Nano CuFe2O4: An efficient, magnetically separable catalyst for transesterification of β-ketoesters

Article abstract of DOI:10.1039/c4ra15014g

The preparation of a variety of β-ketoesters was achieved in high yields from methyl acetoacetate under neutral conditions through the utilization of magnetic CuFe₂O₄ nanoparticles as catalyst. Recycling of the catalyst was performed up to eight times without significant loss in activity. The catalyst was characterized using XRD, XPS, SEM and TEM techniques.

Full text of DOI:10.1039/c4ra15014g

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Nano CuFe2O4: an efficient, magnetically separable
catalyst for transesterification of b-ketoesters
Cite this: RSC Adv., 2015, 5, 18972
Mukut Gohain,a Vinod Kumar,b Johannes H. van Tonder,a Hendrik C. Swart,b
b
a
*
Odireleng M. Ntwaeaborwa and Barend C. B. Bezuidenhoudt
Received 21st November 2014
Accepted 5th February 2015
DOI: 10.1039/c4ra15014g
www.rsc.org/advances
The preparation of a variety of b-ketoesters was achieved in high yields zirconia.16 A recent report by Romanski et al.17 claim the
from methyl acetoacetate under neutral conditions through the transesterication of sterically hindered esters under elevated
utilization of magnetic CuFe2O4 nanoparticles as catalyst. Recycling of pressures which requires special equipment and precautions to
the catalyst was performed up to eight times without significant loss in perform safely.
activity. The catalyst was characterized using XRD, XPS, SEM and TEM
Magnetic nanoparticles have recently emerged as a useful
group of heterogeneous catalysts nding numerous applica-
tions in synthesis and catalysis.18 The separation of magnetic
nanoparticles is simple and economical which represents an
attractive alternative to ltration or centrifugation. Magnetic
separation diminishes the loss of catalyst resulting in enhanced
reusability making these catalysts cost-effective and promising
for industrial applications. In addition, they exhibit high cata-
lytic activity due to their large surface area, relatively low prep-
aration costs as well as low toxicity making them highly
desirable and promising catalysts.19 Super magnetic copper
ferrite, CuFe2O4, is an example of a catalyst which has demon-
strated application in various organic transformations.20 The
efficacy of CuFe2O4 motivated the current study. Herein we
report, for the rst time, the utilization of CuFe2O4 as magnetic
heterogeneous catalyst for the effective transesterication of b-
ketoesters with various alcohols to produce versatile, diversied
esters in moderate to excellent yields.
techniques.
Introduction
b-Ketoesters are important synthons prepared by reacting
highly reactive and unstable diketenes with various alcohols.1
They can be transformed into chiral building blocks by chem-
ical and enzymatic transformation and functions as a tool to
perform chain-extension reactions.2
Transesterication is one of the most effective methods for
the preparation of esters and thus possess wide application in
both academic and industrial research.3 This transformation
can be accelerated by protic4 and Lewis acids5 as well as alkaline
catalysis.6 Among the variety of catalysts7 effecting trans-
esterication, DMAP,8 tetrabutyldistannoxane,9 NBS,10 Ag(OTf)11
and Yb(OTf)3 (ref. 12) represent a few homogeneous examples.
Some of the drawbacks of homogeneous transesterication are
troublesome isolation of the product, catalysts not always
recyclable and large volumes of undesirable waste generated
during workup. Furthermore, selectivity is oen low resulting in
unwanted mixtures of products.
Experimental
General
The crystalline structure of the CuFe2O4 was analyzed using a
Bruker D8 Advance powder diffractometer (XRD; X-ray diffraction).
The X-ray photoelectron spectroscopic (XPS) analysis of CuFe2O4
was carried out with a PHI 5000 Versaprobe Scanning ESCA
Microprobe, while scanning electron microscopy (SEM) images
were obtained on a Shimadzu SSX-550 Superscan scanning elec-
tron microscope. Transmission electron microscopy (TEM) images
were obtained on a Philips (FEI, The Netherlands) CM100 trans-
mission electron microscope. Nuclear magnetic resonance (NMR)
The majority of protic acid catalysts are highly corrosive and,
hence, not ecofriendly which lead to the development of various
heterogeneous analogues for the transesterication of methyl
and ethyl acetoacetates in particular. Examples of acidic
heterogeneous catalysts for the transesterication of b-ketoest-
ers include, among others,13 zeolites,14 kaolinic clay15 and yttria–
aDepartment of Chemistry, University of the Free State, P.O. Box 339, Bloemfontein,
ZA9300, South Africa. E-mail: bezuidbc@ufs.ac.za
1
spectra for H (600 MHz) and 13C (150 MHz) were recorded on a
bDepartment of Physics, University of the Free State, P.O. Box 339, Bloemfontein,
Bruker AM-600 FT spectrometer using CDCl3 as solvent with tet-
ramethylsilane (TMS) as the internal reference. Gas-
ZA9300, South Africa
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chromatography mass-spectrometry (GCMS) was performed on a
Shimadzu GCMS-QP2010 instrument.
Catalyst preparation
A mixture of Cu(OAc)2 (0.91 g, 5 mmol) and Fe(NO3)3$9H2O
(4.04 g, 10 mmol, 2 eq.) was dissolved in an aqueous 0.1 M
ꢀ
NaOH (50 mL) solution and heated to 90 C with mild stirring
for 2 hours. The black precipitate was ltered off, washed with
ethanol (3 ꢁ 50 mL) and water (50 mL) and annealed at 650 ꢀC
for 1 hour in air to yield CuFe2O4 as magnetic nanoparticles.21,22
Catalyst characterization
XRD spectrum of CuFe2O4 is presented in Fig. 1(a). All the
diffraction peaks in the XRD pattern can be indexed to those of
the tetragonal structure of copper ferrite CuFe2O4 according to
JCPDS le no. 35-0425 and is in agreement with the reported
spectra.22 The XPS survey of CuFe2O4 nanoparticles is shown in
to Fig. 1(b) which reveals only copper, iron, and oxygen core
levels.
Fig. 2 SEM image of (a) fresh CuFe2O4 and (b) spent CuFe2O4
nanoparticles. TEM images of (c) fresh CuFe2O4 and (d) spent CuFe2O4
nanoparticles.
The crystallite size (D) of CuFe2O4 is calculated using
Scherer's formula23 given below
mol%) in toluene (10 mL) was heated to reux. Upon comple-
tion of the reaction (assessment by TLC), the catalyst was
immobilized by employing an external magnet and washed with
ethyl acetate. The organic solution was dried over anhydrous
Na2SO4 and the solvent removed under reduced pressure.
Purication of the crude product was achieved through column
chromatography on silica gel by utilizing n-hexane–ethyl acetate
(10 : 1) as eluent to afford the pure b-ketoester (3). The isolated
Kl
D ¼
b cos q
where, l is the wavelength of Cuka radiation (0.1514 nm), K is
correlation factor (0.94), b is FWHM (full width at half
maximum) of peaks and q is Bragg's diffraction angle. Crystal-
lite size of the CuFe2O4 nanoparticles was observed to be ca. 50
nm according to the TEM image [Fig. 2(c)]. The SEM micro-
graphs for CuFe2O4 [Fig. 2(a)] with a 10 mm eld of view displays
the very ne and homogenous nanosphere particles containing
some degree of agglomeration. The observed morphology is the
same as what has been reported in the literature for CuFe2O4.22
The agglomeration of particles can be related to many factors
such as shape factor, surface area, porosity, and density.24 The
SEM [Fig. 2(b)] and TEM [Fig. 2(d)] images of CuFe2O4 nano-
particles recovered aer several cycles of transesterication
reactions reveal similar characteristics.
1
product was then subjected to H NMR, 13C NMR and GCMS
analysis for structural conrmation.
Results and discussion
The reaction conditions (reux, 6 hours) was established with a
mixture of methyl acetoacetate (1) (2 mmol), benzyl alcohol (2a)
(2.1 mmol) and CuFe2O4 in toluene (10 mL) (Scheme 1).
The catalyst loading of CuFe2O4 was evaluated and 10 mol%
(Table 1; entry 1) was identied as the optimal loading as it was
superior to 5 mol% (entry 8) and yielded comparable results to
15 mol% (entry 9). Several other nano-magnetic catalysts like
Fe3O4, Fe2O3, NiFe2O4, CoFe2O3 and MoFe2O3 were also
compared to CuFe2O4 (entries 2–7) and it was found that
although similar product yields were obtained for these cata-
lysts, CuFe2O4 required shorter reaction times (6 hours vs. 8
hours). The CuFe2O3 catalyst was also compared to other widely
used non-magnetic heterogeneous catalysts like ZnO, CuO,
Al2O3 and V2O5 and found to be superior also to these
compounds with higher yield being obtained aer the same
reaction time (entries 11–14).
General method for transesterication
A mixture of methyl acetoacetate (1) (2.0 mmol), the corre-
sponding alcohol (2) (2.1 mmol) and CuFe2O4 catalyst (10
Fig. 1 (a) XRD spectra and (b) XPS survey spectrum of CuFe2O4 Scheme 1 Transesterification of methyl acetoacetate (1) with benzyl
nanoparticles.
alcohol (2a) to produce benzyl acetoacetate (3a).
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Table 1 Optimization of transesterification reaction conditions with Table 2 Transesterification of methyl acetoacetate (1) with various
methyl acetoacetate (1) and benzyl alcohol (2a)a
alcoholsa
Entry
Catalyst
Time (h)
Yieldb (3a) (%)
1
2
3
4
5
6
7
8
CuFe2O4
CoFe2O4
MoFe2O4
NiFe2O4
ZnFe2O4
Fe2O3
6
8
8
8
8
8
8
6
6
6
6
6
6
94
92
93
92
92
92
92
80
93
80
70
75
88
Entry
R
Productb
Time (h)
Yieldc (%)
1
2
3
4
5
C6H5-CH2
(3a)26
(3b)27
(3c)26
(3d)28
(3e)28
6
8
6
6
6
94
90
91
91
84
4-NO2-C6H4-CH2
4-OCH3-C6H4-CH2
3-OCH3-C6H4-CH2
2-OCH3-C6H4-CH2
Fe3O4
CuFe2O4 (5 mol%)
CuFe2O4 (15 mol%)
ZnO
CuO
Al2O3
9
11
12
13
14
6
7
8
(3f)28
(3g)26
(3h)26
6
6
8
80
78
85
V2O5
a
Conditions: 1 (2 mmol), 2a (2.1 mmol) and catalyst (10 mol%) in
reuxing toluene (10 mL). b Isolated yield.
Due to the affinity of copper for oxygen, the trans-
esterication reaction could be envisaged as the result of the Cu
center in the catalyst forming a bi-dentate complex (A) with the
substrate (1) (Scheme 2). This would render a more electrophilic
ester moiety that could be attacked by the alcohol nucleophile
giving the tetrahedral intermediate (B) and subsequent
hydrogen transfer (C) to liberate methanol and the product
ester 3.25 Continuous removal of the low boiling alcohol
(methanol) at the elevated temperature would then shi the
equilibrium toward the desired b-ketoester (3).
The scope of the reaction was evaluated next by employing
a variety of alcoholic substrates (Table 2). Substituents on the
benzyl alcohol substrates included an electron-withdrawing
nitro group (entry 2) and electron-donating methoxy
substituents (entries 3–5). Although similar product yields
could be achieved, the less nucleophilic nitro containing
benzyl alcohol required an additional 2 hours of reaction
time for the reaction to go to completion. Of particular
importance is the product selectivity obtained for 2- and 4-
methoxybenzylalcohol (entries 3 and 5). Despite the tendency
of these substrates to undergo dehydroxylation to benzylic
carbocations (as a result of benzoquinone formation),24 no
carbon–carbon bond formation to the a-carbon of the
9
(3i)29
(3j)26
8
8
70
67
10
11
12
13
(3k)26
(3l)30
8
8
8
90
90
80
(3m)30
14
15
16
17
C5H11
C6H13
C7H15
C8H17
(3n)31
(3o)32
(3p)32
(3q)29
8
8
8
8
95
94
92
90
a
Conditions: 1 (2 mmol), 2 (2.1 mmol) and CuFe2O4 (10 mol%) in
b
reuxing toluene (10 mL). Structures where conrmed by comparing
1H NMR spectra to reported data. c Isolated yields.
ketoester (1) was observed and only the transesteried b-
ketoesters (3c) and (3e) were obtained as products.
Other nucleophilic substrates subjected to these conditions
include 2-thiophenyl-methanol and 2-furanyl-methanol (entries 6
and 7), which gave their respective b-ketoester in ca. 80% yield.
Aliphatic alcohols were also highly effective and produce the
corresponding b-ketoesters in excellent yields (entries 14–17). A
drop in yield to 80% was, however, observed for the sterically
more hindered 2-butanol analogue (entry 13). Allylic alcohols
(entries 8 and 12) were also very effective (85 and 90%) with
propargyl alcohol (entry 9) and crotyl alcohol (entry 10) producing
the lowest yields at ca. 70%. Since the latter alcohols (3i and 3j)
are notorious with respect to instability towards acid catalysts,
the low yields obtained for these substrates are probably expli-
cable in terms of polymerization, atmospheric oxidation and/or
other side reactions.
Scheme 2 Proposed CuFe2O4 catalyzed mechanism for the trans-
esterification of methyl acetoacetate (1).
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these nanoparticles highly attractive as a transesterication
catalyst.
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¨
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