TETRAHEDRON
Pergamon
Tetrahedron 56 (2000) 8561±8566
Ultrasound Assisted Heterogeneous
Permanganate Oxidations
Ï
M a ria Me cÏ iarova, Stefan Toma and Andrea Heribanov aÂ
*
Department of Organic Chemistry, Faculty of Natural Sciences, Comenius University, Mlynska dolina CH-2 SK-842 15 Bratislava,
Slovak Republic
Received 9 June 2000; revised 17 August 2000; accepted 30 August 2000
AbstractÐThe ultrasound effect on the heterogeneous permanganate oxidation of benzyl alcohol and alkylarenes to the corresponding
carbonyl compounds is examined. Application of ultrasonic irradiation leads to shorter reaction times as well as lower reaction temperatures.
KMnO
4
supported by copper sulfate pentahydrate in CH
q 2000 Elsevier Science Ltd. All rights reserved.
2
Cl
2
is possible to use for selective oxidation of benzyl alcohol to benzaldehyde.
6
K 10 montmorillonite. Chromium (VI) dioxide with
Introduction
periodic acid oxidizes alkylbenzenes in a-position to the
corresponding ketones, but toluene is oxidized to benzoic
Potassium permanganate is an oxidant widely used in
organic syntheses. KMnO4 is soluble in water and in
mixtures of water and miscible organic solvents such as
7
acid. Alkylarenes are oxidized to ketones within
10±30 min using KMnO impregnated on alumina under
4
acetone, acetic acid, acetonitrile and pyridine. KMnO can
4
microwave activation in dry media, instead of several
days under classical conditions.
8
also be used as an oxidant in nonpolar organic solvents, but
the aid of phase-transfer agents or adsorbtion onto a solid
support such as hydrated copper (II) sulfate, alumina or
An interesting paper dealing with heterogeneous per-
manganate oxidations was published recently.9 Alkyl-
benzenes are oxidized at the benzylic position when
treated under heterogeneous conditions with potassium per-
1
silica is necessary.
Inorganic salts adsorbed onto solid supports are in general
very useful agents in organic synthesis. Alkylbenzenes are
oxidized to the corresponding carbonyl compounds with
manganate adsorbed on a solid support such as CuSO
and moist Al O , respectively. Using the substrate:oxidant
3
´5H O
4 2
2
2
sodium bromate on the surface of cerium dioxide. Active
molar ratio 1.4:10, yields of the products reached 30±95%
but the reaction time necessary for such good yields was
manganese dioxide supported on alumina is a very good
oxidant for the selective oxidation of alcohols to aldehydes
or ketones and in the case of a,b-unsaturated alcohols, a,b-
48±72 h in the case of CuSO
66±328 h in the case of moist Al
´5H O as a solid support and
2
4
O .
3
2
3
unsaturated carbonyl compounds arise. Benzyl ethers are
oxidized to benzoate esters by potassium permanganate in
dichloromethane with phase-transfer catalysis by triethyl4-
benzylammonium chloride in fair to excellent yields.
Allylic, benzylic and saturated secondary alcohols are ef®-
ciently oxidized to the corresponding aldehydes or ketones5
by a solid mixture of K FeO , Al O and CuSO ´5H O.
Oxidations of alcohols to aldehydes or ketones, aromatic
amines to their corresponding azo derivatives and coupling
of thiophenol into diphenyl disul®de were ef®ciently
performed by potassium ferrate (VI) in the presence of the
The advantages of the reactions on surfaces are as follows:
easy separation of the product from the reagents or catalysts,
new or better selectivities, faster reactions, lower reaction
temperatures, safer, less toxic ef¯uent can be used and other
waste problems can be minimized. Problems encountered
with these reactions, especially for large scale applications,
include dif®cult heat and mass transport and dif®cult
2
4
2
3
4
2
1
0
suspension stirring. These dif®culties and disadvantages
can be eliminated by application of ultrasonic irradiation.
The main goals of this paper were:
Keywords: ultrasound; heterogeneous oxidation; alkylarenes; alcohols;
supported potassium permanganate.
* Corresponding author. Tel.: 142-17-796208; fax: 142-17-796690;
e-mail: toma@fns.uniba.sk
1
. to examine if it is possible to shorten the reaction time as
well as to lower the reaction temperature by application
of ultrasonic irradiation;
0040±4020/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(00)00798-5