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OH
OH
OH
O
Air : 74% yield (24 h)
OH
OH
OH
OH
d
n
4
r
4
n
g
|
2
OH
O
OH
OH
OH
O
H
OH
O
OH
OH
OH
OH
OH
OH
OH
OH
(R,R)-
(S)-
(R)-
(S,S)-
O
2
: 86-87% yield, 97-98%
ee
(20 h)
t
Bu
t
Bu
OH
OH
O
OH
O
2
: 99% conversion, 83% yield,
90% selectivity (8 h)
O
OH
t
Bu
OH
t
Bu
OH
O
2
: 75% conversion, 60% yield,
85% selectivity (8 h)
OH
OH
O
t
Bu
t
Bu
t
Bu
+
OH
.
O
OH
O
2
: 70% conversion, 65% yield,
50% selectivity (11 h)
Selective oxidation of polyols by Waymouth and co-workers.
159
General
conditions: 10 mol% [(neocuproine)Pd(m-OAc)]
2
(OTf)
2
, 9 : 1 (v/v) MeCN-
H
2
O, 25 1C.
Figure 4.15
mixtures of primary and secondary alcohols, the observed preference for the
oxidation of the primary alcohols was believed to be due to pre-equilibria
favouring primary alkoxides. At the present time, the catalyst loadings are
relatively high when it is used aerobically, although it was found that this
could be dramatically reduced when benzoquinone was used as the terminal
oxidant (as low as 0.12 mol% on a 10 g scale). Nonetheless, such studies are
extremely valuable in the design of the next generation of chemoselective
aerobic catalysts. Indeed, the selective oxidation of polyols is very useful and
it is an area that will surely continue to be explored. In this area, it is also
worth highlighting the work of Oberhauser and co-workers, who reported
the chemoselective oxidation of unprotected diols using neutral and cationic
Pd(
II
) complexes in the presence pyridine and pyridine derivatives.
159
This
utilized a readily accessible catalyst system that could oxidize diols to the
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