Activity of palladium on charcoal catalysts in cross-coupling reactions

Article abstract of DOI:10.1016/j.tetlet.2010.07.170

Comparison of the activity of several commercially available Pd/C catalysts in C-C, C-N, and C-S bond forming cross-coupling reactions has demonstrated the importance of the choice of the catalyst source. Investigations showed marked difference in activity between the catalysts. Moreover, the catalytic activity of each catalyst varies with respect to the coupling. The first Pd/C catalyzed Hiyama coupling is reported.

Full text of DOI:10.1016/j.tetlet.2010.07.170

Tetrahedron Letters 51 (2010) 5411–5414
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Tetrahedron Letters
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Activity of palladium on charcoal catalysts in cross-coupling reactions
⇑
Anna Komáromi, Fruzsina Szabó, Zoltán Novák
Eötvös Loránd University, Institute of Chemistry, Pázmány Péter Stny. 1/a, Budapest, Hungary
a r t i c l e i n f o
a b s t r a c t
Article history:
Comparison of the activity of several commercially available Pd/C catalysts in C–C, C–N, and C–S bond
forming cross-coupling reactions has demonstrated the importance of the choice of the catalyst source.
Investigations showed marked difference in activity between the catalysts. Moreover, the catalytic activ-
ity of each catalyst varies with respect to the coupling. The first Pd/C catalyzed Hiyama coupling is
reported.
Received 7 June 2010
Revised 20 July 2010
Accepted 30 July 2010
Available online 6 August 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Cross-coupling
Solid supported catalysis
Charcoal
Palladium
Multi-walled carbon nanotubes
Palladium on charcoal,1 is a widely used solid-supported cata-
lyst for various cross-coupling reactions,2 which is frequently used
in synthetic chemistry. The charcoal as metal carrier is easily
accessible from commercial sources, is inexpensive, and ensures
a resistant, stable support with relatively high surface area. Filtra-
tion of this support from reaction mixtures is a simple and rapid
process which is beneficial for industrial applications. Additionally,
the transition metals deposited onto the surface of the charcoal can
be easily recovered and recycled.
Additionally, to demonstrate the effect of the support we pre-
pared two different Pd/C catalysts under the same conditions7
using Norit A, and multi-walled carbon nanotubes (MWCNT) as
the carbon support. Transition electron microscopy (TEM) images
Although there are several examples of the use of Pd/C catalysts
in Heck,3 Suzuki4 and Sonogashira5 reactions, application of this
catalyst in carbon-heteroatom bond forming reactions is scarce.6
Moreover, there is no procedure for coupling silanes with aromatic
halides.
In the case of solid-supported catalysis, the type, commercial
source, and origin of the Pd/C catalyst is very important for a suc-
cessful coupling. This issue, in most cases, is not examined, or men-
tioned in research reports. Obviously, besides the palladium
content of the catalysts, the size of the palladium particles and their
distribution on the surface also determine the catalytic activity of
the transition metal. The type and the surface area of the charcoal
support, and the conditions for the preparation of the catalyst could
also be important issues. For a synthetic chemist, who generally fol-
lows a synthetic procedure for cross-coupling, the only given infor-
mation is the wt % palladium content of the Pd/C catalyst.
In this context, we aimed to compare the catalytic activity of
some commercially available 10 wt % Pd/C catalysts in four differ-
ent types of cross-coupling reactions.
⇑
Corresponding author. Tel.: +36 1 372 2500x1610; fax: +36 1 372 2909.
E-mail address: novakz@elte.hu (Z. Novák).
Figure 1. TEM image of Pd/MWCNT catalyst.
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.07.170

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A. Komáromi et al. / Tetrahedron Letters 51 (2010) 5411–5414
I
O
1% Pd/C, 1% XPhos
1% Pd/C, 1% DiCyJohnPhos
tBuONa, tBuOH,
O
Cl
N
Ph
Ph
HN
º
DMA, 110 C, 1 h
º
80 C, 1 h
100
60
50
40
30
80
60
40
20
0
20
10
0
SH
I
Br
Pd/C, KOH
DMSO,
Si(OCH3)3
S
1%Pd/C, 4% PPh3
TBAF, DMF,
º
100 C, 1 h
º
110 C, 1 h
100
100
80
60
40
20
0
80
60
40
20
0
Figure 2. Graph A (top left): chlorobenzene (0.5 mmol), phenylacetylene (0.75 mmol), K2CO3 (0.75 mmol), DMA (0.25 mL), Pd/C (10 wt %, 1 mol %), and XPhos (1 mol %), at
110 °C under an Ar atmosphere. Graph B (top right): iodobenzene (0.5 mmol), morpholine (0.75 mmol), tBuONa (0.7 mmol), Pd/C (10 wt %, 1 mol %),and L = DiCyJohnPhos
(1 mol %) in tBuOH (0.5 mL) at 80 °C under an Ar atmosphere. Samples were taken after 1 h. Graph C (bottom left): iodobenzene (0.5 mmol), thiophenol (0.55 mmol), KOH
(0.75 mmol), Pd/C (10 wt %, 0.01 mmol, 2 mol %),and DMSO (0.5 mL) at 110 °C under an Ar atmosphere. Samples were taken after 1 h. Conversions were determined by GC
from the average of four runs. Graph D (bottom right): 3-bromotoluene (0.5 mmol), phenyltrimethoxysilane (0.75 mmol), TBAF (0.75 mmol), Pd/C (10 wt %, 0.005 mmol,
1 mol %), PPh3 (0.02 mmol, 4 mol %mol %), DMF (0.75 mL),and DMSO at 110 °C under an Ar atmosphere. Samples were taken after 1 h. Conversions were determined by GC
from the average of four runs.
of the latter catalyst showed that the palladium catalyst deposited
onto the carbon nanotubes consisted of 4–10 nm metal particles
(Fig. 1).
age catalytic activity compared to other palladium sources. Addi-
tionally to previous studies, we compared the catalytic activity of
the Pd/MWCNT catalyst with Pd/C (Norit A) in Sonogashira and
Buchwald–Hartwig reactions. We found marked differences in
the activity of the catalysts in both coupling reactions. Palladium
deposited onto the nanotube support gave almost full conversions,
but Norit A did not show significant catalytic activity. This finding
demonstrates the importance of the choice of carbon carrier.
For further investigations we chose the palladium-catalyzed
thiolation10 and the Hiyama coupling2b of aryl halides as additional
examples. The C–S bond-forming reaction catalyzed by palladium
on charcoal was demonstrated for the first time by Lin and
co-workers,6b but there was no precedent for the solid-supported
In our earlier reports on the Pd/C-catalyzed Sonogashira cou-
pling of aryl chlorides8 and the Buchwald–Hartwig amination,9
we found that the choice of the solid-supported catalyst was very
important to achieve coupling efficiently. Comparison of these
results showed that some catalysts had completely different activ-
ity in Sonogashira, and amination reactions (Fig. 2, graphs A and B).
The Evonik 196 WN/D, Dutral and Panreac Pd/C catalysts showed
no or low catalytic activity in the Buchwald–Hartwig coupling of
iodobenzene and morpholine, but for the Sonogashira coupling of
chlorobenzene with phenylacetylene these catalysts showed aver-

A. Komáromi et al. / Tetrahedron Letters 51 (2010) 5411–5414
5413
Table 1
Once we had appropriate reaction conditions for the coupling in
hand we next examined the activity of different palladium on char-
coal catalysts in the reaction of bromotoluene, and phenyltrimeth-
oxysilane. Comparative studies showed that the Merck catalyst had
superior activity in this reaction. In contrast, this catalyst was
average in the thiolation and was completely ineffective in the
Buchwald–Hartwig amination and Sonogashira couplings. This
phenomenon was also observed in the case of Evonik 196WN/D.
In the Hiyama reaction Pd on Norit A catalyst also showed poor
reactivity, but Pd/MWCNT had excellent activity.12
Following these optimization and comparative studies, we syn-
thesized several substituted biaryls using solid-supported palla-
dium via the Hiyama reaction (Table 1). Aromatic bromides with
electron-withdrawing and electron-releasing groups and hetaryl
bromides were applicable for the Hiyama coupling under the
developed conditions, and we were able to prepare the products
in moderate to good yields (45–76%).
Hiyama coupling of aryl bromides in the presence of Pd/C Selcat Q6 catalyst.a
Si(OCH3)3
1% Pd/C, 4% PPh3
Br
R
TBAF, DMF,
R
1
2
3
º
110 C, 6 h
Entry
1
Aryl bromide
Product
Yieldb (%)
68
Br
Br
OHC
OHC
H3CO
2
3
H3CO
64
76
Br
H2N
H2N
4
72
Br
In conclusion, we have shown that the source and type of palla-
dium on charcoal catalyst strongly determine the efficiency of
cross-coupling reactions. Moreover, the catalytic activity of the
various Pd/C catalysts was found to be different in cross-coupling
reactions. These comparative studies provide important informa-
tion for synthetic chemists for applications of solid-supported
palladium catalysts in different cross-coupling reactions. On the
basis of our studies the Selcat catalyst family proved to be the most
reliable source of palladium for cross-coupling reactions. We have
also demonstrated that the choice of the carbon support deter-
mines the activity of the catalyst. Our palladium on MWCNT cata-
lyst showed enhanced activity in all the coupling reactions with
the exception of the C–S bond forming reaction. Additionally, for
the first time, we have developed conditions for the Pd/C-catalyzed
Hiyama coupling of aryl halides and aryl silanes.
F3C
Br
Br
F3C
59
48
5
6
F3CO
N
F3CO
N
7
8
76
45
Br
Br
S
S
a
Aryl bromide (1 mmol), phenyltrimethoxysilane (1.5 mmol), TBAF (1.5 mmol),
Pd/C Selcat Q6 (10 wt %) (0.01 mmol, 1 mol %), PPh3 (0.04 mmol, 4 mol %), DMF
(1.5 mL), 110 °C, 6 h.
b
Isolated yield.
palladium-catalyzed coupling of silyl reagents and aryl halides. In
our preliminary investigations, we have found that coupling iodo-
benzene and thiophenol in the presence of 2% Pd/C took place in
30 min at 110 °C under similar conditions to those applied by Lin
and co-workers. As Lin’s procedure required 9 h for completion
of the reaction, we envisaged that the type of supported catalyst
would be very important in this kind of cross-coupling. Next, we
compared the catalytic activity of several Pd/C catalysts in the cou-
pling of iodobenzene and thiophenol. The reactions were con-
ducted in DMSO in the presence of 1.5 equiv KOH at 100 °C, and
we used 2 mol % of palladium in each reaction. The results of these
studies, summarized in Figure 2, graph C, show that there were sig-
nificant differences in the catalytic activity of the solid-supported
catalysts. After 60 min reaction time the conversions were in the
range of 7–90%. The catalyst sold by Panreac proved to be the sec-
ond most efficient catalyst after Selcat A6. However, this catalyst
gave moderate activity in the Sonogashira coupling previously,
and it was not significantly active in the amination. The Pd/C cata-
lyst supplied by Merck (sold as the oxidized form of Pd) was also
ineffective in the amination, but the thiolation took place with
good efficiency in the presence of this catalyst.
To the best of our knowledge, among the cross-coupling reac-
tions, the Hiyama coupling of silanes, and aryl halides has not
yet been achieved with palladium on charcoal catalysts. Therefore,
we developed conditions for this synthetic transformation. We
found that in the presence of 1% Pd/C and 4% PPh3 as a ligand,
iodobenzene could be arylated with phenyltrimethoxysilane in
DMF at 100 °C with the addition of tetrabutylammonium fluoride
(TBAF) as an activator. The choice of DMF and TBAF was crucial
for the reaction; other solvents and fluoride sources proved to be
unsuitable for this coupling. The developed conditions were also
applicable for the coupling of aryl bromides. However, coupling
of aryl chlorides was unsuccessful, even with bulky phosphane
ligands.11
Acknowledgments
The authors thank Professor K. Torkos, Dr. Zs. Eke, and Mr. L.
Tölgyesi for providing analytical support, and Professor I. Dódony
at Eötvös University, Department of Mineralogy for TEM measure-
ments. Financial support and a Research Grant (Z.N.) from the
Magyary Zoltán Felsooktatási Közalapítvány, EEA and Norway
Grants are gratefully acknowledged.
}
Supplementary data
Supplementary data (procedures, optimization studies and
characterization of the materials) associated with this article can
be found, in the online version, at doi:10.1016/j.tetlet.2010.07.170.
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