Iron catalyzed Michael addition: Chloroferrate ionic liquids as efficient catalysts under microwave conditions

Article abstract of DOI:10.1007/s11426-012-4657-z

The iron containing ionic liquid 1-butyl-3-methylimidazolium tetrachloroferrate ([C4mim]Cl/FeCl₃; χ = 0.5) proved to be an efficient catalyst for the solvent-free metal catalyzed Michael addition of ?-keto esters and enones. Due to the ionic st

Full text of DOI:10.1007/s11426-012-4657-z

SCIENCE CHINA
Chemistry
August 2012 Vol.55 No.8: 1614–1619
doi: 10.1007/s11426-012-4657-z
• ARTICLES •
· SPECIAL ISSUE · Ionic Liquid and Green Chemistry
Iron catalyzed Michael addition: Chloroferrate ionic liquids as
efficient catalysts under microwave conditions
VASILOIU Maria, GAERTNER Peter & BICA Katharina*
Institute of Applied Synthetic Chemistry, Vienna University of Technology, Vienna, A-1060, Austria
Received February 28, 2012; accepted April 22, 2012; published online July 4, 2012
The iron containing ionic liquid 1-butyl-3-methylimidazolium tetrachloroferrate ([C4mim]Cl/FeCl3;  = 0.5) proved to be an
efficient catalyst for the solvent-free metal catalyzed Michael addition of β-keto esters and enones. Due to the ionic structure of
the catalyst, a significant acceleration of the reaction rate was observed under microwave conditions. Furthermore, recycling of
the ionic liquid catalyst could be performed after simply distilling off the products.
ionic liquids, iron, catalysis, michael addition, lewis acid, microwave chemistry
pollutant metals on earth the scope for iron catalyzed or-
1 Introduction
ganic reactions is of great interest and constantly growing
[5]. During the past few years, Lewis-acidic ILs with the
general structure [Cnmim]Cl/MexCly have emerged from
their role as mere reaction media toward a role as novel
catalytic systems. Chloroferrate ionic liquids in particular
offer a novel approach to sustainable and environmentally
benign catalysis and can be applied in a surprising diversity
of transformations [6, 7]. The system [Cnmim]Cl/FeCl3 in
various ratios has been previously used for Friedel-Crafts
alkylation and acylation [8], hydroxy methylation [9] as
well as for cross-coupling reactions [10], yet other examples
on C–C bond forming reactions remain rare.
The Michael addition is one of the most useful C–C
bond-forming reactions and has wide synthetic applications
in organic synthesis [11]. This reaction is usually catalyzed
by strong bases such as alkali hydroxides or tertiary amines
that can often result in undesirable side reactions, particu-
larly when functionalized substrates are used. Transition
metal catalysis under neutral and mild conditions can solve
these issues, and the iron(III)-catalyzed Michael addition of
keto esters to enones is one of the most prominent examples
for iron catalysis [12]. Other transition metals including Cu,
Co and Ni have been investigated as catalysts, yet iron has
repeatedly proven to be the most efficient [13]. Several pa-
Ionic liquids (ILs) are low melting salts (< 100 °C) that
have attracted considerable attention as alternatives to or-
ganic solvents in synthesis and catalysis [1]. While the
original motive for the interest in ionic liquids as solvents
for synthesis and catalysis has been the intrinsic lack of va-
pour pressure, some of the most notable results describe
enhancements in chemical reactivity or catalytic perfor-
mance that result of the solvent characteristics of the IL,
rather than just reductions in solvent losses [2, 3]. In many
cases, the focus is put on rendering a homogenous reaction
into a biphasic system for catalyst immobilization and recy-
cling [4]. However, the comparably high price of pure ILs,
as well as their substantial viscosity is still a major draw-
back for their application in synthesis or catalysis. We are
particularly interested to extend the application of ILs be-
yond the state of being mere reaction media and towards
becoming novel catalysts or parts of catalytic systems, thus
giving access to stable and recyclable catalysts with ionic
structures.
Since iron is one of the most inexpensive and non-
*Corresponding author (email: kbica@ioc.tuwien.ac.at)
© Science China Press and Springer-Verlag Berlin Heidelberg 2012
chem.scichina.com www.springerlink.com

Vasiloiu M, et al. Sci China Chem August (2012) Vol.55 No.8
1615
pers applying FeCl3∙6H2O as catalyst have been published
[14], and other iron (III) salts such as Fe(ClO4)3, Fe(acac)3
All products 4, 10, 11, 12, 13, 14 from Michael addition
of -keto esters 2, 5, 6, 7 and enones 3, 8, 9 have been pre-
viously reported in literature, and our analytical data were
found to be identical [13, 21, 22, 23].
and
Fe(III)-exchanged
fluorotetrasilicic
mica
or
Fe-exchanged montmorillonite K10 have been successful-
ly used [15, 16]. Mechanistical studies on this reaction
were done by Christoffers et al. [17] who used DFT cal-
culations as well as Raman, UV-Vis, EXAFS and XANES
studies and by Trage et al. who used electrospray mass
spectroscopy to get insight in the catalytic species [18].
There have also been published two papers dealing with
imidazolium-based ionic liquids as solvents for transition
metal catalyzed Michael addition, which investigated the
influence of the ionic liquids’ impurities on the catalytic
activity of the iron and nickel salts [19]. Recently, the
basic ionic liquid 1-butyl-3-methylimidazolium hydroxide
([C4mim]OH) has been reported as catalyst for conjugate
additions [20].
2.2 Preparation of chloroferrate ionic liquids as cata-
lyst for Michael addition
1-Butyl-3-methylimidazolium tetrachloroferrate ([C4mim]
FeCl4) (1)
1-Butyl-3-methylimidazolium chloride (5.00 g, 28.6 mmol)
and FeCl3∙6H2O (7.74 g, 28.6 mmol) were mixed in a round
bottom flask and stirred for 5 minutes. Almost immediately
an endothermic reaction took place and provided the
chloroferrate ionic liquid 1 as a lower layer. The upper
aqueous layer was carefully removed and the remaining
ionic liquid was dried under stirring for 2 d at 80 °C and
0.01 mbar to yield 8.96 g (93%) of [C4mim]FeCl4 as dark
brown oil.
In this article, we describe the use of the chloroferrate IL
1-butyl-3-methylimidazolium tetrachloroferrate ([C4mim]Cl/
FeCl3 with  = 0.5) 1 as efficient and recyclable catalyst for
iron catalyzed Michael addition of keto esters and enones
with a particular focus on microwave-assisted catalysis.
2.3 Iron-catalyzed Michael addition under convention-
al heating
-Oxo ester (5 mmol) and [C4mim]FeCl4 1 (16.8 mg–2.00 g,
see Table 1) were mixed in a 10 mL round-bottom flask.
Freshly distilled enone (7 mmol) was added to the violet
solution and the reaction mixture was sealed with a glass
stopper equipped with a Teflon ring. The mixture was re-
acted in a pre-heated oil bath for the time given in Table 1.
Remaining volatile material was removed under reduced
pressure, and the reaming oil was directly subjected to
Kugelrohr distillation affording the alkylated -oxo ester as
colourless liquids.
2 Experimental
2.1 General experimental section
All chemicals unless otherwise stated were obtained from
Aldrich and used without further purification. 1-Butyl-3-
methylimidazolium chloride was prepared according to lit-
erature and repeatedly crystallized from EtOAc/CH3CN to
obtain colourless crystals. Methyl vinyl ketone 3 and
acrolein 9 were distilled prior to use.
1H and 13C NMR spectra were recorded on a Bruker AC
200 at 200 and 50 MHz or on a Bruker AC 400 at 400 and
100 MHz, respectively, using the solvent peak or TMS as
reference. 13C NMR spectra were run in proton-decoupled
mode and multiplicities from DEPT were referred as s (sin-
glet), d (doublet), t (triplet), q (quartet), quin (quintet), sext
(sextet), sept (septet) and m (multiplet).
2.4 Iron-catalyzed Michael addition under microwave
irradiation
-Oxo ester (5 mmol) and [C4mim]FeCl4 1 (16.8 mg, 0.05
mmol) were mixed in a microwave vial and sealed with a
Teflon septum. Freshly distilled enone (7 mmol) was added
to the violet solution. Irradiation in a CEM Explorer at
125 °C at 50 W for the time given in Table 2 gave a yellow
solution which was directly subjected to flash chromatog-
raphy (50 g silica, light petrolether:ethyl acetate) to afford
the alkylated -oxo ester as colourless liquid.
GC-MS analyses were conducted on a VOYAGER
Quadrupol (Thermo Finnigan) directly interfaced to a GC
8000 TOP gas chromatograph using a BGB-5 (30 m × 0.32
mm i.d., 1.0 μm film thickness) cross-bonded dimethyl pol-
ysiloxane capillary column. The electron energy was set to
70 eV and the ion source temperature to 200 °C. The oven
program temperature was 80 °C (2 min)//10 °C/min//280 °C
(3 min). Source and transfer line temperatures were set at
200 and 280 °C, resp.
2.4.1
Methyl 2-oxo-1-(3-oxobutyl)cyclopentanecarbox-
ylate (4)
Prepared from keto ester 2 (710.8 mg, 5 mmol) and methyl
vinyl ketone 3 (490.6 mg, 7 mmol) according to the Section
Raman spectra were recorded on a LabRAM HR spec-
trometer (Jobin Yvon, Benzheim, Germany) using a 632.8
nm HeNe laser line.
1
2.3. Yield: 0.99 g of 4 as colorless liquid (94%). H-NMR
(200 MHz, CDCl3):  3.69 (s, 3H), 2.68 (ddd, 1H, J1 =
17.85 Hz, J2 = 9.44 Hz, J3 = 6.02 Hz), 2.55–2.25 (m, 4H),
2.21–1.78 (m, 5H), 2.12 (s, 3H).
Microwave synthesis was performed on a CEM Explorer
PLS microwave unit with an external IR sensor for temper-
ature control. Reported reaction times are hold times.

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Vasiloiu M, et al. Sci China Chem August (2012) Vol.55 No.8
2.4.2 Ethyl 2-oxo-1-(3-oxobutyl)cyclohexanecarboxylate
arated [6].
(10)
The formation of the chloroferrate anion was proven via
Raman spectroscopy, and a characteristic peak at 333 cm1
Prepared from keto ester 5 (851.1 mg, 5 mmol) and methyl
vinyl ketone 3 (7 mmol) according to the Section 2.3. Yield:
1.69 g of 10 as colorless liquid (89%). 1H-NMR (200 MHz,
CDCl3):  4.19 (q, 2H, J = 7.11 Hz), 2.68–2.25 (m, 5H),
2.12 (s, 3H), 2.17–1.35 (m, 7H), 1.26 (t, 3H, J = 7.14 Hz).

from the symmetric Fe–Cl stretch vibration indicates FeCl4
as sole anion.
For initial studies, we investigated the reaction of methyl
vinyl ketone 2 and keto ester 3 in the presence of ionic liq-
uid 1 as solvent and as catalyst (Scheme 2).
2.4.3 Ethyl 2-acetyl-5-oxohexenoate (11)
Both starting materials 2 and 3 were soluble in the sol-
vent [C4mim]FeCl4 1, and therefore a homogenous reaction
could be done without additional co-solvent. Immediately
after the catalyst was mixed with -keto ester 2, a deep pur-
ple color was observed due to formation of an iron(III)-ato
complex (Figure 1). The significant color of these Fe(III)
complexes with 1,3-dicarbonyls is well known, and iron is
assumed to facilitate deprotonation at the activated carbon
of the coordinated dicarbonyl thus increasing the reactivity
towards Michael acceptors [18].
Complete conversion could be easily recognized by a
color change from deep purple to a clear yellow solution.
The reaction proceeded smoothly in 2 hrs at 100 °C and
gave product 4 in 81% yield which could be easily separat-
ed by distillation from the non-volatile ionic liquid (Table 1,
entry 1). Thus encouraged we lowered the amount of ionic
liquid from solvent conditions to a catalytic amount of 10
mol% and could still obtain good conversion. The isolated
yield was only slightly decreased to 77% (Table 1, entry 2).
Surprisingly, the best results were obtained with a lower
catalyst loading applying 1 mol% [C4mim]FeCl4. Complete
conversion could be achieved by heating the sample at
100 °C overnight with a slight excess of methyl vinyl ke-
tone and a good isolated yield of 89% was obtained (Table 1,
Prepared from keto ester 6 (650.1 mg, 5 mmol) and methyl
vinyl ketone 3 (490.6 mg, 7 mmol) according to the Section
2.3. Yield: 0.71 g of 11 as colorless oil (71%). H-NMR
(200 MHz, CDCl3):  3.69 (s, 3H), 2.68 (ddd, 1H, J1 =
17.85 Hz, J2 = 9.44 Hz, J3 = 6.02 Hz), 2.55–2.25 (m, 4H),
2.21–1.78 (m, 5H), 2.12 (s, 3H).
1
2.4.4 Ethyl 2-benzoyl-5-oxohexenoate (12)
Prepared from keto ester 7 (961.1 mg, 5 mmol) and methyl
vinyl ketone 3 (490.6 mg, 7 mmol) according to the Section
2.2. Yield: 1.03 g of 12 as colorless foam (79%). H-NMR
(200 MHz, CDCl3):  4.14 (q, 2H, J = 7.11 Hz), 3.43 (t, 1H,
J = 7.14 Hz), 2.44 (t, 2H, J = 7.04 Hz), 2.99 (s, 3H), 2.12 (s,
3H), 2.02 (m, 2H), 1.21 (t, 3H, J = 7.14 Hz).
1
2.4.5 Ethyl 3-oxo-2-(3-oxocyclohexyl)-3-phenylpropanoate
(13)
Prepared from keto ester 7 (961.1 mg, 5 mmol) and enone 8
(672.9 mg, 7 mmol) according to the Section 2.3. Yield:
1
1.03 of 13 as colorless oil (88%). H-NMR (200 MHz,
CDCl3):  7.99 (m, 2H), 7.51 (m, 3H), 4.27 (dd, 1H, J1 =
8.71 Hz, J2 = 4.99 Hz), 4.13 (m, 2H), 3.05–1.32 (m, 9H),
1.17 (t, 3H, J = 7.14 Hz).
2.4.6 Ethyl 2-benzoyl-5-oxo-pentanoate (14)
Prepared from keto ester 7 (961.1 mg, 5 mmol) and acrolein
9 (392.4 mg, 7 mmol) according to the Section 2.3. Yield:
1
0.65 g of 4 as colorless liquid (88%). H-NMR (200 MHz,
CDCl3):  7.99 (m, 2H), 7.51 (m, 3H), 4.27 (dd, 1H, J1 =
8.71 Hz, J2 = 4.99 Hz), 4.13 (m, 2H), 3.05–1.32 (m, 9H),
1.17 (t, 3H, J = 7.14 Hz).
Scheme 2 Iron catalyzed Michael addition under thermal conditions.
3 Results and discussion
The chloroferrate IL [C4mim]FeCl4 was easily prepared in a
solid state reaction by mixing commercially available solid
[C4mim]Cl and FeCl3∙6H2O, yielding the hydrophobic ionic
liquid as a lower phase and water, which can be easily sep-
Figure 1 Fe(III)-ato complex of [C4mim]FeCl4 1 and methyl 2-oxocy-
clopentancarboxylate 2 and color change observed after complete conver-
sion.
Scheme 1 Preparation of the ionic liquid catalyst [C4mim]FeCl4.

Vasiloiu M, et al. Sci China Chem August (2012) Vol.55 No.8
1617
Table 1 Various Michael additions using 1 as solvent or catalyst under
thermal conditions
Entry a)
Catalyst
2 g
Conditions
2 h, 100 °C
2 h, 100 °C
24 h, 100 °C
24 h, 100 °C
24 h, 100 °C
24 h, 100 °C
24 h, 100 °C
Yield b)
81
1
2
3
4
5
6
7
10 mol%
1 mol%
1 mol% c)
1 mol% c)
1 mol% c)
1 mol% c)
77
89
86
83
38
11
a) All reactions were performed using 5 mmol keto ester 2 and 7 mmol
methyl vinyl ketone 3 at 100 °C; b) isolated yield after distillation; c) 2nd,
3rd , 4th and 5th run, respectively
Scheme 3 Selection of different -keto esters (Michael donors) and
enones (Michael acceptors) for the [C4mim]FeCl4-catalyzed Michael addi-
tion under thermal and microwave conditions.
entry 3). This higher yield with lower catalyst loading was
most likely caused by less product remaining in the ionic
liquid layer after distillation.
side reaction. In general, we observed comparable yields in
microwave irradiation; however, the reaction time could be
reduced significantly and shortened to 15–90 min depend-
ing on the reactivity of the starting materials. Various other
-keto esters were tested as Michael donors, and each gave
good to excellent yields.
The use of cyclohexenone as Michael acceptor instead of
methyl vinyl ketone required in general longer reaction
times but still good yields were achieved (Table 2, entry 8).
Acrolein was also suitable as Michael acceptor and gave an
excellent yield of 91% when reacted with keto ester 2. If
less reactive keto esters, e.g. ethyl 3-oxo-3-phenylpropano-
ate 7 were used in combination with acrolein as Michael
acceptor, the yield is dropping significantly which we think
is a result of polymerization of the α,β-unsaturated aldehyde
as a side reaction (entry 9, 10).
Since ionic liquids have proved to be excellent solvents
for the immobilization of transition metal catalysis, we were
particularly interested if the chloroferrate ionic liquid 1
could be recycled if used as catalyst itself without further
immobilization. After the product was isolated by distilla-
tion, the remaining black residue was directly subjected to
the next run without further work-up or purification. Fresh
starting materials were added and the reaction was success-
fully run again showing only a minor decrease in yield from
89% to 86% in the second run and 83% in the third one, re-
spectively (Table 1, entry 5, 6). However, conversion was
dropping significantly in further runs, which is probably caused
by the accumulation of polymerized methyl vinyl ketone 3
in the remaining ionic liquid catalyst (Table 1, entry 7, 8).
Apart from their application in catalysis, ionic liquids
have recently attracted considerable attention as solvents or
additives for microwave chemistry. Based on the inherent
ionic structure and high polarity that can be easily tuned
depending on the cation and anion, ionic liquids have been
successfully used as aids for microwave assisted heating of
non-polar solvents [24]. Ley et al. could significantly in-
crease the reaction rate and yields of microwave assisted
reactions run in toluene by adding a small quantity of an
ionic liquid to the reaction mixture [25]. Excessive studies
of ionic liquid mediated microwave heating of various or-
ganic solvents were done by Leadbeater et al. [26]. Thus,
we were particularly interested in running the ionic liquid
catalyzed Michael addition under microwave conditions:
Due to the highly polar nature of the iron containing ionic
liquid, microwave irradiation should occur directly in the
catalyst and thus lead to a strong acceleration of the reaction
rate compared to thermal heating.
Since different conditions under conventional heating
and microwave irradiation proved to be optimum for the
iron catalyzed Michael addition (oil bath: 100 °C; MW
125 °C) the yields could not be directly compared. However,
Table 2 Various Michael additions using 1 mol% of 1 as catalyst under
microwave conditions
Entry a)
Keto ester
Enone
Conditions b,c)
Yield d)
94%
89%
94%
71%
72%
79%
91%
88%
52%
48%
1
2
2
5
5
6
6
7
2
7
7
7
3
3
3
3
3
3
9
8
9
9
MW, 125 °C/15 min
MW, 125 °C/30 min
oil bath, 100 °C/24 h
MW, 125 °C/30 min
oil bath, 100 °C/24 h
MW, 125 °C/90 min
MW, 125 °C/90 min
MW, 125 °C/90 min
MW, 125°C/90 min
oil bath, 100 °C/24 h
3
4
5
6
7
8
Reactions were thus performed on a CEM Explorer PLS
microwave unit at a low power rate (50 W) to avoid initial
overheating. A reaction temperature of 125 °C (measured
via an external IR sensor) proved to be optimal for the mi-
crowave reaction, since elevated temperature lead in some
cases to partial polymerization of the Michael acceptor as
9
10
a) All reactions were performed in a CEM Explorer PLS microwave
unit using 5 mmol ketoester and 7 mmol methyl vinyl ketone; b)tempera-
ture control via external IR sensor; c) hold times; d) isolated yield after
flash column chromatography.

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Vasiloiu M, et al. Sci China Chem August (2012) Vol.55 No.8
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and microwave heating and to get insight in the influence of
microwave irradiation, the reaction between ketoester 2 and
methyl vinyl ketone 3 was further investigated via GC-MS
under similar conditions.
7
8
9
Samples with 1 mol% and 10 mol% catalyst loading
were irradiated at 125 °C (controlled via an external IR
sensor) and the conversion was monitored by GC-MS. The-
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subjected to conventional heating in a preheated oil bath
temperature at 125 °C. Under microwave irradiation (10 and
1 mol% catalyst, respectively) a strong acceleration of the
reaction rate was observed: After a reaction time of 1 min
using 1 mol% of [C4mim]FeCl4 1, the conversion in case of
thermal heating was only 1%, whereas 17% conversion
could be observed using microwave irradiation under simi-
lar conditions (closed vessel, 125 °C). The difference is
even bigger in case of 10 mol% catalyst: After 1 min a
conversion of 5% was obtained under thermal condition, in
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4 Conclusions
We have established a successful approach to iron catalyzed
Michael addition of various keto esters to unsaturated alde-
hyds and ketones using a chloroferrate ionic liquid. We
could demonstrate that the use of ionic liquids as catalysts
instead of solvent is superior and that the ionic liquid cata-
lyst can be easily recovered. While improvements for cata-
lyst recycling might be necessary, we could show that ionic
liquids could not only be used as additive or solvent under
microwave addition, but that the use of ionic liquids as cat-
alysts itself exhibits a strong acceleration of the reaction
rate. Further aspects including stereoselective Michael addi-
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