Epoxidation of α-pinene catalyzed by methyltrioxorhenium(VII): Influence of additives, oxidants and solvents

Article abstract of DOI:10.1016/j.molcata.2011.03.017

The epoxidation of α-pinene employing methyltrioxorhenium as catalyst is examined. The influence of mono- and bidentate Lewis basic additives (e.g. ^tbutylpyridine, 4,4′-dimethyl-2,2′-bipyridine, and Schiff-bases) is investigated. Additionally the impact of the oxidant (H ₂O₂ in water or urea-hydrogen peroxide (UHP)) on the catalytic performance is studied. The effect of the solvent is also examined in order to determine the optimal conditions for the epoxidation of α-pinene. The best and straightforwardly applicable result is obtained when a ratio α-pinene:MTO:^tbutylpyridine:UHP of 200:1:40:600 is applied at 0 °C in nitromethane. In this case, α-pinene oxide is formed with 95% yield after 3 h with a turnover frequency (TOF) of 610 h^-1.

Full text of DOI:10.1016/j.molcata.2011.03.017

Journal of Molecular Catalysis A: Chemical 340 (2011) 9–14
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Editor’s Choice paper
Epoxidation of ␣-pinene catalyzed by methyltrioxorhenium(VII): Influence of
additives, oxidants and solvents
Typhène Michela, Daniel Betzb, Mirza Cokojab, Volker Sieberc, Fritz E. Kühna,b,∗
a Molecular Catalysis, Catalysis Research Center, Technische Universität München, Ernst-Otto-Fischer-Str. 1, D-85747 Garching bei München, Germany
b Chair of Inorganic Chemistry, Catalysis Research Center, Technische Universität München, Ernst-Otto-Fischer-Str. 1, D-85747 Garching bei München, Germany
c Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB, Project Group BioCat, c/o Chair of Biogenic Resources, Technische Universität München,
Schulgasse 16, D-94315 Straubing, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
The epoxidation of ␣-pinene employing methyltrioxorhenium as catalyst is examined. The influence of
mono- and bidentate Lewis basic additives (e.g. tbutylpyridine, 4,4ꢀ-dimethyl-2,2ꢀ-bipyridine, and Schiff-
bases) is investigated. Additionally the impact of the oxidant (H2O2 in water or urea–hydrogen peroxide
(UHP)) on the catalytic performance is studied. The effect of the solvent is also examined in order to deter-
mine the optimal conditions for the epoxidation of ␣-pinene. The best and straightforwardly applicable
result is obtained when a ratio ␣-pinene:MTO:tbutylpyridine:UHP of 200:1:40:600 is applied at 0 ◦C in
nitromethane. In this case, ␣-pinene oxide is formed with 95% yield after 3 h with a turnover frequency
Received 30 January 2011
Received in revised form 21 March 2011
Accepted 21 March 2011
Available online 31 March 2011
Keywords:
Homogeneous catalysis
Epoxidation
Methyltrioxorhenium
Terpenes
(TOF) of 610 h−1
.
© 2011 Elsevier B.V. All rights reserved.
Pinene oxide
1. Introduction
prevent diol formation nor does it improve the epoxide yields
[9,10]. A change of the oxidant to urea hydrogen peroxide (UHP)
and addition of the base (+)-2-aminomethylpyrrolidine does not
lead to a better selectivity according to the literature reports
[11]. The reported conversion is still very low (<7%), when MTO
is anchored to polymers such as poly(vinylpyridine) (PVP) or
poly(vinylpyridine N-oxide) (OPVP) [12]. Although some reports
claim that with MTO as catalyst, nearly quantitative yields of ␣-
pinene oxide can be reached [13], we were unable to reproduce
these results. Accordingly, the need for a straightforward, unprob-
lematic procedure towards ␣-pinene oxide still remains. A wide
range of other metals has been applied for the epoxidation of ␣-
pinene. Molybdenum(VI) complexes have been studied for this
reaction [14–20]. Co(III) supported material on hexagonal meso-
porous silica (HMS) or Co supported by zeolite (NaCoY93) was
investigated [21,22]. The synthesis of ␣-pinene oxide was also
carried out with [Cu(pyridine)2Cl2] and the[CrO3(2,2ꢀ-bipyridine)]
as catalysts [23]. Lipase activity towards epoxidation of ␣-pinene
has been studied [24,25]. [Ru(salophen)Cl-PSI] (PSI = polystyrene-
bound imidazole) with NaIO4 has also been employed for the
formation of ␣-pinene oxide [26]. The epoxidation of ␣-pinene
using (N,N-bis(salicylidene)ethylenediammine)Ti-salan complex
is reported to lead to 85% epoxide yield after 7 h reaction time
[27]. Based on these results, we set out to re-examine MTO as cata-
lyst in the presence of Lewis basic additives, since such MTO based
systems proved to be superior to other catalyst systems in many
epoxidation reactions, provided optimal conditions were found.
␣-Pinene is a monoterpene which is an important ingredient
of some flavoring plants such as mint, lavender and ginger. It is
extracted from turpentine oil (350,000 t/year), which is a product
of the paper pulp industry [1]. ␣-Pinene is particularly utilized
as starting material in terpene chemistry to obtain flavors and
fragrances. It is also an intermediary species in the synthesis of
Taxol®, an anticancer drug [2–4]. The epoxidized product of ␣-
pinene, ␣-pinene oxide is used to synthesize campholeic aldehyde,
an intermediate in the synthesis of the sandalwood fragrance [1].
␣-Pinene oxide is additionally employed in the synthesis of trans-
carveol, which is an important component for perfume bases and
food flavor compositions (Scheme 1) [5].
The epoxidation of ␣-pinene in homogeneous phase has been
studied in some detail in the past. Among the homogeneous
catalysts applied for this reaction, methyltrioxorhenium [6–8]
(MTO) is among the most widely used. However, in almost all
cases the epoxidation suffers from either low conversion or low
yield because of the formation of ␣-pinene diol as byproduct.
Using the MTO–Lewis base adducts as catalysts does neither
∗
Corresponding author at: Chair of Inorganic Chemistry / Molecular Catalysis,
Catalysis Research Center, Technische Universität München, Ernst-Otto- Fischer-
Str. 1, D-85747, Garching bei München, Germany. Tel.: +49 89 289 13081;
fax: +49 89 289 13473.
E-mail address: fritz.kuehn@ch.tum.de (F.E. Kühn).
1381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.03.017

10
T. Michel et al. / Journal of Molecular Catalysis A: Chemical 340 (2011) 9–14
2.3. Epoxidation of ˛-pinene in different solvents
2.3.1. Method A
MTO was dissolved in the solvent and the solution brought to the
appropriate reaction temperature. The ligand, the two standards
(mesitylene: 100 ␮L and ethylbenzene: 100 ␮L), and the oxidant
(H2O2 35% or UHP) were added to the solution. ␣-Pinene was then
added to the reaction. The different catalyst:ligand:oxidant ratios
are given in Table 1. Samples were taken after 5 min, 10 min, 15 min,
30 min, 60 min, 90 min, 3 h, 5 h and 24 h. For each sample, 200 ␮L of
the reaction mixture was taken and mixed with a catalytic amount
of MnO2 to decompose excess H2O2. The mixture was then filtered
through MgSO4 in order to remove H2O. CH2Cl2 (1.8 mL) was then
added and the solution was analyzed by GC or stored in the freezer
for several hours.
Scheme 1. Reaction of ␣-pinene oxide to campholeic aldehyde and to trans-carveol.
In this method, the samples are not stable, consequently, it is
important to measure them directly in GC or to store them in the
freezer. It is not possible to do several reaction in the same time.
Consequently, a new method was found to stabilized the GC sam-
ples and allowed measurement over night.
Additionally, MTO requires H2O2 as oxidant (Scheme 2), which
has the advantage of being environmentally benign and cheap in
comparison to other conventional oxidants used in epoxidations,
e.g. meta-chloroperoxybenzoic acid (mCPBA).
Nevertheless, due to the high Lewis acidity of the Re(VII) center
in presence of H2O (the byproduct formed during the oxidation),
ring opening of (sensitive) epoxides to diols occurs. It was shown
that employing Lewis basic additives (Lewis base ligands) decreases
the acidity of the rhenium center [8] and allows the synthesis of
sensitive epoxides [9,28–33]. Particularly the use of pyridine and
some of its derivatives, as well as Schiff base ligands leads in many
cases to both good activity and high selectivity towards epoxidation
of olefins (Scheme 2) [34–42].
In this work, we investigated the optimal and reproducible
conditions for the epoxidation of ␣-pinene with MTO as catalyst.
For this purpose, several mono- and bidentate base adducts were
applied. Additionally, to eliminate potential problem sources, the
effect of using different oxidants and reaction media, including
ionic liquids, was examined.
2.3.2. Method B
MTO was dissolved in the solvent and the solution brought to the
appropriate reaction temperature. The ligand, the two standards
(mesitylene: 250 ␮L and ethylbenzene: 250 ␮L), and the oxidant
(H2O2 35% or UHP) were added to the solution. ␣-Pinene was then
added to the reaction. The different catalyst:ligand:oxidant ratios
are given in Table 2. Samples were taken after 5 min, 30 min, 2 h, 5 h
and 24 h. For each sample, 1.5 mL of the reaction mixture was taken
and mixed with a catalytic amount of MnO2 to decompose excess
H2O2. The mixture was then filtered and extracted 4 times with
1.5 mL of water to remove MTO. The organic layer was then dried
over MgSO4 and filtered. 0.18 mL (for H2O2) or 0.2 mL (for UHP)
was taken from this solution and diluted with 1.3 mL of CH2Cl2.
The solution was then analyzed by GC.
2.4. Epoxidation of ˛-pinene in ionic liquids
2. Experimental
MTO (0.024 mmol) and tbutylpyridine (5 equiv.) were dis-
solved in 1-butyl-3-methylimidazoliumhexafluorophosphate
([BMIM]PF6) (0.2 mL). In a separate flask, mesitylene (100 ␮L),
ethylbenzene (100 ␮L) and ␣-pinene (100 equiv.) were mixed
together and a blank sample was taken from this solution. UHP
or aqueous H2O2 (300 equiv.) was then added to the ionic liquid
followed by the solution of ␣-pinene. The sampling time is the
same as for conventional solvents. At defined times, 41 ␮L of the
organic phase was poured in a vial containing MnO2 in order to
destroy excess oxidant, dried over MgSO4 and filtered. 2 mL of
CH2Cl2 were then added and the sample was analyzed by GC.
2.1. Starting materials
Hydrogen peroxide was used 35% in water (Aldrich). (1R)-(+)-
␣-pinene was purchased from Aldrich. Urea Hydrogen Peroxide
(UHP) contained 35 wt.% H2O2 (Acros organics). Methyltriox-
orhenium and the ionic liquid 1-butyl-3-methylimidazolium
hexafluorophosphate [BMIM]PF6 were synthesized according to
literature procedures [43–46].
2.2. Gas chromatography
3. Results and discussion
Gas chromatography was performed using a DB23 column
(30 m, 0.25 mm, 0.25 ␮m film thickness). The isothermal temper-
ature profile is: 60 ◦C for the first 2 min, followed by a 10 ◦C/min
temperature gradient to 105 ◦C for 10 min, then by 4 ◦C/min to
155 ◦C and finally 20 ◦C/min to 260 ◦C. The injector temperature
was 320 ◦C. Chromatography grade helium was used as the carrier
gas.
3.1. Optimization of the conditions for the epoxidation of
˛-pinene
3.1.1. Lewis base concentration and temperature effects
In
a previous work, optimal conditions for cyclooctene
epoxidation with the system MTO/tbutylpyridine/H2O2 were
determined [35]. The best result was obtained when a molar ratio
cyclooctene:MTO:tbutylpyridine:H2O2 of 100:1:5:300 in CH2Cl2 at
25 ◦C was applied.
The same conditions were used for the epoxidation of ␣-pinene.
However, the formation of ␣-pinene diol was observed. From this
observation it was deduced that ␣-pinene oxide appears to be more
acid sensitive than cyclooctene oxide, being easily transformed to
␣-pinene diol in the presence of MTO and water (Scheme 3).
Scheme 2. Reaction of a MTO-Lewis base complex with H2O2 forming a mono- and
bis(peroxo) complex, L = electron donor ligand [9,28–33].

T. Michel et al. / Journal of Molecular Catalysis A: Chemical 340 (2011) 9–14
11
Table 1
Summary of the different ratios employed in the epoxidation of ␣-pinene.
Entry
␣-Pinene ratio
MTO ratio (mmol)
tbutylpyridine ratio
H2O2 ratio
CH2Cl2 (mL)
1
2
3
4
100
100
100
100
1 (0.048)
1 (0.048)
1 (0.048)
1 (0.048)
5
10
20
20
300
300
300
150
2.3
2.3
2.3
1.2
Table 2
Summary of the different ratios employed in the epoxidation of ␣-pinene.
Entry
␣-Pinene ratio
MTO ratio (mmol)
Liganda ratio
H2O2 ratio
Solventb (mL)
1
2
3
4
100
100
100
100
1 (0.12)
20
10
2
150
150
150
150
7.6
7.6
7.6
7.8
0.5 (0.06)
0.1 (0.012)
1 (0.06)
20
Entry
␣-Pinene ratio
MTO ratio (mmol)
Liganda ratio
UHP ratio
Solventb (mL)
5
6
7
8
100
100
100
100
1 (0.12)
0.5 (0.06)
1 (0.12)
20
20
20
20
150
150
300
300
8.4
8.4
8.4
8.4
0.5 (0.06)
a
tbutylpyridine, 4,4ꢀ-dimethyl-2,2ꢀ-bipyridine or two Schiff-bases.
CH2Cl2, MeNO2, CHCl3 or THF.
b
3.1.2. Oxidant concentration effects
Another method to reduce the diol formation is to decrease the
concentration of H2O2 in the reaction (see Fig. 2).
In the case of a molar MTO:H2O2 ratio of 1:300, the epoxide yield
reaches a maximum (82%) after 1.5 h (conversion ␣-pinene = 96%)
and then decreases, whereas in the case of a ratio of 1:150, the con-
version is somewhat slower and reaches a maximum epoxide yield
of 81% after 3 h (conversion ␣-pinene = 84%). Follow up diol for-
mation is largely suppressed in the latter case. It is therefore more
economical to use the lower concentration of hydrogen peroxide
for the epoxidation of ␣-pinene.
Scheme 3. Reaction from ␣-pinene to ␣-pinene diol in presence of MTO/H2O2 and
water.
Accordingly, the Lewis acidity of the system has to be reduced by
addition of an appropriate Lewis-base. Due to the beneficial effects
of excess Lewis-base, originating from the weak Re–N interaction
[47], an excess of tbutylpyridine was applied. To strengthen the
base effect the reaction temperature was kept low.
All kinetic curves depicted in Fig. 1 show a feature which has
not yet been described in detail in the previous reports on ␣-pinene
epoxidation. The formation of ␣-pinene oxide is fast at the begin-
ning of the reaction but after some time, the product yield not only
becomes stagnant but decreases, reflecting the somewhat slower
reaction to the respective diol becoming dominant. The formation
of ␣-pinene diol is observed for all MTO:tbutylpyridine ratios. How-
ever, a ratio of 1:20 at 0 ◦C leads to the highest yield of ␣-pinene
oxide observed. Besides, the diol formation is less pronounced in
comparison to the other cases. Thus, in the following reactions, the
molar ratio between MTO and the ligands was kept at 1:20 and the
reactions were performed at 0 ◦C.
3.1.3. Catalyst concentration effects
The concentration of MTO was decreased to establish the min-
imum concentration of catalyst needed to perform this reaction
without activity loss. The ratio ␣-pinene:tbutylpyridine:H2O2 used
was 10:2:15.
Decreasing the concentration of MTO in the reaction from
1 mol% to 0.5 mol% leads to quite similar results (Fig. 3). The forma-
tion of ␣-pinene oxide is slightly higher after 2 h with 1 mol% (84%
yield) than with 0.5 mol% (77% yield). Yet, the long-term activity
(>1 d) of the catalyst is decreasing for concentrations below 1 mol%;
the activity decreases more significantly when the concentration of
MTO in the reaction is reduced to 0.1 mol%. To ensure both max-
imum yield and good activity the concentration of MTO was kept
at a 1 mol% level for all following reactions. The poor activity at a
catalyst concentration of 0.1 mol% may be due to several effects:
Fig. 1. Kinetics of the ␣-pinene epoxidation with different molar ratios of
MTO:tbutylpyridine at 0 ◦C and 25 ◦C, ratio ␣-pinene:MTO:H2O2 is 100:1:300 in
CH2Cl2.
Fig. 2. Kinetics of the ␣-pinene epoxidation at 0 ◦C with a molar ratio of ␣-
pinene:MTO:tbutylpyridine 100:1:20 at different MTO:H2O2 ratios in CH2Cl2.

12
T. Michel et al. / Journal of Molecular Catalysis A: Chemical 340 (2011) 9–14
Table 3
Comparison of the influence of ligands 1–4 on the epoxidation of ␣-pinene.
Ligand
Yield ␣-pinene
TOF (h−1
)
Observation
oxide (time)
1
2
3
4
85% (5 h)
81% (5 h)
6% (5 h)
390
200
20
Minor ␣-pinene diol formation
Minor ␣-pinene diol formation
␣-pinene diol as main product
␣-pinene diol as main product
11% (5 h)
20
the formation of ␣-pinene oxide (Table 3). In both examined cases,
the main product is ␣-pinene diol. Consequently, tbutylpyridine
(1) appears to be the best of the examined ligands as additive for
the epoxidation of ␣-pinene. It was thus used for the following
experiments.
Fig. 3. Kinetics of the epoxidation of ␣-pinene with different MTO concentrations
in CH2Cl2.
first, MTO can decompose to methanol and perrhenate to a higher
degree in the presence of large excess of oxidant [31]. Second, the
Lewis base can also be oxidized in the presence of H2O2 [43,44,48].
Oxygen donor adducts of MTO, however, which are formed during
this ligand oxidation process are less active than N-base adducts
[43,44,48]. Third, impurities of oxidant and solvent have a stronger
impact on catalyst deactivation when very low catalyst amounts are
applied. In practical applications, however, a rigorous cleaning of
the ingredients of the reaction mixture might be counterproductive
with respect to reaction efficiency and costs.
3.2.2. Oxidant effect
In all the previous experiments, ␣-pinene diol formation was
observed. It was thus important to investigate other oxidants,
which might prevent this unwanted follow-up reaction. Several
reports describe the utilization of urea hydrogen peroxide (UHP)
as an alternative oxidant to aqueous H2O2 in metal complex cat-
alyzed olefin epoxidation reactions when acid sensitive epoxides
are formed [49,50].
The catalytic reaction undertaken with the system MTO:UHP
has approximately the same activity as the MTO:H2O2 system (see
Fig. 4). However, the formation of ␣-pinene diol is only observed
when H2O2 is used. UHP as oxidant does not lead to byproduct
formation. However, the use of UHP can be problematic due to
its solubility: it is barely soluble in most common organic sol-
vents. Therefore, the real concentration of UHP in solution is hardly
predictable, and the reaction cannot be described as really homo-
geneous. Based on these considerations, both UHP and H2O2 have
certain advantages as oxidants for the epoxidation of ␣-pinene and
will be used in the following experiments.
3.2. Ligand-, oxidant-, and solvent-influence on the epoxidation
of ˛-pinene
3.2.1. Ligand effects
The influence of
bipyridine), two different Schiff base ligands (2-[(E)-(phenylimino)
methyl]phenol and 2,4-dichloro-6-[[(2-chlorophenyl)imino]
a
bidentate ligand (4,4ꢀ-dimethyl-2,2ꢀ-
methyl]phenol) on the ␣-pinene oxide yield was examined
(Scheme 4). The epoxidation of ␣-pinene was originally performed
with a ratio substrate:MTO:ligand:H2O2 (100:1:20:150) in CH2Cl2
at 0 ◦C. The results are summarized in Table 3.
3.2.3. Solvent effects
The activity of the catalytic system MTO:tbutylpyridine
Nitromethane, the ionic liquid ([BMIM]PF6), CHCl3 and THF
were selected as solvents. Sharpless et al. demonstrated that the
use of nitromethane and dichloromethane as solvent leads to simi-
lar efficiency in the epoxidation of cyclohexene [33]. Previous work
[36,51–56] on epoxidation reactions in ionic liquids shows in many
cases a yield improvement compared to organic solvents [52].
The epoxidation of ␣-pinene is favored when dichloromethane
is used as solvent at 0 ◦C. The use of other solvents such as
nitromethane, chloroform or THF leads to lower formation of
␣-pinene oxide. However, the turnover frequencies measured
for dichloromethane and nitromethane are close (see Table 4).
In the case of the ionic liquid [BMIM]PF6, the measurements
proved to be problematic due to a significant amount of solid
residue. Nevertheless, ␣-pinene diol appears to be the main prod-
(1):H2O2 (TOF = 390 h−1
) is higher than the catalytic sys-
tem MTO:4,4ꢀ-dimethyl-2,2ꢀ-bipyridine (2):H2O2 (TOF = 200 h−1).
Moreover, the formation of ␣-pinene diol is less pronounced when
tbutylpyridine (1) is employed as Lewis basic additive, compared
to 4,4ꢀ-dimethyl-2,2ꢀ-bipyridine (2). In contrast to pyridine-based
Lewis bases, Schiff-base ligands 3 and 4 lead to a low selectivity in
Scheme 4. Lewis base ligands used in this study: 4-tert-butylpyridine (1),
4,4ꢀ-dimethyl-2,2ꢀ-bipyridine (2), 2-[(E)-(phenylimino)methyl]phenol (3) and 2,4-
dichloro-6-[[(2-chlorophenyl)imino]methyl]phenol (4).
Fig. 4. Kinetics of the ␣-pinene epoxidation in CH2Cl2 at 0 ◦C employing a ratio
␣-pinene:MTO:tbutylpyridine:oxidant (100:1:20:150) with different oxidants.

T. Michel et al. / Journal of Molecular Catalysis A: Chemical 340 (2011) 9–14
13
Table 4
support of the Graduate School of Chemistry of the Technis-
che Universität München. D. Betz is thankful to the Bayerische
Forschungstiftung for a PhD grant and T. Michel is acknowl-
edged to the Fraunhofer society for the financing of her PhD
position.
Comparison of the influence of H2O2 and UHP in different solvents.a
Entry
Solvent
Yield % (TOF h−1
H2O2
)
UHP
1
2
3
4
CH2Cl2
MeNO2
CHCl3
THF
85 (400)
51 (330)
87 (100)
47 (170)
93 (150)
63 (290)
83 (100)
29 (120)
References
a
Reaction
condition:
ratio
␣-pinene:MTO:tbutylpyridine:oxidant
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Further experiments were done to determine the optimal condi-
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