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Polymer-supported macrocyclic Schiff base palladium complex as an efficient catalyst for the Heck reaction.

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Full Paper
Received: 27 January 2011
Revised: 18 May 2011
Accepted: 22 July 2011
Published online in Wiley Online Library: 7 September 2011
(wileyonlinelibrary.com) DOI 10.1002/aoc.1839
Polymer-supported macrocyclic Schiff base
palladium complex as an efficient catalyst
for the Heck reaction
Ying He and Chun Cai∗
A polymer-supported macrocyclic Schiff base palladium complex has been synthesized. In the Heck reaction of aryl iodides and
bromides with ethyl acrylate or styrene, the complex has been proved to give the corresponding products in good to excellent
yields. The reaction proceeded smoothly in the presence of 0.5 mol% of catalyst in DMF within 1–4 h. Recycling studies have
shown that the catalyst can be readily recovered and reused for four cycles with only a slightly decrease in its activity. Copyright
c 2011 John Wiley & Sons, Ltd.
Keywords: supported catalyst; polymer; Heck reaction; heterogeneous catalysis
Introduction
The palladium-catalyzed reaction of organic halides with alkenes,
i.e. the Heck reaction,[1,2] has become one of the best methods
for C–C bond formation in organic synthesis.[3 – 5] The reaction
generally proceeds in the presence of palladium catalysts
associated with phosphine or N-containing ligands, which stabilize
the palladium species. However, homogeneous catalytic processes
suffer from problems concerning separation of the catalyst from
the reaction mixture, their reuse and palladium contamination in
the products. In addition, most of ligands for Heck reactions are
undesirable in the industrial chemistry owing to their toxicity,
high price and air sensitivity. Supported palladium catalysts,
however, have emerged in recent years as an alternative, with
the advantages of heat stability and easy separation from the
reaction systems compared to homogeneous catalysts.[6 – 13]
Schiff bases[14] are an extensively studied class of ligands[15]
that are known for their selectivity and sensitivity toward various
metal ions. These ligands have also been applied successfully to
C–C bond formation in Suzuki and Heck reactions.[16,17] GonzalezArellano et al. have heterogenized Pd(II)–Schiff base complexes
on MCM-41 and delaminated zeolites, finding they are efficient
and recyclable catalysts for Heck reactions.[18] Homogeneous
palladium non-symmetrical salen-type Schiff base complexes have
been prepared and proved effective in heterogeneous catalytic
C–C cross-coupling reactions.[19] In continuation of our work,[20,21]
we recently developed heterogeneous palladium catalysts for
C–C bond formation reaction based on polymer-supported
Schiff base palladium complex for Suzuki cross-coupling at room
temperature.[22] In this paper, we wish to report the Heck reactions
in DMF at 100 or 125 ◦ C using a polymer-supported macrocyclic
Schiff base palladium complex as a catalyst (Scheme 1).
Experimental
1.0 mmol g−1 Cl, grain size range 100–200 mesh) was obtained
from GL Biochem (Shanghai) Ltd (Shanghai, China).
NMR analyses
All 1 H NMR and 13 C NMR experiments were performed in CDCl3 or
DMSO-d6 and recorded on a Bruker Avance III 500 MHz analyzer. 1 H
spectra were collected at 500 MHz using a 10 330 Hz spectral width,
a relaxation delay of 1.0 s, 65 536 data points, 11.5 µs pulse width,
and trimethylsilane (TMS) (0.00 ppm) as the internal reference. 13 C
NMR spectra were collected 125 MHz using a 29 762 Hz spectra
width, a relaxation delay of 2.0 s, 65 536 data points, 9.6 µs pulse
width, and TMS (0.00 ppm) as internal reference.
Fourier transform infrared spectroscopy (FTIR)
IR spectra were recorded in KBr disks with a Shimadzu IRPrestige21 FTIR spectrometer. Optical-grade, random cuttings of KBr
(International Crystal Laboratories, Garfield, NJ, USA) were ground
with 1.0 wt% of the sample to be analyzed.
Elementary analysis (EA)
Analysis of C, H, and N elements were done using a Vario EL III
(Elementar, Germany) element analyzer.
Inductively coupled plasma (ICP) analysis
Palladium content of the catalyst was measured by ICP on a
PE5300DV analyzer. The resin beads (30 mg) were treated with
a mixture (5 ml) of hydrochloric acid and nitric acid (3 : 1, v/v) at
∗
All chemicals were reagent grade and used as purchased. Chloromethylated polystyrene resin (1% divinylbenzene,
Chemical Engineering College, Nanjing University of Science and Technology,
Nanjing 210094, People’s Republic of China
Appl. Organometal. Chem. 2011, 25, 799–803
c 2011 John Wiley & Sons, Ltd.
Copyright 799
General Remarks
Correspondenceto:Chun Cai,ChemicalEngineeringCollege,NanjingUniversity
of Science and Technology, Nanjing 210094, People’s Republic of China.
E-mail: c.cai@mail.njust.edu.cn
Y. He and C. Cai
Scheme 1. Heck coupling reaction of aryl halides and olefins.
100 ◦ C for 4 h. Following this, the resulting orange-colored solution
was filtered, and the recovered resin beads were washed with
distilled water (2.5 ml × 6). The filtrate was diluted to 50 ml with
distilled water and analyzed by ICP–atomic emission spectrometry
(AES).
Gas chromatography–mass spectrometry (GC-MS) analyses
GC-MS analyses were performed on a Thermo Trace DSQ mass
spectrometer, under the following conditions. Helium was used
as a carrier gas at a flow rate of 1.2 ml min−1 . GC was conducted
using an RTX-5 MS column (15 m × 0.25 mm × 0.25 mm, Restek
Corp., USA). The column temperature was programmed from 50 ◦ C
(1.5 min hold) to 270 ◦ C at 20 ◦ C min−1 . The injector temperature
was set at 220 ◦ C with a split ratio of 20 : 1. The interface
temperature and ion source temperature were both at 250 ◦ C. The
column outlet was inserted directly into the electron ionization
source block, operating at 70 eV.
The contents were refluxed for 24 h. The color of the polymer
beads changed from white to pale yellow, indicating attachment
of o-phenylenediamine. The beads were filtered, washed with
distilled water, dichloromethane and methanol, and then dried
under vacuum. The formation of macrocycle Schiff base of the
polymer resin was confirmed using analytical and IR spectral data.
The macrocycle Schiff base content of the functional beads was
0.68 mmol g−1 according to the result of elemental analyses based
on elemental N.
Preparation of the catalyst
The functional beads (0.35 g) were swollen in toluene (10 ml) for
about 2 h. To this was added Pd(OAc)2 (0.25 mmol, 0.056 g) and
the mixture was stirred at 90 ◦ C for about 12 h. The color of the
beads changed from yellow to dark brown. The resulting beads
were filtrated and washed with methanol, and dried at 100 ◦ C
under vacuum overnight. The palladium content was 5.8% as
determined by ICP.
Scanning electron microscopy (SEM)
SEM analyses were performed with a JEOL JSM-6380LV instrument,
operating at 30 kV.
The functionalized polymer was prepared and characterized as
we described previously.[22] The preparation of the catalyst was
designed by the sequence of reactions given in Scheme 2.
Preparation of Catalyst
Preparation of 2-[3-(2- formylphenoxy)-2-hydroxypropoxy]
benzaldehyde
Salicylaldehyde (0.11 mol, 11.36 g) was added to 100 ml aqueous
sodium hydroxide (0.11 mol, 4.4 g) and heated to 60 ◦ C under
nitrogen. Then, epichlorohydrin (0.05 mol, 4.36 g) was added
dropwise within 2 h. The mixture was stirred continuously for
another 4 h. After cooling to room temperature, the yellow
precipitate was filtrated off, washed with water for several
times, and dried under vacuum. It was recrystallized with
methanol–water (8 : 1, v/v) affording a yellow oily solid, and then
several portions of water were added to afford the desired product
to a yield of 34%.
General Procedure for the Heck Reaction
In a typical reaction, a mixture of aryl halide (1.0 mmol), styrene
(1.5 mmol), K2 CO3 (2.0 mmol), DMF (6 ml) and catalyst (0.5 mol%
Pd) was stirred at 100–125 ◦ C for a certain time and the progress
of the reaction was monitored by GC-MS. After the completion of
the reaction, the mixture was cooled to room temperature. The
solid catalyst was separated by filtration, washed with water to
remove base and salt, and finally with dichloromethane to remove
adsorbed organic substrate, and dried at 100 ◦ C under vacuum
for the next cycle. The filtrate was diluted with water, followed by
extraction with dichloromethane. The combined organic phase
was washed with brine and dried over anhydrous Na2 SO4 . The
solvent was removed and the crude products were purified by
flash chromatography with n-hexane/EtOAc as eluent to afford the
corresponding products. All the products were known compounds
and were identified by comparison of their physical and spectra
data with those of authentic samples.
Results and Discussion
Synthesis and Characterization of the Palladium Catalyst
Preparation of polymer-supported macrocycle Schiff base
800
Pre-washed chloromethylated polystyrene resin 1.0 g (1.0 mmol
Cl) was allowed to swell in DMF (15 ml) for 24 h. Then, 2-[3(2-formylphenoxy)-2-hydroxypropoxy]benzaldehyde (1.5 mmol,
0.45 g), K2 CO3 (0.276 g, 2.0 mmol), and 18-crown-6 (0.053 g,
0.2 mmol) were added to the mixture. The mixture was stirred
at 100 ◦ C for 24 h. The polymer beads were filtered off, washed
with distilled water, dichloromethane and ethanol and then dried
under vacuum. After that, the beads obtained were swollen in
methanol (5 ml) for 2 h, and a solution of o-phenylenediamine
(1.0 mmol, 0.108 g) and a drop of concentrated HCl were added.
wileyonlinelibrary.com/journal/aoc
The catalyst was designed using the sequence of reactions given
in Scheme 2. In order to ascertain the functionalized polymer and
its corresponding Pd complex, IR spectra were recorded separately
at different stages of preparation. As can be seen from Figure 1,
the spectrum of chloromethylated polystyrene resin shows an absorption band at 1265 cm−1 , which is attributed to the C–Cl bond,
was weakened after the introduction of 2-[3-(2-formylphenoxy)-2hydroxypropoxy]benzaldehyde. Correspondingly, a strong band
at 1689 cm−1 in curve B was assigned to the C O vibration.
Moreover, a bond at 1689 cm−1 disappeared in curve C after the introduction of o-phenylenediamine. The range of 1600–1500 cm−1
c 2011 John Wiley & Sons, Ltd.
Copyright Appl. Organometal. Chem. 2011, 25, 799–803
Macrocyclic Schiff base palladium complex as an catalyst for the Heck reaction
Scheme 2. Preparation of polymer-supported macrocyclic Schiff base palladium complex.
Figure 1. IR spectra of (A) chloromethylated polystyrene resin, (B) polymer-supported 2-[3-(2-formylphenoxy)-2-hydroxypropoxy]benzaldehyde,
(C) polymer-supported macrocyclic Schiff base, and (D) polymer-supported macrocyclic Schiff base palladium complex.
corresponds to the υ(C C) and υ(C N) stretch of aromatic rings.
In the polymer-supported palladium complexes, both υAr – O – C and
υC N undergo a slight positive shift, indicating the palladium is
chelated with the nitrogen and oxygen atom.
A scanning electron micrograph was recorded to understand
the morphology of the surface of the support and catalyst. As can
easily be seen from Figure 2, the resin beads have different size and
roughness. The presence of Pd caused changes, as demonstrated
by polymer particle size decrease and roughness of the surface
(Figure 2).
Optimization of the Reaction Conditions
Appl. Organometal. Chem. 2011, 25, 799–803
c 2011 John Wiley & Sons, Ltd.
Copyright wileyonlinelibrary.com/journal/aoc
801
To check the potency of the chloromethylated polystyrene resinsupported macrocyclic Schiff base palladium catalyst, it was used
in Heck reactions. The effects of base, solvent and reaction
temperature were screened to optimize reaction conditions
(Table 1). Initially, K2 CO3 , NaOAc, NaOH, K3 PO4 , and NEt3 were
investigated as bases. As a result, K2 CO3 was found to be the
most effective base (Table 1, entries 1–5). Slightly lower yields and
selectivities were obtained when K3 PO4 , NaOH or NEt3 was used
as base (Table 1, entries 2, 4 and 5). However, NaOAc gave lower
yields than other bases even after 4 h reaction time (Table 1, entry
3). Next, the effects of different solvents were studied. According
to publications from Stambuli et al.,[23] Zapf and Beller,[24] and
Böhm and Herrmann,[25] polar, aprotic solvents tend to give the
best results for Heck coupling. Among the evaluated polar and
non-polar solvents, DMF was the most productive (Table 1, entry
1). Lower catalyst activities were found in other solvents such
as DMAc, CH3 CN and toluene (Table 1, entries 6–8). We further
evaluated the effects of temperature on the reaction. As illustrated
in Table 1 (entries 9 and 10), reducing the reaction temperature also
Y. He and C. Cai
Figure 2. SEM images of (a) polymer-supported macrocyclic Schiff base; (b) the fresh palladium catalyst.
Table 1. Screening reaction conditions for the Heck reaction of
iodobenzene and ethyl acrylatea
◦
Entry
Solvent
Base
Temperature ( C)
Time (h)
Yield (%)b
1
2
3
4
5
6
7
8
9
10
DMF
DMF
DMF
DMF
DMF
DMAc
Toluene
CH3 CN
DMF
DMF
K2 CO3
NaOH
NaOAc
K3 PO4
NEt3
K2 CO3
K2 CO3
K2 CO3
K2 CO3
K2 CO3
100
100
100
100
100
100
100
80
80
50
2
2
4
2
2
2
4
4
2
2
99
98
43
96
92
90
12
28
85
57
a Reaction conditions: iodobenzene (1.0 mmol), ethyl acrylate
(1.5 mmol), base (2.0 mmol); catalyst (0.5 mol%) in 6 ml solvent.
b Isolated yield.
led to a decrease in yield. Therefore, based on the above results,
we selected K2 CO3 as the base, DMF as solvent, and 0.5 mol% with
respect to Pd of catalyst at 100 ◦ C as the best conditions for the
Heck reaction.
Heck Reaction of Aryl Halides with Styrene Catalyzed by the
Supported Catalyst
802
Encouraged by the efficiency of the reaction protocol described
above, we investigated the substrate scope. As can be seen
in Table 2, a range of aryl iodides and bromides were found
to give the desired products in high yields. Aryl iodides with
electron-withdrawing or electron-donating groups underwent
efficient couplings with alkenes for reaction times of 1–2 h at
100 ◦ C (Table 2, entries 1–7). As for aryl bromides with electronwithdrawing groups in the para positions, satisfactory yields were
obtained when the reaction was carried out for 2–4 h at 125 ◦ C
wileyonlinelibrary.com/journal/aoc
Table 2. Heck reaction of aryl halides and olefinsa
Entry
X
R
R
Temperature
(◦ C)
Time
(h)
All
yield (%)b
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
I
I
I
I
I
I
I
Br
Br
Br
Br
Br
Br
Br
Br
Br
H
p-NO2
p-Me
m-Me
o-Me
H
m-Me
p-CF3
p-NO2
p-CHO
p-COMe
p-Cl
p-NO2
p-COMe
p-Me
o-Me
COOEt
COOEt
COOEt
COOEt
COOEt
Phenyl
Phenyl
COOEt
COOEt
COOEt
COOEt
COOEt
Phenyl
Phenyl
COOEt
COOEt
100
100
100
100
100
100
100
125
125
125
125
125
125
125
140
140
2
1
2
2
2
2
2
2
2
2
2
4
3
3
5
5
99
99
96
94
92
97(88/12)c
95(83/17)c
98
99
95
93
82
87(93/7)c
84(9/1)c
24
Trace
a
Reaction conditions: aryl halides (1.0 mmol), olefin (1.5 mmol), K2 CO3
(2.0 mmol); catalyst (0.5 mol%) in 6 ml DMF.
b Isolated yield.
c E/Z ratio shown in parentheses.
(Table 2, entries 8–14). However, aryl bromides with electrondonating groups in the para and ortho positions were less reactive
and even 140 ◦ C was required (Table 2, entries 15 and 16).
Reusability of the Catalyst
Reusability is an important feature that determines the applicability of heterogeneous catalysts. Therefore, the recovery and
reusability of the catalyst were investigated using the reaction of
iodobenzene with ethyl acrylate as a model system. After the first
use of the catalyst in the Heck reaction, the reaction mixture was
c 2011 John Wiley & Sons, Ltd.
Copyright Appl. Organometal. Chem. 2011, 25, 799–803
Macrocyclic Schiff base palladium complex as an catalyst for the Heck reaction
Table 3. Study on the reusability of the catalyst in the Heck reactiona
negligible for the original catalyst, which was also confirmed by
the excellent recoverability and reusability of this heterogeneous
catalyst (Table 3).
Conclusions
Entry
Run
Yields (%)b
1
2
3
4
5
1st
2nd
3rd
4th
5th
99
95
96
90
82
We have successfully applied a polymer-supported macrocyclic
Schiff base palladium complex in the Heck reaction. The catalyst
exhibits high activity, affording a diverse range of products in
good to excellent yields. Furthermore, the catalyst is stable to
the reaction conditions and can be recycled without a significant
loss in activity. The easy separation and availability make such
supported palladium catalysts an interesting alternative to the
homogeneous catalysts.
References
a
Reaction conditions: iodobenzene (1.0 mmol), ethyl acrylate
(1.5 mmol), K2 CO3 (2.0 mmol); catalyst (0.5 mol%) was added initially
in 6 ml DMF, 100 ◦ C.
b Isolated yield.
treated as described in the Experimental section. The recovered
catalyst was used successfully for next four subsequent reactions
and exhibited consistent catalytic activity, which indicated the
excellent reusability of this heterogeneous catalyst (Table 3).
Hot Filtration Experiments
Although extensive studies have been carried out to elucidate
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filtrate was determined by ICP. It was shown that less than 0.5%
of the total amount of the original palladium species was lost into
solution during the course of a reaction. This leaching level was
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c 2011 John Wiley & Sons, Ltd.
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