Effective oxidation of benzylic and aliphatic alcohols with hydrogen peroxide catalyzed by a manganese(III) Schiff-base complex under solvent-free conditions

Article abstract of DOI:10.1016/j.tetlet.2006.02.007

A variety of alcohols were oxidized efficiently into the corresponding ketones and carboxylic acids in excellent yields with hydrogen peroxide using a manganese(III) Schiff-base complex as a catalyst under solvent-free and mild conditions. The oxidation procedure is very simple and the products are easily isolated in excellent yields.

Full text of DOI:10.1016/j.tetlet.2006.02.007

Tetrahedron Letters 47 (2006) 2349–2352
Effective oxidation of benzylic and aliphatic alcohols with
hydrogen peroxide catalyzed by a manganese(III)
Schiff-base complex under solvent-free conditions
Hamid R. Mardani and Hamid Golchoubian*
Department of Chemistry, University of Mazandaran, PO Box 453, Babolsar, Iran
Received 20 December 2005; revised 22 January 2006; accepted 1 February 2006
Available online 20 February 2006
Abstract—A variety of alcohols were oxidized efficiently into the corresponding ketones and carboxylic acids in excellent yields with
hydrogen peroxide using a manganese(III) Schiff-base complex as a catalyst under solvent-free and mild conditions. The oxidation
procedure is very simple and the products are easily isolated in excellent yields.
Ó 2006 Elsevier Ltd. All rights reserved.
The oxidation of alcohols is important in organic syn-
thesis and the development of new oxidation processes
continues to attract attention in spite of the availability
In continuation of our work on the selective oxidation
3
6
of hydrocarbons, we wish to report a very efficient
and selective oxidation of benzylic and aliphatic alco-
hols into the corresponding ketones and carboxylic acids
using hydrogen peroxide catalyzed by the Mn(III)
Schiff-base complex 1, under mild conditions. The com-
plex was prepared by a procedure similar to that used by
1
,2
of numerous oxidizing agents.
Recent demand for
eco-friendly chemical processes has encouraged the
3
–15
development of clean reactions
and awaits further
development of high yielding and economical methods
1
0
37
for the oxidation of alcohols.
Karmaker et al., however, 1,2-diaminoethane was
used instead of 1,3-diaminopropane.
Hydrogen peroxide has many advantages as an oxidant
because water is the only by-product, which is easy to
deal with after reactions. On the other hand, solvent-free
reactions are of general interest because of their poten-
tial in combinational chemistry, simple processes, easy
work-up, low cost and reduction in noxious waste mate-
rials. However, only a limited number of such useful oxi-
+
N
N
O
III
Mn
-
ClO4
1
1–14
O
dation procedures have been reported.
Although, a
variety of different catalytic systems for the hydrogen
peroxide-based oxidation of alcohols have been devel-
1
5–21
oped,
new efficient metal catalysts for this system.
the hydrogen peroxide oxidation of hydrocarbons cata-
there is ongoing interest in the search for
9
,22–26
Whilst
1
2
7,28
lyzed by Mn(IV) has been reported,
lyzed with Mn(III) has received less attention.
oxidation cata-
2
9–35
To the best of our knowledge, there is no literature
report on the oxidation of alcohols using a Mn(III)
catalyst under our conditions. Oxidation of alcohols
was performed at room temperature in the presence of
a catalytic amount of the Mn(III) complex using H O
2
2
as the oxidant (Scheme 1) under solvent-free conditions.
Keywords: Oxidation; Hydrogen peroxide; Catalyst; Selective oxida-
tion; Mn(III) Complex; Alcohols.
*
Corresponding author. Tel./fax: +98 1125242002; e-mail:
h.golchobian@umz.ac.ir
Benzhydrol was selected as a model substrate for
optimization of the process. A typical experimental
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.02.007

2350
H. R. Mardani, H. Golchoubian / Tetrahedron Letters 47 (2006) 2349–2352
OH
O
procedure was as follows: to the Mn(III) complex
Complex 1
(0.1 mmol) was added the substrate alcohol (1 mmol),
then hydrogen peroxide (10 mmol, 30%) was slowly
added. The reaction mixture was stirred at room tem-
perature until TLC indicated the reaction was com-
plete. The residue was then washed with a minimal
volume of solvent (methylene chloride, hexane or
ether). Distillation of the solvent gave the product of
acceptable purity. Oxidation took place to afford ben-
zophenone in a 94% yield in the presence of 1 and
10 equiv of 30% H O in 4.5 h (Table 1, entry 1). The
large excess of hydrogen peroxide required was a result
of its decomposition in the presence of the Mn(III) cat-
alyst. The oxygen released in the decomposition played
little role in the oxidation of the alcohols. The oxida-
tion occurred only in poor yield by simply bubbling
molecular oxygen through reaction mixture under simi-
lar reaction conditions (Table 1, entry 2). To evaluate
the catalytic effect of complex 1, the oxidation of benz-
hydrol was carried out under similar reaction condi-
tions in the absence of catalyst and only the starting
alcohol was recovered (Table 1, entry 3). Alternatively,
the reaction was carried out at 50 °C under the same
conditions in which, the oxidation was complete after
2.5 h (Table 1, entry 4). Further, when the oxidation
of benzhydrol was carried out using less catalyst 1,
+
H O
2 2
Solvent-free
and R.T.
R1
Scheme 1.
R (or H)
2
R1
R2 (or OH)
a
Table 1. Oxidation of benzhydrol to benzophenone
Entry
Reaction time (h)
Conversion (%)
1
4.5
12
12
2.5
12
12
100
<20
NR
100
100
NR
2
2
b
2
c
d
3
e
4
f
5
g
6
a
Unless otherwise indicated all reactions were carried out with 1 mmol
of benzhydrol at room temperature with 0.1 mmol of Mn(III) catalyst
1
and 10 mmol of 30% aqueous H
2 2
O .
b
The reaction was carried out under an atmosphere of O
2
instead of
2 2
H O .
c
d
e
f
The reaction was carried out without Mn(III).
NR: reaction did not occur.
The temperature was increased to 50 °C.
The concentration of Mn(III) was decreased to 0.05 mmol.
The reaction was carried out with 0.1 mmol of Mn(III) acetate in
g
place of complex 1.
a
Table 2. Oxidation of alcohols with hydrogen peroxide catalyzed by the Mn(III) complex under solvent-free and room temperature conditions
b
c
Entry
Substrate
OH
Product
Yield (%)
Time (h)
O
1
94
93
4.5
OH
O
2
4.5
O
O
OH
O
O
O
O
O
3
4
5
6
97
95
95
97
4.0
5.5
4.5
5.0
O
O
OH
O
CH3
CH3
O
OH
CH3
CH3
Cl
Cl
OH
O
O
Et
Et
OH OH
CH3
OH
7
93
4.5
CH3

H. R. Mardani, H. Golchoubian / Tetrahedron Letters 47 (2006) 2349–2352
2351
Table 2 (continued)
b
c
Entry
Substrate
OH
Product
Yield (%)
Time (h)
5.0
O
8
9
92
H
OH
O
OH
H
OH
90
5.0
Cl
Cl
O
O
O
OH
1
1
0
1
(80:16)
(83:14)
4.5
5.0
+
O
O
OH
+
O
O
O
OH
1
1
2
3
+
(80:12)
98
5.0
O
d
OH
O
14.0
d
1
1
4
5
95
95
14.0
5.0
OH
O
OH
O
a
b
c
1
2 2
mmol of substrate at room temperature with 0.1 mmol of Mn(III) catalyst 1 and 10 mmol of 30% aqueous H O .
The products were identified by comparison of physical and spectroscopic properties with authentic compounds.
Isolated yields.
An extra 10 mmol of H O was added after 4.5 h. Without this extra addition a yield of 73% was obtained after 4.5 h.
2 2
d
the reaction was found to reach completion but over a
much longer time (Table 1, entry 5). When other
Mn(III) salts were used, such as Mn(III) acetate as a
catalyst in place of complex 1, no reaction took place
and the starting material was recovered (Table 1, entry
required the addition of an extra 10 mmol % of H O
after 4.5 h.
2
2
As shown, the system was found to be selective and both
aliphatic and benzylic alcohols were oxidized in excellent
yields. The yields were confirmed after the products were
isolated by short column chromatography using appro-
priate combinations of ethyl acetate/hexane. Although,
almost quantitative conversion was observed by TLC,
typically, 3–10% of product was lost during the work-
up procedure.
6
).
The oxidation of other benzylic alcohols was then exam-
ined using the optimized reaction conditions (Table 2).
No oxidation was observed on the aromatic rings of
the benzylic substrates. However, the alcohols in entries
1
0–12 behaved differently and benzylic oxidation also
occurred to generate diketones as by-products (Table
, entries 10–12).36 The yields for the oxidation of
aliphatic alcohols were about 95% (Table 2, entries
3–15). However, the reactions of cyclohexanol and
borneol (Table 2, entries 13 and 14, respectively),
Our results indicate that complex 1 is an effective cata-
lyst for the hydrogen peroxide oxidation of alcohols
under solvent-free conditions. An important advantage
of this method aside from the solvent-free conditions
is that it does not require a phase-transfer catalyst.
2
1

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H. R. Mardani, H. Golchoubian / Tetrahedron Letters 47 (2006) 2349–2352
17. Bouquillon, S.; A ¨ı t-Mohand, S.; Muzart, J. Eur. J. Org.
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