An efficient and selective oxidation of benzylic and aromatic allylic alcohols with manganese dioxide supported on kieselguhr under solvent-free conditions

Article abstract of DOI:10.1007/s11164-012-0945-7

An efficient and selective oxidation of benzylic and aromatic allylic alcohols to the corresponding aldehydes and ketones with manganese dioxide supported on kieselguhr at 50-55 C under solvent-free conditions in yields between 82 and 96 % is described. All the reactions are completed within 4 h. The merits of the present procedure by comparing with those reporting manganese dioxide oxidation are listed and the possible oxidation mechanism is also described. Aliphatic primary alcohols remain intact by this method.

Full text of DOI:10.1007/s11164-012-0945-7

Res Chem Intermed
DOI 10.1007/s11164-012-0945-7
An efficient and selective oxidation of benzylic
and aromatic allylic alcohols with manganese dioxide
supported on kieselguhr under solvent-free conditions
•
Li-Yun Zhu Zhiyuan Lou Jianan Lin
•
•
•
Wenjin Zheng Changhe Zhang Ji-Dong Lou
•
Received: 3 October 2012 / Accepted: 20 November 2012
Ó Springer Science+Business Media Dordrecht 2012
Abstract An efficient and selective oxidation of benzylic and aromatic allylic
alcohols to the corresponding aldehydes and ketones with manganese dioxide
supported on kieselguhr at 50–55 °C under solvent-free conditions in yields
between 82 and 96 % is described. All the reactions are completed within 4 h. The
merits of the present procedure by comparing with those reporting manganese
dioxide oxidation are listed and the possible oxidation mechanism is also described.
Aliphatic primary alcohols remain intact by this method.
Keywords Alcohols Á Kieselguhr Á Manganese dioxide Á Oxidation Á Solvent-free
Introduction
The selective oxidation of alcohols to the corresponding carbonyl compounds such
as aldehydes and ketones is one of the most important reactions in synthetic organic
chemistry, and it has received increased attention over the years [1–6]. Although
many useful procedures for this oxidation have been described, the general problem
L.-Y. Zhu Á J. Lin Á W. Zheng Á J.-D. Lou
College of Life Sciences, China Jiliang University, Hangzhou 310018, Zhejiang, China
Z. Lou
Department of Nutrition & Food Science, University of Maryland, College Park, MD 20742, USA
C. Zhang
Centre for the Research and Technology of Agro-Environmental and Biological Sciences (CITAB)/
´
Department of Biology and Environment, Universidade de Tras-os-Montes e Alto Douro (UTAD),
Apartado 1013, 5001-801 Vila Real, Portugal
J.-D. Lou (&)
Sirnaomics, Inc, 401 Professional Drive, Gaithersburg, MD 20879, USA
e-mail: jidonglou@sirnaomics.com
123

L.-Y. Zhu et al.
cannot be considered definitely settled. The key drawbacks of some reported
procedures are that the over-oxidation of aldehydes to carboxylic acids and the
degradation of unsaturated substrates are often unavoidable side reactions.
Furthermore, the relative difficulty in preparation of the reagents and in the
working-up of the reaction mixture exists during the reaction. In addition to the lack
of selectivity, oxidants like chromium(VI)-based reagents [7] are corrosive, irritants,
toxic, etc. Because of their significant role, the introduction of new methods or
reagents for this oxidative transformation is still fraught with experimental
challenges, especially when considering the search for versatile and selective
reagents.
Results and discussion
Manganese dioxide has been extensively used for the oxidation of a variety of
functional groups [8]. However, to the best of our knowledge, only a few
publications have reported the use of manganese dioxide reagents for selective
oxidation of primary alcohols into aldehydes: for example, solid manganese dioxide
under heterogeneous conditions [9–11], manganese dioxide under solvent-free
conditions [12, 13], and supported manganese dioxide reagent with [14, 15] or
without [16–18] microwave irradiation, while the rest are limited to the oxidation of
secondary alcohols to ketones. Because manganese dioxide is a relatively
environmentally friendly agent, the development of newer manganese dioxide-
based reagents is attracting much current interest in spite of the lack of availability
of procedures reported in the literature.
More recently, we have described that manganese dioxide supported on
kieselguhr is a good reagent that can successfully oxidize primary alcohols into
aldehydes [16] and benzoins to benzils under heterogeneous conditions [19], and
oxidize oximes to aldehydes and ketones under solvent-free conditions [20]. As part
of our ongoing program related to developing new oxidation methods, and based on
our previous investigations [16, 19, 20], we now report here a simple procedure for
efficient and selective oxidative transformation of benzylic and aromatic allylic
alcohols (1) into the corresponding aldehydes and ketones (2) using this supported
reagent at 50–55 °C under solvent-free conditions and grind (Scheme 1). It is well
known that recently considerable attention has been paid to solvent-free reactions
[21–23]. Solvent-free reactions are not only of interest from an ecological point of
view but they also offer considerable synthetic advantages in terms of yield,
selectivity, and simplicity of the reaction procedures. In many cases, solvent-free
reactions (or solid state reactions) occur more efficiently and more selectively than
do their solution counterpart, since molecules in a crystal are arranged tightly and
regularly. These factors are especially important in the industry. Therefore, some of
the traditional organic synthetic methods, which have long been carried out in
solvents, may be modified to more modern, elegant, and safe versions, as, for
example, solvent-free reactions.
In our case, a 1:2 molar ratio of the substrate to the oxidant [16] is employed.
After contact of alcohols with manganese dioxide supported on kieselguhr reagent
123

An efficient and selective oxidation of benzylic and aromatic
O
OH
R2
MnO2 - Kieselguhr
Solvent-free, 50-55oC, Grind
R1
R2
R1
(1)
Scheme 1 Solvent-free oxidation of alcohols to carbonyl compounds
(2)
at 50–55 °C under solvent-free conditions and grind, the expected products are
obtained. All the reactions are completed within 4 h. The progress of the reaction is
monitored with thin-layer chromatography (TLC), and the product is purified by
preparative TLC. The obtained products are all known compounds and identified by
spectroscopic comparison with authentic samples. The results, which are described
in Table 1, show that the present reagent is generally applicable to a range of
alcohols and gives the corresponding aldehydes and ketones in good yields.
Oxidation of aliphatic primary alcohols under the same reaction conditions were
also studied; however, they remain intact.
The main advantages of the present manganese dioxide oxidation are that the
insoluble solid support, kieselguhr, provides a particular reaction environment
capable of enhancing the reaction selectivity and reactivity. Moreover, it is well
known that, for the reaction with manganese dioxide in the absence of support, the
isolation of products from the ensuing residues is rendered more difficult and the
oxidation is also accompanied by the formation of minor contaminants. However, in
the current procedure, it turns out to be very profitable in the work-up, which
becomes reduced to a mere filtration. In addition, some data for the oxidation of
benzyl alcohol to benzaldehyde, as a representative example, with manganese
dioxide or supported manganese dioxide reagents under different reaction condi-
tions are listed in Table 2. It is clearly shown that, by comparison with the reported
work, the present procedure has the additional merits, for instance, the amount of
manganese dioxide used is quite reduced (entries 2, 4, 5, and 8), the yields of the
products are higher (entries 2–7), and it is no need to use microwave equipment
(entry 5). We propose that the present oxidation is a free radical mechanism
(Scheme 2), which is the same as that previously described by Goldman [24].
Experimental
Oxidation of benzyl alcohol to benzaldehyde: typical procedure
A mixture of benzyl alcohol (108 mg, 1 mmol) and manganese dioxide supported
on kieselguhr reagent [19] (2,600 mg, 2 mmol, kieselguhr purchased from Tianjin
Fu Chen Chemical Reagent Factory) was well ground with a pestle and mortar at
50–55 °C for 3 h. The progress of the reaction was monitored by TLC (silica gel
GF254). The solid was washed and filtered with dichloromethane (3 9 5 ml). The
combined filtrates were evaporated to give crude product, which was purified by
preparative TLC (silica gel GF254) with hexane:ethyl acetate (7:3) to afford 101 mg
(95 %) benzaldehyde.
123

L.-Y. Zhu et al.
Table 1 Oxidation of benzylic and aromatic allylic alcohols to aldehydes and ketones with manganese
dioxide supported on kieselguhr under solvent-free conditions
Entry Substratea
Productb
Reaction time
(h)
Yieldc
(%)
1
3
95
CHO
OH
2
3
90
CHO
OH
H3C
H3C
3
3
3
3
4
3
92
86
89
92
86
CHO
OH
H3CO
H3CO
4
CHO
OH
HO
HO
5
CHO
OH
O2N
O2N
6
CHO
OH
OH
OH
CHO
7
OH
8
9
4
2
82
96
CHO
OH
OH
O
O
10
3
90
OH
Cl
Cl
Cl
Cl
a
All substrates are commercially available
b
All products were identified by comparison of their physical and spectral data with those of authentic
samples
c
Yield of isolated pure product
123

An efficient and selective oxidation of benzylic and aromatic
Table 2 Comparison of oxidation of benzyl alcohol to benzaldehyde with manganese dioxide under
different reaction conditions
Entry Supports
Reaction
conditions
Microwave Molar ratio
irradiations substrate to
manganese
Reaction Yield References
time
(%)
dioxide
1
Kieselguhr Solvent-
free/
No
1:2
3 h
95
Present
work
50–55 °C/
grind
2
3
No
No
CHCl3/RT
No
No
1:11.5
1:1.5
23 h
5 h
89
92
[9]
Solvent-
free/
[12]
90 °C/
agitation
4
5
6
7
8
No
Solvent-
free/RT
No
Yes
No
No
No
1:8
1:5
1:2
1:2
1:3
48 h
77
88
90
92
96
[13]
[14]
[16]
[17]
[18]
Silica gel
Solvent-
free
20 min
10 h
Kieselguhr CH2Cl2/
reflux
Graphite
CH2Cl2/
reflux
10 h
Aluminum CH2Cl2/
silicate
12 h
reflux
MnO2
C6H5 - CH2OH
C6H5 - CH2OH/MnO2
Adsorption
1
OH
OH
O
Mn(IV)
OH
•
C6H5 - CHO
Mn(III)
C6H5 - CH2O
Coordination
Radical formation
OH
OH
C6H5 - CHO
2
MnO/H2O
+
C6H5 - CHO/Mn(II)
Scheme 2 Mechanism of the oxidation
123

L.-Y. Zhu et al.
Conclusion
An efficient and selective oxidation of benzylic and aromatic allylic alcohols to the
corresponding aldehydes and ketones with manganese dioxide supported on
kieselguhr at 50–55 °C under solvent-free conditions in the yields between 82
and 96 % is described, and all the reactions were completed within 4 h. The merits
of the present procedure by comparison with reported manganese dioxide oxidations
are that the amount of manganese dioxide used is quite reduced, the yields of the
products are higher, and there is no need to use microwave equipment. The possible
oxidation mechanism is described. Oxidation of aliphatic primary alcohols under
the same reaction conditions were also studied; however, they remain intact.
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