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O
44
(0.1 mol % Pd)
NaHCO
3
, NaOH
H
2
O, 100 °C, 24 h
OH
HO
I
+
SCH
2
CH
2
C
8
F
17
HO
O
O
O
N
Pd
Si
HN
N
N
OH
alcohol dehydrogenase
NaDPH
H
2
O/
i
-PrOH (3:1), pH 7
rt, 24 h
SCH
2
CH
2
C
8
F
17
44
HO
(
R
)-(-)-rhododendrol
90% yield, 2 steps
> 99% ee
Scheme 14.42
recovered magnetically and used for five reaction cycles during which the
yield decreased from 91% to 80%. Analysis of reaction filtrates by ICP-MS
showed no palladium. Palladium nanoparticles supported on CeO
2
catalyze
the Suzuki coupling of aryl bromides in water at room temperature.
138
The
catalyst was very stable. After the reaction, it could be filtered, washed and
dried in air and then reused. Over 11 reaction cycles, the yields ranged from
99 to 97% using 1 mol% palladium.
Surface-modified silica gel has been used to support palladium nano-
particles in aqueous-phase cross-coupling reactions. Palladium nano-
particles supported on a silica-starch composite are active catalysts for
aqueous-phase Heck and Sonogashira couplings of aryl iodides, bromides
and chlorides at 100 1C.
139
The supported catalyst could be used for six
reaction cycles without a decrease in catalyst performance. Analysis of
the recovered support by ICP-MS showed a decrease in palladium loading
from 23.8 ppm prior to use to 22.1 ppm after the Sonogashira coupling.
A triazine modified with fluorous alkyl tails was attached to silica gel co-
valently through a trialkoxysilane substituent (44, Scheme 14.42). Palladium
nanoparticles supported on the resulting materials were effective catalysts
for the Sonogashira, Suzuki and Heck couplings of aryl iodides and
bromides.
140
Up to 15 reaction cycles could be achieved using the supported
catalysts (0.1 mol% Pd) with minimal decrease in reaction yield for both
catalyst supports. The catalyst was applied to the tandem Heck coupling of
4-iodophenol and 3-buten-2-ol followed by enzymatic reduction of the
resulting ketone to give (R)-(-)-rhododendrol in 90% yield and 499% ee.
141
14.2.3.2 Supported Palladium Complexes
Supported palladium nanoparticles often provide highly recyclable catalysts
for aqueous-phase cross-coupling reactions. There are few examples of
catalysts that show high activity for deactivated aryl bromides and aryl
chlorides, however. In order to combine the advantages of ligand-supported
catalysts and heterogeneous systems, there has been significant interest in
supported molecular catalysts in which the ligand is attached to an in-
organic or polymeric support. The challenge in these systems is to maintain
the high catalyst activity of the homogeneous system in the heterogeneous
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