Chemistry Reference
In-Depth Information
Ir
•L-15
H
2
(30 bar)
scCO
2
, 40°C
N
Ph
HN
Ph
Ph
CH
3
Ph
CH
3
81% ee
+
CF
3
O
-
P
N
B
Ir
CF
3
4
Ir
•L-15
Scheme 1.28.
Asymmetric hydrogenation also proceeds smoothly in scCO
2
by using the perfl uoro-
alkyl - substituted ligand (
R
,
S
) - 3 - H
2
F
6
- BINAPHOS (
L - 16
) and Rh complex (Scheme
1.29 ) [43] . Another Rh •
L - 17
complex for asymmetric hydrogenation in scCO
2
has also
been reported (Scheme 1.30) [44]. In these cases, the catalysts are soluble in scCO
2
to
form homogeneous systems during the reactions.
O
O
[Rh(cod)
2
]BF
4
/
L-16
H
CO
2
Me
H
CO
2
Me
H
2
(30 bar), CO
2
, 40°C
97.2% ee
Rf
P
O
O
P
O
Rf
Rf = CH
2
CH
2
(CF
2
)
5
CF
3
(
R
,
S
)-3-H
2
F
6
-BINAPHOS (
L-16
)
Scheme 1.29.
On the other hand, the continuous fl ow scCO
2
system [45] has been applied to asym-
metric hydrogenation using [Rh(COD)
2
]
+
[BF
4
]
-
/H
3
O
40
PW
12
/alumina (CATAXA ® ) and
Josiphos 001 ligand (
L - 18
) as an immobilized chiral catalyst [46]. A mixture of H
2
, CO
2
,
and a substrate (dimethyl itaconate) in 2-propanol is poured into a hydrogenation
reactor with the chiral Rh catalyst, and the product is obtained with 83% ee (Scheme
1.31). In this system, neither perfl uoroalkyl-substituted ligands nor counter anions with
