Efficient and copper-free Pd(OAc)2/DABCO-catalyzed Sonogashira cross-coupling reaction

Article abstract of DOI:10.1055/s-2005-861805

An efficient and copper-free palladium-catalyzed Sonogashira cross-coupling reaction protocol is presented. In the presence of 3 mol% of Pd(OAc) ₂ and 6 mol% of DABCO, cross-coupling of various aryl halides (iodides and bromides) with terminal

Full text of DOI:10.1055/s-2005-861805

804
PAPER
Efficient and Copper-Free Pd(OAc)2/DABCO-Catalyzed Sonogashira Cross-
Coupling Reaction
P
d(OAc
)
/
2i
D
abco
n
S
-
onogash
H
ira
C
ross-Coupling
e
R
eaction ng Li,* Xu-Dong Zhang, Ye-Xiang Xie
Key Laboratory of Chemical Biology & Traditional Chinese Medicine Research, College of Chemistry and Chemical Engineering, Hunan
Normal University, Changsha 410081, P. R. of China
Fax +86(731)8872260; E-mail: jhli@hunnu.edu.cn
Received 18 October 2004; revised 24 November 2004
Pd(OAc)2,
DABCO
Abstract: An efficient and copper-free palladium-catalyzed Sono-
gashira cross-coupling reaction protocol is presented. In the pres-
ence of 3 mol% of Pd(OAc)2 and 6 mol% of DABCO, cross-
coupling of various aryl halides (iodides and bromides) with termi-
nal alkynes afforded the corresponding alkynes in moderate to
excellent yields. Moreover, high TONs (turnover numbers, up to
720 000 for the reaction of 1-iodo-4-nitrobenzene with phenylacet-
ylene) for the Sonogashira cross-coupling reaction were observed.
X
B(OH)2
+
R
K2CO3
Acetone,
110 °C
R'
R
R'
TONs: up to 950 000
Equation 1
Pd(OAc)2/DABCO
+
X
R'
Key words: Pd(OAc)2/DABCO, Sonogashira cross-coupling reac-
tion, aryl halide, terminal alkyne, turnover number
R'
Cs2CO3 (3 equiv)
MeCN, N2
R
R
3–12
1
2
TONs: up to 720 000
Equation 2
Palladium-catalyzed cross-coupling reaction including
Sonogashira cross-coupling reaction is one of the most
important methods for the formation of carbon–carbon
bonds.1 Particularly, alkynes as the Sonogashira cross-
coupling products, are useful building blocks in organic
synthesis and a recurring functional group in many natural
products and bioactive compounds.2 Therefore, consider-
able effort has been directed to the development of new
and efficient palladium catalytic systems for Sonogashira
cross-coupling reaction.1,3–5 Generally, this reaction is
catalyzed by palladium/phosphines/CuI catalytic system.
However, many phosphine ligands are air-sensitive and
expensive, which places significant limits on their syn-
thetic applications.6 On the other hand, the presence of
CuI can result in the formation of some Cu(I) acetylides in
situ that can undergo oxidative homocoupling reaction of
alkynes readily.7 For these reasons, the development of ef-
ficient copper- and phosphine-free methods remains an
area of current interest.5 Very recently, we found that
DABCO was a highly efficient ligand for the palladium-
catalyzed Suzuki–Miyaura cross-coupling reaction pro-
viding high TONs [up to 950 000 TONs for the reaction
of PhI and p-chlorophenylboronic acid) (Equation 1)].8
Thus, we expected to extend the use of this type of ligand
to other palladium-catalyzed transformations. Here, we
report a mild and efficient Pd(OAc)2/DABCO-catalyzed
Sonogashira cross-coupling reaction of aryl halides with
terminal alkynes to form internal alkynes (Equation 2).
Initially, the efficiency of DABCO as the ligand for the pal-
ladium-catalyzed Sonogashira cross-coupling of 4-iodo-
toluene (1a) with phenylacetylene (2a) was evaluated, and
the results are summarized in Table 1. The results showed
that DABCO was an efficient ligand. Without DABCO,
only 35% yield of the desired product 3 was isolated after
15 hours in the presence of Pd(OAc)2 (3.0 mol%) and
K2CO3 (3.0 equiv) under N2 (entry 1). Whereas the yield
of 3 was increased sharply to 90% when 6.0 mol% of
DABCO was present (entry 2). A series of bases were then
examined, and both Cs2CO3 and K2CO3 provide the high-
est yield (entries 2–8). The use of various tertiary amines
as the ligands was also tested, and the results demonstrat-
ed that DABCO was the most effective due to its flexible
steric bulk (entries 3,5 versus 9–14).9
As shown in Table 2, Pd(OAc)2/DABCO was an efficient
catalytic system for cross-coupling of a number of aryl ha-
lides 1a–f with terminal alkynes 2a–d. In the presence of
Pd(OAc)2 (3.0 mol%), DABCO (6 mol%), and Cs2CO3 (3.0
equiv), treatment of aryl iodides 1a–e and activated aryl
bromide 1f with alkynes 2a–d afforded the corresponding
cross-coupled products 4–12, respectively, in moderate to
excellent yields. The desilylated 1-ethynyl-4-nitroben-
zene (11) was obtained exclusively in 76% yield when 1e
was reacted with 2c (entry 8).10 Unfortunately, the
Pd(OAc)2/DABCO catalytic system was ineffective with
the unactivated aryl bromides 1g and 1h even at high re-
action temperature (entries 12–15).
The catalytic efficacy of Pd(OAc)2/DABCO was further
evaluated, and the results are summarized in Table 3. For
coupling of aryl iodides 1c and 1e with 2a, respectively,
the catalyst loadings could be decreased to 0.0001 mol%,
and desired yields could still be obtained after prolonged
SYNTHESIS 2005, No. 5, pp 0804–0808
Advanced online publication: 09.02.2005
DOI: 10.1055/s-2005-861805; Art ID: F14804SS
© Georg Thieme Verlag Stuttgart · New York
x
x
.
x
x
.2
0
0
5

PAPER
Pd(OAc)2/DABCO-Catalyzed Sonogashira Cross-Coupling Reaction
805
Table 1 Palladium-Catalyzed Sonogashira Cross-Coupling Reac-
tion of 1a with 2aa
Further decrease of the catalyst loading to 0.1 mol% led to
low yield (47%, TONs = 470).
3 mol%
Pd(OAc)2
6 mol%
ligand
In summary, a mild and efficient Pd(OAc)2/DABCO cata-
lytic system for the palladium-catalyzed Sonogashira
cross-coupling reaction has been developed. In the pres-
ence of Pd(OAc)2 (3.0 mol%), DABCO (6 mol%), and
Cs2CO3 (3.0 equiv), the reactions of aryl halides with ter-
minal alkynes were carried out smoothly in excellent
yields. Furthermore, high TONs for the palladium-cata-
lyzed Sonogashira cross-coupling reaction were observed
(up to 720 000 TONs for the reaction of 1-iodo-4-ni-
trobenzene with phenylacetylene). Currently, further ef-
forts to apply this Pd(OAc)2/DABCO catalytic system in
other transformations are underway in our laboratory.
+
Me
I
Me
base
(3 equiv)
N2, r.t.
3
1a
2a
Entry
1
Ligand
–
Base
Yield (%)b
K2CO3
DABCO
K2CO3
K2CO3
Cs2CO3
Na2CO3
NaOAc
KF
35
61
90
55
93
41
37
73
8
2
–
3
DABCO
DABCO
DABCO
DABCO
DABCO
DABCO
Et3N
4c
5
Palladium-Catalyzed Sonogashira Cross-Coupling Reaction;
General Procedure
6
A mixture of aryl halide 1 (1 mmol) and alkyne 2 (1.2 mmol),
Pd(OAc)2 (3.0 mol%), DABCO (6 mol%), Cs2CO3 (3 equiv), and
MeCN (5 mL) was charged in a tube under N2 and sealed. Then the
mixture was stirred at indicated reaction temperature for the desired
time (Table 1) until complete consumption of starting material as
judged by TLC analysis. The mixture was filtered and the solvent
evaporated to give a residue, which was purified by flash column
chromatography (hexane or hexane–EtOAc) to afford 3–12
(Table 2).
7
8
9
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
10
11
12
13
14
(i-Pr)2NEt
13
23
45
81
38
(c-C6H12)2NMe
TMEA
1-(2-p-Tolylethynyl)benzene (3)
1H NMR (300 MHz, CDCl3): d: = 7.54–7.50 (m, 2 H), 7.43 (d,
J = 8.1 Hz, 2 H), 7.35–7.31 (m, 3 H), 7.15 (d, J = 7.8 Hz, 2 H), 2.36
(s, 3 H).
HMPA
(–)-sparteine
13C NMR (100 MHz, CDCl3): d = 138.7, 132.9, 131.9, 129.6, 129.5,
128.8, 128.7, 128.4, 89.1, 83.4, 21.9.
a Unless otherwise indicated, the reaction conditions were as follows:
1a (1 mmol), 2a (1.2 mmol), Pd(OAc)2 (3.0 mol%), ligand (6.0
mol%), base (3 equiv), and MeCN (5 mL) at r.t. under N2 for 15 h.
b Isolated yield.
1-(Dec-1-ynyl)-4-methylbenzene (4)
1H NMR (400 MHz, CDCl3): d = 7.28 (d, J = 8.0 Hz, 2 H), 7.08 (d,
J = 8.0 Hz, 2 H), 2.38 (t, J = 7.2 Hz, 2 H), 2.32 (s, 3 H), 1.61–1.55
(m, 2 H), 1.46–1.42 (m, 2 H), 1.31–1.29 (m, 8 H), 0.89 (t, J = 7.2
Hz, 3 H).
c In a mixture of H2O (1 mL) and MeCN (4 mL).
13C NMR (100 MHz, CDCl3): d = 137.4, 131.4, 128.9, 121.0, 89.6,
stirring. For example, when the catalyst loadings were de-
creased to 0.0001 mol%, 72% yield of 9 was isolated from
the reaction of 1e with 2a after 48 hours (TONs = 720 000).
For coupling of aryl bromides, the catalytic efficiency de-
creased to some extent. For example, when the catalyst
loadings were decreased to 1 mol%, activated aryl bro-
mide 1f was also efficiently coupled with phenylacetylene
(2a) for 24 hours to give the product 9 in high yield (87%).
80.6, 31.9, 29.2, 29.1, 28.9, 28.8, 23.7, 21.4, 18.4, 14.1.
1-(2-o-Tolylethynyl)benzene (5)
1H NMR (400 MHz, CDCl3): d = 7.53–7.52 (m, 3 H), 7.35–7.33 (m,
4 H), 7.23–7.22 (m, 2 H), 2.52 (s, 3 H).
13C NMR (100 MHz, CDCl3): d = 140.2, 132.6, 131.9, 131.6, 129.5,
129.2, 128.4, 128.3, 128.2, 125.6, 88.4, 81.7, 20.7.
Table 2 Palladium-Catalyzed Sonogashira Coupling Reaction in the Presence of DABCOa
Entry
1
ArX
Alkyne
Time (h)
15
Product
Yield (%)b
91
4
n-C8H17
Me
I
(2b)
(1a)
2
16
5
89
I
(2a)
Me
(1b)
Synthesis 2005, No. 5, 804–808 © Thieme Stuttgart · New York

806
J.-H. Li et al.
PAPER
Table 2 Palladium-Catalyzed Sonogashira Coupling Reaction in the Presence of DABCOa (continued)
Entry
3
ArX
Alkyne
Time (h)
16
Product
Yield (%)b
86
6
MeO
(1c)
MeO
(1c)
I
I
(2a)
4
21
12
6
7
53
Me3Si
(2c)
5
8
91
I
(2a)
(1d)
O2N
(1e)
O2N
(1e)
O2N
(1e)
O2N
6
9
ca. 100
ca. 100
76
I
(2a)
7
6
10
11
9
n-C8H17
(2b)
I
8c
9
21
24
24
24
24
25
24
25
Me3Si
I
(2c)
95
Br
Br
Br
(2a)
(1f)
O2N
(1f)
O2N
(1f)
10
11
12
13d
14
15e
10
12
8
93
n-C8H17
(2b)
71
CH2OH
(2d)
9
Br
Br
(2a)
(2a)
(2a)
(2a)
(1g)
8
40
(1g)
-
-
MeO
Br
Br
(1h)
7
<5%
MeO
(1h)
a Unless otherwise indicated, the reaction conditions were as follows: 1 (1 mmol), 2 (1.2 mmol), Pd(OAc)2 (3.0 mol%), DABCO (6.0 mol%),
Cs2CO3 (3 equiv), and MeCN (5 mL) at r.t. under N2.
b Isolated yield.
c The desilylated 1-ethynyl-4-nitrobenzene was obtained as the product.
d At 80 °C.
e At 110 °C.
1-Methoxy-4-(2-phenylethynyl)benzene (6)
1H NMR (400 MHz, CDCl3): d = 7.52–7.51 (m, 2 H), 7.48 (d,
J = 9.0 Hz, 2 H), 7.34–7.32 (m, 3 H), 6.88 (d, J = 8.8 Hz, 2 H), 3.83
(s, 3 H).
13C NMR (100 MHz, CDCl3): d = 159.6, 133.0, 131.4, 128.3, 127.9,
123.6, 115.4, 114.0, 89.4, 88.1, 55.3.
Synthesis 2005, No. 5, 804–808 © Thieme Stuttgart · New York

PAPER
Pd(OAc)2/DABCO-Catalyzed Sonogashira Cross-Coupling Reaction
807
Table 3 Catalytic Efficiency of the Palladium-Catalyzed Sonogashira Coupling Reaction of 1 with 2aa
Entry
ArX
1c
1c
1e
1e
1f
Pd(OAc)2 (mol%) Time (h)
Product
Yield (%)b
TON
870
1
2
3
4
5
6
0.1
17
48
14
48
24
48
6
6
9
9
9
9
87
65
92
72
87
47
0.0001
0.1
650 000
920
0.0001
1
720 000
87
1f
0.1
470
a Unless otherwise indicated, the reaction conditions were as follows: 1 (1 mmol), 2a (1.2 mmol), Pd(OAc)2/DABCO (1: 2), Cs2CO3 (3 equiv),
and MeCN (5 mL) at r.t. under N2.
b Isolated yield.
(4-Methoxyphenylethynyl)trimethylsilane (7)
first. Then portions of the solutions of Pd(OAc)2 in MeCN (10 mL)
and DABCO in MeCN (10 mL) were added to a mixture of 1-iodo-
4-nitrobenzene (1e; 1.0 mmol), phenylacetylene (2a; 1.2 mmol),
Cs2CO3 (3 equiv), and MeCN (5 mL) in a tube by syringe and sealed
under N2. The mixture was stirred at r.t. for 48 h until complete con-
sumption of starting material as judged by TLC analysis. After fil-
tering the mixture and evaporation of the solvent, the residue was
purified by flash column chromatography (hexane–EtOAc) to af-
ford 160 mg (72%) of 9 (TONs: 720 000).
1H NMR (400 MHz, CDCl3): d = 7.20 (d, J = 8.4 Hz, 2 H), 6.61 (d,
J = 8.8 Hz, 2 H), 3.60 (s, 3 H), 0.03 (s, 9 H).
13C NMR (100 MHz, CDCl3): d = 161.0, 133.1, 115.2, 113.8, 85.6,
78.4, 56.0, –1.1.
1,2-Diphenylethyne (8)
1H NMR (300 MHz, CDCl3): d = 7.60–7.51 (m, 4 H), 7.39–7.26 (m,
6 H).
13C NMR (75 MHz, CDCl3): d = 132.0, 128.7, 128.6, 123.7, 89.7.
Acknowledgment
1-[2-(4-Nitrophenyl)ethynyl]benzene (9)
We thank the National Natural Science Foundation of China (No.
20202002), Education Department of Hunan Province (No.
02C211) and Hunan Normal University for financial support
(2001).
1H NMR (300 MHz, CDCl3): d = 8.21 (d, J = 8.8 Hz, 2 H), 7.65 (d,
J = 8.8 Hz, 2 H), 7.58–7.54 (m, 2 H), 7.40–7.38 (m, 3 H).
13C NMR (75 MHz, CDCl3): d = 141.3, 132.6, 132.2, 130.6, 129.6,
128.8, 124.0, 122.5, 95.0, 87.9.
1-(Dec-1-ynyl)-4-nitrobenzene (10)
References
1H NMR (400 MHz, CDCl3): d = 8.15 (d, J = 8.8 Hz, 2 H), 7.51 (d,
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1.43 (m, 2 H), 1.31–1.26 (m, 8 H), 0.89 (t, J = 7.2 Hz, 3 H).
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Synthesis 2005, No. 5, 804–808 © Thieme Stuttgart · New York

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J.-H. Li et al.
PAPER
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Synthesis 2005, No. 5, 804–808 © Thieme Stuttgart · New York