DAB-Cy as an inexpensive and effective ligand for palladium-catalyzed homocoupling reaction of aryl halides

Article abstract of DOI:10.1016/j.jorganchem.2006.09.024

A novel catalytic system of PdCl₂(CH₃CN)₂ with N,N′-dicyclohexyl-1,4-diazabutadiene (DAB-Cy) ligand was successfully used in reductive coupling of aryl halides.

Full text of DOI:10.1016/j.jorganchem.2006.09.024

Journal of Organometallic Chemistry 691 (2006) 5697–5700
www.elsevier.com/locate/jorganchem
DAB-Cy as an inexpensive and effective ligand for palladium-catalyzed
homocoupling reaction of aryl halides
*
Ning Ma, Zheng Duan, Yangjie Wu
Chemistry Department, Henan Key Laboratory of Chemical Biology and Organic Chemistry, Key Laboratory of Applied Chemistry of Henan
Universities, Zhengzhou University, Zhengzhou 450052, PR China
Received 27 July 2006; received in revised form 21 August 2006; accepted 14 September 2006
Available online 19 September 2006
Abstract
A novel catalytic system of PdCl2(CH3CN)2 with N,N0-dicyclohexyl-1,4-diazabutadiene (DAB-Cy) ligand was successfully used in
reductive coupling of aryl halides.
Ó 2006 Elsevier B.V. All rights reserved.
Keywords: Homocoupling reaction; Aryl halides; N,N0-dicyclohexyl-1,4-diazabutadiene (DAB-Cy); PdCl2(CH3CN)2
1. Introduction
dicyclohexyl-1,4-diazabutadiene (DAB-Cy) [8] acts as an
effective ligand for palladium-catalyzed Ullmann coupling
reactions.
Biaryls constitute the central skeleton in a great number
of natural products and are important pharamacophores
in a variety of biologically active compounds [1]. Moreover,
by virtue of their excellent physical and chemical properties,
they are also widely applied in monomers of conductive
polymers [2], chiral host molecules for inclusion compounds
[3], and chirality-recognition reagents for chromatography
[4]. As for the synthesis of symmetrical biaryls, the main
method is the Ullmann reaction [5], however, which requires
equimolar amounts of copper and harsh reaction conditions
(neat, >200 °C). In the last decades, there were notable
improvements to the Ullmann reaction. Recently, more
and more attentions have been paid in Pd-catalyzed homo-
coupling reactions [6], which are usually conducted in the
presence of the reducing agents, such as zinc [6a,6b,6c],
hydrogen gas [6d], formate salts [6e,6f], hydroquinone(HQ)
[6g], alcohols [6h,6i,6j,6k,6l] or carbonic oxide gas [6m].
Only P(o-tol)3, As(o-tol)3 [6g] and P(2-furyl)3 [7] had
been used as ligands in palladium catalyzed homocoupling
reactions. Here, we would like to report the simple N,N0-
In order to optimize the reaction conditions, we examined
the homocoupling of p-bromoanisole in different com-
bination and ratios of palladium salts and DAB-Cy. The
results are summarized in Table 1. The best result was
obtained when the reaction carried out in DMF at 135 °C
with a ratio of PdCl2(CH3CN)2: DAB-Cy = 3:4.5 (entry
5). When the reaction carried out at lower temperature,
the yield reduced from 86% to 48% (entries 5 and 6). The
addition of n-Bu4NBr was essential to the successful trans-
formation (entries 4 and 5). The role of n-Bu4NBr probably
strengthened the stabilization of low coordinated Pd(0) spe-
cies, and was the phase transfer catalyst for the inorganic
base/polar solvent/organic substrates/product phases [9].
Under these optimized reaction conditions, we next
examined the scope and limitation of this method. The
results are summarized in Table 2.
Both electron rich and electron poor aryl bromides could
be used to provide the homocoupling products in moderate
to good yields. Aryl iodide showed higher reactivity (entry
2). Reductive coupling of 2-bromopyridine (1n, entry 14)
and 2-bromothiophene (1p, entry 16) proceeded smoothly,
the coupling product 2n and 2p were isolated in 80% and
*
Corresponding author. Tel./fax: +86 371 67979408.
E-mail address: wyj@zzu.edu.cn (Y. Wu).
0022-328X/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2006.09.024

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N. Ma et al. / Journal of Organometallic Chemistry 691 (2006) 5697–5700
Table 1
Homocoupling reaction of p-bromoanisolea
Pd-cat. DAB-Cy
K2CO3, HQ, 135
H3CO
Br
H3CO
OCH3
Entry
Catalyst/ligand
Ratio (10ꢀ2 mmol)
Temperature (°C)
Yieldb (%)
1
2
3
4
5c
6c
a
Pd(OAc)2/DAB-Cy
PdCl2/DAB-Cy
PdCl2(CH3CN)2/DAB-Cy
PdCl2(CH3CN)2/DAB-Cy
PdCl2(CH3CN)2/DAB-Cy
PdCl2(CH3CN)2/DAB-Cy
3/4.5
3/4.5
3/4.5
4/6
3/4.5
3/4.5
135
135
135
135
135
105
8
18
49
50
86
48
Reaction conditions: p-bromoanisole (1 mmol), K2CO3 (1 mmol), HQ (0.5 mmol), DMF (3 ml), 135 °C, 24 h, under the N2 atmosphere.
b
c
Isolated yield.
Addition of n-Bu4NBr (0.5 mmol).
86% yields, respectively. Under similar conditions, 3-bro-
However, this catalytic system is sensitive to the steric
hindrance. When ortho-substituted aryl bromides (1e, 1l,
1s) were used, only a trace amount of the desired products
were obtained.
In summary, DAB-Cy is an indeed effective ligand for
palladium-catalyzed homocoupling of aryl halides. Both
mopydine (1o, entry 15) and 3-bromothiophene (1q, entry
17) were converted into the corresponding reductive cou-
pling products 2o and 2q only in moderate yields (67% and
71%, respectively). When electron deficient aryl chloride
(1j) was used, 21% of biaryl compound was obtained.
Table 2
Homocoupling of aryl halidesa
PdCl2(CH3CN)2,
Ar - X
DAB-Cy
Ar - Ar
K2CO3, HQ, n-Bu4NBr, 135 ˚C, 24h
1
2
Entry
Aryl halide 1
Yield 2b (%)
H3CO
X
1
X = Br
X = I
X = Cl
1a
1b
1c
1d
86
95
–
2c
3d
4
e
70
H3C
Br
5
1e
tracee
CH3
Br
6
7
1f
49
71
Me
N
Br
Me
1g
Br
H3COC
X

N. Ma et al. / Journal of Organometallic Chemistry 691 (2006) 5697–5700
5699
Table 2 (continued)
Entry
Aryl halide 1
Yield 2b (%)
8
9d
10
X = Br
X = Cl
1h
1i
1j
86
21
86
NC
Br
Br
11
12
1k
1l
85
O2N
tracee
NO2
Br
13
14
1m
1n
69
80
Cl
Br
Br
Br
N
15
1o
67
N
16
17
1p
1q
86
71
Br
S
Br
S
18
1r
76
Br
19
1s
tracee
Br
Me
a
Reaction conditions: aryl halide (1 mmol), PdCl2(CH3CN)2 (0.03 mmol), DAB-Cy (0.045 mmol), K2CO3 (1 mmol), HQ (0.5 mmol), n-Bu4NBr
(0.5 mmol), DMF (3 mL), 135 °C, 24 h, under the N2 atmosphere.
b
Isolated yield.
Temperature is 120 °C, and reaction time is 6 h.
PdCl2(CH3CN)2/DAB-Cy is 0.05/0.075 mmol, and reaction time is 48 h.
Most of the starting material was recovered.
c
d
e

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N. Ma et al. / Journal of Organometallic Chemistry 691 (2006) 5697–5700
electron-rich and electron-deficient aryl bromides are appli-
cable substrates to the Pd/ DAB-Cy catalyzed homocou-
pling reactions.
3,30-Bithiophenyl (2q) [6l]: 1H NMR (400 MHz, CDCl3,
TMS) d 7.37 (d, 2H, J = 1.4 Hz), 7.34 (m, 4H).
1
1,10-Binaphthyl (2r) [6g]: H NMR (400 MHz, CDCl3,
TMS) d 7.95 (m, 4H), 7.59 (m, 2H), 7.47 (m, 4H), 7.38
(d, 2H, J = 8.3 Hz), 7.28 (m, 2H).
2. Experimental
2.1. General procedure for symmetrical functionalized
biaryls
Acknowledgments
We are grateful to the National Natural Science Foun-
dation of China (Project 20472074) and the Innovation
Found for Outstanding Scholar of Henan Province (Pro-
ject 0621001100) for the financial support given to this
research.
To a mixture of aryl halide (1 mmol), K2CO3 (1 mmol),
HQ (0.5 mmol), and n-Bu4NBr (0.5 mmol) was added a
DMF solution (3 mL) of PdCl2(CH3CN)2 (0.03 mmol)
and DAB-Cy (0.045 mmol). The mixture was stirred under
nitrogen atmosphere at 135 °C for 24 h. After cooling to
room temperature, water and ethyl acetate were added.
The organic phase was washed with water and dried over
Na2SO4. The solvent was evaporated under vacuum. The
biphenyl product was purified by preparative thin layer
chromatography.
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