Chemistry Reference
In-Depth Information
'easy' substrates such as sulfides or electron-rich
olefins [97] but, as noted earlier, these are not of
much interest for industrial chemical synthesis. In
principle, it should be possible to attach XW
2
com-
plexes covalently to solid supports. This remains a
serious and worthwhile research target, but success-
ful catalysts based on this feature have yet to be
reported.
A much more recently discovered (by Herrmann
et al
.) catalyst based on peroxo-metal chemistry is
methylrhenium(VII) oxide (MeReO
3
, MTO) [98].
The alkyl substitution is critical to catalytic activity,
which is mainly lost on degradation owing to for-
mation of the unreactive rhenate ion ReO
4
-
. The
MTO is a stronger Lewis acid than molybdenum or
tungsten complexes, and catalyses many reactions as
such, including olefin metathesis. Its strong elec-
trophilic nature makes the peroxo complexes good
oxygen-transfer species to olefins, etc. but the acidity
also increases ring-opening, giving diol rather than
epoxide as the product. However, unusual effects of
azine ligands, including pyridines, bipyridines, etc.,
have been found, which appear to accelerate epoxi-
dation but inhibit acid-catalysed ring-opening [99].
The system is complicated further by gradual oxida-
tion of these ligands to N-oxides [100], which is a
common reaction of peroxo-metal oxidants. A full
explanation is still lacking but a lot of good infor-
mation has been generated [99,101].
The MTO is less tolerant of water than Mo and W
systems and requires strong to anhydrous H
2
O
2
for
best results. The ratio of H
2
O
2
to H
2
O and catalyst
influences the equilibria between mono and diper-
oxo complexes, which can exhibit different activi-
ties—more so than molybdenum, for example,
where the monoperoxo tends to disproportionate.
Degradation probably occurs via loss of a proton
from the methyl group and then electrophilic attack
on the H
2
C=Re bond.
In addition to forming a fibrous solid polymer
itself, monomeric MTO has been supported success-
fully on a range of solid surfaces, retaining its cat-
alytic properties [102]. None of these catalysts has
been developed for industrial use to date and gradual
degradation of MTO is obviously a concern if long
lifetimes are to be reached, but there remains scope
for further research to this end.
Somewhat different properties to the above are
offered by vanadium(V) complexes [103], which
O
O
O
O
M
O
X
O
O
O
M
O
O
O
O
Fig. 11.13
Likely active function in Venturello and similar
complexes.
(to reflux in water), but this is not a good option for
epoxidation owing to hydrolytic ring-opening. A key
discovery by Venturello [61a] was the use of phos-
phate/tungstate mixtures that epoxidised terminal
olefins at moderate temperatures (Figs 11.7 and
11.13). This boosts activity by making the peroxo
intermediate asymmetric via non-bonded tung-
sten-oxygen interaction, facilitating oxygen transfer
[92]. Industrial use of this system is certainly
feasible [93].
Since the discovery of this structural feature it has
been reproduced in many other complexes, includ-
ing XM
2
and XM
3
types (X=P, As, S, Si) [94]. There
are drawbacks, however. The non-bonded interac-
tion is the basis for the stability of the complex,
which therefore dissociates after the peroxo group is
lost, suggesting that a significant excess of H
2
O
2
be
maintained. This can lead to further oxidation by
Hock reaction with the epoxide, giving C-C cleavage
(although this itself can be a desirable transforma-
tion [95]). Hydrolytic ring-opening can be a problem
for sensitive epoxides, owing to acidity of the
medium and/or Lewis acid character of the d
0
metal
centre.
Excess H
2
O
2
with tungsten, and particularly
molybdenum, complexes can lead to the liberation
of singlet oxygen under some conditions, which is
a particularly convenient and controllable source of
oxygen [22,96].
Simple immobilisation of molybdenum or tung-
sten complexes on solids gives materials that oxidise
