Oxidative esterification of methylarenes and 1,3-dicarbonyls catalysed by copper chloride immobilized on magnetic nanoparticles

Article abstract of DOI:10.1002/aoc.3674

Superparamagnetic CuCl nanoparticles were prepared and identified as an effective heterogeneous catalyst for the tandem transformation of methylarenes and 1,3-dicarbonyl compounds into the corresponding enole esters by the use of tert-butyl hydroperoxide as an external oxidant. The catalyst can be removed from the reaction medium simply by use of an external magnet. As a consequence, various derivatives of enol esters were synthesized in moderate to good yields.

Full text of DOI:10.1002/aoc.3674

Received: 20 August 2016
DOI 10.1002/aoc.3674
Revised: 23 September 2016
Accepted: 1 October 2016
F U L L PA P E R
Oxidative esterification of methylarenes and 1,3‐dicarbonyls
catalysed by copper chloride immobilized on magnetic
nanoparticles
Athar Nakisa | Meghdad Karimi | Elahe Yazdani | Akbar Heydari
Organic Laboratory, Department of Chemistry,
Superparamagnetic CuCl nanoparticles were prepared and identified as an effective
Tarbiat Modares University, Tehran, Iran
heterogeneous catalyst for the tandem transformation of methylarenes and
Correspondence
Akbar Heydari, Organic Laboratory, Department
1,3‐dicarbonyl compounds into the corresponding enole esters by the use of
tert‐butyl hydroperoxide as an external oxidant. The catalyst can be removed from
the reaction medium simply by use of an external magnet. As a consequence, various
of Chemistry, Tarbiat Modares University, PO Box
14155‐4838, Tehran, Iran.
Email: heydar_a@modares.ac.ir
derivatives of enol esters were synthesized in moderate to good yields.
KEYWORDS
1,3‐dicarbonyl, copper(I) chloride, heterogeneous catalyst, magnetic nanoparticles,
oxidative esterification
1
|
INTRODUCTION
Among these strategies, direct oxidative esterification has
emerged as a powerful tool for ester bond formation because of
the use of simple and inexpensive reagents, which make the
oxidative reaction eco‐friendly and economic. In this regard,
direct esterification of aldehydes and alcohols,[24–28] benzyl
alcohol and alcohols[29] and toluene and alcohols[26,30,31] with
various types of strategies have been reported.
In most of these methods, homogeneous catalysts were
more used than heterogeneous ones because of their high
efficiency,[32] but the cost, transition metal toxicity and lack
of reusability restrict the use of homogeneous catalysts for
academic research and industrial applications. To overcome
these problems, heterogeneous catalysts or heterogenized
homogeneous catalysts (immobilizing homogeneous cata-
lysts on organic or inorganic support for use in the liquid
phase) have been developed.[33,34] So we decided to immobi-
lize CuCl on magnetic nanoparticle support with the linkage
of ionic liquid. This achievement prevents Cu(I) toxicity and
allows reuse of the catalyst four times, the catalyst is cost‐
effective, simple to handle and easily separated, and finally
reaching a greener and more eco‐friendly method for oxida-
tive esterification reactions.
Ester, asagroup, isoneofthemostimportantfunctionalgroups
in medicinal, biological, pharmaceutical and other significant
compounds.[1,2] In addition, it has been applied in a large num-
bers of industries, such as polymer, healthcare and cosmetics,
perfume, etc.[3] Traditionally, esters were prepared through a
two‐step reaction: converting hydroxyl of carboxylic acid into
agoodleavinggroup(acylchloridemostly), thencouplingwith
alcohol.[4] Thedisadvantagesof thosemethodswerepooratom
economy, generation of troublesome wastes and use of hazard-
ous reagents. These challenging problems led lots of interested
scientists to propose some innovative strategies to make the
synthesis of the ester bond greener, including the use of non‐
toxic reagents like N,N′‐dicyclohexylcarbodiimide (Steglich
esterification),[5] diazomethane[5] and trialkyl or aryl phos-
phate‐diazo (Mitsunobu reaction),[6,7] and other methods like
Baeyer–Villiger oxidation.[8–13]
However, because of poor atom economy and difficulty
of product isolation in those procedures, metal‐catalysed
methods like oxidative carbonylation esterification of
alcohols,[14–16] alkenes and alkynes[17–22] and arenes[23] have
been developed in recent years.
Toluene, an accessible compound which is obtained from
the petrol industry, can be oxidized to benzyl alcohol,[35]
benzaldehyde[36] and benzoic acid[37] with homogeneous
and heterogeneous catalytic systems. So, it is considered as
This article is dedicated to memory of Khosro Nakisa
Appl Organometal Chem 2016; 1–6
wileyonlinelibrary.com/journal/aoc
Copyright © 2016 John Wiley & Sons, Ltd.
1

2
NAKISA ET AL.
a source of carbonyl compounds. Also, 1,3‐dicarbonyl com-
pounds are some of the most interesting compounds because
of their versatile applications in synthesis. The active centre
is Cα as nucleophile (with base catalysis) and carbon of
carbonyl group as electrophile (with acid catalysis). Yoo
and Li reported CuBr‐catalysed oxidative esterification of
aldehydes with β‐dicarbonyl compounds.[38] Recently Yin
and co‐authors reported the synthesis of α‐ketoesters from
β‐diketones via vanadium‐mediated tandem transformation
under an oxygen atmosphere.[39] Kim and co‐workers
reported a direct access to phenol esters and enol esters for
2‐acyl phenols and 1,3‐dicarbonyl compounds with ethers
through a copper‐catalysed oxidative method.[40]
In continuation of our interest in using magnetic nanopar-
ticles, we here report enol ester synthesis by oxidative
coupling of 1,3‐dicarbonyls with toluene in the presence
of CuCl–dimethyloctadecyl[3‐(trimethoxysilyl)propyl] ionic
liquid immobilized on the surface of magnetic nanoparticles
as an efficient and magnetically separable catalyst. In the
presence of the new inorganic–organic ionic liquid function-
alized magnetic nanocatalyst, various derivatives of enol
esters were synthesized in moderate to good yields via the
oxidative coupling of 1,3‐dicarbonyls with toluene.
solution (25% w/w, 30 ml) was added to the mixture at room
temperature until the pH was raised to 11. The addition of
NH4OH solution followed to maintain the reaction pH
between 11 and 12 at which a black suspension was formed.
The resulting black dispersion was continuously stirred for
1 h at room temperature and then refluxed for 1 h. Coating
of a layer of silica on the surface of the γ‐Fe2O3 nanoparticles
was achieved by adding ethanol (40 ml) to purified nanopar-
ticles then heated for 1 h at 40 °C. Subsequently, tetraethyl
orthosilicate (TEOS; 10 ml) was charged to the reaction ves-
sel, and the mixture was continuously stirred for 24 h. The
silica‐coated nanoparticles (Fe3O4@SiO2) were collected
using a magnet, followed by washing five times with EtOH
and diethyl ether and drying at 100 °C in vacuum for 12 h.
2.2.2
| Immobilization of ionic liquid
In a separate container, 3 mmol of CuCl was added to 3 mmol
of [3‐(triethoxysilyl)propyl)]octadecyldimethylammonium
chloride, 72% in ethanol, at 80 °C for 8 hr under argon. After
that 1 g of Fe3O4@SiO2 was added to the prepared ionic
liquid in 20 ml of toluene–n‐hexane (6:1) and the mixture
was stirred for 24 h at room temperature. Finally, the resulting
nanoparticles were separated using an external magnet,
washed with n‐hexane and diethyl ether several times and
dried at 50 °C. The synthesis is shown in Scheme 1.
2
| EXPERIMENTAL
2.3 | General procedure for oxidative esterification
2.1 | Chemicals, instrumentation and analysis
catalysed by Fe3O4@SiO2@Ionic Liquid.CuCl
All solvents and chemicals purchased were of analytical
grade and used without further purification. Fourier trans-
form infrared (FT‐IR) spectra were obtained in the region
400–4000 cm−1 using a Nicolet IR100 instrument with spec-
troscopic grade KBr. The morphology of the catalyst was
studied using scanning electron microscopy (SEM; Philips
XL 30 and S‐4160) with coated gold equipped with energy‐
dispersive X‐ray (EDX) spectroscopy capability. Powder
X‐ray diffraction (XRD) spectra were recorded at room tem-
perature with a Philips X‐Pert 1710 diffractometer using Co
Kα radiation (λ = 1.78897 Å) at a voltage of 40 kV and
current of 40 mA to define the crystalline structure of
Fe3O4@SiO2@Ionic Liquid.CuCl nanoparticles. Data
were collected from 10° to 90° (2θ) with a scan speed of
0.02° s−1. The magnetic properties were measured with a
vibrating magnetometer/alternating gradient force magne-
tometer (MD Co., Iran, www.mdk‐magnetic.com). Thermo-
gravimetric analysis was performed using thermal analyser
with a heating rate of 20 °C min−1 over a temperature range
of 25–1100 °C under flowing compressed nitrogen.
According to the reaction optimization results summarized
in Table 1, the first reaction was carried out as follows. A
mixture of catalyst (20 mg) in toluene (3.0 ml) was placed
in a round‐bottom flask at room temperature. Then, tert‐
butyl hydroperoxide (TBHP; 70 wt% in water, 2 mmol)
was added dropwise to the reaction mixture and the mixture
was placed in an oil bath with magnetic stirring and stirred
for 1 h at 80 °C. In a second step, 1,3‐dicarbonyl (1 mmol)
and TBHP (70 wt% in water, 4 equiv.) were added. After
completion of the reaction (TLC monitoring), the catalyst
was removed using an external magnet. Then 1 N HCl
(5 ml) and EtOAc (5 ml) were added. The mixture was
extracted with EtOAc and the combined organic phase was
washed with a saturated aqueous solution of NaHCO3
(10 ml) and brine (10 ml), and dried over anhydrous
2.2 | Catalyst preparation
2.2.1
| Modification of Fe3O4@SiO2
Amounts of 5 mmol of FeCl3⋅6H2O and 2.5 mmol of
FeCl2⋅4H2O salts were dissolved in 100 ml of deionized
water under vigorous stirring (800 rpm). Then NH4OH
SCHEME 1 Preparation of Fe3O4@SiO2@Ionic Liquid.CuCl

NAKISA ET AL.
3
TABLE 1 Optimization of reaction conditionsa
Liquid, the C─H stretching bands (2800–2950 cm−1) appear
and indicate that the ionic liquid is attached to Fe3O4@SiO2.
Entry
Catalyst (mg)
Oxidant
Temp. (°C)
Yield (%)b
1
10
10
10
10
20
30
40
10
10
10
—
10
H2O2
mCPBA
UHP
r.t.c
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
60
<10
<10
<10
30
3.2 | XRD patterns
2
3
In the XRD pattern there are six characteristic peaks for
Fe3O4 (2θ = 30.1°, 35.4°, 43.0°, 53.5°, 57.0° and 62.5°).
These are related to six crystal faces. As shown in Figure 2,
the XRD pattern of Fe3O4@SiO2@Ionic Liquid.CuCl
matches with the peaks of Fe3O4 and this shows that after
surface modification of Fe3O4, the crystalline structure of
Fe3O4 does not change.
4
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
—
5
50
6
50
7
50
8
60
9
80
75
10
11
12
100
80
75
n.d.d
3.3 | SEM analysis
80
n.d.
The morphology of the synthesized nanoparticles at different
stages is depicted in Figure 3. As is clear, the morphology
does not change between each step.
aAll reactions were run with methyl acetoacetate (1 mmol), oxidant (6 equiv.),
solvent (3 ml), under an argon atmosphere for 24 h.
bIsolated yield.
cRoom temperature.
dNot detected.
3.4 | EDX analysis
The existence and amount of all elements were investigated
using EDX. The results (Figure 4) confirm that CuCl salt is
anchored in Fe3O4@SiO2.
3.5 | Vibrating sample magnetometry
SCHEME 2 Oxidative esterification reaction
The magnetic property of the resulting nanoparticles of
Fe3O4 was measured using vibrating sample magnetometry.
In Figure 5 the magnetization (emu g−1) is plotted as a func-
tion of applied magnetic field (Oe) in the range − 10 000 to
10 000 Oe. Zero remanence and coercivity of the magnetiza-
tion curve demonstrate that these nanoparticles have
superparamagnetic properties.
Na2SO4. After removal of the solvent under vacuum, the
crude product was purified by column chromatography to
afford the desired product (Scheme 2).
3
| RESULTS AND DISCUSSION
3.1 | FT‐IR spectroscopy
3.6 | Thermogravimetric/differential thermal analyses
To demonstrate anchoring organic compounds and com-
plexes, FT‐IR spectroscopy was used. The FT‐IR spectra of
Fe3O4, Fe3O4@SiO2 and Fe3O4@SiO2@Ionic Liquid are
shown in Figure 1. The typical peak of the Fe─O stretching
vibration appears in 562 cm−1 in the spectrum of Fe3O4
nanoparticles. The wide peak at 1090 cm−1 related to Si─O
stretching vibration can demonstrate anchoring of SiO2 to
Fe3O4. In the FT‐IR spectrum of Fe3O4@SiO2@Ionic
The amount of materials that modified the surface of the Fe3O4
was measured using thermogravimetric analysis. As shown in
Figure 6, the first step that occurs under 100 °C corresponds to
the presence of water molecules in the structure (2 wt%). The
next step, from approximately 200 to 400 °C, is associated
with the loss of organic portion (10 wt%, ionic liquid and
FIGURE
2
XRD patterns of (a) Fe3O4, (b) Fe3O4@SiO2 and (c)
FIGURE 1 FT‐IzR spectra
Fe3O4@SiO2@Ionic Liquid.CuCl

4
NAKISA ET AL.
FIGURE 4 EDX analysis for (a) Fe3O4@SiO2 and (b) Fe3O4@SiO2@Ionic
Liquid.CuCl
FIGURE 5 Magnetization curve of Fe3O4@SiO2@Ionic Liquid.CuCl
FIGURE 3 SEM analysis of (a) Fe3O4@SiO2, (b) Fe3O4@SiO2@Ionic
Liquid.CuCl before reaction and (c) Fe3O4@SiO2@Ionic Liquid.CuCl after
reaction
copper salt). And the last step between 450 and 750 °C is
attributed to inorganic moiety (5 wt%, silica coating).
3.7 | Inductively coupled plasma analysis
To determine the actual amount of copper ions, inductively
coupled plasma analysis was applied. The amount was
measured as 0.006 mmol g−1
.
3.8 | Catalytic activity
After reaction, the nanocatalyst was washed and dried and
reused in the next run of the reaction. It is found that the mod-
ified magnetic nanoparticles can catalyse the reaction four
FIGURE 6 Thermogravimetric analysis of Fe3O4@SiO2@Ionic Liquid.
CuCl

NAKISA ET AL.
5
times without significant reduction in yield, but after that a
marked decrease occurs. The results are summarized in
Figure 7.
TABLE 2 Preparation of various 1,3‐dicarbonyl compounds with toluenea
3.9 | Optimization
The reaction between methyl acetoacatate and toluene was
chosen as a model reaction to optimize the conditions of
oxidative esterification reaction (Scheme 2). The results are
summarized in Table 1. The first reaction was carried out
under the following conditions: toluene (3 ml),
1,3‐dicarbonyl (1 mmol), Fe3O4@SiO2@Ionic Liquid.CuCl
as heterogeneous catalyst (10 mg), H2O2 as oxidant (30% in
water, 3 equiv.) at 100 °C. The resultant product is obtained
in less than 10% yield (Table 1, entry 1).
To improve the efficiency of the reaction, other oxidants
such as meta‐chloroperoxybenzoic acid (mCPBA), urea–
hydrogen peroxide (UHP) and TBHP were tried (Table 1,
entries 1–4). TBHP proves to be the best with a noticeable
increase in yield (Table 1, entry 4). In the next step, the amount
of catalyst was investigated. The yield is significantly
increased after adding 20 mg of catalyst, beyond which there
is noimprovement, so 20 mg was selected as optimized amount
of catalyst (Table 1, entries 4–7). The effect of temperature
was investigated in the next step. Increasing the temperature
to 80 °C has a distinguishable effect on the yield of product,
but to 100 °C does not, 80 °C gives the best results (Table 1,
entries 7–11). Finally, the effect of oxidant and catalyst was
investigated. In the absence of either one of them no product
is obtained and this result demonstrates the importance of both
and ties the two together (Table 1, entries 12 and 13).
To demonstrate the general applicability of the
obtained conditions, various 1,3‐dicarbonyl compounds
were tested in oxidative esterification reaction with
toluene, the considered catalyst and TBHP. The achieved
products are given in Table 2. To determine how the
designed catalyst can act in the same reaction, the
reaction of toluene with 1‐(2‐hydroxyphenyl)ethanone was
also investigated (Table2).
aReaction conditions for synthesis of compounds 1a–12a: toluene (2 ml), 1,3‐
dicarbonyl (1 mmol), catalyst (20 mg), TBHP (3 equiv.) under argon at 80 °C
for 24 h. Reaction conditions for synthesis of compounds 1b–4b: toluene
(2 ml), phenol (1 mmol), TBHP (3 equiv.), catalyst (20 mg) under argon at
80 °C for 24 h.
Following the successful oxidative esterification of toluene
with 1,3‐dicarbonyl compounds, we were encouraged to extend
this process to the oxidative esterification of toluene with
2‐hydroxyacetophenone to form phenol esters. The reaction
between toluene and 2‐hydroxyacetophenone was used as the
model reaction. The conditions were the same as those
described above (Scheme 2). Hence in this case the optimum
conditions for preparing phenol esters were as follows: toluene
(2 ml), TBHP (3 equiv.), 20 mg of catalyst at 100 °C under an
argon atmosphere (Table 2). All synthesized products were
characterized by recording melting point (in some cases) and
from FT‐IR, 1H NMR and 13C NMR spectra. The spectral data
of the known compounds are provided in the references and the
two new compounds are in supporting information.[38][41–43]
The proposed Cu(II)/Cu(I) catalytic cycle is depicted in
Figure 8. It begins by formation of benzaldehyde from its
aromatic analogue, toluene. In the next step, Cu(II)
complexes with 1,3‐dicarbonyl compound, then decomposes
and the target product is obtained.[38]
FIGURE
7
Catalytic activity of Fe3O4@SiO2@Ionic Liquid.CuCl in
FIGURE
8
Proposed mechanism for Cu(I)‐catalysed oxidative
successive reaction runs
esterification

6
NAKISA ET AL.
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4
| CONCLUSIONS
We have immobilized Cu(I) on Fe3O4@SiO2 via [3‐
(trimethoxysilyl)propyl)]octadecyldimethylammonium chlo-
ride (as ionic liquid) and have demonstrated the magnetic
nanoparticle structure and characterization using various
methods. Then we investigated the catalytic activity in two
different oxidative esterification reactions. We recovered the
magnetic nanoparticles by applying an external magnet and
reused them four times without significant loss of activity.
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ACKNOWLEDGMENT
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We acknowledge Tarbiat Modares University for partial
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How to cite this article: Nakisa, A., Karimi, M.,
Yazdani, E., and Heydari, A. (2016), Oxidative esteri-
fication of methylarenes and 1,3‐dicarbonyls catalysed
by copper chloride immobilized on magnetic nanopar-
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aoc.3674
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