Guanidine complex of copper supported on boehmite nanoparticles as practical, recyclable, chemo and homoselective organic–inorganic hybrid nanocatalyst for organic reactions

Article abstract of DOI:10.1002/aoc.5901

Boehmite (BO) nanoparticles (NPs) were prepared via the injection of aqueous NaOH solution to aqueous aluminum nitrate solution at room temperature. Afterwards, a new complex of copper was immobilized on BO-NPs (Cu-Guanidine@BO-NPs). This heterogeneous nanocatalyst was used as a practical, recyclable, chemo and homoselective nanocatalyst in the organic processes, i.e. the preparation of tetrazole five-membered heterocycles and chemoselective sulfoxidation of sulfides using H₂O₂ as oxidant. In this sense, the prepared nanocatalyst was characterized by AAS, N₂ adsorption–desorption isotherms, WDX, EDS, SEM, and TGA techniques. The reusability of this catalyst was investigated in the described organic reactions for several runs without notable loss of its catalytic activity. Moreover, all of the tetrazole and sulfoxide derivatives were isolated in high Turn Over Number (TON) and Turn Over Frequency (TOF) numbers indicating the high activity and selectivity of Cu-Guanidine@BO-NPs in the described reactions.

Full text of DOI:10.1002/aoc.5901

Received: 25 December 2019
DOI: 10.1002/aoc.5901
Revised: 3 June 2020
Accepted: 5 June 2020
F U L L P A P E R
Guanidine complex of copper supported on boehmite
nanoparticles as practical, recyclable, chemo and
homoselective organic–inorganic hybrid nanocatalyst for
organic reactions
Fariba Jafari1,2 | Arash Ghorbani-Choghamarani2,3
| Neda Hasanzadeh2
1Department of Chemistry, Khozestan
Boehmite (BO) nanoparticles (NPs) were prepared via the injection of aqueous
NaOH solution to aqueous aluminum nitrate solution at room temperature.
Afterwards, a new complex of copper was immobilized on BO-NPs
(Cu-Guanidine@BO-NPs). This heterogeneous nanocatalyst was used as a
practical, recyclable, chemo and homoselective nanocatalyst in the organic
processes, i.e. the preparation of tetrazole five-membered heterocycles and
chemoselective sulfoxidation of sulfides using H2O2 as oxidant. In this sense,
the prepared nanocatalyst was characterized by AAS, N2 adsorption–
desorption isotherms, WDX, EDS, SEM, and TGA techniques. The reusability
of this catalyst was investigated in the described organic reactions for several
runs without notable loss of its catalytic activity. Moreover, all of the tetrazole
and sulfoxide derivatives were isolated in high Turn Over Number (TON) and
Turn Over Frequency (TOF) numbers indicating the high activity and selectiv-
ity of Cu-Guanidine@BO-NPs in the described reactions.
Science and Research Branch, Islamic
Azad University, Ahvaz, Iran
2Department of Chemistry, Ahvaz Branch,
Islamic Azad University, Ahvaz, Iran
3Department of Chemistry, Faculty of
Science, Ilam University, P.O. Box
69315516, Ilam, Iran
Correspondence
A. Ghorbani-Choghamarani, Department
of Chemistry, Faculty of Science, Ilam
University, P.O. Box 69,315,516, Ilam,
Iran.
Email: a.ghorbani@ilam.ac.ir
Funding information
Islamic Azad University
K E Y W O R D S
Boehmite nanoparticles, copper, guanidine, sulfoxides, Tetrazoles
1 | INTRODUCTION
loading capacity on NPs, we witness a sort of high cata-
lytic efficiency.[14,15] In this regard, various NPs were
The recoverability, stability, selectivity and catalytic effi-
ciency of the catalysts are important factors in catalytic
process.[1–4] Therefore, heterogeneous catalysts have
been widely applied in academic researches and indus-
trial applications due to their easy reusability.[5–10]
Unfortunately, heterogeneous catalysts have lower activ-
ity and selectivity than homogenous catalysts.[11] During
recent years, in order to achieve the advantages of both
heterogeneous catalysts (recoverability and stability) and
homogeneous catalysts (catalytic efficiency and selectiv-
ity), NPs have been applied as support for the fabrica-
tion of nanocatalysts.[4,12–14] Because of the fact that
NPs have high surface area, leading to high catalysts
widely used in order to prepare heterogeneous catalysts.
In this sense, it is worth mentioning that carbon
nanotubes,[16] polymers,[17] silica nanomaterials,[18,19]
graphene oxide,[20,21] Metal–organic framework,[22] iron
oxide,[23] heteropolyacid,[24] ionic liquids,[25,26] boehmite
NPs[27,28] etc. were used as support for the immobiliza-
tion of the homogeneous catalysts. Among them,
boehmite NPs is inert in various chemical conditions,
and has excellent stability.[29] High aggregation of -OH
groups on the inert surface of boehmite NPs allowed
them to be used as support for the stabilization of the
catalysts.[30,31] Moreover, boehmite NPs can be obtain
from inexpensive and available materials such as
Appl Organomet Chem. 2020;e5901.
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https://doi.org/10.1002/aoc.5901

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JAFARI ET AL.
sodium hydroxide and aluminum nitrate in aqueous
conditions.[32] Therefore, the application of boehmite
NPs has the advantage of being environment
friendly.[33,34] In addition, boehmite NPs have funda-
mental advantages i.e. non-toxicity, good surface area,
good stability and ease of surface modification.[35] In this
sense, several experimental procedures which have been
presented for the synthesis of boehmite NPs have
focused on the properties of boehmite NPs.[36–41] Despite
all advantages of boehmite NPs, it was rarely used as
heterogeneous support during recent years.[42,43] There-
fore, we report a complex of copper on boehmite NPs
(Cu-Guanidine-@BO-NPs) as efficient and heteroge-
neous catalyst for the chemoselective sulfoxidation of
sulfides using H2O2 as oxidant reagent and also for the
preparation of four-membered heterocycles such as
tetrazoles via [3 + 2] cycloaddition of NaN3 to the
nitriles. Tetrazoles as nitrogen rich structures of organic
compounds were employed in pharmaceuticals, medi-
cine, biochemistry, photographic agents and especially
in explosives.[44–46] Moreover, tetrazoles serve in coordi-
nation chemistry and have also been applied in several
useful biochemical transformations.[47–49] In addition,
tetrazoles can be used as stable replacements for carbox-
ylic acid groups in drug design.[47] Tetrazole derivatives
are usually prepared by [2 + 3] cycloaddition reaction of
nitriles with azides which gives 5-substituted 1H-
tetrazoles.[42,47] Likewise, the selective sulfoxidation of
sulfides is a great transformation in organic chemis-
try[29,31] which emerged in the separation of radioactive,
enzymes activation, and less common Metals.[29,50]
Among several oxidant reagents for sulfoxidation of sul-
fides, H2O2 offers excellent advantages such as being
readily available and environmentally friendly with the
only by-product of water.[23,27] Therefore, to keep pace
with green chemistry principles, herein, we reported
copper catalyst on boehmite nanoparticles as a stable
and reusable catalyst for the oxidation of sulfides using
hydrogen peroxide as oxidant.
FESEM-TESCAN MIRA3 Scanning Electron Micro-
scopes. Besides, this Scanning Electron Microscopes was
employed for the component elemental analysis (EDS
and WDX) of nanoparticles. In addition, the exact con-
tent of copper in the catalyst structure was measured by
AAS technique using 400p-novAA instrument from
Analytik Jena Company. Nitrogen adsorption isotherms
were recorded by a standard gas manifold at 77 K to the
textural properties of the catalyst using a BELSORP MINI
II device. Moreover, the catalyst sample was degassed at
ꢀ
120 C for 2 hr using a BEL PREP VAC II device before
analysis. NMR spectra of products were recorded using
Bruker DRX-400 spectrometer at 100–400 MHz. Melting
points were measured with an Electrothermal 9,100
apparatus.
2.2 | Preparation of cu-guanidine@BO-
NPs nanocatalyst
The modified boehmite NPs by 3-choloropropyltrimtoxy-
silane (nPr-Cl@BO-NPs) were prepared based on the
resent reported method.[12] Afterwards, nPr-Cl@BO-NPs
(1 g) was dispersed in 50 ml of toluene by bath sonication
and, then, guanidine nitrate (2.5 mmol) and the three
ethyl amine (4 ml) were combined to the reaction mix-
ture. Subsequently, the obtained suspension was stirred
ꢀ
under inert N2 atmosphere for 72 hr at 100 C and, then,
the modified nanomaterials (Guanidine@Boehmite) were
washed with EtOH. Finally, 1 g of Guanidine@Boehmite
was dispersed in EtOH (25 ml) by bath sonication, and,
then, Cu (NO3)2.9H2O (2.5 mmol) was added to the
resulting suspension. The reaction mixture was refluxed
and stirred under inert N2 atmosphere for 20 hr. The final
products (Cu-Guanidine@BO-NPs) were isolated by fil-
tration and washed with EtOH and H2O.
2.3 | Experimental procedure for the
synthesis of tetrazoles
2 | EXPERIMENTAL
A mixture of benzonitrile compounds (1 mmol) and
NaN3 (1.6 mmol), in the presence of Cu-Guanidine@BO-
NPs (40 mg, containing 0.44 mol% of copper), was mag-
netically stirred in PEG-400 as green solvent at 120 ꢀC. In
addition, the reaction process was controlled by TLC.
Finally the nanocatalyst was removed by filtration after
cooling the mixture. The recovered catalyst was washed
by ethyl acetate and, in continuation, the products were
extracted with ethyl acetate and aqueous solution of
hydrochloric acid (4 N). The organic solvent was
decanted and dried by anhydrous Na2SO4. Finally, pure
products were isolated by evaporation of ethyl acetate.
2.1 | Materials and instruments
Chemicals and solvents which are employed in this work
were purchased from Aldrich, Fluka, or Merck compa-
nies and used without further purification. Ther-
mogravimetric analyses (TGA) of the catalyst were
recorded by a SDT Q600 V20.9 Build 20 device under air
atmosphere in the temperature range of 30–800 ꢀC and at
a heating rate of 10 C min−1. The morphology and size
ꢀ
of the catalyst were obtained by SEM images using

JAFARI ET AL.
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2.4 | Experimental procedure for the
2.5.5 | 4-(1H-tetrazol-5-yl)benzonitrile
sulfoxidation of sulfides
1H NMR (400 MHz, DMSO): δH = 8.22–8.19 (d,
J = 12 Hz, 2H), 8.09–8.06 (d, J = 12 Hz, 2H) ppm.
Cu-Guanidine@BO-NPs (10 mg, containing 0.11 mol% of
copper) were mixed to 1 mmol of sulfide and 0.5 ml of
H2O2 (33%). Subsequently, the resulting mixture was
magnetically stirred at room temperature under solvent
free conditions. The reaction process was controlled by
TLC. Afterwards, the catalyst was removed by filtration
and, then, the mixture was dilute with H2O and ethyl
acetate. The mixture of the products and ethyl acetate
was dried by anhydrous Na2SO4. Finally, pure products
were isolated by evaporation of ethyl acetate. The
selected spectral data may be found online in the
supporting information.
2.5.6 | 5-phenyl-1H-tetrazole
1H NMR (400 MHz, DMSO): δH = 8.05–8.01 (m, 2H),
7.62–7.57 (m, 3H) ppm.
2.5.7 | 5-(4-nitrophenyl)-1H-tetrazole
1H NMR (400 MHz, DMSO): δH = 8.45–8.42 (d, J = 12 Hz,
2H), 8.30–8.28 (d, J = 12 Hz, 2H) ppm. 13C NMR
(100 MHz, DMSO): δC = 148.7, 130.6, 128.2, 124.6 ppm.
2.5 | Selected spectral data
2.5.1 | 1-(4-(1H-tetrazol-5-yl)phenyl)
2.5.8 | 5-(2-chlorophenyl)-1H-tetrazole
ethanone
1H NMR (400 MHz, DMSO): δH = 7.81–7.79 (d, J = 8 Hz,
1H), 7.72–7.69 (d, J = 12 Hz, 1H), 7.66–7.55 (m, 2H), ppm.
1H NMR (400 MHz, DMSO): δH = 17.12 (b, 1H),
8.22–8.20 (d, J = 8 Hz, 2H), 8.20–8.18 (d, J = 8 Hz, 2H),
2.67 (s, 3H) ppm. 13C NMR (100 MHz, DMSO): δC = 197.9,
155.7, 138.9, 129.6, 128.8, 127.7, 27.4 ppm.
2.5.9 | 4-(1H-tetrazol-5-yl)phenol
1H NMR (400 MHz, DMSO): δH = 10.16 (br, 1H), 7.86–7.84
(d, J = 8 Hz, 2H), 6.95–6.93 (d, J = 8 Hz, 2H) ppm.
2.5.2 | 2-(1H-tetrazol-5-yl)benzonitrile
1H NMR (400 MHz, DMSO): δH = 8.13–8.09 (t,
J = 8 Hz, 2H), 7.99–7.94 (t, J = 8 Hz, 1H), 7.83–7.79
(t, J = 8 Hz, 1H) ppm. 13C NMR (100 MHz, DMSO):
δC = 156.0, 135.4, 134.3, 131.9, 130.2, 128.1, 117.7,
110.7 ppm.
2.5.10 | 2-(phenylsulfinyl)ethanol
1H NMR (400 MHz, CDCl3): δH = 7.70–7.65 (m, 2H),
7.62–7.49 (m, 3H), 5.35 (br, 1H), 4.21–4.14 (m, 1H),
4.03–3.96 (m, 1H), 3.18–3.12 (m, 1H), 2.96–2.90 (m,
1H) ppm.
2.5.3 | 5-(3-nitrophenyl)-1H-tetrazole
1H NMR (400 MHz, DMSO): δH = 8.83–8.81 (t, J = 4 Hz,
1H), 8.48–8.39 (m, 2H), 7.92–7.87 (t, J = 8 Hz, 1H) ppm.
13C NMR (100 MHz, DMSO): δC = 155.0, 148.3, 133.1,
131.2, 126.2, 125.6, 121.5 ppm.
2.5.11 | (methylsulfinyl)benzene
1H NMR (400 MHz, CDCl3): δH = 7.68–7.65 (m, 2H),
7.61–7.56 (m, 1H), 7.55–7.52 (m, 2H), 2.74 (s, 3H) ppm.
13C NMR (100 MHz, CDCl3, ppm) δ = 131.0, 129.4, 127.3,
123.5, 43.9 ppm.
2.5.4 | 2-(1H-tetrazol-5-yl)phenol
1H NMR (400 MHz, DMSO): δH = 12.04 (br, 1H),
8.00–7.96 (d, J = 8 Hz, 1H), 7.42–7.36 (t, J = 8 Hz,
1H), 7.08–7.04 (d, J = 8 Hz, 1H), 7.02–6.96 (t,
J = 8 Hz, 1H), 3.49 (br, 1H) ppm. 13C NMR (100 MHz,
DMSO): δC = 155.3, 151.6, 132.6, 129.0, 119.7, 116.3,
110.5 ppm.
2.5.12 | 1-(butylsulfinyl)butane
1H NMR (400 MHz, CDCl3): δH = 2.72–2.55 (m, 2H),
1.77–1.67 (quint, J = 8 Hz, 2H), 1.55–1.36 (m, 2H),
0.96–0.91 (t, J = 8 Hz, 3H) ppm. 13C NMR (100 MHz,
CDCl3, ppm) δ = 52.0, 24.5, 22.0, 13.6 ppm.

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JAFARI ET AL.
2.5.13 | 1-(methylsulfinyl)dodecane
images are illustrated in Figure 1. Homogeneous and
nearly cubic orthorhombic structures of Cu-Guanidine-
@BO-NPs are shown in SEM images. Moreover, SEM
images indicate that the particles of this catalyst are
between 40–60 nm in size.
1H NMR (400 MHz, CDCl3): δH = 2.80–2.63 (m, 2H), 2.57
(s, 3H), 1.81–1.70 (m, 2H), 1.39–1.26 (m, 18H), 0.90–0.86
(t, J = 8 Hz, 3H) ppm. 13C NMR (100 MHz, CDCl3, ppm)
δ = 54.3, 38.1, 31.4, 29.1, 29.0, 28.89, 28.86, 28.7, 28.3,
22.2, 22.1, 13.6 ppm.
In order to show the elements content, EDS analysis
was performed for Cu-Guanidine@BO-NPs. As shown in
Figure 2, this catalyst was formed from Al (9.07%A), O
(57.11%A), Si (6.19%A), C (15.07%A), N (10.77%A) and
Cu (1.78%A) species. In addition, as shown in Figure 3,
WDX was used for the distributions of elements in the
catalyst structure. In this regard, Figure 3 illustrates a
good homogeneous distribution of the elements for this
catalyst. Besides, the exact condensation of Cu in Cu-
Guanidine@BO-NPs which was calculated by AAS was
3 | RESULTS AND DISCUSSION
In this research project, an environmentally friendly and
economical route is reported for a copper catalyst on the
modified boehmite NPs (Cu-Guanidine@BO-NPs). In
this sense, boehmite NPs were prepared by dropwise
addition of NaOH aqueous solution to Al (NO3)3Á9H2O
aqueous solution.[42] Afterwards, the hydroxyl groups
on the surface of boehmite NPs were modified by
3-chloropropyltrimethoxysilane (CPTMS) by which nPr-
Cl@BO-NPs were prepared. In the next step, the terminal
Cl groups in nPr-Cl@BO-NPs were successfully replaced
by guanidine through substation reaction in toluene at
found to be 0.112 × 10−3 mol g−1
.
In order to determine the amount of organic layers
which were immobilized on boehmite NPs, TGA
analysis was performed. TGA-DTG-DTA diagrams of
Cu-Guanidine@BO-NPs are shown in Figure 4. These
diagrams show three weight losses between 25 ꢀC–
800 ꢀC. The Frist loss as displayed before 200 ꢀC was
related to the removal of the adsorbed solvents (about
5%) which were physically adsorbed in the catalyst.[23]
The second of one was observed between 200–500 ꢀC
because of the calcination of organic layers on boehm-
ite NPs which was approximately 34% of the weight
loss. The final weight loss in TGA diagram of Cu-
ꢀ
100 C under basic conditions of triethylamine (Et3N). It
is worth mentioning that guanidine has been supported
on BO-NPs (Guanidine@BO-NPs) within 72 hr. The final
catalyst (Cu-Guanidine@BO-NPs) was prepared by stabi-
lization of copper on the surface of the modified boehm-
ite NPs by guanidine (Scheme 1).
In the next step, Cu-Guanidine@BO-NPs were char-
acterized by SEM, WDX, EDS, TGA, BET and AAS
techniques.
ꢀ
Guanidine@BO-NPs was observed above 500 C, which
may be related to transformation of thermal phase
of boehmite NPs, that it is 4% of weight loss.[29] As
shown, Cu-Guanidine@BO-NPs were stable up to
200 ꢀC confirming the fact that guanidine complex
of copper was covalently fabricated on the surface of
boehmite nanoparticles. Therefore, this catalyst can
be employed in various organic reactions under
hard conditions.
3.1 | Catalyst characterizations
The diameter size and morphology of Cu-Guanidine-
@BO-NPs were obtained by SEM technique whose
SCHEME 1 Synthesis of Cu-
Guanidine@BO-NPs

JAFARI ET AL.
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FIGURE 1 SEM images of Cu-Guanidine@BO-NPs
FIGURE 2 EDX spectrum of Cu-Guanidine@BO-NPs
Nitrogen adsorption–desorption isotherms of Cu-
Guanidine@BO-NPs are shown in Figure 5. Based on the
IUPAC classification, boehmite NPs display an isotherm
of mesoporous material.[51] The obtained surface area of
Cu-Guanidine@BO-NPs is 63.39 m2/g which is lower
than boehmite NPs (122.8 m2/g[27]). The decrease in the
surface area of the catalyst, greater than parent boehmite
NPs, is due to the immobilization of organic content and
Cu-complex on boehmite NPs.[27] Moreover, BET analy-
sis of Cu-Guanidine@BO-NPs which indicated a lower
pore volume (0.36 cm3 g−1) than the free boehmite
nanoparticles (0.589 cm3 g−1[52]) confirmed the successful
grafting of guanidine and copper complex on the surface
of boehmite nanoparticles.

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JAFARI ET AL.
FIGURE 3 Elemental mapping of (a) Al,
(b) O, (c) Si, (d) C, (e) N, and (f) Cu for Cu-
Guanidine@BO-NPs
3.2 | Catalytic study of cu-guanidine@BO-
NPs
examined in the selected reaction (Table 1, entries 1–4).
When this reaction was tested without Cu-Guanidine-
@BO-NPs, the reaction did not take place (Table 1, entry
1). Meanwhile, the progress of the reaction was improved
upon increasing in the amount of Cu-Guanidine@BO-
NPs catalyst. As displayed in Table 1, 40 mg (containing
0.44 mol% of copper) of Cu-Guanidine@BO-NPs
(Table 1, entry 4) was obtained as the optimum amount
of the catalyst. Moreover, the progress of the reaction
shows a significant dependence to nature and the
amount of NaN3. Therefore, the effect of solvents
(Table 1, entries 6–10) and sodium azide concentration
(Table 1, entries 4–8) were studied in the selected reac-
tion. As shown in Table 1, 1.6 mmol of sodium azide in
PEG-400 as solvent showed a better conversion of
The catalytic activity of Cu-Guanidine@BO-NPs was
applied in the synthesis of 5-substituted tetrazoles and
chemoselective sulfoxidation of sulfides.
Firstly, the catalytic synthesis of tetrazoles was
studied in the presence of Cu-Guanidine@BO-NPs. The
synthesis of 5-substituted tetrazoles from [3 + 2] cycload-
dition of NaN3 to the benzonitriles which is shown in
Scheme 2 was catalyzed by Cu-Guanidine@BO-NPs. In
order to find the best conditions for the preparation of
tetrazoles, the [3 + 2] cycloaddition of NaN3 to the ben-
zonitrile was studied in various conditions (Table 1). In
this regard, different amounts of the catalyst were

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FIGURE 4 TGA-DTG-DTA diagrams of Cu-Guanidine@BO-NPs
FIGURE 5 N2 adsorption–desorption
isotherm of Cu-Guanidine@BO-NPs
SCHEME 2 Synthesis of 5-substituted
tetrazole derivatives in the presence of Cu-
Guanidine@BO-NPs

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JAFARI ET AL.
TABLE 1 Optimization of the Synthesis of 5-phenyl-1H-tetrazole in the presence of Cu-Guanidine@BO-NPs
Entry
Catalyst (mg)
Solvent
PEG
NaN3 (mmol)
Temperature (ꢀC)
Time (min)
720
Yield (%)[a]
1
-
30
35
40
40
40
40
40
40
40
40
1.5
1.5
1.5
1.5
1.6
1.4
1.6
1.6
1.6
1.6
1.6
120
-
92
90
93
95
88
85
91
87
83
90
2
PEG
120
360
3
PEG
120
315
4
PEG
120
180
5
PEG
120
70
6
PEG
120
245
7
DMSO
DMF
H2O
120
150
8
120
190
9
Reflux
Reflux
100
210
10
11
EtOH
PEG
260
420
aIsolated yield.
benzonitrile to 5-phenyl-1H-tetrazole (Table 1, entry 5).
Finally, the effect of temperature was examined by which
120 ꢀC was selected.
4-hydroxybenzonitrile, 4-nitrobenzonitrile, and 4-acetyl-
benzonitrile. As shown, the calculated TOF numbers
for these derivatives are 45.8, 8.7 and 6.2 h−1, respec-
tively. Therefore, their reactivity can be sorted as:
4-OH > 4-NO2 > 4-COCH3 which means that the
functionalized benzonitriles with an electron-donating
groups are reactive than the functionalized benzonitriles
with an electron-withdrawing group.
Homoselectivity which is a great and imprtant prop-
erty in the synthesis of various organic compounds is also
observed in the same functional groups with similar elec-
tronic nature and steric hindrance.[54,55] Homoselectivity
of Cu-Guanidine@BO-NPs was investigated in the
[3 + 2] cycloaddition of NaN3 with terephthalonitrile
(Table 2, entry 7) and phthalonitrile (Table 2, entry 8).
Terephthalonitrile has two quite similar cyano functional
groups on para position and phthalonitrile has two quite
In order to confirm the extend capability of this pro-
cedure, the reaction of NaN3 with various electron-poor
and electron-rich substituted nitriles was investigated in
the presence of Cu-Guanidine@BO-NPs (Table 2). In
addition, different aliphatic (Table 2, entries 11) and aro-
matic (Table 2, entries 1–10) nitriles were studied for the
synthesis of tetrazoles whose corresponding products
were isolated in exellent yields and high TON and TOF
values. Moreover, several benzonitriles bearning ortho,
meta and para functional groups were investigated for
senthesis of the corresponding tetrazoles whose products
were obtaied in good yields. The effect of electronic
nature of aromatic ring in benzonitriles was compared to
the reaction speed and the calculated TOF number for
TABLE 2 Catalytic preparation of tetrazoles in the presence of Cu-Guanidine@BO-NPs
Entry
nitrile
Time (min)
Yield (%)a
TON
216
218
218
223
214
209
218
220
207
211
193
TOF (h−1
185.1
45.8
)
Melting point (ꢀC)
211 –213
230–232
Reported M.P.
210–212 [19]
231–233 [19]
149–152 [42]
224–226 [42]
120–123 [19]
219–221 [53]
254–257 [42]
209–210 [19]
180–183 [42]
173–176 [42]
41–43 [53]
1
Benzonitrile
70
285
270
7
95
96
96
98
94
92
96
97
91
93
85
2
4-hydroxybenzonitrile
3-Nitrobenzonitrile
2-Hydroxybenzonitrile
2-phenylacetonitrile
4-Nitrobenzonitrile
Terephthalonitrile
Phthalonitrile
3
48.4
149–151
4
1911
49.4
223–225
5
260
24 h
340
580
350
34 h
780
121–122
6
8.7
221–223
7
38.5
253–255
8
22.7
210–211
9
2-Chlorobenzonitrile
4-Acetylbenzonitrile
Pentanenitrile
35.5
181–183
10
11
6.2
173–175
14.8
41–43
aIsolated yield.

JAFARI ET AL.
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similar cyano functional groups on ortho position of aro-
matic ring. Interestingly, this procedure afforded the
mono-addition in the presence of Cu-Guanidine@BO-
NPs (Scheme 3) which showed a good homoselectivity of
this catalyst in the synthesis of tetrazole deraiivatives.
A cyclic mechanism for the synthesis of tetrazoles in
the presence of Cu-Guanidine@BO-NPs is outlined in
Scheme 4.[54,56] The first step is the interaction of catalyst
to the nitrile which provides the conditions for the nucle-
ophilic addition of azide. This step is screened as interme-
diate I in Scheme 4. Subsequently, the [3 + 2]
cycloaddition of NaN3 to the benzonitrile gives interme-
diate II. Finally, intermediate II which is protonated by
HCl produced the stable product IV and regenerated the
catalyst, which can continue the catalytic cycle.
SCHEME 4 A cyclic mechanism for the synthesis of
In the next study, the catalytic application of Cu-Gua-
nidine@BO-NPs was performed in the chemoselective
sulfoxidation of sulfides using hydrogen peroxide as
green oxidant because it is very inexpensive and gives
only water as the by-product (Scheme 5). In order to
define the ideal conditions, the sulfoxidation of
methyl phenyl sulfide by H2O2 as oxidant and Cu-
Guanidine@BO-NPs as the catalyst was selected as the
model reaction. Different amounts of this catalyst and
several solvents were studied in the selected reaction at
room temperature. The obtained results were displayed
in Table 3. The selected reaction did not proceed in the
absence of Cu-Guanidine@BO-NPs even after 600 min.
Meanwhile, the speed of the reaction was increased upon
increasing the catalyst concentration. Finally, 10 mg
(containing 0.11 mol% of copper) of Cu-Guanidine@BO-
NPs was selected as the best amount of the catalyst as the
increasing amount of the catalyst to 12 mg (Table 3, entry
5) had no significant effect on the reaction speed or prod-
uct yield. Moreover, the nature of the solvent was exam-
ined by which it was revealed that solvent free condition
showed better results. The summarized results in Table 3
tetrazoles in the presence of Cu-Guanidine@BO-NPs
illustrated that the best conditions for sulfoxidation of
sulfides is 10 mg (containing 0.11 mol% of copper) of Cu-
Guanidine@BO-NPs under solvent free conditions.
Catalytic application of Cu-Guanidine@BO-NPs for
sulfoxidation of sulfides was extend for a wide range
of sulfides (Table 4). Various sulfoxides; including,
aromatic and aliphatic sulfoxides were successfully syn-
thesized in the presence of Cu-Guanidine@BO-NPs.
Over-oxidation of sulfides to sulfone was not observed
and pure sulfoxides were synthesized as the main prod-
ucts (Scheme 6). Chemoselectivity has been regarded as
very significant in the synthesis of organic compounds
such as druges, natural products and biological mole-
cules. In this regard, among the few reactions that
compete with each other, only one reaction is performed.
The chemoselectivity of Cu-Guanidine@BO-NPs was
investigated in the oxidation of 2-Phenylthioethanol
(Table 4, entry 2) in which hydroxyl group can compete
in the oxidation reaction with sulfide. As shown
in Scheme 6, hydroxyl groups were remained intact
under optimized conditions which confirms the
chemoselectivity of this catalyst. Moreover, synthesis of
sulfones by over oxidation can compete with the synthe-
sis of sulfoxides. Meanwhile, sulfoxidation of sulfides was
observed without over-oxidation to generate any by-
product such as sulfone.
A plausible reaction mechanism for the sulfoxidation
of sulfides to sulfoxides in the presence of Cu-Guanidine-
@BO-NPs is outlined in Scheme 7.[57,58] At first, hydro-
gen peroxide is active by Cu-Guanidine@BO-NPs which
gives intermediate A and further is converted to interme-
diate B as active oxidant. Next, nucleophilic attack of
the sulfur of sulfide on this intermediate gives cation
C which, in turn, produces the sulfoxide products and
regenerates the catalyst.
SCHEME 3 Homoselectivity of Cu-Guanidine@BO-NPs in
the synthesis of tetrazole derivatives

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JAFARI ET AL.
SCHEME 5 Sulfoxidation of sulfides in the
presence of Cu-Guanidine@BO-NPs
TABLE 3 Optimization conditions for the sulfoxidation of sulfides in the presence of Cu-Guanidine@BO-NPs
Entry
Catalyst (mg)
Amount of H2O2 (ml)
Solvent
Time (min)
600
Yield (%)[a]
1
-
5
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.5
0.6
Solvent Free
Solvent Free
Solvent Free
Solvent Free
Solvent Free
EtOH
-
55
80
94
96
75
45
63
12
98
95
2
395
3
8
395
4
10
12
10
10
10
10
10
10
395
5
385
6
395
7
CH2Cl2
395
8
DMSO
395
9
EtOAC
395
10
11
Solvent Free
Solvent Free
375
370
aIsolated yield.
TABLE 4 Sulfoxidation of sulfides in the presence of Cu-Guanidine@BO-NPs
Entry
Sulfide
Time (min)
Yield (%)a
TON
891
854
836
864
827
TOF (h−1
142.5
)
Melting point (ꢀC)
Reported M.P.
30–32 [31]
Oil [29]
61–64 [23]
Oil [51]
1
2
3
4
5
Methyl phenyl sulfide
2-Phenylthioethanol
Methyl (undecyl)sulfane
Diallylsulfane
375
85
98
94
92
95
91
30–32
Oil
602.8
420
85
119.4
59–62
Oil
609.8
Dibutylsulfane
330
150.3
Oil
Oil [29]
aIsolated yields.
SCHEME 6
Chemoselective sulfoxidation of
sulfides in the presence of Cu-
Guanidine@BO-NPs
3.3 | Reusability of the catalyst
(Figure 6). As the experiment reveals, it is at the end of
the each reaction that the remained catalyst was recov-
ered by centrifugation and being washed with ethyl ace-
tate. The products were isolated using the extraction with
H2O and ethyl acetate and, then, the remained catalyst
was reused in same reaction up to 6 times. The average
isolated yields of products for 6 runs is 92.33%, which
shows the good reusability of Cu-Guanidine@BO-NPs.
The separating and reusing the remaining catalysts are
important steps in catalytic processes. Therefore, as
screened in Table 1 (entry 4), the recyclability of Cu-
Guanidine@BO-NPs was studied in the synthesis of
5-phenyl-1H-tetrazole with the [3 + 2] cycloaddition of
NaN3 to the benzonitrile under the optimized conditions

JAFARI ET AL.
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SCHEME 7 Proposed mechanism for
oxidation of sulfides in the presence of Cu-
Guanidine@BO-NPs
FIGURE 6 Reusability of Cu-
Guanidine@BO-NPs in the synthesis of
5-phenyl-1H-tetrazole
3.4 | Comparison of the catalyst
best results were reached in the presence of Cu-
Guanidine@BO-NPs as the catalyst in comparison to
other catalysts. In this regard, the isolated yield, and TOF
and TON values in the presence of Cu-Guanidine@BO-
NPs are higher than other catalysts. Moreover, several
reported procedures utilized the organic solvent (Table 5,
entries 2–10) or employed the high temperature (Table 5,
entries 6 and 11) which are expensive and not environ-
ment friendly; meanwhile, PEG was used as green sol-
vent in the present work. In some other procedures,
The efficiency of Cu-Guanidine@BO-NPs was compared
to old literatures. The obtained results for the reaction of
benzonitrile with sodium azide in the presence of Cu-
Guanidine@BO-NPs were compared to previous cata-
lysts. This comparison is summarized in Table 5. In order
to show the practically of Cu-Guanidine@BO-NPs, the
reaction condition, isolated yields, reaction time and
TON and TOF values were compared. As shown, the

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JAFARI ET AL.
TABLE 5 Comparison of Cu-Guanidine@BO-NPs in the preparation of 5-phenyl-1H-tetrazole with previously reported catalysts
Time
(min)
TOF
Yield a (%)
[Reference]
Entry Catalyst
Reaction conditions
PEG-400, 120 ꢀC
DMF, 110 ꢀC
(h−1
)
1
2
3
Pd-Arg@boehmite
420
360
420
7.7
97 [42]
97 [59]
90 [60]
(NH4)Ce (NO3)6
1.61
Fe3O4@SiO2/Salen Cu
(II)
DMF, 120 ꢀC
-
4
5
6
7
8
AgNO3
DMF, 120 ꢀC
DMF, 120 ꢀC
DMF, 120–130 ꢀC
DMF, 120 ꢀC
DMF, 110 ꢀC
300
720
720
720
240
1.66
0.32
-
83 [61]
98 [62]
90 [63]
82 [64]
93 [65]
Cu (OAc)2
Nano ZnO/Co3O4
CuFe2O4
-
CAN supported
HY-zeolite
0.77
9
FeCl3–SiO2
DMF, 120 ꢀC
720
15
-
79 [66]
98 [67]
93 [68]
10
11
Md-Pt NPs@AC
P2O5–SiO2
DMF, 90 ꢀC, microwave irradiation (140 W)
23.76
1.86
Solvent-Free, 130 ꢀC or ultrasonic sonication,
45 ꢀC
150
12
Cu-Guanidine@BO-NPs PEG-400, 120 ꢀC
70
185.1
95 [this work]
ORCID
microwave irradiation (Table 5, entry 10) or ultrasonic
sonication (Table 5, entry 11) were used in order to pre-
pare the tetrazole derivatives.
Arash Ghorbani-Choghamarani https://orcid.org/0000-
0002-7212-1317
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How to cite this article: Jafari F, Ghorbani-
Choghamarani A, Hasanzadeh N. Guanidine
complex of copper supported on boehmite
nanoparticles as practical, recyclable, chemo and
homoselective organic–inorganic hybrid
nanocatalyst for organic reactions. Appl Organomet
Chem. 2020;e5901. https://doi.org/10.1002/aoc.
5901
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