Palladium Loaded Dendronized Polymer as Efficient Polymeric Sustainable Catalyst for Heck Coupling Reaction

Article abstract of DOI:10.1007/s10562-021-03767-6

The palladium incorporated amine-functionalized dendronized polymer was synthesized by the addition of palladium acetate to dendronized polymer in methanol at room temperature. Palladium species are immobilized onto the dendritic structure by their coordination with amino functional groups. The newly developed dendritic system showed high palladium content in the low generation level itself, which was found to be 4.19?mmol/g. This was fairly higher than, the other palladium-based catalysts. Energy dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy, UV–Visible spectroscopy, and X-ray photoelectron spectroscopy were used to confirm the successful synthesis of the new catalyst. It was used as a homogeneous palladium catalyst for Heck coupling reaction between olefins and differently substituted aryl halides and the products were isolated in high yield. The products isolated were in trans configuration, which indicated the selectivity of the newly developed catalytic system. Also, this catalyst system was reused up to nine times without a significant decrease in its catalytic activity. The easy accessibility of catalytic sites, stability, resistance to metal leaching, high catalytic activity and remarkable stereoselectivity with a low amount of catalyst are all due to the dendritic support. The docking study was carried out for all the stilbene derivatives obtained by the Heck coupling reaction against DprE1 protein to study its potential antitubercular activity. All the compounds displayed superior docking score values over the range ??6.5 to ??8.2?kcal/mol, compared to the standard drug isoniazid with docking score of ??6.1?kcal/mol against DprE1. Graphic Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-021-03767-6

Catalysis Letters
https://doi.org/10.1007/s10562-021-03767-6
Palladium Loaded Dendronized Polymer as Efcient Polymeric
Sustainable Catalyst for Heck Coupling Reaction
K. Hiba1 · G. Anjali Krishna1 · S. Prathapan1 · K. Sreekumar1
Received: 15 April 2021 / Accepted: 3 August 2021
© The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature 2021
Abstract
The palladium incorporated amine-functionalized dendronized polymer was synthesized by the addition of palladium acetate
to dendronized polymer in methanol at room temperature. Palladium species are immobilized onto the dendritic structure
by their coordination with amino functional groups. The newly developed dendritic system showed high palladium content
in the low generation level itself, which was found to be 4.19 mmol/g. This was fairly higher than, the other palladium-
based catalysts. Energy dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy, UV–Visible spectroscopy,
and X-ray photoelectron spectroscopy were used to confrm the successful synthesis of the new catalyst. It was used as a
homogeneous palladium catalyst for Heck coupling reaction between olefns and diferently substituted aryl halides and the
products were isolated in high yield. The products isolated were in trans confguration, which indicated the selectivity of
the newly developed catalytic system. Also, this catalyst system was reused up to nine times without a signifcant decrease
in its catalytic activity. The easy accessibility of catalytic sites, stability, resistance to metal leaching, high catalytic activity
and remarkable stereoselectivity with a low amount of catalyst are all due to the dendritic support. The docking study was
carried out for all the stilbene derivatives obtained by the Heck coupling reaction against DprE1 protein to study its potential
antitubercular activity. All the compounds displayed superior docking score values over the range − 6.5 to − 8.2 kcal/mol,
compared to the standard drug isoniazid with docking score of − 6.1 kcal/mol against DprE1.
Graphic Abstract
Keywords Dendronized polymer · Homogeneous catalyst · Heck coupling · Reusability
Extended author information available on the last page of the article
Vol.:(0123456789)
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K. Hiba et al.
have reported a Pd catalyzed arylation reaction of olefnic
compounds with aryl iodides and potassium acetate in
methanol at 120 °C [32]. Later, in 1972 Heck indepen-
dently discovered this reaction under diferent reaction
conditions [6]. Recently, a wide variety of homogeneous
and heterogeneous supported palladium catalysts have
been reported for Heck coupling reaction [33–37]. Most
of these supported catalysts display one or more defects
such as low loading of palladium, severe palladium leach-
ing, poor swelling property of the catalysts in common
organic solvents, use of phosphine ligands, harsh reaction
conditions, higher amount of catalyst loading, higher reac-
tion time and lower product yield.
1 Introduction
Dendronized polymers, which mimic the architecture of
dendrimers, have attracted considerable attention in recent
years due to their unique supramolecular and controlled
hierarchical structure. They are produced by combination
of polymers, which may be linear or branched and den-
dritic molecule as side chain pendant moieties [1, 2]. Even
though dendronized polymers contain structural imperfec-
tion, they retain the most desired properties of dendrim-
ers. A great amount of functionality can be achieved with
these polymers in the low-generation level, which out-
perform the traditional higher-generation dendrimers [3].
Dendronized polymers can be synthesized by convergent
“graft-to” method, the divergent graft from method and
the macromonomer method [4, 5]. These novel polymers
enthused chemists to develop some applications, catalysis
being one of them.
A number of dendrimer-supported palladium catalysts
were also reported for Heck coupling reaction. Liu et al.
reported the synthesis of hyperbranched polyethylene
encapsulated Pd(11) species [38]. Meanwhile, Isfahani et al.
developed a palladium nanosilica triazine dendritic poly-
mer stabilized palladium nanoparticles for the Heck cou-
pling reaction of aryl halides with styrenes under normalized
heating condition and microwave irradiation [39]. Nanosilica
thiolated dendritic polymer-supported palladium nanoparti-
cles were developed by Zakeri et al. [40]. In another case,
magnetic dendritic supports have been used to stabilize Pd-
nanoclustures [24, 41, 42]. Further study of using hyper-
branched polytriazoles with diferent molecular weights
with palladium acetate has proved that polytraizole clusters
were boosting ligands for the catalytic Heck reaction [43].
Prolonged reaction time [40], lower substrate scope [38,
43], Lower amount of palladium content in the supported
catalyst, and increasing risk and prolonged process devel-
opment through heterogenization are some disadvantages
of these reported dendritic catalysts. Therefore, the devel-
opment of an alternative, stable and economic homogene-
ous dendritic Pd(II) catalyst is in high demand. Notably, the
present catalyst TMP-PECH-Amine-G1-Pd is thermally and
chemically stable, highly selective, easy to separate from the
reaction mixture and retains good activity for nine succes-
sive runs in Heck coupling reaction without any additional
activation treatment. To the best of our knowledge, this is
the frst report on the preparation of palladium incorporated
amine-functionalized dendronized polymer and its use in
Heck coupling reaction.
Palladium catalysts have good activity and play a sub-
stantial role in carbon–carbon coupling reactions such as
the Heck [6–8], Suzuki [9, 10], Stille [11, 12], Sonoga-
shira [13, 14], and Hiyama reactions [15]. The high cost
of palladium metal salt, eviction of the metal waste, the
catalyst recovery from the reaction mixture and reusability
are the main challenges addressed by palladium cataly-
sis. This problem can be solved upon the incorporation
of palladium into various organic and inorganic polymer
supports [16–20]. However, the catalyst deactivation due
to unfavourable interactions between the active sites and
the surface of solid support, and metal leaching, are the
key disadvantages of the polymer supported catalysts
[21]. Dendritic polymers, due to their three-dimensional
branched nature, are well suited for hosting palladium
with coordination occurring potentially at the periphery,
interior or both. Thus they may serve as useful polyfunc-
tional ligands, and metal ion sequestering agent [22]. In
recent years, a number of palladium complexed dendritic
catalysts were reported for palladium catalysed reactions,
especially Suzuki coupling [23], Heck coupling [24, 25],
epoxidation of olefns [26], oxidation of sulphides to sul-
foxides [27] etc.
In this work, frst generation tris(hydroxymethyl)pro-
pane initiated polyepichlorohydrin cored amine-func-
tionalized dendronized polymer-supported Pd(II) catalyst
(TMP-PECH-Amine-G1-Pd) was prepared. This catalyst
exhibited high activity for the Heck cross-coupling reac-
tion of aryl halides and olefns. Heck reaction is one of
the most powerful carbon–carbon coupling reactions
for the synthesis of aryl-substituted olefns, and it fnds
applications in natural product synthesis [28], synthesis
of pharmaceutically active compounds [29, 30], and in
material science [31]. In 1971, Mizoroki and coworkers
Molecular docking is an in silico drug designing method,
useful in predicting the preferable binding mode and afn-
ity of the ligand in the active site of the target protein. It
has an important role in pharmaceutical research and
development to reduce the time span and expense of the
drug discovery process by fnding appropriate lead mol-
ecules for the drug target [44, 45]. In this study, all the
synthesized stilbene derivatives obtained by the Heck
coupling reaction were subjected to docking study against
DprE1 protein to study its potential antitubercular activity.
1 3

Palladium Loaded Dendronized Polymer as Efcient Polymeric Sustainable Catalyst for Heck…
Decaprenylphosphoryl-β-d-ribose 2′-epimerase (DprE1) is a
vital enzyme for cell wall synthesis, and it is a highly vulner-
able, fully validated tuberculosis drug target [46].
polymer-supported Pd(II) catalyst (TMP-PECH-Amine-
G1-Pd). The synthesis, characterization and catalytic
application of amine-functionalized tris(hydroxymethyl)
propane initiated polyepichlorohydrin cored dendronized
polymer (TMP-PECH-Amine-G1) was previously reported
[47]. The synthetic procedure for the complexation of pal-
ladium with TMP-PECH-Amine-G1 is shown in Scheme 1.
The complexation reaction with palladium acetate in metha-
nol at room temperature gave rise to palladium containing
dendritic structure with palladium acetate moieties bound
2 Results and Discussion
The present article outlines the synthesis and characteri-
zation of the tris(hydroxymethyl)propane initiated poly-
epichlorohydrin cored amine-functionalized dendronized
Scheme 1 Synthesis of TMP-
PECH-Amine-G1-Pd
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K. Hiba et al.
on the periphery. Palladium incorporated dendritic struc-
ture appeared as black powder and it was characterized by
the following techniques: inductively coupled plasma-mass
spectrometry (ICP-MS), feld emission scanning electron
microscopy (FE-SEM), energy dispersive X-ray spectros-
copy (EDAX), high-resolution transmission electron micros-
copy (HR-TEM), UV–Vis spectroscopy, Fourier transform
infrared spectroscopy (FT-IR), X-ray photoelectron spec-
troscopy (XPS) and thermogravimetric analysis (TG/DTA).
palladium nanoparticles and palladium content was deter-
mined by ICP-XRF as follows: G1DenP-Pd, 0.61 mmol/g
(6.46 wt.%); G2DenP-Pd, 0.56 mmol/g (5.96 wt.%); and
G3DenP-Pd, 0.49 mmol/g (5.15 wt.%) (Table 1, entry 4).
Overall comparison led to the conclusion that, the palla-
dium content in the present catalyst TMP-PECH-Amine-
G1-Pd was higher than the other reported ones. This is one
of the highlights of the present work.
2.1 ICP‑MS Analysis
2.2 FE‑SEM and Energy Dispersive X‑ray (EDAX)
Analysis
Inductively coupled plasma-mass spectrometry (ICP-
MS) analysis revealed the palladium content per gram of
the catalyst TMP-PECH-Amine-G1-Pd as 4.19 mmol/g
(44.48 wt.%), which directly resulted from the higher
amine functionality of the dendronized polymer support
(TMP-PECH-Amine-G1). The amount of palladium con-
tent in the present catalyst was compared with, that in
various reported dendrimer supported homogeneous pal-
ladium based catalysts and the results are summarized
in Table 1. In 2020 Gan et al. reported palladium-loaded
hyperbranched polytriazole with phenyl terminal group as
a catalyst for Heck coupling reaction between iodobenzene
and styrene with 0.79 mmol/g palladium, determined by
ICP-OES analysis (Table 1, entry 1). Similarly, Liu et al.
developed hyperbranched polyethylene (HBPE) encapsu-
lated Pd(II) catalyst in one pot synthetic methodology and
the presence of palladium in their catalyst was quantita-
tively confrmed with Atomic absorption spectrometry as
0.14 mmol/g (Table 1, entry 2). Nanocomposite based on
hyperbranched pyridylphenylene polymer (PPP) and Pd0
nanoparticles (PPP-Pd) were synthesized by Kuchkina
et al. by mixing the polymer with palladium acetate in
toluene, followed by reduction using tetrahydro-1,4-oxa-
zine borane. They have obtained PPP-Pd-3 nanocomposite
with 3.30 mmol/g (35 wt.%) palladium content (Table 1,
entry 3). Wu et al. in 2008, developed frst- to the third-
generation nanosized dendritic phosphine oxide stabilized
In order to study the microstructure and surface morphol-
ogy of the catalyst, TMP-PECH-Amine-G1-Pd, SEM
image was recorded. The FE-SEM micrograph shown in
Fig. 1a, revealed that the palladium complexed dendritic
structure possessed spherical morphology with some
metallic lusture. The presence of palladium in TMP-
PECH-Amine-G1-Pd was also supported by the energy
dispersive X-ray analysis (EDAX) (Fig. 1b), which is a
technique of elemental analysis associated with electron
microscopy based on the generation of characteristic
X-rays that could reveal the presence of elements present
in the samples. The signifcant peaks corresponding to car-
bon, oxygen, and nitrogen can also be seen in the EDAX
spectrum of the catalyst TMP-PECH-Amine-G1-Pd.
2.3 Transmission Electron Microscopy (TEM)
Analysis
The High resolution transmission electron microscopy
images of the palladium incorporated dendritic catalyst
TMP-PECH-Amine-G1-Pd showed small aggregated clus-
ters (Fig. 2a–c). The small cluster formation is due to the
coordination of the amino group of the dendronized poly-
mer to palladium salt.
Table 1 Palladium content in various reported dendritic polymer based homogeneous catalysts
Entry
Dendritic polymer based homogeneous palladium catalyst
Palladium content
(mmol/g)
Ref
1
2
3
4
Palladium-loaded hyperbranched polytriazole (HBP300-Ph-Pd)
Hyperbranched polyethylene (HBPE) encapsulating Pd(II) (HBPE4)
Hyperbranched pyridylphenylene polymer and palladium nanoparticle (PPP-Pd)
Dendritic phosphine oxide-stabilized palladium nanoparticle
G1DenP-Pd
G2DenP-Pd
G3DenP-Pd
0.79
0.14
3.30
[43]
[38]
[48]
[49]
0.61
0.56
0.49
4.19
5
TMP-PECH-Amine-G1-Pd (present catalyst)
1 3

Palladium Loaded Dendronized Polymer as Efcient Polymeric Sustainable Catalyst for Heck…
Fig. 1 a FE-SEM image of TMP-PECH-amine-G1-Pd, b EDAX spectrum of TMP-PECH-Amine-G1-Pd
Fig. 2 TEM images of the
catalyst TMP-PECH-Amine-
G1-Pd (a–c)
1 3

K. Hiba et al.
Fig. 3 UV–Vis spectra of the catalyst TMP-PECH-Amine-G1-Pd
Fig. 4 FT-IR spectra of TMP-PECH-amine-G1-Pd
2.4 UV–Vis Spectroscopy
2.6 X‑ray Photoelectron Spectroscopy
The coordination of the palladium ions to the amino groups
of the dendritic structure was analysed by UV–Vis spectros-
copy. In the absence of the dendritic structure, the absorp-
tion spectrum of the palladium acetate showed a band at
270 nm, which corresponded to ligand to metal charge
transfer transition [50]. After the formation of TMP-PECH-
Amine-G1-Pd complex, the band at 270 nm disappeared and
a new band was evident at 350 nm, which corresponded
to LMCT associated with complexation of Pd(II) to amino
group of dendritic structure (Fig. 3).
XPS analysis was utilized to clarify the surface chemical
composition, incorporation of the palladium and also to
evaluate the oxidation state of Pd in the catalyst TMP-
PECH-Amine-G1-Pd. The XPS wide spectrum of the cat-
alyst TMP-PECH-Amine-G1-Pd showed binding energy
corresponding to the O 1s, N 1s, Pd 3d and C 1s, con-
frming the overall chemical composition of the catalyst
(Fig. 5a). The XPS deconvoluted spectrum of Pd (Fig. 5b),
showed binding energy values at 335.4 eV and 340.7 eV
corresponding to Pd(II) 3d5/2, and Pd(II) 3d3/2 respectively,
consistent with the existence of Pd in divalent oxidation
state. This value is lower than the expected binding energy
of Pd(II) state. The lowering of binding energy value is
due to the efect of neighbouring nitrogen ligand coor-
dinated to palladium metal salt. Thus XPS analysis also
supported the incorporation of palladium in the periphery
of dendritic polymer.
2.5 FT‑IR Spectral Studies
The FT-IR spectrum of the catalyst TMP-PECH-Amine-
G1-Pd and its comparison with TMP-PECH-Amine-G1
dendronized polymer confrmed the successful synthesis of
the above mentioned catalyst. In the FT-IR spectrum of the
TMP-PECH-Amine-G1, the bands appearing at 3411 cm−1
and 1572 cm−1 were attributed to the stretching and bending
vibrations of N–H groups, respectively [47]. The presence
of N–H vibration bands of TMP-PECH-Amine-G1 in the
FT-IR spectrum of TMP-PECH-Amine-G1-Pd proved the
successful synthesis of the present catalyst. The occurrence
of slight shift in stretching and bending N–H bands in TMP-
PECH-Amine-G1-Pd is due to the chemical environment of
Pd–N coordination [51]. In addition to the above mentioned
bands, the complex showed a band at 1621 cm−1 associated
with the C=O asymmetric stretching vibrations of the ace-
tate groups and the band at 1383 cm−1 which corresponded
to the C=O symmetric stretching vibration of the acetate
groups, which indicated the presence of acetate groups in
TMP-PECH-Amine-G1-Pd (Fig. 4) [52, 53].
2.7 TG‑DTG Analysis
Thermogravimetry was carried out to study the thermal
stability of the catalyst TMP-PECH-Amine-G1-Pd and
the thermogram is presented in (Fig. 6). TG analysis of
the TMP-PECH-Amine-G1-Pd was examined from 26 to
400 °C and it exhibited two main weight losses. The frst
weight loss of about 7% occurred below 100 °C which
was attributed to the desorption of moisture. The second
weight loss of 15% occurred at the temperature range of
172–197 °C due to degradation of the polymer backbone.
1 3

Palladium Loaded Dendronized Polymer as Efcient Polymeric Sustainable Catalyst for Heck…
Fig. 5 a XPS wide spectrum of the catalyst TMP-PECH-Amine-G1-Pd, b XPS deconvoluted spectrum of Pd(II)
Table 3 Optimization of temperature, solvent, and base
Entry
Temperature Solvent
(°C)
Base
Yield (%)a
1
2
3
4
5
6
7
8
30
60
80
100
120
80
80
80
80
80
DMF
DMF
DMF
DMF
DMF
DMSO
DMAc
NMP
DMF
DMF
DMF
DMF
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Et3N
Trace
65
96
96
96
92
93
94
92
9
10
11
12
NaOAc
NaOH
Na2CO3
75
60
65
80
80
Fig. 6 TG-DTG curve of the catalyst TMP-PECH-Amine-G1-Pd
Table 2 Optimization of the amount of catalyst
Reaction conditions: bromobenzene (2 mmol), styrene (3 mmol),
base (3 mmol), catalyst (10 mg), solvent, 3 h
aIsolated yield
Entry
Amount of catalyst Amount of catalyst
Yield (%)b
(mg)
(mol%)a
2.8 Catalytic Activity
1
2
3
4
5
0
5
8
10
12
–
–
1.05
1.68
2.09
2.46
72
86
96
96
After successful synthesis and characterization of the struc-
ture of the catalyst, its catalytic activity was evaluated in
the Heck coupling reaction between diferently substituted
aryl halides and olefns under nitrogen atmosphere. Ini-
tially, the diferent reaction conditions such as amount of
catalyst, reaction temperature, efect of solvent, and diverse
bases were optimized using styrene and bromobenzene as
the model substrates. First, the catalyst amount was varied
from 1.05 mol% (5 mg) to 2.46 mol% (12 mg). It was found
that 2.09 mol% (10 mg) of the catalyst TMP-PECH-Amine-
G1-Pd was optimum for the product formation (Table 2,
entry 4). To establish the necessity of the catalyst in this
Reaction conditions: bromobenzene (2 mmol), styrene (3 mmol),
K2CO3 (3 mmol), DMF, 80 °C, 3 h
aCatalyst (based on bromobenzene)
bIsolated yield
1 3

K. Hiba et al.
reaction, a blank reaction was conducted without any cata-
lyst, in which no product was formed even after long time
(Table 2, entry 1). The high palladium content and coopera-
tive efect of the catalyst due to amine-functionalized den-
dritic backbone contributed towards higher product yield.
Similar experiments by changing the temperature, sol-
vent, and base were carried out using 2.09 mol% of catalyst
(Table 3). It was found that, with increasing temperature
from room temperature (RT) to 80 °C, a substantial increase
in the product yield was observed (Table 3, entries 1–3).
However, the further increase of reaction temperature from
80 to 120 °C did not show any signifcant change in the prod-
uct yield (Table 3, entries 3–5). The solvent plays a signif-
cant role in the Heck coupling reaction and usually dipolar
aprotic solvents are used [54]. The reaction was performed
in diferent dipolar solvents such as DMF, DMSO, DMA and
NMP (Table 3, entries 3, 6–8). Among them DMF was the
best solvent giving slightly higher yield (96%) compared to
other selected solvents. Therefore, DMF was selected for the
further study of the reaction. In order to fnd a suitable base,
that would afect the desired reaction, several bases such as
K2CO3, Et3N, NaOAc, NaOH and Na2CO3 were screened
(Table 3, entries 3, 9–12). It was found that, the base played
a central role in Heck coupling reaction and the best result
was obtained with K2CO3 as base.
its ground state energy, structure of the products were opti-
mized using B3LYP method and 6-31G basis set. The opti-
mized structure and ground state energy of all the products
are depicted in Table 4. Based on the ground state energy
value, the compound (E)-3-Styrylthiophene (Table 4, entry
9) has least energy value (− 861.33), thus we expect higher
product yield compared with others. But the experimental
results showed 80% yield. While, in some other cases, the
experimental values are agreeable with the theoretical val-
ues. These results showed that, the formation of stilbene
derivatives was not only predicted by ground state energy
values alone, but also, other factors such as kinetic and ther-
modynamic factors have to be considered in order to predict
the formation of the product theoretically.
The catalyst TMP-PECH-Amine-G1-Pd also revealed
good activity towards diferent olefns (Table 5). The cou-
pling of 1-Iodo-4-nitrobenzene with methyl acrylate, acry-
lonitrile, acrylic acid and methyl methacrylate generated the
corresponding products in good yield.
All alkene products obtained by Heck coupling reaction
are in trans confguration; any cis-isomer was not observed,
which indicated the remarkable stereo-selectivity of the
catalyst TMP-PECH-Amine-G1-Pd (see SI, 1H NMR spec-
trum of the products). The high selectivity of the catalyst is
mainly due to the co-operative interaction of catalytically
active groups owing to the presence of dendritic backbone
[56]. In the 1H NMR spectrum of the stilbene, the absence
of the signal at δ=6.60 ppm proved that cis-isomer was not
formed in the reaction (Fig. 7) [57].
To demonstrate the scope and generality of the meth-
odology, several stilbene derivatives were prepared from
substituted aryl halides and styrene (Table 4). The products
1
were characterized by GC–MS, H NMR and 13C NMR
techniques. Heck coupling reaction generally preferred less
crowded structure, and hence trans product was usually
favoured [55]. Reactions with aryl bromide and iodide were
equally efcient for coupling. Reactions with aryl chlorides
were slow, afording the desired product in slightly lower
yield (Table 4, entries 1 and 2). This may be due to the
weaker leaving group ability of the chloride ion. Aryl bro-
mides with electron withdrawing substituents in the para
position can couple with styrene to aford the correspond-
ing substituted stilbenes in excellent yields (Table 4, entries
3, 4 and 6). The reaction of aryl bromides having electron
releasing substituents in the para position (Table 4, entries
5 and 7) gave slightly lower yield compared to aryl bro-
mides having electron withdrawing substituents. The aryl
bromides having electron donating substituents at meta posi-
tion also aforded the corresponding stilbene in high yields
(Table 4, entry 8). Heterocyclic aryl halides such as 3-bro-
mothiophene and 2-bromothiophen e were also tested in the
Heck coupling with styrene and gave 80% and 97% products,
respectively (Table 4, entries 9 and 10). Therefore, it can
be concluded that the catalyst TMP-PECH-Amine-G1-Pd
exhibited excellent catalytic activity in the Heck coupling
reaction between styrene and aryl halides. In order to fnd
whether the yield of the stilbene derivatives depended on
2.9 Leaching of Palladium and Reusability Study
When the reaction was completed, the product could be
isolated from the reaction mixture by simple extraction
technique with diethyl ether, leaving the catalyst TMP-
PECH-Amine-G1-Pd in water medium. The water medium
containing the catalyst could be washed with ethyl acetate
and it could be concentrated to reduce the volume. The cen-
trifugation of washed water by the application of rotation
speed of 1800 rpm for 40 min, resulted in the settling down
of the catalyst [58, 59]. The reusability of the catalyst TMP-
PECH-Amine-G1-Pd was checked by using bromobenzene
and styrene as model substrates. The catalyst was reused
upto nine cycles of reaction without loss of signifcant activ-
ity (Fig. 8). According to ICP-MS analysis, the amount of
palladium content present in the catalyst at the initial stage
and after 9th reaction cycle was found to be 44.48% and
44.20%, respectively. The result indicated that only 0.28%
of total palladium content in the catalyst was leached out
after 9th cycle of reaction. This indicated the stability of the
present catalyst, here the support of dendronized polymer
contributed to the efective stability of the catalyst.
1 3

Palladium Loaded Dendronized Polymer as Efcient Polymeric Sustainable Catalyst for Heck…
Table 4 Heck coupling reaction of various substrates
Entry
1
X
Y
H
Optimized Structure
− 540. 59 a.u
Yield (%)a
Br
Cl
96
83
2
3
I
4-CHO
4-NO2
98
97
85
Br
Cl
− 653.88 a.u
I
99
− 745.01 a.u
− 632.80 a.u
4
5
Br
Br
4-CN
98
92
4-OCH3
− 655.08 a.u
6
7
Br
Br
4-COCH3
4-NH2
97
88
− 693.19 a.u
− 595.93 a.u
1 3

K. Hiba et al.
Table 4 (continued)
Entry
X
Y
Optimized Structure
Yield (%)a
96
8
Br
3-COOC2H5
− 807.70 a.u
− 861.33 a.u
9
H
H
80
97
10
− 861.33 a.u
Reaction conditions: arylhalide (2 mmol), styrene (3 mmol), K2CO3 (3 mmol), catalyst (10 mg), DMF, 80 °C, 3 h
aIsolated yield
Table 5 Heck coupling reaction of 1-Iodo-4-nitrobenzene with diferent olefns
Entry
1
Olefns
Product
Yield (%)a
99
2
3
95
93
4
92
Reaction conditions: 1-Iodo-4-nitrobenzene (2 mmol), olefn (3 mmol), K2CO3 (3 mmol), catalyst (10 mg), DMF, 80 °C, 3 h
aIsolated yield
1 3

Palladium Loaded Dendronized Polymer as Efcient Polymeric Sustainable Catalyst for Heck…
Fig. 7 1H NMR spectrum of the
stilbene
regenerated the coordinated alkene and migration of
2.10 Mechanism of the Reaction
aryl group from Pd4+ to coordinated alkene followed by
β-hydrogen migration, generated the product. The base
induced removal of hydrogen halide regenerated the Pd2+
catalyst TMP-PECH-Amine-G1-Pd.
A possible mechanism for Heck coupling reaction catalyzed
by TMP-PECH-Amine-G1-Pd is shown in Scheme 2. The
mechanism is proposed on the basis of mechanism suggested
by Shaw et al. [60, 61]. Here Pd2+/Pd4+ is involved instead
of Pd0/Pd2+ cycle. In the frst step olefn was coordinated to
the supported Pd2+, which underwent nucleophilic attack
to give a σ-aryl complex. The resultant electron-rich Pd2+
complex further underwent oxidative addition with aryl
halides to give a Pd4+ complex. The loss of nucleophile
2.11 Comparison with Previous Reports
In order to compare the activity of the present catalyst TMP-
PECH-Amine-G1-Pd with recently reported palladium-
based catalysts for Heck reaction, the reaction between
bromobenzene and styrene was chosen. The results are sum-
marized in the Table 6. It is clear that each catalyst reported
for this reaction has its own benefts. However, the special
efect of soluble dendritic support in the new catalyst TMP-
PECH-Amine-G1-Pd was refected in catalysis as low cata-
lyst amount, stability and high selectivity of the catalyst,
short reaction time, low temperature and higher product
yield.
2.12 Docking Study
Molecular docking study was performed to find out the
interaction between all the stilbene derivatives (ligands) and
‘druggable’ protein involved in vital physiological function
in mycobacterium tuberculosis, namely DprE1. The docking
score for each ligand towards DprE1 protein was obtained
from Autodock Vina sotware. The best docking score (lowest
binding energy value) indicated the more potent binding afn-
ity between the receptor (protein) and ligand molecules. From
the resultant docked structures (see SI, Fig. 1), it could be clear
Fig. 8 Recyclability of TMP-PECH-Amine-G1-Pd
1 3

K. Hiba et al.
Scheme 2 Probable mechanism
Table 6 Comparison of the
activity of TMP-PECH-Amine-
G1-Pd with previous reports
Entry
1
Catalyst
Reaction condition
Yield (%)
91
Ref
GO-Fe3O4-Cellulose-Pd
K2CO3, 7 h, 100 °C,
DMAC: Gly (1:2)
[62]
2
3
4
5
6
CL-salen‐Pd(II)
K2CO3, 3 h, refux, ethanol
NaOAc, 120 °C , 2.5 h, NMP
Et3N, 4 h, 80 °C , DMF,
Na2CO3, 3 h, 100 °C , PEG
K2CO3, 3 h, 80 °C , DMF
97
93
92
25
96
[63]
[64]
[65]
[66]
MNP@SiO2-SBA-PCA-Pd(II)
Pd-AcAc-Am-Fe3O4@SiO2
Pd@{Mo368}
TMP-PECH-Amine-G1-Pd
Present work
Reactants used: bromobenzene and styrene
that all the ligand molecules easily ftted into the active site of
DprE1 and showed good afnity. The binding energy values of
each ligand–receptor complex, their hydrogen bonding inter-
actions and interacting residues are depicted in Table 7. All
the compounds displayed superior docking score values over
the range − 6.5 to − 8.2 kcal/mol, compared to the standard
drug isoniazid with docking score of − 6.1 kcal/mol against
DprE1(Table 6). (E)-Ethyl-3-styrylbenzoate displayed the
best docking score (− 8.2 kcal/mol) towards DprE1 (Table 7,
entry 8; SI, Fig. 1, entry 8). The hydrogen bonding interac-
tions of (E)-Ethyl-3-styrylbenzoate with GLY 55 residue of
protein was not only the reason for enhanced binding afnity,
but also, other interactions such as strong pi-alkyl interactions
with VAL 121, ILE 131, ALA 417, PRO 116 residues, and
carbon hydrogen bond interaction with ALA 53 residue of the
protein contributed towards higher binding afnity. The com-
pound E-4-Nitrostilbene (Table 7, entry 3; SI, Fig. 1, entry 3)
displayed four hydrogen bonding interactions with THR 122,
LEU 56, GLY 57, ARG 58 and showed good docking score
of − 7.9 kcal/mol. Therefore, the strong binding interactions
of all the stilbene derivatives with DprE1 protein indicated the
future scope of the ligands as anti-TB drug.
1 3

Palladium Loaded Dendronized Polymer as Efcient Polymeric Sustainable Catalyst for Heck…
Table 7 Binding afnity,
Entry Ligand
Hydrogen
bonding inter-
actions
Interacting residues
THR 122, LEU 56
THR 122, LEU 56, GLY 57, ARG 58 − 7.9
THR 122, LEU 56
GLY 55
Binding
energy (kcal/
mol)
hydrogen bonding interaction of
protein DprE1 with ligands
1
(E)-Stilbene
(E)-4-Styrylbenzaldehyde
(E)-4-Nitrostilbene
–
2
4
2
1
2
–
1
–
–
3
− 6.9
− 7.7
2
3
4
5
6
(E)-4-Styrylbenzonitrile
(E)-4-Methoxystilbene
(E)-4-Acetylstilbene
(E)-4-Styrylaniline
− 7.9
− 7.4
− 8.1
− 7.0
− 8.2
− 6.5
− 6.5
− 6.1
THR 122, LEU 56
7
8
9
10
11
(E)-Ethyl-3-styrylbenzoate
(E)-3-Styrylthiophene
(E)-2-Styrylthiophene
Isoniazid
GLY 55
GLY 125, ALA 53, ILE 184
scientifc evolution 201 spectrophotometer with INSIGHT
software in the wavelength range 200–900 nm. Palladium
content was quantitatively analyzed by inductively coupled
plasma-mass spectrometry analysis with Thermo scientifc
single quadrupole (SQ) ICP-MS analyzer (STIC CUSAT).
X-ray photoelectron spectroscopy measurements were car-
ried out using Thermo scientifc ESCALAB XI X-ray pho-
toelectron spectrometer (XPS) microprobe (CLIF, Kerala
University). Thermogravimetric analysis (TG/DTG) was
conducted using Perkin elmer diamond thermogravimetry/
diferential thermal analysis (TG/DTA) instrument with
continuous fow of nitrogen and a heating rate of 10 °C/min
using ceramic pan. Transmission electron microscopy image
was recorded using JEM-2100, 200 kV HRTEM instrument
(STIC CUSAT). Energy dispersive X-ray analysis was per-
formed using JEOL model SEM-EDAX instrument with
energy resolution 136 eV and 30 mm2 EDAX detector area
(STIC CUSAT). Field emission scanning electron micros-
copy images of the sample were taken with the instrument
Nova nanoSEM 450, Czech Republic (Department of Phys-
ics, CUSAT). 1H Nuclear magnetic resonance (1H NMR) and
13C Nuclear magnetic resonance (13C NMR) spectra were
recorded on a Bruker 400 MHz spectrometer and chemical
shifts were recorded down feld of TMS (IIRBS-MG Uni-
versity). Gas chromatography–mass spectrometry analysis
of the samples were recorded on a 1200 L single quadrupole,
Varian gas chromatograph model by using helium as carrier.
3 Conclusion
As shown here, dendritic catalysis is a rapidly growing
feld. In this study, palladium was loaded at the periphery
of tris(hydroxymethyl)propane initiated polyepichlorohydrin
cored amine-functionalized dendronized polymer, which
brought about a large number of catalytically active spe-
cies in a small volume. The XPS results confrmed that the
incorporated palladium species were in the divalent oxida-
tion state. The main advantage of this catalytic system was
the high palladium content in the low generation level itself.
The resulting catalyst (TMP-PECH-Amine-G1-Pd) was
found to be highly competent for the synthesis of aryl sub-
stituted olefns via Heck coupling reaction between olefns
and aryl halides. All the products were formed with excel-
lent yield (80–99%) in short period of time at lower tem-
perature. The catalyst has maintained nine cycles of reuse
with very low catalyst loss by leaching. Moreover, the cata-
lyst TMP-PECH-Amine-G1-Pd exhibited thermal stability,
chemical stability, and high selectivity. A comparative study
was also performed to show the superiority of the present
work. Molecular docking studies of stilbene derivatives with
the target protein DprE1 were studied. The docked ligand-
receptor complex provided an insight into the binding activ-
ity of stilbene derivatives with DprE1 enzyme on its binding
region and provided high docking scores.
4 Experimental
4.2 Synthesis of TMP‑PECH‑Amine‑G1 Dendronized
Polymer
4.1 Materials and Methods
The detailed synthesis of TMP-PECH-Amine-G1 den-
dronized polymer was previously reported [47]. The prepa-
ration of TMP-PECH-Amine-G1 is briefy described here.
Epichlorohydrin (11.80 mL 0.15 mol) was added drop
wise to a mixture containing tris(hydroxymethyl)propane
All the solvents and chemicals used were purchased from
local suppliers. Fourier transform infrared spectra of samples
as KBr pellets were recorded on JASCO model 4100 FT-IR
spectrometer. UV–Visible spectra were recorded on Thermo
1 3

K. Hiba et al.
(0.67 g, 0.005 mol), BF3-etherate (0.30 mL 2.50 mmol) and
dichloromethane (20 mL) and was stirred for 5 h at room
temperature. The product (TMP-PECH) (3 g) in chloroform
was allowed to react with p-toluenesulfonyl chloride (3.43 g
0.02 mol) in the presence of pyridine (1.90 mL, 0.03 mol)
frst at 0 °C for 4 h, then at room temperature overnight. The
tosylated TMP-PECH (3 g) was reacted with sodium azide
(1.75 g, 0.03 mol) and tetrabutylammonium bromide (0.03 g,
0.10 mmol) in DMF at 110 °C for 48 h. The poly (glyci-
dylazide) formed (2 g) was dissolved in dry THF (20 mL)
and a slurry of LiAlH4 (0.76 g 0.02 mol) in THF (10 mL)
was slowly added. The reaction mixture was stirred initially
at cold condition for 2 h, then at room temperature and then
refuxed for 1 h. The TMP-PECH-Amine-G0 (2 g) obtained
was added to acrylonitrile (10 mL, 0.15 mmol) at room
temperature under dark room condition. After the initial
exothermic phase, the reaction mixture was heated at 80 °C
for 2 days to complete the addition reaction. After the reac-
tion, the excess acrylonitrile was removed under vacuum. In
the fnal step, the above polymer (2 g) was dissolved in dry
THF and a slurry of LiAlH4 (1.52 g, 0.04 mol) in dry THF
was added drop by drop with constant stirring. The reaction
mixture was stirred in ice bath containing crushed ice for
2 h. The temperature was allowed to rise to 30 °C and the
reaction mixture was refuxed for 24 h to ensure complete
reduction of nitrile group to amino group. After the each
reaction step, the product was washed well with suitable
solvent.
mixture, and extracted with diethyl ether (3×10 mL). The
organic phase was washed several times with water and dried
over anhydrous MgSO4. After evaporation of the solvent, the
pure product was obtained in high yield. The isolated prod-
ucts were characterized by GCMS, 1H NMR and 13C NMR.
4.5 Molecular Docking
All the geometries were optimized using B3LYP/6-31G level
of theory using Gaussian 09 software and geometries were
visualized by Gauss View. The crystal structure of DprE1
protein (PDB ID: 6G83) was re-claimed from the RCSB
PDB. All the coordinated water molecules and unwanted
attached ligands were excluded from the protein using Bio-
via Discovery Studio 2016 64-bit. The ligands were docked
into the binding site region of the receptor using Autodock
Vina Software and for 2D and 3D visualization by using
Biovia Discovery Studio 2016 64-bit.
4.6 The Spectral Data for Selected Product are
Presented as Follows
4.6.1 (E)‑Stilbene [67]
Table 5, entry 1; white solid; MP-123 °C (lit.
1
123.9–124.6 °C); GC–MS (m/z): 180; H NMR (CDCl3,
400 MHz): δ (ppm) 7.56 (d, J=7.5 Hz, 4H), 7.41 (t, J=7.5,
4H), 7.32–7.27 (m, 2H), 7.16 (s, 2H); 13C NMR (CDCl3,
100 MHz): δ (ppm) 137.4, 128.7, 128.7, 127.7, 126.6.
4.3 Synthesis of TMP‑PECH‑Amine‑G1‑Pd
Tris(hydroxymethyl)propane initiated polyepichlorohy-
drin cored amine-functionalized dendronized polymer
(TMP-PECH-Amine-G1) (0.25 g) in methanol was taken in
100 mL round-bottom fask. A solution of palladium acetate
in methanol (0.67 g, 3 mmol) was added. The reaction mix-
ture was stirred at 30 °C for 24 h. After completion of the
reaction, the product was fltered and was washed with an
excess amount of acetone and chloroform. The palladium
incorporated dendronized polymer was obtained as a black
powder. The palladium content obtained as per ICP-MS
analysis was 4.19 mmol/g.
4.6.2 (E)‑4‑Styrylbenzaldehyde [67]
Table 5, entry 2; white solid; MP-116 °C (lit.
1
116.0–116.8 °C); GC–MS (m/z): 208; H NMR (CDCl3,
400 MHz): δ (ppm) 10.00 (s, 1H), 7.88 (d, J=8.0 Hz, 2H),
7.66 (d, J = 8.0 Hz, 2H), 7.56 (d, J = 7.4 Hz, 2H), 7.41 (t,
J = 7.3 Hz, 2H), 7.35–7.30 (m, 1H), 7.26 (d, J = 16.3 Hz,
1H), 7.15 (d, J=16.3 Hz, 1H); 13C NMR (CDCl3, 100 MHz):
δ (ppm) 191.7, 143.4, 136.6, 135.3, 132.2, 130.3, 128.9,
128.5, 127.3, 126.9.
4.4 General Procedure for the Heck Coupling
Reaction
4.6.3 (E)‑4‑Nitrostilbene [67]
Table 5, entry 3; yellow solid; MP-155 °C (lit.
1
The aryl halide (2 mmol), olefn (3 mmol), K2CO3 (3 mmol)
and the catalyst (TMP-PECH-Amine G1-Pd) were taken in
a 50 mL RB fask and DMF was added as solvent. The reac-
tion mixture was stirred at 80 °C under a nitrogen atmos-
phere. The progress of the reaction was monitored by thin-
layer chromatography (TLC). After completion, the reaction
mixture was cooled, distilled water was added to the reaction
154.2–155.0 °C); GC–MS (m/z): 225; H NMR (CDCl3,
400 MHz): δ (ppm) 8.23 (d, J = 8.8 Hz, 2H), 7.64 (d,
J = 8.8 Hz, 2H), 7.57 (d, J = 7.3 Hz, 2H), 7.43–7.30 (m,
3H), 7.27 (d, J = 16.3 Hz, 1H), 7.16 (d, J = 16.3 Hz, 1H);
13C NMR (CDCl3, 100 MHz): δ (ppm) 143.9, 136.2, 133.3,
128.9, 128.3, 127.0, 126.9, 126.3, 124.4, 124.2.
1 3

Palladium Loaded Dendronized Polymer as Efcient Polymeric Sustainable Catalyst for Heck…
Supplementary Information The online version contains supplemen-
4.6.4 (E)‑4‑Styrylbenzonitrile [67]
tary material available at https://doi.org/10.1007/s10562-021-03767-6.
Table 5, entry 4; white solid; MP-114 °C (lit.
Acknowledgements Hiba K. gratefully acknowledges UGC, New
Delhi, India for fnancial support in the form of a Senior Research Fel-
lowship and for the support under SAP DRS-III. The authors are thank-
ful to the Sophisticated Analytical Instrumentation Facility (SAIF),
CUSAT, Kochi for EDX, and Department of Physics for FE-SEM
analysis. We thank CLIF, University of Kerala for XPS, and IIRBS,
Mahatma Gandhi University for NMR analysis.
1
114.9–115.2 °C); GC–MS (m/z): 205; H NMR (CDCl3,
400 MHz): δ (ppm) 7.64 (d, J = 8.1 Hz 2H), 7.59 (d,
J=8.2 Hz, 2H), 7.55 (d, J=7.5 Hz, 2H), 7.41 (t, J=7.3 Hz,
2H), 7.35–7.27 (m, 1H), 7.23 (d, J=16.3 Hz, 1H), 7.10 (d,
J = 16.3 Hz, 1H); 13C NMR (CDCl3, 100 MHz): δ (ppm)
141.9, 136.3, 132.5, 128.9, 128.7, 126.9, 126.8, 126.7,
119.0, 110.6.
Funding This work was supported by the University Grants Commis-
sion (Grant No. Ref. No: 21/12/2014 (ii) EU-V).
4.6.5 (E)‑4‑Methoxystilbene [67]
Declarations
Table 5, entry 5; white solid; MP-134 °C (lit.
1
Conflict of interest The authors declare that they have no confict of
135.3–135.9 °C); GC–MS (m/z): 210; H NMR (CDCl3,
interest.
400 MHz): δ (ppm) 7.52–7.47 (m, 4H), 7.37 (t, J=7.6 Hz,
2H), 7.27–7.24 (m, 1H), 7.09 (d, J=16.3 Hz, 1H), 7.00 (d,
J=16.3 Hz, 1H), 6.92 (d, J=8.8 Hz, 2H), 3.85 (s, 3H); 13
C
NMR (CDCl3, 100 MHz): δ (ppm) 159.3, 137.7, 130.2,
128.7, 128.2, 127.7, 127.2, 126.6, 126.3, 114.2, 55.3.
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Authors and Afliations
K. Hiba1 · G. Anjali Krishna1 · S. Prathapan1 · K. Sreekumar1
1
* K. Sreekumar
Department of Applied Chemistry, Cochin University
kskpolymer.cusat@gmail.com
of Science and Technology, Kochi 682022, India
1 3