Activation of iron (III) and bismuth (III) nitrates by tungstophosphoric acid: Solvent-free oxidative deprotection of oximes to carbonyl compounds

Article abstract of DOI:10.1081/SCC-200031036

Tungstophosphoric acid (H₃PW₁₂O₄₀· 6H₂O) has been used as a catalyst for a simple, rapid, selective, and solvent-free cleavage of oximes to carbonyl compounds using iron (III) and bismuth (III) nitrates as oxidants.

Full text of DOI:10.1081/SCC-200031036

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Activation of Iron (III) and
Bismuth (III) Nitrates by
Tungstophosphoric Acid:
Solvent‐Free Oxidative
Deprotection of Oximes to
Carbonyl Compounds
Habib Firouzabadi a , Nasser Iranpoor a & Kamal
Amani a
a Chemistry Department, College of Sciences ,
Shiraz University , Shiraz, 71454, Iran
Published online: 10 Jan 2011.
To cite this article: Habib Firouzabadi , Nasser Iranpoor & Kamal Amani (2004)
Activation of Iron (III) and Bismuth (III) Nitrates by Tungstophosphoric Acid:
Solvent‐Free Oxidative Deprotection of Oximes to Carbonyl Compounds, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic
Organic Chemistry, 34:19, 3587-3593, DOI: 10.1081/SCC-200031036
To link to this article: http://dx.doi.org/10.1081/SCC-200031036
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SYNTHETIC COMMUNICATIONSw
Vol. 34, No. 19, pp. 3587–3593, 2004
Activation of Iron (III) and Bismuth (III)
Nitrates by Tungstophosphoric Acid:
Solvent-Free Oxidative Deprotection
of Oximes to Carbonyl Compounds
*
Habib Firouzabadi, Nasser Iranpoor, and Kamal Amani
*
Chemistry Department, College of Sciences, Shiraz University,
Shiraz 71454, Iran
ABSTRACT
.
Tungstophosphoric acid (H3PW12O40 6H2O) has been used as a catalyst
for a simple, rapid, selective, and solvent-free cleavage of oximes to car-
bonyl compounds using iron (III) and bismuth (III) nitrates as oxidants.
Key Words: Bismuth(III)nitrate; Carbonyl compounds; Iron(III)nitrate;
Oximes; Solvent-free; Tungstophosphoric acid.
Oximes are extensively used as preferred derivatives for protection,
purification, and characterization of carbonyl compounds.[1] They play an
*
Correspondence: Habib Firouzabadi and Nasser Iranpoor, Chemistry Department,
College of Sciences, Shiraz University, Shiraz 71454, Iran; E-mails: firouzabadi@
chem.susc.ac.ir; Iranpoor@chem.susc.ac.ir.
3587
DOI: 10.1081/SCC-200031036
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com
Copyright # 2004 by Marcel Dekker, Inc.

3588
Firouzabadi, Iranpoor, and Amani
important role as synthetic intermediates en route to nitriles,[2] nitro com-
pounds,[3] nitrones,[4] amines,[5] isoxazolines,[6] and also as an a-activating
group in synthetic organic chemistry.[7] Oxidative deoximation methods that
have been developed in recent years[8] consist of using toxic metals such as
Ce (IV), Pb (IV), and Cr (VI).
Regeneration of carbonyl compounds from oximes by different methods
has been extensively reviewed.[9] Some of these reagents are expensive and in
some cases high temperatures are required, which lead to by-products. Devel-
opment of the new procedures in order to eliminate the problems encountered
by the existing methods for this purpose is still of interest to organic chemists.
Various metal nitrates have been used for this purpose in different reaction
conditions. They consist of copper (II) nitrate supported on silica gel,[10]
clay-supported ammonium nitrate (Clayan) in solvent or under micro-
wave irradiation,[11] Bismuth (III) nitrate,[12] zinc nitrate,[13] thallium (III)
nitrate,[14] and cerium ammonium nitrate (CAN).[15] Some other reported
procedures suffer from using corrosive mineral acids such as HNO3 or
H2SO4, some of them from long reaction times and the other from being
unreactive towards aldoximes.[11,12,14,15]
Heteropoly acids (HPAs) have been extensively studied as acid and oxi-
dation catalysts for many reactions and found industrial application in several
processes.[16] HPAs are promising solid acids to replace environmentally
harmful liquid acid catalysts such as H2SO4.[16c–f] The Keggin type
H3PW12O40 is more active than conventional solid acids such as SiO2-
Al2O3, H3PO4/SiO2, and zeolites.[16c,f]
In continuation of our work on new applications of tungstophosphoric
acid in organic reactions,[17] we here report another useful catalytic appli-
.
cation of H3PW12O40 6H2O (H3PW 6H2O) for the oxidative deprotection
.
of various types of oximes to their corresponding carbonyl compounds
.
.
using Fe(NO3)3 9H2O and Bi(NO3)3 5H2O as oxidants in the absence of
solvent (Sch. 1).
To the best of our knowledge, deprotection of oximes by iron (III)
nitrate nonahydrate has not been reported in the literature. We studied
Scheme 1.

Activation of Iron (III) and Bismuth (III) Nitrates
3589
deoximation of acetophenone oxime, as a model compound in the presence
of iron (III) nitrate nonahydrate at room temperature. For the reaction
performed at room temperature on 1 mmol of the substrate in the absence
.
of solvent, the optimum oxime/iron (III) nitrate/H3PW 6H2O molar ratio
was found to be 1 : 0.5 : 0.05. The reaction was completed in 15 min and acet-
ophenone was isolated in 91% yield. In the absence of tungstophosphoric
acid, acetophenone was isolated in only 48% yield under similar reaction con-
ditions. We also investigated the reaction in the absence of iron (III) nitrate
.
nonahydrate for deprotection of acetophenone oxime with H3PW 6H2O.
Formation of a trace amount of the carbonyl compound was observed after
30 min. A similar reaction was also investigated in various solvents such
as: n-hexane, dichloromethane, chloroform, tetrahydrofuran, acetone, aceto-
nitrile, ethylacetate, ethanol, and petroleum ether with the similar molar ratio
.
of oxime/iron (III) nitrate/H3PW 6H2O at room temperature. The reaction
was performed well in n-hexane in comparison with the other solvents we
used in this investigation and acetophenone was produced with a slower
rate (60 min) and lower yield (78%). Therefore, we applied this procedure
.
[oxime/iron (III) nitrate/H3PW 6H2O (1 : 0.5 : 0.05)] to a series of structu-
rally different oximes (Table 1, method A). Most aromatic and aliphatic
aldoximes and ketoximes were oxidized in good to excellent yields.
Aromatic aldoximes with strong electron-withdrawing groups were oxidized
with difficulties and the corresponding carbonyl compounds were isolated
in moderate yields (Table 1, entry 7). Due to the oxidation of phenolic
groups, oxidation of oximes derived from carbonyl compounds bearing
such groups is not usually straightforward.[14] Oxidation of salicylaldehyde
oxime was successfully performed (method A) in a good yield (Table 1,
entry 11). We found that this method was not suitable for the oxidation of
benzoin oxime and the extensive cleavage of carbon-carbon bond was
observed (Table 1, entry 13). Oxidation of an a,b-unsaturated ketoxime
(Table 1, entry 14) was also studied by this method. We found that this
compound was unreactive towards oxidation and the starting material was
recovered unchanged after 60 min. In all oxidation reactions we have
studied (method A), over-oxidation of the products was not observed.
Recently, bismuth (III) nitrate pentahydrate has been applied for depro-
tection of ketoximes in a mixture of acetone/H2O (9 : 1) under reflux con-
ditions.[12] This procedure suffers, in some cases, from long reaction times
and also is sluggish for the oxidation of aldoximes. Therefore, we decided
.
to apply bismuth (III) nitrate/H3PW 6H2O system for the oxidation of
oximes under solvent-free conditions. For reactions performed at 40–458C
on 1 mmol of acetophenone oxime under solvent-free conditions, the
.
optimum oxime/bismuth (III) nitrate/H3PW 6H2O molar ratio was found
to be similar to iron (III) nitrate nonahydrate (1 : 0.5 : 0.05). Therefore, we

3590
Firouzabadi, Iranpoor, and Amani
.
.
Table 1. Oxidation of oximes using Fe(NO3) 3 9H2O (method A) or Bi(NO3)3 5H2O
(method B) at 40–458C under solvent-free conditions catalyzed by H3PW12O40 6H2O
.
.
(H3PW 6H2O) and their comparison by other reported methods.
Time
(min)
Yielda
(%)
Yield (%) (other
metal nitrates)
Entry
1
Oxime
Method
A
15
10
91b
85
(88)c, (85)d,
(90)e, (95)f
B
2
3
4
5
A
B
15
10
93b
95
A
B
10
10
90
91
(80)c, (86)d,
(79)e, (92)f
A
B
10
10
89
90
(o)c, (88)d, (80)f
A
B
10
10
95
93
6
7
A
B
60
15
84
88
A
B
120
120
45
67
8
9
A
B
10
20
83
86
(80)c
A
B
15
15
83
77
(25)c, (72)e, (85)f
(80)c, (92)d, (88)e
10
A
B
10
60
95g
NRh
(continued)

Activation of Iron (III) and Bismuth (III) Nitrates
3591
Table 1. Continued.
Time
(min)
Yielda
(%)
Yield (%) (other
metal nitrates)
Entry
11
Oxime
Method
A
B
30
60
73b
70
(00)d
12
A
B
60
60
87i
78i
j
j
13
14
A
B
15
15
A
B
60
60
NRh
NRh
aRefers to isolated yield.
bAt rt.
cConditions: Bi(NO3)3 5H2O(0.5 mmol), Cu(OAc)2 xH2O(0.1 mmol), Mont. K 10,
.
.
2–21 h, reflux, acetone-H2O (9 : 1).
dConditions: Tl(NO3)3. (0.5 mmol), H2SO4, rt., a few min., MeOH.
eConditions: CAN (4 mmol, 0.3 M in HNO3), MeCO2H, 08C to rt., 30–40 min,
(glc yield).
fConditions: Clayan (1.125 g, 7 mmol of NH4NO3), CH2Cl2, N2, rt., 5–24 h.
gGC yield.
hNR ¼ no reaction.
iWith 1 mmol of the oxidant.
jQuantitative C-C bond cleavage was occurred and benzaldehyde and benzonitrile was
produced.
applied this ratio to the other oximes in this study. The results (Table 1,
method B) show that oxidative deprotection of oximes by this system is
also very similar to iron (III) nitrate nonahydrate with only one exception,
in which cyclohexanone oxime remained almost intact [Table 1, entry 10
(method B)].
In conclusion, these methods are simple, usually rapid and work in the
absence of solvent for the oxidation of both aldoximes and ketoximes to
their carbonyl compounds. The reagents are not toxic and expensive, and
are easily available compounds. The methods are mild and over-oxidation
of aldehydes to carboxylic acids has not been observed.

3592
Firouzabadi, Iranpoor, and Amani
EXPERIMENTAL
All starting materials and products are known compounds.
.
.
H3PW12O40 6H2O (H3PW 6H2O) is commercially available and its purifi-
cation was performed according to the reported procedure.[18]
General Procedure for the Oxidation of Oximes
.
in the Absence of Solvent by Fe(NO3)3 9H2O and
.
Bi(NO3)3 5H2O Catalyzed by H3PW12O40 6H2O
.
.
(H3PW 6H2O)
Iron (III) nitrate nonahydrate (0.202 g, 0.5 mmol) or bismuth (III) nitrate
.
pentahydrate (0.243 g, 0.5 mmol) and H3PW 6H2O (0.15 g, 0.05 mmol) were
mixed and ground gently in a test tube. Oxime (1 mmol) was added to
the resulting solid mixture and was occasionally agitated mechanically for
the appropriate reaction time at 40–458C (Table 1). After completion of the
reaction, which was clearly visible by discontinuation of the evolution of
reddish nitrogen dioxide fumes and also by monitoring the progress of the
reaction by TLC or GC, the test-tube was laid aside for five min in order to
expel nitrogen dioxide fumes. The resulting mixture was washed repeatedly
with CH2Cl2 (ca. 25 mL) followed by washing with a saturated solution of
NaHCO3 (5 mL) and H2O (2 Â 25 mL).The organic phase was separated
and dried over anhydrous Na2SO4. Filtration and evaporation of the solvent
gave the corresponding carbonyl compound, which was purified by silica
gel column chromatography (60–230 mesh) using n-hexane/EtOAc (8 : 2)
mixture as an eluent to afford pure carbonyl compound (Table 1).
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Received in the UK March 18, 2003