Synthesis of dichlorobis(1,4-dimesityl-1H-1,2,3-triazol-5-ylidene)palladium [PdCl2(TMes)2] and its application to suzuki-miyaura coupling reaction

Article abstract of DOI:10.1246/cl.2010.920

The palladium complex bearing a novel NHC-ligand derived from 1,4-dimesityl-1, 2, 3-tirazole was prepared. The complex catalyzed the Suzuki-Miyaura coupling reaction with aryl chloride successfully to give biaryls in excellent yields, particularly very effective for the sterically hindered reaction between o-substituted arylchlorides and o-substituted phenylboronic acids.

Full text of DOI:10.1246/cl.2010.920

doi:10.1246/cl.2010.920
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Published on the web July 17, 2010
Synthesis of Dichlorobis(1,4-dimesityl-1H-1,2,3-triazol-5-ylidene)palladium [PdCl (TMes) ]
2
2
and Its Application to Suzuki­Miyaura Coupling Reaction
Tatsuhito Nakamura, Kenichi Ogata, and Shin-ichi Fukuzawa*
Department of Applied Chemistry, Institute of Science and Engineering, Chuo University,
1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551
(
Received June 14, 2010; CL-100560; E-mail: orgsynth@kc.chuo-u.ac.jp)
The palladium complex bearing a novel NHC-ligand derived
Mes
CuSO4
Na ascorbate
H 2
from 1,4-dimesityl-1,2,3-tirazole was prepared. The complex
catalyzed the Suzuki­Miyaura coupling reaction with aryl
chloride successfully to give biaryls in excellent yields,
particularly very effective for the sterically hindered reaction
between o-substituted arylchlorides and o-substituted phenyl-
boronic acids.
Mes
N
Mes
MeI
Mes N3
N
N
t-BuOH, H O
CH CN
2
3
rt, 36 h
6
0 °C, 12 h
1
3: 72%
Me
N
N
N Mes
I
Mes
As an extremely useful class of ligands for use in transition-
metal complexes, N-heterocyclic carbenes (NHCs) have
emerged. It is well recognized that replacement of phosphines
Ag O
2
Mes
N
Mes
Me
Ag
AgI2
Mes
1
N
N
CH2Cl2
rt, 15 h
Mes
N
by NHCs can provide complexes with enhanced thermal
stability and catalytic performance due to the higher electron-
donor ability. The majority of NHC ligands used are imidazol-2-
ylidenes A or 1,2,4-triazol-5-ylidenes B (Figure 1). In contrast,
since Crabtree et al. first reported an NHC complex which
N
N
Me
4
: TMes HI 98%
5: 98%
Me
N
N
2
Mes
Cl Pd Cl
Mes Mes
Me
N Mes
features an imidazole ring bound at the C5 position, various
complexes bearing abnormal NHC ligands such as imidazol-5-
PdCl2(CH3CN)2
3
ylidene C, pyrazolin-4-ylidenes D and 1,2,3-triazol-5-ylidene
4
N
N
E have also been reported by several research groups. Although
CH2Cl2
rt, 15 h
N
these series of abnormal NHC ligands and their complexes have
5
,6
been researched, catalytic merit of abnormal NHC has yet to
6
2
: PdCl (TMes)2
98%
7
be developed in detail. Moreover synthesis of 1,2,3-triazole
NHC complexes, and application to transition metal catalytic
reactions have scarcely been studied.
Scheme 1. Synthesis of PdCl (TMes) complex.
2
2
Since Albrecht et al. first reported novel abnormal N-
heterocyclic carbene (abnormal NHC) ligands based on a 1,2,3-
triazole framework, Sankararaman and Bertrand independently
attractive ligand for coupling reactions because TMes is an
analog of IMes [1,3-bis(2,4,6-trimethylphenyl)imidazol-2-yli-
dene] which is known as an effective imidazole carbene ligand
due to electronically rich and steric hindered mesityl groups.
4
disclosed two other types of abnormal NHCs and their metal
8
,9
complexes. In view that 1,2,3-triazoles are readily accessible
by the reaction of an organic azide and an alkyne, i.e., click
chemistry, the modularity of the click chemistry would lead to
diversity of 1,2,3-triazole-based abnormal NHC ligands, and to
alternative NHC ligands.
1
0
Following a general procedure reported by Sharpless et al.,
1,4-dimesityl-1,2,3-triazole (3) was readily prepared by the
reaction of mesitylazide 1 with mesitylacetylene 2 using
copper(I) catalyst in the first step (Scheme 1). The triazole was
converted into triazolium salt 4 quantitatively by treatment with
Novel abnormal carbene metal complexes may be predicted
to have a potential for a unique catalytic activity in organic
reactions because abnormal NHC ligands are stronger donors
4
CH3I in excess according to Albrecht’s method. The reaction of
the 1,2,3-triazolium salt with silver oxide successfully afforded
silver 1,2,3-triazol-5-ylidene complex 5 as a stable white solid in
a quantitative yield. The formation of 5 might be supported by
9
than conventional NHC ligands. Thus, Sankararaman’s pyroli-
dinyl 1,2,3-triazol-5-ylidene palladium complex worked effec-
tively in the Suzuki­Miyaura coupling with aryl bromide, but
not with aryl chloride. We envisioned that TMes (triazole-based
NHC ligand bearing mesityl groups) might be a promising and
1
disappearance of triazolium proton (¤H = 9.38 ppm) in H NMR,
the appearance of a metal-bond carbon signal at 155.2 ppm in
1
3
4­6
C NMR. This is consistent with the previous studies of the
synthesis of Ag­1,2,3-triazol-5-ylidene complex. The Ag com-
plex 5 was converted successfully into the corresponding
palladium carbene complex on treatment with PdCl (MeCN)
in CH2Cl2 at room temperature and dichlorobis(1,2,3-triazol-5-
ylidene)palladium 6 [PdCl2(TMes)2] was obtained as an air-
stable pale yellow complex. The complex was purified by
column chromatography on basic alumina and characterized by
R
R
R
R
N
N
N
N
N
N
N
N
2
2
N
N
N
N
R
R
R
R
R
R
R
R
R
R
A
B
C
D
E
Figure 1.
Chem. Lett. 2010, 39, 920­922
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

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Table 1. The 6-catalyzed Suzuki­Miyaura couplinga
Mes
Cl Pd Cl
Mes Mes
N
N Mes
6
(1 mol%)
Cl
B(OH)2
Mes
N
N Mes
Cs CO3 (2.0 equiv)
2
R2
+
N
N
R1
R2
EtOH, reflux, 15 h
R1
IMes
8
9
10
2
7: PdCl (IMes)2
Entry Aryl chloride
Boronic acid
Yield/%b
1
2
c
Figure 2. IMes and its Pd complex.
1
2
3
4
5
6
7
8
9
0
8a (R = p-MeO)
8a
9a (R = H)
99
60
d
c
9a
9a
9a
9a
9a
9a
9a
6
(1 mol%)
1
CO2tBu
8b (R = m-MeO)
93
98
98
97
94
93
98
99
97
95
68
99
93
98
64
83
83
Br
t
CO2 Bu NaOAc (2.0 equiv)
1
8c (R = o-MeO)
+
1
DMA, 150 °C, 15 h
8d (R = p-Me)
1
11
12
1
3
8e (R = o-Me)
1
99%
8f (R = p-COMe)
1
8g (R = o-COMe)
1
Scheme 2. The 6-catalyzed Mizoroki­Heck coupling.
8h (R = 1-naphthyl) 9a
1
1
8i (R = 2-naphthyl) 9a
1
1
1
8j (R = 2-pyridyl)
9a
9a
obtained in excellent yields regardless of substituents; stereo and
electronic properties of substituent of benzene hardly affected on
yields. Electronically deactivated aryl chlorides and ortho-
substituents tolerated the reaction; the corresponding biaryl-
products were obtained in excellent yields (Entries 1 and 3­8).
The coupling with other aromatic rings such as 1- and 2-
naphthyl chlorides (Entries 9 and 10) gave the corresponding
biaryls in quantitative yields as well. Such heteroaromatic
chlorides as 2- and 3-pyridyl (Entries 11 and 12) coupled with
1
12
13
14
15
16
17
18
19
8k (R = 3-pyridyl)
2
8c
8e
8h
8j
8e
8h
8j
9b (R = o-Me)
9b
9b
9b
2
9c (R = o-MeO)
9c
9c
9
a to provide the corresponding products in nearly quantitative
yields. Reactions using o-methylphenylboronic acid (9b: R =
o-Me, Entries 13­16) or o-methoxyphenylboronic acid (9c: R =
a8 (0.5 mmol),
1.0 mmol), EtOH (1.5 mL); reflux, 15 h. Isolated yields.
9
(0.6 mmol),
6
(0.005 mmol), Cs2CO3
2
b
(
2
c
d
Determined by GC. 7 was used instead of 6.
o-MeO, Entries 17­19) were also performed, and the yields were
as good as those of phenylboronic acid 9a. It is noteworthy that
the reaction between o-substituted aryl chloride and o-substi-
tuted phenylboronic acids proceeded to give the sterically
crowded biaryls in good to excellent yields. The reaction of o-
chlorotoluene (8e) with o-methoxyphenylboronic acid (9c) was
the exception giving the biaryl in a moderate yield (Entry 17).
These results suggest that the 6-catalyzed reaction has a broad
substrate scope. The complex was also effective for the
Mizoroki­Heck reaction of bromobenzene with acrylic ester
(Scheme 2). Detailed study will be reported in due course.
In conclusion, the PdCl2(TMes)2 complex 6 was an efficient
catalyst for the Suzuki­Miyaura coupling reaction with aryl and
heteroaryl chlorides, especially for the sterically hindered
reaction between o-substituted arylchlorides and o-substituted
phenylboronic acids.
1H and 13C NMR, and combustion analysis: the metal-bond
13
carbon was observed at 163.0 ppm in C NMR. According to
4
,8,9
the literature,
complex tends to appear around 160 ppm, the signal at
63.0 ppm demonstrates the formation of the palladium carbene
carbene-carbon of Pd­1,2,3-triazol-5-ylidene
1
complex. Yet to make a single crystal for X-ray analysis failed,
unsuitable crystalline solid being obtained instead.
Palladium catalysts bearing NHC ligands have been used for
such cross coupling reactions as the Suzuki­Miyaura coupling.
NHC ligands are a good alternative to conventional phosphine
ligands in view of thermal stability.11 Thus, we evaluated the
PdCl2(TMes)2 6 in the Suzuki­Miyaura coupling, and optimized
1
the representative reaction between p-chloroanisole (8a: R = p-
2
MeO) with phenylboronic acid (9a: R = H). The reactions were
usually performed in the presence of 6 (1 mol %). After many
optimization experiments, the optimum conditions are revealed
to be refluxing in an ethanol solution for 15 h with Cs CO as a
This study was financially supported by a Grant-in-Aid,
No. 22550044, for Scientific Research from the Japan Society
for the Promotion of Science (JSPS) and a Chuo University
Grant for Special Research.
2
3
base, affording 4-methoxybiphenyl in a quantitative yield
1
2
(
Table 1, Entry 1, and see Supporting Information). The
efficiency of bisimidazolylidene palladium dichloride
7
7
a
[
PdCl (IMes) ] (Figure 2) was tested for comparison, 7 was
References and Notes
2
2
less effective than 6, giving 60% yield of the coupling product
1
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are summarized in Table 1. The coupling products with 9a were
Chem. Lett. 2010, 39, 920­922
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

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Chem. Lett. 2010, 39, 920­922
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/