Ethanol-promoted reductive homocoupling reactions of aryl halides catalyzed by palladium on carbon (Pd/C)

Article abstract of DOI:10.1002/aoc.1635

Homocoupling reactions of aryl bromides or iodides proceeded smoothlywith palladium on carbon (Pd/C) catalyst, ethanol and base in dimethyl sulfoxide (DMSO) to afford exclusively symmetric biaryls in good to excellent yields. Ethanol was first used as a reducing agent in situ to reduce the Pd ²⁺/C species into Pd⁰/C active species to complete the catalytic redox cycle. It was found that ethanol can promote the Pd/C-catalyzed reductive homocoupling of aryl iodides and bromides efficiently in the presence of base. A reaction mechanism has been put forward and discussed. Copyright

Full text of DOI:10.1002/aoc.1635

Full Paper
Received: 22 September 2009
Revised: 13 December 2009
Accepted: 22 January 2010
Published online in Wiley Interscience: 24 February 2010
(www.interscience.com) DOI 10.1002/aoc.1635
Ethanol-promoted reductive homocoupling
reactions of aryl halides catalyzed by palladium
on carbon (Pd/C)
Linjun Shaoa, Yijun Dub, Minfeng Zenga, Xiudong Lia, Wenting Shena,
Shufeng Zuoa, Yueqing Lua, Xian-Man Zhanga and Chenze Qia∗
Homocoupling reactions of aryl bromides or iodides proceeded smoothly with palladium on carbon (Pd/C) catalyst, ethanol and
base in dimethyl sulfoxide (DMSO) to afford exclusively symmetric biaryls in good to excellent yields. Ethanol was first used as a
reducing agent in situ to reduce the Pd2+/C species into Pd0/C active species to complete the catalytic redox cycle. It was found
that ethanol can promote the Pd/C-catalyzed reductive homocoupling of aryl iodides and bromides efficiently in the presence
c
of base. A reaction mechanism has been put forward and discussed. Copyright ꢀ 2010 John Wiley & Sons, Ltd.
Keywords: palladium; biaryls; aryl halides; homocoupling; ethanol
Introduction
(Pd/C) is the most commonly used Pd-based catalyst due to its
commercialavailability,lowcostandeasyregeneration.Thus,Pd/C
canbeusedasanalternativechoicetoconventionalhomogeneous
catalysts in the synthesis of symmetrical biaryls.[12–15] Cravotto
and his coworkers have reported that high-intensity ultrasound
can promote the Pd/C-catalyzed coupling reaction and give
the corresponding biaryls in good yields.[12] Besides ultrasound,
addition of metal, crown ether and phosphorus ligands can
also be used to promote the Pd/C-catalyzed homocoupling
reactions.[13–15] However, all these methods have difficulties in
industrial application.
Biaryls and their heteroaromatic analogues have specific physic-
ochemical properties which are widely used in the synthesis of
conductive polymers, liquid crystals and important biaryl natural
products.[1–3] The Ullmann reductive homocoupling reactions of
aromatic halides have been widely used to synthesize symmetri-
cal biaryls (Scheme 1).[4] However, the high reaction temperature
(over 200 ◦C) and the stoichiometric amount of copper in the tra-
ditional Ullmann reaction limit its utility in the synthesis of biaryls
with functional groups.
Based on the wide application of palladium (Pd)-catalyzed
reductive coupling reactions in the formation of carbon–carbon
bonds, a number of Pd-catalyzed Ullmann reactions have also
beenproposedaseco-friendlyreplacementsforthestoichiometric
Ullmann protocol.[5,6] Pd-catalyzed Ullmann reactions are carried
out under elevated temperatures in the presence of reducing
agents, which facilitates the synthesis of functional biaryls.
Normally, a reducing agent is required in situ to regenerate
the Pd0 active species from Pd2+ species and complete the
Pd0 < − > Pd2+ redox cycle. Until now, various reducing agents
have been developed for the regeneration of Pd0, including
hydrogen gas,[7] zinc,[8] indium,[9] alcohol[6,10] and formate salt.[11]
Both homogeneous and heterogeneous Pd-based catalysts
have been successfully applied in the Ullmann coupling reactions.
The main drawback of homogeneous catalysis is the recycling
of the catalyst, leading to difficulty in product purification and
loss of expensive metals and ligands. In contrast, heterogeneous
Pd catalysts can be easily handled, recovered from the reaction
mixture by simple filtration and reused. Palladium on carbon
Our recent work has demonstrated an efficient homocoupling
of aryl iodides and bromides catalyzed by Pd2+ (dppf) (dppf
= 1,1ꢁ-bis(diphenylphophino)ferrocene).[16] The study shows that
the Pd0(dppf) active species were regenerated from Pd2+ (dppf)
species by dimethyl sulfoxide (DMSO) without the need of any
additional reducing agent. Pd/C was also studied with DMSO
as a reducing agent, but the corresponding yield of biaryls was
low under these reaction conditions. In this article, ethanol is
first applied as a reducing agent to promote the Pd/C-catalyzed
homocoupling reaction. It is well known that ethanol is nontoxic,
cheapandeasilydisposed.Byusingethanolasareducingagent,we
have developed a low-cost, simple and promising Pd/C-catalyzed
system for further industrial application.
∗
Correspondence to: Chenze Qi, Institute of Applied Chemistry, Shaoxing
University, Shaoxing 312000, People’s Republic of China.
E-mail: qichenze@zscas.edu.cn
Catalyst, Reductant
X
a
Institute of Applied Chemistry, Shaoxing University, Shaoxing, Zhejiang
Province 312000, People’s Republic of China
Base
R
R
R
X = I, Br, Cl
b
College of Yuanpei, Shaoxing University, Shaoxing, Zhejiang Province 312000,
People’s Republic of China
Scheme 1. Ullmann homocoupling reactions of aromatic halides.
Appl. Organometal. Chem. 2010, 24, 421–425
c
Copyright ꢀ 2010 John Wiley & Sons, Ltd.

L. Shao et al.
Table 1. Effect of ethanol amount on Pd/C-catalyzed reductive
Table 2. Effects of base on Pd/C-catalyzed reductive homocoupling
homocoupling of iodobenzenea
of iodobenzenea
Yield (%)
Entry
Base
Biphenyl yieldb (%)
Ethanol
(mmol)
Conversionb
(%)
Entry
Biphenylc
Benzeneb
1
2
No base
CsF
0
85.79
19.76
7.60
84.80
77.88
34.01
28.85
71.75
44.71
0
1
2
0
43.24
98.57
100
39.60
68.90
85.79
78.63
74.94
69.90
63.30
64.14
65.65
61.40
36.47
2.76
26.92
11.12
12.62
18.40
25.12
30.20
30.40
27.60
32.10
13.12
3
KF
0.50
0.86
1.71
2.57
3.43
6.00
8.56
12.0
17.1
5.00d
4
NaF
3
5
Cs2CO3
K2CO3
Na2CO3
NaHCO3
KOAc
KOH
4
100
6
5
100
7
6
100
8
7
100
9
8
100
10
11
12
13
9
100
KCl
10
11
100
KBr
0
61.83
Et3N
54.46
a Reactionconditions:1.0 mmoliodobenzene,0.05 mmolPd/Ccatalyst,
7.5 mmol CsF and predetermined amount of ethanol in 5 ml DMSO at
120 ◦C for 3.0 h.
a Reactionconditions:1.0 mmoliodobenzene,0.05 mmolPd/Ccatalyst,
0.86 mmol ethanol, 7.5 mmol base in 5 ml DMSO at 120 ◦C for 3.0 h.
b Biphenyl yields were determined from the GC/MS measurements by
the external standard method.
b Iodobenzene conversions and phenyl yields were determined from
the GC/MS measurements by the area normalization method.
c Biphenyl yield was determined from the GC/MS measurements by
the external standard method.
d A 5 ml aliquot of ethanol was used as the solvent.
is determined by their solubility and basicity, which is associated
with the size of their metal cations.[16]
The cesium salts are similarly good at promoting the Pd/C-
catalyzed homocoupling reaction of iodobenzene. The biphenyl
yieldsare85.79%forCsF(entry2ofTable 2)and84.80%forCs2CO3
(entry 5 of Table 2), respectively. However, when potassium and
sodium salts were used as the base, the corresponding yields of
biaryls varied obviously. This is due to the fact that the solubility
and basicity of cesium salts are mainly affected by the cesium
cation in DMSO, while those of potassium and sodium salts are
affected by their anion. Examination of entries 3, 6 and 9–12
of Table 2 shows that the carbonate anion is the most effective
anion at promoting the homocoupling reaction of iodobenzene.
The alkali chloride and bromide salts have no effect due to their
low basicity (entries 11 and 12 of Table 2). Triethylamine was also
studied for its excellent solubility in DMSO but was not as ideal as
expected due to its weak basicity (entry 13 of Table 2).
Besides the basicity and solubility, the amount of base also
plays an important role. In theory, stoichiometric quantity of a
base is enough for the Pd/C-catalyzed homocoupling reaction
of aryl halides. However, due to the incomplete solubility of the
base, stoichiometric quantity of a base is not enough. As can be
seen from Fig. 1, the iodobenzene conversion and the biphenyl
yield increased rapidly with the amount of CsF when the base was
consumed in three molar equivalents relative to iodobenzene.
The further addition of CsF had negligible enhancement of the
coupling biphenyl yields, suggesting that three molar equivalents
of base is enough to neutralize the acidic products in the
Pd/C-catalyzed homocoupling reactions. As the number of molar
equivalents of CsF was increased to 6.0, biphenyl was obtained in
good yield and the addition of more molar equivalents of CsF had
no obvious positive effect in the yield of biphenyl.
Results and Discussion
A series of palladium catalysts have been studied for the reductive
homocoupling of iodobenzene in DMSO.[16] Unfortunately, when
Pd/Cwasusedasthecatalyst, theyieldsofbiarylswerecomparably
low. Isopropanol has been reported to be an effective reducing
agent for the homogenous palladium catalyst.[6,10] Compared
with isopropanol, ethanol is cheap, nontoxic and easily removed.
Thus, ethanol was studied as the reducing agent to promote
the Pd/C-catalyzed homocoupling reaction of aryl halides in this
article.
First, we examined the Pd/C-catalyzed homocoupling reaction
of iodobenzene to biphenyl by varying the amount of ethanol
(Table 1). It was found that both the biphenyl yield and the
iodobenzene conversion could be greatly increased by a small
amount of ethanol (0.86 mmol). However, further addition of
ethanol could lead to an increase in self-reduction product
(phenyl). When ethanol was used as the solvent, both the biphenyl
yield and the iodobenzene conversion decreased. Therefore,
0.86 mmol of ethanol is clearly the best amount for this reaction
and was chosen for the subsequent studies.
In the palladium-catalyzed reductive reactions, base is indis-
pensable to neutralize the generated acidic byproduct, otherwise
the reaction will be suppressed due to the increased medium
acidity.[17] It was indeed observed that the pH value of the reaction
system decreased as the reaction proceeded. As shown in entry 1
of Table 2, no homocoupling product (biphenyl) was formed in the
absence of base, indicating that a suitable base is indispensable
for successful progress in palladium-catalyzed reductive reactions.
Interestingly, acetates, carbonates, bicarbonates and hydroxides
can also be used as bases to promote the Pd/C-catalyzed ho-
mocoupling reaction of iodobenzene in DMSO. Examination of
entries 2–7 of Table 2 shows that the cesium salts are better than
the corresponding potassium salts, which in turn are better than
the corresponding sodium salts at promoting the Pd/C-catalyzed
homocoupling reactions. The relative effectiveness of these salts
Having realized the optimal type and quantity of base for
the coupling reaction, attention was turned to the evaluation
of the optimal reaction temperature and time. As shown in
Fig. 2, we found that high temperature was beneficial to both
the iodobenzene conversion and the biphenyl yield. At 60 ◦C, the
reactionproceededslowlyandtheconversionofiodobenzenewas
c
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Copyright ꢀ 2010 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2010, 24, 421–425

Ethanol-promoted reductive homocoupling reactions
120
100
80
120
100
80
60
40
20
0
60
Iodobenzene Conversion
Biphenyl Yield
Iodobenzene Conversion
Biphenyl Yield
40
20
0
0
1
2
3
4
5
0
1
2
3
4
5
6
7
8
Reaction Time (h)
Amount of Base (mmol)
Figure 3. Time dependence of Pd/C-catalyzed reductive homocoupling
of iodobenzene. Reaction conditions: 1.0 mmol iodobenzene, 0.05 mmol
Pd/C catalyst, 0.86 mmol ethanol, 6.0 mmol CsF in 5 ml DMSO at
120 ◦C. Iodobenzene conversions were determined from the GC/MS
measurements by the area normalization method and biphenyl yields
were determined from the GC/MS measurements by external standard
method.
Figure 1. Effect of base amount on Pd/C-catalyzed reductive homo-
coupling of iodobenzene. Reaction conditions: 1.0 mmol iodobenzene,
0.05 mmol Pd/C catalyst, 0.86 mmol ethanol and predetermined amount
of CsF in 5 ml DMSO at 120 ◦C for 3.0 h. Iodobenzene conversions were
determined from the GC/MS measurements by the area normalization
method and biphenyl yields were determined from the GC/MS measure-
ments by the external standard method.
homocoupling reaction had almost completed. Thus, ethanol
again proved to be an effective reducing agent for promoting the
Pd/C-catalyzed homocoupling reaction of iodobenzene.
120
100
80
Finally, having determined the optimal conditions for the
homocoupling reaction, the scope and limitations of this Pd/C-
catalyzed homocoupling protocol were also studied. Several
substituted aromatic halides were tested as substrates having
electron-withdrawingandelectron-donatinggroupsinortho,meta
and para positions. Reaction conditions were modified according
tothenatureofsubstituents.TheresultsaresummarizedinTable 3.
As described above, 0.86 mmol ethanol is the optimal amount
for the Pd/C-catalyzed homocoupling reaction of iodobenzene.
However, this amount of ethanol is not as good as we have
assumed for the Pd/C-catalyzed homocoupling reactions of some
aryl bromides and iodides. Examination of entries 6, 7, 12 and
15 in Table 3 shows that the addition of ethanol can increase
the yields of biaryls and decrease the reaction time for some
aryl bromides and iodides. Because of steric hindrance, the yields
of 2,2ꢁ-methoxyl biphenyl are very low as the homocoupling
productsfromtheortho-methoxylgroupsubstitutediodobenzene
and bromobenzene (entries 4 and 10 of Table 3). However, the
steric effect of small substituents (ortho-methyl, ortho-fluoro) is
negligible and excellent biaryl yields could also be obtained
from the Pd/C-catalyzed homocoupling of the corresponding
substituted aromatic halides (entries 9 and 12 of Table 3).
Furthermore, entries 17–19 show that this Pd/C-catalyzed system
is also effective for heteroaryl iodides and bromides. In general, the
reaction is tolerant for both electron-rich and electron-deficient
aryl substrates to provide the corresponding biaryls in good to
excellent yields by using the appropriate amount of ethanol.
Attempts to extend this catalytic system to chlorobenzene and
fluorobenzene failed due to the much stronger carbon–chlorine
and carbon–fluorine bond strength.[19,20] Therefore, it is not
unexpected to find that 4,4ꢁ-dichlorobiphenyl formed as the
sole homocoupling product from 4-chloroiodobenzene (entry
8 of Table 3) and 4-chlorobromobenzene (entry 16 of Table 3).
Similarly, difluorobiphenyl formed from the corresponding fluoro-
60
Iodobenzene Conversion
Biphenyl Yield
40
20
0
60
80
100
120
140
160
Temperature (°C)
Figure 2. Effect of temperature on Pd/C-catalyzed reductive homo-
coupling of iodobenzene. Reaction conditions: 1.0 mmol iodobenzene,
0.05 mmol Pd/C catalyst, 0.86 mmol ethanol, 6.0 mmol CsF and in 5 ml
DMSO for 3.0 h. Iodobenzene conversions were determined from the
GC/MS measurements by the area normalization method and biphenyl
yields were determined from the GC/MS measurements by external stan-
dard method.
found to be only 9.23% after 3 h, while at 100 ◦C, iodobenzene was
completely consumed within the same time. Since low reaction
temperature facilitates the synthesis of biaryls with functional
groups, finally a reaction temperature of 120 ◦C was determined
for the following experiments.
AsshowninFig. 3,thehomocouplingreactionproceededslowly
in the first half hour and then rapidly in the next half hour. Pacho´n
et al.[18] have reported that there was an induction period for the
palladium-catalyzed coupling reaction due to the involvement
of palladium clusters. After the formation of ‘optimal particle
size’ of palladium clusters in the first half-hour, the reaction
could be greatly enhanced in the next half-hour. After 2 h, the
c
Appl. Organometal. Chem. 2010, 24, 421–425
Copyright ꢀ 2010 John Wiley & Sons, Ltd.
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L. Shao et al.
carried out under argon and in a waterless environment. After
careful examination of the reaction product mixture, small
amounts of acetaldehyde and ethyl acetate were detected by
gas chromatography/mass spectroscopy (GC/MS). Therefore, it
can be concluded that ethanol indeed acted as the reducing agent
in the reaction as expected.
Table 3. Pd/C-catalyzed reductive homocoupling of various aryl
iodides and bromidesa
Ethanol
Entry
Substrate
C6H5I
(mmol) Time (h) Biaryl yieldb (%)
1
2
3
4
5
6
0.86
8.56
0.86
8.56
0.86
0.86
8.56
0.86
8.56
8.56
8.56
8.56
8.56
0.86
8.56
0.86
0.86
8.56
0.86
8.56
8.56
0.86
0.86
8.56
3
85.79c
90.43
99.79
46.94
93.50
71.38
100.00
84.66
96.90
98.10
84.74
64.58
83.65
84.88
96.34
84.12
76.50
94.54
86.89
90.58
94.49
100.00
90.61
99.67
Based on the above experimental results, a mechanism for
the Pd/C-catalyzed homocoupling reaction of aromatic halides is
proposed (Scheme 2). It was initially thought that formation of the
biaryl product is achieved by ligand exchange of two molecules of
intermediate 1, which is similar to the literature.[8,11,13,16] However,
after careful density functional theory (DFT) calculations,[21]
the ligand exchange of two molecules of intermediate 1 is
unlikely to happen due to the high energy requirement and
the proposed mechanism (path I in Scheme 2) is a much more
feasible mechanism. The Pd2+ species is reduced into Pd0
active species by ethanol, followed by oxidative insertion of
aromatic halide and then group exchange of X− by CH3CH2O−
to produce intermediate 2. From intermediate 2, formation of
intermediate 3 occurs and the biaryl product is finally formed
via the reductive elimination process with the regeneration of
Pd0 active species in situ to furnish the catalytic redox cycle. In
addition, with alcohol as the reducing agent, Pd0 was detected
in the reaction system by X-ray photoelectron spectroscopic
(XPS).
Although the dehalogenation product is negligible for this
reaction, we also investigated the mechanism of the formation
of dehalogenation product (path II in Scheme 2). DFT calcula-
tion shows that intermediate 2 is also formed for the formation
of dehalogenation product, and then the hydrogen transfer of
intermediate 2 occurs to give intermediate 4. Finally, reductive
elimination occurs and enables the formation of the dehalogena-
tion product and regeneration of Pd0 active species at the same
time.
C6H5Br
22
4-CH3-C6H5I
2-CH3O-C6H4I
3-CH3O-C6H4I
4-CH3O-C6H5I
8.5
66
22
9.5
4
7
4-F-C6H5I
14.5
14.5
52.5
40.5
66
8
9
4-Cl-C6H4I
2-CH3-C6H4Br
2-CH3O-C6H4Br
4-CH3O-C6H4Br
2-F-C6H4Br
10
11
12
22
66
40.5
28
13
14
3-F-C6H4Br
4-F-C6H4Br
66
40.5
66
15
4-F3C-C6H4Br
14.5
40.5
28
16
17
18
19
4-Cl-C6H4Br
2-I-C5H4Nd
2-Br-C5H4Ne
2-CH3O-5-Br-C5H4Nf
28
14.5
a Reaction conditions: 1.0 mmol aryl iodides or bromides, 0.05 mmol
Pd/C catalyst, 6.0 mmol CsF and predetermined amount of ethanol in
5 ml DMSO at 120 ◦C.
b Biaryl yields were determined from the GC/MS measurements by the
area normalization method.
c This data is from the entry 3 of Table 1.
d 2-Iodopyridine.
e 2-Bromopyridine.
Conclusion
f 2-methoxyl-5-bromopyridine.
In this work, ethanol was first used as an efficient reducing
agent to promote the Pd/C-catalyzed homocoupling reactions
of aryl bromides and iodides. This Pd/C-catalyzed system was
successfully applied to a variety of aryl bromides and iodides
with electron-donating and electron-withdrawing groups in
relatively mild reaction conditions. According to the experimental
results, a mechanism for the catalytic cycle was proposed.
The ease of product separation and catalyst recycling promote
this method as a promising Pd/C-catalyzed system in industrial
applications.
Ar-Ar
Ar
Pd(0)
Ar-X
Ar-Pd-H
4
Ar-Pd-Ar
3
Ar-Pd-X
1
CH3CH2O-
HX
+
-
X
CH3CHO
CH3CHO
Experimental Section
Ar-X
Ar-Pd-OCH2CH3
2
Chemicals
Path II
Path I
All solvents and Chemicals were analytical grade. Palladium on
carbon (Pd/C, 5 mol%) was purchased from Zhejiang Metallurgical
Research Institute. DMSO solvent was treated with potassium
hydroxide (KOH) pellet overnight and then distilled under a
reduced pressure before use.
Scheme 2. Mechanism for the Pd/C-catalyzed reactions of aryl halides.
substituted iodobenzene (entry 7 of Table 3) and bromobenzene
(entries 12–14 of Table 3).
As reported by Hassen et al.,[6] when the isopropanol was used
as the reducing agent, the isopropanol was oxidized into acetone
in situ to regenerate the Pd0 active species from Pd2+ species.
However, the oxidation product of ethanol is acetaldehyde,
which is more reactive than ethanol and the reactions were
Typical Experimental Procedure for the Pd/C-catalyzed
Homocoupling Reactions
In a 20 ml tubular reactor containing DMSO (5.0 ml), aryl halide
(1.0 mmol), Pd/C-catalyst (0.05 mmol), CsF (6.0 mmol) and ethanol
c
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Copyright ꢀ 2010 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2010, 24, 421–425

Ethanol-promoted reductive homocoupling reactions
(0.86 mmol) were added under argon atmosphere. The reactor
was placed in a preheated oil bath at 120 ◦C under vigorous
stirring. TLC was used to monitor the homocoupling reaction
until the disappearance of the aryl halide starting materials for
Table 3. After reaction, the reaction mixture was cooled to room
temperature. The samples for GC/MS measurements were filtrated
from the reaction mixture directly.
Acknowledgments
This work was generously supported by the Natural Science
Foundation of Zhejiang Province, People’s Republic of China (no.
Y-4080450) and Shaoxing University. We would like to thank
Professor Q. X. Guo and Dr H. Z. Yu for their kind help in Gaussian
03 calculations.
The product isolation was as follows: after the reaction was
completed, the reaction mixture was cooled to room temperature
and quenched with 10 ml of water, and then extracted three
times with ethyl acetate (3 × 20 ml). The combined organic
layers were washed with water (3 × 20 ml) and saturated brine
(3 × 20 ml), and then dried over anhydrous Na2SO4. Solvents were
evaporated under reduced pressure, and the residue was purified
by chromatography on silica gel with a mixture of petroleum ether
and ethyl acetate (v : v, 1 : 1) as eluent to afford the corresponding
coupling product. All the biaryls were characterized by their
spectroscopic data (1H NMR and GC/MS) and melting point, which
were identical to those found in the literature.[16,21,22]
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Characterizations
The quantitative analysis was performed on an Agilent GC/
MS instrument (Agilent 6890N/5975I) with a programmable
split/splitless injector. The injector-port temperature was set at
270 ◦C. The oven-temperature program was initially set at 140 ◦C
and ramped to 270 ◦C at 10 ◦C/min, and maintained for 2 min at
every step. NMR spectra were recorded in CDCl3 on an Avance
III 400 MHz spectrometer. 1H NMR chemical shifts are reported in
parts per million relative to TMS, with the residual solvent peak as
the internal reference. Melting points were measured on a Buchi
510 in open capillary tubes and were not calibrated.
[22] G. Cahie, C. Chaboche, F. Mahuteau-Betzer, M. Ahr, Org. Lett. 2005,
7, 1943.
c
Appl. Organometal. Chem. 2010, 24, 421–425
Copyright ꢀ 2010 John Wiley & Sons, Ltd.
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