Tandem imine formationviaauto-hydrogen transfer from alcohols to nitro compounds catalyzed by a nanomagnetically recyclable copper catalyst under solvent-free conditions

Article abstract of DOI:10.1039/d1ra02347k

A direct imination reaction was developed by tandem reaction of alcohols and nitro compounds in the presence of Cu-isatin Schiff base-γ-Fe₂O₃as a nanomagnetically recyclable catalyst under solvent-free conditions. By this method, various imines were prepared in good to high yields from one-pot reaction of various alcohols (primary aromatic and aliphatic) and nitro compounds (aromatic and aliphatic)viaan auto-hydrogen transfer reaction. Use of an inexpensive and easily reusable catalyst, without requiring any additives or excess amounts of benzyl alcohol as the reaction solvent are the other advantages of this method. This catalytic system has the merits of cost effectiveness, environmental benignity, excellent recyclability and good reproducibility.

Full text of DOI:10.1039/d1ra02347k

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Tandem imine formation via auto-hydrogen
transfer from alcohols to nitro compounds
catalyzed by a nanomagnetically recyclable copper
catalyst under solvent-free conditions†
Cite this: RSC Adv., 2021, 11, 19121
a
a
Hadis Hosseini Moghadam,a Seyed Ruhollah Derakhshan
*
Sara Sobhani,
b
and Jose Miguel Sansano
´
A direct imination reaction was developed by tandem reaction of alcohols and nitro compounds in the
presence of Cu-isatin Schiff base-g-Fe2O3 as a nanomagnetically recyclable catalyst under solvent-free
conditions. By this method, various imines were prepared in good to high yields from one-pot reaction
of various alcohols (primary aromatic and aliphatic) and nitro compounds (aromatic and aliphatic) via an
auto-hydrogen transfer reaction. Use of an inexpensive and easily reusable catalyst, without requiring
any additives or excess amounts of benzyl alcohol as the reaction solvent are the other advantages of
this method. This catalytic system has the merits of cost effectiveness, environmental benignity,
excellent recyclability and good reproducibility.
Received 24th March 2021
Accepted 16th May 2021
DOI: 10.1039/d1ra02347k
rsc.li/rsc-advances
regard, the borrowing-hydrogen methodology (auto-hydrogen
transfer), which transfers hydrogen from readily available
Introduction
alcohols to nitro compounds to produce amines as well as
valuable aldehydes, provides a promising alternative to the
existing method for the synthesis of imines.8 Some complexes
and compounds derived from transition metals of the second
and third rows, such as ruthenium,9 palladium,10 silver11
iridium12 and gold13 have been introduced as promoters of this
strategy of the imine synthesis. However, the toxicities, pricing,
and stability of these metals prohibit their general use for
industrial purposes.14 Surprisingly, there is only one report on
the use of copper for imines formation via auto-hydrogen
transfer,15 although copper is a low cost, easily available and
Imines are crucial intermediates in the synthesis of biologically
active nitrogen compounds, such as b-lactams, dyes, fragrances,
pharmaceuticals, fungicides, and agricultural chemicals.1 The
C]N bond in imines has electrophilic properties and is widely
used in organic transformations such as reduction, addition,
cyclization, and aziridination reactions.2 Traditionally, imines
are produced from the condensation of primary amines with
carbonyl compounds.3 In recent years, much attention has been
paid to tandem processes as an attractive synthetic concept for
improving overall process efficiency and reducing waste
production by converting simple starting materials into more
complex products in a single reaction vessel.4 In this regard,
tandem synthesis of imines from alcohols and nitro
compounds is a more advantageous method than traditional
procedures, because alcohols are much more stable and readily
available starting materials.5 The process involves selective
oxidation of benzyl alcohol to benzaldehyde followed by the
coupling reaction of amines with benzaldehyde.6 From the
standpoint of sustainable development, an atom-economic,
“green” and operationally convenient method, the synthesis
of imines from nitro compounds is highly desirable.7 In this
aDepartment of Chemistry, College of Sciences, University of Birjand, Birjand, Iran.
E-mail: ssobhani@birjand.ac.ir; sobhanisara@yahoo.com
b
´
´
´
Departamento de Quımica Organica, Facultad de Ciencias, Centro de Innovacion en
´
´
´
Quımica Avanzada (ORFEO-CINQA), Instituto de Sıntesis Organica (ISO),
Universidad de Alicante, Apdo. 99, 03080-Alicante, Spain
† Electronic supplementary information (ESI) available: The synthesis and
analysis of the catalyst. See DOI: 10.1039/d1ra02347k
Scheme 1 Preparation of Cu-isatin Schiff base-g-Fe2O3.
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relatively environmentally benign metal. This reported method
suffers from severe problems related to the use of a toxic organic
solvent, excess amount of the base, high temperature, and
requiring prolonged reaction time (3 days).
In continuation of our efforts on the introduction of new
heterogeneous catalysts for organic transformations,16 we have
recently synthesized Cu-isatin Schiff base-g-Fe2O3 (Scheme 1) and
used it as a new heterogeneous catalyst for the synthesis of
bis(indolyl)methanes and bis(pyrazolyl)methanes.17 Herein, in this
paper, we have expanded the scope of application of this catalyst
for the tandem imine formation by auto-hydrogen transfer from
alcohols to nitro compounds under solvent-free conditions.
Fig. 2 XRD pattern of Cu-isatin Schiff base-g-Fe2O3.
Result and discussion
g-Fe2O3. N–H stretching band of the amide group in isatin
overlapped with the broad O–H band, which was found at
3390 cmꢀ1. In the FT-IR spectra of Cu-isatin Schiff base-g-Fe2O3
(Fig. 1c), the C]N and C]O stretching frequencies were shif-
ted to the lower wave numbers (1600, 1704 cmꢀ1), which
showed the successful coordination of nitrogen and oxygen to
the metal center.
Cu-isatin Schiff base-g-Fe2O3 as a nanomagnetically heteroge-
neous catalyst was synthesized according to our previously re-
ported method.16 Cu-isatin Schiff base-g-Fe2O3 as
a nanomagnetically heterogeneous catalyst, was synthesized by
functionalization of g-Fe2O3 with 3-aminopropyltriethoxysilane
and then the reaction with isatin to produce isatin Schiff base-g-
Fe2O3 followed by the reaction with dissolving CuCl2$2H2O in
methanol (Scheme 1).
As presented in Fig. 2, the reection planes of (2 2 0), (3 1 1),
(1 1 1), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) at 2q ¼ 30.3ꢁ, 35.7ꢁ, 39.9ꢁ,
43.4ꢁ, 53.8ꢁ, 57.4ꢁ and 63.0ꢁ were readily recognized in the XRD
pattern of Cu-isatin Schiff base-g-Fe2O3. These characteristic
peaks matched with those of standard g-Fe2O3 (JCPDS le no.
04-0755). The observed diffraction peaks was indicated that g-
Fe2O3 mostly exists in face-centered cubic structure. In addi-
tion, the diffraction plane of (111) at 2q ¼ 37.4ꢁ in the XRD
pattern of Cu-isatin Schiff base-g-Fe2O3 is ascribed to Cu.
The thermogravimetric analysis (TGA) of Cu-isatin Schiff
base-g-Fe2O3 was used to determine the thermal stability and
content of organic functional groups on the surface of magnetic
nano particles (Fig. 3). TG curve of the catalyst showed the
weight loss at around 182 ꢁC, which was related to the adsorbed
water molecules on the support. The organic parts were
decomposed completely in the temperature range of 200–
The catalyst was characterized by a series of techniques such
as FT-IR, XRD, TGA, TEM, SEM, VSM, ICP and elemental
analysis.
The FT-IR spectra of (a) amino-functionalized g-Fe2O3, (b)
isatin Schiff base-g-Fe2O3 and Cu-isatin Schiff base-g-Fe2O3 are
exhibited in Fig. 1. The FT-IR spectrum of amino-functionalized
g-Fe2O3 exhibited a broad band at around 550–670 cmꢀ1 due to
the stretching vibrations of Fe–O. The appeared peaks at 1085,
3438, 3480 and around 2869–2906 cmꢀ1 in the FT-IR spectrum
of amino-functionalized g-Fe2O3 are related to C–N, N–H and
C–H stretching modes of the alkyl chain, respectively. N–H
bending was observed at 1631 cmꢀ1 (Fig. 1a). New bands at
1455, 1608 and 1718 cmꢀ1 in the FT-IR spectrum of isatin Schiff
base-g-Fe2O3 (Fig. 1b) refer to the C]C, C]N and C]O
stretching vibrations, respectively. These bands proved that
isatin has been successfully reacted with amino-functionalized
ꢁ
600 C.
The metal content of the complex was determined by ICP-
OES, and showed a value of 0.11 mmol gꢀ1. Elemental anal-
ysis showed that the loading of isatin-Schiff base on g-Fe2O3,
was 0.31 mmol gꢀ1 based on the nitrogen and carbon amounts
(0.86% and 3.95%, respectively).
Fig. 1 FT-IR spectra of (a) amino-functionalized g-Fe2O3, (b) isatin
Schiff base-g-Fe2O3 and (c) Cu-isatin Schiff base-g-Fe2O3.
Fig. 3 TGA diagram of Cu-isatin Schiff base-g-Fe2O3.
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Fig. 4 (a) TEM image of Cu-isatin Schiff base-g-Fe2O3 and (b) particle
size distribution histogram of Cu-isatin Schiff base-g-Fe2O3. (c) SEM
image of Cu-isatin Schiff base-g-Fe2O3.
The size and morphology of the synthesized catalyst were
characterized using TEM and SEM (Fig. 4). The TEM image is
clearly showed that Cu-isatin Schiff base-g-Fe2O3 exhibits
spherical morphology with relatively good monodispersity
(Fig. 4a). The particle size distribution of Cu-isatin Schiff base-
g-Fe2O3 was evaluated using TEM and showed that the average
diameter of the particles was 13 nm (Fig. 4b). The SEM image
analysis indicates that the-g-Fe2O3 nanoparticles are spherical
in shape (Fig. 4c).
The saturation magnetization values for g-Fe2O3 and Cu-
isatin Schiff base-g-Fe2O3 were 68.9 and 65.2 emu gꢀ1, respec-
tively (Fig. 5). A slight decrease of the saturation magnetization
of Cu-isatin Schiff base-g-Fe2O3 was due to the immobilization
of Cu complex on the surface of g-Fe2O3.
Cu-isatin Schiff base-g-Fe2O3 was also analyzed by XPS
spectroscopy (Fig. 6). The observed characteristic peaks in the
XPS elemental survey are attributed to carbon (C 1s), nitrogen
(N 1s), oxygen (O 1s), iron, silicon and copper (Fig. 6a).18 The C
1s spectrum (Fig. 6b) showed binding energies of 284.5 (C–C),
285.4 (C]N), 286.4 (C–N) and 288.3 eV (C]O).19 Furthermore,
high resolution XPS of N 1s region conrmed the presence of N–
Fig. 6 (a) XPS patterns of the Cu-isatin Schiff base-g-Fe2O3, (b) C (1s),
(c) N (1s), (d) Cu.
H, imine and –NH2 by revealing three peaks at 400.8, 399.6 and
398.3 eV, respectively (Fig. 6c).20 In the XPS spectrum of Cu
(Fig. 6d), the presence of binding energies at 933.3 and 940.8 eV
Fig. 5 Magnetization curves of (a) Cu-isatin Schiff base-g-Fe2O3 and
(b) g-Fe2O3.
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Table 1 Optimization of the tandem imine formation via auto-hydrogen transfer from benzyl alcohol to nitrobenzene
Temperature
Isolated yield
(%)
Entrya
Catalyst (mol%)
Solvent
Base
(ꢁC)
Time (h)
1
2
3
4
5
6
7
8
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
1
0.5
0
1
1
1
1
1
1
—
—
—
—
—
—
EtOH
H2O
Acetonitrile
Benzyl alcohol
n-Hexane
Toluene
K2CO3
Et3N
Na2CO3
KOH
NaOH
—
100
100
100
100
100
100
65
80
68
100
50
90
5
42
Trace
35
95
86
30
22
0
24
24
9
9
6
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
3
24
24
7
5
3
24
24
7
10
6
24
24
24
10
10
10
12
9
Trace
81
35
10
11
12
13
14
15
16
17
18
19b
20c
21d
22f
22g
23h
32
Ethyl acetate
60
r.t.
65
Trace
Trace
56
92
63
—
—
—
—
—
—
—
—
—
—
—
100
100
100
100
100
100
100
100
100
21
43
30
0e
0
90
87
a
Reaction conditions: nitrobenzene (0.5 mmol, except for entry 21), benzyl alcohol (0.5 mmol, except for entry 22), base (0.5 mmol, except for entry
b
c
d
6), solvent (3 mL, entries 7–13). CuCl2$2H2O was used as a catalyst. Isatin Schiff base-g-Fe2O3 as a catalyst. Without nitrobenzene.
e
f
Benzaldehyde was obtained as the only product in 60% yield. Without benzyl alcohol. g Under O2. h Under Ar.
(Cu 2p3/2) along with the binding energies at 952.9 and 963.1 eV reaction was performed in the absence of the catalyst and also
(Cu 2p1/2) could be ascribed to Cu with (I) oxidation state. The in the presence of CuCl2 and isatin Schiff base-g-Fe2O3 as
peaks centered at 934.9, 936.9 (Cu 2p3/2) and 954.2, 961.3 eV (Cu a copper less analogue of the catalyst (Table 1, entries 18–20). It
2p1/2) are attributed to the existence of Cu(II) in the catalyst.21
was found that the desired product was obtained in low yield
Aer catalyst characterization, its catalytic activity was aer 24 h. These ndings indicated that a desirable activation
studied in the tandem imines formation by auto-hydrogen of copper was occurred by complexation in the catalyst. The
transfer from alcohols to nitro compounds. At rst, to nd reaction of benzyl alcohol was studied in the presence of the
out the best reaction conditions, a set of factors including base, catalyst under optimized reaction conditions (Table 1, entry 21).
solvent, temperature and the amount of the catalyst were The results showed that benzyl alcohol was oxidized and
screened in the reaction of benzyl alcohol and nitrobenzene as produced benzaldehyde in 60% yield aer 10 h. The reaction of
the model reaction. The study of the inuence of the base, as nitrobenzene in the presence of the catalyst was also studied
well as its absence, showed that KOH rendered the best results and any product was not obtained (Table 1, entry 22). This
(Table 1, entry 4). The solvent dependency of the model reaction should be attributed to the lack of benzyl alcohol as a driving
was investigated (Table 1, entries 7–13) and found that the force for subsequent reduction of nitro groups. Performing the
reaction proceeded efficiently under solvent-free conditions as model reaction under O2 and Ar atmosphere proceeded with no
green and sustainable reaction conditions. In an effort to opti- change in the product yields (Table 1, entries 22 and 23)
mize the reaction temperature, temperatures below 100 ꢁC were compared with the optimal conditions (Table 1, entry 16), which
found to minimize the imines formation (Table 1, entries 14 showed that alcohol dehydrogenized without requiring specic
and 15). Screening the amount of the catalyst showed that atmospheric conditions.
decreasing the catalyst amount from 2 to 1 mol% led to
With an optimized catalytic system in hand (Table 1), we set
a negligible decrease in the yield of the product (Table 1, entry out to probe versatility of the present method in the direct imine
16). Poorer activity was obtained by further lowering the catalyst synthesis of various alcohols and nitro compounds by auto-
amount (Table 1, entry 17). To show the role of the catalyst, the hydrogen transfer strategy. As depicted in Table 2, the
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Table 2 Tandem imines formation via auto-hydrogen transfer from
alcohols to nitro compounds catalyzed by Cu-isatin Schiff base-g
Fe2O3
Entrya R1
R2
Product Time (h) Yield (%)
1
2
3
4
5
6
7
8
C6H5
C6H5
C6H5
C6H5
C6H5
a
b
c
d
e
f
g
h
i
j
k
l
m
n
o
10
8
8
92
90
84
87
71
91
68
85
83
75
72
84
70
72
85
4-OH–C6H4
4-OMe–C6H4
4-Me–C6H4
4-Cl–C6H4
C6H5
4-Cl–C6H4
4-OMe–C6H4
4-Me–C6H4
C6H5
4-Cl–C6H4
4-OMe–C6H4
4-Me–C6H4
CH3–CH2–
Fig. 8 FT-IR spectrum of the Cu-isatin Schiff base-g-Fe2O3 after six
times reuse.
9
C6H5
12
13
12
10
12
12
13
11
14
10
12
4-Me–C6H4
4-Me–C6H4
4-Me–C6H4
4-Me–C6H4
4-OMe–C6H4
4-OMe–C6H4
4-OMe–C6H4
4-OMe–C6H4
C6H5
The recyclability of Cu-isatin Schiff base-g-Fe2O3 was inves-
tigated in a model reaction of benzyl alcohol and nitrobenzene
in the auto-hydrogen transfer reaction, under optimized reac-
tion conditions. EtOAc was added to the reaction mixture aer
10 h (Fig. 7a). The catalyst was isolated by an external magnet
(Fig. 7b), washed with EtOAc and EtOH (2 ꢂ 10 mL) and dried
under vacuum. The catalyst was successfully recycled for six
times. Loss of the catalytic activity was not considerably
observed for Cu-isatin Schiff base-g-Fe2O3 in these reactions
(Fig. 7c).
9
10
11
12
13
14
15
CH3–CH3–CH2– C6H5
a
Reaction conditions: nitro compound (1 mmol), alcohol (1 mmol),
KOH (1 mmol), solvent-free, catalyst (1 mol%), 100 ꢁC. Melting points
of the solid products were compared with the reported ones in the ESI
(Table S1).
Comparison of FT-IR of the reused catalyst (Fig. 8) with the
freshly prepared one (Fig. 1c) indicated that any signicant
changes in the chemical structure of the catalyst was not
observed.
Moreover, TEM and FE-SEM images illustrated that the
nanoparticles were still spherical in shape even aer six times
reuse (Fig. 9a and b) and the mean diameter size of the recycled
catalyst was 14 nm (Fig. 9c).
reaction of various nitrobenzenes and benzyl alcohols catalyzed
by Cu-isatin Schiff base-g-Fe2O3 gave expected imines (a–m) in
good to high yields (Table 2, entries 1–13). The reaction of
benzyl alcohol with nitroethane as an aliphatic compound and
also the reaction of nitrobenzene with 1-butanol as an aliphatic
alcohol proceeded well under optimized reaction conditions
(Table 2, entries 14 and 15). The imine products in the current
study have widespread usages in the synthesis of biologically
active compounds with antibacterial and antifungal activities.22
To study the synthetic applications and practical activity of
this protocol, the reaction of benzyl alcohol (50 mmol) and
nitrobenzene (50 mmol) was assessed under the optimized
Fig. 7 (a) Reaction mixture after adding EtOAc, (b) isolation of the
catalyst by an external magnet (c) reusability of Cu-isatin Schiff base-
g-Fe2O3 in the reaction of nitrobenzene and benzyl alcohol at 100 ꢁC Fig. 9 (a) TEM image, (b) FE-SEM image and (c) size distribution of the
in 10 h.
Cu-isatin Schiff base-g-Fe2O3 after six times reuse.
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from the alcohol to the nitro compounds and also the imine
formation from the in situ produced aldehyde and amine. To
investigate the mechanism in detail, the reaction of N-phenyl-
hydroxylamine and benzyl alcohol under optimized reaction
conditions was employed. The imine was obtained in 80% yield
aer 8 h. The reaction of benzaldehyde with aniline was also
studied and the desired imine was produced in 93% yield aer
6 h.
The applicability of this method was also examined for the
one-pot reaction of 2-nitroaniline with benzyl alcohol under the
optimized reaction conditions at the last part of our studies.
Benzimidazole (p) was produced in 82% yields by auto-
hydrogen transfer reaction in the presence of Cu-isatin Schiff
base-g-Fe2O3 (Scheme 3).
Scheme 2 A plausible mechanism for the tandem reaction catalyzed
by Cu-isatin Schiff base-g-Fe2O3.
To show the merits of the present protocol for the synthesis
of imines, we have compared the catalytic activity of Cu-isatin
Schiff base-g-Fe2O3 with those of some reported catalysts in
the auto-hydrogen transfer reactions (Table 3). As depicted in
Table 3, Cu-isatin Schiff base-g-Fe2O3 is the most effective
catalyst for the reaction of benzyl alcohols and nitrobenzenes.
The reported synthetic routes have certain limitations such as
requiring high temperature, long reaction times, expensive
catalysts, additives, large amount of the catalyst, bimetallic
catalysts, especial atmospheric conditions, toxic organic
solvents and most importantly use of excess amounts of benzyl
alcohol as the reaction solvent. More importantly, our catalyst is
a magnetic heterogeneous catalyst and can be easily separated
from the reaction mixture using an external magnet.
Scheme 3 Tandem reaction of 2-nitroaniline with benzyl alcohol
under the optimized reaction conditions.
reaction conditions and the desired product was isolated in
90% yield aer 13 h.
Based on our experiment results and previous literature
reports,10b,12,13,14,26 a reaction mechanism to rationalize the
direct imines formation from the reaction of nitroarenes with
alcohols was depicted in Scheme 2. First, the alcohol is dehy-
drogenated to aldehyde in the presence of Cu-isatin Schiff base-
g-Fe2O3 as a catalyst promoted by base, releasing proton and
hydride (step I). Simultaneously, the hydride is transferred to
the copper complex to give a copper–hydride complex. This
copper–hydride complex reduces the nitro group to amine via N-
phenylhydroxylamine as a well-known intermediate (step II).23
In the next step, the nucleophilic attack of the amine to the
activated aldehyde by the copper catalyst followed by water
elimination gives the imine product (step III). In this mecha-
nism the copper catalyst promoted both the hydrogen transfer
Experimental
General procedure for the tandem synthesis of imine from
alcohols and nitro compounds
Cu-isatin Schiff base-g-Fe2O3 (1 mol%) was added to a mixture
of alcohol (1 mmol), nitro compound (1 mmol) and KOH (1
ꢁ
mmol) and the resulting mixture was stirred at 100 C for an
appropriate time (Table 2). Aer cooling the reaction mixture to
Table 3 Comparison of catalytic activity of Cu-isatin Schiff base-g-Fe2O3 with some reported catalysts for the auto-hydrogen transfer reactions
of alcohols with nitro compounds
Entry (ref.)
Catalyst (mol%)
Reaction conditions
Toluene, 130 ꢁ
Time (h)
Yield (%)
1 (ref. 10a)
2 (ref. 11)
3 (ref. 24)
4 (ref. 25)
5 (ref. 26)
6 (ref. 14)
7 (ref. 12)
8 (ref. 15)
9 (ref. 10b)
10 (ref. 13)
11 (ref. 9)
12 (This work)
Pd/HTa (2)
C
24
12
22
18–42
15–22
15
3–20
3 d
24
35–93
70–99
57
12–100
4–99
57–87
16–92
58–84
51–95
60–98
72–99
65–92
Ag-MCP-1b (25 mg)
Toluene, K2CO3, glycerol, 120 ꢁ
Benzyl alcohol (solvent), Cs2CO3, 100 ꢁ
Solvent free, N2 atmosphere, 160 ꢁ
Benzyl alcohol (solvent), Cs2CO3, 100 ꢁC, aerobic condition
Toluene, tBuOK, 120 ꢁ
Benzyl alcohol (solvent), Cs2CO3, 110 ꢁ
Toluene, NaOH, 130 ꢁ
C
Rh/Au (0.5)
C
Co–N–C/CNT@ACc (0.4 g)
IrIII–AuI heterodimetallic complex (1)
CoOX@NC-800d (10)
Ir–Pd heterodimetallic catalyst (2)
CuO–Fe3O4 (1.3)
C
C
C
C
Pd/DNA (2.9)
Water, LiOH$H2O, 50 ꢁC, N2 balloon
Au/Ag–Mo-NRe (40 mg)
RuCl3 (3)
Toluene, K2CO3, glycerol, Ar, 120 ꢁ
K2CO3, glycerol, N2 atmosphere
C
24
24
8–14
Cu-isatin Schiff base-g-Fe2O3 (1)
Solvent-free, KOH, 100 ꢁ
C
a
Hydrotalcite. b Polyacrylic acid. c Co, N and C composite encapsulated carbon nanotube grown in situ on the surface of activated carbon. d Cobalt
nanoparticles modied with N-doped hierarchical porous carbon derived from biomass. e Au/Ag–Mo nano-rods.
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room temperature, EtOAc (5 mL) was added to the reaction
mixture. The catalyst was separated by an external magnet,
washed with EtOAc (2 ꢂ 10 mL) and EtOH (2 ꢂ 10 mL), dried in
vacuum and reused. The solvent of the combined organic layer
was evaporated under vacuum. Pure products were obtained by
recrystallization in EtOH or by column chromatography eluted
with n-hexane : EtOAc (5 : 1 or 2 : 1). 1H NMR spectra of the
products (a, c, d, e, g, k and p) have been presented in the ESI.†
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Conclusion
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In summary, in this paper, an efficient method for the synthesis
of imines by tandem reaction of various alcohols (primary
aromatic and aliphatic) and nitro compounds (aromatic and
aliphatic) was developed via auto-hydrogen transfer reaction,
catalyzed by Cu-isatin Schiff base-g-Fe2O3 as a nanomagneti-
cally reusable and inexpensive catalyst under solvent-free
conditions. Good to high yields of imines were achieved
under mild reaction conditions without requiring any additive.
Aer EtOAc was added to the reaction mixture, the catalyst was
easily isolated by using an external magnet and reused
successfully for six runs without any signicant changes on the
chemical structure of the catalyst. This catalytic system had the
merits of cost effectiveness, environmental benignity, excellent
recyclability and good reproducibility for the direct synthesis of
imines from alcohols and nitro compounds without requiring
high temperature, long reaction times, expensive catalysts,
additives, large amounts of the catalyst, bimetallic catalysts,
especial atmospheric conditions, hydrogen gas, toxic organic
solvents and most importantly use of excess amounts of benzyl
alcohol as the reaction solvent.
16 (a) S. Sobhani, A. Habibollahi and Z. Zeraatkar, Org. Process
Res. Dev., 2019, 23, 1321; (b) H. H. Moghadam, S. Sobhani
and J. M. Sansano, ACS Omega, 2020, 5, 18619; (c)
A. Khazaee, R. Jahanshahi, S. Sobhani, J. Skibsted and
J. M. Sansano, Green Chem., 2020, 22, 4604; (d) The
method for the preparation of the catalyst is explained in
the ESI.†
Conflicts of interest
There are no conicts to declare.
17 S. Sobhani, S. Asadi, M. Salimi and F. Zari, J. Organomet.
Chem., 2016, 822, 154.
18 M. Niakan, Z. Asadi and M. M. Farahani, Appl. Surf. Sci.,
2019, 481, 394.
19 B. Huang, J. Chen, S. Zhan and J. Ye, J. Electrochem. Soc.,
2016, 163, G26.
Acknowledgements
Financial support of this project by University of Birjand
Research Council is acknowledged. Access to the XPS facilities
of University of Alicante is appreciated.
20 X. Zhang, X. Hu, P. Guan, N. Zhang, J. Li and C. Du, J. Mater.
Sci., 2017, 52, 4713.
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© 2021 The Author(s). Published by the Royal Society of Chemistry
RSC Adv., 2021, 11, 19121–19127 | 19127