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236
Chemistry Letters Vol.36, No.10 (2007)
Oxidation of Alcohols with Hydrogen Peroxide
Catalyzed by Molybdenum(VI)–Peroxo Complex under Solvent-free Conditions
1
;2
Ã1
3
1
2
Yi Luan, Ge Wang, Rudy L. Luck, Mu Yang, and Xiao Han
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, P. R. China
1
2
Beijing University of Chemical Technology, Beijing 100029, P. R. China
Department of Chemistry, Michigan Technological University, Houghton, MI 49931, U. S. A.
3
(Received July 2, 2007; CL-070701; E-mail: gewang@mater.ustb.edu.cn)
The oxidation of primary, secondary, allylic, and aromatic
Table 1. Oxidation of benzyl alcohol with hydrogen peroxide
a
alcohols to the corresponding carbonyl compounds proceeds in
high yield catalyzed by molybdenum(VI)–peroxo complex with
H2O2 under solvent-free conditions. Secondary –OH group are
selectively oxidized even in the presence of a primary –OH
group. Products/catalyst separation can be easily carried out
by simple extraction and the catalytic system can be reused
without much loss of activity.
in various solvents
CH OH
CHO
2
Complex 1
+
H O
2 2
Solvent-free
Entry
Solvent
Yield/%
Selectivity/%
1
2
3
4
—
Toluene
Dichloromethane
Acetonitrile
— (no catalyst)
96
34
22
67
98
99
99
99
Over the years, the oxidation of alcohols to the correspond-
ing carbonyl compound has been one of the most frequently used
synthetic reactions in organic synthesis. However, such trans-
formations have been generally performed with environmentally
1
b
5
N. R.
a
2
Reaction conditions: Benzyl alcohol (1 mmol), 1 (0.01 mol,
1 mol %), 30% aqueous H2O2 (2 mmol), 6 h, 80 C. Yields
were determined by gas chromatography with an internal
standard technique, and based on benzyl alcohol. Reaction
was carried out in the absence of catalyst.
hazardous inorganic oxidants, notably chromium(VI) reagents.
Therefore, from both the economic and environmental view-
ꢀ
3
points, there is a definite need for catalytic oxidation employing
hydrogen peroxide as the stoichiometric oxidant. Because
hydrogen peroxide is environmentally friendly, cheap, and read-
ily available. Recently, several different catalytic systems for
the hydrogen peroxide oxidation of alcohols under solvent-free
b
4
in the yields of 34 and 22%, respectively (Table 1, Entries 2
and 3), and water-miscible polar acetonitrile gave a 67% yield
(Table 1, Entry 4). Controlled experiments without complex 1
showed no oxidation under the same reaction conditions
(Table 1, Entry 5).
To evaluate the scope of this reaction, the oxidation of a se-
lection of primary, secondary, allylic, and aromatic alcohols was
carried out with H2O2 in the presence of a catalytic amount of 1.
The results obtained are summarized in Table 2. Besides benzyl-
ic alcohols, the substrate having electron-releasing substituents,
such as 4-methylbenzyl alcohol, was compatible with this sys-
tem (Table 2, Entry 2). Benzyl alcohols could be oxidized to
benzaldehydes in good yield with no over-oxidation. This behav-
ior is simple to explain because of the reactivity of benzylic
alcohols. The oxidations of allylic and secondary cyclic were
also examined upon treatment with complex 1–H2O2; then they
were converted to the corresponding aldehydes (Table 2, Entries
3–5).
The yields of oxidation of primary aliphatic alcohols were
very low in the same condition. As shown, longer periods of
reaction, greater quantities of catalyst and oxidant were needed
for the oxidation of primary aliphatic alcohols (Table 2, Entries
6 and 7). But the catalytic oxidation could be successfully per-
formed efficiently with secondary aliphatic alcohols (Table 2,
Entries 8 and 9). It was notable that the secondary –OH group
was chemoselectively oxidized to ketone even in the presence
of a primary –OH group (Table 2, Entry 10). Trost and
5
conditions have been developed. For example, a variety of alco-
6
hols can be oxidized by a manganese(III) Schiff-base complex.
The DmpzHFC(cat.)–H2O2 system transforms primary, secon-
dary, and allylic alcohols into their corresponding carbonyl com-
7
pounds with good yields at room temperature. Many molybde-
8
num- and tungsten-based catalytic systems have been reported.
Noyori et al. have described tungstate-based biphasic systems,
which are excellent in the oxidation of alcohol by H2O2 using
9
a phase-transfer catalyst.
In continuation of our work on the MoVI complexes,10
VI
we now report that the new Mo oxo–diperoxo complex
[
(
MoO(O2)2(TEDA)2] (TEDA = 1,4-diazabicyclo[2.2.2]octane)
1), obtained through a simple approach of MoO3 with H2O2
1
5
and TEDA, catalyzes the oxidation of a variety of alcohols to
the corresponding carbonyl groups in high yields by H2O2. Al-
though it is documented that under organic solvent conditions
a variety of olefins,11 sulfides, alcohols, and alkylbenzenes
12
13
14
VI
can be oxidized by Mo oxo–diperoxo complex and H2O2,
to the best of our knowledge, there is no report regarding the
oxidations of alcohols with H2O2 under solvent-free conditions.
Benzyl alcohol was first examined as a standard substrate
with H2O2 in various solvents in the presence of a catalytic
amount of 1, indicating that the system without any solvent
was the most effective one. The reaction was allowed to reflux
ꢀ
at ca. 80 C in the presence of 1 mol % of 1 for 6 h, which gave
1
6
a 96% yield, 98% selectivity to benzaldehyde (Table 1, Entry 1).
Using the non-polar solvents, such as toluene and dichlorome-
thane (organic/aqueous biphasic system), gave benzaldehyde
Masuyama found the same selectivity in a molybdenum-cata-
lyzed alcohol oxidation by hydrogen peroxide. It is assumed that
selective oxidation of a secondary alcohol can be performed
Copyright Ó 2007 The Chemical Society of Japan
Luan, Yi
