Direct coupling of benzene with olefin catalyzed by Pd(OAc)2 combined with heteropolyoxometalate under dioxygen

Article abstract of DOI:10.1021/ja028903a

The oxidative coupling reaction of benzenes with alkenes was successfully achieved by the Pd(OAc)₂/molybdovanadophosphoric acid (HPMoV)/O₂ system. For example, the reaction of benzene with ethyl acrylate by the above catalytic system in acetic acid afforded ethyl cinnamate as a major product in satisfactory yield. Copyright

Full text of DOI:10.1021/ja028903a

Published on Web 01/18/2003
Direct Coupling of Benzene with Olefin Catalyzed by Pd(OAc)2 Combined with
Heteropolyoxometalate under Dioxygen
Takahiro Yokota, Masayuki Tani, Satoshi Sakaguchi, and Yasutaka Ishii*
Department of Applied Chemistry, Faculty of Engineering, Kansai UniVersity, Suita, Osaka 564-8680, Japan
Received October 11, 2002 ; E-mail: ishii@ipcku.kansai-u.ac.jp
Table 1. Oxidative Coupling Reaction of 1 with 2 Catalyzed by
The Heck-Mizoroki arylation of olefins with aryl halides and
Pd(OAc)2 Combined with HPMoV under Dioxygen Atmospherea
triflates is a highly versatile synthetic transformation, but the
yield (%)
formation of undesired waste is unavoidable. In recent years, there
has been a growing demand for the development of a catalytic
method which can activate the C-H bond of arenes such as benzene
instead of halobenzenes in organic synthesis.1 Since the success of
the activation of aryl C-H bonds through ortho-chelation by Ru
complexes,2 such C-H bond activations of substituted arenes have
been made by Ru, Pd, and Rh complexes.1 Although several
transition metal compounds are reported to activate stoichiomet-
rically the C-H bond of nonsubstituted aromatic compounds, it is
still very difficult to carry out this catalytically.3 Fujiwara and co-
workers have reported the stoichiometric and catalytic oxidative
coupling of arenes with alkenes through the cleavage of aromatic
C-H bonds assisted by Pd compounds.4 In 1978, Matveev et al.
reported the Pd(II)-catalyzed arylation of ethylene in the presence
of molybdovanadophosphoric acid.5 Several authors have reported
catalysis by Pd compounds using peroxides and benzoquinones as
oxidants in the coupling of benzene with olefins.4,6 Quite recently,
a few important papers have appeared on the oxidative arylation
of alkenes using dioxygen as a terminal oxidant. Matsumoto et al.
showed that styrene was synthesized from the reaction of benzene
with ethylene catalyzed by an Rh complex in acetic acid with
elevated pressure of dioxygen.7 The Ru-catalyzed reaction of
benzene with acrylates under O2 (2 atm) and CO (6.1 atm) at 180
°C was reported by Milstein et al.8 However, these reactions call
for severe reaction conditions, high reaction temperature, and high
pressurized O2 and CO. Therefore, it has been desired to develop
a new catalytic system for the coupling reaction of olefins with
arenes via the C-H bond activation which can operate under normal
O2 pressure and at up to 100 °C.
run
HPMoV
base
conv. (%)
3
4
5
1
2
3
4
5
6
7b
8
9
10
11
12
13
H7PMo8V4O40
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
91
11
98
93
99
37
69
34
76
90
22
30
53
90
46
100
63
7
5
12
2
11
11
10
8
23
8
14
13
6
0
11
10
15
0
<1
0
1
3
0
3
1
3
0
9
10
73
10
H
6PMo9V3O40
66
63
63
23
28
12
46
54
17
19
34
54
24
50
72
8
H5PMo10V2O40
H4PMo11V1O40
H3PMo12O40
H7PMo8V4O40
H7PMo8V4O40
H7PMo8V4O40
H7PMo8V4O40
H7PMo8V4O40
H7PMo8V4O40
H7PMo8V4O40
H7PMo8V4O40
H7PMo8V4O40
H7PMo8V4O40
H7PMo8V4O40
H7PMo8V4O40
H7PMo8V4O40
LiOAc
CsOAc
KOAc
NH4OAc
Na2CO3
Na2CO3
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
6
14
13
14
10
1
14c
15d
16d,e
17f,g
18f
<1
1
19f,h
72
a 1 (15 mmol) was allowed to react with 2 (1.5 mmol) in the presence
of Pd(OAc)2 (0.1 mmol), HPMoV (43-47 mg, ca. 0.02 mmol), acetylac-
etone (0.1 mmol), and base (0.08 mmol) in acetic acid (5 mL) at 90 °C for
6 h under O2 (1 atm). Conversion and yields were based on 2 used. b 1
(7.5 mmol) was used. c Na2CO3 (0.04 mmol) was used. d In the absence of
acetylacetone. e Pd(acac)2 was used instead of Pd(OAc)2. f The reaction was
carried out using 1 (30 mmol) in propionic acid (5 mL). g The reaction
was carried out for 3 h. h The reaction was carried out under air (1 atm).
mixture was stirred at 90 °C for 6 h. The products were isolated
by column chromatography (230-400 mesh silica gel).
Table 1 summarizes the representative results for the reaction
of 1 with 2. The reaction of 1 with 2 catalyzed by the Pd(OAc)2/
H7PMo8V4O40/acetylacetone/NaOAc system afforded ethyl cin-
namate (3) (63%) and ethyl â-phenylcinnamate (4) (5%) together
with ethyl 3-acetoxyacrylate (5) (12%) derived from the reaction
of 2 with acetic acid (run 1). The reaction had difficulty taking
place in the absence of HPMoV (run 2). Several trends are apparent
from the data in the Table 1. Product distributions were slightly
influenced by the use of HPMoV differing in Mo and V content
(runs 3-5). The combined catalytic system of Pd(OAc)2 with 12-
molybdophosphoric acid afforded a less satisfactory result than that
with HPMoV (run 6). When the amount of 1 was halved, a
considerable amount of 5 as a byproduct was obtained (run 7). The
reaction without a base resulted in low yields of products (run 8).
Among the bases examined, sodium acetate was found to give the
best yield of 3 (66%). Lithium and cesium acetates led to 3 in 46
and 54% yields, respectively, but potassium and ammonium acetates
were inert (runs 9-12). Sodium carbonate gave 3 in moderate yields
(runs 13 and 14).
In the course of our study on the biphenyl synthesis from benzene
by Pd(II) using molybdovanadophosphate and O2 as a reoxidation
system,9 we found that the oxidative coupling reaction of benzenes
with acrylates is facilitated by the use of a catalytic amount of Pd-
(OAc)2 combined with molybdovanadophosphoric acid (HPMoV)
under dioxygen as a terminal oxidant (eq 1).
A typical reaction procedure is as follows: To an acetic acid
solution (5 mL) of Pd(OAc)2 (0.1 mmol), H7PMo8V4O40‚nH2O (43
mg), acetylacetone (0.1 mmol), and NaOAc (0.08 mmol) were
added benzene (1, 15 mmol) and ethyl acrylate (2, 1.5 mmol). The
flask was equipped with a balloon filled with O2 (1 atm). The
The reaction in the absence of acetylacetone resulted in the
decrease of 3 (run 15). It is known that the oxidative arylation of
9
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J. AM. CHEM. SOC. 2003, 125, 1476-1477
10.1021/ja028903a CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Table 2. Oxidative Coupling Reaction of Various Arenes with
Alkenes Catalyzed by Pd(OAc)2 Combined with H7PMo8V4O40
under Dioxygen Atmospherea
5). Styrene, however, reacted with difficulty to give trans-stylbene
in 32% yield (run 6). The reaction of toluene with 2 afforded a
regioisomeric mixture of coupling products in 69% yield (run 7).
Chlorobenzene coupled with 2 in the same fashion as benzene 1
giving a mixture of coupling products (o:m:p ) 27:40:33) in 68%
yield (run 8). Furane gave the corresponding product in satisfactory
yield even at 30 °C (run 9).
To obtain mechanistic information on the present reaction, a 1:1
mixture of benzene 1 and benzene-d6 1-d was reacted with 2 under
the same conditions as in Table 1, run 1. The ratio of the resulting
3 and 3-d was found to be approximately 3/3-d ) 4. A kinetic
isotope effect for the coupling of 1 with methyl acrylate is reported
to be kH/kd ) 2.8 Furthermore, no scrambling of deuterium was
observed. These facts indicate that the cleavage of the C-H bond
of 1 is the slowest step in a sequence of reactions. Because of the
complexity of the present catalytic system, it seems difficult to make
an accurate assessment about the reaction mechanism in the
coupling reaction. However, we thought that the reaction proceeds
via a reaction pathway similar to that proposed by Fujiwara et al;4b
that is, σ-aryl-Pd(II) complex reacts with alkene 2 followed by
â-hydride elimination to give aryl alkene 3 and Pd-H or Pd(0),
which is reoxidized by HPMoV/O2.10
In conclusion, we have documented a highly efficient Pd(II)-
catalyzed direct coupling reaction of benzenes with acrylates using
molecular oxygen as a terminal oxidant under mild conditions.
Further mechanistic investigations as well as synthetic applications
of the present catalytic system are in progress.
Acknowledgment. This work was partially supported by a
Grant-in-Aid for Scientific Research (S) (KAKENHI) (No. 13853008)
from the Japan Society for Promotion of Science.
References
a Arene (30 mmol) was allowed to react with alkene (1.5 mmol) in the
presence of Pd(OAc)2 (0.1 mmol), H7PMo8V4O40 (43 mg, ca. 0.02 mmol),
acetylacetone (0.1 mmol), and NaOAc (0.08 mmol) in propionic acid (5
mL) at 90 °C for 3 h under O2 (1 atm). Conversion and yields were based
on alkene used. b For 10 h. c For 2 h. d The product was obtained as an
isomeric mixture (o:m:p ) 14:42:44). e For 6 h. f The product was obtained
as an isomeric mixture (o:m:p ) 27:40:33). g Furane (3 mmol) was reacted
at 30 °C for 12 h in acetic acid (5 mL).
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adding a small amount of acetylacetone and that the formation of
a byproduct like vinyl acetate is depressed.7 However, Pd(acac)2
was employed in place of Pd(OAc)2 to form 3 and 4 in relatively
good yield even in the absence of acetylacetone (run 16).
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viewpoint, it is very important to note that the reaction in propionic
acid took place even under air (1 atm) to give 3 in 72% yield (run
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in Table 1. Representative results are shown in Table 2. 1 smoothly
reacted with several acrylate derivatives to form the corresponding
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(runs 1-4). The arylation of 1-phenylbuten-3-one under these
reaction conditions gave 1,1-diphenylbuten-3-one in 70% yield (run
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JA028903A
9
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