Efficient combination of task-specific ionic liquid and microwave dielectric heating applied to synthesis of a large variety of nitrones

Article abstract of DOI:10.1002/hc.20581

Under mild microwave irradiation conditions and without any additional organic solvents, a large variety of nitrones were prepared in a weak Lewis base phosphinite task-specific ionic liquid (TSILOPPh2) in excellent yields. This catalytic TSIL was also re

Full text of DOI:10.1002/hc.20581

Heteroatom Chemistry
Volume 21, Number 2, 2010
Efficient Combination of Task-Specific Ionic
Liquid and Microwave Dielectric Heating
Applied to Synthesis of a Large Variety
of Nitrones
Hassan Valizadeh1,2
1Department of Chemistry, Islamic Azad University, Mianeh Branch, Mianeh, Iran
2Department of Chemistry, Faculty of Sciences, Azarbaijan University of Tarbiat Moallem,
Tabriz, Iran
Received 6 October 2009; revised 28 January 2010
has been focused on the synthesis of functionalized
ABSTRACT: Under mild microwave irradiation con-
ionic liquids (FILs), through incorporation of addi-
ditions and without any additional organic solvents, a
tional functional groups as a part of the cation and/or
large variety of nitrones were prepared in a weak Lewis
anion, so-called task-specific ionic liquids (TSILs),
base phosphinite task-specific ionic liquid (TSIL-
and their applications in chemical research. They
OPPh2) in excellent yields. This catalytic TSIL was
have shown great promise not only as alternative
also recyclable. Under optimized microwave irradia-
green solvents but also as reagents or catalysts in
tion conditions, the reaction occurred at very shorter
organic transformations [1,4]. Wang and Li used
reaction time and higher yields in comparison with
TSIL as Lewis base, ligand and reaction medium
ꢀ
C
conventional heating conditions. 2010 Wiley Periodi-
for the palladium-catalyzed Heck reaction [4]. Paun
cals, Inc. Heteroatom Chem 21:78–83, 2010; Published on-
et al. reported the basic ionic liquid catalyzed Kno-
line in Wiley InterScience (www.interscience.wiley.com).
evenagel condensation reaction [6]. Very recently,
DOI 10.1002/hc.20581
we used the TSIL-OPPh2 as weak Lewis base catalyst
and reaction medium for the Biginelli and Knoeve-
nagel reactions [7,8].
INTRODUCTION
Nitrones are highly valuable synthetic interme-
diates in organic synthesis [9,10] and are employed,
for instance, in stereoselective formation of syn-
thetically useful isoxazolidines by their 1,3-dipolar
cycloaddition reaction with alkenes [11–13]. Re-
cently, novel reactivities have been described [14,15].
For the preparation of nitrones, the most popu-
lar method is the condensation of aldehydes or ke-
tones with N-monosubstituted hydroxylamines [16].
Colacino et al. reported the synthesis of various C-
aryl and C-alkyl-nitrones under solvent-free condi-
tions in a ball-mill apparatus [17]. Kim et al. pre-
pared the α-phenyl-N-tert-butyl nitrones and studied
the protective effects of these compounds against
The importance of green reactions in organic syn-
thesis has encouraged scientists to explore the use
of room-temperature ionic liquids (RTILs) as a pow-
erful alternative to conventional molecular organic
solvents or catalysts due to their particular prop-
erties, such as undetectable vapor pressure, wide
liquid range, as well as the ease of recovery and
reuse [1–5]. More recently, much more attention
Correspondence to: Hassan Valizadeh; e-mail: h-valizadeh@
azaruniv.edu.
c
ꢀ
2010 Wiley Periodicals, Inc.
78

Efficient Combination of Task-Specific Ionic Liquid and Microwave Dielectric Heating Applied to Synthesis of a Large Variety of Nitrones 79
microvascular damages in schemia/reperfusion in
the “hamster cheek pouch” assay [18].
of nitrones. In continuation of our work in studying
organic reactions in ionic liquids, water, or solvent-
less systems as a green reaction medium [22–26], we
report the synthesis of nitrones via the condensation
of N-(aryl and alkyl)hydroxylamines with arylalde-
hydes in TSIL-OPPh2 under microwave irradiation
(Scheme 1).
The use of microwave irradiation has been em-
ployed for a number of organic syntheses to reduce
the reaction time and rate enhancement, and to
increase the selectivity and yields. Microwave irra-
diation has also been used for the synthesis of α-(5-
substituted-2-hydroxyaryl)-N-aryl nitrone [19]. Re-
cently, Bortolini and Maiuolo reported the synthesis
of nucleoside derivatives by the microwave-assisted
1,3-cycloaddition reaction of vinyl nucleobases with
nitrones under solvent-free conditions [20]. Very re-
cently, we reported the high yield synthesis of a large
variety of nitrones over MgO at room temperature
under solvent-free conditions [21]. To the best of our
knowledge, no reports are available using ionic liq-
uids for the synthesis of nitrones. In general, the es-
sential requirement for the synthesis of nitrones in-
cludes a base and a reaction medium. This promoted
us to use a task-specific ionic liquid, IL-OPPh2, which
acts as a weak Lewis base as well as an environmen-
tally benign reaction medium in an ion-pair unity,
for the synthesis of nitrones. Very recently, we used
IL-OPPh2 as an efficient and recyclable basic cata-
lyst and reaction medium [7–8]. Here, we report the
development of this ionic liquid, which exhibits a
very high activity and recyclability to the synthesis
EXPERIMENTAL
General Information
All reagents were purchased from Merck (Germany)
and were used without further purification. 1H NMR
spectra were obtained in CDCl3 solution from Bruker
Avance AC-400 MHz (or 300 MHz) and 13C NMR
spectra at 100 MHz (or 75 MHz) on the afore-
mentioned instruments. Mass spectra were recorded
on a Platform II (Micromass, Manchester, UK)
quadrupole mass spectrometer fitted with an elec-
trospray interface. Elemental analyses were carried
out on a Perkin-Elmer 240C elemental analyzer and
are reported in percent atomic abundance. All melt-
ing points are uncorrected and are measured in an
open glass-capillaries using Stuart melting point ap-
paratus. Microwave experiments were conducted in
a Milestone MicroSYNTH apparatus.
_
PF6
OPPh2
+
N
N
IL-OPPh2 =
_
R2
O
H
+
N
O
IL-OPPh2
R2NHOH
+
R1
H
Solvent-free, MW or Thermal heating
R1
1
2
3a-s
3j: R1 = 4-Nitrophenyl, R2 = Ph
3k: R1 = 4-Methoxyphenyl, R2 = Ph
3a: R1 = 2-Naphthyl, R2 = Me
3b: R1 = 2-Furyl, R2 = t-Butyl
3c: R1 = 2-Pyridyl, R2 = t-Butyl
3d: R1 = 2-Furyl, R2 = Me
3e: R1 = 4-Cyanophenyl, R2 = Me
3f: R1 = 4-Methylphenyl, R2 = Me
3g: R1 = 2-Hydroxyphenyl, R2 = Ph
3l: R1 = 4-Hydroxy-3-MethoxyPhenyl, R2 = t-Butyl
3m: R1 = 2-Hydroxy-4-methoxyphenyl, R2 = t-Butyl
3n: R1 = 5-Bromo-2-hydroxyphenyl, R2 = t-Butyl
3o: R1 = 2-Hydroxy-5-methoxyphenyl, R2 = t-Butyl
3p: R1 = 4-Pyridyl, R2 = t-Butyl
3q: R1 = 2-Thiophenyl, R2 = Methyl
3h: R1 = 5-Bromo-2-hydroxyphenyl, R2 = Ph
3i: R1 = 2,3-Dihydroxyphenyl, R2 = Ph
3r: R1 = 2-Furyl, R2 = Benzyl
3s: R1 = 2-Naphthyl, R2 = Benzyl
SCHEME 1 Synthesis of nitrones in IL-OPPh2 as a catalyst and reaction medium.
Heteroatom Chemistry DOI 10.1002/hc

80 Valizadeh
TABLE 1 TSIL-OPPh2 Catalyzed Synthesis of Nitrones under Microwave and Thermal Heating Conditionsa
Time
Microwave (min)
Yieldc (%)
Mp (◦ C)
Entry
Productb
Thermal (h)
Microwave
Thermal
Found, Lit. [Ref.]
1
2
3
4
5
6
7
8
3a
3b
3c
3d
3e
3f
3g
3h
3i
5
4.5
5
4
3.5
5
4
4
5
5
4.5
4
3.5
4
5
5
4.5
4
4
2.5
3
3
2.5
3
3.5
3
3
3
2.5
3
2.5
3
3
3.5
3
3.5
3
96
95
93
98
96
92
95
99
98
97
93
99
99
98
96
99
99
97
99
87
89
83
91
84
81
87
90
89
84
80
88
92
88
87
87
90
89
90
106–109, 108–110 [17]
67–68, 65.5 [18]
64–66, 63–65 [17]
88–90, 90 [28]
183–185, 187 [29]
117–120, 118–20 [30]
129–131, 129–131 [21]
173–176, 173–176 [21]
159–161, 159–161 [21]
181–183, 183–184 [28]
113–116, 115–116 [28]b
193–195, 196 [18]
13–116, 115.2–116 [31]
115–118, 118.7 [31]
69–71, 70.4–70.6 [29]
83–85, 84–86 [17]
9
10
11
12
13
14
15
16
17
18
19
3j
3k
3l
3m
3n
3o
3p
3q
3r
113–115, 115–117 [17]
84–86, 85–88 [17]
102–105, 104–106 [17]
3s
3
aReaction conditions: 10 mmol of arylaldehyde, 10 mmol of N-monosubstituted hydroxylamine, and 10 mmol of TSIL.
bProducts were characterized by elemental analysis and by comparison of their spectroscopic data (1H NMR, 13C NMR, and IR) and melting
points with those reported in the literature.
c Isolated yields after recrystallization.
or ethyl acetate. The extracts were concentrated on
a rotary evaporator, and the crude mixture was pu-
rified by recrystallization using (EtOH/EtOAc) to af-
ford corresponding nitrones.
Preparation of Nitrones in TSIL-OPPH2 under
Microwave Irradiation
General Procedure. In a 10-mL glass microwave
vessel, arylaldehyde derivatives (10 mmol), N-
monosubstituted hydroxylamine (10 mmol), and
TSIL-OPPh2 (10 mmol) were placed. The mixture
was subjected to microwave irradiation at 60 W
for a few minutes (depending on the reactants; see
Table 1). The completion of reaction was moni-
tored by TLC using (EtOAc/petroleum 1:8) as eluent.
After completion of the reaction, the mixture was
cooled to room temperature and the mixture was
extracted with ether or ethyl acetate. The extracts
were concentrated on a rotary evaporator, and the
crude mixture was purified by recrystallization us-
ing (EtOH/EtOAc) to afford corresponding nitrones
(3a–s).
Selected Spectroscopic Data
C-(2-Furyl)-N-(t-butyl)-nitrone (3b). IR νmax
/
cm−1: 3102, 2981, 2936, 1578 (C N), 1150 (N O)
and 1052 (C O). 1H NMR (400 MHz, CDCl3): δ 7.75
(dd, J = 3.91 Hz, J = 1.7 Hz, 1H), 7.69 (s, 1H, nitronyl
H), 7.48 (dd, J = 2.70, J = 1.7 Hz, 1H), 6.60 (dd, J =
3.91, J = 2.70 Hz, 1H), 1.58 (s, 9H); 13C NMR (CDCl3,
75 MHz): δ 148.20, 143.55, 123.12, 115.17, 113.32,
68.50, 29.01; EI-MS [MH]+ m/z 168; Anal. Calcd (%)
for C9H13NO2: C, 64.65; H, 7.84; N, 8.38. Found (%):
C, 64.81; H, 7.79; N, 7.21.
Preparation of Nitrones in TSIL-OPPH2 under
Thermal Conditions
C-(2-Hydroxyphenyl)-N-phenyl-nitrone (3g). IR
νmax/cm−1: 3500–3150 (OH), 1586 (C N), 1159
(N O), and 1033 (C O). 1H NMR (400 MHz, CDCl3):
δ 12.40 (s, 1H, OH), 7.92 (s, 1H, nitronyl H), 7.60–
7.39 (m, 6H, ArH), 7.23 (t, J = 8.9 Hz, 1H), 7.04 (d,
J = 11.15 Hz, 1H), 6.95 (t, J = 8.9 Hz, 1H); 13C NMR
(CDCl3, 75 MHz): δ 134.72, 134.69, 129.21, 128.20,
126.26, 125.39, 125.32, 121.16, 118.99, 118.95,
117.98; EI-MS [MH]+ m/z 214; Anal. Calcd (%) for
General Procedure. The selected arylaldehyde
derivative (10 mmol), N-monosubstituted hydroxy-
lamine (10 mmol), and TSIL-OPPh2 (10 mmol) were
stirred at 70◦C for appropriate time (Table 1). The
completion of reaction was monitored by TLC us-
ing (EtOAc/petroleum) as eluent. After completion
of the reaction, the mixture was extracted with ether
Heteroatom Chemistry DOI 10.1002/hc

Efficient Combination of Task-Specific Ionic Liquid and Microwave Dielectric Heating Applied to Synthesis of a Large Variety of Nitrones 81
C13H11NO2: C, 73.22; H, 5.20; N, 6.57. Found (%): C,
72.98; H, 5.13; N, 6.48.
out by thermal heating conditions as shown in
Table 1. The optimization of the process by varying
temperature, time, and TSIL-OPPh2 was done to get
products in high yields and purity. The yields of the
products obtained by microwave irradiation verses
thermal heating are higher with remarkable reduc-
tion in reaction time due to homogeneous heating
(as a result of strong agitation of reactant molecules)
throughout the reaction media by microwave irradi-
ation as compared to convection currents in ther-
mal heating. This methodology avoids the use of any
base, solvents and requires equimolar amount of the
ionic liquid to promote the reaction.
C-(2,3-Dihydroxyphenyl)-N-phenyl-nitrone (3i).
IR νmax/cm−1: 3450–3000 (OH), 1569 (C N), 1151
(N O), and 1035 (C O). 1H NMR (400 MHz, CDCl3):
δ 12.40 (s, 1H, OH), 8.00 (s, 1H, nitronyl H),
7.77–7.75 (m, 2H, ArH), 7.52–7.47 (m, 5H, ArH and
OH), 7.30 (d, J = 2.3 Hz, 1H), 6.90 (d, J = 8.88 Hz,
1H); 13C NMR (CDCl3, 75 MHz): δ 159.04, 145.92,
139.56, 137.07, 134.38, 130.82, 129.46, 122.29,
121.79, 118.70, 110.70; ESI-MS [MH]+ m/z 230; Anal.
Calcd (%) for C13H11NO3: C, 68.11; H, 4.84; N, 6.11.
Found (%): C, 68.05; H, 4.83; N, 6.09.
To determine whether the ionic liquid was
an essential factor to promote this process,
the reaction of N-phenylhydroxylamine with 4-
methoxybenzaldehyde as a model was carried out
using several butylmethylimidazolium-based ionic
liquids (ILs), [bmim]X, with varying anions such as
C-(4-Pyridyl)-N-(t-butyl)-nitrone (3p). IR νmax
/
cm−1: 1588 (C N), 1189 (N O), and 1099 (C O). 1H
NMR (300 MHz, CDCl3): δ 8.86 (m, 2H), 8.15 (m,
2H), 7.58 (s, 1H, nitronyl H), 1.54 (s, 9H); 13C NMR
(CDCl3, 75 MHz): δ 148.28, 137.18, 128.47, 124.44,
74.34, 51.40, 27.35, 27.20, 27.15; ESI-MS [MH]+ m/z
179; Anal. Calcd (%) for C11H15NO: C, 74.54; H, 8.53;
N, 7.90. Found (%): C, 75.01; H, 8.41; N, 7.75.
−
Cl−, BF4−, and PF6 under microwave irradiation.
With these ionic liquids, the reaction times were
longer and the yield of product 3k was lower than
those in TSIL-OPPh2 under the same conditions.
Next, to improve the yields, we performed the re-
actions using different quantities of reagents. The
best results were obtained with a 1:1:1 ratio of ary-
laldehyde, N-monosubstituted hydroxylamine, and
C-(2-Furyl)-N-benzyl-nitrone (3r). IR νmax/cm−1:
1582 (C N), 1137 (N O), and 1009 (C O). 1H NMR
(300 MHz, CDCl3): δ 7.79 (m, 1H), 7.62–7.30 (m,
7H), 7.64 (m, 1H), 5.16 (s, 2H); 13C NMR (CDCl3,
75 MHz): δ 145.13, 143.09, 133.51, 129.01, 128.14,
128.04, 125.14, 116.30, 113.44, 69.02; ESI-MS [MH]+
m/z 202; Anal. Calcd (%) for C12H11NO2: C, 71.63; H,
5.51; N, 6.96. Found (%): C, 71.72; H, 5.42; N, 6.81.
IL-OPPh2, respectively. For example,
a com-
plete conversion was obtained for the reaction
of 2-hydroxy-4-methoxybenzaldehyde and 4-pyridyl
aldehyde after 3.5 and 5 min, respectively, under mi-
crowave irradiation (Table 1, entries 13 and 16).
A large variety of nitrones were successfully
synthesized in high yields by following the above
method. The products can be separated from the
ionic liquid system by simple extraction with ether
and ethyl acetate in all examined cases. The results of
the synthesis of different nitrones 3a–s are presented
in Table 1. To examine the versatility of this proce-
dure, the reactions of N-monosubstituted hydroxy-
lamine with arylaldehydes containing electron-rich,
electron-poor, or electron-neutral substituents were
examined. The results are listed in Table 1. It
shows that yields (>92% yields) were satisfactory
under microwave irradiation conditions. On the
other hand, the relatively hindered aldehydes, i.e.,
2-naphtylaldehyde, 2,3-dihydroxybenzaldehyde, and
2-hydroxy-5-methoxybenzaldehyde also led to the
good yields of the related nitrones (entries 9, 15, 19).
The reactions worked well with aromatic aldehydes.
However, aliphatic aldehydes, such as propionalde-
hyde and butyraldehyde did not react efficiently with
N-phenylhydroxylamine or N-methylhydroxylamine
under similar conditions in the presence of task-
specific ionic liquid, TSIL-OPPh2, and significant
C-(2-Naphthyl)-N-benzyl-nitrone (3s). IR νmax
/
cm−1: 1572 (C N), 1161 (N O), and 1119 (C O).
1H NMR (300 MHz, CDCl3): δ 9.20 (s, CH=N, 1H),
7.89–7.74 (m, 4H), 7.62–7.28 (m, 8H), 5.10 (s, 2H);
13C NMR (CDCl3, 75 MHz): δ 136.13, 135.21, 132.32,
132.01, 129.16, 129.06, 129.00, 128.21, 128.40,
127.83, 127.64, 127.46, 127.30, 126.41, 125.45,
70.32; ESI-MS [MH]+ m/z 262; Anal. Calcd (%) for
C18H15NO: C, 82.73; H, 5.79; N, 5.36. Found (%): C,
82.75; H, 5.76; N, 5.32.
RESULTS AND DISCUSSION
We have recently introduced an imidazolium-based
phosphinite ionic liquid for the Horner–Wadsworth–
Emmons-type reaction [27]. In the present re-
port, an equimolar quantity of the phosphinite-
functionalized imidazolium salt, TSIL-OPPh2, as a
Lewis base, gives clean nitrones by the condensa-
tion of aldehydes with N-monosubstituted hydroxy-
lamines in high yields under microwave irradiation.
For comparison, the reaction has also been carried
Heteroatom Chemistry DOI 10.1002/hc

82 Valizadeh
TABLE 2 Comparison of Efficiency of IL-OPPh2 in Synthesis of Nitrones (3a, 3b, 3g, and 3r) after Five Times Recycling
Q1
Yield (%)a
Microwave
Thermal
Run
3a
3b
3g
3r
3a
3b
3g
3r
1
2
3
4
5
96
95
95
93
93
95
95
94
94
93
95
94
94
92
92
99
97
97
96
96
87
87
86
86
85
89
88
88
87
86
87
85
86
85
85
90
90
88
88
87
aYields of isolated product.
amount (>45%) of unreacted reactants were isolated
when the reactions were carried out for several min-
utes under microwave irradiation or at 70◦C for sev-
eral hours under thermal heating conditions. As it
can be seen from Table 1, no significant differences
in yields of products were observed by varying the
substituent at nitrogen atom in hydroxylamines in
this procedure. All products are known compounds,
characterized by mp, IR, 1H NMR, 13C NMR spectra,
and elemental analysis.
Ease of recycling is a useful feature of ionic liq-
uids. In the present study, we investigated that IL-
OPPh2 can be recycled. After removal of the mixture
of product, possible impurities and unreacted mate-
rials, the remaining IL was reused for consecutive
five cycles. In the reactions for the preparation of
nitrones (3a, 3b, 3g, and 3r), no significant loss of
product yields was observed when TSIL-OPPh2 was
used after five times recycling (Table 2). In addition,
compared the IR of the reused TSIL-OPPh2 with that
of the new synthesized ionic liquid, the difference
was small. That is to say that in the reaction, the
structure of the catalyst did not change. Compared
with the traditional solvents and catalysts, the easy
recycling is an attractive property of the TSIL-OPPh2
for the environmental protection and economic
reasons.
cyclability of the catalyst are prominent features of
this new procedure. Therefore, we believe that the
new synthetic method reported here would greatly
contribute to the environmentally greener and safer
process.
ACKNOWLEDGMENTS
The partial financial assistance from the Research
Vice Chancellor of Azarbaijan University of Tarbiat
Moallem is gratefully acknowledged.
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