Use of a recyclable poly(N-vinyl carbazole) palladium(II) complex catalyst: Heck cross-coupling reaction under phosphine-free and aerobic conditions

Article abstract of DOI:10.1007/s11243-010-9354-1

A novel pathway for cross-coupling reaction of terminal alkenes with aryl halides (Heck cross-coupling) has been described using a new phosphine-free poly(N-vinyl carbazole) anchored palladium(II) complex as catalyst in aerobic conditions. The catalyst was found to be highly active for the couplings of a variety of substituted and non-substituted aryl halides with terminal alkenes and to smoothly afford the corresponding desired products in good to excellent yields. The catalyst can be reused at least six times without noticeable decrease in catalytic activity. Springer Science+Business Media B.V. 2010.

Full text of DOI:10.1007/s11243-010-9354-1

Transition Met Chem (2010) 35:491–499
DOI 10.1007/s11243-010-9354-1
Use of a recyclable poly(N-vinyl carbazole) palladium(II) complex
catalyst: Heck cross-coupling reaction under phosphine-free
and aerobic conditions
•
Manirul Islam Paramita Mondal
•
•
Anupam Singha Roy Kazi Tuhina
Received: 11 February 2010 / Accepted: 18 March 2010 / Published online: 3 April 2010
Ó Springer Science+Business Media B.V. 2010
Abstract A novel pathway for cross-coupling reaction of
terminal alkenes with aryl halides (Heck cross-coupling)
has been described using a new phosphine-free poly(N-
vinyl carbazole) anchored palladium(II) complex as cata-
lyst in aerobic conditions. The catalyst was found to be
highly active for the couplings of a variety of substituted
and non-substituted aryl halides with terminal alkenes and
to smoothly afford the corresponding desired products in
good to excellent yields. The catalyst can be reused at least
six times without noticeable decrease in catalytic activity.
non-phosphine ligands [19–30] is now of practical interest
and thus has been receiving great attention. With the
development of green chemistry, the design and prepara-
tion of green catalysts have attracted much attention
recently, among which heterogeneous catalysts have been
widely employed [31–36]. The immobilization of metal
catalysts on solid supports has become a valuable tool for
the efficient separation of organic products and catalyst
recycling. Polymers play a significant role in this area,
offering different ways of metal attachment to the polymer
matrix via covalent or non-covalent bonding, through
hydrogen bridges, as well as through ionic, hydrophobic or
fluorous interactions [37–41].
Introduction
The recent discovery of polymer-based Schiff base
ligands and of their effectiveness as catalysts offers an
opportunity to develop phosphine-free heterogeneous
catalysis. Several authors have recently reported polymer-
anchored palladium complexes active for the Heck reaction
under phosphine-free conditions. Schubert and coworkers
reported stabilized palladium nanoparticles in star-shaped
block copolymers [42]. Steel et al. described palladium N-
heterocyclic carbine complexes anchored to polymers, and
it was claimed that a true heterogeneous recyclable catalyst
was functioning [43].
The Heck reaction [1–3], a Pd-catalyzed C–C coupling
between aryl halides and terminal alkenes, is a robust and
efficient method for carbon–carbon bond formation and
remains a flourishing area of research [4, 5]. The reaction
has been applied to many areas, including natural products
[6–8] and fine chemicals synthesis [9–11]. Usually, aryl
halides are reacted with alkenes in the presence of palla-
dium and a suitable base. Soluble palladium compounds,
generally phosphine palladium complexes, are the most
efficient catalysts for the Heck reaction [12–18]. However,
phosphine ligands are expensive, toxic, unrecoverable and
sensitive to air and moisture and therefore environmentally
unfavorable. The development of catalyst systems with
The excellent catalytic activity of poly (N-vinyl carba-
zole) anchored Pd(II) complex for the hydrogenation
reactions of various organic substrates prompted us to
extend our investigations into other catalytic reactions. In
the present work, we have examined the catalytic activity
of poly (N-vinyl carbazole) anchored Pd(II) complex in the
Heck cross-coupling reaction between aryl halides and
terminal alkenes under phosphine-free reaction conditions.
The effects of various reaction parameters for optimiza-
tion of the reaction conditions were also studied. This
polymer-anchored metal complex exhibited high catalytic
M. Islam (&) Á P. Mondal Á A. S. Roy
Department of Chemistry, University of Kalyani, Kalyani,
Nadia 741235, West Bengal, India
e-mail: manir65@rediffmail.com
K. Tuhina
Department of Chemistry, B.S. College, S-24 P.G.S.,
Tangrakhali 743329, West Bengal, India
123

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Transition Met Chem (2010) 35:491–499
O2N
NO2
H2N
NH2
HNO3/AcOH
SnCl2/HCl
NaOH
N
N
N
n
-CH-CH2-
(2)
-CH-CH2-
n
-CH-CH2-
(1)
n
(3)
-CH-CH2-
PhHC=N
N=CHPh
N
PhCHO
Pd(PhCN)2Cl2
MeOH
N
N
N
Cl
Cl
Pd
Pd
-CH-CH2-
(4)
PhCN
NCPh
n
(5)
n
Scheme 1 Synthesis of the polymer-anchored Pd(II) complex
performance and can be easily recovered and reused
without significant loss of catalytic activity.
(2.0 mmol), K2CO3 (1.5 mmol), 0.5 mol% of catalyst and
dodecane as internal standard were added, and the reaction
mixture was stirred at 80 °C under air. To study the pro-
gress of the reaction, the reaction mixtures were collected
at different time intervals and quantified by GC analysis.
At the end of the reaction, the catalyst was separated by
simple filtration. The filtrate was dried over Na2SO4, fil-
tered, concentrated, and the residue was purified by flash
column chromatography on silica gel. The product was
analyzed by GC–MS. All the prepared compounds are
known and were compared with authentic samples.
Experimental
All the reagents were analytical grade and used as such
without further purification. Solvents were purified and dried
according to standard procedures. Poly (N-vinyl carbazole)
(Art.No. 368350–5) was purchased from Aldrich; [Pd
(PhCN)2Cl2], Pd(OAc)2 and PdCl2 were procured from
Arora Matthey. Other reagents were purchased from Merck.
The palladium content was determined by Varian, USA,
AA240 atomic absorption spectrophotometer (AAS). A
Perkin–Elmer, USA, 2400C elemental analyzer was used to
collect microanalytical data (C, H and N). Surface mor-
phology of functionalized polystyrene ligand and metal
complex was analyzed using a scanning electron microscope
(ZEISS EVO40, England) equipped with EDX facility.
Fourier transform infrared (FTIR) spectra for the catalyst and
its precursors were recorded over the wave number range
from 400 to 4,000 cm-1 on a Perkin–Elmer, USA, FTIR 783
spectrophotometer using KBr pellets. UV–Vis spectra were
taken using a Shimadzu, Japan, UV-2401PC double beam
spectrophotometer having an integrating sphere attach-
ment for solid samples. The thermal stability of the immo-
bilized catalyst was determined using a Mettler Toledo,
Switzerland, TGA/DTA 851e instrument. The reaction
products were analyzed using a Varian, USA, 3400 gas
chromatograph equipped with a 30 m CP-SIL8CB capillary
column and a flame ionization detector. All reaction prod-
ucts were identified by using an Agilent, USA, GC–MS
(QP-5050) equipped with a 30 m HP-5 ms capillary column.
Preparation and characterization of poly (3,6-
dibenzaldimino-N-vinylcarbazole) Pd(II) complex
catalyst
The outline for the preparation of the poly (N-vinylcarbaz-
ole) anchored palladium(II) complex, [Pd(C6H4CH=N-
Pol)(PhCN)Cl] (Pol=poly-N-vinylcarbazole) is shown in
Scheme-1. The functionalized poly(N-vinyl carbazole amine)
(3) was prepared according to the method of King and Sweet
[44]. The polymer-anchored ligand (4) was prepared by the
reaction of poly(N-vinyl carbazole) amine (3 g) with benz-
aldehyde (10 mL) in dry toluene (30 mL) medium under
reflux condition for 72 h in nitrogen atmosphere. For the
preparation of poly (N-vinyl carbazole) anchored palla-
dium(II) complex (5), a methanolic solution (25 mL) of
Pd(PhCN)Cl (0.7 g) was mixed with poly(3,6-dibenzaldi-
mino N-vinyl carbazole) ligand (2 g), and the reaction
mixture was first stirred for 24 h at room temperature and
then refluxed for 12 h [45].
Due to insolubility of the [Pd(C6H4CH=N-Pol)(PhCN)-
Cl] complex in all common organic solvents, its structural
investigation was limited only to its physico-chemical
properties, SEM-EDX, TGA-DTA, IR and UV–Vis spectral
data (Scheme 1). The complete incorporation of the organic
substructure in the material was confirmed by elemental
General experimental procedure for Heck reaction
To a suspension of polymer-anchored palladium(II) cata-
lyst in DMF (10 mL), aryl halide (1.0 mmol), alkene
123

Transition Met Chem (2010) 35:491–499
493
Table 1 Analytical data of the
functionalized polymer and the
Pd(II) complex
Compound
Color
Cl %
C %
H %
N %
Pd %
Pol
Faint Yellow
Yellowish brown
Yellow
87.1
76.0
73.9
81.8
84.7
73.9
5.7
4.5
5.2
5.6
5.3
4.4
7.2
Pol-NO2
10.2
10.0
11.1
8.9
Pol-NH3Cl
7.9
3.0
Pol-NH2
Yellow
[(C6H4CH=N-Pol)(PhCN)Cl]
[Pd(C6H4CH=N-Pol)(PhCN)Cl]
Yellow
Yellowish brown
8.2
12.8
Pol = poly-N-vinylcarbazole
analysis (Table 1). The metal content of [Pd(C6H4CH=N-
Pol)(PhCN)Cl] complex determined by AAS suggested
12.80 wt% metal loading in the immobilized palladium
complex. The SEM images of polymer-anchored ligand and
metal complex clearly show the morphological change,
which occurred on the surface of the polymer matrix after
loading of the metal. EDX data also support the metal
attachment on the surface of polymer matrix (Fig. 1).
The catalyst is thermally stable up to 280 °C. The IR
spectrum of poly(3,6-dibenzaldimino N-vinyl carbazole)
showed characteristic IR peak at 1,630 cm-1 that may be
assigned to the C=N stretching vibration of imine, which
on complexation shifted toward lower frequency to
1,620 cm-1 suggesting bond formation between Pd and
ligand. Other characteristic peaks at 1,590 cm-1 (m-C=C-
stretching, aromatic), 720 cm-1 (orthometallation) [46],
Fig. 1 EDX spectra of poly
(3,6-dibenzaldimino N-vinyl
carbazole) ligand (a) and poly
(3,6-dibenzaldimino N-vinyl
carbazole) palladium(II)
complex (b)
123

494
Transition Met Chem (2010) 35:491–499
2,290 cm-1 (m-C:N of benzonitrile), 455 cm-1 (m Pd–N)
[47] and 355 cm-1 (m Pd-Cl) [48] also support the forma-
tion of the palladium(II) complex. The UV–Vis spectra
provided further evidence for the presence of palladium on
polymer support. The absorption maxima at 305 nm may
be attributed to the p ? p* transition in the carbazole and
the phenyl moieties, and the absorption at higher range
(374, 434 nm) is due to the p ? p* and n ? p* transitions
of the imine system in conjugation with the aromatic
nuclei. The Pd(II) carbazole complex shows the bands at
reaction. Since the performance of a successful metal-cat-
alyzed cross-coupling reaction is known to be governed by
a number of factors, at first the effects of various metal
precursors, bases, solvents, temperatures and catalyst con-
centrations on the Heck cross-coupling reaction employing
[Pd(C6H4CH=N-Pol)(PhCN)Cl] catalyst were surveyed.
All the reactions were carried out under air (Scheme 2).
The influences of various metal salts, Pd(OAc)2, PdCl2
and Pd(PhCN)2Cl2 anchored with poly(3,6-dibenzaldimino
N-vinylcarbazole) ligand were first studied. The use of
Pd(OAc)2 or PdCl2 as a metal precursors resulted in mod-
erate conversion of bromobenzene, whereas Pd(PhCN)2-
Cl2 exhibited complete conversion at 80 °C (Table 2,
entries 1–3). Therefore, Pd(PhCN)2Cl2 was taken as a
metal precursor, and further catalytic runs were studied
with this catalyst, [Pd(C6H4CH=N-Pol)(PhCN)Cl]. Using
metal salts without any ligand gave partial conversion of
the desired product (Table 2, entries 4–6). No coupling
reaction was observed without catalyst (Table 2, entry 7).
As the palladium compounds are so expensive, the
amount used is an important criterion to estimate the
value of a catalyst. In order to observe effect of catalyst
concentrations on the conversion of bromobenzene and
yield of desired product, the reactions catalyzed by
[Pd(C6H4CH=N-Pol)(PhCN)Cl] at different concentrations
from 0.1 to 1.0 mol% have been studied. The results are
summarized in Table 2. From these results, it is seen that
reactions at lower catalyst concentration (0.1 mol% of Pd)
resulted in poorer conversion (Table 2, entry 8). The cat-
alytic activity increased with an increase in the palladium
concentration, and best results were obtained at catalyst
concentration of 0.5 mol% Pd (Table 2, entry 3). A further
increase in catalyst concentration (from 0.5 to 1.0 mol%
1
1
465 and 340 nm which may be assigned to A1g ? A2g
1
1
and A1g ? B1g transitions, respectively [49].
Heck cross-coupling reactions
To explore the catalytic activity of the present catalyst, we
examined the Heck cross-coupling reaction. Generally,
Heck reactions are conducted with a tertiary phosphine
ligand. The toxicity of phosphines make them unfavorable
and environmentally unacceptable. To address these con-
cerns, we performed the Heck reaction under phosphine-
free reaction condition. We began our investigation into
Heck coupling reaction with the present catalyst using the
coupling of bromobenzene with styrene as a model
R'
Catalyst
+
R'
X
base, solvent
R
R
X = Cl, Br, I
R = H, CH3, OCH3, COMe, NO2, CN
R' = C6H5, COOMe
Scheme 2
Table 2 Effect of metal precursor and catalyst concentration in Heck cross-coupling reactions
Entry
Catalyst
Cat. Conc. (mol% of Pd)
Conversiona (%)
1.
[Pd(C6H4CH=N-Pol)Cl2]
[Pd(C6H4CH=N-Pol)(OAc)2]
[Pd(C6H4CH=N-Pol)(PhCN)Cl]
PdCl2
0.5
0.5
0.5
5.0
5.0
5.0
–
71
2.
78
3.
90
4.
45
5.
Pd(OAc)2
39
6.
Pd(PhCN)2Cl2
46
7.
None
No reaction
8.
[Pd(C6H4CH=N-Pol)(PhCN)Cl]
[Pd(C6H4CH=N-Pol)(PhCN)Cl]
[Pd(C6H4CH=N-Pol)(PhCN)Cl]
[Pd(C6H4CH=N-Pol)(PhCN)Cl]
0.1
0.25
0.75
1.0
8
9.
67
90
90
10.
11.
Reaction conditions: Bromobenzene (1.0 mmol), styrene (2.0 mmol), K2CO3 (1.5 mmol), DMF (6 mL), catalyst (0.5 mol% of Pd); All the
reactions were carried out in air
a
Conversion of reactant was determined by GC and GCMS analysis using dodecane as internal standard
123

Transition Met Chem (2010) 35:491–499
495
when the reaction temperature was 80 °C (Table 3,
entry 6).
Table 3 Heck cross-coupling reactions using [Pd(C6H4CH=N-
Pol)(PhCN)Cl] complex at various solvents
Solvent plays a crucial role for effective cross-coupling
reaction. To verify the solvent effect in Heck cross-cou-
pling reactions, a series of reactions were investigated by
taking the model reaction in different solvents. The reac-
tion conducted in polar solvent medium like DMF or
DMSO was found to be most effective (Table 3, entries
6, 7). The use of THF, MeOH and ACN as solvents led to
slower reactions (Table 3, entries 8–10), and no desired
cross-coupling products were observed while reactions
were carried out in toluene and dichloromethane (Table 3,
entries 11, 12). Consequently, DMF was chosen as the
medium of choice for this coupling.
Entry
Solvent
Time (h)
Temperature (°C)
Conv.a (%)
1.
DMF
DMF
DMF
DMF
DMF
DMF
DMSO
THF
8
8
30
0
2.
40
09
3.
8
50
24
4.
8
60
56
5.
8
70
82
6.
8
80
90
7.
8
80
88
8.
12
12
12
24
24
Reflux
70
21
9.
MeOH
ACN
46
10.
11.
12.
Reflux
Reflux
Reflux
38
Toluene
DCM
Trace
Trace
To determine the reactivity of the catalyst in various
bases, bromobenzene was chosen as the initial coupling
partner in combination with styrene as a model reaction for
investigation into the optimal base. Various inorganic and
organic bases such as Na2CO3, K2CO3, NaOH, NaOAc,
KOH, Et3N and NaOtBu were examined. As shown in
Table 4, the best performance was observed with K2CO3
(Table 4, entry 2). Other inorganic bases like Na2CO3,
NaOH, NaOAc and KOH gave reasonable conversions
leading to 35–74% yields (Table 4, entries 1, 3–5). The
yield was noticeably low when organic bases such as Et3N
and NaOtBu were employed (Table 4, entries 6, 7). No
conversion of the reactants was observed in the absence of
base (Table 4, entry 8). The catalytic performance of the
coupling reaction was also greatly affected by the base–
substrate molar ratio employed. The Heck cross-coupling
reaction with the present catalyst was carried out under
various base–substrate molar ratios and conditions. The
results are shown in Table 4 (entries 2, 9–11). The best
Reaction conditions: Bromobenzene (1.0 mmol), styrene (2.0 mmol),
K2CO3 (1.5 mmol), DMF (6 mL), catalyst (0.5 mol% of Pd); All the
reactions were carried out in air
a
Conversion of reactant was determined by GC and GCMS analysis
using dodecane as internal standard
Pd) did not significantly improve the conversion (Table 2,
entries 10, 11).
Reaction temperature is another important factor for
Heck reaction. In general, Heck cross-coupling reaction
requires high reaction temperature. It was found that a
temperature below 80 °C was not suitable for this cross-
coupling as a poor conversion was obtained at 40–70 °C
(Table 3, entries 2–5). At room temperature, no cross-
coupled product was detected in the reaction medium
(Table 3, entry 1). However, good efficiency was reached
Table 4 Heck cross-coupling reactions using [Pd(C6H4CH=N-Pol)(PhCN)Cl] complex at various bases and base–substrate ratios
Entry
Base
Base–substrate ratio
Time (h)
Temperature (°C)
Conv.a (%)
1.
Na2CO3
K2CO3
NaOH
NaOAc
KOH
1.5:1
1.5:1
1.5:1
1.5:1
1.5:1
1.5:1
1.5:1
–
8
8
8
8
8
8
8
8
8
8
8
80
80
74
2.
90
3.
80
38
4.
80
49
5.
80
35
6.
Et3N
100
100
80
16
7.
NaOtBu
No base
K2CO3
K2CO3
K2CO3
11
8.
No reaction
9.
0.5:1
1:1
80
25
59
90
10.
11.
80
2:1
80
Reaction conditions: Bromobenzene (1.0 mmol), styrene (2.0 mmol), K2CO3 (1.5 mmol), DMF (6 mL), catalyst (0.5 mol% of Pd); All the
reactions were carried out in air
a
Conversion of reactant was determined by GC and GCMS analysis using dodecane as internal standard
123

496
Transition Met Chem (2010) 35:491–499
yield was achieved with 1.5:1 base–substrate ratio (Table 4,
entry 2).
good to excellent yields. As shown in Table 5, the coupling
of activated and deactivated aryl iodides and alkenes pro-
ceeded in high yields and more rapidly than aryl bromides.
Non-substituted aryl bromide gave moderate yield of
coupled product (Table 5, entries 7, 23). Activated aryl
bromide like p-bromonitrobenzene and p-bromoacetophe-
none bearing electron withdrawing group reacted with
styrene and methyl acrylate to generate the corresponding
On the basis of the above optimized reaction conditions,
the coupling reactions between a variety of aryl halides and
alkenes were investigated (Scheme 2). The results are
summarized in Table 5. Both aryl iodides and aryl bro-
mides could react with aromatic or aliphatic terminal
alkenes to give the corresponding coupling products with
Table 5 Heck reaction of aryl halides with terminal alkenes using [Pd(C6H4CH=N-Pol)(PhCN)Cl] complex catalyst
R'
Catalyst
+
R'
X
°
K2CO3, DMF, 80 C
R
R
Entry
Aryl halides
Alkenes
Producta
Time (h)
Conv.b (%)
Yeildc (%)
1.
Iodo benzene
Styrene
Styrene
Styrene
Styrene
Styrene
Styrene
Styrene
Styrene
Styrene
Styrene
Styrene
Styrene
Styrene
Styrene
Styrene
Styrene
Stilbene
5
6
100
96
97
99
99
98
90
88
87
95
96
95
81
86
85
Trace
100
94
90
93
96
91
86
80
84
88
91
92
80
76
100
96
97
99
99
97
90
87
87
95
95
94
78
86
83
Trace
100
94
90
93
96
91
86
80
84
88
91
92
80
76
2.
p-Iodotoluene
4-Methylstilbene
3.
p-Iodoanisole
4-Methoxystilbene
4-Nitrostilbene
6
4.
p-Iodonitrobenzene
p-Iodoacetophenone
p-Cyanoiodobenzene
Bromobenzene
5
5.
4-Acetylstilbene
5
6.
4-Cyanostilbene
5
7.
Stilbene
8
8.
p-Bromotoluene
4-Methylstilbene
12
12
8
9.
p-Bromoanisole
4-Methoxystilbene
4-Nitrostilbene
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
29.
30.
p-Bromonitrobenzene
p-Bromoacetophenone
p-Cyanobromobenzene
2-Bromopyridine
1-Bromonaphthalene
o-Bromonitrobenzene
Chlorobenzene
4-Acetylstilbene
8
4-Cyanostilbene
10
16
16
24
24
6
2-Styrylpyridine
1-Styrylnaphthalene
2-Nitrostilbene
Stilbene
Iodo benzene
Me acrylate
Me acrylate
Me acrylate
Me acrylate
Me acrylate
Me acrylate
Me acrylate
Me acrylate
Me acrylate
Me acrylate
Me acrylate
Me acrylate
Me acrylate
Me acrylate
Methylcinnamate
p-Iodotoluene
Methyl-4-methylcinnamate
Methyl-4-methoxycinnamate
Methyl-4-nitrocinnamate
Methyl-4-acetylcinnamate
Methyl-4-cyanocinnamate
Methylcinnamate
6
p-Iodoanisole
6
p-Iodonitrobenzene
p-Iodoacetophenone
p-Cyanoiodobenzene
Bromobenzene
6
6
6
12
12
12
10
10
12
16
24
p-Bromotoluene
Methyl-4-methylcinnamate
Methyl-4-methoxycinnamate
Methyl-4-nitrocinnamate
Methyl-4-acetylcinnamate
Methyl-4-cyanocinnamate
p-Bromoanisole
p-Bromonitrobenzene
p-Bromoacetophenone
p-Cyanobromobenzene
1-Bromonaphthalene
o-Nitrobromobenzene
Methyl-(3-naphthalene-5-yl)acrylate
Methyl-3-nitrocinnamate
Reaction conditions: arylhalide (1.0 mmol), alkene (2.0 mmol), K2CO3 (1.5 mmol), DMF (6 mL), catalyst (0.5 mol% of Pd); all the reactions
were carried out in air
a
b
c
1
Products were identified by comparison of their IR and HNMR spectral data those reported in the literature (whenever possible)
Conversion of reactant was determined by GC and GCMS analysis using dodecane as internal standard
In all the coupling reactions, only E-stereochemistry was observed
123

Transition Met Chem (2010) 35:491–499
497
Table 6 Comparison of catalytic activity of the present catalyst in the Heck cross-coupling reaction with other related reported systems
Reaction
Catalyst
Reaction conditions
Yield% Reference
(Bromobenzene ? Styrene)
[Pd(C6H4CH=N-Pol)(PhCN)Cl] K2CO3, DMF, 0.5 mol% catalyst, 80 °C, 8 h
90
70
20
Present
19a
Silylated PdNHC complex
MCM-41 Pd
Et3N, 0.013 mmol Pd, reflux DMF
Bu3N, 0.0187 mol% Pd, 170 °C, 72 h, NMP
19b
Si-As-Pd (0)
Bu3N, p-xylene, 140 °C, 12 h, 0.03 mmol of Pd 54
19c
(Bromobenzene ? Me acrylate) [Pd(C6H4CH=N-Pol)(PhCN)Cl] K2CO3, DMF, 0.5 mol% catalyst, 80 °C, 10 h
86
48
82
60
Present
19d
NHC-Pd(II)
Na2CO3, DMA, 1.0 mol% catalyst,160°C, 18 h
Et3N, 0.013 mmol Pd, reflux DMF
Silylated PdNHC complex
Pd(TMHD)2
19a
K2CO3, DMF, 12 h, 80 °C, 5 mol% catalyst
19e
product in good yields (Table 5, entries 10, 11, 26, 27). For
deactivated aryl bromides, p-bromoanisole and p-bromo-
toluene bearing electron donating group in their para
position, lower yields were obtained under the present
reaction conditions (Table 5, entries 8, 9, 24, 25). The
coupling of ortho-substituted bromobenzene having nitro
group gave the corresponding coupling product with sty-
rene and methyl acrylate in 83 and 76% yield, respectively
(Table 5, entries 15, 30). Sterically hindered 1-iodonaph-
thalene was found to react efficiently under the present
reaction conditions (Table 5, entries 14, 29). Heteroaryl
bromides, such as 2-bromopyridine, also coupled effectively
with styrene providing 78% coupled product (Table 5, entry
13). However, reactivity toward chloroarenes, which was
observed by others [50, 51], was not achieved with our
catalyst system. Displacement of styrene to methyl acry-
late gave lower yields. The entire obtained product was
E-stereochemistry, which was identified by GC–MS and
1HNMR.
continuing the reaction for 4 h, the catalyst was removed
by filtration using Whatmann filter paper, and the resulting
filtrate was subjected to heating for further 4 h reaction.
Gas chromatographic analysis of the filtrate solution
showed that there was no significant increase in conver-
sion, whereas the catalyst containing portion gave 90%
conversion after 4 h reaction (Fig. 2). Palladium leaching
was also studied by atomic absorption spectroscopic anal-
ysis, indicating that the liquid part of Heck reaction mix-
ture contained less than 0.5 ppm palladium. These results
mean that the catalysis by leached palladium is negligible.
The efficient catalytic performance is due to the polymer-
anchored metal complex not due to leached palladium
metal in the solvent.
100
Usual reaction
Comparison of catalytic activity with other reported
systems
Reaction of filtrate
after removal of catalyst
80
Table 6 provides a comparison of the results obtained for
our present catalytic system with those reported in the lit-
erature. From the table, it is seen that present catalyst
exhibited higher conversions and yields compared to the
other reported systems [52–56]. Reactions conducted at
lower temperature and shorter reaction time were required
for the Heck cross-coupling reactions using our catalyst.
60
40
20
0
Heterogeneity test
0
2
4
6
8
Time (h)
Heterogeneity and the palladium leaching of the present
catalyst were examined by the ‘‘Hot filtration test’’ for
the Heck reaction of 4-bromonitrobenzene and styrene.
Figure 2 shows the leaching experiment during the reac-
tion over polymer-anchored palladium catalyst. After
Fig. 2 Leaching experiment of poly(N-vinyl carbazole) anchored
Pd(II) complex catalyst toward the Heck cross-coupling reaction.
Reaction conditions: 4-Bromonitrobenzene (1.0 mmol), styrene
(2.0 mmol), catalyst (0.5 mol% Pd), K2CO3 (1.5 mmol), DMF
(6 mL)
123

498
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with this reaction was the reuse of the catalyst. Recycling
studies were performed with the present catalyst using the
reaction of 4-bromonitrobenzene with styrene. After the
first run, the solid catalyst was separated from the reaction
medium by simple filtration, washed with dichloromethane
and finally dried at 40 °C under reduced pressure. After
separation and washing, the heterogeneous catalysts were
used for the same reactions under the same reaction con-
ditions as for the initial run without any regeneration pro-
cedure. The catalyst was recycled six times to give yields
of 95, 93, 93, 91, 91 and 89% consecutively. This reaction
conversion shows that the immobilized catalyst can be
repeatedly used without any apparent decrease in its cata-
lytic activity and selectivity.
¨
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heterogeneous palladium catalyst [Pd(C6H4CH=N-
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coupling reactions of aryl halides with terminal alkenes. The
catalyst was found to be air-stable, active and non-polluting
under the various reaction conditions screened. The mild
reaction conditions, simple experimental procedure, rapid
conversion, excellent yields and reusability of the catalyst
are notable advantages of the method. High efficiency and
economy gain by simple reaction processing and easy
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Acknowledgments We acknowledge the Department of Science
and Technology (DST), Council of Scientific and Industrial Research
(CSIR) and the University Grant Commission (UGC), New Delhi,
India for funding. One of the authors, K.T., is thankful to the Uni-
versity Grants Commission (Eastern Region), India, for financial
support. We also thank the DST and UGC, New Delhi, India for
providing instrumental support under the FIST and SAP program.
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