Chemistry Reference
In-Depth Information
As such the design of water-compatible catalysts will be the key to the future of this work.
Asymmetric aerobic oxidation in water is an important research target for the future. It
will not be necessary to mention the importance of environmentally benign oxidation
processes to this readership, but due to the high heat capacity and stability of water, the
process is promising. The use of water-soluble small molecules such as formaldehyde and
ammonia will be another critical area. These molecules are inexpensive and potentially
useful carbon and nitrogen building blocks; however, they are not well utilized in conven-
tional organic synthesis in organic solvents. Asymmetric catalysis using formaldehyde
and ammonia will be a key project not only in academia but also in industry.
1.3. CATALYTIC ASYMMETRIC SYNTHESIS IN ALTERNATIVE
REACTION MEDIA
Since many chiral catalysts are valuable and indeed expensive, recovery and reuse of the
catalysts is especially important in industry. For this purpose, nonconventional solvents
have been investigated. In this section, fl uorous solvents, SCFs, and ILs are described.
1.3.1. Fluorous Solvents
Fluorous compounds with appropriate melting and boiling points can be used as sol-
vents. Interestingly, these fl uorous compounds are remarkably different from the cor-
responding hydrocarbons and form bilayers with conventional organic solvents. In terms
of recovery and reuse of catalysts, fl uorous media is of great interest. Since fl uorous
solvents tend to mix poorly with common organic solvents, some catalysts can be immo-
bilized in fl uorous solvents in biphasic systems. In addition, one of the characteristic
points in fl uorous-organic biphasic systems is that some combination of fl uorous and
organic solvents demonstrates increased miscibility at elevated temperature, and that
heating can result in a completely homogeneous mixture in such cases. This is remark-
ably different from water-organic biphasic systems. Accordingly, in fl uorous - organic
biphasic systems, it is possible to carry out reactions under homogeneous conditions at
elevated temperature, and after the reactions occur, the mixture is cooled to become
two phases. While products are separated from organic solvent phases, catalysts can be
recovered from fl uorous solvent phases.
Since the fi rst report of fl uorous biphasic systems (FBSs) [31], reactions using fl uorous
solvents have been recognized as green reactions due to simple procedures and use of
chemically inert and low toxicity fl uorous solvents. In the past decade, FBSs have been
widely applied to asymmetric reactions.
A typical and very successful application of FBS is chiral phase transfer catalyst for
α
- tetrasubstituted symmetrical
chiral phase-transfer catalysts promote asymmetric alkylation of
t
- butylglycinate - ben-
zophenone schiff base [32]. The substituent of the 4,4
-amino acid synthesis. Synthetically convenient 4,4
′
,6,6
′
- positions of binaphthyl effects
on chiral effi ciency and the SiMe
2
(CH
2
CH
2
C
8
F
17
)-substituted catalyst designed as a
recyclable fl uorous chiral phase-transfer catalyst can promote the reaction's good chiral
effi ciency and reusability (Scheme 1.26).
The chiral fl uorous complex tetrakis - dirhodium(II) - (
S
) -
N
- (n - perfl uorooctylsulfonyl)
prolinate (
L - 13
) displays good chemo- and enantioselectivity in intermoleculer cyclo-
propanation and C-H bond activation reactions (Scheme 1.27). The catalyst can be
simply and thoroughly separated from the reaction mixture and is recyclable [33].
′
,6,6
′
