Chemistry Reference
In-Depth Information
attractive alternative. We refer to such procedures
in organometallic catalysis as CESS (
c
atalysis and
e
xtraction using
s
upercritical
s
olutions) processes.
After changing from homogeneous to multiphase
conditions by small changes of the process parame-
ters, the product is removed selectively from the
reaction system with CO
2
by methods closely related
to supercritical fluid extraction (SFE), leaving the
catalyst in the reactor in its active form.
The first example of a combination of homoge-
neous organometallic catalysis and SFE was
described for ring-closing olefin metathesis (RCM)
reactions in scCO
2
using ruthenium carbene catalysts
(Scheme 21.20) [25]. These catalysts showed a very
limited solubility in scCO
2
, the bulk of material
remaining undissolved under the reaction condi-
tions. Simple extraction of the reactor content with
scCO
2
allowed the products to be removed selectively
and the residue in the reactor was still active for
RCM in subsequent runs. Similar observations were
made for palladium-catalysed Heck (Scheme 21.8),
Suzuki and Sonogashira C-C coupling reactions in
scCO
2
[31]. In the enantioselective hydrovinylation
shown in Scheme 21.6, the extraction of the volatile
products resulted not only in separation from the
catalyst but also in purification from less-volatile
by-products formed via secondary reactions of the
product olefin [29].
A special situation is encountered for the iridium-
catalysed enantioselective hydrogenation of imines
in scCO
2
using cationic iridium catalysts (Scheme
21.19) [61]. Detailed control experiments revealed
that the catalyst showed sufficient solubility when
the substrate was present in the reaction mixture but
it was almost insoluble in mixtures with only the
product. Thus, it was possible to remove the reaction
product selectively from the catalyst simply by
purging the reactor with compressed CO
2
after
complete conversion and no change of conditions
between the reaction and extraction stage was
necessary. The catalyst remaining in the reactor
was recycled seven times, exhibiting a certain loss of
activity but no decrease of enantioselectivity (Fig.
21.4).
Another intriguing example for the CESS process
was described for the homogeneous hydroformyla-
tion of long-chain terminal alkenes in scCO
2
(Scheme 21.9) [38]. In the currently used conven-
tional industrial processes, the high-boiling aldehy-
des obtained from these reactions have to be
separated from the catalysts by tedious distillation
and extraction procedures. In scCO
2
, the reaction can
be performed with high rates and selectivities using
rhodium catalysts modified with perfluoroalkyl-
substituted phosphorus ligands. The solubility
properties of these ligands ensure homogeneous
single-phase reaction mixtures for a wide range of
substrates at various temperatures and pressures, but
still allow for the identification of efficient separation
conditions even with very-high-boiling products.
Fig. 21.4
Catalyst recycling in the
enantioselective hydrogenation of
imines (Scheme 21.19) using the CESS
procedure.
