Chromium(III)-salen complex nanoparticles on AlPO4: as an efficient heterogeneous and reusable nanocatalyst for mild synthesis of highly functionalized piperidines, 2-arylbenzimidazoles, and 2-arylbenzothiazoles

Article abstract of DOI:10.1007/s13738-017-1097-x

Abstract: A simple, convenient, and highly efficient multicomponent one-pot synthesis of highly functionalized piperidines has been developed via tandem reactions of β-keto esters, aromatic aldehydes, and various amines in ethanol at room temperature using catalytic amount (10?mol%) of a chromium(III)-salen complex nanoparticles supported on AlPO₄. The heterogeneous catalyst exhibited excellent activity and reusability (up to 8 times) in the synthesis of highly functionalized piperidines. Also, synthesis of 2-arylbenzimidazoles and 2-arylbenzothiazoles have been efficiently developed under mild condition from o-phenylenediamines or 2-aminothiophenol with aryl aldehydes via one-step process using catalytic amount (2.0?mol%) of nanocatalyst in air atmosphere as a green oxidant. The heterogeneous catalyst was characterized by scanning electron microscopy, atomic force microscopy, inductively coupled plasma spectrometry, thermogravimetry for analysis of nitrogen adsorption, and FT-IR spectroscopy. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s13738-017-1097-x

J IRAN CHEM SOC
DOI 10.1007/s13738-017-1097-x
ORIGINAL PAPER
Chromium(III)‑salen complex nanoparticles on AlPO4:
as an effcient heterogeneous and reusable nanocatalyst
for mild synthesis of highly functionalized piperidines,
2‑arylbenzimidazoles, and 2‑arylbenzothiazoles
Hashem Sharghi1 · Mahdi Aberi1 · Mohammad Mahdi Doroodmand1,2
Pezhman Shiri1
·
Received: 19 December 2016 / Accepted: 13 March 2017
© Iranian Chemical Society 2017
Abstract A simple, convenient, and highly effcient mul-
ticomponent one-pot synthesis of highly functionalized
piperidines has been developed via tandem reactions of
β-keto esters, aromatic aldehydes, and various amines
in ethanol at room temperature using catalytic amount
(10 mol%) of a chromium(III)-salen complex nanoparticles
supported on AlPO4. The heterogeneous catalyst exhib-
ited excellent activity and reusability (up to 8 times) in the
synthesis of highly functionalized piperidines. Also, syn-
thesis of 2-arylbenzimidazoles and 2-arylbenzothiazoles
have been effciently developed under mild condition
from o-phenylenediamines or 2-aminothiophenol with
aryl aldehydes via one-step process using catalytic amount
(2.0 mol%) of nanocatalyst in air atmosphere as a green
oxidant. The heterogeneous catalyst was characterized
by scanning electron microscopy, atomic force micros-
copy, inductively coupled plasma spectrometry, thermo-
gravimetry for analysis of nitrogen adsorption, and FT-IR
spectroscopy.
Electronic supplementary material The online version of this
article (doi:10.1007/s13738-017-1097-x) contains supplementary
material, which is available to authorized users.
* Hashem Sharghi
shashem@chem.susc.ac.ir
1
Department of Chemistry, Shiraz University, Shiraz 71454,
Islamic Republic of Iran
2
Nanotechnology Research Institute, Shiraz University,
Shiraz, Islamic Republic of Iran
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J IRAN CHEM SOC
Graphical Abstract
antimalarial [11, 13]. Owing to their importance in bioor-
ganic and medicinal chemistry, there are several reports
about synthesis of these compounds [14–17].
Keywords Chromium(III)-salen complex · Heterogeneous
catalyst · Highly functionalized piperidines ·
2-Arylbenzimidazoles · 2-Arylbenzothiazoles ·
Nanoparticles
The synthesis of highly substituted piperidines have
been developed using several approaches such as intra-
molecular Michael reactions [14], tandem cyclopropane
ring-opening/Conia-ene cyclization [15], aza-Prins-
cyclizations [16, 17], intramolecular Mannich reaction
onto iminium ions [18], imino Diels–Alder reactions [19,
20], cyclohydrocarboxylation [21], aziridine ring expan-
sion [22], and radical cyclization [23]. An alternative
strategy for the preparation of these compounds is using
MCRs. The highly substituted piperidines have been
reported using MCRs strategy by employing calix[n]
arenes [11], l-proline/TFA [13], tetrabutylammonium
tribromide (TBATB) [24], CAN [25], picric acid [26],
Fe@Si-Gu-Prs [27], l-proline nitrate [28], ZnO NPs [29],
and ZrP2O7 nanoparticles [30]. However, many of these
methods have some drawbacks such as low yield, expen-
sive and excess amount of catalysts, long reaction time
and non-recyclability of the catalyst. Therefore, there is
a need for highly effcient, versatile, and eco-friendly
synthetic protocol to obtain functionalized piperidines in
excellent yields.
Introduction
In recent years, one-pot multi-component reactions (MCRs)
have emerged as powerful synthetic tools in organic syn-
thesis due to their advantages over the conventional mul-
tistep synthesis [1–6]. They provide major advantages
including shorter reaction time, lower cost, energy saving,
high atom-economy, inexpensive purifcation, and lack of
waste products [7–10]. The MCRs can be good candidates
for improving synthesis routes in terms of minimal waste
and feasible reactions which satisfy some of the principles
of ‘Green Chemistry’ [2].
Piperidine heterocyclic systems are common in a num-
ber of biologically important natural products and as
important units in pharmaceuticals [11, 12]. These com-
pounds have been extensively studied due to their wide
range of application, e.g., biological activities such as
antiparasitic, antiviral, anticancer, antimicrobial, and
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J IRAN CHEM SOC
The benzimidazole and benzothiazole derivatives are
found in various bioactive compounds having antiulcer
[31, 32], antihistaminic [33], antiviral [34], antihyper-
tensive [35], and anticancer [36, 37] properties. In addi-
tion, benzimidazoles are very important intermediates in
organic reactions [38, 39]. Therefore, the facile synthesis
of benzimidazoles and benzothiazole has gained consid-
erable attention in recent years [40–43].
One of the classical methods for the synthesis of
2-arylbenzimidazoles and 2-arylbenzothiazoles is the
reaction of o-phenylenediamines or 2-aminophenol with
various aldehydes [44]. Various oxidative reagents, such
nitrobenzene (high-boiling oxidant/solvent) [45, 46],
1,4- benzoquinone [47], DDQ [48], air [44], benzofuro-
xan [49], MnO2 [50], In(OTf)3 [43], H2O2/HCl [51],
NaHSO3 [52, 53], and Na2S2O5 [54], cobalt(III)-salen
complex supported on activated carbon [55], Cu(II) com-
plex [56], copper nanoparticles on activated carbon [57],
vanadium(IV)-salen complex nanoparticles immobilized
onto silica [58], sodium perborate [59] and surfactant
(DBSA) as catalyst and I2 as co-catalyst [60] have been
employed in procedures.
in pure deuterated solvents with tetramethylsilane as an
internal standard. The IR spectra were obtained using a
Shimadzu FT-IR 8300 spectrophotometer. Scanning elec-
tron micrographs were obtained by SEM instrumentation
(SEM, XL-30 FEG SEM, Philips, at 20 kV). An atomic
forced microscope model DME-SPM, version 2.0.0.9 was
used for taken AFM images. Mass spectra were deter-
mined on a Shimadzu GCMS-QP 1000 EX instruments
at 70 or 20 eV. Melting points were determined in open
capillary tubes in a Büchi-535 circulating oil melting
point apparatus. The thermogravimetry (TG) of the sam-
ples was analyzed using a laboratory-made TGA instru-
ment. The ICP analysis data were obtained using a Var-
ian Vista-pro analyzer. The purity determination of the
substrates and reaction monitoring were accomplished by
TLC on silica gel PolyGram SILG/UV 254 plates. Col-
umn chromatography was carried out on short glass col-
umns packed with silica gel 60 (70–230 mesh). The Schiff
base N,N′-salicylaldehyde)ethylenediamine (Salen) was
prepared according to a reported procedures [58, 75]. The
Cr(III)-salen complex was synthesized based on a previ-
ously reported procedure [76, 77]. A mixture of Salen and
CrCl3-6H2O was refuxed, while stirring, in ethanol. The
solid product was fltered, washed with cold ethanol, dried
under vacuum, and fnally was recrystallized in chloro-
form. The AlPO4 and AlPO4-ethylenediamine were syn-
thesized by following a reported procedure [78–80].
The most important topics in modern synthetic organic
chemistry is development of effcient reagents and reus-
able catalytic systems [61]. Although homogeneous cata-
lysts have some limitations such as instability, diffculty
in application capabilities, corrosion problems and non-
recyclability, their immobilization can facilitate recovery
which is of considerable attention in both research and
industrial activities [62].
Immobilization of Cr(III)‑salen complexes onto silica
Both supported and unsupported chromium salen com-
plex catalysts show wide applicability such as oxida-
tion of benzaldehyde [63], selective oxidation of benzyl
alcohol [64], asymmetric ring opening of epoxides [65],
enantioselective intramolecular addition of tertiary ena-
mides to ketones [66], asymmetric hetero Diels–Alder
reaction [67], and enantioselective alkylation of acyclic
α,α-disubstituted tributyltin enolates [68].
In continuation to our previous study on preparation and
applications of effcient heterogeneous catalysts [69–74],
this paper reports a simple, mild, and highly effcient one-
pot methodology for synthesis of highly substituted piperi-
dines, 2-arylbenzimidazoles and 2-arylbenzothiazoles.
Chromium(III)-salen complex (0.1 g) was added to
AlPO4-ethylenediamine (1.0 g) and then was vigor-
ously stirred followed by ultrasonic treatment for 10 min
to ensure uniform dispersion of Cr(III)-salen complex on
AlPO4-ethylenediamine surface. Aliquot of 75 mL of dry tol-
uene was added to the mixture while was stirred vigorously
and refuxed for 6 h. The resulting solid was fltered, soxhlet-
extracted with ethanol/acetonitrile (v/v = 1) as solvent for
12 h to remove the homogeneous complexes adsorbed on the
support before being dried at 80 °C for overnight.
General procedure for the synthesis of highly
functionalized piperidines in the presence
of chromium(III) salen complex as a homogeneous
catalyst
Experimental section
To a solution mixture containing β-keto ester (1.0 mmol),
aromatic aldehyde (2.0 mmol) and amine (2.0 mmol) in
5 mL of ethanol, chromium(III) salen complex (10 mol%)
was added followed by stirring at room temperature. After
completion of the reaction, the solvent was evaporated to
give the crude product, which was purifed by silica gel
Instrumentation, analysis and starting material
Chemical materials and solvents were purchased from
either Fluka, Aldrich or Merck Companies. The NMR
spectra were recorded on a Bruker Avance DPX-250 (1H-
NMR 250 MHz and 13C NMR 62.9 MHz) spectrometer
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J IRAN CHEM SOC
column chromatography (short column) employing n-hex-
gel column chromatography with n-hexane/ethyl acetate
ane/ethyl acetate (10:1) as eluent.
(10:1) to afford pure 2-arylbenzothiazole.
General procedure for the synthesis of highly
functionalized piperidines in the presence
of chromium(III) salen complex nanoparticles
supported on AlPO4 as a heterogeneous catalyst
Spectroscopic data
Methyl 1,2,6‑triphenyl‑4‑(phenylamino)‑1,2,5,6‑
tetrahydropyridine‑3‑carboxylate (1)
To a mixture of β-keto ester (1.0 mmol), aromatic alde-
hyde (2.0 mmol) and amine (2.0 mmol) in ethanol
(5 mL), chromium(III) salen complex supported on AlPO4
(10 mol%) as a heterogeneous catalyst was added. The
mixture was stirred at room temperature. The progress of
the reaction was monitored by TLC using n-hexane/ethyl
acetate (10:1) as eluent. After completion of the reac-
tion, the mixture was centrifuged and rinsed with CH2Cl2
(3 × 15 mL). The recovered solid (heterogeneous) cata-
lyst was dried and stored for another consecutive runs. The
product was concentrated by evaporating the solvent which
was subsequently purifed through fltering and washing
with aqueous ethanol. To ensure purifcation, silica gel col-
umn chromatography (short column) was employed using
n-hexane/ethyl acetate (10:1) as eluent.
White solid; mp 194–196 °C; (Lit. [25] mp 194–195 °C).
IR (KBr): 694, 748, 925, 979, 1033, 1072, 1257, 1373,
1450, 1496, 1589, 1658, 2862, 2954, 3016, 3240,
3440 cm−1 1H NMR (CDCl3, 250 MHz):δ 2.84 (dd,
.
J1 = 15.0 Hz, J2 = 2.5 Hz, 1 H), 2.96 (dd, J1 = 15.0 Hz,
J2 = 5.5 Hz, 1 H), 4.01 (s, 3 H), 5.23–5.24 (m, 1 H), 6.34–
6.37 (m, 2H), 6.56 (s, 1 H), 6.62 (d, J = 8.0 Hz, 2 H), 6.68
(t, J = 7.25 Hz, 1 H), 7.10–7.17 (m, 5 H), 7.23–7.43 (m, 10
H), 10.36 (s, 1 H). 13C NMR (CDCl3, 62.9 MHz): δ 33.8,
51.2, 55.3, 58.3, 98.1, 113.1, 116.4, 126.0, 126.5, 126.8,
127.3, 128.4, 128.6, 128.8, 129.0, 129.1, 138.0, 142.9,
144.1, 147.1, 156.4, 167.7. Mass m/z (%): 462 (M+ + 2,
5.3), 461 (M+ + 1, 12.4), 460 (M+, 12.6), 384 (100.0), 351
(23.6), 278 (34.1), 246 (25.2), 220 (21.0), 180 (21.4), 104
(15.6), 77 (29.7). Anal. Calcd for C31H28N2O2 (460.574):
C, 80.84; H, 6.13; N, 6.08; found: C, 80.69; H, 6.22; N,
6.19.
General procedure for the synthesis
of 2‑arylbenzimidazoles in the presence
of chromium(III) salen complex nanoparticles
supported on AlPO4
Methyl 4‑(p‑toluidino)‑2,6‑diphenyl‑1‑p‑tolyl‑1,2,5,6‑
tetrahydropyridine‑3‑carboxylate (2)
A mixture of o-phenylenediamine (1.0 mmol), aryl alde-
hyde (1.0 mmol) and chromium(III) salen complex nano-
particles supported on AlPO4 (2.0 mol%) in ethanol (5 mL)
was stirred at room temperature. After completion of the
reaction (TLC), the mixture was centrifuged and rinsed
with ethanol (3 × 15 mL). The recovered nanocatalyst
was dried and stored for another consecutive runs. The sol-
vent was evaporated to give the crude product, which was
purifed by silica gel column chromatography employing
n-hexane/ethyl acetate (8:1) as eluent.
White solid; mp 222–224 °C; (Lit. [11] mp 220–222 °C).
IR (KBr): 702, 1072, 1257, 1319, 1380, 1450, 1512, 1596,
1
1658, 3425 cm−1. H NMR (CDCl3, 250 MHz): δ 2.17 (s,
3 H), 2.27 (s, 3 H), 2.74 (dd, J1 = 15.0 Hz, J2 = 2.5 Hz,
1 H), 2.89 (dd, J1 = 15.0 Hz, J2 = 5.5 Hz, 1 H), 3.94
(s, 3 H), 5.12–5.14 (m, 1 H), 6.17 (d, J = 8.2 Hz, 2 H),
6.43–6.46 (m, 3 H), 6.87–6.92 (m, 4 H), 7.17–7.35 (m,
10 H), 10.19 (s, 1 H). 13C NMR (CDCl3, 62.9 MHz): δ
20.2, 20.9, 33.6, 51.0, 55.2, 58.3, 56.6, 97.5, 112.9, 125.1,
126.0, 126.3, 126.5, 126.7, 127.1, 128.3, 128.7, 129.5,
135.2, 135.7, 143.1, 144.3, 144.9, 156.7, 168.7. Mass m/z
(%):490 (M+ + 2, 0.5), 489 (M+ + 1, 2.9), 488 (M+, 4.6),
414 (3.2), 397 (33.6), 290 (14.0), 261 (11.8), 235 (12.7),
196 (48.1), 145 (21.4), 118 (13.7), 99 (18.1), 83 (100.0), 57
(74.9). Anal. Calcd for C33H32N2O2 (488.627): C, 81.12; H,
6.60; N, 5.73; found: C, 81.23; H, 6.71; N, 5.82.
General procedure for the synthesis
of 2‑arylbenzothiazoles in the presence
of chromium(III) salen complex nanoparticles
supported on AlPO4
A mixture of 2-aminothiophenol (1.0 mmol) and arylalde-
hyde (1.0 mmol) was stirred in 5.0 mL of ethanol in the
presence of nanocatalyst (2.0 mol%) at 50 °C. After the
reaction was completed, the mixture was centrifuged and
rinsed with ethanol (3 × 15 mL). The solvent was evapo-
rated to give the crude product, which was purifed by silica
Methyl 1‑(4‑methoxyphenyl)‑4‑(4‑methoxyphenylamino)‑
2,6‑diphenyl‑1,2,5,6‑tetrahydropyridine‑3‑carboxylate (3)
White solid; mp 225–227 °C; (Lit. [25] mp 224–225 °C).
IR (KBr): 702, 840, 1033, 1072, 1180, 1242, 1450, 1512,
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J IRAN CHEM SOC
1604, 1651, 2839, 3255, 3417 cm−1. H NMR (CDCl3,
250 MHz): δ 2.65 (dd, J1 = 15.2 Hz, J2 = 2.5 Hz, 1 H),
2.81 (dd, J1 = 15.7 Hz, J2 = 5.7 Hz, 1 H), 3.67 (s, 3 H),
3.75 (s, 3 H), 3.93 (s, 3 H), 5.07–5.08 (m, 1 H), 6.21 (d,
J = 8.7 Hz, 2 H), 6.36 (s, 1 H), 6.46 (d, J = 9.0 Hz, 2
H), 6.61–6.69 (m, 4 H), 7.18–7.31 (m, 10 H), 10.12 (s, 1
H). 13C NMR (CDCl3, 62.9 MHz): δ 33.6, 50.9, 55.4, 55.6,
55.7, 58.3, 97.0, 114.0, 114.5, 126.2, 126.5, 126.8, 127.1,
127.9, 128.2, 128.6, 130.6, 141.5, 143.3, 144.3, 150.9,
157.0, 157.8, 168.6. Mass m/z (%): 522 (M+ + 2, 0.2), 521
(M+ + 1, 1.3), 520 (M+, 1.6), 427 (1.9), 308 (13.2), 210
(16.1), 83 (100.0). Anal. Calcd for C33H32N2O4 (520.625):
C, 76.13; H, 6.20; N, 5.38; found: C, 76.05; H, 6.31; N,
5.46.
529 (M+, 1.9), 417 (32.6), 385 (11.9), 312 (23.9), 254
(27.9), 216 (16.6), 120 (13.1), 83 (100.0). Anal. Calcd
for C31H26Cl2N2O2 (529.464): C, 70.32; H, 4.95; N, 5.29;
found: C, 70.41; H, 5.08; N, 5.35.
1
Methyl 2,6‑bis(4‑methoxyphenyl)‑1‑phenyl‑
4‑(phenylamino)‑1,2,5,6‑tetrahydropyridine‑3‑carboxylate
(6)
White solid; mp 184–186 °C; (Lit. [25] mp 182–183 °C).
IR (KBr): 694, 756, 1033, 1072, 1180, 1249, 1373, 1450,
1504, 1596, 1651, 2839, 2954, 3232, 3448 cm−1. 1H NMR
(CDCl3, 250 MHz): δ 2.78 (dd, J1 = 15.0 Hz, J2 = 2.7 Hz,
1 H), 2.89 (dd, J1 = 15.0 Hz, J2 = 5.2 Hz, 1 H), 3.80 (s,
3 H), 3.81 (s, 3 H), 3.95 (s, 3 H), 5.11–5.12 (m, 1 H),
6.37–6.40 (m, 3 H), 6.56 (d, J = 8.0 Hz, 2 H), 6.63 (t,
J = 7.2 Hz, 1 H), 6.86 (d, J = 8.5 Hz, 4 H), 7.06–7.15 (m,
7 H), 7.23–7.27 (m, 2 H), 10.31 (s, 1 H). 13C NMR (CDCl3,
62.9 MHz): δ 33.8, 51.1, 54.6, 55.2, 55.3, 57.6, 98.2, 113.0,
113.7, 114.1, 116.2, 125.8, 127.5, 127.8, 129.0, 134.7,
135.9, 138.0, 147.1, 156.4, 158.2, 158.8, 168.7. Mass m/z
(%): 522 (M+ + 2, 0.6), 521 (M+ + 1, 1.8), 520 (M+, 2.1),
414 (6.3), 308 (15.4), 210 (17.9), 83 (100.0). Anal. Calcd
for C33H32N2O4 (520.625): C, 76.13; H, 6.20; N, 5.38;
found: C, 76.01; H, 6.04; N, 5.46.
Methyl 1‑phenyl‑4‑(phenylamino)‑2,6‑dip‑tolyl‑1,2,5,6‑
tetrahydropyridine‑3‑carboxylate (4)
White solid; mp 219–220 °C; (Lit. [25] mp 215–216 °C).
IR (KBr): 694, 748, 1072, 1188, 1257, 1319, 1373,
1442, 1504, 1589, 1658, 2862, 2947, 3016, 3232 cm−1
.
1H NMR (CDCl3, 250 MHz): δ 2.35 (s, 3 H), 2.37 (s, 3
H), 2.79 (dd, J1 = 15.0 Hz, J2 = 2.5 Hz, 1 H), 2.91 (dd,
J1 = 15.0 Hz, J2 = 5.5 Hz, 1 H), 3.95 (s, 3 H), 5.14–
5.15 (m, 1 H), 6.31–6.35 (m, 2 H), 6.44 (s, 1 H), 6.56 (d,
J = 8.0 Hz, 2 H), 6.62 (t, J = 7.2 Hz, 1 H), 7.06–7.13 (m,
11 H), 7.24 (d, J = 8.0 Hz, 2 H), 1029 (s, 1 H). 13C NMR
(CDCl3, 62.9 MHz): δ 21.1, 21.2, 33.8, 51.1, 55.0, 58.0,
98.2, 113.0, 116.1, 125.7, 125.9, 126.4, 126.7, 128.9,
129.0, 129.1, 129.4, 135.9, 136.7, 138.0, 139.8, 141.1,
147.2, 156.4, 168.7. Mass m/z (%): 490 (M+ + 2, 0.1),
489 (M+ + 1, 1.3), 488 (M+, 2.1), 413 (3.9), 196 (28.7),
145 (13.8), 99 (13.7), 83 (100.0), 57 (61.2). Anal. Calcd
for C33H32N2O2 (488.627): C, 81.12; H, 6.60; N, 5.73;
found: C, 81.24; H, 6.71; N, 5.65.
Methyl 1‑(4‑chlorophenyl)‑4‑(4‑chlorophenylamino)‑
2,6‑dip‑tolyl‑1,2,5,6‑tetrahydropyridine‑3‑carboxylate (7)
White solid; mp 219–221 °C; (Lit. [7] mp 218–220 °C).
IR (KBr): 756, 810, 1072, 1188, 1265, 1334, 1373, 1458,
1496, 1604, 1651, 2869, 2947, 3240, 3448 cm−1. 1H NMR
(CDCl3, 250 MHz): δ 2.35 (s, 3 H), 2.38 (s, 3 H), 2.73(dd,
J1 = 15.2 Hz, J2 = 2.2 Hz, 1 H), 2.90 (dd, J1 = 15.0 Hz,
J2 = 5.7 Hz, 1 H), 3.97 (s, 3 H), 5.10–5.12 (m, 1 H), 6.23
(d, J = 8.5 Hz, 2 H), 6.38 (s, 1 H), 6.48 (d, J = 9.0 Hz, 2
H), 7.00–7.22 (m, 12 H), 10.25 (s, 1 H). 13C NMR (CDCl3,
62.9 MHz): δ 21.1, 21.2, 33.6, 51.2, 55.2, 58.1, 98.7,
114.1, 121.1, 126.3, 126.5, 127.0, 128.7, 128.9, 129.0,
129.1, 129.5, 131.3, 136.2, 136.5, 137.1, 139.2, 140.2,
145.6, 155.6, 168.5. Mass m/z (%): 559 (M+ + 2, 5.9), 558
(M+ + 1, 14.6), 557 (M+, 11.5), 445 (73.3), 413 (27.0),
326 (71.6), 268 (57.9), 230 (49.4), 165 (10.3), 125 (19.0),
91 (100.0). Anal. Calcd for C33H30Cl2N2O2 (557.517): C,
71.09; H, 5.42; N, 5.02; found: C, 71.18; H, 5.54; N, 5.13.
Methyl 2,6‑bis(4‑chlorophenyl)‑1‑phenyl‑4‑(phenylamino)‑
1,2,5,6‑tetrahydropyridine‑3‑carboxylate (5)
White solid; mp 225–227 °C; (Lit. [25] mp 225–226 °C).
IR (KBr): 694, 748, 856, 979, 1072, 1188, 1265, 1326,
1372, 1404, 1504, 1596, 1651, 2869, 3039, 3240 cm−1
.
1H NMR (CDCl3, 250 MHz): δ 2.78 (dd, J1 = 15.0 Hz,
J2 = 2.5 Hz, 1 H), 2.88 (dd, J1 = 15.2 Hz, J2 = 5.2 Hz,
1 H), 3.96 (s, 3 H), 3.14–3.15 (m, 1 H), 6.42–6.46 (m, 3
H), 6.50 (d, J = 8.2 Hz, 2 H), 6.68 (t, J = 7.2 Hz, 1 H),
7.08–7.28 (m, 13 H), 10.31 (s, 1 H). 13C NMR (CDCl3,
62.9 MHz): δ 33.8, 51.3, 54.8, 57.4, 97.6, 113.0, 116.8,
125.8, 126.1, 127.9, 128.2, 128.5, 128.9, 129.1, 129.2,
132.2, 132.9, 137.7, 141.0, 142.5, 146.5, 156.1, 168.3.
Mass m/z (%): 531 (M+ + 2, 1.4), 530 (M+ + 1, 3.8),
Methyl 4‑(p‑toluidino)‑2,6‑bis(4‑methoxyphenyl)‑
1‑p‑tolyl‑1,2,5,6‑tetrahydropyridine‑3‑carboxylate (8)
White solid; mp 191–193 °C; (Lit. [11] mp 181 °C).IR
(KBr): 802, 1033, 1072, 1172, 1249, 1458, 1512, 1604,
1
1658, 2839, 3008, 3232, 3440 cm−1. H NMR (CDCl3,
250 MHz): δ 2.18 (s, 3 H), 2.29 (s, 3 H), 2.70–2.76 (m, 1
1 3

J IRAN CHEM SOC
H), 2.83 (dd, J1 = 15.0 Hz, J2 = 4.7 Hz, 1 H), 3.79 (s, 3
H), 3.81 (s, 3 H), 3.93 (s, 3 H), 5.06–5.08 (m, 1 H), 6.26
(d, J = 7.2 Hz, 2 H), 6.34 (s, 1 H), 6.46 (d, J = 7.5 Hz,
2 H), 6.83 (d, J = 8.7 Hz, 4 H), 6.91 (t, J = 9.0 Hz, 4 H),
7.09 (d, J = 7.5 Hz, 2 H), 7.24 (d, J = 9.2 Hz, 2 H), 10.22
(s, 1 H). 13C NMR (CDCl3, 62.9 MHz): δ 20.2, 20.9, 33.7,
50.9, 54.7, 55.2, 55.3, 57.5, 97.6, 113.0, 113.6, 114.0,
125.0, 125.9, 127.5, 127.8, 129.4, 129.5, 135.0, 135.3,
135.6, 136.2, 144.9, 156.8, 158.1, 158.7, 168.7. Mass m/z
(%): 550 (M+ + 2, 7.4), 549 (M+ + 1, 14.1), 548 (M+,
15.3), 458 (22.2), 425 (34.6), 323 (66.5), 264 (41.4), 210
(100.0), 123 (34.1), 83 (51.0). Anal. Calcd for C35H36N2O4
(548.679): C, 76.62; H, 6.61; N, 5.11; found: C, 76.51; H,
6.79; N, 5.24.
150.8, 157.0, 157.7, 168.7. Mass m/z (%): 550 (M+ + 2,
3.4), 549 (M+ + 1, 8.0), 548 (M+, 12.7), 458 (14.8), 425
(24.4), 322 (56.1), 264 (38.3), 225 (100.0), 123 (32.1), 83
(74.8). Anal. Calcd for C35H36N2O4 (548.679): C, 76.62; H,
6.61; N, 5.11; found: C, 76.51; H, 6.69; N, 5.22.
Methyl 1‑(4‑chlorophenyl)‑4‑(4‑chlorophenylamino)‑2,
6‑diphenyl‑1,2,5,6‑tetrahydropyridine‑3‑carboxylate (11)
White solid; mp 218–219 °C. IR (KBr): 694, 732, 802,
1080, 1188, 1260, 1319, 1333, 1450, 1496, 1596, 1651,
1
3255, 3448 cm−1. H NMR (CDCl3, 250 MHz): δ 2.68–
2.74 (m, 1 H), 2.88 (dd, J1 = 15.0 Hz, J2 = 5.0 Hz, 1 H),
3.96 (s, 3 H), 5.12–5.13 (m, 1 H), 6.18 (d, J = 8.2 Hz, 2
H), 6.41–6.47 (m, 3 H), 7.00–7.30 (m, 15 H), 10.22 (s,
1 H). 13C NMR (CDCl3, 62.9 MHz): δ 33.5, 51.3, 55.3,
58.3, 98.5, 114.1, 121.2, 126.3, 126.5, 126.7, 127.1, 127.5,
128.5, 128.8, 128.9, 129.1, 131.5, 136.4, 142.3, 143.2,
145.5, 155.6, 168.5. Mass m/z (%): 531 (M+ + 2, 0.6),
530 (M+ + 1, 2.0), 529 (M+, 1.1), 417 (20.8), 312 (13.5),
254 (14.7), 216 (10.3), 120 (19.6), 83 (100.0). Anal. Calcd
for C31H26Cl2N2O2 (529.464): C, 70.32; H, 4.95; N, 5.29;
found: C, 70.41; H, 5.08; N, 5.19.
Methyl 4‑(p‑toluidino)‑2,6‑bis(4‑chlorophenyl)‑1‑p‑tolyl‑
1,2,5,6‑tetrahydropyridine‑3‑carboxylate (9)
White solid; mp 214–215 °C; (Lit. [11] mp 213–215 °C).
IR (KBr): 794, 979, 1010, 1072, 1180, 1257, 1512, 1581,
1
1650, 2862, 3016, 3255, 3442 cm−1. H NMR (CDCl3,
250 MHz): δ 2.19 (s, 3 H), 2.30 (s, 3 H), 2.72 (dd,
J1 = 15.0 Hz, J2 = 3.0 Hz, 1 H), 2.81 (dd, J1 = 15.0 Hz,
J2 = 5.0 Hz, 1 H), 3.93 (s, 3 H), 5.06–5.08 (m, 1 H), 6.30
(d, J = 8.2 Hz, 2 H), 6.34 (s, 1 H), 6.38 (d, J = 8.7 Hz, 2
H), 6.90 (d, J = 8.7 Hz, 2 H), 6.97 (d, J = 7.5 Hz, 2 H),
7.08 (d, J = 8.5 Hz, 2 H), 7.24–7.28 (m, 6 H), 10.20 (s, 1
H). 13C NMR (CDCl3, 62.9 MHz): δ 20.2, 21.0, 33.6, 51.1,
54.9, 57.4, 97.0, 113.0, 125.8, 125.9, 127.9, 128.1, 128.4,
128.8, 129.6, 132.1, 132.8, 135.0, 136.0, 141.2, 142.7,
144.4, 156.4, 168.4. Mass m/z (%): 559 (M+ + 2, 1.2), 558
(M+ + 1, 5.6), 557 (M+, 4.6), 445 (36.0), 413 (10.7), 326
(41.4), 268 (24.8), 229 (27.4), 125 (12.7), 83 (100.0). Anal.
Calcd for C33H30Cl2N2O2 (557.517): C, 71.09; H, 5.42; N,
5.02; found: C, 71.23; H, 5.31; N, 5.14.
Ethyl 1‑phenyl‑4‑(phenylamino)‑2,6‑dip‑tolyl‑1,2,5,6
‑tetrahydropyridine‑3‑carboxylate (12)
White solid mp 233–235 °C; (Lit. [25] mp 230–231 °C).
IR (KBr): 694, 748, 948, 1026, 1072, 1172, 1257, 1325,
1373, 1504, 1596, 1651, 2869, 2923, 2977, 3240 cm−1
.
1H NMR (CDCl3, 250 MHz): δ 1.50 (t, J = 7.0 Hz, 3
H), 2.36 (s, 3H), 2.37 (s, 3H), 2.80 (dd, J1 = 15.0 Hz,
J2 = 2.5 Hz, 1 H), 2.92 (dd, J1 = 15.0 Hz, J2 = 5.5 Hz,
1 H), 4.33–4.53 (m, 2 H), 5.15–5.16 (m, 1 H), 6.32–6.36
(m, 2 H), 6.46 (s, 1 H), 6.56–6.66 (m, 3 H), 7.07–7.14
(m, 11 H), 7.27 (d, J = 8.0 Hz, 2 H), 10.34 (s, 1 H). 13C
NMR (CDCl3, 62.9 MHz): δ 14.9, 21.1, 21.2, 33.8, 55.0,
58.0, 59.7, 98.5, 113.0, 116.1, 125.6, 125.8, 126.4, 126.7,
128.9, 128.9, 129.0, 129.4, 135.8, 136.7, 138.1, 139.8,
141.2, 147.2, 156.2, 168.4. Mass m/z (%): 504 (M+ + 2,
0.5), 503 (M+ + 1, 3.5), 502 (M+, 3.7), 411 (21.7), 306
(19.2), 234(11.6), 194 (28.1), 97 (42.7), 57 (100.0). Anal.
Calcd for C34H34N2O2 (502.654): C, 81.24; H, 6.82; N,
5.57; found: C, 81.33; H, 6.94; N, 5.68.
Methyl 1‑(4‑methoxyphenyl)‑4‑(4‑methoxyphenylamino)‑2,
6‑dip‑tolyl‑1,2,5,6‑tetrahydropyridine‑3‑carboxylate (10)
White solid; mp 223–225 °C; (Lit. [25] mp 226–227 °C).
IR (KBr): 810, 1033, 1072, 1180, 1249, 1458, 1520,
1604, 1658, 2839, 2939, 3255, 3441 cm−1 1H NMR
.
(CDCl3, 250 MHz): δ 2.33 (s, 3 H), 2.36 (s, 3 H), 2.64 (dd,
J1 = 15.2 Hz, J2 = 2.7 Hz, 1 H), 2.80 (dd, J1 = 15.0 Hz,
J2 = 5.7 Hz, 1 H), 3.67 (s, 3 H), 3.76 (s, 3 H), 3.91 (s, 3 H),
5.02–5.03 (m, 1 H), 6.23 (d, J = 8.7 Hz, 2 H), 6.29 (s, 1 H),
6.46 (d, J = 9.0 Hz, 2 H), 6.61–6.69 (m, 4 H), 7.04–7.12
(m, 6 H), 7.20 (d, J = 8.0 Hz, 2 H), 1012 (s, 1 H). 13C
NMR (CDCl3, 62.9 MHz): δ 21.1, 21.2, 33.6, 50.9, 55.4,
55.6, 57.9, 97.1, 113.9, 114.0, 114.5, 126.4, 126.7, 127.9,
128.8, 129.2, 130.8, 135.7, 136.5, 140.2, 141.3, 141.7,
Methyl 1‑(4‑chlorophenyl)‑4‑(4‑chlorophenylamino)‑2,6‑
di(pyridin‑2‑yl)‑1,2,5,6‑tetrahydropyridine‑3‑carboxylate
(13)
White solid; mp 201–203 °C. IR (KBr): 748, 802, 1080,
1188, 1257, 1319, 1434, 1496, 1596, 1651, 2947, 3078,
3263, 3442 cm−1. 1H NMR (CDCl3, 250 MHz): δ 3.14 (dd,
J1 = 15.2 Hz, J2 = 2.2 Hz, 1 H), 3.58 (dd, J1 = 15.5 Hz,
1 3

J IRAN CHEM SOC
J2 = 6.2 Hz, 1 H), 3.91 (s, 3 H), 5.25–5.27 (m, 1 H), 6.31–
6.43 (m, 5 H), 6.98 (d, J = 9.0 Hz, 2 H), 7.09–7.19 (m, 5
H), 7.45 (d, J = 7.7 Hz, 1 H), 7.53–7.66 (m 2 H), 10.27
(s, 1 H). 13C NMR (CDCl3, 62.9 MHz): δ 32.5, 51.2, 57.4,
59.9, 96.5, 113.7, 121.1, 121.4, 121.6, 121.7, 122.2, 126.4,
128.7, 129.1, 131.2, 136.3, 136.4, 136.8, 145.5, 149.5,
156.3, 162.0, 162.9, 168.3. Mass m/z (%): 533 (M+ + 2,
4.0), 532 (M+ + 1, 5.6), 531 (M+, 5.3), 452 (12.4), 405
(5.3), 373 (8.8), 300 (11.1), 241 (8.0), 127 (10.7), 83
(100.0). Anal. Calcd for C29H24Cl2N4O2 (531.439): C,
65.54; H, 4.55; N, 10.54; found: C, 65.43; H, 4.59; N,
11.06.
Methyl 2,6‑bis(3‑nitrophenyl)‑1‑phe‑
nyl‑4‑(phenylamino)‑1,2,5,6‑tetrahydropyridine‑3‑carbox‑
ylate (16)
Pale yellow solid; mp 182–183 °C; (Lit. [25] mp 181–
182 °C). IR (KBr): 694, 748, 1072, 1188, 1257, 1350,
1496, 1535, 1596, 1658, 2862, 3417 cm−1 1H NMR
.
(CDCl3, 250 MHz): δ 2.89 (d, J = 3.7 Hz, 2 H), 3.99 (s,
3 H), 5.33–5.35 (m, 1 H), 6.39–6.50 (m, 5 H), 6.70 (t,
J = 7.2 Hz, 1 H), 7.07–7.17 (m, 5 H), 7.45–7.51 (m, 3
H), 7.67 (d, J = 7.5 Hz, 1 H), 7.95 (s, 1 H), 8.09–8.15
(m, 2 H), 8.23 (s, 1 H), 10.31 (s, 1 H). 13C NMR (CDCl3,
62.9 MHz): δ 33.7, 51.5, 55.1, 57.0, 96.7, 113.1, 117.7,
121.4, 121.5, 121.9, 122.5, 125.6, 126.6, 129.1, 129.3,
129.7, 132.5, 137.1, 144.4, 145.7, 146.3, 148.5, 148.6,
155.5, 168.0. Mass m/z (%): 551 (M+ + 1, 0.2), 550
(M+, 1.0), 428 (8.1), 396 (3.1), 307 (2.0), 265 (1.2), 196
(1.5), 118 (2.3), 83 (100.0). Anal. Calcd for C31H26N4O6
(550.567): C, 67.63; H, 4.76; N, 10.18; found: C, 67.55;
H, 4.84; N, 10.30.
Methyl 2,6‑di(thiophen‑2‑yl)‑1‑p‑tolyl‑4‑(p‑tolylamino)‑1,
2,5,6‑tetrahydropyridine‑3‑carboxylate (14)
White solid; mp 205–207 °C. IR (KBr): 694, 794, 1072,
1257, 1458, 1512, 1604, 1658, 3255, 3432 cm−1. 1H NMR
(CDCl3, 250 MHz): δ 2.21 (s, 3 H), 2.32 (s, 3 H), 2.89 (dd,
J1 = 15.2 Hz, J2 = 3.0 Hz, 1 H), 3.09 (dd, J1 = 15.2 Hz,
J2 = 5.2 Hz, 1 H), 3.91 (s, 3 H), 5.36–5.40 (m, 1 H), 6.41
(s, 1 H), 6.50 (d, J = 8.0 Hz, 2 H), 6.69 (d, J = 8.7 Hz,
2 H), 6.81–7.04 (m, 8 H), 7.12–7.15 (m, 2 H), 10.38 (s, 1
H). 13C NMR (CDCl3, 62.9 MHz): δ 20.3, 21.0, 34.2, 51.0,
52.8, 53.6, 96.8, 113.6, 116.7, 123.5, 124.0, 124.2, 124.3,
124.4, 125.8, 126.3, 126.5, 126.6, 129.4, 129.5, 129.7,
130.0, 135.4, 135.7, 144.0, 147.6, 149.7, 156.6, 168.2.
Mass m/z (%): 502 (M+ + 2, 2.4), 501 (M+ + 1, 6.6), 500
(M+, 11.0), 299 (24.5), 266 (22.5), 200 (11.2), 97 (42.0),
57 (100.0). Anal. Calcd for C29H28N2O2S2 (500.672): C,
69.57; H, 5.64; N, 5.60; found: C, 69.70; H, 5.56; N, 5.71.
2‑(4‑Methylphenyl)‑1H‑benzimidazole (17) [55]
1H NMR (DMSO-d6, 250 MHz): δ 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): δ 20.9, 121.9, 126.3, 127.4, 128.9, 129.4,
139.5, 151.3.
2‑(4‑Chlorophenyl)‑1H‑benzimidazole (22) [55]
1H NMR (DMSO-d6, 250 MHz): δ 7.18–7.21 (m, 2 H),
Ethyl 2,6‑bis(4‑chlorophenyl)‑1‑phe‑
nyl‑4‑(phenylamino)‑1,2,5,6‑tetrahydropyridine‑3‑carbox‑
ylate (15)
7.60 (m, 4 H), 8.17 (d, J = 8.6 Hz, 2H), 12.99 (s, 1 H). 13
C
NMR (DMSO-d6, 62.9 MHz) δ 111.4, 118.9, 121.9, 122.7,
128.1, 129.0, 134.5, 150.1.
White solid; mp 229–231 °C; (Lit. [26] mp 228–230 °C).
IR (KBr): 694, 748, 1010, 1064, 1172, 1257, 1326, 1365,
4‑(1H‑Benzimidazol‑2‑yl)phenol (28) [55]
1496, 1596, 1651, 2869, 3047, 3232 cm−1
.
1H NMR
1H NMR (DMSO-d6, 250 MHz): δ 6.93 (dd, J1 = 8.5 Hz,
J2 = 1.2 Hz, 2H), 7.10–7.22 (m, 2H), 7.52–7.53 (m, 2H),
(CDCl3, 250 MHz): δ 1.47 (t, J = 7.0 Hz, 3 H), 2.76 (dd,
J1 = 15.0 Hz, J2 = 2.7 Hz, 1 H), 2.86 (dd, J1 = 15.2 Hz,
J2 = 5.0 Hz, 1 H), 4.31–4.51 (m, 2 H), 5.09–5.11 (m, 1 H),
6.39–6.50 (m, 5 H), 6.67 (t, J = 6.8 Hz, 1 H), 7.06–7.27 (m,
13 H), 10.32 (s, 1 H). 13C NMR (CDCl3, 62.9 MHz): δ 14.8,
33.7, 54.7, 57.4, 59.9, 97.8, 113.0, 116.8, 125.7, 126.0,
127.8, 128.1, 128.4, 128.8, 129.0, 129.1, 129.3, 129.6,
132.1, 132.9, 137.7, 141.0, 142.5, 146.5, 155.8, 168.0. Mass
m/z (%): 544 (M+ + 1, 2.3), 543 (M+, 2.6), 431 (58.7),
385 (18.2), 326 (23.7), 254 (26.0), 216 (25.4), 137 (20.1),
83 (100.0). Anal. Calcd for C32H28Cl2N2O2 (543.491): C,
70.72; H, 5.19; N, 5.15; found: C, 70.64; H, 5.25; N, 5.28.
8.02 (d, J = 8.6 Hz, 2H), 10.07 (s, 1H), 12.65 (s, 1H). 13
C
NMR (DMSO-d6, 62.9 MHz): δ 115.7, 121.0, 121.6, 128.1,
151.8, 159.1.
2‑(2,5‑Dimethoxyphenyl)‑1H‑benzimidazole (31) [55]
1H NMR (DMSO-d6, 250 MHz): δ 3.78 (s, 3H), 3.95
(s, 3H), 6.99–7.20 (m, 4H), 7.65 (m, 2H), 8.0.01 (d,
J = 3.1 Hz, 1H), 12.14 (s, 1H). 13C NMR (DMSO-d6,
62.9 MHz): δ 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.
1 3

J IRAN CHEM SOC
2‑(Pyridin‑4‑yl)‑1H‑benzo[d]imidazole (34)
124.1, 124.7, 126.2, 128.5, 129.0, 134.1, 153.6, 160.4,
167.5.
White solid; mp 217–219 °C; IR (KBr): 686 (s), 748 (s),
825 (s), 995 (s), 1319 (s), 1434 (s), 1604 (s), 3047 (w)
3‑(Benzo[d]thiazol‑2‑yl)phenol (41)
1
cm−1. H NMR (CDCl3, 250 MHz): δ 7.25–7.37 (m, 2
1H NMR (DMSO, 250 MHz): δ 6.93–6.97 (m, 1H), 7.34–
7.49 (m, 5H), 8.00–8.10 (m, 2H), 10.00 (s, 1H). 13C NMR
(DMSO, 62.9 MHz) δ 113.4, 118.2, 118.5, 122.0, 122.7,
125.3, 126.5, 130.4, 134.0, 134.3, 153.4, 157.9, 167.4.
H), 7.67–7.71 (m, 2 H), 7.99 ((dd, J = 4.5, J = 1.6 Hz,
2 H), 8.75 ((dd, J = 4.5, J = 1.6 Hz, 2 H). 13C NMR
(DMSO-d6, 62.9 MHz): δ 120.3, 122.8, 137.1, 148.7,
150.4. Mass m/z (%): 195 (M+, 69.1), 168 (18.1), 149
(31.9), 94 (41.5), 73 (50.0), 57 (100.0). Anal. Calcd for
C12 H9 N3 (195.223): C, 73.83; H, 4.65; N, 21.52. Found:
C, 73.81; H, 4.68; N, 21.49.
2‑(2‑Chlorophenyl)benzo[d]thiazole (42)
1H NMR (CDCl3, 250 MHz): δ 7.27–7.41 (m, 5H),
7.83 (d, J = 7.8 Hz, 1H), 8.02–8.11 (m, 2H). 13C NMR
(CDCl3, 62.9 MHz) δ 121.4, 123.5, 125.4, 126.3, 127.1,
130.8, 131.1, 131.7, 132.2, 132.7, 136.1, 152.5, 164.0.
2‑(1H‑Benzo[d]imidazol‑2‑yl)‑6‑(morpholinomethyl)
phenol (33)
White solid; mp 185–187 °C; IR (KBr): 748 (s), 864
(s), 1002 (s), 1111 (s), 11,257 (m), 1458 (s), 1519 (s),
1
1612 (s), 3202 (w) cm−1. H NMR (CDCl3, 250 MHz):
δ 2.65 (s, 4 H), 3.81 (s, 6 H), 7.91 (t, J = 7.8, 1 H), 7.12
(d, J = 7.4, 1 H), 7.24 (m, 2 H), 7.64 (m, 2 H), 8.36
(d, J = 7.9, 1 H), 10.94 (br, 2 H). 13C NMR (CDCl3,
62.9 MHz): δ 52.8, 61.0, 66.6, 119.9, 121.5, 122.5,
128.3, 130.7, 156.0. Mass m/z (%): 309 (M+, 25.8), 280
(10.6), 251 (25.5), 224 (100.0), 195 (32.8), 135 (19.6),
111 (12.6), 86 (23.6), 57 (75.4). Anal. Calcd for C18 H19
N3 O2 (309.366): C, 69.88; H, 6.19; N, 13.58. Found: C,
69.83; H, 6.24; N, 13.55.
Results and discussion
In continuation of our study on the synthesis and forma-
tion of metal-salen complexes with different molecules
[57, 58, 69, 70, 75, 81–84], this paper reports the pos-
sibility of synthesizing highly substituted piperidines
using chromium(III)-salen complex as catalyst. Fol-
lowing a previously reported procedures [78–80, 85],
chromium(III)-salen complex as a homogeneous cata-
lyst was prepared. CrCl3-6H2O and salen was mixed in
ethanol. The mixture was then refuxed for 6 h. After
the reaction was completed, the solid product was fl-
tered, washed with cold ethanol, dried under vacuum and
recrystallized in chloroform.
2‑p‑Tolylbenzo[d]thiazole (34) [58]
1H NMR (CDCl3, 250 MHz): δ 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) δ 21.5, 121.6, 123.1, 125.0,
126.2, 127.8, 129.7, 131.0, 135.0, 141.3, 154.3, 168.2.
First of all, the effects of catalyst, solvent, and the
molar ratio of the reagents were investigated by using a
reaction mixture of benzaldehyde, aniline, and methyl-
acetoacetate as the model reaction (Scheme 1).
The catalytic effciency of chromium(III)-salen
for synthesis of product 1 by using methyl acetoac-
etate (1.0 mmol), benzaldehyde (2.0 mmol) and aniline
(2.0 mmol) in various mol% of chromium(III)-salen
2‑(2,5‑Dimethoxyphenyl)benzo[d]thiazole (35) [58]
1H NMR (CDCl3, 250 MHz): δ 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) δ 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.
4‑(Benzo[d]thiazol‑2‑yl)phenol (40)
1H NMR (DMSO, 250 MHz): δ 6.90 (d, J = 8.6 Hz, 2H),
7.32–7.49 (m, 2H), 7.87–8.02 (m, 4H), 10.34 (s, 1H). 13
C
Scheme 1 The model reaction of synthesis of highly functionalized
NMR (DMSO, 62.9 MHz) δ 115.5, 116.0, 121.8, 122.2,
piperidines
1 3

J IRAN CHEM SOC
Table 1 Effect of different solvents on the synthesis of compound 1
from the reaction of benzaldehyde, aniline, and methylacetoacetate at
room temperature using chromium(III)-salen complex (10 mol%)
Entry]
Solvent
Time (h)
Yield (%)a
1
2
3
4
5
6
7
8
CHCl3
Dioxane
THF
24
24
24
24
24
24
16
24
<10
<10
<10
25
DMSO
H2O
–
CH3CN
EtOH
Neat
45
81
31
a
Isolated yield
was investigated. The results indicated that 10 mol%
chromium(III)-salen complex was suffcient to obtain
a high yield of the desired product in a short time. At
chromium(III)-salen complex amounts higher than
10 mol%, there was no signifcant change in the reaction
yield. The chemical structure of product 1 was character-
ized by the spectroscopic techniques and was confrmed
based on available literature data. In the absence of
chromium(III)-salen complex, no product resulted which
indicated that the presence of the catalyst was clearly
required for the reaction to proceed.
The effect of solvents on the synthesis of highly sub-
stituted piperidines from the reaction of aromatic alde-
hydes, β-keto esters, and amines at room temperature
using chromium(III)-salen complex was also investi-
gated as shown in Table 1. It was found that the progress
of these reactions largely depended on the nature of the
solvent; however, ethanol has been more effective than
the other solvents. When the reaction was carried out
under solvent-free conditions, the product was obtained
in low yield (31%). As a result, the stoichiometric ratio
of 2:2:1 (aldehyde: amine: β-keto ester) in the presence
of 10 mol% chromium(III)-salen complex in ethanol was
found to be the optimum conditions for obtaining the
maximum yield.
Using the optimized experimental conditions, highly
substituted piperidines was synthesized by performing a
one pot multi-component reaction between β-keto esters,
aromatic aldehydes, and various amines in the presence
of Cr(III) salen complex as the catalyst. A mechanism
for this synthesis is proposed (Scheme 2) similar to that
established in the literature [2, 7, 24, 25]; however, the
exact explanation for this mechanism is not clear yet. A
possible explanation could be based on the reaction of
aniline with aromatic aldehyde or β-ketoester yielding
the corresponding imine (I) or enamine (II), respectively.
Scheme 2 Proposed mechanism for the Cr(III)-salen complex-cata-
lyzed synthesis of highly substituted piperidines
The catalyst probably facilitates formation of imine (I)
or enamine (II) by increasing the electrophilicity of the
carbonyl group. The imine (I) reacts with enamine (II) to
give intermediate (III) through inter-molecular Mannich-
type reaction. The intermediate (III) reacts with aromatic
aldehyde to give intermediate (IV) through eliminating
a water molecule. The intermediate (IV) tautomerizes to
give the intermediate (V); the intramolecular hydrogen
bond is a spontaneous tendency for tautomerization. The
tautomer (V) undergoes intramolecular Mannich-type
reaction to form intermediate (VI) and this tautomer-
izes to give the product due to conjugation with the ester
group.
Preparation of Cr(III)‑salen complex supported
on AlPO4 nanoparticles as a heterogeneous
nanocatalyst
Immobilization of a homogeneous catalyst can in princi-
ple facilitate its reuse and recovery which is of consider-
able interest in modern synthetic organic chemistry [62].
Since chromium(III) salen catalyzes the synthesis of
highly substituted piperidines under mild condition from
β-keto esters, aromatic aldehydes and various amines, a
heterogeneous catalyst was set out simply by impregnat-
ing Cr(III)-salen complex on AlPO4 nanoparticles as the
support.
1 3

J IRAN CHEM SOC
H2N
NH
H
-NH
2N-(CH2)2
Microwave
2
+
H
AlCl3.6H2O
3PO4
AlPO4
AlPO4
Cl
Cr
N
O
N
O
Cl
N
N
O
H2N
Cr
O
NH
ref. 6h
Toluene,
AlPO4
Fig. 1 Characterization of Cr(III)-salen complex with a SEM, b
AFM images, c voltage profle, and d histogram
Scheme 3 Preparation of immobilized Cr(III)-salen catalyst on eth-
ylenediamine functionalized AlPO4
The amorphous AlPO4 (Al/P = 1) nanoparticles, as
support, were synthesized following a reported proce-
dure [78–80]. The AlPO4-ethylenediamine was synthe-
sized according to the literature [78, 79] and used as a
support for the immobilization of Cr(III)-salen complex.
In a typical experiment, 15.0 g AlPO4, 40 mL diethyl
ether and 2 mL ethylenediamine, were stirred at room
temperature for 1 h. Then, diethyl ether was removed and
the product was subsequently heated for 1.5 h at 100 °C.
The mixture was placed in the center of a domestic
microwave oven and was heated up for 10 min (190 W,
5 min and 380 W, 5 min) to ensure reaction completion.
After irradiation, the mixture was cooled and washed
with ethanol to remove the unreacted ethylenediamine;
the product was then vacuum-dried and separated.
Fig. 2 Thermogram of Cr(III)-salen complex immobilized on AlPO4
support
The reaction pathway for the preparation of
the immobilized catalyst is shown in Scheme 3.
Chromium(III)-salen complex (0.1 g) was added to
AlPO4-ethylenediamine (1.0 g) at room temperature and
agitated with vigorous stirring followed by ultrasonic
treatment for 10 min to ensure uniform dispersion of
Cr(III)-salen complex on AlPO4-ethylenediamine sur-
face. Then, 75 mL of dry toluene was added to the mix-
ture. The resulting suspension was refuxed while vig-
orously stirred for 6 h. The solid product was fltered,
soxhlet-extracted with ethanol and acetonitrile (v/v = 1)
as solvent for 12 h to remove the residue of the homo-
geneous complex that had been adsorbed on the surface
of the support. It was dried at 80 °C for overnight. The
Cr(III)-salen catalyst was successfully immobilized
on AlPO4 and was subsequently used for the catalytic
synthesis of highly substituted piperidines under mild
conditions.
Characterization of Cr(III)‑salen complex supported
on AlPO4
In this experiment, different techniques were adopted
to characterize the immobilized Cr(III)-salen complex.
The scanning electron microscopic (SEM) and atomic
forced microscopic (AFM) images (Fig. 1a, b) revealed
that AlPO4 was an appropriate support for deposition
of Cr(III)-salen complex. The voltage profle image
of Cr(III)-salen complex, corresponding to the AFM
image (Fig. 1b), has also been shown in Fig. 1c. Based
on the voltage profle image, large quantity with nar-
row size distribution (<100 nm) was observed as shown
in Fig. 1d. In accordance with the histogram, the aver-
age size of Cr(III)-salen complex was estimated to be
~40 nm.
1 3

J IRAN CHEM SOC
100
90
80
70
60
50
40
30
1
2
3
4
5
6
7
8
Runs
Fig. 5 Recyclability of chromium(III)-salen complex nanoparticles
supported on AlPO4 in the synthesis of highly substituted piperidine
Fig. 3 Nitrogen adsorption percentage of a pure AlPO4, and b AlPO4
when was used as support for Cr(III)-salen complex
The active surface area of the immobilized Cr(III)-salen
complex was compared with pure AlPO4 via following the
nitrogen adsorption capacity of each solid sample as shown
in Fig. 3. The result (Fig. 3) revealed that an increase of
about 350 m2 kg−1 was observed for the active surface area
of AlPO4 when was used as the support of Cr(III)-salen
complex.
For further characterization of Cr(III)-salen com-
plex, spectroscopic techniques such as FT-IR spectrom-
etry and inductively coupled plasma (ICP) were also
adopted. Figure 4 exhibits the FT-IR spectra of Cr(III)-
salen complex (Fig. 4a) supported on the AlPO4 func-
tionalized with -NH2 (Fig. 4b). According to the FT-IR
spectra (Fig. 4a), the strong peak at ~650 cm−1 is related
to the Cr–O bond. Formation of Cr–N and Cr–O bonds
is evidenced according to the stretching bonds based on
the weak absorption shoulder observed at ~402 cm−1
(Fig. 4a), which is in good agreement with results pre-
viously reported [85–87]. In accordance with the FT-IR
spectra (Fig. 4a, b), a signifcant shift of ~21 cm−1 (from
1415.9 to ~1395 cm−1) clearly reveals the formation of
new Cr–N bond during the chemically immobilization of
Cr(III)-salen complex on the AlPO4 functionalized with
–NH2– support.
As shown in Fig. 5, the recyclability of the immobilized
catalyst was tested. The catalytic activity of the catalyst
was largely unchanged even after eight consecutive runs.
The heterogenized catalyst exhibited good activities and
excellent reusabilities (up to eight runs) in the synthesis of
highly substituted piperidines.
To check the leaching, the immobilized catalyst sus-
pension in ethanol was stirred at room temperature for
10 h. The catalyst was removed, and the fltrate was used
for the synthesis of highly substituted piperidine under
the optimized conditions. No reaction was observed to be
Fig. 4 The FT-IR spectra of a immobilized Cr(III)-salen complex
and b –NH2-functionalized AlPO4 support
The thermal behavior of Cr(III)-salen complex was
also investigated using a home-made thermogravimet-
ric (TG) analyzer as shown in Fig. 2. According to this
fgure, the signifcant quench in the weight percentage
of the complex at ~110 °C is related to the desorption of
water and organic species from the surface of the immo-
bilized Cr(III)-salen complex. The increase in the weight
percentage at ~160 °C is related to the oxidation phos-
phorous and formation of Cr2O3 and P2O5 proving that
Cr(III)-salen complex is decomposed at this tempera-
ture. The oxidation of the phosphorous compounds is so
exothermic that the temperature ramp of the TG instru-
mentation system could not be controlled. This causes
oxidation of aluminum and formation of Al2O3 at a tem-
perature of ~380 °C.
1 3

J IRAN CHEM SOC
Scheme 5 Model reaction for synthesis of 4-methylbenzaldehyde
methyl acetoacetate under identical reaction conditions.
Also, heteroaryl aldehydes, such as thiophene-2-carboxal-
dehyde and pyridine-2-carboxaldehyde, were well tolerated
under these conditions (Table 2, entries 13 and 14). The
percentage yield and reaction time for each highly substi-
tuted piperidines are shown in Table 2. It should be men-
tioned that the developed procedure was not applicable to
aliphatic aldehydes and aliphatic amines for synthesis of
highly substituted piperidines. However, various 1,3-dicar-
bonyl compounds such as ethyl acetoacetate and methyl
acetoacetate as well as different aromatic aldehydes and
anilines produced desired piperidine derivatives in good
yields. The products were characterized by melting point,
Scheme 4 Synthesis of highly substituted piperidines from different
aryl aldehydes, anilines, and β-keto esters
proceeded after 16 h. The results demonstrated that the het-
erogeneous catalyst was stable after its practical utility.
Homogeneous and heterogeneous catalytic system
for synthesis of highly substituted piperidine
Under the optimized experimental conditions, the reaction
between β-keto esters, aromatic aldehydes, and various
amines was carried out in the presence of both homogene-
ous and heterogeneous catalysts in ethanol at room tem-
perature. Both catalysts offered high selectivity and good
reactivity toward synthesis of highly substituted piperidines
under mild condition.
To explore the generality and scope of this method, a
variety of β-keto esters, aromatic aldehydes, heterocyclic
aldehydes, and various amines (Scheme 4) were used for
synthesis of the corresponding highly substituted piperi-
dines. Various anilines and aromatic aldehydes with sub-
stituents such as Me, OMe, Cl, and NO2 were treated with
1
IR, H NMR, 13C NMR, mass spectrometry and elemental
analysis which showed similar results to those reported in
the literature for anti-compounds.
In the next step, we expanded the catalytic system to
the synthesis of benzimidazoles and benzothiazoles. To
exploit simple and suitable conditions for synthesis of
2-arylbenzimidazoles, the reaction of o-phenylenedi-
amine with 4-methylbenzaldehyde was chosen as a model
(Scheme 5).
Table 2 Homogeneous
Entry Aldehyde
R1 R2
Product Homogeneous
catalyst
Heterogeneous
catalyst
and heterogeneous Cr(III)–
salen complex-catalyzed
multicomponent reactions
for the synthesis of highly
Time (h) Yield (%)a Time (h) Yield (%)a
1
C6H5CHO
Me
H
1
2
3
4
5
6
7
8
9
16
17
17
17
15
16
17
18
16
15
18
21
22
21
18
24
81
85
82
83
85
82
82
81
81
79
78
79
65
70
77
73
15
15
16
16
14
15
16
17
15
14
17
19
22
20
18
21
82
85
84
84
85
84
84
83
81
81
78
80
67
73
78
74
substituted piperidines
2
C6H5CHO
Me 4-Me
3
C6H5CHO
Me 4-OMe
4
4-MeC6H4CHO
4-ClC6H4CHO
4-MeOC6H4CHO
4-MeC6H4CHO
4-MeOC6H4CHO
4-ClC6H4CHO
4-MeC6H4CHO
C6H5CHO
Me
Me
Me
H
H
H
5
6
7
Me 4-Cl
Me 4-Me
Me 4-Me
8
9
10
11
12
13
14
15
16
Me 4-OMe 10
Me 4-Cl
Et
11
12
13
14
15
16
4-MeC6H4CHO
H
2-Pyridinylcarboxaldehyde Me 4-Cl
2-Thiophenyl
Me 4-Me
4-ClC6H4CHO
3-NO2C6H4CHO
Et
H
H
Me
a
Isolated yields
1 3

J IRAN CHEM SOC
Entry
1
Product
Time (h)
55 min
Yielda
95
References
[25]
Table 3 Condensation reaction
of o-phenylenediamine
(1.0 mmol) with different
aromatic aldehydes
(1.0 mmol) in the presence of
chromium(III)-salen complex
nanoparticles supported on
AlPO4 (2 mol%) in ethanol at
room temperature
N
CH3
N
H
17
N
2
3
1
96
96
[25]
[25]
N
H
18
H
3C
N
40 min
N
H
19
O
C
N
HO
4
5
3
93
92
[25]
[30]
N
H
20
O
C
N
EtO
50 min
N
H
21
N
Cl
6
7
2
90
91
[25]
[25]
N
H
22
N
2.5
N
H
Cl
23
24
25
N
OCH3
CN
8
9
1.5
5
93
87
93
[25]
[25]
[25]
N
H
N
N
H
CH3
CH
N
10
1
CH3
N
H
26
N
11
12
3
2
90
91
[25]
[25]
N
H
HO
27
N
OH
N
H
28
N
13
14
4
5
89
87
[25]
[25]
N
H
NO2
29
N
S
N
H
30
3CO
H
N
15
16
2
5
92
92
[30]
N
H
OCH3
N
31
N
–
N
H
32
N
N
H
17
6
86
–
HO
N
a
O
Isolated yield
1 3

J IRAN CHEM SOC
In order to ascertain the optimum conditions of the reac-
tion, we optimized various parameters including solvent
and the amount of catalyst for the synthesis of 4-methylb-
enzaldehyde. The best results were obtained with the use
of 2 mol% of chromium(III)-salen complex nanoparticles
supported on AlPO4. The effect of solvents on the conden-
sation reaction was also investigated. Among different sol-
vents examined, ethanol clearly stands out as the solvent
of choice with its fast reaction rate, shorter reaction time,
environmental acceptability and excellent yield of product.
The use of other solvent gave low yield of product. Under
optimized reaction conditions, we studied the generality
and selectivity of nanocatalyst for the condensation of elec-
tronically divergent aromatic aldehydes with o-phenylen-
ediamines (Table 3). Furthermore, aliphatic aldehydes are
not applicable for the synthesis of 2-arylbenzimidazoles
in the presented procedure. Therefore, chromium(III)-
salen complex nanoparticles supported on AlPO4 possess
good catalytic activity for the synthesis of 2-arylbenzimi-
dazoles using various o-phenylenediamines and aromatic
aldehydes.
similar to the case in a smaller scale (Table 3, entry 1), and
the desired product was obtained in 95% isolated yield.
Therefore, we believe that it will fnd wide range of appli-
cations in organic synthesis as well as in industry.
Herein, we proposed
a
mechanism for the
chromium(III)-salen complex nanoparticles supported
on AlPO4 catalyzed preparation of 2-arylbenzimida-
zoles (Scheme 6), which is in analogy to the established
mechanism as reported in the literature [51, 55, 57].
We believe that the synthesis of 2-arylbenzimidazoles
under these conditions follows through the known imine
VII by the reaction of an aromatic aldehyde and o-phe-
nylenediamine in the presence of nanocatalyst by the
elimination of a water molecule, which exists in equi-
librium with the cyclic hydrobenzimidazoles VIII. The
chromium(III)-salen complex nanoparticles supported on
AlPO4 probably facilitates the formation of imine VII
by increasing the electrophilicity of the carbonyl group.
Subsequently, in the presence of nanocatalyst and air, the
cyclic hydrobenzimidazoles VIII oxidized by O2 to give
2-arylbenzimidazoles.
In order to assess the feasibility of applying this pro-
cedure on a preparative scale, we performed the cou-
pling of o-phenylenediamine with 4-methylbenzaldehyde
under the optimized conditions in a 50 mmol scale using
chromium(III)-salen complex nanoparticles supported
on AlPO4. As expected, the reaction proceeded smoothly,
We then applied these reaction conditions to the synthe-
sis of 2-p-tolylbenzo[d]thiazole (Scheme 7). We studied
the effect of reaction temperature. The reaction of benza-
ldehyde (1.0 mmol) and 2-aminothiophenol (1.0 mmol)
using catalytic amount (2.0 mol%) of a chromium(III)-
salen complex nanoparticles supported on AlPO4 was
carried at different temperature. At room temperature,
the reaction rate and yield was found to be very slow,
and it increased with temperature. At 50 °C, the reaction
rate was found to be a maximum, and further increases
in temperature did not show any◦ enhancement. Thus,
stirring the reaction mixture at 50 C in the presence of
chromium(III)-salen complex nanoparticles supported
on AlPO4 was optimal for the synthesis of benzothiazole
derivatives.
The scope and generality of this procedure using
chromium(III)-salen complex nanoparticles supported on
AlPO4 was explored with a variety of aryl aldehydes and
2-aminothiophenol. In all cases, condensation reactions
afforded the desired benzothiazole derivatives in good
yields. (Table 4).
Scheme 6 A possible pathway for the synthesis of 2-arylbenzimida-
zoles
Conclusions
A novel, highly selective, and effcient methodology for
synthesis of highly substituted piperidines via multi-com-
ponent one-pot process under mild conditions has been
developed. To do this, β-keto esters, aromatic aldehydes
and various amines were used, while 10 mol% homoge-
neous chromium(III)-salen complex and heterogeneous
Scheme 7 Model reaction of synthesis 2-p-tolylbenzo[d]thiazole
1 3

J IRAN CHEM SOC
Table 4 Condensation reaction of 2-aminobenzenethiol (1.0 mmol) with different aromatic aldehydes (1.0 mmol) in the presence of
chromium(III)-salen complex nanoparticles supported on AlPO4 (2 mol%) in ethanol at room temperature
Entry
1
Product
Time (h)
2
Yielda
96
References
[25]
N
S
CH3
34
N
2
3
3
95
92
[25]
[25]
S
35
N
OCH3
2.5
S
36
H
3CO
N
S
4
4
87
[25]
OCH3
37
38
39
40
CH3
CH
CH
N
S
5
6
3
93
91
[25]
[30]
N
S
4.5
Cl
N
7
8
3
94
95
[25]
[25]
OH
S
N
2.5
S
OH
41
N
9
2
95
[25]
S
Cl
42
a
Isolated yield
chromium(III)-salen complex (supported on AlPO4 nano-
particles) were used as catalysts. The advantages of the
present procedure are relatively mild reaction condi-
tions, high diastereoselectivity productions, recyclability
of the catalyst, high atom-economy, and excellent yields
which make it a useful alternative to the existing proce-
dures reported for synthesis of highly substituted piperi-
dines. Also, we have developed a general, simple and
effcient one-pot synthetic rout to 2-arylbenzimidazoles
and 2-arylbenzothiazoles from o-phenylenediamines and
2-aminothiophenol with aryl aldehydes, respectively,
using chromium(III)-salen complex nanoparticles sup-
ported on AlPO4 (2 mol%) as the reusable nano cata-
lyst under mild reaction conditions. The mild reaction
conditions, the fast reaction times, large scale synthesis,
easy and quick isolation of products and excellent yields
are main advantages of this procedure which makes
it an attractive and useful contribution to the present
1 3

J IRAN CHEM SOC
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