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Indian Journal of Chemistry
Vol. 44B, July 2005, pp. 1524-1526
Note
CrO3/Al2O3: Rapid oxidation of alcohols to
carbonyl compounds in a solvent- free system
A R Kiasat*, F Kazemi* & K Nourbakhsh
Chemistry Department, College of Science,
Shahid Chamran University, Ahvaz Iran
6137-4-3169, Iran, e-mail: [email protected]
Received 24 March 2004; accepted (revised) 21 December 2004
Primary and secondary alcohols are efficiently oxidized to the
corresponding carbonyl compounds using CrO3 supported onto
alumina under solvent- free conditions. It has been found that the
presence of small amount of t-butanol in the reaction media is
essential. Over-oxidation of aldehydes to carboxylic acids and
cleavage of carbon–carbon double bond is not observed by this
method.
IPC: Int.Cl.7 C 07 C 49/00
Oxidation of primary and secondary alcohols to the
corresponding carbonyl compounds has attracted
considerable attention during recent years1,2. The use
of high oxidation state transition metals3 such as
chromium (VI) reagents for alcohol oxidation is well
known. Since the chromium residues are
environmentally hazardous and are potentially
dangerous (ignition or explosion) during product
isolation and waste disposal,4 it would be
advantageous to develop oxidizing methods which
use chromium trioxide on some solid supports.
CrO3 supported on resin,5 graphite,6 halosilane,7
silica gel,8 aluminum silicate,9 kieselguhr,10 and
zirconium (IV) oxide11 have been previously reported
as oxidizing agents in the presence of solvents.
Recently, application of CrO3 supported on wet
alumina,12 wet silica gel3 and zeolite HZSM-513 have
been reported as oxidizing agents for oxidation of
alcohols to carbonyl compounds under microwave
irradiation in solvent-less system. It has been shown
that in the absence of inorganic supports the oxidation
reactions under microwave irradiation are sluggish
and considerable amounts of starting materials are
recovered unchanged. Very recently we reported14
that CrO3 in the presence of copper sulphate
efficiently oxidized alcohols under free solvent
conditions. We report conditions whereby various
types of primary and secondary alcohols can be
conveniently oxidized to the corresponding carbonyl
compounds under mild reaction conditions by CrO3
supported onto Al2O3 in solvent-free conditions.
Chromium trioxide supported on alumina was
prepared by simply cogrinding alumina with CrO3 in
the ratio 10:1 (w/w) in an agate mortar. In this simple
and efficient method the alcohols were oxidized to the
corresponding carbonyl compounds when the former
was ground in a mortar with a pestle in the presence
of supported CrO3 on Al2O3 and a few drops of tBuOH (Scheme I). The feasibility of the oxidation
was first examined using benzyl alcohol as a model
substrate. Thus, benzyl alcohol was thoroughly mixed
with 2 equivalents of CrO3/Al2O3 and small amount of
t-BuOH in an ambient air environment at room
temperature and benzaldehyde was obtained in 81%
isolated yield within 7 min. It is noteworthy that the
oxidation did not proceed to completion even after
prolonged hours of reaction when less than 2
equivalents of the oxidant were used.
With the first successful result in hand, oxidation
of other alcohols with CrO3/Al2O3 were carried out
under same conditions. The results obtained are
presented in Table I. As shown in Table I, primary
and secondary saturated aliphatic, benzylic, and the
heterocyclic alcohols were oxidized to the
corresponding carbonyl compounds in good to
excellent isolated yields in a very short time (510 min). Over-oxidation of primary benzylic alcohols
to carboxylic acids, which is common in the some
other reagents, was not observed. One of the
interesting features of this reagent was its ability to
convert hydrobenzoin to benzoin contrasting to the
results obtained by many other reagents, which
cleaved the carbon-carbon bond to give the
corresponding aldehyde or acid. An α,β-unsaturated
alcohol, 4-phenyl-but-3-en-2-ol was easily converted
to 4-phenyl-but-3-en-2-one in 90% yield showing that
carbon-carbon double bonds were not prone to
cleavage in this method and no isomerization of
double bonds were observed.
R1
OH
R2
CrO3 / Al2O3
Solvent free
Grinding
Scheme I
R1
O
R2
NOTES
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Table I ⎯ Conversion of alcohols to carbonyl compounds using CrO3 supported onto Al2O3a
Entry
Substance
Product
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
1-heptanol
1-octanol
benzyl alcohol
4-methoxybenzyl alcohol
4-chlorobenzyl alcohol
2-hydroxybenzyl alcohol
3-nitrobenzyl alcohol
cyclohexanol
cycloheptanol
2-isopropyl-4-methoxycyclohexanol
4-phenylcyclohexanol
1-phenylethanol
benzhydrol
4-phenyl-but-3-en-2-ol
hydrobenzoin
heptanal
octanal
benzaldehyde
4-methoxybenzaldehyde
4-chlorobenzaldehyde
2-hydroxy benzaldehyde
3-nitrobenzaldehyde
cyclohexanone
cycloheptanone
2-isopropyl-4-methoxycyclohexanone
4-phenylcyclohexanone
1-phenylethanal
benzophenone
4-phenyl-but-3-en-2-one
benzoin
Time
(min)
Yieldb,c
(%)
6
6
7
5
5
7
10
7
7
7
8
5
7
7
8
68
73
81
85
91
71
93
67
92
95
87
79
89
90
87
a
Molar ratio of reagent to substrate was 2:1. b Yields refer to pure isolated products. c products were
characterized by comparison of their physical data, IR, 1H NMR spectra with authentic samples17.
The promoting effect of Al2O3 was confirmed by
comparing our results with those previously reported
by Ji-Dong Lou et al15, who applied CrO3 in the
absence of inorganic support for the oxidation of
primary alcohols to the corresponding aldehydes in
solvent-free conditions at room temperature. The
oxidation of benzyl alcohol with CrO3 was completed
after 3 hrs, while with CrO3/Al2O3, the time required
for completion of oxidation reaction was 7 min.
The rapid and selective formation of products
demonstrates the efficiency of this new method. The
structures of all the products were settled from their
analytical and spectral (IR, 1H NMR) data and by
direct comparison with authentic samples.
In summary, oxidation of alcoholic groups with
CrO3/Al2O3 in solvent-less system has advantages
over previously reported methods as it provides a
facile, useful, and inexpensive technique addition to
the existing method of oxidations. The advantages of
this environmentally friendly protocol are mild
reaction condition, simple work-up procedure and
short reaction time.
Experimental Section
All oxidation products were known compounds and
were identified by comparison of their physical and
spectral data with those of authentic samples.17
Alcohols, CrO3, and Al2O3 were purchased from
Fluka and Merck. The purity of the products and
reaction monitoring were accomplished by TLC on
silica gel polygram SILG/ UV 254 plates.
General procedure for the oxidation of alcohols
by CrO3/Al2O3. A mortar was charged with Al2O3 (2
g) and CrO3 (0.2 g, 2 mmol) and the mixture was
ground with a pestle for 1 min. Then alcohols (1
mmol) and about seven drops of t-butanol were added
to the mixture. The reaction mixture was ground for
the time specified in Table I. The progress of reaction
was monitored by TLC using CCl4-ether (4:1). The
reaction mixture was poured into a mixture of ether
(20 ml) and water (10 ml). The organic layer was
passed through a small bed of alumina and evaporated
to dryness to give the pure carbonyl compounds
(Table I) (Caution: CrO3 is a highly toxic agent. All
chromium (VI) reagents must be handled with care16).
Acknowledgement
Authors acknowledge the partial support of this
work by Chamran University Research Council.
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