Deprotection chemistry mediated by ZrOCl2 · 8H 2O: An efficient, mild, and green method for the conversion of oximes to carbonyl compounds in aqueous acetone

Article abstract of DOI:10.1080/00397911003797783

(Chemical Equation Presented) Less-toxic, moisture-stable, inexpensive, and ecofriendly zirconium oxychloride octahydrate (ZrOCl₂ · 8H₂O) in aqueous acetone (1:1) mediates the conversion of oximes to carbonyl compounds in moderate to good yields. This green methodology is applicable to both aldoximes and ketoximes with tolerance to >C=C<, -NO₂, -OH, and -Cl groups. The reaction and workup are simple. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397911003797783

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Deprotection Chemistry Mediated by
ZrOCl · 8H O: An Efficient, Mild, and
2
2
Green Method for the Conversion of
Oximes to Carbonyl Compounds in
Aqueous Acetone
a
a
a
Lakhinath Saikia , Subrata Das & Ashim Jyoti Thakur
a
Department of Chemical Sciences, Central University, Napaam,
Tezpur, Assam, India
Published online: 03 Mar 2011.
To cite this article: Lakhinath Saikia , Subrata Das & Ashim Jyoti Thakur (2011) Deprotection
Chemistry Mediated by ZrOCl · 8H O: An Efficient, Mild, and Green Method for the Conversion of
2
2
Oximes to Carbonyl Compounds in Aqueous Acetone, Synthetic Communications: An International
Journal for Rapid Communication of Synthetic Organic Chemistry, 41:7, 1071-1076, DOI:
10.1080/00397911003797783
To link to this article: http://dx.doi.org/10.1080/00397911003797783
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Synthetic Communications , 41: 1071–1076, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911003797783
DEPROTECTION CHEMISTRY MEDIATED BY
ZrOCl ꢀ 8H O: AN EFFICIENT, MILD, AND GREEN
2
2
METHOD FOR THE CONVERSION OF OXIMES TO
CARBONYL COMPOUNDS IN AQUEOUS ACETONE
Lakhinath Saikia, Subrata Das, and Ashim Jyoti Thakur
Department of Chemical Sciences, Central University, Napaam, Tezpur,
Assam, India
GRAPHICAL ABSTRACT
Abstract Less-toxic, moisture-stable, inexpensive, and ecofriendly zirconium oxychloride
octahydrate (ZrOCl
2
ꢀ 8H
2
O) in aqueous acetone (1:1) mediates the conversion of oximes
to carbonyl compounds in moderate to good yields. This green methodology is applicable to
, -OH, and -Cl groups. The
both aldoximes and ketoximes with tolerance to >C=C<, -NO
reaction and workup are simple.
2
Keywords Carbonyl compounds; deprotection; green; protection; zirconium oxychloride
octahydrate
INTRODUCTION
Continuing our interest in protection and deprotection chemistry, we were
[
1]
[
2,3]
curious about oximes, which constitute one of the most useful protecting groups,
especially in multistep organic synthesis, and are extensively used for purification
[
4]
and characterization of carbonyl compounds. The oxime family not only
represents a convenient series of derivatives of carbonyl compounds, but also,
Received December 7, 2009.
Address correspondence to Ashim Jyoti Thakur, Department of Chemical Sciences, Central
University, Napaam, Tezpur 784028, Assam, India. E-mail: ashim@tezu.ernet.in
1
071

1072
L. SAIKIA, S. DAS, AND A. J. THAKUR
importantly, noncarbonyl compounds can be converted to the carbonyl derivatives
[5]
via oxime formation. In addition, oximes may be used as intermediates for the
[
6]
preparation of amides by Beckmann rearrangement. These requirements and
demands for oximes has inspired the development of several deoximation reagents
[
such as trimethylammonium chlorochromate, dinitrogen tetroxide, pyridinium
7]
[8]
[
9]
[10]
[11]
chloroformate, trimethylsilyl chloroformate,
sulfophenyl phosphonate,
titanium silicate-1,
[13]
Photosensitized reaction also
zirconium
[12]
and N-haloamides.
[
14,15]
finds application in the regeneration of carbonyl compounds from oximes.
As far as the application of zirconium oxychloride octahydrate
(
ZrOCl ꢀ 8H O) in organic synthesis is concerned, it has recently attracted attention
2
2
because of its high water tolerance, ready commercial availability, low cost, and low
[16]
toxicity (LD50 ¼ 2950 mg=Kg, oral rat). To the best of our knowledge, oxidative
cleavage of oximes to parent carbonyl compounds using ZrOCl ꢀ 8H O has not been
2
2
reported so far.
RESULTS AND DISCUSSION
Herein, we divulge an experimentally simple, facile deoximation method of oxi-
mes 1 using ZrOCl ꢀ 8H O in aqueous acetone (1:1) under refluxing conditions as
2
2
depicted in Scheme 1. In fact, this is the first example of deprotection chemistry
mediated by ZrOCl ꢀ 8H O. Choice of acetone as one reaction medium was crucial.
2
2
Other common solvents (CHCl , CH Cl , tetrahydrofuran, etc.) failed to carry out
3
2
2
the conversion. Moreover, the presence of acetone is essential, established by the fact
that water alone could not bring about the conversion. Acetone probably serves a
dual role. First, it dissolves the organic reactants, providing a homogeneous aqueous
system, and second, it drives the reaction toward the right-hand side, acting as an
exchange reagent.
To start with a simple model reaction, we preferred to select benzaldoxime
Table 1, entry 1a) as the substrate. The reaction was complete within 2 h with
(
80% yield. This preliminary observation encouraged us to extend the procedure to
other oximes as well. Several optimized examples illustrating the scope and limita-
tions of this new procedure for the conversion of a variety of other oximes 1 (entries
1b–o) to their parent carbonyl compounds 2 in moderate to good yields (60–87%) are
summarized in Table 1. This methodology is equally applicable to ketoximes (entries
1
g, h, and i, Table 1) and aldoximes (entries 1a–f and j, Table 1). It was pleasing to
Scheme 1. Deoximation of acyalic oximes 1 using zirconium oxychloride octahydrate.

DEPROTECTION MEDIATED BY ZrOCl
2
ꢀ 8H
2
O
1073
a
Table 1. Deprotection of acyclic oximes 1a–o via Scheme 1
b,c
Entry
R
1
R
2
Reaction time (h)
Yield (%)
a
b
c
d
e
f
-C
p-ClC
p-BrC
p-OHC
p-MeC
p-NO
m-NO
p-OMeC
-CH
6
H
5
-H
2
80
76
78
85
84
82
78
73
74
70
71
80
87
71
77
6
H
4
-H
-H
-H
-H
-H
-H
-H
-H
-H
-H
-H
4
6
H
4
4
6
H
4
2.5
1
6
H
4
2
C
6
H
4
5
g
h
i
2
C
6
H
4
5
6
H
4
4
2
¼CH-C
6
H
5
4
j
-Thiophenyl
-Furfuryl
4.1
4.2
3
k
l
-3-Formylchromone
p-MeC
m
n
o
6
H
4
-Me
-C
-Me
4.5
4
-C
-C
6
H
H
5
6
H
5
6
5
2
a
Oximes 1 were synthesized according to the standard procedure and characterized by comparison of
their reported spectroscopic and physical data.
b
Isolated yield.
All compounds gave satisfactory IR, H NMR, and C NMR spectroscopic data, and finally their
meeting and boiling points were compared with those of authentic samples.
c
1
13
observe that no Beckmann rearrangement products were found. The -NO , -OH, -Cl
2
groups were found to be tolerated under the reaction conditions. Interestingly, the
presence of electron-withdrawing groups in the oxime tends to increase the reaction
time. The a,b-unsaturated aldoxime, cinnamaldehyde oxime (entry 1k, Table 1), also
afforded the aldehyde smoothly without affecting the carbon–carbon double bond.
Notably, aldehydes regenerated from aldoxime did not undergo further oxidation
under the reaction conditions. At room temperature, conversion was very poor
and required a longer time. The best results were obtained when the amount of
ZrOCl ꢀ 8H O used was twofold with respect to the oximes. In the case of cyclic oxi-
2
2
mes (entries 3a–e, Table 2), such as cyclopentanone (entry 3a, Table 2) and cyclohex-
anone (entry 3b, Table 2) conversions and yields were very poor (40–45%)
(
Scheme 2). We did not notice any polymerization product(s), which was observed
[14]
in the case of photolysis of oximes in acetonitrile. Further increasing the reaction
time gave no significant improvement in yields.
CONCLUSION
In conclusion, we have developed a mild and efficient new procedure for
cleavage of oximes using ZrOCl ꢀ 8H O in nontoxic aqueous solvent system of
2
2
acetone–water (1:1) under refluxing conditions.
EXPERIMENTAL
Melting points and boiling points were determined on a B u¨ chi 504 apparatus
and are uncorrected. Infrared (IR) spectra were recorded in KBr pallets on a Nicolet

1074
L. SAIKIA, S. DAS, AND A. J. THAKUR
a
Table 2. Deprotection of cyclic oximes 3a–e via Scheme 2
b,c
Entry
Oxime 3
Carbonyl compound 4
Reaction time (h)
Yield (%)
a
7
42
b
8
8
45
47
c
d
7.5
48
43
e
8
a
Oximes 3 were synthesized according to the standard procedure and characterized by comparison of
their reported spectroscopic and physical data.
b
Isolated yield.
c
1
13
All compounds gave satisfactory IR, H NMR, and C NMR spectroscopic data, and finally their
melting and boiling points were compared with those of authentic samples.
1
Impact 410) Fourier transform (FT)–IR spectrophotometer. H NMR spectra were
(
recorded on a Varian Mercury Plus 400-MHz NMR spectrophotometer. All the
chemicals were used as received. Acetone and water were distilled before use.
Typical Procedure for the Deoximation
The benzaldoxime 1a (200 mg, 1.65 mmol) along with ZrOCl ꢀ 8H O (1.06 g,
2
2
3
.3 mmol) were taken in 40 ml of acetone and water (1:1) in a round-bottomed flask
and allowed to reflux for 2 h. Progress of the reaction was monitored by thin-layer
chromatography (TLC), and iodine was used as a visualizing agent. After
completion of the reaction as indicated by TLC, acetone was evaporated in

DEPROTECTION MEDIATED BY ZrOCl
2
ꢀ 8H
2
O
1075
Scheme 2. Deoximation of cyclic oximes 3 using zirconium oxychloride octahydrate.
vacuum; the organic content was extracted with ethyl acetate (3 ꢁ 20 ml), washed
with brine, dried over Na SO , and concentrated down in a rotary evaporator;
2
4
and finally column chromatography was performed to afford the pure product, ben-
zaldehyde 2a (140 mg, 80% yield). Similarly, other oximes (1b–o and 3a–e) were
subjected to the same reaction conditions to get the deoximation products.
ACKNOWLEDGMENTS
LNS and AJT are grateful to the Council of Scientific and Industrial Research
CSIR), New Delhi, India, for financial support. SD thanks Tezpur University for
(
the institutional fellowship. The support to record NMR spectra at the Indian Insti-
tute of Technology (IIT), Guwahati, is gratefully acknowledged.
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3
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7
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