Chemistry Reference
In-Depth Information
studied for the Suzuki coupling of iodobenzene and phenyl trihydroxyborate.
A continuous decrease in yield was seen for successive cycles, from 95% in
cycle 1 to 75% in cycle 6. Analysis of the catalyst system using hot filtration
testing and a silica gel-supported catalyst poison suggested that the reaction
occurs on the heterogeneous catalyst, rather than through soluble palladium
species. A related polymer prepared by radical polymerization of triallyl-
amine can also serve as a support for palladium.
156
The resulting material
catalyzes Heck couplings of aryl iodides and bromides and shows modest
activity for aryl chlorides in water.
Silica and other metal oxides also provide the potential to serve as sup-
ports for metal-ligand complexes. Whereas polymeric supports allow for a
wide range of attachment chemistries, they are often physically fragile. In
contrast, metal oxide materials can be more robust, but the attachment
strategies are limited. A silica-supported acetophenone oxime palladacycle
(62) was prepared by reacting alkene-functionalized palladacycle 61 with
thiol-modified silica (60) (Scheme 14.51).
157
The silica gel-supported catalyst
gave superior results in the Suzuki coupling of 4-chloroacetophenone com-
pared with the same palladacycle attached to polystyrene, poly(ethyl acrylate)
or MCM-41. The heterogeneous catalyst could be recovered by filtration,
washed, dried and reused up to eight times with no decrease in perform-
ance. Silica-supported palladium ketoiminate complex 63 catalyzes the
Suzuki, Stille and Sonogashira coupling of heteroaryl chlorides, including
furans, using 0.5 mol% palladium at 60 1C in water-TBAB (Scheme 14.52).
158
The catalyst was used for six reaction cycles with no decrease in product
yield. Palladium leaching to the reaction phase ranged from 0.003 to
0.15 ppm per cycle.
Magnetic iron oxide particles are interesting catalyst supports because
they can be magnetically separated from reaction solutions, rather than
using lower throughput processes, such as filtration or centrifugation.
A palladium complex of bis(di-tert-butylphosphinomethyl)amine
(64,
OH
N
OH
O
Pd
Cl
N
L
O
Pd
Cl
61
8
L
O
O
O
O
SiO
2
SiO
2
Si
OMe
SH
Si
OMe
S
AIBN, CHCl
3
9
60
62
Scheme 14.51
OEt
Si
(HO)
2
B
63
(0.5 mol %)
K
2
CO
3
, TBAB
H
2
O, 60 °C, 8 h
Cl
O
O
+
N
O
N
86%
SiO
2
Pd
S
N
Ph
3
P
Cl
S
63
Scheme 14.52
Search WWH ::
Custom Search