Iron-catalyzed cross-coupling reaction: Recyclable heterogeneous iron catalyst for selective olefination of aryl iodides in poly(ethylene glycol) medium

Article abstract of DOI:10.1039/c3gc36761d

An environmentally friendly iron-based catalyst supported on acac-functionalized silica was successfully prepared and evaluated as a heterogeneous catalyst for Mizoroki-Heck reaction of aryl iodides and olefins. Our catalytic system showed good activities that were comparable to that of palladium catalysts. The catalyst was simply recovered from the reaction mixture and recycled five times. Furthermore, the reaction was carried out in poly(ethylene glycol) as a green solvent. Interestingly, using this catalyst, aryl iodides were selectively olefinated in the presence of aryl bromides.

Full text of DOI:10.1039/c3gc36761d

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Iron-catalyzed cross-coupling reaction: recyclable
heterogeneous iron catalyst for selective olefination of
aryl iodides in poly(ethylene glycol) medium†
Cite this: DOI: 10.1039/c3gc36761d
a,b
b
Abdol Reza Hajipour* and Ghobad Azizi
An environmentally friendly iron-based catalyst supported on acac-functionalized silica was successfully
prepared and evaluated as a heterogeneous catalyst for Mizoroki–Heck reaction of aryl iodides and
olefins. Our catalytic system showed good activities that were comparable to that of palladium catalysts.
The catalyst was simply recovered from the reaction mixture and recycled five times. Furthermore, the
reaction was carried out in poly(ethylene glycol) as a green solvent. Interestingly, using this catalyst, aryl
iodides were selectively olefinated in the presence of aryl bromides.
Received 7th November 2012,
Accepted 10th February 2013
DOI: 10.1039/c3gc36761d
www.rsc.org/greenchem
side effects exist. Also, iron(III) catalysts are air and moisture
stable and easy to store for long periods under normal labora-
Introduction
The palladium-catalyzed Mizoroki–Heck cross-coupling reac- tory conditions.
tion represents one of the most valuable methods for carbon–
carbon bond formation in organic synthesis. Thus, a number derivatives,
of efficient catalytic systems have been developed for this reactions
In the field of C–C bond formation via Grignard
1
8–12
13,14
Suzuki–Miyaura
and
Sonogashira
1
5–19
a variety of works were accomplished and the
cross-coupling reaction. The catalyst in the standard Heck iron-catalyzed cross-coupling method is able to compete with
reaction is a Pd(0) species stabilized by two phosphine ligands. the palladium-catalysed one. However, to the best of our
However, phosphine ligands have some drawbacks; they are knowledge, only one work on iron catalyzed Heck cross-coup-
2
0
expensive, toxic, unrecoverable, and sensitive to oxygen and ling reaction has been reported and heterogeneous iron cata-
2
water. On the other hand, due to the high cost of palladium, lysts have never been used in Heck reaction.
2
0
researchers are also interested in the Heck reactions catalysed
The recent report by Vogel and co-workers on iron-cata-
3
,4
5,6
7
by inexpensive transition metals such as Ni, Co, Cu and lyzed Heck reaction urges us to present our own studies on this
4
Mn.
topic. They demonstrated a novel iron-catalysed arylation of styr-
(20 mol%) along with
Nevertheless, the stress of modern efficiency criteria has enes using a rather large amount of FeCl
2
t-
prompted the search for alternative catalysts that address the picolinic acid as ligand (80 mol%) and BuOK as base (4 mol%)
economic and ecological disadvantages associated with the in DMSO under mild conditions at 60 °C. The presence of
use of palladium, cobalt and nickel catalysts.
strong base resulted in the polymerization of acrylate esters.
In contrast to the above mentioned metals, iron is one of
Here, we report a simple, cost-effective, environmentally
the most abundant metals on earth, and one of the most inex- benign, recyclable, and efficient covalently silica supported
pensive and environmentally friendly ones. Iron catalysts have iron(III)-acac catalytic system for the Heck reaction of aromatic
recently been introduced to address these economic and eco- iodides and olefins (styrene and acrylates) under ligand-less
logical challenges. Many biological systems rely on the rich conditions (Scheme 1). In this work, only a catalytic amount of
chemistry of iron-containing enzymes. It is also a vital metal iron in poly(ethylene glycol) is used as a green solvent.
for life, and thus constitutes an attractive metal for a vast array
of chemical synthetic transformations as no severe toxicity and
a
Pharmaceutical Research Laboratory, Department of Chemistry, Isfahan University
of Technology, Isfahan 84156, IR Iran. E-mail: haji@cc.iut.ac.ir;
Fax: +98 311 391 2350; Tel: +98 311 391 3262
b
Department of Neuroscience, University of Wisconsin, Medical School,
1
†
300 University Avenue, Madison, 53706-1532 WI, USA
Electronic supplementary information (ESI)
0.1039/c3gc36761d
available. See DOI:
1
Scheme 1 Iron catalysed Heck reaction.
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3
Fig. 1 FT-IR spectrum of catalyst compared with Fe(acac) .
Scheme 2 Preparation of silica supported iron catalyst.
confirms the presence of highly dispersed iron deposited on
the silica matrix.
Results and discussion
In the initial attempts to improve this iron-catalyzed Heck
The synthesis of the silica-acac was performed in three steps cross-coupling reaction, iodobenzene was chosen as a test sub-
using grafting-methodology (Scheme 2). First, the reaction of strate to optimize the reaction conditions. A DMF solution of
3
-chloropropyl trimethoxysilane and NaI was carried out in iodobenzene was allowed to react with methyl acrylate (MA)
acetone. In the second step, 3-iodopropyl trimethoxysilane (1.1 equivalents) at 130 °C in the presence of a catalytic
reacts with silica gel in dry toluene. Then, modified silica amount of iron catalyst (10 mg catalyst, 6 × 10− mmol Fe) and
treated with acacH and K CO as base activator in order to 1.5 equivalents of NaHCO . After warming at 130 °C for 3 h,
4
2
3
3
obtain the silica-acac support.
the reaction afforded the cross-coupling product with complete
Iron catalyst formed when silica-acac was mixed with a conversion of the iodobenzene.
FeCl ·6H O aqueous solution and extensively washed until no To get information on the optimal catalyst conditions, we
more iron leaching was observed. The resulting dark orange carried out intensive investigations to define the best solvent
3
2
III
solid was assigned as silica-Fe (acac).
for this transformation. Seven different solvents were tested for
The presence of the organic phase in the silica-acac was 3 h using 10 mg catalyst. Table 1 shows the data collected for
confirmed by FT-IR spectroscopy, XPS and CHN elemental the synthesis of methyl cinnamate as a model reaction. Among
analysis. In the FT-IR spectrum shown in Fig. 1, bands at 1569, the different solvents screened, in some solvents such as
−
1
1
523, 1422, 1362 and 661 cm correspond to the diketone toluene, dioxane and ethanol, only low conversions were
ligand coordinated to iron. Other bands at 1273, 1188, 1021, observed (Table 1, entries 1, 4 and 6). DMF was shown to be
1
9
28, and 801 cm− covered with strong and broad bands of the best solvent for this transformation.
silica support. The inorganic silica component can also be Our initial goal was to use a green media for the Heck reac-
identified by the presence of the typical silica overtone bands tion. Fortunately, the reaction takes place in excellent conver-
of 1864 cm−
The presence of the organic phase in the silica-acac support PEG instead of DMF as the solvent.
was also confirmed by elemental analysis. The total amount of The nature of the base is also crucial to the Heck reaction.
carbon was 77 mmol g . The metal immobilization was con- When NEt , NEt (OH) or NaOAc were used as base (1.5 equiva-
firmed by the EDX and XPS analyses (see ESI†). lents) instead of NaHCO , the formation of lower amount of
In the XRD spectrum of the catalyst (see ESI†), a broad the desired product was detected on GC, but K CO afforded a
(Table 1, entries 8 and 9). The
1
.
sion in poly(ethylene glycol) (PEG) as solvent. Thus we choose
−1
3
4
3
2
3
maximum is observed instead of typical Fe(acac)
3 3
peaks. This higher yield than NaHCO
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Table 1 Optimization of reaction conditions
Table 2 Scope of iodides in iron-catalyzed coupling reaction with olefins
a,b
Entry Solvent
Base (equiv.)
Catalyst (mg) Conversion (%)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
Dioxane NaHCO
3
3
3
3
3
3
3
(1.5) 10
(1.5) 10
(1.5) 10
(1.5) 10
(1.5) 10
(1.5) 10
(1.5) 10
1.2
DMAc
DMF
EtOH
NMP
NaHCO
NaHCO
NaHCO
NaHCO
33.3
99.5
4.0
54.0
5.0
92.4
95.0
99.0
92.4
68
79.3
58.0
99.0
99.5
Entry
R′
Olefin
MA
Time (h)
Yield
Toluene NaHCO
1
H
1.5
1.5
1.5
2
2
3
85
85
85
84
80
77
80
83
77
79
80
80
82
85
80
82
79
69
76
78
70
77
81
79
82
80
72
PEG
PEG
PEG
PEG
PEG
PEG
PEG
PEG
PEG
NaHCO
Run 2
Run 3
Run 4
Run 5
K
K
2
CO
CO
3
(1.5)
(2)
10
10
10
10
10
5
2
3
0
1
2
3
5
6
NaOAc (1.5)
Et
Et
3
NOH (1.5)
N (1.5)
Run 6
3
2
3
4
5
6
7
8
9
1
1
1
1
4-NO
3-NO
4-OCH
4-CH
3-CH
4-Br
4-Cl
4-CN
H
4-NO
3-NO
4-OCH
4-CH
3-CH
4-Br
4-Cl
4-CN
H
2
MA
MA
MA
MA
MA
MA
MA
MA
Styrene
Styrene
Styrene
Styrene
Styrene
Styrene
Styrene
Styrene
Styrene
MMA
MMA
MMA
MMA
1
K
K
K
2
CO
3
3
3
(2)
(2)
(2)
CO
CO
15
25
2
1.2
2.5
2.5
2.5
2
2
1.5
7
5
5
6
5
5
5
5
5
6
6
6
6
2
3
2
3
a
b
GC conversion. Reaction conditions: iodobenzene (0.25 mmol,
3
30 μL), MA (1.2 equiv.), base, solvent (1.5 mL), 130 °C, 1.5 h.
0
1
2
3
2
optimal amount of K
aryl halide (entry 9). Another experiment was set up to deter-
mine the optimal amount of catalyst. A control experiment in 14
the absence of iron catalyst confirmed the crucial role which
iron catalyst played in the described reaction as the reaction
did not occur. A 25 mg catalyst (1.5 × 10 mmol Fe) resulted 18
in the complete conversion in 1.5 hours at 130 °C (Table 1,
entry 16).
2 3
CO was 2 equivalents with respect to
2
3
3
1
1
1
5
6
7
3
−
3
1
2
2
9
0
1
4-NO
4-CN
4-CH
2
We next examined the scope and limitations of this iron- 22
3
catalyzed cross-coupling reaction with various types of aryliodo
2
1
derivatives and olefins (Table 2). All reactions were carried
out using 25 mg catalyst in PEG-200 at 130 °C in the presence
2 3
of 2 equivalents of K CO .
In order to investigate the effect of substituent groups on
iodobenzene, various substituted iodobenzenes were exam-
ined. The cross-coupling reaction of iodoaryl derivative was
amenable to both electron-rich and electron-poor iodoaryl
derivatives and the reaction gave the corresponding coupling
compounds in good to excellent yields. As Vogel et al.
2
0
reported, 4-iodonitrobenzene is destroyed in the presence
of FeCl in the Heck reaction. It is noteworthy that the
2
cross-coupling reaction works successfully with 4-iodonitro-
benzene under our reaction conditions (Table 2, entries 2, 3,
Scheme 3 Study of selectivity between iodo and bromoarenes.
1
1 and 12). Methyl methacrylate and styrene had longer reac-
tion times under our reaction conditions (Table 2, entries first reaction shown in Scheme 3, iodobenzene was the only
0–22).
reagent participating in reaction and bromobenzene was fully
It should also be noted that aryl iodides can be selectively recovered after the purification. On the other hand, in the
1
olefinated in the presence of aryl bromides. Interestingly, the second reaction, 1-bromo-4-iodobenzene was converted to
Heck reaction, in many cases, shows selectivity for aryl bro- methyl-4-bromocinnamate as the only product. The reusability
mides and aryl iodides in the presence of aryl chlorides. of the catalyst was tested using iodobenzene and MA as the
However, to date, no selectivity has been reported between aryl substrates. After each run, the catalyst was recovered by decan-
bromides and aryl iodides.
tation, followed by washing with acetone (3 mL × 3). After
In this work, the competing reactions of bromobenzene drying, the catalyst was reused directly for the next run. The
and iodobenzene with methyl acrylate were studied to evaluate results for the five repeated runs are also presented in Table 2.
the selectivity of our catalyst in the Heck reaction. Another sub- The activity of the catalyst remained unchanged after it was
strate which contains both iodo and bromo substituents, 1- reused five times, indicating that the catalyst was not only very
bromo-4-iodobenzene, was also tested in this reaction. In the active, but also very stable.
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thiocyanate solution. The solid was dried at 80 °C for 5 h. The
iron content of the catalyst was 6 × 10− mmol g based on
2
−1
atomic absorption spectroscopy.
General experimental procedure for the cross coupling
reactions
A 5 mL vial equipped with a stirrer bar was charged with the
requisite aryl halide (0.25 mmol), K
2 3
CO (0.5 mmol), methyl
acrylate (0.3 mmol), 0.01 g of SiO -Fe(acac) catalyst and 1.5 mL
2
PEG-200. The reaction mixture was heated at 130 °C. The pro-
gress of the reaction was monitored by GC (small sample of
reaction mixture was extracted by hexane). After completion of
the reaction, the reaction mixture was extracted with hexane.
The combined organic layers were dried with anhydrous
Scheme 4 Possible mechanism for vinylation of aryl iodides in the presence of
Na
crude product, which was purified by column chromatography
hexane : EtOAc, 75 : 25) to yield the expected product.
2 4
SO . The solvent was evaporated under vacuum to give the
iron catalyst.
(
The mechanism of the vinylation of iodobenzenes with sup-
ported iron(III) catalyst in PEG solvent with Na CO is pro-
2 3
posed, which is shown in Scheme 4. At the first step, we
believe that the reaction started with the oxidation of alcoholic
Conclusions
groups of PEG by iron complex to form reduced iron(I) species, We have demonstrated that silica supported iron(III) complex is
followed by an oxidative addition of aryl iodide. Thereafter, the an efficient and recyclable catalyst for the coupling of aryl
reaction continues to go according to the accepted mechanism iodides and olefins in the presence of PEG as solvent. The iron
of the Heck reaction.
catalyst is highly active and easily recyclable by a simple fil-
tration. This reaction provides a novel environmentally friendly
and economical route for the Heck reaction.
Experimental
Synthesis of silica-acac support
Notes and references
(
3-Chloropropyl) trimethoxysilane (5 mmol) was added to a
solution of anhydrous sodium iodide (5 mmol) in dry acetone
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4
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