SELECTIVE OXIDATION OF BENZOINS
497
O
HO
Cr
Ar1
H
O
OH
O
O
Ar2
Ar2
O
Ar2
Ar1
Ar1
2-
CrO3
+
+
CrO3
H
O
O
O
H
H
2
1
3
SCH 2.
aluminum silicate at room temperature under viscous conditions molecular collision to react and is an extension of our previous
(Scheme 1), which can overcome the problems existed in the work as well.
common solvent-free reactions of the difficulty for the solid
molecular collision to react. The present procedure offers a sim-
REFERENCES
ple and efficient oxidation method for the preparation of benzils,
and is an extension of our previous work as well.
1. Matsushita Electric Industrial Co. Ltd. Photohardenable resin compositions.
Jpn. Kokai Tokkyo Koho 1981, 8198203 (Chem. Abstr. 1981, 95, 188163u).
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Pergamon: New York, 1991, Vol. 3, pp 821–838.
In the present study, a 1.5 to 1 molar ratio of chromic acid to
substrate is employed.[28] First, the solid substrate is dissolved
with a very minimum amount of dichloromethane to form a
viscous liquid, and then the oxidant is added with care. The
mixture is shaken magnetically at room temperature until TLC
analysis indicates a completed reaction, and all the reactions are
completed within 3 hours. 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 efficient oxi-
dation of solid benzoins, and gives the corresponding benzils in
high yields. The oxidized products are all known compounds
and identified by spectroscopic comparison with authentic
samples.
The mechanism of the current oxidation that we consider is
the same as the well-known one that is the oxidation of the sec-
ondary alcohols by chromium (VI) reagents, which involves the
nucleophilic oxygen reacting with the chromium (VI) reagent
to produce an intermediate, chromate ester (3), followed by de-
composition of the ester to afford ketones (Scheme 2) described
previously in the literature.[20,33]
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The main advantages of the present procedure are that un-
der viscous conditions, the oxidation of the solid substrates can
be carried out very efficiently with mild process, and owing to
the reaction, using a very minimum amount of solvents, com-
bustion, toxicity, and environmental pollution of the solvents
are quite reduced. Therefore it is, probably, over previous re-
ported oxidation routes with chromium (VI)-based reagents as
oxidants.
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under solvent-free conditions. J. Chem. Res. (S), 1998, 656–657.
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adsorbed on alumina as oxidant for alcohols and benzoins, Youji Huaxue,
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CONCLUSION
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A simple, versatile, and efficient reagent for oxidation of alcohols in non-
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solvent-free oxidation of organic compounds with potassium dichromate in
the presence of lewis acids. Molecules, 2001, 6, 900–908.
An efficient procedure for oxidation of benzoins to corre-
sponding benzils using chromic acid supported on aluminum
silicate reagent under viscous conditions at room temperature
is described, which can overcome the problems existed in the
common solvent-free reactions of the difficulty for the solid