Olefin epoxidation with a new class of ansa-molybdenum catalysts in ionic liquids

Article abstract of DOI:10.1002/cssc.201000008

Is that a finalansa? Different room temperature ionic liquids (RTILs) are examined as solvents in the biphasic epoxidation of selected olefins with the ansa compounds 1 and 2 as catalyst precursors. Turnover frequencies of up to 44?000 h-1 are reached and

Full text of DOI:10.1002/cssc.201000008

DOI: 10.1002/cssc.201000008
Olefin Epoxidation with a New Class of Ansa-Molybdenum Catalysts in Ionic
Liquids
Daniel Betz,[a] Alexander Raith,[b] Mirza Cokoja,[a] and Fritz E. Kꢀhn*[a, b]
Epoxidation reactions of C=C double bonds are of high interest
for the synthesis of fine chemicals such as pharmaceuticals and
flavor and fragrance molecules.[1] A vast number of coordina-
tion compounds have been applied as catalysts for this type of
reaction. Beside the well-known and highly active methyltriox-
orhenium (MTO), several other ReVII, TiIV, VIII, MoVI, and MnIII
compounds have been found to catalyze olefin epoxidations.[2]
Many other catalytic systems based on molybdenum and tung-
sten have been extensively studied and applied in asymmetric
epoxidation catalysis.[3] Compounds of the type [MoO2X2L2]
(L=Lewis base) and [Cp‘Mo(CO)3Cl] can easily be transformed
to catalytically active oxo-peroxo species for olefin epoxidation
with tert-butyl hydroperoxide (TBHP).[4] [Cp‘Mo(CO)3R] (R=
alkyl) complexes show a catalytic activity comparable to their
chloro analogues.[5] Among this class of compounds, we are
currently exploring ansa complexes of the general type
[Mo(h5-C5H4(CH(CH2)n)-h1-CH)(CO)3] as epoxidation catalysts.[6–9]
We have previously reported the synthesis of new molybde-
num and tungsten ansa complexes with cyclopentyl moieties
and exceptionally stable bridging units, and performed first
studies of these compounds as epoxidation catalysts.[6f]
catalyst, where the activity in RTILs was also higher than in
conventional solvents or under solvent-free conditions.[14] The
catalytic activities of [Cp‘Mo(CO)3R] compounds (Cp‘=Cp, Cp*;
R=Cl, Me) have as well been investigated in RTILs. Among all
examined catalysts, the best epoxide yields were obtained
with [BMIM]NTf2 as the RTIL (BMIM=1-butyl-3-methylimidazoli-
um), as shown by both Valente et al. and our group.[14f, g,15]
Compounds 1 and 2 (Figure 1) were synthesized according
to literature procedures[6f,9a] and tested as catalyst precursors
Figure 1. Compounds tested as catalysts for olefin epoxidation in ionic liq-
uids.
for the epoxidation of cis-cyclooctene, 1-octene, and cis-stil-
bene with tert-butyl hydroperoxide (TBHP) as oxidant. All cata-
lytic investigations were performed at room temperature and
under laboratory atmosphere. The RTILs were water equilibrat-
ed. The course of the reaction was monitored by using GC–MS
and GC–FID, respectively.
Herein, we report the remarkable activity of ansa-bridged
MoVI complexes for the epoxidation of different olefins in room
temperature ionic liquids (RTIL) as solvents. Due to their high
stability towards oxidation, imidazolium salt type RTILs are em-
ployed.[10,11] In the last decade, a plethora of catalytic reactions
has been performed in RTILs as reaction media.[12] The unique
physical properties of RTILs such as low volatility, low flash
point, thermal stability, and high polarity are an attractive alter-
native to organic solvents.[10] Inorganic or organometallic com-
plexes which are insoluble in hydrocarbons are often soluble
in RTILs. Ionic liquids therefore provide a nonaqueous version
of two-phase catalysis, in which the catalyst is dissolved in the
ionic liquid phase and can be easily separated from the prod-
uct. Less than a decade ago, Song and Roh were the first to
report the asymmetric epoxidation in an ionic liquid catalyzed
by a manganese(III) salen complex.[13] Since then, RTILs have
been successfully applied as reaction media for olefin epoxida-
tion, for example, with FeIII porphyrin complexes, inorganic di-
oxomolybdenum(VI) systems, or methyltrioxorhenium (MTO) as
In literature, cyclooctene is reported to be the most fre-
quently used substrate for catalytic epoxidation test reactions.
Compounds 1 and 2 were found to be even more active cata-
lysts for the epoxidation of cyclooctene than the comparable
[CpMo(CO)3CH3] at room temperature.[6d–f] At a catalyst concen-
tration of 1 mol% of catalyst 1, nearly quantitative conversions
(>90% epoxide yield) were reached after 5 min both in
[BMIM]NTf2 and [BMIM]PF6 (Figure 2). The turnover frequencies
(TOF) are in the range of 3000–4000 hÀ1. With compound 2 as
catalyst, the reactions proceeded somewhat slower (Figure 3).
[a] D. Betz, M. Cokoja, Prof. F. E. Kꢀhn
Catalysis Research Center-Chair of Inorganic Chemistry
Technische Universitꢁt Mꢀnchen
Lichtenbergstrasse 4, 85747 Garching (Germany)
Fax: (+49)89 289 13473
E-mail: fritz.kuehn@ch.tum.de
[b] A. Raith, Prof. F. E. Kꢀhn
Catalysis Research Center-Molecular Catalysis
Technische Universitꢁt Mꢀnchen
Figure 2. Reaction kinetics of cyclooctene epoxidation in the presence of 1
~
&
^
*
using [BMIM]BF4 ( ), [BMIM]PF6 ( ), [BMIM]NTf2 ( ), or [C8MIM]PF6 ( ) as sol-
Lichtenbergstrasse 4, 85747 Garching (Germany)
vent.
ChemSusChem 2010, 3, 559 – 562
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
559

lyst molecules is actually involved in the catalytic process and
the number of active centers is accordingly low. At a concen-
tration of 0.05 mol%, more catalytic centers are involved so
that a more realistic turnover frequency (activity) is obtained.
At even lower concentrations (0.01 mol%), deactivation pro-
cesses are seemingly playing a larger role. The exact nature of
the deactivation is currently under investigation; there are indi-
cations that a certain amount of water in the solvent is leading
to decomposition of the catalyst at low catalyst concentra-
tions. To test the stability of the catalytic species and to probe
the convenience of catalyst recycling in a biphasic system, the
RTIL solution containing the catalyst was separated from the
reaction medium by simple decantation via a cannula after the
first run and a further batch of substrate and oxidant was
added (see the Experimental Section). The catalyst remained
active for at least three catalytic runs, associated with a minor
drop in the activity (to 80%; Figure 4). Because leaching effects
could not be observed and the catalyst was stable under the
applied conditions, this activity drop was usually ascribed to
an increasing concentration of water (from the oxidant and/or
substrate) in the ionic liquid which was being added to the
system after each catalytic run.
Figure 3. Reaction kinetics of cyclooctene epoxidations in the presence of 2
~
&
^
*
using [BMIM]BF4 ( ), [BMIM]PF6 ( ), [BMIM]NTf2 ( ), or [C8MIM]PF6 ( ) as sol-
vent.
In [BMIM]NTf2, a quantitative conversion to cyclooctene epox-
ide was reached after 30 min, whereas in [C8MIM]PF6 (C8MIM=
1-octyl-3-methylimidazolium) after 30 min, the epoxide yield
was 85%; 100% were reached after 4 h. However, in both
cases, after 5 min reaction time, the TOFs were 3880 and
4130 hÀ1, respectively. Catalytic tests in the RTIL [BMIM]BF4 re-
sulted in all cases in very poor yields because of the high
water content of this RTIL. For this reason, this RTIL was ex-
cluded in the further studies. Table 1 summarizes the results of
Table 1. Catalytic results for the epoxidation of cyclooctene with com-
pounds 1 and 2 (c=1 mol%) as catalyst precursors and different water-
equilibrated RTILs as solvent in laboratory atmosphere. TBHP as oxidizing
agent.
1
2
Solvent
Yield after Yield after TOF[a] Yield after Yield after TOF[a]
4 h [%]
24 h [%]
[hÀ1
]
4 h [%]
24 h [%]
[hÀ1
]
Figure 4. Epoxide yield for the first, second, and third cycle after 2 h and 4 h
reaction time in the presence of 2. [BMIM]NTf2 was used as solvent (labora-
tory atmosphere with water equilibrated RTIL and cyclooctene as substrate).
CH2Cl2
[BMIM]BF4
[BMIM]PF6 98
[BMIM]NTf2 100
[C8MIM]PF6 55
100
2
100
2
100
100
66
900
27
3100 87
3700 100
1540 100
100
1
100
2
94
100
100
900
38
2410
4130
3880
[a] Determined after 5 min reaction time.
Experiments with 1-octene resulted in low to moderate
yields (52% after 24 h in the case of 1 in [BMIM]NTf2 and 28%
for 2, respectively), which is not particularly surprising, because
it is known that 1-octene is generally less prone to epoxidation
than cyclooctene. However, no diol formation could be detect-
ed (Table 2).
the catalytic epoxidation of cis-cyclooctene with 1 mol% of
molybdenum compounds 1 and 2. In order to determine the
actual catalytic activity of compound 2, the catalyst concentra-
tion was decreased. With
a catalyst concentration of
0.05 mol%, the TOF increased to an exceedingly high value of
44000 hÀ1. To the best of our knowledge, this is the highest
TOF ever reported for a molecular epoxidation catalyst both in
conventional solvents and in ionic liquids. Interestingly, when
using compound 1 under the same conditions, the TOF was
much lower.
Table 2. Catalytic results for the epoxidation of 1-octene with com-
pounds 1 and 2 (c=1 mol%) in CH2Cl2 and different water-equilibrated
RTILs under laboratory atmosphere. TBHP was applied as oxidizing agent.
1
2
Solvent
Yield
after
Yield after
24 h [%]
Yield after
4 h [%]
Yield after
24 h [%]
A
further decrease of the catalyst concentration to
0.01 mol% resulted in a decrease of the TOF to 8900 hÀ1. Com-
plex 2 was employed as homogeneous catalyst precursor in
CH2Cl2 as solvent, which resulted in a TOF of 3800 hÀ1 at a con-
centration of 0.1 mol%.[6f] These results show that at a catalyst
precursor concentration of 1 mol%, only a fraction of the cata-
4 h [%]
CH2Cl2
[BMIM]PF6
—
8
19
17
52
35
—
10
27
11
60
19
28
19
[BMIM]NTf2 32
[C8MIM]PF6 26
560
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ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 2010, 3, 559 – 562

A comparison between the biphasic catalytic system in RTILs
tailed studies to reach even better catalyst stabilities are cur-
rently under investigation in our laboratories.
[6d–f]
and homogeneous catalysis in CH2Cl2
shows that for the
two-phase heterogeneous catalysis, the catalyst activities in
[BMIM]NTf2 are significantly higher than under homogeneous
conditions for compound 1.
Experimental Section
TBHP was purchased from Aldrich as a 5.0–6.0m solution in n-
The catalytic epoxidation of cis-stilbene is rather challenging
in epoxidation catalysis because it is difficult to prevent diol
formation in this case. It is noteworthy that no diols were de-
tected during the course of the measurements (catalyst con-
centration: 1 mol%). The catalytic results are given in Table 3.
Compound 1 afforded 64% cis-stilbene epoxide, whereas
compound 2 afforded 39% epoxide after 4 h in [BMIM]NTf2.
The turnover frequencies were in the range of 1300 hÀ1 (for
compound 1) and 750 hÀ1 (for compound 2).
decane and used after drying over molecular sieves to remove
1
water. H, 13C, and 19F NMR spectra were obtained using a Bruker
Avance DPX-400 spectrometer. Catalytic runs were monitored by
GC methods on a Hewlett–Packard instrument HP 5890 Series II
equipped with a FID, a Supelco column Alphadex 120 and a Hew-
lett–Packard integration unit HP 3396 Series II.
Synthesis of ionic liquids
The RTILs [BMIM]PF6, [C8MIM]PF6, [BMIM]NTf2, and [BMIM]BF4 were
prepared and purified as described in the literature.[10,11] Their spec-
troscopic data were in accordance with the data reported previ-
ously. Compounds 1 and 2 were prepared according to literature
procedures.[6f,9a]
Table 3. Catalytic results for the epoxidation of cis-stilbene with com-
pounds 1 and 2 (c=1 mol%) in CH2Cl2 and different water-equilibrated
RTILs under laboratory atmosphere. TBHP was applied as oxidizing agent.
1
2
Solvent
CH2Cl2
[BMIM]PF6 46
[BMIM]NTf2 64
[C8MIM]PF6 31
Yield after Yield after TOF[a] Yield after Yield after TOF[a]
Catalytic reactions
4 h [%]
24 h [%]
[hÀ1
]
4 h [%]
24 h [%]
[hÀ1
]
All catalytic reactions were performed under laboratory atmos-
phere (under air at RT) in a reaction vessel equipped with a mag-
netic stirrer.
—
9
—
340
1290 39
450 23
37
28
—
36
50
31
—
51
68
33
590
750
270
Cis-cyclooctene: Olefin (800 mg, 7.3 mmol), mesitylene (500 mg, in-
ternal standard), and the catalyst [(1 or 2; 1 mol%, 73 mmol); 2
(0.05 or 0.01 mol%)] dissolved in RTIL (0.5 mL), were added to the
reaction vessel. Afterwards, the reaction was initiated by adding
TBHP (2.65 mL, 5.5m in n-decane).
[a] Determined after 5 min reaction time.
Conclusions
1-Octene: Olefin (800 mg, 7.3 mmol), mesitylene (500 mg, internal
standard), and the catalyst (1 or 2; 1 mol%, 73 mmol) were added
to the reaction vessel and diluted in RTIL (0.5 mL). Afterwards, the
reaction was initiated by adding TBHP (2.65 mL, 5.5m in n-decane).
The biphasic epoxidation of selected olefins (namely cis-cyclo-
octene, 1-octene, and cis-stilbene), with the ansa-compounds
Mo(h5-C5H4(CH2-h1-CH)(CO)3)
1
and Mo(h5-C5H4(CH(CH2)n)-h1-
CH)(CO)3 2 as catalyst precursors, were examined. As solvents,
four different RTILs ([BMIM]BF4, [BMIM]PF6, [BMIM]NTf2, and
[C8MIM]PF6) were used. For all investigated reactions, both the
yields and the TOFs for catalysis carried out in RTILs were
much higher than in all conventional solvents. The biphasic
catalytic systems described herein clearly outperform previous-
ly published homogeneous catalyst systems containing com-
pounds 1 and 2. Quantitative yields without formation of diols
were observed and TOFs up to 44000 hÀ1 were reached. In
contrast to the homogeneous system, the catalyst is immobi-
lized in the ionic liquids and the product phase can be easily
separated from the biphasic system. Catalyst 2 can be reused
for at least three subsequent runs with only a minor loss of ac-
tivity. In all cases, catalytic reactions in [BMIM]NTf2 yielded the
best results, correlating well with other reported Mo sys-
tems.[ 14g,15] This is most likely due to the low water content of
this RTIL which affects the lifetime of the slightly water sensi-
tive MoVI species. In accord with its high water content,
[BMIM]BF4 leads to the lowest product yield. Accordingly re-
placing the conventional solvents CH2Cl2 by more sustainable
ionic liquids in lab-scale reactions seems to be recommendable
and a scale-up of such reactions appears to be promising. Syn-
thesis and examination of tailor-made ionic liquids and de-
Cis-stilbene: Olefin (0.291 g, 1.6 mmol), 4-methylbenzophenone
(0.360 g, internal standard), and the catalyst (1 or 2; 1 mol%,
16 mmol) were added to the reaction vessel and diluted in RTIL
(0.5 mL). The reaction was initiated with the addition of TBHP
(0.6 mL, 5.5m in n-decane).
The course of the reaction was monitored by quantitative GC anal-
ysis. Samples taken were treated with MgSO4 and MnO2 to remove
water and destroy excess peroxide. Afterwards, the sample was di-
luted with CH2Cl2 and the resulting slurry was filtered and the fil-
trate was injected into a GC column. The conversions of cis-cyclo-
octene, 1-octene, cis-stilbene, and the formation of their respective
oxides were calculated from calibration curves (r2 >0.999) recorded
prior to the start of the reaction. For the second and third run, the
upper phase was removed from the reaction vessel by means of
cannulation. Additionally, oil pump vacuum allowed the removal of
tBuOH from the RTIL phase before further cyclooctene (0.800 g,
7.3 mmol), mesitylene (0.500 g) and TBHP (2.65 mL, 5.5m in n-
decane) were added.
Acknowledgements
D. B. is thankful to the Bayerische Forschungsstiftung for his
Ph.D. grant. The help of Susanna Vazac, Ilona Krꢀger, and Sebas-
ChemSusChem 2010, 3, 559 – 562
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org
561

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Keywords: alkenes · epoxidation · homogeneous catalysis ·
ionic liquids · molybdenum
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