Preparation of novel palladium nanoparticles supported on magnetic iron oxide and their catalytic application in the synthesis of 2-imino-3-phenyl-2,3-dihydrobenzo[d]oxazol-5-ols

Article abstract of DOI:10.1002/aoc.4263

Novel Pd nanoparticles were prepared in five successive stages: 1) preparation of the Fe₃O₄ magnetic nanoparticles (Fe₃O₄ MNPs), 2) coating of Fe₃O₄ MNPs with SiO₂ (Fe₃

Full text of DOI:10.1002/aoc.4263

Received: 10 October 2017
DOI: 10.1002/aoc.4263
Revised: 8 December 2017
Accepted: 10 December 2017
F U L L P A P E R
Preparation of novel palladium nanoparticles supported on
magnetic iron oxide and their catalytic application in the
synthesis of 2‐imino‐3‐phenyl‐2,3‐dihydrobenzo[d]oxazol‐5‐ols
Davood Habibi
| Somayyeh Heydari | Mina Afsharfarnia | Fatemeh Moghimpour
Department of Organic Chemistry, Faculty
of Chemistry, Bu‐Ali Sina University,
Hamedan 6517838683, Iran
Novel Pd nanoparticles were prepared in five successive stages: 1) preparation
of the Fe3O4 magnetic nanoparticles (Fe3O4 MNPs), 2) coating of Fe3O4 MNPs
with SiO2 (Fe3O4@SiO2), 3) functionalization of Fe3O4@SiO2 with
3‐chloropropyltrimethoxy‐ silane (CPTMS) ligand (Fe3O4@SiO2@CPTMS), 4)
further functionalization with 3,5‐diamino‐1,2,4‐triazole (DAT) ligand
Correspondence
Davood Habibi, Department of Organic
Chemistry, Faculty of Chemistry, Bu‐Ali
Sina University, Hamedan 6517838683,
Iran.
(Fe3O4@SiO2@CPTMS
@DAT),
and
5)
the
complexation
of
Email: davood.habibi@gmail.com
Fe3O4@SiO2@CPTMS@DAT with PdCl2 (Fe3O4@SiO2@CPTMS@ DAT@Pd).
Then, the obtained Pd nano‐catalyst characterized by different methods such
as the elemental analysis (CHN), FT‐IR, XRD, EDX, SEM, TEM, TG‐DTA
and VSM. Finally, the Pd catalyst was applied for the synthesis of various
2‐imino‐3‐phenyl‐2,3‐dihydrobenzo[d]oxazol‐5‐ols.
KEYWORDS
3,5‐diamino‐1,2,4‐triazole, 3‐chloropropyltrimethoxysilane, Fe3O4 Magnetic nano‐particles, Pd
complex, SiO2
1 | INTRODUCTION
component coupling reaction of aldehydes, amines and
phenylacetylenes under solvent free conditions in ionic
Application of Pd catalyst has been extensively studied in
organic reactions. For example, F. Liu and coworkers
modified the Na+ montmorillonite with L‐cystine, and
subsequently Pd was loaded via an ion‐exchange reaction
under microwave irradiation to obtain the Pd@Mont cat-
alyst. The catalyst was subsequently applied for the
Suzuki cross‐coupling reaction to obtain the correspond-
ing products in good to excellent yields.[1] N. Telvekar
and coworker applied a simple and efficient method for
the synthesis 2‐phenyl pyridine via cyclization of aryl
ketone with 1,3‐diaminopropane using Pd(OAc)2 as cata-
lyst.[2] The Pd(II) complexes were obtained by P.
Viswanathamurthi and coworker from the reaction of
[PdCl2(PPh3)2] with pyridoxal thiosemicarbazone and
pyridoxal N‐methyl thiosemicarbazone, respectively in
ethanol. To screen the catalytic properties of the synthe-
sized complexes, Pd(II) complexes catalyzed three‐
liquid at 80 °C to give propargylamines in high yields.[3]
Trans [PdCl2(PPh3)2] complex was synthesized, character-
ized and applied by Ali Naghipour and coworker as an
efficient catalyst for the amination of aryl halides to afford
primary amines and also in the catalytic Stille cross‐cou-
pling reaction with satisfactory results.[4] A. M. Trzeciak
and coworkers synthesized and characterized two Pd
complexes of the type [PdL2Cl2] containing chiral
imidazole ligands (L = 1‐bornyloxymethylene imidazole
or 1‐fenchyloxymethylyne imidazole) were synthesized.
They tested the two obtained catalysts in various C‐C
bond forming reactions, namely Suzuki–Miyaura,
carbonylative Suzuki–Miyaura, asymmetric Heck‐type
coupling reactions and asymmetric conjugate addition of
phenylboronic acid to heterocyclic acceptors.[5] U. Bora
and coworker prepared the gallic acid‐derived Pd(0) nano-
particles catalyst for the Sonogashira reactions in ethanol
Appl Organometal Chem. 2018;e4263.
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https://doi.org/10.1002/aoc.4263

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HABIBI ET AL.
under optimum thermal conditions.[6] M. Rocamora and
coworker prepared series of cationic allyl Pd complexes
[Pd(η3‐CH3‐C3H5)(P‐P)]X (X = PF6 and X = BPh4) and
[Pd(η3‐1,3‐Ph2‐C3H3)(P‐P)]X (X = PF6 and X = BPh4)
and applied them in the asymmetric allylic substitution
reaction of the benchmark substrate rac‐3‐acetoxy‐1,3‐
diphenyl‐1‐propene with dimethyl malonate and
benzylamine as nucleophiles in order to test their catalytic
potential.[7] T. E. Klimova and coworker prepared the Pd
catalysts supported on hydrogen titanate nanotubes by
adsorption of Pd(OAc)2 from CH2Cl2 solutions. Catalytic
activity was tested in the Heck reaction between 4‐
bromobenzaldehyde or 4‐bromostyrene and styrene. Only
the E‐isomers of the corresponding cross‐coupled prod-
ucts were obtained.[8] B. J. V. Tongol and coworkers syn-
thesized a catalyst consisting of Pd–Ni supported on
graphene oxide composite. The catalytic activity was
tested for ethanol oxidation reaction in half‐cell using
cyclic voltammetry and subsequently it used as an anode
material in a direct ethanol fuel cell.[9] Polycarbosilane
was synthesized by K. Sreekumar and coworker via the
Furthermore, in continuation to our synthesis of the
nitrogen‐containing compounds, we intend to report a
mild, efficient and convenient procedure for the synthesis
of
a range of 2‐imino‐3‐phenyl‐2,3‐dihydrobenzo[d]
oxazol‐5‐ol by the sue of this Pd nano‐catalyst in acetoni-
trile at room temperature (Scheme 2).
2 | EXPERIMENTAL
2.1 | General
All reagents were purchased from the Merck and
Aldrich chemical companies and used without further
purification. The NMR spectra were recorded in DMSO
1
or CDCl3. HNMR and 13C NMR spectra were recorded
on a Bruker Avance DRX 400 and 300 MHz instru-
ments. The chemical shifts are reported in parts per mil-
lion relative to TMS as an internal standard and J values
are given in hertz. FT‐IR (KBr) spectra were recorded
on a Perkine Elmer 781 spectrophotometer. Melting
points were taken in open capillary tubes with a BUCHI
510 melting point apparatus and are uncorrected. Ele-
mental analysis was performed using Heraeus CHNeO‐
Rapid analyzer. The ICP measurements for the metal
content evaluation were performed using a Perkin‐
Elmer ICP/6500. TLC was performed on silica gel
polygram SIL G/UV 254 plates. The TEM images were
recorded on a Zeiss‐EM10C‐80 KV transmission electron
microscope, and the SEM images were recorded on a
Philips XL‐30 scanning electron microscope. The XRD
measurements were done by a Bruker D8 Advance pow-
der diffractometer, using Cu Kα (λ=1.54 A°) as the inci-
dent radiation. Magnetic measurements were carried out
at room temperature using an Iranian Meghnatis
Daghigh Kavir Co Vibrating Sample Magnetometer
(VSM).
polycondensation
of
trichloromethylsilane
and
trimethoxyvinylsilane in the presence of sodium metal.
Then, Pd(OAc)2 was attached to the polycarbosilane and
its catalytic activity investigated in the C–C coupling Heck
reaction between aryl halides or vinyl halides and
activated.[10]
Since magnetic nanoparticles have a large surface‐to‐
volume ratio, their catalytic activities can be improved sig-
nificantly.[11–13] Among magnetic nanoparticles, Fe3O4 is
the most widely used for catalyst supports as it has less
toxicity compared to other metallic counterparts.[14]
Herein, we report preparation of a novel Pd nano‐par-
ticle catalyst via synthesis of the Fe3O4 magnetic nanopar-
ticles, its coating with SiO2, functionalization with 3‐
chloropropyltrimethoxysilane and 3,5‐diamino‐1,2,4‐tri-
azole ligands, and complexation with PdCl2 (Scheme 1).
SCHEME 1 Synthesis of novel Pd nano‐catalyst

HABIBI ET AL.
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magnet, washed repeatedly with ethanol and water
and dried in vacuo, and
• Stage 5: Complexation of Fe3O4@SiO2@CPTMS@DAT
with
PdCl2
(Fe3O4@SiO2@CPTMS@DAT@Pd).
Fe3O4@SiO2@CPTMS@DAT (0.1 g, prepared in the
Stage 4) was added to a dispersed PdCl2 (0.88 g, 5 mmol)
in toluene and stirred vigorously for 24 h under nitro-
gen atmosphere at 70 °C. Then, the resulting Pd cata-
lyst was separated with strong magnet, washed with
ethanol and air‐dried.
SCHEME
dihydrobenzo[d]oxazol‐5‐ols
2
Synthesis
of
various
2‐imino‐3‐aryl‐2,3‐
2.2 | Preparation of Pd nano‐catalyst
A novel Pd nano‐catalyst was prepared in the following
five successive stages:
2.3 | General Procedure for the synthesis
of 2‐imino‐3‐phenyl‐2,3‐dihydrobenzo[d]
oxazol‐5‐ol derivatives
• Stage 1: Preparation of the Fe3O4 magnetic nanoparti-
cles (Fe3O4 MNPs). The Fe3O4 MNPs was prepared
according to the literature.[15] Briefly, the mixture of
FeCl3.6H2O (11.44 g) and FeCl2.4H2O (4.3 g) was dis-
solved in water (100 ml), and the solution stirred for
0.5 h in 80 °C. The solution of 37% ammonia was then
added dropwise with vigorous stirring which a black
solid product obtained when a reaction media reached
to pH 10. The mixture was heated for 0.5 h at 70 °C
and the black magnetite solid product filtered, washed
with water and dried at 80 °C for 12 h.
• Stage 2: Coating of the Fe3O4 magnetic nano‐particles
with SiO2 (Fe3O4@SiO2). Fe3O4@SiO2 was synthe-
sized according to the literature.[16] Briefly, Fe3O4
MNPs (0.2 g, prepared in the Stage 1) were dispersed
into a solution containing ethanol and distilled water
(250 ml, V/V = 4:1) and NH3.H2O (3 ml) under
ultrasonication. Then, tetraethyl orthosilicate (TEOS,
2 ml) was slowly added dropwise and the mixture
stirred for further 6 h. Fe3O4@SiO2 was obtained by
centrifugation, washed with water and ethanol for sev-
eral times, and dried in vacuo.
p‐Benzoquinone (0.108 g, 1.0 mmol), the requisite
phenylcyanamide (1.0 mmol) and the Pd nano‐particle
catalyst (20 mg) were combined in acetonitrile (5 ml).
The mixture was stirred at room temperature and comple-
tion of the reaction monitored with TLC. Then, the Pd
catalyst was separated by a magnet, the solid washed with
acetone, the solvent removed in vacuo and the obtained
solid recrystallized in ethanol.
3 | RESULTS AND DISCUSSION
3.1 | Characterization of the catalyst
Formation of the Pd nano‐catalyst was verified using the
CHN analysis, inductively coupled plasma (ICP), FT‐IR,
X‐ray diffraction (XRD), dispersive X‐ray spectroscopy
(EDX), scanning electron microscopy (SEM), transmis-
sion electron microscopy (TEM), thermogravimetric‐dif-
ferential thermal analysis (TGA‐DTA) and vibrating
sample magnetometer (VSM).
• Stage 3: Functionalization of the coated Fe3O4
3.1.1 | Characterization of the Pd catalyst
by elemental analysis (CHN) and ICP
with
3‐chloropropyltrimethoxysilane
(CPTMS)
ligand (Fe3O4@SiO2@CPTMS). (3‐Chloropropyl)
trimethoxysilane (CPTMS) (1.0 ml, 5 mmol) dis-
solved in dry toluene (100 ml) and Fe3O4@SiO2
(1.0 g, prepared in the Stage 2) added and stirred
for 18 h at 60 °C. Then, the obtained product
was separated with strong magnet, washed with
toluene and dried in vacuo.
The elemental analysis report for the C, H and N elements
was about 7.74, 0.32 and 1.98 respectively. The ICP analy-
sis of the catalyst showed that the Pd content of
Fe3O4@SiO2@CPTMS@ DAT@Pd was about 35%.
3.1.2 | Characterization of the catalyst by
IR spectroscopy
• Stage 4: Further functionalization with 3,5‐diamino‐
1,2,4‐triazole (DAT) ligand (Fe3O4@SiO2@CPTMS
@DAT). 3,5‐Diamino‐1,2,4‐triazole (DAT, 0.495 g,
5 mmol) and K2CO3 (0.69 g, 5 mmol) in toluene
(60 ml) were added to Fe3O4@SiO2@CPTMS (0.1 g,
prepared in the Stage 3) and refluxed for 12 h. Then,
the obtained product was separated with strong
Figure 1 shows the five FT‐IR spectra of A) Fe3O4
MNPs, B) Fe3O4@SiO2, C) Fe3O4@SiO2@CPTMS,
D)
Fe3O4@SiO2@CPTMS@DAT
and
E)
Fe3O4@SiO2@CPTMS@DAT@Pd. Curve A exhibits basic
characteristic peak at about 575 cm‐1 which is attributed

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HABIBI ET AL.
FIGURE
2
The XRD patterns of Fe3O4@SiO2 (blue),
Fe3O4@SiO2@CPTMS (red), Fe3O4@SiO2@CPTMS@DAT (green)
and Fe3O4@SiO2@CPTMS@DAT@Pd (purple)
FIGURE 1 Comparison of the FT‐IR spectra of five compounds
(A, B, C, D and E)
to the presence of the Fe–O stretching vibrations. Curve B
shows a broad band near 1100 cm‐1 which indicates the
silica coated magnetite nanoparticles. Curve C shows a
new peak at about 635 cm‐1 indicating presence of the
C‐Cl bond. Curve D shows two new peaks at 1384 and
1638 cm−1 which are attributed to the C‐N and C=N
bonds, respectively. Curve E shows a shift from 1638 to
1627 cm−1 which is related to the new interaction of Pd
with the nitrogen of the C=N bond. Consequently, com-
parison of the IR spectra confirms the successful stages
of the catalyst preparation, namely preparation of Fe3O4
MNPs, modification, functionalization and complexation.
FIGURE 3 The EDX analysis of the catalyst
3.1.5 | Characterization of the catalyst by
the SEM and the TEM images
The morphology and the particle size of the Pd nano‐cat-
alyst were determined by the SEM and TEM images
(Figure 4). According to these images, the sizes of the Pd
nano‐catalyst particles are in the nanometer ranges
(between 26.61‐88.74 nm).
3.1.3 | Characterization of the catalyst by
the XRD patterns
The XRD pattern (Figure 2, B, blue) at about 2θ = 10, 30,
35, 45, 53, 57, 63 and 74, confirms the silica‐coated mag-
netite nanoparticles (Fe3O4@SiO2) structure. Appearance
3.1.6 | Characterization of the catalyst by
the TGA‐DTA technique
of the new peaks at 2θ = ̴37, ̴50, ̴70 and 87 are attributed
to the Pd species (Figure 2, E, purple).[17]
The thermo‐gravimetric analysis curves of the catalyst
show the mass loss of the organic materials as they
decompose upon heating (Figure 5). It can be observed
that the catalyst shows about five weight loss steps in
the temperature ranges of 145, 245, 340, 400 and 450 °C,
respectively. The initial weight loss at 145 °C is probably
due to the residual water, the second step at 245 °C is
attributed to the thermal decomposition of the complex,
the third step is probably due to the thermal decomposi-
tion of DAT and CPTMS ligands, the forth weight loss is
3.1.4 | Characterization of the catalyst by
the EDX analysis
The chemical composition of the Pd nano‐catalyst was
determined by the EDX analysis (Figure 3). The results
confirm the presence of the anticipated elements in the
structure of the catalyst, namely C (12.41 %), N (3.34 %),
O (22.41 %), Si (2.99 %), Fe (11.33 %) and Pd (28.53 %).

HABIBI ET AL.
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FIGURE 4 The SEM (left) and the TEM (right) images of the catalyst
related to the thermal decomposition of the SiO2 coating
layer and the fifth weight loss is attributed to the thermal
decomposition of Fe3O4. On the basis of these results,
modification and the well grafting of ligand groups is ver-
ified and indicated that the Pd catalyst has approximately
a good thermal stability which is probably due to the
strong interactions between the SiO2 coating layer, the
ligands and the Fe3O4 magnetic nano particles.
and solvent. The best result was obtained with 1.0 mmol
of 1: 1 of p‐benzoquinone: 2,5‐dichlorophenylcyanamide
with 20 mg of the Pd nano‐particle catalyst in acetonitrile
at room temperature (Table 1).
3.3 | Synthesis of various 2‐imino‐3‐
phenyl‐2,3‐dihydrobenzo[d]oxazol‐5‐ol
derivatives
3.1.7 | Characterization of the catalyst by
the VSM technique
Applying the optimized results from the model reaction,
various 2‐imino‐3‐phenyl‐2,3‐dihydro‐ benzo[d]oxazol‐5‐
ol derivatives were synthesized from the reaction of
benzoquinone with phenylcyanamides in acetonitrile at
room temperature with good to excellent yields
(Table 2). As can be seen, those phenylcyanamides
with electron donating groups require shorter reaction
times.
The VSM analyses of the five compounds namely A)
Fe3O4 MNPs, B) Fe3O4@SiO2, C) Fe3O4@SiO2@CPTMS,
D)
Fe3O4@SiO2@CPTMS@DAT,
and
E)
the
Fe3O4@SiO2@CPTMS@DAT@Pd nano‐catalyst were
performed in order to demonstrate and compare their
magnetic properties (Figure 6). As can be seen, all five
compounds have magnetic properties; and show a nice
decrease from A to E (70, 35, 30, 27 and 12 emu/g,
respectively). This can be explained by considering the
probable reduction in the dipolar–dipolar interactions
between the magnetic nanoparticles after their modifica-
tion and complexation which cause the more coating of
Fe3O4 MNPs.
3.4 | Characterization of the 2‐imino‐3‐
phenyl‐2,3‐dihydrobenzo[d]oxazol‐5‐ols
All known compounds were characterized by comparing
their physical and spectroscopic data with those
reported in the literature. The structures of all products
were in agreement with their IR and NMR spectra. In
the IR spectra, the sharp N‐H (about 3300 cm‐1) and
C≡N (about 2300 cm‐1) peaks disappeared and were
replaced by strong absorption bands for O‐H stretching
at 3300 cm‐1. Peaks at 1680 and 1000‐1100 cm‐1 can be
attributed to the new formed C=N and C‐O bands,
respectively.
3.2 | Optimization
The catalyst activity was investigated in a model reaction
for the synthesis of 3‐(2,5‐dichlorophenyl)‐2‐imino‐2,3‐
dihydrobenzo[d]oxazol‐5‐ol from the reaction of p‐benzo-
quinone with 2,5‐dichlorophenyl‐ cyanamide under dif-
ferent conditions of temperature, amount of the catalyst

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HABIBI ET AL.
FIGURE 5 TGA‐DTA patterns of the catalyst in N2 atmosphere
131.5, 131.3, 129.4, 127.6, 109.6, 107.7, 96.5; CHN: Anal.
Calcd for C13H8N2O2Cl2: C, 51.78; H, 2.62; N, 8.82. Found:
C, 52.91; H, 2.73; N, 9.49.
3.5 | Selected spectral data
3.5.1 | 3‐(2‐Chlorophenyl)‐2‐imino‐2,3‐
dihydrobenzo[d]oxazol‐5‐ol (2a)
M.p. 199‐202 °C, FT‐IR (KBr, cm‐1): 3329, 3068, 1687,
3.5.3 | 3‐(4‐Chlorophenyl)‐2‐imino‐2,3‐
dihydrobenzo[d]oxazol‐5‐ol (2c)
M.p. 210‐212 °C, FT‐IR (KBr, cm‐1): 3361, 3088, 1674,
1620, 1589, 1497, 1477, 1439, 1406, 1306, 1189, 1108,
1
1066, 1010, 838, 818, 768, 721, 703, 658, 620; H NMR
(DMSO‐d6, 300 MHz): δ = 9.26 (br, 1H), 7.73 (t, J = 4.4,
1H), 7.66 (t, J = 4.4, 3H), 7.55 (t, J = 4.4, 2H), 7.03 (d, J
= 8.4, 1H), 6.40 (d, J= 8.4, 1H), 5.95 (s, 1H); 13CNMR
(DMSO‐d6, 75 MHz,): δ = 155.2, 154.5, 137.5, 134.1,
132.7, 132.2, 131.6, 131.4, 131.2, 129.3, 109.6, 107.6, 96.4;
CHN: Anal. Calcd for C13H9N2O2Cl: C, 59.86; H, 3.08;
N, 10.75. Found: C, 59.90; H, 3.48; N, 10.75.
1620, 1499, 1466, 1382, 1275, 1188, 1159, 1087, 1003,
1
970, 811, 713, 628; HNMR (300 MHz, DMSO‐d6): δ =
9.35 (br, 1H), 7.61 (q, J = 8.8, 4H), 7.00 (d, J = 8.4, 1H),
6.60 (br, 1H), 6.45 (d, J = 8.4, 1H), 6.37 (s, 1H); 13CNMR
(75 MHz, DMSO‐d6): δ = 155.2, 154.4, 137.3, 134.2,
133.4, 132, 129.9, 127.9, 109.5, 107.8, 96.6; CHN: Anal.
Calcd for C13H9N2O2Cl: C, 60.45; H, 3.24; N, 10.96.
Found: C, 59.90; H, 3.48; N, 10.75.
3.5.2 | 3‐(2,5‐Dichlorophenyl)‐2‐imino‐2,3‐
dihydrobenzo[d]oxazol‐5‐ol (2b)
3.5.4 | 3‐(2,4‐Dichlorophenyl)‐2‐imino‐2,3‐
dihydrobenzo[d]oxazol‐5‐ol (2d)
M.p. 231‐234 °C., FT‐IR (KBr, cm‐1): 3332, 3096, 2683,
1681, 1621, 1601, 1560, 1490, 1476, 1426, 1401, 1306,
1194, 1099, 1014, 848, 826, 784, 712, 661, 613, 582, 438.
1H NMR (DMSO‐d6, 300 MHz): δ = 8.00‐7.40 (m, 3H),
7.25 (br, 1H), 6.97 (d, J = 8.1, 1H), 6.72 (br, 1H), 6.35 (d,
J = 8.1, 1H), 5.96 (s, 1H); 13C NMR (DMSO‐d6,
75 MHz,): δ = 154.6, 154.4, 137.4, 133.6, 133.0, 132.4,
M.p. 216‐218 °C, FT‐IR (KBr, cm‐1): 3333, 3100, 2875,
2683, 1678, 1618, 1587, 1561, 1487, 1473, 1306, 1251,
1216, 1192, 1163, 1029, 1017, 898, 818, 778, 711, 677,
1
616, 563, 541, 441, 408; H NMR (300 MHz, DMSO‐d6):
δ = 7.93(s, 1H), 7.80, (m, 1H), 7.62 (s, 1H), 7.30, (s, 1H),

HABIBI ET AL.
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FIGURE 6 The VSM analyses of A, B, C, D and E
7 (d, J = 8.4, 1H), 6.90 (br, 1H), 6.40 (d, J = 8.4); 13C NMR
(DMSO‐d6, 75 MHz): δ = 154.6, 154.3, 137.1, 135.5, 132.8,
131.6, 129.7, 129.4, 127.9, 125.9, 109.6, 108.1, 96.8; CHN:
Anal. Calcd for C13H8N2O2Cl2: C, 52.34; H, 2.37; N, 8.91.
Found: C, 52.91; H, 2.73; N, 9.49.
(m, 3H), 7.00 (d, J = 8.5, 1H), 6.80 (br, 1H), 6.41 (d, J =
8.2 Hz, 2H); 13C NMR (DMSO‐d6, 75 MHz): δ = 154.4,
154.7, 137.3, 136.9, 133.2, 131.7, 130.5, 128.9, 124.9,
122.2, 109.6, 107.9, 96.6; CHN: Anal. Calcd for
C13H9N2O2Br: C, 51.45; H, 2.70; N, 9.26. Found: C,
51.17; H, 2.97; N, 10.49.
3.5.5 | 3‐(3‐Bromophenyl)‐2‐imino‐2,3‐
dihydrobenzo[d]oxazol‐5‐ol (2e)
M.p. 164‐167 °C, FT‐IR (KBr, cm‐1): 3330, 3065, 1674,
1618, 1590, 1488, 1478, 1438, 1394, 1307, 1199, 1168,
1116, 1073, 1014, 780, 744, 724, 713, 699, 653, 624. H
3.5.6 | 2‐Imino‐3‐p‐tolyl‐2,3‐
dihydrobenzo[d]oxazol‐5‐ol (2f)
M.p. 207‐209 °C, FT‐IR (KBr, cm‐1): 3360, 3064, 1675,
1618, 1580, 1516, 1494, 1475, 1442, 1408, 1382, 1272,
1225, 1000, 998, 970, 821, 811, 793, 713, 689, 646, 619,
1
NMR (DMSO‐d6, 300 MHz): δ = 9.40 (br, 1H), 7.64‐7.48
TABLE 1 Optimization of the reaction
Entry
1
p‐Benzoquinonea
Amount of catalyst
Solvent
Time (min)
50b
Yield %
1.5 mmol
40 mg
EtOH
20
2
3
4
5
1.5 mmol
1.0 mmol
1.5 mmol
1.0 mmol
30 mg
20 mg
30 mg
40 mg
MeOH
MeCN
MeCN
MeCN
50b
8c
10b
12d
12
82
40
35
a2,5‐Dichlorophenylcyanamide (1 mmol),
breflux,
croom temperature, and
dat 50 °C.

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HABIBI ET AL.
TABLE 2 Synthesis of various 2‐imino‐3‐phenyl‐2,3‐dihydrobenzo[d]oxazol‐5‐ols
Entry
Phenylcyanamide
Product
Time (min)
Yield (%)
1
110
88
2
3
450
250
82
90
4
150
80
5
6
120
70
80
82
7
250
92
8
9
25
87
85
100
10
35
78
1
608; H NMR (DMSO‐d6, 300 MHz): δ = 9.35 (br, 1H),
7.37 (d, J = 7.6, 2H), 7.34 (d, J = 7.6, 2H), 6.99 (d, J =
8.4, 1H), 6.40 (d, J = 8.4, 1H), 6.30 (s, 1H), 2.35 (s,
3H); 13C NMR (DMSO‐d6, 75 MHz): δ = 155.9, 154.5,
137.7, 137.7, 137.3, 134.1, 132.6, 130.5, 126.1, 109.4,
107.6, 96.4, 21.1; CHN: Anal. Calcd for C14H12N2O2: C,
69.31; H, 4.76; N, 11.45. Found: C, 69.99; H, 5.03; N,
11.66.

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3.5.7 | 2‐Imino‐3‐(4‐iodophenyl)‐2,3‐
dihydrobenzo[d]oxazol‐5‐ol (2g)
3.5.11 | 3‐(2,4‐Dichlorophenyl)‐2‐imino‐
2,3‐dihydrobenzo[d]ox‐azol‐5‐yl 4‐
methylbenzenesulfonate (3d)
M.p. 248‐251 °C, FT‐IR (KBr, cm‐1): 3358, 3059, 1676,
1627, 1617, 1583, 1566, 1499, 1404, 1381, 1273, 1190,
Yield 85%; mp 164‐166 °C; FT‐IR (KBr, cm‐1): 3339, 3038,
1711, 1693, 1597, 1491, 1476, 1369, 1221, 1194, 1171, 1130,
1093, 1078, 1163, 999, 900, 853, 812, 797, 748, 723, 666,
1
1158, 1058, 1001, 969, 820, 711, 684, 634, 622, 501; H
NMR (DMSO‐d6, 300 MHz): δ = 9.33 (br, 1H), 7.88 (d, J
= 8.1, 2H), 7.41 (d, J = 8, 2H), 6.99 (d, J = 8.4, 1H), 6.70
(br, 1H), 6.39 (t, J = 8.8, 2H); 13C NMR (DMSO‐d6,
75 MHz): δ = 155.1, 154.3, 138.8, 137.3, 135.2, 133.3,
128.2, 109.5, 107.9; CHN: Anal. Calcd for C13H9N2O2I: C,
44.64; H, 2.40; N, 8.22. Found: C, 44.34; H, 2.58; N, 7.96.
1
649, 632, 598, 553; H NMR (DMSO‐d6, 400 MHz): δ =
7.72 (d, J = 8.0 Hz, 2H), 7.67 (d, J = 8.4 Hz, 2H), 7.44‐
7.41 (dd, J = 2.4, 2.0 Hz, 1H), 7.37 (d, J = 8.0 Hz, 3H),
7.19 (d, J = 8.8 Hz, 1H), 6.77‐6.75 (d, J = 8.8 Hz, 1H),
6.61 (d, J = 2.4 Hz, 1H), 2.49 (s, 3H); 13C NMR (DMSO‐
d6, 100 MHz): δ = 156.4, 145.9, 145.8, 142.8, 134.1, 133.2,
132.6, 132.4, 131.9, 131.8, 129.9, 128.6, 127.7, 125.2,
117.0, 110.1, 104.0, 21.8; CHN: Anal. Calcd for
C20H14Cl2N2O4S: C, 53.46; H, 3.14; N, 6.23. Found: C,
53.81; H, 3.21; N, 6.32.
3.5.8 | 3‐(2,6‐Dimethylphenyl)‐2‐imino‐2,3‐
dihydrobenzo[d]oxazol‐5‐ol (2h)
M.p. 204‐207 °C, FT‐IR (KBr, cm‐1): 3347, 3315, 2954,
1862, 2688, 1679, 1616, 1475, 1379, 1296, 1178, 1101,
1
1003, 837, 798, 778, 728, 709, 634, 446; H NMR (DMSO‐
3.5.12 | 3‐(3‐Bromophenyl)‐2‐imino‐2,3‐
dihydrobenzo[d]oxazol‐5‐yl 4‐
d6, 300 MHz): δ = 9.30 (br, 1H), 7.27 (br, 3H), 7.02 (d, J
= 8.2, 1H), 6.43 (br, 1H), 6.36 (d, J = 7.57, 1H), 5.79 (d, J
= 2.33, 1H), 2.07 (s, 6H); 13C NMR (DMSO‐d6, 75 MHz):
δ = 154.7, 154.2, 137.7, 133.9, 132.2, 129.5, 129.1, 109.5,
107.1, 95.6, 17.7; CHN: Anal. Calcd for C15H14N2O2: C,
69.95; H, 5.33; N, 10.84. Found: C, 70.85; H, 5.55; N, 11.02.
methylbenzenesulfonate (3e)
Yield 87%; mp 189‐190 °C; FT‐IR (KBr, cm‐1): 3334, 3086,
1702, 1590, 1495, 1478, 1430, 1388, 1363, 1221, 1192, 1171,
1135, 1092, 1071, 996, 888, 868, 850, 818, 784, 742, 722,
1
700, 684, 665, 653, 617, 594, 554, 520; H NMR (DMSO‐
d6, 400 MHz): δ = 7.72 (d, J = 8.4 Hz, 2H), 7.64‐7.61 (m,
2H), 7.45‐7.44 (m, 2H), 7.37 (d, J = 8.4 Hz, 3H), 7.09 (d,
J = 8.8 Hz, 1H), 6.69 (d, J = 8.8 Hz, 1H), 6.59 (d, J =
2.4 Hz, 1H), 2.49 (s, 3H); 13C NMR (DMSO‐d6,
100 MHz): δ = 156.2, 145.7, 145.6, 142.9, 135.3, 133.2,
131.9, 131.6, 131.3, 129.9, 128.7, 128.6, 124.2, 123.3,
116.1, 109.4, 103.5, 21.8; CHN: Anal. Calcd for
C20H15BrN2O4S: C, 52.30; H, 3.29; N, 6.10. Found: C,
52.42; H, 3.37; N, 6.21.
3.5.9 | 2‐Imino‐3‐(4‐methoxyphenyl)‐2,3‐
dihydrobenzo[d]oxazol‐5‐ol (2i)
M.p. 177‐180 °C, FT‐IR (KBr, cm‐1): 3336, 3089, 1673, 1617,
1579, 1499, 1479, 1414, 1294, 1196, 1161, 1090, 1011, 969,
833, 811, 744, 706, 625, 501; 1H NMR (DMSO‐d6,
300 MHz): δ = 9.30 (b, 1H), 7.45 (d, J = 8.4, 2H), 7.90 (d, J
= 7.8, 2H), 6.98 (d, J = 8.4, 1H), 6.55 (br, 1H), 6.34 (d, J =
8.1, 1H), 6.18 (s, 1H); 13C NMR (DMSO‐d6, 75 MHz): δ =
158.9, 155.5, 154.3, 137.2, 134.4, 127.9, 115.2, 109.3, 107.2,
96.1, 55.8; CHN: Anal. Calcd for C14H12N2O3: C, 65.36; H,
4.61; N, 11.11. Found: C, 65.62; H, 4.72; N, 10.93.
3.5.13 | 3‐(4‐Iodophenyl)‐2‐imino‐2,3‐
dihydrobenzo[d]oxazol‐5‐yl 4‐
methylbenzenesulfonate (3f)
Yield 84%; mp 199‐201 °C; FT‐IR (KBr, cm‐1): 3344, 3043,
3.5.10 | 3,3'‐(1,4‐Phenylene)bis(2‐imino‐
2,3‐dihydrobenzo[d]oxazol‐5‐ol) (2j)
1704, 1496, 1480, 1355, 1170, 1132, 1092, 893, 857, 819,
1
753, 592, 553; H NMR (DMSO‐d6, 400 MHz): δ = 7.89
(d, J = 8.4 Hz, 2H), 7.72 (d, J = 8.0 Hz, 2H), 7.36 (d, J =
8.0 Hz, 3H), 7.24 (d, J = 8.4 Hz, 2H), 7.09 (d, J = 8.8 Hz,
1H), 6.66 (d, J = 8.8 Hz, 1H), 6.58 (d, J = 2.4 Hz, 1H),
2.50 (s, 3H); 13C NMR (DMSO‐d6, 100 MH): δ = 156.2,
145.7, 145.6, 142.9, 139.2, 133.7, 133.2, 131.9, 129.9,
128.7, 127.2, 116.0, 109.4, 103.6, 93.4, 21.8; CHN: Anal.
Calcd for C20H15IN2O4S: C, 47.44; H, 2.99; N, 5.53.
Found: C, 47.52; H, 3.10; N, 5.64.
M.p. 231‐234 °C, FT‐IR (KBr, cm‐1): 3329, 3069, 1674,
1618, 1521, 1480, 1396, 1292, 1190, 1161, 1110, 1007,
1
971, 810, 715, 640, 521; H NMR (DMSO‐d6, 300 MHz):
δ = 9.35 (b, 2H), 7.77 (s, 4H), 7.15‐6.50 (m, 4H), 6.40 (d,
J = 8.1 Hz, 4H); 13C NMR (DMSO‐d6, 75 MHz): δ =
154.9., 154.4, 137.3, 134.0, 133.5, 127.1, 109.5, 107.7, 96.5;
CHN: Anal. Calcd for C20H14N4O4: C, 63.58; H, 3.35; N,
14.24. Found: C, 64.17; H, 3.77; N, 14.97.

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HABIBI ET AL.
SCHEME 3 Suggested mechanism for
the synthesis of 2‐imino‐3‐aryl‐2,3‐
dihydrobenzo[d]oxazol‐5‐ols
conditions is shown in the presence of the Pd nano‐catalyst
(Scheme 3). The proposed mechanism is probably due to
the Lewis acidity of the Pd nano‐catalyst by attaching to
the carbonyl group to facilitate the nucleophilic attack of
the nitrogen atom to form the intermediate with the subse-
quent cyclization to get the final product.
3.5.14 | 3‐(2,6‐Dimethylphenyl)‐2‐imino‐
2,3‐dihydrobenzo[d]ox‐azol‐5‐yl 4‐
methylbenzenesulfonate (3g)
Yield 86%; mp 121‐123 °C; FT‐IR (KBr, cm‐1): 3329, 3052,
1699, 1609, 1490, 1369, 1212, 1191, 1172, 1091, 988, 956,
864, 840, 822, 722, 745, 665, 652, 630, 599, 556, 525; H
1
NMR (DMSO‐d6, 400 MHz): δ = 7.68 (d, J = 8.4 Hz,
2H), 7.36 (t, J = 7.6 Hz, 1H), 7.31 (d, J = 8.4 Hz, 2H),
7.25 (d, J = 7.6 Hz, 2H), 7.16 (d, J = 8.8 Hz, 3H), 6.73
(d, J = 8.8 Hz, 1H), 6.17 (d, J = 2.4 Hz, 1H), 2.46 (s, 3H),
2.09 (s, 6H); 13C NMR (DMSO‐d6, 100 MHz): δ = 156.8,
145.9, 145.6, 143.0, 137.2, 133.2, 131.8, 130.3, 129.8,
129.6, 129.4, 128.6, 115.9, 109.8, 103.2, 21.7, 17.7; CHN:
Anal. Calcd for C22H20N2O4S: C, 64.69; H, 4.94; N, 6.86.
Found: C, 64.78; H, 4.86; N, 6.94.
3.7 | The reusability of the Pd nano‐
catalyst
Recovery and reusability of the catalyst are important
aspects, so the recyclability of the Pd nano‐catalyst
(Fe3O4@SiO2@CPTMS@DAT@Pd) was investigated in a
model reaction (synthesis of 3‐(2,5‐dichlorophenyl)‐2‐
imino‐2,3‐dihydrobenzo[d]oxazol‐5‐ol from the reaction
of
p‐benzoquinone
(1.0
mmol)
with
2,5‐
dichlorophenylcyanamide (1.0 mmol) with 20 mg of the
Pd nano‐particle catalyst in acetonitrile at room tempera-
ture. Therefore, the catalyst was separated by filtration
after the first run, washed with ethanol and dried at
120 °C under vacuum and then used for the next succes-
sive runs under the same conditions. No significant
loss of the activity was observed, indicating that the
applied catalyst is capable and has the high stability
during the synthesis of various 2‐imino‐3‐phenyl‐2,3‐
dihydrobenzo[d]oxazol‐5‐ols.
3.5.15 | 3,3’‐(1,4‐Phenylene)bis(2‐imino‐
2,3‐dihydrobenzo[d]ox‐azole‐5,3‐diyl)bis(4‐
methylbenzene‐ sulfonate) (3h)
Yield 85%; mp 115 °C dec; FT‐IR (KBr, cm‐1): 3338, 3067,
1705, 1617, 1598, 1519, 1485, 1454, 1381, 1292, 1257, 1219,
1193, 1180, 1132, 1091, 991, 956, 895, 862, 835, 814, 742,
1
664, 594, 552, 529; H NMR (DMSO‐d6, 400 MHz): δ =
7.76 (d, J = 8.4 Hz, 4H), 7.66 (s, 4H), 7.39 (d, J = 8.4 Hz,
6H), 7.04 (d, J = 8.8 Hz, 2H), 6.72 (s, 2H), 6.67 (d, J =
8.8 Hz, 2H), 2.50 (s, 6H); 13C NMR (DMSO‐d6,
100 MHz): δ = 156.2, 145.7, 145.5, 142.9, 133.7, 133.2,
132.0, 129.9, 128.7, 126.7, 116.1, 109.4, 103.7, 21.8; CHN:
Anal. Calcd for C34H26N4O8S2: C, 59.81; H, 3.84; N, 8.21.
Found: C, 59.90; H, 3.87; N, 8.27.
4 | CONCLUSION
In conclusion, a novel Pd nano‐catalyst was successfully
prepared, characterized and applied for the synthesis of
a range of 2‐imino‐3‐phenyl‐2,3‐dihydrobenzo[d]oxazol‐
5‐ols.
3.6 | The plausible mechanism
The plausible mechanism for the synthesis of various
2‐imino‐3‐phenyl‐2,3‐dihydrobenzo[d]oxazol‐5‐ol from
the reaction of benzoquinone with different
phenylcyanamides at room temperature under solvent‐free
ACKNOWLEDGEMENT
The authors are grateful to the Bu‐Ali Sina University,
Hamedan Iran, for the financial support of this work.

HABIBI ET AL.
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Davood Habibi http://orcid.org/0000-0002-2386-8502
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