A comparative homocoupling reaction of aryl halides using monomeric orthopalladated complex of 4-methoxybenzoylmethylenetri-phenylphosphorane under conventional and microwave irradiation conditions

Article abstract of DOI:10.1002/aoc.1804

The activity of [Pd(C₆H₄CH₂NH ₂-K²-C-N)PPh₃MOBPPY]OTf complex, A (MOBPPY = 4-methoxybenzoylmethylenetriphenylphosphoraneylide) was investigated in the homocoupling reaction of a vast range of aryl halides under both conventional and microwave irradiation conditions and their results were compared. The complex was active and showed high efficiency in the formation of new C-C bonds. The yields were excellent using a catalytic amount of [Pd(C₆H ₄CH₂NH₂-K²-C-N)PPh ₃MOBPPY]OTf complex in N, N-dimethylformamide at 120 °C. In comparison to conventional heating conditions, the reactions under microwave irradiation gave higher yields in shorter reaction times.

Full text of DOI:10.1002/aoc.1804

Full Paper
Received: 29 August 2010
Revised: 14 March 2011
Accepted: 13 April 2011
Published online in Wiley Online Library: 1 June 2011
(wileyonlinelibrary.com) DOI 10.1002/aoc.1804
A comparative homocoupling reaction of aryl
halides using monomeric orthopalladated
complex of 4-methoxybenzoylmethylenetri-
phenylphosphorane under conventional
and microwave irradiation conditions
Abdol R. Hajipoura,b∗, Kazem Karamia and Ghazal Tavakolia
The activity of [Pd(C6H4CH2NH2-κ2-C-N)PPh3MOBPPY]OTf complex, A (MOBPPY = 4-methoxybenzoylmethylenetriphenyl-
phosphoraneylide) was investigated in the homocoupling reaction of a vast range of aryl halides under both conventional and
microwave irradiation conditions and their results were compared. The complex was active and showed high efficiency in the
formation of new C-C bonds. The yields were excellent using a catalytic amount of [Pd(C6H4CH2NH2-κ2-C-N)PPh3MOBPPY]OTf
complexinN, N-dimethylformamideat120◦C.Incomparisontoconventionalheatingconditions,thereactionsundermicrowave
c
irradiation gave higher yields in shorter reaction times. Copyright ꢀ 2011 John Wiley & Sons, Ltd.
Keywords: homocoupling; catalyst; palladacycle; microwaves
Introduction
is one of the most useful and straightforward methods for biaryl
synthesis.[21–23]
Owing to the excellent physical and chemical properties of
biaryls, they can be used in the synthesis of a vast range
of chemical compounds, such as monomers for constructing
polymers,[1] supramolecular compounds,[2] receptor macrocyclic
molecules[3] and ligands for organometallic catalysts.[4] Biaryls
are also found as building blocks in a large number of natural
products[5] and in a variety of biologically active compounds such
as pharmacophores.[6] Therefore, in recent years, much effort has
been devoted to developing new methods for their synthesis and
many researchers continue to improve the reaction conditions.
Applicationsoftransitionmetalcatalystsintheconstructionofnew
C–C bonds comprise the most powerful method in this field.[7–9]
Transition metal-catalyzed homocoupling of aryl halides or
aryl sulfonates is a very efficient catalytic process for biaryl
synthesis, and applications of palladium,[10] nickel,[11] barium,[12]
indium,[13] copper and rhodium[14] have been reported in these
reactions. Furthermore, symmetrical biaryls can be produced via
copper-catalyzed homocoupling of organosilicon halides[15] and
palladium- and gold-catalyzed homocoupling of aryl boronic
acids.[16]
Application of microwave irradiation for promoting reaction
condition has been becoming a versatile tool in organic
synthesis.[24] This technique can cause increased yields of
reactions and decreased total time for completion of the reactions
and is also advantageous for enabling reproducible and scalable
chemistry development in research laboratories and industrial
processes.[25,26] Unlike traditional heating conditions that transfer
the heat through the reaction vessel, in this procedure heat trans-
fer occurs in a direct manner and the reaction mixture absorbs
the microwave irradiation directly, saving time and energy.[27]
During recent decades, many researchers have focused mainly
on the development of milder reaction conditions using new
catalysts and reaction systems.[28] In continuation this work,[29,30]
we report a new catalytic system for homocoupling reaction of
variousarylhalidesundermicrowaveirradiation. Ourcatalystisnot
sensitive to oxygen and shows high efficiency for biaryl synthesis.
Copper-mediated homocoupling of aryl halides, known as the
Ullmann reaction, is the conventional method for the synthesis
of symmetrical biaryls that was discovered by Ullmann about
100 years ago.[7a,17,18] However, this reaction requires very harsh
conditions, such as elevated temperature (sometimes more than
200 ◦C), and consumption of an equimolar amount of copper
restricts the reaction conditions, because many functional groups
cannot survive such drastic reaction conditions.[19,20] Therefore,
improvements to this method have emerged in recent years.
Among them, palladium-catalyzed homocoupling of haloarenes
∗
Correspondence to: Abdol R. Hajipour, Pharmaceutical Research Laboratory,
Department of Chemistry, Isfahan University of Technology, Isfahan 84156, I. R.
Iran. E-mail: haji@cc.iut.ac.ir
a
Pharmaceutical Research Laboratory, Department of Chemistry, Isfahan
University of Technology, Isfahan 84156, I. R. Iran
b
Department of Pharmacology, University of Wisconsin, Medical School, 1300
University Avenue, Madison, WI 53706-1532, USA
c
Appl. Organometal. Chem. 2011, 25, 567–576
Copyright ꢀ 2011 John Wiley & Sons, Ltd.

A. R. Hajipour, K. Karami and G. Tavakoli
Scheme 1. Synthesis of palladacycle complex [Pd (C6H4CH2NH2-κ2-C-N)PPh3MOBPPY]OTf (A).
Table 1. Optimization of base and solvent for homocoupling reaction under microwave irradiationa
Conversion (%)
Time (min)
Solvent
Base
Entry
95
2
2
2
2
2
2
2
2
2
NMP
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Cs2CO3
Na2CO3
Et3N
1
2
3
4
5
6
7
8
9
100
85
DMF
DMSO
Methanol
Toluene
DMF
55
25
80
65
DMF
Trace
Trace
DMF
DMF
KOH
a Reaction conditions: bromobenzene, 1 mmol; base, 1 mmol; solvent, 3 ml; palladacycle A, 0.002 mmol; 120 ◦C, 600 W.
Results and Discussion
pyrrolidone (NMP) and dimethyl sulfoxide (DMSO), and the best
solvent was DMF (Table 1, entry 2).
Preparation of palladacycle complex [Pd(C6H4CH2NH2-κ2-C-
N)PPh3MOBPPY]OTf, A, as a catalyst was performed according
to our previous work (Scheme 1).[29,31] Then we applied this com-
plex in a homocoupling reaction of various aryl halides under both
traditional and microwave irradiation conditions. To optimize the
reaction conditions, we chose the homocoupling reaction of bro-
mobenzene as the test substrate under microwave irradiation.
Initially, different types of bases were examined using 0.2 mol%
of catalyst and N, N-dimethylformamide (DMF) as a base at 120 ◦C
and 600 W. As can be seen in Table 1, K2CO3 as the base gave the
best results (Table 1, entry 2).
Furthermore, we repeated these reaction conditions for
optimization of catalyst concentration using DMF as the solvent,
K2CO3 as a base and various amount of catalyst loading. The data
clearly showed that 0.1 mol% of catalyst gave the highest yields
(Table 2, entry 5). Therefore, these optimized reaction conditions
were employed for homocoupling reactions of a vast range of
aryl halides at 120 ◦C. Furthermore, our catalyst was insensitive to
oxygen and therefore, all of the reactions were performed under
an air atmosphere. The results are listed in Tables 3 and 4.
As demonstrated in Tables 3 and 4, this catalytic system can be
used for homocoupling reactions of different types of aryl iodides,
bromides and also some reactive aryl chlorides, but the times
required for completion of these reactions are longer than for
similar aryl bromides and iodides under both reaction conditions
(Tables 3 and 4, entries 3, 14 and 17). We also examined the
Then we applied several organic solvents under the same
reaction conditions by employing 0.2 mol% of palladacycle A.
The data clearly showed that the best results were obtained
using microwave-active polar solvents such as DMF, N-methyl-2-
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A comparative homocoupling reaction of aryl halides
Table 2. Optimization of catalyst concentration in homocoupling reaction under microwave irradiation
Conversion (%)
Time (min)
Catalyst (mol %)
Entry
0
90
90
90
90
90
90
None
0.005
0.01
0.05
0.1
1
2
3
4
5
6
10
25
50
95
100
0.2
a Reaction conditions: bromobenzene, 1 mmol; K2CO3, 1 mmol; DMF, 3 ml; palladacycle A, 120 ◦C, 600 W.
electronic and steric effects on the resultant yields of the reactions.
For this purpose we used 2-, 3- and 4-bromoacetophenone
under optimized reaction conditions (Table 3, entries 5–7). The
homocoupling reaction of 4-bromoacetophenone was performed
completely and its resultant yield was excellent. In the case of
3-bromoacetophenone, although the reaction was carried out
quantitatively, completion of the reaction required a longer
reactiontimethan4-substitutedderivatives.However,thereaction
of 2-bromoacetophenone, which has greater steric effects, gave
a moderate yield in a much longer reaction time. Therefore,
increasing the hindrance in the vicinity of leaving group can cause
a decrease in the reaction conversion. On the other hand, the
homocoupling reaction was performed using aryl halides bearing
both electron-donating and electron-withdrawing substituents
and their results are comparable. Therefore, under the defined
conditions, the reactions did not show any sensitivity to electronic
effects. Application of 1-bromo-3-chlorobenzene as the aryl halide
causedtheformationof3, 3ꢁ-dichlorobiphenylastheonlyproduct.
This means that the reaction acts chemoselectively and just Br as
the best leaving group leaves the substrate (Table 3, entry 4).
2-Bromopyridine was used as a heterocyclic aryl halide and its
homocouplingreactionwasexamined.However,inthiscase,again
the reaction was not performed quantitatively (Table 3, entry 15).
Furthermore, the homocoupling reaction of 4-iodonitrobenzene
was not completed quantitatively (Table 3, entry 16) and only a
moderate yield of 4,4ꢁ-dinitrobiphenyl was obtained. The data
in Tables 3 and 4 clearly show that application of microwave
irradiation can decrease the time of completion of the reactions
and also in many cases cause better conversion and yields to be
obtained.
produce the corresponding products in excellent yields and high
chemoselectivity in short reaction times. Reactions were carried
out under both conventional heating and microwave irradiation
conditions.However,thiscatalyticsystemcouldnotcoupletheless
reactive aryl chlorides. The results showed that, under microwave
irradiation conditions, the reactions were completed in shorter
reaction times and also the yields of the reactions were increased.
Experimental Section
General
All melting points were taken on a Gallenkamp melting appara-
tus and are uncorrected. 1H-NMR spectra were recorded using
400 MHz in CDCl3 solutions at room temperature (TMS (Tetram-
ethylsilane) was used as an internal standard) on a Bruker, Avance
500 instrument (Rheinstetten, Germany) and Varian 400 NMR.
FT-IR spectra were recorded on a spectrophotometer (Jasco-680,
Japan). Spectra of solids were determined using KBr pellets. Vi-
brational transition frequencies are reported as wavenumbers
(cm−1). We used the Milestone Microwave (Microwave Labstation-
MLS GmbH-ATC-FO 300) for synthesis. Furthermore, we used gas
chromatography (Beifin 3420 gas chromatograph equipped a Var-
ian CP SIL 5CB column, 30 m, 0.32 mm, 0.25 µm) for examination
of reaction completion and yields. Palladium acetate, aryl halides
and all chemicals were purchased from Merck and Aldrich and
were used as received.
Synthesis of Palladacycle Complex [Pd(C6H4CH2NH2-κ2-C-
N)PPh3MOBPPY]OTf (A)
A mechanistic description of the homocoupling reaction is
given in Scheme 2. Initially, Pd(II) converts to Pd(0),[32] followed
by the oxidative addition of aryl halid to Pd(0) to form aryl
palladium (II) intermediate 1. An exchange between the two
molecules of intermediate 1 produces compounds 2 and 3.
Finally, reductive elimination of intermediate 2 causes the desired
coupling products.
[Pd(C6H4CH2NH2-κ2-C-N)PPh3MOBPPY]OTfasapalladacyclecom-
plex was prepared according to our previous work (Scheme 1).[29]
MOBPPY was prepared according to the Ramirez et al. method
in two steps.[31] Interaction of 2-bromo-4ꢁ-methoxyacetophenone
and PPh3 gave phosphonium salts, which then reacted with a
weak base in H2O, causing the formation of MOBPPY. This ylide
was not sensitive to air and moisture.
Then a freshly prepared solution of [Pd(C6H4CH2NH2-κ2-
C-N)PPh3(Cl)]OTf reacted with MOBPPY (1 : 1 molar ratio) in
Tetrahydrofuran (THF) or Me2CO at 0 ◦C, which after 30 min of
stirring caused the precipitation of white solids of stoichiometry
[Pd(C6H4CH2NH2-κ2-C-N)PPh3(MOBPPY)]OTf (A) in good yields.
Conclusions
In summary, here we repot a highly active and efficient catalyst for
promoting the homocoupling reaction of various aryl halides to
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A. R. Hajipour, K. Karami and G. Tavakoli
Table 3. Homocoupling reaction of various aryl halides under optimized reaction conditions under microwave irradiationa
Yield (%)b
93
Time (min)
1
Product
ArX
Entry
1
94
90
92
90
1
10
2
2
3
4
5
6
89
9
6
62
20
7
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A comparative homocoupling reaction of aryl halides
Table 3. (Continued)
Yield (%)b
91
Time (min)
8
Product
ArX
Entry
8
92
10
9
83
90
5
10
11
10
89
4
12
85
7
13
87
20
14
c
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A. R. Hajipour, K. Karami and G. Tavakoli
Table 3. (Continued)
Yield (%)b
55
Time (min)
20
Product
ArX
Entry
15
69
85
20
20
16
17
a Reaction conditions: aryl halide, 1 mmol; K2CO3, 1 mmol; palladacycle A, 0.1 mol%; DMF, 3 ml; 120 ◦C, 600 W.
b Isolated yield.
Table 4. Homocoupling reaction of various aryl halides under optimized reaction conditions in an oil batha
Yield (%)b
92
Time (h)
1
Product
ArX
Entry
1
92
88
90
1.2
4.5
2.3
2
3
4
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A comparative homocoupling reaction of aryl halides
Table 4. (Continued)
Yield (%)b
85
Time (h)
4
Product
ArX
Entry
5
83
5.5
6
56
89
8.5
7
8
5
92
6.5
9
84
90
5
10
11
5.3
c
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A. R. Hajipour, K. Karami and G. Tavakoli
Table 4. (Continued)
Yield (%)b
88
Time (h)
3.5
Product
ArX
Entry
12
87
7
13
80
11
14
50
64
10
10
15
16
82
10
17
a Reaction conditions: aryl halide, 1 mmol; K2CO3, 1 mmol; palladacycle A, 0.1 mol%; DMF, 3 ml; 120 ◦C.
b Isolated yield.
General Procedure for Homocoupling Reaction of Aryl Halides
hexane and water. The organic layer was washed with brine, dried
over MgSO4 and concentrated under reduced pressure using a
rotary evaporator. The residue was purified by recrystallization
from ethanol and water or column chromatography (EtOAc–n-
hexane, 30 : 70).
In a round-bottom flask equipped with a condenser for refluxing
and a magnetic stirring bar, aryl halide (1 mmol), K2CO3 (1 mmol),
complex A (0.001 mmol) and DMF (3 ml) were added and heated
at 120 ◦C under an air atmosphere. In the case of microwave-
assisted reactions, the mixture was irradiated in a microwave oven
at 120 ◦C and 600 W. The mixture was stirred under these reaction
conditions and monitored by both thin-layer chromatography
(EtOAc–n-hexane, 30 : 70) and gas chromatography. In each case,
after completion of the reaction, the mixture was diluted with n-
3,3ꢀ-Diacetylbiphenyl (entry 6, Table 3)
The compound was characterized by comparing m.p. and 1H NMR
with the values found in the literature.[33]
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A comparative homocoupling reaction of aryl halides
Scheme 2. Mechanism of homocoupling reaction.
4,4ꢀ-Dimethoxybiphenyl (entry 9, Table 3)
A. L. Monteiro, Tetrahedron Lett. 2002, 43, 2327; c) S. Venkatraman,
T. Huang, C. J. Li, Adv. Synth. Catal. 2002, 344, 399; d) M. A. Brimble,
M. Y. Lai, Org. Biomol. Chem. 2003, 1, 2084; e) J.-H. Li, Y.-X. Xie,
D. L. Yin, J. Org. Chem. 2003, 68, 9867; f) Q. Li, J. Nie, F. Yang,
R. Zheng, G. Zou, X. J. Tang, Chin. J. Chem. 2004, 22, 419; g) J.-
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The compound was characterized by comparing m.p. and 1H NMR
with the values found in the literature.[34]
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We gratefully acknowledge the funding support received for this
project from the Isfahan University of Technology, I. R. Iran. Further
financial support from Center of Excellence in Sensor and Green
Chemistry Research is gratefully acknowledged.
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Copyright ꢀ 2011 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2011, 25, 567–576