Selective oxidation of benzylic alcohols using can supported onto silica gel under microwave irradiation

Article abstract of DOI:10.1080/10426500490485084

Cerium ammonium nitrate (CAN) adsorbed on HNO₃/silica gel is a mild reagent for selective oxidation of benzylic alcohols to the corresponding aldehydes under microwave irradiation in solventless system.

Full text of DOI:10.1080/10426500490485084

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SELECTIVE OXIDATION OF
BENZYLIC ALCOHOLS USING
CAN SUPPORTED ONTO SILICA
GEL UNDER MICROWAVE
IRRADIATION
Majid M. Heravi a , Hossein A. Oskooie a ,
Pegah Kazemian a , Fatemeh Drikvand a & Mitra
Ghassemzadeh b
a Department of Chemistry, School of Sciences,
Azzahra University , Tehran, Iran
b Chemistry & Chemical Engineering Research Center
of Iran , Tehran, Iran
Published online: 16 Aug 2010.
To cite this article: Majid M. Heravi , Hossein A. Oskooie , Pegah Kazemian ,
Fatemeh Drikvand & Mitra Ghassemzadeh (2004) SELECTIVE OXIDATION OF BENZYLIC
ALCOHOLS USING CAN SUPPORTED ONTO SILICA GEL UNDER MICROWAVE IRRADIATION,
Phosphorus, Sulfur, and Silicon and the Related Elements, 179:11, 2341-2344, DOI:
10.1080/10426500490485084
To link to this article: http://dx.doi.org/10.1080/10426500490485084
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Phosphorus, Sulfur, and Silicon, 179:2341–2344, 2004
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ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500490485084
SELECTIVE OXIDATION OF BENZYLIC ALCOHOLS
USING CAN SUPPORTED ONTO SILICA GEL
UNDER MICROWAVE IRRADIATION
Majid M. Heravi,a Hossein A. Oskooie,a Pegah Kazemian,a
Fatemeh Drikvand,a and Mitra Ghassemzadehb
Department of Chemistry, School of Sciences, Azzahra
University, Vanak, Tehran, Iran;a and Chemistry & Chemical
Engineering Research Center of Iran, Tehran, Iranb
(Received February 20, 2004; accepted April 29, 2004)
Cerium ammonium nitrate (CAN) adsorbed on HNO3/silica gel is a
mild reagent for selective oxidation of benzylic alcohols to the corre-
sponding aldehydes under microwave irradiation in solventless system.
Keywords: Benzylic alcohols; CAN; microwave irradiation; oxidation
The oxidation of alcohols to cabonyl compounds is an important trans-
formation of organic synthesis, and several methods have been explored
to accomplish such a conversion.1 However, there is a constant demand
to develop more selective synthetic methods that are able to discrim-
inate efficiently between various functional groups and be relatively
ecofriendly. Therefore, selective methods allowing for oxidation of pri-
mary benzylic alcohols to aldehydes without overoxidation to carboxylic
acids and without competitive oxidation of secondary or nonbenzylic
alcohols remain challenging. So far relatively few procedures are avail-
able for this type of selectivity.2
During the past few years, cerium ammonium nitrate (CAN) has be-
come a synthetic reagent and catalyst of growing importance.3 Recent
years have witnessed a phenomenal growth in the use of high surface
area inorganic solids as reaction media for organic transformation.4
The advantages of the reactions are that they provide enhanced re-
action rates, greater selectivity, and manipulative simplicity. The use
of microwave irradiation to deliver energy for organic reactions has in-
creased in popularity in recent times.5 We6 and others7 have found that
Address correspondence to Majid M. Heravi, Department of Chemistry, School of
Sciences, Azzahra University, Vanak, Tehran, Iran. E-mail: mmheravi@azzahra.ac.ir
2341

2342
M. M. Heravi et al.
the coupling of microwave irradiation with the use of solid supports fa-
cilitates numerous chemical transformations.
In this article we describe a new method for the selective oxidation of
benzylic alcohols to the corresponding aldehydes using CAN supported
onto HNO3/silica gel as a support and cocatalyst under microwave irra-
diation. As a starting point we used CAN as an oxidant for oxidation of
benzylic alcohol under microwave irradiation in solventless system. In
this case, thin layer chromatography (TLC) analysis indicated the pres-
ence of unreacted alcohol. Solvent-free conditions, using 2 equiv. of CAN
supported onto silica gel under microwave irradiation gave a mixture of
benzaldehyde and unreacted benzyl alcohol. However, when we used a
drop of HNO3 on silica gel under the above condition benzyl alcohol was
quatitatively oxidized to benzaldehyde without any noticeable overoxi-
dation to benzoic acid, even with an excess of CAN. Moreover, primary
aliphatic and allylic alcohols, and even secondary benzylic alcohols, are
oxidized with much lower efficiency under the same reaction condi-
tions (Table I, entries 5 and 6). This observation suggested a possible
chemoselective oxidation of primary benzylic alcohols in the presence
of alyphatic and secondary ones. Indeed the present oxidation system
is prone to be exceptionally chemoselective. The reaction of 1:1 mixture
of benzyl alcohol and 1-phenyl ethanol with our reagent resulted in the
complete conversion of the benzyl alcohol to benzaldehyde, whereas vir-
tually no ketone could be detected by TLC. To establish the generality of
TABLE I Oxidation of Benzylic Alcohols to Aldehydes
Reaction time
(min)
Yielda
(%)
Entry
1
Substrate
Product
5
89
2
3
4
5
91
90
4
8
75
5
6
5
5
Trace
Trace
aYields refer to isolated products.

Oxidation of Alcohols
2343
the method for benzylic alcohols, various benzylic alcohols were success-
fully converted to their corresponding aldehydes using CAN supported
onto HNO3/silica gel in short reaction time and excellent yields.
In summary, this methodology allows a convenient, high-yielding,
relatively benign selective and chemoselective procedure for oxidation
of benzylic alcohols. The ease of workup and availability of reagents are
the other advantages of this method. These features should make it of
significant synthetic value.
EXPERIMENTAL
CAN, alcohols, and silica gel were purchased and used as received. All
products were known and identified by comparison of their physical
data with those of authentic samples.
General Procedure for the Oxidations
In a beaker a drop of conc. HNO3 was added to silica gel (1 g) and mixed
thoroughly using a spatula. To this HNO3/silica gel, CAN (2 mmol) and
neat benzylic alcohol were added and mixed thoroughly. The beaker was
placed in a household microwave oven for the indicated time (Table I).
The progress of reaction was monitored by TLC using hexane:EtOAc
(8:2) as eluent. After the completion of the reaction, to the residue
CH2Cl2 (10 ml) was added and the reaction mixture was filtered off.
The filtrate was evaporated to dryness under reduced pressure, and
the crude was directly subjected to column chromatography using hex-
ane:EtOAc (8:2) as eluent to yield the corresponding aldehydes (Table I).
CAN
−→
ArCH2OH
ArCHO
HNO3/ silica gel
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