An easy and fast method for conversion of oximes to the corresponding carbonyl compounds under microwave irradiation

Article abstract of DOI:10.1081/SCC-120001502

1-Benzyl-4-aza-1-azoniabicyclo[2.2.2]octane dichromate has been found to be an efficient and new reagent for the conversion of oximes to the corresponding carbonyl compounds. The reaction is performed under microwave irradiation.

Full text of DOI:10.1081/SCC-120001502

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AN EASY AND FAST METHOD FOR CONVERSION
OF OXIMES TO THE CORRESPONDING CARBONYL
COMPOUNDS UNDER MICROWAVE IRRADIATION
a
A. R. Hajipour, S. E. Mallakpour & S. Khoee
a
Pharmaceutical Research Laboratory, College of Chemistry, Isfahan University of
Technology, Isfahan, IR, 84156, Iran
Version of record first published: 21 Aug 2006.
To cite this article: A. R. Hajipour, S. E. Mallakpour & S. Khoee (2002): AN EASY AND FAST METHOD FOR CONVERSION OF
OXIMES TO THE CORRESPONDING CARBONYL COMPOUNDS UNDER MICROWAVE IRRADIATION, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 32:1, 9-15
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SYNTHETIC COMMUNICATIONS, 32(1), 9–15 (2002)
AN EASY AND FAST METHOD
FOR CONVERSION OF OXIMES TO
THE CORRESPONDING CARBONYL
COMPOUNDS UNDER
MICROWAVE IRRADIATION
A. R. Hajipour,* S. E. Mallakpour,* and S. Khoee
Pharmaceutical Research Laboratory,
College of Chemistry, Isfahan University of
Technology, Isfahan, 84156, IR, Iran
ABSTRACT
1-Benzyl-4-aza-1-azoniabicyclo[2.2.2]octane dichromate has
been found to be an efficient and new reagent for the conver-
sion of oximes to the corresponding carbonyl compounds.
The reaction is performed under microwave irradiation.
Oximes are highly crystalline and very useful for purification and
characterization of carbonyl compounds. These derivatives are also useful
1
as efficient protecting groups for aldehydes and ketones. Conversion of
oximes back to their corresponding carbonyl compounds is a very important
process in synthetic organic chemistry. Several oxidative deoximation
methods have been developed and have found some advantages over the
2
–12
classical hydrolysis method.
*
Corresponding authors. E-mail: haji@cc.iut.ac.ir
9
Copyright & 2002 by Marcel Dekker, Inc.
www.dekker.com

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HAJIPOUR, MALLAKPOUR, AND KHOEE
Much important work has been performed on the use of microwave
dielectric heating effects. These include the preparation of samples for
1
3–16
17–19
organic chemistry,
to waste treatment,
preparation of samples for analysis,
polymer technology,
26
application
24
drug release targeting,
2
0,21
22–23
2
5
ceramics and alkane decomposition.
Heterogeneous reactions that are facilitated by supported reagents on
various solid inorganic surfaces have received attention in recent years.
1
3–16
The advantage of these methods over conventional homogenous reactions is
that they provide greater selectivity, enhanced reaction rates, cleaner prod-
ucts and manipulative simplicity. In continuation of our ongoing program
1
4
to develop environmentally benign methods using solid supports, we now
wish to report the synthesis of 1-benzyl-4-aza-1-azoniabicyclo[2.2.2]octane
dichromate (BAABCD). This reagent is readily prepared by reaction of an
aqueous solution of 1-benzyl-4-aza-1-azoniabicyclo[2.2.2]octane chloride (2)
with CrO in 3N HCl at room temperature as shown in Scheme 1.
3
Scheme 1.
The resulting orange powder, which can be stored for months without
loss of activity, is soluble in acetonitrile, acetone and N,N-dimethylform-
amide and slightly soluble in chloroform, ethyl acetate and dichloromethane,
but is not soluble in carbon tetrachloride, n-hexane and diethylether.
BAABCD (3) is an efficient and novel reagent for the oxidative clea-
vage of the C¼N bond of oximes under microwave irradiation. Using this
reagent under microwave irradiation various oximes have been successfully
converted to their corresponding aldehydes and ketones in good to excellent
yields.
First, the deoximation was performed by mixing one equivalent of
the reagent with different oximes in a mortar and ground with pestle until
the formation of a homogeneous mixture. Then, the reaction mixture was
transferred to an Erlenmeyer flask and irradiated in a microwave oven, until
TLC showed complete disappearance of the starting material. However,
the reagent was destroyed gradually and the starting material remained
intact during the solid phase irradiation. So, we decided to use a solvent,
which can play three important roles: 1) as a primary absorber, 2) as a

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11
solvent for both the reagent and the product (3) as a factor for increasing the
dielectric constant of the medium. Because by increasing the dielectric con-
stant, the coupling with microwave increased. We tested several solvents
such as CCl , hexane, toluene, o-cresol, acetonitrile and dichloromethane.
4
The reaction was not carried out in hexane and toluene because these
solvents are microwave inactive solvents and therefore did not absorb the
heat of microwave irradiation. o-Cresol and acetonitrile are microwave
active solvents but during the reaction, the oximes and solvent are both
affected by the reagent and produce several by-products. Since dichloro-
1
a
methane is a microwave active solvent and also does not affect by the
reagent during the reaction progress, we chose this solvent in these reac-
tions. Therefore 1.5 ml of dichloromethane was added to one equimolar
amount of oximes (4a–4o) and reagent (3) and the reaction mixture was
irradiated in a domestic microwave oven until TLC showed complete dis-
appearance of starting materials (Scheme 2). Monitoring the reaction by
TLC showed that by disappearance of starting material spot, the corre-
sponding carbonyl compound appeared as the only product and no
by-product was observed on the TLC plate. The reaction completed
within 25–40 s and the pure carbonyl compound was obtained by a simple
work-up without further purification.
Scheme 2.
Comparing the results obtained from the microwave method by pre-
2
vious catalytic method showed that in the latter method aliphatic oximes
8
did not react with (3), even in the presence of AlCl , whereas in this new
3
method they react easily (Table 1). Consequently, it seems that under micro-
wave irradiation the reaction losses its selectivity. Therefore, it is interesting
to investigate the competitive reaction of aliphatic and benzylic oximes with
this reagent. When equimolar amounts of cyclohexanal oxime (4k) and
3,4-dimethoxybenzaldehyde oximes (4b) were treated with (3), only the
benzylic oxime (4b) was selectively oxidized to its corresponding aldehyde
whereas aliphatic oxime (4k) remained unchanged (Scheme 3). Finally we
can say that the selectivity of the reaction did not decrease but its opera-
tional domain was extended.

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HAJIPOUR, MALLAKPOUR, AND KHOEE
a
Table 1. Conversion of Oximes to Carbonyl Compounds
1
The compounds (5a–5o) were characterized by H NMR and IR
analysis. Notably, aldehydes did not undergo further oxidation to their
carboxylic acids under the reaction conditions.
In order to evaluate the effect of microwave irradiation in this reaction,
we tried the reaction of acetophenone oxime with reagent (3) without using
any microwave irradiation. The reaction did not proceed at all after 60 min

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13
grinding of the reaction mixture. The mechanism of the reaction is the same,
2
which has been reported by us for the oxidation of alcohols by this reagent.
7
Another noteworthy advantage of the reagent lies exclusive oxidation
of oximes in the presence of semicarbazone. When a mixture of an equimo-
lar amount of 3,4-dimethoxybenzaldehyde oxime and 2,5-dimethoxy ben-
zaldehyde semicarbazone was treated with reagent (3), only the oxime was
selectively oxidized to the corresponding carbonyl compound and the semi-
carbazone remained unchanged (Eq. 1).
Equation 1.
When we treated a,b-unsaturated oxime only the C¼N bond was
selectively oxidized to the corresponding carbonyl compounds and the
reagent was ineffective in oxidizing C¼C double bond (Table 1).
In conclusion, we are reporting a new and efficient methodology for
the regeneration of aldehydes and ketones from oximes under microwave
irradiation. The stability and easy preparation of BAABCD, easy work-up
procedure, and high yields of the products, make this method a novel and
useful one relative to the reported method for regeneration of carbonyl
compounds from their derivatives. These results make BAABCD a bench-
top oxidizing agent.
EXPERIMENTAL SECTION
Deprotection of Oximes (4a-4o) with Reagent (3)
Under Microwave Irradiation
The deprotection of benzophenone oxime is representative for depro-
tection of oximes. Benzophenone oxime (0.27 g, 1.39 mmol) was added to
1-benzyl-4-aza-1-azoniabicyclo[2.2.2]octane dichromate (0.87 g, 1.39 mmol)

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HAJIPOUR, MALLAKPOUR, AND KHOEE
and the mixture was ground with a pestle in a mortar to form a homoge-
neous mixture. After transfer of the mixture to an Erlenmeyer flask and
adding 1.5 ml CH Cl , it was irradiated with microwave oven, until TLC
2
2
showed complete disappearance of starting materials (Table 1). Then CCl4
15 ml) was added to the reaction mixture and after vigorous stirring, the
(
mixture was filtered off and the solvent was evaporated in vacuo. The pure
benzophenone was obtained.
ACKNOWLEDGMENT
The authors are thankful of the Isfahan University of Technology
IUT), I.R. Iran for the financial support.
(
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Received in Japan October 10, 2000

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