Preparation of polymer anchored Pd-catalysts: Application in Mizoroki-Heck Reaction

Article abstract of DOI:10.1016/j.molcata.2010.08.023

A series of polymer anchored Schiff bases were prepared from chloromethylated styrene-divinylbenzene copolymer beads and loaded with PdCl₂ to get air and water stable supported catalysts. The catalysts were characterized and utilized in Mizoroki-Heck reaction of aryl bromides and iodides.

Full text of DOI:10.1016/j.molcata.2010.08.023

Journal of Molecular Catalysis A: Chemical 332 (2010) 70–75
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Preparation of polymer anchored Pd-catalysts: Application in Mizoroki–Heck
Reaction
S.A. Patela, K.N. Patelb, S. Sinhaa, B.V. Kamathb,∗, A.V. Bedekarb,∗
a Gujarat Alkalies and Chemicals Limited, Vadodara 391 346, India
b Department of Chemistry, Faculty of Science, M. S. University of Baroda, Vadodara 390 002, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 April 2010
Received in revised form 23 August 2010
Accepted 26 August 2010
A series of polymer anchored Schiff bases were prepared from chloromethylated styrene–divinylbenzene
copolymer beads and loaded with PdCl2 to get air and water stable supported catalysts. The catalysts were
characterized and utilized in Mizoroki–Heck reaction of aryl bromides and iodides.
© 2010 Elsevier B.V. All rights reserved.
Keywords:
Heterogeneous catalysis
Phosphine free polymer supported ligand
Aqueous Mizoroki–Heck reaction
Recyclable catalyst
1. Introduction
ane [20,21], self-supported polymeric catalysts [22], nano-particles
[23], metals loaded on clays and zeolites [24,25], metal oxides,
Development of chemical transformations utilizing environ-
mentally compatible and safe methodologies has gained renewed
importance. Many efforts are underway to modify the existing
important reactions to meet ever increasingly stringent envi-
ronmental norms. In this effort, immobilization of toxic metal
complexes on polymer supports and explore their use in many
reactions is of vital consideration [1,2]. The distinct advantage
of polymer anchored metal catalysts in conventional reactions is
of several folds: simple methods of immobilization, easy recov-
erability, reusability, effortless extraction, high activity, solvent
compatibility, economy when costly metals are used etc. Some of
the metals used for catalytic reactions are toxic or undesirable in
sensitive products such as pharmaceutical intermediates where
the use of heterogeneous catalysts can help to keep the levels of
their contamination low and to the acceptable level. The concept
of polymer supported metal complexes is widely explored by sev-
eral researchers and the subject is extensively covered in the recent
literature [3–9]. Since the first report by Merrifield on the concept of
polymer supported peptide synthesis [10] several significant devel-
opments have taken place to use a host of polymeric materials as
supports for synthesis and catalysis [11–18]. These include the use
of soluble polymer supports [13,14], dendrimers [18,19], polysilox-
mesoporous materials [7], etc.
Polymer prepared by crosslinking of polystyrene and divinyl-
benzene remains an easily available and well studied support for
facile functionalization and hence a support of choice by many
groups of researchers. In this paper we report our findings on
the synthesis of polymer bound Schiff base ligands, their palla-
dium complexes and applications of the Pd-loaded catalysts for
Mizoroki–Heck coupling reaction [26]. There are a few reports on
polymer bound phosphine ligands [27–29], but very few of phos-
phine free N,N type polymer attached Pd complexes have been
developed for this reaction [30].
2. Experimental
2.1. Material
Chloromethylated poly(styrene–divinyl benzene) copolymer
spherical beads (0.3–1.2 mm) with 5 or 8% crosslink were sup-
plied by Ion-Exchange (India) Limited, Mumbai. Palladium chloride,
1,3-diaminopropane, 4,4ꢀ-diaminobiphenyl and iodobenzene were
obtained from Aldrich and used without further purification.
Styrene (Fluka) was distilled before use, 4-iodo anisole was pre-
pared from 4-anisidine [31], and other chemicals and solvents of
analytical grade were purchased from local suppliers.
2.2. Preparation of polymer anchored chelating ligand
∗
Corresponding authors. Tel.: +91 265 2795552.
E-mail addresses: kamathbv1@rediffmail.com (B.V. Kamath),
Commercial sample of chloromethylated resin beads were first
purified by soxhlet extraction with methanol to remove the sol-
avbedekar@yahoo.co.in (A.V. Bedekar).
1381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.08.023

S.A. Patel et al. / Journal of Molecular Catalysis A: Chemical 332 (2010) 70–75
71
1
Cl
THF
H2N
NH2
THF
H2N
H2N
3
2
N
H
N
H
NH2
4
NH2
5
CHO
OH
PhMe, 80 oC, 6 h
PhMe, 80 oC, 6 h
6
N
N
N
H
H
HO
7
8
N
HO
Scheme 1. Synthesis of polymer anchored Schiff bases.
uble impurities. Purified resin was then stirred for several hours
with appropriate quantity of diamines, viz 1,3-diaminopropane 2
and 4,4ꢀ-diaminobiphenyl 3 dissolved in tetrahydrofuran such that
one of the amino groups is alkylated and other remained free. The
beads of product 4 or 5 obtained from the two reactions were
separated and washed carefully with the same solvent and then
successively with methanol, deionised water, dioxane and again
with dry methanol and dried at 50 ◦C under reduced pressure. Hav-
ing established the presence of amino group by IR spectral analysis
samples of 4 and 5 were heated with excess of salicylaldehyde 6 in
toluene at 80 ◦C [32]. The formation of Schiff base anchored onto
the polymer 7 and 8 was established by analytical and IR analysis.
estimated using Perkin-Elmer Atomic Absorption Spectrometer,
Model Zeeman ZL-4100, for which a known weight of catalyst was
digested with conc. HCl and then diluted to constant volume for the
analysis. Surface area of the polymer beads before and after com-
plexation with metal was determined using the BET method with
a Carlo-Erba Surface Area Analyzer. The IR spectra in the range of
50–4000 cm−1 were recorded on a Nicolet Manga-550 Spectropho-
tometer. Thermogravimetric analyses of the catalysts were carried
out on Shimadzu DT-30 Analyzer at a heating rate of 10 ◦C min−1
up to 600 ◦C under the atmosphere of nitrogen. Diffuse reflectance
spectra(200–400 nm)were recordedon aShimadzu UV-240 instru-
ment using optical grade BaSO4 as the reference. Scanning electron
micrographs of the catalysts and the supports were taken on Jeol
JSM-T 300 instrument. Swelling behavior of the freshly prepared
catalysts in polar and non-polar solvents and their bulk densities
at 27 ◦C were measured as described previously [33].
2.3. Loading of Pd(II) on the polymer anchored ligand
The polymer beads of 7 or 8 were kept in ethyl alcohol for one
hour in a round bottom flask. These were then treated with 1%
(w/v) solution of PdCl2 in the same solvent, initially at 50 ◦C for
45 min with occasional shaking and then left on mechanical shaker
for 15 days at room temperature. After this time the color of beads
changed to brown giving a preliminary indication of loading of the
metal on the resin giving polymer anchored Pd-catalyst beads. The
beads were separated, washed with ethyl alcohol and dried under
reduced pressure at 60–65 ◦C for 24 h. By this procedure the fol-
lowing polymer anchored Pd-catalysts were prepared: A-1 and B-1
with 5% crosslink and A-2 and B-2 with 8% crosslink, where A-1 and
A-2 were obtained from 7 and B-1 and B-2 are from 8.
The structures of various stilbenes obtained by catalytic cou-
pling reaction were established by comparing the m.p. with known
or reported values and H NMR spectra.
2.5. Procedure for catalytic Heck reaction
The reaction of aryl halide and olefin, usually attached with
an electron withdrawing group in presence of Pd-catalyst and a
suitable base, Mizoroki–Heck reaction, is an extremely important
method of synthesis of substituted stilbene and cinnamic acid
derivatives [34–36]. Separation of the used catalysts for further
reactions is crucial for viable applications. Also the contamination
of palladium in the sensitive products becomes a matter of con-
cern. The variants of catalysts A and B were taken in solvent and
allowed to swell for 30 min, and charged with the reagents for
Mizoroki–Heck reaction. Two solvent systems were identified: i.
water with small quantity of cetyltrimethylammonium bromide
(CTAB) as promoter and ii. dimethylacetamide (DMA) for this reac-
tion, while the third solvent toluene was not very effective. The
mixture is heated for several hours and the progress of reaction
The present study involves characterization and screening of
these four catalysts for the Mizoroki–Heck reaction of aryl halides
with styrene to prepare stilbene derivatives.
2.4. Analysis of catalyst
Elemental analysis of the polymer anchored Schiff bases and
the corresponding Pd-catalysts were carried out on Coleman Ana-
lyzer. The metal contents of the polymer anchored catalysts were

72
S.A. Patel et al. / Journal of Molecular Catalysis A: Chemical 332 (2010) 70–75
N
N
N
H
H
HO
7
8
N
HO
PdCl2, EtOH, r.t. 15 d
PdCl2, EtOH, r.t. 15 d
Catalyst A-1 (5 % cross linking in 1)
Catalyst A-2 (8 % cross linking in 1)
Catalyst B-1 (5 % cross linking in 1)
Catalyst B-2 (8 % cross linking in 1)
Scheme 2. Loading of Pd on polymer supported Schiff bases.
B-1: 5% poly(S-DVB)Pd(II)(4,4ꢀ-DABP-SB)
B-2: 8% poly(S-DVB)Pd(II)(4,4ꢀ-DABP-SB)
Catalyst
CTAB (10 %)
I
K2CO3
+
3.1. Catalyst characterization
100 oC, 40 h
Scheme 3. Standard Mizoroki–Heck reaction.
The physical properties of supports and catalyst samples were
studied and the data is presented in Table 1. The slightly higher
surface area observed for B-2 might be due to relative difference in
size of the Schiff base formed from 4,4ꢀ-diaminobiphenyl [38].
The elemental analysis data of the supports and the catalysts
are presented in Table 2. Substantial decrease in the chlorine con-
tent is observed when the Schiff bases 7 and 8 are formed from
chloromethylated resin beads 1. Formation of Schiff base was fur-
ther confirmed by detection of nitrogen. The loading of palladium
in the catalysts was found to be in the range of 1.0–2.5 × 10−4 g/g
of resin.
The choice of suitable solvent for catalytic reactions is very
crucial for maximizing the activity of supported catalysts. Better
swelling behavior results in easier access of the reagents to the
active sites to undergo efficient catalytic cycle of the required con-
version. It is seen that polar solvents are generally better swelling
agents than aliphatic or aromatic non-polar solvents (Table 3).
Interestingly, water exhibited maximum degree of swelling. In fact,
the present study of Mizoroki–Heck reaction needs polar solvents.
The swelling behavior of the catalysts showed water and dimethy-
lacetamide (DMA) to be suitable.
Thermogravimetric analysis of the supported Pd-catalysts
reveal that they are quite stable up to 300–325 ◦C (Table 4), but
for a small initial weight loss which might be due to traces of mois-
ture. The weight loss above 300 ◦C may be due to the dissociation
of anchored ligand and/or breaking of polymeric chain. The present
analysis shows that the catalysts can be safely used in any stan-
dard Mizoroki–Heck reaction where the reaction temperature is
maintained around 140 ◦C.
Although the scanning electron micrographs are featureless,
they do exhibit change in morphology while going stepwise from
chloromethylated beads to the final polymer anchored catalysts.
The UV–vis reflectance spectra of the synthesized catalysts in
BaSO4 matrix exhibit weak intensity absorption bands in the region
i.e. formation of stilbene was monitored by thin layer chromatog-
raphy (see Scheme 3). The products, various substituted stilbenes
were separated by column chromatography and their structures
were confirmed by comparison of melting point and also from H
NMR and mass analysis [37].
2.5.1. Typical procedure for the Mizoroki–Heck reaction
Synthesis of trans-4-methoxy-4ꢀ-methylstilbene (Table 7, entry-3):
A two neck round bottom flask was charged with 4-iodoanisole
(0.1 g; 0.427 mmol), catalyst A-2 (0.091 g; 0.000213 mmol Pd,
0.05 mol%), dry potassium carbonate (0.118 g; 0.855 mmol) and
dimethylacetamide (6 mL) under the nitrogen atmosphere. This
mixture was slowly heated. As soon as the temperature reaches
65 ◦C, a solution of 4-methylstyrene (0.076 g; 0.64 mmol) in DMA
(2 mL) was introduced. The reaction mixture was then heated to
140 ◦C and continued for 40 h. The reaction mixture was quenched
with water and extracted with ethyl acetate (3× 25 mL). The
combined organic phase was washed with water and dried over
anhydrous sodium sulfate. Solvent was removed in vacuum and
the crude product was purified by column chromatography on sil-
ica gel to afford trans-4-methoxy-4ꢀ-methylstilbene (0.09 g, 94.7%)
as white solid. m.p. = 164 ◦C (reported 163–164 ◦C).
3. Results and discussion
The chemical modification of chloromethylated resin and sub-
sequent loading of palladium is outlined in Schemes 1 and 2. Four
different catalysts studied in the present work are designated as:
A-1: 5% poly(S-DVB)Pd(II)(1,3-DAP-SB)
A-2: 8% poly(S-DVB)Pd(II)(1,3-DAP-SB)
Table 1
Physical properties of polymers investigated.
No
Polymer (% crosslink)
Surface area (m2 g−1
)
Moisture content (wt.%)
Bulk density (g cm−1
)
Pore size (cm3 g−1
)
1
2
3
4
5
6
1 (5%)
1 (8%)
A-1 (5%)
A-2 (8%)
B-1 (5%)
B-2 (8%)
39.9
52.2
30.5
38.2
34.6
45.4
–
–
0.31
0.48
0.30
0.25
–
–
0.45
0.49
0.45
0.52
0.20
0.21
0.12
0.15
0.10
0.13

S.A. Patel et al. / Journal of Molecular Catalysis A: Chemical 332 (2010) 70–75
73
Table 2
Elemental analyses of chloromethylated polymer 1, their Schiff bases 7 and 8 and supported Pd(II) catalysts. All a for 5% crosslink, b for 8% crosslink.
No
Sample
% C
% H
% Cl
% N
Pd (g/g of resin)
1
2
3
4
5
6
7
8
9
1a (with 5% crosslink)
1b (with 8% crosslink)
7a (with 5% crosslink)
7b (with 8% crosslink)
8a (with 5% crosslink)
8b (with 8% crosslink)
A-1
A-2
B-1
B-2
74.2
72.8
81.6
78.1
80.1
75.6
71.9
72.1
72.0
70.8
6.0
6.0
7.3
6.2
7.0
6.1
7.1
6.0
6.9
6.3
15.6
27.6
10.9
13.4
9.7
13.9
–
–
–
–
–
–
–
–
–
–
–
–
4.9
4.7
4.9
2.4
4.0
3.2
3.0
2.3
1.0 × 10−4
2.5 × 10−4
2.0 × 10−4
1.7 × 10−4
10
Table 3
indicating the coordination of metal with ligand to form the com-
plex on polymer support (Table 5).
Swelling data of supported catalysts (mol%).
No
Solvent
A-1
A-2
B-1
B-2
1
2
3
4
5
6
7
8
9
Water
Methanol
Ethanol
Acetonitrile
Dichloromethane
Dioxane
DMF
Acetone
THF
Toluene
6.67
5.06
4.06
3.73
3.80
2.99
3.12
2.84
1.94
1.59
2.68
0.82
6.15
4.45
3.72
3.36
3.40
3.15
2.60
3.15
2.18
1.79
1.80
1.31
6.79
5.39
3.41
3.12
3.01
2.66
2.46
2.36
1.90
1.65
1.59
1.51
6.26
4.32
5.02
3.43
3.10
3.02
2.74
2.68
1.63
1.41
1.41
0.84
3.2. Catalytic Heck reaction
The main goal to support metal complex on polymer matrix is
to enhance the life of resulting catalyst, ensure easy separation
of it from reaction products, make it feasible to reuse the cata-
lyst and hence increase its overall efficacy. This requires necessary
modification of the surface of polymer in order to easily complex
metal ions on it. One effective way to do this is by attaching a
chelate of appropriate Schiff base to a suitable organic polymer
[39–44] or modified silica [45], chitosan [46] and zeolite [47]. In the
present study chloromethylated styrene–divinyl benzene copoly-
mer is attached with a Schiff base with two kinds of linker groups
CH2–CH2–CH2 in series-A and C6H4–C6H4 in series-B. The size and
shape of linker group is one variation being studied in order to be
able to fine tune the solid catalyst for particular application.
The polymer anchored Pd-catalysts were screened for the stan-
dard reaction of synthesis of stilbene by reaction of iodobenzene
and styrene in presence of base (Scheme 3). The results are pre-
sented in Table 6.
All the four catalysts prepared were quite effective and the prod-
ucts were obtained in good yield with respectable TON for the
reactions. Catalyst A-2 was slightly more active since its crosslink
and loading is also higher. The reaction was carried out in water
at 100 ◦C with a small quantity of CTAB to increase solubility of
reagents. While toluene gave poor yield probably due to the poor
swelling of beads and hence ineffective penetration of reagents to
reach the active sites, the other solvents, DMA and DMF are found
to be active for this reaction.
10
11
12
Cyclohexane
n-Heptane
Table 4
Thermogravimetric data of support and palladium anchored catalysts.
No
Sample
Degradation temp. (◦C)
Wt. loss (%)
1
2
3
4
5
6
1a (5% crosslink)
1b (8% crosslink)
A-1
A-2
B-1
B-2
478
469
380
348
397
350
33.0
31.0
16.8
15.2
22.6
12.0
330 nm for palladium supported catalysts, which are assigned to
the d–d transition of the metal. Ligand to metal charge transfer
bands (LMCT) are not seen in this region.
The mid IR (4000–400 cm−1) and far IR (500–50 cm−1) spectra
of polymer supported palladium complexes at different stages of
the synthesis were studied to elaborate on the nature of coordi-
nation of the metal ion to the polymer. The peaks in the region of
3449–3407 cm−1 is due to ꢀ(N–H) stretching frequency, which show
slight shift to lower wave number due to the coordination. Medium
intensity band at 1639–1633 cm−1 corresponding to ꢀ(C N) (azome-
thine) in polymer anchored Schiff base is shifted to lower wave
numbers (1606–1600 cm−1) on complexation with palladium indi-
cating “N” coordination of the ligand to the Pd(II) metal ion. The
spectra also shows a weak intensity bands at 490–450 cm−1 for
ꢀ(Pd–N), around 300 cm−1 for ꢀ(Pd–Cl) and 380–320 cm−1 for ꢀ(Pd–O)
Having established that the catalyst A-2 and B-1 are more effec-
tive among catalysts studied, number of different aryl iodides and
aryl bromides were screened with styrene and its derivatives to
see the generality of the system. The results are summarized in
Table 7. For this study a slightly higher quantity of the catalyst is
taken (0.05 mol% of Pd) and a combination of water and DMA is
used. The choice of solvent is based on the solubility of aryl halides
and styrene. From the point of view of environmental safety it is
beneficial to explore the possibility to use water as solvent, prefer-
Table 5
IR spectral data of 7, 8 and metal complexes.
Sample
ꢀ(N–H) (cm−1
)
ꢀ(C N) (cm−1
)
ꢀ(Pd–O) (cm−1
)
ꢀ(Pd–N) (cm−1
)
ꢀ(Pd–Cl) (cm−1
)
7a (5% crosslink)
7b (8% crosslink)
8a (5% crosslink)
8b (8% crosslink)
A-1
A-2
B-1
B-2
3449br
3432br
3407br
3409br
3436br
3409br
3429br
3430br
1633s
1639s
1634s
1638s
1633s
1606s
1633s
1600s
320
354
350
375
485
479
462
456
302
303
302
302

74
S.A. Patel et al. / Journal of Molecular Catalysis A: Chemical 332 (2010) 70–75
Table 6
Screening of supported catalysts for standard Mizoroki–Heck reaction.a
No
Catalyst
Pd contentb (g/g resin)
Crosslink (%)
Yieldc (%)
TON
1
2
3
4
A-1
A-2
B-1
B-2
1.0 × 10−4
2.5 × 10−4
2.0 × 10−4
1.7 × 10−4
5
8
5
8
73
96
93
89
7301
9580
9294
8897
a
All reactions were run for 40 h at 100 ◦C with CTAB (10 mol%).
Pd content (0.01 mol%) for all reactions.
Isolated product, pure trans isomer.
b
c
Table 7
Application of the catalyst for palladium catalyzed Mizoroki–Heck reaction. a
No.
ArX
Olefin
Solvent {ratio} [Temp. ◦C]
Catalyst A-2 [mol% of Pd]
Stilbene [yield/%]b
TON
I
1
DMA [140]
[0.001]
88,408
9089
1886
1675
1290
1784
1878
[89]
H2Oc [100]
[0.01]
[0.05]
[0.05]
[0.05]
[0.05]
[0.05]
MeO
MeO
MeO
MeO
I
I
2
3
4
5
6
7
[91]
Me
Me
DMA [140]
[95]
N
N
I
DMA–H2O {2:1} [120]
DMA–H2O {2:1} [120]
DMA [140]
[83]
N
MeO
N
I
MeO
[65]
NO2
[89]
Me
[94]
NO2
I
Me
I
I
DMA [140]
COOtBu
[85]
COOtBu
8
9
DMA [140]
DMA [140]
[0.05]
[0.05]
1700
1660
COOtBu
[83]
COOtBu
MeO
MeO
I
Br
10
DMA [140]
DMA [140]
[0.05]
812
[41]
Me
11
[0.05]d
1004
Me
Br
[50]
a
All reactions run for 40 h with K2CO3 (2 eq.).
Isolated yield, mostly trans isomer.
With CTAB (10%).
b
c
d
With B-1.

S.A. Patel et al. / Journal of Molecular Catalysis A: Chemical 332 (2010) 70–75
75
Recycle study
Acknowledgments
100
80
60
40
20
We thank the management of Gujarat Alkalies and Chemicals
Limited, Vadodara, and The Maharaja Sayajirao University of Bar-
oda, Vadodara, for adequate assistance to this work.
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