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Cl
Cl
Pd
N
N
OH
S
Cl
22
(0.02 mol %)
NaOH, TBAB, 100 °C, 7 h
+ PhB(OH)
2
O
O
92%
Scheme 14.19
i
-Pr
Br
(HO)
2
B
23
(1 X 10
-5
mol %)
KOH, H
2
O, 100 °C, 15 h
NaO
3
S
N
+
Cl
Pd
MeO
MeO
95%
i
-Pr
23
2
Scheme 14.20
couplings of aryl bromides and chlorides.
54
With 4-bromoacetophenone,
over 100 000 turnovers with a turnover frequency (TOF) of 50 000 h
1
were
achieved at 100 1C in water-TBAB, but the palladacycle could not be effect-
ively recycled. In contrast, a polymer-supported version attached through an
oxime ether could be used for four reaction cycles before seeing a decrease in
conversion to product.
A recyclable catalyst was obtained from the cyclopalladated complex of
anthracene-9-carbaldehyde imine containing a sulfonated aryl group on the
imine nitrogen (23, Scheme 14.20). Up to 9 000 000 turnovers were achieved
in the Suzuki coupling of 4-bromoanisole at 100 1Cin15h.
55
The catalyst
system could be used for up to six cycles, but the catalyst performance de-
graded, requiring longer reaction cycles. Analysis of the reaction mixture by
TEM showed the formation of nanoparticles, which may be the true active
species.
14.2.1.4 Copper- and Iron-Catalyzed Coupling in Aqueous
Media
Although palladium has been the dominant metal used in cross-coupling
reactions for the past several decades, there is a resurgence in interest in the
use of the more Earth-abundant, and less costly, first-row transition metals,
such as copper and iron.
56
Copper-catalyzed cross-coupling has a long his-
tory, going back to the early work of Ullmann and Goldberg.
57
The devel-
opment of more active ligand-supported copper catalysts has promoted a
resurgence of interest in copper.
58
With this renewed interest in alternative
cross-coupling catalysts, examples of their application in aqueous solvents
have been reported. Because the ligands used in these reactions are often
more hydrophilic than typical ligands used in palladium-catalyzed reactions,
it is often not necessary to introduce additional hydrophilic groups.
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