Chromium trioxide supported onto wet silica gel: Rapid oxidation of alcohols to carbonyl compounds under microwave irradiation in solventless system

Article abstract of DOI:10.1080/10426500214883

In an environmentally benign system, alcohols are rapidly oxidized to carbonyl compounds using CrO₃ supported onto wet silica gel as an oxidant under microwave irradiation.

Full text of DOI:10.1080/10426500214883

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Chromium Trioxide Supported onto Wet Silica Gel:
Rapid Oxidation of Alcohols to Carbonyl Compounds
Under Microwave Irradiation in Solventless System
Majid M. Heravi a , Nasrin Farhangi a , Yahya Sh. Beheshtiha a , Karim Assadolah a , Mitra
Ghassemzadeh b & Koroush Tabar-Hydar b
a Azzahra University , Vanak, Tehran, Iran
b Chemistry and Chemical Engineering Research Center of Iran , Tehran, Iran
Published online: 27 Oct 2010.
To cite this article: Majid M. Heravi , Nasrin Farhangi , Yahya Sh. Beheshtiha , Karim Assadolah , Mitra Ghassemzadeh &
Koroush Tabar-Hydar (2002) Chromium Trioxide Supported onto Wet Silica Gel: Rapid Oxidation of Alcohols to Carbonyl
Compounds Under Microwave Irradiation in Solventless System, Phosphorus, Sulfur, and Silicon and the Related Elements,
177:12, 2883-2885, DOI: 10.1080/10426500214883
To link to this article: http://dx.doi.org/10.1080/10426500214883
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Phosphorus, Sulfur and Silicon, 2002, Vol. 177:2883–2885
C
Copyright 2002 Taylor & Francis
1042-6507/02 $12.00 + .00
DOI: 10.1080/10426500290146929
CHROMIUM TRIOXIDE SUPPORTED ONTO WET
SILICA GEL: RAPID OXIDATION OF ALCOHOLS
TO CARBONYL COMPOUNDS UNDER MICROWAVE
IRRADIATION IN SOLVENTLESS SYSTEM
Majid M. Heravi,a Nasrin Farhangi,a Yahya Sh. Beheshtiha,a
Karim Assadolah,a Mitra Ghassemzadeh,b
and Koroush Tabar-Hydarb
Azzahra University, Vanak, Tehran, Irana and Chemistry and
Chemical Engineering Research Center of Iran, Tehran, Iranb
(Received April 4, 2002)
In an environmentally benign system, alcohols are rapidly oxidized to
carbonyl compounds using CrO3 supported onto wet silica gel as an
oxidant under microwave irradiation.
Keywords: Carbonyl compounds; chromium trioxide; oxidation of
alcohols; silica gel
The oxidation of alcohols to carbonyl compounds is an important
transformation in organic chemistry attracting much interest.1–3 Al-
though a large number of methods are known in the literature for
such a transformation, there still appears a need either to improve
the existing oxidation methods4 or to introduce newer reagents to per-
mit better selectivity under milder conditions and with easy work-up
procedures.5
Chromium based reagents have been used extensively in organic
synthesis.6 A drawback against such oxidants and their use in multi-
stage organic synthesis in spite of their power is their lake of selectiv-
ity, for example, overoxidation of aldehydes to carboxylic acids and the
degradation of unsaturated substrates are often unavoidable side re-
actions. Moreover oxidants based on chromium are corrosive and they
are irritants for the skin and for sensitive parts of the body such as
eyes.7 Introduction of reagents on solid supports have shown some of
Address correspondence to Majid M. Heravi, Department of Chemistry, School of
Sciences, Azzahra University, Vanak, Tehran, Iran. E-mail: mmheravi@azzahra.ac.ir
2883

2884
M. M. Heravi et al.
TABLE I Oxidation of Alcohols Using CrO3/Wet Silica Gel under Microwave
Irradiation in Solvent-Free Conditions
R1
R1
CrO -silica gel
3
e
%
→
e
OH
O
MW
%
R2
Substrate
Benzylalcohol
2-Nitrobenzylalcohol
2-Methylbenzylalcohol
Benzohydrol
R2
Entry
Time (sec)
Product
Yielda (%)
1
2
3
4
5
6
7
8
60
60
120
60
180
120
180
180
Benzaldehyde
96
98
98
98
70
98
96
96
2-Nitrobenzaldehyde
2-Methylbenzaldehyde
Benzophenone
Cinnamaldehyde
Cyclohexanone
Cinnamylalcohol
Cyclohexanol
n-Amylalcohol
n-Amylaldehyde
Iso-Amylaldehyde
Iso-Amylalcohol
aUnoptimized yields of isolated products that exhibited physical and spectral properties
in accur with the assigned structure.
these problems, but still affords an attractive route in organic synthesis
in view of the selectivity and associated ease of manipulation.8 In con-
tinuation of our investigations on organic reactions in solventless sys-
tems under microwave irradiation, we report an efficient and rapid
oxidation of alcohols to the corresponding carbonyl compounds us-
ing CrO3-wet silica gel under microwave irradiation in a solventless
system.
The reaction is conducted by mixing finely ground wet silica gel-
chromium (VI) oxide with neat alcohols and exposed the mixture on
microwave irradiation for a very short time (Table I). We discovered
that in the absence of wet silica gel the reactions do not proceed and in
the presence of dry silica gel the reactions are sluggish and considerable
amounts of alcohols are recovered unchanged even after reaction for
extended periods of time. The reactions are relatively clean with no
tar formation which is typical for many chromium trioxide oxidations.
Interestingly, no overoxidation to carboxylic acids was observed.
The oxidation of cinnamyl alcohol with this method gave a moderate
yield of cinnamaldehyde (70%) and benzaldehyde (30%) showing the
carbon-carbon bonds are prone to cleavage by this procedure.
In conclusion, this communication affords a valuable extension of the
use of chromium trioxide as oxidant. High yields, very short reaction
time, easy work-up procedure, and solventless conditions are advan-
tages of this methodology.

Chromium Trioxide Supported onto Wet Silica Gel
2885
EXPERIMENTAL SECTION
All oxidation products are known compounds. They were identified by
their physical and spectroscopic data.
Wet Silica Gel Supported Chromium (VI) Oxide
Silica gel (10 g, Aldrich Brockmann 60) was shaken with distilled water
(2 mL). This mixture (2.4 g) was mixed with chromium trioxide (0.8 g,
8 mmol) using a pistle and mortar.
Oxidation of Alcohols to Carbonyl Compounds
General Procedure
In a beaker, neat alcohol (1 mmol) was mixed with the above catalyst
(1 mmol). An exothermic reaction started with darkening of the orange
color of the reagents. The completion of the reaction was confirmed
by TLC (hexane: EtOAc; 8:2). The product was extracted with CH2Cl2
and was passed through a small bed of alumina (1 cm) to afford the
corresponding carbonyl compound (Table I). (Caution: Although this
reaction worked safely in our hand, using an efficient hood is strongly
recommended.)
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