Oxidation of benzoins to benzils with chromium trioxide under viscous conditions

Article abstract of DOI:10.1080/00397911.2010.492071

An efficient and selective oxidation of benzoins into corresponding benzils using chromium trioxide under viscous conditions at room temperature is described. The present oxidations are completed within 2 h with the yield of 86-96%. It can overcome the problems of the common solvent-free reactions, in which it is difficult for the solid molecular collision to react.

Full text of DOI:10.1080/00397911.2010.492071

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Oxidation of Benzoins to Benzils with
Chromium Trioxide Under Viscous
Conditions
Li-Hong Huang a , Qiang Wang b , Yi-Chun Ma a , Ji-Dong Lou b &
Changhe Zhang c
a College of Life Sciences , China Jiliang University , Hangzhou ,
Zhejiang , China
b Sirnaomics, Inc. , Gaithersburg , Maryland , USA
c Centre for the Research and Technology of Agro-Environmental
and Biological Sciences (CITAB)/Department of Biology and
Environment , Universidade de Trás-os-Montes e Alto Douro (UTAD) ,
Vila Real , Portugal
Published online: 20 Apr 2011.
To cite this article: Li-Hong Huang , Qiang Wang , Yi-Chun Ma , Ji-Dong Lou & Changhe Zhang
(2011) Oxidation of Benzoins to Benzils with Chromium Trioxide Under Viscous Conditions, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,
41:11, 1659-1663, DOI: 10.1080/00397911.2010.492071
To link to this article: http://dx.doi.org/10.1080/00397911.2010.492071
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Synthetic Communications1, 41: 1659–1663, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2010.492071
OXIDATION OF BENZOINS TO BENZILS WITH
CHROMIUM TRIOXIDE UNDER VISCOUS CONDITIONS
Li-Hong Huang,1 Qiang Wang,2 Yi-Chun Ma,1 Ji-Dong Lou,2
and Changhe Zhang3
1College of Life Sciences, China Jiliang University, Hangzhou, Zhejiang,
China
2Sirnaomics, Inc., Gaithersburg, Maryland, USA
3Centre 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), Vila Real, Portugal
GRAPHICAL ABSTRACT
Abstract An efficient and selective oxidation of benzoins into corresponding benzils using
chromiumtrioxideunderviscous conditionsatroomtemperature isdescribed. Thepresentoxi-
dations are completed within 2 h with the yield of 86–96%. It can overcome the problems of the
common solvent-free reactions, in which it is difficult for the solid molecular collision to react.
Keywords Benzils; benzoins; chromium trioxide; oxidation; viscous conditions
INTRODUCTION
The oxidation of alcohols to the corresponding carbonyl compounds with
hexavalent chromium derivatives is widely used in organic synthesis, and a large
number of procedures have been described.[1–3] It is well known that carbonyl
compounds in the chemical industry are precursors with wide applicability. How-
ever, the established chromium oxidation methods still have some disadvantages:
mainly the chromium-based reagent must often be prepared. With methods involv-
ing use of aqueous acid and miscible organic solvents such as acetone[4] and dimethyl
sulfoxide,[5] the solvent may be attacked by chromic acid, and sometimes thick
reaction mixtures are produced, which hinder isolation of the product. Therefore,
new experimental procedures for conducting oxidation with chromium reagents
are still of interest, especially in the search of versatile and selective methods.
Received December 11, 2009.
Address correspondence to Ji-Dong Lou, Sirnaomics, Inc., 401 Professional Drive, Gaithersburg,
MD 20879, USA. E-mail: jidonglou@sirnaomics.com
1659

1660
L.-H. HUANG ET AL.
RESULTS AND DISCUSSION
Benzils are synthetically important because of their wide practical applicability.
They also find use as photosensitive agents in photocurable coatings and as interest-
ing precursors for some pharmaceutically important compounds such as the antiepi-
leptic phenytoin and the anticonvulsant dilantin.[6–11]
Mostly, benzils can be prepared by the way of oxidation of benzoins with
chromium reagents. Up to now, several hexavalent chromium derivatives have been
employed for this oxidation transformation, for instance, chromyl chloride,[12]
quinaldinium dichromate,[13] poly(2-vinyl-quinolinium) dichromate,[13] ammonium
chlorochromate,[14–18] chromium trioxide,[19–22] trimethylammonium chlorochro-
mate,[23] pyridinium chlorochromate,[9] Jones reagent,[24] potassium dichromate,[25]
quinoxalinium bromochromate,[26] pyridinium fluorochromate,[27,28] and piperdi-
nium chlorochromate.[29] Although many diverse chromium methods are available
for this oxidation, convenience, selectivity, and mild procedures are important
objectives of current relevance.
Considerable attention has been paid to the solvent-free reactions in organic
synthesis. Solvent-free reactions are not only of interest from 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. These
factors are especially important in industry. Unfortunately, under solvent-free con-
ditions, these do not occur for solid substrates, such as benzoins, performed at room
temperature because molecules, substrates, and reagents are in crystal forms, which
make it difficult for the collision to reach the reaction, so such reactions are normally
carried out at a temperature near or over the substrate melting point by either heating
or other technologies. This dissolves the solid substrates into liquid forms, which
increases the reaction rate. Therefore, some of the solvent-free reactions, especially
for those solid substrates, may be modified to more modern and elegant versions.
Previously, we described oxidation of alcohols with chromium trioxide under
solvent-free conditions[30] and oxidation of solid benzoins with manganese dioxide under
viscous conditions,[31] respectively. In continuation of these investigations, we report
here a more efficient procedure for the oxidation of solid benzoins (1) into the corre-
sponding benzils (2) using chromium trioxide under viscous conditions at room tempera-
ture (Scheme 1), which can overcome the problems in the common solvent-free reactions.
In the present procedure, a 1 to 2 molar ratio of the substrate to chromium
trioxide is employed. The oxidation is very simple: first the solid substrate is
Scheme 1.

OXIDATION OF BENZOINS TO BENZILS
1661
Table 1. Oxidation of benzoins with chromium trioxide under viscous conditions
Mp (ꢀC)
Reported[32]
Reaction Yieldb
Substrate
Producta
time (h)
(%)
Found
93–95
2
96
95
2
2
92
86
103–104
130–132
104–105
131–133
2
2
93
89
194–196
159–161
197–199
162
aAll the products are known compounds and were identified by comparison of their melting points with
the literature values as well as by 1H NMR and IR.
bYields of isolated pure products.
dissolved with a 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 thin-layer chromatographic (TLC) analysis indicates a completed
reaction, and all the reactions are completed within 2 h. Finally the residue is
washed, and the product is then purified by preparative TLC. The results in
Table 1 show that this method efficiently oxidizes solid benzoins and gives the cor-
responding benzils in good yields. The oxidized products are all known compounds
and identified by spectroscopic comparison with authentic samples.
The main advantages of the present procedure is that under viscous conditions
the oxidation of the solid substrates can be carried out very efficiently using a shak-
ing machine with mild process, and therefore it is clearly better than other previously
reported chromium(VI)-based reagents. In addition, because the reaction uses a
minimum amount of solvents, combustion, toxicity, and environmental pollution
of the solvents are reduced. Likewise, compared with the oxidation using chromium
trioxide–Kieselguhr under viscous conditions[21] that we reported before, the present
procedure has no need of preparation for the supported reagent.
EXPERIMENTAL
Oxidation of Benzoin to Benzil: Typical Procedure
A mixture of benzoin (212 mg, 1 mmol) (dissolved with a minimum amount
of dichloromethane (0.2 mL) to form a viscous liquid in advance) and chromium

1662
L.-H. HUANG ET AL.
trioxide (200 mg, 2 mmol) in a normal test tube is shaken mechanically at room
temperature for 2 h with care. The progress of the reaction is monitored by TLC
(plates: aluminum-backed silica gel Merck 60 GF254) using hexane–ethyl acetate
(7:3) as eluent. The reaction mixture is then washed with dichloromethane
(3 Â 10 mL). The combined filtrates are evaporated to give the crude product, which
is purified by preparative TLC with hexane–ethyl acetate (7:3) to afford benzil
(201 mg; 96%).
CONCLUSION
In conclusion, an efficient procedure for oxidation of benzoins to correspond-
ing benzils using chromium trioxide under viscous conditions at room temperature is
described, which can overcome the problems in the common solvent-free reactions
that make it difficult for the solid molecules to react.
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