Synthesis of amidoalkyl naphthols via AlCl3 functionalized silica-coated Fe2O3 magnetic nanoparticles as a new, efficient, and eco-friendly catalyst

Article abstract of DOI:10.1007/s13738-014-0560-1

Abstract Nano γ-Fe₂O₃@SiO₂-AlCl₂ is synthesized and applied as a recyclable catalyst for cost-effective synthesis of amidoalkyl naphthols, from aldehydes, 2-naphthols, and amides (or urea). This heterogeneous ca

Full text of DOI:10.1007/s13738-014-0560-1

J IRAN CHEM SOC
DOI 10.1007/s13738-014-0560-1
ORIGINAL PAPER
Synthesis of amidoalkyl naphthols via AlCl functionalized
3
silica‑coated Fe O magnetic nanoparticles as a new, efficient,
2
3
and eco‑friendly catalyst
M. Ghader · M. Z. Kassaee
Received: 30 June 2014 / Accepted: 15 November 2014
©
Iranian Chemical Society 2015
Abstract Nano γ-Fe O @SiO –AlCl is synthesized and
For instance, amidoalkyl-2-naphthol derivatives can
be easily converted to important biologically active com-
pounds such as aminoalkyl-2-naphthols by an amide
hydrolysis reaction. Several Lewis and Brønsted acids
have been applied for this transformation including Ce
(SO ) [9], Montmorillonite K10 [10], iodine [11], cation-
2
3
2
2
applied as a recyclable catalyst for cost-effective synthe-
sis of amidoalkyl naphthols, from aldehydes, 2-naphthols,
and amides (or urea). This heterogeneous catalyst enhances
product purity and can efficiently be removed by an exter-
nal magnet. Characterization of the catalyst is carried out
by SEM, XRD, ICP, TGA, and VSM. The thermal solvent-
free procedure offers advantages such as shorter reaction
times, simple work-ups, excellent yields, and reusability of
the catalyst.
4
2
exchanged resins [12], NaHSO ∙H O [13], Fe (HSO )
4 3
4
2
[14], sulfamic acid/ultrasound [15], HClO /SiO [16], cya-
4
2
nuric chloride [17], and K CoW O ∙3H O [18]. However,
5
12 40
2
many of the reported methods suffer from one or more of
the following drawbacks: (1) low product yield, (2) pro-
longed reaction time, (3) the use of large amount of rea-
gent, (4) the use of toxic reagents, and (5) incompatibility
with the green chemistry. Therefore, search for finding a
protocol for the synthesis of aminoalkyl-2-naphthols that
are not associated with the above disadvantages is still
relevant.
Keywords Nano ∙ Amidoalkyl naphthols ∙ Recyclable
catalyst ∙ Maghemite ∙ Green chemistry
Introduction
Multicomponent reactions (MCRs) are special types of
synthetically useful processes in which three or more dif-
ferent starting materials react to give a final product in a
one-pot procedure [1–5]. MCRs have drawn a great deal
of attention for increasing the efficiency by combining
several operational steps without any need for isolation of
intermediates or changing the reaction conditions. MCRs
are becoming powerful tools in modern synthetic chemis-
try due to their efficiency, atom economy, and convenience
in the construction of multiple new bonds in one-pot pro-
cesses. They play powerful roles in approaching complex
structures and promote ‘green chemistry’ [6–8].
Following our work on ‘‘green chemistry’’ [19–21], we
describe an environmentally benign solvent-free approach
for the synthesis of amidoalkyl naphthols from aldehydes,
2-naphthols, and amides (or urea), using aluminum chlo-
ride functionalized silica-coated Fe O magnetic nanopar-
2
3
ticles, under solvent-free conditions.
Experimental
Samples are characterized by X-ray diffraction (XRD,
Philips Xpert MPD, Cu Kα irradiation, λ = 1.54056 A),
scanning electron microscope (SEM, VEGA, TESCAN
microscope with an accelerating voltage of 20 kV), energy
dispersive X-ray (EDX, INCA, Oxford Instrument). Also,
γ-Fe O @SiO –AlCl were characterized by thermo-gravi-
M. Ghader ∙ M. Z. Kassaee (*)
Department of Chemistry, Tarbiat Modares University,
P.O. Box 14155-4838, Tehran, Iran
2
3
2
2
metric analysis (TGA, PerkinElmer) from 25 °C to 900 °C
with heating rate of 5 °C under air, ICP, and VSM.
e-mail: kassaeem@modares.ac.ir
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J IRAN CHEM SOC
Scheme 1 Preparation of
AlCl -functionalized magnetic
3
Fe O nanoparticles: a Fabrica-
2
3
tion of γ-Fe O , b Core–shell
2
3
formation between γ-Fe O and
2
3
tetraethyl orthosilicate (TEOS),
c Fictionalization of γ-Fe O @
2
3
SiO with AlCl and formation
2
3
of the γ-Fe O @ SiO –AlCl
2
2
3
2
Synthesis of aluminum chloride functionalized
of NH OH ammonia is added, and the resulting mixture is
4
silica-coated γ-Fe O magnetic nanoparticles
stirred for 30 min, at 40 °C. Subsequently, tetraethyl ortho-
silicate (TEOS, 1.0 mL) is charged to the reaction ves-
sel, and the mixture is continuously stirred for 24 h. The
silica-coated nanoparticles (SiO @Fe O ) are collected
by a permanent magnet, followed by washing three times
with EtOH, diethyl ether and drying in vacuum for 24 h, at
100 °C.
2
3
1
.99 g of FeCl ∙4H O and 3.25 g of FeCl ∙6H O are sepa-
2 2 3 2
rately dissolved in 20 ml water. Mixing is carried out by vig-
orous stirring (800 rpm). After addition of 0.6 M, 200 mL of
2
2
3
NH OH, a black precipitate forms at room temperature. To
4
maintain the reaction pH between 11 and 12, a concentrated
(
25 %, w/w) solution of NH OH (30 mL) is added. The
10 g of SiO @Fe O is dried for 24 h at 120 °C. It is
4
2
2
3
black dispersion is continuously stirred for 1 h at ambient.
Heating and refluxing it for 1 h yields brown dispersion of
γ-Fe O Nps, which is purified through 3 cycles of repeated
centrifugation (3000–6000 rpm, 20 min), decantation, and
redispersion, until a stable brown magnetic dispersion is
obtained (pH 9.4).
added to a mixture of aluminum trichloride (2.25 g) in
70 mL sodium-dried toluene and stirred at reflux for 2 h.
The solvent is removed under vacuum at room temperature
(Scheme 1). According to the past TGA reports [30, 31],
SiO @Fe O nanoparticles is stable under reflux condition
2
3
2
2
3
for loading AlCl . Therefore we can do this treatment with-
3
γ-Fe O Nps (8.5 %, w/w, 20 mL) is dispersed in metha-
out any problem and get γ-Fe O @SiO –AlCl as a nano-
2
3
2
3
2
2
nol (80 mL) for 1 h at 40 °C. To it, a concentrated solution
catalyst powder.
1
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J IRAN CHEM SOC
Scheme 2 Synthesis of amidoalkyl naphthol over γ-Fe O @SiO –AlCl
2
2
3
2
Table 1 One-pot synthesis
of amidoalkyl naphthols by
reaction of aldehydes, amides,
and 2-naphthol in presence of
the γ-Fe O @SiO –AlCl
Entrya R1
R2
b
Product Time (min) Yield (%) mp (°C) mp (lit)
References
1
2
3
4
5
6
7
8
H
Ph
Ph
Ph
2a
2b
2c
5
13
5
92
96
81
82
78
96
89
95
240
240
214
216
208
239
240
237
239–241 [23]
3-Nitro
242–243 [23]
214–216 [23]
222–223 [25]
205–207 [26]
238–240 [24]
238–240 [24]
237–238 [23]
2
3
2
2
4-Methyl
4-Methyl
CH3 2d
20
30
8
a
Reaction conditions: aldehyde
1 mmol), 2-naphthol (1 mmol),
4-Hydroxy CH3 2e
(
3-Nitro
H
CH3 2f
CH3 2g
CH3 2h
amide (1.2 mmol), catalyst
10
7
(10 mg), 120 °C
4-Nitro
b
Isolated yield
General procedure for synthesis of amidoalkyl naphthol
alkali solution. In so doing, FeCl ·4H O and FeCl ·6H O
2
2
3
2
over γ-Fe O @SiO –AlCl
2
are dissolved in water separately and then mixed under vig-
2
3
2
orous stirring. After addition of NH OH, a black precipi-
4
To a mixture of aldehyde (1 mmol), 2-naphthol (1 mmol),
tate forms at room temperature. To maintain the reaction
and amide/urea (1.2 mmol) is added, γ-Fe O @SiO –AlCl
pH between 11 and 12, a concentrated solution of NH OH
2
3
2
2
4
(
10 mg) (Scheme 2; Table 1). The mixture is stirred mag-
is added. The black dispersion is continuously stirred at
ambient. Heating and refluxing it yields brown dispersion
of γ-Fe O Nps, which is purified through three cycles of
netically at 120 °C while the reaction progress is monitored
by thin layer chromatography (TLC). After precipitation of
the resulting amidoalkyl naphthol, the reaction mixture is
cooled to room temperature, and the product is washed with
acetone. The catalyst is separated with the aid of an external
magnet, and the residue is recrystallized from ethanol.
2
3
repeated centrifugation, decantation, and redispersion, until
a stable brown magnetic dispersion is obtained at pH 9.4.
In order to coat γ-Fe O Nps with a layer of silica, the
2
3
above colloidal γ-Fe O Nps is dispersed in methanol and a
2
3
concentrated solution of NH OH ammonia is added. Sub-
4
sequently, tetraethyl orthosilicate (TEOS) is charged to the
reaction vessel, and the mixture is continuously stirred. The
silica-coated nanoparticle (SiO @Fe O ) is collected by a
Result and discussion
2
2
3
Here we present data pertaining to a new, efficient, and
permanent magnet, followed by washing with EtOH, die-
thyl ether and drying in vacuum.
eco-friendly catalyst (γ-Fe O @SiO –AlCl ) for synthesis
2
3
2
2
of amidoalkyl naphthols. The results include characteriza-
tion of the catalyst by SEM, XRD, ICP, TGA and VSM.
Then we discuss application of γ-Fe O @SiO –AlCl as a
Adding dried SiO @Fe O to a mixture of alu-
minum trichloride in sodium-dried toluene and stirring at
2
2
3
reflux gives γ-Fe O @SiO –AlCl as a flowable powder
2
3
2
2
2
3
2
2
recyclable catalyst for synthesis of amidoalkyl naphthols,
from aldehydes, 2-naphthols and amides (or urea).
(Scheme 1).
Characterization of aluminum chloride functionalized
Synthesis of aluminum chloride functionalized
silica-coated γ-Fe O magnetic nanoparticles
2
3
silica-coated γ-Fe O magnetic nanoparticles
2
3
The fair dispersity and spherical morphology of nanocata-
A brown maghemite nanoparticle (γ-Fe O Nps) is synthe-
lyst γ-Fe O @SiO –AlCl is indicated by its SEM image
2
3
2
3
2
2
sized by co-precipitation of ferric and ferrous ions in an
(Fig. 1). The nanocatalyst γ-Fe O @SiO –AlCl is further
2 3 2 2
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J IRAN CHEM SOC
Fig. 1 The SEM image of the synthesized γ-Fe O @SiO –AlCl (a), and its EDX analysis (b)
2
3
2
2
Fig. 2 XRD patterns of
γ-Fe O , γ-Fe O @SiO and
2
3
2
3
2
γ-Fe O @SiO –AlCl
2
2
3
2
characterized by EDX analysis which shows peaks corre-
sponding to Fe, Si, Cl, O, and Al elements with weight per-
centages of 37.4, 8.97, 6.29, 41.4, and 5.94 %, respectively.
The X-ray diffraction patterns of γ-Fe O , γ-Fe O @
appear at 60 and 27 emug−1, respectively (Fig. 3). Hence,
the functionalized magnetic NPs drop upon saturation mag-
netization. This might be due to the effect of amorphous
silica layer coated on the γ-Fe O . In addition, a minimized
2
3
2
3
2
3
SiO , and γ-Fe O @SiO –AlCl are consistent with their
value of coercivity and negligible magnetization hysteresis
found for each sample exhibits superparamagnetic behavior
at room temperature. This indicates that the functionalized
magnetic NPs are without aggregation.
2
2
3
2
2
structures (Fig. 2). Specifically, the position and relative
intensities of all peaks confirm well with standard XRD
pattern of γ-Fe O (JCPDS card No.39-1346) indicating
2
3
retention of the crystalline cubic structure during function-
Thermal stability of γ-Fe O @SiO –AlCl is probed by
2
3
2
2
alization of γ-Fe O . A weak broad band (2θ = 18−27°)
TG–DTA in air (Fig. 4). There are three distinct weight loss
2
3
appears in γ-Fe O @SiO and γ-Fe O @SiO –AlCl
stages in the TGA curve of γ-Fe O @SiO –AlCl sample.
2
3
2
2
3
2
2
2
3
2
2
which may be assigned to the amorphous silane shell
formed around the magnetic cores.
The first weight loss which ranges from 44 to 242 °C may
be attributed to the evaporation of residual solvents, mainly
to the evaporation of the absorbed water. At the second stage,
the weight loss from 250 to 548 °C region can be attributed
to the combustion/decomposition of TEOS or APTES mole-
cules. The third stage which appears between 550 and 700 °C
Magnetic properties of γ-Fe O and γ-Fe O @SiO –
2
3
2
3
2
AlCl NPs are measured by applying an external magnetic
2
field at room temperature. The saturation magnetization
values of γ-Fe O3 NPs and γ-Fe O @SiO –AlCl NPs
2
2
3
2
2
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J IRAN CHEM SOC
Table 2 Optimization of the γ-Fe O @SiO –AlCl catalyzed model
2
3
2
2
reaction for synthesis of amidoalkyl naphthols
Entrya
Catalyst (mg)
Time (min)
Yield (%)
b
1
2
3
4
5
6
No catalyst
60
10
10
10
9
–
2
57
65
89
89
53
5
10
20
AlCl (0.5 mol %)
20
3
a
Benzaldehyde (1 mmol), acetamide (1.2 mmol), 2-naphthol
1 mmol), catalyst, 120 °C
(
b
Isolated yield
catalyst loading on γ-Fe O @SiO –AlCl . Results indicate
2
3
2
2
that percent by weight of the active catalytic species and the
amount of catalyst loading are 5.47 and 0.02, respectively.
Evaluation of the catalytic activity of γ-Fe O @SiO –
2
3
2
AlCl through synthesis of amidoalkyl naphthols
2
Catalytic activity of γ-Fe O @SiO –AlCl is investigated
2
3
2
2
as a heterogeneous catalyst in one-pot synthesis of ami-
doalkyl naphthols through three-component reaction of
aldehydes, amides, and 2-naphthol (Scheme 2). For this
purpose, the reaction of benzaldehyde, acetamide, and
Fig. 3 VSM patterns of γ-Fe O (a), and γ-Fe O @SiO –AlCl (b)
2
3
2
3
2
2
2
-naphthol as a simple model reaction is probed to estab-
lish the feasibility of the strategy and optimize the reaction
conditions. Various catalysts are evaluated for this model
reaction (Table 2).
The reaction can tolerate a range of aromatic aldehydes
carrying either electron-donating or electron-withdrawing
substituents in the Meta and Para positions. Electron-with-
drawing aldehydes, such as 3-nitro benzaldehyde work well
without formation of any side product and give the ami-
doalkyl naphthol derivative in 96 % yield (Table 1, entry
2
). In contrast, aliphatic aldehydes don’t expedite the reac-
tion and their yields are very low. Aliphatic amides such as
acetamide as well as aromatic amides give good yields of
the product (Table 1). All products are characterized on the
1
13
basis of their spectroscopic data such as FT-IR, H and
C
NMR spectra.
Fig. 4 TGA plot of γ-Fe O @SiO –AlCl
2
2
3
2
1
13
H and C NMR spectra of products
is associated with the condensation of silanols in the silica
shell or the loss of the remaining H O molecules embedded
N‑((2‑hydroxynaphtalen‑1‑yl) (phenyl) methyl) benzamide
2
within the crystal structure [22]. There is little loss of weight
(2a)
in the TGA curves above 700 °C, showing that the γ-Fe O @
2
3
1
SiO –AlCl are stable in this temperature range.
HNMR (500 MHz, DMSO-d ): δ = 10.30 (s, 1H), 9.00 (d,
2
2
6
ICP detection is developed to determine the percent by
weight of the active catalytic species and the amount of
J = 8.6, 1H), 7.84 (m, 3H), 7.78 (d, J = 8.75, 1H), 7.54 (m,
1H), 7.46 (m, 3H), 7.27 (m, 8H).
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J IRAN CHEM SOC
1
3
13
CNMR (125 MHz, DMSO-d ): δ = 165.66, 153.10,
CNMR (125 MHz, DMSO-d ): δ = 170.17, 153.38,
6
6
1
1
1
41.93, 134.27, 132.24, 131.35, 129.29, 128.54, 128.43,
28.31, 128.11, 127.05, 126.68, 126.46, 126.36, 122.60,
18.61, 118.27.
148.23, 133.33, 132.65, 130.83, 130.37, 130.03, 129.17,
128.89, 127.25, 123.25, 123.10, 121.70, 120.92, 118.94,
118.31, 48.13, 22.73.
N‑((2‑hydroxynaphtalen‑1‑yl) (3‑nitrophenyl) methyl)
N‑((2‑hydroxynaphtalen‑1‑yl) (phenyl) methyl) acetamide
benzamide (2b)
(2g)
1
1
HNMR (500 MHz, DMSO-d ): δ = 10.40 (s, 1H), 9.12 (s,
HNMR (500 MHz, DMSO-d ): δ = 9.94 (s, 1H), 8.40 (s,
6
6
1
H), 8.09 (s, 3H), 7.85 (d, J = 8.7, 4H), 7.70 (s, 1H), 7.51
1H), 7.72 (m, 3H), 7.34 (s, 1H), 7.19 (m, 7H), 1.98 (3H).
1
3
(m, 3H), 7.31 (m, 3H).
CNMR (125 MHz, DMSO-d ): δ = 169.62, 153.56,
6
13
CNMR (125 MHz, DMSO-d ): δ = 166.17, 153.37,
143.03, 132.75, 129.63, 128.94, 128.37, 126.71, 126.46,
123.67, 122.79, 119.29, 118.89, 48.26, 23.08.
6
1
1
1
47.74, 144.48, 133.95, 133.23, 132.14, 131.49, 129.92,
29.69, 128.67, 128.38, 128.33, 127.30, 126.97, 122.75,
22.45, 121.57, 120.89, 118.55, 117.22, 48.89.
N‑((2‑hydroxynaphtalen‑1‑yl) (4‑nitrophenyl) methyl)
acetamide (2h)
N‑((2‑hydroxynaphtalen‑1‑yl) (p‑tolyl) methyl) benzamide (2c)
1
HNMR (500 MHz, DMSO-d ): δ = 10.2 (s, 1H), 8.6 (d,
6
1
HNMR (500 MHz, DMSO-d ): δ = 10.27 (s, 1H), 8.96
J = 6.5, 1H), 8.11 (m, 2H), 7.79 (m, 3H), 7.34 (m, 3H),
7.27 (m, 1H), 7.20 (d, J = 8.1, 1H), 2.00 (s, 3H).
6
(s, 1H), 8.04 (s, 1H), 7.80 (m, 4H), 7.49 (m, 4H), 7.26 (m,
1
3
3
H), 7.10(m, 4H), 2.22(s, 3H).
CNMR (125 MHz, DMSO-d ): δ = 169.56, 151.17,
6
1
3
CNMR (125 MHz, DMSO-d ): δ = 165.55, 153.03,
145.84, 132.13, 129.75, 128.57, 128.29, 127.06, 126.60,
123.13, 122.75, 122.43, 118.42, 117.73, 47.84, 22.46.
The catalytic activity and the ability of γ-Fe O @SiO –
6
1
1
1
38.94, 135.54, 134.34, 132.22, 131.31, 129.19, 128.66,
28.51, 128.44, 128.31, 126.99, 126.63, 126.33, 122.58,
18.64, 118.40, 49.01, 20.47.
2
3
2
AlCl to be recycled and reused are studied (Fig. 5). The
2
catalyst is separated by an external magnet and is reused
as such for subsequent experiments under similar reaction
conditions. Yields of the product decrease only slightly
after five attempts.
N‑((2‑hydroxynaphtalen‑1‑yl) (p‑tolyl) methyl) acetamide (2d)
1
HNMR (500 MHz, DMSO-d ): δ = 9.94 (s, 1H), 8.39 (d,
6
J = 8.25, 1H), 7.78 (d, J = 8, 1H), 7.74 (d, J = 8.85, 1H),
A comparison for the efficiency of the catalytic activity
of γ-Fe O @SiO –AlCl is made with six other methods
7
.33 (m, 1H), 7.24 (m, 1H), 7.19 (d, J = 8.8, 1H), 7.06 (d,
2
3
2
2
J = 8.25, 1H), 7.03 (m, 4H), 2.21 (s, 3H), 1.95 (s,3H).
(Table 3). The results show that this method is superior to
some of the earlier methods in terms of yields and reaction
times.
13
CNMR (125 MHz, DMSO-d ): δ = 169.04, 152.99,
6
1
1
39.48, 134.93, 132.22, 129.04, 128.45, 128.42, 126.15,
25.90, 122.26, 118.91, 118.38, 47.52, 22.59, 20.46.
N‑((2‑hydroxynaphtalen‑1‑yl) (4‑hydroxyphenyl) methyl)
Conclusion
acetamide (2e)
Covalent functionalization of AlCl3 onto the magnetic
Fe O nanoparticles is successfully achieved by a multiple
1
HNMR (500 MHz, DMSO-d ): δ = 9.91 (s, 1H), 9.16 (s,
6
2
3
1
2
H), 8.35 (m, 1H), 7.77 (m, 3H), 7.34 (s, 1H), 7.21 (m,
H), 6.98 (m, 3H), 6.62 (m, 2H), 1.93 (s, 3H).
1
3
CNMR (125 MHz, DMSO-d ): δ = 169.40, 156.18,
100
6
1
1
2
53.48, 133.04, 132.80, 129.42, 128.94, 127.74, 126.63,
23.83, 122.79, 119.64, 119.02, 115.24, 115.16, 48.04,
3.19.
8
6
4
2
0
0
0
0
0
N‑((2‑hydroxynaphtalen‑1‑yl) (3‑nitrophenyl) methyl)
acetamide (2f)
1
HNMR (500 MHz, DMSO-d ): δ = 10.12 (s, 1H), 8.62 (s,
1
2
3
4
5
6
1
1
H), 8.02 (m, 2H), 7.81 (m, 3H), 7.55 (m, 2H), 7.40 (m,
H), 7.22 (m, 3H), 2.02 (s, 3H).
Fig. 5 Reusability of the γ-Fe O @SiO –AlCl catalyst
2
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2
2
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J IRAN CHEM SOC
Table 3 Comparison of
catalytic activity of γ-Fe O @
Entrya
b
Catalyst
Condition
Time
Yield (%)
Reference
2
3
SiO −AlCl with known
2
2
1
2
3
4
5
6
7
8
9
Montmorillonite K10 clay
p-TSA
Solvent-free/125 °C
Solvent-free/125 °C
Solvent-free/80 °C
Solvent-free/125 °C
Solvent-free/80 °C
Solvent-free/125 °C
Solvent-free/ 100 °C
Solvent-free/80 °C
Solvent-free/110 °C
Solvent-free/120 °C
Solvent-free/120 °C
Solvent-free/120 °C
1.5 h
5 h
89
88
94
90
88
89
91
80
92
85
73
89
[18]
[27]
[28]
[18]
[29]
[15]
[17]
[32]
[33]
[34]
[35]
−
catalysts
Copper p-toluenesulfunate
K CoW O , 3H O
1.5 h
2 h
5
12 40
2
Thiamine HCl (0.5 mmol)
I2 (5 mol %)
4 h
15 h
Wet-cyunaric chloride
Zwitterionic salt
H4SiW12O40
10 min
2 h
a
Benzaldehyde (l mmol),
acetamide (1.2 mmol),
20 min
5–7 min
15 min
10 min
10
11
12
Zeolite H-BEA
H BO
2
1
b
-naphthol (l mmol), catalyst,
20 °C
3
3
γ‑Fe O @SiO –AlCl
2
2
3
2
Isolated yield
synthetic procedure which is confirmed with XRD, ICP,
TGA, and SEM. The most interesting features of the pre-
sent work include durability as well as efficient catalytic
activity for one-pot synthesis of amidoalkyl naphthols via
three-component solvent free coupling reactions of alde-
hydes, amides, and 2-naphthol. This method offers several
advantages including high yield, short reaction time, simple
work-up procedure, ease of separation, and recyclability
of the magnetic catalyst, as well as the ability to tolerate a
wide variety of substitutions in the reagents.
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1
1
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