Amination of aryl iodides catalyzed by a palladium-copper complex supported by a chelate-bridging ligand

Article abstract of DOI:10.1021/om300902t

Palladium-copper complexes supported by a new chelate-bridging ligand designed for a cooperative effect of two metals were synthesized. The bimetallic complexes exhibited higher activities as catalysts for amination of aryl iodides than mononuclear catalyst.

Full text of DOI:10.1021/om300902t

Communication
pubs.acs.org/Organometallics
Amination of Aryl Iodides Catalyzed by a Palladium−Copper
Complex Supported by a Chelate-Bridging Ligand
Naofumi Tsukada,* Nozomi Ohnishi, Shiori Aono, and Fusako Takahashi
Department of Chemistry, Graduate School of Science, Shizuoka University, Shizuoka, 422-8529 Japan
S
* Supporting Information
ABSTRACT: Palladium−copper complexes supported by a new
chelate-bridging ligand designed for a cooperative effect of two metals
were synthesized. The bimetallic complexes exhibited higher activities
as catalysts for amination of aryl iodides than mononuclear catalyst.
oth palladium and copper complexes are widely used as
Bcatalysts in various transformations of organic compounds.
Scheme 1. Stepwise Synthesis of Palladium−Copper
Complexes 2
Figure 1. Molecular structure of 1, showing 50% thermal ellipsoids.
Hydrogen atoms are omitted for clarity.
(Buchwald−Hartwig aminations3), amines are first activated
by a metal center in copper-catalyzed reactions (Ullmann
condensations4,5). Therefore, palladium and copper are
expected to act in a cooperative manner by simultaneous
activation of two substrates. It is easily imagined that these
cooperative effects are emphasized by fixing two metals in close
proximity by an appropriate bridging ligand.6 However,
synthesis of palladium−copper complexes for this purpose
has been rather limited, and their use as a catalyst has not been
reported yet.7 Herein we report the synthesis of a new
palladium−copper complex and the cooperative effect of the
two metals in the reaction of aryl iodides with anilines.
Although each of these two metals works alone in most
reactions, these metals also cooperate in some important
catalysis to enable transformations difficult for a single metal.
For example, copper serves as a reoxidizing agent of palladium
in the Wacker oxidation1 and activates a terminal alkyne and
delivers it to palladium in the Sonogashira coupling reaction.2
The cooperative effect is expected also in reactions that are
catalyzed by both metals and mechanisms of which are different
from each other. For example, the coupling reaction of aryl
halides and amines, which is a useful and powerful method for
the preparation of arylamines, is catalyzed by palladium or
copper complexes.3,4 While aryl halides react with a metal
center prior to amines in palladium-catalyzed reactions
Received: September 21, 2012
Published: October 22, 2012
© 2012 American Chemical Society
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Organometallics
Communication
Table 2. Coupling Reaction of Aryl Iodides and Amines in
a
the Presence of 2
b
entry
R
R′
product
yield (%)
1
2
H
Ph
Ph
Ph
Ph
Ph
Ph
Ph
3
99
93
97
90
93
90
71
94
93
81
91
86
95
68
95
94
82
86
83
82
90
98
p-CH3
6
3
p-OCH3
7
4
p-NO2
8
5
p-CN
9
6
p-COCH3
10
11
6
7
p-CO2CH3
Figure 2. Molecular structure of 2c, showing 50% thermal ellipsoids.
Hydrogen atoms are omitted for clarity.
8
H
p-C6H4CH3
p-C6H4OCH3
p-C6H4N(CH3)2
p-C6H4NO2
p-C6H4CN
p-C6H4COCH3
p-C6H4CO2CH3
Ph
9
H
7
10
H
12
8
b
Table 1. Coupling Reaction of Iodobenzene and Aniline in
11
H
a
c
the Presence of Palladium and/or Copper Catalysts
12
H
9
13
14
H
10
11
12
12
13
14
15
16
17
18
H
d
15
o-CH3
H
16
o-C6H4CH3
o-C6H4CH3
benzyl
e
17
o-CH3
H
f
b
18
yield
f
entry
catalyst (mol %)
(%)
19
H
n-butyl
f
20
H
n-hexyl
1
2a (1)
2b (1)
2c (1)
none
92
92
97
0
f
21
H
1-phenylethyl
cyclohexyl
2
f
22
H
3
a
4
A mixture of aryl iodide (0.50 mmol), amine (0.50 mmol), and
sodium tert-butoxide (0.60 mmol) in toluene (2 mL) was heated at 40
5
1 (1)
0
b
c
°C for 24 h in the presence of 2a (2.0 mol %). Isolated yields. 100
°C. 60 °C. 42 h. 100 °C, 18 h, 2c was used instead of 2a.
6
Pd2(dba)3·CHCl3 (1) + dpqfamH (2)
Pd2(dba)3·CHCl3 (1)
0
d
e
f
7
1
8
CuI (2) + dpqfamH (2)
3
9
CuI (2)
5
quinoline rings. Addition of copper(I) iodide in wet acetone
under an oxygen atmosphere divided the dimer 1 in half,
yielding hydroxo-bridged palladium(II)−copper(II) complex
2a. Addition of copper(I) bromide or copper(I) chloride gave
the corresponding complexes 2b and 2c, which were not
obtained by the reaction with copper(II) salts. The molecular
structure of 2c is shown in Figure 2. Palladium and copper are
bridged by a hydoxo ligand and the chelating ligand, dpqfam.
Each metal center has a square-planar geometry.
10
11
12
13
14
15
16
17
1 (1) + CuI (2)
99
92
14
0
Pd2(dba)3·CHCl3 (1) + CuI (2) + dpqfamH (2)
Pd2(dba)3·CHCl3 (1) + CuI (2) + 4 (2)
Pd2(dba)3·CHCl3 (1) + CuI (2) + 4 (4)
Pd2(dba)3·CHCl3 (1) + CuI (2) + 5 (2)
Pd2(dba)3·CHCl3 (1) + CuI (2) + 5 (4)
Pd2(dba)3·CHCl3 (1) + CuI (2) + 8-aminoquinoline (2)
3
0
0
Pd2(dba)3·CHCl3 (1) + CuI (2) + 5 (2) + 8-
aminoquinoline (2)
0
Next, we investigated the ability of the palladium−copper
complexes 2a−c as catalysts in the coupling reaction of
iodobenzene and aniline. The reaction proceeded smoothly at
80 °C in the presence of 1 mol % of 2a−c, giving
diphenylamine 3 in 92−97% yields (Table 1, entries 1−3). It
is possible that single-metal complexes that are generated by
decomposition of 2 are actually active species. However, single-
metal catalysts with or without dpqfam(H) exhibited much
lower catalytic activities (entries 5−9). It is not necessary to
prepare and isolate the palladium−copper complex 2 in
advance: direct addition of the palladium complex 1 and
copper iodide into the reaction mixture was also effective for
the reaction, giving 3 in 99% yield (entry 10). Similar results
were obtained by addition of various copper salts such as CuBr,
CuCl, CuCl2, and Cu(OAc)2 instead of CuI, although the yields
of 3 were slightly decreased (80−93%). Furthermore,
preparation of 1 is also unnecessary: the reaction proceeded
smoothly by direct addition of Pd2(dba)3·CHCl3, CuI, and
dpqfamH into the reaction mixture as catalysts (entry 11). Two
chelate moieties of dpqfamH are important for the high activity
a
A mixture of iodobenzene (0.50 mmol), aniline (0.50 mmol), and
sodium tert-butoxide (0.60 mmol) in THF (2.0 mL) was heated at 80
°C for 3 h in the presence of catalyst. GC yields.
b
On the basis of our previous study of dinuclear palladium
complexes,8,9 a new chelate-bridging ligand, N-(2-
diphenylphosphino)phenyl-N′-8-quinolinylformamidinate
(dpqfam), was designed for a palladium−copper complex.
Treatment of a protonated form of the ligand (dpqfamH) with
one equivalent of (CH3)2Pd(tmeda) generated palladium
complex 1, which has two palladium centers and two chelate-
bridging ligands (Scheme 1). The molecular structure of 1 is
shown in Figure 1. The ligand chelates palladium by the
phosphinoaniline moiety and bridges two palladium by the
amidine moiety. No metal coordinates to nitrogen atoms of the
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Organometallics
Communication
(6) (a) Shibasaki, M.; Yamamoto, Y. Multimetallic Catalysis in Organic
Synthesis; Wiley-VCH: Weinheim, 2004. (b) Rowlands, G. J.
Tetrahedron 2001, 57, 1865. (c) van den Beuken, E. K.; Feringa, B.
L. Tetrahedron 1998, 54, 12985. (d) Steimhagen, H.; Helmchen, G.
Angew. Chem., Int. Ed. 1996, 35, 2339.
(7) (a) Karshtedt, D.; Bell, A. T.; Tilley, T. D. Organometallics 2003,
22, 2855. (b) Son, K.; Pearson, D. M.; Jeon, S.-J.; Waymouth, R. M.
Eur. J. Inorg. Chem. 2011, 4256.
(8) Tsukada, N.; Tamura, O.; Inoue, Y. Organometallics 2002, 21,
2521.
(9) (a) Tsukada, N.; Mitsuboshi, T.; Setoguchi, H.; Inoue, Y. J. Am.
Chem. Soc. 2003, 125, 12102. (b) Tsukada, N.; Setoguchi, H.;
Mitsuboshi, T.; Inoue, Y. Chem. Lett. 2006, 35, 1164. (c) Tsukada, N.;
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of the bimetallic catalyst. Compounds 4, 5, and 8-aminoquino-
line, which lack one of the two chelate moieties, proved much
less effective than dpqfamH as ligands (entries 12−16).
Addition of both chelate moieties 5 and 8-aminoquinolines
instead of dpqfamH was also ineffective (entry 17).
The reaction proceeded at 40 °C by using toluene as solvent
instead of THF. Table 2 summarizes the results of the coupling
reaction of aryl iodides and amines in toluene. The reaction of
various aryl halides bearing electron-donating or electron-
withdrawing substituents afforded corresponding diarylamines
in high yields (entries 1−7). Various substituted anilines were
also reactive, although higher temperature was required in the
case of nitro- or cyano-substituted anilines (entries 8−14).
Sterically hindered diarylamines bearing ortho-substituents were
also obtained (entries 15−17). Although higher temperature
was required, the reaction of iodobenzene and primary
alkylamines gave alkylarylamines in high yields (entries 18−22).
In summary, we designed a new chelate-bridging ligand for
enhancement of a cooperative effect of palladium and copper,
synthesized dinuclear palladium−copper complexes 2 by using
the ligand, and found that 2 proved higher in activity in the
amination of aryl iodides than mononuclear catalyst. This result
shows that palladium and copper in 2 work in a cooperative
manner. Detailed mechanistic studies to understand how the
two metals cooperate and fine-tuning of the ligand for higher
catalytic activity are in progress.
ASSOCIATED CONTENT
* Supporting Information
■
S
Detailed experimental procedures, characterization data for new
compounds, and crystallographic data (CIF). This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Phone/Fax: +81-54-238-4759. E-mail: sntsuka@ipc.shizuoka.
ac.jp.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was carried out using instruments of the Center for
Instrumental Analysis of Shizuoka University.
■
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■
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