Highly efficient palladacycle/dihydroimidazolium chloride system for the suzuki-miyaura cross-coupling of aryl halides (I, Br, Cl) with arylboronic acids

Article abstract of DOI:10.1002/cjoc.201190014

A combination of a tertiary amine-based palladacycle and an N-heterocyclic carbene ligand precursor (1, N,N-bis-mesityl-4,5-dihydroimidazolium chloride) has been applied to catalyze the Suzuki-Miyaura cross-coupling of aryl halides with arylboronic acids. The substrate scope is general: a variety of electron rich and deficient aryl halides (I, Br, Cl) and arylboronic acids were found to undergo the cross-coupling reaction in good to excellent yields at low catalyst loading of 0.01-1 mol%. A combination of a tertiary amine-based palladacycle and an N-heterocyclic carbene ligand precursor has been applied to catalyze the Suzuki-Miyaura cross-coupling of aryl halides with arylboronic acids. Varieties of electron rich and deficient aryl halides (I, Br, Cl) and arylboronic acids were found to undergo the cross-coupling reaction in good to excellent yields at low catalyst loading of 0.01-1 mol%.

Full text of DOI:10.1002/cjoc.201190014

FULL PAPER
Highly Efficient Palladacycle/Dihydroimidazolium Chloride
System for the Suzuki-Miyaura Cross-Coupling of
Aryl Halides (I, Br, Cl) with Arylboronic Acids
a
a
a
Wang, Haiming (汪海明)
Wang, Jinping (王晋萍)
Qiu, Wenwei (仇文卫)
,a
,b
a
Yang, Fan* (杨帆)
Liu, Xiaofeng* (刘晓峰)
Tang, Jie (汤杰)
a
Institute of Medicinal Chemistry, Department of Chemistry, East China Normal University,
Shanghai 200062, China
b
Reata Pharmaceuticals, Inc. 2801 Gateway Drive Suite 150, Irving, Texas 75063, USA
A combination of a tertiary amine-based palladacycle and an N-heterocyclic carbene ligand precursor (1,
N,N-bis-mesityl-4,5-dihydroimidazolium chloride) has been applied to catalyze the Suzuki-Miyaura cross-coupling
of aryl halides with arylboronic acids. The substrate scope is general: a variety of electron rich and deficient aryl
halides (I, Br, Cl) and arylboronic acids were found to undergo the cross-coupling reaction in good to excellent
yields at low catalyst loading of 0.01— 1 mol%.
Keywords Suzuki-Miyaura reaction, N-heterocyclic carbine, palladacycle, aryl halides
Introduction
palladacycle and an NHC ligand has been rarely ex-
plored and only a limited amount of examples have been
reported. Herein, we report a novel combination of a
During the last three decades, the Suzuki-Miyaura
reaction has been demonstrated as an efficient and ver-
satile method to synthesize dienes, polyenes, styrenes,
and biaryls, which are common structural subunits in
complex natural products, pharmaceuticals and func-
8
tertiary amine-based palladacycle and an NHC precursor
(
1, N,N-bis-mesityl-4,5-dihydroimi-dazolium chloride)
as a highly efficient catalyst system for the Suzuki-
Miyaura cross-coupling of aryl halides with arylboronic
acids.
1
tional materials. Traditionally, Pd-phosphine com-
plexes have been proven as powerful catalysts for the
Suzuki-Miyaura reaction according to numerous re-
2
ports. However, the air-sensitivity of phosphine ligands
poses significant difficulty to the application of these
3
catalysts to certain synthetic efforts. Therefore, it re-
mains active interest in developing new catalysts that
provide improved stability and activity.
Since Herrmann reported the first Palladium/
N-heterocyclic carbene (NHC) complex catalyzed Heck
4
reaction in 1995, NHCs have been shown as another
class of highly effective and widely applicable ligands
5
for Pd-based catalyst systems. NHCs are air-stable, low
2 2 2
Figure 1 Structures of palladacycle Pd (dmba) Cl and NHC
toxic and readily tunable, therefore, they have attractive
advantages of easy storage and handling. On the other
hand, palladacycles, with air and moisture-stability and
high catalytic activity, are also alternative catalysts
ligands.
Results and discussion
(
precatalysts) proven to be efficient for cross-coupling
We began our investigation with the cross-coupling
of bromobenzene and phenylboronic acid in N,N-
dimethylacetamide (DMA) (Table 1). The easily acces-
sible palladacycle Pd (dmba) Cl and N-heterocyclic
carbene ligand precursor N,N-bis-mesityl-4,5-dihydro-
imidazolium chloride 1 were chosen to in situ form the
6
reactions. Among them, tertiary amine-based pallada-
cycles developed by Bedford and others including our
group have been demonstrated as effective precatalysts7
for the Heck reaction and the Suzuki-Miyaura reaction.
However, the combination of a tertiary amine-based
2
2
2
*
E-mail: fyang@chem.ecnu.edu.cn; liu@reatapharma.com; Fax: 0086-021-62232100
Received July 31, 2010; revised September 27, 2010; accepted October 8, 2010.
Project supported by the National Natural Science Foundation of China (No. 20772023).
2416
© 2010 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2010, 28, 2416— 2420

Highly Efficient Palladacycle/Dihydroimidazolium Chloride System
active catalyst under the same reaction condition. The
impact of the base on this reaction was first investigated
Entry 2 vs. 1). Lack of the NHC ligand precursor 1 also
led to a dramatically reduced yield, although the pal-
(
Table 1, Entries 1—7). In the presence of 0.1 mol%
ladacycle Pd
2 2 2
(dmba) Cl itself has been known as an
catalyst, the reaction turned out to be very sluggish
while Et
effective catalyst for a variety of cross-coupling reac-
tions employing aryl iodides as coupling partners.
7
c,7e-7g
3
N was used as the base (Table 1, Entry 1).
However, to our delight, the reactions employing inor-
ganic bases performed much better even with signifi-
cantly reduced catalyst loading (0.01 mol%) (Table 1,
Entry 2—7). In general, the yield improved with the
a
Table 2 Effect of Pd sources and ligand on catalytic efficiency
3 4 2
increasing strength of the base and K PO •3H O gave
the best result (76%, Table 1, Entry 7). Although the
reaction tolerates moisture, addition of extra amount of
water exhibited deleterious effect on the yield (Table 1,
Entry 8 vs. 7). Further increasing the steric hindrance of
the ligand by replacing the mesityl group with a bulkier
adamantly group dramatically decreased the yield (Ta-
ble 1, Entry 9 vs. 7). Most importantly, excellent yield is
obtained for cross-coupling of bromobenzene with
b
Entry
[Pd]/mol%
(dmba) Cl (0.01)
PdCl (0.02)
Pd (dmba) Cl (0.01)
1/mol%
0.02
Yield /%
1
2
3
Pd
2
2
2
89
32
21
2
0.02
2
2
2
None
a
General reaction condition: PhBr (1 mmol), PhB(OH)
2
(1.2
b
mmol), DMA (2 mL), K PO •3H O (1.5 mmol). Isolated yield,
3
4
2
average of two runs.
2 2 2
phenylboronic acid catalyzed by Pd (dmba) Cl /1 (0.01
mol%) at 120 ℃ (Table 1, Entry 10). Furthermore, the
reaction could proceed at room temperature without a
significantly negative impact on the yield, although a
higher catalyst loading was required (Table 1, Entry 11).
Following the optimized reaction condition, the
scope of the Suzuki-Miyaura reaction catalyzed by
Pd (dmba) Cl /1 system was investigated. A wide range
2 2 2
of aryl halides bearing substituents of various electronic
properties were found to undergo the cross-coupling
reaction with phenylboronic acid in consistently excel-
lent yields with a catalyst loading of 0.01 mol% (Table
Table 1 Preliminary reaction condition survey for the Su-
zuki-Miyaura cross-coupling of bromobenzene with phenylbo-
a
ronic acid
3
, Entry 1—8). More challenging aryl halide such as
p-methoxyphenyl bromide (Table 3, Entry 10 vs. 9) also
coupled well with higher catalyst loading (1 mol%).
Most importantly, the coupling of aryl chlorides with
b
phenylboronic acid catalyzed by the Pd
system proceeded smoothly in good to excellent yields
2 2 2
(dmba) Cl /1
Entry
Base
Et
Catalyst/mol% T/℃ Time Yield /%
1
2
3
4
5
6
7
3
N
0.1
140 60 min
140 70 min
140 70 min
140 70 min
140 70 min
140 70 min
140 70 min
140 70 min
140 70 min
120 8 h
34
36
54
65
61
60
76
63
47
89
76
(
Table 3, Entry 11—14), mostly within 1 h. It is note-
NaOAc
Na CO
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.1
worthy that aryl chlorides are usually challenging
“unreactive” cross-coupling partners although they are
readily available, versatile substrates with relatively low
2
3
2 3
K CO
9
cost. Attempts to facilitate a cross-coupling of a deac-
Cs CO
2
3
tivated aryl chloride with phenylboronic acid had, un-
fortunately, limited success (Table 3, Entry 15). To fur-
ther demonstrate the scope of this reaction, a series of
arylboronic acid with various electronic properties as the
other coupling partner were tested in the cross-coupling
with bromobenzene. In general, these arylboronic acids
underwent the cross-coupling reaction successfully to
give high yields with a negligible difference among
them (Table 3, Entries 16—18).
KF
K
3
PO
4 2
•3H O
c
8
K PO /H O
3
4
2
d
9
K PO •3H O
3
4
2
1
0
1
K
3
PO
4
•3H
2
O
1
K PO •3H O
r.t. 48 h
3
4
2
a
2
General reaction condition: PhBr (1 mmol), PhB(OH) (1.2
mmol), DMA (2 mL), base (1.5 mmol), Pd (dmba) Cl /1＝1∶2.
2
2
2
In conclusion, we have demonstrated a novel and
highly efficient catalyst system with a combination of a
b
c
Isolated yield, average of two runs.
2
H O (0.044 mmol) was
d
added. 1,3-Bis(1-adamantyl)-4,5-dihydroimidazolium chloride
2 2 2
tertiary amine-based palladacycle Pd (dmba) Cl and an
(
2) was used as the ligand.
N-heterocyclic carbene ligand precursor (1, N,N-bis-me-
sityl-4,5-dihydroimidazolium chloride) for the Su-
zuki-Miyaura cross-coupling of aryl halides with arylbo-
ronic acids. A variety of aryl halides (I, Br, Cl) and aryl-
boronic acids bearing various electronic properties were
found to undergo the cross-coupling reaction in good to
excellent yields at low catalyst loading of 0.01—1 mol%.
We conjectured that both the palladacycle
Pd
cursor N,N-bis-mesityl-4,5-dihydroimidazolium chloride
might be critical for this cross-coupling process. In
order to test our hypothesis, we initially chose an alter-
native Pd source, PdCl , to replace Pd (dmba) Cl and
indeed observed a marked yield decreasing (Table 2,
2 2 2
(dmba) Cl and N-heterocyclic carbene ligand pre-
1
2
2
2
2
2 2 2
Furthermore, both of the palladacycle Pd (dmba) Cl
Chin. J. Chem. 2010, 28, 2416— 2420
© 2010 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
2417

Wang et al.
FULL PAPER
a
Table 3 The Suzuki-Miyaura cross-coupling of aryl halides with arylboronic acids
b
Entry
1
R
H
X
I
R'
H
Time
1.5 h
3 h
Catalyst/mol%
0.01
Yield /%
97
2
3
4
5
6
7
8
9
p-Me
p-MeO
o-Me
H
I
H
0.01
99
I
H
12 h
5 h
0.01
94
I
H
0.01
98
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
Cl
Cl
Br
Br
Br
H
8 h
0.01
89
p-CH CO
H
4 h
0.01
99
3
p-NO2
p-Me
H
4 h
0.01
99
H
24 h
32 h
50 min
50 min
45 min
45 min
5 h
0.01
85
p-MeO
p-MeO
H
0.01
54
1
0
1
2
3
4
5
6
7
8
H
1.00
99
1
1
1
1
1
1
1
1
p-CH CO
H
1.00
98
3
2,4-diNO2
H
1.00
99
p-CF3
H
H
1.00
76
H
1.00
62
p-Me
H
H
10 h
8h
1.00
42
MeO
Me
0.01
90
H
8h
0.01
88
H
CH CO
8h
0.01
85
3
a
General reaction condition: aryl halides (1.0 mmol), arylboronic acid (1.2 mmol), DMA (2 mL), K
3
PO
4
•3H
2
O (1.5 mmol),
b
Pd
2
(dmba)
2
Cl
2
/1＝1 : 2. Isolated yield, average of two runs.
and the dihydroimidazolium chloride 1 are readily ac-
cessible, insensitive to air, moisture and heat, and suit-
able for long-term storage.
General procedure for synthesis of biaryls
A mixture of aryl halide (1 mmol), arylboronic acid
(
1.2 mmol), K
Pd (dmba) Cl (0.055 mg, 110 mmol or 5.5 mg, 0.01
mmol) and N,N-bis-mesityl-4,5-dihydroimidazolium
3 4 2
PO •3H O (399 mg, 1.5 mmol),
－
4
2
2
2
Experimental
－
4
chloride 1, (0.069 mg, 2×10 mmol or 6.9 mg, 0.02
All reagents and solvents were purchased from
commercial sources and were used without further puri-
mmol) in DMA (2 mL) was stirred under N
2
at 120 ℃
1
for an indicated time (Table 3), cooled to room tem-
perature. The solution was diluted with water (5 mL)
and extracted with diethyl ether (10 mL×3). The com-
fication. The melting points are uncorrected. H NMR
spectra were measured at 500 MHz with TMS as the
internal standard.
2 4
bined organic layer was dried with anhydrous Na SO
Preparation of N,N-bis-mesityl-4,5-dihydroimida-
zolium chloride
and concentrated under reduced pressure. The residue
was purified by flash column chromatography (silica gel,
using mixture of hexane/ethyl acetate as eluent) to af-
ford the biphenyl product.
A
mixture of N,N'-bis-(2,4,6-trimethylphenyl-
amino)ethane dihydrochloride (1.85 g, 5.0 mmol), 16
mL of triethyl orthoformate, and two drops of formic
acid was refluxed for 3 h. The ethanol was then distilled.
The temperature of the reaction mixture reached 130 ℃
for 30 min. Upon cooling to room temperature a color-
less solid precipitated which was collected by filtration.
Recrystallization of the crude product with acetoni-
trile/ether afforded 1.26 g of N,N-bis-mesityl-4,5-dihy-
1
0
Biphenyl 1 White crystal, m.p. 69—70 ℃ [Lit.
6
7
4
9—70 ℃]; H NMR (CDCl
3
, 500 MHz) δ: 7.34 (t, J＝
.0 Hz, 2H), 7.42—7.45 (m, 4H), 7.59 (d, J＝7.0 Hz,
H).
4
-Methylbiphenyl White powder, m.p. 48—50 ℃
1
0
1
[
(
7
Lit. 46—47 ℃]; H NMR (CDCl
3
, 500 MHz) δ: 2.43
s, 3H), 7.27—7.29 (m, 2H), 7.36 (t, J＝7.0 Hz, 1H),
.44—7.47 (m, 2H), 7.53 (d, J＝7.0 Hz, 2H), 7.62 (dd,
droimidazolium chloride. Yield 74%, m.p.＞260 ℃;
1
J＝7.0, 2.0 Hz, 2H).
2
H NMR (D O, 500 MHz) δ: 2.29 (s, 6H), 2.36 (s, 12H),
4
-Methoxybiphenyl White crystal, m.p. 87—88
4
.47 (s, 4H), 7.09 (s, 4H), 9.14 (s, 1H).
1
0
1
℃
[Lit. 87 ℃]; H NMR (CDCl
3
, 500 MHz) δ: 3.84
2
418
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© 2010 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2010, 28, 2416— 2420

Highly Efficient Palladacycle/Dihydroimidazolium Chloride System
(
7
s, 3H), 6.97 (d, J＝7.0 Hz, 2H), 7.29—7.31 (m, 1H),
11, 1184.
.35—7.42 (m, 2H), 7.52—7.56 (m, 4H).
(h) Kantam, M. L.; Srinivas, P.; Yadav, J.; Likhar, P. R.;
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For recent reviews on palladium-N-heterocyclic carbene
catalysts, see:
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2
-Methylbiphenyl White crystal, m.p. 254—255
1
1
1
℃
[Lit. 255 ℃]; H NMR (CDCl
3
, 500 MHz) δ: 2.25
3
4
5
(
7
s, 3H), 7.22—7.25 (m, 4H), 7.30—7.32 (m, 3H), 7.38—
.39 (m, 2H).
4
-Acetylbiphenyl Colorless crystal, m.p. 121—
1
2
1
1
23 ℃ [Lit. 120—121 ℃]; H NMR (CDCl
3
, 500
MHz) δ: 2.64 (s, 3H), 7.42 (d, J＝7.0 Hz, 1H), 7.46 (t,
J＝7.0 Hz, 2H), 7.63 (dd, J＝7.0, 2.0 Hz, 2H), 7.68—
7
.70 (m, 2H), 8.04 (dd, J＝7.0, 2.0 Hz, 2H).
4
-Nitrobiphenyl Pale yellow crystal, m.p. 112—
1
3
1
1
14 ℃ [Lit. 113—115 ℃]; H NMR (CDCl
3
, 500
MHz) δ: 7.39—7.46 (m, 3H), 7.56—7.57 (m, 2H), 7.67
(
dd, J＝7.5, 2.0 Hz, 2H), 8.24 (dd, J＝7.5, 2.0 Hz, 2H).
2
,4-Dinitrobiphenyl Yellow crystal, m.p. 107—
1
4
1
1
7
7
2
08 ℃ [Lit. 108 ℃]; H NMR (CDCl
3
, 500 MHz) δ:
.32—7.34 (m, 2H), 7.46—7.48 (m, 3H), 7.68 (d, J＝
.5 Hz, 1H), 8.42 (dd, J＝7.5, 2.0 Hz, 1H), 8.69 (d, J＝
.0 Hz, 1H).
4
-Trifluoromethylbiphenyl White solid, m.p. 66—
10
1
6
7 ℃ [Lit. 67—68 ℃]; H NMR (CDCl
3
, 500 MHz)
δ: 7.42—7.43 (m, 3H), 7.56—7.58 (m, 2H), 7.65 (m,
4
H).
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Acknowledgment
6
We thank Lab of Organic Functional Molecules, the
Sino-French Institute of ECNU for supports.
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FULL PAPER
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