Selective epoxidation of (+)-limonene employing methyltrioxorhenium as catalyst

Article abstract of DOI:10.1016/j.molcata.2012.03.011

This report presents a study of the epoxidation of limonene employing methyltrioxorhenium (MTO) as catalyst. The influence of base ligands, namely t-butylpyridine, 4,4′-dimethyl-2,2′-bipyridine and pyrazole on the catalytic activity was investigated. The choice of the oxidant (H ₂O₂ in water or H₂O₂ stabilized by urea) was also examined. The effect of the solvent has been studied in order to determine optimal conditions for the epoxidation of (+)-limonene. The best result was obtained when a molar ratio (+)-limonene:MTO:H₂O ₂:t-butylpyridine of 100:0.5:10:150 was used at 25 °C in dichloromethane. 1,2-Limonene oxide was formed with 77% yield and 96% selectivity after 1 h with a TOF of ca. 900 h^-1.

Full text of DOI:10.1016/j.molcata.2012.03.011

Journal of Molecular Catalysis A: Chemical 358 (2012) 159–165
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Selective epoxidation of (+)-limonene employing methyltrioxorhenium as
catalyst
Typhène Michela, Mirza Cokojab, Volker Sieberc, Fritz E. Kühna,b,∗
a Molecular Catalysis, Catalysis Research Center, Technische Universität München, Ernst-Otto-Fischer Straˇe 1, D-85747 Garching bei München, Germany
b Chair of Inorganic Chemistry, Catalysis Research Center, Technische Universität München, Ernst-Otto-Fischer Straˇe 1, D-85747 Garching bei München, Germany
c Chair of Chemistry of Biogenic Resources, Straubing Science Centre, 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:
This report presents a study of the epoxidation of limonene employing methyltrioxorhenium (MTO)
as catalyst. The influence of base ligands, namely t-butylpyridine, 4,4ꢀ-dimethyl-2,2ꢀ-bipyridine and
pyrazole on the catalytic activity was investigated. The choice of the oxidant (H2O2 in water or H2O2
stabilized by urea) was also examined. The effect of the solvent has been studied in order to deter-
mine optimal conditions for the epoxidation of (+)-limonene. The best result was obtained when a molar
ratio (+)-limonene:MTO:H2O2:t-butylpyridine of 100:0.5:10:150 was used at 25 ◦C in dichloromethane.
Received 12 November 2011
Received in revised form 9 March 2012
Accepted 12 March 2012
Available online 21 March 2012
Keywords:
Homogeneous catalysis
Epoxidation
1,2-Limonene oxide was formed with 77% yield and 96% selectivity after 1 h with a TOF of ca. 900 h−1
.
© 2012 Published by Elsevier B.V.
Methyltrioxorhenium
Terpenes
1,2-Limonene oxide
N-Base adduct
1. Introduction
particularly used as chiral precursors [6]. 1,2-Limonene oxide has
also being used as a bio-renewable monomer in the formation of
biodegradable polymers via copolymerization with CO2 [1,7].
Epoxidation reactions have been extensively studied in the
past. The organic peroxyacids, such as m-chloroperbenzoic acid
are still the most widely used epoxidation agents, employing the
stoichiometric peracid route. However, they are economically and
environmentally undesirable as they produce waste, containing
their corresponding acids, and are not selective in the formation of
epoxides and their cleaved products, especially in the preparation
of acid-sensitive epoxides [8,9]. According to literature reports on
limonene epoxidation employing homogeneous catalysts, the for-
mation of 1,2-limonene oxide (1) is challenging, since it competes
with the formation of byproducts, for example 8,9-limonene oxide
(2) and 1,2–8,9-limonene dioxide (3, also named dipentene diox-
ide). Moreover, other byproducts such as carvone (4) and carveol
(5) are also observed depending on the catalyst used. This problem
is different from difficulties encountered during the epoxidation of
␣-pinene. In the latter case, the major issue was to decrease the
acidity of the rhenium center in order to avoid ring opening of the
epoxide [10] (Scheme 2).
Monocyclic monoterpene hydrocarbons occur in many essen-
tial oils and their byproducts. Limonene is one of the most common
monoterpenes and a widely used feedstock (30,000 tons per year)
[1]. For example, limonene can be found in cosmetics, as fragrance
in perfume and as a flavoring to mask the bitter taste of alka-
loids. It is used as a precursor of carvone in chemical synthesis
and applied as solvent in cleaning products. It exists in two opti-
cally active forms: d-limonene, possessing a strong orange smell is
the main component of citrus oil, and l-limonene, which is found
in pinewood and has a piney, turpentine-like odor [2,3]. Racemic
limonene is known as dipentene (Scheme 1).
Terpene oxides such as 1,2-limonene oxide have many applica-
tions in synthetic chemistry. They are, in fact, the most important
members of the terpene family for the perfume industry and
are widely used as raw materials in the manufacture of a range
of important commercial products [4,5]. Optically pure epoxides,
such as 1,2-limonene oxide and their corresponding 1,2-diols are
important building blocks in asymmetric synthesis. They are more
The epoxidation of limonene with Al2O3 as catalyst leads to
rather low conversion (max. 70% after 4 h) and the formation of
both monoepoxides and diepoxide [11]. The selectivity towards
1,2-limonene oxide is in the best case around 90%. Cobalt base
complexes, employed in the oxidation of limonene, lead to low
∗
Corresponding author at: Molecular Catalysis, Catalysis Research Center, Tech-
nische Universität München, Ernst-Otto-Fischer Stra␤e 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 © 2012 Published by Elsevier B.V.
doi:10.1016/j.molcata.2012.03.011

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T. Michel et al. / Journal of Molecular Catalysis A: Chemical 358 (2012) 159–165
In the present work, the optimal conditions for the regioselec-
tive epoxidation of (+)-limonene to 1,2-limonene oxide employing
MTO as catalyst were investigated. For this purpose, several mono-
and bidentate base adducts have been added to MTO. Additionally,
the effect of using different oxidants and different reaction media
was examined in order to study the optimal reaction conditions
for the synthesis of 1,2-limoneneoxide in high selectivity. Under
the conditions examined, only dipentene dioxide was detected as
byproduct. Another part of the work is dedicated to the study of
the conditions favoring the formation of dipentene dioxide. It is
of interest to investigate formation conditions of the byproduct in
order to optimize the reaction conditions in a way to produce only
one single product or one product in very large excess.
Scheme 1. Structures of (S)-(−) and (R)-(+)-limonene.
conversion (40%) and the formation of carvone and carveol as
main products [12,13]. Dimethyldioxirane allows the formation of
1,2-limonene oxide with 94% selectivity [14]. However, the con-
version remains rather low (71%). Iron-based catalysts employed
in the epoxidation of limonene yield a mixture of carvone, carveol
and 1,2-limonene oxide [15]. Jacobsen’s catalyst allows high con-
versions of limonene (up to 100%) with a very good selectivity
(90%) towards dipentene dioxide [16]. However, employing N-
methylimidazole as additive leads to a decrease in conversion
(70%) associated with enhanced selectivity towards 1,2-limonene
oxide (74%) [17]. Begué et al. used Mn(OAc)3·2H2O as catalyst with
O2/pivalaldehyde as oxidant in perfluoro-2-butyltetrahydrofuran,
with 1,2-limonene oxide formed in 96% yield after 1 h [18]. Employ-
ing Mo based catalysts in limonene epoxidation leads, in general,
to either low conversions or low selectivities towards 1,2-limonene
oxide. A microwave-assisted study of the epoxidation of limonene
employing [CpMo(CO)3CH3] as catalyst reports the formation of
1,2-limonene oxide with 93% selectivity at 80% conversion [19].
The best results reported to date are, however, heterogeneously
catalyzed processes. For instance, polyoxometalate catalysts lead
to the formation of 1,2-limonene oxide with 98% selectivity at 99%
conversion after 30 min at room temperature [20]. As seen in the
previously published results of the epoxidation of limonene, the
main challenge remains to favor the formation of 1,2-limonene
oxide in high selectivity. Methyltrioxorhenium (MTO) is well estab-
lished as an very efficient catalyst for olefin epoxidation reactions
[21–23]. Some studies described the reactivity of MTO towards
the epoxidation of limonene to 1,2-limonene oxide [24]. The best
results known up to date were reported by Rudler et al., reaching
a conversion of limonene of 98% and a 1,2-limonene oxide selec-
tivity of 86% after 2 h at 4 ◦C [25]. However, despite all efforts we
were unable to reproduce these results (see Section 2.3). Under
the same conditions, we observed several other byproducts besides
DPO, and the yield of the main product decreased constantly with
the reaction time. Thus, we set out to reinvestigate the optimal
conditions for an efficient and selective epoxidation of limonene to
1,2-limonene oxide employing MTO as catalyst. Hydrogen peroxide
was chosen as oxidant because of its environmental and economic
advantages [5]. Its only drawback, the ring opening of sensitive
epoxides at the Lewis acidic Re center, can be overcome by the use of
nitrogen containing Lewis bases such as pyridine and derivatives,
suppressing the formation of diols by reducing the Lewis acidic
properties of MTO [26].
2. Experimental
2.1. Starting materials
All commercial products were of the highest grade available and
were used as such. A 35% solution of H2O2 in water (Aldrich) was
used for the catalytic test reactions. (+)-Limonene was obtained
from Aldrich. UHP contains 35 wt.% H2O2 (Acros Organics).
Methyltrioxorhenium was synthesized according to the literature
[27].
2.2. Gas chromatography
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 4 ◦C/min to
140 ◦C and finally 10 ◦C/min to 260 ◦C. The injector temperature
was 320 ◦C. Chromatography grade helium was used as carrier
gas.
2.3. Literature conditions [25]
Limonene (6 mmol) was dissolved in CH2Cl2 (5 mL) and cooled
to 4 ◦C. MTO (1 mol%/olefin) and then 150 equiv. H2O2 (10% in
water) were added to the solution. The reaction was stirred for
2 h, quenched with MnO2 and analyzed by GC. Result: Conversion
limonene = 99%; yield 1,2-limonene oxide = 47%; yield dipentene
dioxide = 23%. Moreover, other byproducts (29%), among them 8,9-
limonene oxide and the corresponding diols of the found epoxides,
were observed in the GC spectrum which explain the low 1,2-
limonene oxide yield obtained compared to the conversion of
limonene.
2.4. Epoxidation of (+)-limonene in different solvents
MTO was dissolved in the solvent and the solution was let at
25 ◦C or cooled at 0 ◦C. The ligand, the 2 standards (mesitylene:
0.5 mL and naphthalene (solution 2 g in 10 mL CH2Cl2): 0.5 mL),
Scheme 2. Possible products of the oxidation of limonene.

T. Michel et al. / Journal of Molecular Catalysis A: Chemical 358 (2012) 159–165
161
Table 1
Summary of the different molar ratio employed in the epoxidation of (+)-
limonene at-butylpyridine, 4,4ꢀ-dimethyl-2,2ꢀ-dipyridine, pyrazole bH2O2, UHP
cCH2Cl2, MeNO2, CHCl3, nBuOH or THF.
T (◦C)
Reaction condition: molar ratio
(+)-Limonene
MTO
t-Butylpyridine
H2O2
CH2Cl2 (mL)
25 and 0
25
25
25
25
25
25
25
25
100
100
100
100
100
100
100
100
100
100
100
100
1
1
1
1
1
1
0.1
1
1
20
20
20
10
5
300
150
100
150
150
150
150
300
300
600
600
600
6.4
7.1
7.3
7.2
7.2
7.3
7.3
6.3
6.1
4.9
4.8
4.6
0
2
40
60
20
40
60
25
25
25
1
1
1
Fig. 1. Kinetics of the oxidation of (+)-limonene.
3.2. Epoxidation of (+)-limonene: formation of 1,2-limonene
oxide
T (◦C)
Reaction condition: molar ratio
(+)-Limonene
100
MTO
0.5
Liganda
10
Oxidantb
150
Solventc (mL)
7.2
3.2.1. Oxidation of (+)-limonene, determination of the byproducts
25
The formation of 1,2-limonene oxide (LO) and byproducts is
investigated. For this purpose, the molar ratio (+)-limonene:MTO:t-
butylpyridine:H2O2 of 100:1:20:300 is applied in CH2Cl2 at 25 ◦C.
As depicted in Fig. 1, under the applied conditions the formation
of 1,2-limonene oxide (LO) is fast at the beginning of the reaction.
The yield of 1,2-limonene oxide reaches a maximum after 15 min
and then decreases. The byproduct observed during the epoxida-
tion of (+)-limonene (L) employing MTO as catalyst is dipentene
dioxide (DPO). Other byproducts, e.g. 8,9-epoxide or (+)-limonene
diol, were not observed.
1 equiv. MTO correspond to 0.06 mmol; 0.5 equiv. MTO correspond to 0.03 mmol;
0.1 equiv. MTO correspond to 0.006 mmol.
and the oxidant (H2O2 35% or UHP) were added to the solution.
(+)-Limonene was then added to the reaction. 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, 1 mL of the reaction mixture was taken and neu-
tralized by MnO2. The mixture was dried over MgSO4. 200 ␮L from
the dry solution was taken, 0.5 mL of a solution of 1 mL of methyl-
naphthalene in 50 mL isopropanol and 0.8 mL of isopropanol were
mixed in a vial and analyzed by GC (Table 1).
3.2.2. Optimization of the reaction conditions
The synthesis of 1,2-limonene oxide is optimized employing
MTO as catalyst, t-butylpyridine as Lewis base adduct and aque-
ous hydrogen peroxide as oxidant in CH2Cl2. The influence of the
concentration of each reactant is investigated as well as the effect
of temperature.
3. Results and discussion
3.1. Background
3.2.2.1. Temperature influence. Applying
a
molar ratio (+)-
The epoxidation of olefins employing MTO as catalyst has been
extensively studied [28–33]. The influence of different ligands, oxi-
dants and solvents on the activity of MTO was reported in some
detail for a variety of substrates [34–36]. The most commonly
applied oxidant for the epoxidation of olefins utilizing MTO as cat-
alyst is aqueous hydrogen peroxide. It is an efficient oxidant, cheap
and environmentally friendly since the only byproduct formed is
water. However, the MTO/H2O2 system can lead to the formation
of diols by ring opening of the epoxide at the Re center, due to its
strong Lewis-acidic character [37]. This is particularly the case for
acid-sensitive epoxides. It was shown that the ring opening can be
prevented when the Lewis acidity of Re(VII) is reduced by coordina-
tion of ␴-donor ligands to Re, which are typically aromatic N-bases,
such as for example pyridine derivatives [28,29,37]. The Lewis base
adducts of MTO inducing the best activity in olefin epoxidation
are pyridine based such as t-butylpyridine or 4,4ꢀ-dimethyl-2,2ꢀ-
bipyridine [34,38,39]. Pyrazole was also described as an efficient
Lewis-base additive [40,41]. To avoid water as solvent, anhydrous
hydrogen peroxide adducts, e.g. UHP (urea hydrogen peroxide) can
be applied [35,36]. The solvents are also known to have an influ-
ence on the activity of the catalytically active complexes. The most
efficient solvents in the epoxidation of olefin employing MTO as
catalyst are non- or weakly coordinating solvents such as CH2Cl2
and CHCl3 not competing with Lewis-base and substrate for coordi-
nation sites [28,42,43]. The preliminary study on the epoxidation of
limonene described in this paper was thus undertaken with these
reactants.
limonene:MTO:t-butylpyridine:H2O2 of 100:1:20:300 in CH2Cl2
leads to slower oxidation of (+)-limonene at 0 ◦C than at 25 ◦C (see
Fig. 2). The temperature was decreased in an attempt to avoid the
formation of the diepoxide.
From the structure of (+)-limonene, it is possible to deduce that
the oxidation of the (1–2) double bond is easier than the (8–9) dou-
ble bond. Moreover, the activity of the catalytic complex decreases
with lower temperature. This property could have lead to the only
formation of 1,2-limonene oxide avoiding the epoxidation of the
second double bond, which cannot be easily oxidized at low tem-
perature. However, as depicted in Fig. 1, the selectivity towards
1,2-limoneneoxide is not improved at lower temperature. At the
highest yield of 1,2-limonene oxide under both conditions (30 min
for T = 0 ◦C and 15 min for T = 25 ◦C), the selectivity towards LO is
higher at 25 ◦C (S = 77% for 25 ◦C and S = 60% for 0 ◦C). This temper-
ature is therefore applied in all following experiments.
3.2.2.2. Influence of the oxidant concentration. Again, a molar ratio
(+)-limonene:MTO:t-butylpyridine:H2O2 of 100:1:20:X is applied
in CH2Cl2 at 25 ◦C.
The kinetics of the epoxidation of limonene depicted in Fig. 3-1
show how important it is to stop the reaction after a certain time
to obtain the best ratio of yield vs selectivity towards 1,2-limonene
oxide. In the case of the molar ratio MTO:oxidant 1:150, the selec-
tivity towards 1,2-limonene oxide is optimal (100%) after 10 min,
however, the yield is low (45%). If the reaction is stopped after

162
T. Michel et al. / Journal of Molecular Catalysis A: Chemical 358 (2012) 159–165
Fig. 2. Kinetics of the epoxidation of limonene at: (1) 0 ◦C and (2) 25 ◦C (yields are determined by GC analysis).
30 min, good yields of 1,2-limonene oxide can be reached (75%),
however the selectivity is already beyond its optimum (77%). Stop-
ping at the time with the best selectivity/yield ratio is therefore a
prerequisite. In this example, the optimal time is 15 min with 63%
yield and 92% selectivity towards 1,2-limonene oxide. The optimal
time for the molar ratio MTO:H2O2 1:300 and 1:100 are respec-
tively 15 min and 2 h. The yield and selectivity showed in Fig. 3-2
are taken for each condition at this optimal time.
The oxidant concentration leading to the highest yield is
MTO:H2O2 1:150 (Fig. 3-1). The selectivity reaches also a maxi-
mum when a ratio MTO:H2O2 of 1:150 is used (Fig. 3-2). A higher
concentration of oxidant enhances the activity of the catalytic sys-
tem. As shown in Fig. 3-1, the formation of 1,2-limonene oxide is
faster during the first 15 min. In the case of a 1:300 ratio, however,
after 15 min, the yield of 1,2-limonene oxide already decreases due
to byproduct formation. In the case of a 1:100 ratio, the formation
of LO and DPO are both slow. Consequently, the MTO:H2O2 ratio of
1:150 leads to the highest yield and selectivity within a reasonable
time reaction and is thus applied for the following experiments.
epoxidation [40,44]. The complex formed by the N-oxide ligand
with MTO is also catalytically less active than the N-base-MTO
adduct. It is therefore important to use a significant excess of
the Lewis base adduct. As depicted in Fig. 4, the highest yield
and selectivity towards 1,2-limonene oxide are reached when a
molar ratio MTO:t-butylpyridine of 1:20 is applied. When no N-
base adduct is added to the reaction, besides dipentene dioxide
several other byproducts could be observed. It can reasonably be
assumed that the byproducts are 8,9-limonene oxide and the cor-
responding diols to the found epoxides. The latter observation
shows that employing the N-base adduct prevents the formation
of other byproducts. A MTO:t-butylpyridine ratio of 1:20 leads
also to a fast formation of dipentene dioxide since the yield of
1,2-limonene oxide begins to decrease early (30 min). As a con-
sequence, a high concentration of Lewis base adduct favors the
formation of 1,2-limonene oxide. The optimal 1,2-limonene oxide
yield was obtained after 15 min with a yield of 63% and a selectivity
of 92%. Finally, the concentration of t-butylpyridine was increased
to a MTO:t-butylpyridine ratio of 1:40. Yield and selectivity towards
1,2-limonene oxide, however, decrease at this latter ratio. Conse-
quently, the ratio MTO:t-butylpyridine of 1:20 appears to lead to
the formation of 1,2-limonene oxide with the highest yield and
selectivity.
3.2.2.3. Influence of the ligand concentration. It is known that the
activity of the catalytic system increases, within certain border
lines, with the quantity of the Lewis base adduct [34,38]. The ligand
is useful to prevent the formation of byproducts forming due to
the Lewis acidity of the rhenium center [26]. The influence of
ligand concentration on the regioselectivity of the formation of 1,2-
limonene oxide is therefore important to be determined. For this
purpose, a molar ratio (+)-limonene:MTO:t-butylpyridine:H2O2 of
100:1:X:150 was applied in CH2Cl2 at 25 ◦C.
The activity of the catalytic system increases when the concen-
tration of the Lewis base increases from 1:5 to 1:20 [40]. However,
the activity of the examined catalytic system is better when no
Lewis base adduct is added than for MTO:ligand ratios of 1:5 and
1:10. The MTO/H2O2 in some cases oxidizes the N-base ligand to
N-oxide. This reaction, however, is usually slower than the olefin
3.2.2.4. Influence of the catalyst concentration. A molar (+)-
limonene:MTO:t-butylpyridine:H2O2 ratio of 100:X:20:150 is
applied in dichloromethane at 25 ◦C (Fig. 5).
The activity of the system appears to be best at the lowest
applied concentration of 0.1 mol% catalyst. At higher concentration
most likely not all catalyst molecules are involved in the reac-
tion. Additionally, a catalyst concentration of 1 mol% leads to fast
byproduct formation since the catalyst molecule not involved in
olefin epoxidation may instead promote its transfer to diepox-
ide. An optimal selectivity and yield (96 and 76%) is reached with
0.5 mol% catalyst after 1 h.
Fig. 3. Influence of the oxidant concentration on the (1) yield; (2) highest selectivity of (+)-1,2-limonene oxide at yieldoptimal. Molar ratio MTO:H2O2 (a) 1:300/topt = 15 min;
(b) 1:150/topt = 15 min; (c) 1:100/topt = 2 h.

T. Michel et al. / Journal of Molecular Catalysis A: Chemical 358 (2012) 159–165
163
Fig. 4. Influence of the ligand concentration on the (1) yield; (2) highest selectivity of 1,2-limoneneoxide at yieldoptimal. Molar ratio MTO:t-butylpyridine: (a) 1:20/topt = 15 min;
(b) 1:10/topt = 30 min; (c) 1:5/topt = 1 h; (d) 1:0/topt = 15 min.
3.2.2.5. Influence of the oxidant H2O2 vs UHP. A molar (+)-
limonene:MTO:t-butylpyridine:oxidant ratio of 100:0.5:10:150 is
applied in dichloromethane at 25 ◦C.
3.2.2.7. Solvent influence. In the epoxidation of olefins, the sol-
vent plays a crucial role. In this study, the influence of CH2Cl2,
CHCl3, THF and MeNO2, which are often used in olefin epoxida-
tion, was studied. Additionally, the alcohol n-butanol is also used
to study the influence of a one-phase system, which could be of
practical interest as well. For this purpose, the molar ratio (+)-
limonene:MTO:t-butylpyridine:H2O2 of 100:0.5:10:150 is applied
in different solvents at 25 ◦C.
In n-butanol as well as in nitromethane, the formation of 1,2-
limonene oxide is fast (5 min) but the yield does not increase
significantly any more (Fig. 8-1). Selectivity towards 1,2-limonene
oxide is ca. 60% for both solvents, however, due to byproduct
formation the conditions are not very promising. An explana-
tion for the fast activity decrease when n-butanol is used is the
presence of a one-phase system. As mentioned before, the sys-
tem MTO/H2O2 catalyzes the transformation of t-butylpyridine
to t-butylpyridine-N-oxide. In a two phase system, the N-oxide
adduct dissolves in the water phase, avoiding the formation
of less active MTO/N-oxide base adduct. In the case of a one-
phase system, a MTO/N-oxide base adduct is formed leading
to an activity decrease (see Fig. 8-1). CH2Cl2 is the solvent
allowing the highest activity of the catalytic system followed
by CHCl3 and THF (Fig. 8-1). As depicted in Fig. 8-2, the latter
order is maintained for the selectivity towards the 1,2-limonene
oxide.
Fig. 6 shows a feature which is quite different compared to
the epoxidation of ␣-pinene [26]. As for ␣-pinene, employing
MTO:UHP as catalytic system does not decrease the velocity of
the epoxidation compared to the catalytic system MTO:H2O2 but
increases the selectivity towards the desired epoxide. In case of
(+)-limonene epoxidation, the catalytic system MTO:UHP is less
efficient than MTO:H2O2 (Fig. 6-1). This decrease in catalytic activ-
ity can be explained by the fact that UHP is not soluble in most
organic solvents. The access of the oxidant and the formation of the
catalytic species are consequently slower. The selectivity towards
1,2-limonene oxide formation is not enhanced when UHP is used
as oxidant (Fig. 6-2). The highest yield (77%) and selectivity (96%)
are observed when aqueous hydrogen peroxide is used as oxidant.
3.2.2.6. Ligand
influence. t-Butylpyridine,
4,4ꢀ-dimethyl-2,2ꢀ-
bipyridine and pyrazole are applied as ligands. A molar ratio
(+)-limonene:MTO:ligand:H2O2 of 100:0.5:10:150 is applied in
dichloromethane at 25 ◦C.
It is known that N-base adduct such as t-butylpyridine and
4,4ꢀ-dimethyl-2,2ꢀ-bipyridine can be oxidized to the corresponding
N-oxide by the MTO/H2O2 system [40,44]. Pyrazole was found to be
a good alternative to the pyridine-based ligand since it is not easily
oxidized by MTO/H2O2 and its MTO adducts shows good reactivity
towards epoxidation activity[41]. As seen in Fig. 7-1, both 4,4ꢀ-
dimethyl-2,2ꢀ-bipyridine and pyrazole lead to a somewhat higher
rate of 1,2-limonene oxide formation than t-butylpyridine. How-
ever, as depicted in Fig. 7-2, the use of t-butylpyridine as N-base
adduct leads to the best selectivity towards 1,2-limonene oxide
and allows the highest product yield. Hence, in the following, t-
butylpyridine was used as additive.
3.3. Epoxidation of (+)-limonene: formation of 1,2–8,9-limonene
dioxide (DPO)
Parallel to the experiments on 1,2-limonene oxide formation,
some experiences are executed to synthesize the byproduct dipen-
tene dioxide in high yield and selectivity. Despite the main goal of
this work being to synthesize the mono-epoxide in high selectivity,
Fig. 5. Influence of the catalyst concentration on the (1) yield; (2) highest selectivity of 1,2-limonene oxide at yieldoptimal. (a) 1 mol% MTO/topt = 15 min; (b) 0.5 mol%
MTO/topt = 1 h; (c) 0.1 mol% MTO/topt = 1 h30.

164
T. Michel et al. / Journal of Molecular Catalysis A: Chemical 358 (2012) 159–165
Fig. 6. Effect of the different oxidants on the (1) yield; (2) highest selectivity of 1,2-limonene oxide at yieldoptimal. (a) Hydrogen peroxide/topt = 1 h; (b) UHP/topt = 2 h.
Fig. 7. Effect of the different Lewis bases on (1) yield; 2) (highest selectivity of 1,2-limonene oxide at yieldoptimal. (a) t-Butylpyridine/topt = 1 h; (b) pyrazole/topt = 30 min; (c)
4,4ꢀ-dimethyl-2,2ꢀ-bipyridine/topt = 30 min.
Fig. 8. Effect of the different solvents on the (a) yield; (b) highest selectivity of 1,2-limonene oxide at yieldoptimal. (a) CH2Cl2/topt = 1 h; (b) CHCl3/topt = 1 h; (c) THF/topt = 4 h;
(d) nBuOH/topt = 30 min; (e) MeNO2/topt = 30 min.
it is of interest to investigate the conditions which can lead to the
formation of the byproduct dipentene dioxide in high yields and
selectivity.
is too high (Table 3). The formation of dipentene oxide competes
with other byproducts when the molar ratio MTO:H2O2 of 1:600 is
applied leading to lower dipentene oxide yields.
A systematic study of the formation of dipentene dioxide
during the investigation of 1,2-limonene oxide synthesis shows
that a highly active catalytic system favors the byproduct for-
mation. Therefore, high concentrations of Lewis base adduct and
oxidant should allow high yield formation of dipentene oxide.
Dichloromethane was applied with a catalyst concentration of
1 mol%. In the following experiments, the effect of the concentra-
tion of either t-butylpyridine or aqueous hydrogen peroxide on the
oxidation of (+)-limonene was studied.
The conditions leading to the formation of dipentene diox-
ide in highest yield and selectivity are found applying a molar
ratio (+)-limonene: MTO:t-butylpyridine:H2O2 of 100:1:40:300 in
dichloromethane at 25 ◦C.
Table 2
Influence of Lewis base adduct concentration towards the formation of dipentene
dioxide.
As depicted in Table 2, the formation of dipentene oxide is
favored when the concentration of Lewis base adduct increases
from a MTO:ligand ratio 1:20 to 1:60. As seen in the previous para-
graph, the yield of dipentene dioxide increases with the oxidant
concentration between the molar ratio 1:100 and 1:300. How-
ever, this relation does not apply when the oxidant concentration
Conditions: molar ratio
Yieldmax DPO
Time (h)
(+)-Limonene
MTO
t-Butylpyridine
H2O2
100
100
100
1
1
1
20
40
60
600
600
600
55%
70%
83%
24
24
24

T. Michel et al. / Journal of Molecular Catalysis A: Chemical 358 (2012) 159–165
165
Table 3
Hauser, P. Altmann and T. Schröferl are acknowledged for helpful
discussions. T.M. gratefully acknowledges the support of the Grad-
uate School of Chemistry of the Technische Universität München.
Influence of oxidant concentration towards the formation of dipentene dioxide.
Conditions: molar ratio
Yieldmax DPO
Time (h)
(+)-Limonene
MTO
t-Butylpyridine
H2O2
References
100
100
1
1
40
40
300
600
90%
70%
24
24
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This work was supported by the Fraunhofer-Gesellschaft in the
frame of the project “Stereospecific Epoxidation Of Terpenes”. S.