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Farhadi & Ansari
photosensetizers [25-33]. Most of these methods produce a
large amount of metal waste and thus cause environmental
problems. Therefore, the development of simple, fast,
inexpensive, widely applicable and environmentally benign
oxidants/procedures is an ongoing area of research.
2.45 GHz).
General
Procedure
for
the
Oxidative
Decarboxylation of Carboxylic Acids under
Microwave Irradiation
In recent years, the application of microwaves has become
popular among chemists both as a means to improve the
classical organic reactions (shortening reaction times and/or
improving yields) and promote new reactions [34]. Numerous
organic reactions such as acylation and alkylation reactions,
aromatic and nucleophilic substitutions, condensations,
cycloadditions, protection and deprotection reactions,
esterifications and transesterifications, heterocyclizations,
rearrangements, organometallic reactions, oxidations and
reductions assisted by microwave heating have been reported
[35,36]. To date, no report has been presented concerning the
decarboxylation of carboxylic acids involving the use of
microwaves as an energy source.
In continuation of our research to in explore green methods
for the organic transformations [37-40], we decided to
examine the decarboxylation of arylacetic acids using
CoIIIW12O405- as an oxidant under microwave irradiation. This
Co(III)-containing heteropoly compound is an excellent
oxidizing agent (E° = 1.01 V vs. NHE). Unlike most reported
oxidants, this compound is non-toxic and its reduction to
CoIIW12O406- occurs easily retaining the original structure. This
reagent has been recently applied to various organic
transformations [41-45]. Herein, we wish to report on the
microwave-induced decarboxylation of various α-aryl- and
α,α-diarylcarboxylic acids using a stoichiometric amount of
K5CoIIIW12O40 in 1:1 v/v% H2O-CH3CN as the mixed solvent
which resulted in the formation of the corresponding
aldehydes and ketones in high yields.
In a 100 ml Teflon-bottle was placed carboxylic acid (1
mmol) dissolved in 10 ml acetonitrile and K5CoIIIW12O40 (4
mmol) dissolved in 10 ml of water. The vessel was irradiated
in a laboratory microwave oven at a power of 180 W (20%).
The progress of the reaction was monitored by TLC and GC-
MS. After completion of the reaction, the mixture was cooled
to room temperature, the organic layer extracted with ethyl
acetate (2 × 20) and concentrated under a reduced pressure.
The residue was subjected to silica-gel plate or column
chromatography using carbon tetrachloride-ethyl acetate as an
eluent to give the pure product. The results are presented in
Tables 1 and 2. All products were commercially available and
were identified through the comparison of their physical and
1
spectral data (m.p., TLC, FT-IR, GC-MS and H NMR) with
those of authentic samples or reported data.
General
Procedure
for
the
Oxidative
Decarboxylation of Carboxylic Acids under
Conventional Heating
In order to make a comparison, the decarboxylation of
acids was also carried out under the conventional heating
conditions in a similar manner outlined above except that the
reaction mixture was refluxed for the appropriate time in an
oil-bath rather than microwave irradiation. The results are
compared in Tables 1 and 2.
RESULTS AND DISCUSSION
In the initial experiments when a mixture of 2-phenyl-
EXPERIMENTAL
propionic acid (1
, 1 mmol) and K5CoIIIW12O40 (4 equiv.) in
aqueous acetonitrile solution (20 ml, 1:1 v/v%) was irradiated
at the initial microwave power of 180 W (20%), acetophenone
Materials and Methods
All the carboxylic acids employed in this study were
commercially available and were used without further
purification. All solvents were spectroscopic grade.
K5CoIIIW12O40 was prepared according to the literature method
[46]. Microwave irradiation was carried out in a laboratory
(
2
) was formed as the sole product in 95% isolated yield after
16 min (Scheme 1). On the other hand, this reaction under
refluxing conditions gave with 93% isolated yield in a much
2
longer reaction time of 1.25 h.
It is interesting to point out that as the reaction progressed,
the color of the reaction mixture gradually changed into blue-
modified microwave oven (LG-30L, 900 W, MW frequency
471