Synthesis and applications of a new palladacycle as a high active catalyst in the Suzuki couplings

Article abstract of DOI:10.1016/j.jorganchem.2008.07.003

The reaction of biphenyl-based phosphine P(o-C₆H₄Me)Ph₂ (1) with Pd(OAc)₂ in toluene affords the air and water stable palladacycle (2) as a binuclear compound which has been characterized by multi-nuclear NMR spectroscopy and elemental analysis as a mixture of cis and trans isomers with relative intensity of 1:3, respectively. This palladacycle is a highly efficient catalyst precursor for the coupling of aryl boronic acids and aryl halides. Both activated and deactivated aryl bromides and chlorides are efficiently coupled in the presence of 2 to furnish the corresponding cross-coupled products in excellent yields, and a wide variety of functional groups are tolerated in aryl halides. This methodology has also been extended for the coupling of bromoarylphosphines and bromoarylphosphine oxides with aryl boronic acids for the generation of hindered corresponding products.

Full text of DOI:10.1016/j.jorganchem.2008.07.003

Journal of Organometallic Chemistry 693 (2008) 3135–3140
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis and applications of a new palladacycle as a high active catalyst in the
Suzuki couplings
*
Mohammad Joshaghani , Marzieh Daryanavard, Ezzat Rafiee, Shirin Nadri
Department of Chemistry, Faculty of Science, Razi University, Kermanshah, Iran
Kermanshah Oil Refining Company, Kermanshah, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 May 2008
Received in revised form 18 June 2008
Accepted 1 July 2008
Available online 8 July 2008
The reaction of biphenyl-based phosphine P(o-C6H4Me)Ph2 (1) with Pd(OAc)2 in toluene affords the air
and water stable palladacycle (2) as a binuclear compound which has been characterized by multi-
nuclear NMR spectroscopy and elemental analysis as a mixture of cis and trans isomers with relative
intensity of 1:3, respectively. This palladacycle is a highly efficient catalyst precursor for the coupling
of aryl boronic acids and aryl halides. Both activated and deactivated aryl bromides and chlorides are effi-
ciently coupled in the presence of 2 to furnish the corresponding cross-coupled products in excellent
yields, and a wide variety of functional groups are tolerated in aryl halides. This methodology has also
been extended for the coupling of bromoarylphosphines and bromoarylphosphine oxides with aryl boro-
nic acids for the generation of hindered corresponding products.
Keywords:
Palladacycle
Carbon–carbon bond forming
Suzuki coupling
Biphenyl-based phosphine
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
both electronically activated and deactivated aryl bromides
[16,17].
Organopalladium complexes play an important role in carbon–
carbon bond forming reactions because of their versatility, com-
patibility with most of functional groups and relative low toxicity
[1]. Among them, palladacycles have merited special attention as
their enormous supremacy in many aspects [2]. The facility of
preparation and versatility for modification of their steric and
electronic properties combined with their usually high air and
thermal stability, infer them a variety of applications in organic
synthesis, new materials, bio-organometallic chemistry, and orga-
nometallic catalysis [3–9]. Since Hermann and Beller demon-
strated the extremely high catalytic activity of cyclopalladated
tri-o-tolylphosphine in Heck reaction [10], many palladacycles
have been synthesized and used as highly efficient catalysts in
carbon–carbon bond forming reactions [2,10–13]. Of note is that
the best results were obtained with phosphapalladacycles or with
palladacycles modified with carbenes or phosphorus containing
ligands. This might be rationalized by the extra stability provided
by these ligands for the stabilization of the low-ligated catalyti-
cally active Pd species involved in the main catalytic cycle
[14,2,15]. Also Bedford and co-workers recently investigated the
possibility of using related palladated triarylphosphite complexes
and found extremely high activity in Suzuki cross coupling with
We have already reported the synthesis of biphenyl-based
phosphines by Suzuki coupling of arylboronic acids with bromoa-
rylphosphine oxides using Pd(dba)2 and PPh3 (Scheme 1) [18].
Moreover we have recently demonstrated that biphenyl-based
phosphine 1 which was made based on Buchwald’s instructive
and pincer works [19], has been successfully applied to the palla-
dium catalyzed Suzuki coupling of aryl halides as well as bromoa-
rylphosphines and bromoarylphosphine oxides, with low catalyst
loading and good to excellent conversions [20]. This excellent
activity in coupling reactions using palladacycles as well as our
idea that the extraordinary activity of the phosphine 1 may be
due to formation of palladacycle intermediate encouraged us to
synthesis the corresponding palladacycle 2 from the phosphine 1
and investigate its catalytic activity in the Suzuki coupling reac-
tions. Therefore we report herein the synthesis of palladacycle 2,
which was easily prepared from the biphenyl-based phosphine 1
on addition of Pd(OAc)2 in toluene as a binuclear complex with
two acetate-bridged ions (Scheme 2). To test the reactivity of the
synthesized palladacycle in the Suzuki reaction, it was employed
in the reaction of a series of aryl halides with arylboronic acids.
This palladacycle has also been used for the coupling of bromoaryl-
phosphines and bromoarylphosphine oxides with arylboronic
acids (Scheme 3).
In order to compare the relative activity of palladacycle 2 to
phosphine 1, all coupling reactions were carried out in obtained
optimized conditions for phosphine 1 [20].
* Corresponding author. Tel./fax: +98 831 4274559.
E-mail address: mjoshaghani@razi.ac.ir (M. Joshaghani).
0022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.07.003

3136
M. Joshaghani et al. / Journal of Organometallic Chemistry 693 (2008) 3135–3140
Table 1
O
Suzuki coupling of aryl halides using palladacycle 2a
PPh2
PPh2
Br
Entry
1
Aryl halide
Arylboronic acid
Yieldb (%)
100
10ꢀ3 Ton
100
i) (o-tolyl)B(OH)2, Pd(dba)2, PPh3
(OH)2B
Br
ii)HSiCl3
Me
1
Scheme 1. The synthesis route for preparation of biphenyl-based phosphine 1.
2
100
75
100
75
COMe
NO2
(OH)2B
(OH)2B
(OH)2B
(OH)2B
(OH)2B
(OH)2B
(OH)2B
(OH)2B
(OH)2B
(OH)2B
(OH)2B
(OH)2B
Br
Br
Br
Ph
H2
C
PPh2
Ph
3
OAc
Pd(OAc)2
P
Pd
Pd
P
OAc
C
H2
Me
4
100
65
100
65
Ph
CH3
1
Ph
2
Scheme 2. The synthesis route for preparation of palladacycle 2.
CH3
5
Br
Br
R
R
Y
Y
B(OH) 2
X
+
6
100
90
100
90
OMe
R'
R'
R = H, Me; R' = H, Me, Br, COMe, COPh, OMe, Ph, NO2;
X = Cl, Br; Y = H, PPh2, OPPh2
7
Br
Br
Scheme 3. The Suzuki cross coupling reaction.
8
60
60
2. Results and discussion
2.1. Synthesis and characterization of the palladacycle 2
9
65
65
Cl
The palladacycle 2 was easily obtained on 85% yield as white
powder from the reaction 2-diphenylphosphino-20-methylbiphe-
nyl, phosphine 1, with Pd(OAc)2 in toluene via coordination of pal-
ladium followed by C–H activation which was as a binuclear
compound with two acetate ion bridge (Scheme 2). Structure of
this palladacycle has been ascertained by means of elemental anal-
ysis and multi-nuclear NMR spectroscopy as a mixture of cis and
trans isomers with relative intensity 1:3 and 1 mole ether of crys-
tallization. The NMR data are similar to another acetate-bridged
palladacycle, which is previously prepared by Herrmann on addi-
tion of P(o-tolyl)3 to Pd(OAc)2 [21].
10
11
12
13
14
100
100
95
100
100
95
Cl
Cl
COMe
COMe
Cl
OMe
2.2. Suzuki coupling reactions of aryl halides
Cl
90
90
To test the reactivity and the catalytic properties of the synthe-
sized palladacycle, it was first employed in the Suzuki coupling
reaction of a series of aryl halides with phenylboronic acid and
o-tolylboronic acid. The obtained results are presented in Table 1.
This table shows that palladacycle 2 is a high reactive catalyst,
which can catalyze the Suzuki cross coupling of even extremely
electron-rich aryl halides such as 3-chloroanisole (Table 1, entry
12). As expected, the catalytic activity depends on type of the ha-
lide. Similar conversions were obtained with high activate substi-
tuted aryl bromides and chlorides (e.g. Table 1, entries 2, 10).
However, usually less reactive aryl chlorides resulted poorer con-
version and yields compared to the corresponding aryl bromides
(Table 1, entries 1, 9). The reactivity increases with increasing
the reactivity of aryl chloride; reaches to 100% in the case of high
reactive functional groups such as acyl (Table 1, entry 10).
Me
100
100
Br
Br
Br
(OH)2B
Me
15
16
100
100
100
100
COMe
(OH)2B
Me
CH3
(OH)2B
In the case of less-bulky aryl halides, the rates and conversions
were independent to steric effects of the studied boronic acids. For

M. Joshaghani et al. / Journal of Organometallic Chemistry 693 (2008) 3135–3140
3137
Table 1 (continued)
on the conversion. This is probable due to increasing the contribu-
tion of transmetallation step in the rate of reaction.
Entry
17
Aryl halide
Arylboronic acid
Yieldb (%)
85
10ꢀ3 Ton
85
Me
Br
2.3. Suzuki coupling reactions of bromoarylphosphine oxides and
bromoarylphosphines
(OH)2B
Me
We have extended the experimental protocol for the prepara-
tion of biphenyl-based phosphines and biphenyl-based phosphine
oxides. Table 2 summarizes the results of the Suzuki cross coupling
of bromoarylphosphine oxides and bromoarylphosphines with two
arylboronic acids. Monobromophosphine oxide was coupled with
1.2 equiv. of arylboronic acids (Table 2, entries 1, 3) to give the cor-
responding biphenylphosphine oxides. Dibromophosphine oxide
was coupled with 2.4 equiv. of arylboronic acids (Table 2, entries
2, 4) to give the corresponding terphenylphosphine oxides as a sole
product without biphenyl byproduct which was expected to be
formed. This method is also efficient for coupling arylboronic acids
with haloarylphosphines instead of their oxides.
18
19
60
85
25
60
85
25
Cl
Cl
(OH)2B
Me
COMe
(OH)2B
Me
Br
Me
20
(OH)2B
a
Reaction conditions: 1.0 mmol aryl halide, 1.5 mmol ArB(OH)2, 0.001 mol% pal-
ladacycle, 2.0 mmol K3PO4, 5 mL toluene, 1 mL water, 100 °C, 1 h (6 h for aryl
chlorides).
2-Diphenylphosphinobromobenzene, [PPh2(o-C6H4Br)] (Table
2, entry 5) and 5-diphenylphosphino-1,3-dibromobenzene,
[PPh2(C6H3Br2)], (Table 2, entry 6) were coupled with phenylbo-
ronic acid to give 1 and terphenylphosphine in 75 and 70% yield,
b
Isolated yield.
ortho-substituted aryl halides such as 2-bromotoluene (Table 1, en-
try 20) however, the ortho effects of two ortho methyls on the o-
tolylboronic acid and 2-bromotoluene led to a remarkable decrease
respectively according to 31P NMR. This method negates
a
Table 3
Comparison of Suzuki coupling using palladacycle 2 and phosphine 1a
Table 2
Suzuki coupling of bromoarylphosphine oxides and bromoaryl-phosphines using
Entry Aryl halide
Arylboronic acid
Yieldb (%)
Phosphine 1 Palladacycle
palladacylce 2a
Entry
1
Substrate
Arylboronic acid
Yieldb (%)
85
Ton
1
20(90)
75
4250
O
NO2
CH3
(OH)2B
Br
(OH)2B
PPh2
Br
2
20(25)
50(70)
100
(OH)2B
(OH)2B
Br
O
PPh2
CH3
2
3
4
5
82
65
40
75
70
4100
3250
2000
3750
3500
3
0
65
(OH)2B
Br
Br
Br
O
4
50
100
90
(OH)2B
(OH)2B
(OH)2B
(OH)2B
(OH)2B
OMe
Br
Me
PPh2
(OH)2B
Br
5
40
Br
O
Br
PPh2
Me
6
0(30)–(20)
10(50)
40(70)
60
(OH)2B
Br
Br
7
65
95
Cl
PPh2
Br
Me
Cl
(OH)2B
8
COMe
PPh2
Cl
6
Br
(OH)2B
9
20(60)
90
(OH)2B
Br
a
Reaction conditions: 1.0 mmol aryl halide, 1.5 mmol ArB(OH)2, 0.001 mol%
Pd(OAc)2 (or palladacycle 2 in the absence of phosphine 1), 0.002 mol% phosphine 1,
2.0 mmol K3PO4, 5 mL toluene, 1 mL water, 100 °C, 1 h (6 h for aryl chlorides using
palladacycle).
a
Reaction conditions: 1.0 mmol phosphine oxide (or phosphine), 1.2 mmol
ArB(OH)2 (2.4 mmol for entries 2, 4, and 6), 0.02 mol% palladacycle 5, 2.0 mmol
K3PO4, 5 mL toluene, 1 mL water, 5 h.
Isolated yield according to 31P NMR, not optimized.
b
b
Isolated yield. The numbers in parentheses are yields after 24 h.

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M. Joshaghani et al. / Journal of Organometallic Chemistry 693 (2008) 3135–3140
Table 4
Comparison of Suzuki coupling of phenylboronic acid and different aryl halides using palladacycle 2 and other palladacycles
Entry
Ar-X
Palladacycle
[Pd] (%)
Base
Solvent
T (°C)
Time (h)
1
Yield (%)
100
Ref.
_
1
2
0.001
K3PO4
Toluene
100
COMe
Br
CH3
2
2
0.001
K3PO4
K3PO4
K2CO3
K3PO4
K3PO4
K2CO3
K2CO3
K2CO3
K2CO3
Toluene
100
130
110
130
130
130
130
130
130
1
100
69
93
97
74
59
92
92
74
_
Br
t-Bu
CH3
S
3
1
DMF
1
[23]
[24]
[23]
[24]
[17]
[17]
[26]
[26]
Pd Cl
S
Br
t-Bu
Ph
COMe
N
OH
4
0.1
Toluene
0.5
Pd
Cl
Br
2
COMe
Cl
5
0.2
DMF
2
Pd
2
S
t-Bu
Me
Me
Br
CH3
Cl
6
0.002
0.001
0.001
0.05
0.001
DMF
13
18
18
20
20
Pd
2
S
t-Bu
Br
O
O
PPh2
COMe
7
Toluene
Toluene
o-Xylene
o-Xylene
PdTFA
PPh2
Br
O
PPh2
COMe
Me
PdTFA
PPh2
8
O
Br
COMe
Ac
O
Pd
9
2
2
P
(o-tolyl)2
Br
COMe
Ac
O
Pd
10
P
(o-tolyl)2
Br

M. Joshaghani et al. / Journal of Organometallic Chemistry 693 (2008) 3135–3140
3139
reduction step and if established generally could be an alternative
3.2. Preparation of palladacycle 2
Schlenk tube was charged with phosphine
and efficient method for synthesis a wide variety of phosphines.
The results show that phenylboronic acid gives higher conver-
sion in comparison with o-tolylboronic acid (Table 2, entries 1–
4). Same results were reported in other Suzuki coupling systems
[22]. This feature shows that rate of conversion is depended to
the type of arylboronic acid and so suggests that transmetallation
step may be contributed in the rate of reaction.
A
1
(1.0 g,
2.85 mmol) in toluene (60 mL). A solution of Pd(OAc)2 (0.45 g,
2 mmol) in toluene (30 mL) was added from dropping funnel drop
wise. The solution turned pale yellow immediately. The mixture
was heated to 50 °C under nitrogen for 3 min and cooled quickly.
The volume of mixture was reduced to a quarter in vacuo. A white
powder was obtained on addition of hexane (60 mL) together with
a few black particles which was filtered and the solvent was evap-
orated using rotary evaporator. The crude product was resolved in
toluene (10 mL) and was purified over cellite. The solvent was
evaporated and the white powder was crystallized with toluene/
ether, yield was 85%.
2.4. Comparison of Suzuki coupling using palladacycle 2 and
phosphine 1
In order to investigate the relative activity of the palladacycle 2
compared to the phosphine 1, some similar Suzuki cross coupling
reactions using palladacycle 2 and phosphine 1 which we have re-
cently demonstrated [21], are listed in Table 3.
The results show that the palladacycle 2 is very reactive than
phosphine 1 in the Suzuki coupling reactions. It may be due to
the mechanism aspects. The Suzuki coupling for phosphine 1 sys-
tem proceeds by a mechanism in which the formation of a mono-
phosphine palladacycle intermediate contribute significantly in the
rate of reaction.
1H NMR (400 MHz, CDCl3): d 1–1.5 (m, 16, CH3 of ether, CH3 of
acetate bridges, CH2 coordinated to palladium), 3.48 (q, 4, CH2 of
ether), 6.5–7.8 (m, 36, aromatic protons); 13C NMR (200 MHz,
CDCl3): 15.5, 21.4, 22.2, 25.6, 28.1, 63.4, 125.1, 125.6, 126.3, 127,
2, 127.5, 128.2, 128.9, 129.4, 132,5, 133.8, 135,2, 136,3, 138,4,
139,6, 31P{1 H} NMR (162 MHz, CDCl3): d 15.40 (s), 27.69 (s) (for
both cis and trans isomers with relative intensity of 1:3). Anal. Calc.
for C54H46O4P2Pd2 ꢁ C4H10O: C, 62.87, H, 5.05, P, 5.57, Pd, 19.19.
Found: C, 63.01, H, 4.95, P, 5.65, Pd, 18.94%.
2.5. Comparison of Suzuki coupling using palladacycle 2 and other
palladacycles
3.3. General procedure for the Suzuki coupling of aryl halides
High activity of palladacycle 2 in the Suzuki coupling reactions
enforced us to compare the obtained results with palladacycle 2
and other similar reported palladacycle systems. Table 4 summa-
rizes this comparison study.
Table 4 illustrates that the palladacycle 2 gives higher conver-
sion in lower catalyst loading in comparison with similar systems
(Table 4, entries 1–5). Also in comparatively similar system using
same catalyst loading, palladacycle 2 gives higher conversion with
lower reaction times (Table 4, entries 1–2, 6–8).
In conclusion we have demonstrated that this new palladacycle
is a highly efficient catalyst precursor for the coupling of arylbo-
ronic acids with aryl halides and, from the standpoint of yields, it
ranks with the best reported systems in the literature, especially
when compared with the analogous phosphine adducts of halo-
gen-bridged dimer palladacycles [25]. Palladacycle 2 is also effi-
cient catalyst for the generation of biphenyl-based phosphines
and biphenyl-based phosphine oxides. In addition, compared to
other similar reported systems, our suggested system has some
advantages such as higher activity, lower catalyst loading, lower
reaction times, etc.
Reaction tube was charged with PhB(OH)2 (1.5 mmol), K3PO4
(2 mmol) under a dry nitrogen atmosphere. A solution of 4-bromo-
acetophenone (1.0 mmol in 2 mL of freshly dried toluene) along
with a solution of palladacycle 2 (0.001 mmol in 3 mL of freshly
dried toluene) were added through a rubber septum. After addition
of water (1 mL), the resulting mixture was heated at 100 °C for 1 h.
After extraction with ether, the organic phase was dried over
MgSO4. The solvent was evaporated and a crude product was ob-
tained which was characterized by its 1H NMR spectrum and melt-
ing point. To isolate the product, the crude product was purified by
chromatography with EtOAc/hexane (1:8).
3.4. General procedure for the coupling of bromoarylphosphines and
bromophosphine oxides
Reaction tube was charged with PhB(OH)2 (1.2 mmol), K3PO4
(2.0 mmol), PPh2(o-C6H4Br) or OPPh2(o-C6H4Br) (1.0 mmol), palla-
dacycle 2 (0.02 mmol) in freshly dried toluene (5 mL) under a dry
nitrogen atmosphere. After addition of water (1 mL), the resulting
mixture was heated at 100 °C for 5 h. After cooling to room tem-
perature, the mixture was diluted with water and extracted with
chloroform (3 ꢂ 10 mL), the combined organic extracts were
washed with brine, dried over MgSO4. The solvent was evaporated
and a crude product was obtained. The crude product was purified
by flash chromatography (1:4 EtOAc/hexane).
3. Experimental
3.1. General remarks
All reactions were performed under an atmosphere of either dry
nitrogen or argon. All chemicals were purchased commercially
from Fluka and/or Merck companies that were used without fur-
ther purification. Solvents were treated using standard procedures
and were distilled under an atmosphere of nitrogen before use. The
biphenyl-based phosphine 1 was prepared according to a modifica-
tion of our previously work [18].
Acknowledgements
The authors are grateful to the Kermanshah Oil Refining Com-
pany and the Razi University Research Council for the financial
support of this work.
1H (400 MHz), 13C (100 MHz) and 31P (162 MHz) NMR spectra
recorded on a Bruker Avance Spectrometer. Elemental analysis
was performed using CHN Herause rapid model. Shimadzu GC
14-A and thin layer chromatography on precoated silica gel Fluo-
rescent 254 nm (0.2 mm) on aluminum plates were used for mon-
itoring the reactions. Conversions were determined by GC, based
on bromoacetophenone. The cross coupling products were charac-
terized by their 1H NMR spectra and melting points.
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