Chemistry Reference
In-Depth Information
product (or the excess of a reagent) is tagged, then
the switches are called scavengers, and in this case
the product preserves the organic character (Fig.
22.22b) [82]. The components can be separated sim-
ilarly to the previous attempts by two- or three-
phase liquid-liquid extraction or by fluorous solid-
phase extraction. The detachment of the fluorinated
tagging unit is followed by simple phase separation.
could be made fluorous soluble by attaching fluorous
ponytails to the catalyst core in appropriate size and
number [114], mostly transition metal complexes
have been converted to fluorous soluble through
ligand modification. Fluorous-ligand-modified
biphasic catalysts [4,14,15] have been used success-
fully in allylic nucleophilic substitutions [115],
cross-coupling reactions [116], Diels-Alder reaction
[117], epoxidation of olefins [44,48,68,83,118-120],
Friedel-Crafts acylation [117], Heck heterocoupling
[121], hydrogenation [35,81,122-124], hydroformy-
lation [8,9,64], hydroboration [125], hydrosilylation
[126], intramolecular cyclisation of unsaturated
esters [69], Kharasch addition reaction [127], living
radical polymerisation [128], oligomerisation [78],
oxidation of alcohols [129], oxidation of aldehydes
[120], oxidation of alkanes [48,77], oxidation of sul-
fides [9,120], Stille couplings [103,130], and Wacker
oxidation of alkenes [55]. In addition, several chiral
catalysts have been developed recently for alkylation
of aromatic aldehydes [131,132] and epoxidation of
olefins [39]. The common feature of these catalytic
systems is the easy separation of the product(s) and
the facile recycling of the fluorous catalyst.
9 Fluorous Biphasic Catalysis
Simple and efficient separation of products from the
catalyst under mild conditions is crucial to the ap-
plication of homogeneous catalysts in industrial
processes. The use of biphasic systems, in which one
of the phases contains the dissolved catalyst, could
allow particularly easy separation of the products
(Fig. 22.1). A biphasic catalyst is designed to dissolve
preferentially in the
catalyst phase
, although having a
large partition towards the catalyst phase also could
be appropriate for some applications. The required
solubility can be achieved by attaching
solubilising
groups
to the catalyst. If the products (
P
) have limited
solubility in the catalyst phase, a second,
product
phase
could form during conversion of the reactants
(
A
and
B
), which can be separated easily.
The fluorous biphasic catalysis concept was orig-
inally developed during the search for a novel
catalytic system for the selective oxidation of me-
thane to methanol, utilising molecular oxygen
[9,14]. The fluorous medium is especially suitable for
oxidation reactions because the solubility of dioxy-
gen is very high and the fluorous solvents are
remarkably resistant to oxidation. Furthermore, as
the conversion level of an oxidation reaction
increases, the amount of polar products increases,
thus enhancing product separation. Because fluo-
rous solvents readily dissolve methane and have
limited miscibility with methanol, the over-oxidation
of the product methanol could be avoided provided
that a catalyst could be designed to dissolve prefer-
entially in the fluorous phase. The attachment of flu-
orous ponytails (L=R
fn
=F(CF
2
)
n
) to oxidation
catalysts, such as phthalocyanines or porphyrins,
was shown [9] to provide high fluorous solubility
and facile catalyst separation. Although this
approach has not resulted in a new methane oxida-
tion catalyst yet, it has led to the development of a
wide range of fluorous biphasic catalysts.
Although most soluble homogeneous catalysts
10 Relationship between Fluorous and
Supercritical Carbon Dioxide Media
Supercritical carbon dioxide (scCO
2
) has a pressure-
and temperature-dependent solubility parameter
(d=18.2 ¥r
sc
/r
liq
MPa
1/2
) and will be miscible
either with low-polarity organic solvents or fluoro-
carbons under appropriate conditions [133,134]. In
general, fluorous-soluble reagents and catalysts also
are soluble in supercritical carbon dioxide. Conse-
quently, a fluorous reagent or catalyst could be tested
in fluorous solvents at much lower operating pres-
sure and the most promising one developed further
in supercritical carbon dioxide for commercial appli-
cations. It should be noted that perfluoroaryl-
containing reagents and catalysts soluble in super-
critical carbon dioxide might not dissolve in fluorous
solvents.
11 Economical Feasibility of Fluorous
Biphasic Chemistry
Fluorous solvents generally are considered to be too
expensive for large-scale commercial processes.
However, their non-toxic nature and the significant
