Microwave assisted oxidation of benzaldehydes under solvent-free condition using oxone/wet-alumina

Article abstract of DOI:10.1080/10426500490463655

The solventless oxidation of benzaldehydes to carboxylic acids using oxone/wet-alumina under microwave irradiations is described.

Full text of DOI:10.1080/10426500490463655

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Phosphorus, Sulfur, and Silicon and the Related
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MICROWAVE ASSISTED OXIDATION OF BENZALDEHYDES
UNDER SOLVENT-FREE CONDITION USING OXONE®/WET-
ALUMINA
Mir M. Alavi Nikje a & Mohammad A. Bigdeli b
a Imam Khomeini International University , Qazvin, Iran
b Teacher Training University , Tehran, Iran
Published online: 16 Aug 2010.
To cite this article: Mir M. Alavi Nikje & Mohammad A. Bigdeli (2004) MICROWAVE ASSISTED OXIDATION OF BENZALDEHYDES
UNDER SOLVENT-FREE CONDITION USING OXONE®/WET-ALUMINA, Phosphorus, Sulfur, and Silicon and the Related Elements,
179:7, 1445-1448, DOI: 10.1080/10426500490463655
To link to this article: http://dx.doi.org/10.1080/10426500490463655
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Phosphorus, Sulfur, and Silicon, 179:1445–1448, 2004
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Copyright Taylor & Francis Inc.
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500490463655
MICROWAVE ASSISTED OXIDATION
OF BENZALDEHYDES UNDER SOLVENT-FREE
CONDITION USING OXONEꢀR /WET-ALUMINA
Mir M. Alavi Nikjea and Mohammad A. Bigdelib
Imam Khomeini International University, Qazvin, Iran;a
and Teacher Training University, Tehran, Iranb
(Received October 30, 2003; in final form November 25, 2003)
The solventless oxidation of benzaldehydes to carboxylic acids using
oxone/wet-alumina under microwave irradiations is described.
Keywords: Benzaldehydes; eco-friendly; microwave irradiation; oxone/
wet-alumina; solventless
The oxidation of aldehydes to carboxylic acids is an important transfor-
mation in organic synthesis and several methods have been developed
to accomplish this conversion.1 In the past, the use of oxone(2KHSO5,
KHSO4, K2SO4) in the presence of various solvents was developed,2
and although all previous methods are not detrimental to the envi-
ronment, organic solvents or heavy metal based oxidants are used in
this transformation. Organic solvents are not only expensive but often
are flammable, toxic, and environmentally hazardous. Motivated by re-
cent interest in carring out microwave assisted reactions in solvent-free
media, emphasis on green chemistry,3 and in the continuation of our
experiments for the use of oxone/wet-alumina in solventless oxidation
reactions,4 we decided to re-examine this procedure using microwave
irradiations for the oxidation of benzaldehydes.
Oxone is a convenient, inexpensive, and powerful oxidant used for the
transformation of a wide range of functional groups,2 including cleav-
age of oximes using glacial acetic acid in the range of 1–5 h in good
yields.5
Application of microwave irradiation currently is under intensive
examination.6 The important effects are decreasing reaction time (up
to 3 orders of magnitude) and in some cases, cleaner reactions with
Address correspondence to Mir M. Alavi Nikje, Department of Chemistry, School of Sci-
ences, Imam Khomeini International University, Qazvin, Iran. E-mail: alavi-m@ikiu.ac.ir
1445

1446
M. M. A. Nikje and M. A. Bigdeli
easier work up. Organic solvents are not only expensive, but they can
not be placed in microwave oven in open vessels (avoiding the risk of
high pressures and explosion). Coupling of microwave irradiation and
dry media therefore is organic chemists delight.
We investigated the use of oxone as an oxidant for oxidation reactions
under microwave irradiation and found it uncontrollable due to explo-
sions and decomposition of materials. Reagents deposited on mineral
supports have gained popularity in organic synthesis due to their se-
lectivity and ease of manipulation.7 Association of oxone with mineral
supports has been pioneered by Marimoto et al.8 We mixed alumina
with oxone in equal weight and made and intimate a homogenous mix-
ture. When neat benzaldehyde was mixed with the pure reagent and
placed in microwave oven, a sluggish oxidation was observed. How-
ever, when this mixture was moistened, a rapid and efficient oxidation
of benzaldehyde took place. To assess the generality of the method a
wide variety of benzaldehydes regardless of the electron donating and
electron accepting nature of the substituents on the phenyl group were
subjected to oxidation using oxone/wet-alumina under microwave irra-
diation (Table I).
In this article, we report the mixture of oxone/wet-alumina can serve
as an efficient, selective, mild, and eco-friendly reagent for the oxida-
tion of benzaldehydes under microwave irradiation. The best ratio of
substrate/oxone was found to be 1/1.1. Use of excess reagent affected
neither the yields nor the time of reactions.
Oxone/wet-alumina
ArCHO −−−−−−−−−−−−−−→ ArCOOH
Microwave Irradiations
TABLE I Oxidation of Benzaldehydes Using
Oxone/Wet-Alumina Under Microwave Irradiationa
Entry
Ar
Yield (%)b
1
2
3
4
5
6
7
8
9
C6H5
98
96
96
92
95
91
86
74
82
4-CH3C6H4
4-OCH3C6H4
2-NO2C6H4
3-NO2C6H4
4-NO2C6H4
3-ClC6H4
4-ClC6H4
2-OHC6H4
aIrradiation time 9 min for all benzaldehydes.
bAll products are known and were characterized by
comparing their physical and spectroscopic data with
those of authentic samples.

Benzaldehydes
1447
As indicated in the Table I, the time of reaction is very short and
yields are high to excellent.
In conclusion, oxidation of benzaldehydes with oxone/wet-alumina
under microwave irradiation in solvent less system is a rapid, manip-
ulatively simple, inexpensive, and selective protocol and can be added
to organic synthesis methodology.
EXPERIMENTAL
All compounds are known and were characterized by comparison of
their physical and spectroscopic data with those of authentic sam-
ples. All substrates were purchased from Merck company. Microwave
irradiation was carried out in a domestic instrument at full power.
The reagent was prepared by grinding 5 g of alumina 60G (Type
E) purchased from Merck and 5 g of oxone and premoistened before
use.
Oxidation of Benzaldehyde
General Procedure
An appropriate amount of benzaldehyde (3 mmol) and oxone/wet-
alumina (3.3 mmol based on oxone) were mixed intimately in a pyrex
beaker. To this mixture a drop of water was added and stirred to make
a homogenous mixture. The beaker was placed in a house hold mi-
crowave oven for 9 min for all benzaldehydes. The progress of the re-
action was monitored by TLC. After completion of the reaction, CHCl3
(2 × 5 ml) was added and the mixture was filtered. The solvent was
evaporated to dryness to afford the corresponding carboxylic acids
(Table I).
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