Chemistry Reference
In-Depth Information
NMe
2
Cl
Cl
Cl
P
P
P
Pd
Pd
Pd
P
P
P
Cl
Cl
Cl
Me
2
N
P
Cl
Cy
2
P
2
O
Pd
Fe
Pd
Cl
Cl
Ph
2
P
PPh
2
Cl
Pd
P
Cl
Cl
Figure 3.13 Examples of
some of
the prominent new-generation L
2
PdX
2
precatalysts.
NEt
2
X
β
-hydride
r
eductive
X
Et
2
NPdL
2
X
L
2
Pd
+
HX
Et
3
N
HPdL
2
X
elimination
elimination
H
r
eductive
R
2R-M
L
2
Pd
+
RR
L
2
PdX
2
+2M-X
PdL
2
elimination
R
r
eductive
2RO
L
OR
L
"LPd(OR) "
+
OR
L
Pd
+2X
RO
elimination
R = e.g. Ac or H
Scheme 3.14 Reduction of L
2
PdX
2
to Pd(0).
on both small and large scales. However, these air-stable catalysts become
activated to L
n
Pd(0) in catalysis, hence the coupling reactions have to be
conducted under inert conditions to maximize the optimal results, such as
low loading and shorter reaction time.
One of the earliest known L
2
PdX
2
precatalysts is trans-dichlorobis-
(triphenyl)phosphine, (Ph
3
P)
2
PdCl
2.
As with (Ph
3
P)
4
Pd, this catalyst also
generates (Ph
3
P)
2
Pd(0), but the mechanism of activation is speculative. The
reducing agent could be one of the coupling partners (such as boronic acid)
or solvent (such as alcohol,
64
etc.; Scheme 3.14).
There are a number of early examples where the preformed Pd(II) catalyst,
(Ph
3
P)
2
PdCl
2
,
65
has been used as a commercial catalyst. Although it is con-
sidered to be an old-generation catalyst, it is still very commonly used in
many large-scale processes, mainly because of its relatively low cost in
comparison with the newly developed advanced catalysts.
The catalyst
26,66,67
is an air-stable Pd(II) d
8
16-electron complex, therefore
displaying a square-planar conformation. Its preparation was reported by
Miyaura and Suzuki, involving reaction of PdCl
2
with Ph
3
P ligand in the
presence of PhCN (Scheme 3.15),
65
although newer processes are available.
The successful use of (Ph
3
P)
2
PdCl
2
even in recent years can be exemplified
by the large-scale preparation of eniluracil (Scheme 3.16).
68
Here, the catalyst
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