An efficient heterogeneous Pd catalyst for the Suzuki coupling: Pd/Al 2O3

Article abstract of DOI:10.1246/cl.2007.918

Pd^II-loading alumina catalyst, which is simply prepared through impregnation of γ-alumina with Pd(OAc)₂ followed by calcination in the air, shows a high catalytic activity for the Suzuki coupling of aryl bromides with arylboronic acids under phosphine ligand-free conditions. Only 0.25 mol% of palladium is sufficient for the promotion of the couplings in ethanol. Copyright

Full text of DOI:10.1246/cl.2007.918

918
Chemistry Letters Vol.36, No.7 (2007)
An Efficient Heterogeneous Pd Catalyst for the Suzuki Coupling: Pd/Al2O3
Daisuke Kudo,1 Yoichi Masui,2 and Makoto OnakaÃ2
1Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033
2Graduate School of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902
(Received April 10, 2007; CL-070389; E-mail: conaka@mail.ecc.u-tokyo.ac.jp)
PdII-loading alumina catalyst, which is simply prepared
Table 1. Suzuki coupling with various bases and catalystsa
through impregnation of ꢀ–alumina with Pd(OAc)2 followed
by calcination in the air, shows a high catalytic activity for the
Suzuki coupling of aryl bromides with arylboronic acids under
phosphine ligand-free conditions. Only 0.25 mol % of palladium
is sufficient for the promotion of the couplings in ethanol.
Pd cat.
base
MeO
Br
+
PhB(OH)2
MeO
Ph
EtOH
1a (1 mmol)
2a (1.5 mmol)
3a
Base
Temp
Time
/h
Yieldb
/%
Entry
Catalyst
/mmol
/K
The Suzuki coupling,1 which is one of the most commonly
used methods for biaryls synthesis, has conventionally been
performed by homogeneous palladium catalysts in the presence
of various phosphine ligands. Such catalysts are highly active
for the coupling, but the separation of the catalyst components
from products is often a troublesome operation. By contrast,
heterogeneous palladium catalysts are much easier upon the
separation. Among various immobilized Pd catalysts, palladium
on activated carbon (Pd0/C and PdII/C) is readily available and
recognized highly active.2 Generally, more than 2–3 mol % of
Pd are employed for Pd/C-catalyzed reactions except a few
examples,3 and the catalysis of Pd/C is sensitive to the air,
reaction solvents, and reaction temperatures.4
1
2
Pd/ALO-7
Pd/ALO-7
Pd/ALO-7
Pd/ALO-7
Pd/ALO-7
Pd/ALO-7
Pd/ALO-7
Pd/ALO-7
Pd/ALO-2
Pd/ALO-3
Pd/ALO-7
Pd/Cd
K2CO3 (2)
Cs2CO3 (2)
KF (4)
333
333
333
333
333
333
333
333
318
318
318
318
318
3
79
89
96
93
89
15
8
3
3
3
4
CsF (4)
K3PO4 (1.3)
NaOH (4)
Et3N (4)
none
3
5
3
6
3
7
3
8
3
0
9c
10c
11c
12c
13c
KF (4)
2.5
2.5
2.5
2.5
2.5
73
43
59
31
35
KF (4)
KF (4)
KF (4)
Pd(OAc)2
KF (4)
Palladium particles in Pd/C, which are finely but weakly
fixed on activated carbon, are apt to be reduced to form less
active palladium agglomerates (palladium black) under harsh
conditions. To suppress such agglomeration, porous oxides such
as Al2O3 and SiO2 which strongly interact with palladium would
be favored as a Pd support. Such heterogeneous palladium cata-
lysts have been employed not only for selective hydrogenation
of olefins,5 but also for a C–C bond formation like the Heck
reaction.6 Recently, Hell et al. reported the Suzuki coupling with
palladium loaded on magnesium–lanthanum mixed oxides.7
Here, we report that palladium-loading alumina, Pd/Al2O3, is
a highly effective catalyst for the Suzuki coupling.
The Pd/Al2O3 catalyst was prepared via the impregnation of
three kinds of ꢀ–Al2O3 (ALO-2, ALO-3, and ALO-7)8 with
Pd(OAc)2 in toluene followed by calcination in the air at
573 K. One wt % of palladium to the alumina was loaded. The
color of the impregnated catalyst was light orange, and quite dif-
ferent from that of palladium black derived from the calcination
of Pd(OAc)2 without alumina, implying that the palladium
species on Al2O3 existed mainly in the PdII oxidation state,
which was also confirmed by XPS. X-ray diffraction of the
Pd/Al2O3 did not match that of commercial Pd black as well
as PdO. Nitrogen adsorption analysis of the Pd/Al2O3 indicated
the pore structure of the parent ꢀ–alumina was almost main-
tained.
aThe reaction was carried out with 1.0 mmol of 1a, 1.5 mmol of 2a, and
0.005 mmol of palladium-including catalyst in 5 mL of ethanol.
bIsolated yield. cA smaller amount of palladium (0.0015 mmol) was
used. d10 wt % Pd K-Type from N. E. Chemcat was used.
organic base, NEt3, gave a very low yield (Entry 7).9
More interestingly, the catalysis of Pd/Al2O3 was strongly
influenced by the impurities included in the alumina supports:
Acidic ALO-2 containing sulfate ions was found more effective
than basic ALO-3 having Na ions and neutral ALO-7 (Entries 9–
11). And the catalysis of Pd/ALO-2 was obviously superior to
that of Pd0/C and Pd(OAc)2 (Entries 9, 12, and 13). Examining
solvent effects, we found alcoholic solvents, especially ethanol,
are suitable for the reaction. In water, the reaction also proceed-
ed, but the yield was much lower due to insolubility of reactants
1a and 2a in water.
Using KF as a base, we then applied the Pd/ALO-2 catalyst
to various couplings of aryl bromides with arylboronic acids in
EtOH (Table 2). Most of the couplings with electron-rich and
poor aryl bromides proceeded well to yield the corresponding
biaryls 3 in good to quantitative yields by use of only
0.25 mol % Pd. Only 2-nitrobenzene gave an unsatisfactory re-
sult, probably due to the strong coordination of a nitro group
to palladium. In the coupling with 4-methoxybromobenzene
(1a), the use of 0.01 mol % palladium promoted the reaction with
a turnover number (TON) of 8400 (Entry 1). We also examined
the catalysis of Pd(OAc)2 and commercial Pd/C under the same
conditions. Toward electron-rich aryl bromides, these catalysts
were less effective. Especially, with Pd(OAc)2, rapid reduction
We performed the coupling of 4-bromoanisole (1a) with
phenylboronic acid (2a) in EtOH in the presence of concomitant
base under phosphine ligand-free conditions (Table 1). Among
inorganic bases (K2CO3, Cs2CO3, KF, CsF, K3PO4, and NaOH),
potassium fluoride (KF) was found the best (Entry 3), while an
Copyright Ó 2007 The Chemical Society of Japan

Chemistry Letters Vol.36, No.7 (2007)
919
Table 2. Suzuki coupling of aryl bromides with arylboronic
acidsa
We compared the amount of Pd leaking in EtOH for Pd/
ALO-2 and commercial Pd/C during the reaction of 1a with
2a. After the catalysts were filtered off, ICP analysis of the EtOH
solutions showed that less than 3 wt % of Pd was contained in the
solutions, indicating that the Pd leakage took place to almost the
same extent for the both Pd-loading catalysts. When the filtered
Pd/ALO-2 was reused in the same reaction, the yield of 3a was
80%.
In summary, we discovered that the Pd/ALO-2 catalyst
prepared by the simple impregnation–calcination of Al2O3 with
Pd(OAc)2 was an effective catalyst for the Suzuki coupling using
aryl bromide. The feature of the catalyst is that 1) a very low
amount of Pd is sufficient for the coupling, 2) there is no need
to use phosphine-based organic additives, 3) the coupling
smoothly proceeds in pure EtOH without the aid of water, and
4) since PdII species are loaded on the alumina support, Pd/
ALO-2 is stabilized and safe to handle in the air.
R2
R2
Pd cat.
KF
R1
Br
R4 B(OH)2
R1
+
R4
EtOH
333 K, 3-12 h
2a R4 = Phenyl
2b R4 = p-Tolyl
R3
R3
1
3
Entry
1
R1
R2 R3
2
Time/h Catalyst
Yield/%b
Pd/ALO-2 3a 97 (84c)
1
2
1a MeO
H
H
H
H
H
2a
3
Pd/Cd
50
31
3
Pd(OAc)2
4
1b
1c
1d
Me
H
2a
2a
3
Pd/ALO-2 3b 97
5
Pd/Cd
68
67
6
Pd(OAc)2
7
OMe
6
Pd/ALO-2 3c 73
8
Pd/Cd
28e
29e
9
Pd(OAc)2
Pd/ALO-2 3d 64e
The authors are indebted to Dr. Kouhei Okitsu of the
University of Tokyo for his help in XPS analysis. This study
was financially supported by Development of Microspace and
Nanospace Reaction Environment Technology for Functional
Materials Project of NEDO, Japan.
10
11
12
13
14
15
16
17
18
19
20
H
Me Me 2a
12
Pd/Cd
tracee
17e
Pd(OAc)2
1e
1f
H
H
Me
H
H
H
H
H
H
H
H
H
H
H
H
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2b
3
3
3
3
3
3
3
3
3
3
3
Pd/ALO-2 3e 96
Pd/ALO-2 3f 95
Pd/ALO-2 3g 96
Pd/ALO-2 3h<10e
Pd/ALO-2 3i 96
Pd/ALO-2 3j 99
Pd/ALO-2 3k 98
Pd/ALO-2 3l 96
Pd/ALO-2 3m 82
Pd/ALO-2 3n 98
Pd/ALO-2 3b 96
1g
1h
1i
CN
H
H
This article is dedicated to the late Professor Yoshihiko Ito.
NO2
H
Ac
References and Notes
´
1j CF3
H
1
a) J. Hassan, M. Sevignon, C. Gozzi, E. Shultz, M. Lemaire,
1k
1l
H
F
CF3
F
Chem. Rev. 2002, 102, 1359. b) N. Miyaura, A. Suzuki,
Chem. Rev. 1995, 95, 2457. c) A. F. Littke, G. C. Fu, Angew.
Chem., Int. Ed. 2002, 41, 4176.
21 1m CO2H
H
22
23
1n OH
1f
H
2
3
4
a) F.-X. Felpin, T. Ayad, S. Mitra, Eur. J. Org. Chem. 2006,
2679. b) L. Yin, J. Liebscher, Chem. Rev. 2007, 107, 133,
and references cited therein.
a) R. G. Heidenreich, K. Kohler, J. G. E. Krauter, J. Pietsch,
¨
´
Synlett 2002, 1118. b) G. Lu, R. Franzen, Q. Zhang, Y. Xu,
Tetrahedron Lett. 2005, 46, 4255.
a) C. R. LeBlond, A. T. Amdrews, Y. Sun, J. R. Sowa, Jr.,
Org. Lett. 2001, 3, 1555. b) H. Sajiki, T. Kurita, A. Kozaki,
G. Zhang, Y. Kitamura, T. Madgawa, K. Hirota, Synthesis
2005, 537.
H
H
aCarried out with 1.0 mmol of 1, 1.2 mmol of 2, and 4 mmol of KF in the
presence of palladium (0.0025 mmol) in 5 mL of ethanol, for 3 h. bIsolated
yield. c0.0001 mmol (0.01 mol %) of Pd/ALO-2 was used. TON = 8400.
e
d10 wt % Pd K-Type from N. E. Chemcat was used. NMR yield.
occurred and the deposition of palladium black was observed.
In order to obtain the active Pd catalyst, the calcination
process is requisite: The Pd/ALO-2 catalyst which had been
prepared via loading Pd(OAc)2 on the alumina followed by the
thermal treatment in the air was highly active, while a simply
loading catalyst of Pd(OAc)2/ALO-2 which had not underwent
the calcination only showed as low catalytic activities as unload-
ed Pd(OAc)2. The tight interaction of the oxide anions in ALO-2
with PdII ions upon the calcination seems to be indispensable
for an efficient catalyst precursor.
`
5
6
a) C. R. Lederhos, P. C. L’Argentiere, N. S. Figoli, Ind. Eng.
Chem. Res. 2005, 44, 1752. b) R. Strobel, F. Krumeich, W. J.
Stark, S. E. Pratsinis, A. Baiker, J. Catal. 2004, 222, 307.
c) M. Burgener, R. Furrer, T. Mallat, A. Baiker, Appl. Catal.,
A 2004, 268, 1.
a) A. Biffis, M. Zecca, M. Basato, Eur. J. Inorg. Chem. 2001,
1131. b) S. S. Prockl, W. Kleist, M. A. Gruber, K. Koehler,
¨
The strong coordination of the alumina to PdII would also
enhance the stabilities of the Pd species in the catalytic system.
To compare the stability between Pd/ALO-2 and Pd(OAc)2, we
immersed the two catalysts in an EtOH solution of 1a and 2a in
the absence of KF for a specified period (1 or 6 h) followed by
addition of KF: Although no coupling occurred without KF,
the reaction started to yield 3a just upon addition of KF. No
change in the activities of Pd/ALO-2 was observed after the
catalyst was immersed in EtOH for 1 and 6 h: 90% yield of 3a
for 1 h and 90% yield for 6 h. By contrast, Pd(OAc)2 gradually
lost its activities depending on the time in EtOH in the absence
of KF: 20% of 3a for 1 h and 14% for 6 h.
Angew. Chem., Int. Ed. 2004, 43, 1881. c) K. Okumura,
K. Nota, K. Yoshida, M. Niwa, J. Catal. 2005, 231, 245.
A. Cwik, Z. Hell, F. Figueras, Org. Biomol. Chem. 2005, 3,
4307.
7
8
The alumina samples were supplied by the Catalysis Society
of Japan, and contain the following impurities: ALO-2:
2À
0.03% Fe2O3, 0.22% SiO2, 0.04% Na2O, 0.50% SO4
;
ALO-3: 0.01% Fe2O3, 0.01% SiO2, 0.3% Na2O, 0.01%
TiO2; ALO-7: < 0.01% Fe2O3, < 0.1% SiO2, < 0.0015%
Na2O; < 0.1% ClÀ.
Q. Yao, E. P. Kinney, Z. Yang, J. Org. Chem. 2003, 68,
7528.
9
Published on the web (Advance View) June 23, 2007; doi:10.1246/cl.2007.918