Chemistry Reference
In-Depth Information
CO
2
Me
Rh(NOD)
2
(BF
4
)
3
+
L-22
or
L-23
CO
2
Me
R
R
S/C 200, H
2
(1 bar), rt, 10 min
TBME/[bmim]BF
4
R = NHAc, CH
2
CO
2
Me
100% conv., 99% ee
PCy
2
N
R'
2
P
Fe
N
N
H
(CF
3
SO
2
)
2
N
O
L-22
: (R)-(S) R = Ph
L-23:
(R)-(S) R = 3,5-(CF
3
)
2
C
6
H
3
Scheme 1.36.
Ru-BINAP and its derivative complexes are very common reagents for asymmetric
hydrogenation [58] . [Ru(O
2
CMe)
2
• ((
R
) - tolBINAP)] in [bmim][PF
6
]/H
2
O system pro-
motes the hydrogenation of tiglic acid to give the corresponding product in excellent
conversion with good ee (Scheme 1.37). The enantioselectivity depends on hydrogen
pressure, and water enhances enantioselectivity at high pressure, while there is no effect
of adding water at low pressure. To avoid the use of organic solvents, supercritical
carbon dioxide (scCO
2
) can be applied to this reaction to recover the organic product
from the reaction mixture. Since Ru catalyst is not soluble in scCO
2
, the product can be
extracted from IL by scCO
2
and is contaminated only by water. The catalyst can be
immobilized to the IL layer, therefore effi ciently reused multiple times without loss of
activity and even with enhanced enantioselectivity [59] .
CO
2
H
Ru(O
2
CMe)
2
•(
R
)-tolBINAP, H
2
(5 bar)
[C
4
C
1
im ][PF
6
]/H
2
O (3/2)
CO
2
H
*
Extract with scCO
2
99% conv., 85% ee
Scheme 1.37.
1.3.3.2. Asymmetric Diels-Alder Reaction in ILs
Copper catalysts based on imidaz-
olium-tagged bis(oxazolines) enhance the reaction rate and enantioselectivity of the
corresponding product in the reaction of
N
- acyloyl and
N
- crotonyloxazolidinones with
cyclopentadiene and 1,3 - cyclohexadiene (Scheme 1.38 ) [60] .
The introduction of an imidazolium tag into bis(oxazolines) signifi cantly improves
the recovery and reuse of the catalyst for reactions performed in IL. The catalyst can
be recycled at least 10 times without loss of activity and enantioselectivity. Furthermore,
the imidazolium-functioned ligand does not leach into the organic phase [61,62].
1.3.3.3. Asymmetric Epoxidation in ILs
Mn - salen complex
11
catalyzes asymmetric
epoxidation of alkenes effi ciently in IL [63,64]. Since IL is solidifi ed at the reaction
temperature, the reaction requires the use of DCM to form homogenous solution. IL
