Chemistry Reference
In-Depth Information
O
OH
Polyaniline-supported VO(acac)
2
, (2.3 mol% eq. V), O
2
toluene, 100
o
C
R
R'
R
'
55-99%
R = alkyl, aryl; R' = alkyl, aryl, H
d
n
4
r
4
n
g
|
5
CH
2
Cl
2
+ VO(acac)
2
VO(acac)
2
rt, 48 h
polyaniline
Polyaniline-supported VO(acac)
2
O
O
O
O
O
O
O
O
O
V
V
O
N
N
N
N
n
Proposed structure of polyaniline-supported VO(acac)
2
Scheme 6.27 Aerobic oxidation of alcohols catalyzed by recyclable polyaniline-sup-
ported VO(acac)
2
.
Polyaniline-supported VO(acac)
2
(2.3 mol% eq. V), O
2
O
Ph
H
toluene, 100
o
C, 9 h
98%
Ph
OH
O
2.5 mol% VO(acac)
2
, O
2
O
O
+
+
toluene, 100
o
C, 9 h
Ph
H
Ph
OH
Ph
O
h
50%
20%
20%
.
Scheme 6.28 Oxidation of benzyl alcohol catalyzed by polyaniline-supported and -
unsupported VO(acac)
2
.
The proposed reaction mechanism is depicted in Scheme 6.29.
32
The
alcohol, followed by O
2
, may coordinate with the vanadium catalyst to
generate a superoxo complex (a) that can readily undergo intramolecular
hydrogen abstraction to give an intermediate (b), followed by elimination of
H
2
O
2
to complete the catalytic cycle.
Sobczak and co-workers developed a procedure for the oxidation of
alcohols with O
2
catalyzed by N-hydroxyphthalimide (NHPI) and vanadium
compounds (Scheme 6.30).
33
Various alcohols (primary and secondary) were
selectively oxidized to the corresponding carboxylic acids and ketones in
high yields under an oxygen atmosphere in the presence of 5 mol% NHPI,
0.2 mol% VO(acac)
2
and 0.2 mol% LiCl. Two other catalytic systems based on
NHPI-VO(acac)
2
-Bn
4
NCl and NHPI-(Bn
4
N)VO
3
were also shown to be
effective.
A plausible mechanism for the oxidation of cyclohexanol to cyclohexanone
was shown to involve both a radical and a molecular pathway (cycles A and B,
respectively) (Scheme 6.31).
34
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