Palladium on activated carbon - A recyclable catalyst for Suzuki-Miyaura cross-coupling of aryl chlorides in water

Article abstract of DOI:10.1055/s-2006-926305

Aryl chlorides are efficiently coupled with aryl boronic acids using ligandless Pd/C in water. The catalyst is easily recovered via filtration and reused up to four times with an overall yield of 73%. In addition the catalytic activity and selectivity of a variety of homogeneous and heterogeneous palladium sources have been compared. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2006-926305

692
PAPER
Palladium on Activated Carbon – A Recyclable Catalyst for Suzuki–Miyaura
Cross-Coupling of Aryl Chlorides in Water
Pd/C – A
Catalyst for
S
o
uzuki–Miya
r
ura Cros
t
s-Coup
e
ling of Ar
y
n
l
Chlorides in Wat
L
er ysén,*a,b Klaus Köhlera
a
Department Chemie, Anorganische Chemie, TU München, Lichtenbergstr. 4, 85747 Garching, Germany
The Danish University of Pharmaceutical Sciences, Universitetsparken 2, 2100 Copenhagen, Denmark
Fax +4535306040; E-mail: mly@dfuni.dk
b
Received 24 August 2005
persion and a high degree of Pd(II), as it was shown to be
the most effective in the Heck reaction of chlorobenzene
with styrene.9 We employed this catalyst and demonstrat-
ed that aryl chlorides with various sensitive functional
Abstract: Aryl chlorides are efficiently coupled with aryl boronic
acids using ligandless Pd/C in water. The catalyst is easily recov-
ered via filtration and reused up to four times with an overall yield
of 73%. In addition the catalytic activity and selectivity of a variety
of homogeneous and heterogeneous palladium sources have been groups like nitrile and ester couple readily under the envi-
compared.
ronmentally friendly conditions developed (Table 1). We
now wish to report our results on the comparison of this
catalyst with other palladium sources as well as the at-
tempts to recycle the catalyst.
Key words: aryl chlorides, water, palladium, cross-coupling, bi-
aryls
As summarized in Table 1 various electron-deficient as
well as electron-rich aryl chlorides were suitable sub-
strates for the reaction, and the reactivity of the aryl chlo-
rides is reflected in the reaction conditions. All reactions
were performed under an ambient atmosphere as homo-
coupling was minimal. Activated aryl chlorides reacted
smoothly at low palladium concentrations (0.2–0.5
mol%), and with only 1.1 equivalents of phenylboronic
acid. The deactivated chloroarenes required higher cata-
lyst concentrations (2.0 mol%) and longer reaction times
(six hours). Addition of TBAB was essential to stabilize
the catalyst10 and 0.5 equivalent was found to be optimal
as lower concentrations resulted in diminished yields.
Screening of several bases proved NaOH to be superior
and 2.5–5 equivalents were found to be the most favor-
able. For the reaction of chlorobenzonitriles and methyl
esters (Table 1, entries 2–5) it was necessary to change the
base due to substantial hydrolysis to the benzoic acid.
K2CO3 and K3PO4 both led to benzoic acid formation but
KF11 proved efficient to activate the phenylboronic acid
without causing hydrolysis. Chlorobenzene coupled with
4-tolylboronic acid at 140 °C after a reaction time of six
hours, using only 0.5 mol% Pd (Table 1, entry 7). 2- and
4-Chlorotoluene gave high conversions using 2 mol% Pd
(Table 1, entries 8–10). 4-Chloroanisole (Table 1, entry
11) gave a high yield (83%) and even 4-chlorophenol and
4-chloroaniline (Table 1, entries 12 and 13) could be con-
verted, though in moderate yield. Attempts to couple 2-
chloropyridine (Table 1, entry 14) under these conditions
failed and changing the base to K2CO3 or KF did not in-
duce the reaction. Tagata and Nishida12 found it necessary
to use PPh3 as ligand, DME as solvent, and heating over-
night in order to couple 2-chloropyridine with phenylbo-
ronic acid. Attempts to couple 2-chloropyridine by the
addition of ligands was not pursued.
The coupling of aryl boronic derivatives with aryl halides
known as the Suzuki–Miyaura1 reaction has become the
most popular way of accessing biaryls. The literature is
flooded with reports of different catalytic systems and
conditions that transform the boronic nucleophiles to
valuable fine chemicals and pharmaceutically interesting
compounds.2 Much effort has been invested in preparing
catalytic systems that allow the activation of aryl chlo-
rides as these are generally cheaper and more abundant
than aryl iodides and bromides. A vast number of ligands
have been developed that enhance the coupling of aryl
chlorides, and likewise, a lot of palladium catalysts such
as palladacycles3 have evolved. Some major drawbacks
are the handling and the availability of these catalysts and
ligands, as not many are commercially accessible and
some are air-sensitive.
Suzuki coupling in water has received much attention
since water as a solvent is very attractive and many cata-
lytic systems have been reported.4 The use of ligandless
palladium is also well documented in the coupling of aryl
iodides and bromides.5 However, only a few examples
have appeared, proving that ligandless palladium can be
efficiently applied to the coupling of aryl chlorides in wa-
ter.6
In our preliminary study7 we demonstrated that Pd/C in
combination with TBAB in water is an excellent system
for the coupling of aryl chlorides. Our reason for using a
heterogeneous palladium source was the prospect of at-
taining much cleaner reactions as palladium can be re-
moved by simple filtration,8 thus opening the possibility
of recycling the catalyst. We investigated a commercially
available Pd/C source that possesses high palladium dis-
SYNTHESIS 2006, No. 4, pp 0692–0698
Advanced online publication: 19.01.2006
DOI: 10.1055/s-2006-926305; Art ID: P13205SS
x
x
.
x
x
.2
0
0
6
The use of aryl bromides and aryl iodides as coupling
partners is well-documented but Badone5a reported prob-
© Georg Thieme Verlag Stuttgart · New York

PAPER
Pd/C – A Catalyst for Suzuki–Miyaura Cross-Coupling of Aryl Chlorides in Water
693
Table 1 Cross-Coupling of Aryl Chlorides in Water Using Pd/C as Catalyst
Pd/C
TBAB, NaOH
Cl
B(OH)2
R
R
+
H2O
Entry
R
Pd
(mol%)
Temp
(°C)
Time
PhB(OH)2 NaOH
Product
Isolated yield
(equiv)
(equiv)
(%)b
O
117
4-COCH3
4-CN
0.2
100
100
2 h
3 h
1.1
2.5
99
218
0.5
0.5
1.1
1.1
2.5
2.5
98c
89c
NC
CN
319
2-CN
140
140
3 h
3 h
O
O
420
4-CO2CH3
0.5
0.5
1.1
1.1
2.5
2.5
87c
O
O
521
2-CO2CH3
140
3 h
63c,d
619
722
822
922
4-F
0.5
0.5
2.0
2.0
140
140
140
140
3 h
6 h
6 h
6 h
1.1
1.1
1.1
1.5
2.5
2.5
2.5
5
87
73e
70
81
F
H
H3C
H3C
H3C
4-CH3
4-CH3
CH3
1023
2-CH3
2.0
140
6 h
1.5
5
74f
1122
1217
1324
14
4-OCH3
4-OH
2.0
2.0
2.0
2.0
140
140
140
140
6 h
6 h
6 h
6 h
1.5
1.5
1.5
1.5
5
5
5
5
83
MeO
HO
56
4-NH2
66g
trace
H2N
2-Cl-pyridine
N
a Reaction conditions: aryl halide (4 mmol), TBAB (2 mmol), H2O (6 mL), pressure tube, air.
b Yield of chromatographically pure products.
c KF used as base.
d Product contaminated with 6% 2-chlorobenzoic acid methyl ester.
e 4-Tolylboronic acid was used as a nucleophile.
f Product contaminated with 5% biphenyl.
g Determined by 1H NMR spectroscopy.
Synthesis 2006, No. 4, 692–698 © Thieme Stuttgart · New York

694
M. Lysén, K. Köhler
PAPER
lems when coupling aryl iodides under aqueous condi- seen in Table 3 they generally coupled efficiently produc-
tions as the reaction stopped after one hour, resulting in ing a high yield of the biaryls. Attempts to couple 2-meth-
incomplete conversion. Using our standard conditions oxyphenylboronic acid and 2-chloronitrobenzene
with low palladium loading (0.2 mol%) we found that 4- (Table 3, entry 2) resulted in a lower yield (53%) probably
iodobenzene and 4-iodophenol coupled readily and gave due to steric interactions. Heating the reaction for six
an excellent yield (ca. 90%) after two hours at 70 °C hours gave a similar conversion and higher palladium
(Table 2, entries 1 and 2) whereas 4-iodoanisole only gave concentrations were not tested. The coupling of the 2-cy-
trace amounts of coupling product (Table 2, entry 3). ano boronic ester (Table 3, entry 6) led to no product for-
However, all aryl iodides could be completely converted mation probably due to hydrolysis of the nitrile. Changing
to the corresponding biaryls by raising the temperature to the base to KF only produced trace amounts of product.
100 °C for two hours.
All the potassium trifluoroborates tested were readily cou-
pled resulting in high yields of the biaryls (Table 3, entries
7–9).
Under the same conditions aryl bromides coupled effi-
ciently and gave full conversion within two hours with ex-
cellent isolated yield (95–99%; Table 2, entries 4–6). Using these aqueous conditions, alternative palladium
Even 2-bromoaniline gave a high yield (89%) requiring sources were tested to see if this was a general reaction or
only 0.5 mol% Pd and a reaction time of three hours at due to the use of the optimized catalyst (Table 4). Hetero-
100 °C to reach completion (Table 2, entry 7).
geneous palladium catalysts together with homogenous
PdCl2 and Pd(OAc)2 all showed excellent activity. This
indicates that the heterogeneous palladium sources simply
leach palladium into solution during the reaction as previ-
ously reported.8 When homogenous palladium was em-
ployed a low concentration (Table 4, entries 9 and 11)
resulted in quantitative conversion, however, at higher
concentrations (Table 4, entries 8 and 10) conversion de-
creases. This is probably due to the formation of palladi-
Traditionally, boronic acids were the only organoboron
compounds utilized in the cross-coupling but in the last
ten years several other boron derivatives like esters13 and
trifluoropotassium salts14 have found wide application.
Conditions that allow coupling of these derivatives have
been developed but only in organic solvents. Coupling of
different arylboronic acids, esters, and potassium trifluo-
roborates under these aqueous conditions were tested. As
Table 2 Cross-Coupling of Aryl Iodides and Aryl Bromides
R2
Pd/C
Hal
B(OH)2
TBAB, NaOH
R1
R1
+
R2
H2O
Entrya
1
R1
H
Hal
I
R2
Product
Temp (°C)
Conversion (%)b
Yield (%)b
70
100
94
100
90
>99
CH3
H
CH3
70
100
90
100
90
>99
2
4-OH
I
HO
70
100
5
100
5
>99
3
4-OMe
4-OMe
4-CH3
4-OH
I
H
MeO
MeO
H3C
HO
422
522
617
Br
Br
Br
H
100
100
100
nd
nd
nd
99c
95c
97c
H
H
NH2
725
2-NH2
Br
H
100
nd
89c,d
a Reaction conditions: aryl halide (2 mmol), arylboronic acid (2.2 mmol), Pd (0.2 mol%), TBAB (1 mmol), NaOH (5 mmol), H2O (4 mL), 2 h,
100 °C.
b Determined by GC with diethylene glycol dibutyl ether as internal standard; nd = not determined.
c Yield of chromatographically pure products.
d Experiment performed with Pd/C (0.5 mol%) at 100 °C for 3 h.
Synthesis 2006, No. 4, 692–698 © Thieme Stuttgart · New York

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Pd/C – A Catalyst for Suzuki–Miyaura Cross-Coupling of Aryl Chlorides in Water
695
Table 3 Cross-Coupling of Various Aryl Boronic Derivatives
R2
Pd/C
BR
TBAB, NaOH
R1
R2
+
R1
H2O
Cl
Entry
126
ArBR
R2
Product
Yield (%)a
97
B(OH)2
B(OH)2
2-NO2
NO2
NO2
O
O
227
2-NO2
2-NO2
53
98
B(OH)2
328
F
NO2
F
O
B
426
2-NO2
4-NO2
96
98
O
O
O
NO2
NO2
F
F
O
B
528
NO2
CN
CN
O
B
6
4-NO2
2-NO2
4-NO2
tracec
BF3K
726
89
NO2
NO2
BF3K
829
98
F
F
NO2
BF3K
930
4-NO2
97
O
O
a Reaction conditions: aryl chloride (1 mmol), ArBR (1.2 mmol), TBAB (0.5 mmol), NaOH (2.5 mmol), H2O (2 mL), 100 °C, 2 h.
b Yield of chromatographically pure product.
c KF used as base.
um clusters and precipitation of palladium black, thereby the reaction fails to reach completion within two hours
decelerating the reaction. Pd/C from different suppliers all (Table 4, entries 5 and 6). As expected, palladium immo-
showed the same reactivity and selectivity (Table 4, en- bilized on Zeolite15 (Table 4, entry 7) displayed similar
tries 1–4), whether reduced to Pd(0) or dried. It is also ev- activity as it is also able to release palladium under these
ident that at catalyst concentrations lower than 0.2 mol% reaction conditions.
Synthesis 2006, No. 4, 692–698 © Thieme Stuttgart · New York

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M. Lysén, K. Köhler
PAPER
Table 4 Screening of Various Palladium Sources
Table 5 Palladium Sources under More-Demanding Conditions
O
O
CH3
Pd
CH3
B(OH)2
B(OH)2
Pd
TBAB, NaOH
TBAB, NaOH
+
H2O
Cl
Cl
H2O
Entrya
Pd
Amount
(mol%)
Conversion Yield
Entrya Pdb
Amount
Conversion Yield
(%)b
(%)b
14
10
10
25
19
12
8
(mol%)
0.22
0.24
0.22
0.21
0.08
0.04
0.22
3.45
0.56
3.10
0.28
(%)c
(%)c
1
2
3
4
5
6
7
Pd/C (A)
Pd/C (B)
Pd/C (C)
Pd/C (D)
Pd/NaY
PdCl2
0.53
0.58
0.58
0.54
0.46
0.79
0.75
31
1
2
Pd/C (A)
98
95
23
Pd/C (B)
Pd/C (C)
Pd/C (D)
Pd/C (D)
Pd/C (D)
Pd/NaY
PdCl2
100
100
100
64
>99
>99
>99
62
10
3
34
4
19
5
21
6
68
67
Pd(OAc)2
10
7
100
86
>99
59
a Reaction conditions: 4-chlorotoluene (2 mmol), phenylboronic acid
(3 mmol), TBAB (1 mmol), NaOH (10 mmol), H2O (4 mL), 140 °C,
8
6 h.
b Determined by GC with diethylene glycol dibutyl ether as internal
standard.
9
PdCl2
100
79
>99
55
10
11
Pd(OAc)2
Pd(OAc)2
100
>99
a Reaction conditions: 4-chloroacetophenone (2 mmol), phenylboron-
ic acid (2.2 mmol), TBAB (1 mmol), H2O (4 mL), 100 °C, 2 h.
b A: Johnson-Mattey 5% Pd/C (H2O, 49.37%); B: Aldrich 5% Pd/C;
C: Degussa 5% Pd/C dried and reduced to Pd(0); D: Degussa 5% Pd/
C (H2O, 51.90%).
lyst preparation, pre-treatment) becomes more relevant
for its catalytic properties.
All the palladium sources tested under these conditions
proved excellent in the cross-coupling of aryl chlorides
and we chose to continue with catalyst C to investigate the
scope and limitations of recycling the catalyst.
c Determined by GC with diethylene glycol dibutyl ether as internal
standard.
Although a low palladium loading is sufficient to achieve
In order to couple electron-rich aryl chlorides, extended high conversions it is desirable to have a catalyst that can
reaction times and higher temperatures were needed and easily be separated from the reaction and reused.
therefore the stability of the palladium could be ques-
Recovery of the catalyst was accomplished by simple fil-
tioned. The palladium sources listed in Table 4 were in-
tration through celite or by centrifugation. As shown in
vestigated to identify if the palladium source could have a
Table 6, the procedure by which the palladium is recov-
major influence on the success of the reaction. As is evi-
ered is of great importance. In the reaction where the cat-
dent from Table 5, all the palladium sources showed sim-
alyst was reused without any activator, whether it was
ilar activity under these reaction conditions. Employing
recovered by centrifugation or filtration, it had lost most
0.5 mol% Pd in the reaction of 4-chlorotoluene with phe-
of its activity by the time it was reused for a third reaction.
nylboronic acid, using NaOH as base and heating to
Although the selectivity is still very high when the cata-
140 °C for six hours all led to less than 30% yield. The
lyst is first reused, satisfactory yields of 80% and 67% are
heterogeneous palladium sources (Table 5, entries 1–7)
obtained, subsequent use only gives 38% and 32% yield
showed slightly better activity than PdCl2 and Pd(OAc)2
(Table 6, entries 1 and 3).
(Table 5, entries 6 and 7) at this temperature. The unre-
De Vries et al.16 have shown that palladium activity can be
duced and wet Pd/C-catalysts from Johnson-Mattey
restored by addition of iodine which (partially) re-oxidiz-
(Table 5, entry 1) and Degussa (Table 5, entry 4) both
es palladium to Pd(II). Addition of 0.1 mmol iodine to the
showed the best reactivity but the Degussa catalyst dis-
catalyst retrieved by centrifugation did not restore the re-
played superior selectivity. The Pd/C catalyst from Ald-
activity as the yield after it is first reused is only 60%
rich (Table 5, entry 2) proved less reactive and selective
(Table 6, entry 2) and then only 20% after its second re-
under these conditions and the reduced and dried Pd/C
use. The low yield of the reaction could be due to incom-
catalyst (Table 5, entry 3), though exhibiting excellent se-
plete recovery of the catalyst by centrifugation. The
lectivity, showed poor activity. The palladium immobi-
catalyst retrieved by filtration on the other hand exhibited
lized on Zeolite again shows good reactivity and excellent
full activity by addition of iodine in the first reuse. This
selectivity. The experiments demonstrate that, under more
activity was retained as the catalyst was further reused.
demanding conditions, the nature of the palladium (cata-
Synthesis 2006, No. 4, 692–698 © Thieme Stuttgart · New York

PAPER
Pd/C – A Catalyst for Suzuki–Miyaura Cross-Coupling of Aryl Chlorides in Water
697
Table 6 Catalyst Recovery and Reuse
Pd/C
TBAB, NaOH
additive
Cl
B(OH)2
+
H2O
O
O
Entrya
Catalyst recyclingb Additivec
Original use 1st Reuse
2nd Reuse
3rd Reuse
4th Reuse
Overall yield (%)d
1
2
3
4
5
Centrifugation
Centrifugation
Filtration
–
99
99
99
99
99
80
60
65
99
99
38
20
32
99
80
–
–
72
60
65
71
73
I2
–
–
–
–
–
Filtration
I2
I2
25
47
32
41
Filtration
a Reaction conditions: 4-chloroacetophenone (4 mmol), phenylboronic acid (4.4 mmol), Pd/C (0.2 mol%), NaOH (10 mmol), H2O (6 mL), 2 h,
100 °C.
b See experimental section for details.
c Additive was added prior to the reaction only when the catalyst was reused; I2 (0.1 mmol).
d Yield was determined by GC using diethylene glycol dibutyl ether as internal standard.
purified by flash chromatography to yield the corresponding biaryl.
The spectroscopic data of the biaryls were in agreement with those
previously reported.
When reused for the third time, the activity dropped re-
markably giving only 26% conversion but activity was
maintained even after the fourth time, resulting in an over-
all yield of 71% over five reactions. A second sequence
gave similar results, a quantitative yield was obtained
when the catalyst was reused twice, then the yield starts to
decrease resulting in an overall yield of 73%.
4-Fluoro-2¢-nitrobiphenyl28
1H NMR (300 MHz, CDCl3): d = 7.84 (dd, 1 H, J = 7.9, 1.3 Hz),
7.59 (dt, 1 H, J = 7.5, 1.3 Hz), 7.47 (dt, 1 H, J = 7.9, 1.5 Hz), 7.39
(dd, 1 H, J = 7.6, 1.5 Hz), 7.30–7.23 (m, 2 H), 7.13–7.05 (m, 2 H).
In conclusion, we have demonstrated that different com-
mercially available ligandless palladium sources, in com-
bination with TBAB, are efficient catalysts for the
coupling of aryl chlorides in water. Furthermore we have
proven that Pd/C can be recovered by simple filtration and
reused several times with high overall yield.
Recycling of the Pd/C Catalyst – Centrifugation
The reaction was carried out as above, after cooling to r.t. and addi-
tion of CH2Cl2 (5 mL) the reaction was transferred to a centrifuge
flask, CH2Cl2 (4 mL) and H2O (4 mL) were added and the resulting
mixture was centrifuged at 8000 rpm for 20 min. The liquids were
decanted and the Pd/C was washed with CH2Cl2 (4 mL) and centri-
fuged again. After decantation the Pd/C was transferred to a clean
pressure tube and the experiment was repeated on the same scale.
The reaction mixture was analyzed by GC; the yield and conversion
were calculated with diethylenglycol dibutylether as the internal
standard.
Flash column chromatography was performed using silica gel
(Merck, 40–63 mesh). TLC was performed using Merck silica gel
60 F254 aluminum sheets. GC analysis was performed on a HP6890
with diethylene glycol dibutyl ether as internal standard. NMR
spectra were recorded in CDCl3 on a 400 MHz Bruker spectrometer
with TMS/CDCl3 as internal standards.
In the sequence where iodine was added as activator, I2 (25 mg, 0.1
mmol) was added with the other reagents and the reaction and work-
up was as above.
All solvents and reagents were obtained from Fluka or Aldrich and
used without further purification. Pd/C was purchased from Degus-
sa AG (E105CA/W), Johnson-Mattey, and Aldrich and used as re-
ceived. The palladium on NaY-zeolite was prepared as previously
reported.15 H2O was passed through a standard universal ion-
exchange and used without further purification.
Recycling of the Pd/C Catalyst – Filtration
The reaction was carried out as above, after cooling to r.t. and addi-
tion of CH2Cl2 (5 mL), the reaction was vacuum-filtered through a
3 cm plug of celite in a glass filter funnel (pore 4, ∆ 210 mm). The
celite was washed with CH2Cl2 (5 mL). The Pd/C was scraped off
and transferred to a clean pressure tube and the experiment was re-
peated on the same scale. The remaining celite was washed with
CH2Cl2 (10 mL) and the reaction mixture was analyzed by GC; the
yield and conversion were calculated with diethylene glycol dibutyl
ether as the internal standard.
Pd/C-Catalyzed Suzuki Cross-Coupling; General Procedure
A pressure tube was charged with Pd/C, E105CA/W (0.2–2 mol%),
TBAB (2 mmol), NaOH (2.5–5 equiv), phenylboronic acid (1.2–1.5
equiv), and aryl halide (4 mmol). H2O (6 mL) was added and the
flask was sealed with a Teflon screw cap and stirred in a preheated
oil bath for the time indicated. After cooling to r.t., CH2Cl2 (10 mL)
was added, and the mixture was filtered through a plug of celite to
remove the catalyst, washed with CH2Cl2 (2 × 10 mL), the com-
bined organic phase was dried (MgSO4), adsorbed onto celite, and
In the sequence where iodine was added as activator, I2 (25 mg, 0.1
mmol) was added with the other reagents and the reaction and work-
up was as above.
Synthesis 2006, No. 4, 692–698 © Thieme Stuttgart · New York

698
M. Lysén, K. Köhler
PAPER
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Acknowledgment
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