Chemistry Reference
In-Depth Information
effect of conventional solvents and favouring the
intramolecular pathway. Using density instead of
concentration has the big advantage that 'dilution'
can be achieved at constant volume, making the
reaction rate the only decisive parameter for the
space-time yield.
5.2 Improvement of changes in
stereoselectivity
One of the earliest examples of a homogeneously
metal-catalysed reaction in scCO
2
was the hydro-
formylation of propene using [Co
2
(CO)
8
] (Scheme
21.9) [36]. In the original work, a significant
improvement in regioselectivity for the desired linear
aldehyde was noted at rates comparable with those
obtained in classical solvents. The results of a later
study suggest that the selectivity can be affected
further by the CO
2
pressure (or density) of the reac-
tion medium [62]. The linear/branched ratio was
reported to increase from 2.7 : 1 to 4.3 : 1 as the
pressure doubled from 92 to 184 bar. A higher
linear/branched ratio also was reported for the
hydroformylation of hex-1-ene in scCO
2
using
rhodium trialkylphosphine [41]. Moreover, different
chemoselectivities from those observed in organic
solvents were obtained in this case.
The enantioselective hydroformylation using
rhodium catalysts bearing 'CO
2
-philic' chiral phos-
phine ligands gave unprecedented high regioselec-
tivities towards the branched isomers for several
substrates in scCO
2
[13]. Taking into account the
regio- and enantioselectivity, the overall selectivity
towards the desired single stereoisomer was signifi-
cantly higher than with the optimised system in con-
ventional solvents (Fig. 21.3). In this particular case,
detailed control experiments revealed that the high
regioselectivity originated mainly from the introduc-
tion of the perfluoroalkyl side chains into the ligand
framework rather than from the specific properties
of the reaction medium.
A remarkable effect of scCO
2
was observed in the
heterogeneously catalysed diastereoselective oxida-
tion of cysteine derivatives (see Scheme 21.21) [63].
The reaction, which proceeds without any stereo-
selective differentiation in conventional solvents,
could be optimised to reach up to 95% of diastereo-
selectivity in scCO
2
. Under conditions well beyond
the critical data of CO
2
, the selectivity exhibited a
dramatic dependence on pressure (density) of the
Scheme 21.1
8
Design of 'CO
2
-philic' anthraquinones.
Scheme 21.19
Enantioselective hydrogenation of
N
-(1-phenylethylidene)aniline in scCO
2
.
reaction profiles were strongly different in the two
media: the turnover rate was largely independent of
conversion in scCO
2
whereas the reaction slowed
down considerably as it proceeded in the organic
solvent.
It has been mentioned already that the high com-
pressibility of scCO
2
allows its bulk density to be
changed continuously from gas- to liquid-like values
with small variations of pressure and/or tempera-
ture. For example, the density of scCO
2
at 37°C is
0.33 g ml
-1
at 80 bar, but it increases to 0.80 g ml
-1
at
150 bar. Such density variations can have a strong
impact on chemical equilibria and reaction rates,
providing sometimes additional control over yield
and/or selectivity that is not available in conven-
tional liquids. For example, the product distribution
of the ring-closing metathesis (RCM) in scCO
2
cata-
lysed by ruthenium complexes could be controlled
entirely by adjusting the density of the supercritical
medium (see Scheme 21.20) [25]. The desired
macrocyclic product was produced with excellent
yields at densities above 0.65 g ml
-1
, whereas at lower
densities only oligomer products resulted from
acyclic diene metathesis (ADMET). Increasing the
density of the medium leads to an increased number
of solvent molecules, thus mimicking the dilution
