Chemistry Reference
In-Depth Information
O
O
O
H
+
/ H
2
O
OH
H
2
O
2
OH
O
O
tungstate
OH
OH
O
Fig. 11.19
Production of
epoxysuccinic acid.
O
O
phase systems sometimes can be achieved by the use
of lower alcohols (if these are not attacked) and ace-
tonitrile (in the absence of base). Acetone is some-
times proposed, but there are severe safety issues
related to ketone peroxide build-up that are often
apparently neglected.
Sheldon [165] recently has described the use of tri-
fluoroethanol as a solvent for methyltrioxorhenium
(MTO) catalysis of epoxidation. The solvent helps
water tolerance by the catalyst and is proposed as a
good 'partner' for H
2
O
2
. In this case, the reaction is
apparently quite slow for some terminal olefins but
high yields are obtained. The Ishii-Venturello system
has been mentioned already and continues to be
understood and developed further [94,97]. An
industrially relevant example [166] is the mono-
epoxidation of cyclododecatriene using phospho-
tungstic acid and and a phase-transfer catalyst, with
solvent-free continuous reaction.
Berkessel [167] has shown that the Mn/tmtacn/
ascorbic acid complex (Fig. 11.20) is an efficient cat-
alyst for epoxidation of methyl acrylate (97% yield)
and of 1-octene (83% yield) in water/MeCN. The
ascorbic acid is, not surprisingly, oxidised signifi-
cantly in the system but is necessary to the function
of the Mn catalyst.
Polyoxometallate catalysis also has been develop-
ing in recent years as the ability to probe structure
and reactivity has advanced. Mizuno [168] has
described g-[SiW
10
(Fe(OH
2
))
2
O
38
]
6-
for epoxidation
of many olefins. Excess substrate is used through-
out and although epoxidation predominated, some
allylic oxidation also is seen for 1- and 2-octene, and
much more for cyclohexene. When 1 : 1 sub-
strate/H
2
O
2
is used with cyclohexene, more diol than
epoxide formed and there was less allylic oxidation,
which suggests acid-catalysed ring-opening. The
two Fe sites may be cooperating in the oxidation, in
Me
N
L
Me
N
Mn
O
O
O
N
Me
HOH
2
C
OH
Fig. 11.20
The Mn/tmtacn/ascorbate complex.
line with an emerging pattern for many transition-
metal-substituted polyoxometallates. For example
Ishikawa [169] epoxidised cyclooctene in aqueous
acetonitrile with a-[PTi
2
W
10
O
38
(O
2
)
2
]
7-
, which is said
to occur via a di-Ti-h
2
-peroxo intermediate, where
each of these interacts with neighbouring tungsten
sites. The olefin is not of interest, but cooperation of
adjacent metal centres now appears widespread in
intact polyoxometallate catalysis. Hydrogen peroxide
decomposition was also catalysed—about 3 : 1 excess
was needed to get a high epoxide yield, even with
this very reactive olefin.
For the most part epoxidation is electrophilic, but
some homogeneous nucleophilic oxidations are of
importance, most notably those of enones. Such oxi-
dations can be achieved by alkaline H
2
O
2
[170] using
various bases, from simple caustic alkali to milder
bases including tertiary amines. Effective stabilisa-
tion of H
2
O
2
at high pH is important, to avoid waste-
