Combination of AlCl3, ionic liquid, and microwaves: An efficient method for dehydration and 1,3-dipolar cycloaddition: An unusual observation in the presence of acrylonitrile

Article abstract of DOI:10.1080/00397910801997694

Use of a combination of ionic liquids in the presence of aluminium chloride and microwave irradiation was shown to be a very efficient and high-yielding method for the dehydration of aldoximes and also for 1,3-dipolar cycloadditions. Surprisingly, the presence of acrylonitrile rendered an unusual formation of substituted amide. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910801997694

Synthetic Communications1, 38: 1952–1957, 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910801997694
Combination of AlCl3, Ionic Liquid, and
Microwaves: An Efficient Method for
Dehydration and 1,3-Dipolar Cycloaddition: An
Unusual Observation in the Presence
of Acrylonitrile
Radhika S. Kusurkar, Nilesh H. Naik, and Prajakta N. Naik
Department of Chemistry, University of Pune, Pune, India
Abstract: Use of a combination of ionic liquids in the presence of aluminium
chloride and microwave irradiation was shown to be a very efficient and high-
yielding method for the dehydration of aldoximes and also for 1,3-dipolar
cycloadditions. Surprisingly, the presence of acrylonitrile rendered an unusual
formation of substituted amide.
Keywords: AlCl3; Aldoxime; Ionic liquid; Microwave; Nitrile
Recently, room-temperature ionic liquids have been used very effectively
for various reactions because of their solvating ability, simple workup
procedure, and recyclability. Ionic liquids are highly polar but noncoor-
dinating, and they catalyze the reactions, giving better selectivity.[1]
Advantages such as rapid reactions, high yields, and selectivity in micro-
wave-assisted reactions have become well known in recent years. We have
been using this methodology for some other reactions.[2] In our earlier
work,[3] indole-2-carboxamide was obtained when Vilsmeier–Haack
reagent was treated with indole-2-aldoxime followed by treatment with
boiling NaOH. However, treatment with only water furnished the corre-
sponding nitrile. In view of our interest in microwave-assisted reactions
and the importance of ionic liquids as solvents, we decided to use them
in combination for a dehydration reaction of aldoximes, and the results
Received August 18, 2007
Address correspondence to Radhika S. Kusurkar, Department of Chemistry,
University of Pune, Pune 411007, India. E-mail: rsk@chem.unipune.ernet.in
1952

1953

1954
R. S. Kusurkar et al.
are described herein. Ionic liquids, being polar and ionic in character,
couple to microwave irradiation efficiently and consequently may be
ideal microwave-absorbing entities for organic reactions. There are
reports of preparation of ionic liquids using microwave irradia-
tion,[4] and many ionic liquids are effectively used for various organic
transformations.[5,6]
Furan-2-aldoxime was chosen as substrate to investigate the dehy-
dration with AlCl3 in ionic liquid under microwave irradiation (MW).
Table 1 describes the results of this investigation. The reaction carried
out using ionic liquid in the presence of AlCl3 and xylene in the micro-
wave oven was extremely rapid, and in a few minutes the corresponding
nitrile was obtained in very good yield. However, in this reaction, use of
only ionic liquid gave the product in less yield. The formation of nitrile
was generalized using five different aldoximes (Table 1). p-Methoxyben-
zaldoxime gave substantial demethylation when the reaction was carried
out without ionic liquid. With ionic liquid, the demethylated product was
in minor amount. Surprisingly, the demethylation was not observed in
the reaction of veratraldoxime. All these results indicated that combi-
nation of ionic liquids and microwave irradiation in xylene constitutes
a very good method for dehydration of oximes.
It was then envisaged to study the 1,3-dipolar reaction of oximes with
conventional dipolarophiles, using the combination of ionic liquids and
microwave irradiation. Thus the reaction of methyl acrylate as a dipolar-
ophile with furan-2-aldoxime and AlCl3 in [Bmim]þBrꢀ using microwave
irradiation furnished 1,3-dipolar cycloadduct 1d (Table 2). The use of the
combination of previously mentioned conditions furnished 1,3-dipolar
cycloadduct 1d in a short time and in better yield than the earlier report.[7]
In the case of oximes 4a and 5a, the reactions using methyl acrylate gave
nitriles 4b and 5b respectively instead of the expected 1,3 dipolar cyclo-
adducts. This indicated rapid dehydration of these aldoximes.
Surprisingly, when acrylonitrile was used instead of methyl acrylate,
substituted amide 1c resulted from the reaction (Table 3). The structure
Table 2. Reaction conditions and yield of microwave-assisted cycloaddition
reaction of oxime 1a to 1d using methyl acrylate
Sr. no.
Catalyst
No catalyst
AlCl3
[Bmim]þBrꢀ
AlCl3=[Bmim]þBrꢀ
Time (min)
Yield (%)
1
2
3
4
74
5
30
5
90
71
89
85

AlCl3, Ionic Liquid, and Microwaves
1955
Table 3. Reaction conditions and yields of amides using acrylonitrile
Sr. no. Substrate
Catalyst
Method Time Product Yield (%)
1
2
3
4
5
1a
1a
1a
1a
4a
AlCl3
MW
15 min
5.5 h
30 h
2.5 min
5 min
1c
1c
1c
1c
4c
69
80
48
90
80
AlCl3=[Bmim]þBrꢀ 120 ꢁC
AlCl3=[Bmim]þBrꢀ RT
AlCl3=[Bmim]þBrꢀ MW
AlCl3=[Bmim]þBrꢀ MW
of the amide was confirmed by spectral data. The formation of substi-
tuted amide can be explained using initial Beckmann rearrangement,
followed by addition to acrylonitrile (Scheme 1). In the case of
Scheme 1. Microwave assisted reactions and mechanism using ionic liquid.

1956
R. S. Kusurkar et al.
veratraldehyde oxime, a small amount of nitrile was also produced along
with the substituted amide during this reaction. Conversion of nitrile (iso-
lated from the earlier reaction) to the substituted amide 4c was not
observed under the same reaction conditions, indicating that nitrile might
not be the intermediate for the formation of substituted amide. The dif-
ference in the reaction of methyl acrylate and that of acrylonitrile could
be attributed to the better dipolarophilic ability of methyl acrylate than
acrylonitrile.
CONCLUSION
Dehydration of aldoximes was achieved efficiently using a combination
of microwave-assisted reactions in xylene and ionic liquid in the presence
of aluminium chloride. The same combination also proved to be effective
for 1,3-dipolar reaction of furfuraldoxime. However, unusually, acryloni-
trile furnished substituted amide instead of the 1,3-dipolar cycloadduct,
indicating its lower reactivity as a dipolarophile.
EXPERIMENTAL
Thin-layer chromatographic analysis (TLC) was performed using acti-
vated silica gel on precoated aluminium plates and was detected under
1
UV. All recorded melting points are uncorrected. The H NMR spectra
were recorded as d values in CDCl3 with reference to TMS as internal
standard on a Varian Mercury instrument (300 MHz). FT-IR spectra
were recorded on a Perkin-Elmer 1600. GC-MS spectra were recorded
on a Perkin-Elmer Clarus 500.
General Procedure for Conversion of Aldoximes to Nitriles and
1,3-Dipolar Adducts
Anhydrous AlCl3 (1.2 mmol) was stirred with ionic liquid (1 mmol) in
20 ml of either xylene or acrylonitrile or methyl acrylate for 30 min.
The aldoxime (1 mmol) was added to it, and the reaction mixture was stir-
red for 1 h. The reaction mixture was irradiated in a microwave oven
(750 W). Completion of reaction was monitored by TLC. Solvent was
distilled out, and the reaction mixture was extracted with ethyl acetate.
After usual workup, the product was purified by column chromato-
graphy using hexane and ethyl acetate as an eluent.

AlCl3, Ionic Liquid, and Microwaves
ACKNOWLEDGMENTS
1957
We are grateful to A. P. Gadgil and J. P. Chaudhari for providing IR
and NMR spectra, respectively.
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