Chemistry Reference
In-Depth Information
An inverted scCO
2
/aqueous biphasic system has been used as reaction media for
Rh-catalyzed asymmetric hydrogenation. Chiral CO
2
- philic catalysts are effi ciently
immobilized in scCO
2
as the stationary phase, while polar substrates and products are
contained in water as a mobile phase. The catalyst phase is reused several times with
high conversion and product recovery of more than 85%. The chiral ligand (
R
,
S
) - 3 - H
2
F
6
-
BINAPHOS (
L - 16
) allows highly enentioselective hydrogenation of itaconic acid and
methyl - 2 - acetamidoacrylate under these conditions [48] .
1.3.3. IL
ILs are defi ned as materials that are composed of ions and have melting points below
100°C (the temperature may be fl exible). ILs have many properties that lend themselves
to clean chemical synthesis, perhaps most notably that they are nonvolatile and are
therefore not lost to the atmosphere. The main advantages are that catalyst lifetimes
can be extended, often considerably, and rates of reactions are accelerated in many
cases. In addition, recovery and reuse of catalysts is possible using biphasic or multipha-
sic catalyst systems. Effi cient reactions with organic compounds under homogeneous
conditions in an IL, followed by extraction of the products with scCO
2
, provide an ideal
process without any volatile organic solvents. Typical ILs are shown in Scheme 1.33.
R
1
R
2
N
N
[NR
n
H
4-n
]
+
X
-
+
N
R
N
-alkylpyridinium salts
X
X
N,N'
-dialkylimidazolium salts
Alkyl ammonium salts
X = hal, BF
4
, PF
6
, OTf, OAc, NTf
2
, SbF
6
, etc
R
1
R
2
Abbreviations
C
4
C
1
im
:
R
1
= C
4
H
9
, R
2
= CH
3
C
4
C
2
im
:
R
1
= C
4
H
9
, R
2
= C
2
H
5
C
2
C
1
im
:
R
1
= C
2
H
5
, R
2
= CH
3
N
N
+
X
Scheme 1.33.
Major classes of ionic liquids.
1.3.3.1. Asymmetric Hydrogenation in ILs
The catalytic enantioselective hydrogena-
tion of unsaturated bonds such as C= C, C =O, and C=N in the presence of molecular
hydrogen or hydrogen donor is one of the most useful chemical transformations. Since
molecular hydrogen is used as the reactant in many cases, it is clear that the solubility
of hydrogen gas in ILs is an important factor in catalyzed reactions. It has been suggested
that increased reaction rates in biphasic hydrogenation reactions in ILs could be due to
high solubility of hydrogen in the IL, and the correlation of ILs and the solubility of
molecular hydrogen has been studied [49].
Since the fi rst report of enantioselective metal-catalyzed reaction in ILs [50], asym-
metric hydrogenation of carbon-carbon double bonds and carbon-heteroatom double
bonds has been the most investigated transformation. Several combinations of transition
metals, chiral ligands, and ILs have been studied for these reactions [51].
