Chemistry Reference
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5 mol% [Imim-TEMPO][FeCl
4
]
5 mol% NaNO
2
, O
2
(0.2 MPa), H
2
O, 30-100
o
C
OH
O
R
2
R
1
R
2
R
1
d
n
4
r
4
n
g
|
5
O
N
N
[Imim-TEMPO][FeCl
4
] =
N
O
O
FeCl
4
Scheme 6.8 Aerobic oxidation catalyzed by Fe and TEMPO supported in IL.
8 mol% Fe(NO
3
)
3
.
6H
2
O, 0.1-1 mol% SBA-15-TEMPO
10 mol% NaNO
2
, O
2
(1 atm), toluene, 25
o
C
OH
O
R
2
R
1
R
2
R
1
50~99%
Scheme 6.9 Aerobic oxidation catalyzed by FeCl
3
6H
2
O-SBA-15-TEMPO/NaNO
2
.
high yields. The catalyst SBA-15-TEMPO can be reused for 10 reaction cycles
without a significant loss of catalytic activity.
6.2 Oxidations Catalyzed by Vanadium Compounds
6.2.1 Aerobic Oxidation of a-Hydroxycarbonyls and Related
Compounds
In 1999, Kirihara et al. reported that a-hydroxycarbonyls were eciently
oxidized to a-dicarbonyls using a catalytic amount of VOCl
3
in acetonitrile
under an oxygen atmosphere (Scheme 6.10).
15
Various a-hydroxy ketones
and a-hydroxy esters afforded the desired a-dicarbonyls in almost quanti-
tative yield. On the other hand, alcohols lacking a carbonyl moiety at the
a-position did not react under the same conditions.
Toste and co-workers developed the oxidative kinetic resolution of
a-hydroxy esters by their aerobic asymmetric oxidation catalyzed by
VO(OiPr)
3
and chiral O,N,O-chelating tridentate ligands (Scheme 6.11).
16
Several racemic aryl-, vinyl- and alkyl-substituted a-hydroxy esters were
eciently resolved with selectivity factors (k
rel
) ranging from 12 to 450.
Chen and co-workers synthesized a series of chiral vanadyl carboxylates
derived from N-salicylidene-
L
-a-amino acids and V(
V
).
17,18
These chiral
catalysts were effective in the aerobic oxidative kinetic resolution of a variety
of racemic a-hydroxy esters,
17
amides
17
and phosphonates
18
(Scheme 6.12).
During the course of their investigations, Chen and co-workers found that
a pentanuclear C
4
-symmetric cluster complex was formed for the 3,5-di-tert-
butyl analog when potassium salts were employed.
19
The cluster complex
was utilized to facilitate the asymmetric aerobic oxidations of various
.
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