Samarium powder-catalyzed palladium-free and ligand-free sonogashira coupling reactions

Article abstract of DOI:10.1002/adsc.200900258

A study of samarium powder-catalyzed cross-coupling reactions of aryl halides with terminal alkynes is described. The couplings performed in the polyethylene glycol PEG-600 provided the corresponding coupling products in good yields. The first example of palladium-free, copper-free and amine-free catalytic system for Sonogashira couplings is presented in the absence of ligand.

Full text of DOI:10.1002/adsc.200900258

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DOI: 10.1002/adsc.200900258
Samarium Powder-Catalyzed Palladium-Free and Ligand-Free
Sonogashira Coupling Reactions
a,
a
a
a,
Jincheng Mao, * Minyan Wu, Guanlei Xie, and Shunjun Ji *
a
Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials
Science, Suzhou University, Suzhou 215123, Popleꢀs Republic of China
Fax : (+86)-512-6588-0089; e-mail: jcmao@suda.edu.cn
Received: April 14, 2009; Revised: July 14, 2009; Published online: September 8, 2009
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200900258.
[
2]
Abstract: A study of samarium powder-catalyzed
cross-coupling reactions of aryl halides with termi-
nal alkynes is described. The couplings performed
in the polyethylene glycol PEG-600 provided the
corresponding coupling products in good yields.
The first example of palladium-free, copper-free
and amine-free catalytic system for Sonogashira
couplings is presented in the absence of ligand.
biologically active substances. For example, the con-
struction of arylalkynes plays an important part in
prepation of macrocyclic peptide (A), which has
[
3]
gained pharmaceutical interest. In recent years, aryl-
alkynes (B) have been investigated as conjugated
oligomers for reasearch on electrical conductance at
the single molecule level. Thus, it is obvious to show
the importance and practicablity of such a coupling
reaction as shown in Scheme 1.
AHCTUNGTRENNUNG
[
4]
Keywords: aryl halides; cross-coupling; ligand-free;
samarium powder; terminal alkynes
Since its discovery, this alkynylation of aryl halides
has gained great attention and various effective cata-
lytic systems have been developed. The original Sono-
gashira coupling is carried out in the presence of
PdCl ACHTUNGTRENGUNN( PPh ) , PdCl /PPh , or Pd ACHTURENGNUNG( PPh ) together with
2 3 2 2 3 3 4
CuI as the cocatalyst and a larger amount of amines
[
5]
Transition metal-catalyzed cross-coupling reactions as the solvents or cosolvents. Based on this founda-
are the very useful processes for forming carbon- tion, several modified protocols have been presented,
[7]
[
1]
[6]
carbon bonds. The Sonogashira-type coupling be- including copper-free, Pd/Ni bimetallic systems,
[
8]
[9]
tween terminal alkynes with ary or vinyl halides is Au complex and Ru/Al O systems. However, they
2
3
one of the important and widely used procedures for were all limited for industrial use due to the high
the synthesis of molecules containing an acetylenic costs. To solve this problem, a low loading of palldium
À4
[10]
moiety, which is found in many fine chemicals and catalyst (10 mol%) was employed. On the other
Scheme 1. Compounds containing an arylalkyne moiety.
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Table 1. Screening catalytic conditions for samarium powder-catalyzed Sonogashira coupling between 4-iodoanisole and
[a]
phenyl
ACHTUNGTRENNUNGa cetylene.
[b]
Entry
Cat. [mol%]
Base
Solvent
1:2
Yield [%]
1
2
Sm (10)
–
K CO
PEG-400
PEG-400
PEG-400
PEG-200
PEG-600
PEG-1000
DMF
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.5
1:2
44
2
3
3
3
3
3
3
3
3
3
3
3
K CO
trace
trace
18
84
62
trace
trace
trace
trace
68
trace
trace
12
NR
trace
67
34
17
75
40
31
trace
32
2
[
c]
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
Sm (10)
Sm (10)
Sm (10)
Sm (10)
Sm (10)
Sm (10)
Sm (10)
Sm (10)
Sm (10)
Sm (10)
Sm (10)
Sm (10)
Sm (10)
Sm (10)
Sm (10)
Sm (10)
Sm (10)
Sm (20)
Sm (30)
Sm (50)
Sm (10)
Sm (10)
Sm (10)
Sm O (10)
K CO
2
K CO
2
K CO
2
K CO
2
K CO
2
K CO
DMSO
2
K CO
C ACHTUNGTRENNUG[ min]PF
2
6 6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
K CO
glycol
2
[
d]
K CO
PEG-600
PEG-600
PEG-600
PEG-600
PEG-600
PEG-600
PEG-600
PEG-600
PEG-600
PEG-600
PEG-600
PEG-600
PEG-600
PEG-600
PEG-600
PEG-600
PEG-600
2
K PO
3
4
Cs CO
2
3
KF
Et N
3
KOH
K CO
2
3
3
3
3
3
3
3
3
3
3
3
K CO
2
K CO
1:3
2
K CO
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
2
K CO
2
K CO
2
[
[
e]
f]
K CO
2
K CO
2
g]
K CO
NR
NR
NR
2
K CO
2
3
2
SmI (10)
K CO
2
2
[
a]
Reaction conditions: Ar, Sm (ca. 350 mm powder from Aldrich, 99% purity, 0.05 mmol), 4-iodoanisole (0.5 mmol), phenyl-
acetylene (0.6 mmol), base (1.0 mmol), solvent (2 mL), 1308C, 24 h.
Isolated yield based on 4-iodoanisole.
The catalytic reaction was performed in air.
Sm (ca. 550 mm powder) was used instead of Sm (ca. 350 mm powder).
I2 (10 mol%) was employed as the additive.
[
[
[
[
[
[
b]
c]
d]
e]
f]
TBAB (10 mol%) was employed as the additive.
LiCl (10 mol%) was employed as the additive.
g]
hand, the use of a cheaper and lower-toxicity metal alkynylation of aryl halides without using any ligands
instead of palladium provides another possible and co-catalysts.
[11]
route.
Thus, copper salts together with various li-
Polyethylene glycol (PEG) is widely considered as
gands as novel catalytic systemes have been report- a good recyclable solvent with a low environmental
[
12]
ed. Recently, Bolm and co-workers have reported impact. It is interesting to find that PEG usually af-
the FeCl -catalyzed Sonogashira coupling reaction forded better results than the commonly used sol-
3
[
13]
[16]
using DMEDA as the ligand. Meanwhile, we and vents. This is perhaps attributable to its special role
other groups have demonstrated efficient Fe/Cu co- in the catalysis relative to classical phase-transfer cat-
catalyzed alkynylations of aryl halides. Differently, alysts. Thus, the coupling between 4-iodoanisole and
Wang has disclosed that the Sonogashria coupling was phenylacetylene as the model reaction was carried
investigated in the presence of ultrafine nickel out in PEG-400 in the presence of 10 mol% of sama-
[
14]
[
14]
powder together with CuI and PPh3. Based on our rium powder (ca. 350 mm). The respective results are
interest on coupling reactions using easily available listed in Table 1. To our delight, only the exclusive de-
[12i,j,14a,15]
catalysts,
herein, we describe our recent find- sired coupling product was obtained in promising
ing that samarium powder could directly catalyze the yield (44%) (Table 1, entry 1). However, as we know
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Samarium Powder-Catalyzed Palladium-Free and Ligand-Free Sonogashira Coupling
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that for the copper-mediated alkynylation of aryl hal- reagents including aryl halides, alkynes, K CO and
2
3
ides, it is unavoidable to get the self-coupling of al- PEG-600 had the low levels of Pd (below 10 ppb) as
[
17]
kynes as the possible by-products. Therefore, the cou- measured by ICP-MS. Therefore, we will apply this
pling with our protocol was highly selective. When optimal catalytical system to the couplings of different
the reaction was carried out in air or in the absence substrates.
of base, no product was obtained (entries 2 and 3).
A wide array of substrates was subjected to the op-
PEGs with different molecular weights have been em- timized reaction conditions as shown in Table 2. To
ployed in the reaction (entries 4–6) and PEG-600 our delight, we found that for the aryl iodides, all
gave the best result of 84% yield (entry 5). Other transformations proceeded well, giving the corre-
common solvents including DMF, DMSO and glycol sponding products 3–12 with moderate to good yields
were sceened and only traces of product were ob- (Table 2, entries 1–15). In regard of the alkynylation
tained (entries 7–9). In addition, an ionic liquid, such of aryl bromides, promising results were also obtained
as C ACHTUNGTRENNUNG[ min]PF , was a suitable choice for the possible (entries 16 and 17).
6 6
solvent (entry 10). Different sizes of the samarium
powder did not give enhanced results (entry 11). Sub- also carried out based on the alkynylation of 4-iodo-
sequently, various inorganic and organic bases have anisole. The way to recover the catalytic system was
Subsequently, catalyst recycle experiments were
AHCTUNGTRENNUNG
been investigated in the coupling reaction. However, very simple, only by adding diethyl ether to the reac-
none of them gave better result than K CO (en- tion and decanting the upper layer. The residue was
2
3
tries 12–16). Changing the ratios between the two sub- added to the substrates for the second run. All the
strates did not favor the reactions (entries 17–19). In- work-up should be performed under an atmosphere
crease of the samarium loading did not result in en- of argon. However, to our astonishment, only 40%
hanced results (entries 20–22). Various additives, such yield of the desired coupling product was obtained.
as iodine, TBAB (n-Bu NBr) and LiCl, have been
Scanning electron microsope (SEM) studies of both
4
employed in the reactions and disappointing results fresh and used catalysts were carried out to under-
were obtained (entries 23–25). Other forms of samari- stand the shape and size of the particles. Figure 2a
um were not favorable for the catalytic reaction (en- and Figure 2b show the SEM images of the fresh and
tries 26 and 27).
used catalyst after the first cycle, respectively. It is ob-
Under the same conditions, different common viously seen that after the reaction the size of samari-
metal powders have been used in the alkynylation of um powder became smaller, which shows that smaller
4-iodoanisole. From Figure 1 it can be seen that be- size is not beneficial for the efficiency of catalyst.
sides samarium, copper, zinc, nickel, iron, indium
powders also showed promising catalytic activities.
Compartively, samarium gave the best result. In order
to control for issues of potential metal contamination,
we carried out experiments in new flasks with new
stirring bars and new caps. We also ensured that all of
Figure 1. Various metal powders were employed in the
Sonogashira coupling of 4-iodoanisole and phenylacetylene.
Figure 2. SEM images of samarium powders.
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Jincheng Mao et al.
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[a}
Table 2. Sm powder-catalyzed Sonogashira coupling of alkynes and aryl halides.
o
[b]
Entry
RX
R’CꢀCH
Temperature [ C]
Yield [%]
1
2
3
4
5
6
7
8
9
3
130
140
130
130
130
130
140
140
140
84
64
86
84
87
63
76
87
82
4
5
6
7
8
8
9
10
10
11
140
62
1
1
1
1
1
1
1
1
2
3
4
5
6
7
12
12
12
7
130
140
150
140
150
140
140
78
80
83
80
85
41
46
7
4
6
[
a]
Reaction conditions: Ar, Sm (ca. 350 mm powder from Aldrich, 99% purity, 0.05 mmol), aryl halide (0.5 mmol), alkyne
0.6 mmol), K CO (1.0 mmol), PEG-600 (2 mL), 130–1508C, 24 h.
Isolated yield based on aryl halide (average of three runs).
(
2
3
[
b]
However, on the other hand, it suggested a practical without generating homocoupling of alkynes as the
and alternative way to prepare the metal powders possible by-products; 4) the procedure is simple to
with smaller size.
perform in the absence of any additives. The scope,
In summary, we have developed a very practical mechanism, and synthetic applications of this reaction
and efficient protocol for the alkynylation of aryl io- are under further investigation.
dides and bromides catalyzed by samarium powder in
PEG-600. Features worth noting include 1) the cou-
pling reactions between various alkynes and substitut-
ed aryl halides proceeded efficiently to afford valua-
ble compounds; 2) the catalytic system is ligand-free,
palladium-free, copper-free and amine-free; 3) the
coupling reactions afforded the exclusive product
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Samarium Powder-Catalyzed Palladium-Free and Ligand-Free Sonogashira Coupling
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Experimental Section
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as monitored by TLC. After that the mixture was poured
into ether, then washed with water, extracted with ethyl ace-
7
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4
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3
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2
We are grateful to the grants from the National Natural Sci-
ence Foundation of China (No. 20802046) and the Key Labo-
ratory of Organic Synthesis of Jiangsu Province for financial
support. We thank Dr. Marc Creus from University of Basel,
Switzerland for helpful discussions.
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