Silica-coated nano-Fe3O4-supported iminopyridine palladium complex as an active, phosphine-free and magnetically separable catalyst for Heck reactions

Article abstract of DOI:10.1002/aoc.3608

A novel magnetic nanoparticle-supported iminopyridine palladium complex was successfully prepared by attaching palladium acetate to iminopyridine ligand-functionalized silica-coated nano-Fe₃O₄. The as-prepared catalyst was well characterized and was evaluated in Heck reactions in terms of activity and recyclability. It was found to be highly efficient for the reactions of various aryl iodides and aryl bromides having electron-withdrawing groups with olefins under phosphine-free and inert atmosphere-free conditions. Moreover, the catalyst could be conveniently recovered using an external magnet, and the recyclability was influenced by the base in the Heck reaction. The catalyst could be reused at least six times with no significant loss in activity when triethylamine acted as the base.

Full text of DOI:10.1002/aoc.3608

Received: 18 July 2016
DOI 10.1002/aoc.3608
Revised: 1 August 2016
Accepted: 15 August 2016
F U L L PA P E R
Silica‐coated nano‐Fe O ‐supported iminopyridine palladium
3
4
complex as an active, phosphine‐free and magnetically separable
catalyst for Heck reactions
Qiang Zhang1*
| Xin Zhao | Huai‐Xin Wei | Ji‐Hang Li | Jun Luo
1
1
1
2*
1
Jiangsu Key Laboratory of Environmental
Functional Materials, School of Chemistry,
Biology and Material Engineering, Suzhou
University of Science and Technology, Suzhou
A novel magnetic nanoparticle‐supported iminopyridine palladium complex was
successfully prepared by attaching palladium acetate to iminopyridine ligand‐func-
tionalized silica‐coated nano‐Fe O . The as‐prepared catalyst was well characterized
3
4
2
15009, China
and was evaluated in Heck reactions in terms of activity and recyclability. It was
found to be highly efficient for the reactions of various aryl iodides and aryl bro-
mides having electron‐withdrawing groups with olefins under phosphine‐free and
inert atmosphere‐free conditions. Moreover, the catalyst could be conveniently
recovered using an external magnet, and the recyclability was influenced by the base
in the Heck reaction. The catalyst could be reused at least six times with no signif-
icant loss in activity when triethylamine acted as the base.
2
School of Chemical Engineering, Nanjing
University of Science and Technology, Nanjing
10094, China
2
Correspondence
Qiang Zhang, Jiangsu Key Laboratory of
Environmental Functional Materials, School of
Chemistry, Biology and Material Engineering,
Suzhou University of Science and Technology,
Suzhou 215009, China. Jun Luo, School of
Chemical Engineering, Nanjing University of
Science and Technology, Nanjing 210094, China.
Email: zhangqiang005@gmail.com; luojun@njust.
edu.cn
KEYWORDS
Heck reaction, heterogeneous catalyst, iminopyridine palladium, magnetic
nanoparticle
1
|
INTRODUCTION
Magnetic nanoparticles (MNPs) have attracted much
interest as novel support materials due to high surface area,
The palladium‐catalyzed Heck reaction of aryl halides with
good stability, facile functionalization and magnetic separa-
[
18,19]
olefins is one of the most important and powerful strategies
tion, low cost and low toxicity.
Because of these attrac-
[
1–3]
for C─C bond formation in modern organic synthesis.
tive features, a variety of MNP‐supported catalysts have
[
20,21]
And the substituted olefins are versatile and valuable building
blocks in the fields of pharmaceuticals, natural products and
been designed and widely applied in coupling reactions.
For example, Du et al. prepared an efficient MNP‐supported
[4,5]
[22]
functional materials.
Usually, the reaction is catalyzed
phosphine nano‐Pd catalyst for Suzuki and Heck reactions.
by palladium salts associated with ligands, such as phos-
Zolfigol et al. prepared an active magnetically separable Pd
[
6,7]
phines,
which stabilize the palladium species and prevent
catalyst by the immobilization of PdCl
nano‐Fe treated with ClPPh and used it in coupling reac-
And Li et al. designed another phosphine Pd
2
on silica‐coated
the appearance of inactive palladium black. However, homo-
geneous catalytic processes suffer from the tedious separation
of the catalyst in order to avoid contamination of residual
3
23]
O
4
2
[
tions.
complex for C─C coupling reactions.
group developed phosphanamine‐functionalized MNPs
[
24]
Lately, Panahi's
[
8]
metals in the final products, especially for pharmaceuticals.
[
25]
Additionally, in view of economic and environmental
concerns, the recovery and reuse of the expensive palladium
catalyst are highly desirable, particularly in large‐scale syn-
thesis. Thus, many efforts have focused on the development
of environmentally benign and highly efficient heterogeneous
for the Pd‐catalyzed Heck reaction.
Shaikh et al. used
MNP‐supported ferrocenylphosphine–Pd as a reusable
[
26]
catalystforHeckolefination. AndGholinejadetal.designed
a new phosphinite‐functionalized Fe ‐supported Pd catalyst
O
3 4
[
27]
for Suzuki coupling. Phosphines are some of the most pop-
ular ligands. However, they are air‐sensitive, expensive and
virulent, which limits wide use in large‐scale application. So,
it is still desirable to develop phosphine‐free MNP‐supported
Pd catalysts with high activity, excellent stability and low cost.
[9]
palladium catalysts for this coupling. A series of organic
[
10,11]
and inorganic materials, such as polymers,
ionic
[
12‐,3]
[14,15]
[16]
[17]
liquids,
silica,
carbon and metal oxides, have
been explored for immobilizing palladium catalysts.
Appl Organometal Chem 2016; 1–9
wileyonlinelibrary.com/journal/aoc
Copyright © 2016 John Wiley & Sons, Ltd.
1

2
ZHANG ET AL.
Schiff bases are known as types of powerful organic
ligands, which are widely applied in transition metal‐cata-
2.2 | Synthesis of Pd(OAc) @MNP
2
2
.2.1 | Synthesis of silane‐functionalized iminopyridine ligand
[
28,29]
lyzed reactions.
Iminopyridine bidentate ligands, a
[42]
(
2)
new type of Schiff base, have appeared as attractive ligands
A mixture of pyridine‐2‐carbaldehyde (5.35 g, 50.0 mmol),
p‐aminophenol (5.46 g, 50.0 mmol) and methanol (25 ml)
was stirred for 3 h under reflux. Then, the mixture was cooled
to ambient temperature and the iminopyridine ligand 1 was
obtained immediately by filtration (6.44 g, 65%).
[
30]
[31]
for Pd‐catalyzed polymerization,
cyclization
or Heck
[
32]
reactions. Previous interesting works on these N,N‐chelat-
ing ligand‐modified silica‐ or biomaterial‐supported Pd cata-
lysts for C─C couplings by Clark's group
[
33–35]
motivate
further exploration of better heterogeneous Pd catalysts.
More recently, some similar Pd complexes immobilized on
Compound 1 (3.97 g, 20.0 mmol) and NaH (0.72 g,
3
0.0 mmol) were added into 30 ml of anhydrous
dimethylformamide (DMF) and the resulting mixture was
stirred for at room temperature. 3‐
[
10]
[36,37]
[38–40]
polymer,
porous silica
or magnetic supports
have emerged as active catalysts for Pd‐catalyzed C─C cou-
pling reactions.
2
h
Chloropropyltriethoxysilane (5.06 g, 21 mmol) was then
added dropwise and the mixture was stirred at 85°C under
nitrogen atmosphere for 24 h, and then cooled and filtered.
The filtrate was concentrated under vacuum. The desired
product 2 was obtained by purification with silica gel chro-
matography (hexane–EtOAc, 5:1) as a yellow oil (5.8 g,
Driven by the unique properties of MNPs and the valu-
able applications of iminopyridine bidentate ligands and in
continuation of our efforts in designing heterogeneous cata-
[
41–44]
lysts,
herein we present a silica‐coated nano‐Fe O ‐
3
4
supported iminopyridine Pd complex (Pd(OAc) @MNP;
2
Scheme 1) as a new magnetically separable catalyst for the
Heck reaction of aryl halides with olefins.
7
2%).
2
.2.2 | Synthesis of iminopyridine‐functionalized MNPs (3)
A mixture of SiO @Fe O (1.0 g) and dry toluene (50 ml)
2
3 4
2
2
|
EXPERIMENTAL
was sonicated for 1 h, and compound 2 (0.5 g, 1.24 mmol)
and pyridine (0.25 ml, 3.08 mmol) were then added. After
stirring for 24 h under reflux in nitrogen atmosphere, the
mixture was cooled to ambient temperature. The product 3
was collected using a permanent magnet, washed several
times with toluene and acetone and then dried under vacuum
overnight.
.1 | General remarks
Silica‐coated nano‐Fe O (SiO @Fe O ) was prepared
according to our previously reported procedure.
vents used were strictly dried according to standard opera-
tions and stored over 4 Å molecular sieves. Other chemicals
3
4
2
3 4
[41]
All sol-
(AR grade) were commercially available and used without
further purification. Infrared (IR) spectra were obtained with
a Nicolet spectrometer (KBr). NMR spectra were recorded
2
2
.2.3 | Synthesis of palladium catalyst Pd(OAc) @MNP (4)
Powder 3 (1.0 g) was added into an acetone (50 ml) solu-
tion of palladium acetate (67.2 mg, 0.30 mmol), and the
mixture was stirred at room temperature for 24 h under
nitrogen atmosphere. Then, the solid was collected using
a permanent magnet, washed several times with acetone
and then dried under vacuum overnight to afford the
Pd(OAc) @MNP catalyst. The content of Pd was 1.57%
as determined using inductively coupled plasma atomic
emission spectrometry (ICP‐AES).
with
a
Bruker DRX500 (500 MHz) spectrometer.
Thermogravimetric analysis (TGA) was carried out under
nitrogen using a Shimadzu TGA‐50 spectrometer. Elemental
analysis was performed with an Elementar Vario EL III
recorder. Transmission electron microscopy (TEM) images
were obtained with a JEM‐2100 instrument. Palladium con-
tent of the catalyst was measured using inductively coupled
plasma (ICP) analysis with an L‐PAD analyzer (Prodigy).
The magnetization curve was obtained with a vibrating sam-
ple magnetometry (VSM) instrument (JDM‐13 T). Quantita-
tive analysis of the conversion yield was conducted using GC
2
2
.3 | General procedure for heck reactions
(Varian CP‐3800).
Aryl halide (1.0 mmol), olefin (1.5 mmol), Pd(OAc) @MNP
2
(
(
35.0 mg, 0.5 mol%), triethylamine (2.0 mmol) and DMF
5 ml) were added to a tube and sealed. The reaction mixture
was stirred at 100°C for an appropriate time. After comple-
tion of the reaction, the catalyst was separated using a perma-
nent magnet, and washed sequentially with water, ethanol
and ether, and dried under vacuum. The obtained DMF solu-
tion was diluted with water (15 ml) and extracted with ether
(2 × 10 ml). The combined ether fractions were dried over
SCHEME 1 Synthetic route to MNP‐supported palladium catalyst
Pd(OAc)2@MNP
Na SO , filtered and concentrated. The residue was purified
2
4

ZHANG ET AL.
3
by flash chromatography on silica gel (petroleum ether–ethyl
acetate, 10:1) to afford the corresponding products.
J = 16.0 Hz, 1H), 4.28 (q, J = 7.1 Hz, 2H), 2.39 (s, 3H),
1.36 (t, J = 7.1 Hz, 3H).
1
(
E)‐Ethyl 3‐(4‐acetylphenyl)acrylate (l).
H
NMR
(
500 MHz, CDCl , δ, ppm): 7.97–7.95 (m, 2H), 7.69 (d,
3
J = 16.0 Hz, 1H), 7.61–7.59 (m, 2H), 6.52 (d, J = 16.0 Hz,
H), 4.28 (q, J = 7.1 Hz, 2H), 2.61 (s, 3H), 1.34 (t,
J = 7.1 Hz, 3H).
2
.4 | Analytical data for products
1
1
(
E)‐Methyl cinnamate (a). H NMR (500 MHz, CDCl , δ,
3
1
ppm): 7.72 (d, J = 16.0 Hz, 1H), 7.61–7.48 (m, 2H), 7.41–
.39 (m, 3H), 6.46 (d, J = 16.0 Hz, 1H), 3.83 (s, 3H).
(E)‐Ethyl 3‐(4‐formylphenyl)acrylate (m). H NMR
7
(500 MHz, CDCl , δ, ppm): 9.99 (s, 1H), 7.86 (d,
3
1
(
E)‐Ethyl cinnamate (b). H NMR (500 MHz, CDCl , δ,
J = 8.2 Hz, 2H), 7.68–7.63 (m, 3H), 6.51 (d, J = 16.0 Hz,
1H), 4.25 (q, J = 7.1 Hz, 2H), 1.31 (t, J = 7.1 Hz, 3H).
3
ppm): 7.71 (d, J = 16.0 Hz, 1H), 7.55–7.53 (m, 2H), 7.41–
.38 (m, 3H), 6.46 (d, J = 16.0 Hz, 1H), 4.29 (q,
J = 7.1 Hz, 2H), 1.36 (t, J = 7.1 Hz, 3H).
1
7
(E)‐Ethyl 3‐(3‐formylphenyl)acrylate (n). H NMR
(500 MHz, CDCl , δ, ppm): 10.01 (s, 1H), 7.99 (s, 1H),
3
1
(
E)‐Butyl cinnamate (c). H NMR (500 MHz, CDCl , δ,
7.86 (d, J = 7.6 Hz, 1H), 7.74 (d, J = 7.7 Hz, 1H), 7.69 (d,
J = 16.1 Hz, 1H), 7.54 (t, J = 7.7 Hz, 1H), 6.50 (d,
J = 16.0 Hz, 1H), 4.25 (q, J = 7.1 Hz, 2H), 1.32 (t,
3
ppm): 7.71 (d, J = 16.0 Hz, 1H), 7.56–7.53 (m, 2H), 7.41–
.38 (m, 3H), 6.47 (d, J = 16.0 Hz, 1H), 4.23 (t,
J = 6.7 Hz, 2H), 1.71–1.66 (m, 2H), 1.48–1.44 (m, 2H),
7
J = 7.1 Hz, 3H).
1
0.99 (t, J = 7.4 Hz, 3H).
(E)‐Ethyl 3‐(4‐(trifluoromethyl)phenyl)acrylate (o).
H
1
(
E)‐Stilbene (d). H NMR (500 MHz, CDCl , δ, ppm):
NMR (500 MHz, CDCl , δ, ppm): 7.68 (d, J = 16.0 Hz,
3
3
7
2
.54 (d, J = 7.4 Hz, 4H), 7.38 (t, J = 7.7 Hz, 4H), 7.28 (m,
1H), 7.64–7.60 (m, 4H), 6.50 (d, J = 16.0 Hz, 1H), 4.28 (q,
H), 7.14 (s, 2H).
J = 7.1 Hz, 2H), 1.34 (t, J = 7.1 Hz, 3H).
1
1
(
E)‐Butyl 3‐(4‐bromophenyl)acrylate (e). H NMR
(E)‐Ethyl 3‐(4‐nitrophenyl)acrylate (p).
H
NMR
(500 MHz, CDCl , δ, ppm): 7.61 (d, J = 16.0 Hz, 1H),
(500 MHz, CDCl , δ, ppm): 8.25 (d, J = 8.6 Hz, 2H),
3
3
7
.52 (d, J = 8.3 Hz, 2H), 7.39 (d, J = 8.4 Hz, 2H), 6.43 (d,
J = 16.0 Hz, 1H), 4.21 (t, J = 6.7 Hz, 2H), 1.71–1.65 (m,
H), 1.44–1.40 (m, 2H), 0.96 (t, J = 7.4 Hz, 3H).
7.73–7.67 (m, 3H), 6.56 (d, J = 16.0 Hz, 1H), 4.30 (q,
J = 7.1 Hz, 2H), 1.36 (t, J = 7.1 Hz, 3H).
1
2
(E)‐Ethyl 3‐(4‐cyanophenyl)acrylate (q).
H NMR
1
(
E)‐Butyl 3‐(4‐nitrophenyl)acrylate (f).
H
NMR
(500 MHz, CDCl , δ, ppm): 7.69–7.65 (m, 3H), 7.61 (d,
3
(500 MHz, CDCl , δ, ppm): 8.26–8.24 (m, 2H), 7.69–7.67
J = 8.3 Hz, 2H), 6.52 (d, J = 16.0 Hz, 1H), 4.29 (q,
3
(m, 3H), 6.57 (d, J = 16.0 Hz, 1H), 4.25 (t, J = 6.7 Hz,
J = 7.1 Hz, 2H), 1.35 (t, J = 7.1 Hz, 3H).
2
H), 1.74–1.68 (m, 2H), 1.47–1.43 (m, 2H), 0.98 (t,
J = 7.4 Hz, 3H).
E)‐Butyl 3‐(4‐hydroxyphenyl)acrylate (g). H NMR
500 MHz, CDCl , δ, ppm): 7.63 (d, J = 16.0 Hz, 1H),
1
(
3
| RESULTS AND DISCUSSION
(
3
7
1
2
.45–7.42 (m, 2H), 6.86–6.83 (m, 2H), 6.31 (d, J = 16.0 Hz,
H), 5.69 (s, 1H), 4.21 (t, J = 6.7 Hz, 2H), 1.71–1.67 (m,
The Pd(OAc) @MNP catalyst was prepared via the proce-
2
dure shown in Scheme 1. Firstly, the precursor silane‐func-
tionalized iminopyridine ligand 2 was synthesized by
nucleophilic substitution of iminopyridine ligand 1 and (3‐
chloropropyl)triethoxysilane under basic conditions. Sec-
ondly, a condensation reaction of SiO @Fe O with precur-
H), 1.48–1.43 (m, 2H), 0.97 (t, J = 7.4 Hz, 3H).
1
(
E)‐Butyl 3‐(4‐aminophenyl)acrylate (h). H NMR
(500 MHz, CDCl , δ, ppm): 7.59 (d, J = 15.9 Hz, 1H), 7.34
3
(d, J = 8.5 Hz, 2H), 6.66–6.63 (m, 2H), 6.24 (d, J = 15.9 Hz,
2
3 4
1
H), 4.18 (t, J = 6.7 Hz, 2H), 3.94 (s, 2H), 1.70–1.64 (m, 2H),
sor 2 afforded iminopyridine ligand‐functionalized MNPs
(3). Ultimately, the obtained 3 was reacted with palladium
acetate in acetone at room temperature for 24 h to afford
1.43–1.40 (m, 2H), 0.96 (t, J = 7.4 Hz, 3H).
1
(
E)‐Butyl 3‐(4‐methoxyphenyl)acrylate (i). H NMR
(
500 MHz, CDCl , δ, ppm): 7.63 (d, J = 16.0 Hz, 1H),
the target catalyst, Pd(OAc) @MNP.
3
2
7
1
2
.46–7.44 (m, 2H), 6.89–6.87 (m, 2H), 6.30 (d, J = 16.0 Hz,
H), 4.18 (t, J = 6.7 Hz, 2H), 3.80 (s, 3H), 1.68–1.64 (m,
In order to confirm the successful functionalization of
MNPs, IR spectroscopy was employed to allow a detailed
investigation of SiO @Fe O , precursor 2 and functionalized
H), 1.44–1.39 (m, 2H), 0.95 (t, J = 7.4 Hz, 3H).
2
3 4
1
(
E)‐Ethyl 3‐(2‐methoxyphenyl)acrylate (j). H NMR
MNPs 3 (Figure 1). The Si─O─Si and Fe─O vibrations of
(
7
500 MHz, CDCl , δ, ppm): 8.00 (d, J = 16.2 Hz, 1H),
.51 (dd, J = 7.6, 1.2 Hz, 1H), 7.36–7.32 (m, 1H), 6.96 (t,
SiO @Fe O can be obviously observed at 1090.8 and
3
2
3
−
4
1
578.6 cm , respectively. The IR spectrum of precursor 2
−1
J = 7.5 Hz, 1H), 6.91 (d, J = 8.3 Hz, 1H), 6.53 (d,
J = 16.2 Hz, 1H), 4.27 (q, J = 7.1 Hz, 2H), 3.88 (s, 3H),
1
shows typical bands at around 1589 cm (C═N vibration),
−
1
1497 cm (C═C vibration of aryl ring), 2926 and 2879 cm
−1
.34 (t, J = 7.1 Hz, 3H).
(alkyl chain stretching vibrations). While in the spectrum
1
(
E)‐Ethyl 3‐p‐tolylacrylate (k). H NMR (500 MHz,
of functionalized MNPs 3, these characteristic peaks are at
similar wavenumbers, with a slight offset due to the interac-
tion with the support. Nevertheless, none of these significant
CDCl , δ, ppm): 7.68 (d, J = 16.0 Hz, 1H), 7.44 (d,
J = 8.1 Hz, 2H), 7.21 (d, J = 8.0 Hz, 2H), 6.41 (d,
3

4
ZHANG ET AL.
FIGURE 1 IR spectra of (a) SiO2@Fe3O4, (b) precursor 2 and (c) func-
tionalized MNPs 3
features can be observed in the spectrum of the SiO @Fe O
3 4
2
support. Moreover, the nitrogen content of the functionalized
MNPs 3 determined by elemental analysis is 0.56%, which
shows that the loading amount of the iminopyridine ligand
−
1
is 0.20 mmol g . These results indicate that the
iminopyridine ligand was successfully grafted onto
SiO @Fe O .
2
3 4
The resulting functionalized MNPs 3 were treated with an
acetone solution of palladium acetate at ambient temperature
to provide the desired catalyst Pd(OAc) @MNP (4). The Pd
2
content measured using ICP‐AES is 1.57%, which indicates
−
1
the loading amount of Pd is 0.15 mmol g . The morphology
of Pd(OAc) @MNP was investigated using TEM. The dark
2
nano‐Fe O cores are surrounded by grey silica shell and
3
4
the mean grain size is 20–25 nm (Figure 2a).
The stability of the Pd(OAc) @MNP catalyst was also
2
investigated using TGA (Figure 3). The TGA curve indicates
an initial weight loss of 1.1% up to 120°C, owing to the
adsorbed water catalyst on the support. The organic fractions
are decomposed in the range 150–500°C. The obvious ther-
mal degradation of the catalyst occurs after 200°C, which
reveals good stability. The main weight loss of the organic
moieties is about 8.0%, which is well consistent with the ele-
mental analysis.
FIGURE 2 TEM images of (a) fresh catalyst, (b) catalyst reused six times
with triethylamine as the base and (c) catalyst reused three times with
potassium carbonate as the base
The magnetic property of Pd(OAc) @MNP was evalu-
2
ated using VSM at room temperature. The VSM magnetiza-
tion curve goes through the zero point (Figure 4). The
phenomenon of no magnetic hysteresis indicates that the as‐
prepared catalyst is superparamagnetic. As a result, the cata-
lyst can be simply separated from the reaction system using
an external magnet.
The activity of Pd(OAc) @MNP was initially investi-
2
gated in the Heck reaction of iodobenzene and butyl acrylate.
The effects of base, solvent, temperature and Pd loading were
screened to optimize reaction conditions (Table 1). Initially,
K CO , K PO , NaOAc, KF, NEt , NBu and pyridine were
2
3
3
4
3
3
investigated as bases. As a result, NEt is found to be the best
FIGURE 3 TGA of Pd(OAc)2@MNP
3

ZHANG ET AL.
5
out under solvent‐free conditions, which afford the product
with 76% yield (Table 1, entry 13). Finally, the temperature
and Pd loading were evaluated. Reducing the reaction tem-
perature or Pd loading leads to a decrease in yield (Table 1,
entries 14 and 16). Interestingly, Pd(OAc) @MNP
2
can promote the Heck reaction more efficiently than
[
42]
Pd(OAc) @MCM‐41
(Table 1, entry 17), which indicates
2
that the smaller particles allow more diffusion effects and
then accelerate the reaction.
Under the optimized reaction conditions, the Heck reac-
tions of a variety of aryl halides with olefins were investi-
gated, and the results are summarized in Table 2. Aryl
iodides with electron‐withdrawing or electron‐donating
groups undergo efficient couplings with olefins at 100°C
for 1–4 h, which afford the corresponding products in good
to excellent yields ranging from 80 to 98% (Table 2, entries
FIGURE 4 Magnetization curve of Pd(OAc)2@MNP
1
–11). As for aryl bromides with electron‐withdrawing
a
TABLE 1 Optimization of reaction conditions
groups, satisfactory yields (84–93%) are also obtained when
the coupling reactions are carried out at 120°C for 4 h
(
Table 2, entries 12–17). However, for other aryl bromides
b
Entry
Base
Solvent
DMF
Time (h)
Temp. (°C)
Yield (%)
and aryl chlorides as substrates, the catalytic system is less
effective even for a prolonged time or at higher temperature
(Table 2, entries 18–20). Since the Heck reaction with
1
2
3
4
5
6
7
8
9
K
K
2
3
CO
3
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
3
100
100
100
100
100
100
100
100
100
80
95
90
56
18
96
92
<5
90
56
22
28
62
76
72
98
67
82
PO
4
DMF
DMF
DMF
DMF
DMF
DMF
NMP
PhMe
EtOH
[
45]
NaOAc
KF
unsubstituted olefins is rarely investigated,
a preliminary
study of the coupling reactions with simple olefins was also
undertaken. Unfortunately, there is no substantial improve-
ment in the Heck reaction of iodobenzene with 1‐octene or
cyclohexene for our catalytic system (Table 2, entries 21
and 22).
NEt
NBu
Pyridine
3
3
NEt
NEt
NEt
NEt
NEt
NEt
NEt
NEt
NEt
NEt
3
3
3
3
3
3
3
3
3
3
To further evaluate the catalytic performance of
1
1
0
1
3
Pd(OAc) @MNP, a comparison was made of the activity
of various supported Pd catalysts in the Heck reaction of
iodobenzene with ethyl acrylate (Table 3). The as‐prepared
catalyst Pd(OAc) @MNP is an equally or more efficient
catalyst with respect to yield and turnover frequency
(TOF) value than previously reported ones.
The recyclability of the MNP‐supported Pd catalyst was
then investigated using the Heck reaction of iodobenzene
with butyl acrylate under the optimal conditions. After com-
pletion of the reaction, the catalyst was conveniently recov-
ered using an external magnet (Figure 5), washed
sequentially with water, ethanol and ether, and dried under
vacuum for the next run. As can be seen from Figure 6, the
catalyst can be reused six times with no significant loss in
activity. As we know, an excellent yield can also be obtained
when K CO acts as the base (Table 1, entry 1); thus another
2
CH
3
CN
3
80
c
1
2
DMF–H
None
DMF
DMF
DMF
DMF
2
O
3
100
100
80
1
1
3
1.5
1.5
1
2
4
d
1
5
6
100
100
100
e
1
1
6
f
7
1.5
a
Reaction conditions: iodobenzene (1.0 mmol), butyl acrylate (1.5 mmol), base
2.0 mmol), Pd(OAc) @MNP (0.5 mmol% Pd), solvent (5 ml).
(
b
2
Isolated yield.
c
1
:1, v/v.
d
2
Pd(OAc) @MNP (1.0 mmol% Pd).
e
Pd(OAc)
2
@MNP (0.1 mmol% Pd).
f
Pd(OAc)
2
@MCM‐41 (0.5 mmol% Pd).
2
3
base (Table 1, entry 5). Slightly lower yields are obtained
when K CO , K PO or NBu is used as the base (Table 1,
entries 1, 2 and 6). However, when NaOAc acts as the base,
a moderate yield is obtained and only trace product is
detected with pyridine as the base (Table 1, entries 3 and
recycling experiment using potassium carbonate as the base
was conducted. The catalyst loses its activity gradually
(Figure 6). After long‐term heat stress, some corrosion occurs
slowly due to the weak interactions between the silica layer of
the catalyst and the strong alkali K CO in strongly polar sys-
2
3
3
4
3
2
3
7
). Subsequently, various solvents were screened, DMF giv-
tem, then the appearance of Pd leaching or aggregation leads
to catalyst deactivation, which might be the main reason for
loss of activity. In contrast, with triethylamine as an organic
weak base, the corrosion of the silica layer could be
ing the best result (Table 1, entry 5). But, the reaction is less
efficient when hydrous DMF is used as the solvent (Table 1,
entry 12). It is worth noting that the reaction can be carried

6
ZHANG ET AL.
a
2
TABLE 2 Heck reactions of aryl halides with olefins catalyzed by Pd(OAc) @MNP
1
2
b
Entry
R
X
R
Product
Time (h)
Temp. (°C)
Yield (%)
1
H
H
I
CO
CO
CO
2
Me
1
100
97
98
96
2
3
I
I
2
Et
1.5
1.5
100
100
n
H
H
2
Bu
4
5
I
I
Ph
2
2
120
100
90
92
n
4‐Br
CO
2
2
Bu
n
6
7
8
9
4‐NO
4‐OH
4‐NH
2
I
I
I
CO
CO
CO
Bu
2
100
100
100
94
86
80
n
2
2
Bu
3.5
4
n
2
Bu
n
4‐OMe
2‐OMe
I
I
CO
CO
2
2
Bu
3
4
100
100
91
84
1
1
1
0
1
2
Et
Et
Et
4‐Me
I
CO
CO
2
2
2
4
100
120
93
88
4‐COCH
3
Br
1
1
3
4
4‐CHO
3‐CHO
Br
Br
CO
CO
2
2
Et
Et
4
4
120
120
92
84
(Continues)

ZHANG ET AL.
7
TABLE 2 (Continued)
Entry
R1
4‐CF
X
R2
Product
Time (h)
Temp. (°C)
Yield (%)b
1
5
3
Br
CO
2
Et
4
120
87
1
6
4‐NO
2
Br
CO
2
Et
4
120
93
17
18
19
20
4‐CN
H
Br
Br
Br
Cl
CO
CO
CO
CO
2
2
2
2
Et
Et
Et
Et
4
8
120
120
120
130
91
56
37
<5
4‐Me
4‐NO
12
12
2
c
2
1
2
H
H
I
I
n‐C
6
H
13
4
4
120
100
11
c
2
–C
4
H
8
–
6
a
Reaction conditions: aryl halide (1.0 mmol), olefin (1.5 mmol), NEt3 (2.0 mmol), Pd(OAc)2@MNP (0.5 mmol% Pd), DMF (5 ml).
b
Isolated yield.
c
Conversion yield of iodobenzene based on GC analysis.
2
TABLE 3 Comparison of Pd(OAc) @MNP with other catalysts for the Heck reaction of iodobenzene with ethyl acrylate
Entry
Catalyst (mol%)
SBA‐15/PrSO Pd (0.5)
TiO @Pd NPs (1)
Pd(OAc) /[HQ‐PEG‐DIL][BF
Pd–Schiff base complex/PS (0.5)
Pd‐DABCO‐γ‐Fe (1)
PdCl /[DEA][HAc] (1.5)
Conditions
Yield (%)
TOF (h−1)
[
[
[
[
[
[
[
[
[
14]
1
2
3
4
5
6
7
8
9
3
Et
Et
Et
3
3
3
N, toluene, 80°C, 4 h
N, DMF, 140°C, 10 h
N, 100°C, 2 h
95
93
94
99
88
95
78
98
100
99
90
48
9
17]
12]
11]
46]
13]
47]
48]
49]
2
2
4
] (1)
47
99
176
5
K
2
CO
3
, DMF, 100°C, 2 h
2
O
3
Et
3
N, DMF, 100°C, 0.5 h
2
100°C, 12 h
Pd(0)‐PVP (1)
K
K
2
CO
3
3
, EtOH, 80°C, 4 h
, DMF, 100°, 3.1 h
20
38
4
Pd/ZnO NPs (0.8)
2
CO
Pd/MNP@IL‐SiO
Pd@MOF‐3 (1.25)
Pd/Nf‐G (0.3)
2
(1)
Et
Et
Et
Et
3
3
3
3
N, DMAc, 110°C, 24 h
n
[50]
51]
10
11
12
N, BuOH, 80°C, 24 h
3
[
N, DMF, 120°C, 2 h
N, DMF, 100°C, 1.5 h
150
131
Pd(OAc)
2
@MNP (0.5)
98
negligible. Excitingly, TEM analysis of these two reused cat-
alysts confirms the hypotheses. Apparently, the morphology
of the catalyst reused six times with triethylamine as the base
is similar to that of the fresh catalyst (Figure 2b). However, as
can be seen from the morphology of the catalyst reused three
times with potassium carbonate as the base, parts of grey

8
ZHANG ET AL.
4
| CONCLUSIONS
In summary, we have developed a novel MNP‐supported
iminopyridine Pd complex as a new magnetically separable
catalyst for the Heck reaction of aryl halides with olefins.
High activity was observed and the catalyst could be conve-
niently recovered using an external magnet. Interestingly,
the recyclability of Pd(OAc) @MNP was obviously
2
influenced by the base in the Heck reaction. The catalyst
could be reused at least six times with no significant loss in
activity when triethylamine acted as the base.
ACKNOWLEDGMENTS
We are grateful for financial support from the Natural Sci-
ence Foundation of Jiangsu Province of China (no.
BK20150282) and the Scientific Research Foundation of
Suzhou University of Science and Technology (no.
XKQ201417). And we are grateful to Excellent Innovation
Team in Science and Technology of University in Jiangsu
Province and Collaborative Innovation Center of Technology
and Material of Water Treatment for discussions.
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How to cite this article: Zhang, Q., Zhao, X., Wei,
H.‐X., Li, J.‐H., and Luo, J. (2016) Silica‐coated
nano‐Fe3O4‐supported iminopyridine palladium com-
plex as an active, phosphine‐free and magnetically sep-
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