Oxidation of alkyl aromatics to ketones by tert-butyl hydroperoxide on manganese dioxide catalyst

Article abstract of DOI:10.1016/j.tetlet.2012.03.091

The catalytic activity of the manganese oxide was investigated for the oxidative functionalization of alkylaromatics to benzylic ketones using tert-butyl hydroperoxide (TBHP) as an oxidant. Manganese oxides of different types were tested for this reaction. Of all the oxides, the nano amorphous manganese dioxide exhibited significant catalytic activity and selectivity for the reaction. The nano amorphous MnO₂/TBHP catalytic system could also be reused for six consecutive cycles with no considerable loss in catalytic activity.

Full text of DOI:10.1016/j.tetlet.2012.03.091

Tetrahedron Letters 53 (2012) 2989–2992
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Oxidation of alkyl aromatics to ketones by tert-butyl hydroperoxide on
manganese dioxide catalyst
⇑
Anand S. Burange, Sandip R. Kale, Radha V. Jayaram
Department of Chemistry, Institute of Chemical Technology, Matunga, Mumbai 400 019, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The catalytic activity of the manganese oxide was investigated for the oxidative functionalization of
alkylaromatics to benzylic ketones using tert-butyl hydroperoxide (TBHP) as an oxidant. Manganese oxi-
des of different types were tested for this reaction. Of all the oxides, the nano amorphous manganese
dioxide exhibited significant catalytic activity and selectivity for the reaction. The nano amorphous
MnO2/TBHP catalytic system could also be reused for six consecutive cycles with no considerable loss
in catalytic activity.
Received 14 January 2012
Revised 21 March 2012
Accepted 22 March 2012
Available online 5 April 2012
Keywords:
Oxidation
Ó 2012 Elsevier Ltd. All rights reserved.
Heterogeneous catalyst
Alkyl aromatics
Nano metal oxide
Manganese dioxide
Aromatic ketones are key intermediates for the synthesis of
insecticides, photo initiators, perfumes, and pharmaceuticals and
also integral part of various natural products.1 Insertion of carbonyl
functionality at the benzylic position of the corresponding alkyla-
renes producing aryl ketones as valuable building blocks for special
chemicals is an important reaction.2,3 There are some relevant
synthesis of aryl ketones. In heterogeneous catalyst system, the
catalyst is present in a separate phase as that of the reactants
and the catalytic reactions occur on the surface of the catalyst. Het-
erogeneous catalysis in synthetic chemistry has attracted the
attention of organic chemists because of their reusability, easy
separation, and their applications.18 Recently, we have reported
the oxidation of alkyl aromatics using TBHP as oxidant over LaMO3
perovskites (M@Cr, Co, Fe, Mn, Ni) as catalysts.19 In this Letter, we
describe the catalytic activity of a single metal oxide, MnO2 for the
oxidation of alkylaromatics to desired ketones (Scheme 1). It was
found that the method of preparation of the oxide strongly influ-
enced the percentage conversion of the desired ketones. In addi-
tion, the catalyst system was effective for the synthesis of wide
range of substrates under mild reaction conditions.
Manganese dioxide catalysts were prepared by different
reported methods such as calcination of manganese nitrate,20
and reduction of potassium permanganate.21 The manganese diox-
ide (MnO2) obtained by the reduction of potassium permanganate
was found to be of nano size and amorphous by SEM and XRD
techniques (Fig. 1). The activity of the prepared manganese diox-
ides was compared with commercial samples of MnO2 (Table 1).
examples including oxidation of
D
5-steroids to corresponding bio-
logically interesting
D
5-7-ketone derivative4 and benzylic oxidation
of xanthene to xanthone.5 Traditionally, aromatic ketones are syn-
thesized by the Friedel–Craft’s acylation using homogeneous Lewis
acids or strong protonic acids. These processes require harsh reac-
tion conditions and stoichiometric amounts of catalysts and also
lead to the generation of large volumes of corrosive and toxic
wastes.6 As a clean alternative various catalyticsystems are reported
for the synthesis of benzylic ketones. This includes CrMCM-41/
H2O2,7 NaBiO3/AcOH,8 NaIO4/H+/LiBr,9 NaClO2/NHPI,10 in situ gen-
erated Bi(0) as catalyst/TBHP/picolinic acid,11 HBr–H2O2,12
RuCl2(PPh3)–t-BuOOH,13 Mn(III) salen complex/iodosobenzene,14
Co(II)phthalocyanine/[bmim]Br,15 and Mn-containing porous sili-
cates.16 Presently, the industrial production of benzylic ketones is
carried out by oxidation using molecular oxygen and a cobalt cyclo-
alkane carboxylate or cobalt acetate catalyst in acetic acid.17
Most of the above listed methods suffer from disadvantages
such as the use of non benign solvents, non recoverable homoge-
neous catalysts, etc. Thus, the use of a heterogeneous catalyst sys-
tem in the liquid phase remains an attractive alternative for the
O
MnO2
OOH
+
80 oC, Solvent
⇑
Corresponding author. Tel.: + 91 22 3361 2607; fax: +91 22 3361 1020.
E-mail address: rv.jayaram@ictmumbai.edu.in (R.V. Jayaram).
Scheme 1. Oxidation of DPM to benzophenone over MnO2.
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.03.091

2990
A. S. Burange et al. / Tetrahedron Letters 53 (2012) 2989–2992
Figure 1. A-XRD, B-SEM images of amorphous nano MnO2 (ANMnO2).
Table 1
Table 2
Comparison of catalytic activities of manganese
Effect of amount of amorphous nano MnO2 catalyst on
dioxidesa
oxidation of diphenylmethanea
Entry
Metal oxide
Conversion (%)
Entry
Catalyst (mol %)
Conversionb (%)
b
1
2
3
MnO2
36
38
92
1
2
3
5
10
15
20
—
65
92
86
80
—
c
MnO2
ANMnO2
4
a
5c
Reaction conditions: Diphenylmethane (1 mmol),
70% TBHP (3 mmol), amorphous nano MnO2
(10 mol %), temperature (80 °C), time (10 h).
a
Reaction conditions: Diphenylmethane (1 mmol),
70% TBHP (3 mmol), ANMnO2 (10 mol %), temperature
(80 °C), time (10 h).
b
Commercial manganese dioxide.
MnO2 synthesized by calcination of manganese
c
b
nitrate.
Conversion and selectivity determined by GC and
GC–MS analysis.
c
Without TBHP.
It was observed that amorphous nano MnO2 (ANMnO2) showed
better activity compared to other two samples of MnO2 hence fur-
ther reaction was carried out with this catalyst. To optimize the
reaction condition,24 the reactions of diphenylmethane (DPM) with
TBHP on ANMnO2 were carried out by varying mole ratio of TBHP,
temperature, and solvents. The effect of catalyst loading has strong
influence on the percentage conversion of desired ketones. The ef-
fect of catalyst loading was optimized by employing the reaction
with 5, 10, 15, and 20 mol % of catalyst. It was observed that
10 mol % of catalyst provided excellent conversion (92%) and selec-
tivity (100%) toward desired product while further increase in the
catalyst loading had slight decrease in catalyst activity (Table 2,
entry 2).
The effect of solvents on oxidation of alkyl aromatics was stud-
ied and it was observed that nature of solvent affected the conver-
sion of the reaction. In non-polar solvents like toluene (62%) and
CCl4 (76%) slightly lower conversion of desired product was ob-
served (Table 3, entries 8 and 9), whereas the polar aprotic solvents
such as ACN, and DMSO (Table 3, entries 3 and 7), provided 92% and
80% conversion, respectively, under optimized reaction conditions.
Table 3
Effect of temperature and solvent on the oxidation of diphenylmethane to
benzophenonea
Entry
Temperature (°C)
Solvent
Conversionb (%)
1
2
3
4
5
6
7
8
9
60
70
80
90
80
80
80
80
80
ACN
ACN
ACN
ACN
40
65
92
94
30
46
80
62
76
THF
DMF
DMSO
Toluene
CCl4
a
Diphenylmethane (1 mmol), 70% TBHP (3 mmol), ANMnO2 (10 mol %), time(10 h).
Conversion determined by GC and GC–MS analysis.
b
Maximum activity was observed at an optimum reaction tempera-
ture 80 °C in acetonitrile solvent (Table 3, entry 3) while on further

A. S. Burange et al. / Tetrahedron Letters 53 (2012) 2989–2992
2991
increase in reaction temperature no noticeable change in % conver-
In comparison with other alkylaromatics, toluene did not react be-
cause of the formation of a stable benzylic radical as intermediate.
In case of tetralene and isobutyl benzene formation of naphtha-
lene-1,4-dione and benzoic acid was observed in trace amount
respectively. In order to check the effect of aromatic ring, the reac-
tion was also carried out with 2,6-dimethyl pyridine but in this
case no oxidation product was obtained.
The oxidation of DPM was also carried out with different oxi-
dants such as molecular oxygen and hydrogen peroxide under
the optimized reaction conditions. However, these oxidants were
found to be not effective. To confirm the heterogeneous nature of
a catalytic system, the catalyst was taken in ACN solvent in clean
round bottom flask and the system was heated to 80 °C for 1 h
and the catalyst was filtered out. To the filtrate diphenylmethane
and TBHP were added and subjected to the reaction. Oxidation to
benzophenone was not observed in this case which proves the het-
erogeneity of the reaction. The oxidation reaction was inhibited
when 2,6-bis(1,1-dimethylethyl)-4-methyl phenol, a radical scav-
enger was added to the reaction system. This confirms the function
of the catalyst which is to generate OH free radicals from TBHP.22
The probable mechanism of oxidation involves cleavage of oxidant
c
sion was observed. It was found that commercial MnO2b and MnO2
have larger particle size and crystalline in nature. The better activity
of ANMnO2 may be due to the small particle size and amorphous
nature. Hence, the final optimized reaction parameters for the syn-
thesis of aryl ketones were DPM (1 mmol), catalyst loading
(10 mol %), TBHP (3 mmol), temperature (80 °C), and reaction time
(10 h).
To expand the scope and generality of the developed protocol
and to check the efficacy of TBHP/ANMnO2 catalytic system, oxida-
tion of various alkylaromatics was performed under the same opti-
mized conditions. Cyclic alkylaromatics like fluorene (Table 4,
entry 4), anthrone (Table 4, entry 6) and xanthene (Table 4, entry
8) were readily oxidized with 100% conversion and 100% selectiv-
ity. Tetralene (Table 4, entry 2) when reacted under same reaction
conditions gave 92% conversion with 80% selectivity. Acenaph-
thene (Table 4, entry 5) also showed 100% selectivity with 88% con-
version. Indene (Table 4, entry 3) showed 100% selectivity for the
formation of Indone with 89% conversion. Alkylaromatics with
open chain alkyl group such as isobutyl benzene (Table 4, entry
7) underwent a reaction with 95% selectivity and 85% conversion.
Table 4
ANMnO2 catalyzed oxidation of various alkylarenesa
Entry
Reactant
Product
Conversionb (%)
92, 83c
Selectivityb (%)
100
O
O
1
2
3
92
89
80
O
100
O
4
5
100
88
100
100
O
O
O
6
100
100
O
O
7
8
85
95
O
O
100
100
O
a
b
c
Reaction conditions: Diphenylmethane (1 mmol), 70% TBHP (3 mmol), nano amorphous MnO2 (10 mol %), temperature (80 °C), time (10 h).
Conversion and selectivity determined by GC and GC–MS analysis.
Isolated yield.

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A. S. Burange et al. / Tetrahedron Letters 53 (2012) 2989–2992
References and notes
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24. All the chemicals were purchased from standard sources and used without
further purification. The XRD data was obtained using an X-ray diffractometer
(Philip 1050) with radiation. The conversions of reactants oxidation were
based on the GC analysis using a capillary column (Chemito1000). Products
were identified by GC/MS analysis (Schimadzu, GC–MS).
over the catalyst to generate radicals, which abstract benzylic
hydrogen to form benzylic radical.23 The benzylic radical readily
combines with hydroxyl radical to form benzhydrol. Further oxida-
tion of benzhydrol gives benzophenone.
A further set of experiment was carried out to check the reus-
ability of the catalyst for the oxidation of diphenylmethane. The
catalyst was separated after each run by filtration, washed 2–3
times by using methanol and then heated at 100 °C for half an
hour. The catalyst could be recycled six times with no significant
loss in activity. During reusability studies sixth run provided 89%
conversion and 100% selectivity in favor of benzophenone (Fig. 2).
In conclusion, we have developed an efficient and clean proto-
col for the oxidation of alkylaromatics using TBHP as an oxidant
over nano amorphous MnO2 catalyst. The prepared catalyst is
found to be useful for the selective conversion of alkylaromatics
into aromatic ketones with high conversion. As far as our consider-
ation, the present work firstly demonstrates the oxidation of alkyl-
aromatics using nano size amorphous material as a catalyst. The
developed methodology was effectively applicable for various
alkylaromatics. Hence, it will be useful to explore the area of amor-
phous nano metal oxides as oxidation catalysts in the synthesis of
industrially important chemicals.
General experimental procedure: To a clean dry 10 mL round-bottom flask
containing 1.0 mmol alkyl aromatics, 3.0 mmol equivalent of 70% TBHP was
added. This was followed by the addition 10 mol % of amorphous manganese
dioxide catalyst and 2 mL acetonitrile solvent. The mixture was stirred at 80 °C.
The progress of the reaction was monitored by GC. After completion of the
reaction, the reaction mixture was cooled to room temperature and the
catalyst was separated by filtration method. The products were identified by
GC–MS. The ANMnO2 catalyst was prepared by the reported method. In a
typical method with; 1.7 mL of triethanolamine was diluted to 100 mL distilled
water to which 200 mL 0.03 mol lÀ1 KMnO4 was added dropwise under
vigorous stirring at room temperature. The brown precipitate obtained was
filtered, washed with distilled water until the pH of washed water is 7.0; the
precipitate was dried at 100 °C for 24 h and then calcined at 300 °C for 2 h.
Catalyst MnO2 is prepared by simple calcination of Mn(NO3)2Á4H2O for 13 h at
145 °C by reported procedure.
Acknowledgment
The author (A.S.B.) is thankful to UGC for providing Junior Re-
search Fellowship (Green Tech -JRF).
Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.tetlet.
2012.03.091.