Chemistry Reference
In-Depth Information
yields into their corresponding oxidation products
(Table 21.1). This epoxidation is particularly efficient
in the case of linear or cyclic substrates containing
internal double bonds. The presence of the steel walls
initiates a radical chain process and no additional
catalyst is required.
In general, the synthetic utility of oxidations
with molecular oxygen in scCO
2
is still limited, but
some of the advantages associated with the use of
this medium have become apparent. The potential
of scCO
2
for modern synthetic chemistry already is
more elaborated in hydrogenation processes, where
similar safety arguments apply as for oxidations. In
addition to the catalytic hydrogenation processes dis-
cussed in Section 5, several highly enantioselective
hydrogenation reactions of C=C double bonds were
reported [15,16,55]. The asymmetric homogeneous
hydrogenation of dehydroamino acid derivatives
could be achieved using cationic rhodium/phosphine
complexes with lipophilic counter-ions such as
the BARF anion (BARF = tetrakis[3,5 bis(trifluoro-
methyl)phenyl]borate) to solubilise the catalyst in
Table 21.1
Epoxidation of various olefins with molecular oxygen and 2-methyl-propionaldehyde in scCO
2
(
T
=
55°C,
d
co
2
=
0.75 g ml
-
1
, [S]/[Ald]
=
1/2, [S]/O
2
=
1/10,
t
=
1
8
h)
Substrate
Conversion (%)
Product(s)
Selectivity (%)
>
99
Epoxycyclooctane
>
99
95
Epoxycyclohexane
91
46
1,2-Epoxyoctane
8
7
3,4-Epoxyoctane
5
96
trans
-3,4-Epoxyhexane
>
9
8
73
Propylene oxide
14
Acetone
67
32
2-(2-Oxoethyl)-benzaldehyde
59
2-Indanone
21
1,2-Indandiol
11
51
cis
-1,2-Epoxide
59
trans
-1,2-Epoxide
30
8
,9-Epoxides
9
16
Epoxyisophorone
30
Data taken from Ref. 54.
