342 J. CHEM. RESEARCH (S), 1997
J. Chem. Research (S),
1997, 342–343†
Selective Oxidation of Arenes in Dry Media under Focused
Microwaves†
Abdelouahad Oussaid and Andre´ Loupy*
Laboratoire des Re´actions Se´lectives sur Supports, CNRS UA 478, ICMO, Universite´ Paris XI,
Baˆtiment 410, 91405 Orsay Ce´dex, France
Arenes are oxidized into ketones within 10–30 min using KMnO4 impregnated on alumina under microwave activation in
dry media, instead of several days under classical conditions.
The use of solid supports in synthetic chemistry is a well
established and environmentally friendly technology.1–3 The
oxidation of organic compounds with potassium permanga-
nate adsorbed on solid supports (alumina, bentonite, silica
gel) has especially been proved to be of noticeable effi-
ciency.3–7 The most significant advantages lie in increases in
reactivity and selectivity when compared to homogeneous
conditions. For instance, primary alcohols are oxidized into
aldehydes without subsequent oxidation to carboxylic
acid.3,6
microwaves and to have a continuous temperature control
followed by IR detection.15 The main results are indicated in
Table 1.
The oxidations gave excellent yields within very short reac-
tion times. For instance, the reaction time was reduced from
118 h to 10 min for oxidation of fluorene into fluorenone
(Scheme 1) and from 282 h to 30 min for diphenylmethane
oxidation into benzophenone.
KMnO4–alumina
It was recently shown that KMnO4 adsorbed onto alumina
could lead to selective oxidations of arenes at the benzylic
position.7 However, although the selectivity of this reagent is
noteworthy, the reactions suffer from very long reaction
times (up to 328 h) when performed at room temperature in
the presence of 1,2-dichloroethane (DCE) as solvent. The
same reaction has also been carried out using aqueous
KMnO4 in the presence of ultrasound.8
no solvent; microwaves
O
100% (10 min)
Scheme 1
In summary, our method describes a noticeable improve-
ment in reaction conditions for the selective oxidation of
arenes and takes advantage of both dry media technology and
microwave activation (under the same conditions of time and
temperature, yields were reduced to 40–70% by conventional
heating). We have also shown that, in the case of 9,10-di-
hydroanthracene oxidation, anthracene is an intermediate
product on the way to anthraquinone formation. Unfortuna-
tely, at the present time, the efficiency of this system is
limited to diaryl compounds as its extension to indan, phtha-
lan or toluene is problematic and will need further investi-
gation.
In this paper, we report a procedure where the oxidation
was performed in dry media in order to improve the condi-
tions and to prevent problems connected with solvent use
(cost, handling, safety, pollution, decrease in reactivity by
dilution of the reactants). Microwave activation rather than
conventional heating was preferred, as solid supports are
rather poor thermal conductors (with consequently an
important gradient in temperature inside the materials),
whereas they behave as strong microwave absorbents (and
therefore with a better homogeneity in temperature,9,10).
Consequently, reactions were generally faster and the
obtained products more pure.10,11 In this context we should
point out that the permanganate oxidation of toluene to
benzoic acid in basic solution gives only a poor yield (40%)
and is reported to result in a violent explosion due to high
pressures developed.12 Oxidations of dihydropyridines, esters
and alcohols have been subsequently described in ‘dry media’
using MnO2 on bentonite under microwave irradiation.13,14
For these reasons, we considered the behaviour of KMnO4
on alumina in ‘dry media’ in a monomode reactor (Synthe-
wave 402, Prolabo) in order to take advantage of focused
Experimental
Preparation of Oxidative System.sFinely ground potassium per-
manganate (50 g) dissolved in water (100 ml) was added to alumina
(acidic or neutral, Merck activity I, 63–200 nm; 200 g). After shak-
ing for 15 min, the majority of the water was removed by evapora-
tion under reduced pressure and the obtained powder was dried
under microwave irradiation for 5 min.
Fluoren-9-one.sIn a Pyrex matrix adapted to a Synthewave 402
monomode reactor, fluorene (2 mmol, 0.332 g) was added to the
KMnO4–alumina mixture (6 mmol, 4.74 g). After 5 min of mechan-
Table 1 Permanganate oxidations in dry media under monomode microwave activation (emitted power = 150 W)
Experimental conditions
Lit. (DCE)b
Reactant
Product
t/min
T/°Ca
Yield (%)
t/h
Yield (%)
Fluorene
Fluoren-9-one
10
12
140
120
107
110
103
110
100
118
67
100
92
100
92
2 (98)
2 (98)
Xanthene
Xanthone
100
Anthrone
Diphenylmethane
9,10-Dihydroanthracene
Anthraquinone
Benzophenone
Anthracene (1)ǹAnthraquinone (2)
Anthracene (1)ǹAnthraquinone (2)
10
86
97
75
30
30
120
282
121
121
1 (70)ǹ2 (9)
1 (6)ǹ2 (84)
aFinal temperature measured by IR detection. bRef. 7.
ical stirring, the mixture was irradiated (under stirring) at 150 W
for 10 min. At the end of exposure to microwaves, the mixture was
cooled to room temperature and eluted with diethyl ether (50 ml).
After filtration and solvent removal, the crude product was identi-
fied by comparison (GC and NMR) with an authentic sample.
*To receive any correspondence (e-mail: aloupy@icmo.u-psud.fr).
†This is a Short Paper as defined in the Instructions for Authors,
Section 5.0 [see J. Chem. Research (S), 1997, Issue 1]; there is there-
fore no corresponding material in J. Chem. Research (M).
Oussaid, Abdelouahad
