Selective oxidation of alkylarenes in dry media with potassium permanganate supported on montmorillonite K10

Article abstract of DOI:10.1016/S0040-4039(02)00976-0

The solvent-free oxidation of alkylarenes with KMnO₄ supported on montmorillonite K10 is reported. The beneficial effects of microwave and ultrasound irradiation on the reactions is described.

Full text of DOI:10.1016/S0040-4039(02)00976-0

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 5165–5167
Selective oxidation of alkylarenes in dry media with potassium
permanganate supported on montmorillonite K10
a,
a
a
b,
Ahmad Shaabani, * Ayoob Bazgir, Fatemeh Teimouri and Donald G. Lee *
a
Chemistry Department, Shahid Beheshti University, PO Box 19395-4716, Tehran, Iran
Chemistry Department, Regina University, Regina, Saskatchewan, Canada S4S 0A2
b
Received 25 March 2002; revised 8 May 2002; accepted 17 May 2002
Abstract—The solvent-free oxidation of alkylarenes with KMnO supported on montmorillonite K10 is reported. The beneficial
4
effects of microwave and ultrasound irradiation on the reactions is described. © 2002 Elsevier Science Ltd. All rights reserved.
1
. Introduction
those obtained under heterogeneous conditions where
the reductant is dissolved in an inert solvent; however,
the reaction times are reduced from a few days to a few
hours at room temperature. As indicated in Scheme 1,
the reaction is equally effective when oxygen is present
in the side chain. Excellent yields of the corresponding
lactones are realized when the side chain is an exocyclic
ether.
Adsorption onto solid supports is known to cause a
change in the properties of many reagents. One of the
most striking examples of a change in chemical reactiv-
ity and selectivity that has been reported to date is with
the well-known oxidant potassium permanganate.
1
2
It was recently shown that alkylarenes are oxidized at
the benzylic position when treated under heterogeneous
conditions with KMnO adsorbed on a solid support
Some alkylarenes such as indan, xanthene, fluorene and
tetralin are very readily oxidized to the corresponding
carbonyl compounds (entries 9–12), while the others
such as o-xylene are completely resistant to oxidation
4
3
3,4
5
such as CuSO ·5H O, Al O , or zeolite. Product
4
2
2
3
yields were relatively good, but reaction times necessary
for these yields were very long. For example 48–72 h
when CuSO ·5H O is the solid support and 66–328 h in
(
entry 7), possibly because of an unfavorable steric
4
2
effect. The decreased reactivity of p-bromotoluene sug-
gests that electron-withdrawing groups are deactivating.
As similar observation has previously been made for
the corresponding reactions carried out under heteroge-
the case of moist Al O and 96 h in the case of zeolite.
2
3
In connection with our previous work on solid state
oxidation, we wish to report the results obtained from
6
3
neous conditions. However, the results reported in
a study of the oxidation of alkylarenes with KMnO4
adsorbed on the very inexpensive and readily available
support, montmorillonite K10. The procedure gives
products in good yields and avoids problems connected
with solvent use (cost, handling, safety and pollution).
Decreased reaction times are also realized because of
increased reactivity of the oxidant in the solid state.
Table 1 indicate that much shorter reaction times are
required for the solvent free reactions, particularly if
they are assisted by microwave or ultrasonic
irradiation.
As can be seen from the results reported in Table 1,
application of ultrasonic technology in these reactions
decreases the time required to obtain good yields by a
factor of about 10. Use of microwave irradiation
decreases the required time by another order of magni-
tude. For example, the time required to convert
fluorene into 9-fluorenone in 85–90% yield, 1440 min at
room temperature in the absence of irradiation, is
reduced to 120 min when ultrasonic irradiation is
applied and to 10 min when microwave irradiation is
used. As a consequence, it is possible to select the most
As can be seen from Table 1, alkylarenes are converted
into the corresponding a-ketones in good yields under
very mild conditions. The products are identical to
Keywords: alkylarenes; oxidation; ketones; permanganate; montmo-
rillonite K10.
*
Corresponding authors. Fax: +98-21-2403041; e-mail: ashaabani@
cc.sbu.ac.ir
0
040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)00976-0

5
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A. Shaabani et al. / Tetrahedron Letters 43 (2002) 5165–5167
Table 1. The oxidation of alkylarenes by potassium permanganate adsorbed on montmorillonite K10 under solvent free
a
conditions. Effect of microwave and ultrasonic on yield and times required
Entry
Alkylarene
Product
Yield/timeb
Yield/timec
Yield/timed
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
Diphenylmethane
Ethylbenzene
Propylbenzene
Butylbenzene
Toluene
p-Xylene
o-Xylene
p-Bromotoluene
Tetralin
Indan
Xanthene
Fluorene
Isochroman
Phthalane
2,3-Dihydrobenzofurane
Benzophenone
Acetophenone
Propiophenone
1-Phenyl-1-butanone
Benzaldehyde
p-Methylbenzaldehyde
No reaction
p-Bromobenzaldhyde
a-Tetralone
1-Indanone
86%/20 h
64%/19 h
57%/20 h
60%/20 h
60%/18 h
67%/20 h
81%/2.5 h
52%/3.5 h
54%/2.5 h
61%/3 h
52%/1.7 h
53%/2.7 h
78%/20 min
44%/12 min
50%/20 min
63%/25 min
54%/12 min
65%/35 min
20%/20 h
87%/20 h
85%/20 h
81%/22 h
89%/24 h
81%/20 h
90%/20 h
12%/2 h
82%/2.5 h
83%/2.5 h
86%/2 h
87%/2 h
80%/2 h
86%/2 h
15%/30 min
82%/25 min
78%/30 min
86%/10 min
86%/10 min
80%/10 min
87%/15 min
0
1
2
3
4
5
Xanthone
9-Fluorenone
Isochroman-1-one
Phthalone
No reaction
a
Isolated yields. Product identification is carried out by comparison of physical properties (melting points, mass spectra, infrared and NMR
spectra) with literature values.
Temperature=23°C.
Reaction subjected to ultrasonic irradiation.
Reaction promoted by microwave irradiation.
b
c
d
lonite K10 can be recommended as a practical oxidant
for benzylic oxidations in the solid state at room
temperature.
2
. Experimental
2
.1. Oxidation of indan under classical condition
Potassium permanganate (3.16 g, 20 mmol) and mont-
morillonite K10 (6 g) were ground together in a mortar
until a fine homogeneous powder was obtained.
Indan (0.24 g, 2 mmol) was added to this KMnO /K10
4
mixture (4.5 g, 9.9 mmol) in a 25 mL round bottomed
flask and mixed magnetically at room temperature until
TLC (eluent: hexane–ethyl acetate) analysis indicated a
completed reaction (20 h). The residue was then washed
with CH Cl (2×20 mL). After filtration and removal of
Scheme 1.
appropriate conditions for a particular application. If
time is of little importance, the reaction can be com-
pleted without expenditure of additional energy. How-
ever, if time is an important factor, energy can be
applied in the form of either ultrasonic or microwave
irradiation.
2
2
the solvent, the crude product was chromatographed on
silica gel (eluent: hexane–ethyl acetate) to give purified
product (0.22 g, 1.7 mmol, 85%) which was identified
by comparison with authentic 1-indanone using TLC,
8
1
melting point (39–41°C; lit. 42°C), H NMR (500
It was recently shown that KMnO4 adsorbed onto
alumina could lead to selective oxidation of alkylarenes
at the benzylic position under MW irradiation. How-
3
3
MHz, CDCl ): l 2.63 (t, J 6.1 Hz, 2H), 3.09 (t, J
3
HH
HH
6
.1 Hz, 2H), 7.08–7.80 (m, 4H) and MS: m/z (%)=132
+
7
(
M , 100), 104 (98), 78 (15), 51 (10).
ever, the efficiency of that system was limited to diaryl
compounds and extension of it to indan, phthalene and
toluene was problematic. In this work, we were able to
improve the previous method for oxidation of low
boiling compounds by using a Teflon vessel. As indi-
cated in Table 1, the oxidation of both diaryl and aryl
alkyl compounds have been carried out in relatively
high yields in a Teflon vessel irradiated by microwaves.
2
.2. Oxidation of isochroman in the presence of ultra-
sound irradiation
Isochroman (0.27 g, 2 mmol) and KMnO /K10 (4.5 g,
4
9.9 mmol) were finely mixed together and the reaction
mixture irradiated in a 25 mL beaker. The progress of
the reaction was monitored by TLC. When complete
(
150 min), the reaction mixture was washed with
In summary due to its good reactivity and ease of
CH Cl (2×20 mL). After filtration and solvent removal
the crude product was chromatographed on silica gel
2
2
product separation, KMnO adsorbed on montmoril-
4

A. Shaabani et al. / Tetrahedron Letters 43 (2002) 5165–5167
5167
(
(
(
(
(
eluent: hexane–ethyl acetate) to give purified product
research of Shahid Beheshti University and the Natural
Sciences and Engineering Research Council of Canada.
1
0.24 g, 1.6 mmol, 80%). Identification was by H NMR
3
500 MHz, CDCl ): l 3.10 (t, J
8.57 Hz, 2H), 4.56
3
HH
3
t, J
8.57 Hz, 2H), 7.20–8.20 (m, 4H) and MS: m/z
HH
+
%)=148 (M , 68), 118 (100), 90 (76), 63 (25), 39 (25).
.3. Oxidation of tetralin under microwave irradiation
References
2
1
. (a) Smith, K., Ed. Solid Supports and Catalysis in Organic
Synthesis; Ellis Harwood and PTR Prentice Hall: New
York, 1992; (b) Laszlo, P. In Comprehensive Organic Syn-
thesis; Trost, B. M., Ed.; Pergamon: New York, 1991; Vol.
7, p. 839.
In a 25 mL Teflon beaker, tetralin (0.26 g, 2 mmol) was
added to KMnO /K10 (4.5 g, 9.9 mmol). After 3 min of
4
mechanical stirring, the mixture was irradiated at
medium power for 25 min. At the end of exposure to
microwave irradiation, the mixture was cooled to room
temperature and eluted with CH Cl (2×20 mL). After
filtration and solvent removal the crude product was
chromatographed on silica gel (eluent: hexane–ethyl
2. Lee, D. G. In Encyclopedia of Reagents for Organic Syn-
thesis; Paquette, L. A., Ed.; Wiley: New York, 1995; pp.
4274–4281.
3. Noureldin, N. A.; Zhao, D.; Lee, D. G. J. Org. Chem.
1998, 62, 8772.
4. Zhao, D.; Lee, D. G. Synthesis 1994, 915.
5. Sreekumar, R.; Padmakumar, R. Tetrahedron Lett. 1997,
38, 5143.
2
2
acetate) to give purified product (0.24 g, 1.7 mmol,
1
8
2%). Identification was by H NMR (500 MHz,
CDCl ): l 2.01–2.18 (m, 2H), 2.50–2.65 (m, 2H), 2.80–
3
3
(
.01 (m, 2H), 7.20–8.10 (m, 4H) and MS: m/z (%)=145
+
M −1, 15), 130 (100), 91 (58), 57 (25).
6. Shaabani, A.; Lee, D. G. Tetrahedron Lett. 2001, 42,
5833.
7. Oussaid, A.; Loupy, A. J. Chem. Res. (S) 1997, 342.
Acknowledgements
8. CRC Handbook of Chemistry and Physics, Weast, R. C.,
Ed.; The Chemical Rubber Co.: Boca Raton, 1975–1976;
p. C342.
We gratefully acknowledge financial support from the