Chemistry Reference
In-Depth Information
Me
3
SiN
3
Cr•(
R
,
R
)-
L-26
(3 mol %)
CSA
MeOH
[C
4
C
1
im][PF
6
]/[C
4
C
1
im][OTf] = (5/1)
20°C, 28 h
OSiMe
3
OH
N
3
N
3
O
1st: 68%, 94% ee
2nd: 72%, 93% ee
3rd: 85%, 93% ee
4th: 75%, 94% ee
5th: 76%, 93% ee
H
H
NN
Cr
Bu
t
t
Bu
O
O
Cl
t
Bu
Bu
t
Cr•(
R
,
R
)-
L-26
Scheme 1.40.
Co•(
R
,
R
)-
L-26•
OAc (0.5 mol %)
H
2
O
THF-[C
4
C
1
im][PF
6
], 20°C
24-48 h
O
H
O
O
+
Cl
Cl
Cl
OH
>99% ee
even after 10th use of the catalyst
H
H
NN
Co
t
Bu
O
t
Bu
O
t
Bu
t
Bu
Co•(
R
,
R
)-
L-26
Scheme 1.41.
against reduction to Co(II) complex. The catalyst can be reused 10 times without loss
of ee and reactivity [66] (Scheme 1.41 ).
1.3.3.5. Asymmetric Dihydroxylation in ILs
Osmium - catalyzed asymmetric dihy-
droxylation constitutes a versatile method for the synthesis of chiral vicinal diols from
alkenes. Its industrial utility suffers from several disadvantages, mainly the high cost of
osmium and chiral ligands and the toxicity of the metal, which may contaminate the
optically active product. To address these issues, many efforts to modify these catalysts
have been made. The main way to utilize chiral OsO
4
catalysts effi ciently is to immobilize
the catalyst to polymers [67], porous resins [68], and the other various solid supports
[69]. IL can also provide functions to support the catalysts [70]. In the asymmetric dihy-
droxylation of
trans
- stilbene catalyzed by OsO
4
(1.5 mol %) and ligand
L - 27
(2.0 mol %)
in the presence of N - methylmorpholine N - oxide (NMO) (2.6 mol %) and [C
4
C
1
im][PF
6
]
(2 mL for 2-mmol scale) in acetone-water (v/v, 10/1) at 0°C, the catalyst can be recovered
in IL phase and reused up to three times without signifi cant loss of activity and ee with
