Heterogeneous High-Loading Hyperbranched Polyglycidol with Peripheral NHC–Pd Complex: Synthesis and Application as Catalyst in Suzuki Coupling Reaction

Article abstract of DOI:10.1007/s10562-019-02784-w

Abstract: A new polymer-supported highly functionalized dendritic polyether with peripheral NHC–Pd complex was developed by the simple anionic ring opening polymerization of glycidol on bis(3-hydroxypropyl) amine grafted Merrifield resin and its subsequent functional modification of hydroxyl group to NHC–Pd complex. The polymeric material had high stability and swelling capacity in a wide range of solvents, even in water. Similar to supported higher generation dendrimer, the newly developed system showed high palladium content with uniformly distributed and well isolated metal nanocentres. The palladium content was found to be 2.02?mmol/g which was comparatively higher than other heterogeneous NHC–Pd based catalysts and the polymer was used as heterogeneous palladium catalyst for Suzuki coupling reaction. Various factors affecting the catalyst’s performance were studied. The high availability and increased accessibility of catalytic sites, stability and dendrimer like property of the polymeric support, reusability, better resistance to metal leaching, easy synthesis etc. are the important features of this newly synthesized catalytic system. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-019-02784-w

Catalysis Letters
https://doi.org/10.1007/s10562-019-02784-w
Heterogeneous High‑Loading Hyperbranched Polyglycidol
with Peripheral NHC–Pd Complex: Synthesis and Application
as Catalyst in Suzuki Coupling Reaction
J. K. Anjali1 · K. Sreekumar1
Received: 8 January 2019 / Accepted: 7 April 2019
© Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract
A new polymer-supported highly functionalized dendritic polyether with peripheral NHC–Pd complex was developed by
the simple anionic ring opening polymerization of glycidol on bis(3-hydroxypropyl) amine grafted Merrifeld resin and its
subsequent functional modifcation of hydroxyl group to NHC–Pd complex. The polymeric material had high stability and
swelling capacity in a wide range of solvents, even in water. Similar to supported higher generation dendrimer, the newly
developed system showed high palladium content with uniformly distributed and well isolated metal nanocentres. The pal-
ladium content was found to be 2.02 mmol/g which was comparatively higher than other heterogeneous NHC–Pd based
catalysts and the polymer was used as heterogeneous palladium catalyst for Suzuki coupling reaction. Various factors afect-
ing the catalyst’s performance were studied. The high availability and increased accessibility of catalytic sites, stability and
dendrimer like property of the polymeric support, reusability, better resistance to metal leaching, easy synthesis etc. are the
important features of this newly synthesized catalytic system.
Graphical Abstract
Keywords Hyperbranched polymer · Suzuki coupling · N-heterocyclic carbene · Heterogeneous catalyst
Electronic supplementary material The online version of this
1 Introduction
article (https://doi.org/10.1007/s10562-019-02784-w) contains
supplementary material, which is available to authorized users.
In recent years, multifunctional branched architectures have
* K. Sreekumar
emerged as promising materials in various felds like drug
delivery, biosensors, metal adsorption, catalysis etc. [1,
2]. Dendrimers were used widely for a long period. Now,
increased attention is given to the irregular counterpart,
kskpolymer.cusat@gmail.com
1
Department of Applied Chemistry, Cochin University
of Science and Technology, Kochi, Kerala 682022, India
Vol.:(0123456789)
1 3

J. K. Anjali, K. Sreekumar
hyperbranched polymer mainly due to the ease of synthesis
and the structural similarity to dendrimers. Hyperbranched
polymers represent an attractive alternative to dendrimers
especially in the feld of catalysis [3–6]. The hyperbranched
polymers show higher fexibility than the dendritic structure,
which is one of the desired qualities for a novel support car-
rying catalytically active material.
Several N-heterocyclic carbene–palladium complexes
(NHC–Pd) were developed which have got the same σ-donor
and low π-acceptor ability compared to phosphine ligands
[27]. Besides, the NHC can form stronger bonds with major-
ity of metals and these metal complexes show high air and
moisture stability [29]. These NHC–Pd based catalysts show
better resistance to metal leaching during catalysis reaction
involving high temperature [30–32]. Polystyrene, polysily-
lethers, mesoporous silica, cellulose, etc. are some of the
reported solid supports used to immobilize NHC–Pd com-
plex [33–40].
The most studied hyperbranched polymer is polyglyc-
erol. The multipurpose applications of polyglycerol have
been revealed recently [7]. The major applications are in
drug delivery systems and also in catalysis. The biomedi-
cal application [8, 9] of polyglycerol is mainly due to its
biocompatibility, immunogenicity, oxidation stability and
low toxicity [10–12]. In organic synthesis, polyglycerol has
extreme importance as support for catalysts and reagents
[13]. The special interest for polyglycidol as support mate-
rial is mainly due to the following factors such as, the high
thermal and chemical stability, easy functionalisation, non
coordinating nature, fexibility and also the close resem-
blance with perfect dendrimer in terms of globular structure
and high functional group capacity [14–17]. Tommaso et al.
reported the application of (S)- Tyrosine based imidazolidin-
4-one grafted polyglycerol as chiral organo catalyst [18].
Polyglycerol supported fuoroalcohol is another example of
hyperbranched polymer based catalytic system [19]. Polyg-
lycerol was also used as a high-loading support for the costly
reagent boronic acid in Suzuki coupling reaction [20].
The main drawbacks of classical crosslinked resins used
for catalyst immobilization are their limited loading capac-
ity, nonavailability of the catalytic sites due to poor swelling
in desired solvent system and also indefnable incorpora-
tion of catalysts within the polymeric support [21]. Graft-
ing of hyperbranched polymer to the resin is a versatile
method for the introduction of a large number of functional
groups to the solid support within one step. Due to graft-
ing, swelling property of solid support can be modifed [22].
Haag et al. have introduced heterogeneous hyperbranched
polymers and extended their application from catalysis to
multistep organic synthesis [23]. Recently, hyperbranched
poly(phenylacetylene)-supported palladium catalyst was
introduced as a novel heterogeneous catalyst for multiple
cross-coupling reactions [24]. Except for a few polymer-
stabilized metal nanoparticle based catalytic systems, there
are not much reports on the heterogeneous hyperbranched
polymers [25, 26].
In this paper, the synthesis of a new polymer-supported
highly functionalized hyperbranched polyol by the anionic
ring opening polymerization of glycidol on bis(3-hydroxy-
propyl) amine grafted Merrifeld resin is reported. Among
the various methods of surface grafting, easy and single
step hypergrafting method is used for the heterogenization
of polyglycidol on polystyrene support [22]. Among the
two monomers, 3-hydroxymethyl-3-ethyloxetane and gly-
cidol available for the preparation of hyperbranched aliphatic
polyether polyols, glycidol was selected for the present syn-
thesis [41]. The resulting polymer-supported hyperbranched
polyglycidol was conveniently decorated with NHC–Pd
complex at the periphery and its catalytic applicability in
Suzuki cross-coupling reaction for the synthesis of biaryls
was studied.
2 Materials and Methods
Merrifield resin (Chloromethyl polystyrene, 2% DVB,
200–400 mesh size, chlorine capacity 1 mmol/g), bis (3-ami-
nopropyl)amine, glycidol, propargyl bromide and 1-methyl
imidazole were purchased from Sigma-Aldrich. Infrared
(IR) spectra of polymer samples were recorded with JASCO
FTIR 4100 spectrometer using KBr pellets. Thermogravim-
etry (TG) was carried out using Perkin Elmer Pyris Dia-
mond6 thermogravimetric/diferential thermal analyzer with
a heating rate of 10 °C min−1 under N2. Energy Dispersive
X-Ray Analysis (EDAX) was performed on Bruker with
energy resolution < 126 eV and a 30 mm2 active window
area. Field emission scanning electron microscopy (FESEM)
image of the sample was taken using the instrument Nova
nanoSEM 450, Czech Republic. Cross Polarized Magic
Angle Spinning 13C Nuclear Magnetic Resonance (CP-
MAS 13C NMR) spectra were recorded on a JEOL (ECX
400) 400 MHz instrument from IISc Bangalore. Palladium
content was analyzed by inductively coupled plasma-optical
emission spectrometry (ICP-OES) analysis with Thermo
Fischer ICAP instrument. Transmission electron micros-
copy (TEM) image of the sample was recorded using JEM
2100 HRTEM instrument. X-ray photoelectron spectros-
copy (XPS) was conducted using Kratos Axis Ultra X-ray
Palladium catalysts have been recognized as power-
ful synthetic tools in multiple organic transformations
[27]. Suzuki cross-coupling reaction of aryl halides with
arylboronic acids is one of the most important palladium
based reactions to selectively construct biaryl compounds
in organic synthesis. The polymer supported forms are
also available. Polymer-supported PCP pincer palladium
complex was a typical novel catalyst in this category [28].
1 3

Heterogeneous High-Loading Hyperbranched Polyglycidol with Peripheral NHC–Pd Complex:…
photoelectron spectroscope, UK (AIMS, Cochin). GC–MS
analysis was carried out on a 1200 L single Quadrupole,
Varian Gas Chromatograph model. 1H and 13C NMR spectra
were recorded using Bruker 400 MHz instrument.
out [42]. From these values, the amount of amino groups per
gram of the resin was calculated as 1.3 mmol/g.
3.5 Conversion of Amino Group to Hydroxyl Groups
by Diazotization (PS–D–OH)—4
3 Experimental Procedures
The amino terminated resin (1 g) was mixed with HCl
(20 mL, 2 M) and it was kept at 0 °C for 15 min. To this ice-
cold resin, chilled sodium nitrite solution (33.3 mL, 5 M)
was added in drop wise manner with constant stirring. After
the addition of sodium nitrite solution, the reaction was
allowed to continue for 1 h in ice cold condition and brought
back the reaction to room temperature. The resin beads were
washed with excess of hot water (5×10 mL), NaOH solution
(3×10 mL, 2 M), ethanol (5×10 mL), methanol (5×10 mL)
and ether (5 ×10 mL) and dried. It was qualitatively veri-
fed using ninhydrin and the yield of the resin obtained was
0.93 g (93%). The hydroxyl group capacity of the resulting
resin was 1.28 mmol/g.
3.1 Synthesis of Schif’s Base of Bis(3‑aminopropyl)
amine‑1
The Schif’s base of bis(3-aminopropyl)amine was prepared
by mixing one molecular equivalent of bis(3-aminopropyl)
amine (1.4 mL, 10 mmol) with two molecular equivalents
of benzaldehyde (2.1 mL, 20 mmol) at room temperature.
The reaction was continued until the appearance of an oily
viscous liquid and it was used as such for further reaction
without any purifcation [42].
3.2 Incorporation of Schif’s Base to Merrifeld Resin
(PS–D–PNH2)—2
3.6 Hypergrafting of Glycidol on Hydroxy
Terminated Resin (Het–PG)—5
Schif’s base incorporated resin was prepared by adding the
viscous Schif base (2.32 mL, 80 mmol) and triethylamine
(250 µL) to pre-swelled chloromethylated PS–DVB resin
(1 g, 1 mmol/g) in 1,4-dioxane and the mixture was heated
at 100 °C for 48 h with occasional stirring. The solid materi-
als were fltered of, washed well with water and dioxane.
After soxhlet extraction with dioxane for 48 h, the resin was
washed with ether and dried under vacuum. The yield of
Schif’s base incorporated resin obtained was 1.14 g (87%).
Hydroxy terminated resin beads (2.25 g) were suspended in
dry diglyme (60 mL), taken in a 500 mL three-necked fask.
After swelling for 1 h, the solution of potassium tertiary
butoxide (2 g) in THF was added to deprotonate the hydroxyl
groups. The mixture was stirred at 40 °C over 12 h. THF
and the byproduct tert-butyl alcohol were distilled of. After
adding dry DMF (120 mL) to the residue in RB fask, the
temperature was raised to 120 °C and a solution of freshly
distilled glycidol (45 mL) in dry DMF (75 mL) was added
drop wise to the reaction mixture. The mixture was stirred
for 12 h at 120 °C and neutralized with HCl (1 M). The
solid material was fltered, washed with water and MeOH
and after soxhlet extraction with methanol, it was dried
under vacuum to give pale yellow resin beads with a yield
of 13 g. The hydroxyl group capacity of the resulting resin
was 15.3 mmol/g.
3.3 Deprotection of Amino Group by Schif’s Base
Cleavage (PS–D–NH2)—3
The phenyl groups of Schif base resin was removed by stir-
ring the resin (1 g) with hydrochloric acid (6 M) at 60 °C
for 12 h. After the reaction, the yellow coloured resin
beads formed were fltered, washed with NaOH solution
(5 × 7.5 mL, 1 M) and with water (5 × 7.5 mL) until the
solution was neutral. The polymer beads formed were suc-
cessively washed with methanol and ether and dried under
vacuum. The amino group loading capacity obtained was
1.3 mmol/g. The yield of amino terminated resin was 96%.
3.7 Determination of Hydroxyl Group Capacity
The resin 100 mg was acetylated with standard acetylat-
ing agent acetic anhydride-piperidine mixture (1:4, 3 mL)
for 6 h. After adding distilled water (10 mL), the mixture
was refuxed for an additional 3 h to ensure the complete
hydrolysis of the unreacted acetic anhydride. The mixture
was cooled and fltered. The acetic acid formed was back
titrated with standard NaOH solution (0.1 M) using phenol-
phthalein as indicator. A blank titration was also carried out
[43]. From the titre values hydroxyl group capacity of the
resin was found to be 15.3 mmol/g.
3.4 Estimation of Amino Group Capacity
The amino terminated resin (250 mg) was suspended in HCl
(40 mL, 0.1 M) for 24 h with occasional stirring. The resin
was fltered and washed well with distilled water. The fltrate
and washings were collected. The unreacted HCl was esti-
mated by titration against standard Na2CO3 solution using
methyl orange indicator. A blank titration was also carried
1 3

J. K. Anjali, K. Sreekumar
vacuum to give the black Het–PG–NHC–Pd complex with
a yield of 0.68 g.
3.8 Synthesis of Heterogeneous O‑Mesyl
Polyglycidol (Het–PG–OMS)—6
3.12 General Procedure for Suzuki Coupling
Reaction
Polymer-supported hyperbranched polyglycidol (3.5 g,
15.3 mmol/g—OH group) was suspended in pyridine
(70 mL), taken in a two-necked RB fask. After cooling the
mixture to 0 °C using an ice/NaCl bath, fve equivalents
of mesyl chloride (20.9 mL) was added dropwise without
increase in temperature. The mixture was stirred overnight
at room temperature. After 24 h it was fltered and the
resin was washed with water and acetone and it was dried
to obtain 5.80 g (77%) honey coloured resin.
The aryl halide (1 mmol), phenyl boronic acid (1.2 mmol),
K2CO3 (3 mmol), catalyst (Het–PG–NHC–Pd) (8 mg) and
the solvent DMF: water (1:1, 5 mL) were added to a 10 mL
RB fask and it was stirred at 80 °C. The progress of the
reaction was monitored by thin layer chromatography (TLC).
After the completion of the reaction, catalyst was fltered and
the reaction mixture was added to brine and extracted with
diethyl ether. The isolated products were characterized using
GCMS and 1H NMR.
3.9 Synthesis of Heterogeneous Polyglycidylazide
(Het–PG–N3)—7
4 Results and Discussion
Polymer supported O-mesyl polyglycidol (5.8 g,
53.5 mmol) was suspended in dry DMF (60 mL) taken in
a 500 ml one-necked fask with refux condenser. To the
mixture, fve equivalents of NaN3 was added and it was
heated at 80 °C for 3 days. After cooling it was fltered,
washed with water, methanol and chloroform. It was dried
to obtain honey coloured resin with a yield of 83%.
4.1 Preparation and Characterization of Catalyst
Merrifeld resin based solid phase methodology was used
for the entire reaction sequence. Synthetic procedure is sum-
marized in Scheme 1. The bifurcated ligand Bis(3-amino-
propyl)amine was introduced to the resin in order to increase
the functional group capacity and to increase the accessibil-
ity of functional group by extending the active site from
the hydrophobic polystyrene-DVB matrix. Since the direct
attachment of the diamine cause many unwanted coupling
through diferent amino groups in the ligand, the diamine
Bis(3-aminopropyl)amine was converted to its Schif base
by treatment with benzaldehyde for protecting the termi-
nal amino groups and it was incorporated to chloromethyl-
ated PS–DVB resin (chlorine loading 1 mmol g−1) uniquely
through secondary amine of Schif’s base ligand moiety by
heating at 100 °C in the presence of triethylamine. Protec-
tion of amino group was removed by stirring with 6 M HCl
at 60 °C for 12 h. The amine capacity of the resulting resin
(PS–D–NH2) was determined by volumetric method and it
was found to be 1.3 mmol g−1. The terminal amino group
was converted to hydroxyl group by diazotization reaction.
(Scheme 2).
3.10 Synthesis of Imidazolium‑Loaded
Heterogeneous Polyglycidol (Het–PG–IM)—8
N-methyl-N´-propargyl-imidazolium bromide was syn-
thesized by heating the mixture of N-methylimidazole
(100 mmol) and propargyl bromide (80% in toluene,
120 mmol, 1.2 eq.) in dry DMF (25 mL) at 80 °C for 12 h.
To the cooled reaction mixture, heterogeneous polyglycer-
ylazide (1 g), copper(I) iodide (0.2 eq), and DIPEA (5 mL)
were added along with DMSO–THF (1:1) (10 mL). The
suspension was stirred at 40 °C for 24 h. The resin was
collected by fltration and it was washed with, acetoni-
trile, pyridine, MeOH and CH2Cl2. After drying yellow
coloured resin was obtained with a yield of 96%.
Each stage of the resin modifcation can be easily veri-
fied using IR spectrum of respective resin as shown in
Figs. 1 and 2. The attachment of schifs base of Bis(3-ami-
nopropyl)amine which is a golden yellow viscous liquid
to the resin can be easily verifed by the presence of peak
at 1640 cm−1 corresponding to (–C=N–) in the IR spec-
trum of the resin PS–D–PNH2. The formation of amine
terminated resin after Schif’s base cleavage was noted by
the absence of peak at 1640 cm−1 (–C=N–) and an addi-
tional peak appear at 3424 cm−1 corresponding to (–NH2)
in the IR spectrum (PS–D–NH2). IR spectrum of the resin
3.11 Synthesis of NHC–Pd Complex Loaded
Heterogeneous Polyglycidol (Het–PG–NHC–
Pd)—9
Imidazolium-loaded heterogeneous polyglycidol (0.5 g)
was added to a solution of Pd(OAc)2 (0.50 g, 2.2 mmol) in
dry DMF and it was stirred in a RB fask at 80 °C for 24 h.
After cooling to room temperature, the reaction mixture
was fltered and the solid product was washed with DMF,
MeOH, and ether. The resulting material was dried under
1 3

Heterogeneous High-Loading Hyperbranched Polyglycidol with Peripheral NHC–Pd Complex:…
Scheme 1 Synthesis of polystyrene supported NHC-Pd complex
(PS–D–OH) after diazotization reaction showed a shift in
peak position to 3470 cm−1 which indicated the conver-
sion of –NH2 to –OH group. Complete conversion was
confrmed by the negative ninhydrin test with the resin. In
order to increase the loading capacity of the resin to a higher
extent, anionic polymerization of glycidol was carried out
on –OH terminated resin (PS–D–OH). This heterogene-
ous hypergafting method of glycidol on the resin support
allowed easy separation of soluble polyglycerol formed by
self polymerization of glycidol. In order to get high con-
version during surface grafting, the complete metalation of
the initiator (PS–D–OH) followed by very slow monomer
1 3

J. K. Anjali, K. Sreekumar
Scheme 2 Suzuki coupling reaction
Fig. 2 IR spectra of various stages of modifcation of heterogeneous
hyperbranched polyglycidol
volumetric method using acetic anhydride and it was found
to be 15.3 mmol/g. The IR spectrum (Het–PG) showed sharp
peaks at 1100 cm−1, and 3400 cm−1 corresponding to ether
linkage and hydroxyl group in the resin. Successful polymer
grafting was further confrmed by the sudden increase in
oxygen content in EDAX data as shown in Fig. 3.
Fig. 1 IR spectra of various stages of Het–PG synthesis
addition method was used for the synthesis of heterogene-
ous hyperbranched polyglycidol (Het–PG). Almost 6 times
increase in weight was observed after polymerization and
the resulting resin (Het–PG) showed high swelling capac-
ity in common organic solvents, especially DMF, methanol
and water. So a core shell type structure was expected for
(Het–PG) with a small crosslinked polystyrene core (17%
by weight) and a large hydrophilic polyglycidol shell (83%
by weight) which extended out in polar reaction media. The
crosslinked polystyrene core prevents the complete disso-
lution in solvents and (Het–PG) can be used as a normal
solid support for solid phase organic synthesis. The large
polyglycidol shell facilitates the homogeneous like reaction
in polar medium. Due to dense packing of polyglycidol on
polystyrene resin it may have a three dimensional globular
structure similar to supported higher generation dendrimer.
The hydroxyl group capacity of the resin was determined by
CP–MAS 13C NMR spectrum gave an insight to the
structure of hypergrafted resin (Het–PG). In CP–MAS 13
C
NMR spectrum of the resin (Het–PG), as shown in Fig. 5,
the sharp peaks at 128 ppm and the peak at 145 ppm were
attributed to aromatic carbon of benzene in the polystyrene
backbone. The broad signal at 30–52 ppm was due to the
combined signal of methylene of the polystyrene backbone,
and the two carbon atoms of bis-(3-aminopropyl)amine
segment. The peaks in between 60–80 ppm was assigned
to aliphatic carbon of polyglycerol graft (63.9 ppm—CH2
of primary terminal alcohol, 71.6 ppm—CH of secondary
terminal alcohol, 70.0 ppm—CH of secondary alcohol in
the chain, and 73.2 ppm—neighboring carbon to secondary
alcohol in the chain, 80.3 ppm–CH near to primary alcohol
participated in branching) as well as methylene carbon in
between benzene ring and tertiary amine [23]. The FESEM
1 3

Heterogeneous High-Loading Hyperbranched Polyglycidol with Peripheral NHC–Pd Complex:…
Fig. 3 a EDS data of various
(b)
(a)
stages of Het-PG synthesis, b
EDS spectrum of Het-PG
C
N
O
image of the resin (Fig. 8) showed agglomerated spherical
particles and the surface roughness of spherical particle was
an evidence for hypergrafting. From the TG result, as in
Fig. 8, it is clear that the resin Het–PG was thermally stable
up to 350 °C.
In the IR spectrum of Pd complex, Het–PG–NHC–Pd, the
intensities of the bands corresponding to the imidazolium
units (1164 cm−1 and 623 cm−1) were sharply reduced
after reaction with palladium acetate which indicated the
formation of NHC–Pd complex. The palladium content in
the EDAX data of Het–PG–NHC–Pd (Fig. 4) gave a strong
evidence for the formation of palladium complex.
Het–PG was converted to its azide form by a two step
process. The hydroxyl group of Het–PG was activated
using methyl sulfonyl chloride in pyridine and the result-
ing Het–PG–OMS was easily transformed to its azide form
Het–PG–N3 using excess NaN3. The bands at 1359 cm−1
and 1177 cm−1 corresponded to the O=S asymmetric
and symmetric stretching vibration in the IR spectrum of
Het–PG–OMS, clearly indicated the conversion of hydroxyl
group to –OMS group [44]. The IR spectrum of azide ter-
minated polystyrene Het–PG–N3 showed a sharp peak at
2100 cm−1 which clearly indicated the conversion of pol-
ymer-supported polyol (Het–PG) to azide terminated resin
Het–PG–N3. Propargyl imidazolium bromide was syn-
thesized and it was attached to azide terminated support
(Het–PG–N3) using click chemistry and the attachment was
identifed by the absence of the peak corresponding to azide
group (2100 cm−1) in the IR spectrum of respective mate-
rial (Het–PG–Im). The additional peaks such as 1579 cm−1,
1164 cm−1, in the IR spectrum of Het–PG–Im were due to
the stretching vibration of N–sp2 C bond, N–sp3 C bond in
imidazole ring and 623 cm−1 was due to the vibration of the
imidazole ring [44]. Bromine content in the EDAX spec-
trum of Het–PG–Im (Fig. 4) also gives an evidence for the
successful attachment of propargyl imidazolium bromide to
Het–PG–N3. Incorporation of propargyl imidazolium bro-
mide was also verifed using CP–MAS 13C NMR spectrum.
In the CP–MAS 13C NMR spectrum (Fig. 5), the peaks at
142.7 ppm,140.9 ppm, 137.2 ppm and 126.4 ppm were
assigned to carbons present in imidazole and triazole rings
in the system [45, 46]. The resulting Het–PG–Im was coor-
dinated with palladium using palladium acetate solution in
DMF to give the fnal catalyst Het–PG–NHC–Pd complex.
From the ICP-OES analysis, the palladium content was
found to be 2.02 mmol/g which was comparatively higher
than other reported heterogeneous NHC–Pd catalysts. The
palladium content in various reported polystyrene based het-
erogeneous catalysts is summarized in Table 1. It is clear
that the functional group capacity of the resin-supported
hyperbranched polyether was similar to the supported higher
generation dendrimer, synthesized by tedious multiple steps
and the palladium content in the present support was almost
10 times higher than the polystyrene-supported NHC–Pd
system.
Low level Imidazolium loaded polystyrene itself shows
very good swelling capacity in polar solvents, especially, in
water [35]. The present catalyst showed dramatically high
swelling capacity in water. This may be attributed to the
high polyglycerol content due to hypergrafting and subse-
quent high imidazolium loading on polystyrene-supported
polyglycidol [23, 33, 35–37]. So the present system is very
suitable for water based catalysis.
The high resolution transmission electron microscopy
(HRTEM) images (Fig. 6) of the catalyst Het–PG–NHC–Pd
showed uniformly distributed and well isolated palladium
nanocentres of about 6 nm size on the peripheral polygly-
cidol shell. The TEM image showed spherical nanoparticles
of uniform size and shape similar to the palladium nano-
particle generated by homogeneous polyglycidol. But the
size was slightly higher than that generated by homogeneous
polyglycerol (2.4 nm) [17]. This indicated that the palladium
coordination site in the new support Het–PG–Im was well
solvated and uniformly exposed to the reagents similar to
1 3

J. K. Anjali, K. Sreekumar
Fig. 4 EDS spectra of Het-PG-Im and Het-PG-NHC-Pd
its homogeneous counterpart. This observation pointed out
to homogenous like catalysis in polar solvent. Similar to
supported higher generation dendrimer the large portion of
polyglycidol graft in the present system not only contributes
to palladium content, size and shape of nanocentre but also
acts against aggregation and leaching of metal centres [25,
47].
palladium (2) was reduced to palladium (0) by the presence
of polymeric backbone [17, 44]. Quantitative estimation of
palladium content in the sample using XPS analysis also sup-
ported the ICP-OES result.
Thermogravimetry (TG) was carried out to study the ther-
mal stability of Het–PG–NHC–Pd (Fig. 8). The weight loss on
heating from 40 to 150 °C might be attributed to the release
of physisorbed water and loosely bound volatiles on the sur-
face. Thermal degradation of NHC–Pd complex occurred after
250 °C which implied the stability.
As shown in Fig. 7, the XPS spectrum of the catalyst
Het–PG–NHC–Pd showed four binding energy peaks at
335.5 eV, 340.8 eV, 339.5 eV, 342.1 eV which were assigned
to the electron transitions of Pd(0) 3d5/2, Pd(0) 3d3/2, Pd(2)
3d5/2, and Pd(2) 3d3/2 respectively [44]. The XPS spectrum
clearly suggests that palladium (0) is also present along
with palladium in +2 oxidation state. That means some of
Fig. 5 a CP-MAS 13C NMR spectrum of Het-PG resin, b CP-MAS 13C NMR spectrum of Het-PG-Im
1 3

Heterogeneous High-Loading Hyperbranched Polyglycidol with Peripheral NHC–Pd Complex:…
Table 1 Palladium content in
Sl. No.
Reported polystyrene based heterogeneous palladium catalysts
Palladium
content
various reported polystyrene
based heterogeneous catalysts
(mmol/g)
1.
2.
3.
4.
5.
6.
7.
8.
Polystyrene-supported NHC–Pd complex
0.29 [35]
0.11 [36]
0.1 [33]
0.1 [38]
0.11 [37]
0.05 [37]
1.12 [47]
2.02
N-Heterocyclic carbene–palladium complex on polystyrene resin surface
Polystyrene-Supported N-Heterocyclic Carbene–Palladium Catalyst
Polystyrene with 6-chlorohexanol spacer supported NHC–Pd complex
Amphiphilic polymer(Polystyrene-PEG 200) supported NHC–Pd complex
Amphiphilic polymer(Polystyrene-PEG 600) supported NHC–Pd complex
Polystyrene-supported G3 PAMAM-Pd(II) complexes
Het–PG–NHC–Pd (Present catalyst)
Fig. 6 TEM images of the cata-
lyst Het-PG-NHC-Pd (a–d)
1 3

J. K. Anjali, K. Sreekumar
Fig. 7 a XPS wide spectrum of catalyst Het-PG-NHC-Pd, b XPS deconvoluted spectrum of Pd
Fig. 8 a TG data of Het-PG and the catalyst Het-PG-NHC-Pd, b FESEM image of Het-PG
investigation, the reaction was carried out in the absence of
catalyst under refux condition. No product was obtained in
the absence of catalyst. The reaction was carried out in the
presence of catalyst and after various trials, as in Table 2,
the amount of catalyst was optimized as 8 mg. Due to high
palladium content in the catalyst, small amount of catalyst
was enough for the reaction.
4.2 Catalysis by Polymer Supported Dendritic
NHC–Pd Complex
Considering all the favourable factors of the new material
such as high swelling capacity in polar medium, high pal-
ladium content, dendrimer like structure and its well iso-
lated palladium nanocentres etc., the catalytic activity of the
Het–PG–NHC–Pd in Suzuki coupling reaction was investi-
gated by taking the coupling between 4-bromoanisole and
phenylboronic acid as the model reaction. As a preliminary
The efect of temperature on the reaction was studied
(Table 3). When the reaction was carried at room tem-
perature, the yield was very low. To increase the yield, the
1 3

Heterogeneous High-Loading Hyperbranched Polyglycidol with Peripheral NHC–Pd Complex:…
Table 2 Optimization of amount of catalyst
Table 5 Suzuki coupling reaction of various substrates
Amount of catalyst (mg) Amount of catalyst (mol%) Yield (%)a
Entry
X
Y
R
Yield (%)a,b
0
5
7
8
–
–
1
2
3
4
5
6
7
8
9
10
Cl
Br
I
H
H
H
H
H
H
H
H
H
H
H
H
H
84
93
95
93
93
92
91
87
85
95
1.0
1.4
1.6
2.0
70
85
90
90
I
4-NO2
4-COOEt
4-CHO
4-CN
4-OH
4-NH2
4-OMe
10
Br
Br
Br
Br
Br
Br
Reaction conditions: 4-bromoanisole (1 mmol), Phenylboronic acid
(1.2 mmol), 80 °C, DMF, 3 h, K2CO3 (3 mmol), aGC yield
Table 3 Optimization of
Temperature (°C) Yield (%)a
Reaction conditions: aAryl halides (1 mmol), Phenylboronic acid
(1.2 mmol), 80 °C, DMF:H2O (`1:1), 3 h, K2CO3 (3 mmol), catalyst
(8 mg), bisolated yield
temperature of reaction
RT
60
22
75
90
90
80
100
Table 6 Recycling study
No.of cycles
Yield (%)a,b
Reaction conditions: 4-bro-
moanisole (1 mmol), Phe-
nylboronic acid (1.2 mmol),
catalyst 8 mg, DMF, 3 h, K2CO3
(3 mmol), aGC yield
1
2
3
4
93
92
90
88
Reaction condition: aBromoben-
zene (1 mmol), Phenylbo-
ronic acid (1,2 mmol), 80 °C,
DMF:H2O (`1:1), 3 h, K2CO3
Table 4 Infuence of solvent and base in Suzuki coupling reaction
Yield (%)a
b
Entry
Solvent
Base
(3 mmol), catalyst (8 mg), iso-
lated yield
1
2
3
4
5
6
DMF
DMF
DMF
DMF
DMF–H2O(1:1)
DMF–H2O(2:1)
Na2CO3
K2CO3
CsCO3
Et3N
K2CO3
K2CO3
82
90
85
78
95
92
in DMF, DMF was selected for the present study. The reac-
tion was performed in the presence of diferent bases such
as Na2CO3, K2CO3, CsCO3, Et3N etc. The best result was
obtained in the presence of K2CO3. Water has an important
role in the reaction. As the water content increased, yield
also increased and the maximum yield was obtained at the
water to DMF volume ratio 1:1. This may be attributed to
better solubility of base and phenyl boronic acid in the sol-
vent mixture. The results are summarized in Table 4.
With the optimised protocol for the Suzuki cross-coupling
reaction in hand, the scope of the reaction was explored by
using various aryl halide substrates. The results are given in
Table 5. All the selected arylhalides with electron-rich and
electron-poor groups were converted to the corresponding
biaryls with excellent yields. From the table, it is clear that
aryl iodide showed better reactivity than aryl bromide and
chlorides and the reaction proceeded smoothly even in the
presence of other functional groups in the substrate. The
electron donating and withdrawing groups in the substrate
had no special efect on the reaction yield. All the products
were characterized and compared with literature data. The
details are given as supporting information.
Reaction conditions: 4-bromoanisole (1 mmol), Phenylboronic acid
(1.2 mmol), catalyst 8 mg, 3 h, 80 °C, Base (3 mmol), aGC yield
reaction was conducted at higher temperature conditions.
The temperature for the reaction was optimized as 80 °C and
the reaction was completed within 3 h. The rapid reaction
may be due to the high accessibility of catalytically active
sites which were attached at the periphery of heterogeneous
hyperbranched polyglycidol. The polyglycidol part might
protrude out from the dense crosslinked polymeric matrix
as a high functional group containing network to reaction
media making it available for the adsorption of large number
of reactants at a time.
The infuence of solvent and base in Suzuki coupling
reaction was studied. Since most of the Suzuki coupling
reactions were reported in polar solvents, especially, DMF
and the prepared catalyst showed very good dispersibility
1 3

J. K. Anjali, K. Sreekumar
Table 7 Detailed comparison of the present catalyst with the analogous Polystyrene supported and Polystyrene-PEG supported NHC–Pd based
catalysts for Suzuki–Miyaura cross coupling reaction
Sl no Reactants
Catalyst
Catalyst Amount
(mol%) of catalyst
(mg)
Time (h) Yield (%) References
1
2
3
Phenyl boronic acid and 4-methoxybromobenzene PS–NHC–Pd
1.2
2
1.6
41.3
400
8
12
12
3
68.5
76
95
[35]
[37]
This work
Phenyl boronic acid and 4-methoxyiodobenzene
PS–PEG(600)NHC–Pd
Phenyl boronic acid and 4-methoxybromobenzene Het–PG–NHC–Pd
In order to establish the heterogeneous nature of the cata-
5 Conclusion
lyst, 150 mg catalyst was frst heated to 80 °C for a period
of 3 h in the solvent (DMF: Water, 1:1) and the fltrate was
used for the reaction. It was noted that at least 1 mol% cata-
lyst was enough for considerable reaction. No product was
obtained during that attempt which clearly indicated the
heterogeneous nature of the catalyst. After each reaction,
the catalyst was recycled by washing it with ethyl acetate
and methanol followed by drying at room temperature under
vacuum. From the reusability study of the catalyst, as shown
in Table 6, it is clear that, the catalyst can be reused at least
four times without considerable loss of activity. As per ICP-
OES analysis, the amount of palladium present in the cata-
lyst at the initial stage and after 4th cycle was 21.54% and
21.52% respectively. The result indicated a considerably low
palladium leaching of 0.1% (That means only 0.1% of total
palladium content in the catalyst was leached out after 4th
cycle). As expected, N-heterocyclic carbene ligand provided
better resistance to metal leaching and similar to dendrimers,
the globular hypebranched architecture can also produce iso-
lated palladium centers which prevent aggregation leading
to poor catalytic activity during repeated cycles [25, 29–32,
47]. The catalyst do not demand special inert atmosphere for
the reaction process.
A new polymer-supported hyperbranched polyglycidol
decorated with NHC–Pd complex at the periphery based
catalytic system was developed and its catalytic activity
in Suzuki cross coupling reaction was studied. The hybrid
catalyst combined the beneft of crosslinked polystyrene
such as easy recovery, and also that of hyperbranched poly-
ether such as high functional group density within one step,
increased accessibility of functional sites, etc. The polygly-
cidol shell grafted on polystyrene contributed to dendritic
structure and properties and also to water compatibility.
The resulting catalyst was found to be highly competent
for the synthesis of biaryl derivatives via Suzuki reaction
under mild conditions. Furthermore, the catalyst exhibited
excellent thermal stability, chemical stability and reproduc-
ibility without metal leaching. All the special properties of
the new polymer-supported dendritic NHC–Pd catalyst such
as high swelling capacity in polar medium, high palladium
content, dendrimer like structure and its well isolated palla-
dium nanocentres etc. were positively refected in catalysis.
The present supported dendritic NHC–Pd complex signifes
a novel class of highly loaded heterogeneous catalyst with
wide range of solvent compatibility necessary to facilitate a
more or less homogeneous like catalysis.
The special efects of hypergrafted polyether in the new
catalyst Het–PG–NHC–Pd were refected in catalysis as high
yield, low reaction time, low amount of catalyst and high
reusability etc. compared to other polystyrene-supported
NHC–Pd based catalysts. It is clear from the Table 7, show-
ing the detailed comparison of the present catalyst with the
analogous polystyrene supported and PS–PEG supported
NHC–Pd based catalysts. The catalytic activity of most of
polystyrene supported NHC–Pd based catalysts decrease
drastically after frst run due to high palladium leaching
[35, 36].
6 Supporting Information
1H NMR, 13C NMR and, GC–MS data of compounds.
Acknowledgements The authors gratefully acknowledge STIC-
CUSAT, IISc- Bangalore, and NCESS-TVM, for the assistance in
various spectral analysis. One of the authors (K. J. A) acknowledges
the CSIR, Govt. of India for fnancial assistance.
The general three stage mechanism (oxidative addition,
transmetallation, reductive elimination) of Suzuki coupling
reaction is expected and NHCs facilitate both the initial oxi-
dative addition and the fnal reductive elimination steps of
the catalytic cycle [48].
Compliance with Ethical Standards
Conflict of interest The authors declare that they have no confict of
interest.
1 3

Heterogeneous High-Loading Hyperbranched Polyglycidol with Peripheral NHC–Pd Complex:…
27. He Y, Cai C (2011) Chem Commun 47:12319
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