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all cases, remarkable improvements were observed, and enantioselectivity higher than
80% ee was obtained in the oxidation of aryl methyl sulfi des. From the observed positive
nonlinear effect and the necessity of a half equivalent of the carboxylic acid/carboxylate
additive with respect to iron, a monocarboxylate-bridged diiron(III) complex has been
proposed as a key intermediate in the catalytic cycle.
Egami and Katsuki also reported an iron-based catalyst for the reaction (Scheme
11.61 ) [109] . Fe(salan) complex
52
serves as an effective catalyst, with aqueous hydrogen
peroxide as oxidant. The method employs the most favorable solvent, water, and can be
performed even in the absence of a surfactant. It is noteworthy that not only alkyl aryl
sulfi des but also dialkyl sulfi des underwent oxidation with high enantioselectivity. Scarso
and Strukul also reported the platinum-catalyzed asymmetric sulfi de oxidation in water,
but their system requires the addition of a surfactant [110].
O
-
52
(1 mol
%)
R
1
S
30% H
2
O
2
(1.5 equiv.)
+
R
2
R
1
S
+
H
2
O, 20°C
Me
M
e
R
2
N
N
81-96% ee
Fe
O
O
Cl
O
-
O
-
Ph
Ph
O
-
O
-
S
+
S
+
S
+
S
+
C
12
H
25
92%, 96% ee
78%, 81% ee
93%, 87% ee
82%, 94% ee
52
Scheme 11.61.
11.4.6. Aluminum Catalyst
Recently, Katsuki and coworkers identifi ed aluminum(salalen) complexes as effi cient
catalysts for asymmetric sulfi de oxidation using aqueous hydrogen peroxide (Scheme
11.62 ) [111] . Al(salalen) complex
53
promotes the oxidation of a wide variety of sulfi des
with high enantioselectivity even under aqueous conditions. Aryl methyl sulfi des undergo
highly enantioselective oxidation, and ethyl phenyl sulfoxide and benzyl methyl sulfox-
ide are also obtained with high enantioselectivity. Other sulfur-containing compounds
such as cyclic sulfi des and thioacetals are also good substrates for the oxidation system
53
(2 mol
%)
pH 7.4 phosphate buffer
O
−
R
1
S
+
30% H
2
O
2
(1.1 equiv.)
R
2
R
1
S
+
M
e
MeOH
R
2
N
N
Al
Cl
O
O
S
O
−
S
O
−
P
h
Ph
O
−
+
+
S
S
S
+
O
−
O
−
Ph
Ph
86%, 98% ee
80%, 91% ee
83%, 80% ee
92%, 98% ee
89%, 99% ee
53
Scheme 11.62.
