Chemistry Reference
In-Depth Information
Rh and Ru-chiral phosphine or Ru-chiral diamine complexes are often combined
with
N
,
N
′
- dialkylimizazolium salts [52,53] . The Rh - catalyzed hydrogention of C = C bond
such as
-acetoamide cinnamic acid and related enamides works well to give the cor-
responding products with high enantioselectivities (Scheme 1.34). The catalyst can be
reused, and IL can suppress the catalyst aging in some cases; however, in other cases,
the catalyst loses its activity in the process of recycling, probably because of leaching or
deactivation [54] .
α
Rh•
L-20
H
2
(2 bar), 25°C
[C
4
C
2
im][PF
6
]-
i
PrOH
CO
2
Me
CO
2
Me
NHAc
83% conv., 93% ee
NHAc
OTf
P
P
Rh
Rh•
L-20
Scheme 1.34.
Imidazolium moieties were introduced to chiral diphosphine ligand (
L - 21
) in order
to attach the catalyst to the IL phase. The modifi ed catalyst can work effi ciently in the
hydrogenation and can be recovered and reused without signifi cant loss of activity
(Scheme 1.35 ) [55] .
[Rh(cod)•
L-21
](BF
4
)
3
H
2
(1 bar), 20°C
[C
4
C
2
im][PF
6
]-
i
PrOH
NHAc
NHAc
O
N
N
N
N
N
N
Ph
2
P
PPh
2
L-21
Scheme 1.35.
With a similar concept, Josephos ligands with an imidazolium tag can be applied to
the asymmetric hydrogenation of methyl acetamidiacrylate and dimethyl itaconate in
biphasic cosolvent/IL combinations (Scheme 1.36) [56,57]. The introduction of an imid-
azolium group in the Josephos ligand improves the affi nity of the Rh complex for
the IL to lead the suppression of catalyst leaching in the combination of t-butyl methyl
ether (TBME)/[bmim]BF
4
. The catalyst can be recycled and reused with effi cient product
isolation.
