Ionically tagged benzimidazole palladium(II) complex: Preparation and catalytic application in cross-coupling reactions

Article abstract of DOI:10.1016/j.tetlet.2011.05.079

An imidazolium chloride tagged palladium(II) complex has been conveniently prepared and structurally analyzed. It is active toward cross-coupling of arylboronic acid with aryl halide and benzoyl chloride, giving moderate to high yield of the desired biaryls and aryl ketones, respectively. The present phosphine-free (N-N)Pd(II) complex could be efficiently recycled at least four times with minor decrease of activity in the aqueous Suzuki-Miyaura coupling reaction.

Full text of DOI:10.1016/j.tetlet.2011.05.079

Tetrahedron Letters 52 (2011) 3897–3901
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Ionically tagged benzimidazole palladium(II) complex: preparation
and catalytic application in cross-coupling reactions
a,b
a
a
a
a,
⇑
Li Zhang , Junliang Wu , Lijun Shi , Chungu Xia , Fuwei Li
a
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 73000, PR China
Graduate School of the Chinese Academy of Sciences, Beijing 100039, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
An imidazolium chloride tagged palladium(II) complex has been conveniently prepared and structurally
analyzed. It is active toward cross-coupling of arylboronic acid with aryl halide and benzoyl chloride, giv-
ing moderate to high yield of the desired biaryls and aryl ketones, respectively. The present phosphine-
free (N–N)Pd(II) complex could be efficiently recycled at least four times with minor decrease of activity
in the aqueous Suzuki–Miyaura coupling reaction.
Received 22 March 2011
Revised 12 May 2011
Accepted 17 May 2011
Available online 24 May 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Phosphine-free
Cross-coupling
Recyclable catalyst
Crystal structure
Pd-catalyzed Suzuki–Miyaura cross-coupling of arylboronic
acid with aryl halide provides a very important tool for the synthe-
sis of unsymmetrical biaryls.1 Aromatic ketone can also be effec-
tively prepared by another cross-coupling of arylboronic acid
The route for the synthesis of the ionically tagged palladium
catalyst (2) is illustrated in Scheme 1. Direct N-alkylation of
commercially available 1,2-dimethyl imidazole with 2-chloro-
methyl-1-ethyl-benzenimidazole prepared according to our re-
ported procedure afforded the desired methyl imidazolium
chloride salt supported benzimidazole ligand (1) as a white solid
2
with acid chloride, and such method can avoid the disadvantages
of other strategies such as the low region-selectivity of Friedel–
3
11
Crafts acylation and handling the toxic carbon monoxide in the
in 87% yield. Subsequently, the new Pd(II) complexes were ob-
tained in 85% yield as yellow crystals from an aqueous methanolic
4
Pd-catalyzed carbonylation of arylboronic acid and aryl halide.
Furthermore, separation and reuse of the expensive transition me-
tal catalyst and commonly used excess phosphine ligand in the
cross-coupling reactions is a particularly attractive feature from
the viewpoint of the reduction of catalyst expense and the contam-
ination to product.
solution of K
2 4
[PdCl ] with the ligand at room temperature. Forma-
tion of this ionically tagged ligand and its Pd(II) complex has been
confirmed by H NMR, C NMR and ESI-MS. For 2 there is an addi-
tional group resonance with a ratio of about 1:3.5 for the protons of
the ligand’s alkyl moieties in its H NMR spectrum, this points to
1
13
1
Adding an ionic moiety such as imidazolium salt onto the neu-
tral donor ligand to make the resultant metal complex more solu-
ble in water or ionic salt is an ideal solution for immobilization and
recycling of the expensive transition metal catalyst, and product
could be conveniently extracted from catalyst system by using
the presence of a secondary species in solution condition apart
from the expected main trans–trans-Pd(II) complex, which is more
stable because of the less steric hindrance between the two li-
gands. Single crystals suitable for X-ray crystallography of 2 were
2
conveniently obtained by slow diffusion of Et O into its MeOH
5
desirable aqueous or ionic liquid solvent system. Recently, a wide
solution. X-ray diffraction analysis revealed an essentially
square-planar Pd(II) coordination geometry with two cationic
ligands trans to each other [Fig. 1, depository number: CCDC813731].
Pd-catalyzed Suzuki–Miyaura cross-coupling of aryl bromide or
range of catalysts designed by this methodology have been suc-
6
7
cessfully applied in the cross-coupling, olefin metathesis, oxida-
8
9
10
tion, polymerization reaction and asymmetric catalysis.
1
2
Herein, we report the preparation of a structurally defined imi-
dazolium salt functionalized benzimidazole palladium(II) complex
iodine with boronic acid is well documented. However, the
development of efficient phosphine-free and recyclable14 palla-
dium catalyst system still remains to be developed further. Inter-
estingly, we found that 2 was a highly active and recyclable
catalyst toward the Suzuki–Miyaura cross-coupling reaction of aryl
halides with arylboronic acid.
13
(2) and its catalytic applications in the C–C cross-coupling of aryl-
boronic acid with aryl halide and acid chloride, respectively.
⇑
Corresponding author. Fax: +86 931 4968129.
E-mail address: fuweili@licp.cas.cn (F. Li).
0
040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.05.079

3
898
L. Zhang et al. / Tetrahedron Letters 52 (2011) 3897–3901
Cl
N
N
N
N
N
N
N
N
K PdCl4
2
N
Cl Pd Cl
N
THF,reflux
N
Cl
N
N
N
(
2)
(
1)
Cl
N
N
Cl
Scheme 1. Synthesis of the Pd(II) complex (2) from imidazolium salt precursor (1).
transfer catalyst and stabilizer to improve activity of the metal
1
5
catalyst in the aqueous catalytic system. Consequently, the yield
was promoted from 68% to 89% by addition of certain amount of
TBAB (entry 3), raising the temperature to 60 °C gave the excellent
yield of 97% (entry 4). Then the aqueous TBAB/H
0 °C were selected as the optimized solvent system and reaction
temperature, respectively. Lower yield (77%) was obtained in the
presence of organic base such as triethyl amine (NEt ) (entry 5),
2
O mixture and
6
3
NaOAc and 4-dimethylamiopryidine (DMAP) even gave quite low
yield of the product (entries 6 and 7).
With the optimized reaction condition in hand, we next
extended scope of the present protocol to the cross-coupling of
other aryl halides with the substituted phenylboronic acid (Table
2). Phenyl bromides with an electronic withdrawing group
(
EWG) (3b–3f) could couple efficiently with general high isolated
yields (>90%) of the desired product (5b–5f) (entries 1–5). Coupling
of biphenyl bromide with phenylboronic acid also proceeded
smoothly giving an 88% yield of 5g (entry 6). The deactivated
1
-bromo-4-methylbenzene (3a) worked well toward the phenyl-
boronic acid with a EWG (4b and 4c) (entries 7 and 8). 4-Nitro-
phenyl bromide could react with boronic acid having either a
EWG or electronic donating group (EDG), resulting in the corre-
sponding substituted biaryls (5j–5l) in 92–98% yield (entries
9–11). The catalyst was also effective toward the coupling of
activated aryl bromide with 2- or 3-methyl substituted phenylbo-
ronic acid with excellent yield of the desired products (5m–5o,
entries 12–14). Interestingly, the present water soluble catalyst
also worked well toward the coupling of aryl chloride giving a
Figure 1. Molecular structure of trans–trans-Pd(II) complex (2) showing 30%
probability ellipsoids. Selected bond distances [Å] and angles [°]: Pd1–N1
2
.015(3), Pd1–Cl1 2.297(8), N1–Pd1–N1A 180.0(2), N1–Pd1–Cl1 89.4(8), N1–Pd1–
ClA 90.6(8), Cl1–Pd1–Cl1A 180.0.
Cross-coupling of deactivated 1-bromo-4-methylbenzene (3a)
with phenylboronic acid was selected as a model reaction to screen
the reaction conditions of the catalyst, and the results are shown in
Table 1. Because of the good water solubility of complex 2, pure
water was initially used as a solvent resulting in a 68% isolated
yield of the desired biaryl (5a) at room temperature within 12 h
7
0% yield of 5a (entry 15).
The reusability of expensive metal catalyst is very important
from the viewpoints of economy and the reduction of catalyst con-
tamination to the product. Suzuki–Miyaura coupling of deactivated
3
2
a with phenylboronic acid was selected to test the recyclability of
, because of its air stability and insolubility with less polar organic
in the presence of K
be improved to 72% by extending the reaction time to 24 h (entry
), which probably indicates the reaction duration is not a decisive
2 3
CO as a base (entry 1), and yield could only
2
solvent such as ether, the catalyst (H O–TBAB phase containing
2
catalyst) could be easily separated from the product phase (nonpo-
lar organic solvent phase) by extraction three times with diethyl
ether, subsequently the catalyst phase can be directly reused in
the next run with fresh addition of base, and it was found that
the present phosphine-free ionic Pd(II) complex showed high sta-
bility and could be used at least five times with the fifth run giving
a 86% isolated yield of the desired 4-methylbiphenyl (5a) (for more
details about reuse experiment, see Supplementary data).
factor to achieve a higher yield of the product. Tetrabutyl ammo-
nium bromide (TBAB) has been approved to be a useful phase
Table 1
Screening experiments for the Suzuki–Miyaura cross-coupling reaction catalyzed by
a
2
Entry
Solvent
Base
T (°C)
t (h)
Yieldb (%)
Another interesting application of 2 was the cross-coupling of
arylboronic acid with acid chloride to yield aryl ketones, which
are extensively used in pharmaceuticals fragrance and agrochemi-
1
2
3
4
5
6
7
H
H
2
O
O
K
K
K
K
2
2
2
2
CO
CO
CO
CO
3
3
3
3
25
25
25
60
60
60
60
12
24
12
12
12
12
12
68
72
89
97
77
23
19
2
TBAB/H
TBAB/H
TBAB/H
TBAB/H
TBAB/H
2
O
2
O
2
O
2
O
2
O
2
a,16
cals.
Firstly, reaction of phenylboronic acid with benzoyl chlo-
NEt
NaOAc
DMAP
3
ride was chosen to optimize reaction conditions (Table 3). Typical
reaction was carried out in the presence of 1.0 mol % Pd(II) catalyst
(2) at room temperature with water as solvent. Unfortunately, ben-
a
zophenone (7a) was obtained as a main product only in 43% yield
with carboxylic anhydride (17%) as the only by-product, which led
to the low yield of the desired 7a (entry 1). In 2008, Xin reported
Reaction condition: phenylboronic acid (0.6 mmol), 1-bromo-4-methylbenzene
0.5 mmol), TBAB (1.0 g), H O (1.0–1.5 mL), base (0.8 mmol), catalyst 2 (1.0 mol %).
Isolated yield after column chromatography.
(
2
b

L. Zhang et al. / Tetrahedron Letters 52 (2011) 3897–3901
3899
Table 2
a
Pd(II) complex catalyzed Suzuki–Miyaura cross-coupling of aryl halides with arylboronic acids in aqueous TBAB solution
R2
R1
R2
1.0 mol % 2
X
B(OH)2
+
°
C
R1
60
5
3
X =Cl,Br
4
Entry
1
R2
CF
R1
Product
Yieldb (%)
CF
3
, 3b
H, 4a
H, 4a
H, 4a
H, 4a
H, 4a
H, 4a
4-F, 4b
3
98
98
89
96
90
88
81
79
98
97
5
b
CN
2
3
4
5
6
7
8
9
CN, 3c
5
c
NO
F
NO
2
, 3d
2
5
d
F, 3e
5
e
Cl, 3f
Ph, 3g
Cl
5
f
Ph
5
g
CH
3
, 3a
, 3a
F
CH
3
5
h
CH
3
4-CF
3
, 4c
F
3
C
CH
3
5
i
F
NO
2
NO
2
, 3d
4-F, 4b
5
j
F C
NO
2
1
0
1
NO
2
, 3d
4-CF
3
, 4c
3
5
k
NO
2
1
NO
2
, 3d
3-CH
3-CH
2-CH
2-CH
3
, 4d
, 4d
, 4e
, 4e
92
90
93
H C
3
5
l
F
1
1
1
2
3
4
F, 3e
3
3
3
H C
3
5
m
F
F, 3e
CH
3
5n
CN
CN, 3c
93
70
CH
3
5
o
5c
CH
H, 4a
CH
3
1
3
, X = Cl, 3h
5
a
a
b
c
Reaction condition: aryl halide 3 (0.5 mmol), arylboronic acid 4 (0.6 mmol), TBAB (1.0 g), H
Isolated yield after column chromatography.
Temperature is 80 °C.
2 2 3
O (1.0–1.5 mL), K CO (0.8 mmol), 12 h.
that reaction of arylboronic acid with carboxylic anhydride to
afford ketone can be promoted in good to excellent yield by using
a mixture of acetone/H
der to convert the carboxylic anhydride by-product in our catalytic
system to ketone, coupling reaction was performed in acetone/H O
2
Table 3
O (the best rate is 1:1) as solvent.17 In or-
2
Screening experiment for the cross-coupling reaction of benzoyl chloride with
phenylboronic acida
b
Entry
Solvent
T (°C)
t (h)
Yield (%)
aqueous mixture and a surprising promotion of yield to 62% was
observed in 4 h at room temperature (entry 2), and there was no
by-product (carboxylic anhydride) detected any more from GC–
MS analysis. Raising temperature to 60 °C can increase the catalytic
activity further, and an optimized yield of 85% could be achieved
by extending the reaction time to 12 h (entries 4 and 5).
1
2
3
4
5
H
2
O
25
25
25
60
60
12
4
12
4
43
62
65
70
85
Acetone + H
Acetone + H
Acetone + H
Acetone + H
2
2
2
2
O
O
O
O
12
a
Reaction condition: phenylboronic (1.2 mmol), benzoyl chloride (1.0 mmol),
acetone (1.5 mL), H O (1.5 mL), Na CO (1.6 mmol).
Isolated yield after column chromatography.
2
2
3
To further understand the scope and limitations of this catalytic
system for cross-coupling reaction, we have investigated the cou-
b

3
900
L. Zhang et al. / Tetrahedron Letters 52 (2011) 3897–3901
Table 4
Table 4 (continued)
Coupling reaction of acid chlorides with arylboronic acidsa
Entry
R1
R3
Cl
Product
Yieldb (%)
O
O
O
1
.0 mol % 2
B(OH)2
R1
Cl
R1
°
60
C
R3
14
3-CH
3
3
71
R3
4
6
7
Cl
Cl
CH
3
7
n
o
O
Entry
R1
R3
Product
Yieldb (%)
15
2-CH
Cl
86
O
CH
3
7
1
2
H
H
H
85
98
a
Reaction condition: acid chloride 6 (1.0 mmol), arylboronic acid 4 (1.2 mmol),
CO (1.6 mmol), 12 h.
Isolated yield after column chromatography.
7
a
H
2
b
O (1.5 mL), acetone (1.5 mL), Na
2
3
O
4-F
F
pling of representative acid chlorides and arylboronic acids under
7
b
1
8
O
the optimized reaction conditions (Table 4). As shown in Table
, the reaction of benzoyl chloride with arylboronic acids bearing
electron-withdrawing (–F, –CF ) and electron-donating (–CH
4
3
4
4-CF
3
H
H
70
82
3
3
)
F3C
groups proceeded the corresponding ketones (7b–7e) in good to
high isolated yield, which indicated that the effect of substituent
of arylboronic acids on the catalytic activity is not obvious. In order
to gain more information about the effect of substituents on an-
other coupling partner (benzoyl chloride), p-toluoyl chloride and
4-chlorobenzoyl chloride were selected to react with a range of
arylboronic acids. As seen in Table 4 (entries 6–15), it was observed
that either 4-chlorobenzoyl chloride or p-toluoyl chloride could
7
c
O
O
3-CH
3
CH
3
7
d
5
6
7
8
2-CH
H
3
H
73
51
45
63
couple smoothly with arylboronic acids, resulting in the corre-
CH
3
19
7
e
sponding substituted aryl ketones (7f–7o) in 45–90% yield.
A
O
sterically hindered 2-methylphenylboronic acid could also react
with acid chlorides giving 47–86% yield of the desired products
Me
Me
Me
(
entries 5, 10, and 15). As found in other catalyst systems for the
CH3
7
f
cross coupling of acid chloride and boronic acid, the present
ionically tagged Pd(II) complex did not show obvious substituent
O
2
c
effect on its catalytic performance.
4-F
In summary, we have prepared a structurally defined Pd(II)
complex which demonstrated efficient activities toward the
cross-couplings of aryl halides with representative phenylboronic
acids under aqueous catalytic system. This imidazolium salt sup-
ported (N–N)Pd(II) complex could be conveniently recycled up to
four times keeping high reactivity under aerobic conditions. Its
utility can be extended to another cross-coupling of arylboronic
acids with acid chlorides affording moderate to good yields of
the desired aryl ketones. Investigations on preparation of active
and stably recyclable catalyst in matching aqueous solvent system
are ongoing in our lab.
F
CH
3
7
g
O
4-CF
3
F
3
C
CH
3
7
h
O
O
9
3-CH
3
Me
90
CH3
CH3
7
i
Acknowledgments
10
11
12
13
2-CH
H
3
Me
Cl
47
68
51
55
CH3
O
CH
Cl
3
7
j
We acknowledge the National Natural Science Foundation of
China (21002106 and 20625308) and the Chinese Academy of
Science for financial support.
7
k
Supplementary data
O
4-F
Cl
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2011.05.079.
F
F
Cl
7
l
O
References and notes
4-CF
3
Cl
1.
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C
Cl
7
m
1736–1742; (c) Goossen, L. J.; Goossen, K.; Stanciu, C. Angew. Chem., Int. Ed.
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8
9
.
.
(1.5 mL) and acetone (1.5 mL) were added into a 25 mL schlenk flask, then the
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2 4
chromatography using silica gel (petroleum ether/ethyl acetate = 20:1)
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9): 1H NMR(CDCl
3
, 400 MHz): d 7.71–7.69 (d, J = 7.70 Hz, 2H), 7.59 (s, 1H),
7.55–7.53 (d, J = 7.54 Hz, 1H), 7.37–7.30 (m, 2H), 7.26–7.24 (d, J = 7.25 Hz, 2H),
2.41 (s, 3H), 2.39 (s, 3H); 13C NMR(CDCl
, 100 MHz): d 195.6, 142.1, 137.0,
136.9, 134.0, 131.9, 129.3, 129.2, 127.9, 127.0, 126.1, 20.6, 20.3.
1
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3