Chemistry Reference
In-Depth Information
O
CH
3
CH
3
H
2
O
2
catalyst
Fig. 11.26
Preparation of menadione
by the oxidation of an arene to a
quinone.
O
CHBr
2
CH
3
CH
2
Br
H
2
O
2
/HBr
hν
H
2
O
2
/HBr
h
ν
X
X
X
H
2
O
Co
II
/ Br
-
H
2
O
H
2
O
2
/
HOAc
CH
2
OAc
CO
2
H
CH
2
OH
CHO
[O]
[O]
Fig. 11.27
Range of useful products
from aromatic side-chain oxidation.
X
X
X
X
pressure in general-purpose plant with H
2
O
2
as
oxidant is useful for fine chemicals. In this case, the
second oxidation on a side chain is faster than the
first, tending to give toluic acid or aldehyde inter-
mediates if a low excess of oxidant is used. Another
side-chain oxidation system uses H
2
O
2
and hydrogen
bromide irradiated with visible light [227,228],
the active species being atomic bromine. Here, the
second oxidation is slower than the first, offering a
selective route to dialcohols from xylenes, for
example. Also, although acetophenones and ben-
zolphenones can be oxidised to the intermediate
carbinol by control of H
2
O
2
stoichiometry, the
ketones can be produced directly by using excess
H
2
O
2
with only catalytic levels of HBr.
Many useful intermediates can be obtained via
these benzylic oxidations, as shown in Fig. 11.27.
Benzylic oxidation of ethylbenzene over Ti-, V- and
Sn-containing MFI zeolites has been reported [229]
and
p
-chlorotoluene is oxidised to the benzaldehyde
over vanadium zeolites [230].
Alcohol oxidations
A recent review [231] describes methods for alcohol
oxidation with both H
2
O
2
and O
2
. It is becoming
increasingly difficult to sustain the use of traditional
methods involving stoichiometric oxidation with
chromium(VI). Peroxo-metal and peroxygen-driven
metal redox systems both now figure strongly.
Regarding the peroxo-metal systems, the combi-
nation of H
2
O
2
with simple molybdate salts is a very
effective catalytic system for oxidising secondary
alcohols to ketones, and benzylic alcohols to aldehy-
des [91] (Fig. 11.28). Unlike epoxidation, molybde-
num(VI) generally catalyses alcohol oxidation as
well as tungsten(VI). However, there have been a
couple of salient recent papers about the tungsten
system. One of these, from Noyori [232], describes a
practical method for alcohol oxidation using 3-30%
H
2
O
2
without solvent: sodium tungstate is the cata-
lyst and the phase-transfer agent is methyltrioctyl
ammonium bisulfate. A similar method assisted by
