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O
NO
2
Fe(II)-TEMPOH
R
2
R
1
d
n
4
r
4
n
g
|
5
OH
O
2
NO
Fe(III)-TEMPO
R
1
R
2
H
2
O
TEMPO=
TEMPOH=
N
OH
N
O
Scheme 6.3 A plausible reaction mechanism for FeCl
3
-TEMPO-NaNO
2
-catalyzed
aerobic oxidation.
O
N
5 mol% FeCl
3
,
8 mol% NaNO
2
0.1-3 mol% CH
3
C
N
O
RCH
2
OH
RCHO
ClCH
2
CH
2
Cl, O
2
(0.4 MPa), 50
o
C
Scheme 6.4 Aerobic oxidation catalyzed by FeCl
3
-4-acetamido-TEMPO-NaNO
2
.
In order to improve the selectivity of the oxidation of aliphatic primary
alcohols to aldehydes, Liang and co-workers further examined the reaction
conditions and found that using 4-acetamido-TEMPO instead of TEMPO
provided the best results (Scheme 6.4).
6
Several primary alcohols were
selectively oxidized to the corresponding aldehydes in high yields in the
presence of 5 mol% FeCl
3
, 0.1-3 mol% TEMPO and 8 mol% NaNO
2
in 1,2-
dichloroethane under an oxygen atmosphere at 50 1C. This reaction system
was applicable to a broad range of primary and secondary alcohols, which
could contain carbon-carbon double bonds, and also N, O and S
heteroatoms.
Wang and Liang also reported that a combination of Fe(NO
3
)
3
and
4-OH-TEMPO eciently catalyzed the oxidation of alcohols under an
ambient air atmosphere in acetonitrile at room temperature (Scheme 6.5).
7
Recently, Ma and co-workers reported an Fe(NO
3
)
3
-TEMPO-catalyzed
aerobic oxidation in the presence of a catalytic amount of NaCl in 1,2-
dichloroethane, which eciently converted essentially all types of alcohols,
including compounds bearing different unsaturated carbon-carbon bonds,
within a couple of hours at room temperature (Scheme 6.6).
8-11
The exact
role of NaCl is not clear. This reaction system is applicable to allenols,
8
propargylic alcohols,
9
allylic alcohols
10
and indole carbinols.
11
Gao et al. recently reported that silica gel not only enhances the catalytic
reaction rate but also increases selectivity for the products in the aerobic
oxidation of alcohols catalyzed by FeCl
3
6H
2
O-TEMPO (Scheme 6.7).
12
.
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