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P
P d P
Pd
P
Pd
P
P
N
P
Pd
P
P
Pd
P d
P
P
Fe
Ph
Fe
Ph
Ph
Ph
Ph
Ph
Ph
Ph
N
Ph
Ph
Figure 3.10 Commercially available preformed L
2
Pd(0) catalysts.
2NaOH
Step I
H
0
o
C-rt
MeOH
OMe
Br
0
o
C-rt
2
PdBr
2
+ 2NaOMe
Pd
Pd
+2NaBr
MeO
Br
H
3
Step II
H
Br
Br
OMe
4PCy
3
Cy
3
P
+
Pd
Pd
Pd
2
PCy
3
OCH
3
0
o
C-rt
H
MeO
Br
H
β
-H elimination
3
4
Step III
H
75 °C
Cy
3
P
Pd
+
NaOH/MeOH
(PCy
3
)
2
Pd
+ NaBr
+CH
3
OH
Br
PCy
3
4h
4
Scheme 3.9 Mechanism of information of L
2
Pd(0) complexes.
22a
Reprinted with permission from Organic Letters. Copyright 2010 Ameri-
can Chemical Society.
As the 18-electron rule predicts, all the new L
2
Pd(0) catalysts (14-elec-
tron species) are air sensitive and should be stored and handled under an
inert atmosphere. They are typically white/off-white in color and slowly
turn brown or gray-black when exposed to oxygen. The less bulky species
such as (Cy
3
P)
2
Pd are more susceptible to degradation and should be
stored under an inert atmosphere at low temperatures to extend their
shelf-life.
The new Pd(0) precatalysts have been evaluated in a number of different
coupling reactions. An analysis of the applications of these catalysts can help
identify which catalyst should be used in a specific reaction.
(t-Bu
3
P)
2
Pd has found widespread use as an effective precatalyst for
a number of cross-coupling reactions. Limanto et al. made use of a
(t-Bu
3
P)
2
Pd-catalyzed Heck alkynylation (copper-free Sonogashira coupling)
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