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= "HSA"
N
SO
3
H
OH
d
n
4
r
4
n
g
|
2
R
1
R
1
0.2 - 0.5 mol% HSA/Pd(OAc)
2
OH
O
R
2
R
2
5 mol% [N
4444
][OAc],100
o
C
toluene or DMSO:water (1:1)
30 bar air or O
2
/CO
2
(8:92)
Figure 4.17 HSA/Pd(OAc)
2
-catalysed oxidation of secondary alcohols.
comparatively slow, the amount of acetic acid was reduced or even replaced
with tetrabutylammonium acetate ([N
4444
][OAc]). This resulted in improved
yields of products, but the use of the acetate salt, especially in high
concentrations, increased the extent of catalyst decomposition observed.
Compared with other areas of homogeneous catalysis, Pd(
II
) alcohol
oxidations have utilized a relatively narrow selection of ligand structures. It
is therefore worth highlighting the use of N,O-ligands. Muldoon and co-
workers screened a selection of commercially available N,O-ligands and
found that 8-hydroxy-2-quinolinesulfonic acid (HSA) gave good perform-
ances.
165
The system (Figure 4.17) was most suitable for secondary alcohols,
but impressively it could oxidize even unactivated aliphatic alcohols with
loadings as low 0.2 mol%.
165b
Similar to the work of Sheldon and co-workers
(see Table 4.2), they attempted to switch from using pressurized air to safer,
more dilute O
2
mixtures (e.g. 8% O
2
).
When 0.5 mol% of catalyst was employed, it was found that better per-
formance could be achieved when O
2
-CO
2
(8:92) was used rather than O
2
-N
2
(8:92). It was thought that the CO
2
mixture resulted in superior O
2
solubility
and mass transfer, resulting in a performance comparable to that when the
equivalent pressure of air was used. O
2
-N
2
(8:92) resulted in slower reaction
rates and greater catalyst decomposition owing to the system being mass
transfer limited in O
2
.
C´mpora and co-workers also investigated the use of N,O-ligands
and prepared a range of well-defined Pd complexes of the type
[Pd(CH
2
CMe
2
Ph)(N-O)(L)], where L
¼
pyridine or a pyridine derivative and
N-O is a bidentate, anionic ligand (e.g. 2-pyridinecarboxylate).
166
Oxidation
reactions generally required a catalyst loading of 1 mol%, 100 1C and 12 h;
however, they also carried out mechanistic investigations as part of their
study and these gave further insight into the influence of such N,O-ligands.
It appears that having ligands with an acidic group means that the ligand
can become involved in key steps in the catalytic cycle, 'facilitating the
proton transfer from the substrate (alcohol) to the final electron acceptor
(oxygen)'.
166
This conclusion also fits well the studies of Muldoon and co-
workers, as they found that the best performing ligands of those screened
had acidic groups in positions adjacent to where the Pd would bind.
165
.
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