Methyltrioxorhenium-catalysed oxidation of secondary amines to nitrones in ionic liquids

Article abstract of DOI:10.2478/s11696-012-0208-5

Nitrones serve as starting materials for the synthesis of many heterocycles. Oxidation of secondary amines using hydrogen peroxide and the catalytic amount of methyltrioxorhenium in ionic liquids is a useful method for the preparation of nitrones. Ultrasonic irradiation and ionic liquids have a positive influence on the reaction. The nitrones required were isolated in good yields. Corresponding hydroxylamines, which can be easily oxidised to nitrones, often accompanied the main products. Methyltrioxorhenium in ionic liquids was re-used in several reaction cycles without any deteriorating effect on the course of the reaction.

Full text of DOI:10.2478/s11696-012-0208-5

Chemical Papers 67 (1) 51–58 (2013)
DOI: 10.2478/s11696-012-0208-5
ORIGINAL PAPER
Methyltrioxorhenium-catalysed oxidation of secondary amines
to nitrones in ionic liquids
Mária Mečiarová, Melinda Mojzesová, Radovan Šebesta*
Department of Organic Chemistry, Faculty of Natural Sciences, Comenius University,
Mlynská dolina CH-2, 842 15 Bratislava, Slovakia
Received 1 March 2012; Revised 27 April 2012; Accepted 30 April 2012
Dedicated to Professor Štefan Toma on the occasion of his 75th birthday
Nitrones serve as starting materials for the synthesis of many heterocycles. Oxidation of secondary
amines using hydrogen peroxide and the catalytic amount of methyltrioxorhenium in ionic liquids
is a useful method for the preparation of nitrones. Ultrasonic irradiation and ionic liquids have a
positive influence on the reaction. The nitrones required were isolated in good yields. Corresponding
hydroxylamines, which can be easily oxidised to nitrones, often accompanied the main products.
Methyltrioxorhenium in ionic liquids was re-used in several reaction cycles without any deteriorating
effect on the course of the reaction.
c
ꢀ 2012 Institute of Chemistry, Slovak Academy of Sciences
Keywords: nitrone, amine, methyltrioxorhenium, oxidation, ionic liquid
Introduction
factor. Consequently, those catalytic oxidations are
preferable that use cheap and environmentally benign
oxidants such as oxygen or hydrogen peroxide in the
presence of suitable catalysts. Hydrogen peroxide is
usually employed as an oxidant in combination with
various catalysts, such as Na2WO4 (Co¸skun & Parlar,
2005; Evans et al., 2006; Mitsui et al., 1984; Mura-
hashi et al., 1985, 1990; Radivoy et al., 2001), salts of
Mo(VI), W(VI), or SeO2 (Ballini et al., 1992; Marcan-
toni et al., 1995; Nguyen et al., 2009; Yamada et al.,
2002), Pt(II) (Colladon et al., 2008), Ti(IV) (Zonta
et al., 2008), Ru(IV) (Goti et al., 1994). The effective
oxidation of secondary amines to nitrones was also
performed with alkyl hydroperoxides as oxidants in
Ti(IV) catalysis (Buonomenna et al., 2006; Forcato
et al., 2010, 2003). Molecular oxygen acted as the pri-
mary oxidant of secondary amines with cyclohexanone
mono-oxygenase (Colonna et al., 2004) or methyltri-
oxorhenium (Yamazaki, 1997) as catalysts.
Nitrones are useful and versatile intermediates in
the synthesis of heterocyclic compounds (Gothelf &
Jørgensen, 1998). Enantioselective 1,3-dipolar cyclo-
additions of nitrones to α,β-unsaturated carbonyl
compounds provide versatile synthons for the con-
struction of biologically active products, especially
pharmaceuticals and agrochemicals (Chow et al.,
2007; Jen et al., 2000; Kano et al., 2005; Puglisi et
al., 2004; Weselin´ski et al., 2009).
Nitrones can be prepared by condensation of N-
alkyl- or N-arylhydroxylamines with aldehydes (An-
drade et al., 2008; Dicken & DeShong, 1982; Gauthe-
ron-Chapoulaud et al., 2001; Hassan et al., 1997; Robl
& Hwu, 1985) or by oxidation of secondary amines
and the corresponding hydroxylamines, respectively.
Oxidation procedures using stoichiometric amounts
of dioxirane (Murray & Singh, 1990; Murray et al.,
1992), oxaziridines (Cicchi et al., 2000), or Oxoneꢀ
(2KHSO5 · KHSO4 · K2SO4) (Gella et al., 2009) usu-
ally suffer from poor atom economy and a high E-
Methyltrioxorhenium (CH3ReO3, MTO) is re-
garded as a particularly useful catalyst in this respect.
The first reaction catalysed by MTO was an epoxida-
*Corresponding author, e-mail: radovan.sebesta@fns.uniba.sk

52
M. Mečiarová et al./Chemical Papers 67 (1) 51–58 (2013)
tion of alkenes with hydrogen peroxide (Herrmann et
al., 1991). At present, MTO-catalysed oxidation with
H2O2 is a practical and universal method for oxidis-
ing a variety of substrates such as alkenes, alcohols,
sulphides, amines, carbonyl compounds, halides (Es-
penson, 1999). Oxidation of secondary amines with
a stoichiometric amount of a urea-hydrogen perox-
ide complex under MTO catalysis also proceeds with
good yields. These reactions can be performed in a
range of non-aqueous media, such as CH2Cl2, MeOH,
EtOH, or MeCN (Goti & Nannelli, 1996; Murray et
al., 1996; Owens et al., 2000; Zauche & Espenson,
1997). Secondary amines were also efficiently oxidised
to the corresponding nitrones by the MTO/H2O2 sys-
tem without urea in MeOH and EtOH, respectively
(Yamazaki, 1997). A range of tertiary, secondary, as
well as primary amines were selectively oxidised to
their corresponding N-oxides, nitrones, and nitro com-
pounds with molecular oxygen under MTO catalysis
in acetonitrile (Sharma et al., 2003). MTO chemically
bound to the silica support is also an effective catalyst
for the oxidative conversion of aldehydes and amines
to nitrones using a solid urea/H2O2 complex as the ox-
idant. Immobilisation of the catalyst prevents it from
leaching as well as enables its recovery (Singh et al.,
2010).
Fig. 1. Oxidation of dibenzylamine (I). Reactions conditions:
i) MTO, H2O2, ionic liquid.
vakia); Re content was 71.0–76.0 %. NMR spectra
were recorded on a Varian Mercury plus instrument
(300 MHz for 1H NMR and 75 MHz for 13C NMR, Var-
ian, USA). Chemical shifts (δ) are given in ppm rel-
ative to tetramethylsilane. IR spectra were measured
on a Thermo Scientific Nicolet iS10 (Thermo Scien-
tific, Czech Republic) in the range of 500–4000 cm−1
.
GC-MS data were recorded on an Agilent Technol-
ogy 6890N gas chromatograph (Agilent Technology,
USA) with MSD 5973 Network (Agilent Technology,
USA), EI 70 eV, full scan 33–350 m/z, DB-5MS 30 m
× 0.25 mm × 0.25 µm. Flash chromatography was per-
formed on Merck silica gel 60. Thin-layer chromatog-
raphy was performed on Merck TLC-plates silica gel
60, F-254.
Typical procedure for synthesis of nitrones
Methyltrioxorhenium is also attractive in connec-
tion with non-conventional solvents (Crucianelli et al.,
2010). MTO immobilised in ionic liquid [bmim]BF4 (1-
butyl-3-methylimidazolium tetrafluoroborate) served
as a catalyst in the Baeyer–Villiger oxidation of cyclic
ketones with hydrogen peroxide. This reaction system
permitted recycling of the catalyst in the ionic liq-
uid (Bernini et al., 2003). MTO/H2O2 in ionic liquids
[bmim]BF4 and [bmim]PF6 is also an efficient reaction
system for oxidation of hydroxylated and methoxy-
lated benzaldehydes and acetophenones to the corre-
sponding phenols (Bernini et al., 2005).
Although there are several methods available for
nitrone synthesis, only a few conform to the princi-
ples of sustainable chemistry. Furthermore, rhenium
is rare and MTO is relatively expensive. As a conse-
quence, we decided to study the MTO-catalysed ox-
idation of secondary amines from the perspective of
possible catalyst recycling. This paper shows that sec-
ondary amines can be oxidised with hydrogen peroxide
in various ionic liquids under conventional stirring as
well as under ultrasonic irradiation.
MTO (30 mg, 0.12 mmol) in 30 % H2O2 (4 mL) was
added to a stirred solution of amine (3.5 mmol) in ionic
liquid (3 mL) and the reaction mixture was stirred in
an air atmosphere for the time as listed in the tables.
The sonochemical experiments were performed in an
ultrasonic cleaning bath operating at 20 kHz. The re-
action mixture was then extracted with tBuOMe. The
organic layer was collected and dried over anhydrous
Na2SO4. The drying agent was removed by filtration
and the solvent was evaporated. The crude product II
was purified by crystallisation from hexanes and char-
1
acterised by comparison of H NMR spectra with the
literature data. Compounds Va and VIIIa–VIIIc were
purified by column chromatography on silica gel with
CH2Cl2–MeOH (ϕr = 95 : 5) as an eluent.
Recycling of ionic liquids
The reaction products were extracted with
tBuOMe. The remaining ionic liquid solution was not
purified or dried and it was charged directly with a
new portion of amine and H2O2, thus repeating the
reaction cycle.
Experimental
All reactions were carried out in air atmosphere.
Solvents were purified by standard methods before
use (Armarego & Chai, 2003). Ionic liquids were pur-
chased from Merck (Slovakia) or IoLiTech (Germany)
and were used without any purification. The purity
Results and discussion
As a benchmark reaction, we chose the oxidation
of dibenzylamine (I ) to nitrone II (Fig. 1). Murray et
al. (1996) described an 85 % yield of nitrone II using
MTO in ethanol. In order to verify the suitability of
ionic liquids as reaction media for the MTO-catalysed
1
of ionic liquids was confirmed by H NMR as agree-
able. MTO was purchased from Sigma–Aldrich (Slo-

M. Mečiarová et al./Chemical Papers 67 (1) 51–58 (2013)
Table 1. Spectral data of the compounds prepared
53
Compound
Spectral data
Reference
II
IR (neat), ν˜/cm−1: 689, 712, 749, 946, 1025, 1153, 1210, 1453, 1458, 1497,
Evans et al. (2006)
1581, 2977, 3031, 3062
1H NMR (CDCl3), δ: 5.08 (s, 2H, CH2), 7.37–7.42 (m, 7H, HCPh), 7.46–7.51
+
(m, 2H, CH
, HCPh), 8.19–8.22 (m, 2H, HCPh
)
—
N
—
13C NMR (CDCl3), δ: 71.3 (CH2), 128.5 (CH), 128.6 (CH), 129.0 (CH), 129.2
(CH), 130.4 (CH), 130.5 (CH), 133.2 (CPq h), 134.2 (CqPh
)
III
Va
IR (neat), ν˜/cm−1: 1454, 1495, 2858, 3028, 3055, 3213
1H NMR (CDCl3), δ: 3.88 (s, 4H, 2 CH2), 7.27–7.40 (m, 10H, HCPh), OH
Fernandes et al. (2008)
Colacino et al. (2008)
proton signal was not observed
IR (neat), ν˜/cm−1: 692, 754, 846, 943, 1164, 1405, 1594, 2859, 2932, 3058
1
+
—
H NMR (CDCl3), δ: 3.88 (s, 3H, CH3), 7.37–7.43 (m, 4H, CH
N
, HCPh),
—
8.19–8.22 (m, 2H, HCPh
13C NMR (CDCl3), δ: 54.4 (CH3), 128.5 (CH), 130.5 (CH), 135.3 (CPq h
)
)
VIIIa
VIIIb
IR (CHCl3), ν˜/cm−1: 1153, 1163, 1427, 1451, 1585, 2984, 3066, 3579
Colonna et al. (2004)
Colonna et al. (2004)
1H NMR (CDCl3), δ: 1.54 (t, J = 7.3 Hz, 3H, CH3), 3.96 (q, J = 7.3 Hz, 2H,
CH2), 7.40–7.42 (m, 4H, HCPh, CH
+), 8.22–8.25 (m, 2H, HCPh
—
N )
—
13C NMR (CDCl3), δ: 13.6 (CH3), 62.0 (CH2), 128.7 (CH), 129.9 (CH), 130.5
(CPq h
)
IR (CHCl3), ν˜/cm−1: 689, 930, 1091, 1149, 1175, 1301, 1308, 1365, 1379,
1453, 1577, 2938, 2980, 3061
1H NMR (CDCl3), δ: 1.48 (d, J = 6.6 Hz, 6H, 2CH3), 4.22 (m, 1H,
CH(CH3)2), 7.38–7.49 (m, 4H, HCPh, CH
+), 8.23–8.27 (m, 2H, HCPh
—
N )
—
13C NMR (CDCl3), δ: 20.8 (CH3), 67.5 (CH), 128.4 (CH), 128.6 (CH), 130.1
(CH), 130.7 (CH), 132.3 (CqPh
)
VIIIc
IR (neat), ν˜/cm−1: 696, 758, 835, 905, 1117, 1192, 1364, 1444, 1562, 1579,
2925, 2973, 3082
Colacino et al. (2008)
Kluge et al. (1996)
1H NMR (CDCl3), δ: 1.62 (s, 9H, 3 CH3), 7.40–7.42 (m, 3H, HCPh), 7.55 (s,
1H, CH
+), 8.28–8.31 (m, 2H, HCPh
—
N )
—
13C NMR (CDCl3), δ: 28.3 (CH3), 70.8 (C(CH3)), 128.4 (CH), 128.8 (CH),
129.8 (CH), 130.1 (CH), 131.0 (CqPh
)
XIa
H NMR (CDCl3), δ: 1.3 (d, J = 7.1 Hz, 6H, 2 CH3), 1.88 (s, 3H, CH3C
+),
—
N
—
1
1.89 (s, 3H, CH3C
+), 3.33 (m, 1H, CH(CH3)2)
—
N
—
XIIa
XIII
1H NMR (CDCl3), δ: 1.37 (d, J = 6.4 Hz, 6H, 2 CH3), 2.15 (s, 3H,
Kluge et al. (1996)
Gella et al. (2009)
+
CH3C
N
), 2.17 (s, 3H, CH3C
+), 4.45 (m, 1H, CH(CH3)2)
—
N
—
—
—
1H NMR (CDCl3), δ: 2.25 (m, 2H, CH2), 2.74 (m, 2H, CH2), 3.94 (m, 2H,
CH2), 6.93 (br s, 1H, NCH)
a) CompoundsXI and XII were isolated as a mixture and the ratio of XI : XII was determined by measuring the integrals of the
signals at δ = 1.89 and δ = 2.17. Spectral data are consistent with those in literature (Kluge et al., 1996).
oxidation of secondary amines, we selected a number
of common and commercially available ionic liquids
with varying cations and anions.
The investigation started with water-miscible ionic
liquids, which produce a homogeneous mixture upon
the addition of aqueous H2O2. The oxidation of
dibenzylamine (I) with H2O2–MTO in 1-butyl-3-
methylimidazolium tetrafluoroborate ([bmim]BF4) af-
forded nitrone II, in a 66 % yield as a single product
(Table 2, entry 1). Interestingly, when the reaction was
performed in an ultrasonic cleaning bath (20 kHz), ox-
idation products were isolated in a slightly higher yield
(78 %), but a small amount of hydroxylamine III was
also formed (Table 2, entry 2). Similar results were ob-

54
M. Mečiarová et al./Chemical Papers 67 (1) 51–58 (2013)
Table 2. Oxidation of dibenzylamine (I)
Entry
Solvent
Conditions
Yield II + III/%a
II : IIIb
1
2
3
4
5
6
7
8
9
[bmim]BF4
[bmim]BF4
2 h, silent, r.t.
2 h, ultrasound
2 h, ultrasound
2 h, silent, 50◦C
2 h, ultrasound
2 h, ultrasound
2 h, ultrasound
2 h, ultrasound
2 h, ultrasound
66
78
100 : 0d
93 : 7
[emim]EtOSO3
[emim]EtOSO3
[bmim]CF3SO3
[bmpyr]N(CN)2
[bmim]MeOCO2
[bmim]PF6
76 (51)c
70d
53
80 : 20 (29 : 71)c
50 : 50
34 : 66
77 : 23
100 : 0
100 : 0
62d
73
65
96d
[empyr]EtOSO3
91 : 9
a) Isolated sum of II and III; b) determined by 1H NMR; c) reactions without MTO; d) yield and ratio are average values from 2
parallel runs.
Table 3. Recycling of MTO in the oxidation of I
Entry
Solvent
Reaction
Run
Yield II + III/%
II : IIIa
1
2
3
4
1.
2.
3.
4.
66
44
57
50
100 : 0
90 : 10
94 : 6
[bmim]BF4
Silent, r.t.
100 : 0
5
6
7
8
1.
2.
3.
4.
94
94
67
61
93 : 7
100 : 0
46 : 54
91 : 9
[empyr]EtOSO3
[bmim]PF6
Ultrasound
Ultrasound
9
10
1.
2.
65
68
100 : 0
28 : 72
a) Determined by 1H NMR.
tained in 1-ethyl-3-methylimidazolium ethylsulphate
([emim]EtOSO3); however, the ratio of II : III was
slightly worse than in [bmim]BF4 (Table 2, entry 3).
To distinguish between the effect of the ultrasound
itself and the heating of a reaction medium, the ox-
idation was also performed under conventional heat-
ing. After completion of the ultrasonic reaction, the
bath temperature was usually 50◦C, so the reaction
in [emim]EtOSO3 was performed at this temperature.
The reactions with and without the ultrasound treat-
ment gave similar chemical yields but the ratio of
II : III was much higher under ultrasonic treatment
(cf. Table 2, entries 3 and 4). Interestingly, hydrogen
peroxide itself, even without MTO, can partially oxi-
dise amine I in [emim]EtOSO3 under ultrasonic irradi-
ation (51 % combined yield of II and III). In the non-
catalysed reaction, however, hydroxylamine III pre-
dominated as the major product.
The reactions in ionic liquids 1-butyl-3-methyl-
imidazolium triflate ([bmim]CF3SO3) and 1-butyl-3-
methylpyridinium dicyanamide ([bmpyr]N(CN)2)
gave somewhat inferior results. Lower yields of ox-
idation products and higher portions of hydroxy-
lamine III were obtained (Table 2, entries 5 and
6). On the other hand, interesting results were ob-
tained in 1-butyl-3-methylimidazolium methylcarbon-
ate ([bmim]MeOCO2) and 1-butyl-3-methylimidazo-
lium hexafluorophosphate ([bmim]PF6). The reaction
in these ionic liquids afforded pure nitrone II in ac-
ceptable yields (Table 2, entry 7 and 8). Ionic liquid
[bmim]PF6 is also of interest, because it is immiscible
with water, thus the reaction proceeded in two phases.
Best results were obtained using ionic liquid 1-ethyl-
3-methylpyrridinium ethylsulphate ([empyr]EtOSO3)
as a solvent. The oxidation reaction in this ionic liquid
proceeded smoothly and afforded a high yield of the
main oxidation products with high II : III ratio (96 %,
91 : 9) (Table 2, entry 9). With several ionic liquid,
a higher amount of MTO (5 mole %) was also exam-
ined but the yields of oxidation products were similar
to the experiments in which 3.5 mole % of MTO was
used.
Oxidation of secondary amines in ionic liquids
with MTO–H2O2 is reproducible. Some experiments
were repeated several times. Table 2, entries 4, 6,
and 9, reports the average yields and II : III ra-
tios. To verify the synthetic utility of this method,
oxidation of dibenzylamine (I) with MTO–H2O2 in
[empyr]EtOSO3 was performed under ultrasonic irra-
diation for 4 h in 7 mmol scale. After the reaction,
nitrone II was isolated in 84 % yield. A small amount
of hydroxylamine III (9 %), was oxidised to nitrone II
with Cu(OAc)2 in 50 % yield. Thus, the total yield of
nitrone II was 86 % (1.25 g).

M. Mečiarová et al./Chemical Papers 67 (1) 51–58 (2013)
55
Fig. 2. Oxidations of amines IVa–IVc. Reaction conditions: i) MTO, H2O2, ionic liquid.
Fig. 3. Oxidations of alkyl(benzyl)amines VIIa–VIIb. Reaction conditions: i) MTO, H2O2, ionic liquid, 2 h, ultrasound.
Methyltrioxorhenium, as a polar organometallic
compound is readily soluble in ionic liquids and is
not extracted by organic solvents with lower polarity.
This property enables the recyclability of all ionic liq-
uids containing MTO. Four reaction cycles were per-
formed in [bmim]BF4 (Table 3, entries 1–4). Accept-
able yields of oxidation products were obtained with
high ratios of II : III., Oxidation products II and III
in [empyr]EtOSO3 were isolated in higher yields in
the first and second runs. The ratio of II : III was
high, but both yields and selectivity decreased in sub-
sequent runs (Table 3, entries 5–8). Recycling of MTO
in [bmim]PF6 was less successful due to the formation
of a large amount of undesired hydroxylamine III (Ta-
ble 3, entries 9 and 10) in the second reaction cycle.
Recycling was also attempted in other ionic liquids
but the results were inferior. For instance, the reac-
tion in [bmim]MeOCO2 proceeded with a high yield
(73 %) and, in the first run, nitrone II was the only
product. However, in the second, third and fourth re-
action cycles, hydroxylamine III was isolated as the
main product and nitrone II yields from all the cycles
were lower than 20 %.
This oxidation procedure was applied to a range
of secondary amines. The oxidation of benzyl(methyl)
amine (IVa) proceeded with 100 % conversion, but
a mixture of products was obtained (Fig. 2). When
[emim]EtOSO3 was used as a solvent, benzaldehyde
oxime (VIa) was detected (1H NMR, GC-MS) as the
major product under conventional conditions as well
as under ultrasonic irradiation. The reactions per-
formed at 5◦C or with a lower amount of hydrogen per-
oxide (3.5 eq.) did not affect the reaction course. Ben-
zoic acid anhydride and traces of benzaldehyde and ni-
trone Va were also detected. Nitrone Va was isolated in
yields of 29 % (1st cycle), and 24 % (2nd cycle), respec-
tively, when [empyr]EtOSO3 was used as a solvent.
The above by-products were also detected in this re-
action mixture. In comparison, Goti (Goti & Nannelli,
1996) described the oxidation of benzyl(methyl)amine
(IVa) at ambient temperature using an alternative
method, methanol with MTO/UHP, which afforded
a 95 % yield of nitrone Va after 24 h.
The oxidation of substituted benzyl(methyl)
amines IVb and IVc in [emim]EtOSO3 under ultra-
sonic irradiation proceeded with 100 % conversion,
but again several products were obtained (Fig. 2). The
corresponding oximes VIb (65 % yield) and VIc (58 %
yield) were the major products. 4-Chlorobenzoic acid
anhydride was isolated (10 % yield) as a by-product of
amine IVb oxidation, and 4-methoxybenzoic acid an-
hydride was isolated in 22 % yield as a by-product of
the amine IVc oxidation. Thus, the MTO–H2O2 oxi-
dation in ionic liquids is less useful for the oxidation of
arylmethyl(methyl)amines. Presumably, the reaction
is more rapid with the methyl group and the resulting
—
—
C
N double bond is then cleft on oxidation.
On the other hand, the oxidation of benzyl(ethyl)
amine (VIIa) in [empyr]EtOSO3 under ultrasonic ir-
radiation proceeded chemo-selectively and also regio-
selectively (Fig. 3). Nitrone VIIIa was isolated as the
only product in 51 % yield from the 1st reaction cycle.
The yield of nitrone VIIIa decreased slightly in subse-
quent runs (2nd cycle, 48 %; 3rd cycle, 43 %; 4th cycle,
42 %). Although Colonna et al. (2004) reported the
oxidation of benzyl(ethyl)amine by molecular oxygen
and cyclohexanone monooxygenase with 68 % conver-
sion, only 30 % of VIIIa was isolated. Thus, the recy-
clable procedure as presented, using MTO in the ionic
liquid, is an interesting alternative from a practical
perspective. The oxidation of benzyl(isopropyl)amine
(VIIb) gave similar results; the desired nitrone VIIIb
was isolated as the sole product (Fig. 3). Oxidation
of amine VIIc, however, afforded nitrone VIIIc only
in small quantities with hydroxylamine IXc (Fig. 3)
as the main product. Increasing the steric bulk of
the substrate probably impedes the access of MTO
and the product results from a non-catalysed reaction
with H2O2. Nevertheless, a mixture of nitrone VIIIc
and hydroxylamine IXc was oxidised to nitrone VIIIc
with a 67 % yield by another reaction with Cu(OAc)2.
The results of oxidation of amines VIIa–VIIc are sum-
marised in Table 4.
Oxidation of diisopropyl amine (X) proceeded in
[emim]EtOSO3 as well as in [empyr]EtOSO3 with low
yields (Fig. 4). The combined yields of nitrone XI

56
M. Mečiarová et al./Chemical Papers 67 (1) 51–58 (2013)
Table 4. Oxidation of benzyl(alkyl)amines VIIa–VIIc
completely. This result suggests, as was also described
in the literature (Espenson, 1999), that MTO is de-
composed in the presence of proton donors and by the
action of H2O2. However, the ionic liquid solution still
retained some of the oxidising activity. Partial leach-
ing of rhenium species during extractions can also be
expected (Saladino et al., 2008). On the other hand, a
successive addition of hydrogen peroxide can, to some
extent, compensate for loss of the catalyst.
Amine
Ionic liquid
Run
Yield of nitrone/%
VIIa
VIIa
VIIa
VIIa
1.
2.
3.
4.
51
48
43
42
[empyr]EtOSO3
VIIb
VIIb
VIIb
VIIb
1.
2.
3.
4.
41
80
61
42
[empyr]EtOSO3
[emim]EtOSO3
Conclusions
VIIc
1.
9 (57)a
The oxidation of various secondary amines was
studied using the MTO/H2O2 oxidation system in
different ionic liquids under conventional conditions
and under ultrasonic irradiation. The methodology
is useful for the oxidation of dibenzyl and ben-
zyl(alkyl)amines. Reactions proceeded with higher
yields in ionic liquids than in conventional solvents.
Furthermore, ionic liquids enable recycling of the cat-
alyst for several reaction cycles. Although the recy-
cling experiments are less consistent within runs, the
method remains applicable and attractive from a prac-
tical perspective. The application of ultrasonic irradi-
ation improved yields of the products.
a) Yield of IXc; ratio VIIIc : IXc = 13 : 87.
Fig. 4. Oxidation of diisopropylamine (X). Reaction condi-
tions: i) MTO, H2O2, [emim][EtOSO3], 1 h, stirring,
ambient temperature.
Acknowledgements. This publication is the result of imple-
mentation of project 26240120025 supported by the Research
& Development Operational Programme funded by the ERDF.
Financial support from the Slovak Grant Agency VEGA, grant
No. 1/0543/11 is also acknowledged. NMR measurements were
conducted within the framework of Slovak State Programme
project No. 2003SP200280203. The authors wish to thank Prof.
Dr. Štefan Toma for his valuable discussions.
and oxaziridine XII were only 25 % in [emim]EtOSO3
and 10 % in [empyr]EtOSO3. Nitrone XI was detected
as the major product in both ionic liquids. However,
compounds XI and XII were isolated as an insepa-
rable mixture. The only product of the oxidation of
diisopropylamine (X ) in the 2nd reaction cycle was
aziridine XII (45 %). The combined yields ofXI and
XII decreased to 35 % (ratio XI : XII = 56 : 44) in
the 3rd cycle and to 33 % (ratio XI : XII = 57 : 43)
in the 4th cycle, respectively. Similarly, Kluge et al.
(1996) reported the oxidation of diisopropylamine to
the mixture of nitrone and oxaziridine, using sulphonic
peracid as an oxidant.
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