Fluorinated solvents in methyltrioxorhenium-catalyzed olefin epoxidations

Article abstract of DOI:10.1002/ejic.201200120

Epoxidations with methyltrioxorhenium(VII) (MTO) as catalyst precursor and additives are examined in fluorinated solvents under various conditions. These fluorinated solvents have a strong influence on the ¹H NMR, ¹³C NMR, and ¹⁷O NMR spectroscopic data of the catalyst precursor, apparently contributing significantly to the catalytic activity of such systems. Accordingly, the highest turnover frequencies (TOF) obtained with MTO-containing systems have been determined, reaching up to 39000 h ^-1. Despite the high initial activity, such systems show a pronounced sensitivity to water leading also to faster catalyst decomposition. The high activity as well as the high sensitivity of the catalyst may be due to its solvent-enhanced higher Lewis acidity, leading to quantitative epoxide yields only when quite stable epoxides are the end products and very high amounts of Lewis base additives are applied.

Full text of DOI:10.1002/ejic.201200120

FULL PAPER
DOI: 10.1002/ejic.201200120
Fluorinated Solvents in Methyltrioxorhenium-Catalyzed Olefin Epoxidations
Philipp Altmann,[a] Mirza Cokoja,[a] and Fritz E. Kühn*[a]
Keywords: Epoxidation / Homogeneous catalysis / Peroxides / Rhenium / Perfluorinated solvents
Epoxidations with methyltrioxorhenium(VII) (MTO) as cata- mined, reaching up to 39000 h–1. Despite the high initial ac-
lyst precursor and additives are examined in fluorinated sol-
vents under various conditions. These fluorinated solvents
have a strong influence on the 1H NMR, 13C NMR, and 17O
NMR spectroscopic data of the catalyst precursor, apparently
contributing significantly to the catalytic activity of such sys-
tems. Accordingly, the highest turnover frequencies (TOF)
tivity, such systems show a pronounced sensitivity to water
leading also to faster catalyst decomposition. The high ac-
tivity as well as the high sensitivity of the catalyst may be
due to its solvent-enhanced higher Lewis acidity, leading to
quantitative epoxide yields only when quite stable epoxides
are the end products and very high amounts of Lewis base
obtained with MTO-containing systems have been deter- additives are applied.
Introduction
Fluorinated solvents have been successfully applied in
many homogeneous catalyzed reactions.[1] Among these re-
actions, the application of fluorinated solvents in epoxid-
ation reactions is a topic of continuing interest.[2] The epox-
idation of olefins in fluorinated alcohols such as hexa-
fluoroisopropanol is possible even without the addition of
another catalyst, as the solvent alone is able to activate the
oxidant.[2b–2e,2g–2j] In combination with methyltrioxorheni-
um(VII) (MTO), which is one of the most efficient olefin
Scheme 2. (1) Adduct formation of MTO with additive (A) or sol-
vent (S). (2) Reaction of the MTO·A/S adduct with H2O2 to form
catalytically active species A and B.
oxidation catalysts,[3] highly active catalytic media are
formed.[2a,2f] Generally, MTO-catalyzed olefin epoxidations
proceed according to Scheme 1. Solvents and additives play
major roles during the catalytic reaction, as both of them
influence and/or stabilize catalytically active species A and
B (Scheme 2), leading to higher yields and selectivi-
ties.[3e,3f,3i,3j,4]
(1), 4-tert-butylpyridine (2), and 4,4Ј-dimethyl-2,2Ј-bipyr-
idine (3) are superior with substrates such as 1-octene and
cyclooctene (Figure 1).[3e,3f,3j,4b,6]
Scheme 1. Oxidation of olefins to epoxides.
Lewis base additives usually reduce the Lewis acidity of
the rhenium center, which inhibits ring-opening of the epox-
ide. Additionally, reaction velocity and yield are increased
in their presence. The performance of different types of
Lewis bases depends additionally on the substrate used.
Schiff base ligands, for example, 4, show better activities
when trimethylbenzene is used as the substrate.[5] Pyrazole
Figure 1. Additives used in the oxidation of cyclooctene.
Although, the system MTO/fluorinated solvent has al-
ready attracted some attention,[2a,2f] optimized reaction pro-
tocols and meaningful comparisons are largely lacking.
Various reaction parameters are therefore examined in this
work under identical reaction conditions by using cyclo-
octene as the benchmark substrate and trifluoroethanol
(TFE) and hexafluoroisopropanol (HFIP), two fluorinated
alcohols, as standard solvents.
[a] Chair of Inorganic Chemistry/Molecular Catalysis,
Catalysis Research Center, Technische Universität München,
Ernst-Otto-Fischer-Str. 1, 85747 Garching bei München,
Germany
Fax: +49-89-289-13473
E-mail: fritz.kuehn@ch.tum.de
Eur. J. Inorg. Chem. 2012, 3235–3239
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3235

P. Altmann, M. Cokoja, F. E. Kühn
FULL PAPER
Results and Discussion
NMR Spectroscopy
MTO.[2a,2f,3e,3f,3j,4b,4c,5,6b] Blank experiments without MTO
lead only to yields of 3% in TFE and 7% yield in HFIP at
room temperature. At 0 °C, no yield is detectable in either
solvent. Without any additive to MTO in the catalytic sys-
tem, the reaction displays a TOF of 1640 h–1 in HFIP and
a yield of 48% after 3 h. The reaction is slower in TFE
without an additive (TOF = 660 h–1) than in HFIP, but the
system is active over a longer period of time with a yield of
67% after 3 h. This is even superior to 4-tert-butylpyridine
or Schiff base 4 as additives in HFIP (Figure 2). Comparing
MTO (0.1 mol-%) together with different additives (100-
fold excess with respect to MTO), pyrazole and 4-tert-butyl-
pyridine show the best activities with quantitative yields af-
ter 1 h. Pyrazole led to a slightly higher initial activity re-
sulting in a TOF of 7800 h–1 compared with 7000 h–1 for 2.
The use of Schiff base 4 and 4,4Ј-dimethyl-2,2Ј-bipyridine
resulted in more or less equal yields of ca. 50% after 3 h
(Figure 2). Interestingly, Schiff base 4 showed a much better
performance than expected, based on previously published
results.[6b]
The 1H NMR, 13C NMR, and 17O NMR chemical shifts
of MTO are summarized in Table 1. Comparison between
the fluorinated and non-fluorinated solvents shows the pro-
nounced influence of TFE and HFIP on the electronic
characteristics of MTO. Pyrazole was examined in the
NMR study, as it leads to the most active MTO/Lewis base
catalyst system in this type of olefin epoxidation catalysis.
Adduct formation of pyrazole with MTO can be observed
by the chemical shifts in the data presented in Table 1. The
high-field shift of the methyl protons is typical for such a
coordination.[7] Additionally, a shift of the pyrazole protons
is visible when MTO is added to the reaction mixture (TFE:
6.68 to 6.62 ppm and 7.79 to 7.83 ppm; HFIP: 6.48 to
1
6.38 ppm and 7.41 to 7.56 ppm) The H NMR values of
the MTO methyl group are low-field shifted by 0.2–0.5 ppm
compared to those observed in CDCl3, indicating a more
pronounced electron deficiency at the methyl protons. It is
1
noteworthy that the H NMR shift of the H3C-Re moiety
of the pyrazole adduct in fluorinated solvents is comparable
to the shift of the H3C-Re moiety in free MTO in CDCl3.
The 17O NMR chemical shifts for the oxygen atoms of un-
coordinated MTO in TFE and HFIP are 824 and 813 ppm,
respectively. Whereas the shift in TFE is in the range of that
observed in toluene and benzene (823 ppm),[3g] the shift of
813 ppm in HFIP is the most high-field shifted MTO signal
observed so far in any solvent. Furthermore, the 17O NMR
signal of the MTO base adducts in fluorinated solvents is
more high-field shifted than that of free MTO itself, which
is opposite to the effect observed in conventional solvents.
Additionally, the 17O NMR shift difference between free
and coordinated MTO is somewhat larger in fluorinated
solvents than in CDCl3, indicating that the ligand effect of
pyrazole on the oxygen atoms is more pronounced in fluori-
nated solvents than in CDCl3. The opposite is the case for
the carbon and hydrogen atoms. In this case, the shift differ-
ence between free and coordinated MTO in CDCl3 is more
pronounced than in fluorinated solvents. Moreover, in both
Figure 2. Oxidation of cyclooctene with different additives. Cyclo-
octene (12 mmol), MTO (0.1 mol-%), H2O2 (27%, 24 mmol),
HFIP, 0 °C, additive (10 mol-%) (♦ = 3, ᭿ = 2, ᭹ = 1, ᭡ = 4).
For all following experiments, pyrazole was the additive
of choice, as it performs as good as 4-tert-butylpyridine and
the results obtained here are easier to compare to published
work.[2a,2f]
1
the 13C NMR and H NMR spectra, the chemical shifts of
The concentration of the additive plays an important role
in the MTO-catalyzed epoxidation of olefins. Figure 3
clearly shows the correlation between pyrazole concentra-
tion and yield. Pyrazole stabilizes the catalytically active
species of MTO better when more pyrazole is present
[Equation (2) in Scheme 2]. The low catalyst concentration
of 0.01 mol-% reveals the beneficial effect of the additive
even more than the normally used concentrations between
MTO and its pyrazole adduct are not reversed in fluori-
nated solvents with respect to field strength. Nevertheless,
the shift values of the pyrazole adduct in fluorinated sol-
vents are almost identical to those of free MTO in CDCl3.
Effect of Additives
The Lewis bases shown in Figure 1 have proven to be
1
and 0.1 mol-%.[2a,2f,3e,3h,3n,4a,6a,7–8] With 100 equiv.
very efficient as additives in olefin epoxidation with (1 mol-%) pyrazole per molecule MTO, the reaction stops
Table 1. NMR spectroscopic data (in ppm) of MTO with and without pyrazole in different solvents.
CDCl3 (ref.[6a]
)
TFE
HFIP
δ(1H)
δ(13C)
δ(17O)
δ(1H)
δ(13C)
δ(17O)
δ(1H)
δ(13C)
δ(17O)
MTO
MTO + pyrazole
2.67
2.18
19.03
24.62
829
832
2.88
2.70
17.87
19.63
824
817
2.84
2.64
18.49
19.18
813
807
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Eur. J. Inorg. Chem. 2012, 3235–3239

Fluorinated Solvents in Methyltrioxorhenium-Catalyzed Olefin Epoxidations
after 30 min due to catalyst decomposition. The TOF is solvent, the reaction in nitromethane is slower in the begin-
also the lowest at 8200 h–1. The use of 1000 equiv. (10 mol- ning of the reaction. However, in nitromethane the catalysis
%), 3000 equiv. (30 mol-%), and 5000 equiv. (50 mol-%) led proceeds over a longer period of time with good activity,
to a constant increase in yield and TOF peaking in quanti- which is in contrast to TFE, so that the yields after 3 h are
tative conversion of cyclooctene after 3 h and a TOF of nearly identical, namely, 90–93%. Reasons for this behavior
39000 h–1 for 5000 equiv. of pyrazole. This is, to the best of are most likely the reduced stability of the catalytically
our knowledge, the first quantitative conversion of cyclooc- active species in TFE[2f] and the positive effect nitrometh-
tene into cyclooctene oxide with such a small concentration ane has on the oxidation of olefins with MTO as a catalyst.
of MTO, and it is also the highest TOF ever reported for a The energy barrier for the coordination of olefin and the
MTO-catalyzed epoxidation of an olefin. So far, the best peroxo species of MTO is much lower in nitromethane than
conversion with 0.01 mol-% MTO was 72%, achieved by in other common solvents.[5]
Yamazaki in a 35% H2O2/3-methylpyrazole/CH2Cl2 sys-
Table 3. Oxidation of cyclooctene in various solvents.[a]
tem.[9] Blank experiments with 50 mol-% pyrazole and
without MTO showed no activity in both solvents. Reduc- Entry
ing the catalyst concentration to 0.005 mol-% with
Solvent
Yield after 30 min Yield after 3 h TOF
[%]
[%]
[h–1]
5000 equiv. of pyrazole led to a yield of about 50%, which
remained constant after 1 h. Most likely, MTO is too dilute
in this solution to maintain a highly stabilized and active
catalytic system.
1
2
3
4
CH2Cl2
CH3NO2
TFE
28
48
66
89
71
93
90
1100
2300
5600
7800
HFIP
100
[a] Cyclooctene (12 mmol), MTO (0.1 mol-%), H2O2 (27%,
24 mmol), 0 °C, pyrazole (10 mol-%).
H2O2 Concentration and Temperature Effects
The catalytic reactions carried out at various tempera-
tures are depicted in Table 4. It is obvious that the catalytic
transformation is quantitative (Table 4, Entry 1) or almost
quantitative (Table 4, Entry 2) or that the catalytic system
shows poor activity (Table 4, Entry 3) after 30 min at the
selected temperatures. As mentioned before, the reason is
the decomposition of the catalyst at elevated temperatures.
In fluorinated solvents, this temperature-induced decompo-
sition seems to be significantly faster than in other sol-
vents.[6b] The difference in yield between –10 and 0 °C is
much smaller than the gap from 0 °C to room temperature.
The TOF at –10 °C is the highest and stands for the most
active system. Nevertheless, after 1 h, the yields at both –10
and 0 °C are identical and quantitative. It has to be consid-
ered if the additional effort to cool the system to –10 °C is
worth the benefit of the faster starting period.
Figure 3. Effect of concentration of pyrazole on the oxidation of
cyclooctene with MTO. Cyclooctene (12 mmol), MTO (0.01 mol-
%), H2O2 (27%, 24 mmol), HFIP, 0 °C, 1 (᭹ = 50 mol-%, ᭺ =
25 mol-%/0.005 mol-% MTO, ᭡ = 30 mol-%, ᭿ = 10 mol-%, ♦ =
1 mol-%).
When TFE is used as the solvent instead of HFIP, the
effect of the pyrazole concentration is similar but the overall
results (with respect to yield and TOF) are lower (see
Table 2).
Table 4. Oxidation of cyclooctene at different temperatures.[a]
Table 2. Oxidation of cyclooctene in TFE.[a]
Entry
T
[°C]
Yield after 30 min
[%]
Yield after 1 h
[%]
TOF
[h–1]
Entry
Pyrazole
[equiv.]
Yield after 1 h
[%]
TOF
[h–1]
1
2
3
–10
0
r.t.
99
89
56
99
98
57
9000
7800
6600
1
2
3
10
100
1000
26
25
46
5500
7000
16600
[a] Cyclooctene (12 mmol), MTO (0.1 mol-%), HFIP, H2O2 (27%,
24 mmol), pyrazole (10 mol-%).
[a] Cyclooctene (12 mmol), MTO (0.01 mol-%), TFE, 0 °C, H2O2
(27%, 24 mmol).
The concentration of the aqueous hydrogen peroxide
solution, that is, the water concentration, also has an influ-
ence on the catalytic reaction in fluorinated solvents. Car-
ried out in TFE, the yield with 27% H2O2 solution is 57%
Solvent
The experimental results match those of Iskra et al. for and with 50% H2O2 solution it is 64% at room temperature
TFE, HFIP, and CH2Cl2 (Table 3).[2f] It is very interesting after 3 h. The TOFs also show this effect with 4700 h–1 in
that the results in nitromethane (CH3NO2) are as good as 27% H2O2 solution and 6100 h–1 in 50% H2O2 solution. In
those obtained in TFE after 3 h. In contrast to TFE as this case, the differences after 3 h are relatively small. The
Eur. J. Inorg. Chem. 2012, 3235–3239
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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3237

P. Altmann, M. Cokoja, F. E. Kühn
FULL PAPER
Table 5. Oxidation of 1-octene.[a]
Entry
Solvent
T
[°C]
Conc. of H2O2
[%]
Yield after 30 min
[%]
Yield after 3 h
[%]
TOF
[h–1]
1
2
3
4
5
6
TFE
HFIP
TFE
HFIP
TFE
0
0
0
0
r.t.
r.t.
27
27
50
50
50
50
4
26
33
41
52
65
89
200
330
400
1700
400
4900
20
16
40
27
71
HFIP
[a] 1-Octene (12 mmol), MTO (0.1 mol-%), H2O2 (27%, 24 mmol), pyrazole (10 mol-%).
effect can be observed more clearly with 1-octene as the
substrate (Table 5). Generally, the lower the water concen-
Conclusions
A comprehensive comparison between the two most
widely applied fluorinated solvents, trifluoroethanol and
hexafluoroisopropanol, has been made. The main substrate
was cyclooctene, which is used as a benchmark in most
studies dealing with olefin epoxidation. From the common
organic solvents, only nitromethane is competitive with at
least one fluorinated solvent (TFE). 4-tert-Butylpyridine
and pyrazole are more or less equal additives in the oxi-
dation under the applied conditions. Pyrazole affords a
slightly higher TOF. The amount of pyrazole plays a decis-
ive role with respect to catalyst stability. With 5000 equiv.
of additive it is possible to apply catalyst concentrations of
only 0.01 mol-% and still reach quantitative yields. A TOF
of 39000 h–1 is the highest ever obtained for epoxidations
with MTO. The amount of water in the system, controlled
by the hydrogen peroxide concentration, leads to small in-
creases in yield and TOF for lower water concentrations. In
contrast to cyclooctene as substrate, 1-octene requires
higher temperatures to form epoxides in sufficient amounts.
The overall activation of the catalytic system seems to play
a more important role than the increased decomposition of
the catalytically active species at elevated temperatures.
tration, the higher the activity. At higher water concentra-
tions, the equilibrium of the stabilized MTO–additive com-
plex is shifted more to an MTO–water complex (Scheme 2).
This MTO–water complex can lead to decomposition of
MTO.[3h]
Epoxidation of Other Olefins
Cyclooctene is easy to oxidize, as it forms a quite stable
epoxide. It is, however, well known that open-chain, ter-
minal olefins such as 1-octene are more difficult to epoxi-
dize and that they have a stronger tendency to form di-
ols.[3a,3e,3j,4b,4c] Therefore, we investigated the effect of
fluorinated solvents on the MTO-catalyzed epoxidation of
1-octene in TFE and HFIP at different temperatures and
different H2O2 concentrations (Table 5). Product yields
were not detected in blank experiments run without MTO.
No significant diol formation was observed in any of the
following experiments. Again, HFIP appeared to be the sol-
vent of choice. Under identical conditions, the TOFs and
yields derived in HFIP were superior to those from TFE.
Application of an even more highly concentrated hydrogen
peroxide solution led, similar to the case of cyclooctene, to
higher yields. This difference is more pronounced for 1-oct-
ene than for cyclooctene. Hence, the water concentration
plays a more significant role in the oxidation of 1-octene
than in the oxidation of cyclooctene. The oxidation of 1-
octene is much slower than the oxidation of cyclooctene
(see TOFs in Table 5) and competing reactions including
water have a greater impact. The most interesting point is
the influence of the temperature. It has an opposite effect
to that observed in the epoxidation of cyclooctene. At room
temperature, the yields and TOFs are higher, especially in
HFIP (Table 5, Entries 3–6). A reason for this is most likely
the lower reactivity of 1-octene. The faster decomposition
of the catalyst at higher temperatures has a smaller effect
than the increase in the general activity of 1-octene
achieved by higher temperatures.
Experimental Section
General Methods: All solvents, as well as compounds 1–3, were
purchased from commercial sources and used without further puri-
fication. Compound 4 was synthesized according to a literature
procedure.[5] HFIP and TFE are toxic solvents and should be han-
dled with great care and with the appropriate safety precautions.
MTO-Catalyzed Epoxidation: To a solution of MTO was added the
ligand, the solvent (8 mL), the substrate (12 mmol, 1 equiv.), and
H2O2 (27%, 24 mmol, 2 equiv.). The reaction was stirred at a se-
lected temperature and samples were taken at different reaction
times. The samples were treated with MnO2 to destroy hydrogen
peroxide and subsequently dried with MgSO4, filtered and analyzed
by gas chromatography (standards: indene/p-xylene).
17O NMR: NMR spectra were recorded with a Bruker Avance
DPX-400 spectrometer. An encapsulated water sample inside the
NMR tube was used as reference.
The use of limonene as a substrate led to 37% yield of
the product in TFE and 46% yield in HFIP. Both yields
remain constant after 5 min. This indicates a very fast de-
composition of the catalyst in the presence of this substrate.
Only 8–10% of α-pinene is oxidized with a follow-up diol
formation. The fluorinated solvents used here seem to cre-
ate a media too active for such labile epoxides.
Acknowledgments
P. A. thanks TUM-Graduate School for financial support.
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Received: February 7, 2012
Published Online: May 22, 2012
Eur. J. Inorg. Chem. 2012, 3235–3239
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
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