Chemistry Reference
In-Depth Information
OsO
4
(0.2 mol
%)
62
or
63
(2 mol
%)
O
OH
O
OR'
+ NMO
(1.1 equiv.)
OR'
t
BuOH/H
2
O, pH 5, RT
OH
R
R
40-70% ee
K
2
[OsO
2
(OH)
4
] (1 mol
%)
62
or
63
(2-5 mol
%)
NHTs
OH
R"
R"
R"
+ TsN(Na)Cl•3H
2
O
(1 equiv.)
+
t
BuOH/H
2
O, pH 8, RT
OH
NHTs
30-59% ee
24-55% ee
N
HTs
OH
CO
2
H
CO
2
H
or
OH
NHTs
6
6
Scheme 11.74.
In the common procedures of the Sharpless asymmetric dihydroxylation, NMO or
potassium ferricyanide/potassium carbonate is used as an oxidant to give the active
osmium tetroxide. However, the cogeneration of stoichiometric amount of NMM or
ferrocyanide is problematic from the viewpoint of atom economy. Thus, alternative
methods for the catalytic regeneration of osmium tetroxide from an osmate
VI
species
have been studied. Beller and coworkers achieved the asymmetric dihydroxylation using
molecular oxygen without the addition of mediators (Scheme 11.75) [137]. While the
elevated reaction temperature led to somewhat decreased enantioselectivity, optically
active 1,2-diols could be obtained with reasonable enantioselectivity. Bäckvall and
coworkers also disclosed that fl avin
64
can mediate the osmium-catalyzed dihydroxyl-
ation in the presence of aqueous hydrogen peroxide as the terminal oxidant (Scheme
11.76 ) [138] . The reaction proceeded effi ciently under the mild conditions, and high
enantioselectivity was observed.
K
2
[OsO
2
(OH)
4
]
(DHQD)
2
PHAL
OH
+ O
2
(1 bar)
OH
Ph
Ph
pH 10.4 buffer/
t
BuOH, 50°C
52%, 90% ee
Scheme 11.75.
The Sharpless asymmetric dihydroxylation displays excellent enantioselectivity for a
wide range of olefi ns and is frequently utilized in organic synthesis. However, a major
drawback of the Sharpless method is the toxicity of the osmium reagent and waste
materials derived from it. Thus, the development of an alternative method using more
environmentally benign metals has been urged.
Que and coworkers for the fi rst time discovered an iron-based catalyst for enantiose-
lective
cis
- dihydroxylation of olefi ns. In the presence of aqueous hydrogen peroxide as
