Chemistry Reference
In-Depth Information
Fig. 22.6
Synthesis of a chiral BINOL derivative. (From Ref.
8
4.)
precursor acid chlorides with
i
-Pr
3
SiH, followed by
phase separation at -20°C from the side product
i
-Pr
3
SiCl [87]; and the reaction of difluorotris(per-
fluoroalkyl)phosphoranes, F
2
P(R
f
n
)
3
, with excess of
P(SiMe
3
)
3
leads to the formation of the correspond-
ing tris(perfluroalkyl)-phosphines, P(R
f
n
)
3
, in high
yield [88]. Fluorous crown ethers can be obtained
under cryogenic conditions by elemental fluorina-
tion of the corresponding hydrocarbon crown ethers
supported on NaF. These fluorous macrocycles are
very weak bases and not soluble in common organic
solvents [89].
Owing to the great importance of alkyl and aryl
phosphines in homogeneous transition metal cata-
lysis, it is not surprising that their fluorous analogues
are among the most studied ligands in fluorous
biphasic catalysis [90].
It is important to note that the reactivity of
R
f
(CH
2
)
n
X (
n
= 0, 1, 2; X = Cl, Br, I) derivatives
appears to be anomalous when compared with
the parent hydrocarbon derivatives. For example,
perfluoroalkyl iodides (
n
= 0) are attacked at the
iodine atom when reacted with organolithium
anions (LiR) [91]. The rather low reactivity of
CF
3
CH
2
X (X = Cl, Br, I) compounds [92] as well as
the easy HX elimination from perfluoroalkylethyl
halides, R
f
(CH
2
)
2
X, are well documented [56]. Prac-
tically, compounds having (CH
2
)
3
or higher spacers
between the perfluoroalkyl groups (
R
f
n
) and the
reaction centre will show normal reactivities but
these can be coupled with solubility problems when
longer perfluoroalkyl chains are involved.
5 Fluorous Extraction
Generally a large number of longer fluorous pony-
tails are necessary to achieve high fluorous phase
separation by liquid-liquid extraction. The attach-
ment of longer fluorous ponytails, however,
will increase the molecular weight so much that
commercial applicability is beyond reality. Because a
trifluoromethyl group could serve as a superdense
fluorophilic moiety, replacement of some of the
longer fluorous ponytails with trifluoromethyl
groups could significantly lower the molecular
weight and maintain fluorous solubility [33].
Extraction processes can be classified as simple
or reactive. In the former type every entity parti-
tions according to its phase behaviour, which is
determined by the composition and structure.
Simple fluorous extractions are involved in fluorous
catalyst recovery processes [11] and fluorous syn-
theses [19,93]. Examples for reactive extractions are
the synthesis of various fluorous catalysts by
the reaction of organic-soluble metal precursors
and fluorous ligands such as phosphines [9] and
the synthesis of fluorous fullerene adducts [94].
The preparation of
trans
-Pt[P(CH
2
CH
2
R
f6
)
3
]
2
Cl
2
com-
plex is a typical example for reactive extraction. A
suspension of
cis-
Pt(C
6
H
5
CN)
2
Cl
2
in CH
3
CN and
a solution of P(CH
2
CH
2
R
f6
)
3
in CF
3
C
6
F
11
was stirred
overnight. As a result, a biphasic mixture was
obtained consisting of a colourless CH
3
CN phase and
a very pale yellow CF
3
C
6
F
11
phase. The phases then
were separated and the CF
3
C
6
F
11
was removed
in
