Chemistry Reference
In-Depth Information
O
O
45
(X = Br) (3 mol
%)
O
Ar
+
13% NaOCl
Ar
Ar
X
−
R
R'
R
R'
Toluene, 0°C
OH
89-99% ee
N
+
O
O
O
O
O
O
OH
Ph
Ph
Ph
Ar
Ar
Ph
Ar
Ar = 3,5-Ph
2
-C
6
H
3
45
99%, 96% ee
91%, 99% ee
98%, 96% ee
Scheme 11.54.
O
-
46
(2 mol
%)
R
1
S
+
UHP
(1 equiv.)
R
2
R
1
S
+
MeOH, 0°C
R
2
N
N
92-99% ee
Ti
O
O
O
-
O
-
O
-
P
h
Ph
O
-
S
+
S
+
S
+
S
+
Cl
O
78%, 98% ee
88%, 99% ee
91%, 93% ee
72%, 93% ee
2
46
Scheme 11.55.
11.4. ASYMMETRIC SULFIDE OXIDATION
11.4.1. Introduction
Since the development of the titanium/tartrate-catalyzed asymmetric oxidation of sul-
fi des that was independently reported by the Kagan and Modena groups in the early
1980s, signifi cant efforts have been made to expand the scope of asymmetric sulfi de
oxidation [92-94]. There are a number of powerful catalysts for specifi c sulfi des today.
Current interests are directed toward the development of more sustainable methods
using greener hydrogen peroxide as the oxidant and more enantioselective catalysts for
challenging substrates such as dialkyl sulfi des.
11.4.2. Titanium Catalyst
While a number of asymmetric sulfi de oxidations using titanium-based catalysts have
been developed, there were few reports that used hydrogen peroxide as oxidant and
achieved both high enantioselectivity and a wide substrate scope [95]. In 2001, Saito and
Katsuki demonstrated that di-μ - oxo titanium(salen) complex
46
catalyzed asymmetric
oxidation of sulfi des with high enantioselectivity in the presence of UHP as the oxidant
(Scheme 11.55) [96]. Not only aryl methyl sulfi des but also ethyl phenyl sulfi de and
benzyl methyl sulfi de gave high enantioselectivity. Ti(salen) complex
46
was also
