Selective method for the conversion of oximes to their corresponding carbonyl compounds under microwave irradiation by N-bromo-N-phenylpara- toluenesulfonamide

Article abstract of DOI:10.1002/jccs.200700145

In this work, oximes are converted to their corresponding carbonyl compounds in good yields using N-bromo-N-phenyl-para-toluenesulfonamide, under microwave irradiation. The simple work-up procedure minimizes loss of product.

Full text of DOI:10.1002/jccs.200700145

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Journal of the Chinese Chemical Society, 2007, 54, 1011-1015
1011
Selective Method for the Conversion of Oximes to their Corresponding
Carbonyl Compounds under Microwave Irradiation by N-Bromo-N-phenyl-
para-toluenesulfonamide
Ardeshir Khazaei,a* Minoo Sadrib and Hassan Hosseinic
aDepartment of Chemistry, Faculty of Sciences, University of Bu-Ali Sina,
P. O. Box 65178-4119, Hamadan, Iran
bEducation & Research Center of Science & Biotechnology, Malek Ashtar University, Tehran, Iran
cDepartment of Chemistry, Faculty of Sciences & Engineering, University of Imam Hossein, Tehran, Iran
In this work, oximes are converted to their corresponding carbonyl compounds in good yields using
N-bromo-N-phenyl-para-toluenesulfonamide, under microwave irradiation. The simple work-up proce-
dure minimizes loss of product.
Keywords: Carbonyl compounds; Oximes; N-Bromo-N-phenyl-para-toluenesulfonamide; Selec-
tive.
INTRODUCTION
In recent years, oxime compounds have been used as
by polar molecules, non-polar molecules being inert to
MW dielectric loss.15-16
In microwave dielectric heating, the microwave en-
ergy is introduced into the chemical reactor remotely and
direct access by the energy source to the reaction vessel is
obtained. The microwave radiation passes through the
walls of the vessel and heats only the reactants and solvent,
not the reaction vessel itself. If the apparatus is properly de-
signed, the temperature increase will be uniform through-
out the sample, which can lead to less by-products and/or
decomposition products. In order to understand why this
phenomenon occurs, it is necessary to comprehend the un-
derlying mechanisms of microwave dielectric heating. As
with all electromagnetic radiation, microwave radiation
can be divided into an electric field component and a mag-
netic field component. The former component is responsi-
ble for the dielectric heating, which is effected via the fol-
lowing mechanism.
intermediates for a number of synthetic products, and have
also been proved to be important and useful reagents in or-
ganic syntheses.1-3 These compounds are also useful pro-
tecting groups in organic syntheses4 and have found exten-
sive application in the isolation of carbonyl compounds.5-6
Several methods based on reductive, hydrolytic, and oxida-
tive reactions have been developed for the cleavage of ox-
imes.7-9 Owing to the considerable importance of such
compounds there is still scope for newer methods as the ex-
isting oxidative methods suffer from disadvantages such as
long reaction time,10 formation of over oxidation products
and difficulties in isolation of products.11 Also many de-
oximating reagents are relatively expensive and difficult to
prepare. We have previously reported convenient methods
for the deoximation of oximes to their corresponding car-
bonyl compounds.12 We now describe a new method for the
selective cleavage of oximes to their carbonyl compounds
under microwave irradiation with the use of N-bromo-N-
phenyl-para-toluenesulfonamide as an effective oxidizing
agent that overcomes the disadvantages associated with ox-
idative methods developed so far. Microwave-promoted re-
actions occur with dramatic decreases in reaction times13-14
and in some cases cleaner reactions with easier work-ups
than observed when using conventional heating. Micro-
wave reactions involve selective absorption of MW energy
One of the interactions of the electric field component
with the matrix is called the dipolar polarization mecha-
nism. For a substance to generate heat when irradiated with
microwaves it must possess a dipole moment, as has a wa-
ter molecule. A dipole is sensitive to external electric fields
and will attempt to align itself with the field by rotation.
The applied field provides the energy for this rotation. Un-
der low frequency irradiation, the molecule will rotate in
phase with the oscillating electric field. The molecule gains
some energy by this behaviour, but the overall heating ef-

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1012 J. Chin. Chem. Soc., Vol. 54, No. 4, 2007
Khazaei et al.
fect by this full alignment is small. Alternatively, under the
influence of a high frequency electric field the dipoles do
not have sufficient time to respond to the oscillating field
and do not rotate. Since no motion is induced in the mole-
cules, no energy transfer takes place and therefore no heat-
ing occurs. If the applied field is in the microwave radiation
region, however, a phenomenon occurs between these two
extremes. In the microwave radiation region, the frequency
of the applied irradiation is low enough so that the dipoles
have time to respond to the alternating electric field and
therefore rotate. The frequency is, however, not high enough
for the rotation to precisely follow the field. Therefore, as
the dipole re-orientates to align itself with the electric field,
the field is already changing and generates a phase differ-
ence between the orientation of the field and that of the di-
pole. This phase difference causes energy to be lost from
the dipole by molecular friction and collisions, giving rise
to dielectric heating.17
aldoximes were converted to the corresponding aldehydes
and no carboxylic acid was formed due to overoxidation of
the regenerated aldehyde (entries 3, 6).
Interestingly, camphor oxime was also converted to
the corresponding carbonyl compound as a model for deox-
imation of the sterically hindered ketone oxime (entry 10).
In addition, benzoin oxime (entry 1) was cleaved to the cor-
responding carbonyl compounds smoothly without affect-
ing the other functionality. Therefore this method is useful
for the chemoselective oxidative deoximation of oximes in
the presence of alcohols or for oximes that contain an OH
functional group. Thus we also used this procedure for the
selective deoximation of acetophenone oxime in the pres-
ence of an equimolar amount of benzyl alcohol. The only
observed product was acetophenone in 94% conversion.
We observed the competitive oxidation of oximes in the
presence of alkenes. For this purpose, equimolar mixtures
of cyclohexanone oxime and cyclohexene in acetone and
water were allowed to react with N-bromo-N-phenyl-para-
toluenesulfonamide under microwave irradiation. The
deoximation of cyclohexanone oxime took place (96%
conversion), whereas the cyclohexene was recovered un-
changed.
RESULTS AND DISCUSSION
In continuation of our studies on application of N-
halo compounds in organic chemistry,18 we now found a
novel and efficient protocol for the deoximation of a vari-
ety of oximes using N-bromo-N-phenyl-para-toluenesul-
fonamide under microwave irradiation conditions. This re-
agent was prepared from N-phenyl-para-toluenesulfon-
amide (Scheme I).
This reagent 2 is stable for a couple of months at 25oC
and can be stored in the refrigerator almost permanently.
One of the advantages of this method is that, after the reac-
tion was completed, N-bromo-N-phenyl-para-toluenesul-
fonamide 2 converted to the N-phenyl-para-toluenesul-
fonamide 1, which can be recovered, brominated and re-
used many times.
The reagent 2 has low toxicity, low cost and is easily
prepared. The best results are achieved using aqueous ace-
tone as the solvent; also the use of an inert atmosphere is
not required for this reaction (Scheme II).
CONCLUSION
Deoximation of different types of oximes (acyclic,
cyclic, and benzylic) and sterically hindered oximes were
investigated. The results are summarized in Table 1. The
In conclusion, in this study, we have demonstrated the
efficiency of N-bromo-N-phenyl-para-toluenesulfonamide
Scheme I

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Selective Method for the Preparation of Carbonyl Compounds
J. Chin. Chem. Soc., Vol. 54, No. 4, 2007 1013
Scheme II
Table 1. Deoximation with N-bromo-N-phenyl-para-toluenesulfonamide under microwave
irradiation
Entry
Substrate
benzoin oxime
4-methyl acetophenone oxime 4-methyl acetophenone
Carbonyl compound
Time (min) Yield (%)a,b
1
2
benzoin
1.6
1
90
95
94
94
96
93
92
93
90
89
90
3
benzaldehyde oxime
acetophenone oxime
cyclohexanone oxime
cinnamaldehyde oxime
diisopropyl ketone oxime
benzaldehyde
1
4
acetophenone
1
5
cyclohexanone
cinnamaldehyde
diisopropyl ketone
1
6
1.5
1.4
1.3
2
7
8
isobutyl methyl ketone oxime isobutyl methyl ketone
9
cyclopentanone oxime
camphor oxime
cyclopentanone
camphor
10
11
2.1
2
ethyl methyl ketone oxime
ethyl methyl ketone
a Products were characterized by their physical constants, comparision with authentic samples and by
their IR and 1H NMR spectra.
b Isolated yields.
towards the cleavage of oximes to their corresponding car-
low cost reagent, easy preparation, easy work up, very
short reaction times and no formation of over oxidation
products due to the chemoselectivity and mild nature of re-
bonyl compounds. The notable special features of this
methodology are: selectivity, excellent yields of products,

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1014 J. Chin. Chem. Soc., Vol. 54, No. 4, 2007
Khazaei et al.
agent 2.
was monitored by TLC. After completion of the reaction,
the solvent was removed under reduced pressure, and then
Et2O (15 mL) was added to the mixture and stirred for 15
min. The sulfonamide was removed by filtration and the
product was purified by column chromatography [(hex-
ane-Et2O)(4:1)] (Table 1). Products were characterized by
their physical constants, by comparison with authentic
samples and by their IR and 1H NMR spectra.
EXPERIMENTAL
IR and 1H NMR spectra were recorded using a Shimadzu
435-U-04 spectrophotometer (KBr pellets) and a 90 MHz
Jeol FT-NMR spectrometer, respectively. 1H NMR chemi-
cal shifts were measured relative to TMS (int; 1H).
Synthesis of N-phenyl-para-toluenesulfonamide
DATA OF THE PRODUCTS
In a beaker (10.0 g, 53 mmol) of 4-methylbenzenesul-
fonyl chloride was placed. The beaker was heated on a wa-
ter bath (75oC) until 4-methylbenzenesulfonyl chloride be-
came a liquid. Then aniline (4.9 mL, 53 mmol) was added
dropwise, with stirring. The mixture was stirred for 40 min-
utes at 60 oC. The mixture was cooled and distilled water
(70 mL) was added. The product was collected by suction
on a Büchner funnel, and washed with cold distilled water
(30 mL). Recrystallizaton with ethanol afforded pure prod-
uct in 90% yield. mp 120-121 oC; IR (KBr): 3260, 2920,
1: Benzoin, mp 131-133 oC, IR: 3416, 3380, 3030,
1670, 1573, 1460, 1375, 1275, 1190, 980, 850, 610 cm-1; 1H
NMR (DMSO-d6/TMS): d 6 (s, 1H), 6.1 (s, 1H), 7-7.5 (m,
8H), 8 (d, 2H).
2: 4-Methyl acetophenone, bp 219-221oC, IR: 3300,
3080, 3000, 2960, 1680, 1640, 1420, 1210, 954, 840, 750,
690 cm-1; 1H NMR (DMSO-d6/TMS): d 2.3 (s, 3H), 2.7 (s,
3H), 7.2 (d, 2H), 7.8 (d, 2H).
3: Benzaldehyde, bp 176-178 oC, IR: 3050, 3030,
2850, 2710, 1950, 1900, 1820, 1705, 1450, 1250, 920, 830,
740 cm-1, 1H NMR (CDCl3/TMS): d 7.5 (m, 2H), 7.6 (m,
1H), 7.9 (m, 2H), 10 (s, 1H).
4: Acetophenone, bp 81-83 oC, IR: 3600, 3345, 3050,
3020, 2960, 2850, 1680, 1635, 1580, 1450, 1260, 1070,
960, 730, 620 cm-1, 1H NMR (CDCl3/TMS): d 2.6 (s, 3H),
7.2-7.7 (m, 3H), 7.9 (m, 2H).
1
1593, 1465, 1325, 1148, 1080, 816 cm-1; H NMR (ace-
tone-d6/TMS): d = 2.35 (s, 3H), 6.90 (1H), 7.37-7.82 (m,
9H); Anal. Calcd. for C13H13NO2S: C, 63.15; H, 5.26; N,
5.66. Found: C, 63.35; H, 5.50; N, 5.85%.
Synthesis of N-bromo-N-phenyl-para-toluenesulfon-
amide
N-phenyl-para-toluenesulfonamide (10 g, 40.5 mmol)
was dissolved in a solution of NaOH (0.06 mol, 2.4 g) in
water (40 mL) at room temperature. Then a solution of bro-
mine (6.4 mL) in CCl4 (12 mL) was added with vigorous
stirring. The resulting precipitate was filtered and washed
with cold distilled water (20 mL) and then recrystallization
with ethanol afforded pure reagent in 87 % yield, mp
129-130 oC. IR (KBr): 2910, 1590, 1436, 1350, 1161, 812
5: Cyclohexanone, bp 152-155 oC, IR: 3600, 3420,
2950, 2600, 1850, 1800, 1760, 1710, 1670, 1440, 1320,
1220, 1060, 990, 750 cm-1, 1H NMR (CDCl3/TMS): d 1.5-2
(m, 6H), 2.5 (m, 4H).
6: Cinnamaldehyde, bp 124-127 oC, IR: 3330, 3060,
2990, 2800, 1670, 1620, 1606, 1570, 1350, 1210, 1070,
970, 750, 620 cm-1, 1H NMR (CDCl3/TMS): d 6.5 (m, 1H),
7 (m, 1H), 7.1-7.7 (m, 5H), 10 (d, 1H).
1
o
cm-1; H NMR (acetone-d6/TMS): d = 2.35 (s, 3H),
7: Diisopropyl ketone, bp 120-122 C, IR: 3600,
7.25-7.80 (m, 9H); Anal. Calcd. for C13H12NO2SBr: C, 48;
H, 3.69; N, 4.30. Found: C, 48.13; H, 3.76; N, 4.47%.
3450, 3100, 2870, 1710, 1630, 1450, 1220, 1130, 980, 890,
1
610 cm-1, H NMR (CDCl3/TMS): d 1 (d, 12H), 2.5 (m,
2H).
General procedure for deoximation of oximes
8: Isobutyl methyl ketone, bp 114-116 oC, IR: 3350,
2940, 1710, 1450, 1290, 1110, 970, 830, 620 cm-1,1H NMR
(CDCl3/TMS): d 0.9 (d, 6H), 2-2.2 (m, 4H), 2.4 (d, 2H).
9: Cyclopentanone, bp 126-129 oC, IR: 3400, 2960,
2800, 1970, 1740, 1620, 1450, 1230, 950, 890, 810, 710,
460 cm-1, 1H NMR (CDCl3/TMS): d 1.7-2.4 (m, 8H).
10: Camphor, mp 165-168 oC, IR: 3450, 2940, 2870,
A mixture of the oxime (3 mmol), N-bromo-N-phen-
yl-para-toluenesulfonamide (1.1 g, 3.3 mmol), acetone (12
mL) and distilled water (2 mL) were placed in a 250 mL
round-bottomed flask and refluxed under microwave irra-
diation in a microwave oven at a power output of 200 W for
the appropriate time as indicated in Table 1. The reaction