Tetrachloropyridine: A new reagent for the dehydration of aldoximes under microwave

Article abstract of DOI:10.1081/SCC-120006011

Dehydration of aldoximes to nitriles using tetrachloropyridine under microwave in dry media is described. The procedure is applicable to a variety of aldoximes and the reagent can be recycled and reused.

Full text of DOI:10.1081/SCC-120006011

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SYNTHETIC COMMUNICATIONS
Vol. 32, No. 15, pp. 2391–2394, 2002
TETRACHLOROPYRIDINE: A NEW
REAGENT FOR THE DEHYDRATION OF
ALDOXIMES UNDER MICROWAVE*
y
Nagarapu Lingaiah and Ravirala Narender
Organic Chemistry Division-II, Indian Institute of
Chemical Technology, Hyderabad-500 007, India
ABSTRACT
Dehydration of aldoximes to nitriles using tetrachloropyridine
under microwave in dry media is described. The procedure is
applicable to a variety of aldoximes and the reagent can be
recycled and reused.
Dehydration of aldoximes to nitriles is an important transformation in
[
1–9]
organic synthesis. The classical methods
of dehydration include
trifluoroacetic anhydride, chlorosulphonyl isocyanate, diphosphorus tetra-
iodide, selenium dioxide, 4,6-diphenyl-2-methylthiopyrylium tetrafluorobo-
rate, copper(II)acetate and the triphenylphosphine carbon tetrachloride
system. Most of those methods require excess of solvent for reaction and
[
10,11]
[12]
[13–18]
the catalyst remains as a waste.
reported as reusable catalysts but they suffer from harsh reaction conditions
However, zeolites
or clay
are
*
IICT Communication No. 4622.
y
Corresponding author. Fax: þ91-040-7173757; E-mail: nagarapu@iict.ap.nic.in
2
391
DOI: 10.1081/SCC-120006011
Copyright & 2002 by Marcel Dekker, Inc.
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392
LINGAIAH AND NARENDER
[19]
and longer reaction time.
Therefore a convenient and eco-friendly
method for dehydration is desirable. Recently microwave assisted reactions
is an area of growing interest because of its distinct advantages like rapid
reaction, non-solvent conditions and operational simplicity with minimum
disposal of waste. We have selected tetrachloropyridine (TCP) as an effec-
[20]
tive catalyst.
Herein, we wish to report for the first time the results of
dehydration of aldoximes to nitriles using TCP as a reusable catalyst under
microwave under non-solvent conditions (Scheme 1).
Scheme 1.
The reaction of aldoximes and tetrachloropyridine is carried out on
alumina support under microwave irradiation. The products are isolated
only by extraction. The aldoximes were prepared according to the literature
[21]
procedure method. Thus in a typical procedure, the corresponding nitriles
2a–n) were isolated in good yields when a mixture of aldehyde oximes 1 and
(
TCP 10 : 1.5 adsorbed on alumina (finer than 200 mesh, 5 equiv.) and taken in
a pyrextest tube was subjected to microwave irradiation in a microwave at an
output of about 650 W. After cooling the reaction mass to room temperature,
the products were isolated by extracting with dichloromethane (DCM) and
the solvent DCM was removed under vacuum. The crude products were
purified by distillation, crystallisation or column chromatography (Table 1).
The catalyst recovered during workup could be effectively reused.
While the nitrile formation is observed using 10% w/w of TCP in the
reaction, better yields are obtained by increasing the concentration of TCP
to 15% w/w. It is observed that in the case of substrate bearing an electron
withdrawing group (Entry 2g), the reaction is faster with good yield,
whereas an electron donating substrate (Entry 2h) gives lower yield. The
heterocyclic (Entry 2j–m) and polycyclic aldoximes (Entry 2o) are also effec-
tively converted into nitriles in good yields. An important feature of the
reaction is that the catalyst is recovered and reused for two cycles without
substantial loss in the yield of product.
The easy workup, convenient handling and availability of the catalyst
makes the method highly practical for the dehydration of alkyl, aryl, hetero-
cyclic and polynuclear aldehyde oximes. In conclusion, we are reporting a

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DEHYDRATION WITH TETRACHLOROPYRIDINE
2393
Table 1. Dehydration of Aldehyde Oximes 1 to Nitriles 2
M.P. or B.P./Torr
a
Time
(h)
Yield
(%)
c,d
Found
[22,23]
Product
R
Reported
2
2
2
2
2
2
2
2
2
2
a
b
c
d
e
f
g
h
i
Ph
4-(Cl)C H
3
3
3
4
3.5
3
3
3
4
72
80
80
75
75
78
85
79
75
70
189–192/760
93–94
190.7/760
94–96
43–46
144–145
113
217.6/760
149
61–62
6
4
4
e
2-(Cl)C
2,6-Cl
4-(OH)C H
6
H
43–44
142–145
2
C
6
H
3
f
110–112
216–217/760
148
6
4
4-(Me)C
6
H
4
4-(NO )C
2
6
H
4
g
4-(MeO)C H
6
59–61
233–236/760
116–118
4
n-C H
9
235–237/760
118/760
19
j
Pr
4
b
2
2
2
k
4.5
4.5
4.5
65
66
60
147/738
190–192
104.5
146/738
192/760
103
b
l
m
2
o
5
80
174–176
175–177
a
b
c
Yields of isolated products; Purified by column chromatography on silica gel; All
products were identified by comparison of their TLC, IR and NMR spectra with
d
those of authentic samples; Crystallised from EtOH unless otherwise noted; From
e
f
g
2 2
Et O/hexane; From benzene; From Et O.
new catalyst for the facile dehydration of aldoximes to nitrile a reusable
catalyst and dry reaction conditions are remarkable features of the
procedure.
ACKNOWLEDGMENT
The authors are thankful to the Director and Head, Division of
Organic Chemistry-II, for providing facilities and Dr. H. Meshram for
helpful discussion.

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394
LINGAIAH AND NARENDER
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Received in Japan May 22, 2001