Facile one-pot synthesis of novel: N -benzimidazolyl-α-arylnitrones catalyzed by salts of transition metals

Article abstract of DOI:10.1039/c9ra08570j

A novel series of N-benzimidazol-2-yl-α-aryl nitrones 3a-j is synthesized via simple one-pot condensation/oxidation of 2-aminobenzimidazole, an aromatic aldehyde and m-chloro perbenzoic acid (m-CPBA) as an effective oxidant using Mn(NO₃)_2<

Full text of DOI:10.1039/c9ra08570j

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Facile one-pot synthesis of novel N-
benzimidazolyl-a-arylnitrones catalyzed by salts of
transition metals†
Cite this: RSC Adv., 2019, 9, 41851
Mehdi Kalhor, * Sima Samiei and S. Ahmad Mirshokraei
A novel series of N-benzimidazol-2-yl-a-aryl nitrones 3a–j is synthesized via simple one-pot
condensation/oxidation of 2-aminobenzimidazole, an aromatic aldehyde and m-chloro perbenzoic acid
(
m-CPBA) as an effective oxidant using Mn(NO
3
) $6H
2
2
O as an efficient catalyst at room temperature. All
synthesized N-benzimidazolyl nitrones were identified using FTIR, NMR and mass spectroscopy. Also,
1
stability energy theory calculations were performed and H NMR computational spectra were generated
for the isomeric structures of 3a; the results show that the stability order is oxaziridine (4) followed by
the nitrones 3a
E
and 3a
Z
. Also, comparing the computational spectroscopy results with the experimental
. Among the remarkable points of this protocol, stable N-
data shows great accordance with nitrone 3a
E
heterocyclic nitrones were prepared for the first time from raw materials under mild oxidative conditions.
Therefore, they can easily be applied as high-potential intermediates for synthesizing valuable
heterocycles in mild conditions. Due to benefits such as the use of inexpensive and available catalysts,
short reaction times, high yields, facile workup to obtain pure product, and facile separation of the side
product (m-chlorobenzoic acid), this simple protocol complies greatly with the principles of green
chemistry.
Received 20th October 2019
Accepted 25th November 2019
DOI: 10.1039/c9ra08570j
rsc.li/rsc-advances
Resonance structures (1–3) can be considered for nitrone
Fig. 2). All the azometine N-oxide groups are dipolar, and
Introduction
(
The name “nitrone” is a brevity which was proposed by Pfeiffer typical nitrone reactions are dependent on this dipolar state.
in 1916 for a functional group in organic compounds composed Due to the importance of nitrones, various methods for their
1
of an N-oxide of an imine. The term is a contraction of the synthesis have been reported. Generally, these are divided into
2,3
terms “nitrogen ketone”. Nitrones have a 1,3-dipole structure; four main methods, including (1) alkylation of oximes with
4
2,17
they are used in 1,3-dipolar cycloadditions (especially for various nitrogen reagents without using oxidant,
(2) the
5
synthesis of oxaziridine, isoxazolines, oxadiazolidines, etc. ) and condensation reaction of a carbonyl compound and a N-alkyl or
18
formal cycloadditions in the synthesis of many natural prod- aryl mono-substituted hydroxylamine,
(3) oxidation of
6
ucts. Increased reactivity of nitrones with metal derivatives has secondary amines or N,N-substituted hydroxylamines using
19–22
been discovered; therefore, one of the most important appli- oxidizing agents,
and (4) oxidation of imines by oxidizing
cations of nitrones is that some of them can be used as ligands reagents such as m-CPBA, hydrogen peroxide (H
2
O
2
), and alkyl
7
23,24
in inorganic chemistry. Nitrones are also applied as spin traps hydroperoxides
(Fig. 2).
8
in biological studies as well as therapies in age-related
Recently, a few methods have been developed for the prep-
diseases. Many nitrone derivatives have pharmacological aration of nitrones through one-pot condensation/oxidation of
activity and form necessary parts of the molecular structures of primary amines and aldehydes in the presence of methyl tri-
Also, they are forecasted to be effective inhibi- oxorhenium (MTO), Naon-immobilized MoOCl
tors of acidic and microbial corrosion. Nitrones are found in immobilized oxo-rhenium, or graphite oxide (and oxone as
9
10–12
25
26
main drugs.
4
,
silica-
13
27
28
many natural alkaloids, such as Huperzine J and K obtained the oxidant) as the catalyst.
1
4
15
from Huperzia serrata, Notoamide U, and Plakinamine iso-
There are also reports that isomerization of oxaziridines in
the presence of Lewis acids propels the formation of nitro-
16
lated from the genus Corticium (Fig. 1).
2
9
nes. Although each of these synthetic strategies has valuable
properties, in some cases, these methods present major
drawbacks such as low selectivity, procedures that require
harsh conditions, tedious workup and purication, inacces-
sibility of precursors (the hydroxylamines), formation of
oxidation by-products, use of hazardous solvents, and
Department of Chemistry, University of Payame Noor, P. O. BOX 19395-4697, Tehran,
Iran. E-mail: mekalhor@gmail.com; mekalhor@pnu.ac.ir; Fax: +98 2537179170; Tel:
+
†
1
98 2537179170
Electronic supplementary information (ESI) available. See DOI:
0.1039/c9ra08570j
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Fig. 1 Natural structures with nitrone frameworks.
expensive catalysts and oxidants. However, the contribution of
According to the above, this paper reports a facile and clean
new, more efficient procedures in this eld can still be inter- synthetic method for the preparation of novel nitrones con-
esting and benecial. taining the N-benzimidazole moiety under mild conditions for
Fig. 2 Four main pathways of synthesis of nitrones and some of their synthetic applications.
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Scheme 1 The model reaction for optimizing the reaction conditions.
the rst time. These N-heterocyclic nitrones, 3a–j, are obtained
These results created an incentive to develop an efficient and
through one-pot reactions of 2-aminobenzimidazole, various simple method to produce nitrones 3a–j via the reaction of 2-
aromatic aldehydes and m-CPBA as an oxidant reagent by aminobenzimidazole with various aromatic aldehydes con-
applying Mn(NO
3 2 2
) $6H O as an effective catalyst at room taining electron withdrawing groups or electron donating
temperature, with good to excellent yields and simple workup groups and a per-acid (m-CPBA) under the same conditions
compared to previous reports.
(Scheme 2).
The results are summarized in Table 2, which shows that the
yields of the one-pot condensation/oxidation method used for
the production of nitrones from the raw materials are high to
Results and discussion
Firstly, to evaluate and optimize the synthesis conditions of excellent. Moreover, the workup is simple, the amount of cata-
target compounds, the reaction of 2-aminobenzimidazole with lyst used is low and the reaction time is short in comparison to
4
1
-nitro benzaldehyde and m-CPBA in ethanol in the presence of previous methods.
0 mol% Cu(NO
3
)
2
catalyst at room temperature was performed
According to our previous knowledge, here, we suggest
as a model reaction (Scheme 1).
a probable mechanism for the synthesis of the nitrone in
Aer several tests (Table 1), the best result was obtained Scheme 3. Initially, an empty orbital of Mn cation as a Lewis
2
+
when the reaction occurred in the presence of 10 mol% acid activates the carbonyl group of the aromatic aldehyde for
Mn(NO ) $6H
3 2 2
O in ethanol. In this case, the reaction yield and nucleophilic amine (1) attack, followed by catalytic oxidation
time were 95% and 25 minutes, respectively (Table 1, entry 5). and the loss of a water molecule to form the intermediate I.
34
Table 1 Optimizing the reaction conditions for the synthesis of compound 3g
Catalyst loading
(mol%)
ꢀ
a
Entry
Catalyst
Solvent
Temperature ( C)
Time (min)
Yield (%)
1
2
3
4
5
6
7
8
9
Cu(NO
Co(NO
Zn(NO
Ni(NO
Mn(NO
Mn(NO
Mn(NO
Mn(NO
Mn(NO
Mn(NO
3
)
2
$6H
$6H
$6H
$6H
2
O
O
O
O
10
10
10
10
10
5
15
10
10
10
10
10
10
10
10
10
10
10
10
5
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
90
rt
80
80
rt
rt
rt
30
30
60
30
25
40
25
40
35
45
30
50
80
35
35
60
60
50
50
50
50
50
50
65
70
60
87
95
70
80
65
78
70
85
75
50
72
70
0
3
)
2
2
3
)
2
2
3
)
2
2
3
3
3
3
3
3
)
2
$6H
2
O
O
O
O
O
O
)
)
)
)
)
2
2
2
2
2
$6H
$6H
$6H
$6H
$6H
2
2
2
2
2
CH
MeOH
CH Cl
3
CN
1
8
9
1
1
1
1
1
1
1
1
1
1
2
0
2
2
MnCl
CoCl
ZnCl
CuCl
NiCl
2
$6H
$6H
$H
$H
$H
2
O
EtOH
EtOH
EtOH
EtOH
EtOH
2
2
O
0
1
2
3
4
5
6
7
8
9
0
2
2
O
2
2
O
2
2
O
Mn(NO
Mn(NO
Mn(NO
Mn(NO
Mn(NO
3
)
)
)
)
)
2
$6H
$6H
$6H
$6H
$6H
2
2
2
2
2
O
O
O
O
O
H
H
2
O
O
3
3
3
3
2
2
2
2
2
0
—
—
—
EtOH
EtOH
EtOH
20
25
25
0
0
0
b
CAN
10
10
—
Ca(IO
3
)
2
—
a
b
Isolated yield. Cerium(IV) ammonium nitrate.
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Scheme 2 The synthetic pathway for the preparation of nitrones 3a–j.
The corresponding Schiff base, under catalytic activation, Theoretical studies
undergoes nucleophile attack by the third molecule, m-CPBA,
In order to investigate the different probable molecular struc-
instantly leading to the formation of intermediate II. Immedi-
ately, upon the third catalytic activation of the intermediate II
following intermolecular nucleophile attack, cyclization of N-
tures of the nitrones and oxaziridine, a theoretical approach
31,35
was applied.
The optimized structures and relative energy
comparison of these molecules are shown in Fig. 3. The gure
indicates that the relative energies of the isomers are in the
order oxaziridine (4) > 3a > 3a . The stability of 3a can be
29
benzimidazolyl oxaziridines can be accomplished. Finally, in
2
+
the presence of Mn cation as a Lewis acid, the corresponding
E
Z
E
28
nitrone is formed through rearrangement.
attributed to its full conjugation and aromatic rings.
In this project, we also synthesized nitrones 3a–j in accor-
dance with general method 4 (Fig. 2) in two steps, but with lower
yields (42% to 70%), longer reaction times (1.5 to 6 hours) and
more complex and tight separation of the pure product (Scheme
1
Also, H NMR data for the nitrone and oxaziridine molecules
in the gas phase were calculated with the DFT method using the
6-311G (d) basis set. The results and experimental data are
presented in Table 4. The table indicates that obtained experi-
mental results for the N–H (11.07 ppm) bond and C–H (8.68
ppm) bond most resemble the 3aE form of nitrone in the
theoretical approach.
4). The reason for the rapid formation and stability of these
nitrones (compared to the formation of the oxaziridine ring or
amide) may be the presence of intermolecular hydrogen bonds
30
between the hydrogen of the imidazole and the nitrone
31–33
oxygen
(Scheme 4). On the other hand, the intense reso-
Conclusion
nance between nitrone, aryl and the benzimidazole ring accel-
erates this process.
In this project, we succeeded in designing a new procedure for
the mild and facile synthesis of novel N-benzimidazolyl-a-aryl
nitrones for the rst time. These compounds were obtained
through one-pot reactions of 2-aminobenzimidazole, various
aromatic aldehydes and m-CPBA as an effective oxidizing
All the synthesized N-benzimidazolyl nitrones are new, and
their structures were conrmed using FT-IR and NMR spec-
1
troscopy and mass spectral data. In the H-NMR spectra, the
shi of the CH]N (Schiff base) proton signals from 9.38 to 9.78
34
ppm to 7.82 to 8.80 ppm conrmed the formation of the
nitrone structure. In general, in the IR spectrum for a nitrone
system, it is expected that there will be ve characteristic
vibrational frequencies, including C]N, N–O, and C–N
stretching and C–H in-plane and out-of-plane bending
reagent in the presence of a catalytic amount of Mn(NO
3 2
) -
$
6H O at room temperature in good to excellent yields. The
2
method was demonstrated to be simple both in conducting the
reaction and in isolating the pure products; thus, it is a useful
procedure for the synthesis of the target compounds. Also,
31
frequencies. A red shi at the C]NO-vibrational frequency in
theoretical studies showed that the 3a form of nitrone is close
E
ꢁ
1
the N-benzimidazolyl nitrones (1535 to 1581 cm ) relative to
to the neutral structure of oxaziridine in terms of energy
stability and is very similar to the experimental spectral data.
ꢁ1
the corresponding Schiff bases (1601 to 1621 cm ) as well as
the appearance of a relatively strong band for N–O stretching in
ꢁ1
nitrones (1012 to 1103 cm ) conrms the formation of these
products. Other signals were revealed in the expected regions
which are consistent with their structures. In the mass spectra
of compounds 3a–j, molecular ion peaks (5% to 100%) and all
expected fragments consistent with the structures of the
annular products were seen.
Experimental
All the consumed chemicals were purchased from Merck or
Fluka. Melting points were determined using an electro-thermal
digital apparatus and are uncorrected. A Galaxy Series FT-IR
5
000 spectrometer (using KBr discs) was used to record the IR
To show the efficiency of this method, it was compared with
reported results in the literature for the direct synthesis of
nitrones. The obtained results are listed in Table 3. The results
clearly show that this catalytic procedure is superior to other
methods in terms of reaction time and yield, economical
convenience, etc. Therefore, it can be considered as one of the
best choices for facile and direct synthesis of N-heterocyclic
nitrones under mild conditions.
1
13
6
spectra. H and C NMR spectra were acquired in DMSO-d on
a Bruker (300 and 500 MHz) spectrometer. Chemical shis (d)
were presented in ppm using tetramethyl silane (TMS) as an
internal standard. The mass spectra were recorded on an Agi-
lent spectrometer, model 5975C VL MSD, with a Triple-Axis
Detector at 70 eV. The reactions were monitored by thin layer
chromatography (TLC) using silica gel on F254 aluminum sheets
(Merck).
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a
Table 2 Synthesis of the nitrone derivatives 3a–j
o
b
Entry
Compound
Time (min)
30
Mp ( C)
Yield (%)
1
180–182
89
85
2
3
30
40
228
212–214
86
4
5
25
25
241–242
260–262
91
85
6
7
40
25
201–204
210–212
85
95
8
9
35
25
187–188
188–190
93
95
1
0
35
189–190
91
a
3 2 2
Reaction conditions: 0.1 mmol 2-aminobenzimidazole (1) with 0.1 mmol of the aromatic aldehyde in the presence of 10 mol% Mn(NO ) $6H O in
b
ethanol (5 mL), addition of m-CPBA (0.12 mmol) and room temperature. Isolated yield.
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Scheme 3 Proposed mechanism for the synthesis of compounds 3a–j.
Computational methods
calculated with the DFT method using the B3LYP/6-311G (d)
35,36
basis set.
The study of the NMR spectra was carried out
Thermodynamic data and investigation of the IR spectra of the
nitrone and oxaziridine molecules in the gas phase were
using the NMR ¼ giao b3lyp/6-311+g(2d,p) level of theory.
Scheme 4 Synthesis of compounds 3a–j in two steps without catalyst and intermolecular hydrogen bonding (III).
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Table 3 Comparison of the catalytic procedure with other methods for the direct synthesis of N-aryl or alkyl nitrones
Entry
1
Reaction conditions
Time (h)
Yield (%)
2.5–9
15–89 (ref. 25)
2
3.5–8
50–80 (ref. 26)
3
4
3–8
2–8
40–75 (ref. 27)
50–97 (ref. 28)
5
0.4–0.58
85–91 (this work)
1
3
Theoretical calculations were performed using the Gaussian 09 7.98 (s br, 2H, H–Ar), 7.49–7.08 (t br, 6H, H–Ar) ppm, C NMR
3
7
package and the Gauss-View molecular visualization program.
6 C
(DMSO-d , 75 MHz): d 170.1, 153.6, 139.2, 133.2, 132.4, 130.8,
2
8
+
1
1
4
30.2, 129.3, 1 .1, 122.2, 111.5 ppm; MS (m/z, %): 237.2 (M , 5),
56.0 (75), 133.1 (100), 111.1 (58), 85.2 (40), 71.2 (46), 57.2 (62),
3.2 (43).
General preparation of nitrones 3a–j
To a solution of 2-aminobenzimidazole (1 mmol) and the cor-
responding aromatic aldehyde (1 mmol) in ethanol (5 mL),
Mn(NO ) $6H O (10 mol%, 0.027 g) was added. The reaction
mixture was stirred magnetically at room temperature. Then, m-
CPBA (1.2 mmol) was added to the mixture. The reaction
progress was monitored by TLC (petroleum ether : ethyl acetate
N-(1H-Benzo[d]imidazol-2-yl)-a-(2-chlorophenyl) nitrone (3b).
Gray solid. IR (KBr, nmax): 3377 (N–H), 1666, 1631 (C]N), 1579
3
2
2
(
(
(
–C]N–O), 1519, 1435, 1298, 1273 (C]C), 1456 (H–C]N), 1176
ꢁ
1 1
C–N), 1143, 1020 (N–O), 866 (C–Cl), 744 (C–H) cm ; H NMR
DMSO-d , 300 MHz): d
12.27 (br, 1H, NH), 7.68 (d, J ¼ 5.91 Hz,
H, H–C]N –O ), 7.55–7.45 (m, 6H, H–Ar), 7.12 (s br, 2H, H–
6
H
+
ꢁ
1
2
: 1). Aer completion of the reaction, aqueous NaHCO
3
(10%,
13
Ar) ppm, C NMR (DMSO-d
6 C
, 125 MHz): d 167.2, 147.3, 135.8,
1
5 mL) was added to the mixture, and the resulting precipitate
1
35.2, 131.4, 130.2, 129.7, 129.3, 127.0, 121.4, 119.9, 113.8 ppm,
was ltrated to afford pure nitrones 3a–j. For further purity,
these products can be crystallized from a mixture of ether: n-
hexane (1 : 1). Also, increasing the amount of HCl (10%) in the
+
MS (m/z, %): 271.1 (M , 46), 236.1 (52), 208.1 (48), 139.1 (100),
111.1 (71), 75.1 (34).
N-(1H-Benzo[d]imidazol-2-yl)-a-(3-chlorophenyl) nitrone (3c).
Gray solid. IR (KBr, nmax): 3307 (N–H), 1624, 1604 (C]N), 1575

ltrate causes the m-chlorobenzoic acid to precipitate; then, the
extraneous acid product is separated by ltration.
(
(
–C]N–O), 1596, 1344 (C]C), 1473 (H–C]N), 1263 (C–N), 1024
ꢁ
1 1
6
N–O), 918 (C–Cl), 758 (C–H), 740 cm ; H NMR (DMSO-d , 300
Spectroscopic data for nitrones 3a–j
+
MHz): d
H
12.22 (br, 1H, NH), 7.68 (d, J ¼ 5.49 Hz, 1H, H–C]N –
ꢁ
13
N-(1H-Benzo[d]imidazol-2-yl)-a-phenyl nitrone (3a). Light O ), 7.55–7.48 (t br, 6H, H–Ar), 7.14 (s, 2H, H–Ar) ppm, C NMR
yellow solid. IR (KBr, nmax): 3269 (N–H), 3095 (C–H), 1683, 1608 (DMSO-d
, 75 MHz): d
167.2, 147.2, 135.6, 135.0, 131.2, 129.9,
6
C
(
C]N), 1535 (–C]N–O), 1419, 1222 (C]C), 1382 (H–C]N), 129.5, 129.0, 126.9, 124.0, 121.2, 113.6 ppm, MS (m/z, %): 271.1
ꢁ1
1
+
1
263 (C–N), 1029 (N–O), 898, 759 (C–H), 619 cm ; H NMR (300 (M , 21), 243.1 (18), 160.1 (28), 139.1 (100), 111.1 (71), 75.1 (27).
): d 11.07 (br, 1H, NH, the NH protons dis-
N-(1H-Benzo[d]imidazol-2-yl)-a-(4-chlorophenyl) nitrone (3d).
appeared upon D
O addition), 8.68 (s br, 1H, H–C]N –O ), Gray solid. IR (KBr, nmax): 3348 (N–H), 1653, 1641 (C]N), 1571
MHz, DMSO-d
6
H
+
ꢁ
2
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Z E
Fig. 3 The optimized structures and relative energy comparison of the nitrone (3a , ) and oxaziridine (4) molecules.
(
1
–C]N–O), 1591, 1523, 1435 (C]C), 1462 (H–C]N), 1271 (C–N), (–C]N–O), 1531, 1348 (–NO ), 1597, 1469, 1419 (C]C), 1400
2
ꢁ
1 1
093 (N–O), 844 (C–Cl), 750 (C–H) cm ; H NMR (DMSO-d , 500 (H–C]N), 1263 (C–N), 1080 (N–O), 923, 752 (C–H), 731,
6
ꢁ
1 1
MHz): dH 12.33 (br, 1H, NH, the NH protons disappeared upon 715 cm ; H NMR (300 MHz, DMSO-d ): d (the NH proton
D
6
H
+
ꢁ
2
O addition), 8.15 (s br, 1H, H–C]N –O ), 7.89 (s br, 2H, H–Ar), exchanges with deuterium in D
2
O, which is present in DMSO-
), 8.96 (s, 1H, H–C]N –O ), 8.57–8.50 (m, 1H, H–Ar), 8.36 (d,
168.8, 150.1, 136.8, 136.4, 134.3, 132.8, J ¼ 6.66 Hz, 1H, H–Ar), 7.91–7.74 (m, 2H, H–Ar), 7.34 (d, J ¼
1
3
+
ꢁ
7
(
1
.65–7.53 (t br, 4H, H–Ar), 7.15 (s, 2H, H–Ar) ppm; C NMR
DMSO-d , 125 MHz): d
31.0, 130.2, 128.2, 121.8, 119.6, 112.7 ppm, MS (m/z, %): 271.1 7.50 Hz, 2H, H–Ar), 7.15 (br, 2H, H–Ar) ppm; C NMR (DMSO-
d
6
6
C
1
3
+
(M , 48), 236.2 (51), 208.2 (41), 139.2 (100), 111.2 (45), 75.2 (17).
6 C
d , 75 MHz): d 170.1, 137.6, 135.3, 135.0, 131.9, 131.4, 130.1,
N-(1H-Benzo[d]imidazol-2-yl)-a-(4-bromophenyl) nitrone (3e). 128.9, 125.7, 124.4, 123.4, 122.4, 112.5 ppm, MS (m/z, %): 282.1
+
Gray solid. IR (KBr, nmax): 3311 (N–H), 1660, 1624 (C]N), 1550 (M , 50), 160.1 (50), 132.1 (100), 105.1 (50), 76.1 (37).
(
(
–C]N–O), 1571, 1394 (C]C), 1340 (H–C]N), 1273 (C–N), 1012
N-(1H-Benzo[d]imidazol-2-yl)-a-(4-nitrophenyl) nitrone (3g).
ꢁ
1 1
N–O), 808 (C–Br), 769 (C–H), 736 cm ; H NMR (DMSO-d
6
, 500 Red solid. IR (KBr, nmax): 3346 (N–H), 1670, 1651 (C]N), 1575
+
ꢁ
H 2
MHz): d 12.36 (br, 1H, NH), 8.07 (s br, 2H, H–C]N –O and H– (–C]N–O), 1521, 1352 (–NO ), 1597, 1309 (C]C), 1462 (H–C]
Ar), 7.70 (br, 2H, H–Ar), 7.44 (br, 2H, H–Ar), 7.14 (br, 3H, H– N), 1278 (C–N), 1103 (N–O), 850, 740 (C–H), 715 cm ; H NMR
Ar) ppm, C NMR (DMSO-d
1
1
1
ꢁ1 1
1
3
6
, 125 MHz): d
C
169.1, 150.2, 134.8, (DMSO-d
6
, 300 MHz): d
H
12.56 (br, 1H, NH), 8.34 (d, J ¼ 6.99 Hz,
+
ꢁ
32.7, 131.1, 130.4, 129.3, 128.3, 127.7, 125.5, 121.8, 119.0, 3H, H–C]N –O and H–Ar), 8.13 (d, J ¼ 8.07 Hz, 2H, H–Ar),
+
1
3
12.7 ppm, MS (m/z, %): 315.2 (M , 57), 287.1 (53), 185.1 (100), 7.44–7.18 (d br, 4H, H–Ar) ppm; C NMR (DMSO-d , 75 MHz):
6
55.1 (50), 132 (14), 105.2 (28), 76.2 (28).
N-(1H-Benzo[d]imidazol-2-yl)-a-(3-nitrophenyl) nitrone (3f). 123.7, 122.8, 112.5 ppm; MS (m/z, %): 282 (M , 100), 254 (57),
d 170.6, 152.4, 151.0, 149.4, 143.3, 140.4, 131.0, 130.1, 124.6,
C
+
Red solid. IR (KBr, nmax): 3317 (N–H), 1705, 1618 (C]N), 1581 235 (12), 219 (12), 160 (99), 121, 104 (77).
41858 | RSC Adv., 2019, 9, 41851–41860
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1
Table 4 The H NMR data (calculated and experimental) and total energies for the isomeric compounds 3a
a
ꢁ1
b
b
No.
Compound
E (E, kcal mol
)
C–H (ppm) (experimental)
N–H (ppm) (experimental)
ꢁ
779.68669653
ꢁ489253.40207)
6.62
—
8.60
—
(
4
ꢁ
779.69570020
ꢁ489259.05187)
8.52
(8.68)
10.62
(11.07)
(
3
a
E
ꢁ
779.71431871
ꢁ489270.73499)
9.55
(8.68)
10.13
(11.07)
(
3
a
Z
a
b 1
Total energies in hartree. H NMR data in the gas phase.
N-(1H-Benzo[d]imidazol-2-yl)-a-(p-tolyl) nitrone (3h). Gray
solid. IR (KBr, nmax): 3396 (N–H), 2924 (C–H), 1658, 1633 (C]N),
Conflicts of interest
1
1
(
573 (–C]N–O), 1527, 1456, 1313, 1257 (C]C), 1431 (H–C]N), There are no conicts to declare.
ꢁ
1 1
168 (C–N), 1020 (N–O), 842, 767, 750 (C–H), 611 cm ; H NMR
): d (the NH proton exchanges with
O, which is present in DMSO-d
300 MHz, DMSO-d
6
H
Acknowledgements
), 8.08 (d, J ¼
deuterium in D
2
6
+
ꢁ
7
2
1
.50 Hz, 2H, H–C]N –O and H–Ar), 7.08–7.42 (m, 7H, H–Ar),
1
3
The gratitude of the authors goes to the research community of
the Chemistry Department of Payame Noor University, who
provided nancial and technical support for this project.
.36 (s, 3H, CH
3 6 C
) ppm; C NMR (DMSO-d , 75 MHz): d 168.8,
51.1, 142.0, 135.7, 133.0, 129.2, 128.8, 121.3, 113.7 ppm, MS
+
(
(
m/z, %): 251.2 (M , 96), 223.2 (56), 119.1 (100), 91.1 (87), 65.1
25).
N-(1H-Benzo[d]imidazol-2-yl)-a-(4-methoxyphenyl) nitrone (3i).
Gray solid. IR (KBr, nmax): 3327 (N–H), 1660, 1624 (C]N), 1556
References
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+
H–C]N –O ), 7.85 (s, 1H, H–Ar), 7.37–7.28 (d br, 2H, H–Ar),
6
(
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%
6
1
3
.97 (t br, 2H, H–Ar), 6.87–6.78 (m, 5H, H–Ar) ppm; C NMR
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33.2, 131.0, 130.3, 129.3, 128.1, 121.7, 111.6 ppm; EIMS (m/z,
6
C
+
): 253.1 (M , 22), 225.1 (13), 160.0 (17), 121.1 (100), 93.1 (54),
5.1 (82).
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