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5-8 mol% Fe(NO
3
)
3
, 3-8 mol%
HO
NO
OH
O
air, CH
3
CN, rt
d
n
4
r
4
n
g
|
5
R
1
R
2
R
1
R
2
Scheme 6.5 Aerobic oxidation catalyzed by Fe(NO
3
)
3
-4-OH-TEMPO.
1-10 mol% Fe(NO
3
)
3
.
6H
2
O, 1-10 mol% TEMPO
1-10 mol% NaCl, O
2
(1 atm), DCE, 25
o
C
OH
O
R
2
R
1
R
2
R
1
Scheme 6.6 Aerobic oxidation catalyzed by Fe(NO
3
)
3
-TEMPO-NaCl.
8 mol% FeCl
3
.
6H
2
O, 2 mol% TEMPO
silica gel, toluene, O
2
(0.5 MPa) or air
O
OH
R
2
R
1
R
2
R
1
Scheme 6.7 Aerobic oxidation catalyzed by FeCl
3
-TEMPO-silica gel.
The high performance of this silica gel-catalyst systemmay be due to surface
silanol groups. Primary and secondary benzylic alcohols were oxidized
eciently to the corresponding aldehydes and ketones in high yields. Con-
versely, aliphatic alcohols were less reactive under the reaction conditions.
6.1.3 Aerobic Oxidation Catalyzed by Recyclable
Fe(
III
)-TEMPO Derivatives
The first bimagnetic catalyst, Fe and TEMPO supported in an ionic liquid
(IL), was developed by Zhang and co-workers in 2011.
13
A task-specific
bimagnetic imidazolium salt, [Imim-TEMPO][FeCl
4
], containing cooperative
functionalities was easily synthesized and applied to the selective aerobic
oxidation of aromatic alcohols (Scheme 6.8). Quantitative yields with excel-
lent selectivity were achieved under mild conditions without the use of any
organic solvent. It is worth noting that the reaction also performed well even
when using air as an oxidant instead of pure oxygen. The catalyst [Imim-
TEMPO][FeCl
4
] was readily recovered by diethyl ether extraction and could
be used repeatedly by charging with fresh substrate and NaNO
2
. The catalyst
could be reused at least five times with retention of high activity and
selectivity. At least in principle, the use of a magnet to recover this magnetic
IL is also possible.
Gao and co-workers selected a representative mesoporous silica, SBA-15,
as the support for TEMPO and found that FeCl
3
6H
2
O-SBA-15-TEMPO/
NaNO
2
eciently catalyzed the oxidation of alcohols with either O
2
or air as
the terminal oxidant (Scheme 6.9).
14
Various alcohols were oxidized at a low
catalyst loading (0.1-1 mol%) to the corresponding aldehydes and ketones in
.
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