Experimental and theoretical approving of anomeric based oxidation in the preparation of 2-sbstituted benz-(imida, oxa and othia)-zoles using [2,6-DMPy-NO2]C(NO2)3 as a novel nano molten salt catalyst

Article abstract of DOI:10.1039/c6ra13231f

The synthesis of 2-sbstituted benz-(imida, oxa and othia)-zole derivatives were occurred in the presence of 2,6-dimethyl-1-nitropyridin-1-ium trinitromethanide [2,6-DMPy-NO₂]C(NO₂)₃via the condensation reaction between 1,2-phenylenediamine or 2-aminophenol or 2-aminothiophenol and corresponding aldehyde at room temperature under solvent-free conditions respectively. [2,6-DMPy-NO₂]C(NO₂)₃ as a nano molten salt (NMS) catalyst was fully characterized by Fourier transform infrared (FT-IR), nuclear magnetic resonance (¹H NMR and ¹³C NMR), mass, thermal gravimetric (TG), derivative thermal gravimetric (DTG), X-ray diffraction patterns (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis. The described reactions are in close agreement with the green chemistry disciplines and their major advantages are good yields, short reaction time and ease of separation. Our recently new introduced concept entitled "anomeric based oxidation" was proposed for the final step of the described synthesis and it was also approved using theoretical studies.

Full text of DOI:10.1039/c6ra13231f

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PAPER
Experimental and theoretical approving of
anomeric based oxidation in the preparation of 2-
sbstituted benz-(imida, oxa and othia)-zoles using
[2,6-DMPy-NO2]C(NO2)3 as a novel nano molten
salt catalyst†
Cite this: RSC Adv., 2016, 6, 58667
a
a
Mohammad Ali Zolfigol, Ardeshir Khazaei, Saied Alaie,a Saeed Baghery,a
*
*
Farahnaz Maleki,a Yadollah Bayatb and Asiye Asgarib
The synthesis of 2-sbstituted benz-(imida, oxa and othia)-zole derivatives were occurred in the presence of
2,6-dimethyl-1-nitropyridin-1-ium trinitromethanide [2,6-DMPy-NO2]C(NO2)3 via the condensation
reaction between 1,2-phenylenediamine or 2-aminophenol or 2-aminothiophenol and corresponding
aldehyde at room temperature under solvent-free conditions respectively. [2,6-DMPy-NO2]C(NO2)3 as
a nano molten salt (NMS) catalyst was fully characterized by Fourier transform infrared (FT-IR), nuclear
magnetic resonance (1H NMR and 13C NMR), mass, thermal gravimetric (TG), derivative thermal
gravimetric (DTG), X-ray diffraction patterns (XRD), scanning electron microscopy (SEM) and transmission
electron microscopy (TEM) analysis. The described reactions are in close agreement with the green
chemistry disciplines and their major advantages are good yields, short reaction time and ease of
separation. Our recently new introduced concept entitled “anomeric based oxidation” was proposed for
the final step of the described synthesis and it was also approved using theoretical studies.
Received 21st May 2016
Accepted 13th June 2016
DOI: 10.1039/c6ra13231f
www.rsc.org/advances
Ionic liquids (ILs) and molten salts (MSs) are materials that are
completely composed of ions and can be synthesized for different
Introduction
purposes. Attention to these compounds, oen is attracted as the
green, technological media of the future is still increasing
quickly19 and stems from their near-zero vapour pressure,20 their
thermal stability21 and their broadly harmonic properties as
hydrophobicity, polarity and solvent miscibility behaviour by
suitable modication of the cation and the anion. Mainly, ILs and
MSs are dened as those fused salts with a melting point less than
100 ꢀC, with salts with higher melting points mentioned to as
molten salts (melting point more than 100 ꢀC).22 Combination of
various cations and anions allow a wide range of chemical and
physical characteristics to be attained, including volatile23 and in-
volatile systems, and thus the terms “designer” and “task-specic”
ILs and MSs have been developed.24,25 Most recently, Atkin et al.
have extensively reviewed the chemical and physical properties of
nanostructured ILs and MSs.26 Since the energetic materials have
attracted interest over the past decades,27 we decided to design
and synthesize novel and recyclable NMS catalyst. For this
purpose, we have prepared 2,6-dimethyl-1-nitropyridin-1-ium tri-
nitromethanide [2,6-DMPy-NO2]C(NO2)3 as a naval and recyclable
nanostructured molten salt (NMS) (Scheme 1). In our previous
works we reported some theoretical studies that all supported the
anomeric-based oxidation mechanism.28,29 Herein we also report
a theoretical study on above mechanism for the synthesis of
compounds reported here.
2-Substituted benz-(imida, oxa and othia)-zoles have received
remarkable attention in heterocyclic chemistry.1 These hetero-
cyclic compounds display biological and pharmacological
activities such as: antimicrobial,2 antiparkinson,3 antiviral,4
antifungal,5 antibiotic6 and anticancer.7 These compounds are
also used as ligands for asymmetric transformations.8 Synthetic
methods that are common for the preparation of these
compounds usually include the reaction of a carboxylic acid or
its derivatives with an appropriate 1,2-phenylenediamine, 2-
aminophenol or 2-aminothiophenol using a strong acid at high
temperatures.9 Various synthetic processes and catalysts have
been developed for the synthesis of 2-substituted benz-(imida,
oxa and othia)-zoles such as cetylpyridinium bromide,10 PEG
400,11 DDQ,12 NiO2,13 Ba(MnO4)2,14 PCC,15 ZrOCl2$8H2O,16
TMSCl17 and CAN18 because of the importance of this type of
compounds.
aFaculty of Chemistry, Bu-Ali Sina University, Hamedan 6517838683, Iran. E-mail:
zol@basu.ac.ir; mzolgol@yahoo.com; Khazaei_1326@yahoo.com; Fax: +98
8138257407
bFaculty of Chemistry and Chemical Engineering, Malek Ashtar University of
Technology, Tehran, Iran
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra13231f
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829 cmꢁ1
;
1H NMR (400 MHz, DMSO-d6): dppm 2.70 (s, 6H,
–CH3), 7.75 (d, 2H, ArH, J ¼ 8.0 Hz), 8.39 (t, 1H, ArH, J ¼ 7.8 Hz);
13C NMR (100 MHz, DMSO-d6): dppm 18.1, 97.4, 139.3, 160.0,
165.9; MS: m/z ¼ 303 [M]+.
General procedure for the synthesis of 2-aryl benz-(imida, oxa
and othia)-zole derivatives
To a mixture of aromatic aldehydes (1 mmol) and one of the 1,2-
phenylenediamine (1 mmol; 0.108 g), 2-aminophenol (1 mmol;
0.109 g) or 2-aminothiophenol (1 mmol; 0.125 g) in a round
bottom ask, 1 mol% (0.003 g) of [2,6-DMPy-NO2]C(NO2)3 NMS
was added as a catalyst and stirred at room temperature under
solvent-free conditions for appropriate time (Table 3). Aer
completion of the reaction as detected by thin layer chroma-
tography (TLC) (n-hexane/ethyl acetate: 5/2), ethyl acetate
(10 mL) was added to reaction mixture and stirred under reux
condition for 10 minutes. Then, the resulting mixture was
washed with water (10 mL) and decanted to separate catalyst
from other materials (the reaction mixture was soluble in hot
ethyl acetate and NMS catalyst was soluble in water). The
aqueous layer was decanted, separated and its water was
removed to provide the catalyst for another reaction. The
solvent of organic layer was removed and the crude product was
puried by recrystallization from ethanol (95%). More puri-
cation can be performed by recrystallization in n-hexane. In this
study, NMS catalyst was recycled and reused for ve times
without signicant loss of its catalytic activity.
Scheme
1 Synthesis of 2,6-dimethyl-1-nitropyridin-1-ium trini-
tromethanide [2,6-DMPy-NO2]C(NO2)3 as a NMS catalyst.
Experimental
The materials were purchased from Merck, Fluka and Sigma-
Aldrich and were used without any additional purication. All
reactions were tested by thin layer chromatography (TLC) on gel
F254 plates. Spectrometer (1H NMR 400 MHz and 13C NMR 100)
in pure deuterated DMSO with tetramethylsilane (TMS) was
studied as the internal standard. The prepared NMS catalyst
was known by FT-IR, 1H NMR, 13C NMR, mass, X-ray diffraction
patterns (XRD), scanning electron microscopy (SEM), trans-
mission electron microscopy (TEM), thermal gravimetric (TG)
and derivative thermal gravimetric (DTG) analysis. X-ray
diffraction (XRD) patterns of catalyst were attained on a APD
2000, Ital structure with Cu Ka radiation (k ¼ 0.1542 nm)
operating at 50 kV and 20 mA in a 2 h range of 5–90ꢀ with step
size 0.01ꢀ and time step 1.0 s to assess the crystallinity of the
catalyst. Fourier transform-infrared spectra of the samples were
recorded on a Perkin-Elmer FT-IR spectrometer 17259 using
KBr disks. Thermal gravimetric analyses by a Perkin-Elmer TGA
were achieved on catalyst. The SEM analyses were performed
with a TESCAN/MIRA and a maximum acceleration voltage of
the primary electrons between 10 and 15 kV. Transmission
electron microscope (TEM) measurements were carried out on
a Philips CM10 analyzer.
Spectral data analysis for compounds
2-(1-Phenylprop-1-en-2-yl)-1H-benzo[d]imidazole (Table 3,
ꢀ
entry 2). Orange solid; mp: 229–231 C; yield: 87%; IR (KBr): n
1
3417, 3067, 2994, 2973, 1636, 1619, 1449, 1417, 1279 cmꢁ1; H
NMR (400 MHz, DMSO-d6): dppm 2.41 (s, 3H, –CH3), 3.40 (s, 1H,
–CH), 7.23 (t, 3H, J ¼ 7.6 Hz, ArH), 7.38 (d, 1H, J ¼ 7.6 Hz, ArH),
7.48 (t, 2H, J ¼ 7.6 Hz, ArH), 7.52 (d, 2H, J ¼ 7.6 Hz, ArH), 7.58 (s,
1H, –NH), 7.63 (d, 1H, J ¼ 7.6 Hz, ArH); 13C NMR (100 MHz,
DMSO-d6): dppm 15.1, 110.9, 111.0, 118.7, 121.4, 122.5, 127.4,
127.5, 128.1, 128.5, 128.8, 129.2, 130.2, 136.3, 143.4; MS: m/z ¼
234 [M]+.
General procedure for the synthesis of 2,6-dimethyl-1-
nitropyridin-1-ium trinitromethanide [2,6-DMPy-NO2]C(NO2)3
as nano structure molten salt (NMS) catalyst
2-([1,10-Biphenyl]-4-yl)-1H-benzo[d]imidazole (Table 3, entry
ꢀ
To a round-bottomed ask (50 mL) containing 2,6-dime- 3). Yellow solid; mp: 213–215 C; yield: 94%; IR (KBr): n 3462,
1
thylpyridine (3 mmol; 0.322 g) in CH3CN (5 mL), was added 3366, 3025, 1595, 1486, 1371, 1265, 1135 cmꢁ1; H NMR (400
tetranitromethane (3 mmol; 0.588 g) drop wise and stirred over MHz, DMSO-d6): dppm 6.59 (t, 1H, J ¼ 7.6 Hz, ArH), 6.74 (d, 1H, J
a period of 120 min at room temperature. Then, the solvent was ¼ 6.8 Hz, ArH), 7.00 (t, 1H, J ¼ 8.4 Hz, ArH), 7.17 (d, 1H, J ¼ 8.0
removed by distillation under reduced pressure and the product Hz, ArH), 7.43 (t, 1H, J ¼ 7.4 Hz, ArH), 7.53 (t, 2H, J ¼ 7.4 Hz,
ꢀ
was dried under vacuum at 80 C for 120 minutes. The yellow ArH), 7.77 (d, 2H, J ¼ 8.0 Hz, ArH), 7.83 (d, 2H, J ¼ 8.4 Hz, ArH),
solid product was ltered, washed with diethyl ether for three 8.09 (d, 2H, J ¼ 8.4 Hz, ArH), 8.71 (s, 1H, –NH); 13C NMR (100
times, and then dried under vacuum conditions. [2,6-DMPy- MHz, DMSO-d6): dppm 114.6, 116.1, 116.9, 126.7, 126.8, 127.6,
NO2]C(NO2)3 was identied by FT-IR, 1H NMR, 13C NMR, mass, 127.9, 129.0, 129.1, 135.6, 139.3, 142.3, 143.8, 155.7; MS: m/z ¼
thermal gravimetry (TG), derivative thermal gravimetric (DTG), 270 [M]+.
X-ray diffraction patterns (XRD), scanning electron microscopy
(SEM) and transmission electron microscopy (TEM) analysis 5). White solid; mp: 208–210 C; yield: 85%; IR (KBr): n 3350,
(Scheme 1).
4-(1H-Benzo[d]imidazol-2-yl)-2-ethoxyphenol (Table 3, entry
ꢀ
1
3050, 2981, 1593, 1501, 1452, 1271 cmꢁ1; H NMR (400 MHz,
2,6-Dimethyl-1-nitropyridin-1-ium trinitromethanide [2,6- DMSO-d6): dppm 3.94 (t, 3H, J ¼ 7.6 Hz, –CH3), 3.94 (q, 2H, J ¼ 7.2
DMPy-NO2]C(NO2)3. Yellow solid; mp: 145–147 ꢀC; yield: 96% Hz, –CH2), 5.42 (s, 1H, –OH), 6.39 (d, 1H, J ¼ 8.0 Hz, ArH), 6.63
(0.873 g); IR (KBr): n 3511, 3090, 3000, 1662, 1389, 1350, 1175, (s, 1H, ArH), 6.68 (t, 1H, J ¼ 8.2 Hz, ArH), 6.94 (d, 1H, J ¼ 8.0 Hz,
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ArH), 7.17 (s, 1H, –NH), 7.22 (d, 1H, J ¼ 7.6 Hz, ArH), 7.48 (d, 1H,
J ¼ 7.6 Hz, ArH), 7.67 (t, 1H, J ¼ 9.2 Hz, ArH); 13C NMR (100
MHz, DMSO-d6): dppm 14.5, 63.7, 110.8, 111.9, 114.0, 115.5,
118.5, 122.0, 127.8, 135.9, 142.5, 146.5, 146.6, 148.2, 153.4; MS:
m/z ¼ 254 [M]+.
2-(1H-Indol-3-yl)-1H-benzo[d]imidazole (Table 3, entry 6).
Brown solid; mp: 299–301 ꢀC; yield: 83%; IR (KBr): n 3484, 3167,
1
3113, 3042, 2979, 1634, 1576, 1497, 1393, 1243 cmꢁ1; H NMR
(400 MHz, DMSO-d6): dppm 7.20 (s, 1H, ArH), 7.29 (t, 4H, J ¼ 7.2
Hz, ArH), 7.52 (d, 2H, J ¼ 7.6 Hz, ArH), 8.10 (d, 2H, J ¼ 7.2 Hz,
ArH), 8.29 (s, 1H, –NH), 9.93 (s, 1H, –NH); 13C NMR (100 MHz,
DMSO-d6): dppm 114.6, 116.1, 116.9, 123.8, 126.7, 127.4, 127.8,
128.2, 128.5, 130.6, 132.7, 134.2, 134.3, 135.0, 144.0; MS: m/z ¼
233 [M]+.
Scheme 2 Synthesis of 2-substituted benz-(imida, oxa and othia)-
zoles by [2,6-DMPy-NO2]C(NO2)3 as a NMS catalyst.
2-(Pyridin-4-yl)-1H-benzo[d]imidazole (Table 3, entry 8).
Orange solid; mp: 203–205 ꢀC; yield: 87%; IR (KBr): n 3350, 3050,
2981, 1593, 1501, 1452, 1271 cmꢁ1; 1H NMR (400 MHz, DMSO-
d6): dppm 6.06 (d, 3H, J ¼ 6.0 Hz, ArH), 7.22 (d, 3H, J ¼ 9.2 Hz,
ArH), 7.27 (t, 1H, J ¼ 6.0 Hz, ArH), 7.79 (t, 1H, J ¼ 6.8 Hz, ArH),
8.81 (s, 1H, –NH); 13C NMR (100 MHz, DMSO-d6): dppm 111.8,
113.6, 114.4, 116.3, 117.2, 118.5, 127.1, 127.3, 139.3, 152.3,
162.7; MS: m/z ¼ 195 [M]+.
synthesis of 2-sbstituted benz-(imida, oxa and othia)-zoles.
Reaction was proceeded by using 1,2-phenylenediamine or
2-aminophenol or 2-aminothiophenol and corresponding
aldehydes at room temperature under solvent-free conditions
respectively (Scheme 2).
4-(1H-Benzo[d]imidazol-2-yl)benzene-1,2-diol (Table 3, entry
9). Pink solid; mp: 243–245 ꢀC; yield: 85%; IR (KBr): n 3412,
Characterization of 2,6-dimethyl-1-nitropyridin-1-ium
trinitromethanide [2,6-DMPy-NO2]C(NO2)3 as a NMS catalyst
1693, 1610, 1451, 1285 cmꢁ1 1H NMR (400 MHz, DMSO-d6):
;
dppm 5.35 (s, 1H, –NH), 6.35 (d, 1H, J ¼ 8.0 Hz, ArH), 6.39 (s, 1H,
ArH), 6.65 (d, 1H, J ¼ 8.0 Hz, ArH), 6.85 (d, 1H, J ¼ 8.4 Hz, ArH),
7.00 (d, 1H, J ¼ 8.0 Hz, ArH), 7.20 (t, 2H, J ¼ 7.6 Hz, ArH), 7.35 (s,
1H, –OH), 7.65 (s, 1H, –OH); 13C NMR (100 MHz, DMSO-d6):
dppm 110.9, 113.4, 115.4, 115.5, 116.5, 116.6, 117.0, 118.7, 120.3,
121.8, 122.0, 127.7, 132.2; MS: m/z ¼ 226 [M]+.
Characterization of [2,6-DMPy-NO2]C(NO2)3 as a nano molten
salt catalyst was approved using FT-IR, 1H NMR, 13C NMR,
mass, TG, DTG, XRD, SEM and TEM analysis.
In the FT-IR spectrum of [2,6-DMPy-NO2]C(NO2)3 the
absorption bond at 1662 cmꢁ1 and 1389 cmꢁ1 is connected to
vibrational modes of –NO2 bonds. Furthermore, the known
peak at 3090 cmꢁ1 related to C–H stretching group on 2,6-
dimethyl-1-nitropyridin-1-ium ring. Also, the known peak at
3000 cmꢁ1 linked to C–H stretching of methyl group on 2,6-
dimethyl-1-nitropyridin-1-ium ring. The IR spectrum changes of
[2,6-DMPy-NO2]C(NO2)3 in comparison with 2,6-dimethylpyr-
idin and tetranitromethane displayed formation of NMS
catalyst (Fig. 1).
Computational details
Computations were performed using the Gaussian09
program.30 Density functional theory has been used to investi-
gate the reactions of 2-phenyl benz-(imida, oxa and othia)-zole
compounds (7) in the presence of HNO3 and CH(NO2)3 which
in situ have been generated from NMS catalyst. All geometry
optimizations were performed at B3LYP/TZVP level of theory.
Frequency calculations at the same level of theory have also
been performed to identify all of the stationary points as
minima with no imaginary frequencies or transition structures
with one imaginary frequency. In addition, the nature of tran-
sition structures was conrmed by intrinsic reaction coordi-
nates (IRC).31 In order to investigate the mechanism we used the
total electronic energy (Eel + ZPE) and the Gibbs free energies.
The intramolecular interactions were calculated on the basis of
natural bond orbital (NBO)32 analyses.
Results and discussion
In continuation of our studies on development of nano-
structured ionic liquids (NILs),33 molten salts (NSMSs)34,35 in
organic synthesis, herein we wish to introduce a novel nano
molten salt catalyst, namely 2,6-dimethyl-1-nitropyridin-1-ium
Fig. 1 The IR spectrum of tetranitromethane (a), 2,6-dimethylpyridine
trinitromethanide [2,6-DMPy-NO2]C(NO2)3 (Scheme 1) for the (b) and [2,6-DMPy-NO2]C(NO2)3 (c).
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Fig. 4 The mass spectrum of [2,6-DMPy-NO2]C(NO2)3.
Fig. 2 The 1H NMR spectrum of [2,6-DMPy-NO2]C(NO2)3.
The 1H NMR spectrum of the [2,6-DMPy-NO2]C(NO2)3 shows
a triplet at 8.39 ppm and a doublet at 7.75 ppm related to
hydrogens on the aromatic ring of 2,6-dimethyl-1-nitropyridin-
1-ium. Also a singlet at 2.70 ppm linked to hydrogens methyl
group on the aromatic ring of 2,6-dimethyl-1-nitropyridin-1-
ium. The 1H NMR chemical shi changes of [2,6-DMPy-NO2]
C(NO2)3 in comparison with 2,6-dimethylpyridine displayed
creation of described NMS catalyst (Fig. 2).
Appearance of ve signals in the 13C NMR spectrum is in
accordance with the [2,6-DMPy-NO2]C(NO2)3 structure. The
important peak of 13C NMR spectra of NMS catalyst is related to
the –C(NO2)3 group on trinitromethanide counter ion which is
identied at d ¼ 97.4 ppm and corresponded peaks at 139.3,
160.0 and 165.9 ppm are linked to aromatic carbons of 2,6-
dimethyl-1-nitropyridin-1-ium ring. Also a peak at 18.2 ppm
connected to methyl group on the aromatic ring of 2,6-dimethyl-
1-nitropyridin-1-ium. Furthermore the 13C NMR chemical shi
Fig. 5 The thermal gravimetric (TG) (a) and derivative thermal gravi-
metric (DTG) (b) analysis of [2,6-DMPy-NO2]C(NO2)3.
changes of [2,6-DMPy-NO2]C(NO2)3 in comparison with 2,6-
dimethylpyridine and tetranitromethane obviously revealed
preparation of the synthesized NMS catalyst (Fig. 3).
The mass spectrum of the [2,6-DMPy-NO2]C(NO2)3 is in
agreement with the structure of the NMS catalyst and showed
Fig. 3 The 13C NMR spectrum of [2,6-DMPy-NO2]C(NO2)3.
58670 | RSC Adv., 2016, 6, 58667–58679
Fig. 6 The XRD pattern of {[2,6-DMPy-NO2]C(NO2)3}.
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the parent peak at 303 m/z. The noteworthy peak of mass
spectrum of NMS catalyst is linked to trinitromethanide counter
ion which is known at 150 m/z and connected peak at 153 m/z is
related to 2,6-dimethyl-1-nitropyridin-1-ium ring (Fig. 4).
The thermal gravimetric (TG) analysis and derivative thermal
gravimetric (DTG) of [2,6-DMPy-NO2]C(NO2)3 as a NMS catalyst
ꢀ
were also studied at range of 25 to 550 C, with a temperature
increase rate of 10 ꢀC minꢁ1 in a nitrogen atmosphere. The
linked diagrams are displayed in Fig. 5. The rst weight loss curve
around 90–110 ꢀC (3%) is related to the residual physisorbed
water and organic solvents, which were applied in the catalyst
synthesis. The signicant weight loss of the NMS catalyst was
occurred above 187 ꢀC, which can be suitable for the catalytic
uses in organic synthesis.ꢀThe considerable weight loss happened
in the range of 110–187 C, is the main loss of [2,6-DMPy-NO2]
C(NO2)3. The thermal gravimetric (TG) analysis and derivative
thermal gravimetric (DTG) of the NMS catalyst presented
important loss in one step, and decomposed above 187 ꢀC.
Size, shape and morphology of 2,6-dimethyl-1-nitropyridin-1-
ium trinitromethanide [2,6-DMPy-NO2]C(NO2)3 as a mild NMS
catalyst was investigated by X-ray diffraction (XRD) pattern
(Fig. 6), scanning electron microscopy (SEM) and transmission
electron microscopy (TEM) (Fig. 7). To conrm the structure of
[2,6-DMPy-NO2]C(NO2)3, initially its XRD pattern was considered.
As shown in Fig. 6, the XRD patterns of NMS catalyst expose
peaks at 2q z 12.00ꢀ, which was approved using the described
value of scanning electron microscopy (SEM) and transmission
electron microscopy (TEM) (Fig. 7). Peak width (FWHM), size and
inter planer distance related to XRD pattern of [2,6-DMPy-NO2]
C(NO2)3 were investigated in the 12.00ꢀ degree. The average
crystallite size D was studied by the Scherrer formula: D ¼ Kl/
(b cos q), where K is the Scherrer constant, l being the X-ray
wavelength, b is the half-maximum peak width, and q is the
Bragg diffraction angle. Consequently, the average size of the
NMS catalyst achieved from this equation was found to be about
79.91 nm (2q ¼ 12.00ꢀ; peak width [FWHM] ¼ 0.10 degree; size ¼
79.91 nm; inter planer distance ¼ 0.736645 nm), which is mainly
in a good accordance with the scanning electron microscopy and
transmission electron microscopy (Fig. 7). The results of TEM
histogram of [2,6-DMPy-NO2]C(NO2)3 clearly shows that particle
size of NMS catalyst is scattered mainly between 50 and 75 nm.
Fig. 7 Scanning electron microscopy (SEM) (a), transmission electron
microscopy (TEM) (b and c) and TEM histogram (d) of [2,6-DMPy-NO2]
C(NO2)3.
Table
1 Optimization reaction between biphenyl-4-carbaldehyde
and 1,2-phenylenediaminea
Catalyst amount
Entry (mol%)
Temperature (ꢀC) Time (min) Yieldb (%)
1
2
3
4
5
6
7
8
—
—
0.5
0.5
1
1
1
1
2
r.t.
100
r.t.
100
r.t.
50
120
120
30
30
15
15
15
15
15
15
15
15
7
11
71
71
94
94
94
94
94
94
94
94
75
100
r.t.
100
r.t.
100
9
10
11
12
2
5
5
Catalytic application of 2,6-dimethyl-1-nitropyridin-1-ium
trinitromethanide [2,6-DMPy-NO2]C(NO2)3 as a NMS catalyst
a
Reaction condition: biphenyl-4-carbaldehyde (1 mmol, 0.182 g), 1,2-
b
phenylenediamine (1 mmol, 0.108 g). Reaction condition: isolated
Initially, to optimize the reaction conditions, the condensation
reaction between biphenyl-4-carbaldehyde and 1,2-phenyl-
enediamine was selected as a model and different amounts of
yield.
Table 2 Solvent effect in the reaction between biphenyl-4-carbaldehyde and 1,2-phenylenediaminea
Solvent
Solvent-free
H2O
C2H5OH
CH3CN
CH2Cl2
CH3CO2Et
n-Hexane
Reaction time (min)
Yield (%)b
15
94
30
91
15
94
30
93
30
89
45
83
60
74
a
Reaction condition: biphenyl-4-carbaldehyde (1 mmol, 0.182 g), 1,2-phenylenediamine (1 mmol, 0.108 g), NMS catalyst (1 mol%, 0.003 g), solvent
(2 mL). b Reaction condition: isolated yield.
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Table 3 Synthesis of 2-aryl benz-(imida, oxa and othia)-zole derivatives by using [2,6-DMPy-NO2]C(NO2)3 as an efficient catalyst under solvent-
free conditionsa
Entry
Product
Time (min)
Yieldb (%)
Mp (ꢀC) [Lit.]Ref.
1
15
94
263–265 [270–272]36a (orange solid)
2
25
87
229–231 (orange solid)
3
4
15
15
94
93
213–215 (yellow solid)
159–161 (ref. 36b) (yellow solid)
5
6
30
30
85
83
208–210 (white solid)
299–301 (brown solid)
7
8
9
30
25
30
85
87
85
278–280 (ref. 36c) (yellow solid)
203–205 (orange solid)
243–245 (pink solid)
10
10
96
313–315 [315–317]36d (yellow solid)
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Table 3 (Contd.)
Entry
Product
Time (min)
Yieldb (%)
Mp (ꢀC) [Lit.]Ref.
11
20
92
225–227 [231–233]36d (grey solid)
173–175 [175–177]36e (brown solid)
267–269 [268–270]36d (yellow solid)
237–239 (ref. 36b) (yellow solid)
12
13
14
25
20
20
90
92
92
15
25
88
203–205 [200–201]36f (yellow solid)
16
17
18
25
25
15
87
87
95
290–292 [283–285]36g (yellow solid)
342–344 [>300]36f (yellow solid)
291–293 [290–292]36d (yellow solid)
19
20
25
20
88
93
235–237 [238–240]36g (yellow solid)
261–263 [265–267]36h (yellow solid)
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Table 3 (Contd.)
Entry
Product
Time (min)
Yieldb (%)
Mp (ꢀC) [Lit.]Ref.
21
25
91
147–149 [143–145]36h (yellow solid)
221–223 [224–225]36i (yellow solid)
22
25
30
91
90
23
121–123 [116–118]36i (yellow solid)
a
Reaction condition: aldehyde (1 mmol), 1,2-phenylenediaime or 2-aminophenol or 2-aminothiophenol (1 mmol), NMS catalyst (1 mol%).
Reaction condition: isolated yield.
b
ꢀ
NMS catalyst at range of room temperature up to 100 C were a series of aromatic aldehydes underwent electrophilic substi-
investigated under solvent-free conditions (Table 1). As dis- tution reaction with varied range of 2-aryl benz-(imida, oxa and
played in Table 1, the best results were attained when the othia)-zole derivatives in good to excellent yields. The nature
reaction was achieved using 1 mol% of NMS catalyst at room and electronic properties of the substituents on the aromatic
temperature (Table 1, entry 5). No improvement was identied ring affect the conversion rate, and aromatic aldehydes count-
in the yield of reaction via increasing the amount of the NMS ing electron-withdrawing groups on the aromatic ring react
catalyst and temperature (Table 1, entries 6–12). Table 1 clearly faster than electron-releasing groups.
shows that in the absence of NMS catalyst, the product was
attained in low yield aer 120 min (Table 1, entries 1 and 2).
According to the previously reported method,28,29,35 an
appropriate mechanism for the synthesis of 2-aryl benz-(imida,
Also, to compare the result of the solution in comparison oxa and othia)-zole derivatives (3) using [2,6-DMPy-NO2]C(NO2)3
with solvent-free conditions, mixture of biphenyl-4- as a NMS catalyst was suggested (Scheme 3). Firstly, 1,2-phe-
a
carbaldehyde and 1,2-phenylenediamine as a typical reaction, nylenediaime or 2-aminophenol or 2-aminothiophenol (2)
by 1 mol% of NMS catalyst in various solvents for example H2O, attack to aldehyde (4), which is activated with the [2,6-DMPy-
C2H5OH, CH3CN, CH2Cl2, CH3CO2Et and n-hexane were studied NO2]C(NO2)3 as a NMS catalyst to form imine intermediate (6)
at room temperature. From Table 2 it can be identied that this by removing one molecule of HNO3. Then, intermediate (7) is
reaction under solvent-free conditions is obviously the best produced via intra-molecular cyclization and proton exchange
choice. However, in this research solvent-free condition is of imine intermediate (6). Finally, intermediate (7) via anomeric
preferred to water and/or ethanol since solvent-free condition is based oxidation was transferred to 2-aryl benz-(imida, oxa and
green, mild, safe and cheap in comparison with solution. The othia)-zoles (3).36 Previous studies have proposed aerobic auto
obtained results (yield of product and reaction time) in polar oxidation for conversion of intermediate (7) to its correspond-
solvent were more enhanced than nonpolar solvent due to ionic ing 2-aryl benz-(imida, oxa and othia)-zoles (3).37 In contrast to
structure of NMS catalyst (the catalytic activity of NMS catalyst the previously reported mechanistic explanation for the nal
was improved).
step of the above described organic synthesis,37 we believed
Aer nding the optimized reaction conditions, the inves- that, this step might be a progress by uncommon hydride
tigation was proceeded by completing the reaction between transfer as well as Cannizzaro reaction (Scheme 4)38 and H2
a series of aromatic aldehydes and 1,2-phenylenediaime or 2- releasing from tricyclic orthoamide (Scheme 5).39 Recently, we
aminophenol or 2-aminothiophenol. To show the common have suggested an anomeric based oxidation for the nal step
applicability of this procedure, various aromatic and hetero- for the synthesis of 1,4-dihydropyrano-[2,3-c]-pyrazole,28a 2,4,6-
aromatic aldehydes were capably reacted in the same condi- triarylpyridine derivatives28b and 2-amino-3-cyanopyridines29
tions. These results stimulated us to study briey the capability (Schemes 6–8). To achieve this goal, reaction was performed
of this new process for several aromatic and hetero-aromatic under nitrogen atmosphere and in the absence of any molecular
aldehydes under optimized conditions. As shown in Table 3, oxygen. We observed that, the reaction progressed in the
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Scheme 6 Synthesis of 1,4-dihydropyrano-[2,3-c]-pyrazole deriva-
tives via ABO.28a
Scheme 3 The proposed mechanism for the synthesis of 2-aryl benz-
(imida, oxa and othia)-zole derivatives by {[2,6-DMPy-NO2]C(NO2)3} as
a NMS catalyst.
Scheme 7 The synthesis of 2,4,6-triarylpyridines through ABO.28b
absence of any oxygen molecules and under nitrogen atmo-
sphere. This fact approves that the oxidation of intermediate (7)
to its corresponding 2-aryl benz-(imida, oxa and othia)-zoles (3)
did not need the oxygen in the air. By studying the above-
mentioned evidence, conversion of intermediate (7) to its cor-
responding products (3) might be carried out using unusual
hydride transfer and releasing of molecular hydrogen (H2). The
C–H bond is so weakened via electron donation from the
nitrogen's lone pairs into the anti-bonding of C–H (s*C–H
orbital) which can be broken via reaction with a proton to afford
molecular hydrogen. Most recently, we introduced a new term
for this phenomenon entitled “anomeric based oxidation”
Scheme
4 The proposed mechanism for the in situ oxidation–
reduction in Cannizzaro reaction through unusual hydride transfer via
ABO.38
Scheme 5 A striking example which had been observed for an unusual
hydride transfer from tricyclic orthoamide (A) through ABO.39
Scheme 8 The synthesis of 2-amino-3-cyanopyridines by ABO.29
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the formation of transition structures, TS-N, TS-O and TS-S, and
then removing the molecular hydrogen (H2) the intermediate 8
will be formed for all of 2-phenyl benz-(imida, oxa and othia)-
zole compounds 3. According to values of calculated Gibbs
free energies, this reaction for intermediate 8 is about ꢁ2.68
kcal molꢁ1 exothermic but for oxazole and thiazole intermedi-
ates 8 are respectively 8.32 and 11.32 kcal molꢁ1 endothermic.
In the nal step of the reaction the intermediate 8 converts
into 2-phenyl benz-(imida, oxa and othia)-zole (3) through an
exothermic process (DG ¼ ꢁ13.00, ꢁ19.75 and ꢁ20.93 kcal
molꢁ1 for benzimidazole, benzoxazole and benzothiazole,
respectively). In conclusion, the whole process of conversion of
7 to 3 through the releasing molecular hydrogen for all of 2-
phenyl benz-(imida, oxa and othia)-zole is exothermic (DG ¼
ꢁ15.92, ꢁ11.44 and ꢁ9.81 kcal molꢁ1, respectively).
Scheme
9 Different conformers considered for 2-phenyl benz-
(imida, oxa and othia)-zole intermediates 7.
(ABO). The major reason of ABO is driving force of aromatiza-
tion which will be supported via stereo electronic and/or
anomeric effect.
Intriguingly, we found that the released HNO3 during the
formation of intermediate 6, operates similar to CH(NO2)3
moiety of NMS catalyst, so the formation of 2-phenyl benz-
(imida, oxa and othia)-zole compounds occurs using release of
hydrogen molecule (Fig. 9). Comparison of the energy proles
given in Fig. 8 and 9 shows that there are similar amounts of
DG‡ for both CH(NO2)3 and HNO3 groups in all reactions.
However, in the presence of HNO3, in contrast to CH(NO2)3, the
nal step that corresponds to the formation of 2-phenyl benz-
(imida and oxa)-zole 3 through intermediate 8 is endothermic
(DG ¼ 1.89, 0.11 and ꢁ1.89 kcal molꢁ1 for benzimidazole,
benzoxazole and benzothiazole, respectively).
Density functional theory has been used to investigate the
“anomeric based oxidation” (ABO) for the nal step in mecha-
nistic process for the synthesis of 2-phenyl benz-(imida, oxa and
othia)-zole. Starting from 7, three different orientations of two
N–H and adjacent C–H groups on imidazole intermediate 7 with
respect to each other and also two different orientations of
adjacent N–H and C–H groups on oxazole and thiazole inter-
mediates 7 give a to e conformers (Scheme 9). Our calculations
show that the difference in stability between the conformers of
one dened structure is less than 0.6 kcal molꢁ1 for all of the
compounds (for more details see ESI†). So, Fig. 8 shows energy
prole only for conformer a of imidazole intermediate 7 and
d of oxazole and thiazole intermediates 7. As illustrated in
Fig. 8, by addition of CH(NO2)3 moiety of NMS catalyst through
The important step of anomeric based oxidation is the direct
elimination of H2 molecule from intermediate 7 and CH(NO2)3
or HNO3 catalyst (Fig. 8 and 9) in which the C–H bond in
Fig. 8 Energy profile calculated for synthesis of 2-phenyl benz-(imida, Fig. 9 Energy profile calculated for synthesis of 2-phenyl benz-(imida,
oxa and othia)-zole (3) by CH(NO2)3 moiety of NMS catalyst beginning oxa and othia)-zole (3) by HNO3 beginning from compound 7 (see
from compound 7 (see Scheme 4). The relative Gibbs free energies and Scheme 4). The relative Gibbs free energies and the total electronic
the total electronic energies (Eel + ZPE, figures in parentheses) ob- energies (Eel + ZPE, figures in parentheses) obtained from the B3LYP/
tained from the B3LYP/TZVP calculations both are given in kcal molꢁ1
.
TZVP calculations both are given in kcal molꢁ1
.
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Table 4 The main second order perturbation energies (kcal molꢁ1) calculated for different conformers of imidazole, oxazole and thiazole
intermediates 7
LP N1 /
s*C–H
LP X /
s*C–H X ¼ N, O and S
Compounds
Conformer
Total
a
b
c
d
e
d
e
6.35
4.92
5.09
5.84
4.39
7.48
5.09
6.35
4.92
4.84
7.06
6.30
3.88
3.18
12.70
9.84
9.95
12.90
10.69
11.36
8.27
Imidazole
Oxazole
Thiazole
compound 7 is so weakened due to the delocalization of lone from other materials (the product is soluble in hot ethyl acetate
pair of X atoms (X ¼ N, O or S atoms) to s*C–H bond (the and NMS catalyst is soluble in water). The aqueous layer was
anomeric effect) and therefore the activation energy is relatively decanted, separated and applied for alternative reaction aer
minor. The NBO analysis of donor–acceptor interactions which removing water. It is recognized that the catalytic activity of the
are shown in Table 4 identied that the anomeric effect due to NMS catalyst was restored within the limits of the experimental
delocalization of lone pair of X atoms to s*C–H bond for errors for ve continuous runs. The recycled catalyst was also
conformer a of imidazole and conformer d of oxazole and characterized by FT-IR spectrum aer its application in the
thiazole intermediates 7 are maximum and are 12.70, 12.90 and reaction. This spectrum was the same as those of the fresh
11.36 kcal molꢁ1, respectively.
catalyst. The deactivation of the NMS catalyst is low. The reac-
Thus the above theoretical studies support our suggested tion was scaled up to 10 mmol of biphenyl-4-carbaldehyde and
mechanism and shows that releasing molecular hydrogen (H2) 1,2-phenylenediamine using 10 mol% of NMS catalyst at room
is quite possible in such systems. The Cartesian atomic coor- temperature. The yield of the reaction was 94% aer 15 min and
dinates of all compounds involved in our suggested mechanism 84% aer the fourth run. The results were summarized in
are available in ESI.† In theoretical calculations, the solvent Fig. 10.
effect was not investigated, because the reaction was performed
under solvent-free conditions.
Also in using [2,6-DMPy-NO2]C(NO2)3 as a NMS catalyst in
the synthesis of benzimidazole (a), benzoxazole (b) and benzo-
The reusability of the [2,6-DMPy-NO2]C(NO2)3 as a NMS thiazole (c) derivatives, we investigated the efficacy of various
catalyst is an important advantage for any catalytic system. For catalysts for approving suggested mechanism and ABO. To
this purpose, the reaction between biphenyl-4-carbaldehyde optimize the reaction conditions, the reaction between 4-
and 1,2-phenylenediamine was selected as a typical reaction chlorobenzaldehyde and 1,2-phenylenediaime or 2-amino-
by NMS catalyst. Aer completion of the reaction (identied by phenol or 2-aminothiophenol under N2 atmosphere at room
TLC), ethyl acetate was added to the reaction mixture and temperature was applied as a typical procedure (Table 5).
heated to extract product from remained starting materials.
This solution was washed with water to separate NMS catalyst
Table 5 The synthesis of benzimidazole, benzoxazole and benzo-
thiazole derivatives by various catalysts for approving of suggested
mechanism and ABOa
Time (min)
Yieldb (%)
Catalyst
loading
Entry Catalyst
a
b
c
a
b
c
1
2
NMS catalyst
HBF4
1 mol%
5 mol%
15 25 30 95 91 90
30 45 60 87 72 70
3
4
5
6
Fe(HSO4)3
Ca(HSO4)2
Zn(HSO4)2
Oxone
10 mol% 30 45 60 73 55 51
10 mol% 30 45 60 45 31 28
10 mol% 30 45 60 51 37 33
5 mol%
30 45 60 57 42 40
7
Ce(HSO4)3$7H2O 10 mol% 30 45 60 63 47 45
8
9
10
11
Al(HSO4)3
Bi(HSO4)3
CeO2
10 mol% 30 45 60 58 44 41
10 mol% 30 45 60 48 35 30
10 mol% 30 45 60 67 53 49
10 mol% 30 45 60 41 36 32
PbO2
a
Fig. 10 Reusability investigations of [2,6-DMPy-NO2]C(NO2)3 as
a NMS catalyst for the reaction between biphenyl-4-carbaldehyde and
1,2-phenylenediamine in 15 min.
Reaction conditions: 4-chlorobenzaldehyde (1 mmol) and 1,2-
phenylenediaime, 2-aminophenol and 2-aminothiophenol (1 mmol),
under N2 atmosphere, r.t. b Reaction conditions: isolate yield.
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Table 6 Comparison of the results in the synthesis of typical product by [2,6-DMPy-NO2]C(NO2)3 as a NMS catalyst with those reported catalysts
in the literature
Entry
Reaction condition
Catalyst loading
Time (min)
Yield (%)
Ref.
1
2
3
4
5
6
7
8
NMS catalyst, solvent-free, r.t.
[Msim]Cl, EtOAc, 60 ꢀC
1 mol%
10 mol%
10 mol%
0.1 g
0.1 mmol
5 mol%
5 mol%
5 : 2 mL
3 wt%
5 mol%
0.01 mmol
0.2 mmol
20 mL
15
13
8
15
30
300
45
180
60
90
60
95
89
93
93
89
90
88
75
94
94
78
84
46
This work
36d
36d
36b
40a
40b
36a
40c
36f
36c
[Dsim]Cl, EtOAc, 60 ꢀ
Boric acid, H2O, r.t.
BF3$OEt2, r.t.
C
Cu/C, EtOH, r.t.
TBAF, H2O, ultrasonic irradiation
Glycerol : H2O, 90 ꢀ
Scolecite, EtOH, 90 ꢀ
CAN, PEG, 50 ꢀ
CuSO4, EtOH, 60 ꢀ
C
C
9
10
11
12
13
C
C
40d
40e
40f
H5IO6–SiO2, CAN, r.t.
Acetic acid, reux
20
240
To compare the efficiency of described catalyst with some of
those reported catalysts by other researchers in the literature for
the synthesis of benzimidazole derivatives, we have presented
the results of those reported catalysts for the condensation of 4-
chlorobenzaldehyde and 1,2-phenylenediaime in Table 6. As
Table 6 presents, the [2,6-DMPy-NO2]C(NO2)3 as a NMS catalyst
has signicantly improved the synthesis of product in different
terms.
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In summary, naval, mild and recyclable 2,6-dimethyl-1-
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nanostructured molten salt (NMS) was designed, synthesized and
1
fully characterized by FT-IR, H NMR, 13C NMR, mass, thermal
gravimetric (TG), derivative thermal gravimetric (DTG), X-ray
diffraction patterns (XRD), scanning electron microscopy (SEM)
and transmission electron microscopy (TEM) analysis. Catalytic
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synthesis of 2-aryl benz-(imida, oxa and othia)-zole derivatives via
the condensation reaction between various aromatic aldehyde
and 1,2-phenylenediamine or 2-aminophenol or 2-amino-
thiophenol at room temperature under solvent-free conditions.
The advantages of this investigation are simplicity of product
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reusability of the NMS catalyst. A mechanistic method was also
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Acknowledgements
We thank Bu-Ali Sina University, Iran National Science Foun-
dation (INSF) (The Grant of Allameh Tabataba'i's Award, Grant
Number BN093) and National Elites Foundation for nancial
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