A first example of a reaction under viscous conditions: Oxidation of solid benzoins with manganese dioxide

Article abstract of DOI:10.1007/s00706-007-0800-5

A new, general, and efficient technique for the reaction of solid substrates under viscous conditions with shaking at room temperature is described, which can overcome some of the problems existing in common solvent-free reactions solids as the starting materials. In this manuscript we present as a first example of this method the oxidation of solid benzoins into the corresponding benzils using an environmentally friendly and inexpensive oxidant, manganese dioxide.

Full text of DOI:10.1007/s00706-007-0800-5

Monatsh Chem 139, 609–611 (2008)
DOI 10.1007/s00706-007-0800-5
Printed in The Netherlands
A first example of a reaction under viscous conditions:
oxidation of solid benzoins with manganese dioxide
1
1
2
3
4
1
Ji-Dong Lou , Xiu L. Lu , Aizen J. Marrogi , Feng Li , Chun-Ling Gao , Xiaoping Yu
1
College of Life Sciences, China Institute of Metrology, Hangzhou, Zhejiang, China
2
Department of Pathology and Area Laboratory Medicine, Walter Reed Army Medical Center, Washington DC, USA
School of Chemistry, University of Sydney, NSW, Australia
3
4
Center for Prostate Disease Research, Uniformed Service University of Health Science, Bethesda, MD, USA
Received 17 May 2007; Accepted 5 September 2007; published online 21 April 2008
Springer-Verlag 2008
#
Abstract A new, general, and efficient technique for
the reaction of solid substrates under viscous condi-
tions with shaking at room temperature is described,
which can overcome some of the problems existing in
common solvent-free reactions solids as the starting
materials. In this manuscript we present as a first ex-
ample of this method the oxidation of solid benzoins
into the corresponding benzils using an environmen-
tally friendly and inexpensive oxidant, manganese
dioxide.
because both the reacting molecules, substrates and
reagents, are in crystal forms that makes it difficult
for reach collision for reaction. Consequently, such
reactions are normally carried out at a temperature
near or over the substrate melting point by either
heating or other methods in order to dissolve the
solid substrates into liquid forms to increase the re-
action rate. Therefore, some of the reported solvent-
free reactions, especially for those solid substrates,
require modification to improve their profile.
Keywords Viscous conditions; Solid substrate; Solvent-free
reactions; Oxidation; Solid benzoins.
Results and discussion
In continuation of our ongoing program related to
developing new oxidation methods we wish to report
here a novel, general, and more efficient procedure
for solid substrate reactions under viscous conditions
with shaking at room temperature. As a first example
of this approach we report here the oxidation of solid
benzoins 1 into the corresponding benzils 2 using an
environmentally friendly and inexpensive oxidant,
manganese dioxide (MnO2).
Introduction
Considerable attention has been paid to solvent-free
reactions, which have attracted interest not only
from an ecological point of view, but in many cases
also offer considerable synthetic advantages in terms
of yield, selectivity, simplicity of the procedure, and
operation at room temperature [1–3]. These factors
are especially important in industry. Unfortunately,
under solvent-free conditions, it is in most cases not
possible to performed reaction at room temperature
Correspondence: Ji-Dong Lou, College of Life Sciences,
China Institute of Metrology, Hangzhou, Zhejiang 310018,
China. E-mail: lou@cjlu.edu.cn

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J.-D. Lou et al.
Table 1 Oxidation of solid benzoins under viscous conditions with MnO2
a
b
Yield =%
ꢀ
Substrate
Product
Reaction time=h
m.p.= C
Found
94–95
Reported [17]
3
3
3
3
3
96
93
95
92
95
95
102–103
133–134
197–199
161–163
104–105
131–133
197–199
162
a
All the products are known compounds and were identified by comparison of their spectra with the literature values
Yield of isolated product
b
The oxidation of benzoins into benzils is an im-
portant synthetic reaction in organic chemistry [4–
]. Benzils are versatile compounds which can be
solid substrates can be carried out very efficiently
using a shaking machine under mild conditions.
Compared to previous heterogeneous MnO oxida-
6
2
used for the preparation of a variety of molecules,
many of which show a diversity of interesting bio-
logically activities [7–12]. To the best of our knowl-
edge, there are only few reports about the use of
tions [13–15], for example, the yields of the pro-
ducts are higher, the oxidations can be performed
at room temperature without any heating, the sup-
ported reagents are no need of preparation, and the
work-up is easier. On the other hand, owing to the
reaction using a very minimum amount of solvents,
combustion, toxicity, and environmental pollution of
the solvents are quite reduced.
In summary, a new, general and efficient tech-
nique for solid substrate reactions under viscous
conditions with shaking at room temperature is de-
scribed, which can overcome problems known for
common solvent-free solid reactions.
MnO for the oxidation of benzoins to benzils, for
2
instance, with solid MnO [13, 14], and with sup-
2
ported MnO reagent [15], all of which are carried
2
out under heterogeneous conditions at reflux.
In our case, active MnO is prepared from potas-
2
sium permanganate (KMnO ) under basic conditions
4
according to literature [16]. The molar ratio of sub-
strate to MnO is 1:2. The oxidation is very simple:
2
first the solid substrate is dissolved with a very min-
imum amount of dichloromethane to form a viscous
liquid, and then the oxidant is added in one portion.
The mixture is shaken magnetically at room temper-
ature until TLC analysis indicates complete conver-
sion. All the reactions are completed within 3 h.
Finally the residue is washed, and the product is then
purified by preparative TLC. The results, which are
shown in Table 1, show that this method is an effi-
cient oxidation method for solid benzoins and gives
the corresponding benzils in high yields.
Experimental
Oxidation of benzoin to benzil: typical procedure
Benzoin (212 mg, 1 mmol) is dissolved in a minimum amount
3
of dichloromethane (0.22 cm ) to form a viscous liquid, to
which active MnO
a normal test tube. This mixture is shaken mechanically
Horizontal oscillator; Model: HY-2, Zhengji Instrument Co.
(174 mg; 2 mmol) [16] is added in
2
(
Ltd.) at room temperature for 3 h. The progress of the reaction
is monitored by TLC (plates: aluminum-backed silica gel
Merck 60 GF254) using n-hexane:ethyl acetate (7:3) as eluent.
The reaction mixture is then washed with dichloromethane
The main advantage of the present procedure is
that under viscous conditions the oxidation of the

Oxidation of solid benzoins with manganese dioxide
611
3
(
3Â5 cm ). The combined filtrates are evaporated to give
8. a) (1981) Matsushita Electric Industrial Co. Ltd., Jpn
Kokai Tokkyo Koho 8198203; b) Chem Abstr 95:188163v
9. Vollhardt KPC, Schore NE (1994) Organic Chemistry,
2nd edn. Freeman, New York, p 924
10. Mitra AK, De A, Karchaudhuri NJ (1999) Chem Res
(S):246
crude product, which is purified by preparative TLC with
hexane:ethyl acetate (7:3) to afford benzil (202 mg; 96%).
With tetrahydrofuran (THF) as a different solvent for the
oxidation of benzoin the corresponding benzil was obtained in
9
3% yield.
1
1. Vogel A (1978) Textbook of Practical Organic Chemistry
including Qualitative Organic Analysis, 4th edn. Long-
man, New York, p 883
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