Microwave assisted conversion of oximes and semicarbazones to carbonyl compounds using benzyltriphenylphosphonium peroxymonosulfate

Article abstract of DOI:10.1007/s00706-002-0499-2

Benzyltriphenylphosphonium peroxymonosulfate in the presence of catalytic amounts of bismuth chloride was found to be an efficient and mild reagent for the oxidative cleavage of oximes and semicarbazones to the corresponding carbonyl compounds under microwave irradiation.

Full text of DOI:10.1007/s00706-002-0499-2

Monatshefte f €u r Chemie 134, 45–49 (2003)
DOI 10.1007/s00706-002-0499-2
Microwave Assisted Conversion of Oximes
and Semicarbazones to Carbonyl Compounds
Using Benzyltriphenylphosphonium
Peroxymonosulfate
1
;ꢀ
Iraj Mohammadpoor-Baltork , and Hadi Adibi
1
Abdol R. Hajipour , Shadpour E. Mallakpour ,
2
1
1
Pharmaceutical Research Laboratory, College of Chemistry, Isfahan University
of Technology, Isfahan 84156, Iran
2
Department of Chemistry, Isfahan University, Isfahan 81744, Iran
Received March 4, 2002; accepted (revised) April 8, 2002
Published online September 2, 2002 # Springer-Verlag 2003
Summary. Benzyltriphenylphosphonium peroxymonosulfate in the presence of catalytic amounts of
bismuth chloride was found to be an efficient and mild reagent for the oxidative cleavage of oximes
and semicarbazones to the corresponding carbonyl compounds under microwave irradiation.
Keywords. Benzyltryphenylphosphonium peroxymonosulfate; Carbonyl compounds; Microwave
irradiation; Oxime; Semicarbazone.
Introduction
Oximes and semicarbazones are crystalline and stable compounds, which not only
are used for the characterization and purification of carbonyl compounds but also
play an important role in the protection of carbonyl compounds [1]. Thus, the
regeneration of carbonyl compounds from their derivatives under mild conditions
is an important process in synthetic organic chemistry. Recently, extensive studies
on the oxidative cleavage of these derivatives have been carried out using various
oxidants [2–15]. However, many of these methods use expensive, toxic or rare
oxidizing reagents that are difficult to prepare, and they also use a higher molar
ratio of reagent. Therefore, there is a need for a simple, less expensive and safer
method for cleavage of oximes and semicarbazones.
+
Oxone (2KHSO ꢁ KHSO ꢁ K SO ) is a strong, inexpensive, water-soluble
5
4
2
4
and versatile oxidizing reagent that is commercially available, but this reagent is
ꢀ Corresponding author. E-mail: haji@cc.iut.ac.ir

4
6
A. R. Hajipour et al.
insoluble in organic solvents and buffering is needed due to its acidity [16].
Recently, we have reported the potency of benzyltriphenylphosphonium peroxy-
monosulfate (1) as a mild, inexpensive and efficient oxidizing reagent for oxidation
of alcohols to the corresponding carbonyl compounds under solvent-free [17a] or
non-aqueous conditions [17b], oxidation of urazoles to the corresponding tri-
azolinediones under solid-state conditions [17c], oxidative deprotection of tri-
methylsilyl and tetrahydropyranyl ethers and deprotection of ethylene acetals to
the corresponding carbonyl compounds under aprotic conditions [17d], and solid
state oxidation of sulfides and thiols to the corresponding sulfoxides and disulfides
under solventless condition [17e]. In recent years, there has been a growing interest
in the application of microwave irradiation in chemical reaction enhancement [18],
because of its cleaner reactions, decreased reaction times and easier work-up. In
continuation of our ongoing program to develop environmentally benign methods
using solid-state conditions especially under microwave irradiation [19], we now
report a facile oxidative cleavage of oximes and semicarbazones (2) using benzyl-
triphenylphosphonium peroxymonosulfate (1) in the presence of catalytic amounts
of bismuth chloride and acceleration under microwave irradiation. The reaction
proceeds efficiently in good to excellent yields at ambient pressure within a few
minutes.
This reagent is a white powder, which is prepared by the dropwise addition of
+
an aqueous solution of Oxone to an aqueous solution of benzyltriphenylphos-
phonium chloride in quantitative yield at room temperature, and it can be stored
ꢂ
for months without losing its potency [17]. The amounts of HSO5 in this reagent
have been determined by an iodometric titration method [20] and the measure-
ments are consistent with almost 99% (24.24 ꢃ 0.6% by weight) of active oxidizing
agent. The reagent (1) is quite soluble in methylene chloride, chloroform, acetone,
and acetonitrile and insoluble in non-polar solvents such as carbon tetrachloride,
n-hexane, and diethyl ether.
Results and Discussion
Initially, the oxidative cleavage was carried out by mixing one equivalent of the
reagent 1 and bismuth chloride with acetophenone oxime (2c) as a model com-
pound in a mortar and ground the mixture with a pestle until the formation of a
homogeneous mixture (1 min). Then, the reaction mixture was transferred to an
Erlenmeyer flask and irradiated in a microwave oven (5 min). It was found that the
starting material 2c remained intact during solid-phase irradiation. So, we tested
several solvents such as carbon tetrachloride, hexane, toluene, acetonitrile and
dichloromethane. Finally, dichloromethane was found to be a microwave active
solvent.
In order to evaluate the effect of bismuth chloride in this reaction, we carried
out the oxidative deprotection of the model compound 2c with the reagent 1 with-
out using any catalyst. Since oxidative deprotection of 2c under microwave irra-
diation with this reagent failed in the absence of catalyst, the catalytic effect of
several Lewis acids such as ZnCl , FeCl ꢁ 6H O, AlCl , and BiCl was investi-
2
3
2
3
3
gated. Surprisingly, only BiCl was effective. The optimum molar ratio of bismuth
3
chloride was 0.3 mol-equiv. Under these conditions, aldoximes and keto oximes 2

Microwave Assisted Conversion of Oximes
47
Table 1. Conversion of oximes and semicarbazones 2 to carbonyl compounds 3 with reagent 1 un-
a,b
der microwave irradiation
Entry Substrates 2
Products 3
Time
Yield
c
(min) (%)
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
a
b
c
d
e
f
Benzaldoxime
Benzaldehyde
2.5
5
99
90
99
90
95
98
90
90
80
90
85
95
90
93
80
90
85
70
80
75
75
85
70
70
80
70
3-Nitrobenzaldoxime
3-Nitrobenzaldehyde
Acetophenone
Acetophenone oxime
3
Benzophenone oxime
Benzophenone
3
4-Chloroacetophenone oxime
4-Bromoacetophenone oxime
4-Phenylacetophenone oxime
3,4-Dimethoxyacetophenone oxime
5-Methylfurfuraldoxime
3-Methoxybenzaldoxime
Cyclohexanone oxime
4-Chloroacetophenone
4-Bromoacetophenone
4-Phenylacetophenone
3
3
g
h
i
4
3,4-Dimethoxyacetophenone 2.5
5-Methylfurfuraldehyde
3-Methoxybenzaldehyde
Cyclohexanone
3
3
5
2.5
3
3
4
4
5
5
4
4
5
5
5
8
5
5
j
k
l
9-Fluorenone oxime
9-Fluorenone
m
n
o
p
q
r
s
ꢀ-Tetralone oxime
Cinnamaldoxime
ꢀ-Tetralone
Cinnamaldehyde
4-Phenylcyclohexanone
Acetophenone
4-Phenylcyclohexanone oxime
Acetophenone semicarbazone
4-Bromoacetophenone semicarbazone
Benzophenone semicarbazone
4-Bromoacetophenone
Benzophenone
3-Methoxybenzaldehyde semicarbazone 3-Methoxybenzaldehyde
t
3-Chlorobenzaldehyde semicarbazone
4-Acetylpyridine semicarbazone
ꢀ-Tetralone semicarbazone
3-Chlorobenzaldehyde
4-Acetylpyridine
u
v
w
x
y
z
ꢀ-Tetralone
5-Methylfurfural semicarbazone
3-Nitrobenzaldehyde semicarbazone
4-Phenylacetophenone semicarbazone
Cinnamaldehyde semicarbazone
5-Methylfurfuraldehyde
3-Nitrobenzaldehyde
4-Phenylacetophenone
Cinnamaldehyde
a
b
Oxidant=substrate=BiCl (1:1:0.3); confirmed by comparison with authentic samples (IR, H
1
3
c
NMR, TLC) [21]; yield of isolated pure carbonyl compound
were converted to the corresponding carbonyl compounds 3 under microwave
irradiation in high yields and short reaction times. The reagent 1 was also used
to convert semicarbazones 2 to the corresponding carbonyl compounds 3 under
microwave irradiation in the presence of bismuth chloride (0.3 mmol) in good to
high yields (Table 1).
The mildness of the reagent 1 has been shown by the oxidative cleavage of
,ꢁ-unsaturated oximes and semicarbazones. When we treated ꢀ,ꢁ-unsaturated
ꢀ
oxime or semicarbazone, only the C¼N bond was selectively oxidized to the
corresponding carbonyl compound and the reagent was ineffective in oxidizing
the C¼C double bond (Table 1). Notably, aldehydes did not undergo further oxida-
tion to their carboxylic acids under the reaction conditions.
In conclusion, we have reported a new and efficient methodology for the regen-
eration of aldehydes and ketones from oximes and semicarbazones under micro-
wave irradiation. The stability, the easy preparation of the reagent, and the easy

4
8
A. R. Hajipour et al.
Scheme 1
work-up make this method a novel and useful one relative to the present meth-
odologies for regeneration of carbonyl compounds from their derivatives under
microwave irradiation.
Experimental
All of the yields refer to isolated products after purification. All of the products were characterized by
1
comparison of their spectral (IR, H NMR) and physical data (melting and boiling points) with those of
authentic samples [21]. The reagent 1 was prepared according to our previous reported procedures
1
17]. All H NMR spectra were recorded at 90MHz in CDCl relative to TMS as an internal standard
[
3
and IR spectra were recorded on a Shimadzu 435 IR spectrometer. The apparatus used for the oxidative
deprotection was a Samsung domestic microwave oven (2450 MHz, 900 W) without any modification,
but all of the reactions were carried out in a hood with strong ventilation.
Typical procedure for the regeneration of carbonyl compounds 3
from oximes and semicarbazones 2 using reagent 1
under microwave irradiation
Benzophenone oxime 2d (197 mg, 1 mmol), benzyltriphenylphosphonium peroxymonosulfate (1)
(466 mg, 1 mmol) and bismuth chloride (95 mg, 0.3 mmol) were mixed together in a mortar and ground
with a pestle to form a homogeneous mixture (1min). After transferring the mixture to an Erlenmeyer
flask and adding 3 ml of dichloromethane, it was irradiated in a microwave oven (2450 MHz, 900 W)
for 3 min. After completion of the reaction (TLC examination), the reaction mixture was allowed to
reach room temperature, washed with carbon tetrachloride (2ꢄ 15ml) and after vigorous stirring it was
filtered through a sintered glass funnel. The solvent from the combined extracts was evaporated under
vacuum and the crude product was passed through a bed of silica gel (1cm) using carbon tetrachloride
ꢅ
ꢅ
as an eluent to give benzophenone 3d (163 mg, 90%), m.p. 50–51 C [Ref. [21] m.p. 49–51 C].
Acknowledgement
We are thankful to the Isfahan University of Technology (IUT) Research Council for financial support.
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49
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