Isomerization of α-pinene oxide in the presence of methyltrioxorhenium(VII)

Article abstract of DOI:10.1016/j.catcom.2013.02.001

The catalytic performance of methyltrioxorhenium(VII) (MTO) has been investigated for the first time in the isomerization of α-pinene oxide (PinOx) into campholenic aldehyde (CPA). The high isomerization activity of MTO is coupled with high selectivity to CPA: CPA yield of up to 87% (100% conversion) was obtained by using α,α,α-trifluorotoluene as solvent at 15 C. Catalyst recycling is possible in a relatively simple fashion by using MTO coupled to an appropriate ionic liquid. The catalytic application of MTO in the isomerization of PinOx versus the integrated epoxidation- isomerization process of the conversion of α-pinene into CPA is discussed.

Full text of DOI:10.1016/j.catcom.2013.02.001

Catalysis Communications 35 (2013) 40–44
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Isomerization of α-pinene oxide in the presence
of methyltrioxorhenium(VII)
Sofia M. Bruno a, Martyn Pillinger a, Fritz E. Kühn b, Isabel S. Gonçalves a, Anabela A. Valente a,
⁎
a
Department of Chemistry, CICECO, University of Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal
b
Chair of Inorganic Chemistry/Molecular Catalysis, Catalysis Research Center, Technische Universität München, Ernst-Otto-Fischer-Str. 1, 85747 Garching bei München, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
The catalytic performance of methyltrioxorhenium(VII) (MTO) has been investigated for the first time in the
isomerization of α-pinene oxide (PinOx) into campholenic aldehyde (CPA). The high isomerization activity of
MTO is coupled with high selectivity to CPA: CPA yield of up to 87% (100% conversion) was obtained by using
α,α,α-trifluorotoluene as solvent at 15 °C. Catalyst recycling is possible in a relatively simple fashion by using
MTO coupled to an appropriate ionic liquid. The catalytic application of MTO in the isomerization of PinOx
versus the integrated epoxidation–isomerization process of the conversion of α-pinene into CPA is discussed.
© 2013 Elsevier B.V. All rights reserved.
Received 23 January 2013
Received in revised form 6 February 2013
Accepted 7 February 2013
Available online 14 February 2013
Keywords:
Methyltrioxorhenium
Acid catalysis
Isomerization
α-Pinene oxide
Campholenic aldehyde
1. Introduction
cyclotrimerization of aldehydes to give 1,3,5-trioxanes [21], and forma-
tion of aziridines from imines [22]. MTO has been successfully used as a
East Indian sandalwood oil is one of the most precious and widely
used raw materials in the fragrance industry [1]. Dwindling resources
and leaping prices have motivated the search for synthetic substi-
tutes, of which the best are derived from campholenic aldehyde
(CPA), e.g. Sandalore® and Javanol® (Givaudan), Bacdanol® (IFF),
Brahmanol® (Dragoso) and Polysantol® (Firmenich) [1]. CPA is pre-
pared from inexpensive α-pinene, most of which is a byproduct of
the paper industry. Epoxidation of α-pinene gives α-pinene oxide
(PinOx), which rearranges under the influence of an acid catalyst to
give CPA. A wide series of other products such as trans-carveol (TCV),
trans-sobrerol (TSB), trans-pinocarveol (PCV), iso-pinocamphol and
iso-pinocamphone (IPC) can also be formed [2]. In general, the isomer-
ization of PinOx to CPA is favored in the presence of Lewis acids, while
Brønsted acids give mainly TCV, TSB and p-cymene [3]. Although
ZnBr2 in benzene is well known as an effective homogeneous catalyst
leading to CPA selectivity of about 85% [2,4,5], the process suffers from
severe drawbacks, including fast deactivation of the catalyst, require-
ment for a high catalyst/substrate ratio, and water pollution by zinc ha-
lides. This has motivated ongoing research to find better performing
and more environmentally friendly catalyst systems [2,3,5–17].
catalyst for the epoxidation of α-pinene into PinOx (formation of CPA
was not mentioned) [23]. On the other hand, a system comprising
MTO, 3-cyanopyridine and hydrogen peroxide was proposed as a possi-
ble one-pot route to obtain CPA from α-pinene in nitromethane, al-
though selectivity to CPA was low (27% selectivity at 0 °C; conversion
was not mentioned) [24].
Owing to the performance of MTO as a Lewis acid catalyst and the
industrial interest in the synthesis of CPA, we set out to find an opti-
mized system for the MTO-catalyzed isomerization of PinOx to CPA.
By properly selecting the solvent and reaction conditions it is possible
to achieve high yields of CPA, and catalyst recovery and recycling can
be facilitated by using ionic liquids (ILs) as solvents.
2. Experimental
2.1. Materials
MTO was prepared as described previously [25]. 1,2-Dichloroethane
(DCE, Sigma-Aldrich, 99%), acetonitrile (NCMe, Sigma-Aldrich, 99%),
toluene (Sigma-Aldrich, Chromasolv Plus≥99.9%), ethanol (EtOH,
Panreac, Absolute), α,α,α-trifluorotoluene (TFT, Aldrich, >99%), 1-
butyl-3-methylimidazolium chloride ([bmim]Cl, Aldrich, 98%), 1-butyl-
3-methylimidazolium tetrafluoroborate ([bmim]BF4, io-li-tec, 99%)
and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide
([bmim]NTf2, io-li-tec, 99%) were obtained from commercial sources
and used as received.
Methyltrioxorhenium(VII) (MTO) is a well known organometallic
Lewis acid [18,19] and has been applied as a catalyst in a large number
of organic transformations [19], for instance Diels–Alder reactions [20],
⁎
Corresponding author. Fax: +351 234 401470.
E-mail address: atav@ua.pt (A.A. Valente).
1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2013.02.001

S.M. Bruno et al. / Catalysis Communications 35 (2013) 40–44
41
2.2. Catalytic tests
A twofold decrease in the initial concentration of MTO ([MTO]0)
from 0.032 M to 0.016 M led to
a lower initial reaction rate
The catalytic reactions were carried out in a magnetically stirred,
closed borosilicate reaction vessel (10 mL capacity) which was
immersed in an oil bath thermostated at 35 °C or a methanol–
monoethylene glycol bath at 15 °C (controlled using a Lab. Compan-
ion RW-0525G cooling circulating bath). Typically, the micro reactor
was loaded with MTO (8.5 μmol), α-pinene oxide (170 μmol) and
an organic solvent (0.5 mL) or an IL (75 μL). The solvent and MTO
were mixed at the reaction temperature for 10 min prior to addition
of PinOx (taken as the instant reaction began). The course of the reac-
tion was monitored using a Varian 3800 GC equipped with a BR-5
(Bruker) capillary column (30 m×0.25 mm; 0.25 μm) and a flame
ionization detector, using H2 as the carrier gas and cyclododecane ep-
oxide as an internal standard. The reaction products were identified
by GC–MS (Trace GC 2000 Series (Thermo Quest CE Instruments) —
DSQ II (Thermo Scientific)), equipped with a capillary DB-5 column
(30 m×0.25 mm; 0.25 μm) and using He as carrier gas.
(1437 mol h−1 and 765 mol h−1, respectively), although similar CPA
yield and PinOx conversion were reached after 60 min reaction (62%
yield at 100% conversion). As expected for a stable catalyst and a reac-
tion with negligible mass transfer limitations, the turnover frequency
(TOF) was roughly independent of the amount of catalyst (42 and
h−1 for [MTO]0 of 0.032 and 0.016 M, respectively).
−1
45 mol mol
Re
The catalytic performance of MTO for the reaction of PinOx is strongly
dependent on the type of solvent used as shown in Table 1 and Fig. 1 for
DCE, TFT, toluene, EtOH or NCMe as solvent ([MTO]0=0.016 M, 35 °C).
The reaction mixtures were always homogeneous and thus differences
in the solubility of MTO in the reaction media may be ruled out. A com-
parison of the noncoordinating solvents DCE, TFT and toluene indicates
that DCE and TFT are favorable, leading to the highest CPA yields (entries
2, 4, 7 of Table 1, Fig. 1-B). The poorer catalytic performance for toluene
in relation to DCE and TFT may be due to its considerably lower polarity
(dipole moments are 0.38, 1.36 and 2.86 D for toluene, DCE and TFT, re-
spectively [29]). Possibly, the halogenated solvents participate in the fa-
vorable formation of transition states/intermediates through hydrogen
bonding (via their electronegative atoms). The highest CPA yield of
70% was observed with TFT as solvent (100% conversion) (Fig. 1-B). Be-
sides the solvent polarity, the higher hydrophobicity of TFT in relation to
DCE may avoid water in the reaction system and undesirable side reac-
tions. These results are superior to those reported recently by us for mo-
lybdenum and tungsten complexes; the best result was 56% CPA yield at
97% conversion, 3 h reaction, at 35 °C, using higher initial concentration
of catalyst and higher initial molar ratio of catalyst:PinOx [8]. For NCMe
and EtOH (entries 8 and 9, Table 1) complex reaction mixtures com-
posed of several products (many unidentified) formed in relatively
small amounts were obtained; e.g. for NCMe the byproducts included
IPC and TCV formed in less than 2% yield each. The poor catalytic results
for the two solvents may be partly due to their acid/base properties. The
somewhat basic solvent NCMe could level-off the acidity of the catalyst
resulting in the sluggish reaction of PinOx (Fig. 1-A). This hypothesis
was checked in separate catalytic tests in which a Lewis base, specifically
pyridine or 2,2′-bipyridine, was added initially to the reaction mixture
with DCE as solvent. The addition of the Lewis bases had a detrimental
effect on the catalytic performance of MTO: less than 17% conversion
and 4% CPA yield were reached at 20 h reaction. For EtOH as solvent,
the reaction was the fastest (Fig. 1-A), but the CPA yield was the lowest
(entry 8 of Table 1, Fig. 1-B). The catalytic results for NCMe and EtOH
cannot be explained solely on the basis of the coordinating ability of
these solvents since EtOH led to the fastest reaction of all tested solvents.
The (non-coordinating) halogenated and (coordinating) EtOH solvents
may participate in hydrogen bonding, albeit with opposite roles; in con-
trast to the halogenated solvents, EtOH participates as a “hydrogen
donor”. On the other hand, the inappropriateness of EtOH as solvent
for converting PinOx to CPA may be partly related to its slight Brønsted
acidity, which may be enhanced by interaction with MTO [16].
3. Results and discussion
The acid-catalyzed isomerization of α-pinene oxide (PinOx) was
investigated using MTO as catalyst and 1,2-dichloroethane (DCE) as
solvent, at 35 °C. The added-value product campholenic aldehyde
(CPA) was formed in 61% yield at 100% conversion, reached at
30 min reaction. The by-products included trans-carveol (TCV), iso-
pinocamphone (IPC) and trans-pinocarveol (PCV) (Scheme 1). With-
out a catalyst the reaction was slow and poorly selective to CPA (1%
CPA yield at 28% conversion and 1 h reaction), indicating that MTO
promotes the conversion of PinOx into CPA. The catalytic activity re-
sponsible for the target reaction is most likely associated with the
Lewis acidity of the metal center which is in a high oxidation state
(ReVII). The activation of PinOx possibly involves the primary coordi-
nation of PinOx via the epoxide oxygen atom to the metal center
[4,26,27]. The low coordination number of the metal and the small-
sized ligands help avoid steric hindrance and allow enhanced accessi-
bility to the metal center [28].
Table 1
Isomerization of PinOx into CPA.a.
Entry
Solvent
DCE
Reaction time (min)
PinOx conversion (%)
CPA yield (%)
1
2
3
4
5
6
7
8
9
30
60
30
96
100
75
100
84
100
61
100
46
58
62
50
70
73
87
24
16
4
TFT
60
60b
90b
300
30
Toluene
EtOH
NCMe
300
a
Reaction conditions, unless otherwise indicated: MTO:PinOx molar ratio=1:20,
[MTO]0=0.016 M, 35 °C.
b
Scheme 1. Simplified representation of the reaction of PinOx in the presence of MTO.
The reaction temperature was 15 °C.

42
S.M. Bruno et al. / Catalysis Communications 35 (2013) 40–44
Table 2
Isomerization of PinOx using MTO/IL systems at 35 °C.a
IL
Run
Reaction time (min)
Conv. (%)b
Yield (%)
CPA
TCV
[bmim]NTf2
1
2
3
4
1
1
2
2
1
1
1
10
10
10
10
10
30
30
1440
10
60
100
100
100
100
78
100
86
91
60
55
58
56
27
41
29
34
1
5
4
2
0
4
5
6
5
2
[bmim]BF4
[bmim]Cl
58
67
100
1
15
5
50
1440
a
Reaction conditions: MTO:PinOx molar ratio=1:20, [MTO]0=0.083 M.
Conversion of PinOx.
b
The homogeneous catalyst MTO was recycled in a relatively simple
fashion by dissolving the complex in an ionic liquid (IL) and reusing
the MTO/IL mixture after separating the reaction products by solvent
extraction. ILs possessing the 1-butyl-3-methylimidazolium cation
([bmim]+) and different anions (chloride, tetrafluoroborate (BF4−) or
bis(trifluoromethylsulfonyl)imide (NTf2−)) were tested at 35 °C. The
type of IL influenced the reaction of PinOx and CPA yields: [bmim]
NTf2 led to the highest CPA yield of 60% at 100% conversion, whereas
for [bmim]Cl the reaction was the slowest and the main product formed
Fig. 1. Kinetic profiles of the reaction of PinOx (A) and dependence of CPA yield on
PinOx conversion (B) using MTO with different solvents: DCE (◊), TFT (□), toluene
(Δ), EtOH (✱) and NCMe (○), at 35 °C; TFT at 15 °C (+). Reaction conditions: PinOx:
MTO molar ratio=1:20; [MTO]0=0.016 M.
Nevertheless, a better understanding of the solvent effects would require
detailed studies involving for example DFT calculations.
For TFT as solvent, a decrease in the reaction temperature from 35 to
15 °C led to a considerable improvement in the CPA yield (from 70% to
87% at 100% conversion), at the expense of lower initial reaction rate
(entries 4, 6, Table 1; Fig. 1). The catalytic performance of MTO seems
to be fairly good when compared with the literature data for homoge-
neous or heterogeneous catalysts tested in the same reaction: homoge-
neous catalysts such as H3PW12O40 [3], (Indenyl)M(η3-C3H5)(CO)2
with M=Mo, W [8], Cu(BF4)2 [7], Cu(ClO4)2 [5], Cu(OTf)2 with OTf=
bis(trifluoromethanesulfonate) [5], Cu(NO3)2·3H2O [17], Al(NO3)3·
9H2O [17], ZnBr2 [5], SnCl2 [27], and CeCl3 [27]; and heterogeneous cat-
alysts such as MOF-Cu3(btc)2 with btc=benzene-1,3,5-tricarboxylate
[5], H3PW12O40/SiO2 [6], Zn(OTf)2/HMS24 [9], B2O3/SiO2 [10], (FeCl3,
ZnCl2 or H3BO3)/(SiO2 or TiO2 nanoparticles) [12], iron or iron oxides
supported on TiO2 [11], SiO2 [13,14] or Zr-SiO2 [14], H-US-Y zeolite
[15], Fe-MCM-41 [26,30], and (Ce or Sn)/SiO2 [27]. Some of the best re-
sults include 84% CPA yield at 88% conversion using Cu(BF4)2 as homo-
geneous catalyst [7] and 84% CPA yield at 100% conversion using
MOF-Cu3(btc)2 as heterogeneous catalyst (for the two catalysts the
reactions were carried out at room temperature, using DCE as solvent)
[5]. In the former case the catalyst recycling issue was not successfully
addressed [7]; in the latter, the catalytic activity decreased in consecu-
tive runs, and modifications of the catalyst's crystallite sizes and tex-
tural properties occurred [5].
Fig. 2. Kinetic profiles of the reaction of PinOx (A) and dependence of CPA yield on
PinOx conversion (B) using MTO/IL systems at 35 °C: [bmim]Cl (Δ), [bmim]BF4 (run
1 (□) and run 2 (◊)) and [bmim]NTf2 (run 1 (✱), run 2 (×), run 3 (+) and run 4
(▬)). Reaction conditions: PinOx:MTO molar ratio=1:20; [MTO]0=0.083 M.

S.M. Bruno et al. / Catalysis Communications 35 (2013) 40–44
43
Table 3
Reaction of α-pinene (pin) using MTO/H2O2 and comparison with literature data.
MTO:pin:H2O2 molar ratios
Cosolvent
Lewis base (eq.)a
Time (min)/Temp. (°C)b
Conv. (%)c
Yield (%)
PinOx
Ref
CPA
0.5:100:150
0.5:100:150
0.5:100:150
0.5:100:110
0.5:100:230
1:100:150
1:100:150
0.5:100:170
1:100:150
TFT
TFT
[bmim]NTf2
THF
THF
DCM
DCM
DCM
DCM
–
120/15
120/35
10/35
360/0
84/0
120/r.t.
60/r.t.
150/0
300/0
168/0
102/0
1440/0
72
76
100
56
91
70
55
100
–
2
1
0
1
4
8
This work
This work
This work
[24]
[24]
[35]
[34,35]
[24]
[23]
[24]
[24]
–
–
–
5
nmd
nm
nm
nm
nm
nm
nm
py (100)
–
63
35
55
90
85
80
0
bipy (6)
py (24)
tbupy (20)
py (100)
cianopy (24)
tbupy (20)
0.5:100:210
0.5:100:160
1:100:150
MeNO2
MeNO2
MeNO2
100
–
e
–
–
51
nm
[23]
a
Base ligand molar equivalents relative to MTO with py=pyridine, cianopy=3-cianopyridine, tbupy=tert-butylpyridine, and bipy=2,2′-bipyridine.
Reaction time and temperature (r.t.=room temperature).
Conversion of α-pinene.
nm=not mentioned.
CPA formed with 27% selectivity, conversion not mentioned.
b
c
d
e
was TCV (byproducts included PCV and IPC formed in less than 2% yield
performance of MTO was superior using [bmim]NTf2 than [bmim]BF4
or [bmim]Cl. To the best of our knowledge this is the first report on
the coupling of MTO with [bmim]NTf2 for catalytic applications. Similar
CPA yields were obtained for four consecutive 10 min batch runs using
MTO/[bmim]NTf2 at 35 °C. One can envisage an industrial process in
which the reaction products are distilled from the MTO/IL reaction mix-
ture, avoiding the use of volatile organic solvents used in the present
work for extracting the reaction products. On the other hand, supported
MTO catalysts may be promising for carrying out this reaction.
each) (Table 2, Fig. 2). In contrast to that observed for [bmim]BF4, the
MTO/[bmim]NTf2 mixture was effectively recycled leading to a compa-
rable CPA yield for four consecutive batch runs (Table 2, Fig. 2); in the
absence of MTO the reaction of PinOx was sluggish giving 21% conver-
sion at 1 h. The superior catalytic performance of MTO with the IL
[bmim]NTf2 may be partly related to the hydrophobicity of this solvent
compared to the other ILs (somewhat in parallel with the higher CPA
yields observed for hydrophobic TFT in comparison to DCE).
The use of MTO as catalyst in the epoxidation of olefins with H2O2
has been patented [31–33]. It would be interesting to synthesize CPA
via an integrated epoxidation–isomerization of α-pinene using MTO/
H2O2. Only a few studies in the literature deal with the reaction of
α-pinene using MTO/H2O2 and the reported CPA yields are generally
low (Table 3) [23,24,34,35]. To the best of our knowledge there is only
one report on the use of an IL as solvent (specifically 1-butyl-3-
methylimidazolium hexafluorophosphate, [bmim]PF6) in the reaction
of α-pinene with MTO/H2O2 and the main product was α-pinene diol
(a significant amount of solid residue was obtained) [23]. In this work,
the one-pot epoxidation–isomerization reaction system for converting
α-pinene into CPA using the MTO/[bmim]NTf2 at 35 °C was investigat-
ed. The reaction was very fast (100% conversion at 10 min reaction), but
CPA yield was low (8%) and PinOx was not detected (Table 3). Low CPA
yields were also obtained when using MTO/H2O2 and TFT as solvent at
15 and 35 °C (Table 3). Drawbacks of the MTO/H2O2 catalytic system
include the needs to avoid water in the reaction mixture and tune the
catalyst acidity through the addition of Lewis base compounds, in
order to enhance selectivity towards PinOx (intermediate in the forma-
tion of CPA) [23,24,34,35], and the catalyst stability may be critical [26].
Considering that PinOx is an intermediate in the overall reaction of
α-pinene into CPA, and the Lewis acidity requirement of the epoxida-
tion step is less demanding than that of the isomerization one, the use
of MTO and related catalysts for the integrated process is challenging.
Nevertheless, the catalytic application of MTO seems promising for
the second stage of this process.
Acknowledgments
We are grateful to the Fundação para a Ciência e a Tecnologia (FCT),
the Programa Operacional Ciência e Inovação (POCI) 2010, Orçamento
do Estado (OE), Fundo Europeu de Desenvolvimento Regional (FEDER)
and CICECO (PEst-C/CTM/LA0011/2011), for the general funding. The
FCT and the European Social Fund are acknowledged for a post-
doctoral grant to S.M.B. (SFRH/BPD/46473/2008). F.E.K. is grateful to
the German Research Foundation (DFG) for the financial support (Pro-
ject No. KU 1265/8-1).
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