Epoxidation of olefins catalyzed by a molecular iron n-heterocyclic carbene complex: Influence of reaction parameters on the catalytic activity

Article abstract of DOI:10.1002/cctc.201402063

The catalytic epoxidation of olefins by an iron(II) complex bearing a tetradentate bis(pyridyl-N-heterocyclic carbene) ligand was investigated. This is the first example of the use of an organometallic iron compound (i.e., with a Fe-C bond) as an olefin epoxidation catalyst. The catalyst system, used without additives, showed good epoxide yields and selectivity for various olefins after a reaction time of 5 min. It was found that the epoxide yield strongly depended on the amount of the peroxide used and its nature and noticeably increased at lower temperatures.

Full text of DOI:10.1002/cctc.201402063

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DOI: 10.1002/cctc.201402063
Epoxidation of Olefins Catalyzed by a Molecular Iron
N-Heterocyclic Carbene Complex: Influence of Reaction
Parameters on the Catalytic Activity
[
a]
Jens W. Kꢀck, Andreas Raba, Iulius I. E. Markovits, Mirza Cokoja, and Fritz E. Kꢀhn*
The catalytic epoxidation of olefins by an iron(II) complex bear-
ing a tetradentate bis(pyridyl-N-heterocyclic carbene) ligand
was investigated. This is the first example of the use of an or-
ganometallic iron compound (i.e., with a FeÀC bond) as an
olefin epoxidation catalyst. The catalyst system, used without
additives, showed good epoxide yields and selectivity for vari-
ous olefins after a reaction time of 5 min. It was found that the
epoxide yield strongly depended on the amount of the perox-
ide used and its nature and noticeably increased at lower
temperatures.
ligands such as bpmpn [N,N’-bis(2-pyridylmethyl)-1,3-diamino-
propane] were later used by Que et al. to elucidate the struc-
ture–reactivity relationship of aminopyridine ligands in epoxi-
[13]
dation catalysis. In recent catalytic studies, other aminopyri-
dine ligands based on bipiperidine and bipyrrolidine motifs
were used with potential applications in asymmetric epoxida-
[14,18,19]
tion catalysis.
The groups of Valentine, Nam, and Que analyzed the mecha-
nism of iron(II)-catalyzed epoxidations with hydrogen perox-
[
11,13,17,20,21]
ide.
oxo complexes took place.
tude of different reaction pathways for olefin epoxidation and
In all cases, the formation of iron(III)–hydroper-
[11,22]
From this intermediate, a multi-
[20,23]
Epoxidation reactions are important transformations of olefins
with relevance for both bulk chemical synthesis and the pro-
dihydroxylation is possible.
A key factor in the mechanism
of these conversions is the geometry of the free coordination
sites in the ligand sphere. Valentine and Que’s research
showed that a cis conformation of the free bonding sites was
of utmost importance to form an iron(V)–oxohydroxo inter-
[
1,2]
duction of fine chemicals and pharmaceuticals.
Although
the need for
[
3,4]
a plethora of epoxidation catalysts is known,
cheaper and more environmentally friendly epoxidation reac-
tions prevails. The synthesis of iron-based coordination com-
mediate necessary for dihydroxylation. In contrast, free trans
[
5]
[11–13,24]
pounds inspired by nature is a promising approach. In bio-
logical systems, enzymes containing active iron sites oxidize
a broad range of substrates, including methane. Typical exam-
coordination sites favored epoxidation reactions.
Most
catalysts with cis-labile coordination require acetic acid to
[
14,16]
work efficiently,
and very low epoxide yields are usually
[
6]
[25–27]
ples are cytochrome P450, soluble methane monooxyge-
obtained under acid-free conditions.
[
7]
[8]
nase, and Rieske dioxygenases. Mimicking these biological
systems, molecular iron(II) complexes typically involve porphy-
Recent mechanistic investigations showed the formation of
a cis-bidentate ferric peracetate complex from a catalyst bear-
ing cis-labile ligands that went on to form the active iron(V)–
oxo oxidant in high yields, which thus explained the increased
[
9,10]
[11–17]
rin ligand motifs
or non-heme N-donor ligands.
Valen-
tine et al. was the first to report the epoxidation of olefins with
[24,28]
an iron(II) cyclam complex (cyclam=1,4,8,11-tetraazacyclotetra-
epoxide yields.
This effect was not so pronounced for
[11]
decane) with very good selectivities towards epoxides. Other
complexes with trans coordination of the solvent molecules,
which are unable to form the cis-coordinating ferric
peracetate ligand, but they did exhibit good selectivi-
ties towards epoxidation without acid additives. Ac-
cordingly, new trans-labile ligand systems may help
to make iron complexes feasible alternatives to well-
established epoxidation catalysts, such as methyl-
[
29]
[30]
trioxorhenium, Jacobsen’s catalyst, and molybde-
[
31]
num catalysts.
In this work, the catalytic activity of an iron(II) com-
plex coordinated by the tetradentate di(o-imidazol-2-
ylidenepyridine)methane ligand (in the following de-
2 2
Scheme 1. Catalytic epoxidation of olefins by using H O as the oxidant and the iron(II)
complex [Fe(NCCN )(MeCN) ](PF ) (1) as the catalyst.
2 6 2
Me
Me [32]
noted as NCCN ) for the epoxidation of olefins is
reported (Scheme 1). To the best of our knowledge, complex
is the first organometallic iron complex (i.e., with a direct
1
[
a] J. W. Kꢀck, A. Raba, I. I. E. Markovits, Dr. M. Cokoja, Prof. Dr. F. E. Kꢀhn
Chair of Inorganic Chemistry/Molecular Catalysis
Catalysis Research Center
Technische Universitꢁt Mꢀnchen
Ernst-Otto-Fischer Strasse 1, D-85747 Garching bei Mꢀnchen (Germany)
Fax: (+49)89-289-13473
ironÀcarbon bond) to be used as an epoxidation catalyst. Com-
plex 1 is stable towards air and water, which is remarkable
considering the relative thermodynamic lability of FeÀC com-
[26]
pared to FeÀO bonds, and this renders this system a good
E-mail: fritz.kuehn@ch.tum.de
candidate for epoxidation reactions.
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First investigations towards the oxidation ability of complex
agent. Whereas aqueous hydrogen peroxide gave the highest
conversions of 92% with 2 mol% of 1, the use of other oxidiz-
ing agents resulted in significantly lower conversions and 4 to
14% yield. Higher conversions at longer reaction times were
only observed for oxidants other than aqueous hydrogen per-
oxide. With the use of these other oxidants, the reactions were
slower and generally showed lower conversions than those ob-
tained with the use of H O . Reaction times longer than 60 min
1
were performed by using cis-cyclooctene as the model sub-
strate. Commonly used oxidants such as hydrogen peroxide,
tert-butyl hydroperoxide (TBHP), and the urea hydrogen perox-
ide adduct (UHP) were used in these experiments. Alongside
these variations, experiments with different catalyst loadings
were performed (Table 1).
2
2
resulted in no further epoxidation of cis-cyclooctene for all oxi-
dants. The lowered conversions and slower reaction resulting
from the use of UHP are readily explained by the fact that the
solubility of UHP in acetonitrile is lower than that of aqueous
hydrogen peroxide. More interestingly, the conversions were
significantly lowered by the use of the alkyl hydroperoxide
TBHP. In previous mechanistic work with iron(II) epoxidation
catalysts, iron(III)–hydroperoxide complexes were identified as
Table 1. Influence of catalyst loading and oxidant on the conversion of
cis-cyclooctene. Conversions were determined by H NMR spectroscopy
by using internal standards.
1
[
h]
[h]
[i]
Oxidizing
agent
Catalyst loading Conv. [%] Selectivity Epoxide TOF
[
h]
À1
[mol%]
(t [min])
[%]
yield [%] [h
]
[
a]
[d]
H
2
O
2
5
99 (5)
92 (5)
66 (5)
4 (5)
>99
>99
>99
>99
>99
>99
>99
>99
99
92
66
4
2
3
238
552
792
480
12
1.5
72
7
[
[
e]
f]
2
1
0
2
[11,13]
key intermediates towards the active catalysts.
[
g]
.1
[e]
[
b]
The stability of this intermediate towards decomposition is
of high importance to the reaction rate, and the influence of
alkyl substituents on the stability of the OÀO bond directly af-
fected the rate of the catalytic conversion. Given that the OÀO
bond is weaker in alkyl hydroperoxides, a shorter lifespan for
the iron(III)–hydroperoxo intermediate is reasonable, and this
TBHP
2 (5)
3
(60)
12 (5)
4 (60)
[
c]
[e]
UHP
2
12
14
1
2 2
Reaction conditions: cis-Cyclooctene (320 mmol, 100 mol%); [a] H O
(
(
[
50% aq., 486 mmol), [b] TBHP (70% aqueous, 486 mmol), or [c] UHP
486 mmol); complex ([d] 16 mmol, [e] 6.4 mmol, [f] 3.2 mmol, or
g] 0.32 mmol); t=5 min; T=258C. [h] Conversions and selectivities were
1
[11]
has been shown in other systems. Several olefin substrates
1
were examined to show the general catalytic activity of com-
plex 1 to epoxidize C=C bonds. The reactions were performed
under ambient conditions; conversions and yields were deter-
determined by H NMR spectroscopy with benzene as an internal stan-
dard. [i] TOF=epoxide yield/(loadingcat. 1 ꢂt).
1
mined by H NMR spectroscopy. Complex 1 was found to effi-
Complex 1 was found to selectively catalyze the epoxidation
of cis-cyclooctene in high yields without the need for acids or
other additives within a reaction time of 5 min. The first experi-
ment performed with a catalyst loading of 2 mol% afforded
the epoxide in 92% yield after 5 min with no detectable for-
mation of the diol to give a turn-
ciently catalyze epoxidation reactions in high selectivities for
terminal and cyclic olefins without the need for acids or other
additives (Table 2). The TOFs and epoxide yields were highly
dependent on the substitution pattern of the double bond in
the substrate. However, the selectivities were excellent for the
À1
over frequency (TOF) of 552 h .
As expected, increasing the cata-
lyst loading resulted in an in-
crease in the conversion of the
olefin. At a catalyst loading of
Table 2. Epoxidation of various substrates by using complex 1 as a catalyst under ambient conditions. Conver-
sions and selectivities were determined by H NMR spectroscopy by using internal standards.
1
[a]
[c]
[c]
[h]
Substrate
Catalyst loading
[mol%]
Conv. [%]
Selectivity
[%]
Epoxide yield
[%]
TOF
[h
À1
(t [min])
]
5
mol%, almost quantitative con-
version was observed. Lower cat-
alyst loadings resulted in lower
conversions, but high TOFs of up
[d]
2
92 (5)
>99
92
552
[
[
[
[
d]
e]
e]
e]
2
2
2
2
58 (5)
90 (5)
51 (5)
13 (5)
>99
>99
>99
84
58
90
51
11
348
540
306
66
À1
to 792 h were still maintained.
The detection of residual olefin
even at 5 mol% catalyst loading
might be caused by the limited
activity of the catalyst or the fast
decomposition of the catalyst
under the oxidative conditions.
However, the TOFs indicated
that limited activity was not
a likely factor, as the reaction
rates were very high with high
conversions after only 5 min.
[
e]
2
18 (5)
90
16
11
96
–
[b]
[f]
[g]
[i]
1
30 (180)
36
[
a] Reaction conditions, unless noted otherwise: Olefin (160 mmol, 100 mol%), complex 1 (3.2 mmol), H
aq., 243 mmol), t=5 min, T=258C. [b] Reaction conditions: Propylene (10.39 mmol, 380 kPa), complex
(0.1 mmol), H (50% aq., 15.6 mmol), frozen in liquid N before addition of H then thawed and stirred
for 3 h. [c] Conversions and selectivities were determined by H NMR spectroscopy with an internal standard:
2 2
O (50%
1
2
O
2
2
2 2
O
1
[
(
d] benzene, [e] pyridine, [f] toluene. [g] 1,2-Propanediol was obtained in 64% yield. [h] TOF=epoxide yield/
loadingcat. 1 ꢂt). [i] Accurate determination of the TOF was not possible.
The reaction was highly sensi-
tive to changes in the oxidizing
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formation of epoxides and were largely independent of the
substitution pattern of the olefin with the exception of propyl-
ene. High epoxide yields were obtained for alkyl-substituted
olefins, and the best yields were obtained for cyclic olefins
such as cis-cyclooctene, whereas the aromatic olefins styrene
and methyl styrene were less prone to epoxidation. This sug-
gested that the nucleophilicity of the olefin was an important
factor in the epoxidation and conversely that the catalyst re-
acted electrophilically. Rybak-Akimova showed that this effect
correlates well with the oxidation potential of the substituted
conditions, hydrogen peroxide was the limiting reagent in the
epoxidation reaction.
At higher concentrations, this could not be the case, as
there was a certain amount of H O left after the reaction time,
2
2
as indicated by a violent reaction with activated MnO . It is
2
more probable that the activity of the complex was lowered as
a result of partial deactivation of the iron catalyst.
If the concentration of hydrogen peroxide was a limiting
factor after the reaction was finished, there would still be
active catalyst present in the reaction mixture. Two control ex-
periments were conducted to investigate the catalyst stability
towards oxidants in the presence of an excess amount of the
substrate. In the first experiment, 150 equivalents H O was
[
14,33]
olefins.
In other reports on epoxidation catalysis with
iron(II) aminopyridine complexes, cyclic olefins were proven to
give significantly higher yields for epoxidations and dihydroxy-
2
2
[
11,14,25]
lations than terminal olefins.
In none of these studies
used with 100 equivalents substrate and 2 equivalents catalyst.
After incomplete conversion in 5 min, another 150 equivalents
H O was added, which resulted in no further catalytic conver-
was propylene examined; however, it is known from other ho-
mogeneous systems that propylene epoxidation is still a chal-
lenge with most catalysts and that only low or moderate epox-
2
2
sion. In the reciprocal case of 50 equivalents H O with another
2
2
[
29,34,35]
[36]
ide yields
are obtained with only a few exceptions.
50 equivalents added after 5 min, a conversion of 83% was
reached, which showed that the complex was still active in the
presence of an excess amount of the substrate after a first re-
action cycle. Even if 24 h passed before the second addition,
the conversion reached 72%. This indicated that the catalyst
had decomposed only to a slight degree and showed that the
catalyst was remarkably stable in the presence of an excess
amount of the substrate. In principle, this renders the catalyst
recyclable, as it retains most of its activity as long as there is
an excess amount of the substrate. Ligand design to facilitate
recycling and enhance stability towards oxidants therefore
seems worthwhile and investigations are currently underway.
Kinetic studies showed that at room temperature the cata-
lytic conversion was complete after 2 min (Figure 2). As expect-
ed, the overall activity of the catalyst was lowered at decreased
temperatures. The initial TOF at room temperature determined
Propylene epoxidation with complex 1 afforded moderate con-
versions, which proved its activity towards unactivated olefins.
Given that in all previous experiments no quantitative con-
version of the olefin was achieved, the influence of the con-
centration of hydrogen peroxide on the conversion was inves-
tigated (compare Figure 1). During the fast catalytic conver-
À1
by the initial slope was very high at 2624 h , which corre-
sponded to 87% conversion after a reaction time of just 1 min.
However, the epoxidation did not proceed to more than 90%
conversion, which corresponded to a negligible increase in the
Figure 1. Conversion of cis-cyclooctene dependent on the number of equiv-
alents of the oxidant. Reaction conditions: cis-Cyclooctene (320 mmol,
2 2
100 mol%), complex 1 (6.4 mmol, 2 mol%), H O (varying equiv., 50% aq.),
t=5 min, T=258C.
sion, gas evolution from the reaction solution was observed,
which indicated that unproductive hydrogen peroxide decom-
position might have occurred, catalyzed by complex 1, as it is
[
37,38]
well known for other iron(II) compounds.
If less than
1
50 equivalents H O was used, the conversion was lower than
2
2
that expected for complete conversion of the oxidant.
In the range from 150 to 300 equivalents H O , maximum
2
2
conversions were reached. At higher oxidant concentrations,
the conversions decreased again. The conversions for low H O2
2
Figure 2. Effect of lowering the temperature in the catalytic oxidation of cis-
cyclooctene. Reaction conditions: cis-Cyclooctene (319 mmol, 100 mol%),
concentrations and the observed evolution of gas during the
reaction suggested that some of the oxidant decomposed and
thus could not productively react to the epoxide. Under these
complex 1 (6.4 mmol, 2 mol%), and H
2 2
O (50% aq., 486 mmol, 150 mol%),
T=À10 (
&
), 0 (*), 10 (~), and 258C (!).
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turnovers after the initial phase of a high TOF. This complied
with all our previous experiments with H O in which conver-
Experimental Section
2
2
General methods
sions at room temperature were recorded after 5 min with no
further reaction after this short reaction period (Tables 1 and
[32]
Complex 1 was synthesized according to a literature procedure.
2
). At a reaction temperature of 108C, the TOF was lower
Unless otherwise noted, all conversions were determined by cali-
brated GC analysis by using p-xylene as an internal standard. Ex-
periments performed by using different substrates were analyzed
by H NMR spectroscopy for which the specific product signal inte-
grals were compared to the internal standard used.
À1
(1437 h ), which corresponded to a conversion of 96% after
À1
2
min. At 0 and À108C, the TOF was 568 and 368 h , respec-
1
tively, with a maximum conversion of 99% at 08C and 100% at
À108C.
In general, for these kinetic experiments the maximum con-
versions were increased at lower temperatures, whereas the
TOFs of the catalyst were lowered. A viable explanation for this
behavior is the combined influence of the temperature on the
stability and activity of the catalyst. Whereas compound 1 re-
acted to the catalytically active species under oxidative condi-
tions, deactivation of the catalyst was also observed (see
Scheme 1). Both of these pathways were slowed down at de-
creased temperatures; however, the unproductive decomposi-
tion pathway was slowed down to a greater extent than the
productive epoxidation pathway. The resulting increased
period of activity of the catalyst resulted in increased conver-
sions up to total conversion of 100% even though the activity
itself was lowered. Overall, the increased maximum conversion
is therefore a stability effect rather than an activity effect.
Generally, a comparison of literature-reported TOFs was
somewhat problematic owing to the varying reaction condi-
tions and, more importantly, to the great variation of sampling
Catalytic epoxidation of cis-cyclooctene
General procedure: cis-Cyclooctene (35.2 mg, 319 mmol) was added
to a vial charged with a solution of complex 1 (5 mg, 6.4 mmol) in
acetonitrile (1 mL). A solution of H O2 (50% in H O, 27.6 mL,
2
2
4
86 mmol) in acetonitrile (1 mL) was then added. The reaction mix-
ture was stirred for 5 min and activated MnO was added to termi-
nate the reaction. After filtration, GC samples were prepared by di-
luting a portion of the reaction mixture (100 mL) with n-hexane
2
À1
(900 mL) and the GC standard (4 mgmL
p-xylene in iPrOH,
1
5
00 mL). H NMR spectroscopy experiments were conducted in
CD CN. After 5 min of stirring, the internal standard was added,
3
the catalyst was removed by adsorption to alumina, and the
1
H NMR spectra were recorded.
Acknowledgements
J.W.K. thanks the Studienstiftung des Deutschen Volkes for finan-
cial support.
times and reaction times within the data. The highest TOF of
À1
2
5200 h corresponding to 50% yield of cyclooctene oxide
with 0.5 mol% catalyst after 14 s was reported by Que et al. for
2
+
Keywords: epoxidation · homogeneous catalysis · iron ·
carbene ligands · peroxides
the non-heme iron epoxidation catalyst [(bpmen)Fe(MeCN) ]
2
[
20]
.
However, this result was only obtained in the presence of
additives. Under additive-free conditions, the highest reported
À1
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[
20]
a 17% yield after 30 s. Although there are a few reports on
À1 [14]
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[
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2
2
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[
[
(
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4
the reactive intermediates and ligand design to enhance stabil-
ity and activity are currently under investigation in our
laboratories.
[
[
7
1
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