Methionine: A green and efficient promoter for copper-catalyzed Sonogashira cross-coupling reactions

Article abstract of DOI:10.1002/aoc.3361

In the presence of amino acids as environmentally friendly ligands, CuI-catalyzed Sonogashira cross-coupling of various aryl halides with phenylacetylene was conducted to afford the corresponding internal alkynes. l-Methionine was found to be useful for t

Full text of DOI:10.1002/aoc.3361

Full paper
Received: 17 May 2015
Revised: 29 June 2015
Accepted: 14 July 2015
Published online in Wiley Online Library: 28 September 2015
(wileyonlinelibrary.com) DOI 10.1002/aoc.3361
Methionine: a green and efficient promoter for
copper-catalyzed Sonogashira cross-coupling
reactions
Abdol R. Hajipoura,b* and Fatemeh Mohammadsaleha
In the presence of amino acids as environmentally friendly ligands, CuI-catalyzed Sonogashira cross-coupling of various aryl
halides with phenylacetylene was conducted to afford the corresponding internal alkynes. L-Methionine was found to be useful
for this palladium-free and amine-free coupling reaction. It was also found that the solvent system plays an important role in this
reaction, and significantly affects the product formation and reaction rate. Sonogashira coupling of aryl iodides and aryl bromides
in dimethylsulfoxide or dimethylformamide gave the coupled products in good to excellent yields. Copyright © 2015 John Wiley &
Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web site.
Keywords: amino acid; copper catalysis; cross-coupling reaction; Sonogashira; internal alkyne
simultaneously.[36,37] This reaction system results in an increased re-
activity of the reagents and the ability of the reaction to be carried
Introduction
Amino acids are organic compounds that form one of the most
prominent tools of natural products, and they are known as the
building blocks of life.[1–3]
out at room temperature. The copper co-catalyst generates a cop-
per acetylide intermediate that subsequently transmetallates on
the palladium center. Many different palladium-based catalytic sys-
tems, regardless of whether or not a copper co-catalyst is used,
have been applied for this reaction,[38–40] which is problematic
for industrial use due to the cost of palladium. However, the use
of effective palladium-free systems, including copper, zinc or iron,
would obviously be much more interesting in view of modern
organic synthesis owing to the lower cost and easy availability
of these systems.[41–44] A number of important studies have fo-
cused on the development of new catalytic systems including
phosphine- and palladium-free conditions.[45] Various catalytic sys-
tems based on iron,[42] cobalt,[46] nickel[47] and silver,[48] in combina-
tion with different ligands have been reported for Sonogashira
coupling reactions.
Copper-based catalysts are powerful in organic reactions be-
cause they can easily access Cu(0), Cu(I), Cu(II) and Cu(III) oxidation
states,[49] allowing them to act through both radical and/or power-
ful two-electron bond-forming pathways via organometallic inter-
mediates similar to those of palladium catalysts. Recent attention
has been focused on employing copper-only catalytic systems in
Sonogashira couplings. Bolm and co-workers[50] have reported cou-
pling reactions between various aryl iodides and terminal alkynes
Nowadays, a great deal of progress has been made in improv-
ing the reaction conditions and designing ligands for metal-
catalyzed carbon–carbon and carbon–heteroatom bond forming
reactions.[4–6] Combinations of metal salts and ligands are efficient
catalytic systems for the formation of C–O, C–N and C–C
bonds.[7,8] Several groups have reported that some bidentate
compounds like diketones,[9] 1,2-diamines,[10,11] 1,2-diols[12] and
1,10-phenanthroline[13] could serve as ligands to facilitate metal-
catalyzed carbon–carbon and carbon–heteroatom cross-coupling
reactions. A number of important studies have been focused on
the development of synthetic routes that minimize contamination
and pollution in chemical synthesis by use of green reagents in
producing materials.[14,15]
It is known that the structure of α-amino acids as ligands can
accelerate metal-catalyzed reactions.[16–18] Ma et al. have reported
the widespread applications of amino acids as promoters in
Cu-assisted coupling reactions.[19,20] They reported the coupling re-
actions of aryl halides and α-amino acids. That group also found
that N-methylglycine and L-proline were suitable promoters in the
Cu-catalyzed coupling reactions of aryl halides with various
nucleophiles.[21–27]
Transition metal-catalyzed carbon–carbon bond formation has
been used in the synthesis of many pharmaceuticals and natural
and industrial products.[28–30] The Sonogashira coupling reaction
is a powerful synthetic method for the construction of sp2–sp
carbon–carbon bonds by the introduction of a triple bond into aro-
matic systems via cross-coupling of aryl/alkyl halides and terminal
acetylene,[31,32] frequently employed in the synthesis of both bio-
logically and synthetically important aryl acetylenes.[33–35] This reac-
tion generally employs Pd catalyst and Cu(I) salt as co-catalyst
*
Correspondence to: Abdol R. Hajipour, Pharmaceutical Research Laboratory,
Department of Chemistry, Isfahan University of Technology, Isfahan 84156, IR,
Iran. E-mail: haji@cc.iut.ac.ir
a Pharmaceutical Research Laboratory, Department of Chemistry, Isfahan
University of Technology, Isfahan 84156, IR, Iran
b Department of Pharmacology, University of Wisconsin, Medical School, 1300
University Avenue, Madison, WI 53706-1532, USA
Appl. Organometal. Chem. 2015, 29, 787–792
Copyright © 2015 John Wiley & Sons, Ltd.

A. R. Hajipour and F. Mohammadsaleh
using [Cu(N,N′-dimethylethylenediamine)2]Cl2ÁH2O as a catalyst
precursor in dioxane at 135°C in a reaction time of 22 h. Li et al.[51]
investigated the CuI/1,4-diazabicyclo[2.2.2]octane (DABCO) cata-
lytic system in Sonogashira cross-couplings of aryl halides and vinyl
halides. Here, the coupling reactions were carried out using 10 mol
% of CuI catalyst, 20 mol% of DABCO ligand and Cs2CO3 as the base
in dimethylformamide (DMF) at 135–140°C in 4–25 h. Sonogashira
coupling of a variety of aryl halides using 10 mol% of CuI, 30 mol
% of β-diketone ligand and K2CO3 base in DMF at 90–120°C in
24–40 h was reported by Monnier et al.[52] Ma and Liu[53] introduced
a Cu(I)/amino acid catalytic system for Sonogashira coupling reac-
tions. They obtained coupled products using CuI (10 mol%) as cat-
alyst and N,N-dimethylglycine hydrochloride salt (30 mol%) as
promoter in the presence of K2CO3 and DMF at 100°C in 24–36 h.
Xu et al. have recently reported catalytic systems featuring low cop-
per loading (0.5 mol% < Cu < 5 mol%) for copper-catalyzed
Sonogashira-type reactions accelerated by a catalytic amount of ad-
ditives such as polycyclic aromatic hydrocarbon or organophos-
phate as the ligand.[54,55] They found Cu(OTf)2 as an effective
copper source in this reaction system. Based on these reports, it
can be concluded that most of the palladium-free copper-catalyzed
Sonogashira coupling reactions usually require high temperatures
and/or long reaction times.
In continuation of our recent studies on the design of catalytic
systems containing amino acids,[56,57] and as a part of our goal
to develop copper-based catalysts for Sonogashira coupling
reactions,[58,59] in the work reported herein we evaluated the ac-
tions of several combinations of CuI/amino acids in Sonogashira
coupling reactions (Scheme 1). Between the different tested amino
acids (phenylalanine, methionine and proline), methionine showed
the best results. The solvent system was found to have an impor-
tant role in this reaction system. A variety of aryl halides with
phenylacetylene were coupled to afford the corresponding internal
alkynes in good to excellent yields. A catalytic amount of methio-
nine (8 mol%) as a green promoter was used for aryl iodides in this
transformation.
It is found that by employing CuI (10 mol%), K3PO4 (2 equiv.) and
L-proline (30 mol%) in the mixed solvent DMF–H2O (80:20) at 130°C,
the corresponding alkyne product is obtained in 67% yield after 3 h
(Table 1, entry 1). This result clearly indicates that the α-amino acid
has an accelerating effect on the coupling of 4-iodonitrobenzene
with phenylacetylene. Encouraged by this result, we also investi-
gated the efficiency of two amino acids, phenylalanine and
methionine, as the ligands in this reaction. Using phenylalanine as
the promoter, a decreased yield of the desirable product 2a is ob-
tained from the reaction of substrate 4-iodonitrobenzene with
phenylacetylene (46% yield; Table 1, entry 3). However, the yield
of 2a is increased to 83% when using methionine (Table 1, entry
2). These results show that under these reaction conditions, methi-
onine is more effective than phenylalanine and proline.
To further investigate and compare the performance of proline
and methionine as the promoter in this transformation, we also ex-
amined the efficiency of both promoters using 4-iodoanisole as the
substrate. It is found that the treatment of 4-iodoanisole with
phenylacetylene, CuI (10 mol%), proline (8 mol%) and K3PO4 (2
equiv.) in dimethylsulfoxide (DMSO) at 135–140°C affords the
coupled product in 75% GC yield after 7 h. However, methionine
shows a better result under the same reaction conditions, affording
93% GC yield of the desired product. We also investigated the
efficiency of methionine (8 mol%) as the ligand in this transforma-
tion using 4-iodoanisole substrate and employing CuI (10 mol%)
and K2CO3 (3 equiv.) in DMF at 100°C. The corresponding
Sonogashira product is obtained in 10% yield after 7 h and in
65% yield after 15 h.
The accelerating effect of amino acid ligands in the coupling
of aryl bromides with phenylacetylene was also investigated using
the coupling of 4-bromonitrobenzene and phenylacetylene as
a model reaction. As evident from Table 2, all amino acids that
we chose can prompt the coupling reaction by employing
CuI (10 mol%) and K3PO4 (2 equiv) in DMSO at 140°C. Here,
similar to the case of aryl iodides, methionine and proline give
better conversions than phenylalanine. Based on these results,
we applied methionine as the promoter for the CuI-catalyzed
Sonogashira coupling of various aryl halides.
In order to optimize the reaction conditions, we then examined
the effect of different reaction parameters such as solvent, temper-
ature and amino acid concentration on yields of the coupling prod-
ucts as summarized in Tables 1 and 2. The data show that the best
results are obtained using K3PO4 as the base at 135–140°C. It is
noteworthy that the solvent system plays an important role in this
reaction. Among the various solvents tested, such as DMF, DMSO,
poly(ethylene glycol) (PEG), N-methyl-2-pyrrolidone (NMP) and
DMF–H2O (8:2), DMSO gives the best result. However, for some aryl
halide substrates in this reaction system, DMF gives better results
(Table 3, entries 6 and 14). The concentration of methionine was
also examined in this reaction system. Amounts of 8 mol% for aryl
iodide substrates and 15 mol% for aryl bromides are obtained as
the optimum amounts.
Results and discussion
Here, the efficiency of CuI/amino acid has been examined as a cat-
alytic system for the reaction of aryl halides with phenylacetylene.
Initially, we chose the reaction of 4-iodonitrobenzene with
phenylacetylene as a model to begin this investigation (Table 1).
It is found that, in the absence of supporting ligand, the
reaction gives 1,4-diphenylbuta-1,3-diyne as the main product
from homocoupling of phenylacetylene through a Glaser-type
reaction.[60] Many research groups have reported the use of
amino acids as promoter ligands in metal-catalyzed coupling
reactions,[16,19] especially for copper-based catalytic systems. Ac-
cordingly, we examined the efficiency of using CuI catalyst in com-
bination with amino acid ligands for this reaction system.
After optimizing the reaction conditions, we then explored the
scope and limitations of this transformation (Table 3), and the
CuI/amino acid catalytic system was applied in the Sonogashira
coupling of various aryl halides with phenylacetylene. The corre-
sponding internal alkynes are produced in moderate to excellent
yields. In this reaction system, the effect of solvent on the resulting
yields and conversion times of the reactions is important to note. In
most cases, DMSO is an effective medium for the coupling reactions
(Table 3, entries 1, 2, 4, 10, 12). However, for some substrates
(Table 3, entries 6, 14, 15), DMF gives better results. For example,
Scheme 1. Structure of amino acids examined in this study.
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Copyright © 2015 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2015, 29, 787–792

Copper-catalyzed Sonogashira cross-coupling reactions
Table 1. Optimization of reaction conditions for Sonogashira couplinga
Entry
Solvent
Base
Ligand
T
Yield (%)b
(2 equiv.)
(°C)
1
2
3
4
5
6
7
8
9
10
DMF–H2O (80:20)
DMF–H2O (80:20)
DMF–H2O (80:20)
DMF–H2O (80:20)
DMF–H2O (80:20)
DMF–H2O (80:20)
DMSO
K3PO4
L-Proline (30 mol%)
130
130
130
130
130
130
140
140
140
100
67
83
46
82
55
78
95
50
43
12
K3PO4
L-Methionine (30 mol%)
L-Phenylalanine (30 mol%)
L-Methionine (30 mol%)
L-Methionine (30 mol%)
L-Methionine (15 mol%)
L-Methionine (8 mol%)
L-Methionine (8 mol%)
L-Methionine (8 mol%)
L-Methionine (8 mol%)
K3PO4
K2CO3
KOH
K2CO3
K3PO4
DMF
K3PO4
DMF–DMSO (50:50)
DMF
K3PO4
K2CO3 (3 equiv.)
aReaction conditions: 4-nitroiodobenzene (0.1 mmol), phenylacetylene (0.12 mmol), reaction time: 3 h.
bGC yield.
Table 2. Optimization of reaction conditions for Sonogashira couplinga
Entry
Solvent
DMSO
Base (2 equiv.)
Ligand
L-Proline
CuI (mol%)
T (°C)
Yield (%)b
1
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K2CO3
K3PO4
K3PO4
K3PO4
K3PO4
10
10
10
10
10
10
10
10
10
10
5
140
140
140
140
140
140
140
130
130
130
140
90
90
2
DMSO
L-Methionine
L-Phenylalanine
L-Proline
3
DMSO
78
4
DMF
45
5
NMP
L-Proline
15
6
PEG
L-Proline
-
7
DMSO
L-Methionine
L-Methionine
L-Proline
50
8
DMSO
25
9
DMF–H2O (80:20)
DMF–H2O (80:20)
DMSO
Trace
Trace
85
10
11
L-Methionine
L-Methionine
aReaction conditions: 4-nitrobromobenzene (0.1 mmol), phenylacetylene (0.12 mmol), ligand (15 mol%), reaction time: 1 h.
bGC yield.
with 4′-iodoacetophenone as the substrate in DMSO solvent only a
trace amount of the corresponding Sonogashira product is
afforded (Table 3, entry 5); however, the use of DMF as the solvent
system, under the same reaction conditions, increases the yield of
the coupled product strongly (up to 91%).
We examined the electronic and steric effects on the yields and
reaction times of the couplings. This catalytic system is compatible
with a wide range of functional groups such as nitro, cyano,
methoxy and carbonyl on the aryl halides. Electron-poor aryl ha-
lides transform to the corresponding coupled products rather than
electron-rich aryl halides with better conversion and shorter reac-
tion times. The effects of steric hindrance of the procedure were ex-
amined using 2-iodonitrobenzene (Table 3, entry 8). An increasing
hindrance in the vicinity of the leaving group results in a decrease
in the conversion. As evident from Table 3, most aryl iodides are
successfully converted to the corresponding Sonogashira products
in excellent yields. Aryl bromides also react with phenylacetylene to
afford the corresponding alkyne products (Table 3, entries 12–15).
However, the reactivity of aryl bromides is lower than that of aryl
iodides and they require longer times giving lower yields. Aryl chlo-
rides are inactive in this reaction system.
For the Cu(I)/α-amino acid catalytic system, an accelerating
effect is induced by the structure of the α-amino acid in Cu-
catalyzed coupling reactions. It is well known that copper ions
can form chelates with amino acids through carboxyl and amino
groups.[19] The ability of amino acids to promote coupling reac-
tions might be dependent on their coordination ability as
bidentate additives.
To date, several mechanisms for copper-assisted Sonogashira
coupling reactions have been described in the literature.
Appl. Organometal. Chem. 2015, 29, 787–792
Copyright © 2015 John Wiley & Sons, Ltd.
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A. R. Hajipour and F. Mohammadsaleh
Table 3. Sonogashira coupling reactions of aryl halides with phenylacetylenea
Entry
Ar–X
Solvent
Yield (%)b
Time (h)
1
p-O2N–Ph–I
p-MeO–Ph–I
p-MeO–Ph–I
p-NC–Ph–I
DMSO
DMSO
DMF
2
92
90
2
7
15
2
3
75
4
DMSO
DMSO
DMF
98
5
p-MeOC–Ph–I
p-MeOC–Ph–I
m-O2N–Ph–I
o-O2N–Ph–I
p-Me–Ph–I
10
6
Trace
91
6
7
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMF
1.5
98
8
2
20
4
20
9
65
10c
11
12
13d
14
15
16
17e
18
p-I–Ph–I
99
Ph–I
7
80
m-Me–Ph–I
m-Br–Ph–I
15
7
70
81
p-O2N–Ph–Br
p-MeOC–Ph–Br
p-MeOC–Ph–Br
p-NC–Ph–Br
p-O2N–Ph–Cl
1
80
15
15
5
Trace
50
DMF
80
DMSO
10
Trace
aReaction conditions: aryl halide (0.5 mmol), phenylacetylene (0.52 mmol), K3PO4 (1 mmol), methionine (8 mol% for Ar–I and 15 mol% for Ar–Br), solvent,
CuI (10 mol%), nitrogen atmosphere.
bIsolated yield.
c1.2 mmol phenylacetylene was used for 0.5 mmol aryl halide. Final product was 1,4-bis(phenylethynyl)benzene.
d1.2 mmol pPhenylacetylene was used for 0.5 mmol aryl halide. Selected product was 1,3-bis(phenylethynyl)benzene.
eProline (20 mol%) was used as the ligand.
Taillefer and co-workers[52] have reported a mechanism of Cu-
catalyzed Sonogashira coupling in the presence of β-diketone
ligand. They proposed that the resting state Cu(I)–diketone
complex reacts with alkyne in the presence of base to generate
a Cu(I)–acetylide intermediate. This then would react by oxida-
tive addition with the aryl halide to form a four-coordinated Cu
(III) complex which undergoes reductive elimination to expel
the coupled product. Miura and co-workers also suggested a
Cu(I)/Cu(III) mechanistic pathway for copper-catalyzed coupling
reactions.[61]
makes Cu(I) species more reactive toward the oxidative addi-
tion to form a copper(III) complex or stabilizes this intermedi-
ate, promoting the coupling reaction.
According to the mechanism of Bolm and co-workers,[50] this
reaction may also proceed by a concerted breaking of the aryl
halide bond and formation of a new C–C bond, giving the coupled
product.
There is less experimental evidence for mechanisms of aryl halide
activation in Cu(I)-catalyzed coupling reactions. Whether a Cu(III)
complex is ever formed, or the product is formed directly, is under
investigation in mechanistic studies.[63]
As previously mentioned, among the tested amino acids as
promoters in this reaction system (L-phenylalanine, L-proline and
L-methionine), methionine gives the best results. Methionine is a
sulfur-containing amino acid. The greater ability of this amino acid
to promote coupling reactions might be dependent on the pres-
ence of thioether sulfur donors in its structure. Methionine addition
to the N and O donor groups[64] using sulfur groups can also be
chelated, which may lead to more active and stable Cu(I) species
in the reaction medium (Scheme 2).
Bolm and co-workers[50] investigated the coupling reaction
between aryl iodides and terminal arylalkynes using a copper
catalyst being accelerated by diamine ligands at 135°C. They
proposed a mechanism for this coupling reaction based on
kinetic measurements and density functional theory calcula-
tions. That group suggested that the reaction proceeds by
concerted C–I bond dissociation and new C–C bond forma-
tion, and that this is the rate-limiting step. They believed
that in this pathway for the coupled-product formation, there
is no stable Cu(III) intermediate. A similar mechanism was sug-
gested by Sagadevan and Hwang[62] for the Sonogashira C–C
coupling reaction using copper(I) chloride catalyst under blue
light-emitting diode light irradiation at room temperature.
Based on these reports, two mechanisms can be put forward
for our Cu(I)-catalyzed amino acid-promoted Sonogashira
coupling reactions. In the oxidative addition/ reductive elimi-
nation pathway, involving a four-coordinate Cu(III) intermedi-
ate, the complex formation of Cu(I) with an amino acid
Scheme 2. Possible complexations of methionine and copper.
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Copyright © 2015 John Wiley & Sons, Ltd. Appl. Organometal. Chem. 2015, 29, 787–792

Copper-catalyzed Sonogashira cross-coupling reactions
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Aryl halide (0.5 mmol), K3PO4 (1 mmol), methionine (8 mol%,
0.006 g for Ar–I; 15 mol%, 0.011 g for Ar–Br), CuI (10 mol%, 0.01 g)
and phenylacetylene (0.52 mmol) in DMSO or DMF were intro-
duced into a round-bottom flask equipped with a stirring bar and
a reflux condenser under a nitrogen atmosphere. The mixture was
stirred at 135–140°C and the reaction progress was monitored
using TLC and GC. After the reaction was complete, or when the
progress of the reaction had stopped, the cooled resulting mixture
was diluted by adding EtOAc–H2O. The organic layer was sepa-
rated, and the aqueous layer was extracted with EtOAc. The com-
bined organic layers were dried over anhydrous CaCl2, filtered
and concentrated to give the corresponding product, which could
be purified using column chromatography (hexane–EtOAc). The
arylalkyne products were known compounds[52,65,66] and were
1
characterized using Fourier transform infrared, H NMR and 13C
NMR spectroscopies.
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We gratefully acknowledge the funding support received for this
project from the Isfahan University of Technology (IUT), IR Iran,
and Isfahan Science and Technology Town (ISTT), IR Iran. Further fi-
nancial support from the Center of Excellence in Sensor and Green
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