One-pot synthesis of 2-arylbenzimidazole, 2-arylbenzothiazole and 2-arylbenzoxazole derivatives using vanadium(IV)-salen complex as homogeneous catalyst and vanadium(IV)-salen complex nanoparticles immobilized onto silica as a heterogeneous nanocatalyst

Article abstract of DOI:10.1007/s13738-011-0045-4

The efficient synthesis of 2-arylbenzimidazole, 2-arylbenzothiazole and 2-arylbenzoxazole derivatives is described by condensation of aryl aldehydes and o-phenylenediamines, 2-aminothiophenol and 2-aminophenol in a single pot using a catalytic amount of vanadium(IV)-salen complex or vanadium-salen nanoparticles supported on silica (5.0 mol%) in excellent isolated yields. The immobilized catalyst was characterized by powder X-ray diffraction, scanning electron microscopy, atomic force microscopy, transmission electron microscopy, inductively coupled plasma analysis, thermogravimetric instrument for analysis of nitrogen adsorption and FT-IR spectroscopy.

Full text of DOI:10.1007/s13738-011-0045-4

J IRAN CHEM SOC (2012) 9:189–204
DOI 10.1007/s13738-011-0045-4
ORIGINAL PAPER
One-pot synthesis of 2-arylbenzimidazole, 2-arylbenzothiazole
and 2-arylbenzoxazole derivatives using vanadium(IV)–salen
complex as homogeneous catalyst and vanadium(IV)–salen
complex nanoparticles immobilized onto silica as a heterogeneous
nanocatalyst
•
Hashem Sharghi Mahdi Aberi Mohammad Mahdi Doroodmand
•
Received: 4 June 2011 / Accepted: 14 September 2011 / Published online: 10 January 2012
Ó Iranian Chemical Society 2012
Abstract The efficient synthesis of 2-arylbenzimidazole,
2-arylbenzothiazole and 2-arylbenzoxazole derivatives is
described by condensation of aryl aldehydes and o-pheny-
lenediamines, 2-aminothiophenol and 2-aminophenol in a
single pot using a catalytic amount of vanadium(IV)–salen
complex or vanadium–salen nanoparticles supported on
silica (5.0 mol%) in excellent isolated yields. The immo-
bilized catalyst was characterized by powder X-ray dif-
fraction, scanning electron microscopy, atomic force
microscopy, transmission electron microscopy, inductively
coupled plasma analysis, thermogravimetric instrument for
analysis of nitrogen adsorption and FT-IR spectroscopy.
as an anticancer agent [9], an orexin-1 receptor antagonist
[10], a HIV reverse transcriptase inhibitor [11], to name
just a few), enzyme inhibitors [12], in vivo imaging [13],
fluorescence material [14], plant growth regulator [15] and
dyes [16].
While many reports are available for the synthesis of
benzimidazoles, benzothiazoles and benzoxazoles, the
most popular approaches generally involve condensation–
dehydration of o-phenylenediamines, 2-aminothiophenol
and 2-aminophenol with either carboxylic acid derivatives
under strong acid/high temperature conditions [17–19], or
aldehydes with subsequent oxidation using various oxida-
tive reagents such as nitrobenzene (high-boiling point
oxidant/solvent) [20–24], 1,4-benzoquinone [25], I2 [26],
Fe(III)/Fe(II) [27], air [28, 29], In(OTf)3 [31], PhI(OAc)2
[32], Zn praline [33], Sc(OTf)3 [34, 35], Yb(OTf)3 [36],
heteropoly acids [37], thionyl chloride-treatment [38],
DDQ [39, 40], electron-deficient olefins [41], benzofuroxan
[42], MnO2 [43], Pb(OAc)4 [44], oxone [45], NaHSO3 [46,
47], H2O2/HCl [48], Na2S2O5 [49], potassium persulfate–
CuSO4 [50], potassium ferricyanide [51], H2O2/CAN [52],
H2O2/Fe(NO3)3 [53], cobalt(III)–salen complex supported
on activated carbon [54], Cu(II) complex [55], porphyri-
natoiron(III) complex supported on activated silica [56],
NH4OAc [57], and 4-methoxy-TEMPO [8] for the syn-
thesis of benzimidazoles, air [30], Sc(OTf)3 [34], potas-
sium persulfate–CuSO4 [50], H2O2/CAN [52], H2O2/
Fe(NO3)3 [53], on water [58] and 4-methoxy-TEMPO [8]
for the synthesis of benzothiazoles and air [29], MnO2 [43],
Pb(OAc)4 [44], potassium persulfate–CuSO4 [50], potas-
sium ferricyanide [51] and 4-methoxy-TEMPO [8] for the
synthesis of benzoxazoles have been employed in the
synthesis of these products. Nevertheless, there are rare
examples for preparation of all types of these classes of
compounds under similar conditions. Most conditions led
Keywords Vanadium(IV)–salen complexes ꢀ
Nanoparticle heterogeneous catalyst ꢀ Benzimidazole ꢀ
Benzothiazole ꢀ Benzoxazole
Introduction
Benzo-fused heterocyclic systems such as benzimidazole,
benzothiazole, and benzoxazole derivatives are present in
natural products and in synthetic pharmaceutical and
agrochemical compounds [1–8]. These compounds have
been extensively studied due to their wide ranges of
application, e.g., biological and therapeutic activities (such
H. Sharghi (&) ꢀ M. Aberi ꢀ M. M. Doroodmand
Department of Chemistry, Shiraz University, Shiraz 71454,
Islamic Republic of Iran
e-mail: shashem@susc.ac.ir
M. M. Doroodmand
Nanotechnology Research Institute, Shiraz University, Shiraz,
Islamic Republic of Iran
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J IRAN CHEM SOC (2012) 9:189–204
to the formation of one or two classes of these products and
they failed to form other types.
substrates and reaction monitoring were accomplished by
TLC on silica gel PolyGram SILG/UV 254 plates. Chem-
ical materials were purchased from Fluka, Aldrich and
Merck Companies. The used aminopropyl silica gel was
also purchased from Fluka. The Schiff base N,N0-(salicyl-
aldehyde)ethylenediamine (Salen) was prepared by repor-
ted procedures [69–71]. The catalysts, VO–salen 1 and
VCl2–salen 2, were synthesized following a reported pro-
cedure [69, 72]. In a typical reaction for preparation of
VO–salen 1, the salen (1.55 g in 5 mL ethanol) was added
to an ethanolic solution of VOSO4 (1.25 g in 50 mL eth-
anol). The reaction mixture was maintained in reflux for
2 h. After the reaction was completed the solid was filtered
and recrystallized in ethanol. Also, for the preparation of
VCl2–salen 2, a suspension of VO–salen 1 (2.1 g,
6.30 mmol) in toluene (50 mL) was added to SOCl2
(0.53 mL, 7.22 mmol) and the reaction mixture was kept
under reflux during 30 min. After the reaction was com-
pleted the solid was filtered and washed with toluene.
In accordance with the green chemistry principles, the
immobilization of homogeneous metal coordination com-
plexes is a very useful methodology in organic synthesis.
Heterogeneous catalysts are easily separated from the
reaction mixture and can be recovered and reused [59, 60].
Inorganic solids have been widely used as solid supports of
chiral salen complexes [61, 63]. The use of inorganic solid
supports has some advantages over other types of supports,
such as mechanical resistance, chemical and thermal sta-
bility [62].
Supported heterogeneous vanadium catalysts have
shown wide applicability; such as oxidative desulfurization
of model fuel diesel [64], oxidation of sulfides [65], enan-
tioselective cyanosilylation of aldehydes [60] oxidation and
oxidative bromination of organic substrates [66], epoxida-
tion of geraniol [67], and oxidation of cyclohexane [68].
In this paper, we report an efficient, simple and one-pot,
synthesis of 2-arylbenzimidazoles, 2-arylbenzothiazoles
and 2-arylbenzoxazole from various o-phenylendiamines,
2-aminothiophenol and 2-aminophenol with aryl alde-
hydes, respectively, in the presence of homogeneous and
heterogeneous vanadium(IV)–salen complex (5.0 mol%)
under an atmospheric oxygen as a ‘‘green’’ oxidant in
ethanol at room temperature for benzimidazoles and ben-
zothiazoles, and in xylene at 120 °C for benzoxazole.
Anchoring of VCl2(salen) complexes onto silica
3-Aminopropyl functionalized silica gel (1.0 g) and VCl2–
salen 2 (1.5 mmol) were added to CH2Cl2 under stirring at
25 °C and kept stirring for 24 h. The solid products were
separated by centrifuging and washed several times with
CH2Cl2. The solid products then dried at 60 °C in vacuum.
General procedure for the preparation of 2-substituted
benzimidazoles in the presence of vanadium(IV)–salen
complex
Experimental
Instrumentation, analysis and starting material
A mixture of o-phenylenediamine (1.0 mmol) and aryl
aldehyde (1.0 mmol) was stirred in 10.0 mL of EtOH in
the presence of VCl2–salen (5.0 mol%) at room tempera-
ture for the appropriate time as mentioned in Table 2. After
the reaction was completed the solvent was evaporated to
give the crude product, which was purified by silica gel
column chromatography employing n-hexane/ethyl acetate
(8:1) as eluent.
NMR spectra were recorded on a Bruker Avance DPX-250
(1H-NMR 250 MHz and 13C NMR 62.9 MHz) spectrom-
eter in pure deuterated solvents with tetramethylsilane as
an internal standard. IR spectra were obtained using a
Shimadzu FT-IR 8300 spectrophotometer. Mass spectra
were determined on a Shimadzu GCMS-QP 1000 EX
instruments at 70 or 20 eV. Scanning electron micrographs
were obtained by scanning electron microscopy (SEM)
instrumentation (SEM, XL-30 FEG SEM, Philips, at
20 kV). An atomic forced microscope (AFM, DME-SPM,
version 2.0.0.9) was also used for AFM images. Melting
points determined in open capillary tubes in a Bu¨chi-535
circulating oil melting point apparatus. X-ray diffraction
(XRD, D8, Advance, Bruker, axs) were obtained for
characterization of the heterogeneous catalyst. TEM image
was obtained using a transmission electron microscopy
(TEM, CN-10, Philips, 100 kV). The thermogravimetry
(TG) of the samples was analyzed using a lab-made TGA
instrument. The ICP analysis data were obtained using a
Varian Vista-pro analyzer. The purity determination of the
General procedure for the preparation of 2-substituted
benzothiazoles in the presence of vanadium(IV)–salen
complex
A mixture of 2-aminothiophenol (1.0 mmol) and aryl
aldehyde (1.0 mmol) was stirred in 5.0 mL of EtOH in the
presence of VCl2–salen (5.0 mol%) at room temperature
for the appropriate time as mentioned in Table 2. After the
reaction was completed the solvent was evaporated to give
the crude product, which was purified by silica gel column
chromatography employing n-hexane/ethyl acetate (10:1)
as eluent.
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J IRAN CHEM SOC (2012) 9:189–204
191
General procedure for the synthesis of 2-substituted
benzoxazoles in the presence of vanadium(IV)–salen
complex
% vanadium(IV)–salen complex nanoparticles onto silica
at 120 °C. Progress of reactions was monitored by TLC
using n-hexane/ethyl acetate (20:1). After the reaction was
completed, the whole reaction mixture was centrifuged and
rinsed with EtOH (3 9 15 mL). The recovered catalyst
was dried and stored for another consecutive reaction runs
and the combined centrifuge was concentrated by evapo-
rating the solvent to give the crude product, which was
purified by silica gel column chromatography employing n-
hexane/ethyl acetate (20:1) as eluent.
A mixture of 2-aminophenol (1.0 mmol) and aryl aldehyde
(1.0 mmol) was stirred in 1.0 mL of xylene in the presence of
VCl2–salen catalyst (5.0 mol%) at 120 °C for the appropriate
time as mentioned in Table 2. After the reaction was com-
pleted the solvent was evaporated to give the crude product,
which was purified by silica gel column chromatography
employing n-hexane/ethyl acetate (20:1) as eluent.
Spectroscopic data
General procedure for the preparation of 2-substituted
benzimidazoles in the presence of vanadium(IV)–salen
complex nanoparticles onto silica
2-Phenyl-1H-benzimidazole
White solid; mp 290–292 °C (Lit. [38] mp 292 °C). IR
(KBr): 694 (m), 742 (s), 1,113 (m), 1,277 (m), 1,315 (m),
1,412 (s), 1,450 (s), 1,591 (w), 1,620 (m), 3,049 (w), 3,440
(w) cm-1. 1H NMR (DMSO-d6, 250 MHz): d 7.14–7.25 (m,
2H), 7.44–7.61 (m, 5H), 8.20 (d, J = 7.2 Hz, 2H), 12.94 (s,
1H). 13C NMR (DMSO-d6, 62.9 MHz): d 122.1, 126.4,
128.4, 128.9, 129.2, 129.8, 130.1, 151.2. Mass m/z (%): 194
(M?, 97.4), 149 (45.6), 115 (20.2), 97 (27.2), 73 (46.5), 57
(100.0). Anal. Calcd for C13H10N2 (194.235): C, 80.39; H,
5.19; N, 14.42. Found: C, 80.30; H, 5.25; N, 14.39.
For each reaction, o-phenylenediamine (1.0 mmol) with aryl
aldehyde (1.0 mmol) was stirred in 10.0 mL of EtOH (96%)
in the presence of 5.0 mol % vanadium(IV)–salen complex
nanoparticles onto silica (the amount of catalyst has been
measured via ICP results) at room temperature. Progress of
reactions was monitored by TLC using n-hexane/ethyl acetate
(8:1). After the reaction was completed the whole reaction
mixture was centrifuged and rinsed with EtOH (3 9 15 mL).
The recovered catalyst was dried and stored for another con-
secutive reaction runs and the combined centrifuge was con-
centrated by evaporating the solvent to give the crude product,
which was purified by silica gel column chromatography
employing n-hexane/ethyl acetate (8:1) as eluent.
5-Methyl-2-phenyl-1H-benzimidazole
White solid; mp 242–243 °C (Lit. [73] mp 242–143 °C).
IR (KBr): 698 (m), 1,402 (m), 1,450 (s), 2,882 (m), 3,039
1
(m), 3,410 (w) cm-1. H NMR (DMSO-d6, 250 MHz): d
General procedure for the preparation of 2-substituted
benzothiazoles in the presence of vanadium(IV)–salen
complex nanoparticles onto silica
2.35 (S, 3H), 7.00 (d, J = 8.1 Hz, 1H), 7.36–7.55 (m, 5H),
8.14 (d, J = 7.6 Hz, 2H), 12.75 (S, 1H). 13C NMR
(DMSO-d6, 62.9 MHz): d 21.3, 111.1, 118.4, 123.5, 126.2,
128.8, 129.2, 129.6, 130.3, 131.1, 150.9. Mass m/z (%):
209 (M??1, 36.5), 208 (M?, 47.4), 149 (16.1), 104 (32.1),
73 (59.1), 57 (100.0). Anal. Calcd for C14H12N2 (208.262):
C, 80.74; H, 5.81; N, 13.45. Found: C, 80.79; H, 5.77; N,
13.41.
For each reaction, 2-aminothiophenol (1.0 mmol) with aryl
aldehyde (1.0 mmol) was stirred in 5.0 mL of EtOH (96%) in
the presence of vanadium(IV)–salen complex nanoparticles
onto silica (5.0 mol%) at room temperature. Progress of
reactions was monitored by TLC using n-hexane/ethyl acetate
(10:1). After the reaction was completed the whole reaction
mixture was centrifuged and rinsed with EtOH (3 9 15 mL).
The recovered catalyst was dried and stored for another con-
secutive reaction runs and the combined centrifuge was con-
centrated by evaporating the solvent to give the crude product,
which was purified by silica gel column chromatography
employing n-hexane/ethyl acetate (10:1) as eluent.
2-Phenyl-1H-benzimidazole-5-carboxylic acid
White solid; mp 325 °C (Lit. [56] mp 325 °C). IR (KBr):
771 (m), 1,296 (s), 1,687 (s), 2,921 (br), 3,058 (m), 3,340
1
(m) cm-1. H NMR (DMSO-d6, 250 MHz): d 7.47–7.66
(m, 4H), 7.84 (d, J = 8.4 Hz, 1H), 8.17–8.20 (m, 3H),
12.79 (S, 1H), 13.19 (S, 1H). 13C NMR (DMSO-d6,
62.9 MHz): d 122.7, 123.5, 124.0, 125.0, 126.7, 128.3,
129.2, 129.6, 130.3, 142.4, 153.7, 168.0. Mass m/z (%):
239 (M??1, 8.6), 238 (M?, 9.2), 221 (7.1), 193 (1.9), 152
(5.6), 111 (16.2), 83 (41.4), 57 (100.0). Anal. Calcd for
C14H10N2O2 (238.244): C, 70.58; H, 4.23; N, 11.76.
Found: C, 71.52; H, 4.28; N, 11.72.
General procedure for the synthesis of 2-substituted
benzoxazoles in the presence of vanadium(IV)–salen
complex nanoparticles onto silica
2-Aminophenol (1.0 mmol) with aryl aldehyde (1.0 mmol)
was stirred in 1.0 mL of xylene in the presence of 5.0 mol
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J IRAN CHEM SOC (2012) 9:189–204
Phenyl(2-phenyl-1H-benzimidazol-5-yl)methanone
2-(4-Methylphenyl)-1H-benzimidazole
Pale yellow solid; mp 221–222 °C (Lit. [73] mp
221–221.5 °C). IR (KBr): 1,282 (m), 1,321 (s), 1,614 (s),
White solid; mp: 270–272 °C (Lit. [73] mp 270–272 °C).
IR (KBr): 744 (s), 822 (m), 964 (m), 1,273 (m), 1,393 (m),
1,431(s), 1,462 (s), 1,500 (m), 1,554 (w), 2,916 (m), 2,978
1
1,639 (m), 3,116 (m), 3,398 (m) cm-1. H NMR (DMSO-
d6, 250 MHz): d 7.41–7.58 (m, 11H), 8.19 (d, J = 7.4 Hz,
2H), 13.28 (s, 1H). 13C NMR (DMSO-d6, 62.9 MHz): d
124.2, 126.7, 128.3, 129.0, 129.4, 129.5, 130.4, 131.0.
132.0, 138.7, 153.9.1, 195.5. Mass m/z (%): 299 (M??1,
18.5), 298 (M?, 19.2), 221 (43.7), 194 (10.5), 166 (12.6),
149 (15.8), 111 (13.9), 94 (40.8), 71 (43.5), 55 (100.0).
Anal. Calcd for C20H14N2O (298.343): C, 80.52; H, 4.73;
N, 9.39. Found: C, 80.58; H, 4.75; N, 9.25.
(m), 3,095 (m), 3,447 (w) cm-1 1H NMR (DMSO-d6,
.
250 MHz): d 2.35 (s, 3H), 7.15–7.20 (m, 2H), 7.33 (d,
J = 8.1 Hz, 2H), 7.46–7.56 (m, 2H), 8.07 (d, J = 8.1 Hz,
2H), 12.84 (s, 1H). 13C NMR (DMSO-d6, 62.9 MHz): d
20.9, 121.9, 126.3, 127.4, 128.9, 129.4, 139.5, 151.3. Mass
m/z (%): 208 (M?, 66.5), 149 (49.1), 111 (18.0), 83 (44.9),
57 (100.0). Anal. Calcd for C14H12N2 (208.262): C, 80.74;
H, 5.81; N, 13.45. Found: C, 80.69; H, 5.90; N, 13.38.
Ethyl 2-Phenyl-1H-benzimidazole-5-carboxylate
2-(4-Methylphenyl)-1,3-benzothiazole
White solid; mp 183 °C. IR (KBr): 746 (s), 1,229 (s), 1,368
(m), 1,620 (m), 1,730 (s), 3,310 (s) cm-1. 1H NMR (CDCl3,
250 MHz): d 1.37 (t, J = 7.1 Hz, 3H), 4.38 (q, J = 7.1 Hz,
2H), 7.26–7.43 (m, 3H), 7.57 (m, 1H), 7.95 (dd, J = 8.5,
J = 1.3 Hz, 1H), 8.12–8.16 (m, 2H), 8.35 (s, 1H), 12.1 (s,
1H). 13C NMR (CDCl3, 62.9 MHz) d 14.3, 61.1, 124.3,
124.8, 127.1, 129.1, 129.3, 130.7, 154.9, 167.5. Mass m/z
(%): 267 (M??1, 11.3), 266 (M?, 49.4), 221 (100.0), 193
(34.6), 166 (18.2), 149 (11.6), 111 (15.3), 90 (17.0), 63
(35.7). Anal. Calcd for C16H14N2O2 (266.298): C, 72.17; H,
5.30; N, 10.52. Found: C, 72.21; H, 5.35; N, 10.55.
White solid; mp 85 °C (Lit. [74] mp 85 °C). IR (KBr): 761
(s), 816 (m), 959 (m), 1,311 (m), 1,479 (s), 1,603 (w),
1
2,914 (W), 3,049 (w) cm-1. H NMR (CDCl3, 250 MHz):
d 2.29 (s, 3H), 7.15 (d, J = 7.9 Hz, 2H), 7.23–7.36 (m,
2H), 7.74 (d, J = 7.8 Hz, 1H), 7.86 (d, J = 8.2 Hz, 2H),
7.94 (d, J = 8.0 Hz). 13C NMR (CDCl3, 62.9 MHz) d
21.5, 121.6, 123.1, 125.0, 126.2, 127.8, 129.7, 131.0,
135.0, 141.3, 154.3, 168.2;. Mass m/z (%):225 (M?, 100.0),
224 (55.5), 223 (17.7), 190 (3.1), 148 (12.8), 108 (12.8), 91
(6.1), 89 (6.3), 82 (9.7), 69 (22.5), 63 (1.5), 58 (10.1), 57
(8.4), 55 (8.4). Anal. Calcd for C14H11NS (225.308): C,
74.63; H, 4.92; N, 6.22; S, 14.23. Found: C, 74.64; H, 4.89;
N, 6.27; S, 14.26.
2-Phenyl-1,3-benzothiazole
White solid; mp 112 °C (Lit. [8] mp 111–112 °C). IR
(KBr): 686 (s), 762 (s), 962 (m), 1,431 (m), 1,472 (m),
3,060 (w) cm-1. 1H NMR (CDCl3, 250 MHz): d 7.36–7.42
(m, 1H), 7.48–7.53 (m, 4H), 7.91 (d, J = 8.5 Hz, 1H),
8.08–8.13 (m, 3H). 13C NMR (CDCl3, 62.9 MHz) d 121.7,
123.3, 125.2, 126,3, 127.6, 129.0, 131.0, 133.6, 135.1,
154.2, 168.0. Mass m/z (%):211(M?, 8.2), 186 (5.3), 166
(5.5), 80 (39.2), 64 (100.0), 57 (25.8), 55 (44.1). Anal.
Calcd for C13H9NS (221.281): C, 73.90; H, 4.29; N, 6.63;
S, 15.17. Found: C, 73.94; H, 4.32; N, 6.61; S, 15.14.
2-(4-Methylphenyl)-1,3-benzoxazole
White solid; mp 113–114 °C (Lit. [75] mp 113–115 °C).
IR (KBr): 760 (s), 820 (s), 1,055 (s), 1,177 (m), 1,243 (s),
1,501 (m), 1,556 (m), 1,622 (m), 2,916 (m), 3,053
1
(w) cm-1. H NMR (CDCl3, 250 MHz): d 7. 2.43 (s, 3H),
7.20–7.36 (m, 4H), 7.55–7.59 (m. 1H), 7.74–7.78 (m, 1H),
8.15 (d, J = 8.3 Hz, 2H). 13C NMR (CDCl3, 62.9 MHz) d
21.6, 110.4, 119.8, 124.3, 124.8, 127.5, 129.6, 141.9,
142.2, 150.6, 163.2. Mass m/z (%):209 (M?, 100.0), 180
(9.1), 149 (22.5), 91 (27.7), 64 (35.8), 63 (45.6). Anal.
Calcd for C14H11NO (209.247): C, 80.36; H, 5.30; N, 6.69.
Found: C, 80.38; H, 5.35; N, 6.72.
2-Phenyl-1,3-benzoxazole
White solid; mp 96–98 °C (Lit. [8] mp 96–97 °C). IR
(KBr): 692 (s), 744 (s), 1,053 (s), 1,241 (s), 1,447 (s), 1,455
.
(m), 1,545 (s), 1,608 (m), 3,049 (w) cm-1 1H NMR
2-(4-Methoxyphenyl)-1H-benzimidazole
(CDCl3, 250 MHz): 7.23–7.37 (m, 2H), 7.51–7.57 (m, 4H),
7.75–7.85 (m, 1H), 8.25–8.28 (m, 2H). 13C NMR (CDCl3,
62.9 MHz) d 110.6, 120.0, 124.5, 125.0, 127.1, 127.6,
128.8, 129.2, 131.0, 131.4, 142.1, 150.7, 163.0. Mass m/z
(%):195 (M?, 77.4), 167 (27.6), 149 (10.6), 85 (34.2), 70
(25.1), 57 (100.0). Anal. Calcd for C13H9NO (195.220): C,
79.98; H, 4.65; N, 7.17. Found: C, 79.96; H, 4.68; N, 7.16.
White solid; mp 226 °C (Lit. [76] mp 226–227 °C). IR
(KBr): 744 (s), 1,034 (m), 1,180 (m), 1,254 (s), 1,296 (m),
1,400 (m), 1,435 (s), 1,454 (s), 1,477 (m), 1,500 (s), 1,612
(s), 2,948 (m), 3,055 (m), 3,422 (w) cm-1 1H NMR
.
(DMSO-d6, 250 MHz): d 3.78 (s, 3 H), 7.10 (d, J =
8.8 Hz, 2H), 7.16 (q, J = 3.0 Hz, 2H), 7.50 (m, 2H), 8.12
(d, J = 8.8 Hz, 2H), 12.76 (s, 1H). 13C NMR (DMSO-d6,
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J IRAN CHEM SOC (2012) 9:189–204
193
62.9 MHz): d 55.2, 111.0, 114.3, 118.4, 121.5, 122.0,
122.6, 128.0, 151.3, 160.5. Mass m/z (%): 225 (M??1,
86.4), 224 (M?, 92.4), 210 (51.8), 182 (51.0), 149 (30.9),
129 (17.8), 97 (30.4), 69 (100.0). Anal. Calcd for
C14H12N2O (224.261): C, 74.98; H, 5.39; N, 12.49. Found:
C, 74.93; H, 5.42; N, 12.45.
2,828 (w), 3,055 (w) cm-1. 1H NMR (CDCl3, 250 MHz): d
3.90 (s, 3H), 3.98 (s, 3H), 7.00–7.02 (m, 2H), 7.37–7.41(m,
1H), 7.47–7.50 (m, 1H), 7.93 (d, J = 8.6 Hz, 1H),
8.09–8.12 (m, 2H). 13C NMR (CDCl3, 62.9 MHz) d 55.9,
56.3, 112.7, 113.4, 118.7, 121.3, 122.7, 122.8, 124.7,
125.9, 136.3, 151.8, 152.1, 153.9, 162.9. Mass m/z (%):
272 (M??1, 19.6), 271 (M?, 67.9), 238 (47.8), 210 (14.4),
185 (30.6), 159 (13.4), 136 (100.0), 108 (18.2), 69 (37.8).
Anal. Calcd for C15H13NO2S (271.332): C, 66.40; H, 4.83;
N, 5.16; S, 11.82. Found: C, 66.45; H, 4.86; N, 5.20, S,
11.88.
2-(4-Methoxyphenyl)-1,3-benzothiazole
White solid; mp 122 °C (Lit. [77] mp 120–121 °C). IR
(KBr): 760 (s), 832 (s), 968 (m), 1,027 (m), 1,181 (m), 1,221
(m), 1,256 (s), 1,433 (m), 1,477 (s), 1,605 (s), 2,991 (w),
1
3,059 (w) cm-1. H NMR (CDCl3, 250 MHz): d 3.78 (s,
2-(2,5-Dimethoxyphenyl)-1,3-benzoxazole
3H,), 6.84 (d, J = 8.9 Hz, 2H), 7.25–7.39 (m, 2H), 7.78 (d,
J = 7.9 Hz, 1H), 7.91–7.96 (m, 3H). 13C NMR (CDCl3,
62.9 MHz) d 55.4, 114.3, 121.5, 122.8, 124.8, 126.2, 126.4,
129.1, 134.9, 154.2, 161.9, 167.8. Mass m/z (%):241 (M?,
80.2), 240 (33.0), 226 (41.3), 210 (11.4), 198 (38.7), 165
(5.1), 149 (66.1), 73 (31.8), 71 (37.8), 69 (38.6), 57 (100.0).
Anal. Calcd for C14H11NOS (241.307): C, 69.68; H, 4.59; N,
5.80; S, 13.29. Found: C, 69.71; H, 4.60; N, 5.86; S, 13.33.
White solid; mp 70–72 °C. IR (KBr): 751 (m), 1,038 (m),
1,249 (m), 1,538 (m), 1,618 (m), 1,821 (w), 3,012
1
(w) cm-1. H NMR (CDCl3, 250 MHz): d 3.88 (s, 3H),
3.93 (s, 3H), 7.037.06 (m, 2H), 7.33–7.36 (m, 2H),
7.58–7.60 (m, 1H), 7.68 (d, J = 2.8, 1H), 7.80–7.81 (m,
1H). 13C NMR (CDCl3, 62.9 MHz) d 55.9, 56.7, 110.5,
113.7, 115.1, 116.3, 119.0, 120.2, 124.3, 125.0, 142.0,
150.3, 152.9, 153.4, 161.4. Mass m/z (%): 256 (M??1,
23.4), 255 (M?, 93.6), 226 (72.3), 212 (31.9), 195 (12.8),
169 (35.1), 149 (38.3), 120 (94.7), 94 (100.0), 69 (92.6).
Anal. Calcd for C15H13NO3 (255.271): C, 70.58; H, 5.13;
N, 5.49. Found: C, 70.61; H, 5.19; N, 5.51.
2-(4-Methoxyphenyl)-1,3-benzoxazole
White solid; mp 97–98 °C (Lit. [8] mp 97–98 °C). IR (KBr):
741 (m), 831 (m), 1,019 (m), 1,169 (m), 1,255 (m), 1,453 (m),
1
1,500 (s), 1,616 (s), 3,052 (w) cm-1. H NMR (CDCl3,
250 MHz): d 3.85 (s, 3H), 7.01 (d, J = 8.8 Hz, 2H),
7.29–7.33 (m, 2H), 7.52–7.55 (m, 1H), 7.71–7.75 (m, 1H),
8.18 (d, J = 8.8 Hz, 2H). 13C NMR (CDCl3, 62.9 MHz) d
55.3, 110.3, 114.3, 114.7, 119.5, 124.3, 124.5, 129.3, 142.3,
150.6, 162.2, 163.1. Mass m/z (%): 226 (M??1, 11.1), 225
(M?, 30.9), 182 (8.6), 165 (22.2), 111 (16.0), 97 (45.7), 69
(100.0). Anal. Calcd for C14H11NO2 (225.246): C, 74.65; H,
4.92; N, 6.22. Found: C, 74.66; H, 4.97; N, 6.26.
2-(4-Isopropylphenyl)-1H-benzimidazole
White solid; mp 250–251 °C (Lit. [56] mp 250–251 °C).
IR (KBr): 741 (m), 1,273 (m), 1,439 (s), 1,620 (m), 2,951
.
(s), 3,055 (w), 3,417 (w) cm-1 1H NMR (DMSO-d6,
250 MHz): d 1.22 (d, J = 6.9 Hz, 6H), 2.93 (m, 1H), 7.17
(m, 2H), 7.40 (d, J = 8.2 Hz, 2H), 7.51–7.62 (m, 2H), 8.10
(d, J = 8.2 Hz, 2H), 12.83 (s, 1H). 13C NMR (DMSO-d6,
62.9 MHz): d 23.6, 33.3, 111.2, 118.6, 121.5, 122.3, 126.4,
127.7, 150.3, 151.3. Mass m/z (%): 237 (M??1, 40.1), 236
(M?, 67.3), 221 (100.0), 194 (11.6), 110 (17.6), 92 (29.5),
65 (31.2). Anal. Calcd for C16H16N2 (236.316): C, 81.32;
H, 6.82; N, 11.85. Found: C, 81.25; H, 6.90; N, 11.78.
2-(2,5-Dimethoxyphenyl)-1H-benzimidazole
White solid; mp 217–218 °C. IR (KBr): 758 (m), 1,025 (m),
1,420 (m), 1,490 (s), 1,616 (w), 3,112 (br), 3,421 (w) cm-1
.
1H NMR (DMSO-d6, 250 MHz): d 3.78 (s, 3H), 3.95 (s,
3H), 6.99–7.20 (m, 4H), 7.65 (m, 2H), 8.01 (d, J = 3.1 Hz,
1H), 12.14 (s, 1H). 13C NMR (DMSO-d6, 62.9 MHz): d
55.4, 56.0, 112.0, 113.3, 113.6, 117.0, 118.4, 121.5, 122.1,
134.7, 142.6, 148.7, 151.0, 153.1. Mass m/z (%): 255
(M??1, 12.4), 254 (15.0), 149 (14.7), 119 (23.1), 95 (19.2),
69 (100.0). Anal. Calcd for C15H14N2O2 (254.284): C,
70.85 H, 5.55, N, 11.02. Found: C, 70.82, H, 5.61, N, 10.97.
2-(4-Isopropylphenyl)-1,3-benzothiazole
White solid; mp 78–79 °C. IR (KBr): 757 (s), 966 (s),
1,312 (m), 1,435 (m), 1,485 (m), 1,557 (m), 1,624 (m),
1,825 (w), 2,957 (m), 3,048 (w) cm-1. H NMR (CDCl3,
1
250 MHz): d 1.21 (d, J = 6.7 Hz, 6H), 2.84–2.95 (m, 1H),
7.26 (d, J = 8.3 Hz, 2H), 7.28–7.40 (m, 2H), 7.81 (d,
J = 7.9 Hz, 2H), 7.95 (d, J = 8.5 Hz, 2H). 13C NMR
(CDCl3, 62.9 MHz) d 23.5, 34.2, 121.6, 123.1, 125.0,
126.2, 126.6, 127.1, 127.6, 128.0, 131.3, 135.0, 152.2,
154.3, 168.2. Mass m/z (%): 253 (M?, 52.3), 238 (100.0),
223 (23.1), 109 (22.9), 69 (18.8). Anal. Calcd for
2-(2,5-Dimethoxyphenyl)-1,3-benzothiazole
White solid; mp 105–107 °C. IR (KBr): 755 (m), 1,016
(m), 1,177 (m), 1,291 (s), 1,461 (m), 1,504 (s), 1,612 (m),
123

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J IRAN CHEM SOC (2012) 9:189–204
C16H15NS (253.361): C, 75.85; H, 5.97; N, 5.53; S, 12.65.
Found: C, 75.86; H, 6.05; N, 5.57; S, 12.67.
2H), 7.50 (d, J = 8.6 Hz, 2H), 7.56–7.60 (m, 1H),
7.74–7.79 (m, 1H), 8.19 (d, J = 8.6 Hz, 2H). 13C NMR
(CDCl3, 62.9 MHz) d 110.6, 120.1, 124.7, 125.3, 125.6,
128.8, 129.2, 137.7, 142.0, 150.7, 162.0. Mass m/z (%):
231 (M??2, 38.3), 230 (M??1, 36.5), 229 (M?, 100.0),
201 (27.8), 166 (17.8), 111 (10.2), 92 (21.5), 63 (79.0).
Anal. Calcd for C13H8ClNO (229.665): C, 67.99; H, 3.51;
N, 6.10. Found: C, 67.96; H, 3.57; N, 6.12.
2-(4-Isopropylphenyl)-1,3-benzoxazole
White solid; mp 76–78 °C. IR (KBr): 745 (s), 1,011 (m),
1,242 (s), 1,452 (m), 1,496 (m), 1,552 (m), 1,617 (m),
1
2,904 (w), 2,946 (m), 3,050 (w) cm-1. H NMR (CDCl3,
250 MHz): d 1.30 (d, J = 6.9 Hz, 6H), 3.00 (m, 1H),
7.30–7.41 (m, 4H), 7.54–7.59 (m, 1H), 7.74–7.81(m, 1H),
8.19 (d, J = 6.6 Hz, 2H). 13C NMR (CDCl3, 62.9 MHz) d
23.7, 34.2, 110.5, 119.8, 124.5, 124.7, 124.8, 127.0, 127.7,
142.2, 150.7, 152.8, 163.3. Mass m/z (%): 238 (M??1,
17.6), 237 (M?, 48.0), 222 (100.0), 207 (8.1), 193 (6.6),
130 (3.3), 65 (29.7). Anal. Calcd for C16H15NO (237.300):
C, 80.98; H, 6.37; N, 5.90. Found: C, 80.99; H, 6.42;
N, 5.91.
4-(1H-Benzimidazol-2-yl)phenyl 2-propynyl ether
White solid; mp 200–201 °C. IR (KBr): 746 (s), 835 (m),
1,025 (s), 1,178 (m), 1,229 (m), 1,450 (s), 1,495 (s), 1,610
(s), 2,859 (w), 2,919 (w), 3,270 (m), 3,417 (w) cm-1. H
1
NMR (DMSO-d6, 250 MHz): d 3.60 (s, 1H), 4.88 (s, 2H),
7.13–7.19 (m, 4H), 7.54–7.70 (m, 2H), 8.11 (d,
J = 8.8 Hz, 2H), 12.76 (s, 1H). 13C NMR (DMSO-d6,
62.9 MHz) d 55.5, 78.4, 78.9, 115.2, 121.8, 123.4, 127.9,
151.2, 158.5. Mass m/z (%): 249 (M??1, 15.3), 248 (M?,
49.3), 209 (100.0), 181 (63.7), 149 (12.1), 105 (14.9), 77
(24.7), 57 (38.1). Anal. Calcd for C16H12N2O (248.283): C,
77.40; H, 4.87; N, 11.28. Found: C, 77.42; H, 4.91;
N, 11.27.
2-(4-Chlorophenyl)-1H-benzimidazole
White solid; mp 292–293 °C (Lit. [38] mp 292 °C). IR
(KBr): 744 (s), 829 (s), 964 (m), 1,014 (m), 1,088 (m),
1,273 (m), 1,319 (m), 1,427 (s), 1,470 (m), 1,489 (m),
1,626 (w), 3,051 (w) cm-1
.
1H NMR (DMSO-d6,
250 MHz): d 7.18–7.21 (m, 2 H), 7.60 (m, 4 H), 8.17 (d,
J = 8.6 Hz, 2H), 12.99 (s, 1 H). 13C NMR (DMSO-d6,
62.9 MHz) d 111.4, 118.9, 121.9, 122.7, 128.1, 129.0,
134.5 150.1. Mass m/z (%): 230 (M??2, 21.1), 229
(M??1, 36.3), 228 (M?, 54.9), 193 (13.5), 167 (7.0), 149
(25.0), 129 (11.1), 111 (15.4), 85 (33.2), 57 (100.0). Anal.
Calcd for C13H9ClN2 (228.681): C, 68.28; H, 3.97; N,
12.25. Found: C, 68.23; H, 4.05; N, 12.24.
4-(1,3-Benzothiazol-2-yl)phenyl 2-propynyl ether
White solid; mp 131–132 °C. IR (KBr): 756 (s), 841 (m),
1,015 (m), 1,226 (m), 1,245 (m), 1,485 (s), 1,602 (m),
1
3,277 (w) cm-1. H NMR (CDCl3, 250 MHz): d 2.49 (s,
1H), 4.69 (s, 2H), 6.98–7.03 (m, 2H), 7.31–7.40 (m, 2H),
7.80 (d, J = 8.6 Hz, 1H), 7.94–8.00 (m, 3H). 13C NMR
(CDCl3, 62.9 MHz) d 55.9, 76.1, 78.0, 115.3, 121.5, 122.9,
124.9, 126.2, 127.2, 129.1, 134.9, 154.2, 159.7, 167.6.
Mass m/z (%): 266 (M??1, 3.3), 265 (M?, 15.4), 226
(33.8), 210 (27.9), 181 (10.4), 149 (26.7), 121 (12.5), 95
(28.3), 69 (100.0). Anal. Calcd for C16H11NOS (265.329):
C, 72.43; H, 4.18; N, 5.28; S, 12.08. Found: C, 72.49; H,
4.25; N, 5.26; S, 12.13.
2-(4-Chlorophenyl)-1,3-benzothiazole
White solid; mp 110–113 °C (Lit. [8] mp 110–112 °C). IR
(KBr): 731 (m), 828 (m), 964 (m), 1,089 (s), 1,399 (m),
1
1,473 (s), 1,508 (m), 3,012 (w) cm-1. H NMR (CDCl3,
250 MHz): d 7.31–7.43 (m, 4H), 7.82 (d, J = 7.8 Hz, 1H),
7.92–7.97 (m, 3H). 13C NMR (CDCl3, 62.9 MHz) d 121.6,
123.3, 125.4, 126.5, 128.7, 129.2, 130.9, 132.1, 135.0,
137.0,154.1, 166.6. Mass m/z (%): 247 (M??2, 39.3), 246
(M??1, 27.7), 245 (M?, 100.0), 210 (14.6), 108 (58.5), 69
(55.2). Anal. Calcd for C13H8ClNS (245.726): C, 63.54; H,
3.28; N, 5.70; S, 13.05. Found: C, 63.53.28; H, 3.30; N,
5.71; S, 13.12.
4-(1,3-Benzoxazol-2-yl)phenyl 2-propynyl ether
White solid; mp 121–123 °C. IR (KBr): 746 (s), 1,016 (m),
1,173 (m), 1,218 (s), 1,452 (m), 1,498 (s), 1,616 (m), 3,293
1
(w) cm-1. H NMR (CDCl3, 250 MHz): d 2.57 (s, 1H),
4.76 (s, 2H), 7.07–7.12 (m, 2H), 7.30–7.34 (m, 2H),
7.53–7.55 (m, 1H), 7.72–7.75 (m, 1H), 8.18–8.23 (m 2H).
13C NMR (CDCl3, 62.9 MHz) d 55.8, 76.1, 77.9, 110.4,
115.2, 119.7, 120.5, 124.4, 124.7, 129.3, 142.2, 150.6,
160.1, 162.9. Mass m/z (%): 250 (M??1, 22.0), 249
(M?, 80.6), 210 (100.0), 182 (90.9), 149 (12.9), 127 (39.2),
94 (23.7), 63 (25.9). Anal. Calcd for C16H11NO2 (249.268):
C, 77.10; H, 4.45; N, 5.62. Found: C, 77.08; H, 4.49;
N, 5.60.
2-(4-Chlorophenyl)-1,3-benzoxazole
White solid; mp 145–147 °C (Lit. [8] mp 144–145 °C). IR
(KBr): 739 (s), 832 (m), 1,011 (m), 1,056 (m), 1,091 (m),
1,244 (m), 1,452 (m), 1,483 (m), 1,616 (m), 3,057
1
(w) cm-1. H NMR (CDCl3, 250 MHz): d 7.33–7.38 (m,
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J IRAN CHEM SOC (2012) 9:189–204
195
2,935 (w), 1,367 (br) cm-1. 1H NMR (CDCl3, 250 MHz): d
1.22–2.22 (m, 8H), 3.09 (s, 1H), 4.01–4.23 (m, 2H), 5.25
(s, 2H), 7.10 (d, J = 8.9 Hz, 2H), 7.26–7.36 (m, 2H),
7.52–7.56 (m, 1H), 7.71–7.74 (m, 2H). 8.18 (d,
J = 8.9 Hz, 2H). 13C NMR (CDCl3, 62.9 MHz) d 23.9,
24.7, 31.6, 33.8, 62.1, 66.9, 72.6, 110.4, 115.1, 119.6,
122.7, 124.4, 124.7, 129.4, 132.4, 143.9, 151.4, 161.8.
Mass m/z (%) 391 (M??1, 8.3), 290 (M?, 18.4), 211
(78.7), 180 (20.0), 152 (85.4), 134 (13.7), 97 (15.4), 81
(100.0), 56 (45.3). Anal. Calcd for C22H22N4O3 (390.440):
C, 67.68; H, 5.68; N, 14.35. Found: C, 67.72; H, 5.73; N,
14.42.
2-(4-{[4-(1H-Benzimidazol-2-yl)phenoxy]methyl}-1H-
1,2,3-triazol-1-yl)cyclohexanol
White solid; mp: 254–256 °C. IR (KBr): 745 (m), 833 (m),
1,077 (m), 1,178 (m), 1,248 (s), 1,441 (m), 1,499 (m),
1,613 (m), 2,855 (w), 2,933 (m), 3,151 (m), 3,415 (br)
1
cm-1. H NMR (DMSO-d6, 250 MHz): d 1.14–1.32 (m,
2H), 1.69–2.06 (m, 4H), 2.47–2.49 (m, 2H), 3.98–4.26 (m,
2H), 4.98 (s, 1H), 5.20 (s, 2H), 7.15–7.23 (m, 4H),
7.53–7.57 (m, 2H), 8.10 (d, J = 8.7 Hz, 2H), 8.22 (s, 1H).
13C NMR (DMSO-d6, 62.9 MHz) d 23.7, 24.3, 31.9, 34.7,
61.3, 65.8, 71.3, 114.7, 115.0, 121.9, 122.6, 123.9, 128.0,
139.1, 141.5, 151.2, 159.5. Mass m/z (%): 391 (M??2,
4.5), 390 (M??1, 15.5), 389 (M?, 17.4), 360 (1.3), 292
(8.7), 262 (4.5), 248 (2.4), 235 (4.4), 210 (100.0), 181
(23.9), 152 (34.5), 81 (38.6), 54 (20.8). Anal. Calcd for
C22H23N5O2 (389.455): C, 67.85; H, 5.95; N, 17.98.
Found: C, 67.88; H, 6.03; N, 17.93.
Results and discussions
Metal–salen complexes show wide applicability and are
now used as catalyst for a variety of enantioselective
reactions such as oxidation [78–80], sulfimidation [81],
addition of TMSCN to aldehydes [82], conversion of 1,2-
epoxyethanes to 2-haloethanol structures [83], sulfoxida-
tion [84], cyclopropanation [85] and alkyne addition to
ketones [86]. In conjunction with our studies on the syn-
thesis of metal–salen complexes [54, 87–91] and our con-
tinued efforts to utilize heterogeneous catalysts for
developing organic reactions [54, 56, 92–101], herein, we
report a novel and efficient method for the synthesis of
2-substituted benzimidazoles, benzothiazoles and benzox-
azole using a vanadium(IV)–salen complex and vana-
dium(IV)–salen complex nanoparticles supported on silica
as the catalyst.
2-(4-{[4-(1,3-Benzothiazol-2-yl)phenoxy]methyl}-1H-
1,2,3-triazol-1-yl)cyclohexanol
White solid; mp 196–198 °C. IR (KBr): 730 (m), 758 (s),
1,174 (m), 1,253 (s), 1,307 (m), 1,432 (m), 1,481 (s), 1,605
.
(m), 2,853 (w), 2,932 (m), 3,350 (br) cm-1 1H NMR
(DMSO-d6, 250 MHz): d 1.15–1.32 (m, 2H), 1.64–1.96 (m,
4H), 2.47–2.48 (m, 2H), 3.65–3.82 (m, 1H), 4.18–4.22 (m,
1H), 4.96 (s, 1H), 5.22 (s, 2H), 7.22 (d, J = 8.8 Hz, 2H),
7.37–7.53 (m, 2H), 7.98–8.10 (m, 4H), 8.24 (s, 1H). 13C
NMR (DMSO-d6, 62.9 MHz) d 23.7, 24.4, 31.9, 34.7, 61.4,
65.8, 71.3, 115.4, 122.1, 122.4, 123.9, 125.1, 125.7, 126.5,
128.8, 134.2, 141.3, 153.6, 160.6, 166.9. Mass m/z (%):
408 (M??2, 5.8), 407 (M??1, 15.9), 406 (M?, 19.7), 389
(3.8), 292 (2.8), 279 (4.1), 227 (100.0), 210 (33.5), 180
(14.6), 152 (98.2), 134 (12.7), 108 (25.0), 81 (77.3), 55
(45.2). Anal. Calcd for C22H22N4O2S (406.501): C, 65.00;
H, 5.45; N, 13.78; S, 7.89. Found: C, 65.08; H, 5.49; N,
13.77; S, 7.94.
The catalysts, VO–salen 1 and VCl2–salen 2, were
synthesized following a reported procedure [69, 72]. The
reaction pathway for the preparation of the catalysts is
shown in Scheme 1.
In the presence of a catalytic amount of VO(salen) 1 and
VCl2(salen) 2, the reaction of o-phenylenediamine
(1.0 mmol) was first examined with benzaldehyde
(1.0 mmol) at room temperature in ethanol. We obtained
exclusively 2-substituted product 3, and no N-alkylated
product 4 was observed (Scheme 2).
2-(4-{[4-(1,3-Benzoxazol-2-yl)phenoxy]methyl}-1H-1,2,3-
triazol-1-yl)cyclohexanol
According to Table 1, when VO(salen) 1 and VCl2(sa-
len) 2 were used as catalysts in the presence of atmospheric
air as a green oxidant, a comparison between the activity of
White solid; mp 182–184 °C. IR (KBr): 784 (m), 1,035
(m), 1,257 (m), 1,498 (m), 1,616 (m), 1,650 (m), 2,876 (w),
O
Cl
N
N
N
O
N
O
N
O
N
O
VOSO4
V
SOCl2
V
HO
OH
EtOH, reflux
PhCH3, reflux
Cl
1
2
Scheme 1 The reaction pathway for the preparation of the catalysts
123

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J IRAN CHEM SOC (2012) 9:189–204
Scheme 2 Model reaction of
synthesis benzimidazole
derivatives
O
NH2
NH2
N
N
N
Cat.
H
EtOH, rt
N
H
4
3
(exclusive)
(not formed)
these complexes exhibits that the complex 2 results higher
yield of the benzimidazole 3 than the complex 1. While the
condensation reaction of o-phenylenediamine with benz-
aldehyde without any catalyst (Table 1, entry 3) led to low
yield. These observations show that vanadium–salen
employs an important role in synthesis of benzimidazole.
According to Table 1, the best results were obtained with
the use of 5.0 mol % of VCl2(salen) 2. When the amount of
the catalyst was lower, the yield of the product decreased,
whereas raising the catalyst concentration did not lead to an
appreciable increase in the yield. The product was obtained
in low yield if this reaction was operated under nitrogen
atmosphere (Table 1, entry 4). That is to say that O2 plays
an important role in this reaction.
be the oxidative cyclization step [56]. Furthermore, we
found that it is not necessary to prepare Schiff bases in
advance. The vanadium(IV)–salen complex may be oxi-
dized under the oxidation conditions to vanadium(V)–salen
complex [VV(O2) (C)]. The vanadium(V)–salen complex
C attacks the C–H bond of cyclic hydrobenzimidazole
B and dissociates a C–H bond, forming an alkyl radical
intermediate [Rꢀ HOOV (D)], similar to the reported
experiment for the oxidation of adamantane by a VO(a-
cac)2 [102]. Then the oxidative dehydrogenation of adduct
D affords the desired 2-arylbenzimidazole, and the simul-
taneous generation of H2O2, as shown in Scheme 3.
H2O2 as efficient oxidant has been shown to catalyze the
oxidation of some organic reactions [52, 53, 68, 103]. We
thought that, in the presence of H2O2, the rate of this
reaction might increases. As we expected, in the presence
of H2O2 (1.0 mmol), benzaldehyde (1.0 mmol) and o-
phenylenediamine (1.0 mmol) 5.0 mol% of vanadium
(IV)–salen complex in 10 ml ethanol, the reaction time
reduced. The mechanism for the enhancement in the rate of
this reaction, in the presence of H2O2, can be described as
follows. In the presence of H2O2, oxidation of B by
vanadium(V) is enhanced because of the regeneration of
vanadium(V) by H2O2 oxidizing vanadium(IV) [104]. The
redox cycling between vanadium(IV)/vanadium(V) H2O2
also produces a highly reactive OH radical, which can then
attack to D, as shown in Scheme 3.
During our optimization studies, the effect of solvents
was also tested. We found that the solvent plays a signifi-
cant role in terms of the reaction rate and the isolated yield.
Among various solvents tested, ethanol clearly stands out as
the solvent of choice with its fast reaction rate, high yield,
cheapness, selectivity and environmental acceptability.
We proposed a mechanism for the vanadium(IV) salen
complex catalyzed reaction for the synthesis of 2-substi-
tuted benzimidazole derivatives (Scheme 3). The reaction
of amine with aryl aldehyde forms the imine A, which
exists in equilibrium with the cyclic hydrobenzimidazole
B [39–47, 54–56]. To explore a plausible mechanism for
this reaction, aldehyde consumption was monitored by GC
analysis and we found that the Schiff base is formed
quickly and the rate-determining step (RDS) was found to
Evidence related to the proposed mechanism:
To investigate the effect of oxygen on the oxidation state of
vanadium–salencomplex, theprocedurewasfirstfollowedfor
the synthesis of vanadiumcomplex in both air atmosphere and
under oxygen free environment such as nitrogen atmosphere.
Then, to 5.0 ml of water suspension of the sample (0.1 g),
synthesized under oxygen free atmosphere, ozone was bub-
bled with 5 ml min-1. The TG analysis revealed that, in the
synthesized vanadium complex, an ozone molecule was
substituted with a chloride. This was evaluated based on the
difference (1.3%) between the molecular weight of ozone
(48 g mol-1) and atomic weight of chloride (35.5 g mol-1).
Also, thePlatoofthethermogramprovedthatozonewasacted
as a good oxidizing agent for oxidation of V(IV) into V(V).
Therefore, vanadium(V)–salen complex can be easily formed
using oxidizing agents such as oxygen or ozone.
Table 1 The condensation reaction of o-phenylenediamine
(1.0 mmol) with benzaldehyde (1.0 mmol) using either VO–salen 1
and VCl2–salen 2 as catalysts in ethanol at room temperature
Entry
Catalyst
Catalyst
(mol %)
Time
(h)
Yield
(%)a
1
2
3
4
5
6
7
a
1
2
–
2
2
2
2
5
5
2
2
89
93
–
12
2
\10
30b
61
5
3
2
4
2
75
10
2
93
Isolated yield
b
Operated under nitrogen atmosphere
123

J IRAN CHEM SOC (2012) 9:189–204
197
Scheme 3 The proposed
mechanism for the
vanadium(IV)–salen complex
catalyzed the synthesis of
2-substituted benzimidazole
derivatives
O
NH2
NH2
N
H
-H2O
fast
R
NH2
R
A
VIV
O2
N
H2O2
N
H
R
VV(O2)
C
H
N
H
H
N
H
N
.
HOOVIV
R
B
N
H
D
R
H
N
.
H
H+
VIV
N
D
R
N
N
R
H
H
H2O
N
H
.
OH
N
H
H
N
VIV
H2O
.
R
B
N
H
D
R
The synthesis of benzimidazole derivates in the presence
of NaOH crystals lead to the decomposition of vanadium
complex at higher temperature (228 °C) that was 43 °C
higher than the result obtained in the thermogram of
vanadium–salen complex. This difference also points to the
formation of NaOOVIV as intermediate material during the
synthesis of benzimidazole derivatives using vanadium–
salen as catalyst.
According to the voltammograms the sharp peak posi-
tioned at 0.538 V is related to the conversional V(IV)
complex into V(V), whereas the cathodic peak positioned
at 0.485 V reveals to the reverse reduction process of
vanadium complex. The difference between these two
voltages is 53 mV, which reveals one electron exchange
during the oxidation process of V(IV) complex. This shows
that V with 5 oxidation number is considered as an inter-
mediate during the catalytic activity of the catalyst during
the synthesis of organic compounds.
To investigate more on the formation intermediate com-
pounds during the catalytic activity of the catalyst, the
electrochemical behaviors of both the ligand and the vana-
dium complex were investigated in detail as shown in Fig. 1.
In the next step, we expanded the catalytic system to the
oxidative synthesis of benzothiazoles and benzoxazole
123

198
J IRAN CHEM SOC (2012) 9:189–204
Preparation of the heterogeneous supported catalyst:
One of the general trends in catalysis is to design systems
that permit catalysts to be recycled and reused [63, 105,
106].
Since VCl2–salen 2 proved to be the best catalyst among
the vanadium–salens for the synthesis of 2-substitued
benzimidazoles, benzothiazoles and benzoxazoles, we set
out to prepare a new heterogeneous catalyst by a simple
impregnation of VCl2–salen 2 onto silica (3-aminopropyl
functionalized silica gel) [107]. The route to the synthesis
of the immobilized catalyst is shown in Scheme 7. First,
3-aminopropyl functionalized silica gel (1.0 g) and VCl2–
salen 2 (1.5 mmol) were added to CH2Cl2 under stirring at
25 °C and kept stirring for 24 h. The solid product was
separated by centrifuging and washed several times with
CH2Cl2. The solid product was then dried at 60 °C in
vacuum and their vanadium nanoparticles were examined
by an inductively coupled plasma (ICP) analyzer. To
evaluate the quantity of vanadium nanoparticles, the sup-
ported catalyst was treated with concentrated HCl (5.0 M)
and HNO3 (5.0 M), followed by ICP analysis. The vana-
dium nanoparticles were determined to be 4.281% (w/w)
(Scheme 6).
Fig. 1 Cyclic voltammograms of (A) free ligand (2 9 10-4 M) and
complex (2 9 10-4 M) in EtOH (0.1 M TBTB) at glassy carbon
working electrode at scan rate 50 mV s-1 under nitrogen atmosphere
using 2-aminothiophenol and 2-aminophenol, respectively,
with aryl aldehydes as starting materials using VCl2(salen)
complex as the catalyst.
We examined the condensation reactions of benzalde-
hyde with 2-aminothiophenol (Scheme 4). Under the opti-
mized reaction conditions, 2-aminothiophenol (1.0 mmol)
and benzaldehyde (1.0 mmol) with VCl2–salen complex
(0.05 mmol) were taken up in ethanol, and the reaction
mixture was stirred at room temperature. The reaction was
completed after 3 h and 2-phenyl-1,3-benzothiazole was
obtained in high yield (96%).
Characterization of vanadium complex supported
on silica
We have tried to develop this synthetic method for the
preparation of benzoxazoles in the presence of VCl2–salen
complex as the catalyst in ethanol as solvent at room
temperature, but it could not be applied for these com-
pounds. Instead, during the research, we found that VCl2–
salen complex could catalyze the synthesis of benzoxazoles
in xylene at 120 °C. Under the optimized reaction condi-
tions, 2-aminophenol (1.0 mmol) and benzaldehyde
(1.0 mmol) with VCl2(salen) complex (0.05 mmol) was
heated in xylene at 120 °C. The reaction was completed
within 4 h and 2-phenyl-1,3-benzoxazole was obtained in
excellent yield (95%) (Scheme 5).
In this study, vanadium–salen complex in nano scale
dimension was supported on silica substrate according to
the proposed procedure.
Figure 2 shows the scanning electron microscopic
(SEM) and atomic force micrograph (AFM) images of
vanadium–salen nanoparticles, respectively. Good corre-
lations were found between the average size evaluated for
the vanadium nanoparticles based on AFM and SEM
images. The voltage profile image (Fig. 2d) reveals that
this process has capability to synthesize vanadium particles
at nanoscale dimension.
Scheme 4 Synthesis of
benzothiazole derivatives
O
NH2
N
H
VCl2(salen) complex, 5.0 mol %
SH
S
Scheme 5 Synthesis of
benzoxazole derivatives
O
NH2
N
O
VCl2(salen) complex, 5.0 mol %
H
OH
123

J IRAN CHEM SOC (2012) 9:189–204
199
Scheme 6 Preparation of
immobilized VCl2–salen
catalyst with the linkage groups
axially connected to vanadium
atoms
Cl
V
N
O
N
O
NH2
HN
Cl
N
O
N
O
V
Si
O O
Si
Cl
O
O O
O
2
CH2Cl2, rt, 24h
Fig. 2 a SEM image of
vanadium–salen nanoparticles
supported on silica. AFM
images including: b two-
dimensional, c three-
dimensional and d voltage
profile of vanadium–salen
complex supported on silica
Also, the adsorption behavior of nitrogen on both silica
and vanadium complex supported on silica based on ther-
mogravimetric (TG) analysis results has been studied.
Significant increase (*370 m2 kg-1) was estimated in the
active surface area of silica substrate, when supported with
vanadium complexes.
820 °C is related to the decomposition of silica substrate in
air atmosphere. Also, the last part (Plato) of the thermo-
gram (7.6%) reveals the amounts of V2O5 formed
according to the decomposition of vanadium complex.
Figure 3 shows the FT-IR spectrum of vanadium–salen
complex on silica. Formation of vanadium–salen complex
is approved based on the formation of V–O bond as
revealed with accordance with the strong peaks positioned
at *700 and 825 cm-1, in the FT-IR spectrum. Also, the
peak situated at *1,080 cm-1 is related to the formation of
V–N during the supporting of the vanadium–salen complex
on the modified silica [108–110].
The thermal behavior of vanadium–salen complex on
silica was also evaluated using the TG analysis. According
to the thermogram, a significant decrease (11.2%) in the
weight percentage of vanadium complex supported on
silica at around 185 °C is related to the decomposition of
vanadium–salen complex, whereas the decrease at around
123

200
J IRAN CHEM SOC (2012) 9:189–204
related to V(001) was investigated to estimate the theoretical
average size of vanadium nanoparticles using Debye–
´
˚
Scherrer equation. For k = 1.504 A, this value was esti-
mated to around 180 nm, which was in good agreement with
the size observed on the SEM and AFM images.
A transmission electron microscopy (TEM, CM-10,
Philips, 100 kV) was used for obtaining TEM images.
Homogeneous and heterogeneous catalytic
benzimidazole, benzothiazole and benzoxazole
synthesis
VCl2–salen complex homogeneous and immobilized on a
silica surface were tested in the catalytic synthesis of
2-substituted benzimidazoles, benzothiazoles and benzox-
azoles in the presence of atmospheric oxygen as a green
oxidant.
Fig. 3 FT-IR spectrum of vanadium–salen nanoparticles on silica
Supporting the VCl2(salen) complex provided the
chance of recovery of catalyst for the successive uses. The
catalyst could be readily removed from the reaction mix-
ture by simple centrifugation and be used repeatedly for
several times without appreciable loss in activity.
The recovered immobilization of VCl2(salen) complex
was dried and used for another successive reaction run and
the combined centrifuge was collected for product identi-
fication and quantitation. The catalytic activity of immo-
bilized VCl2(salen) complex remained largely unchanged
even after five consecutive runs (Fig. 5).
In this study, the amount of vanadium nanoparticles was
evaluated using TG and ICP analyses. Based on the ICP
results, the quantity of vanadium nanoparticles was eval-
uated to be 4.281%, whereas this value was *4.3% based
on the TG analysis. The small difference between ICP and
TG analysis is probably related to the sample pretreatment
needed for the analysis of vanadium silica sample with ICP
spectrometer.
Figure 4 shows the smoothed XRD pattern on the
vanadium–salen complex supported on activated silica. As
clearly shown in Fig. 4, the strong peak at 2h = 11.9° is
related to the activated silica. The effect of activation
process is clearly revealed from the intensity of the peak.
Also, the strong peaks at 2h = 11.9°, 22.6°, 26.7°, 28.3°,
30.84°, 42.2°, 46.5°, 54.9°, and 60.7° are related to V(200),
V(001), V(101), V(110), V(400), V(002), V(411), V(601)
and (V412), respectively. In this experiment, the peak
Also, the same TEM image was observed (Fig. 6b) for
the vanadium–salen nanoparticles supported on silica after
synthesis by the proposed process and after at least five
time use in the synthesis of organic reaction.
benzimidazole
benzothiazole
benzoxazole
100
96
92
88
84
80
1
2
3
4
5
Fig. 5 Catalyst recyclability studies for 2-phenylbenzimidazole,
2-phenylbenzothiazole in ethanol at room temperature, and for
2-phenylbenzoxazole in xylene at 120 °C (time reaction for benzim-
idazoles, benzothiazoles and benzoxazole, respectively, are 2, 3 and
4 h)
Fig. 4 Smoothed XRD pattern of vanadium–salen nanoparticles
supported on silica
123

J IRAN CHEM SOC (2012) 9:189–204
201
Fig. 6 TEM images of
vanadium nanoparticles
supported on silica a at initial
time, b after at least five time
use in the synthesis of organic
reaction
Scheme 7 The condensation
O
R1
reaction of o-phenylendiamine,
2-aminothiophenol and
2-aminophenol with aryl
aldehydes in the presence of
homogeneous and
R
NH2
XH
R
N
X
VCl2(salen) complexe, 5 mol %
H
R1
heterogeneous catalysts
X = NH, S, O
R = H, CH3, COOH, PhCO, COOEt
To check the leaching, we stirred the immobilized cat-
alyst in EtOH at room temperature for 5 h. The immobi-
lized catalyst was removed and the filtrate was used for the
synthesis of benzimidazole under the described reaction
conditions. In this case, no product was obtained after 2 h.
Finally, under the optimized conditions, the condensa-
tion of o-phenylenediamine, 2-aminothiophenol and
2-aminophenol with aryl aldehydes were carried out in the
presence of homogeneous and heterogeneous catalysts in
ethanol at room temperature for benzimidazoles and ben-
zothiazoles, and in xylene at 120 °C for benzoxazoles
(Scheme 7).
2-alkylbenzimidazoles, 2-alkylbenzothiazoles and 2-alkyl-
benzoxazoles in the presented procedure. The reaction of
3,4-diaminotoluene, 3,4-diaminobenzoic acid, 3,4-diami-
nobenzophenone and ethyl 3,4-diaminobenzoate with
benzaldehyde was also studied. All the substrates consis-
tently afforded the reaction selectively to the corresponding
benzimidazoles in excellent yields (Table 2, entries 2–5).
Also, we have reported the synthesis of new triazole
derivatives of benzimidazole, benzothiazole and benzox-
azole (Table 2, entries 26–28).
To access the feasibility of applying this method on a
preparative scale, we carried out the coupling of o-phen-
ylenediamine with benzaldehyde in 50-mmol scale in the
presence of the homogeneous and heterogeneous catalysts.
As expected, the reaction proceeded smoothly, similar to
the case in a smaller scale (Table 2, entry 1).
We studied the scope of the reaction by varying o-
phenylendiamines, 2-aminothiophenol and 2-aminophenol
for condensation with aryl aldehydes under the optimized
condition and the results are shown in Table 2. In all cases,
the reaction mixture was cleaned and carried out within
In conclusion, we have introduced a simple, mild and
highly efficient method for the synthesis of 2-arylbenzim-
idazole and 2-arylbenzothiazole using catalytic amount
(5.0 mol%) of vanadium(IV)–salen nanoparticles sup-
ported on silica as oxidant in ethanol under an atmospheric
oxygen at room temperature and in xylene at 120 °C for
2-arylbenzoxazole. This catalyst can also be reused in the
1
1–6 h. All products were characterized by IR, H NMR,
13C NMR and mass spectrometry.
As shown in Table 2, the presented method has the
ability to tolerate other functional groups such as methyl,
hydroxy, methoxy and halogen on the aryl aldehyde. Ali-
phatic aldehydes are not applicable for the synthesis of
123

202
J IRAN CHEM SOC (2012) 9:189–204
Table 2 Synthesis of 2-arylbenzimidazoles, 2-arylbenzothiazoles and
2-arylbenzoxazole from various o-phenylendiamine, 2-aminothiophe-
nol and 2-aminophenol (1.0 mmol) with aryl aldehydes (1.0 mmol)
in the presence of homogeneous and heterogeneous VCl2–salen catalyst
(5.0 mol%) in ethanol at room temperature for benzimidazoles and
benzothiazoles, and in xylene at 120 °C for benzoxazole
Table 2 continued
H3CO
N
16
3
92
93
3
92
95
O
OCH3
N
Homogeneous catalyst Heterogeneous catalyst
17
3.5
3.5
N
H
Entry
Product
Time (h) Yield (%)a Time (h) Yield (%)a
N
18
4
4
94
92
4
4
94
92
S
N
1
2
2
93
94
2
94
94
N
H
N
19
O
H3
C
N
1.5
1.5
N
N
H
Cl
20
5.5
91
5.5
90
N
H
O
C
N
HO
Ph
N
3
4
1
6
94
94
1
6
94
95
Cl
Cl
21
6
4
90
94
6
4
92
93
N
H
S
N
O
C
22
N
O
N
H
N
O
23
1.5
94
1.5
94
N
H
O
C
N
EtO
5
6
1.5
3
92
96
1.5
3
92
96
N
N
H
O
O
24
2
4
90
90
2
4
91
92
S
N
S
N
25
O
N
O
N
7
8
4
95
92
4
96
93
O
N
N
OH
OH
N
H
26
27
2
91
89
2
90
89
N
N
CH3
1.5
1.5
N
H
N
S
O
O
N
N
2.5
2.5
N
N
S
CH3
CH3
9
1.5
3
95
94
1.5
3
95
96
N
O
N
N
N
N
O
OH
10
28
4
89
4
90
N
OCH3
11
12
13
3
2.5
4
91
94
92
3
2.5
4
90
96
93
N
H
a
Isolated yield
N
S
OCH3
OCH3
N
O
oxidation of 2-substituted benzimidazoles, benzothiazoles
and benzoxazoles for several times. Hence, we believe that
it will find wide application in organic synthesis as well as
in industry.
H3CO
N
14
15
5
5
85
90
5
5
88
91
N
H
OCH3
Acknowledgments We gratefully acknowledge the support of this
work by the Shiraz University Research Council. We are also grateful
to Mr. H. Sajedian Fard and Mr. M. S. Darvish Tafvizi for their
helpful cooperation.
H3CO
N
S
OCH3
123

J IRAN CHEM SOC (2012) 9:189–204
203
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