Chemistry Reference
In-Depth Information
O
O
O
PCWP (0.5%)
+
+
+
35% H
2
O
2
, CHCl
3
, rt
O
O
O
78%
9%
12%
0.25%
Scheme 10.60
tively in the conversion of cyclohexene to adipic acid
[434]. This fascinating six-step reaction was achieved
using 1 mol.% of catalyst and a 12% excess of hydro-
gen peroxide with gradual heating to 90°C for 6 h.
A yield of 93% of pure product was obtained. The
mechanism apparently involves an epoxidation step,
two alcohol oxidation steps, one Baeyer-Villiger
oxidation and two hydrolysis steps (Scheme 10.62).
This synthesis is an excellent example of a 'green'
chemical process, particularly when compared
with the current industrial method for the fabrica-
tion of adipic acid. Noyori's series of publications
remarkably illustrate the potential of PTC in design-
ing environmentally benign processes. In a further
development, Noyori's system was accelerated by an
order of magnitude when it was carried out under
microwave radiation [435].
Although Noyori's catalyst failed in the synthesis
of styrene oxide, very recently Yadav [436] reported
the efficient epoxidation of styrene using classical
PTC methodology by applying a synergistic combi-
nation of cetyldimethylbenzylammonium chloride
and dodecatungstophosphoric acid at 50°C in a
biphasic 30% H
2
O
2
/dichloroethane system.
Another significant hydrogen peroxide oxidation
process is the Baeyer-Villiger oxidation of ketones.
This has been reviewed recently by Ricci, with par-
ticular reference to PTC methods [437]. Oxychlori-
nation and oxybromination of benzene by sodium
perborate with HCl or HBr, respectively, was cata-
lysed by TBAB [438]. The effect of tetraheptylam-
monium chloride on the rate of oxychlorination of
p
-cresol by HCl/H
2
O
2
was studied by Mukhopadhyay
et al
. [439] A sevenfold rate increase was measured
in the oxidative chlorination system in comparison
with the non-catalytic reaction. Hydrogen peroxide
also was shown to be effective in the removal of the
hydrazine group from a pyridine skeleton under mild
conditions (Scheme 10.63) [440].
Alkyl hydroperoxides also are effectual oxida-
tion reagents. Zawadiak [441] applied 1-methyl-1-
Na
2
WO
4
(0.2%)
[CH
3
(
n-
C
8
H
17
)
3
N]HSO
4
OH
O
30% H
2
O
2
, 90
°
C, 4 h
yield 93%
Scheme 10.61
of terpenes [421] (Scheme 10.60), oxidation of vinyl
ethers and silyl enol ethers [422], epoxidation of
undecylenic acid and its esters [423] and dehomolo-
gation of aldehydes via oxidative cleavage of silyl
enol ethers [424]. The combination of Aliquat 336
with peroxotungstophosphate was used in the epox-
idation of castor oil [425]. These catalytic ion pairs
were claimed to be as active and selective as the
stoichiometric reagent methyltrioxorhenium [426].
Other useful applications of the PTC-tungstate/H
2
O
2
system are the synthesis of nitrones of
N
-alkyl-a-
amino acids [427] and the oxidative desulfurisation
of dibenzothiophene [428].
An apparent breakthrough in H
2
O
2
oxidations was
reported by Noyori
et al
. in 1996. These authors
developed
a totally halide and solvent-free system
based
on an Na
2
WO
4
catalyst combined with trioctyl-
methylammonium hydrogen sulfate as phase-
transfer agent. This system is claimed to be most
effective relative to prior systems (in terms of turn-
over numbers) in the epoxidation of terminal olefins
[429] or functional olefins [430] (including allylic
alcohols, unsaturated esters, ketones and ethers) and
in the oxidation of secondary alcohols to ketones
(Scheme 10.61) [431] and of benzylic alcohols to
benzaldehydes or benzoic acids [432].
Aldehydes were oxidised selectively to carboxylic
acids even in the absence of a metal due to the acidic
properties of the hydrogen sulfate anion [433]. The
Na
2
WO
4
/QHSO
4
system also has been applied effec-
