Chemistry Reference
In-Depth Information
The aldehydes collected by the CESS approach con-
tained less than 1 ppm of rhodium and the catalysts
were recycled five times without any noticeable
change in activity and selectivity. Most recently, this
approach was extended also to enantioselective
hydroformylation reactions [14].
Scheme 21.25). The production of DMF with this
system occurs with outstanding efficiency, providing
quantitative conversion at almost perfect selectivity,
and up to 420 000 mol DMF mol
-1
catalyst could be
obtained. The molecular ruthenium catalysts could
be anchored to silica supports by a sol-gel process
and then were recyclable with negligible leaching
[72,73].
The use of CO
2
as a solvent and a reagent also has
been considered in the synthesis of organic car-
bonates [83] and particularly polycarbonates using
epoxides as the other starting material. The copoly-
merisation of propylene and CO
2
in scCO
2
was
effected using a heterogeneous zinc catalyst [84]. A
CO
2
-soluble zinc catalyst was used to achieve the
copolymerisation with 1,2-epoxycyclohexane, with
polymer yields up to 69%. Up to 400 g of polymer
were formed per gram of zinc and the product
contained more than 90% carbonate linkages (see
Scheme 21.26) [85]. Soluble chromium porphyrin
catalyst proved even more effective (3.9 kg polymer
g
-1
chromium) [86], giving copolymers containing
more than 95% carbonate linkages and narrow
molecular weight distributions.
In an early example for C-C coupling reactions,
CO
2
was used beyond its critical data as a reactant in
the nickel-catalysed synthesis of tetraethyl-2-pyrone
from hex-3-yne (see Scheme 21.27) [87]. A side
reaction of the nickel complexes with CO
2
was iden-
tified as a major deactivation pathway, in this case
with fully soluble catalytic systems [47]. On the
other hand, the reactivity of CO
2
also can help to
protect catalytically active centres by chemical inter-
action with functional groups of the substrate that
otherwise would deactivate the catalyst. This princi-
ple was demonstrated first with ruthenium catalysts
used for RCM in scCO
2
[25]. These catalysts are
deactivated by basic N-H groups and substrates
7 Simultaneous Use of Supercritical
CO
2
as Reaction Medium and Reagent
The use of CO
2
as a cheap, non-toxic and readily
available C
1
-building block seems especially desirable
within the context of 'green chemistry', because it
would allow the use of a small portion of a major
waste material of human activities to replace con-
siderably less benign reagents such as carbon
monoxide or phosgene. Despite its reputation of
being 'unreactive', CO
2
undergoes a variety of
synthetically useful transformations, especially in
the presence of transition metal catalysts [74].
The hydrogenation of CO
2
to formic acid and its
derivatives is an interesting approach to use CO
2
as
a raw material for commodity products, but efficient
catalysts were not available until the early 1990s
[75,76]. At that time, highly active catalysts for the
hydrogenation of CO
2
to formic acid were developed
on the basis of rhodium complexes soluble in polar
organic [77,78] or aqueous [79] solvents. These
catalysts, however, proved not to be compatible with
scCO
2
[76]. The first system operating efficiently in
scCO
2
was based on a ruthenium-alkylphosphine
complex, leading to turnover frequencies (TOFs) of
up to 1400 h
-1
compared with a TOF of 80 h
-1
with
the same catalytic system in tetrahydrofuran [80].
The addition of methanol or dimethylamine to the
system allowed the formation of formic acid to be
coupled to subsequent condensation to methyl
formate [81] or dimethylformamide (DMF) [82] (see
Scheme 21.25
Hydrogenation of scCO
2
to formic acid, methyl formate or DMF.
