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In-Depth Information
H
L
n
Pd
II
X
2
R
2
HO
H
2
O
R
1
d
n
4
r
4
n
g
|
2
Catalyst reoxidation
Alcohol oxidation
R
2
O
+ 2 HX
1/2 O
2
2 HX
+
L
n
Pd
0
R
1
Aggregation
of Pd
0
Pd-black
Figure 4.13 Catalytic cycle for aerobic ligand-modulated Pd(
II
)-catalysed alcohol
oxidation.
undesirable; for example, it can lead to reduced selectivity in some cases. To
address such issues, methods have been developed that utilize redox co-
catalysts other than copper. B¨ckvall has been one of the pioneers in this
area and his group have developed systems which use a series of 'electron-
transfer mediators' to enable O
2
to be used as the terminal oxidant.
142
For
example, Pd(0) is oxidized to Pd(
II
) by 1,4-benzoquinone and the resultant
1,4-hydroquinone is oxidized by a metal complex (e.g. cobalt salophen or
iron porphyrin complexes) which can be oxidized readily by O
2
. Once again,
this is an approach that is still utilized by researchers today, as it can deliver
very impressive results, with reactions having excellent selectivity with low
loadings of Pd. Nonetheless, it would be highly advantageous if it were
possible to use a single Pd(
II
) complex as a catalyst and re-oxidize it directly
with O
2
, as shown in Figure 4.13.
A single complex should be simpler and arguably more attractive for lar-
ger-scale use and, importantly, the use of ligands should permit the tuning
of the catalytic activity and selectivity. Ligand modulation of catalyst per-
formance is well established in other areas of homogeneous catalysis such as
hydrogenation and hydroformylation; however, this paradigm was slower to
take hold in homogeneous palladium oxidations. In this part of the chapter,
we focus on ligand-modulated Pd(
II
) catalysis for alcohol oxidation. It is not
possible to discuss all the catalysts that have been reported to date; there-
fore, we will highlight examples of leading catalyst systems and outline the
current mechanistic understanding for these systems.
.
4.3.2 Examples of Ligand-Modulated Pd(
II
) Catalysts
Perhaps the first report of co-catalyst-free Pd-catalysed aerobic oxidation of
alcohols was in 1977 by Blackburn and Schwartz.
143
In this case, PdCl
2
was
used in conjunction with 5 mol% NaOAc and the reaction rates and turnover
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