Chemistry Reference
In-Depth Information
Scheme 21.1
Asymmetric hydroformylation of styrene in scCO
2
: the large variation in the enantiomeric excess (ee) results from
the complex phase behaviour of the reaction mixture.
amine dimethylamine is used, the phase behaviour
is entirely different because CO
2
and dimethylamine
form the liquid adduct dimethylammonium carba-
mate (dimcarb) under the reaction conditions. A
liquid phase then is present at any stage of the
reaction, whereas single phase conditions can be
achieved at least initially with the tertiary amine.
Such investigations are very important for deter-
mination of the optimal reaction conditions and for
understanding the reaction systems.
The catalytic asymmetric hydroformylation of
styrene (see Scheme 21.1) provides an illustrative
example of the dramatic effects that can result from
the phase behaviour of reaction mixtures, especially
when presumably homogeneous organometallic cat-
alysts are involved. Catalysts for asymmetric hydro-
formylation often are formed in situ from an achiral
rhodium complex and a chiral diphosphine, and
the compatibility of such procedures with scCO
2
was
investigated for the BINAPHOS ligand [11]. High
CO
2
densities (0.70-0.82 g ml
-1
) resulted in single-
phase reaction mixtures, but very low enantioselec-
tivities (5-35%) were obtained for the aldehyde
under these conditions. When the densities were
lowered towards the critical density of pure CO
2
(0.47 g ml
-1
), the enantioselectivity improved up to
66% but a coloured liquid phase was observed
during the reaction. This striking density dependence
results from the poor solubility of the chiral ligand
in scCO
2
: the ligand-containing species inducing
enantioselectivity can operate only when a liquid
phase is formed at lower densities, whereas the cat-
alytic reaction occurs mainly via ligand-free rhodium
species in the supercritical phase.
The solubility of reactants, products and catalyst in
the supercritical phase obviously is a decisive factor
for the reaction design. The situation described above
Fig. 21.2
Fixation of fluorinated chains (wavy lines) to the
aromatic rings of an arylphosphine enhances the solubility of
the ligand and its metal (M) complexes in scCO
2
.
results from the fact that arylphosphine ligands and
their metal complexes—an important class of homo-
geneous catalysts for chemical synthesis—do not
exhibit sufficient solubility for catalysis under homo-
geneous supercritical conditions. One general
approach to overcome this limitation consists of
fixation of perfluoroalkyl groups to the aromatic
rings of these ligands (Fig. 21.2) [12]. Applying this
methodology to the BINAPHOS ligand used for
enantioselective hydroformylation resulted in a
catalytic system that provided over 90% enantio-
meric excess (ee) regardless of the phase behaviour
of the reaction mixture [13,14]. Other possibilities to
enhance the solubility of organometallic catalysts in
scCO
2
include the use of 'CO
2
-philic' (mainly fluori-
nated) counter-ions [15] or the addition of co-
solvents such as fluorinated alcohols [16].
The use of fluorinated groups to render organic
or inorganic materials 'CO
2
-philic' is not restricted to
transition metal catalysts. Other applications include
