G Model
CRAS2C-3960; No. of Pages 5
4
F. Nikbakht, A. Heydari / C. R. Chimie xxx (2014) xxx–xxx
the more favorable benzylic methylene and other regioi-
someric nitrones were not seen (the ratio of isomers
determined by H NMR analysis; Table 2, entries 5–7).
the first run after 5 hours. In second and third runs, the
product yields decreased to 82% and 80%, respectively
(Table 2, entry 1). The nature of the recovered catalyst was
studied by FT-IR spectroscopy and XRD analysis and
assignments were in good agreement with the reported
values [12]. To verify the heterogeneity of the catalyst in
the oxidation of the secondary amines, in an experiment,
the catalyst was separated from the reaction mixture after
30 minutes of the reaction and the filtrate was then stirred
at room temperature for a duration of 10 hours. The results
demonstrated that no further selective conversion of
dibenzylamine was observed in the absence of the catalyst.
1
In a few cases, a small amount of benzaldoxime (4–6%)
was isolated as the minor product according to gas
chromatography (GC), using an authentic sample as the
calibrant (Table 2, entries 2, 6, 7) [15]. The oxidation of
N-phenylbnenzylamine, which has an aromatic ring
directly linked to the nitrogen atom, was messy due to
the competitive electron transfer from the amine to the
oxidant [16].
The oxidation of symmetrical non-benzylic secondary
amines such as dibutylamine (8a) and pyrrolidine (9a)
were carried out and their corresponding nitrones were
obtained as predicted (Table 2, entry 8–9).
3. Experimental
Methyl prolinate was converted into the thermodyna-
mically stable 2-substituted nitrone, regioselectively
3.1. Preparation of [(nBu
3
Sn)
2
4
WO ] coordination polymer
(entry 10).
The C-phenyl nitrone 1b (N-benzylidenebenzylamine
The complex [(nBu
described previously [11]. Five mmol nBu
dissolved in a mixture of water (3 mL) and acetone
(13.5 mL). Then, a solution of 2.5 mmol Na WO . 2H
in 5 mL distilled water was added slowly to the nBu SnCl
3
Sn)
2
WO
4
]
was prepared as
N-oxide), was isolated as the Z-isomer, as usual [17–19]. In
the case of dibutylamine, the formation of Z-isomer was
3
SnCl was
1
also observed as the only product (referring to H NMR
2
4
2
O
data compared to literature data) [5].
3
The oxidant activity of coordination polymer
solution while stirring magnetically. A white precipitate
formed immediately and stirring was continued for a
further 5 min. The resultant solid was filtered, washed with
water and acetone and then dried at 100 8C. FT–IR
[
3 2 4
(nBu Sn) WO ] may be attributed to the formation of
peroxo complexes that have reportedly been isolated and
characterized in several cases [20]. This oxidation process
can follow the steps from amine to hydroxylamine and
subsequently to nitrone.
spectrum (KBr disc) was in agreement with results of
À1
preceding reports. IR (KBr,
n
cm ): 2960 (vs), 2925 (vs),
This study additionally, investigated the oxidation of
primary amines. It was determined using the same
experimental procedure (Table 2). In the presence of
2859 (s), 1512 (w), 1458(s), 1417 (w), 1377 (m), 1342 (w),
1292 (w), 1250 (w), 1182 (w), 1155 (w), 1078 (m), 1049
(w), 1003 (w), 964 (w), 883 (m), 850 (m), 815 (vs), 744 (m),
670 (s), 669 (s), 613 (w), 513 (w).
4
3 2 4
mol% of the catalyst, [(nBu Sn) WO ], the reaction of
a
-phenylethylamine, with 4 equiv of 30% aqueous H O
2 2
resulted in the selective formation of (E)-acetophenone
oxime in 80% yield as the only product (Table 2, entry 11).
In the case of other primary and benzylic amines such as
benzylamine and 4-methylbenzylamine the correspond-
ing oximes were isolated in moderate yields (Table 2,
entries 12–13). The stereochemistries of oximes were
confirmed by comparison on the H NMR spectrum with
reported values [21]. Oximes are important intermediates
for the synthesis of amides via a Beckmann rearrangement
3.2. General experimental procedure for the preparation of
nitrones and oximes
Four mol% (33 mg) of organometallic coordination
3 2 4
polymer [(nBu Sn) WO ], was added to a stirred solution
of methanol (2 mL) and amine (1.0 mmol) and the
resultant suspension was stirred for 2 min. To this, 3–
4 mmol aqueous hydrogen peroxide (30 wt%) was added,
after which the reaction mixture was stirred at r.t., until
completion of the reaction as indicated by thin-layer
chromatography (TLC). The catalyst was filtered and
washed with acetone several times. Excess hydrogen
peroxide was decomposed by adding small portions of
sodium hydrogen sulphite. The solution was extracted
with dichloromethane. The combined organic extracts
were dried over anhydrous sodium sulfate. The mixture
was filtered and the solvent was removed by a rotary
evaporator and the crude residue was purified by
preparative thin-layer chromatography using ethyl acetate
and n-hexane (1:4) as eluents to give corresponding
nitrone or oxime derivatives. All the products are known
compounds (except 7b) and were characterized by
1
[
22] and the selective catalytic oxidation of primary
amines into oximes assumes great importance [23,24].
Less activated cyclohexylamine, a non-benzylic pri-
mary amine, was not converted to cyclohexanone oxime at
all [25]. The oxidation of tert-butylamine was also
performed and the process did not provide any products.
To complete this study, reusability of the catalyst was
investigated in the oxidation of dibenzylamine. A set of
experiments was carried out to examine the catalyst
activity along five oxidation runs. The model reaction was
carried out using 50 mg of the catalyst and the experiment
was suitably scaled up. When the reaction was complete,
the catalyst was filtered and washed with acetone several
times. The remaining catalyst was dried under vacuum and
reused in subsequent reactions. More than 98% of the
catalyst could usually be recovered from each reaction. The
catalyst could be reused for five cycles with no significant
loss of its catalytic activity. The product yield was 85% in
1
13
comparison of their IR, H NMR and C NMR spectroscopic
data and respective melting points with the reported
values.
The spectroscopic data of nitrone 7b is reported since
no literature data are available.
Nikbakht, Fatemeh
