Selective oxidative deoximation with anhydrous Ce(IV) sulfate

Article abstract of DOI:10.1016/j.tetlet.2015.04.111

Anhydrous Ce(SO₄)₂ in chloroform selectively converts oximes into the parent carbonyl compounds in good yields. Hammett correlations helped to indicate a plausible mechanism for the above reaction. The formation of 1-(dinitromethyl)benzene was assumed to be a product of isonitroso hydrogen reaction with arylaldoximes.

Full text of DOI:10.1016/j.tetlet.2015.04.111

Tetrahedron Letters xxx (2015) xxx–xxx
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Selective oxidative deoximation with anhydrous Ce(IV) sulfate
Oktay Asutay a, Nilüfer Hamarat b, Nesimi Uludag b, Necdet Coßskun c,
⇑
a Namik Kemal University, Muratli Vocational School, Department of Chemical Technology, Muratli, Tekirdag, Turkey
b Namik Kemal University, Faculty of Science and Arts, Department of Chemistry, 59030 Tekirdag, Turkey
c Uludag University, Faculty of Science and Arts, Department of Chemistry, 16100 Bursa, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
Anhydrous Ce(SO4)2 in chloroform selectively converts oximes into the parent carbonyl compounds in
good yields. Hammett correlations helped to indicate a plausible mechanism for the above reaction.
The formation of 1-(dinitromethyl)benzene was assumed to be a product of isonitroso hydrogen reaction
with arylaldoximes.
Received 9 February 2015
Revised 10 April 2015
Accepted 28 April 2015
Available online xxxx
Ó 2015 Published by Elsevier Ltd.
Keywords:
Oxidation
Oximes
Reaction mechanism
Hammett correlation
Isonitroso hydrogen
Introduction
R
R
Ce(IV)
-Ce(II)
OH
Oxidative deoximation for the regeneration of aldehydes and
ketones from the corresponding oximes is still a worthwhile area
of study. A survey of the recent literature revealed that a wide
range of oxidizing agents have been used for this purpose with dif-
ferent degrees of success. The main examples from the literature
include N-bromo-(4-methylphenyl) sulfonimide under microwave
irradiation,1 bis-pyridinesilver(I) dichromate,2 Ce(IV) based phase-
transfer oxidants,3 dichloramine-T,4 zeolite supported chromium
trioxide under microwave irradiation,5 catalytic sodium tungstate,6
clay supported bis-(trimethylsilyl) chromate under classical heat-
ing as well as microwave irradiation,7 cerium(IV) sulfate in ace-
tonitrile and alcohol,8 ceric ammonium nitrate (CAN),9
imidazolium fluorochromate,10 tetrabutylammonium peroxydisul-
fate,11 alumina supported ammonium chlorochromate,12 Mn(III)
triacetate,13 and bentonite-supported silver carbonate.14
Photooxygenation has also been reported as a useful method for
deoximation.15
R1
N
R1
O
1
2
Scheme 1. Deoximation of oximes with anhydrous Ce(SO4)2.
Results and discussion
In our initial reactions we compared the reaction rates of
Ce(SO4)2Á4H2O with those of dehydrated Ce(SO4)2. At the beginning
of the reaction the rate seemed to be higher with the hydrated oxi-
dant, however shortly after equilibrium was established, no further
consumption of the oxime was observed. The dehydrated reagent
provided full consumption of the substrate in a relatively short
time; a screen for the optimum solvent revealed chlorinated
hydrocarbons to be the best.
In the case of
a-bromoacetophenone oxime, 0.5 equiv of the
Herein we report a simple method for deoximation at room
temperature using dehydrated Ce(SO4)2 and also work on under-
standing the mechanism of the Ce(IV) mediated oxidative deoxi-
mation (Scheme 1).
oxidant converted the substrate into the corresponding ketone.
However, reactions with aromatic aldoximes under the same con-
ditions did not go to completion even with prolonged reaction
times. Therefore, we decided to investigate the effect of sub-
stituents on the rate of deoximation. After stirring a series of aldox-
imes with anhydrous Ce(SO4)2 for 10 min in CDCl3 the reaction
mixtures were filtered to remove the oxidant and investigated by
1H NMR spectroscopy. Attempts to correlate the initial rate con-
stants based on the formation of oxime Ce(IV) complexes 3 and
⇑
Corresponding author. Tel.: +90 224 294 1725; fax: +90 224 294 1899.
E-mail address: coskun@uludag.edu.tr (N. Coßskun).
http://dx.doi.org/10.1016/j.tetlet.2015.04.111
0040-4039/Ó 2015 Published by Elsevier Ltd.
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2
O. Asutay et al. / Tetrahedron Letters xxx (2015) xxx–xxx
O
O
O
O
O
O
S
S
S
+
O
O
O
OH
HO
OH
H
H
R
O
N
R1
N
O
N
O
R1
R1
4
5
6
R = H
Scheme 3. Oxime to nitrile oxide oxidation by Ce(IV) sulfate.
NOH
NOH
SO3
OH
R1
NOH
+
R1
N
1
7
HO
O
Figure 1. Plot of logKX versus
r
(R2 = À0.97). KX = [3]*[2]/[1].17
H
N
N
O
N
O
R1
S
O
R1
+
S
O
aldehydes 2 were unsuccessful, however K constants were in a
good correlation with Hammett16
substituent constants (Fig. 1).
The Hammett plot (Fig. 1) provided a negative constant which
N
O
H
O
r
8
9
10
O
q
HO
indicated that electron-withdrawing substituents slow down the
reaction, while electron-donating substituents accelerate the deox-
imation reaction. These results allowed us to propose the mecha-
nism depicted in Scheme 2. The dehydrated oxidant coordinates
with the oxime through the nitrogen to give intermediate complex
3, which is detectable by 1H NMR spectroscopy. This may undergo
fragmentation to sulfate ester 4 and CeSO4. Ester 4 is probably in
equilibrium with carbonyl 2 and SO3, as well as the oxidized form
of hydroxylamine, namely isonitroso hydrogen NOH.
The observed substituent effect indicates that the rate deter-
mining step should be the formation of intermediate 3. Attempts
to perform the reaction in the presence of water failed, with the
oxime remaining after several days.
Performing the reaction by the addition of a chloroform solution
of the oxime to a suspension of the oxidant (3 equiv) in chloroform
provided the best selectivity. Using more concentrated solutions,
or adding the oxidant in portions to a solution of oxime led to
the formation of by-products such as oxadiazoles. For example,
the addition of dehydrated Ce(SO4)2 (3 equiv) to a solution of
p-chlorobenzaldoxime in chloroform (9 mL) gave p-chloroben-
zaldehyde (19%), 3,5-bis(4-chlorophenyl)-1,2,4-oxadiazole (3%)
and 1-chloro-4-(dinitromethyl)benzene (7%) after column chro-
matography. Similarly, p-tolylaldoxime gave p-tolylaldehyde
Scheme 4. Possible mechanism for dinitromethylarene formation.
the presence of a species which can deoxygenate the nitrile oxide
dimers (Scheme 4).
The isolation of compounds 9 in the cases of p-chloro and
p-tolylbenzaldoximes provides evidence for the formation of isoni-
troso hydrogen23 which upon reaction with the starting oxime
could give diaziridine 7 which reacts with SO3 to give intermediate
8. Fragmentation of 8 may account for the formation of species 9
and 10 which are assumed to be responsible for deoxygenation
of the corresponding oxadiazole N-oxides.
With these results in hand we illustrated the utility of the
method using the oximes given in Table 1.
Conclusions
The reaction of oximes with anhydrous Ce(SO4)2 was shown to
be a substituent dependent reaction with a negative
q constant of
À1.94. The oxygen source in the formation of the carbonyl was the
sulfate anion which forms a sulfate ester with the N-coordinated
oxime. Fragmentation of the latter intermediate led to Ce(II) sulfate
and the carbonyl compound. The optimized protocol was applied
to the selective conversion of different oximes to give the corre-
sponding carbonyls in high yields at room temperature with rela-
tively short reaction times (exceptions are with strongly
electron-withdrawing substituents) and simple work-up.24 We
suggest that for the first time isonitroso hydrogen was generated
from a hydroxylamine derivative and trapped by chemical means.
Attempts to use the highly reactive intermediates formed during
the deoximation process are underway.
(33%), 3,5-di-p-tolyl-1,2,4-oxadiazole
(22%)
and
1-(dini-
tromethyl)-4-methylbenzene (22%). The formation of nitrile oxides
(5)18 which are the precursors of the oxadiazoles19 formed can be
rationalized by invoking the pathway depicted in Scheme 3.
Intermediate 4 can fragment to give 5 and the hydrogen sulfate
anion. The latter can deprotonate 5 establishing an equilibrium
between 5 and 6. The presence of an oxadiazole in the reaction
mixture instead of the expected N-oxides can be explained by
O
S
O
S
O
O
O
O
R
O
SO3
R
O
R
OH
N
CeSO4
+
CeSO4
+
R1
N
R1
NOH
R1
OH
OH
3
4
2
Scheme 2. Oxime to carbonyl oxidation by Ce(IV) sulfate.
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O. Asutay et al. / Tetrahedron Letters xxx (2015) xxx–xxx
15. Kuo, Y. H.; Jang, T. L. J. Chin. Chem. Soc. 1991, 38, 203–205.
3
Table 1
Ce(SO4)2 mediated oxidative deoximation of oximesa
16. (a) Jaffe, H. H. Chem. Rev. 1953, 53, 191–261; (b) Hansch, C.; Leo, A.; Taft, R. W.
Chem. Rev. 1991, 91, 165–195; (c) March, J. Advanced Organic Chemistry
Reactions Mechanism and Structure, 3rd ed.; Wiley & Sons: New York, 1985. p
244.
Entry
Oxime (1)b
Timec
Carbonyl (2)d
1
2
Benzaldoxime
p-Tolylaldoxime
60
90
1380
35
20
25
120
100
120
140
120
80
97
80
97
67
57
84
98
75
41e
65e
17. The data used in the Hammett correlations are given in the Table below:
3
p-Nitrobenzaldoxime
Substituent effect on the deoximation of arylaldoximesa
4
5
6
7
p-Chlorobenzaldoxime
3,4-Dimethoxybezaldoxime
Thiophene-2-carbaldehyde oxime
trans-Cinnamaldehyde oxime
Ar
[3]
[1]
[2]
K = [3][2]/[1]
8
a-Bromoacetophenone oxime
C6H5
0.012
0.00034
0.014
0.01
0.082
0.0967
0.082
0.088
0.079
0.006
0.003
0.004
0.002
0.005
8.78E-04
1.05E-05
6.83E-04
2.27E-04
1.01E-03
9
10
11
Acetophenone oxime
Cyclopentanone oxime
Cyclohexanone oxime
p-NO2C6H4
p-MeC6H4
p-ClC6H4
p-MeOC6H4
a
Ce(SO4)2 (0.996 g, 3 mmol) in chloroform (5 mL), oxime (1 mmol) in chloroform
0.016
(5 mL), room temperature.
b
The starting oximes were prepared according to known procedures24 and their
a
0.1 mmol of oxime was dissolved in 1 mL of CDCl3 and poured onto
identity and purity was checked by 1H and 13C NMR spectroscopy.
0.3 mmol Ce(SO4)2 and the mixture stirred for 10 min. The insoluble
oxidant was filtered and the mixture transferred to an NMR tube.
c
The reaction time in minutes.
d
The yield in %. The identity of the products was determined by 1H NMR of the
isolated carbonyl compounds.
e
The 1H NMR detected yields were higher than 90%.
18. (a) Itoh, K.; Horiuchi, C. A. Tetrahedron 2004, 60, 1671–1681; (b) Arai, N.;
Iwakoshi, M.; Tanabe, K.; Narasaka, K. Bull. Chem. Soc. Jpn. 1999, 72, 2277–
2285; (c) Béha, S.; Giguére, D.; Patnam, R.; Roy, R. Synlett 2006, 1739–1743.
19. To a solution of p-chlorobenzaldoxime (3 mmol, 0.468 g) in chloroform (9 mL),
dehydrated Ce(SO4)2 (9 mmol, 2.998 g) was added and the reaction mixture
stirred at room temperature for 15 min. The mixture was filtered through a
sintered glass funnel and the organic solvent evaporated. Besides p-
chlorobenzaldehyde (19%), 3,5-bis(4-chlorophenyl)-1,2,4-oxadiazole and 1-
chloro-4-(dinitromethyl)benzene were also isolated in 3% and 7% yields,
respectively after column chromatography. 3,5-bis(4-Chlorophenyl)-1,2,4-
Acknowledgments
Uludag˘ and Namık Kemal Universities Scientific Research
Projects are gratefully acknowledged for the financial support
(Project No NKUBAP.00.M6.AR.12.01).
oxadiazole: mp 181–182 °C Lit20 mp 183 °C. 1H NMR (400 MHz, CDCl3)
d
8.17–8.14 (m, 2H), 8.13–8.10 (m, 2H), 7.57–7.53 (m, 2H), 7.51–7.48 (m, 2H). 1-
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Please cite this article in press as: Asutay, O.; et al. Tetrahedron Lett. (2015), http://dx.doi.org/10.1016/j.tetlet.2015.04.111