Chemistry Reference
In-Depth Information
oxygen source NaIO
4
was located in the aqueous
phase, whereas the ruthenium catalyst migrated
between the two phases (see Scheme 21.10). The
selectivity for adipic acid was very high (99%) but
rapid deactivation of the ruthenium catalyst occurred.
The metal-catalysed oxidation of alkenes with
organic peroxides occurs readily in scCO
2
as the
solvent [45-47]. Three studies showed almost simul-
taneously that good to excellent yields of epoxides
could be obtained with anhydrous
t
BuOOH as the
oxidant using molybdenum-based catalysts. Diols
were obtained as the major products with aqueous
t
BuOOH, most probably due to the in situ hydrolysis
of the epoxide (see Scheme 21.11). Typical yields
of diols often were better in scCO
2
than in con-
ventional solvents and the products generally
were cleaner. Organic hydroperoxides also were
employed for enantioselective epoxidations using a
titanium catalyst in the presence of chiral tartrate
ligands [46].
The selective oxidation of organic compounds with
molecular oxygen is still a major challenge for chem-
ical synthesis and the use of scCO
2
seems to open
very promising new directions in this area. As
mentioned previously, scCO
2
is largely miscible with
oxygen, thus allowing the oxidant/substrate ratio to
increase while considerably improving the safety of
the process due to expanded explosion limits. The
rhodium-catalysed aerobic oxidation of tetrahydro-
furan to g-butyrolactone has been achieved in scCO
2
with moderate turnovers and selectivities [48].
Several studies on catalytic [49] and non-catalytic
[50] free-radical aerobic oxidations of alkanes have
been conducted and the partial oxidation of propane
catalysed by metals impregnated on solid supports
demonstrates the potential of scCO
2
for these reac-
tions [51]. The aerobic oxidation of cyclohexane
to cyclohexanol and cyclohexanone catalysed by
an iron complex bearing perfluorinated porphyrin
ligands has been studied in sub- and supercritical
CO
2
[52]. The results seemed to indicate that the
reaction rate reaches a maximum around the critical
pressure of CO
2
.
Related iron-based catalysts were found to enable
the epoxidation of cyclohexene with molecular
oxygen as the sole oxidant, but yields and selectivi-
ties were moderate [53]. A frequently used method
to activate molecular oxygen for selective epoxida-
tions is the addition of an aldehyde as a sacrificial
co-oxidant. Under these conditions, the oxidation of
alkenes was found to be very efficient in scCO
2
, the
reaction being initiated by the stainless steel of
the reactor walls (see Scheme 21.12) [54] . A large
variety of substrates could be converted with good
Scheme 21.10
Oxidation of
cyclohexene to adipic acid in an
scCO
2
/water biphasic medium.
Scheme 21.11
Molybdenum-catalysed
oxidation of alkenes to form epoxides
or diols using scCO
2
as reaction
medium.
