Palladium nanoparticles generated from allylpalladium chloride in situ: A simple and highly efficient catalytic system for Mizoroki-Heck reactions

Article abstract of DOI:10.1016/j.jorganchem.2011.10.002

The Mizoroki-Heck reactions of aryl halides catalyzed by palladium nanoparticles generated in situ from a simple allyl palladium precursor were investigated in argon. The high turnover numbers of 9,300,000 have been obtained with 4-bromobenzonitrile as substrate and 3500 with 4-nitrochlorobenzene. When the reaction was performed in air, a low yield was given, but it could be improved obviously by addition of PEG-400. The main reason was that inactive Pd(II) species could be rapidly reduced to the active Pd(0) by PEG. In other word, the existence of air and PEG led to a synergistic effect which the oxidation by air prevents the aggregation of Pd NPs and the reduction by PEG maintains the high activity of Pd(0) species.

Full text of DOI:10.1016/j.jorganchem.2011.10.002

Journal of Organometallic Chemistry 697 (2012) 1e5
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Short communication
Palladium nanoparticles generated from allylpalladium chloride in situ: A simple
and highly efficient catalytic system for MizorokieHeck reactions
*
Wei Wang, Qin Yang, Rong Zhou, Hai-Yan Fu, Rui-Xiang Li , Hua Chen, Xian-Jun Li
Key laboratory of Green Chemistry and Technology, Ministry of Education, College of Chemistry, Sichuan University, 610064 Chengdu, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 June 2011
Received in revised form
The MizorokieHeck reactions of aryl halides catalyzed by palladium nanoparticles generated in situ from
a simple allyl palladium precursor were investigated in argon. The high turnover numbers of 9,300,000
have been obtained with 4-bromobenzonitrile as substrate and 3500 with 4-nitrochlorobenzene. When
the reaction was performed in air, a low yield was given, but it could be improved obviously by addition
of PEG-400. The main reason was that inactive Pd(II) species could be rapidly reduced to the active Pd(0)
by PEG. In other word, the existence of air and PEG led to a synergistic effect which the oxidation by air
prevents the aggregation of Pd NPs and the reduction by PEG maintains the high activity of Pd(0) species.
Ó 2011 Elsevier B.V. All rights reserved.
17 September 2011
Accepted 4 October 2011
Keywords:
MizorokieHeck reaction
Palladium nanoparticles
Allyl complex
PEG
1. Introduction
2. Experimental
The palladium-catalyzed coupling reactions of alkenes with aryl
halides, known as the MizorokieHeck (MH) reaction [1e4], are
among the most important methods for carbonecarbon bond
formation [5e7], and have been extensively used in natural product
synthesis, pharmaceutical industry and bioorganic chemistry
2.1. General information
[Pd(C )Cl] was synthesized with the reported method [31].
3
H
5
2
The chemicals from commercial sources were used as received
except DMF, DMA and styrene, which were redistilled prior to use.
All reactions were carried out in oven-dried Schlenk tube in argon
[8e13]. Over the past decades, lots of efforts have been paid to look
1
for the efficient catalytic systems, such as palladium salts [14e18],
palladacycle complexes [19,20], palladium nanoclusters immobi-
lized on supports [21e23], and palladium-phosphine complexes
except the reactions in air. H NMR spectra were recorded on
a Bruker Avance II-400 MHz with reference to TMS as the internal
standard in CDCl
on an Agilent GC 6890
m. Transmission electronic microscopy
3
. Gas chromatograph analysis was performed
[24e26]. Although palladacycle and palladiumephosphine
N
with an SE-30 column of
complex systems show high catalytic performance, the synthesis of
ligands is tedious. The ligand-free palladium catalysts is a real
simple system in practical use [27,28]. For example, Pd nano-
30 m ꢀ 0.32 mm ꢀ 0.25
m
2
(TEM) was analyzed in a Tecnai G F20 S-TWIN microscope with the
operation voltage of 200 kV.
particles (Pd NPs), which were formed with Pd(OAc)
2
[29,30] and
[
Pd(CH CN) Cl ]·6Ph PCl [15], could convert many substrates into
3
2
2
4
the desired products in high yields. However, the high palladium
loadings in these systems are not beneficial to the industrial and
synthetic applications.
It is of current interest to develop a new catalytic system which
meets the goals of simple operation procedure, low catalyst
loading, and high activity toward aryl halides. Herein, we would
2.2. General procedure for the MH coupling reaction
4-Nitrochlorobenzene (158 mg, 1 mmol), Na
2 mmol), and n-Bu NBr (240 mg, 0.75 mmol) were added into
a dried schlenk tube with a magnetic bar, and then styrene (150 L,
1.5 mmol), the solution of [Pd(C )Cl] (100 L, 0.0005 mmol) in
2 3
CO (212 mg,
4
m
H
3 5
2
m
like to report that Pd NPs generated from [Pd(C
performed the high reactivity for MH coupling of aryl halides.
3
H
5
)Cl]
2
in situ,
DMA (3 mL) were added successively into it. The reaction mixture
was heated in 120 C for 4 h with stirring. After the reaction mixture
ꢁ
was cooled to room temperature, water (4 mL) was added into it. The
mixture solution was extracted with ethyl acetate (3 ꢀ 5 mL). The
organic phase was washed with brine (3 ꢀ 5 mL) and dried with
*
Corresponding author. Tel./fax: þ86 28 85412904.
E-mail addresses: liruixiang@scu.edu.cn, scu.ruixiang@gmail.com (R.-X. Li).
2 4
anhydrous Mg SO . The dried solution was filtered and evaporated
0
022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2011.10.002

2
W. Wang et al. / Journal of Organometallic Chemistry 697 (2012) 1e5
under vacuum to give the crude product. The product was purified
by silica gel chromatography (petroleum ether).
3
. Results and discussion
The use of Pd NPs catalysts for the MH reactions is not a new
subject and PdCl
precursors. To our knowledge, [Pd(C
catalyzed MH coupling is not reported, so we tried to investigate the
catalytic performance of [Pd(C )Cl] as the precursor. Firstly, the
2
, Pd(OAc)
2
and Pd(dba)
2
etc. are widely used as the
3 5
H
)Cl] as a precursor for NPs
2
H
3 5
2
effects of solvents and additives on MH coupling were investigated.
Optimization of the reaction conditions showed that DMA was the
best solvent and n-Bu NBr (TBAB) which had been demonstrated to
4
stabilize Pd NPs [32,33] was the most efficient additive. The perfor-
mance of other catalyst precursors was also investigated in detail
(
Table 1). The results indicated that all precursors showed good
activity and afforded the yields of 75e88% with substrate to catalyst
the amount of palladium) ratio (S/C) of 1000 (Table 1, entries 1, 3, 5,
and 7). According to the reported results on Pd NPs generated from
PdCl and Pd(OAc) , the turnover numbers (TON) are between 1000
and 100,000 in general [16,29]. Obviously, our results with PdCl and
Pd(OAc) as precursors are comparable with the reported results
under the similar reaction conditions [27]. However, when S/C was
raised to 5, 000, the reactivity of [Pd(C )Cl] as precursor was
superior to PdCl , Pd(OAc) and Pd(dba) and it gave a TON of 3500
Table 1, entries 2, 4, 6, and 8). Thus, we chose [Pd(C )Cl] as the
catalyst precursor for the following experiments.
Whether [Pd(C )Cl] converted into Pd NPs like PdCl
Pd(OAc) under the reaction conditions? The relationship between
(
Fig. 1. Conversion vs time plot (reaction conditions: 4-O
2
NC
6 4 3 5
H Cl (1 mmol), [Pd(C H )
2
2
Cl]
2
(0.05 mmol%), styrene (1.5 mmol), Na
2
CO
3
(2 mmol), DMA (3 mL), TBAB
ꢁ
(0.75 mmol), in argon, 120 C). (-) Control experiment; (B) Hg(0) (500:1 Hg/Pd)
2
poisoning experiment.
2
H
3 5
2
the various catalyst precursors, TEM was used to detect the micro-
graphs of palladium nanoparticles generated from different palla-
dium precursors in the period of the highest activity (Fig. 2). It
2
2
2
(
H
3 5
2
revealed that Pd NPs formed with [Pd(C
average size of nanoparticles was 2.7 nm after running for 20 min
(Fig. 2a). In contrast, the size of nanoparticles formed with PdCl was
7.5 nm (Fig. 2b) under the same reaction conditions and it was very
close to that formed with [Pd(C )Cl] for 2 h (Fig. 2c). It indicated
that Pd NPs generated from [Pd(C )Cl] were more stable than
those from PdCl and could maintain the small size for a longer time.
We speculate that the allyl group in [Pd(C )Cl] does not dissociate
from palladium even if [Pd(C )Cl] converts into Pd NPs, which
effectively stabilizes Pd NPs and makes them maintain the small size.
According to the reported results, the CeC coupling in
phosphine-free Pd nanoparticle systems can be efficiently carried
out in the presence of PEG in air [36e38]. For the Pd NPs catalyzed
cross-coupling reactions, it also documented that Pd(0) species
would convert into PdO if the nanoparticles were exposed to an
3 5 2
H )Cl] were small and the
3
H
5
2
2
and
2
2
conversion vs. time and the effect of mercury on the reaction were
further investigated (Fig. 1). According to the curve of conversion vs.
time, this catalytic system had the induction period of about 10 min,
and then a high activity was exhibited after running for 15 min. The
addition of mercury resulted in the complete loss of catalytic activity.
These phenomena are typical characters of nanoparticle catalysis
and the reaction process is closely related to heterogeneous catalysis
3
H
5
2
3
H
5
2
2
H
3 5
2
3
H
5
2
[20,32]. According to the reported results, some olefins such as
methyl acrylate could reduce PdCl to Pd NPs [34,35]. So it was
2
reasonable to suggest that the formation of Pd(0) NPs in this system
resulted from the reduction of styrene or n-butyl acrylate to
3 5 2
[Pd(C H )Cl] . To explore the reason of the reactivity difference with
Table 1
a
MH reaction between 4-nitrochlorobenzene and styrene by different catalyst precursors.
Cl
[
Pd] / Na CO3
2
+
O N
o
2
DMA, TBAB, 120 C
2
O N
Entry
[Pd]
Time
[ArX]/[Pd]
Yield (%)
1
2
3
4
PdCl
PdCl
Pd(OAc)
Pd(OAc)
Pd(dba)
Pd(dba)
[Pd(C
[Pd(C
[Pd(C
2
2 h
24 h
2 h
24 h
2 h
24 h
2 h
24 h
20 h
1000
5000
1000
5000
1000
5000
1000
5000
1000
75
34
86
38
80
37
88
70
51
2
2
2
b
5
2
6
7
8
2
3
3
3
H
H
H
5
)Cl]
5
)Cl]
5
)Cl]
2
2
2
c
9
a
ꢁ
2
4-O NC
6
H
4
Cl (1 mmol), styrene (1.5 mmol), Na
2
CO
3
(2 mmol), DMA (3 mL) was degassed, TBAB (0.75 mmol), in argon, 120 C, E-isomer was obtained selectively in all cases
(
>95%), GC yields.
b
dba: dibenzylideneacetone.
c
No TBAB was added.

W. Wang et al. / Journal of Organometallic Chemistry 697 (2012) 1e5
3
Fig. 2. TEM micrographs and nanoparticles size distributions: (a) [Pd(C
3
H
5
)Cl]
2
as catalyst precursor and reaction for 20 min, (b) PdCl
2
as catalyst precursor and reaction for 20 min,
(
c) [Pd(C
3
H
5
)Cl]
2
as catalyst precursor and reaction for 2 h. Conditions: 4-O
2
6 4 2 3
NC H Cl (1 mmol), styrene (1.5 mmol), [Pd] (0.1 mmol%), Na CO (2 mmol), DMA (3 mL), TBAB
ꢁ
(
0.75 mmol), in argon, 120 C.
oxygen-containing environment [33,39]. Furthermore, it suggested
that the oxidized Pd NPs were the real active species and the
binding of oxygen with palladium nanoparticles could inhibit the
aggregation of Pd NPs in PEG-400 [40]. In a sharp contrast, our Pd
NPs catalytic system must be performed in inert gas to keep the
high activity without PEG. In air, a very low yield was given even
the reaction was run for a longer time (Table 2, entry 5). However,
the addition of PEG-400, which is as a stabilizer, could make the
reaction progress rapidly in air (Table 2, entry 6). But the existence
of PEG-400 obviously inhibited the catalytic activity of this system
in argon atmosphere (Table 2, entries 2 and 3) due to the quick
formation of palladium black. According to our results, the oxidized
nanoparticles showed low or no reactivity to CeC coupling reac-
tion, and the real active species should be Pd(0) species.
argon, the peak at 345 nm was disappeared, revealing that full
conversation of [Pd(C )Cl] to Pd(0) NPs [34]. Surprisingly,
3
H
5
2
addition of styrene in air, a new absorption peak was appeared at
350 nm, indicating that the generation of Pd(II). The peak at 350 nm
was disappeared when PEG-400 was added to this mixture in air
[37]. Furthermore, the carbonyl adsorption peak was found by the
FT-IR detection of the reaction solution at the end of the reaction.
These results clearly showed that styrene acted as the reducing
agent for [Pd(C H )Cl] to produce the TBAB-stabilized Pd(0) NPs in
3 5 2
argon, oxygen in air could oxidize Pd(0) NPs to Pd(II), and PEG-400
could quickly reduce Pd(II) to Pd(0) in air. Obviously, the high
In order to investigate the role of PEG-400 and air in the MH
3 5 2
reaction, the UVeVis spectra of [Pd(C H )Cl] under the different
reaction conditions is shown in Fig. 3. An absorption peak was
observed at 345 nm when a mixture of [Pd(C
3
H
5
)Cl]
2
and TBAB at
ꢁ
100 C for 30 min in argon. Addition of styrene to this mixture in
Table 2
The effect of air and PEG-400 on the MH reaction of 4-nitrochlorobenzene and
styrene.a
Entry
1b
Conditions
PEG-400
Time
Yield (%)
Ar
Ar
Ar
air
air
air
None
1 mL
1 mL
None
None
1 mL
2 h
2 h
18 h
2 h
18 h
2 h
88
47
52
21
23
70
b
2
3
b
4
5
6
a
2
4-O NC
6
H
4
Cl (1 mmol), styrene (1.5 mmol), [Pd(C
3
H
5
)Cl]
2
(0.05 mol%), Na
2
CO
3
ꢁ
(
2 mmol), DMA (3 mL), and TBAB (0.75 mmol), 120 C, E-isomer was obtained
selectively in all case (>95%), GC yields.
Fig. 3. UVeVis absorption spectra of the palladium nanoparticles under the different
reaction conditions.
b
DMA was degassed.

4
W. Wang et al. / Journal of Organometallic Chemistry 697 (2012) 1e5
Fig. 4. proposed mechanism of Pd-catalyzed MH reaction under aerobic or inert gas conditions.
The catalytic performance of this system for different aryl
halides and alkenes was investigated (Scheme 1). When S/C was
0,000, the yields of activated aryl bromides with styrene were
1
good to excellent in short time (Table 3, entries 1, 3, 5, 8, and 10).
Although the moderate yield (52%) was obtained when S/C was
increased to 100,000 (Table 3, entry 11), a good yield of 85% was
achieved by extending the reaction time (44 h). Especially, the
reaction of aryl bromides bearing electron withdrawing groups
with n-butyl acrylate could be performed at the very low catalyst
loadings (Table 3, entries 16e21). The maximum TON of 9,300,000
Scheme. 1. Pd-Catalyzed MH Reaction of different substrates.
ꢁ
was achieved for 4-NCC
6
H
4
Br at 130 C (Table 3, entry 21). To the
best of our knowledge, this value is one of the highest TON for MH
coupling in the absence of phosphine ligand. Our catalytic system
works much more efficiently than many Pd systems, including
palladacycles without phosphine [20] and palladium-
tetraphosphine [41]. Furthermore, this system was effective to
some inactivated substrates, such as bromobenzene, 3-
bromoanisole, and 4-bromoanisole (Table 3, entries 13e15). It
activity of palladium particles in the presence of PEG and air is
owing to the cooperation of the oxidation by oxygen and the
reduction by PEG-400. The oxidation process prevents the aggre-
gation of Pd NPs and the reduction process maintains the high
activity of Pd(0) species. The proposed mechanism is shown in
Fig. 4.
Table 3
3 5 2
MH coupling reaction of aryl bromides with alkenes catalyzed by [Pd(C H )Cl]
.a
ꢁ
Yield(%b)
Entry
ArX
Alkene
Base
[ArX]/[Pd]
T ( C)
t (h)
1
2
3
4
5
6
7
4-O
4-O
2
NC
NC
6
H
H
4
Br
Br
Br
Br
Styrene
Styrene
Styrene
Styrene
Styrene
Styrene
Styrene
Styrene
Styrene
Styrene
Styrene
Styrene
Styrene
Styrene
Styrene
n-butyl acrylate
n-butyl acrylate
n-butyl acrylate
n-butyl acrylate
n-butyl acrylate
n-butyl acrylate
n-butyl acrylate
n-butyl acrylate
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
10,000
100,000
10,000
100,000
10,000
100,000
100,000
10,000
100,000
10,000
100,000
100,000
10,000
10,000
10,000
120
130
120
130
120
130
130
120
130
120
130
130
120
120
120
120
130
130
130
130
130
120
120
14
24
14
24
14
24
24
14
24
14
24
24
24
24
24
8
14
24
24
24
44
24
24
100(89)
60
100(89)
71
100(96)
81
96(92)
100(62)
68
2
6
4
4-F
4-F
3
CC
CC
6
H
H
4
3
6
4
4-AcC
4-AcC
3-AcC
4-OHCC
4-OHCC
3-OHCC
3-OHCC
4-NCC
PhBr
4-CH
3-CH
6
6
6
H
H
H
4
Br
4
Br
4
Br
c
8
9
6
H
4
4
4
4
Br
6
6
6
H
H
H
Br
Br
Br
10
11
12
13
14
15
16
17
18
19
20
21
22
23
100(93)
52
6
H
4
Br
100(98)
93(87)
73(66)
97(90)
100(96)
99
100(94)
99(95)
100(96)
93
Na
2
Na
2
Na
2
CO
3
CO
3
CO
3
3
OC
OC
6
H
H
4
Br
Br
3
6
4
4-AcC
4-AcC
6
H
H
4
Br
Br
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
10,000
6
4
100,000
500,000
500,000
5,000,000
10,000,000
10,000
4-O
2
NC
6
H
4
Br
Br
4-OHCC
6
H
4
4-NCC
4-NCC
PhBr
6
H
H
4
Br
Br
6
4
Na
Na
2
CO
CO
3
74(66)
61(47)
4-CH
3
OC
6
H
4
Br
2
3
10,000
a
b
c
ArX (1 mmol), Alkene (1.5 mmol), Base (2 mmol), DMA (3 mL), and TBAB (0.75 mmol), E- isomer was obtained selectively in all case (>95%).
GC yields. Isolated yields were shown in parentheses.
Due to the product being oxidized in air results in the low isolated yield.

W. Wang et al. / Journal of Organometallic Chemistry 697 (2012) 1e5
5
Table 4
MH coupling reaction of aryl chlorides with alkenes catalyzed by [Pd(C
Appendix. Supporting information
3 5 2
H )Cl]
.a
Entry
1c
ArX
Alkene
[ArX]/[Pd]
Yield (%b)
Supporting information related to this article can be found
online at doi:10.1016/j.jorganchem.2011.10.002.
4-O
4-O
3-O
2-O
2
NC
NC
NC
NC
6
6
6
6
H
H
H
H
4
4
4
4
Cl
Cl
Cl
Cl
Styrene
Styrene
Styrene
Styrene
Styrene
Styrene
Styrene
Styrene
1000
5000
1000
500
500
500
500
500
500
500
500
500
500
500
99(92)
70
2
3
4
5
6
7
8
9
2
2
2
90(84)
92(85)
93(91)
96(90)
98(95)
Trace
Trace
99(98)
96(84)
82(77)
60(52)
88
References
4-NCC
2-NCC
4-AcC
PhCl
6
H
H
4
Cl
Cl
[
[
[
[
[
[
[
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[
[
[
[
a
ArX (1 mmol), Alkene (1.5 mmol), Na
2
CO
3
(2 mmol), DMA (3mL), and TBAB
ꢁ
(
0.75 mmol), 120 C, 24 h.
b
GC yields. Isolated yields were shown in parentheses.
Reaction time is 4 h.
c
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2
3
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(
Similarly, the catalyst system is also efficient for activated aryl
chlorides (Table 4), especially, for the sterically hindered 2-
nitrochlorobenzene and 2-chlorobenzonitrile (Table 4, entries 1e7).
Unfortunately, the low yields were obtained for the activated
terminal olefin of n-butyl acrylate (Table 4, entries 12 and 13). After
the amount of palladium precursors was increased to 0.1 mol%, the
reasonable yields could be gained (Table 4, entries 10 and 11). Prob-
ablya palladacycle led tothe lowactivity, because it wasreported that
a transient palladacycle could be formed by reaction of Pd species
with n-butyl acrylate and it was much more difficult to undergo
oxidative addition for inactivated aryl bromides and aryl chlorides
[
[
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[
[
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[
[
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In conclusion, we present a simple and highly efficient catalytic
system for the MH reaction. This catalytic system involving aryl
bromides as substrates has a very good activity as long as the
4
[
[
[
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amount of [Pd(C
. Furthermore, the catalytic system also gives excellent yields
toward activated aryl chlorides.
3 5 2
H )Cl] is in the low range of 0.005e0.000005 mol
%
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