Chemistry Reference
In-Depth Information
containing this type of functional group can be
cyclised in conventional solvents in protected form
only. The conventional synthesis to obtain cyclic
amines by this route therefore involves three steps
(protection/cyclisation/deprotection) and has a low
atom efficiency due to the protecting group manip-
ulation. In scCO
2
, however, the cyclisation is readily
achieved without the need for an additional pro-
tecting group (see Scheme 21.28), because the
reversible formation of the carbamic acid results in a
temporary protection of the amine in the solvent
scCO
2
. Upon pressure release, the carbamic acid is
converted back spontaneously to the free amine,
which is isolated directly in one step.
combination with homogeneous or heterogeneous
catalysts. Additional benefits such as increased reac-
tion rates, tuning of product selectivity, enhanced
catalyst lifetime and the combination of reaction and
extraction often can result from the choice of scCO
2
instead of an organic solvent. Although scCO
2
is far
from being a universal solvent, the design of 'CO
2
-
philic' materials and surfactants is making rapid
progress and will help to broaden the scope of
possible applications in the near future. At present,
the economic evaluation of CO
2
-based catalytic
processes is still difficult because many important
parameters are not yet known with sufficient cer-
tainty. Obviously, working with scCO
2
means signif-
icant capital investment in costly high-pressure
equipment and benefits such as simplified work-up
procedures, enhanced reaction rates and selectivities,
or savings through elimination of organic and
hazardous wastes, must help to counterbalance these
costs. There is, however, increasing evidence that
8 Conclusion
Supercritical carbon dioxide is a highly promising
reaction medium for environmentally benign
chemical synthesis and can be applied widely in
Scheme 21.26
Copolymerisation of 1,2-epoxycyclohexane with scCO
2
.
Scheme 21.27
Coupling of hex-3-yne
with scCO
2
.
Scheme 21.2
8
Use of scCO
2
as solvent
and temporary protecting group
during ring-closing metathesis (RCM)
of amines.
