Microwave promoted rapid oxidation of alcohols using cobalt nitrate hexahydrate supported on silica gel under solvent free conditions

Article abstract of DOI:10.1081/SCC-120015814

A new method for oxidation of alcohols into the corresponding carbonyl compounds by treatment with cobalt nitrate hexahydrate adsorbed on silica gel is described. This reagent can be used as an effective and mild oxidizing agent for this transformation under microwave irradiation in dry media, and high yields. Over oxidation of the formed aldehydes was not observed.

Full text of DOI:10.1081/SCC-120015814

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Microwave Promoted Rapid Oxidation of Alcohols Using
Cobalt Nitrate Hexahydrate Supported on Silica Gel
Under Solvent Free Conditions
A. R. Kiasat a , F. Kazemi a & M. Rafati b
a Chemistry Department, College of Science, Shahid Chamran University, Ahvaz, Iran
b Ghachsaran Islamic Azad University, Ghachsaran, Iran
Published online: 16 Aug 2006.
To cite this article: A. R. Kiasat , F. Kazemi & M. Rafati (2003): Microwave Promoted Rapid Oxidation of Alcohols Using Cobalt
Nitrate Hexahydrate Supported on Silica Gel Under Solvent Free Conditions, Synthetic Communications: An International
Journal for Rapid Communication of Synthetic Organic Chemistry, 33:4, 601-605
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SYNTHETIC COMMUNICATIONSÕ
Vol. 33, No. 4, pp. 601–605, 2003
Microwave Promoted Rapid Oxidation of
Alcohols Using Cobalt Nitrate Hexahydrate
Supported on Silica Gel Under Solvent
Free Conditions
1,
2
A. R. Kiasat,1, F. Kazemi, and M. Rafati
*
*
1Chemistry Department, College of Science,
Shahid Chamran University, Ahvaz, Iran
2Ghachsaran Islamic Azad University,
Ghachsaran, Iran
ABSTRACT
A new method for oxidation of alcoholsinto the correpsonding
carbonyl compoundsby treatment with cobalt nitrate hexahydrate
adsorbed on silica gel is described. This reagent can be used as an
effective and mild oxidizing agent for thistranfsormation under
microwave irradiation in dry media, and high yields. Over oxidation
of the formed aldehydeswasnot observed.
*Correspondence: A. R. Kiasat and F. Kazemi, Chemistry Department, College
of Science, Shahid Chamran University, Ahvaz, Iran; Fax: þ983332040; E-mail:
kiasat_a@cua.ac.ir or kazemi_f@cua.ac.ir.
601
DOI: 10.1081/SCC-120015814
Copyright & 2003 by Marcel Dekker, Inc.
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com

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602
Kiasat, Kazemi, and Rafati
Heterogeneousorganic reactionsthat are facilitated by uspported
reagents on porous solids and assisted by microwaves, in particular,
have developed successfully and in the past few years there has been a
[1,2]
tremendousinterets in thisarea.
Since only the polar reactants
adsorbed on mineral surfaces absorb microwave energy, a variety of
reagent doped on such supports can be utilized for the enhancement of
organic reactions using a household microwave oven. These solvent-free
MW-assisted reactions[3] provide an opportunity to work with open vessels
thus avoiding the risk of high pressure development and increasing the
potential of such reactions to upscale.
The oxidation of alcoholic group to carbonyl functionality isan
important reaction in organic synthesis and several methods are available
to accomplish this conversion under a variety of reaction conditions.
Because of its important role in synthetic chemistry, this reaction
continues to receive attention from the chemists in search for newer
and selective methods of oxidation.[4,5]
Metallic nitratescan be uesd asmild oxidizing agent, but little
attention has been paid to this potential in organic synthesis.[6–9]
Laszlo and Cornelis,[6,7] have reported on appreciably unstable Fe(III)
nitrate impregnated on K10 bentnite clay, asa supported metal nitrato
complex for the oxidation of alcohol to carbonyl compound. They have
reported that, satisfactory results have been obtained with a variety of
secondary aliphatic, alicyclic, and benzyl alcohol, and with primary
benzyl alcohol. With primary aliphatic alcohol, complex mixturesof
[2]
productswasobtained. In 1997, a report by Varma and Dahiya
out-
lined that clayfen can be oxidize alcoholsupon exposure to microwaves
under solvent free conditions, also the limitation with aliphatic alcohol
still persists and the protocol gives a mixture of compounds. Nishiguchi
and Asano[8,9] have supported some metal nitrates on silica gel and used
those as oxidizing agents for the oxidation of benzylic alcohols under
nitrogen atmosphere. They have reported that activity of the oxidizing
agentsdepended on reaction temperature. By thees factsin mind
and also reports[7] about high reactivity of bidentate form of nitrate
anion in some metal nitrates such as cobalt nitrate, we now wish to
report herein a facile and selective oxidation of alcohols to carbonyl
compounds using cobalt nitrate hexahydrate supported on silica gel
(Co(NO3)2/Silica gel) under solvent-free conditions in a process that is
accelerated by microwave irradiation.
The oxidizing reagent was easily prepared by addition of silica gel to
a solution of cobalt nitrate in acetone. Solvent was evaporated and the
powdered reagent was dried under reduced pressure. This oxidizing agent
was stable and could be stored in air at room temperature without losing

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Oxidation with Cobalt Nitrate Hexahydrate
603
Table 1. Oxidation of alcoholsto carbonyl compoundsunder microwave
irradiation.a
Entry
Substrate
PhCH2OH
3-Me OPhCH2OH
4-Me OPhCH2OH
4-Me PhCH2OH
4-BrPhCH2OH
Time (min)
Yield (%)b
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
9
12
8
91
90
93
90
87
89
88
92
91
91
89
85
90
89
87
86
8
14
12
14
10
9
11
14
17
12
12
13
18
2-CIPhCH2OH
CH3(CH2)6CH2OH
Ph–CH¼CH–CH2OH
PhCH(OH)CH3
PhCH(OH)Ph
4-CIPhCH(OH)CH3
PhCH2CH(OH)CH2Ph
CH3(CH2)5CH(OH)CH3
Cyclohexanol
Cycloheptanol
PhCH(OH)COPh
aWere confirmed by comparison with authentic sample (IR and NMR).
bYield of isolated pure carbonyl compound.
itsactivity. The procedure involvesa simple mixing of neat alcoholswith
(Co(NO3)2/Silica gel) and irradiated the mixturesin a microwave oven
for the time being specified in Table 1. Several experiments were
performed, at variouspowersand irradiation times, in order to find the
most adequate condition for this reaction under microwave irradiation.
Oxidizing reagent to substrate ratio of 2:1 and the maximum power out
put of 200 W wasfound to be the optimal condition.
Our experiments show that not only primary and secondary
benzylic alcoholsbut also aliphatic alcoholsunderwent oxidation to the
corresponding carbonyl compounds in high yields with a short time of
irradiation. It isnoteworthy that further oxidation of obtained aldehydes
to their carboxylic acidsor cleavage of double bondsof a-b- unsaturated

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604
Kiasat, Kazemi, and Rafati
Table 2. Oxidation of alcoholsto carbonyl compoundsunder
conventional heating.a
Entry
Substrate
Time (h)
Yield (%)
1
2
3
4
PhCH2OH
5
5
5
5
96
65
80
75
CH3(CH2)6CH2OH
4-CIPhCH(OH)CH3
CH3(CH2)5CH(OH)CH3
aThe alcohol wasthoroughly mixed with the oxidizing agent
and heated at 80ꢀC under solvent free conditions.
alcoholswere not observed. It wasfound that the yield of reaction in the
absence of silica gel was very low.
To show the dramatic acceleration in the rate of oxidation due to
microwave irradiation, under the same experimental condition, several
alcoholswere heated juts by conventional heating. However, only
good yieldsof productswere obtained in comparably very long time
(Table 2).
[7]
Aspreviousreports,
it isprobable that cobalt nitrate hexahydrate
supported on silica gel is serve as a source of nitrosonium ions and the
oxidation reaction presumably proceeds via the intermediacy of nitrous
esters.
In conclusion, the present procedure of solid phase oxidation of
alcoholsprovidesa general methodology. The application of microwave
offersa very quick and clean method for thisconversion. The operational
simplicity, selectivity, use of inexpensive reagent, high yields in signifi-
cantly short reaction time, can make this procedure a useful and attractive
alternative to the currently available methods.
EXPERIMENTAL
General
All yields refer to isolated products. Products were characterized by
1
comparison of their physical data, IR and H NMR spectra with known
samples. The purity determination of the substrates and reaction mon-
itoring were accomplished by TLC on silica gel polygram SILG/UV 254
plates.

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Oxidation with Cobalt Nitrate Hexahydrate
General Procedure for the Oxidation of Alcohols
605
Silica gel wasadded to a os lution of cobalt nitrate hexahydrate in
acetone (1.0 mmol gꢀ1 SiO2). The mixture wasevaporated and the residue
wasfinally dried in the presence of P O5 under vacuum at 45ꢁC overnight.
2
Alcohol (1 mmol) wasthoroughly mixed with the oxidizing agent (2.582 g,
2 mmol). The resulting mixture was then taken in a Pyrex Erlenmeyer flask
(25 mL) and irradiated in a commercial microwave oven (200 W) for the
specified period of time (Table 1). The reaction mixture was then allowed
to reach room temperature. The inorganic support was separated by
filtration after eluting the product with CCl4. The obtained product after
evaporation of the solvent was dried over anhydrous sodium sulfate to
afford the TLC and 1H NMR pure carbonyl compound in 85–93%
isolated yield.
ACKNOWLEDGMENT
We would like to acknowledge the partial support of this work by
Ghachsaran Islamic Azad University Research Council.
REFERENCES
1. Varma, S.R.; Dahiya, R. Tetrahedron Lett. 1997, 38, 7823.
2. Varma, S.R.; Dahiya, R. Tetrahedron Lett. 1997, 38, 2043.
3. Varma, S.R.; Dahiya, R. Synth. Commun. 1998, 28 (21), 4087.
4. Einhorn, J.; Einhorn, C.; Ratajczak, F.; Pierre, J.L. J. Org. Chem.
1996, 61, 7452.
5. Srinivasan, R.; Akila, S.; Caroline, J.; Balasubramanian, K. Synth.
Coumm. 1998, 28 (12), 2245.
6. Cornelis, A.; Laszlo, P. Synthesis 1980, 849.
7. Cornelis, A.; Laszlo, P. Synthesis 1985, 909.
8. Nishiguchi, T.; Asano, F. Tetrahedron lett. 1988, 6265.
9. Nishiguchi, T.; Asano, F. J. Org. Chem. 1989, 54, 1531, and references
cited therein.
Received in the UK December 3, 2001

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