Microwave-assisted suzuki cross-coupling reactions using dimeric ortho-palladated complex of tribenzylamine

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Microwave-assisted Suzuki Cross-
Coupling Reactions using Dimeric ortho-
palladated Complex of Tribenzylamine
Abdol Reza Hajipour a b & Fatemeh Rafiee a
a Pharmaceutical Research Laboratory, Department of Chemistry ,
Isfahan University of Technology , Isfahan , 84156 , IR Iran
b Department of Neuroscience , University of Wisconsin, Medical
School , 1300 University Avenue, Madison , WI , 53706-1532 , USA
Published online: 24 Oct 2013.
To cite this article: Abdol Reza Hajipour & Fatemeh Rafiee (2013) Microwave-assisted Suzuki
Cross-Coupling Reactions using Dimeric ortho-palladated Complex of Tribenzylamine, Organic
Preparations and Procedures International: The New Journal for Organic Synthesis, 45:6, 465-472,
DOI: 10.1080/00304948.2013.834772
To link to this article: http://dx.doi.org/10.1080/00304948.2013.834772
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Organic Preparations and Procedures International, 45:465–472, 2013
Copyright © Taylor & Francis Group, LLC
ISSN: 0030-4948 print / 1945-5453 online
DOI: 10.1080/00304948.2013.834772
Microwave-assisted Suzuki Cross-Coupling
Reactions using Dimeric ortho-palladated Complex
of Tribenzylamine
Abdol Reza Hajipour1,2 and Fatemeh Rafiee1
1Pharmaceutical Research Laboratory,
Department of Chemistry, Isfahan University of Technology, Isfahan 84156,
IR Iran
2Department of Neuroscience, University of Wisconsin, Medical School, 1300
University Avenue, Madison, WI 53706-1532, USA
Biaryls are used as building blocks for a wide range of herbicides,1 pharmaceuticals,2 natu-
ral products,3 biologically active compounds,4 microelectrode array,5 conducting polymers,
and liquid crystal materials.6 Suzuki cross-coupling of aryl halides with aryl boronic acids
is one of the most versatile and utilized synthetic method for the construction of symmetric
and unsymmetric biaryl compounds.7–14 Palladium and its complexes are also efficient
and active catalysts in cross-coupling reactions.15–17 The major benefits of the Suzuki
coupling are mild reaction conditions, tolerance of a broad range of functionalities, com-
mercial availability and ease of handling of organoboron reagents, and formation and
facile disposal of the non-toxic boron-containing by-products.8,18 Although Pd-phosphine
complexes19 and imidazole carbenes20 are among the most active catalysts used for the
Suzuki reaction, they are usually sensitive to air and moisture and/or expensive. Pallada-
cycle complexes that are air- and moisture-stable and possess high catalytic activity have
recently been employed in cross-coupling reactions such as Suzuki reaction21 and much
effort has been expended in the development of efficient and selective catalytic systems for
the Suzuki reaction. Among them, tertiary amine-based palladacycles have been shown to
be as effective pre-catalysts for cross-coupling reactions.22
Transition-metal-catalyzed cross-coupling reactions typically require long reaction
times under an inert atmosphere for completion under traditional heating. Microwave ir-
radiation is a powerful technique by which reactions can be brought to completion in
minutes or seconds rather than hours or days. High-speed Suzuki reactions have been
carried out successfully under controlled microwave conditions.23 The use of homoge-
neous metal catalysts in conjunction with microwaves leads to an increased lifetime
Received December 6, 2012; in final form July 29, 2013.
Address correspondence to Abdol Reza Hajipour, Department of Neuroscience, University of
Wisconsin, Medical School, 1300 University Avenue, Madison, WI 53706-1532, USA. E-mail:
haji@cc.iut.ac.ir
465

466
Hajipour and Rafiee
of the catalyst, saving time and energy, gives high yields, and a decrease in discarded
by-products from thermal side-reactions.24–27 In continuation of our recent investigations
on the synthesis and application of the palladacycle catalysts,28–33 we now report the exten-
sion of [Pd{C6H4(CH2N(CH2Ph)2)(μ-Br)]2 homogeneous complex, as a thermally stable
and oxygen insensitive catalyst for the cross-coupling reaction of various aryl halides with
aryl boronic acids and sodium tetraphenylborate under microwave irradiation.
We recently utilized the dimeric ortho-palladated complex [Pd{C6H4(CH2N
(CH2Ph)2)}(μ-Br)]2 in the Heck coupling and cyanation reaction.34,35 A suitable palladium-
catalyzed reaction requires high catalyst efficiency and activity. Its availability, its cost, and
the price of the organic starting materials are also of great importance. Tribenzylamine as
an N-donor ligand is an available and inexpensive amine. The ortho-palladation reaction
of this substrate is simple36 and leads to an efficient catalyst for coupling reactions even
with less reactive aryl chlorides, which are available and inexpensive substrates. This paper
investigates the efficiency of this catalyst in Suzuki reaction of various aryl halides under
microwave irradiation (Scheme 1).
N(CH2Ph)2
Pd
Br
2
Ar-Ar'
Ar'B(OH)2
ArX
+
or 0.25 eq Ar4'BNa
K2CO3, EtOH
60 °C
Scheme 1
In order to optimize the reaction conditions, we employed the coupling reaction of
4-bromoanisole with phenylboronic acid using catalytic amounts of the dimeric ortho-
palladated complex of tribenzylamine to evaluate the effects of the various bases and
solvents under microwave irradiation. The monitoring system for reaction time, tempera-
ture, pressure, and power in a microwave reactor allows for excellent control of reaction
parameters which generally leads to rapid optimization and more reproducible reaction
conditions. The best results were obtained in ethanol as the high microwave absorbing sol-
vent and K2CO3 as base by employing 0.1 mmol% of the ortho-palladated complex as the
catalyst at 60◦C. Other bases such as Cs2CO3, Na2CO3, K3PO4, NaOAc and NEt3 and also
other solvents e.g. DMF, NMP, THF, 1,4-dioxane, H2O and CH3CN were less effective.
Low palladium concentration usually led to long reaction times and increasing the amount
of palladium catalyst shortened the reaction time; however, the yield of 4-methoxybiphenyl
did not increase. The best result was obtained when the cross-coupling reaction was carried
out with 0.1 mol% of dimeric complex.
These optimized reaction conditions were applied in the Suzuki cross-coupling reaction
of various aryl halides under microwave irradiation. As is shown in (Table 1), the catalyst
can be used for cross-coupling reaction of aryl iodides, bromides and even of less reactive
aryl chlorides with aryl boronic acids and sodium tetraphenylborate in good to excellent
yields. Sodium tetraphenylborate can react with four equivalents of electrophilic reagents.

Suzuki Cross-Coupling Reactions
467
Table 1
Suzuki Reaction using ortho-Palladated Catalyst under Microwave Irradiationa
Time Yield mp (◦C)
Entry Ar-X
Boron Source
PhB(OH)2
Biaryl Product
Ph-Ph
(min) (%)b (lit. mp)
1
Ph-I
1
2
2
2
2
1
1
2
2
3
4
4
5
4
5
1
7
5
7
6
5
5
3
96
97
96
95
94
94
93
94
92
95
94
93
90
90
90
94
93
76
65
70
87
90
92
70–71 (69–70)44
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
p-O2NC6H4I
m-O2NC6H4I
p-MeOC6H4I
p-MeOC6H4I
PhBr
p-MeOC6H4B(OH)2 p-O2NC6H4-C6H4-p-OMe
p-MeOC6H4B(OH)2 m-O2NC6H4-C6H4-p-OMe
111–112 (111—113)44
69–70 (70–71)44
85–87 (86–87)44
85–87 (86–87)44
85–87 (86–87)44
70–71 (69–70)44
85–87 (86–87)44
176–178 (174–176)45
89–91(91–92)46
PhB(OH)2
Ph4BNa
p-MeOC6H4-Ph
p-MeOC6H4-Ph
p-MeOC6H4B(OH)2 p-MeOC6H4-Ph
PhBr
Ph4BNa
Ph-Ph
p-MeOC6H4Br
p-MeOC6H4Br
p-NCC6H4Br
p-HOCC6H4Br
p-NCC6H4Br
p-OHCC6H4Br
PhB(OH)2
p-MeOC6H4-Ph
p-MeOC6H4B(OH)2 p-MeOC6H4-C6H4-p-OMe
PhB(OH)2
PhB(OH)2
Ph4BNa
p-NCC6H4-Ph
p-HOCC6H4-Ph
p-NCC6H4-Ph
p-OHCC6H4-Ph
55–57 (58–59)47
89–91 (91–92)46
55–57(58–59)47
Ph4BNa
p-MeOCC6H4Br p-MeOC6H4B(OH)2 p-MeOCC6H4-C6H4-p-OMe
p-MeOCC6H4Br PhB(OH)2
p-ClC6H4Br
PhCl
p-HOCC6H4Cl
153- 154 (152–153)48
118–120 (117–118)44
75–76 (74–76)44
70–71 (69–70)44
55–57 (58–59)47
118–120 (117–118)44
55–57 (58–59)47
Yellow oil49
p-MeOCC6H4-Ph
p-ClC6H4-Ph
Ph-Ph
p-HOCC6H4-Ph
p-MeOCC6H4-Ph
p-HOCC6H4-Ph
o-O2NC6H4-Ph
o-ClC6H4-Ph
m-ClC6H4-Ph
PhB(OH)2
PhB(OH)2
PhB(OH)2
p-MeOCC6H4Cl PhB(OH)2
p-HOCC6H4Cl
o-O2NC6H4Br
o-ClC6H4Br
Ph4BNa
PhB(OH)2
PhB(OH)2
PhB(OH)2
Colorless oil oil49
Colorless oil oil49
m-ClC6H4Br
aReaction conditions: aryl halide (1 mmol), phenylboronic acid (1.2 mmol) or Ph4BNa (0.25 mmol),
K2CO3 (2 mmol), ortho-palladated catalyst (0.1 mmol%), ethanol (2 ml), 60◦C, 600 W. b) Isolated
yield.
Therefore, from both economical concerns and increased environmental awareness, this is
an atom economical reaction.37
We examined the electronic effects on the resulted yields and conversion times of the
reactions. Aryl bromides and iodides substituted with electron-withdrawing groups are the
most suitable substrates for the Suzuki cross-coupling reactions. Due to the strength of
the C-Cl bond and its reluctance towards oxidative addition to Pd(0), the times required
for conversion of aryl chlorides to biaryls are longer (Tables 1, Entries 17–20). Aryl
halides bearing electron-withdrawing substituents are transformed to the corresponding
coupled products with better conversions and shorter reaction times than those electron-
donating substituents. The chemoselectivity of the procedure was examined using 1-bromo-
4-cholorobenzene. In this reaction, bromide acted as the better leaving group (Table 1, Entry
16). The steric hindrance of the procedure was examined (Table 1, Entries 21–22). Increase
the hindrance in vicinity of leaving group can cause to decrease the reaction conversion.
A general catalytic cycle for the Suzuki reaction is presented in Scheme 2. Initially,
Pd(II) converts to Pd(0),38 followed by the oxidative addition of the aryl halide to Pd(0) to
form aryl palladium (II) intermediate 1. Phenylboronic acid which is activated by base reacts
with intermediate 1; after transmetalation, intermediate 2 is obtained. Finally, reductive
elimination of intermediate 2 produces the desired coupling products. When Ph4BNa is
used instead of phenylboronic acid, intermediate 1 is coupled with one aryl group of

468
Hajipour and Rafiee
Ph4BNa. The generated Ph3B, Ph2B(OH),and PhB(OH)2 will then participate in the next
catalytic cycles in the presence of base to produce the desired coupling products.37
[Pd{C6H4(CH2N(CH2Ph)2)}(µ-Br)]2
Ar-Ar'
ArX
L2Pd (0)
Reductive
Elimination
Oxidative
Addition
Ar
Ar
L2Pd
PdL2
X
Ar'
2
1
K2CO3 + H2O
-
Ar'B(OH)3 K+
+
KHCO3
B(OH)3 + KX
B(OH)3 + KX
Ar'B(OH)2
Ar4'BNa
or
Ar2'BOH
Ar3'B
Ar'B(OH)2
Scheme 2
The high efficiency of the palladacycle catalysts is due to the slow generation of low-
ligated Pd(0) complexes from a stable palladium(II) pre-catalyst. Palladacycles decompose
to liberate catalytic Pd(0) species and show a positive Hg(0) test which was assigned
as probable evidence for catalysis by Pd nanoparticles.39–43 To evaluate the proposed
mechanism, the mercury drop test was employed; when a drop of mercury metal was added
to the reaction mixture of 4-bromoanisole and phenylboronic acid under the optimized
conditions described and the reaction mixture heated, no catalytic activity was observed.
The results summarized in Table 1 show that in comparison to conventional reflux
conditions (Table 2), Suzuki reactions can be carried out successfully by homogenous
metal catalyst in conjunction with microwave irradiation.
Microwave irradiation led to dramatically reduced reaction times and generally to
higher yields. The dramatic rate enhancement is due to the rapid and uniform heating of
the reaction mixture and increased catalyst lifetime by the elimination of wall effects in
microwave compared to oil-bath heating.24 Microwave irradiation raises the temperature of
the whole volume simultaneously (bulk heating) whereas in the oil-heated tube, the reaction
mixture in contact with the vessel wall is heated first.50
Comparison of the efficiency of palladacycle catalyst with other catalytic systems in
the Suzuki coupling reaction of 4-bromoanisole using different catalytic systems under
conventional heating conditions is presented in Table 3. The ortho-palladated catalyst was
found to be more active and efficient than the other catalysts.

Suzuki Cross-Coupling Reactions
469
Table 2
Suzuki Reaction using ortho-Palladated Catalyst under Conventional Reflux Conditionsa
Entry Ar-X
Boron source
Biaryl product
Time
(min)
Yield
(%)b
1
2
3
4
5
6
7
8
9
10
p-O2NC6H4I
p-MeOC6H4B(OH)2 p-O2NC6H4-C6H4-p-OMe
20
60
120
30
180
95
92
90
91
93
90
91
92
80
72
p-MeOC6H4I
p-MeOC6H4Br
p-NCC6H4Br
p-HOCC6H4Br
PhB(OH)2
PhB(OH)2
Ph4BNa
p-MeOC6H4-Ph
p-MeOC6H4-Ph
p-NCC6H4-Ph
p-HOCC6H4-Ph
PhB(OH)2
p-MeOCC6H4Br p-MeOC6H4B(OH)2 p-MeOCC6H4-C6H4-p-OMe 150
p-ClC6H4Br
PhCl
p-HOCC6H4Cl
p-MeOCC6H4Cl PhB(OH)2
PhB(OH)2
PhB(OH)2
Ph4BNa
p-ClC6H4-Ph
Ph-Ph
p-HOCC6H4-Ph
p-MeOCC6H4-Ph
15
240
300
450
aReaction conditions: aryl halide (1 mmol), phenylboronic acid (1.2 mmol) or Ph4BNa (0.25 mmol),
K2CO3 (2 mmol), ortho-palladated catalyst (0.1 mmol%), ethanol (2 ml), 60◦C.
Experimental Section
All melting points were taken on a Gallenkamp melting apparatus and are uncorrected.
1H-NMR spectra were recorded using 400 MHz in CDCl3 at room temperature in the TMS
as an internal standard on a Bruker, Avance 500 instrument (Rheinstetten, Germany) and
Varian 400 NMR. FT-IR spectra were obtained on a spectrophotometer (Jasco-680, Japan);
spectra of solids were determined as using KBr pellets. Vibrational transition frequencies
are reported in wave number (cm−1). A Milestone microwave (Microwave Labstation- MLS
GmbH- ATC-FO 300) was used. A GC (BEIFIN 3420 gas chromatograph equipped a Varian
CP SIL 5CB column- 30 m, 0.32 mm, 0.25 μm) was employed to ascertain completion of
the reactions to check and yields. Palladium acetate, aryl halides and all other chemicals
were purchased from Merck and Aldrich and were used as received.
General Procedure for the Synthesis of the Catalyst. Cyclopalladation of triben-
zylamine was carried out by heating a mixture of the amine (0.4030 g, 1.4 mmol) with
palladium(II) acetate (0.3000 g, 1.34 mmol) in benzene (30 ml) at 80◦C. After one day,
the resulting mixture was filtered and the filtrate was evaporated to dryness to give a solid
was recrystallized from diethyl ether and hexane, mp. 154–156◦C, lit. mp. 156◦C.36 To
a solution of this complex (0.0012 g, 0.2 mmol) in acetone (25 ml) was added sodium
Table 3
Comparison of the Different Catalytic Systems Efficiency with ortho-Palladated Catalyst
Catalytic
system
Zeolite confined
Palladium(0)
Pd/C (10%)52
Pd(OAc)2
(3 mol%),
DABCO
ortho-Palladated
Catalyst
(0.1 mmol%)
Nano-clusters
(0.82 mol%)51
(6 mol%)53
Aryl halide
Time Conversion Time Yield Time Yield Time Conversion
(h)
6
(%)
99.6
(h)
24
(%)
73
(h)
16
(%)
90
(h)
2
(%)
100
p-MeOC6H4Br

470
Hajipour and Rafiee
bromide (0.206 g, 2 mmol) and the suspension was stirred for one day and acetone was
evaporated. The solid residue obtained was dissolved in methylene chloride (15 ml) and the
solution was dried by filtration through a plug of anhydrous magnesium sulfate to remove
insoluble material. Concentration of the filtrate to 2 ml using rotary evaporator at room
temperature, followed by addition of n-hexane (15 ml) precipitated the dimeric complex
catalyst as a suspended solid which was collected and air dried, mp. 169◦C (dec.).
General Procedure for the Suzuki Cross Coupling. A mixture of the aryl halide
(1 mmol), phenylboronic acid (0.1463 g, 1.2 mmol) or Ph4BNa (0.0855 g, 0.25 mmol),
K2CO3 (0.2764 g, 2 mmol), ortho-palladated complex (0.0006 g, 0.1 mmol%) was added
to ethanol (2 ml) in round-bottom flask equipped with condenser and placed into the
Milestone microwave oven. Initially the microwave irradiation was set at 600 W and the
temperature was raised to the desired temperature of 60◦C and the reaction mixture was
held at this temperature until the reaction was completed. The direct control of reaction
mixture temperature was carried out using the IR sensors and software that enables on-line
temperature-pressure control by regulation of microwave power output. The mixture was
stirred continuously during the reaction and monitored by both TLC and GC. After the
reaction was completed, the mixture was cooled to room temperature and was diluted with
ether (2 × 10 ml) and water (10 ml). The organic phase was dried over MgSO4, filtered and
concentrated under reduced pressure using a rotary evaporator. The residue was purified by
silica gel column chromatography (n-hexane:EtOAc, 9:1). The products were characterized
by comparing their mp., IR, 1H, 13C NMR spectra with those reported in the literature.
Also Suzuki cross coupling reaction was carried out successfully in larger scale
(30 mmol of aryl halide) using this ortho-palladated complex (0.018 g).
Acknowledgements
We gratefully acknowledge the funding support received for this project from the Isfahan
University of Technology (IUT), IR Iran and Isfahan Science & Technology Town (ISTT),
IR, Iran. Further financial support from the Center of Excellence in Sensor and Green
Chemistry Research (IUT) is gratefully acknowledged.
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