A comparative Suzuki reaction of aryl halides using a new dimeric orthopalladated complex under conventional and microwave irradiation conditions

Article abstract of DOI:10.1002/aoc.2852

The cross-coupling reaction between phenylboronic acid and various types of aryl halides (Suzuki reaction) was carried out using a catalytic amount of a new phosphine-based catalyst under microwave irradiation. The reaction conditions were optimized and results showed that, by application of this catalytic system, N-methyl-2-pyrrolidone as the solvent and potassium carbonate as the base, reactions could be completed in a short reaction time with high to excellent yields. Copyright 2012 John Wiley & Sons, Ltd. The Suzuki reaction of aryl halides was carried out using a new catalyst under the optimized conditions. Application of microwave irradiation lead to very good results in a short time. Copyright

Full text of DOI:10.1002/aoc.2852

Full Paper
Received: 17 December 2011
Revised: 5 March 2012
Accepted: 5 March 2012
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/aoc.2852
A comparative Suzuki reaction of aryl halides
using a new dimeric orthopalladated complex
under conventional and microwave
irradiation conditions
a,b
a
a
Abdol R. Hajipour *, Kazem Karami and Ghazal Tavakoli
The cross-coupling reaction between phenylboronic acid and various types of aryl halides (Suzuki reaction) was carried out
using a catalytic amount of a new phosphine-based catalyst under microwave irradiation. The reaction conditions were
optimized and results showed that, by application of this catalytic system, N-methyl-2-pyrrolidone as the solvent and
potassium carbonate as the base, reactions could be completed in a short reaction time with high to excellent yields.
Copyright © 2012 John Wiley & Sons, Ltd.
Keywords: Suzuki cross-coupling reaction; catalyst; microwave irradiation
halides (such as triflates) to be effectively coupled with boron
Introduction
reagents. First, some catalysts such as PdCl (PPh ) and Pd(OAc) /
2
3 2
2
Biaryl motifs are very useful organic compounds that have found
many applications in different fields, including synthesis of phar-
maceuticals, herbicides, conducting polymers, natural products
PPh were used for this purpose. However, although these types
3
of catalysts were air stable and would be a good catalyst for the
present reaction, their conversions, yields and chemoselectivities
were not as good as this catalyst. In comparison to PdCl (PPh )
[
1,2]
and liquid crystalline materials.
Therefore, in recent years, an
2
3 2
extraordinary amount of effort has been spent in developing
new straightforward and environment-friendly reactions for
aryl–aryl coupling and this field of science has become a matter
of wide interest.
and Pd(OAc) /PPh , our catalyst has some advantages, e.g. the
2 3
reactions perform in very shorter reaction times and also the
catalyst is air stable and chemoselective. Furthermore, it does not
decompose at high temperatures. Nowadays, many researchers
have focused on preparing new types of palladium-based catalysts
using different ligands with high efficiencies and yields to improve
the efficiency of such types of reactions. Our catalyst excellent and
therefore its application is preferable to other conventional
catalysts.
In the history of organic chemistry, the formation of new
bonds between two carbon atoms has always been one of the
[
3]
fundamental reactions, and therefore much research around
the world has tried to broaden new methodologies that are able
to improve the reaction conditions. Cross-coupling reactions
catalyzed by palladium complexes, encouraged by the 2010
Nobel Laureates’ discoveries, are powerful and suitable one-step
Application of microwave heating in organic synthesis was
[
18]
developed about 50 years ago.
Therefore, the use of this
[
4]
tools for the construction of C-C bonds. Some of the well
technique as an environmentally friendly green synergy source
has grown and microwave irradiation has become a powerful
technique by which the completion of the reaction can be
[
5,6]
[7–13]
known reactions in this category are Stille,
Suzuki
and
[14]
Kumada–Hiyama cross-coupling reactions. The Suzuki reaction
[
19–21]
is one of the most useful and applicable reactions that can be
decreased from hours to minutes.
This technique was used
[
15]
used on both laboratory and industrial scales.
In this method
also to improve the sluggish Suzuki coupling by various research
[
22–25]
an organic electrophile and a nucleophilic organoboron deriva-
tive couples with each other in the presence of a palladium
groups.
Herein, in continuation of our previous investigations,
[
26–32]
we
[8b]
catalyst
(Scheme 1).
wish to report a new phosphine-based catalytic system for Suzuki
cross-coupling reaction under microwave irradiation.
The Suzuki cross-coupling reaction has more advantages than
the other palladium-catalyzed processes, such as functional
group tolerances, stability and availability of many boron deriva-
tives, low toxicity of reagents and intermediates, high thermal
stability and mild reaction conditions with high to excellent yield
*
Correspondence to: Abdol R. Hajipour, Pharmaceutical Research Laboratory,
Department of Chemistry, Isfahan University of Technology, Isfahan 84156,
Islamic Republic of Iran. E-mail: haji@cc.iut.ac.ir
[
16]
with a vast range of substrates.
mixture is simple and inorganic products of the reaction easily re-
moved with water.
Also work-up of the reaction
[12b,17]
a Pharmaceutical Research Laboratory, Department of Chemistry, Isfahan
University of Technology, Isfahan 84156, Islamic Republic of Iran
However, for the success of coupling
reactions, the catalyst should be chosen properly. During the past
few years, very different types of palladium catalyst precursors
have been developed that allow various aryl halides and pseudo
b Department of Pharmacology, University of Wisconsin, Medical School, 1300
University Avenue, Madison, WI 53706-1532, USA
Appl. Organometal. Chem. 2012, 26, 401–405
Copyright © 2012 John Wiley & Sons, Ltd.

A. R. Hajipour et al.
Scheme 1. Suzuki cross-coupling reaction
amounts of catalyst. The results listed in Table 2 (entry 5) show
that the best results were obtained using 0.1 mol% of catalyst 2.
These reaction conditions were applied in the Suzuki cross-
coupling reaction of different types of aryl halides using phe-
nylboronic acid under both traditional heating and microwave
Results and Discussion
The Suzuki cross-coupling reaction of different types of aryl
halides using palladacycle complex [Pd(m-Cl){C,C-[C
6 4 2
H (PPh CHC
(
O)C -4-F) ]}], 2 (Scheme 2) was investigated under both tradi-
6
H
4
2
ꢀ
tional heating and microwave irradiation conditions. Initially, in
order to optimize the reaction conditions, the Suzuki reaction of
irradiation conditions at 120 C.
As demonstrated in Table 3, this catalytic system can be used
for Suzuki cross-coupling reaction of different types of aryl
iodides, bromides and also less reactive aryl chlorides. However,
in comparison to aryl bromides and aryl iodides, the Suzuki reac-
tion using aryl chlorides as substrate needs longer reaction times
for completion under both reaction conditions (Table 3, entries 3,
16 and 17). We also examined the electronic and steric effects on
yields. For this purpose we used 2-, 3- and 4-bromoacetophe-
none under optimized reaction conditions (Table 3, entries 5–7).
The Suzuki reaction of 4-bromoacetophenone was performed
completely and its resulted yield was excellent. In the case of 3-
bromoacetophenone, although the reaction was carried out
quantitatively, completion of the reaction required a longer reac-
tion time than 4-substituted derivatives. However, the reaction of
2-bromoacetophenone, which has more steric effects, gave a
moderate yield in a longer reaction time. On the other hand,
the reaction of 1-bromonaphthalene performed faster with
better yield than 9-bromophenantehrene, as the latter has more
hindrance. Thus the greater the hindrance in the vicinity of the
leaving group, the higher is the decrease in the reaction conversion.
The Suzuki reaction was performed by aryl halides bearing both
electron-donating and electron-withdrawing substituents and the
results are comparable. Therefore, under the defined conditions,
reactions did not show any sensitivity to electronic effects; for
example, 4-bromomethoxybenzene (or 4-methoxyiodobenzene)
4
-bromobenzonitrile as the test substrate using phenylboronic
acid as boron reagent was chosen and the effect of various
parameters on the yields and conversions was examined. For this
purpose, initially this reaction was performed using different
solvents in the presence of 0.2 mol% palladacycle 2 as catalyst
and K
As can be seen in Table 1, the best results were obtained using
N-methyl-2-pyrrolidone (NMP) as the solvent (Table 1, entries 1–5).
We tried these conditions for finding a suitable base for this
2 3
reaction and the results clearly showed that K CO can apply as
the best base (Table 1, entries 6–9).
ꢀ
2
CO
3
as the base at 120 C under microwave irradiation.
For optimizing of catalyst concentration the above reaction
was repeated using NMP as the solvent and K CO as the base
under microwave irradiation at 120 C in the presence of various
2
3
ꢀ
Scheme 2. Synthetic procedure for preparation of palladacycle complex 2
a
Table 1. Optimization of base and solvent for Suzuki cross-coupling reaction under microwave irradiation
b
Entry
Base
Solvent
Conversion (%)
1
2
3
4
5
6
7
8
9
K
K
K
K
K
2
2
2
2
2
CO
CO
CO
CO
CO
3
3
3
3
3
DMF
83
91
NMP
Methanol
Toluene
DMSO
NMP
65
22
74
Cs
Na
Et
NaOAc
2
CO
3
81
2
CO
3
NMP
78
3
N
NMP
Trace
57
NMP
a
ꢀ
Reaction conditions: 1 mmol aryl halide, 1.2 mmol phenylboronic acid, 1 mmol base, 3 ml solvent, 120 C and 0.2 mol% palladacycle 2.
b
Determined by gas chromatography.
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Copyright © 2012 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2012, 26, 401–405

New dimeric orthopalladated complex
a
Table 2. Optimization of catalyst concentration in the Suzuki cross-coupling reaction under microwave irradiation
b
Entry
Catalyst (mol%)
Conversion (%)
1
2
3
4
5
6
None
0.005
0.01
0.05
0.1
0
21
32
55
89
90
0.2
a
ꢀ
2 3
Reaction conditions: 1 mmol aryl halide, 1.2 mmol phenylboronic acid, 1 mmol K CO , 3 ml NMP, 120 C and palladacycle 2.
b
Determined by gas chromatography.
Table 3. Suzuki cross-coupling reaction of phenylboronic acid with various aryl halides under optimized reaction conditions under microwave
irradiation and traditional heating conditions
c
c
Entry
Ar-X
Product
Time (min)
Yield (%)
Time (min)
Yield (%)
a
b
(
MW)
(heating)
1
2
3
4
5
6
7
8
9
Ph-I
Biphenyl
2
1
94
94
84
91
92
90
58
91
89
83
91
92
95
93
89
88
86
90
75
91
90
91
89
87
75
42
87
64
57
86
88
85
82
79
75
71
Ph-Br
Ph-Cl
Biphenyl
Biphenyl
6
210
180
330
510
960
315
960
960
360
360
300
270
405
720
720
1-Bromo-3-chlorobenzene
4-Bromoacetophenone
3-Bromoacetophenone
2-Bromoacetophenone
4-Bromobenzaldehyde
4-Iodonitrobenzene
3-Chlorobiphenyl
4-Acetylbiphenyl
3-Acetylbiphenyl
2-Acetylbiphenyl
4-Formylbiphenyl
4-Nitrobiphenyl
2-Phenylpyridine
4-Methoxybiphenyl
4-Methoxybiphenyl
4-Cyanobiphenyl
1-Phenylnaphtalene
9-Phenylphenanhrene
4-Acetylbiphenyl
4-Formylbiphenyl
2
6
11
25
7
10
15
8
1
1
1
1
1
1
1
1
0
1
2
3
4
5
6
7
2-Bromopyridine
4-Methoxybromobenzene
4-Methoxyiodobenzene
4-Bromobenzonitrile
1-Bromonaphtalene
9
5
4
9-Bromophenanthrene
4-Chloroacetophenone
4-Chlorobenzaldehyde
6
23
22
a
ꢀ
ꢀ
Reaction conditions: 1 mmol aryl halide, 1.2 mmol phenylboronic acid, 1 mmol K
2
CO
CO
3
, 3 ml NMP, 120 C and 0.1 mol% palladacycle 2, 600 W.
b
Reaction conditions: 1 mmol aryl halide, 1.2 mmol phenylboronic acid, 1 mmol K
Determined by gas chromatography.
2
3
, 3 ml NMP, 120 C and 0.1 mol% palladacycle 2.
c
has a comparable result to 4-bromobenzonitrile or 4-bromo-
benzaldehyde (Table 3, entries 8, 11–13). Application of
2-phenylpyridine was achieved under microwave irradiation con-
ditions, whereas the reaction under heating conditions occurred
slowly and after 16 h only a moderate yield was obtained (Table 3,
entry 10). The same results were achieved in the case of the
Suzuki reaction of 4-iodonitrobenzene (Table 3, entry 9) and
again only under microwave irradiation was full conversion to
4-nitrobiphenyl achieved, the reaction being performed quanti-
tatively. The data in Table 3 clearly show that application of
1
3
-bromo-3-chlorobenzene as aryl halide caused formation of
-chlorobiphenyl as the only product. The result indicates
that the reaction acts chemoselectively and only Br substi-
tutes with the phenyl ring from phenylboronic acid (Table 3,
entry 4). 2-Bromopyridine was used as a heterocyclic aryl
halide in the Suzuki reaction and in this case full conversion to
Appl. Organometal. Chem. 2012, 26, 401–405
Copyright © 2012 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

A. R. Hajipour et al.
microwave irradiation can decrease the time of completion of the
reactions and also in many cases improve conversion and yields.
Reportedly, in almost all of the Suzuki coupling reactions, a small
amount of biphenyl as byproduct of the reaction was formed as a
result of the homocoupling reaction of phenylboronic acid pre-
cursor, which is a major drawback of the process. However, in
this research, by application of microwave irradiation, the forma-
tion of this byproduct was reduced and under these conditions
was hardly observed at all; this is another advantage of using
microwave irradiation in organic synthesis.
General Procedure for the Suzuki Reaction of Aryl Halides
In a round-bottom flask equipped with a condenser for refluxing
and a magnetic stirring bar, aryl halide (1 mmol), phenylboronic
acid (1.2 mmol), K CO3 (1 mmol), complex 2 (0.001 mmol) and
2
ꢀ
NMP (3 ml) were added and heated at 120 C under air atmo-
sphere. In the case of microwave-assisted reactions the mixture
ꢀ
was irradiated in a microwave oven at 120 C and 600 W. The mix-
ture was stirred under these reaction conditions and monitored
by both TLC (EtOAc–n-hexane, 25:75) and gas chromatography.
In each case after completion of the reaction, the mixture was di-
luted with n-hexane and water. The organic layer was washed
2
with brine, dried over CaCl , and concentrated under reduced
Conclusion
pressure using a rotary evaporator. The residue was purified by
recrystallization from ethanol and water.
In summary, a new catalytic system for Suzuki cross-coupling re-
action was developed. The reactions were performed under both
conventional heating and microwave irradiation as a green syn-
ergy source and these reaction conditions were compared. The
results clearly show that using microwave irradiation conditions
the required time for completion of the reactions could be mini-
mized from hours to minutes in comparison to conventional
heating conditions. The catalyst was not sensitive to oxygen
and also any or a little amount of biphenyl as the byproduct of
the reaction was formed in each case. The yields were very good
to excellent, making this procedure a good synthetic route for
biaryl synthesis.
All of the compounds have been characterized by comparing
1
13
melting point, H and C NMR with the values found in the
literature:
[
36a]
4
2
4
4
4
-Acetylbiphenyl (Table 3, entry 5)
[
36b]
-Phenylpyrdine (Table 3, entry 10)
-Methoxybiphenyl (Table 3, entry 11)
-Cyanobiphenyl (Table 3, entry 13)
-Phenylbenzaldehyde (Table 3, entry 17)
[
36c]
[
36a]
[
36d]
Acknowledgments
We gratefully acknowledge the funding support received for this
project from the Isfahan University of Technology (IUT), Islamic
Republic of Iran. Further financial support from Center of Excel-
lence in Sensor and Green Chemistry Research is gratefully
acknowledged.
Experimental
General
All melting points were taken on a Gallenkamp melting apparatus
1
and are uncorrected. H NMR spectra were recorded at 400 MHz in
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