An efficient and recyclable ligand-free PEG1000-DIL/toluene temperature-dependent biphasic system for palladium catalysed Heck reactions

Article abstract of DOI:10.3184/003685011X13238914941786

The Heck reaction of aryl halides with terminal olefins was successfully performed by using Pd (OAc)₂ as the catalyst in a ligand-free PEG₁₀₀₀-DIL (PEG₁₀₀₀-based dicationic ionic liquid)/toluene system exhibiting temperature-dependent biphasic behavior. In addition, the isolation of the products is readily performed by a simply decantation, and the catalytic system can be recycled and reused without loss of catalytic activity.

Full text of DOI:10.3184/003685011X13238914941786

JOURNAL OF CHEMICAL RESEARCH 2011
DECEMBER, 731–733
RESEARCH PAPER 731
An efficient and recyclable ligand-free PEG -DIL/toluene temperature-
1
000
dependent biphasic system for palladium catalysed Heck reactions
Xiang Liu, Kai Yao, Yu-Lin Hu and Ming Lu*
College of Chemical Engineering, Nanjing University of Science andTechnology, 200 Xiaoling Wei, Nanjing 210094, P. R. China
The Heck reaction of aryl halides with terminal olefins was successfully performed by using Pd (OAc) as the catalyst
2
in a ligand-free PEG1000-DIL (PEG1000-based dicationic ionic liquid)/toluene system exhibiting temperature-dependent
biphasic behavior. In addition, the isolation of the products is readily performed by a simply decantation, and the
catalytic system can be recycled and reused without loss of catalytic activity.
Keywords: Heck reaction, Pd (OAc) , ligand-free, PEG1000/toluene
2
we now present the first efficient example of a free ligand
The Heck reaction has emerged as an extremely powerful,
palladium-catalysed Heck reaction in temperature-dependent
attractive, and widely utilised method for the construction
1
–7
organic solvent–ionic liquid biphasic system.
of carbon–carbon bonds. The reaction normally requires a
palladium catalyst to be present, particularly in the reactions of
Our group recently demonstrated that a novel PEG1000-based
dicationic ionic liquid (PEG1000-DIL, Scheme 1) not only
exhibited temperature-dependent phase behavior with toluene,
but also performed excellent catalytic activity in some
8
–12
aryl halides with olefins. However, the recovery and reuse
of the catalyst is highly difficult. In view of economical and
environmental reasons, the development of recyclable catalytic
systems is one of the most challenging fields in laboratory and
industry. In the past two decades, ionic liquids have attracted
increasing attention due to their excellent physicochemical
properties, such as reasonable thermal and chemical stability,
2
2,23
reactions.
In continuation of our previous work and to
extend our research in developing temperature-dependent
biphasic systems and their application in transition-metal-
catalysed reactions, we would like to explore the catalysis role
of PEG1000-DIL/toluene temperature-dependent biphasic system
for Heck coupling in the presence of the cheap and air-stable
1
3,14
negligible vapour pressure, and ease of recyclability.
Moreover, carbon–carbon coupling catalysed by a palladium
source in ionic liquids (IL) have been employed successfully.
15–17
Pd (OAc)2.
Initially, investigations were carried out using iodobenzene
However, these processes suffer from some limitations: (i) the
need for the use of toxic and expensive phosphine or N-hetero-
cyclic carbene ligands, and (ii) the workup complexity, because
most ILs show a degree of solubility in commonly used organic
solvents, which causes many difficulties for product separation
and recovery of the IL, especially in homogeneous catalytic
reaction systems. Therefore, the search for new readily
available ligand-free catalytic systems for addressing these
drawbacks remains an important goal.
and methyl acrylate as the reactants for optimising reaction
conditions in the PEG1000-DIL/toluene temperature-dependent
biphasic system. The effects of base, temperature and the
amount of catalyst on the yields in the coupling reactions are
summarised in Table 1.
As demonstrated in Table 1 (Table 1, entry 4), Et N as base
gave the desired product with high yield. Those inorganic
3
bases such as KOH, Na CO and K CO (Table 1, entries 1–3)
2
3
2
3
1
8
were worse bases for the coupling due to their poor solubility
in the biphasic system. We also optimised the concentration of
catalyst and found all the reactions examined led to some
degree of conversions but only the use of 2 mmol% palladium
acetategavethebestresult(Table1, entries4, 8, 9).Temperature
was an important factor in the range 80–110 °C (Table 1,
entries 4–7). The results indicated that the highest yield was
obtained at 110 °C. It is noteworthy that PEG1000-DIL/toluene
could not be a homogenous phase until the temperature was
Multiphase systems, especially liquid–liquid biphasic
1
9
systems, have been developed. Currently, some novel
temperature-dependent ionic liquid biphasic catalytic systems
including organic solvent/ionic liquid and supercritical carbon
2
0,21
dioxide/ionic liquid
have been reported and it has been
found that these catalytic systems have advantages such as
high conversions and selectivity, stability at high temperatures,
reusability in the reaction, etc., which provide a novel route for
separating and recycling of catalysts (Fig. 1). To the best of our
knowledge, there is no report on the palladium catalysed Heck
reaction in temperature-dependent biphasic systems. Hence,
8
0 °C at least.
For practical applications, the separation from reaction
product and reusability of the catalyst are very important
factors in Heck reactions. Easy catalyst separation and
recycling in successive batch operations can greatly increase
the efficiency of the reaction. To demonstrate this issue, the
recycling experiments were conducted for the coupling reaction
of iodobenzene and methyl acrylate. After the product was
isolated from the reaction mixture, the residue was washed
with toluene three times and the organic layer was decanted.
Finally, the ionic layer was concentrated under reduced
pressure to remove traces of toluene and the resulting liquid
Fig. 1 Temperature-dependant PEG1000-DIL catalytic system.
*
Correspondent. E-mail: Alexwpch@163.com
Scheme 1 Structures of PEG1000-DIL.

7
32 JOURNAL OF CHEMICAL RESEARCH 2011
Table 1 Optimised conditions via the coupling of iodobenzene
Table 2 Heck reactions of aryl Iodides with terminal olefinsa
a
with methyl acrylate
Yieldb/
%
Temp./ Conversionb/ Yieldc/
Entry
R1
R2
Time /
h
Entry
Base/
mmol
2
Pd(OAc) /
mmol%
o
C
%
%
1
2
3
4
5
6
7
8
9
1a, H
1a, H
1a, H
1a, H
1a, H
2a, COOMe
2b, COOEt
2c, COOn-Bu
2d, COOH
2e, Ph
4
3a, 95
3b, 94
3c, 91
3d, 98
3e, 96
3f, 94
3g, 92
3h, 91
3i, 95
1
2
3
4
5
6
7
8
9
KOH
2
2
110
110
110
110
80
100
120
110
110
68
75
60
70
73
95
82
88
93
50
70
4
Na
CO
3
2
4
K
2
CO
3
2
78
4
Et
Et
Et
Et
Et
Et
3
3
3
3
3
3
N
N
N
N
N
N
2
100
88
4
2
1b, 4-OMe 2a, COOMe
1b, 4-OMe 2b, COOEt
1b, 4-OMe 2c, COOn-Bu
1b, 4-OMe 2d, COOH
1b, 4-OMe 2e, Ph
1c, 4-NO
1c, 4-NO
1c, 4-NO
1c, 4-NO
1c, 4-NO
6
2
95
6
2
98
6
0.5
1
55
6
74
10
11
12
13
14
15
6
3j, 95
a
Reaction conditions: iodobenzene (1.0 mmol), acrylate (2.0 mmol),
, PEG1000-DIL (2.0 g), toluene (3.0 g).
Determined by GC analysis.
Isolated yield.
2
2
2
2
2
2a, COOMe
2b, COOEt
2c, COOn-Bu
2d, COOH
2e, Ph
2.5
2.5
2.5
2.5
2.5
3k, 99
3l, 96
3m, 95
3n, 99
3o, 99
base (2.0 mmol), Pd (OAc)
2
b
c
a
Reaction conditions: aryl iodides (1.0 mmol), terminal olefins
N (2.0 mmol), Pd(OAc) (2 mmol%), PEG1000-DIL
2.0 g), toluene (3.0 g), 110 °C, the reaction was monitored by
TLC.
was used for the next cycle under the same conditions. The
result in Fig. 2 shows that the catalytic activity is maintained
very well even after five cycles.
(2.0 mmol), Et
(
3
2
b
Isolated yield.
According to the above optimised conditions, we examined
the application of the PEG1000-DIL catalytic system to cross-
coupling various aryl iodides and terminal olefins. As shown
in Table 2, it was found that there were still electronic effects
of substituents on the yields of this reaction; electron-with-
drawing or electron-donating substituents reacted with olefins
rapidly and generated the coupled products with excellent
yields. For the same olefins, aryl iodide with an electron-
reaction time for completing the Heck reaction of aryl bromides
and aryl chloride with methyl acrylate. The results in Table 3
also show the electronic nature of the aryl bromides has a clear
effect on the coupling reactions. For example, the coupling
reaction of electron-rich 4-methyl-bromobenzene with methyl
acrylate provided a 68% isolated yield (Table 3, entry 4), but
for electron-deficient 4-nitro-bromobenzene, the desired
coupled product was obtained in 80% isolated yield (Table 3,
entry 2). However, it was unfortunate that poor yields were
obtained for an aryl chloride even if possessing electron-
drawing groups such as nitro (Table 3, entries 5–6).
withdrawing group such as NO (Table 2, entries 11–15) gave
2
a high yield in a shorter time than those cases with electron-
donating substituents (Table 2, entries 6–10). It is noteworthy
that the reactivity of terminal olefins is also important for Heck
reactions. According to the attached functional groups, the
activity of terminal olefins was found to be: COOH>Ph>
COOMe>COOEt>COOn-Bu, which could be seen from the
corresponding yields of desired products. These results are
consistent with the well-established trend in traditional Heck
reactions using palladium derivatives as catalysts. It should
be pointed out that all of the obtained products were trans-
In conclusion, we have disclosed an efficient and recyclable
ligand-free PEG1000-DIL/toluene catalytic system for the Heck
reaction of aryl halides with terminal olefins, especial aryl
iodides. It is worth noting that the present work is the first
example of a Heck reaction catalysed by Pd(OAc) in a
2
temperature-dependent biphasic system. Moreover, the desired
products were isolated by a simple decantation, and the
catalytic system can be recycled and reused without evident
1
isomers, and were identified by H NMR and melting point.
Encouraged by these results, further investigations were
carried out for extending the scope of this catalytic system, the
reactivity of substituted aryl bromides and aryl chloride was
studied under the same conditions. It is well known that either
C–Br or C–Cl is much more difficult compared to C–I
activation. Therefore, compared to aryl iodides, it took a longer
Table 3 Heck reactions of aryl bromides and aryl chloride with
methyl acrylatea
b
Entry
R1
X
Time / h
Yield /%
1
2
3
4
5
6
H
1d, Br
1e, Br
1f, Br
1g, Br
1h, Cl
1i, Cl
36
30
24
48
48
48
3a, 75
3k, 80
3p, 90
3q, 68
3p, 46
3k, 38
4-NO
2
2,4-NO
2
2
4-CH
2,4-NO
4-NO
3
2
a
Reaction conditions: ArX (1.0 mmol), methyl acrylate (2.0 mmol),
3 2
Et N (2.0 mmol), Pd (OAc) (2 mmol%), PEG1000-DIL (2.0 g),
Fig. 2 RepeatedreactionsusingrecoveredPEG1000-DILcatalytic
system.
toluene (3.0 g), 110 °C, the reaction was monitored by TLC.
b
Isolated yield.

JOURNAL OF CHEMICAL RESEARCH 2011 733
1
decrease in activity. Further efforts to extend the application of
the system in other Pd-catalysed reactions are in progress in
our laboratory.
= 150/1, v/v). H NMR (500 MHz, CDCl ): δ = 7.69 (d, J = 16.0 Hz,
3
1
4
(
(
H), 7.51–7.54 (m, 2H), 7.37–7.39 (m, 3H), 6.45 (d, J = 16.0 Hz, 1H),
.21 (t, J = 6.7 Hz, 2H), 1.65–1.74 (m, 2H), 1.38–1.50 (m, 2H), 0.97
t, J = 7.3 Hz, 3H); GC-MS: m/z (%) = 204 (18) [M ], 148 (74), 131
100), 103 (45), 77 (24).
E)-Methyl 3-(4-nitrophenyl) acrylate (3k): Rf = 0.4 (petroleum
ether/ethyl acetate = 20:1, v/v). Chromatography solvent petroleum
ether/ethyl acetate = 50/1, v/v). White solid; Mp. 137.5–138.7 °C; H
NMR (500 MHz, CDCl ): δ = 8.23–8.25 (m, 2H), 7.66–7.73 (m, 3H),
+
Experimental
26
(
All the chemicals were from commercial sources without any
pretreatment. All reagents were of analytical grade. The ionic liquids
1
23 1
were synthesised according to the literature procedure. H NMR
spectra were recorded on a Bruker 500-MHz spectrometer with
tetramethylsilane (TMS) as an internal standard. Melting points were
recorded on a Buchi R-535 apparatus and are uncorrected. Gas
chromatography (GC) analysis was performed on anAgilent GC-6820
equipped with a 30 m × 0.32 mm × 0.5 µm HP-Innowax capillary
column and a flame ionisation detector. GC-MS analyses were
performed on a Saturn 2000 GC/MS instrument.
3
6.55 (d, J = 16.1 Hz, 1H), 3.83 (s, 3H); GC-MS: m/z (%)= 207 (48)
M ], 176 (100), 146 (88), 130 (78), 118 (63), 102 (69), 90 (41),
6 (40), 51 (30).
+
[
7
Received 29 October 2011; accepted 28 November 2011
Paper 1100960 doi: 10.3184/003685011X13238914941786
Published online: 27 December 2011
Palladium acetate catalysed Heck cross-coupling reaction of aryl
halides with terminal olefins in a PEG1000-DIL/toluene temperature-
dependent biphasic system; general procedure
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1
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