Solvent-free Sonogashira coupling reaction via high speed ball milling

Article abstract of DOI:10.1039/b915669k

Herein, we report on the solvent-free Sonogashira reaction utilizing high speed ball milling. Sonogashira coupling of a variety of para substituted aryl halides were performed with trimethylsilylacetylene or phenylacetylene. We observed that iodo and bromo substituted aromatics successfully undergo Sonogashira coupling. However, chloro and fluoro substituted aryl compounds were unreactive. Conducting the coupling reaction in the absence of copper iodide led to low yields. Alternately, if the reaction is conducted with a copper ball in a copper vial in lieu of copper iodide, the coupling product is observed in high yields. This demonstrates the first report on the use of the vial and ball material as a catalyst in a ball milled chemical reaction. The Royal Society of Chemistry 2009.

Full text of DOI:10.1039/b915669k

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www.rsc.org/greenchem | Green Chemistry
Solvent-free Sonogashira coupling reaction via high speed ball milling
Dennis A. Fulmer, William C. Shearouse, Shareika T. Medonza and James Mack*
Received 24th June 2009, Accepted 30th July 2009
First published as an Advance Article on the web 27th August 2009
DOI: 10.1039/b915669k
Herein, we report on the solvent-free Sonogashira reaction utilizing high speed ball milling.
Sonogashira coupling of a variety of para substituted aryl halides were performed with
trimethylsilylacetylene or phenylacetylene. We observed that iodo and bromo substituted
aromatics successfully undergo Sonogashira coupling. However, chloro and fluoro substituted aryl
compounds were unreactive. Conducting the coupling reaction in the absence of copper iodide led
to low yields. Alternately, if the reaction is conducted with a copper ball in a copper vial in lieu of
copper iodide, the coupling product is observed in high yields. This demonstrates the first report
on the use of the vial and ball material as a catalyst in a ball milled chemical reaction.
4
2-45
Introduction
ionic liquids.
There have been reports of solvent-free Sono-
gashira reactions, but several of these procedures are run for long
reaction times under high temperature conditions.
Recently, the scientific community has taken a proactive
approach to environmental pollution. Within the area of
organic synthesis, solvent-free reactions have gained widespread
46-48
Other
1,2
drawbacks are that these solvent-less methods only react with
49
aryl iodides or strongly electron withdrawing groups on the
3-10
attention for their ability to significantly reduce solvent waste.
48
aryl ring. To this end, we wanted to investigate the Sonogashira
High-speed ball milling (HSBM) under solvent-free conditions
is a method that has been shown to be a viable alternative to
solution based chemistry. HSBM is a technique that works
on the same principles as a mortar and pestle; specifically,
it uses mechanical energy to convert reactants to products
coupling reaction under HSBM conditions.
Results and discussion
Sonogashira coupling reaction under solvent-free aerobic high
speed ball milling conditions
11-14
during the course of the reaction.
Many reports in the
literature have shown that HSBM is effective for various organic
transformations and for the development of environmentally
Initially, we chose to investigate the coupling of iodobenzene
with both phenyl acetylene and trimethyl silyl acetylene under
HSBM conditions. To a custom-made 1/2¢¢ ¥ 2¢¢ stainless steel
vial, we added iodobenzene, catalytic copper iodide, potassium
carbonate and tetrakistriphenylphosphine palladium catalyst
(Scheme 2). These reactions were carried out under aerobic
conditions in the absence of a solvent; typically, Sonogashira
reactions are carried out under inert atmosphere using dry
solvents.
15-23
benign reactions.
Herein, we report our results on the
Sonogashira reaction under solvent-free high-speed ball milling
conditions.
The Sonogashira reaction (which is an adaptation to the
24,25
Castro–Stephens protocol)
is the coupling between a termi-
nal alkyne and an aryl halide or aryl triflate through the use of
26-29
a palladium catalyst (scheme 1).
This reaction is important
in the synthesis of various molecules critical to the development
30
of natural products, pharmaceuticals and organic materials.
Scheme 1 Sonogashira reaction.
Scheme 2 Solvent-free Sonogashira reaction via high speed ball milling
conditions.
Due to the importance of the Sonogashira reaction there has
been a tremendous amount of research investigating various
types of metal catalysts, solvents, reaction times and tempera-
tures. Additionally, a fair amount of research has been geared
towards making this important reaction more environmentally
benign. There has been research conducting the reaction under
After successfully conducting the Sonogashira reaction under
ball milling conditions, we next turned our attention to optimiz-
ing the catalytic loading, the base, type of palladium catalyst
and versatility of other aryl halides.
Through our analysis, we discovered that using 2.5%
of the catalyst was the lowest we could use that gave
high yields of coupling product after 17 hours of milling.
We substituted tetrakistriphenylphosphine palladium with
trans-dichlorobis(triphenylphosphine)palladium and observed
31-36
37-41
aqueous conditions,
microwave conditions
and using
University of Cincinnati, Cincinnati, OH, USA. E-mail: james.mack@
uc.edu; Fax: +1 513 556 9239; Tel: +1 513 556 9249
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similar catalytic activity; hence, each catalyst under these
conditions afforded very high yields of coupling product. It
has been shown that the activated palladium(0) catalyst can
be prepared from cheaper more readily available palladium
catalysts such as palladium chloride and palladium acetate
solution, iodosubstituted aryl groups were more reactive than
the bromosubstituted analogs. Unfortunately, we did not have
success coupling chloro or fluoro aryl compounds, but typically
these halogens are sluggish coupling partners. We next examined
how dihaloaromatics performed under HSBM conditions. Cou-
pling 1,4-bromoiodobenzene or 1,4-chloroiodobenzene with
trimethylsilylacetylene or phenylacetylene, we observed selective
coupling to the iodo- group to give only a mono coupling
product at the iodo position in very high yields. However,
even when a large excess of the acetylene is used, only the
mono addition product is observed. Furthermore, reacting
1,4-dibromobenzene or 1,4-bromochlorobenzene with the
acetylene derivatives gives selective coupling to the bromo
substituted carbon but the yield dramatically decreases, and
again using excess acetylene does not provide high yields of the
disubstituted coupled product. We are currently in the process
of trying to optimize these conditions for dihalogenated aryl
systems to improve the yields of the disubstituted product.
50-56
with the addition of free ligand.
When we used palladium
acetate as the catalyst and free triphenylphosphine, the reac-
tion did not give the coupling product even after extended
milling times.
The size and ball material has often affected the results of
these ball milled reactions. We examined 1/8¢¢ and 1/4¢¢ stainless
steel, brass, bronze, aluminium, tungsten carbide and aluminium
oxide balls in this reaction. Our best yield in the shortest reaction
time occurred when a 1/4¢¢ tungsten carbide ball was used. We
were able to couple the reagents in high yields in a reaction
time of approximately 17 hours. Many of the reactions could
be completed in shorter time durations, however, for the sake
of comparison each reaction was carried out for a 17 hour time
period.
After completing the optimization of the iodobenzene system,
we went on to study the Sonogashira of a variety of haloaro-
matics using trimethylsilyl acetylene and phenylacetylene as
coupling partners (Table 1). Our investigation found that a wide
variety of iodo and bromo aryl compounds could be coupled to
both trimethylsilylacetylene and phenyl acetylene, as indicated
in Table 1. Various functional groups were tolerated; however,
benzyl alcohols and phenols did not provide the coupling
product. In most cases there was little difference in yield coupling
to either trimethylsilyl or phenyl acetylene, which suggests
that the Sonogashira coupling under HSBM conditions should
tolerate a wide range of alkynes. Consistent with the reaction in
Copper-free Sonogashira
One area of Sonogashira reactions that has been the focus
of extensive research has been the use of copper-free reaction
conditions. Copper iodide is needed to activate the alkyne to
increase the rate of coupling. One major drawback to the use
of this co-catalyst is that it increases the amount of alkyne-
alkyne coupling (i.e. Glaser coupling). We wanted to determine
if Sonogashira coupling reactions would be successful under
ball milling conditions without the addition of copper iodide.
When copper iodide was omitted as the co-catalyst, the yield
of the Sonogashira coupling was significantly lower than when
copper iodide is used as the co-catalyst for all examples, except
for the very reactive iodobenzene (Table 2). Although we
had success under copper-free conditions, typically the yields
were lower than when copper iodide was used. It has been
previously reported that there are trace amounts of copper inside
Table 1 Sonogashira reactions of p-substituted benzene with
trimethylsilylacetylene and phenylacetylene
57
commercial sources of palladium catalyst. Therefore, it is very
possible that these “copper-free” reactions aren’t truly copper-
free and the low yield could be attributed to the minute traces
of copper in the tetrakistriphenylphosphine palladium we used
in the reaction.
a
a
X
R
Percent Yield R¢ = Ph
Percent Yield R¢ = TMS
I
I
I
I
I
I
I
OMe
Me
H
Br
Cl
86
84
95
88
87
84
40
93
76
74c
20
Table 2 “Copper-free” Sonogashira reactions of p-substituted benzene
with trimethylsilylacetylene
b
I
I
trace
d
e
f
39
30
Br
Br
Br
Br
Br
Br
Br
Br
Br
OMe
Me
H
Cl
CHO
CHO
trace
33
93
7
trace
26
90
4
a
a
89
90
85
9
8
87
89
66
5
3
X
R
Percent Yield w/CuI
Percent Yield “copper-free”
NO
Br
Br
2
I
I
I
I
I
Br
Br
H
Me
Br
Cl
OMe
CHO
CHO
95
84
88
87
84
87
89
39
17
43
37
58
44
46
g
h
i
h
h
a
b
c
Reported as isolated yields. 1,2 substituted. Isolated 43% disub-
d
e
stituted product. Isolated 8% disubstituted product. Isolated 63%
disubstituted product. Isolated 31% of disubstituted product. Isolated
f
g
b
h
4
1
8% disubstituted product. No disubstituted product was observed.
,2 substituted.
i
a
b
Reported as isolated yields. 1,2 disubstituted.
1
822 | Green Chem., 2009, 11, 1821–1825
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Table 3 Sonogashira reactions of p-substituted benzene with
trimethylsilylacetylene using a 3/32¢¢ copper ball in a stainless steel vial
Table 4 Sonogashira reactions of p-substituted benzene with
trimethylsilylacetylene using a 3/32¢¢ copper ball in a copper vial
a
a
a
X
R
Percent Yield w/CuI
Percent Yield w/Cu ball
Percent Yield w/Cu ball
and Cu vial
a
X
R
Percent Yield w/CuI
I
I
I
I
I
Br
Br
H
Me
Br
Cl
OMe
CHOb
CHO
95
84
88
87
84
87
89
87
46
31
52
40
85
88
I
I
I
I
I
Br
Br
H
Me
Br
Cl
OMe
CHO
CHO
95
84
88
87
84
87
89
88
83
89
86
42
84
90
b
a
b
Reported as isolated yields. 1,2 disubstituted.
a
b
Reported as isolated yields. 1,2 disubstituted.
Using the ball and vial as a source of copper
show that the copper vial was responsible for the increased
reactivity. We machined a 1/2¢¢ ¥ 2¢¢ Teflon vial and conducted
the “copper-free” Sonogashira reaction. Under these conditions,
the reaction gave low yields similar to the reaction without the
inclusion of copper iodide. This further demonstrates that the
copper vial is the source of the copper co-catalyst in the reaction.
We are currently investigating the surface of the copper vial to
determine the catalytic species.
Although “copper-free” conditions were successful under
HSBM conditions, the yields were low. Therefore, the use of a
copper co-catalyst is an important aspect of the reaction under
ballmillingconditions. We decided to use acopper ballas the ball
bearing usedinthereaction. The copper ballnotonlycan be used
to break down the crystal lattice, but also as the reacting catalyst.
58,59
Metal leaching in ball milling reactions is not uncommon;
therefore, if small amounts of copper were introduced into the
reaction environment, it should increase the product yields.
Furthermore, the recovery of the catalyst, in this case the copper
ball, would be as simple as removing the copper ball from
the reaction media. Using a 3/32¢¢ copper ball bearing with
a stainless steel vial, we repeated the Sonogashira reaction on
various substrates. Using the copper ball as the catalyst resulted
in low yields, similar to the reaction conducted in an absence of
copper iodide (Table 3). It has been calculated that using the Spex
Conclusions
In conclusion, we have shown the first Sonogashira coupling
reaction under solvent-free ball milling conditions. Under
these conditions, iodo and bromo aromatics make very good
coupling partners but we had little success with chloro or fluoro
aryl compounds. We investigated the reaction under aerobic
conditions, where traditionally an inert atmosphere is used for
these reactions. Conducting the reaction in the absence of copper
iodide led to moderate yields of the coupling product. However,
substituting the copper iodide and conducting the reaction using
a copper ball and a copper vial allows yields similar to the
use of copper iodide. This demonstrates a novel use of the vial
material as a catalyst for the reaction. Ball milling is a powerful
method that can allow for the formation of environmentally
benign reactions. Further understanding of the chemistry in this
process can lead to a paradigm shift in the way chemists conduct
organic reactions.
60
8
000 M mill there are approximately 142 impacts per second.
This led us to believe that there weren’t enough contacts between
the copper ball and the alkyne. Thus, we decided to machine a
copper vial which would give more surface area for the copper
and presumably be better at activating the alkyne to successfully
undergo the Sonogashira coupling reaction.
We machined a 1/2¢¢ ¥ 2¢¢ screw-capped copper vial to be
used in conjunction with a 3/32¢¢ copper ball bearing. Using
various haloaromatics, we were able to perform ball milled
Sonogashira reactions. Implementing the same conditions and
examining the same substrates, we were able to undergo the
coupling in very high yields (Table 4); yields which equal the
results with copper iodide as co-catalyst under these conditions.
To the best of our knowledge, this represents the first report of
ball milling which uses the ball and vial material as a catalyst
Experimental
General
61
1
for conducting chemical reactions. At the conclusion of each
reaction, the vials and ball bearing were weighed and we noticed
no significant change in mass of either the ball bearing or the
vial. After each use, the copper vial was washed with a small
amount of acetone to ensure a clean reaction surface for the
next reaction. After continual use of the copper vial to conduct
Sonogashira reactions, we observed no depreciation in the yield
of the reactions after using it for more than a month.
H NMR spectra were obtained using a Bruker Avance
400 MHz spectrometer. Deuterated chloroform was obtained
from Cambridge Isotope Laboratories, Inc., Andover, MA.,
and used without further purification. GC-MS data was
obtained using a Hewlett-Packard 6890 series GC-MS with
a Zebron ZB-5, 15 mm ¥ 0.25 mm ¥ 0.25 mm column. All
aryl halide reagents and terminal alkynes were purchased
from Acros Organics and used without further purification.
Copper iodide, palladium(II)chloride, palladium(II)acetate,
All the Sonogashira reactions we investigated were conducted
in either a stainless steel or a copper vial. We wanted to further
trans-dichlorobis(triphenylphosphine)palladium(II),
and
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tetrakis(triphenylphosphine)palladium(0) were purchased from
Strem Chemicals, Inc. and used without further purification.
Potassium carbonate was purchased from Acros Organics.
Tungsten carbide ball-bearings were purchased from Small
Parts Inc. and copper ball-bearings were purchased from
McMaster-Carr Supply. Simriz 486 Perfluoroelastomer O-rings
0.99 mmol), and tetrakis(triphenylphosphine)palladium(0)
(0.028 g, 0.025 mmol) were added to a custom-made 2.0 ¥
0.5 inch screw capped stainless steel vial with inserted per-
fluoroelastomer O-ring, along with a 3/32¢¢ inch copper ball
bearing. After placement of the vial in a Spex Certiprep
8000D mixer/mill, the reagents were ball milled for 17 hours.
The resulting mixture was removed from the vial and the
organic layer was extracted with ethyl acetate using a sep-
aratory funnel and dried over anhydrous magnesium sulfate.
The organic layer was removed under reduced pressure to the
crude product. The crude product was purified by column
chromatography using a cyclohexane/ethyl acetate eluent to
afford trimethylsilylphenylacetylene in 87% yield (0.148 g,
0.085 moles).
(
6/16¢¢ ID ¥ 7/16¢¢ OD ¥ 3/32¢¢ width) were purchased from
Small Parts Inc. The copper rods used to make copper vials
were purchased from McMaster-Carr Supply. The Teflon rods
used to make Teflon vials were purchased from McMaster-Carr
Supply. The stainless steel rods used to make stainless steel vials
were purchased from McMaster-Carr Supply. All measurements
for the vials refer to the inner diameter and the measurements
for the ball bearings refer to its outer diameter. Ball-milling was
carried out in an 8000M SpexCertiprep Mixer/Mill purchased
from Spex certiprep.
Typical procedure for copper ball catalyzed coupling of aryl
halides with trimethylsilyl acetylene
Typical procedure for the coupling of aryl halides with
trimethylsilyl acetylene
Iodobenzene (0.200 g, 0.98 mmol), trimethylsilyl acety-
lene (0.103 g, 1.05 mmol), potassium carbonate (0.137 g,
In all cases, the aryl halide was used as the limiting
reagent. Iodobenzene (0.200 g, 0.98 mmol), trimethylsilyl acety-
lene (0.103 g, 1.05 mmol), potassium carbonate (0.137 g,
0
.99 mmol), and tetrakis(triphenylphosphine)palladium(0)
(0.028 g, 0.025 mmol) were added to a custom-made 2.0 ¥ 0.5
inch screw capped copper vial with inserted perfluoroelastomer
O-ring, along with a 3/32¢¢ inch copper ball bearing. After
placement of the vial in a Spex Certiprep 8000D mixer/mill, the
reagents were ball milled for 17 hours. The resulting mixture was
removed from the vial and the organic layer was extracted with
ethyl acetate using a separatory funnel and dried over anhydrous
magnesium sulfate. The organic layer was removed under
reduced pressure to the crude product. The crude product was
purified by column chromatography using a cyclohexane/ethyl
acetate eluent to afford trimethylsilylphenylacetylene in 88%
yield (0.150 g, 0.086 moles).
0
.99 mmol), copper iodide (0.002 g, 0.01 mmol), and
tetrakis(triphenylphosphine)palladium(0) (0.028 g, 0.025 mmol)
were added to a custom-made 2.0 ¥ 0.5 inch screw capped
stainless steel vial inserted with a perfluoroelastomer O-ring,
along with a 0.250 inch tungsten carbide ball bearing. After
placement of the vial in a Spex Certiprep 8000M mixer/mill,
the reagents were ball milled for 17 hours. The resulting mixture
was removed from the vial and the organic layer was extracted
with ethyl acetate using a separatory funnel and dried over
anhydrous magnesium sulfate. The organic layer was removed
under reduced pressure to give trimethylsilylphenylacetylene in
9
5% yield (0.162 g, 0.093 moles).
Acknowledgements
Typical procedure for the copper-free coupling of aryl halides
with trimethylsilyl acetylene
We thank the National Science Foundation (CHE-0548150) and
the URC at the University of Cincinnati for financial support
of this research. We would like to give a special thanks to Doug
Hurd for assistance with the making of the reaction vials.
Iodobenzene (0.200 g, 0.98 mmol), trimethylsilyl acety-
lene (0.103 g, 1.05 mmol), potassium carbonate (0.137 g,
0
(
0
.99 mmol), and tetrakis(triphenylphosphine)palladium(0)
0.028 g, 0.025 mmol) were added to a custom-made 2.0 ¥
.5 inch screw capped stainless steel vial with an inserted
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