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Cu
I
R
1
d
n
4
r
4
n
g
|
0
R
2
O
N
OH
N
O
1/2O
2
H
2
O
N
O
Cu
II
O
N
R
1
Cu
II
O
N
OH
R
2
H
R
1
OH
R
2
H
Figure 2.4 Proposed mechanism of the aerobic oxidation of alcohols catalyzed by
the CuCl-TEMPO system.
one-electron oxidation of copper(
I
) by TEMPO was proposed in the catalytic
pathway (Figure 2.4). Reaction of this complex with alcohols results in an
alcoholate-coordinated copper(
II
) complex. The latter then reacts with a
second molecule of TEMPO to abstract an H-atom, resulting in the for-
mation of carbonyl compound, TEMPO-H and copper(
I
). TEMPO-H is rapidly
oxidized by air to regenerate TEMPO.
29
Later, Vat`le reported a convenient method for the oxidative rearrange-
ment of tertiary allylic alcohols to b-substituted enones by utilizing a CuCl
2
-
TEMPO catalytic system in the presence of O
2
.
30,31
New copper-based cata-
lytic systems using N-hydroxyphthalimide
32
and N-hydroxyindole
33
(D and F
in Figure 2.2) have been reported for the chemoselective aerobic oxidation
of alcohols.
Following the success of Cu-TEMPO systems, a variety of copper salts and
organic ligands were screened to improve the chemoselectivity of catalytic
alcohol oxidations. In 2003, Sheldon and co-workers reported a simple
catalytic system containing copper(
II
) bromide, TEMPO and 2,2
0
-bipyridine
(bpy) for the aerobic oxidation of alcohols in acetonitrile-water solvent mix-
ture at room temperature (Figure 2.5a).
34
The catalytic system exhibited high
chemoselectivity for primary alcohols under mild conditions. Secondary al-
cohols such as octan-2-ol and 1-phenylethanol were not oxidised. Moreover,
no over-oxidation of the aldehydes to the corresponding acids was observed. It
was noted that all the components were necessary for ecient transformation.
A mechanism similar to that with GO has been proposed in which TEMPO
radical abstracts a hydrogen atom from the metal-coordinated alcoholate.
29
.
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