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arylation of 4-bromobenzaldehyde with the yield decreasing from 90% to
82%. The E-factors of Heck couplings of aryl iodides catalyzed by palladium
on amphiphilic carbon nanospheres under a range of conditions were found
to range between 4 and 6 kg waste kg
1
product.
6b
These E-factors were
similar to those obtained for other heterogeneous palladium catalyst
systems.
Most industrial heterogeneous catalyst systems are supported on in-
organic supports, such as silica, alumina or zeolites. Palladium-exchanged
NaY zeolite provides good activity for the Suzuki coupling of aryl iodides
and bromides, but the catalyst activity rapidly degrades upon recycling.
132
Palladium nanoparticles obtained by reducing the Keggin-type poly-
oxometallate K
5
PPdW
11
O
39
were active for the Suzuki, Stille and Heck
coupling of aryl bromides and activated aryl chlorides in water.
133
This
system provides one of the few examples of a true aqueous-phase Buchwald-
Hartwig amination reactions in the literature. Palladium nanoparticles
supported on an MCM-41 surface modified with an alkylcyanosiloxane (43,
Scheme 14.40) are effective catalysts for the Sonogashira coupling of aryl
iodides in water.
134
The catalyst can be used for five reaction cycles with no
decrease in reaction yield.
Metal-organic frameworks (MOF) are attractive supports because of their
large surface area. The chromium terephthalate-based MOF MIL-101 was
used as a support for palladium nanoparticles.
135
The MIL-101-supported
palladium (0.9 mol%) catalyzed the Suzuki coupling of deactivated aryl
chlorides in excellent yield at 80 1C in water-TBAB. The Pd-MIL-101 catalyst
was also applied to the condensation of 2-iodoanilines with alkynes to give
indoles in excellent yields (Scheme 14.41).
136
The MIL-101-supported catalyst
could be used for 10 reaction cycles with minimal decrease in reaction yield,
whereas an MCM-41-supported catalyst gave lower yields and showed a
significant decrease in activity after six cycles.
Simple metal oxides can also serve as supports for palladium nano-
particles. Magnetic Fe
3
O
4
-supported palladium particles were applied to the
Hiyama coupling of aryl bromides in water.
137
The catalyst could be
Pd
C
N
C
N
C
N
PdCl
2
O
O
O
O
O
H
2
NNH
2
H
2
O
30 °C, 3 h
PdCl
2
acetone
reflux, 72 h
O
Si
S
O
Si
S
Si
S
O
O
41
42
43
Scheme 14.40
Pd-MIL-101 (3 mol % Pd)
CuI (5 mol %)
PPh
3
(2 mol %)
K
2
CO
3
, H
2
O, 90 °C, 15 h
I
+
Ph
Ph
97%
H
MeO
2
C
NH
2
MeO
2
C
Scheme 14.41
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