Preparation and characterization of magnetic graphene nanocomposite containing Cu(proline)2 as catalyst for asymmetric aldol reactions

Article abstract of DOI:10.1007/s11164-019-03755-x

A new catalyst has been prepared via immobilization of Cu(proline)₂ complex onto the surface of magnetic graphene. The fabricated nanocatalyst was characterized by Fourier-transform infrared (FT-IR) spectroscopy, powder X-ray diffraction (PXRD) analysis, vibrating-sample magnetometry (VSM), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) spectroscopy, transmission electron microscopy (TEM), inductively coupled plasma (ICP) techniques, and elemental analysis. Its catalytic performance was investigated in the aldol reaction using a mild and ecofriendly procedure. The synthesized nanocomposite, which contains Cu(II) center as Lewis acid, was found to be an efficient catalyst for asymmetric aldol reactions, affording corresponding aldol products in high yield and excellent enantiomeric excess (> 90?%). The examined catalyst was prepared from low-cost, easily available starting materials and can be readily isolated by magnetic decantation for recycling and reuse in consecutive reactions without significant loss of activity.

Full text of DOI:10.1007/s11164-019-03755-x

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-019-03755-x
Preparation and characterization of magnetic graphene
nanocomposite containing Cu(proline)2 as catalyst
for asymmetric aldol reactions
M. Kooti1 · F. Kooshki1 · E. Nasiri1
Received: 12 September 2018 / Accepted: 28 January 2019
© Springer Nature B.V. 2019
Abstract
A new catalyst has been prepared via immobilization of Cu(proline)2 complex onto
the surface of magnetic graphene. The fabricated nanocatalyst was characterized by
Fourier-transform infrared (FT-IR) spectroscopy, powder X-ray difraction (PXRD)
analysis, vibrating-sample magnetometry (VSM), scanning electron microscopy
(SEM), energy-dispersive X-ray (EDX) spectroscopy, transmission electron micros-
copy (TEM), inductively coupled plasma (ICP) techniques, and elemental analysis.
Its catalytic performance was investigated in the aldol reaction using a mild and eco-
friendly procedure. The synthesized nanocomposite, which contains Cu(II) center
as Lewis acid, was found to be an efcient catalyst for asymmetric aldol reactions,
afording corresponding aldol products in high yield and excellent enantiomeric
excess (>90 %). The examined catalyst was prepared from low-cost, easily available
starting materials and can be readily isolated by magnetic decantation for recycling
and reuse in consecutive reactions without signifcant loss of activity.
Keywords Graphene · Proline · Copper complex · Aldol reactions · Manganese
ferrite · Asymmetric reactions
Introduction
The asymmetric aldol reaction is known as one of the most favored synthetic routes
for carbon–carbon bond formation in organic chemistry. Therefore, a great deal of
efort has been made to develop innovative approaches to improve the performance
of this reaction. Recently, organocatalysis has become a powerful and attractive
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s1116
4-019-03755-x) contains supplementary material, which is available to authorized users.
*
M. Kooti
m_kooti@scu.ac.ir
1
Chemistry Department, Shahid Chamran University of Ahvaz, Ahvaz, Iran
Vol.:(0123456789)
1 3

M. Kooti et al.
methodology for production of useful aldol compounds. Proline is an important
small chiral amino acid and one of the most well-known organocatalysts. It is also
very easily available in pure form with fairly low price. Due to its unique properties,
such as conformational rigidity, proline and its various derivatives have been used as
catalysts for asymmetric aldol reactions [1–9].
Despite the advantages of proline and its derivatives as homogeneous catalysts,
they sufer from some unavoidable problems, mainly difculties in separation from
reaction media and catalyst recovery. Since catalyst recycling and product purity are
important in any chemical industry, heterogeneous catalysts are greatly preferred
over their homogeneous counterparts. In recent years, use of transition-metal com-
plexes supported on inorganic matrices to design heterogeneous organocatalysts has
received remarkable attention. In this regard, proline has been immobilized on vari-
ous supports, including mesoporous materials, metals, and layered compounds, to
improve its efciency for catalyzing asymmetric aldolization [10–12]. Moreover,
application of supported chiral metal complexes for production of specialty and
niche chemicals is of great interest [13–15].
Magnetic nanoparticles (MNPs), especially those of mixed metal oxides such
as the wide class of ferrites, have been extensively explored in recent years. These
MNPs, either in pristine form or modifed with some functional groups, are applied
in many areas, including biomedicine, biotechnology, environmental remediation,
and biosensing as well as catalysis. Magnetic nanoparticles have also been utilized
as new catalyst supports, because the magnetic core allows magnetic manipulation
of the catalyst by simple application of an external magnetic feld [16–23].
Graphene (G), on the other hand, has emerged as a fascinating support for various
catalysts due to its high specifc surface area and good chemical and thermal sta-
bility. Some graphene-based metallic nanocatalysts, such as CoFe2O4, Mn3O4, Au,
Pd, FePt, and PtAu, have shown unique catalytic performance in a wide range of
heterogeneous organic reactions [24–36]. We report herein the synthesis of a novel
heterogeneous nanocatalyst composed of a Cu(proline)2 complex immobilized on
a silica-coated graphene/MnFe2O4(G/MF) composite. The new nanocomposite [G/
MF@SiO2@Cu(proline)2] was found to be an efcient catalyst for asymmetric aldol
reaction without use of organic solvents. Furthermore, the catalyst can be easily sep-
arated from reaction media with the aid of a permanent magnet and fnally recycled.
This green nanocatalyst, which is readily prepared from low-cost materials, can be
considered to be a promising catalyst for asymmetric aldol reactions.
Experimental
Chemicals
All reagents were of analytical grade and used as received. Products were charac-
1
terized by comparison of their physical data, such as FT-IR and H nuclear mag-
netic resonance (NMR) spectra, with those of known samples. NMR spectra were
recorded on a Bruker Advance DPX 400 MHz spectrometer using tetramethylsi-
lane (TMS) as internal standard. FT-IR spectra were obtained using a FT BOMEM
1 3

Preparation and characterization of magnetic graphene…
MB102 spectrophotometer. Powder X-ray difraction (PXRD) patterns were taken
with a Philips X-ray difractometer (model PW1840) over the 2θ range from 10° to
80° using Cu Kα radiation (λ=1.54056 Å). Field-emission (FE)SEM images were
obtained using a Hitachi Japan S4160 scanning electron microscope. The magnetic
properties of the samples were studied by vibrating sample magnetometry (VSM,
Meghnatis Daghigh Kavir Company). The copper content of the G/MF@SiO2@
Cu(proline)2 nanocatalyst was determined using an ICP-atomic emission spectros-
copy (AES) instrument (HORIBA JobinYvon, Longjumeau Cedex, France). Gas
chromatography (GC) experiments were performed with a Shimadzu GC-16A
instrument using a 2-m column packed with silicon DC-200 or Carbowax 20M.
Synthesis of G/MF@SiO2@Cu(proline)2
The copper(II) complex Cu(proline)2 was prepared by adding 2.5 mmol copper(II)
chloride dihydrate to hot aqueous solvent (20 mL) containing 5 mmol proline and
5 mmol sodium hydrogen carbonate [37]. The mixture was then stirred in a water
bath and, after condensation, poured into an ethyl alcohol/acetone mixture (1:1
ratio). The resulting pale-blue precipitate of Cu(proline)2 was fltered, washed with
ethyl alcohol and acetone several times, and dried at room temperature for 5 h. The
amount of main elements in Cu(proline)2 complex was found as follows: Anal.
Calcd. for C10H14CuN2O6: C, 37.31; N, 8.71; H, 4.39 %. Found: C, 37.06; N, 8.65;
H, 4.94 % by elemental analysis. The copper content of the complex as measured
by ICP-AES analysis was 19.46 %. Therefore, according to elemental analysis, the
Cu:ligand molar ratio in the copper complex was 1:2.
In a separate step, G/MF was synthesized by dispersing 0.3 g presynthesized gra-
phene into 20 mL deionized water with sonication for 30 min. Then, Fe(NO3)3·9H2O
(0.4 M, 25 mL) and Mn(NO3)2·6H2O (0.2 M, 25 mL) were added into the graphene
suspension with stirring at room temperature for 5 min, and the pH was adjusted
to 11–12 using 6 M NaOH solution. After addition of 1.0 g polyvinylpyrrolidone
(PVP) as surfactant to this mixture, it was heated at 80 °C for 2 h. The obtained
nanocomposite was then collected by applying a permanent magnet, washed with
hot water–ethanol mixture, and fnely powdered after drying in an oven at 100 °C for
4 h. The G/MF nanocomposite was coated with silica using the Stöber method [38].
These was achieved by adding tetraethylorthosilicate (TEOS) to G/MF in presence
of NH3 to obtain a three-component G/MF@Si nanocomposite. In the subsequent
step, 0.70 g G/MFe@Si nanocomposite was stirred in 35 mL ethanol at room tem-
perature for 2 h. To this suspension, 0.35 g Cu(proline)2 dispersed in 5 mL water
was added, and the whole mixture was stirred at room temperature for 24 h. The
resulting solid [G/MF@SiO2@Cu(proline)2] was separated by magnetic decanta-
tion, washed with ethanol, and dried in an oven at 80 °C for 5 h. The amount of
copper in this nanocomposite was found to be 0.633 mmol g−1 as determined by
ICP-AES analysis.
1 3

M. Kooti et al.
Aldol reactions catalyzed by G/MF@SiO2@Cu(proline)2
A mixture of benzaldehyde and cyclohexanone at molar ratio of 1:2 mmol and nano-
catalyst (0.1 g) was stirred at 40–60 °C; the progress of the condensation reaction
was monitored continuously by thin-layer chromatography (TLC). Ethyl acetate
(10 mL) was then added to the reaction mixture, and the nanocatalyst was magneti-
cally separated and washed with ethyl acetate and dried for reuse in a new aldol
reaction. The remaining liquid was washed with saturated aqueous sodium chloride
solution (5 mL) and water (10 mL). The organic layer, which contains the prod-
uct of the aldol reaction, was separated and dried over anhydrous Na2SO4, followed
by evaporation of the solvent at room temperature. The remaining residue was then
recrystallized in ethanol to give pure aldol product with 98 % yield. The yields in
this and other similar reactions were measured by GC analysis. Identifcation of the
products for a number of selected reactions between cyclohexanone and benzalde-
hyde derivatives was also performed by 1H NMR and GC–mass spectrometry (MS)
techniques. The spectral data of selected products are given below.
Selected spectroscopic data
2-(Hydroxyl(phenyl)methyl)cyclohexan-1-one (Table 1, entry 1): 1H NMR
(250 MHz, DMSO) δ: 1.68–1.73 (4H, m), 1.84–1.90 (1H, m), 2.01–2.08 (1H, m),
2.22–2.77 (2H, m), 2.42–2.49 (1H, m), 2.78–2.89 (1H, d), 4.18 (1H, t), 7.28–7.63
(4H, m), 7.93–7.96 (1H, m). Anti/syn: 69/31, ee value for the anti-isomer: 96 %.
2-(Hydroxyl(4-nitrophenyl)methyl)cyclohexan-1-one (Table 1, entry 2): 1H NMR
(250 MHz, DMSO) δ: 1.53–1. 73 (4H, m), 1.83–1.87 (1H, m), 1.99–2.1 (1H, m),
2.21–2.32 (2H, m), 2.48–2.67 (1H, m), 3.32 (1H, d), 5.21 (1H, s), 7.52–7.61 (2H,
m), 8.13–8.21 (2H, d). Anti/syn: 95/5, ee value for the anti-isomer: 94 %.
2-(Hydroxy(p-tolyl)methyl)cyclohexan-1-one (Table 1, entry 3): 1H NMR
(250 MHz, DMSO) δ: 1.49–1.79 (5H, m), 1.98–2.05 (1H, m), 2.23 (2H, m), 2.35
(3H, s), 2.48–2.53 (2H, m), 5.20 (1H, s), 7.24–7.27 (2H, d), 7.81–7.84 (2H, d).
2-(Hydroxy(3-chlorophenyl)methyl)cyclohexan-1-one (Table 1, entry 4): 1H
NMR (250 MHz, DMSO) δ: 1.71 (4H, m), 1.80–1.89 (1H, m), 2.01–2.08 (1H, m),
2.32–2.50 (3H, m), 3.35 (1H, d), 4.10 (1H, t), 7.28–7.59 (4H, m). Anti/syn: 98/2, ee
value for the anti-isomer: 94 %.
2-(Hydroxy(3-fuorophenyl)methyl)cyclohexan-1-one (Table 1, entry 6): 1H
NMR (250 MHz, DMSO) δ: 1.53–1.69 (4H, m), 1.82–1.90 (1H, m), 2.01–2.11 (2H,
m), 2.21–2.24 (1H, m), 2.44–2.79 (1H, m), 3.38 (1H, d), 4.20 (t, 1H), 7.26–7.33
(3H, m), 7.42–7.57 (1H, m). Anti/syn: 96/4, ee value for the anti-isomer: 93 %.
2-(Hydroxy(p-dimethylaminophenyl)methyl)cyclohexan-1-one (Table 1, entry 7):
1H NMR (250 MHz, DMSO) δ: 1.74 (4H, m), 1. 90 (1H, m), 2.21–2.26 (3H, m),
2.48–2.49 (1H, m), 2.44–2.79 (1H, m), 3.03 (6H, s), 3.32–3.34 (1H, m), 6.75–6.79
(2H, d), 7.65–7.69 (2H, d), 9.65 (OH).
2-((2-Chlorophenyl)(hydroxy)methyl)cyclohexanone (Table 1, entry 8): anti/syn:
95/5, ee value for the anti-isomer: 92 %.
2-(Hydroxy(2-hydroxyphenyl)methyl)cyclohexan-1-one (Table 1, entry 9): 1H
NMR (250 MHz, CDCL3) δ: 1.20 (2H, m), 1.48–1.66 (4H, m), 1.92–2.10 (2H, m),
1 3

Preparation and characterization of magnetic graphene…
Table 1 Aldol reactions using G/MF@SiO2@Cu(proline)2 as nanocatalyst in solvent-free condition
Entry
Aldehyde
ketone
Producta
Time (h)
Yield (%)
%ee (anti:syn)
O
O
OH
CHO
1
5
98
96
69/31
O
OH
CHO
O
O
O
94
95/5
2
3
5
87
NO2
NO2
O
OH
CHO
5
77
-
CH3
CH3
CHO
O
OH
OH
OH
94
Cl
4
5
5
5
5
78
87
87
82
98/2
Cl
O
OCH3
CHO
O
O
O
OCH3
5
6
-
CHO
O
93
96/4
F
F
CHO
O
OH
7
8
-
CH3
N
CH3
N
H3
C
CH3
CHO
O
O
O
O
O
OH
OH
Cl
92
95/5
Cl
5
5
81
98
CHO
OH
9
OH
-
CHO
OH
O
10
11
CH3
H3
C
CH3
24
24
75
70
-
-
CHO
OH
O
O
CH3
H3C
CH3
O2N
NO2
CHO
OH
O
O
O
O
12
13
14
Cl
CH3
H3
C
CH3
24
24
5
78
76
95
-
-
-
Cl
CHO
OH
OH
CH3
H3C
CH3
OH
CHO
O
OH
O
CH3
1 3

M. Kooti et al.
Table 1 (continued)
Entry
Aldehyde
ketone
O
Producta
Time (h)
Yield (%)
%ee (anti:syn)
CHO
OH
O
15
CH3
5
90
-
O2N
NO2
CHO
OH
OH
O
O
16
OH
CH3
5
96
-
CHO
O
O
O
17
18
H3C
CH3
5
85
-
H3C
H3C
O
CHO
O
O
O
H3C
OC2H5
5
97
-
H3C
C2H5
O
O
19b
20
O
O
O
OH
O
O
72
72
72
45
43
75
95
-
H3
C
H3
C
H
OH
O
O
H
H
H
21c
O
OH
O
-
H
O
O
O
O
O
O
OH
22
23
24
72
72
72
61
30
49
-
-
-
H
H
O
OH
H
O
O
O
OH
O
CHO
O
OH
25
5
85
-
aProducts were characterized by comparison of NMR, IR, and mass spectroscopy results with reported
data
bTemperature 20 °C
cTemperature 60 °C
1 3

Preparation and characterization of magnetic graphene…
2.22–2.25 (1H, m), 2.49 (1H, m), 4.18–4.37 (1H, m), 5.01–5.18 (1H, d), 6.66–6.92
(2H, m), 7.03–7.26 (1H, m), 7.43–7.45 (1H, m).
4-Hydroxy-4-phenylbutan-2-one (Table 1, entry 10), 1H NMR (250 MHz, CDCl3)
δ: 2.15–2.19 (1H, br s), 2.38 (3H, s), 2.75–2.97 (2H, d), 5.20 (1H, m), 7.14–7.62
(5H, m).
1
3-Hydroxy-1,3-diphenylpropan-1-one (Table 1, entry 14), H NMR (250 MHz,
CDCl3) δ: 2.69 (1H, br s), 3.46 (2H, d), 5.45 (1H, m), 7.30–7.76 (7H, m), 7.89–8.22
(3H, m).
3-Hydroxy-3-(4-nitrophenyl)-1-phenylpropan-1-one (Table 1, entry 15), 1H NMR
(250 MHz, CDCl3) δ: 1.34 (1H, CH), 2.69–2.71 (2H, CH2), 7.36–8.39 (9H, m).
1
3-Hydroxy-3-(2-hydroxyphenyl)-1-phenylpropan-1-one (Table 1, entry 16), H
NMR (250 MHz, CDCl3) δ: δ: 2.09–2.10 (1H, CH), 2.61 (2H, t), 7.00–7.79 (9H, m).
3-Benzylidenepentane-2,4-dione (Table 1, entry 17): 1H NMR (250 MHz,
CDCl3) δ: 2.24–2.38 (7H, 2CH3, CH), 7.35 (5H, m).
1
Ethyl 2-benzylidene-3-oxobutanoate (Table 1, entry 18): H NMR (250 MHz,
DMSO) δ: 1.30–1.37 (2H, m), 2.33–2.49 (6H, m), 4.30–4.40 (1H, m), 7.31–7.71
(5H, m).
2-(1-Hydroxyethyl)cyclohexan-1-one (Table 1, entry 19): ee value: 95 %.
Results and discussion
The synthesized nanocatalyst is designated as G/MF@SiO2@Cu(proline)2; its step-
wise preparation is shown in Scheme 1. According to the results of various analyses,
a structure for the synthesized nanocatalyst is proposed in Scheme 1.
To characterize the as-synthesized nanocatalyst and confrm immobilization of
the copper complex on its surface, FT-IR spectroscopy and other techniques were
utilized. Figure 1 shows the FT-IR spectra of G/MF (a), Cu(proline)2 (b), and G/
MF@SiO2@Cu(proline)2 (c). The FT-IR spectrum of G/MF (Fig. 1a) shows two
peaks at 463 cm−1 and 582 cm−1, which are assigned to Fe–O stretching at the tet-
rahedral and octahedral sites of MnFe2O4, respectively. Figure 1b shows the FT-IR
spectrum of Cu(proline)2 complex. The peak at 1595 cm−1 is related to C=O
groups, while the peaks appearing at 1042 and 1116 cm−1 are related to stretching
of C–O and C–N bonds. In the spectrum for G/MF@SiO2@Cu(proline)2 (Fig. 1c),
two peaks at 1092 and 796 cm−1 are observed, related to stretching and bending
vibration modes of Si–O–Si bonds. The two peaks of MnFe2O4 (471, 587 cm−1) are
clearly seen in Fig. 1c for G/MF@SiO2@Cu(proline)2, indicating the presence of
this magnetic spinel ferrite in the fnal composite.
The PXRD patterns of MnFe2O4, Cu(proline)2, and G/MF@SiO2@Cu(proline)2
are presented in Fig. 2. The main peaks at 2θ=30.1°, 36.0°, 43.5°, 57.5°, 57.3°, and
63.1°, corresponding to refections from (2 2 0), (3 1 1), (4 0 0), (4 2 2), and (3 3 3)
planes, respectively, all correspond to MnFe2O4 phase. The same peaks are observed
in the PXRD pattern of the G/MF@SiO2@Cu(proline)2 nanocomposite, but with a
slight shift to lower 2θ values due to the coating of MnFe2O4 with Cu-proline com-
plex. This observation reveals that MnFe2O4 retained its structure in the synthesized
composite.
1 3

M. Kooti et al.
Scheme 1 Stepwise preparation of G/MF@SiO2@Cu(proline)2
A TEM image of the prepared nanocatalyst is depicted in Fig. 3a, clearly show-
ing spherical morphology with slight aggregation. This image indicates that MF@
SiO2@Cu(proline)2 (seen as dark spots) is grafted on graphene sheets with particle
size distribution of about 25–30 nm. SEM images of all the synthesized samples are
also presented in Fig. 3. In these images, graphene shows the expected sheet-like
structure (Fig. 3b); after coating with MnFe2O4 only a slight change is observed,
and the nanoparticles of this ferrite are well integrated on the surface of graphene
(Fig. 3c). The SEM image of G/MF@SiO2@Cu(proline)2 (Fig. 3d) reveals a nearly
spherical morphology for this nanocomposite.
1 3

Preparation and characterization of magnetic graphene…
Fig. 1 FT-IR spectra of G/MF (a), Cu(proline)2(b), and G/MF@SiO2@Cu(proline)2 (c)
The composition of the as-fabricated G/MF@SiO2@Cu(proline)2 nanocom-
posite was further confrmed by EDX analysis. The EDX spectrum of this nano-
composite is shown in Fig. 4, revealing the presence of all main elements, i.e.,
Mn, Fe, Si, C, N, O, and Cu, composing the nanocomposite.
The magnetic properties of G/MF and G/MF@SiO2@Cu(proline)2 were deter-
mined by vibrating sample magnetometry (VSM); the magnetic hysteresis loops
are shown in Fig. 5. The VSM curve for the nanocatalyst shows a saturation mag-
netization (Ms) of only 2 emu g−1, indicating typical superparamagnetic behavior
for this composite. The Ms value for this composite is much lower than that of
the two-component G/MF composite (6 emu g−1) or pure MnFe2O4. The lower
Ms value of this composite may be due to the presence of nonmagnetic silica and
Cu(proline)2 complex shells. The magnetization of G/MF@SiO2@Cu(proline)2,
however, is still high enough to enable its magnetic separation.
The thermogravimetric analysis (TGA) curve of G/MF@SiO2@Cu(proline)2
catalyst is shown in Fig. 6. The frst weight loss (80–180 °C, 1.58 %) corre-
sponds to adsorbed water in the catalyst structure. The seconded weight loss
1 3

M. Kooti et al.
Fig. 2 PXRD patterns of MnFe2O4 (a), Cu(proline)2 (b), and G/MF@SiO2@Cu(proline)2(c)
at ~ 190–750 °C is related to release of grafted l-proline of the copper complex
and decomposition of the carbon chain of graphene.
We present a green method for one-pot aldol reaction between various aldehydes
and some ketones in presence of G/MF@SiO2@Cu(proline)2 as catalyst. First, alde-
hydes and ketones were stirred in absence of the catalyst under solventless condi-
tions for 4–5 h. No change in the starting materials was observed according to TLC
monitoring of the reaction. However, with addition of nanocatalyst to the reaction
mixture, under similar conditions, the reaction proceeded in a period of time to give
aldol product in high yield. This result proves that the aldol reaction can only take
place in presence of the catalyst. To further explore the scope and limitations of
this catalytic system, diferent aldehydes were tested in the aldol reaction with some
ketones under optimized conditions. The results obtained in these reactions are pre-
sented in Table 1.
As mentioned above, proline and it derivatives can efciently catalyze asym-
metric aldol reactions. To prove that G/MF@SiO2@Cu(proline)2 acts as a chiral
catalyst, enantiomeric excess (ee) values of some of the products were determined
by chiral high-performance liquid chromatography (HPLC) analysis. According to
the results, G/MF@SiO2@Cu(proline)2 showed excellent performance in the direct
asymmetric aldol reaction. The ee values of anti-isomer, as major enantiomer, for
the reaction of cyclohexanone with aromatic aldehydes in free solvent within 5 h
was over 90 % (see Figs. S14 to S19).
For further confrmation of the catalytic activity of the designed nanocatalyst,
separate experiments for aldol reaction were performed in presence of other sam-
ples instead of G/MF@SiO2@Cu(proline)2. None of the examined materials dis-
played any catalytic activity for aldol reaction, except proline, CuCl2·2H2O, and
1 3

Preparation and characterization of magnetic graphene…
Fig. 3 TEM image of G/MF@SiO2@Cu(proline)2 (a) and SEM images of graphene (b), G/MF (c), and
G/MF@SiO2@Cu(proline)2 (d)
Fig. 4 EDX spectrum of G/MF@SiO2@Cu(proline)2
1 3

M. Kooti et al.
Fig. 5 Hysteresis loops of MnFe2O4, G/MF, and G/MF@SiO2@Cu(proline)2
Fig. 6 TGA curve of G/MF@SiO2@Cu(proline)2
1 3

Preparation and characterization of magnetic graphene…
Table 2 Comparative study
Entry
Nanocatalyst
Yield (%)
of various materials in aldol
reaction
1
Graphene
MnFe2O4
G/MF
G/MF@SiO2
Proline
CuCl2·2H2O
Cu(proline)2
G/MF@SiO2@Cu(proline)2
Trace
Trace
Trace
Trace
59
60
65
98
2
3
4
5
6
7
8
Table 3 Comparison of efciency of G/MF@SiO2@Cu(proline)2 with other catalysts in aldol reaction
Entry Catalyst
Solvent
Tempera- Time (h) Yield (%) Ref.
ture (°C)
1
2
3
4
5
6
Trifuoroacetic acid
l-Alanine on graphene
Siloxy-l-serine
2,4-Dinitrobenzenesulfonic acid Brine
Chiral pheboxerhodium
G/MF@SiO2@Cu(proline)2
H2O
Water
Brine
25
40
R.t.
R.t.
60
72
24
12–48
12–24
72
46
85
71
85
42–75
98
[39]
[40]
[41]
[42]
[43]
Present
Toluene
Solvent-free 40
5
Table 4 Reusability of G/MF@
Run
Fresh
1
2
3
4
SiO2@Cu(proline)2
Yield (%) 98
96 95 93 90 ee: 89 %, anti/syn: 73/27
Cu(proline)2, but with lower yield compared with G/MF@SiO2@Cu(proline)2. The
results of these experiments are presented in Table 2. This observation indicates that
the catalytic activity of the copper complex, Cu(proline)2, was enhanced after it was
immobilized on the surface of the G/MFe@SiO2 composite.
To show the advantages of the G/MF@SiO2@Cu(proline)2 catalyst, we compare
the results of the present work with other recently reported catalysts used for aldol
reactions in Table 3, revealing that the catalyst introduced herein is far superior to
the mentioned catalysts in terms of reaction time and conditions.
A facile lifecycle is a major advantage of any heterogeneous catalyst. Therefore,
to check the nanocatalyst recovery, the direct aldol reaction of benzaldehyde with
cyclohexanone was taken as a model reaction. After reaction completion, the cata-
lyst was separated by applying an external magnetic force and washed with ethyl
acetate for three times. After drying the isolated catalyst at 100 °C for 2 h, it was
reused in a new aldol reaction with fresh substrates. The catalyst was reused up to
fve cycles with only negligible loss of activity, as shown in Table 4. Furthermore,
the copper content of the recycled nanocatalyst measured by ICP-AES analysis
1 3

M. Kooti et al.
Scheme 2 Proposed mechanism for aldol reaction catalyzed by G/MF@SiO2@Cu(proline)2
after fve runs was 0.604 mmol g−1, indicating a loss of only 4.58 % compared
with the original catalyst. Therefore, leaching of the catalyst was not great and had
no signifcant efect on its catalytic activity. Also, the ee value for the anti-isomer
2-(hydroxyl(phenyl)methyl)cyclohexan-1-one in presence of recycled nanocatalyst
after fve runs was 89 % (Fig. S20).
According to previous reports in literature [44] and our observations, a mecha-
nism for the aldol reaction is proposed in Scheme 2. First, the carbonyl group
of cyclohexanone is activated by metal center of the copper complex in G/MF@
SiO2@Cu(proline)2 nanocatalyst (I). This interaction makes the α-hydrogen of
the ketone more acidic, which initiates formation of metal–enolate complex II.
Nucleophilic attack of the carbon–carbon double bond of the enolate moiety on
the carbonyl group of benzaldehyde will subsequently occur to produce interme-
diate III. By reorganization of bonding pairs, intermediate IV is formed, protona-
tion of which yields the aldol product.
Conclusions
We describe a simple protocol for synthesis of aldol compounds using G/MF@
SiO2@Cu(proline)2 as efcient and magnetically responsive catalyst. Some of
the important advantages of this method include mild reaction conditions, high
aldol yield, solvent-free conditions, and easy recovery and reusability of the cata-
lyst. The as-prepared catalyst was reused for fve times without loss of catalytic
1 3

Preparation and characterization of magnetic graphene…
activity. This catalytic procedure performed under solventless conditions with
very low leaching of the catalyst can be considered as an environmentally benign
approach for aldol reactions.
Acknowledgements The authors are grateful for support of this work (grant no. 1395) provided by the
Research Council of Shahid Chamran University of Ahvaz, Iran.
References
1. M.T. Crimmins, B.W. King, E.A. Tabet, K. Chaudhary, J. Org. Chem. 66, 894 (2001)
2. L. Li, L.W. Xu, Y.D. Ju, G.Q. Lai, Synth. Commun. 39, 764 (2009)
3. J. Kofoed, T. Darbre, J.L. Reymond, Chem. Commun. 14, 1482 (2006)
4. N. Yoshikawa, Y.M.A. Yamada, J. Das, H. Sasai, M. Shibasaki, J. Am. Chem. Soc. 121, 4168 (1999)
5. V. Sivamurugan, R.S. Kumar, M. Palanichamy, V. Murugesan, J. Heterocyclic Chem. 42, 969 (2005)
6. B.M. Soares, A.M. Aguilar, E.R. Silva, M.D. Coutinho-Neto, I.W. Hamley, M. Reza, J. Ruokolainene,
W.A. Alves, Phys. Chem. Chem. Phys. 19, 1181 (2017)
7. R. Greco, L. Caciolli, A. Zaghi, O. Pandoli, O. Bortolini, A. Cavazzini, C. De Risi, A. Massi, React.
Chem. Eng. 1, 183 (2016)
8. D.G. Crivoi, R.A. Mirandaa, E. Finocchio, J. Llorca, G. Ramis, J.E. Sueiras, A.M. Segarra, F. Medina,
Appl. Catal. A 519, 116 (2016)
9. Y. Wang, H. Shen, L. Zhou, F. Hu, S. Xie, L. Jiang, Catal. Sci. Technol. 6, 6739 (2016)
10. Z. An, Y. Guo, L. Zhao, Z. Li, J. He, ACS Catal. 4, 2566 (2014)
11. S.M. Islam, A.S. Roy, R.C. Dey, S. Paul, J. Mol. Catal. A Chem. 394, 66 (2014)
12. A. Lu, T.P. Smart, T.H. Epps III, D.A. Longbottom, R.K. O’Reilly, Macromolecules 44, 7233 (2011)
13. M. Lombardo, F. Pasi, S. Easwar, C. Trombini, Synlett 16, 2471 (2008)
14. B. List, Tetrahedron 58, 5573 (2002)
15. D.J. Eyckens, H.L. Brozinski, J.P. Delaney, L. Servinis, S. Naghashian, L.C. Henderson, Catal. Lett.
146, 212 (2016)
16. S.H. Hosseini, A. Asadnia, J. Nanomater. 2012, 1 (2012)
17. Y. Jiang, C. Guo, H. Xia, I. Mahmood, C. Liu, H. Liu, J. Mol. Catal. B 58, 103 (2009)
18. X. Zheng, S. Luo, L. Zhang, J.P. Cheng, Green Chem. 1, 455 (2009)
19. V. Polshettiwar, R. Luque, A. Fihri, H. Zhu, M. Bouhrara, J.M. Basset, Chem. Rev. 111, 3036 (2011)
20. J.H. Lee, Y.M. Huh, Y.W. Jun, J.W. Seo, J.T. Jang, H.T. Song, S. Kim, E.J. Cho, H.G. Yoon, J.S. Suh, J.
Cheon, Nat. Med. 13, 95 (2007)
21. R.F. Majidi, N.S. Sanjani, F. Agend, Thin Solid Films 515, 368 (2006)
22. L.G. Bach, M.R. Islam, J.T. Kim, S.Y. Seo, K.T. Lim, Appl. Surf. Sci. 258, 2959 (2012)
23. M. Bagheri, M. Masteri-Farahani, M. Ghorbani, J. Magn. Magn. Mater. 327, 58 (2013)
24. J. Chen, B. Yao, C. Li, G. Shi, Carbon 64, 225 (2013)
25. S. Sheshmani, R. Amini, Carbohydr. Polym. 95, 348 (2013)
26. V. Georgakilas, M. Otyepka, A.B. Bourlinos, V. Chandra, N. Kim, K.C. Kemp, P. Hobza, R. Zboril,
K.S. Kim, Chem. Rev. 112, 6156 (2012)
27. D.S. Su, S. Perathoner, G. Centi, Chem. Rev. 113, 5782 (2013)
28. M.J. Allen, V.C. Tung, R.B. Kaner, Chem. Rev. 110, 132 (2009)
29. J. Guo, Y. Li, Sh Zhu, Zh Chen, Q. Liu, D. Zhang, W.J. Moon, D.M. Song, RSC Adv. 2, 1356 (2012)
30. X. Yan, J. Chen, Q. Xue, P. Miele, Micropor. Mesopor. Mater. 135, 137 (2010)
31. P. Xiong, G. Hu, Y. Fan, W. Zhang, J. Zhu, X. Wang, J. Power Sources 266, 384 (2014)
32. T. Rakjumar, G.R. Rao, J. Chem. Sci. 120, 587 (2008)
33. L.T.A. Sofa, A. Krishnan, M. Sankar, N.K. Kala Raj, P. Manikandan, P.R. Rajamohanan, T.G. Ajithku-
mar, J. Phys. Chem. C 113, 21114 (2009)
34. Y. Zhang, Y. Zhao, C. Xia, J. Mol. Catal. A 306, 107 (2009)
35. J. Dupont, R.F. de Souza, P.A.Z. Suarez, Chem. Rev. 102, 3667 (2002)
36. P. Virtanen, T.O. Salmi, J.P. Mikkola, Top. Catal. 53, 1096 (2010)
37. Y. Inomata, T. Takeuchi, T. Moriwaki, Inorg. Chim. Acta 68, 187 (1983)
38. W. Stöber, A. Fink, J. Colloid Interface Sci. 26, 62 (1968)
39. N. Mase, Y. Nakai, N. Ohara, H. Yoda, K. Takabe, F. Tanaka, C.F. Barbas III, J. Am. Chem. Soc. 128,
734 (2006)
1 3

M. Kooti et al.
40. M. Sadiq, R. Aman, K. Saeed, M.S. Ahmad, M.A. Zia, Mod. Res. Catal. 4, 43 (2015)
41. Y.C. Teo, P.P.F. Lee, Synth. Commun. 39, 3081 (2009)
42. M. M. Heravi, S. Asadi, Tetrahedron: Asymmetry, 2012, 23, 1431
43. B.M. Trost, C.S. Brindle, Chem. Soc. Rev. 39, 1600 (2010)
44. A. Khalaf-Nezhad, E.S. Shahidzadeh, S. Sarikhani, F. Panahi, J. Mol. Catal. A Chem. 379, 1 (2013)
Publisher’s Note Springer Nature remains neutral with regard to jurisdictional claims in published
maps and institutional afliations.
1 3