Selective oxidation of benzoins with potassium dichromate under viscous conditions

Article abstract of DOI:10.1080/15533171003629097

An efficient procedure for oxidation of benzoins to corresponding benzils using potassium dichromate under viscous conditions at room temperature is described. The procedure can overcome the problems existed in the common solvent-free reactions of the difficulty for the solid molecular collision to react. The present oxidations are completed within 3 hours with the yield between 86-97%.

Full text of DOI:10.1080/15533171003629097

Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 40:160–162, 2010
Copyright © Taylor & Francis Group, LLC
ISSN: 1553-3174 print / 1553-3182 online
DOI: 10.1080/15533171003629097
Selective Oxidation of Benzoins with Potassium Dichromate
Under Viscous Conditions
1
1
2
2
3
Ji-Dong Lou, Yi-Chun Ma, Negin Vatanian, Qiang Wang, and Changhe Zhang
1
College of Life Sciences, China Jiliang University, Hangzhou, Zhejiang, China
Sirnaomics, Inc., Gaithersburg, MD, USA
Instituto de Biologia Experimental e Tecnologia, Universidade Nova de Lisboa, Oeiras, Portugal
2
3
1.2 mmol) in a normal test tube is shaken mechanically (Hori-
An efficient procedure for oxidation of benzoins to correspond- zontal oscillator; Model: HY-2, Zhengji Instrument Co. Ltd.) at
ing benzils using potassium dichromate under viscous conditions room temperature for 3 h with care. The progress of the reac-
at room temperature is described. The procedure can overcome
the problems existed in the common solvent-free reactions of the
Merck 60 GF254) using hexane:ethyl acetate (7:3) as eluent.
difficulty for the solid molecular collision to react. The present
tion is monitored by TLC (plates: aluminum-backed silica gel
The reaction mixture is then washed with dichloromathane
oxidations are completed within 3 hours with the yield between
86–97%.
(3 × 10 mL). The combined filtrates are evaporated to give
crude product, which is purified by preparative TLC with hex-
Keywords benzils, benzoins, oxidation, potassium dichromate, vis- ane: ethyl acetate (7:3) to afford benzil (203 mg; 97%).
cous conditions.
Oxidation of Benzoin to Benzil
1
H NMR (300 MHz, CDCl3): δ (ppm), 7.9–7.8 (4H, m, 2 ×
INTRODUCTION
Ar-H); 7.6–7.4 (6H, m, 2 × Ar-H). FT-IR (KBr): 1680 (C=O),
Benzils are synthetically important because of their wide
1
595 (C=C).
practical applicability. They also find use as photosensitive
agents in photocurable coatings, and as interesting precur-
sors for some pharmaceutically important compounds like the
ꢀ
ꢀ
Oxidation of 4,4 -dimethylbenzoin to 4,4 -dimethylbenzil
1
H NMR (300 MHz, CDCl3): δ (ppm), 7.6 (4H, d, J = 9Hz,
antiepileptic phenytoin and the anticonvulsant dilantin.[
1−6]
2
× Ar-H); 7.2 (4H, d, J = 9Hz, 2 × Ar-H); 2.4 (6H, s, 2 ×
Benzils can be prepared by the way of oxidation of ben-
zoins with hexavalent chromium derivatives. Up to now, sev-
eral chromium reagents have been employed for this oxida-
CH3). FT-IR (KBr): 1680 (C=O), 1600 (C=C).
ꢀ
[
7] Oxidation of 4,4 -dimethoxybenzoin to
tion transformation, for instance, pyridinium fluorochromate,
ꢀ
1
[
8]
4,4 -dimethoxybenzil
ammonium chlorochromate–alumina,
chromium trioxide-
[
9]
[
1
0
]
H NMR (300 MHz, CDCl3): δ (ppm), 7.6 (4H, d, J = 9Hz,
k
i
e
s
e
l
g
h
u
r
,
a
n
d
c
h
r
o
m
i
u
m
t
r
i
o
x
i
d
e
i
n
d
i
m
e
t
h
y
l
s
u
l
f
o
x
i
d
e
,
2
× Ar-H); 6.9 (4H, d, J = 9Hz, 2 × Ar-H); 3.8 (6H, s, 2 ×
etc. Although a diversity of chromium methods is available for
this oxidation, developments of convenient, selective, and mild
procedures are important objective of current relevance.
OCH3). FT-IR (KBr): 1675 (C=O), 1600 (C=C).
Oxidation of 4-chlorobenzoin to 4-chlorobenzil
H NMR (300 MHz, CDCl3): δ (ppm), 7.8–7.3 (9H, m, Ar-
H). FT-IR (KBr): 1675 (C=O), 1595 (C=C).
1
EXPERIMENTAL
Oxidation of Benzoin to Benzil; Typical Procedure
A mixture of benzoin (212 mg, 1 mmol) that is dissolved with Oxidation of 4-methoxybenzoin to 4-methoxybenzil
1
a very minimum amount of dichloromethane (0.22 mL) to form
H NMR (300 MHz, CDCl3): δ (ppm), 7.8–7.4 (7H, m, Ar-
a viscous liquid in advance and potassium dichromate (353 mg, H). 7.0–6.9 (2H, m, Ar-H), 3.6 (3H, s, OCH3). FT-IR (KBr):
675 (C=O), 1590 (C=C).
1
Received 4 October 2009; accepted 17 January 2010.
Address correspondence to Ji-Dong Lou, College of Life Sciences,
China Jiliang University, Hangzhou, Zhejiang 310018, China. E-mail:
lou@cjlu.edu.cn
RESULTS AND DISCUSSION
Recently, considerable attention has been paid to the solvent-
free reactions in organic synthesis. Solvent-free reactions are not
160

SELECTIVE OXIDATION OF BENZOINS
161
O
temperature. These factors are especially important in industry.
Unfortunately, under solvent-free conditions, basically, it is not
satisfied for solid substrates, like benzoins, to be performed at
room temperature because molecules, substrates, and reagents
are in crystal forms that are of difficulty for collision to reach the
reaction, so that such reactions are normally carried out at the
temperature near or over the substrate melting point by either
heating or other technologies in advance in order to dissolve
the solid substrates into the liquid forms to increase the reaction
rate. Therefore, some of the solvent-free reactions, especially
for those solid substrates, may be modified to more modern and
elegant versions.
OH
Potassium dichromate
Viscous conditions, RT
Ar2
Ar2
Ar1
Ar1
O
O
(1)
(2)
1
2
Ar
Ar
H
3
C
3
H C
Previously, we described oxidation of alcohols with
potassium dichromate under solvent-free conditions[
11]
and
H
3
CO
3
H CO
oxidation of solid benzoins with manganese dioxide under
viscous conditions,[ respectively. In continuation of these
investigations, we now report here a more efficient procedure
for the oxidation of solid benzoins (1) into the corresponding
benzils (2) using potassium dichromate under viscous condi-
tions at room temperature (Scheme 1), which can overcome the
problems existed in the common solvent-free reactions of the
difficulty for the solid molecular collision to react as well.
In the present procedure, 1 to 1.2 molar ratio of the substrate
12]
Cl
3
H CO
SCH. 1.
only of interest from ecological point of view, but in many cases to potassium dichromate is employed, and some symmetrical
also offer considerable synthetic advantages in terms of yield, and unsymmetrical benzoins are carried out. The oxidation is
selectivity, simplicity of the procedure, and operation at room very simple: first the solid substrate is dissolved with a very
TABLE 1
Oxidation of benzoins with potassium dichromate under viscous conditions
◦
m.p. ( C)
a
b
Reported[13]
Substrate
Product
Reaction time (h)
Yield (%)
Found
3
3
97
92
89–91
92–93
99–101
102–104
3
3
3
90
86
86
129–131
70–72
131–133
73
58–60
62
a
All the products are known compounds and were identified by comparison of their melting points with the literature values as well as by
1
H NMR and IR.
Yields of isolated pure products.
b

162
J.-D. LOU T AL.
minimum amount of dichloromethane to form a viscous liquid, REFERENCES
and then the oxidant is added in with care. The mixture is shaken
with an oscillator at room temperature until TLC analysis indi-
cates a completed reaction, and all the reactions are completed
within 3 hours. Finally the residue is washed, and the prod-
uct is then purified by preparative TLC. The results, which are
shown in Table 1, show that this method is an efficient oxidation
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 main advantages of the present procedure is that un-
der viscous conditions the oxidation of the solid substrates can
be carried out very efficienty using an oscillator with mild
process, and therefore it is clear over other previous reported
chromium (VI)-based reagents. In addition, owing to the reac-
tion using a very minimum amount of solvents, combustion,
toxicity, and environmental pollution of the solvents are quite
reduced. Likewise, comparing with oxidation using supported
chromium reagents, the present procedure is in no need of prepa-
ration for supported reagent. The mechanism of the present ox-
1. Morrison, H., Danishefsky, S., and Yates, P. Preparation of α-diazo ketones.
J. Org. Chem.1961, 26, 2617–2618.
2. Matsushita Electric Industrial Co. Ltd. Photohardenable resin composi-
tions. Jpn. Kokai Tokkyo Koho 1981, 8198203 (Chem. Abstr. 1981, 95,
1
88163u).
3. Vollhardt, K.P.C., and Schore, N.E. Organic Chemistry, 2nd ed., Freeman:
New York, 1994, pp 924–929.
4. Mitra, A.K., De, A., and Karchaudhuri, N. Microwave-assisted syntheses
of 1,2-diketones. J. Chem. Res. (S) 1999, 246–247.
5. Vogel, A. Textbook of Practical Organic Chemistry including Qualitative
Organic Analysis, 4th. Ed.; Longman: New York, 1978; p. 883.
6. Joul, J.A., Mills, K., and Smith, G.F. Heterocyclic Chemistry, 3rd ed.;
Chapman & Hall: London, 1995.
7. Cainelli, G., and Cardillo, G. Chromium Oxidation in Organic Chemistry.
Springer; Berlin, 1984.
8
. Zhang, G.S., Shi, Q.Z., Chen, M.F., and Cai, K. Ammonium chlorochromate
adsorbed on alumina: a new reagent for the oxidation of alcohols and
benzoins to the corresponding carbonyl compounds. Synth. Commun. 1997,
27(6), 953–956.
9
. Lou, J.D., Li, L., Vatanian, N., Lu, X.L., and Yu, X. Selective oxidation
of α-hydroxy ketones to α-diketones with chromium trioxide supported
on kieselghur reagent, Synth. React. Inorg. Metal Org. Nano-Metal Chem.
2
008, 38(4), 373–375.
idation that we consider is the same as the well-known one that 10. Lou, J.D., Vatanian, N., Zhang, C., and Wang, G.Q. Oxidation of benzoins
is the oxidation of the secondary alcohols by chromium (VI)
with chromium trioxide in dimethyl sulfoxide. Synth. React. Inorg. Metal
Org. Nano-Metal Chem. 2009, 39(3), 121–123.
1. Lou, J.D., and Xu, Z.N. Selective solvent free oxidation of alcohols with
reagents described previously in the literature.[14]
1
potassium dichromate, Tetrahedron Lett. 2002, 43, 8843–8844.
CONCLUSION
12. Lou, J.D., Lu, X.L., Marrogi, A.J., Li, F., Gao, C.L., and Yu, X. A first
example for a reaction under viscous conditions: oxidation of solid benzoins
with manganese dioxide, Monatsh. Chem. 2008, 139, 609–611.
3. Khandekar, A.C., Paul, A.M., and Khadilkar, B.M. Silica supported man-
ganese dioxide: An efficient reagent for oxidation of benzoins. Synth. Com-
mun. 2002, 32(19), 2931–2936.
In conclusion, an efficient procedure for oxidation of ben-
zoins to corresponding benzils using potassium dichromate
under viscous conditions at room temperature is described,
which can overcome the problems that exist in the common
1
solvent-free reactions of the difficulty for the solid molecular 14. Solomons, T.W.G., and Fryhle, C.B. Organic Chemistry, 8th ed., John Wiley
Sons, Inc. 2004, pp 547–549.
&
collision to react.

Copyright of Synthesis & Reactivity in Inorganic, Metal-Organic, & Nano-Metal Chemistry is the property of
Taylor & Francis Ltd and its content may not be copied or emailed to multiple sites or posted to a listserv
without the copyright holder's express written permission. However, users may print, download, or email
articles for individual use.