6850
R. Yanada, Y. Takemoto / Tetrahedron Letters 43 (2002) 6849–6851
Table 1. Effects of solvents and cooxidants on OsO4-catalyzed dihydroxylation of 1,1-diphenylethylene
Run
Solventa
OsO4 mol%
Cooxidant
Yield of 2b
%
Recovery of 1b
%
1
2
3
4
5
6
7
8
[emim]BF4
[emim]BF4
A
[emim]BF4
[emim]BF4
[emim]BF4
[emim]BF4
[emim]PF6
5
1
1
5
5
5
5
5
NMO·H2O
NMO·H2O
NMO·H2O
30%H2O2
Na2CO3·1.5 H2O2
t-BuOOH
100
29
44
2
18
31
6
0
71
56
96
82
18
94
NMO
NMO·H2O
0
Complex mixturec
a [emim]BF4=1-ethyl-3-methylimidazolium tetrafluoroborate; A=[emim]BF4+Et4NOH (0.1 equiv., 10% aqueous solution); [emim]PF6=1-ethyl-3-
methylimidazolium hexafluoroborate.
b Yields were determined by 1H NMR.
c This reaction was carried out at 70°C because the melting point of [emim]PF6 is 61°C.
This new method using immobilized OsO4 in an ionic
liquid was applied to several substrates, including
mono-, di-, and trisubstituted aliphatic olefins, as well
as to aromatic olefins. The results are summarized in
Table 3. In all cases, the desired diols were obtained in
high yields (runs 1–5).
Table 2. Recovery and reuse of OsO4 (5 mol%) in
[emim]BF4 using 1,1-diphenylethylene
Run
1
2
3
4
5
Yield (%)
95
93
96
95
93
In summary, we have demonstrated the usefulness of
recoverable and reusable immobilized OsO4 in
[emim]BF4 for dihydroxylation of several olefins. This
approach is simple and practical. It should be noted
that the volatility and toxicity of OsO4 are greatly
suppressed when the ionic liquid [emim]BF4 is used.11
Examination of the applicability of our method to the
asymmetric dihydroxylation of olefins is now in
progress.
[emin]PF6 afforded no desired product (run 8). As
described above, the reaction conditions in run 1 is the
choice for good yield.
Next, we examined the possibility of recovery and reuse
of OsO4 in [emim]BF4. After the first reaction had been
completed, ethyl acetate was added to the reaction
mixture in order to extract the diol. An orange–brown
ionic liquid containing the catalyst was recovered, and
this could be reused for further catalytic oxidation
reaction. This procedure was repeated five times, and
the results obtained are shown in Table 2 (runs 1–5).
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