A biomimetic iron catalyst for the epoxidation of olefins with molecular oxygen at room temperature

Article abstract of DOI:10.1002/anie.201004623

It's no sacrifice: A bio-inspired iron system, in which a β-keto ester serves as a sacrificial cosubstrate, readily epoxidizes olefins under ambient conditions with air. Aromatic olefins are oxidized in high yields with excellent chemoselectivity. Mechanistic investigations point out substantial differences to well-known radical-based autoxidations.

Full text of DOI:10.1002/anie.201004623

DOI: 10.1002/anie.201004623
Catalytic Oxidation
A Biomimetic Iron Catalyst for the Epoxidation of Olefins with
Molecular Oxygen at Room Temperature**
Kristin Schrꢀder, Benoꢁt Join, Arlin Jose Amali, Kathrin Junge, Xavi Ribas, Miquel Costas,* and
Matthias Beller*
[
17]
The selective oxidation of organic compounds remains a
challenging task fundamentally important to life, the environ-
ment, and synthetic chemistry. Requirements for modern
oxidation methods include cheap and selective catalysts, high
atom efficiency, and environmentally friendly oxidants. In this
context, molecular oxygen, especially in form of air, is the
hydroxylation of b-keto esters,
we herein report the
selective iron-catalyzed oxidation of olefins to epoxides
using air as the oxidant. A bio-inspired b-keto ester is
employed as a sacrificial co-substrate, and oxygen transfer
occurs highly selectively at room temperature. Notably, the
mechanism of this type of O activation is similar to that
occurring in co-substrate-dependent non-heme iron oxygen-
2
[1]
most desirable oxidant. However, efficient nonradical
[
18]
transformations using O are scarce, and novel catalysts that
ases.
2
can activate O under mild conditions for the direct and
While investigating the hydroxylation of b-keto esters
under air, we observed the partial epoxidation of trans-
stilbene in the presence of a combination of iron chloride,
imidazole ligands, and ethyl 2-oxocyclohexanecarboxylate.
As shown in Table 1, all three components of the catalyst
system (Fe/imidazole/b-keto ester) are needed for successful
2
[
2]
[17]
selective oxidation of organic substrates are still needed. In
this respect iron catalysts are particularly attractive because
of the ready availability, low cost, and low toxicity of this
metal. Significant progress has been made towards the
[
3]
application of easy-to-use iron catalysis in the last decade.
[4]
Various methods such as cross-coupling reactions, sulfide
oxidations, allylic alkylations and aminations, alcohol
oxidations, and Michael additions have been successfully
[
5]
[6]
[
7]
[8]
[a]
Table 1: Iron-catalyzed epoxidation of stilbene with air.
developed. Known iron catalysts for O activation include Gif
2
[
9]
systems, tris(2-(acetoacetoxy)ethyl methacrylate)iron com-
plexes with aldehydes as co-substrates,
[10]
macrocyclic iron
[
11]
complexes,
and catecholate mimics, which are able to
[
12]
catalyze cleavage of catechols. The main drawbacks of such
catalysts are their low selectivity, temperature dependence,
and high tendency to react by radical-type autoxidation in the
absence of the metal catalyst. In contrast, based on iron-
catalyzed oxidations of alcohols or olefins to aldehydes,
[
13]
Entry
Metal salt
1
2
Conv.
[%]
Yield
[%]
[
b]
[b]
(equiv)
(equiv)
[14]
[
15]
[16]
[c]
1
FeCl ·6H O
FeCl ·6H O
3 2
0.8
0.8
–
2
3
3
2
2
0.5
2
2
2
2
2
2
2
2
0.2
0.2
0.5
–
1
0.5
1
34
45
1
33
93
55
64
91
21
1
1
0
1
3
1
1
64
26
35
0
arenes to quinones,
and olefins to epoxides,
and the
3
2
2
3
4
5
6
7
8
9
FeCl ·6H O
3
2
[*] Dr. K. Schrꢀder, Dr. B. Join, A. J. Amali, Dr. K. Junge,
FeCl ·6H O
2
3
2
Prof. Dr. M. Beller
FeCl ·6H O
92
51
59
81
16
0
1
0
0
1
3
2
Leibniz-Institut fꢁr Katalyse e.V. an der Universitꢂt Rostock
Albert-Einstein-Strasse 29a, 18059 Rostock (Germany)
E-mail: matthias.beller@catalysis.de
FeCl ·6H O
3
2
FeCl ·6H O
3
2
FeCl ·6H O
0.5
1
3
2
Homepage: www.catalysis.de
FeCl ·6H O
3
2
A. J. Amali
Current address:
Nanomaterials Laboratory, Inorganic & Physical Chemistry Division
Indian Institute of Chemical Technology, Hyderabad (India)
10
11
12
13
–
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
FeCl ·4H O
2
2
NiCl ·4H O
2
2
CoCl ·6H O
2
2
Dr. X. Ribas, Dr. M. Costas
14
15
16
17
CrCl ·H O
3
2
Departament de Quimica, Universitat de Girona
Campus de Montilivi, 17071, Girona (Spain)
E-mail: miquel.costas@udg.edu
RuCl ·6H O
3
2
1
0
28
[
d]
CuCl ·2H O
2
2
MnCl2
[
**] K.S. appreciates financial support provided by the Graduiertenkol-
leg 1213 and the Max-Buchner-Forschungsstiftung. B.J. thanks the
Alexander-von-Humboldt-Stiftung. We thank Dr. S. Enthaler for X-
ray structure analysis, Dr. D. Gꢀrdes for ESI-MS measurements, and
M. Heyken for technical assistance. M.C. thanks MICINN for
project CTQ2009-08464/BQU, Generalitat de Catalunya for an
ICREA Academia Award, and UdG STR for experimental support.
[
(
1
a] Reaction conditions: In a 15 mL reaction tube, the metal source
0.025 mmol), 2 (x equiv), CH CN (10 mL), trans-stilbene (0.5 mmol=
3
equiv), and biphenyl (GC internal standard, ca. 20 mg) were added in
sequence at room temperature in air. The reaction mixture was stirred
and the reaction was initiated by addition of the sacrificial reductant 1
(
x equiv). [b] Conversion and yield were determined by GC analysis.
[c] tert-Amyl alcohol was used as solvent. [d] When bulkier imidazole
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201004623.
ligands like 1-benzylimidazole were used, low amounts of product were
detected (28% yield with 85% selectivity).
Angew. Chem. Int. Ed. 2011, 50, 1425 –1429
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1425

Communications
epoxidation (Table 1, entries 3, 4, and 10). Increasing 1 to
three equivalents with respect to trans-stilbene resulted in
3% conversion with excellent selectivity and 92% yield at
Table 2: Scope and limitations in the epoxidation of olefins with
oxygen.
[
a]
9
room temperature (Table 1, entry 5).
II
Because Iqbal and co-workers reported the use of Co
Entry
Substrate
Conv.
[%]
Yield
[%]
Sel.
[%]
complexes for the hydroxylation of methyl 2-oxocyclopenta-
necarboxylate with concomitant epoxidation of olefins, we
also tested different metal chlorides under our conditions
[
b]
[b]
[c]
[19]
1
2
3
91
28
73
81
89
97
69
[
d]
(
Table 1, entries 11–17). However, no epoxidation was
27
observed in the presence of Ni, Co, Cr, Cu, or Ru chlorides.
50
35
63
Only for MnCl unselective activity was detected. Apparently
2
4
5
48
78
72
81
the selective epoxidation reaction with air is specific for
FeCl ·6H O, an observation which a priori seems incompat-
3
2
ible with a metal-initiated radical autoxidation. The specific
nature of the catalyst system was further studied by variation
of nine structurally related b-keto esters. However, only in the
presence of ethyl 2-oxocyclopentanecarboxylate significant
activity (conversion > 30%) was observed (Table S2). Next,
6
7
89
93
69
82
78
88
1
4 different amine bases were tested. Although a number of
imidazole ligands—except for 2-substituted derivatives—
showed activity in the epoxidation reaction, the best results
were obtained in the presence of inexpensive imidazole 2
8
9
89
(81)
91
(
Table S3).
In Table 2 the general applicability of the room-temper-
ature epoxidation in the presence of air is demonstrated.
trans-Stilbene derivatives showed high yields and excellent
chemoselectivity (Table 2, entries 1 and 6–9); these results are
even better than those with the best previously described
iron-based systems with hydrogen peroxide as the terminal
99
57
96 (84)
97
52
1
0
30
24
[
16]
oxidant. Apart from the desired product, traces of benzal-
dehyde were detected. Aliphatic olefins were also catalyti-
cally epoxidized; however, these reactions generally pro-
ceeded in lower yields with selectivities above 65% (Table 2,
entries 4, 11, and 13). cis Aromatic olefins such as cis-stilbene
and cis-b-methylstyrene (Table 2, entries 2 and 12) were
epoxidized to give the corresponding trans epoxides. This
indicates that the oxygen transfer does not proceed in a
concerted manner and a long-lived radical or carbocationic
intermediate is apparently created before the second CÀO
1
1
1
2
32
40
75
94
[
d]
37
1
3
66
44
66
[a] Reaction conditions: In
a
15 mL reaction tube, FeCl
·6H O
3 2
(
(
0.025 mmol), imidazole 2 (0.25 mmol), CH CN (10 mL), substrate
0.5 mmol) and biphenyl (GC internal standard, ca. 20 mg) were added
3
epoxide bond is formed.
in sequence at room temperature in air. The reaction mixture was stirred
for 20 h and the reaction was initiated by addition of the sacrificial
reductant 1 (1 mmol) by syringe. [b] Conversion and yield were
determined by GC analysis; yields of isolated products are given in
brackets. [c] Selectivity refers to the ratio of yield to conversion in
percentage; [d] The corresponding cis-stilbene and cis-methyl styrene
oxide could not be observed; instead trans-stilbene oxide and trans-
methyl oxide were detected.
To elucidate whether an autoxidation pathway is respon-
sible for this cis-to-trans stereoscrambling, the oxidation of
a-pinene was studied (Table 2, entry 13). It is well known that
radical oxidations, which proceed under thermal autoxidation
conditions, yield mainly allylic oxidation products with this
substrate and only small amounts of the corresponding
[20]
epoxides.
However, with our system minimal amounts
around 2%) of allylic oxidation products were detected
verbenone; 4,6,6-trimethylbicyclo[3.1.1]hept-3-en-2-one)
(
(
and the main product was a-pinene oxide (44% yield, 66%
selectivity). The yields and selectivities exhibited by the
present iron system are significantly higher than those of
reactions catalyzed by cobalt(II) salen complexes, which
Interestingly, the catalyst system is not deactivated after
the reaction. Indeed, after a reaction time of 20 hours,
complete reactivation of the iron catalyst was observed
upon addition of another charge of trans-stilbene
(0.5 mmol) and 2 equiv of keto ester 1. Noteworthy, the
excellent chemoselectivity remained.
To obtain more insight into the molecular nature of the
catalyst, mass spectroscopic measurements were carried out.
The ESI mass spectrum of a freshly prepared reaction mixture
[
21]
operate via free diffusion radicals. Hence, we exclude a
metal-initiated radical autoxidation. In agreement with this
interpretation, ethylbenzene and tetrahydronaphthalene,
which contain weak CÀH bonds and are prone to radical
[
22]
pathways, were not oxidized by our system.
(
5 mol% FeCl ·6H O/2 equiv 1/0.5 equiv imidazol/1 equiv
3 2
1
426
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 1425 –1429

trans-stilbene) under an air atmosphere in acetonitrile
showed in positive-ion mode a series of cluster ions at
m/z 366.0760, 434.1137, and 522.1554, which are assigned to
were observed in any of the reactions studied, and the
conversion of 1 after 24 h was independent of the nature of
the substrate (within experimental error). For the initial
stages of the reaction (conversion of 1 < 50%), 1 was
consumed at similar reaction rates for all three substrates
(Table 3, Figure S2). This strongly suggests that a common
reaction intermediate is formed regardless of the substrate.
Furthermore, the formation of trans-stilbene epoxide oc-
curred at the same rate as the consumption of 1 (Figure 1).
III
+
III
+
III
+
[
Fe (1ÀH) ] , [Fe (1ÀH) (2)] and [Fe (1ÀH) ] + H ,
2
2
3
respectively. In addition, when the experiment is performed
in negative-ion mode, a cluster ion assigned to [FeCl ] is also
observed at m/z 197.8. [FeCl ] has been reported as a kind of
inactive iron-sink complex for Michael additions.
À
4
À
4
[8b,23]
Along
this line, addition of 5 mol% HCl, NaCl, or Bu NCl to our
4
reaction system led to a distinct decrease of activity and
product yield (Table S4 in the Supporting Information).
Therefore, we conclude that inactive [FeCl4] complexes
reduce the iron pool for the formation of active species in our
reaction.
Table 3: Rate constants for the pseudo-first-order oxidation reaction of 1
as a function of the olefinic substrate.
À
Next, the catalytic activity of well-defined isolated com-
[16a]
plexes was investigated. When trans-[FeCl (2) ]Cl
was
2
4
À5 À1
2
Substrate
k1 [10
s
]
R
applied under reaction conditions, only poor reactivity was
observed (< 5% conversion), demonstrating that this com-
plex is not an active precatalyst. Unfortunately, all efforts to
trans-stilbene
styrene
2.88Æ0.13
3.27Æ0.13
2.75Æ0.11
1.83Æ0.09
0.986
0.989
0.990
0.983
1
–
-octene
prepare [Fe(1) ] failed. Nevertheless, a related catalytically
3
active complex [Fe(2-acetylcyclopentanone) ] could be pre-
3
pared and structurally characterized by X-ray diffraction
(
Figure S1). In accordance with the required balance of
imidazole and keto ester 1 (1:3 to 1:4, see Table 1), the latter
complex was studied in the presence of additional 2-acetyl-
cyclopentanone (Table S5), but only poor activity was
observed. Based on these results, and the ESI-MS analysis,
we propose that the active species corresponds to
III
+
III
+
[
Fe (1ÀH) ] , and [Fe (1ÀH) (2)] , and the latter appears
2
2
as the most likely catalyst candidate because of the require-
ment of imidazole ligands under the catalytic conditions.
To shed more light on the reaction mechanism, we studied
the products that arose from the conversion of the sacrificial
reductant 1 during the epoxidation reaction. Interestingly,
both the hydroxylated keto ester 3 and the ring-opened
product 4 are formed (Scheme 1). Alcohol 3 is formed by two-
Figure 1. Consumption of 1 compared to the yield of trans-stilbene
k1
oxide with a single-exponential fit y=aÀbc .
[
17]
electron oxidation of the keto ester.
In contrast, the
A more detailed analysis showed that the ratio of 3 to
trans-stilbene epoxide is constant over the course of the
reaction (Figure S3). Additional NMR experiments after
filtration of the crude mixture showed a ratio of about 1:1
for the formed epoxide and 3. In addition, a 50–60% yield of 4
was recorded (with respect to stilbene). This proves the role
of 1 as a sacrificial reductant and shows that the conversion of
1
is closely connected with the selective formation of trans-
Scheme 1. Main products resulting from the oxidation of keto ester 1.
stilbene epoxide (Figure 1).
Similar results are observed for styrene as substrate. trans-
Stilbene and styrene react at comparable rates, but in the
latter case the reaction proceeded with lower chemoselectiv-
ity towards the epoxide. Investigations in the ratio of products
3 and 4, which arise from two- and four-electron oxidations of
the co-substrate, respectively, could be used to estimate the
relative rates of these two reactions. GC analysis showed that
the ratio 4/3 is substrate independent and decreased during
the reaction time. Apparently, the different oxidations of 1 are
parallel pathways.
formation of the carboxylic acid 4 formally corresponds to a
dioxygenase type of reaction, where two oxygen atoms are
inserted into the b-keto ester, in a formal four-electron
oxidation. Indeed, the latter reaction bears strong resem-
blance to that mediated by acetylacetone dioxygenase
(
Dke1), an iron-dependent enzyme that cleaves various
[25]
diketones and b-keto esters.
Next, time-dependent oxidations of trans-stilbene, sty-
rene, and 1-octene were studied. The consumption of 1 in the
absence of substrate was also analyzed. No induction periods
The origin of the oxygen in the products was studied by
means of isotopic-labeling experiments under an inert
Angew. Chem. Int. Ed. 2011, 50, 1425 –1429
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
1427

Communications
atmosphere of argon with trans-stilbene. As expected, no
Notably, related Fe complexes play an important role in
reaction is observed under these conditions. Afterwards
labeling experiments were carried out in the presence of
oxygen and H2 O. When 20 equiv H2 O (with respect to
trans-stilbene) was added, labeled oxygen was not incorpo-
rated into the product and there was insignificant influence on
the yield and conversion (Scheme 2).
the O activation of acetylacetone dioxygenase (Dke1), an
2
iron-dependent enzyme that cleaves various diketones and
1
8
18
[25]
b-keto esters (Scheme 4). For this enzyme it is known that
2,4-pentadione participates in the O activation by forming an
2
organic peroxide species; however, there is an ongoing debate
on whether the formation of this intermediate takes place
[
26]
through a concerted step or via an iron(III) superoxido
[
27]
complex.
Scheme 4. Cleavage of 2,4-pentadione into methylglyoxal and acetate
by the enzyme Dke1.
1
8
Scheme 2. Labeling studies with H2 O.
On the other hand, while trans-stilbene oxide did not show
any incorporation of labeled oxygen, small amounts of
benzaldehyde (< 5%), which is formed as cleavage product,
displayed incorporation of the labeled oxygen (roughly 5–
Based on these precedents, we envision an iron super-
oxide as the active species in our system. Subsequent radical-
type addition to the olefin leads to an intermediate suscep-
tible to epimerization. Finally, the second CÀO bond forms
and the OÀO bond is cleaved to form the corresponding
1
8
1
0%). O incorporation (approximately 30%) from water
was also observed in the carboxylic acid 4 but not in the
hydroxylated product 3. Since oxygen exchange between
epoxide and 3. Alternatively, along a parallel reaction path,
an oxetane species is formed that gives 4 as the main product.
Stilbenes are unique substrates because there is an excellent
correlation between two-electron oxidation of the co-sub-
strate and formation of the epoxide. This epoxidation
constitutes a unique example of co-substrate-assisted activa-
[
24]
water and a-keto acids is known, we added labeled water
after the reaction was complete (20 h) and then stirred the
reaction mixture for an additional 16 h. Indeed incorporation
1
8
of comparable amounts of O-labeled oxygen into benzalde-
1
8
hyde and 4 was observed together with traces of O-labeled 3
tion of O in a monooxygenase type of reaction.
2
1
8
(
< 5%). Therefore, we conclude that the incorporation of O
In conclusion, we have developed a novel and practical
synthetic method for the epoxidation of olefins with air as the
oxidant and with fair to excellent chemoselectivities under
very mild conditions. Notably, the catalyst can be easily
reactivated. Mechanistic investigations point out substantial
differences to known radical-based autoxidation paths. The
atoms in benzaldehyde and carboxylic acid 4 occurred after
the oxidation event. Clearly, the oxygen atoms incorporated
into the epoxide and 3 or in 4 are derived from O2.
Based on all these investigations our proposed mechanism
is shown in Scheme 3. Despite the inherent difficulty of
analyzing complex reaction mixtures, the results of the ESI-
MS experiments in combination with catalytic experiments
oxygenase-like co-substrate-assisted activation of O gener-
2
ates the active iron species responsible for the epoxidation.
The present methodology exploits a highly challenging four-
III
+
suggest an active [Fe (1-H) (2)] catalyst species. The
2
hydroxylation of 1 and the epoxidation reaction are closely
connected, and the rate of conversion of 1 is independent of
the nature of the olefinic substrate. These observations
provide convincing evidence that 1 acts as a co-substrate,
electron reduction of the O molecule, and by doing so it
differs from the most commonly used peroxides (2e reduced
2
À
versions of O ). Thus, it constitutes a unique biomimetic
2
catalyst system and opens up attractive alternatives for bio-
inspired catalysis.
strictly required for selective O activation, generating the
2
active species that epoxidize the olefin.
Received: July 27, 2010
Published online: January 11, 2011
Keywords: b-keto esters · epoxidation · imidazole · iron · oxygen
.
[
[
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