Oxidation of hydrobenzoins to benzils by ammonium chlorochromate/ montmorillonite K-10 under ultrasound irradiation

Article abstract of DOI:10.1080/00397910701818503

The oxidation of hydrobenzoins to the corresponding benzils was carried out in excellent yield by ammonium chlorochromate (ACC) supported on montmorillonite K10 in CH₂Cl₂ at room temperature under ultrasound irradiation.

Full text of DOI:10.1080/00397910701818503

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Oxidation of Hydrobenzoins
to Benzils by Ammonium
Chlorochromate/
Montmorillonite K-10 Under
Ultrasound Irradiation
a
a
Ji-Tai Li & Xue-Li Sun
a
College of Chemistry and Environmental Science,
Hebei University, Key Laboratory of Analytical
Science and Technology of Hebei Province, Baoding,
China
Published online: 12 Nov 2009.
To cite this article: Ji-Tai Li & Xue-Li Sun (2009) Oxidation of Hydrobenzoins to
Benzils by Ammonium Chlorochromate/Montmorillonite K-10 Under Ultrasound
Irradiation, Synthetic Communications: An International Journal for Rapid
Communication of Synthetic Organic Chemistry, 39:24, 4321-4327, DOI:
10.1080/00397910701818503
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Synthetic Communications , 39: 4321–4327, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910701818503
Oxidation of Hydrobenzoins to Benzils by
Ammonium Chlorochromate/Montmorillonite
K-10 Under Ultrasound Irradiation
Ji-Tai Li and Xue-Li Sun
College of Chemistry and Environmental Science, Hebei University,
Key Laboratory of Analytical Science and Technology
of Hebei Province, Baoding, China
Abstract: The oxidation of hydrobenzoins to the corresponding benzils was
carried out in excellent yield by ammonium chlorochromate (ACC) supported
on montmorillonite K10 in CH
irradiation.
2 2
Cl at room temperature under ultrasound
Keywords: Benzil, hydrobenzoin, oxidation, supported reagent, ultrasound
irradiation
INTRODUCTION
Benzil derivatives are an important class of compounds and have been
[1]
applied as photosensitive agents in photocurable coatings, biologically
[3]
important compounds, inhibitors of the acid corrosion of mild steel,
[2]
[4]
and starting materials in organic and pharmaceutical synthesis. Many
methods were developed for the synthesis of benzils. Among them, the oxi-
dation of 1,2-diols to 1,2-diketones has attracted increasing interest. Many
[
5]
[6]
methods, including Br =CCl =(Bu Sn )O, NaOCl-4-MeO-(TEMPO),
3
2
4
2
[7]
[8]
4-AcNH-TEMPO)-OTs-TsOH, 4-PhCO -TEMPO–electrolysis, n-Bu4
(
N[MoO(O ) (C H N(O)CO )],
2
[
9]
H O -[C H N-(CH ) CH ] {PO [W(O)
2 2 5 5 2 15 3 3 4
O -Co(acac) -N-hydroxyphthalimide,
2
2
2
5
4
2
[10]
O ) ] },
[11]
(
N-bromosuccinimide
2
2 4
3
Address correspondence to Ji-Tai Li, College of Chemistry and Environmental
Science, Hebei University, Wusi East Road No. 180, Baoding 071002, China.
E-mail: lijitai@hbu.cn
4
321

4322
J.-T. Li and X.-L. Sun
Scheme 1. Oxidation of hydrobenzoins to benzils.
[
12]
þ
ꢀ
NBS), PhN Me Br , o-iodoxy-benzoic acid, bromamine-T, and
3
3
[13]
[14]
[15]
(
[
16]
H O , have been used to achieve this transformation. However, in spite of
2
2
their potential utility, some of the reported methods suffer from drawbacks
such as long reaction times, expensive catalysts, and poor yields.
Reagents supported on inorganic substrates have been received
increasing attention in recent years as a means to develop convenient
and environmentally friendly reagents. Montmorillonite K10 is known
to behave as Brønsted acids in organic reactions. The use of montmoril-
lonite K10 as solid support has become very useful in synthetic organic
chemistry because of the enhanced selectivity resulting from its large sur-
face area. The concept of utilizing reagents adsorbed on inert inorganic
supports has been reported and applied, especially to chromium com-
[
17]
pounds. These reagents can oxidize various alcohols to corresponding
carbonyl compounds and have many advantages such as mild conditions,
easy workup procedures, and good yields.
Ultrasound has increasingly been used in modern organic synthesis
in the past three decades. It offers the potential for shorter reaction time
[
18]
and milder conditions.
We have reported the oxidation of hydroben-
[
19]
zoin to benzils by CrO ꢀNH Cl under ultrasound irradiation. Herein,
3
4
we report an efficient oxidation of hydrobenzoins to the corresponding
benzils by ammonium chlorochromate (ACC)=montmorillonite K10
under ultrasound irradiation (Scheme 1).
RESULTS AND DISCUSSION
The results of the oxidation of 1,2-diphenyl-1,2-ethanediol to benzil by
ACC= montmorillonite K10 under ultrasound irradiation are summar-
ized in Table 1. From the results, the optimum reaction conditions are
as follows: under ultrasound irradiation, the 1,2-diphenyl-1,2-ethanediol
is 1 mmol and the oxidant (ACC=montmorillonite K10) is 2.22 g.
To verify the effect of ultrasound irradiation, we have performed
the oxidation of 1,2-diphenyl-1,2-ethanediol in the presence of ACC=
ꢁ
montmorillonite K10 by stirring it for 80 min at 22–27 C without
sonication; the yield of 2a was 89% (Table 1, entry 5). Under ultrasound

Oxidation of Hydrobenzoins to Benzils
4323
Table 1. Effect of the amount of oxidant (ACC=montmorillonite K10) on the
a
oxidation of 1,2-diphenyl-1,2-ethanediol under ultrasound irradiation
ACC=montmorillonite
K10 (g)
Time
(min)
Isolated
yield (%)
Entry
1
2
3
4
5
1.11
1.66
2.22
2.77
2.22
80
70
45
40
80
88
90
95
97
b
89
a
ꢁ
Reaction temperature: 22–27 C; the substrate was 1 mmol; about 0.555 g
ACC=montmorillonite K10 including 1 mmol ACC.
b
The reaction was stirred without ultrasound.
irradiation, the reaction can be completed in 45 min in 95% yield (Table 1,
entry 3). It was clear that ultrasound irradiation improved the reaction
result.
Under this condition, the ACC=montmorillonite K10 system was
developed for use in the oxidation of hydrobenzoins to benzils. The
results are shown in Table 2.
As shown in Table 2, the oxidants ACC=montmorillonite K10 can
oxidize hydrobenzoins to benzils in good yields under ultrasound irradia-
tion. In Khandekar et al.’s report, it took 12 h to oxidize the benzoins to
[
22]
benzils in 80–94% yields using SiO -MnO as the oxidant.
In Zhang
et al.’s reports of using ACC adsorbed on alumina or silica gel, benzoin
2
2
Table 2. Oxidation of hydrobenzoins by ACC=montmorillonite K10 under
a
ultrasound
Time
(min)
Isolated
yield (%)
Mp
ꢁ
ꢁ
Mp ( C)
[Lit.]
Compound
Ar
( C)
[
22]
2
2
2
2
2
2
2
2
a
b
c
d
e
f
C
2-ClC
2,4-Cl C H
2
3-ClC
4-ClC
6
H
5
45
60
60
60
60
60
70
120
95
91
88
92
92
90
91
96
95–96
95
[
[
[
22]
23]
21]
6
H
4
131–132
146–149
140–142
198–199
103–104
123–124
95–96
134–135
147–148
142–143
6
3
6
6
H
H
4
[
22]
4
199
[
22]
21]
4-CH C H
4
104–105
121–122
[22]
95
3
6
[
g
h
4-CH
Benzoin
3
OC
6
H
4
b
a
b
ꢁ
Reaction temperature: 22–27 C.
Benzoin was 1 mmol; the oxidant ACC=montmorillonite K10 was 1.11 g
(
including 2 mmol ACC).

4324
J.-T. Li and X.-L. Sun
ꢁ
[23]
was stirred at 60 C in a water bath for 14–16 h.
However, in the
present procedure, the oxidation of benzoin by ACC=montmorillonite
K10 was completed in 95% yield within 2 h under ultrasound irradiation.
Even the oxidation of hydrobenzoins in our systems was completed in
good to excellent yields within 45–70 min.
CONCLUSION
In conclusion, montmorillonite K10–supported ammonium chlorochro-
mate, as the convenient workup reagent, is easily available and may serve
as an excellent oxidant for the transformation of hydrobenzoins to
benzils under ultrasound irradiation in good yields.
EXPERIMENTAL
[23a]
Preparation of ACC/Montmorillonite K10
Ammonium chloride (10.7 g, 0.2 mol) was added to a solution of chro-
ꢁ
mium trioxide (20 g, 0.2 mol) in water (50 mL) within 15 min at 40 C.
The mixture was cooled until a yellow-orange solid formed. Reheating
ꢁ
it to 40 C gives a solution. Montmorillonite K10 (80 g) was then added
ꢁ
to the solution with stirring at 40 C. After evaporation in a rotary
ꢁ
evaporator, the orange solid was dried for 2.5 h at 50 C. The content
of ACC in ACC=montmorillonite K10 was about 18.1%.
General Procedure for Oxidation of Hydrobenzoin to Benzil
Melting points are uncorrected. Sonication was performed in Shanghai
Branson-25–06 ultrasonic cleaner (with a frequency of 25 kHz and a
nominal power of 250 W). The starting materials, hydrobenzoins, were
[24]
prepared according to literature.
A 50-mL round flask was charged with hydrobenzoins (1, 1 mmol),
ACC=montmorillonite K10 (2.22 g, including ACC 4 mmol), and
CH Cl (8 mL) in one portion. The reaction flask was located in the clea-
2
2
ner bath, where the surface of reactants was slightly lower than the level
of the water. The mixture was irradiated for a period as indicated in
Table 2 (the reaction was monitored by thin-layer chromatography,
TLC). The reaction products were isolated by filtration from the reaction
mixture. Then the solvent was removed by evaporation under reduced

Oxidation of Hydrobenzoins to Benzils
4325
pressure. The authenticity of the product was established by comparing
[
the melting point with that reported in literature.
19,22,23]
ACKNOWLEDGMENT
The project was supported by the Natural Science Foundation of Hebei
Province (B2006000969), China.
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