CH ), 4.00 (q, J = 7.2 Hz,
2H, OCH ), 5.16 (s, 1H, CH), 7.18–7.30 (m, 5H, arom
3
2
3
2
CH), 7.78 (s, 1H, NH), 9.25 (s, 1H, NH); IR (KBr): 3244,
-
1724, 1639 cm ; Anal. calcd. for C H N O : C, 64.60;
1
1
4 16 2 3
H, 6.20; N, 10.79. Found: C, 64.69; H, 6.28; N, 10.90.
(
TPD) of ammonia. Ammonia was chemisorbed at 120 °C
2.2.2 5-Ethoxycarbonyl-6-methyl-4-(2-chloroquinolin-
3-yl)-3,4-dihydropyrimidin-2-one (4l)
and then desorption was carried out up to 700 °C at heating
rate of 10 °C/min. The solvents were distilled before use.
All reagent used were of analytical grade.
1
mp 248–250 °C; H-NMR (DMSO-d ): 1.069 (t, J =
6
7
.2 Hz, 3H, CH CH O), 2.34 (s, 3H, CH ), 3.99
3 2 3
2
.2 Typical Procedure for the Biginelli Reaction
(q, J = 7.2 Hz, 2H, OCH ), 5.37 (s, 1H, CH), 7.10–7.18
2
(m, 1H, arom CH), 7.23 (s, 1H, arom CH), 7.32
All zeolites were activated, by heating at higher temperature
of 773–823 K for 3–4 h, before loading into the reactor.
(d, J = 7.5 Hz, 1H, arom CH), 7.46–7.49 (m, 1H, arom
CH), 7.57 (s, 1H, NH), 7.70 (d, J = 7.5 Hz, 1H, arom CH)
1
23

Synthesis of Dihydropyrimidinones Using Large Pore Zeolites
1543
1
.22 (s, 1H, NH); C-NMR (DMSO-d ): 16.74, 20.53,
3
9
Table 2 Biginelli reaction of benzaldehyde (1a), ethylacetoacetate
and urea using different zeolites
6
4
1.94, 61.73, 98.71, 117.47, 152.76, 154.80, 167.84; IR
-
1
a
(
KBr): 3294, 3217, 1660, 1434, 1566, 754 cm ; MS:
m/z = 349 (M ? 2); Anal. Calcd. for C H ClN O : C,
Catalysts
Time (h)
Yield (%)
Found
1
7
16
3 3
Reference
5
1
9.05; H, 4.66; N, 12.15. Found: C, 59.25; H, 4.62; N,
2.18.
H-BEA (15)
H-MOR (11)
H-Y (2.43)
5–6
10
85
60
75
19
–
In order to investigate reusability, catalyst was separated
–
from the reaction mixture by filtration, washed with etha-
nol, dried overnight at 333 K and re-activated by heating in
a stream of air at 673–773 K for 4 h. This procedure was
followed for each recycle study of the catalyst.
12
80 [17]
21 [17]
H-ZSM-5 (15)
12
Reaction conditions: aldehyde/urea/ethylacetoacetate/catalyst: 1/1/1/2
wt% in toluene as solvent
a
Isolated yields
3
Results and Discussion
12-membered ring (large pore) showed the high yield of
85% (run 1, Table 2) in considerably shorten reaction time
3
.1 Catalysts Characterizations
as compared to other zeolites indicating that higher acid
strength of the acid centre. Thus, the preferential order
to yield DHPM was found to be: H-BEA [ H-Y [
H-MOR [ H-ZSM-5.
Physicochemical characterization data provided in the
Table 1 reveal that the surface area of the zeolite BEA and
dealuminated BEA is higher than the rest of the zeolites and
the concentration of acid centre found for zeolites MOR is
higher than the rest of the zeolites. Moreover, the XRD pat-
terns of all the dealuminated BEA were found to be similar to
the parent zeolite. Moreover, upon dealumination, pore vol-
ume and surface area of zeolite is slightly increases and total
acidity of zeolite decreases as increase in Si/Al ratio.
3.3 Effect of Geometry and Surface Acidity
The observed low activity of medium pore zeolite, ZSM-5,
is probably ascribed to the diffusional limitation of the
pores towards bulkier reactant molecules and geometrical
constraints for the formation of intermediates inside the
pores. Whereas, MOR exhibits largest number of acid sites
compare to other zeolites (Table 1) but it seems that the
unidirectional channels system causes either inherent dif-
fusional limitations or pore blocking owing to strong
adsorption of the reactant or products. Further, in agree-
ment with existing literature [19] it can also be concluded
that, existence of strong acid sites in ZSM-5 and MOR does
not favor the Biginelli reaction due to the possibility of
denseness of the active sites. Moreover, the surface area of
all BEA zeolites is higher than other zeolites. Hence, it is
concluded that low to moderate acidity and higher surface
area of zeolite BEA are prime factors for higher catalytic
activity in the Biginelli reaction.
3
.2 Synthesis of DHPMs
Zeolites with different topologies as solid acid catalysts
were used to elucidate the role of the zeolite channel sys-
tem on their activity and selectivity in the Biginelli reac-
tion. It is observed that, Biginelli reaction did not occur in
the absence of zeolite catalyst. The Biginelli reaction of
DHPM was carried out with 12-member-ring (MR) tridi-
rectional zeolites (H-Y and H-BEA), a 12-MR monodi-
rectional zeolite (H-MOR) and 10-MR bidirectional zeolite
(
ZSM-5) in the toluene as a solvent. Table 2 shows the
effect of various structural features such as, geometry (pore
structure and dimension), acidity and Si/Al ratio of zeolites
on the synthesis of DHPM. H-BEA with 3-dimensional
The most surprising observation was the low activity
shown by H-Y and H-MOR compared to H-BEA (Table 2).
Table 1 Physiochemical
properties of various zeolites
used in Biginelli reaction
Catalysts
Si/Al
Pore volume
(
S
BET
2
(m /g)
Ammonia uptake (mmol/g)
3
cm /g)
Weak
Strong
Total
H-BEA (12)
H-BEA (15)
H-BEA (25)
H-BEA (34)
H-MOR (11)
H-Y (2.43)
12
0.27
0.30
0.34
0.36
0.22
0.34
–
680
710
732
750
412
480
320
0.70
0.57
0.35
0.22
2.02
2.30
1.95
0.89
0.70
0.40
0.30
2.39
–
1.60
1.27
0.75
0.52
4.41
2.30
3.35
15
25
34
11
2.43
15
H-ZSM-5 (15)
1.40
123

1
544
S. R. Mistry et al.
It should be noted in this case that, there is no geometrical
constraints for the diffusion of reactant through the pores
but the strong adsorption of reactants and/or products in the
pores of the catalyst that poisons or block the active sites
and channels. The higher activity of H-BEA may also
ascribed to higher Si/Al ratio compared to rest of the
zeolites consequently leads to higher hydrophobicity.
Furthermore, it appears that we may favor the product
desorption by controlling the parameters such as polarity of
reactant, solvent and catalyst as well. Thus, apart from
acidity and geometry; one should also take into account the
adsorption properties (hydrophobicity and hydrophilicity)
of the zeolites [22].
less polar zeolite with lower concentration of acid sites
performs better. In fact, an 85% yield of 4a was obtained
with 2 wt% of BEA(15) after 6 h. Moreover, the solvent
may also affect the reactivity of the reaction intermediate,
it may also adsorbed strongly and poison the active sites,
cause substrate exclusion from the surface and increase the
resistance to diffusion of the reactants inside the channels.
Therefore, the less polar solvent toluene (dielectric con-
stant = 2.38) is more suitable compared to the ethanol,
ACN and DMF (dielectric constant = 24.5, 37.5 and 38
respectively).
The similar behavior was observed when the Biginelli
reaction of p-hydroxy benzaldehyde (1b), ethylacetoacetate
and urea was carried out with the BEA(15) (Fig. 2). How-
ever, the decrease in activity of Si/Al curve towards more
hydrophobic surface was occurred due to the presence of OH
group at para position of the aromatic ring which increases
the polarity of substrate 1b as compared to 1a, and therefore
in the adsorption/desorption characteristics.
3
.4 Effect of Surface Polarity (Si/Al Ratio) of Zeolites
It is known that, with increasing framework Si/Al ratio, the
catalyst becomes more hydrophobic, the concentration of
acid sites decreases and strength of remaining sites
increases [23]. According to the results and conclusions
presented above, we could expect that the less polar zeolite
should favor the desorption of adsorbed polar product.
Therefore, in order to study the effect of polarity of zeo-
lites, we have carried out Biginelli reaction of benzalde-
hyde (1a), ethylacetoacetate and urea using H-BEA of
varied Si/Al ratio (12, 15, 25 and 34) under various reac-
tion conditions as depicted in Fig. 1. Zeolite BEA with
different Si/Al ratio were prepared by dealumination of
these parent zeolite BEA(12).
In general, the higher activity of BEA zeolite is due to
the higher hydrophobicity and dealumination increases the
diffusivity of the reactants and/or products thereby
enhances accessibility of the acid sites to the reactant
molecules by creating a mesopores in the crystallites.
3.5 Synthesis of Substituted DHPMs
In order to optimized the catalyst concentration, the Bigi-
nelli condensation of benzaldehyde, ethyl acetoacetate, and
urea was carried out with varied amount of BEA(15) (0.5,
1, 2 up to 5 wt%) in toluene as a solvent. The best result
was obtained by carrying out the reaction with 1:1:1 molar
ratios of aldehyde, ethyl acetoacetate, urea and 2 wt% of
zeolites BEA(15) under reflux condition.
The results from Fig. 1 show a maximum in activity for
a sample with Si/Al ratio 15 which presents a much lower
concentration of acid sites than sample with low Si/Al ratio
(
Si/Al = 12) (see Table 1). From these results, it seems
that the larger number of acid sites does not guaranty a
higher catalytic activity due to the inhibiting adsorption
effect of reactant and product. Thus, in the present case, a
Using the optimized reaction conditions, the perfor-
mance and efficiency of these procedures were explored for
9
8
8
7
7
6
6
5
5
4
4
3
3
0
5
0
5
0
5
0
5
0
5
0
5
0
85
80
75
7
6
6
5
5
0
5
0
5
0
Toluene
Ethanol
ACN
Toluene
Ethanol
ACN
45
4
3
0
5
DMF
DMF
30
10
15
20
25
30
35
10
15
20
25
30
35
Si/Al
Si/Al
Fig. 1 Influence of the Si/Al ratio of Zeolite H-BEA on the Biginelli
reaction of benzaldehyde (1a), ethylacetoacetate and urea
Fig. 2 Influence of the Si/Al ratio of Zeolite H-BEA on the Biginelli
reaction of p-hydroxy benzaldehyde (1b), ethylacetoacetate and urea
1
23

Synthesis of Dihydropyrimidinones Using Large Pore Zeolites
1545
Table 3 Zeolite H-beta (H-
BEA) catalyzed synthesis of
DHPMs under reflux conditions
a
Entry
R
Time (h)
Yield (%)
Mp (°C)
Found
Reported [17–19]
4
4
4
4
4
4
4
4
4
a
b
c
d
e
f
C
6
H
5
5–6
5–6
5–6
5–6
5–6
5–6
5–6
5–6
5–6
5–6
5–6
5–8
85
70
80
75
70
79
82
65
75
65
70
48
201–202
224–227
223–226
173–177
200–205
200–203
204–209
233–236
205–209
150–152
171–172
–
202–204
226–228
225–226
176–177
202–205
200–202
207–210
234–236
207–209
153–155
170–172
248–250
4-HO–C
3-NO –C
4-F–C
2-HO–C
4-MeO–C
4-Cl–C
6 4
H
2
6 4
H
6 4
H
6 4
H
6 4
H
g
h
i
6
H
4
6 5
C H CH=CH
3-MeO C
6
H
4
Reaction conditions: aldehyde/
urea/ethylacetoacetate/catalyst:
1/1/1/2 wt% in toluene as
solvent
4j
CH CH CH
2
3
2
4
k
(CH
3
)
2
CH
4
l
9 5
2-Cl–C H N
a
Isolated yields
the synthesis of a wide range of substituted DHPMs. The
results are summarized in Table 3. It is observed that,
aromatic aldehydes with both electron withdrawing and
electron-donating substituents reacted efficiently with urea
and ethyl acetoacetate in the presence of catalytic amount
of H-BEA (2 wt%) under reflux condition to give the
corresponding DHPMs without the formation of any side
products and in high to moderate yields. The aliphatic
aldehydes, which are known to be less reactive under
conventional Biginelli reaction conditions also exhibited
excellent reactivity to afford high yield (Table 3, entry 4j
and 4k). However, bulkier heterocyclic aldehyde gave poor
yield due to the geometrical constraints for the diffusion of
reactant through the pores of zeolite H-BEA (Table 3,
entry 4l).
The catalyst H-BEA expected to adsorb the acylimine
intermediate prior to the addition reaction. Herein, we
implemented GAUSSVIEW energy calculations [24] as
proposed by Blasco and his coworkers [25, 26] by using the
density functional UB3LYP/STO-3G* method to propose
the possible adsorption model. Figure 3a shows the various
proposed models that results from the interaction of acyl-
imine intermediate with the Si(OsiH ) OH and Al(Osi-
3
3
H ) (OH) (SiH ) clusters, as models of the silanol group
2
3
2
3 2
and Bronsted acid sites, respectively. The main distances
obtained from the optimization of the geometry are also
indicated in Fig. 3a. When silanol groups are considered
(Models A and B), the model B was found to be relatively
more stable than C (-4.83 kcal/mol) where the hydrogen-
atom of the silanol interacts with the carbonyl-oxygen and
imine-nitrogen atom of acylimine intermediate. The cal-
culated bond length inferred possible hydrogen atom
transfer occurs from the zeolite hydroxyl group. Inspection
of the relative energy calculated for model C and D indi-
cate that the former is more stable (-2.88 kcal/mol) and no
hydrogen-atom transfer occurs in this case. According to
the theoretical study, the adsorption of acylimine to zeolite
can be proposed through carbonyl-oxygen and imine-
nitrogen atoms (Fig. 3b).
The suggested mechanism of the zeolite catalyzed
transformation is shown in Scheme 2. The first step in the
mechanism is the condensation between the aldehyde and
urea, with some similarities to Mannich condensation. The
acylimine intermediate generated acts as an electrophile for
the nucleophilic addition of the keto enol ether, and the
carbonyl ketone of the resulting adduct undergoes con-
densation with the urea NH to give the cyclized Biginelli
2
product.
Scheme 2 Suggested
mechanism for the synthesis
of DHPMs
O
O
Zeolite
Model A/B/C/D (Fig.3)
NH2
RCHO
+
H N
2
NH2
N
R
O
O
O
R
O
R
OEt
EtO
NH
EtO
NH
O
2
N
O
O
H N
H
123

1
546
S. R. Mistry et al.
Fig. 3 a Optimized structural
parameters of different possible
adsorption models. b Optimized
structure of model D
studied. Zeolite presents cleaner green technology over the
conventional homogeneous acid catalysts used for the
synthesis of DHPMs.
It has been found that the adsorption properties (polar-
ity) of zeolites can be more significant than the total
number of acid sites when the polar reactants and solvents
are involved.
Zeolite BEA and dealuminated BEA (BEA(15) and
BEA(25)) showed higher catalytic activity compared to the
other zeolites. This can be attributed to the higher Si/Al
ratio of zeolites which consequently results in higher
hydrophobicity and thereby the acid strength of the cata-
lyst. Moreover, dealumination increases the diffusivity of
the reactants and/or products thereby enhance the accessi-
bility of the acid sites to the reactant molecules by creating
a mesopores in the crystallites.
Fig. 4 a Reusability of catalyst b XRD patterns of H-BEA fresh
(
H-BEA-1) and after four run (H-BEA-2)
3
.6 Recyclability of Catalyst
The XRD patterns of zeolites before and after using
revealed that the zeolite retained its crystallinity throughout
and thus can be reused. The reusability of the catalyst was
also confirmed by carrying out the reaction of benzalde-
hyde, ethyl acetoacetate, and urea in the presence of 2 wt%
H-BEA under toluene reflux condition. The catalyst can be
separated from the reaction mixture by addition of ethanol
followed by simple filtration. The results indicate that the
catalyst can be used five times without any loss of its
activity (Fig. 4).
Acknowledgment The authors are thankful to the Director, SVNIT,
Surat, for providing research and financial assistance. The authors
would also like to thank Sud-Chemie India Pvt. Ltd., India, for
characterization and gift of samples of zeolites.
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4
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1
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Synthesis of Dihydropyrimidinones Using Large Pore Zeolites
1547
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