Iron/copper-catalyzed C-C cross-coupling of aryl iodides with terminal alkynes

Article abstract of DOI:10.1016/j.tetlet.2008.07.151

Synergic effect of iron and copper salts as catalysts for the Sonogashira-Hagihara cross-couplings of aryl iodides with terminal alkynes is demonstrated. High yields of cross-coupled products are obtained under conditions that are smoother than those using only CuI as catalyst. Furthermore no expensive or/and toxic ligand is required.

Full text of DOI:10.1016/j.tetlet.2008.07.151

Tetrahedron Letters 49 (2008) 5961–5964
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Iron/copper-catalyzed C–C cross-coupling of aryl iodides with terminal alkynes
*
Chandra M. Rao Volla, Pierre Vogel
Laboratoire de Glycochimie et Synthèse Asymétrique (LGSA), Swiss Federal Institute of Technology Lausanne (EPFL), Batochime, 1015 Lausanne, Switzerland
a r t i c l e i n f o
a b s t r a c t
Article history:
Synergic effect of iron and copper salts as catalysts for the Sonogashira–Hagihara cross-couplings of aryl
iodides with terminal alkynes is demonstrated. High yields of cross-coupled products are obtained under
conditions that are smoother than those using only CuI as catalyst. Furthermore no expensive or/and
toxic ligand is required.
Received 24 June 2008
Revised 23 July 2008
Accepted 29 July 2008
Available online 31 July 2008
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Sonogashira–Hagihara coupling
Iron
Copper
Synergic effect
1. Introduction
terminal alkynes undergo C–C cross-coupling reactions in the pres-
ence of iron salt and CuI as catalysts and under relatively smooth
Aryl alkynes are important compounds for material sciences
and medicinal chemistry.1 These compounds are best obtained
by the Sonogashira–Hagihara cross-coupling reaction of terminal
alkynes with aryl halides or triflates, originally using palladium
catalysts together with phosphine or diamine ligands.2 Transition
metal-free Sonogashira–Hagihara reactions of aromatic iodides
and bromides with terminal alkynes have been reported, but they
require very high temperature, phase-transfer catalyst, and micro-
wave activation. Furthermore, they are limited to aromatic al-
kynes.3 Sonogashira–Hagihara cross-couplings have been
reported using less harsh conditions in the presence of copper,4
ruthenium,5 or nickel6 catalysts and adequate ligands. Li and co-
workers7 reported that CuI/DABCO is an effective catalyst for Sono-
gashira–Hagihara cross-couplings of aryl halides and vinyl halides.
Highly dispersed Cu metal on alumina is a good catalyst for the
cross-coupling of aryl iodides with phenylacetylene.8 CuI/ligand
couples are good catalysts for C–C, C–N, and C–O Ullmann-type
coupling reactions.9 The discovery in 1954 by Kharasch and Rein-
muth,10 then in 1971 by Tamura and Kochi11 that Grignard re-
agents and alkyl halides can be cross-coupled in the presence of
iron catalysts has stimulated several studies toward the substitu-
tion of expensive and toxic transition metals and ligands by iron
catalysts in C–C bond forming reactions.12–18 Iron salts are cheap,
non-toxic, and environmentally benign.19 A recent report by Bolm
and co-workers20 on iron-catalyzed Sonogashira reaction urges us
to present our own studies on this topic.21 Aromatic iodides and
conditions that do not require the presence of expensive or toxic
ligand. Iron-catalysis has been recently extended to allylic alkyl-
ation, allylic aminations,22 and carbon-heteroatom cross-coupling
reactions.21,23,24
2. Results
Our initial attempts used 4-iodotoluene and phenylacetylene as
the coupling partners in the presence of iron(III) acetylacetonate
and copper(I) iodide, without any external ligand. The reaction
was carried out in DMF at 140 °C with Cs2CO3 as the base of choice
(Table 1). We find that the reaction is completed in 36 h giving 1-
(4-methylphenyl)-2-phenylethyne in 93% yield (Table 1, entry 1).
The crucial roles played by both iron and copper salts were clear
as reactions done in the absence of either iron or copper led only
to low yields of product (entries 2 and 3). Without iron and copper
catalysts the coupling did not occur at all (entry 4), thus demon-
strating the synergic effect of iron and cuprous salts. This type of
synergic effect of Fe/Cu has been disclosed in the arylmagnesation
of internal alkynes.25 Reaction time was somewhat shorter in
DMSO (entry 6) or NMP (N-methylpyrrolidinone) (entry 7). The
beneficial effect of NMP was also observed for other iron-catalyzed
C–C coupling reaction.13,18 Among several iron salts, (Table 1, en-
tries 9–13) iron(III)acetylacetonate appeared to be the best cata-
lyst. Nakamura has shown the importance of fluoride ion in iron-
catalyzed Corriu–Kumada coupling reaction.17c In our case
FeF3ꢀ(H2O)3 (entry 10) was not as efficient as Fe(acac)3. To our
surprise, although FeCl3 + CuI (entry 9) did catalyze the reaction
almost as well as Fe(OAc)2 + CuI (entry 12), FeCl2 + CuI (entry 11),
* Corresponding author. Tel.: +41 21 693 93 71; fax: +41 21 693 93 50.
E-mail address: pierre.vogel@epfl.ch (P. Vogel).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.07.151

5962
C. M. Rao Volla, P. Vogel / Tetrahedron Letters 49 (2008) 5961–5964
Table 1
Iron/copper-catalyzed coupling of 4-iodotoluene with phenylacetylene
Fe salt, CuI
base (2 eq)
+
I
H
solvent, 140 oC
Entry
Fe source (mol %)
CuI (mol %)
Ligand (15 mol %)
Base
Solvent
Time (h)
Yielda (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Fe(acac)3 (10)
Fe(acac)3 (10)
—
10
—
10
—
—
—
—
—
—
—
—
—
—
—
—
—
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
K3PO4
DMF
DMF
DMF
DMF
DMF
DMSO
NMP
NMP
NMP
NMP
NMP
NMP
NMP
NMP
NMP
NMP
NMP
NMP
36
48
48
36
2.5
21
20
48
48
48
48
48
48
40
40
36
48
21
93
—
6
—
—
Fe(acac)3 (10)
Fe(acac)3 (10)
Fe(acac)3 (10)
Fe(acac)3 (5)
FeCl3 (10)
10
10
10
10
10
10
10
10
—
10
10
10
10
10
94b
87
95
68
55
76
Traces
61
—
89
85
FeF3ꢀ3H2O (10)
FeCl2 (10)
Fe(OAc)2 (10)
Fe(OAc)2 (10)
Fe(acac)3 (7)
Fe(acac)3 (7)
Fe(acac)3 (7)
Fe(acac)3 (10)
Fe(acac)3 (10)
—
TMEDA
DMEDA
HMTA
—
91
71
Traces
—
Et3N
a
Yields were determined after flash chromatography.
b
Reaction was done in
a microwave reactor. acac = Acetylacetonate, DMF = dimethylformamide, DMSO = dimethylsulfoxide, NMP = N-methylpyrrolidone, TME-
DA = N,N,N0,N0-tetramethylethylenediamine, DMEDA = N,N0-dimethylethylenediamine, HMTA = hexamethylenetetramine.
Fe(OAc)2 alone (entry 13) did not catalyze it. This shows the impor-
tance of the oxygenated -ligands of the iron salt. Cahiez et al.
reported the beneficial effect of TMEDA and HMTA as additives
in iron-catalyzed cross-coupling of alkyl halides with Grignard
reagents.13d,e
for a 79% yield. A wide variety of terminal acetylenes with aryl,
r
alkyl, 3-pyridinyl, and cyclohexen-1-yl substituents gave the corre-
sponding products of cross-coupling in good to excellent yields.
In order to extend the scope of the reaction further, we explored
the cross-coupling of various electrophilic partners with phenyl-
acetylene (Table 3). Apart from 4-iodonitrobenzene all aryl iodides
assayed led to good yields of isolated products. With 4-bromo-, 4-
chlorotoluene, and 4-methylbenzenesulfonyl chloride (entries 2–
4) no product of cross-coupling could be observed. In the latter
case, the sulfonyl chloride decomposed rapidly giving a small
amount of products of homocoupling. Interestingly, the cross-cou-
pling reaction was successful for electron-poor (entries 6–10, 15,
Addition of external ligands such as Et3N, TMEDA, DMEDA, or
HMTA did not help the reaction (entries 14–16). Substitution of
Cs2CO3 by K3PO4 retarded somewhat the reaction, whereas Et3N
as base alone gave the product only in trace amounts (entries 17
and 18). When the reaction was carried out in m-xylene in the
presence of 1 equiv of NMP, low yield of the coupling product
was observed. Although the reaction takes 20 h for completion,
the reaction time can be greatly decreased by using a microwave
reactor (2.5 h for nearly same yields) (Table 1, entry 5). Our best
conditions (Table 1, entry 7) were then applied to the cross-cou-
pling of a variety of terminal acetylenes with 4-iodotoluene (Table
2). In most cases, the reaction was over in ca 20 h at 140 °C except
for (4-methoxyphenyl)acetylene which required 36 h (entry 10)
Table 3
Iron/copper-catalyzed cross-coupling of aryl halides with phenylacetylene
Fe(acac)3 (10 mol %),
CuI(10 mol %)
Cs2CO3 (2 equiv.),
NMP, 140 oC
R'-X
+
H
Ph
R'
Ph
Table 2
Entry
R0
X
Time (h)
Yielda (%)
Iron/copper-catalyzed coupling of various terminal alkynes with 4-iodotoluene
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
4-Methylphenyl
4-Methylphenyl
4-Methylphenyl
4-Methylphenyl
Phenyl
4-Fluorophenyl
4-(Trifluoromethyl)phenyl
4-Nitrophenyl
4-Cyanophenyl
4-Acetylphenyl
2-Methylphenyl
3-Methylphenyl
1-Naphthyl
4-Methoxyphenyl
4-Bromophenyl
4-Chlorophenyl
I
Br
Cl
SO2Cl
I
I
I
I
I
I
I
I
I
I
I
I
20
24
24
24
20
18
18
24
18
18
20
20
20
36
18
18
95
0
0
I
H
R
0b
94
96
98
0c
Fe(acac)3(10 mol%), CuI(10 mol%)
Cs2CO3 (2 equiv.), NMP, 140 oC
R
Entry
R
Time (h)
Yielda (%)
87
89
91
85
93
83
96
97
1
2
3
4
5
6
7
8
9
Phenyl
Phenyl
4-Methylphenyl
n-Octyl
n-Hexyl
t-Butyl
n-Butyl
3-Pyridyl
4-Methoxyphenyl
Cyclohexen-1-yl
20
2.5
20
18
18
18
18
21
36
20
95
94b
92
91
96
86
87
92
79
94
a
Yield of isolated products after flash chromatography.
Starting material was decomposed after 3 h under the reactions conditions,
b
10
small amount of homocoupling of phenylacetylene was observed.
a
c
Yields of isolated products after flash chromatography.
Reaction was done in a microwave reactor.
Starting material was decomposed to give a mixture of products (nitrobenzene
b
and aniline).

C. M. Rao Volla, P. Vogel / Tetrahedron Letters 49 (2008) 5961–5964
5963
16) as well as for electron-rich aryl iodides (entries 11–14). 4-Iodo-
nitrobenzene decomposed immediately under our reaction condi-
tions to give a mixture containing nitrobenzene and aniline and
no trace of the desired product of cross-coupling (entry 8). All
products of C–C coupling were obtained pure and were identical
to authentic samples (1H, 13C NMR spectra).
tube was connected to a vacuum line and filled with Ar (3 times),
NMP (2 mL) and phenylacetylene (0.2 mL, 1.84 mmol, 2 equiv)
were added. The reaction mixture was stirred in a microwave reac-
tor at 140 °C for 2.5 h. After cooling to room temperature, the mix-
ture was diluted with diethyl ether, and water was added. The
aqueous layer was extracted again with ether (20 mL * 3 times).
The combined organic phases were dried (MgSO4), filtered, and
concentrated under reduced pressure (rotavap). The residue was
purified by flash chromatography on silica gel.
3. Discussion
More studies are necessary to approach possible mechanisms of
our cross-coupling reactions. Under our conditions that do not use
any ligand apart from solvent (DMF, DMSO, and NMP), we find that
CuI alone catalyzes the homo-coupling very slowly (Table 1, entry
3). On addition of a Fe salt, the reaction is accelerated significantly.
It is known that CuI/Na2CO3 catalyzes the homo-coupling of termi-
nal alkynes with aryliodonium salts.26 Thus, a possible role of the
Fe salt is to activate the oxidative addition of CuI, or Cu acetylide
intermediate (that could arise from Cu(II) salt intermediates
according to the Bohlmann mechanism27), onto aryl iodide by
coordination to the iodo moiety of ArI. Alternatively, the activation
could involve a SET from ArI to the Fe(III) salt giving FeXꢁ3 and the
corresponding radical-cation ArI+Å that undergoes a faster oxidative
addition than neutral ArI. The formation of the Ar–Cu(I)–acetylide
intermediates might be preceded by the formation of Cu acetylides,
or by addition of ArCuI to the terminal acetylenes,28 followed by
base-induced HI elimination. The nature of the base is essential
as Et3N does not induce the cross-coupling reaction as well as
Cs2CO3.
Acknowledgement
We are grateful to the Swiss National Science Foundation (Bern)
and the Roche Research Foundation (Basel). We also thank F. Sep-
ulveda and M. Rey for their technical help.
References and notes
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In a sealable tube dried under vacuum were placed the corre-
sponding ArI (if it is a solid) (1 equiv), Fe(acac)3 (10 mol %), CuI
(10 mol %), and Cs2CO3 (2 equiv) (glove box). Then, the tube was
sealed with the cap. The tube was connected to a vacuum line
and filled with Ar (3 times), NMP (2 mL), the corresponding iodide
(if it is a liquid) (1 equiv), and the corresponding alkyne (2 equiv)
were added. The reaction mixture was stirred at 140 °C for the time
mentioned. After cooling to room temperature, the mixture was di-
luted with diethyl ether, and water was added. The aqueous layer
was extracted again with ether (20 mL * 3 times). The combined
organic phases were dried (MgSO4), filtered, and concentrated un-
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A
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