Ultrasound-promoted ligand-free heck reaction in water

Article abstract of DOI:10.1080/00397911.2010.486511

An ultrasound irradiation-promoted Heck reaction catalyzed by ligand-free palladium acetate was described. The ultrasound-promoted Heck reaction was carried out under mild and environmentally friendly conditions, resulting in the corresponding products with good to excellent chemical yields (up to 96% yield) and high regioselectivities and stereoselectivities (E/Z99:1). Acrylamide and acrylic acid were used for the first time as substrates in the ultrasound-promoted Heck reaction. It was found that the reaction could be finished within 20min.

Full text of DOI:10.1080/00397911.2010.486511

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Ultrasound-Promoted Ligand-Free Heck
Reaction in Water
Guanghui An a , Xiaoyun Ji a , Jianlin Han a & Yi Pan a
a School of Chemistry and Chemical Engineering, Nanjing University,
State Key Laboratory of Coordination , Nanjing , China
Published online: 30 Mar 2011.
To cite this article: Guanghui An , Xiaoyun Ji , Jianlin Han & Yi Pan (2011) Ultrasound-
Promoted Ligand-Free Heck Reaction in Water, Synthetic Communications: An International
Journal for Rapid Communication of Synthetic Organic Chemistry, 41:10, 1464-1471, DOI:
10.1080/00397911.2010.486511
To link to this article: http://dx.doi.org/10.1080/00397911.2010.486511
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Synthetic Communications1, 41: 1464–1471, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2010.486511
ULTRASOUND-PROMOTED LIGAND-FREE HECK
REACTION IN WATER
Guanghui An, Xiaoyun Ji, Jianlin Han, and Yi Pan
School of Chemistry and Chemical Engineering, Nanjing University,
State Key Laboratory of Coordination, Nanjing, China
GRAPHICAL ABSTRACT
Abstract An ultrasound irradiation–promoted Heck reaction catalyzed by ligand-free
palladium acetate was described. The ultrasound-promoted Heck reaction was carried
out under mild and environmentally friendly conditions, resulting in the corresponding pro-
ducts with good to excellent chemical yields (up to 96% yield) and high regioselectivities
and stereoselectivities (E/Z > 99:1). Acrylamide and acrylic acid were used for the first
time as substrates in the ultrasound-promoted Heck reaction. It was found that the reaction
could be finished within 20 min.
Keywords Aqueous media; Heck reaction; palladium acetate; ultrasound
INTRODUCTION
The palladium-catalyzed Heck reaction is one of the most powerful tools for
carbon–carbon bond-formation processes in synthetic organic chemistry and organic
methodology.[1] The Heck reaction has been thoroughly investigated in a variety of
modifications,[2] and its extraordinary generality with a wide range of application for
numerous commercially important products has been demonstrated.[3] However, the
Heck reaction usually is performed under the protection of an inert atmosphere and
needs long reaction times and high temperature.[4] Furthermore, some efficient
catalysts developed for the Heck reaction require toxic solvents together with
air-sensitive bulky phosphine ligands and the presence of hindered amines.[5] In
recent years, the use of ionic liquids has been reported to try to improve the reaction
Received January 7, 2010.
Address correspondence to Jianlin Han or Yi Pan, School of Chemistry and Chemical Engineering,
Nanjing University, State Key Laboratory of Coordination, Nanjing 210093, China. E-mail: molab@
nju.edu.cn; yipan@nju.edu.cn
1464

ULTRASOUND-PROMOTED HECK REACTION
1465
rate. However, even done in such media, the reactions still involved long reaction
times (24–72 h) at temperatures ranging from 80 to 100 ꢀC.[6]
Ultrasound has been reported to have an effect on the reaction rate of many
organic reactions.[7] These reactions with the assistance of ultrasound radiation
usually can be carried out under mild conditions and can be completed within a very
short reaction time. Although in the past decade, some examples of Heck reactions
under ultrasound radiation have been reported,[8,9] ultrasound-promoted Heck reac-
tions have not been well documented and still remain a great challenge in the scope
of substrates, reaction time, reaction temperature, and solvent. During our continu-
ous research about the Heck reaction,[10] we found that the reaction between acryla-
mide and iodobenzene is completed within 20 min under ultrasound irradiation.
Herein, we report a ligand-free Heck reaction in water under ultrasound irradiation.
Acrylamide is studied for the first time in the Heck reaction under ultrasound
irradiation, with the reaction being faster than those reported so far.[8,9,11]
RESULTS AND DISCUSSION
In this work, an ultrasound probe was used to improve the efficiency of the
ultrasound radiation. The iodobenzene and acrylamide were chosen as substrates
for initial study. The ultrasound-promoted reaction was carried out in water with
K2CO3 as base catalyzed by Pd(OAc)2 in the presence of tetrabutylammonium bro-
mide (TBAB) at 80 ꢀC (Scheme 1). After 30 min, the desired product, (E)-
cinnamamide, was observed with 29% yield.
To improve the chemical yield of this ultrasound-promoted reaction, the reac-
tion condition was systematically optimized (Table 1). First, the surfactants for the
current reaction were investigated. Sodium dodecane-1-sulfonate was the most
efficient one among all the surfactants examined, and the best yield was detected
(86%, Table 1, entry 5). However, the previous reported efficient surfactant TBAB
for Heck reaction[8f] seems not a suitable one for the current system, and only
29% yield was obtained (entry 1). Other surfactants, such as benzyltriethylammo-
nium chloride and polyethylene glycol, did not improve the chemical yields (entries
2–4). The bases used in this reaction also have a great effect on the formation of pro-
ducts. Potassium carbonate was an efficient base for this reaction and gave the best
chemical yield (entry 5). When strong inorganic bases such as sodium hydroxide and
potassium hydroxide were used, almost no corresponding product was obtained
(entries 6 and 7). The weak bases KOAc and NaHCO3 also gave very poor yields
of products (entries 8 and 10). The temperature plays an important role on the
chemical yields. When the reaction was carried out at 60 ꢀC, only 47% yield was
observed (entries 11 and 12). A moderate yield was found when the temperature
Scheme 1. Heck reaction between iodobenzene and acrylamide under ultrasound.

1466
G. AN ET AL
Table 1. Optimization of reaction conditionsa
Entry
Surfactant
Base
Temp. (ꢀC) Time (min) E=Zb Yield (%)c
1
2
Tetrabutylammonium bromide (TBAB) K2CO3
80
80
80
80
80
80
80
80
80
80
40
60
70
90
100
90
90
90
90
90
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
5
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
29
21
75
59
86
<10
14
16
27
45
35
47
71
96
82
20
85
91
92
86
Benzyltriethylammonium chloride
PEG-200
PEG-1000
K2CO3
K2CO3
K2CO3
K2CO3
KOH
3
4
5
Sodium dodecane-1-sulfonate
Sodium dodecane-1-sulfonate
Sodium dodecane-1-sulfonate
Sodium dodecane-1-sulfonate
Sodium dodecane-1-sulfonate
Sodium dodecane-1-sulfonate
Sodium dodecane-1-sulfonate
Sodium dodecane-1-sulfonate
Sodium dodecane-1-sulfonate
Sodium dodecane-1-sulfonate
Sodium dodecane-1-sulfonate
Sodium dodecane-1-sulfonate
Sodium dodecane-1-sulfonate
Sodium dodecane-1-sulfonate
Sodium dodecane-1-sulfonate
Sodium dodecane-1-sulfonate
6
7
NaOH
KOAc
8
9
Na2CO3
NaHCO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
10
11
12
13
14
15
16
17
18
19
20
10
15
25
30
aIodobenzene (5 mmol), acrylamide (6 mmol), base (7.5 mmol), Pd(OAc)2 (0.05 mmol), surfactant
(0.5 mmol), and water (20 mL).
bDetermined by 1H NMR. Only E-isomer was detected.
cIsolated yield.
was increased to 70 ꢀC (entry 13). The greatest yield was observed when the reaction
was performed at 90 ꢀC (96% yield, entry 14). However, the chemical yield decreased
to 82% when the temperature was increased to 100 ꢀC (entry 15). The ultrasound-
promoted Heck reaction can be completed within 20 min. Although extending the
reaction time gave no improvement on chemical yields (entries 19 and 20), the yield
decreased to 85% when the reaction was stopped at 10 min (entry 17), and even lower
yield was obtained when the reaction was stopped at 5 min (entry 16).
After getting the optimized reaction conditions, a series of aryl iodides were
used as substrates and the corresponding products were observed in all cases
(Table 2). As shown in Table 2, this reaction has a broad substrate range, including
iodobenzene (Table 2, entry 1), and aryl iodides with electron-donating groups
(entries 2 and 3) or electron-withdrawing groups on aromatic rings (entries 4 and 5).
All reactions of the aryl iodides could give corresponding cinnamamide products
with modest to excellent chemical yields (42–96%). These substrates also showed
excellent stereoselectivities, and only E-isomers were observed for all the cases. How-
ever, in the case of the substrate containing bromine as a substituent on the aromatic
ring (entry 6), only a trace amount of cinnamamide product was observed.

ULTRASOUND-PROMOTED HECK REACTION
1467
Table 2. Heck reaction between aryl iodides and acrylamide under ultrasounda
Entry
Aryl iodide
R1
Products
E=Zb
Yield (%)c
1
2
3
4
5
6
1a
1b
1c
1d
1e
1f
H
o-CH3
m-CH3
p-Cl
2a
2b
2c
2d
2e
2f
>99:1
>99:1
>99:1
>99:1
>99:1
—
96
42
62
71
p-COCH3
p-Br
66
<10
aAryl iodide (5 mmol), acrylamide (6 mmol), K2CO3 (7.5 mmol), Pd(OAc)2 (0.05 mmol), sodium
dodecane-1-sulfonate (0.5 mmol), and water (20 mL) at 90 ꢀC for 20 min.
bDetermined by 1H NMR. Only E-isomer was detected.
cIsolated yields.
Besides acrylamide, three other active olefins (styrene, methyl acrylate, and
acrylic acid) were also found to be suitable for the reaction (Table 3). The
corresponding adducts were obtained in good yields of 54–82% and excellent stereo-
selectivities of > 99:1. A good yield was especially obtained for acrylic acid (82%,
Table 3. Heck reaction between iodobenzene and olefins under ultrasounda
Entry
1
Olefin
Products
E=Zb
Yield (%)c
>99:1
54
2
3
>99:1
>99:1
76
82
aIodobenzene (5 mmol), olefin (6 mmol), K2CO3 (7.5 mmol), Pd(OAc)2 (0.05 mmol), sodium dodecane-
1-sulfonate (0.5 mmol), and water (20 mL), at 90 ꢀC for 20 min.
bDetermined by 1H NMR. Only E-isomer was found.
cIsolated yields.

1468
G. AN ET AL
Table 3, entry 3). To our knowledge, this is also the first report about the acrylic acid
used as olefin substrate in the ultrasound-promoted Heck reaction.
In conclusion, the Heck reaction of aryl iodides and active olefins in water cat-
alyzed by Pd(OAc)2 is considerably accelerated by ultrasound radiation. Acrylamide
and acrylic acid were used for the first time in the ultrasound-promoted Heck reac-
tion and gave good yields and excellent stereoselectivities. The ultrasound promoted
Heck reaction could be completed within 20 min, which is faster than those reported
until now.
EXPERIMENTAL
Reagents and solvents were purchased as reagent grade and used without
further purification unless otherwise stated. All reactions were performed in standard
oven-dried glassware under a nitrogen atmosphere unless otherwise stated. Melting
points were determined using a Gallenkamp (Griffin) melting-point apparatus. Tem-
1
peratures are expressed in degrees Celsius (ꢀC) and are uncorrected. H and 13C
NMR spectra were recorded at 300 and 75 MHz on a Bruker AM 300-MHz spec-
trometer. Chemical shifts (d) are reported in parts per million relative to tetramethyl-
silane(TMS) (d ¼ 0 ppm) as internal standards. Coupling constants (J) are reported
in hertz (Hz). Multiplicities are reported as singlet (s), broad singlet (bs), doublet
of doublets (dd) or multiplet (m).
General Procedures for Ultrasound-Assisted Heck Reaction
A three-necked flask was charge with an ultrasound probe, and then 5 mmol of
iodobenzene, 6 mmol of acrylamide, 0.05 mmol of Pd(OAc)2, 7.5 mmol of K2CO3 as
base, 0.5 mmol of sodium dodecane-1-sulfonate, and 20 mL water were added in the
flask. The reaction mixture was stirring strongly to make a good dispersion. After stir-
ring, the mixture was directly placed in oil bath at 90 ꢀC, and ultrasound radiation was
started at the same time. After 20 min, the reaction mixture was filtered, evaporated,
and directly purified by preparative thin-layer chromatographic (TLC) plate.
Characterized Data for Products 2a–2i
Compounds 2a–2i are all known compounds.[11,12]
(E)-Cinnamamide (2a). White needle crystal. 1H NMR (300 MHz, CDCl3) d
(ppm) 7.67 (d, J ¼ 15.6 Hz, 1H), 7.52–7.55 (m, 2H), 7.38–7.40 (m, 3H), 6.49 (d,
J ¼ 15.6 Hz, 1H), 5.86 (br, 2H).
(E)-3-o-Tolylacrylamide (2b). White needle crystal. 1H NMR (300 MHz,
CDCl3) d (ppm) 7.50–7.52 (d, J ¼ 15.4 Hz, 1H), 7.08–7.14 (m, 2H), 7.01 (m, 1H),
6.91 (d, J ¼ 15.6 Hz, 1H), 5.57 (br, 2 H), 2.34 (s, 3H).
(E)-3-m-Tolylacrylamide (2c). White needle crystal. 1H NMR (300 MHz,
CDCl3) d (ppm) 7.56–7.59 (d, J ¼ 15.4 Hz, 1H), 7.13–7.18 (m, 2H), 7.01 (m, 1H),
6.86 (d, J ¼ 15.6 Hz, 1H), 5.57 (br, 2H), 2.34 (s, 3H).

ULTRASOUND-PROMOTED HECK REACTION
1469
(E)-3-(4-Chlorophenyl)acrylamide (2d). White needle crystal. 1H NMR
(300 MHz, CDCl3) d (ppm) 7.63 (d, J ¼ 15.8 Hz, 1H), 7.45 (d, J ¼ 8.4 Hz, 2H),
7.36 (d, J ¼ 8.4 Hz, 2H), 6.44 (d, J ¼ 15.8 Hz, 1H), 5.54 (br, 2H).
(E)-3-(4-Acetylphenyl)acrylamide (2e). White needle crystal. 1H NMR
(300 MHz, CDCl3) d (ppm) 7.87–7.89 (m, 2H), 7.59–7.64 (d, J ¼ 15.7 Hz, 1 H),
7.50–7.53 (m, 2H), 6.44–6.49 (d, J ¼ 15.7 Hz, 1H), 2.53 (s, 3H), 5.57 (br, 2H).
(E)-1,2-Diphenylethene (2g). White needle crystal. 1H NMR (300 MHz,
CDCl3) d (ppm) 7.51 (d, J ¼ 4.75 Hz, 4H), 7.35 (t, J ¼ 5.0 Hz, 4H), 7.24 (m, 2H),
7.10 (s, 2H).
(E)-Methyl cinnamate (2h). White needle crystal. 1H NMR (300 MHz,
CDCl3) d (ppm) 7.59 (d, J ¼ 15.7 Hz, 1H), 7.29–7.64 (m, 5H), 6.45 (d, J ¼ 15.7 Hz,
1H), 3.81 (s, 3H).
1
(E)-Cinnamic acid (2i). White needle crystal. H NMR (300 MHz, CDCl3) d
(ppm) 7.82 (d, J ¼ 15.5 Hz, 1H), 7.57–7.60 (m, 2H), 7.42–7.44 (m, 3H), 6.44 (d,
J ¼ 15.5 Hz, 1H).
ACKNOWLEDGMENTS
We gratefully acknowledge financial support from the National Natural
Science Foundation of China (No. 20772056 to Y.P.). This work was also partially
supported by research funds from the Qing-Lan Program of Jiangsu Province, the
Kua-Shi-Ji Program of the Education Ministry of China, and the Jiangsu 333
program (for Pan).
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