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OH
O
10 mol% Fe(NO
3
)
3
, 5 mol% FeBr
3
MeCN, air, rt, 24 h
R
2
R
1
R
2
R
1
secondary alcohol
ketone
10 mol% Fe(NO
3
)
3
, 5 mol% FeBr
3
MeCN, air, rt, 24 h
d
n
4
r
4
n
g
|
5
N.R.
CH
3
(CH
2
)
6
CH
2
OH
OH
O
10 mol% Fe(NO
3
)
3
, 5 mol% FeBr
3
MeCN, air, rt, 24 h
OH
OH
74%
Scheme 6.1 Aerobic oxidation catalyzed by Fe(NO
3
)
3
-FeBr
3
.
O
OH
5 mol% FeCl
3
.
6H
2
O, 2 mol% TEMPO, 5 mol% NaNO
2
R
2
R
1
R
2
R
1
PhCF
3
, air, rt
5 mol% FeCl
3
6H
2
O
2 mol% TEMPO
5 mol% NaNO
2
PhSCH
3
PhSCH
3
RCH
2
OH
+
R
CHO
+
PhCF
3
, air, rt
Scheme 6.2 Aerobic oxidation catalyzed by FeCl
3
-TEMPO-NaNO
2
.
6.1.2 Fe(
III
)-TEMPO-Catalyzed Aerobic Oxidation
TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl)
3
and related compounds
4
have
been used as catalysts for the oxidation of organic compounds for many
years. Recently, catalytic systems for alcohol oxidation involving transition
metals and TEMPO have attracted a significant amount of attention owing
to the use of molecular oxygen as the terminal oxidant. In 2005, Liang and
co-workers reported that FeCl
3
-TEMPO-NaNO
2
catalyzed the selective and
mild aerobic oxidation of a broad range of alcohols to the corresponding
aldehydes and ketones (Scheme 6.2).
5
The ideal conditions for the oxidation
used 5 mol% FeCl
3
, 2 mol% TEMPO and 5 mol% NaNO
2
as catalysts in
(trifluoromethyl)benzene (PhCF
3
) at room temperature under ambient air
pressure. Primary and secondary benzylic alcohols, cinnamyl alcohol and
secondary aliphatic alcohols were eciently oxidized to the corresponding
aldehydes and ketones in high yields with excellent selectivity. On the other
hand, aliphatic primary alcohols were converted to the corresponding
aldehydes with modest selectivity accompanied by both acids and esters. It is
notable that sulfide did not interfere with the alcohol and was not oxidized
under the reaction conditions. In fact, when a mixture of alcohols and
methyl phenyl sulfide was reacted with air under these reaction conditions,
the alcohols were selectively oxidized to aldehydes, whereas methyl phenyl
sulfide remained unreacted.
A plausible mechanism for this catalytic oxidation can be described as a
cascade of redox reactions involving two cycles as depicted in Scheme 6.3.
.
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