Sub-Mol % catalyst loading and ligand-acceleration in the coppercatalyzed coupling of aryl Iodides and terminal alkyenes

Article abstract of DOI:10.1002/chem.201000344

(Figure Presented) Just add ligand! The coupling of aryl iodides and terminal alkynes can be catalyzed with sub-mol % amounts of a copper complex. Addition of a large excess of diamine ligand converts the inactive polymeric resting state of the catalyst i

Full text of DOI:10.1002/chem.201000344

COMMUNICATION
DOI: 10.1002/chem.201000344
Sub-Mol% Catalyst Loading and Ligand-Acceleration in the Copper-
Catalyzed Coupling of Aryl Iodides and Terminal Alkyenes
Erik Zuidema and Carsten Bolm*[a]
Aryl alkynes and related conjugated enynes are important
precursors for the synthesis of pharmaceuticals, natural
products, and organic materials.[1] They can conveniently be
produced from terminal alkynes and aryl or vinyl halides or
triflates by the well-known palladium-copper co-catalyzed
Sonogashira–Hagihara reaction.[2] Motivated by the high
price and toxicity of palladium, there has been considerable
effort to minimize the amount of this metal in the reaction
mixture.[3] Already a decade before the discovery of the So-
nogashira–Hagihara protocol,[2a] stoichiometric couplings be-
tween aryl iodides and copper acetylides, in the absence of
palladium, had been reported.[4] The subsequently devel-
oped copper-catalyzed versions of this so-called Castro–Ste-
phens reaction initially involved complexes with phosphane
ligands.[5] Later, copper catalysts with more stable and readi-
ly available nitrogen-[6] or oxygen-based ligands,[7] as well as
unsupported catalysts[8] and copper nanoparticles were intro-
duced for this process.[9] Compared to the palladium- or pal-
ladium/copper-catalyzed versions, these protocols have sig-
nificant drawbacks. First, copper also catalyzes the oxidative
homocoupling of terminal alkynes (the so-called Glaser re-
action)[10] leading to lower yields of the desired cross-cou-
pling products and complicating their purifications. Second,
copper loadings of more than 5 mol% and high reaction
temperatures are generally required to obtain acceptable
yields. Although copper is a cheap and abundant metal,
such high catalyst loadings lead to other problems. Apart
from a more difficult product purification, large amounts of
copper(I) acetylides will be formed, which, depending on
the acetylene used, can be highly explosive.
ed to investigate a possible use of low copper concentrations
in Castro–Stephens couplings of aryl halides and terminal al-
kynes. As a catalyst precursor, the air- and moisture-stable
ACTHNUTRGNEUNG
CuII complex [Cu(DMEDA)2]Cl2·H2O (1)[12] was chosen
(DMEDA= N,N’-dimethylethylenediamine). To minimize
contamination with trace amounts of other catalytically
active metals, this compound was synthesized starting from
99.995% pure CuCl2 and 99%+ pure DMEDA using
HPLC-grade solvents. The remaining metal impurities were
determined by ICP-MS, which showed that the resulting
complex contained 8 ppm of Mn, 1 ppm of Ni, and less than
4 ppb of Pd.
For the initial screening, the reaction between iodoben-
zene and phenylacetylene was selected (Table 1). Based on
previous work on similar catalyst systems,[6a] a combination
Table 1. The coupling of phenylacetylene and iodobenzene: Screening of
catalyst and ligand concentrations.[a]
Entry
x
y
Solvent
Yield [%] (GC)
3 h 22 h
[mol%]
ACHTUNGTRNE[NUNG mol%]
1
2
3
4
5
6
–
–
1
0.5
0.5
0.5
–
30
–
–
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
DMEDA
0
0
4
0
0
20
14
>99
>99
3
30
48
>99
1ꢀ104
[a] Performed in closed vials at 1358C.
Inspired by the recent findings on catalyzed cross-cou-
pling reactions with ppm-level copper loadings,[11] we decid-
of (distilled) 1,4-dioxane as solvent and Cs2CO3 (99.9%+)
as base was chosen. The reactions were performed by using
1 mmol of phenyl acetylene and 1.5 mmol of iodobenzene in
1 mL of solvent, under an argon atmosphere. Glaser-type
homocoupling products were not observed under these con-
ditions. No reaction occurred in the absence of copper com-
plex 1 (Table 1, entries 1 and 2). This demonstrated the es-
[a] Dr. E. Zuidema, Prof. Dr. C. Bolm
Institute for Organic Chemistry, RWTH Aachen University
Landoltweg 1, 52056 Aachen (Germany)
Fax : (+49)241-8092-391
E-mail: Carsten.Bolm@oc.rwth-aachen.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201000344.
Chem. Eur. J. 2010, 16, 4181 – 4185
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4181

sential role of the copper cata-
lyst and revealed that any ppm-
level amounts of metals present
in the base, substrates, solvent,
and ligand did not catalyze the
reaction.
With 0.5 or 1 mol% of the
catalyst the yield of the cou-
pling product was low even
after 22 h. Because previous
À
studies in C X cross-coupling
reactions had shown that the ef-
ficiency of the catalysis was af-
fected by the ligand concentra-
tion,[11,13] the reaction was per-
formed in the presence of
30 mol%
of
additional
DMEDA (60 equiv relative to
1). To our delight, the product
was obtained in 48% and quan-
titative yield after 3 and 22 h,
respectively,
0.5 mol%
using
complex
only
of
1
(Table 1, entry 5). This im-
provement corresponded to a
more than twelve-fold increase
in reaction rate. Without
copper, the presence of ligand
did not lead to the formation of
*
Figure 1. Kinetic measurements on the coupling between iodobenzene and phenylacetylene in 1,4-dioxane (
)
*
and toluene ( ) at 1358C. Dependence of the reaction rate on the iodobenzene (A), phenylacetylene (B),
copper (C), and DMEDA (D) concentration.
diphenylalkyne (Table 1, entry 2). Conceptionally, these ob-
servations indicate that this is a prime example of ligand-ac-
celerated catalysis (LAC).[14]
appeared to be independent of the alkyne concentration
over a concentration range of 0.2–1.6 molLÀ1 (Figure 1, B).
The dependence of the reaction rate on the amount of
copper in the reaction mixture was evaluated at copper con-
centrations of 0–6.3 mmolLÀ1 (0–1 mol% of 1 relative to
alkyne), using the previously described reaction conditions.
In toluene, the reaction rate was first-order dependent on
the copper concentration at very low copper loadings, and
above 0.8 mmolLÀ1 (0.11 mol%), it became independent of
the copper concentration. In dioxane, the reaction rate ex-
hibited a non-linear rate-dependence on the copper concen-
tration over the range studied here (Figure 1, C). The effect
of the ligand on the reaction rate was studied at ligand con-
centrations between 0–1.5 molLÀ1 (0–450 mol% relative to
the alkyne), using the reaction conditions described before.
In the absence of additional ligand, the rate of the reaction
was very low in both solvents (0.008 molLÀ1 product hÀ1 in
dioxane, and 0.003 molLÀ1 product hÀ1 in toluene). The
ligand had a profound accelerating effect (Figure 1, D). In
toluene, a first-order rate dependence was observed over
the entire concentration range. In dioxane, the reaction was
first-order dependent on the concentration of the ligand up
to a concentration of 0.7 molLÀ1, which corresponded to a
ligand-to-metal ratio of 200:1 and a ligand-to-alkyne ratio of
1:1. At higher concentrations, the rate dependence started
to deviate from first order.
Using these reaction conditions, several other base/solvent
combinations that are common in copper-catalyzed cross-
coupling reactions were then investigated (see Table S1 in
the Supporting information). Also relatively mild and cheap
inorganic bases such as K2CO3 and K3PO4 proved highly ef-
fective. In contrast, neither KOH, KOtBu, NaOAc nor, to
our surprise, NEt3 gave the coupling product. As solvent,
1,2-dimethoxyethane and toluene could also be employed,
whereas DMF, DMSO, and 1,2-dichoroethane proved un-
suitable. Although both reactants and the product were in-
soluble in water, the reaction did proceed in its presence.
Performing the reaction in DMEDA, a liquid with a boiling
point of 1188C, had a highly beneficial effect, leading to
complete conversion within 3 h (Table 1, entry 6). This pro-
vided further evidence of the significant rate-enhancement
by this diamine.
To gain more insight into this ligand-accelerated process,
kinetic studies were performed and the dependence of the
reaction rate on the concentrations of the various reaction
components was investigated (Figure 1). In both 1,4-dioxane
and toluene (using 0.005 mmol of 1 and 0.3 mmol of
DMEDA in 1 mL of solvent) the reaction rate showed a
first-order rate dependence on the iodobenzene concentra-
tion over a range of 0–2 molLÀ1 (Figure 1, A). Despite
larger fluctuations in the measurements, the reaction rate
These kinetic measurements shed light on the mechanism
of the reaction. The observed order in copper suggested that
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Chem. Eur. J. 2010, 16, 4181 – 4185

Copper-Catalyzed Coupling of Aryl Iodides and Terminal Alkyenes
the resting state of the catalyst
COMMUNICATION
was
a dimeric or polymeric
complex that was in equilibrium
with an active monomeric cata-
lyst.[15,16] The first order in both
iodobenzene and DMEDA
demonstrated that these two re-
agents were involved early in
the catalytic cycle leading from
the catalyst resting state up to
and including the rate-deter-
mining step. Taking into ac-
count the partial order in
copper, the initial steps in the
Scheme 1. The proposed reaction mechanism for the catalytic Castro–Stephens reaction (black) and the well-
established catalytic cycle for the palladium-copper co-catalyzed Sonogashira–Hagihara reaction (gray); with
M=Cs in this study.
catalytic cycle were likely to involve first, a reversible coor-
dination of the ligand to the polymeric resting state of the
catalyst, converting it into an active monomeric catalyst,
and second, reaction of this monomeric species with iodo-
benzene (Scheme 1). The zeroth order in phenyl acetylene
indicated that this substrate was either already coordinated
to the catalyst in its resting state, or that it was only involved
in the catalytic cycle after the rate-determining step (as it is
the case in the corresponding palladium-catalyzed process).
The nature of the resting state of the catalyst was investi-
gated by performing stoichiometric reactions at 1358C in di-
oxane, systematically omitting components from the reac-
tion mixture. When 1 was combined with phenylacetylene in
the absence of a base, no reaction occurred and 1 was recov-
ered. Heating of 1 in the presence of Cs2CO3, but in the ab-
sence of iodobenzene and phenylacetylene, resulted in a
color change from deep blue to dark green, and the forma-
tion of a black precipitate. A similar behavior was observed
when this reaction was performed in the presence of excess
ligand. When 1 was allowed to react with an over-stoichio-
metric quantity of phenylacetylene in the presence of
Cs2CO3, the reaction mixture rapidly turned bright yellow,
and a yellow precipitate was formed. A similar color change
was observed under catalytic conditions, although there the
concentration of 1 was much lower and the color change
more difficult to detect. The solid was isolated, washed with
water and diethyl ether and then extensively dried. Elemen-
tal analysis revealed that this compound contained no nitro-
gen, and its carbon and hydrogen contents corresponded to
[Cu(phenylacetylene)]n, a known CuI coordination poly-
mer.[5b,17] In the coupling reaction this compound was active
under the previously described conditions.
aryl halide (Scheme 1). With the goal of gaining a deeper in-
sight into the initial steps of the reaction, conversions of sev-
eral electronically different aryl halides were investigated
(Table 2). Reaction rates, expressed in mol product
molÀ1 CuhÀ1, were determined at two different catalyst load-
ings to explore whether all substrates showed the same non-
linear dependence of the reaction rate on the catalyst con-
Table 2. The coupling of aryl acetylenes and aryl iodides: Turnover fre-
quencies at two different catalyst loadings.
R1
Turnover frequencies [hÀ1
]
À
X Ar
Entry
Cu (0.5 mol%)
Cu (0.05 mol%)
1
2
H
H
24
90
29
29
99
3
H
106
4
5
6
H
H
H
33
35
110
115
247
114
7
H
152
631
From these results we deduce the following scenario:
DMEDA acts as reductant of 1, in analogy to other secon-
dary amines that are known to reduce CuII to CuI.[18] Subse-
quently, a CuI acetylide is formed, which upon agglomera-
tion loses its nitrogen ligands. This product is insoluble in
common organic solvents, but it dissolves in pyridine and
DMEDA to give light-green highly oxidation-sensitive solu-
tions. The resting state of the catalyst therefore seems to be
a polynuclear copper–acetylide species, and the role of the
large excess of ligand is to generate a highly electron-rich
monomeric acetylide complex that is able to react with the
8
H
238
310
<1
13
820
9
H
979
10
11
12
13
H
n.d.[a]
n.d.[a]
n.d.[a]
n.d.[a]
OMe
Me
CF3
18
37
[a] n.d. =not determined.
Chem. Eur. J. 2010, 16, 4181 – 4185
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4183

C. Bolm and E. Zuidema
centration as previously observed for iodobenzene. Indeed,
for all substrates the measured turnover frequencies were
approximately three times higher at 0.05 mol% than at
0.5 mol% of copper. This indicated that the large differen-
ces in reaction rate could indeed be attributed to the intrin-
sic reactivities of the aryl halides towards the catalyst, and
were not caused by different amounts of active catalyst in
the reaction mixture.
active catalyst in solution and higher reaction rates/lower re-
action temperatures.
Acknowledgements
This research was supported by the Fonds der Chemischen Industrie and,
in part, by the Cluster of Excellence funded by the Excellence Initiative
of the German federal and state governments. We thank Prof. Dr. W.
Dott, Dr. M. Mçller, and Mr. S. Thorçe (Institut fꢂr Hygiene und Um-
weltmedizin, RWTH Aachen University) for analyses by ICP-MS and ap-
preciate stimulating comments by Prof. Dr. H. J. G. de Vries (DSM) and
Prof. Dr. H. Plenio (TU Darmstadt).
À
As in C N bond-forming reactions catalyzed by similar
catalyst systems,[15a,19] the reaction was accelerated by the
presence of electron-withdrawing substituents on the aryl
iodide. Two substrates fell outside this trend. 4-Fluoroiodo-
benzene (Table 2, entry 1) was less active, while 4-methoxy-
iodobenzene (Table 2, entry 4) was more active than expect-
ed based on the Hammett s values of the two substitu-
ents.[20] In addition, p-NO2 and p-CN substituents on the
arene ring led to extraordinarily high turnover frequencies,
indicative of a significant buildup of negative charge on the
arene during the reaction. Unfortunately, bromobenzene
was not converted under these reaction conditions.
Keywords: alkynes · Castro–Stephens reaction · copper ·
diamines · low catalyst loading · Sonogashira reaction
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Despite the reaction rate being independent of the alkyne
concentration (Figure 1, B), differences were observed when
electronically modified aryl acetylenes were applied
(Table 2, entries 5, 11–13). This behavior indicated that the
coordination of the alkyne to the catalyst took place prior
to the rate-determining step of the reaction, and therefore
the alkyne was already present in the resting state of the
catalyst. The addition of electron-withdrawing substituents
to the alkyne favors the coordination of the donating dia-
mine ligand to the copper–acetylide resting state of the cata-
lyst, leading to a higher concentration of the active mono-
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reaction was indeed catalyzed by copper and not co-cata-
lyzed by any traces of palladium that may be present in the
reaction mixture. In the latter case, the acetylide would only
be transferred to the active palladium catalyst after oxida-
tive addition of the aryl halide to palladium (Scheme 1). If
À
this step or the subsequent C C bond formation would then
determine the overall reaction rate, the kinetic measure-
ments would show a positive order in alkyne. If, on the
other hand, the rate-determining step preceded this trans-
metallation step, the electronic properties of the alkyne
should not influence the overall reaction rate.
In conclusion, we have demonstrated that Castro–Ste-
phens reactions can readily be performed using sub-mol%
copper loadings. The process is a striking example of ligand-
accelerated catalysis, in which the addition of DMEDA
ligand converts the resting state of the catalyst into an un-
precedentedly active monomeric CuI acetylide species. Al-
though a large excess of the ligand is required to activate
the copper catalyst, the relatively low boiling point of
DMEDA allows recovery and recycling of the ligand
through distillation. Further research will be focused on elu-
cidating the exact nature of the rate-determining step of this
process and on developing ligand structures that coordinate
more strongly to CuI acetylide complexes. Based on the pro-
posed mechanism this should lead to increased amounts of
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Chem. Eur. J. 2010, 16, 4181 – 4185

Copper-Catalyzed Coupling of Aryl Iodides and Terminal Alkyenes
COMMUNICATION
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Received: February 9, 2010
Published online: March 18, 2010
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