Palladium(II)-N-heterocyclic carbene complex derived from proline towards Suzuki-Miyaura coupling reaction in water at room temperature

Article abstract of DOI:10.1016/j.jorganchem.2011.03.045

Palladium(II)-N-heterocyclic carbene complex 1 derived from proline has been proved to be a highly effective catalyst in the Suzuki-Miyaura coupling reaction of aryl iodides and bromides with arylboronic acids in water at room temperature. The reactions a

Full text of DOI:10.1016/j.jorganchem.2011.03.045

Journal of Organometallic Chemistry 696 (2011) 2576e2579
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Journal of Organometallic Chemistry
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Palladium(II)-N-heterocyclic carbene complex derived from proline towards
SuzukieMiyaura coupling reaction in water at room temperature
Yi-Qiang Tang, Huan Lv, Jian-Mei Lu, Li-Xiong Shao*
College of Chemistry and Materials Engineering, Wenzhou University, Chashan University Town, Zhejiang Province 325035, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 21 January 2011
Received in revised form
Palladium(II)-N-heterocyclic carbene complex 1 derived from proline has been proved to be a highly
effective catalyst in the SuzukieMiyaura coupling reaction of aryl iodides and bromides with arylboronic
acids in water at room temperature. The reactions are tolerant towards various functional groups in the
substrates. Moreover, the complex 1-catalyzed medium-scale (10.0 mmol) SuzukieMiyaura reactions
were also carried out and it was found that the complex was also effective enough in these cases.
Ó 2011 Elsevier B.V. All rights reserved.
28 March 2011
Accepted 29 March 2011
Keywords:
N-Heterocyclic carbene
Palladium complex
SuzukieMiyaura coupling reaction
Water
Synthetic methods
1. Introduction
So, numerous groups had developed many organic transformations
using water as the solvent [37e48]. In this regard, some reports on
Since the isolation of the first stable free carbene by Arduengo
et al. in 1991 [1], the chemistry of carbene and their tran-
sitionemetal complexes had been dramatically concerned during
the past years [2e13]. Among them, development of donor-
functionalized N-heterocyclic carbene (NHC)-metal complexes
has been attracted much attention and it was found that these
bifunctional carbeneemetal complexes will enhance catalytic
activity in carbonecarbon and carbon-heteroatom bond formations
the SuzukieMiyaura coupling reaction carried out in water were
reported [49e57]. Though some papers have been reported on the
Pd(II)eNHC complexes-catalyzed Suzuki-Miyuara coupling reac-
tion carried out in refluxing water [58e60], it keeps to be a big
challenge for organic chemists to develop the Pd-NHC complexes-
catalyzed SuzukieMiyaura coupling reaction in water under rela-
tively milder conditions. In this laboratory, it was found that Pd(II)e
NHC complex 1 derived from proline is a good catalyst for the
SuzukieMiyaura coupling reaction of aryl iodides and bromides
with arylboronic acids in water at room temperature. It was also
found that the corresponding reactions carried out in medium-
scale (10.0 mmol) can be also achieved with comparable yields.
Herein, we wish to report these results in detail.
[14e22]. Previously, we had prepared some Pd(II)eNHC complexes
derived from proline and their structures were fully determined by
X-ray diffraction. For example, Fig. 1 shows the structure of one of
these complexes [23].
SuzukieMiyaura coupling reaction, the reaction between orga-
noboranes and organic electrophiles, is one of the most powerful
and general methods for carbonecarbon bond formations [24e32].
During the past years, the developments in so called Green Chem-
istry have drawn peoples’ great attention because of their potential
benefits in resource and energy efficiency, health and environ-
mental safety, etc [33e36]. As one of the basic principles in Green
Chemistry, development of safer solvents, arouses synthetic
chemists’ concern with no doubt. In this respect, water becomes
one of the favorable candidates because it is cheap and safe enough.
2. Experimental
2.1. General remarks
1H and 13C NMR spectra were recorded on a Bruker Avance-
00 MHz or Bruker Avance-500 MHz spectrometer for solution in
3
3
CDCl with tetramethylsilane (TMS) as an internal standard;
J-values are in Hz. Commercially obtained reagents were used
without further purification. Flash column chromatography was
carried out using Huanghai 300e400 mesh silica gel at increased
pressure.
*
Corresponding author. Tel./fax: þ86 577 883 73111.
E-mail address: Shaolix@wzu.edu.cn (L.-X. Shao).
0
022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2011.03.045

Y.-Q. Tang et al. / Journal of Organometallic Chemistry 696 (2011) 2576e2579
2577
Table 2
Et
Pd(II)eNHC complex 1 catalyzed coupling reactions of aryl bromides and iodides 2
with arylboronic acids 3.
N
N
N
X
B(OH)2
Pd(II)-NHC 1 (1.0 mol %)
R1
Pd
Br
Ph
Br
+
t
1
KO Bu (2.0 equiv),
H O, rt, 24 h
R1
R2
3
R2
Fig. 1. Pd(II)eNHC complex 1 derived from proline.
2
2
4
2.2. General procedure for the Pd(II)eNHC complex 1-catalyzed
SuzukieMiyaura cross-coupling reaction of arylboronic acids with
aryl iodides and bromides
Entrya
1
3 (R2)
Yield (%)b
2 (R /X)
1
2
3
4
5
6
7
8
9
2a (4-MeO/Br)
2a
3b (4-Me)
3c (3,5-Me
3d (4-Cl)
3e (2-Me)
3a (H)
3f (4-MeO)
3f
3f
3f
3f
3f
3a (H)
3e
4b, 96
4c, 85
4d, 87
4e, 80
4f, 96
4a, 90
4b, 92
4e, 84
4g, 89
4a, 96
4d, 94
4a, 87
4e, 90
2
)
(
If aryl halide is a liquid) under N
2
atmosphere, arylboronic acid
2a
2a
t
3
(0.6 mmol), Pd(II)eNHC 1 (1.0 mol%), KOBu (1.0 mmol) and H
2
O
2b (3-MeO/Br)
2c (H/Br)
2d (4-Me/Br)
2e (2-Me/Br)
2f (4-F/Br)
2g (H/I)
2h (4-Cl/I)
2i (4-MeO/I)
2i
(
(
2.0 mL) were added into a Schlenk tube, then aryl halide 2
0.5 mmol) was added. The mixture was stirred vigorously at room
temperature for 24 h. Then the solvent was removed under reduced
pressure and the residue was purified by a flash column chroma-
10
11
12
13
tography (SiO
If aryl halide is a solid) under N
0.5 mmol), arylboronic acid 3 (0.6 mmol), Pd(II)eNHC 1 (1.0 mol%),
2
) to give the pure product.
(
2
atmosphere, aryl halide 2
(
t
2
KOBu (1.0 mmol) and H O (2.0 mL) were added into a Schlenk
a
All reactions were carried out using 2 (0.5 mmol), 3 (0.6 mmol), KOBut (2.0
reaction tube. The mixture was stirred vigorously at room
temperature for 24 h. Then the solvent was removed under reduced
pressure and the residue was purified by a flash column chroma-
equiv), Pd(II)eNHC 1 (1.0 mol%) in H
2
O (2.0 mL) at room temperature for 24 h.
b
Isolated yields.
tography (SiO
2
) to give the pure product.
conditions were established as using Pd(II)eNHC complex 1
3
. Results and discussion
t
(
1.0 mol%) as the catalyst, KOBu (2.0 equiv) as the base and H
2
O
(
2.0 mL) as the solvent at room temperature [61].
Under the optimized reaction conditions, a wide array of aryl
In initial studies, a model reaction of 4-methoxyphenyl bromide
a (0.5 mmol) with phenylboronic acid 3a (0.6 mmol) in the
2
bromides and iodides 2 can react with arylboronic acids 3 to provide
the corresponding cross-coupling products 4 in good to excellent
yields (Table 2). The reactions can tolerate both electron-donating
and electron-withdrawing groups on both substrates of aryl
halides and arylboronic acids. Moreover, sterically encumbering
orth-substituent on both of the aryl halides and arylboronic acids
only have less effect on the reactions (Table 2, entries 4, 8 and 13).
The Pd(II)eNHC complex 1-catalyzed reaction of hetero-
aromatic iodide 2j with 4-methyoxyphenylboronic acid 3f was also
presence of Pd(II)eNHC complex 1 (1.0 mol%) was carried out in
water at room temperature for 24 h. Investigations into the optimal
base showed that the reactions were significantly influenced by the
bases used (Table 1). Inorganic bases such as NaOH, KOH and KOBut
could provide the coupling product 4a in high yields (Table 1,
entries 1, 2 and 5). For all other inorganic bases such as Cs
K CO , NaOBu , K PO 3H O and KHCO , inferior yields were
2 3 3 4 2 3
2
CO
3
,
t
.
obtained (Table 1, entries 3, 4, 6, 7 and 8). So the optimal reaction
ꢀ
carried out at 60 C under the similar reaction conditions and the
Table 1
corresponding product 4h can be obtained in 96% yield (Scheme 1).
For the reaction involving heteroaromatic boronic acid 3g, the
corresponding coupling product 4i was also formed in 75% yield
(Scheme 1).
Optimization for complex 1 catalyzed SuzukieMiyaura coupling reaction of 2awith 3a.
Br
B(OH)2
Pd(II)-NHC 1 (1.0 mol%)
+
base (2.0 equiv),
B(OH)2
S
H O, rt, 24 h
2
OMe
3a
Pd(II)-NHC 1 (1.0 mol%)
2
a
OMe
+
t
4
a
I
KOBu (2.0 equiv)
S
o
H O, 60 C, 24 h
2
j
2
OMe
3f
(
0.5 mmol)
OMe
4h, 96%
Entrya
Base
Yield (%)b
(
0.6 mmol)
1
2
3
4
5
6
7
8
NaOH
KOH
87
90
53
44
92
63
70
37
O
I
B(OH)2
Cs
2
CO
CO
3
Pd(II)-NHC 1 (1.0 mol%)
K
2
3
+
t
t
KO Bu
NaO Bu
KOBu (2.0 equiv)
t
O
o
H O, 60 C, 24 h
2
K
3
PO
4
.3H
2
O
OMe
2i
3g
(0.6 mmol)
KHCO
3
OMe
4i, 75%
a
(0.5 mmol)
All reactions were carried out using 2a (0.5 mmol), 3a(0.6 mmol), base (2.0
equiv), Pd(II)eNHC 1 (1.0 mol%)in H2O (2.0 mL) at room temperature for 24 h.
b
Isolated yields.
Scheme 1. Reactions involving heteroaromatic iodide or boronic acid.

2578
Y.-Q. Tang et al. / Journal of Organometallic Chemistry 696 (2011) 2576e2579
O
also found that the reactions can be carried out in medium-scale
with less effect on the yields. The SuzukieMiyaura coupling reac-
tion performed in water based on the Pd(II)eNHC complex repor-
ted here are significantly improved with respect to the previous
report [58e60], but are less efficient than the best systems known
in the literature under non-aqueous conditions.
Cl
B(OH)2
OMe
Pd(II)-NHC 1 (5.0 mol%)
KO Bu (2.0 equiv)
t
+
H O (2.0 mL), reflux, 12 h
2
O
3
f
Acknowledgements
2
k
(
0.6 mmol)
OMe
j, 43%
(
0.5 mmol)
4
Financial support from the National Natural Science Foundation
of China (Nos. 21002072 and 20702013), the Department of
Education of Zhejiang Province (No. Y200907072) and the start-up
fund of Wenzhou University is greatly acknowledged. Yi-Qiang
Tang thanks Science and Technology Department of Zhejiang
Province for financial support (No. 2010R424050).
Scheme 2. Pd(II)eNHC complex 1-catalyzed coupling reaction of 4-acetylphenyl
chloride 2k.
Br
B(OH)2
Pd(II)-NHC 1 (0.2 mol%)
Appendix. Supplementary data
4
b
+
t
KO Bu (2.0 equiv)
H O (40 mL), r.t., 29 h
1
.8928 g, 89%
2
Supplementary data related to this article can be found online at
doi:10.1016/j.jorganchem.2011.03.045.
OMe
Me
3b
2a
(10.0 mmol) (12.0 mmol)
Br
B(OH)2
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the Supporting Information for the details.