Chemistry Reference
In-Depth Information
Scheme 21.7
Stille cross-coupling of PhI in scCO
2
.
Scheme 21.
8
Heck cross-coupling of PhI in scCO
2
.
Scheme 21.9
Hydroformylation of
alkenes in scCO
2
.
important hydroformylation in particular has been
investigated intensively, and catalytic systems com-
patible with the use of compressed CO
2
include
unmodified cobalt [36,37] or rhodium [38] catalysts,
phosphine-modified rhodium catalysts [12,38-42],
heterogenised rhodium systems [43] and even
chiral catalysts for asymmetric synthesis [13,14].
The hydroformylation of long-chain terminal and
internal olefins using unmodified rhodium catalysts
occurs with significantly higher rates in scCO
2
as
compared with toluene (see Scheme 21.9) [38]. We
will come back to this reaction in more detail on
several occasions throughout this chapter.
From the few selected examples discussed above,
it can be seen already that a large variety of syn-
thetically useful reactions can be performed in scCO
2
with similar and sometimes even better results than
in conventional organic solvents. The use of CO
2
in
industrial-scale synthesis would imply continuous
processes where the products recovered at the outlet
of the reactor are free of organic solvents and the
gas used as reaction medium would be recycled and
recompressed. In addition to providing a 'green'
alternative to VOCs without producing hazardous or
toxic organic wastes, the use of scCO
2
can increase
the inherent safety of a process, as discussed in the
next section.
4 Use of Supercritical CO
2
for
Safer Processes
The use of non-explosive and non-flammable scCO
2
as a solvent (i.e. in a large excess compared with
all other components) can reduce potential risks and
hazards in many reaction types involving highly
reactive reactants or gases. The inert solvent, which
is used, for example, in fire extinguishers, allows
pressurised gases such as hydrogen, carbon monox-
ide or oxygen to be manipulated under much safer
conditions than applying these reagents in neat
form. Furthermore, the excellent heat transport
capacities of scCO
2
relative to low-density gaseous
mixtures can be very advantageous for the removal
of heat from highly exothermic reactions, thus
avoiding hot spots in reactors.
Following these considerations, scCO
2
provides a
highly attractive medium for oxidation reactions.
In addition, CO
2
itself is of course inert towards
oxidation, thus excluding the formation of any by-
products from oxidation of the solvent. Several
studies have focused on metal-catalysed oxidation of
olefins using organic or inorganic oxidants. The
oxidative cleavage of C=C double bonds using cat-
alytic amounts of ruthenium tetroxide was performed
in a biphasic water/scCO
2
system [44]. The actual
