Zinc (II) incorporated porous organic polymeric material (POPs): A mild and efficient catalyst for synthesis of dicoumarols and carboxylative cyclization of propargyl alcohols and CO2 in ambient conditions

Article abstract of DOI:10.1016/j.mcat.2019.110541

Zinc (II) incorporated porous organic polymer catalyst, Zn@BBAPA has been designed, synthesized and characterized thoroughly by XRD, EDX, SEM, HR-TEM, FTIR spectra, UV–vis spectra, TGA and N₂ absorption desorption isotherm. An eco-friendly porous Zn@BBAPA catalyst exhibits highly efficient catalytic performance for the production of α-alkylidene cyclic carbonates by using propargylic alcohols viaCO₂ fixation.In addition the catalyst shows outstanding efficiency for the synthesis of various dicoumarol derivatives in green solvent medium (water)from 4-hydroxycoumarin and aromatic aldehydes. Presence of super hydrophobic nature and electron-donation characterof NH₂ groups of BBA framework with high surface area is main aspect to be responsible for elevated catalytic performance as well as improvement in catalyst stability and enabling a good recyclability.

Full text of DOI:10.1016/j.mcat.2019.110541

Molecular Catalysis 477 (2019) 110541
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Zinc (II) incorporated porous organic polymeric material (POPs): A mild and
efficient catalyst for synthesis of dicoumarols and carboxylative cyclization
of propargyl alcohols and CO in ambient conditions
2
Sk Safikul Islama,b, Noor Salam , Rostam Ali Molla , Sk Riyajuddin , Nasima Yasmin ,
c
d
e
b
c
e
a,⁎
Debasis Das , Kaushik Ghosh , Sk Manirul Islam
Department of Chemistry, University of Kalyani, Kalyani, Nadia, 741235, W.B., India
Aliah University, Kolkata, 700156, W.B, India
Department of Chemistry, The University of Burdwan, Burdwan, 713104, W.B., India
Department of Science and Humanities, S. N. Bose Govt. Polytechnic College, Ratua, Malda, W.B., India
Institute of Nano Science and Technology, Mohali, Punjab, 160062, India
a
b
c
d
e
A R T I C L E I N F O
A B S T R A C T
Keywords:
Porous organic polymer (POP)
Zinc
CO fixation
2
α-Alkylidene cyclic carbonates
Dicoumarols
Zinc (II) incorporated porous organic polymer catalyst, Zn@BBAPA has been designed, synthesized and char-
acterized thoroughly by XRD, EDX, SEM, HR-TEM, FTIR spectra, UV–vis spectra, TGA and N2 absorption des-
orption isotherm. An eco-friendly porous Zn@BBAPA catalyst exhibits highly efficient catalytic performance for
the production of α-alkylidene cyclic carbonates by using propargylic alcohols viaCO fixation.In addition the
2
catalyst shows outstanding efficiency for the synthesis of various dicoumarol derivatives in green solvent
medium (water)from 4-hydroxycoumarin and aromatic aldehydes. Presence of super hydrophobic nature and
2
electron-donation characterof NH groups of BBA framework with high surface area is main aspect to be re-
sponsible for elevated catalytic performance as well as improvement in catalyst stability and enabling a good
recyclability.
1. Introduction
2
carbonates by coupling reaction of CO with propargylic alcohols via-
carboxylative cyclization.α-alkylidene cyclic carbonates have wide
spread applicability such as intermediates of pharmaceuticals, pesti-
cides and fine chemicals production[5]. There are many methodologies
including transition metal grafted catalyst systems like Ru [6], Co [7],
Cu [8], Pd[9], Ag [10], Zn [11], Ni [12] free metal catalytic environ-
ments such as phosphine[13], bicyclic guanidines[14]and alkoxide-
functionalized imidazolium betaines[15]which accredited as potential
catalytic materials for the synthesis of α-alkylidene cyclic carbonates.
However, the main drawbacks of the reported catalytic systems are
harsh reaction conditions (including high temperature, pressure), dif-
ficulty in catalyst recovery by traditional filtration and use of toxic and
expensive metals.
With rising global concern for climate change, the exploitation of
carbon dioxide (CO ), as a green carbon source for the manufacture of
2
valuable added chemical intermediates has garnered moreattention in
research activities worldwide over the past decades due to sustainable
and environmental convenience arising from the utilization of renew-
able sources and the increasing concentration of the greenhouse gases
[
2
1]. Consequently to reduce the atmospheric gathering of CO , it is very
important to take necessary steps for controlling the emission of the
industrial raw materials by flue-gas capture and convert it into desir-
able economically competitive products [2]. In this respect, researchers
across the globe are diligently working for invention of new protocols
which can effectively take up the atmospheric CO
value-added chemicals [3]. Still, it remains challenging to develop
2
and convert it into
In past decade, few Cu[16] or Ag[17] metal anchored polymer
catalysts were also employed as heterogeneous catalyst for this
highly efficient and green routes for CO
2
fixation reaction under sus-
2
CO fixation reaction under high pressure. However, several syntheses
tainable conditions [4].
Among the several known CO conversions, one of the most effec-
2
tive and attractive strategies is the production of α-alkylidene cyclic
of reusable and efficient catalysts for the reaction under room tem-
perature and atmospheric pressure are still strongly desirable for this
important transformation into value-added fine chemicals.
⁎
Corresponding author at: Department of Chemistry, University of Kalyani, Kalyani, Nadia, 741235, W.B., India.
E-mail address: manir65@rediffmail.com (S.M. Islam).
https://doi.org/10.1016/j.mcat.2019.110541
Received 16 June 2019; Received in revised form 20 July 2019; Accepted 31 July 2019
2468-8231/ © 2019 Elsevier B.V. All rights reserved.

S.S. Islam, et al.
Molecular Catalysis 477 (2019) 110541
Scheme 1. Preparation of the mesoporous Zn@BBAPA complex catalyst.
On the other hand, pharmaceutically dicoumarol is a naturally oc-
2. Experimental section
curring anticoagulant, functioning similar to the warfarin, which is a
competitor of vitamin K. Structurally dicoumarols are bridge sub-
stituted dimers of 4-hydroxycoumarin compound. Dicoumarols have
huge demands as the prevention and treatment of thrombosis and re-
ducing the metastases in animal models[18]. In addition, during the last
few years warfarin sodium therapeutically used as an anticoagulant has
been extensively employed as one of the most crucial classes of drugs
for cancers treatment and has exhibited to improve response rates of
tumor. Furthermore, in field of medical science, this pharmaceutically
valuable compound has been also shown as some activity against me-
tastatic renal cell carcinoma, prostate cancer and malignant melanoma
2.1. Preparation of Catalyst
2.1.1. Synthesis of benzene-benzylamine (BBA)
20 ml anhydrous 1,2-dichloroethane was taken in 250 ml RB; then
benzene (0.312 g, 4 mmol) and benzylamine (0.428 g, 4 mmol) were
mixed to this solution followed by addition of formaldehyde dimethyl
acetal (1.216 g, 16 mmol). Next anhydrous FeCl
was added slowly to the reaction mixture. After that under N
3
(2.596 g, 16 mmol)
atmo-
2
sphere the reaction mixture was heated to 80 °C for 18 h. Then the
mixture was cooled to room temperature, filtered and thoroughly wa-
[
19]. Because of these wide spread application in medicinal field,
shed with CH
part was dried at 80 °C for another 20 h. The solid material was washed
with CH OH by Soxhlet technique for 48 h to provide a chocolate color
material, this material was named as BBA.
3
OH till the filtrate part became colourless. Next the solid
coumarin nucleus have recently been received great attention towards
the researchers [20]. Conversely, there are certain disadvantages such
as long reaction time, expensive reagents, use of toxic catalysts, strin-
gent conditions and low product yields for the production of this cou-
marin derivative. Therefore, an alternative strategy to overcome these
drawbacks is to employ porous polymer in terms of operational sim-
plicity, economic viability and catalyst reusability for the synthesis of
dicoumarol [21].
The porous organic polymers (POPs) are a very demanding material
for a wide range of frontline applications because of their own prop-
erties such as mechanical stability, high hydrothermal nature, tunable
pore size distribution, readily modifiable functionality and high surface
area [22]. The interaction between metal and porous organic polymers
can be smoothly improved by grafting coordination groups into metal
precursors, which provides an extra stability and enhancement of the
catalytic activity of metal as compare to other supported catalyst
3
2.1.2. Synthesis of BBAPA
The chocolate color material (BBA, 2 gm) was taken in 20 ml of
methanol and then 2-pyridine aldehyde was added drop wise to the
stirring solution. The reaction mixture was refluxed for 20 h. Then re-
sulting reaction mixture was allowed to cool at room temperature,-
filteredand washed thoroughly with toluene followed by methanol.
Finally the ligand was dried under vacuum for 24 h. The ligand was
designated as BBAPA.
2.1.3. Synthesis of Zn@BBAPA catalyst
This Polymeric BBAPA ligand (0.5 gm) dissolved in methanol
(
2 2
15 mL) was treated with 0.05 g of ZnI and 0.06 g ofanhydrous ZnCl
[
23–24].
Herein, we report the synthesis of Zn grafted benzene-benzylamine
porous organic polymers (Zn@BBAPA) via alkylation of benzene and
benzylamine with anhydrous FeCl as a promoter by using dimethox-
separately and then the mixtures were stirred for 30 min. Next two
solutions were refluxed at 80 °C for one day. The Zn@BBAPA-1 and
3
Zn@BBAPA-2 were washed with CH OH and dried at 40 °C under va-
3
cuum (Scheme 1).
ymethane as a cross-linker. The Zn@BBAPA material showed excellent
catalytic performance towards the production of α-alkylidene cyclic
2
.1.4. General procedure for the synthesis of α-alkylidene cyclic carbonates
2
carbonates via coupling reaction of CO and propargylic alcohol under
In this catalytic process, 2-methyl-3-butyn-2-ol (1a) was used as the
atmospheric conditions. This Zn based porous organic polymer also
shown outstanding catalytic activity in the convenient synthesis of
some dicoumarols.
starting substrate. A mixture of 1a (1.5 mmol), DBU (1.5 mmol) and
Zn@BBAPA-1 (50 mg) were charged into in a 50 ml stainless-steel au-
toclave reactor under magnetic stirring. Then the stainless-steel reactor
2 2
was purged off air by blowing CO . After the pressure of CO (1 MPa)
was introduced, the reaction mixture was stirred for 12 h at RT. Upon
2

S.S. Islam, et al.
Molecular Catalysis 477 (2019) 110541
the completion of the reaction, stainless-steel reactor was placed in cold
water for 10 min and CO
reaction mixture was extracted with CH
salt, Na SO . Then the mixture was evaporated to obtain the desired
2
balloon wascarefully released slowly. The
2
Cl and dried over anhydrous
2
2
4
crude product. The final product (2a) was obtained by silica gel column
chromatography (using petroleum ether and ethyl acetate solvent).
2.1.5. Typical procedure for the synthesis of dicoumarols
4-hydroxycoumarin (1.5 mmol) and water (10 mL) was transferred
into a 50 ml RB flask together with Zn@BBAPA-2 (35 mg) catalyst and
aldehyde (1 mmol) at 80 °C for particular time (Table 4). The reaction
was monitored by TLC. After the response of the reaction was complete,
the reaction mixture was settled down to RT. The reaction mixture was
2 4
extracted with DCM and dried over anhydrous salt, Na SO .Then the
mixture was evaporated to obtain the desired crude product. The cat-
alyst, Zn@BBAPA-2 was reused after the recovery of the catalyst by
simple separation technique followed by washing.
Fig. 2. Fourier-transform infrared spectra of BBAPA and Zn@BBAPA-1.
3. Results and discussion
3
3
.1. Characterization of Zn@BBAPA complexes
.1.1. FESEM and EDX analysis
In order to investigate the structural morphology of Zn@BBAPA-1,
field emission scanning electron microscopy (FE-SEM) has been per-
formed. Fig. 1 a and b demonstrate morphology at two different mag-
nifications. However Zn@BBAPA-1 showed much bigger rock-like
particle morphology with clear crystal edges. From Figure-b it is no-
ticed that some mesopores present in this material which provided the
presence of inter-particle porosity in material as seen from the BET
analysis.Energy dispersive spectroscopic analysis of X-rays (EDX)
(
supporting information) data of the Zn@BBAPA-2 also denotes the
presence of Zn particles over the polymer matrix surface.
3.1.2. FTIR specta
FT-IR spectra of porous organic polymer BBAPA and Zn@BBAPA-1
materials are provided in the Fig. 2. The absorption bands centered at
Fig. 3. UV–vis spectra of mesoporous BBAPA and Zn@BBAPA-1.
−1
3405 and 3417 cm can be ascribed by the –NH
2
groups present in the
porous organic polymer BBAPA and the Zn@BBAPA-1 materials. The
−1
−1
bond in polymeric samples.
bands at 1603 cm
and 1430 cm
can be assigned to the stretching
vibrations of the aromatic C]C bonds in the benzene rings. The ab-
−
1
−1
3
sorption peaks at 2924 cm and 2907 cm indicate the sp C–H bond
3.1.3. Uv–vis spectra
−1
in Zn@BBAPA-1 and BBAPA materials. The band 1081 cm
corre-
(Zn@
UV-Vis absorbance spectra of BBAPA and Zn@BBAPA-1 were ex-
amined in Fig. 3. The electronic absorption spectrum of the base BBAPA
exhibits two peak at 218 nm and 272 nm for n→π* and π→π*
−1
sponds to the C–N bond. The absorption peaks at 1645 cm
−
1
BBAPA-1) and 1650 cm
(BBAPA) indicates the formation of imine
Fig. 1. FE-SEM image of Zn@BBAPA-1 material.
3

S.S. Islam, et al.
Molecular Catalysis 477 (2019) 110541
Fig. 4. X-ray Powder Diffraction spectra of BBAPA and Zn@BBAPA-1.
Fig. 6. TGA plot of the BBAPA and Zn@BBAPA-1 material.
transitions, respectively. But in case of Zn@BBAPA-1 complex, a no-
ticeable change in absorption spectra was observed. The complex shows
new absorption hump near 300 nm–330 nm. It can be attributed to the
charge transfers between coordinated atoms and the central zinc atom
583.3 m²/g, respectively. Their respective calculated pore volumes are
0.485cc/g and 0.549cc/g. The pore size distributions of the samples
have been drawn from the isotherms by employing the non-local den-
sity functional theory (NLDFT). Two sharp peaks indicate at 2.15 nm
and 1.96 nm which are attributed that they are close to mesopores.
[
25].
3
.1.4. X-ray powder diffraction studies
The XRD analysis of the BBAPA and Zn@BBAPA-1 materials are
3
.1.6. Thermogravimetric analysis
Using TGA, the thermal stability of the polymer (BBAPA) and the
shown in Fig. 4.The XRD pattern displaying a broad diffraction peak
around 2θ value nearly 20-25° indicated the material is amorphous in
nature [26]. From this patterns after Zn(II) loading that no phase cor-
catalyst (Zn@BBAPA) has been examined under the air flow in tem-
perature range of 0–600 °C at a heating rate of 10 °C min. The TGA
curve of a mesoporous Zn@BBAPA-1 and BBAPA polymer are shown in
Fig. 6. The polymer and zinc complex were stable up to approx. 280 °C
after that both started to decompose. Only 20% wt. loss was obtained at
2
responding to ZnI or ZnO was observed in Zn@BBAPA-1.
3
.1.5. N
2
adsorption-desorption study
4
00 °C temperature. The TGA analysis suggests that supported zinc
In order to determine the permanent porosity in the Zn grafted
complex (Zn@BBAPA-1) decomposed at slightly higher temperature
than that of BBAPA.
BBAPA material N
The Fig. 5 reveals N
2
adsorption-desorption isotherms were performed.
adsorption-desorption isotherms of the complexes
2
Zn@BBAPA-1 and Zn@BBAPA-2. In desorption hysterisis of both Zn@
BBAPA-1 and Zn@BBAPA-2 contains large mesoporosity. These iso-
therms showed the typical type IV adsorption isotherms according to
IUPAC with a large H2 hysteresis loop in the high pressure region [27]
of 0.4–0.9 and 0.41-0.8 for Zn@BBAPA-1 and Zn@BBAPA-2 respec-
tively relative pressure (P/P ) of N .The catalysts Zn@BBAPA-1 and
o 2
Zn@BBAPA-2 showed an initial large uptake, followed by a steady rise
in nitrogen adsorption with a large hysteresis, and the steep increases in
3
.1.7. HRTEM analysis
Additionally, the TEM images of Zn@BBAPA-1 and Zn@BBAPA-2
catalysts are exhibited in Fig. 7. As noticed from the TEM images of two
different catalysts it was shown that these are porous in nature and dark
spotted areas and Zn metal is clearly dispersed on the surface of the
porous polymers. In addition to that, no agglomeration of metallic Zn is
found in the whole sample grid and Fig. 7 indicates uniform anchoring
of Zn(II) species at the surface of BBAPA material. Dark spots in the
Fig. 7, the TEM images corresponding to c and d, recommend the ex-
istence of high electron density across the specimen.
the high P/P
mesopores [28]. The surface areas were calculated over the relative
pressure (P/P ) ranging from 0.01 to 1.2. The BET surface areas of the
fresh Zn@BBAPA-1 and Zn@BBAPA-2 complexes are 552.1 m²/g and
o
region implies that the materials consist wide range of
o
Fig. 5. The N
2
adsorption/desorption isotherm of Zn@BBAPA-1(a) and Zn@BBAPA-2(b). The plot of pore size distributions employing NLDFT method is shown in
the inset.
4

S.S. Islam, et al.
Molecular Catalysis 477 (2019) 110541
Fig. 7. The TEM images of Zn@BBAPA-1 and Zn@BBAPA-2 at different scales (a) 200 nm, (b) 50 nm and (c) 200 nm, (d) 100 nm respectively.
3
3
.2. Catalytic performance of Zn@BBAPA material
Table 1
Effect of various catalysts on reaction .
a
.2.1. Organic α-alkylidene cyclic carbonates Synthesis catalyzed by Zn@
Yieldb
Entry
Catalysts
BBAPA material
We have synthesized the catalyst, Zn@BBAPA-1 and checked its
1
2
3
4
5
6
7
8
–
NEt
DBU
Zn@BBAPA-1
Zn(NO
ZnCl /DBU
Zn@BBAPA-2/ DBU
Zn@BBAPA-1/ DBU
0
0
0
< 5
< 1
72
78
98
3
activity for CO
dioxide (Scheme 2). Our main focus was the transformation of pro-
pargylic alcohol to α-alkylidene cyclic carbonates utilizing CO in
presence of a base. The reaction of 2-methyl-3-butyn-2-ol (1a) and CO
to corresponding α-alkylidene cyclic carbonates was studied as a model
reaction under 1 MPa CO pressure at 30 °C under solvent free condi-
2
fixation reaction using propargylic alcohol and carbon
2
3 2
) /DBU
2
2
2
tion. To optimize the reaction conditions with variations of diverse
parameters viz. temperature, time of reaction and amount of catalyst for
getting the better conversion of 2-methylbut-3-yn-2-ol, we have per-
formed number of reactions.
a
Typical reaction conditions: 1.5 mmol of substrate, 1.5 mmol of DBU,
b
5
0 mg of Zn@BBAPA-1, 1 MPa CO
isolated products.
2
, 30 °C, time 6 h. All yields correspond to
We have investigated the catalytic reactivity power of different
catalyst systems on CO fixation reaction of propargylic alcohol at 30 °C
2
for 6 h (Table 1). It is undoubtedly seen that the reaction did not pro-
ceed without the catalyst under same reaction conditions (Table 1,
3
entry 1). The efficiency of the NEt , DBU or Zn@BBAPA-1 was ex-
amined separately under the optimized reaction conditions giving no or
lesser percentage of yields (Table 1, entries 2–4). For propargyl alco-
3 2
hol's cyclization reaction, Zinc catalyzed Zn(NO ) /DBU systems had
low activity of the catalyst for the reaction (Table 1, entries 5). De-
lightedly, the zinc salts along with halogen as the anion were found to
superior than the oxygen acid based zinc salt shown as the Table 1. This
could be attributed to the dissociation power of the anions for the
catalytic activity of the halogenated zinc salts. In comparison to the
other reported metal-free and metal catalysts, the present reaction
conditions were much better and the yields were also higher.
It is speculated that there are enhancement on the yield of the re-
action with the increasing amount of active sites. 50 mg of catalyst was
found to be the optimum amount for the reaction to furnish best yield
(
Table 2). More than 50 mg of Zn@BBAPA-1 catalyst loading did not
have superior catalytic performance in CO
sence of DBU (Table 1, entries 6–8). It was noted that Zn@BBAPA-1/
DBU catalyst system effortlessly underwent the CO fixation reaction to
produce 98% yields within 6 h (entry 8). This optimization study re-
vealed that the catalytic activity of the halogen based zinc salts are
2
fixation reaction in pre-
Table 2
The time and amount of Catalyst effect on α-alkylidene cyclic carbonates syn-
2
a
thesis .
Entry
Time
Catalyst Amount(mg)
Yield(%)b
1
2
3
4
5
6
.
.
.
.
.
.
4
6
8
6
8
10
50
40
40
50
60
40
48
62
90
98
98
92
a
Typical reaction conditions: substrate (1.5 mmol), DBU(1.5 mmol), Zn@
Scheme 2. Synthesis of α-alkylidene cyclic carbonates catalyzed by Zn@
BBAPA-1 material.
b
BBAPA-1 (50 mg), 1 MPa CO
ducts.
2
, 30 °C. All yields correspond to isolated pro-
5

S.S. Islam, et al.
Molecular Catalysis 477 (2019) 110541
Table 3
a
Zn@BBAPA-1 catalyzed synthesis ofα-alkylidene cyclic carbonates .
Entry Substrate
Product
Time (h) Yieldb (%) TON/TOF
−
1
(h
)
1
.
6
98
49/8
2
.
8
98
49/6
Fig. 8. The effect of temperature over the synthesis of α-alkylidene cyclic car-
bonatescatalyzed by Zn@BBAPA-1 material.
3.
9
97
48.5/5
improve conversion further. Influence of reaction duration on the cat-
alytic transformation was also examined ranging from 4 h to 10 h and
the results show that after 6 h reaction duration, maximum conversion
4
5
.
.
12
16
96
0
48/4
0/0
(
98%) was obtained under the given reaction condition indicating re-
action time of 6 h was optimal one at 30 °C in our catalytic system
Table 2).
The effect of temperature over the carboxylative cyclization reac-
(
tion was also checked. At room temperature, a maximum yield was
observed (30 °C). Then gradually increasing reaction temperature from
40 °C to 80 °C it was observed that yield of product decreases with in-
creasing temperature. Therefore, 30 °C was chosen as the standard
temperature for the reaction as shown in Fig. 8.
a
Reaction conditions: 1.5 mmol of substrate, 1.5 mmol of DBU, 50 mg of
b
After the attainment of optimized conditions we tried to extend the
2
Zn@BBAPA-1, 1 MPa CO ,30 °C. All yields correspond to isolated products.
2
scope of various substituted propargylic alcohols with CO to furnish
the yields of the corresponding products as summarized in Table 3.
Different types of alkyl substituted propargylic alcohols at different
positions were used as valuable substrates to provide the respective
carbonates with excellent yields. The presence of steric hindrance of the
Table 4
Optimization of solvent and reaction time for the synthesis of dicoumarol a.
Yield(%)b
Entry
Solvents
Time
1
2
3
4
5
H
2
O
2 min
30 min
40 min
20 h
98.5
85
64
group R
substrate reactivity. The substrate had taken almost 6 h to complete the
2
conversion when R =R =Me. But with bulky alkyl groups R and R ,
1 2
and R in the propargylic alcohols strongly influenced the
MeOH
CH COOH
DCM
3
1
2
1
36
reaction had taken much longer time to complete (entries 2–4). On the
contrary, the internal propargylic alcohol showed no reactivity and
remained inactive towards this reaction, hence no corresponding pro-
duct was detected under described reaction conditions (Table 3, entry
3
CH CN
24h
Trace
a
Reaction
conditions:
4-hydroxycoumarin
(1.5 mmol),
4-hydro-
xybenzaldehyde (1 mmol), Zn@BBAPA-2 (35 mg),water (10 mL) and 80 °C, All
the reactions were monitored by TLC. All yields correspond to isolated pro-
ducts.
b
5).
A probable reaction pathway for production of α-alkylidene cyclic
carbonates in presence of Zn@BBAPA-1 is shown in the Scheme 3. The
two main points responsible for the formation of intermediate and re-
action mechanism are: lewis acidity of Zn metal and stronger dis-
sociation capacity of iodide. The mechanism involvingthe process of
BBAPA-2 complex in another suitable reaction methodology for the
synthesis of the respective dicoumarol compounds from 4-hydro-
xycoumarin and various substituted aromatic aldehydes in a small re-
action time (1–5 min). Initially, 4-hydroxybenzaldehyde and 4-hydro-
xycoumarin were taken as staring materials for this model reaction
which was found to be catalyzed by Zn@BBAPA-2 catalyst (Scheme 4).
Number of reactions was carried out under several conditions by
varying solvents and catalysts in order to investigate the most desirable
reaction conditions for our represented reaction of dicoumarol and the
results of all scenarios are documented in Tables 4 and 5.
In the current study the loading of Zn metal catalyst influences the
effect of the model reaction. Better yield was found with the application
of 35 mg of catalyst providing 98.5% conversion of substrate. When the
reaction process was conducted with less than 30 mg of Zn@BBAPA-2
material, the reactions either remained incomplete or required more
time to complete. This result suggested that Zn@BBAPA-2 material
shows heterogeneity in catalytic nature for synthesis of dicoumarol
CO
lows: The propargylic alcohol was activated by the Zn complex and the
DBU plays a role to activate CO to form an intermediate carbonatesalt
29]. The metal complex catalyst, Zn@BBAPA-1 activated alkyne
2
incorporation into propargylic alcohols was assumed to be as fol-
2
[
moiety, a bond formation take place between Zn and alkyne part of the
intermediate [30]. Then 5-exo-trig cyclization would take place to
generate the desired α-alkylidene cyclic carbonates under optimized
conditions followed by the release of the metal catalyst. The Zn@
BBAPA-1 played a crucial role for activation of the alkyne that will
drive further reaction to obtained the desired product [31].
3.2.2. Synthesis of dicoumarols catalyzed by Zn@BBAPA-2 complex
The additional aspiration of the present work was to employ Zn@
6

S.S. Islam, et al.
Molecular Catalysis 477 (2019) 110541
Scheme 3. Proposed mechanism of α-alkylidene cyclic carbonates synthesis catalyzed by supported Zn catalyst.
Scheme 4. Production of dicoumarol moiety catalyzed by Zn@BBAPA-2 material.
Table 5
a
Influence of various catalysts on the synthesis of dicoumarol in water .
Entry
Catalyst
Time(min)
Yield(%)b
1
2
3
4
5
6
7
Zn@BBAPA-2
2
98.5
90
80
52
30
AlCl
NiCl
CuCl
3
18
20
30
40
40h
48h
2
2
FeCl
ZnCl
3
2
32
Trace
No Catalyst
a
Reaction
conditions:
4-hydroxycoumarin
(1.5 mmol),
4-hydro-
2
xybenzaldehyde (1 mmol), catalyst (35 mg), H O (10 mL) and 80 °C, All the
reactions were monitored by TLC. bAll yields correspond to isolated products.
moiety.This reaction was performed under a wide range of solvents
such as MeOH, CH
company of Zn@BBAPA-2 catalyst. With moderately less polar aprotic
solvent like CH Cl ,the desired product was obtained with very low
2 2 3 2 3
Cl , CH CN, H O and CH COOH (Table 4) in the
Fig. 9. The influence of quantity of the catalyst over the synthesis of dicou-
marol.
2
2
yield (Table 4, entry 4). Either trace amount of product was detected or
no reaction was found with the use of the high polar aprotic solvents,
chloride catalysts were seen to be of the sequence
ZnCl < FeCl < CuCl < NiCl < AlCl < Zn@BBAPA-2. Our initial
2 3 2 2 3
study suggested that Zn@BBAPA-2 complex was more effective with
respect to reaction time and generated the excellent 98.8% of yield at
CH
smoothly in protic polar solvents like CH
the desired product, despite increasing the reaction time period, within
0–40 min. Moreover, in presence of H O, a green and environment
3
CN (Table 4, entry 5). It is noted that the reaction undergoes
3
COOH and MeOH to produce
3
2
3 2 3 2
optimized reaction time. In presence of AlCl , NiCl , FeCl and ZnCl ,
friendly solvent, the product was generated in maximum desired yield
in 2 min (Table 4, entry 1).
the reaction was performed in longer reaction time (10–30 min) as
compared to our catalytic reaction timeand also gave poor to moder-
ately good yield. Moreover copious wasteby-product which creates
environmental troubles was generated from those reactions. When the
reaction was carried out in absence of catalyst trace amount of yield
was obtained even after large extended reaction duration(48 h). It is
thus concluded from Table 5 that Zn@BBAPA-2 shows the highest
These excellent results motivated us to explore the efficiencies of
different metal precursor based catalysts for the synthesis of dicoumarol
derivatives (Table 5). For comparison, different types of catalysts were
investigated to show the benefits Zn@BBAPA-2 complex on other cat-
alysts. Various transition metal lewis acids were used for the ideal re-
action (Table 5). It was observed that the reactivity of several metal
7

S.S. Islam, et al.
Molecular Catalysis 477 (2019) 110541
The effect of temperature over the reaction was also examined. At
room temperature, a poor yield was found (30 °C). Then gradually in-
creasing reaction temperature from 40 °C to 100 °C it was observed that
only at 80 °C temperature the maximum yield was obtained. It was also
seen that no such change in yield wasobserved beyond the temperature
of 80 °C. Thus 80 °C was chosen as the optimal temperature for the
reaction as shown in Fig. 10.
These noteworthy successes in catalytic application of novel meth-
odology encouraged us for further investigation of the efficiency of our
newly porous catalyst to utilize various aldehyde substrates (Scheme 4
and Table 6).This methodology is equally compatible with the sub-
strates having both electron donating and withdrawing groups yielding
well to excellent products of dicoumarol. The reaction of p- and o-NO
2
benzaldehyde underwent smoothly to produce the respective dicou-
marol products with very good yields (entries 3 and 5).Similarly,he-
teroaromatic aldehydes also underwent efficient reaction to give mod-
erate to good yields of desired products (entries 6–7). This coumarin
moiety containing OH group and a double bond (C]C), turned to be
very suitable to react with the carbonyl group of substituted aldehydes
to deliver corresponding dicoumarol.
Fig. 10. The effect of temperature over the synthesis of dicoumarol catalyzed
by Zn@BBAPA-2 material.
catalytic activity under optimized conditions for the change of 4-hy-
droxycoumarin (1) to corresponding dicoumarol.
The catalytic competence of porous Zn@BBAPA-1 and Zn@BBAPA-
2
materials as well as the developed methodology was competitive with
The influence of quantity of the catalyst (from 10 mg to 60 mg) on
the conversion yield of this reaction was also checked (Fig. 9). The
reaction was conducted under same condition in presence of 35 mg of
the catalyst furnishing 98.5% conversion over two minutes. But it was
observed that increasing the amount of the catalyst loading marks no
further conversion. So, this result suggested that 35 mg of catalyst
loading is the optimum amount of catalyst for this reaction.
some other reported catalysts including reaction condition with yields
and the comparative observations have been presented in Table 7. It is
observed from the study that porous Zn@BBAPA-1 and Zn@BBAPA-2
materials showed moderately better catalytic efficiency for both α-al-
kylidene cyclic carbonates using the propargylic alcohol and synthesis
of dicoumarol respectively under optimized conditions.
Table 6
a
Substrate scope for synthesis of various dicoumarols catalyzed by Zn@BBAPA-2under optimized condition .
Entry
Aldehyde
Product
Time/Min
2
Yieldb (%)
98.5
TON/TOF(h−1
36/1080
)
1
.
.
2
2.5
4.5
98.5
94
36/865
35/466
3
.
4
5
.
.
3.5
5
95
95
35/603
35/421
6
7
.
.
4.5
3
94
35/466
36/720
96.5
a
Typical reaction conditions: 4-hydroxycoumarin (1.5 mmol), aldehyde (1 mmol), catalyst (35 mg), H
TLC. bAll yields correspond to isolated products.
2
O (10 mL) and 80 °C, All the reactions were monitored by
8

S.S. Islam, et al.
Molecular Catalysis 477 (2019) 110541
Fig. 11. Recycling efficiency chart of porous Zn@BBAPA-1and Zn@BBAPA-2
material.
3.3. Recyclability of porous Zn@BBAPA materials
The excellent catalytic activity of both the catalysts motivated us to
study their thermal stability, recyclability and recommencement which
are the most important aspect in a heterogeneous catalytic system. In
order to evaluate the recyclability these porous catalysts were re-
covered successfully and utilized to obtain α-alkylidene cyclic carbo-
2
nates using the propargylic alcohol under 1 MPa CO pressure and also
synthesis of dicoumarol in optimal reaction conditions, which are
shown in the Fig. 11. The catalyst was recovered fromthe reaction
mixture after the reaction was completed. The recovery process was
done through the centrifuge process followed by washing methanol,
dried and successively used for next cycle reaction. The porous Zn@
BBAPA materials could be subsequently recycled in successive five runs
without appreciable loss in catalytic activity as indicated in the nominal
change in yield of the products.
3
3
.4. Heterogeneity of catalyst
.4.1. Zn poisoning test
By solid-phase poisoning test [37], we have checked the hetero-
geneous nature of the Zn@BBAPA-1 catalyst. This reaction was carried
out for synthesis of α-alkylidene cyclic carbonates using the propargylic
2
alcohol under 1 MPa CO pressure.We utilized readily available poly(4-
vinylpyridine) as an efficient zinc poison, which selectively binds and
deactivates the leached-out zinc. For the carboxylative cyclization re-
action,
a
mixture
of
2-methyl-3-butyn-2-ol(1.5 mmol),DBU
(
1.5 mmol),CO
2
(1 MPa),0.0075 g poly(4-vinylpyridine) (PVP) and Zn@
BBAPA-1 (0.05 g) was stirred under our optimal reaction conditions. In
this case, there is no change in conversion was found, which indicates
that the Zn@BBAPA-1 catalyst is heterogeneous in nature. If zinc lea-
ched out from the complex in the reaction mixture then PVP formed a
complex with the leached zinc and thus deactivated the active Zn
species, slowed down the reaction rate and conversion was decreased.
From these results it is clear that no leaching of zinc occurred during
the carboxylative cyclization reaction. We have also checked the het-
erogeneous nature of the Zn@BBAPA-2 catalyst for the dicoumarol
synthesis.
3.4.2. Hot filtration
In order to access the heterogeneous nature of our designed Zn@
BBAPA-1 materials we have conducted the hot filtration test under the
optimization condition. In the synthesis of α-alkylidene cyclic carbo-
nates using the propargylic alcohol under 1 MPa CO pressure, the
2
catalyst was separated out from reaction mixture after 1.5 h under the
optimization condition and further reaction was allowed with the fil-
trate. It was noted that no reaction occurred after filtration of catalyst
from mixture. But it was found that the zinc was not leached into the
9

S.S. Islam, et al.
Molecular Catalysis 477 (2019) 110541
solution as measured by AAS analysis. So, our investigation obviously
demonstrates that our porous Zn@BBAPA-1 material was stable and
heterogeneous in nature.
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In conclusion, we have established a novel strategy for the synthesis
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SMI acknowledges Department of Science and Technology, DST-
SERB (Project reference number EMR/2016/004956), New Delhi, Govt.
of India, Board of Research in Nuclear Sciences (BRNS), Project re-
ference number 37(2)/14/03/2018-BRNS/37003, Govt. of India and
Council of Scientific & Industrial Research, CSIR, (Project No 02(0284)/
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16/EMR-II), Date: 06-12-2016; New Delhi, Govt. of India, for providing
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financial support. SSI also thankfully acknowledges UGC, New Delhi,
Govt of India, for providing senior research fellowship (MANF-2015-17-
WES-56540) and also thankful to Mr. Naveen Tiwari for his uncondi-
tional support. NY acknowledges Aliah University. NS wish to ac-
knowledge UGC, New Delhi for financial support through Dr. DS
Kothari postdoctoral research fellowship (Award no. F.4-2/2006(BSR)/
CH/17-18/0176). RAM thankful to S. N. Bose Govt. Polytechnic
Collegefor support.SMI acknowledges DST and UGC, New Delhi, Govt
of India, for providing fund to the Dept. of Chemistry, University of
Kalyani through PURSE, FIST and SAP program.
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