Rapid oxidation of alcohols to aldehydes and ketones with chromium trioxide catalyzed by kieselguhr under solvent-free conditions

Article abstract of DOI:10.1007/s11164-012-0663-1

A rapid and selective oxidation of alcohols to aldehydes and ketones with chromium trioxide catalyzed by kieselguhr under solvent-free conditions in yields between 90 and 98 % is described. The role of kieselguhr in this oxidation is not only as support but also as catalyst, even though the catalytic nature of the kieselguhr is currently unclear. The present oxidation is even faster than (or the same as) those assisted with microwave irradiations. Meanwhile, the influence of the water presence during the preparation of the reagent is considered.

Full text of DOI:10.1007/s11164-012-0663-1

Res Chem Intermed (2013) 39:1015–1020
DOI 10.1007/s11164-012-0663-1
Rapid oxidation of alcohols to aldehydes and ketones
with chromium trioxide catalyzed by kieselguhr
under solvent-free conditions
Ai He
Changhe Zhang
•
Jie Zhang
•
•
Wenjin Zheng
Lihong Huang
•
Chen Huang
Ji-Dong Lou
•
•
Received: 24 April 2012 / Accepted: 26 May 2012 / Published online: 22 June 2012
Ó Springer Science+Business Media B.V. 2012
Abstract A rapid and selective oxidation of alcohols to aldehydes and ketones
with chromium trioxide catalyzed by kieselguhr under solvent-free conditions in
yields between 90 and 98 % is described. The role of kieselguhr in this oxidation is
not only as support but also as catalyst, even though the catalytic nature of the
kieselguhr is currently unclear. The present oxidation is even faster than (or the
same as) those assisted with microwave irradiations. Meanwhile, the influence of
the water presence during the preparation of the reagent is considered.
Keywords Oxidation Á Chromium trioxide Á Kieselguhr Á Catalysis
Introduction
The oxidation of alcohols to the corresponding aldehydes and ketones is an
important reaction in synthetic organic chemistry. Chromium trioxide (CrO ), an
3
A. He Á J. Zhang Á W. Zheng Á L. Huang Á J.-D. Lou
College of Life Sciences, China Jiliang University, Hangzhou 310018, Zhejiang, China
C. Huang
China Pharmaceutical University, Nanjing 210009, Jiangsu, China
C. Zhang
Centre for the Research and Technology of Agro-Environmental and Biological Sciences
(
CITAB)/Department of Biology and Environment, Universidade de Tr a´ s-os-Montes e Alto Douro
UTAD), Apartado 1013, 5001-801 Vila Real, Portugal
(
J.-D. Lou (&)
Sirnaomics, Inc., 401 Professional Drive, Gaithersburg, MD 20879, USA
e-mail: jidonglou@sirnaomics.com
123

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A. He et al.
oxidizing agent used in organic chemistry for over a century, is one of the most
versatile and vigorous of the commonly used oxidants in this transformation [1–4].
Unfortunately, it is well known that the main problems for the traditional chromium
trioxide in this oxidation are that the overoxidation of aldehydes to carboxylic acids
and the degradation of unsaturated substrates are often unavoidable side reactions.
Such oxidations are normally performed in acidic aqueous solution. Under the
reaction condition, the oxidizing reagent can further oxidize the aldehyde to the
acid, because in an aqueous medium the aldehyde product can be hydrated to
the geminal diol, which is further oxidized. In addition, under acidic medium, the
aldehyde product and unreacted alcohol react to give a hemiacetal that is rapidly
oxidized to an ester [5]. Over the years, chromium trioxide has been continually
developed and modified to overcome these key problems that occur for the
reactions, especially in the overoxidation that exists during the selective preparation
of aldehydes from the corresponding alcohols.
Reactions using supported reagents have attracted much attention. A certain
reagent onto or into an organic polymeric material or an inorganic porous material
or a layered support material not only exerts modified reactivity of the reagent
and/or specific product selectivity, but considerably simplifies product isolation.
Thus, a great variety of supported reagent systems are currently in wide use in
organic synthesis [6, 7]. Furthermore, reagent recovery or generation can also be
very easy. Supported reagents are relatively safe to handle owing to the full
chemisorption of the possible toxic chemicals. In most cases, solid supported
reagents have been found superior to the non-supported reagents. So far, there
have been some papers describing the selective oxidation of alcohols using
chromium trioxide-based reagents with or without microwave irradiations [8–12].
Since oxidation with supported chromium trioxide reagents would appear to be a
way to circumvent the overoxidation and the ester formation, in recent years the
traditional chromium trioxide oxidation has been modified to more modern and
elegant versions.
Experimental
Oxidation of benzyl alcohol to benzaldehyde: typical procedure
Chromium trioxide (600 mg, 6 mmol) and kieselguhr (720 mg; purchased from
Tianjin Fu Chen Chemical Reagent Factory) were mixed with a pestle and mortar.
Benzyl alcohol (540 mg; 518 mL; 5 mmol) was added. The reaction mixture was
well ground at room temperature using a pestle. The progress of the reaction was
monitored by TLC (glass backed silica gel GF254 plates; Branch of Qingdao
Haiyang Chemical) using hexane:ethyl acetate (7:3) as eluent. The reaction was
immediately (0.5 min) completed. The solid was filtered and washed with
dichloromethane (3 9 5 mL). The combined filtrates were evaporated to give
crude product, which was purified by preparative TLC (silica gel GF254) with
hexane:ethyl acetate (7:3) to afford 520 mg (98 %) benzaldehyde.
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Rapid oxidation of alcohols
1017
Results and discussion
We have found that kieselguhr is a good inorganic solid support for organic
synthesis [13–27], which has also been used as chromium trioxide supports in the
selective oxidation of alcohols to the corresponding aldehydes and ketones under
heterogeneous conditions [13] and solvent-free conditions [16], respectively. In
continuation of our previous investigations, we now report here a facile, rapid, and
selective oxidation of alcohols (1) to aldehydes and ketones (2) with chromium
trioxide catalyzed by kieselguhr under solvent-free conditions (Scheme 1), which
can overcome the overoxidation of aldehydes to carboxylic acids and the
degradation of unsaturated substrates existing in the traditional chromium trioxide
oxidation.
In the present work, benzyl alcohol as a representative substrate has been studied.
A 1 to 1.2 molar ratio of benzyl alcohol to chromium trioxide is employed. After
mixing with a pestle and mortar under solvent-free conditions at room temperature,
the corresponding benzaldehyde is obtained immediately (0.5 min) in 98 % yield.
Under this reaction condition or even after 2 h oxidation, no over-oxidation has
been observed. Our results are listed in the Table 1.
Some data for the oxidation of benzyl alcohol to benzaldehyde with supported
chromium trioxide reagents under different reaction conditions are described in
Table 2. It is clearly shown that by comparing with the reported work without
microwave irradiations (entries 2 and 3), the main advantages for the present
procedure (entry 1) are that the product yields are higher, and/or the molar ratio of
substrate to chromium trioxide are lower, and/or the reaction times are shorter.
Interestingly, the present oxidation is even faster than (or the same as) those assisted
with microwave irradiations (entries 4 and 5). We discovered that the present
procedure greatly expedited the reaction rate with excellent yields without needing
microwave irradiations. Therefore, we consider that the role of kieselguhr in the
oxidation is not only as support but also as catalyst, even though the catalytic nature
of the kieselguhr is currently unclear. In addition, the promoting effect of kieselguhr
can also be confirmed by comparing the current results with those previously
reported, which applied chromium trioxide in the absence of inorganic support for
the oxidation of primary alcohols to the corresponding aldehydes in solvent-free
conditions at room temperature. For example, the oxidation of benzyl alcohol with
chromium trioxide was completed under solvent-free condition after 180 min (entry
6), while with the current kieselguhr reagent, the required time for completion of
oxidation reaction is around just 0.5 min.
In the present investigation, we also found a high influence if the water is present
during the preparation of the chromium trioxide supported on the kieselguhr
reagent. From Table 3, it can be seen that kieselguhr cannot be touched with water
Scheme 1 Oxidation of
alcohols
OH
R2
CrO3 - kieselguhr
O
R1
solvent-free, RT, 30-40 sec
R1
R2
(1)
(2)
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A. He et al.
Table 1 Oxidation of alcohols to aldehydes and ketones with chromium trioxide catalyzed by kieselguhr
under solvent-free conditions
Reaction
c
Yield
a
b
Entry
Substrate
Product
time
(
%)
(
min)
0.5
CHO
OH
1
2
3
4
5
6
98
96
94
94
96
90
CHO
OH
0.5
0.7
0.5
0.7
0.7
H3C
H C
3
CHO
OH
H3CO
H CO
3
CHO
OH
HO
HO
CHO
OH
O2N
2
O N
CHO
OH
OH
OH
CHO
OH
7
8
9
0.5
0.5
0.5
96
98
98
OH
O
O
OH
Cl
Cl
Cl
Cl
a
All the substrates are commercially available
b
All products were identified by comparison of their physical and spectral data with those of authentic
samples
c
Yield of isolated pure product
during the preparation of the reagent; otherwise, the prepared reagent may
considerably decrease its activity (entries 2 and 3).
Conclusion
In this work, a facile, rapid, and selective oxidation of alcohols to aldehydes and
ketones with chromium trioxide catalyzed by kieselguhr under solvent-free
1
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Rapid oxidation of alcohols
1019
Table 2 Comparison of oxidation of benzyl alcohol to benzaldehyde with supported chromium trioxide
reagents under solvent-free conditions with or without microwave irradiations
Entry Supports
Reaction
conditions
Microwave Molar ratio Reaction Yield References
irradiations substrate to time (min) (%)
chromium
trioxide
1
2
Kieselguhr
Solvent-free, No
RT, grind
1:1.2
1:2
0.5
98
Current
work
a
Wet alumina
Solvent-free, No
RT, grind
Immediacy 76
(no date
indicated)
[8]
3
4
5
6
a
Copper sulfate
Wet silica gel
Solvent-free, No
RT, grind
1:2
7
75
96
99
90
[9]
Solvent-free, Yes
grind
1:1
1
[10]
[11]
[28]
HZSM-5 zeolite Solvent-free, Yes
grind
1:1.2
1:1.5
0.5
180
No
Solvent-free, No
RT, stirring
In the references [8, 12], it was not clearly indicated whether or not the reaction was performed under
microwave irradiations, but from the scheme, it was shown that the oxidation was carried out under
microwave irradiations. Here, we assume it was without assistance by microwave irradiations
Table 3 Comparison of oxidation of benzyl alcohol to benzaldehyde with different methods to prepare
chromium trioxide supported on kieselguhr
Entry Media for
Reaction
Molar ratio
(substrate to
chromium trioxide) (min)
Reaction Yield References
preparation conditions
of reagent
time
(%)
1
2
Dry
Solvent-free, RT, grind
1:1.2
1:3
0.5
60
98
83
Current work
[13]
Water
Heterogeneous,
RT, stirring
3
Water
Solvent-free, RT, stirring 1:2
10
94
[16]
conditions in excellent yields is described. The present oxidation is even faster than
or the same as) those assisted with microwave irradiations. We consider that the
(
role of kieselguhr in the oxidation is not only as support but also as catalyst, even
though the catalytic nature of the kieselguhr is currently unclear. Meanwhile, the
influence of the water presence during the preparation of the reagent is considered.
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