A cheap, efficient, and environmentally benign synthesis of the versatile catalyst methyltrioxorhenium (MTO)

Article abstract of DOI:10.1002/anie.200703017

Avoiding toxic starting materials such as (CH₃)₄Sn, a cheap, environmentally benign high-yield synthesis has been developed for the multitalented catalyst methyltrioxorhenium(VII) (MTO; see picture: black C, white H, blue Re, red O). This novel approach is applicable on a large scale and for several derivatives of MTO, such as ethyltrioxorhenium(VII). (Figure Presented)

Full text of DOI:10.1002/anie.200703017

Angewandte
Chemie
DOI: 10.1002/anie.200703017
Catalysts
A Cheap, Efficient, and Environmentally Benign Synthesis of the
Versatile Catalyst Methyltrioxorhenium (MTO)**
Wolfgang A. Herrmann,* Alexandra M. J. Rost, Josef K. M. Mitterpleininger, Normen Szesni,
Stefanie Sturm, Richard W. Fischer, and Fritz E. Kühn*
Dedicated to the memory of Ernst Otto Fischer and to Süd-Chemie on the occasion of its 150th anniversary
Organometallic oxides in high oxidation states play a key role
as catalysts in oxygen-transfer reactions.[1] The catalytic
potential of methyltrioxorhenium (MTO, 1) is unrivalled in
numerous reactions.[2] MTO was first synthesized in a quite
time-consuming (weeks) and low-scale (milligrams) process.[3]
Based on extensive studies in academia and industry, MTO
has been recognized to be the most active available catalyst
for olefin epoxidation. Aside from its application as an
oxidation catalyst, MTO is also used to catalyze other
reactions, for example, olefin metathesis and aldehyde
olefination. However, its synthetic access suffers from
severe drawbacks with respect to larger scale applications:[4]
a) Direct alkylation of dirhenium heptoxide (Re2O7) with
the non-reducing alkyl-transfer reagent tetramethyltin[5]
wastes half of the rhenium by concomitant formation of
Re2O7 is in principle the most straightforward starting
material for the synthesis of MTO since both the pure metal
and inorganic perrhenates are prepared from Re2O7. The
disadvantage of the above methods is that good yields of
MTO are obtained only if the volatile, highly toxic, and
carcinogenic tetramethyltin or related, still quite toxic
derivatives are applied. Thus, both the synthesis and purifi-
cation of MTO is expensive, with complex safety precautions
as well as cumbersome purifying procedures being manda-
tory. This prevented any reasonable industrial application of
MTO (and any of its derivatives).
Alkylrhenium(VII) oxides are also available via dialkyl
zinc precursors.[8] However, dimethylzinc entails significant
reduction of ReVII when exposed to Re2O7 and related ReVII
precursor compounds.[5,9] We have now discovered unexpect-
edly that clean methylation without any reduction occurs
when methylzinc acetate[10] is allowed to react with perrhe-
nylacetate [Eq. (1)]. The yields of methyltrioxorhenium
(catalytically
inactive)
trimethyltin
perrhenate,
(CH3)3SnOReO3.
b) Alkylation of Re2O7 in the presence of trifluoroacetic
anhydride (“anhydride route”) results in nearly quantita-
tive yields of MTO.[6] However, toxic methyltin reagents
are required.
c) Preparation from inorganic perrhenates via the covalent
trimethylsilyl perrhenate[7] once again requires the use of
tetramethyltin.
isolated are about 90%. Under nonoptimized conditions, a
50 g-scale synthesis yielded around 75% of the pure MTO
catalyst. The new synthesis is not only efficient but also cheap.
Methylzinc acetate is available from inexpensive readily
available chemicals according to Equation (2). As the han-
[*] Prof. Dr. Dr. h.c. mult. W. A. Herrmann,A. M. J. Rost,
J. K. M. Mitterpleininger
Lehrstuhl für Anorganische Chemie
Department für Chemie
Technische Universität München
Lichtenbergstrasse 4,85747 Garching (Germany)
Fax: (+49)89-289-13473
E-mail: wolfgang.herrmann@ch.tum.de
Homepage: http://aci.anorg.chemie.tu-muenchen.de
Prof. Dr. F. E. Kühn
Molekulare Katalyse
dling of trimethylaluminum is no significant obstacle for a
synthetic chemist, the synthesis of methylzinc acetate accord-
ing to Equation (2) is superior to that following the most
common method given in Equation (3).
Department für Chemie
Technische Universität München
Lichtenbergstrasse 4,85747 Garching (Germany)
Fax: (+49)89-289-13473
E-mail: fritz.kuehn@ch.tum.de
Dr. N. Szesni,S. Sturm,Dr. R. W. Fischer
Süd-Chemie AG
R&D Catalytic Technologies
Waldheimer Strasse 13,83052 Bruckmühl (Germany)
The new synthesis proceeds conveniently as follows: First,
Re2O7 is dissolved in acetonitrile to quantitatively form
perrhenylacetate upon treatment with acetic anhydride. The
[**] The authors are grateful to the Margarethe-Ammon Foundation
(PhD grant for A.M.J.R.) and the Bayerische Forschungsstiftung
(PhD grant for J.K.M.M.).
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Communications
=
methylating reagent CH3Zn[OC( O)CH3] is then added
slowly at À108C [Eq. (1)]. For purification, the resulting
solution is separated from the precipitated zinc acetate. After
several washing steps, pure MTO is obtained in about 90%
yield. Table 1 shows the results of various experiments in
used, but to date silver perrhenate shows the best results.
More work is now underway to optimize this route. The
combination of the surprisingly efficient alkylating agent
methylzinc acetate and the new rhenium source, based on
perrhenates, elegantly eliminates two of the major “trouble
makers” at the same time, namely the sensitive and less easy
to handle dirhenium heptoxide, and the expensive, highly
toxic tinalkyls. Both issues were a severe problem, which—up
to now—hampered all efforts to bring MTO to industrial use
beyond laboratory-scale reactions. We are now in a position to
develop a pilot-plant synthesis of MTO. The ongoing fine-
tuning of the methyl-transfer agent is well set to enable
commercial synthesis of MTO on a multi-kilogram scale.
Our new approach not only provides a route for the
production of the catalyst MTO, but also promises a general
economic and simple high-yield access to alkylrhenium(VII)
oxides by means of nontoxic, non-reducing, and robust
alkylating reagents.
Table 1: Synthesis of alkylrhenium(VII) oxides.
Reactive ReVII precursor
Alkylating reagent
Yield [%][a]
=
=
CH3C( O)OReO3
CH3ZnOC( O)CH3
90[b] (R=CH3)
95[b] (R=CH3)
60[c] (R=C2H5)
=
=
CF3C( O)OReO3
CH3ZnOC( O)CH3
=
=
CH3C( O)OReO3
CH3CH2ZnOC( O)CH3
[a] Yield of RReO3. [b] Yield of isolated product. [c] Yield quantified by
1H NMR spectroscopy.
which different carboxylic acid anhydrides were applied for
the activation of Re2O7 and alkylzinc acetates to prepare
MTO and its derivative ethyltrioxorhenium(VII) (ETO).
For MTO, the overall preparation follows the net
Equation (4). Zinc serves only as the alkyl mediator metal.
Experimental Section
All experiments were carried out under an oxygen- and water-free
argon atmosphere using standard Schlenk techniques. All solvents for
use in an inert gas atmosphere were purified by a solvent purification
system MB SPS (MBraun).
This novel procedure avoids the use of toxic organotin
reagents and is much cheaper and easier to perform than all
previously known routes. The synthesis of MTO according to
Equation (4) should now facilitate wide industrial application
of the catalyst MTO.
=
=
Methylzinc acetate, CH3ZnOC( O)CH3: Powdered Zn[OC(
O)CH3]2·2H2O (11.1 g, 60.6 mmol) was dried for 3 h at 708C. The loss
of water was gravimetrically determined. Anhydrous zinc acetate was
suspended in toluene (50 mL). At À108C, Al(CH3)3 (20 mmol) in
toluene was added dropwise, and the reaction mixture was stirred for
5 h at À108C. After removal of the solvent in vacuo, methylzinc
Nevertheless, another drawback for the synthesis of MTO
in large quantities (and thus for the application of MTO in
industrial processes) is the sensitivity of Re2O7 towards traces
of water; all manipulations described above have to be
performed under strict exclusion of moisture. In contrast,
inorganic perrhenates are stable towards air and water, and
can be stored without decomposition over long periods of
time. In addition, they are readily available, also on a large
scale, from rhenium metal. Therefore, it would be highly
desirable to replace dirhenium heptoxide as the starting
material by simple and more cost-efficient perrhenates. Along
these lines, we found that the reaction of silver perrhenate
with one equivalent of acetyl chloride delivers perrhenyl
acetate in 98% yield which, after alkylation with methylzinc
acetate, likewise delivers MTO in high yields [ > 90%,
Eq. (5)].
1
acetate was obtained as a white solid. Yield: 80% (6.7 g); H NMR
(400 MHz, CDCl3, 258C): d = 2.14 (s, 3H, C-CH3), À0.68 ppm (s, 3H,
13
=
Zn-CH3); C NMR (400 MHz, CDCl3, 258C): d = 180.4 (s, C O),
À15.2 ppm (s, Zn-CH3).
Methyltrioxorhenium(VII), CH3ReO3: Re2O7 (10 g, 22.0 mmol)
was suspended in acetonitrile (50 mL), and then one equivalent of
acetic anhydride was added. The reaction mixture was stirred for
30 min. Two equivalents of methylzinc acetate dissolved in acetoni-
trile were then added dropwise at À108C to the resulting clear
solution. After a reaction time of 30 min, the solution was separated,
and the solvent was removed in vacuo. The resulting solid was washed
several times with small portions of cold (À208C) n-pentane to obtain
pure MTO. The analytical data are identical to the previously
published data and confirm that the compound is pure.[5] Yield: 90%
(10.9 g).
=
Ethylzinc acetate, CH3CH2ZnOC( O)CH3: Dimethylzinc
(20 mmol) in n-hexane was added dropwise to a suspension of
acetic acid (1.2 g, 20 mmol) in n-hexane (100 mL) at À788C. The
reaction mixture was allowed to warm up to ambient temperature.
After removal of the solvent in vacuo, and a washing step with cold
acetonitrile, a white solid was obtained. Yield: 93% (2.85 g).
Ethyltrioxorhenium(VII),
CH3CH2ReO3:
Re2O7
(4.88 g
(10 mmol) was suspended in acetonitrile (100 mL), and then acetic
acid anhydride (1.03 g, 10 mmol) was added. After a reaction time of
20 min, ethylzinc acetate (2.09 g, 20 mmol) in acetonitrile was slowly
added at À108C. After removal of the solvent in vacuo,
ethyltrioxorhenium(VII) was obtained. The analytical data corre-
spond to those in literature.[8]
The reaction of silver perrhenate with acetyl chloride is
fast and occurs with a quantitative precipitation of silver
chloride. After filtration of the by-product, the reaction
solution can directly be treated with methylzinc acetate
without further purification. The silver chloride can be
recovered quantitatively. Other perrhenates can also be
Received: July 6, 2007
Published online: August 21, 2007
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[4] A. M. J. Rost, W. A. Herrmann, F. E. Kühn, Tetrahedron Lett.
Keywords: alkylation · catalysts · rhenium · synthetic methods ·
zinc
.
2007, 48, 1775; F. E. Kühn, K. R. Jain, M. Zhou, Rare Met. 2006,
25, 411.
[5] W. A. Herrmann, J. Kuchler, J. K. Felixberger, E. Herdtweck, W.
Wagner, Angew. Chem. 1988, 100, 420; Angew. Chem. Int. Ed.
Engl. 1988, 27, 394.
[1] A. M. Al-Ajlouni, J. H. Espenson, J. Am. Chem. Soc. 1995, 117,
9243; J. Rudolph, K. L. Reddy, J. P. Chiang, K. B. Sharpless, J.
Am. Chem. Soc. 1997, 119, 6189; F. E. Kühn, A. M. Santos, M.
Abrantes, Chem. Rev. 2006, 106, 2455; C. C. Rom¼o, F. E. Kühn,
W. A. Herrmann, Chem. Rev. 1997, 97, 3197; A. M. Santos, F. E.
Kühn in Homogeneous Multiphase Catalysis, Vol. 2 (Eds.: B.
Cornils, W. A. Herrmann, I. T. Horvµth, W. Leitner, S. Mecking,
H. Olivier-Bourbigou, D. Vogt), Wiley-VCH, Weinheim, 2005,
p. 807.
[6] W. A. Herrmann, F. E. Kühn, R. W. Fischer, W. R. Thiel, C. C.
Rom¼o, Inorg. Chem. 1992, 31, 4431.
[7] W. A. Herrmann, R. M. Kratzer, R. W. Fischer, Angew. Chem.
1997, 109, 2767; Angew. Chem. Int. Ed. Engl. 1997, 36, 2652.
[8] W. A. Herrmann, C. Rom¼o, R. W. Fischer, P. Kiprof, C.
De Meric de Bellefon, Angew. Chem. 1991, 103, 183; Angew.
Chem. Int. Ed. Engl. 1991, 30, 185; W. A. Herrmann, F. E. Kühn,
C. C. Rom¼o, H. Tran Huy, M. Wang, R. W. Fischer, W. Scherer,
P. Kiprof, Chem. Ber. 1993, 126, 45.
[9] A. M. J. Rost, A. Scherbaum, W. A. Herrmann, F. E. Kühn, New
J. Chem. 2006, 30, 1599.
[10] G. E. Coates, D. Ridley, J. Chem. Soc. 1965, 1870.
[2] F. E. Kühn, A. Scherbaum, W. A. Herrmann, J. Organomet.
Chem. 2004, 689, 4149; F. E. Kühn, A. M. Santos, W. A.
Herrmann, Dalton Trans. 2005, 2483; W. A. Herrmann, F. E.
Kühn, Acc. Chem. Res. 1997, 30, 169.
[3] I. R. Beattie, P. J. Jones, Inorg. Chem. 1979, 18, 2318.
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