Chemistry Reference
In-Depth Information
-1
-1
Scheme 21.16
Cyclisation of 1,5-pentanediol and 1,6-hexanediol in scCO
2
.
high flow rates of substrates do not necessarily
require high flow rates of scCO
2
, which could be very
interesting for future scale-up of such continuous
and solvent-free syntheses.
The dehydration of alcohols to ethers with solid
acid catalysts was studied in the same continuous-
flow reactor system [60]. The results on the forma-
tion of cyclic ethers from a,w-diols showed that the
yield of the desired product could be adjusted nicely
by very small variations in the reaction parameters.
Optimised conditions led to high selectivity for the
cyclic alkyl ethers with very little rearrangement to
branched ethers, which is a typical side reaction in
classical syntheses (see Scheme 21.16).
Another promising example for the successful
transposition of a process from conventional solvents
to scCO
2
is the heterogeneously catalysed synthesis
of hydrogen peroxide [21,22]. The actual generation
of H
2
O
2
occurs via a sequential hydrogenation/oxi-
dation cycle of anthraquinones as a redox relay (see
Scheme 21.17).
In the present technology, the anthraquinones
are dissolved in an organic solvent and first hydro-
genated over a palladium catalyst at 30-50°C in a
three-phase reactor. The anthrahydroquinone then
is fed to a second two-phase reactor, where it is
oxidised back to the anthraquinone under produc-
tion of H
2
O
2
. The process is carried out continuously
and is inherently safe because hydrogen and oxygen
are not mixed directly and mild temperatures are
used. Nevertheless, it also has some severe solvent
constraints because it is mass transfer limited during
the hydrogenation step and deep hydrogenation of
the aromatic ring leads to by-products that are resis-
tant to the back oxidation. The work-up procedure
requires a tedious liquid-liquid extraction step to
isolate the H
2
O
2
. A transposition of this process in
scCO
2
thus seems very attractive but is hampered by
the very low solubility of the anthraquinones in
Scheme 21.17
The anthraquinone-based
hydrogenation/oxidation process for the production of H
2
O
2
.
scCO
2
(<0.1 mmol l
-1
). To overcome this limitation,
specially functionalised anthraquinones containing
CO
2
-philic fluorinated ponytails were designed (see
Scheme 21.18) [21] and tested as substrates for the
hydrogenation step in scCO
2
[22]. Indeed, enhanced
and kinetically controlled reaction rates were
observed and deep hydrogenation leading to unde-
sirable by-products was reduced, resulting in clean
formation of the fluorinated anthrahydroquinones.
A remarkable 20-fold increase in catalytic effi-
ciency was observed when the hydrogenation
of
N
-(1-phenylethylidene)aniline to
N
-phenyl-1-
phenylethylamine was carried out in scCO
2
instead
of CH
2
Cl
2
(see Scheme 21.19) [61]. The best results
were obtained with BARF as the counter-ion for the
chiral iridium catalyst and enantioselectivities were
fully comparable with those obtained in CH
2
Cl
2
. The
