Methyltrioxorhenium-catalyzed epoxidations in ionic liquids

Article abstract of DOI:10.1039/b001661f

Alkenes and allylic alcohols have been epoxidized in an ambient- temperature ionic liquid for the first time using methyltrioxorhenium (MTO) and urea hydrogen peroxide; excellent conversions and selectivities for the epoxides of a wide number of substrates were observed.

Full text of DOI:10.1039/b001661f

Methyltrioxorhenium-catalyzed epoxidations in ionic liquids
Gregory S. Owens and Mahdi M. Abu-Omar*
Department of Chemistry and Biochemistry, University of California, Los Angeles, CA 90095-1569, USA.
E-mail: mao@chem.ucla.edu
Received (in Irvine, CA, USA) 28th February 2000, Accepted 11th May 2000
Alkenes and allylic alcohols have been epoxidized in an
ambient-temperature ionic liquid for the first time using
methyltrioxorhenium (MTO) and urea hydrogen peroxide;
excellent conversions and selectivities for the epoxides of a
wide number of substrates were observed.
In this work, the advantageous properties of the MTO–UHP
oxidation system and [emim]BF4 have been combined to give
an exceptionally clean environment for catalytic oxidations.
One major advantage of this system is that both UHP and MTO
are soluble in the ionic liquid, as are the peroxorhenium species.
This gives an oxidation solution that is completely homoge-
neous. The formation of the peroxorhenium species is evident
by the appearance of an intense yellow color in the solution; this
color is attributable to the monoperoxo- and the diperoxo-
rhenium intermediates. UV–Vis experiments have confirmed
the presence of the diperoxorhenium complex, as indicated by
its absorption maximum at 360 nm.
The results in Table 1 show that several different olefinic
substrates have been oxidized to epoxides, with yields ranging
from fair (in the case of 1-decene) to excellent. Aqueous
hydrogen peroxide (30%) was used in one experiment (entry 4)
to illustrate the ring-opening of sensitive epoxides in the
presence of large amounts of water, as previously reported in
molecular solvents.5,11,12 In contrast, the reaction in entry 3 was
performed with molecular sieves to ensure the complete
removal of water from the system. As a result, only the epoxide
One of the pressing issues for chemists in the twenty-first
century is the pursuit of ‘clean’ or ‘green’ chemical transforma-
tions.1 A particular source of chemical waste that is often taken
for granted is the use of volatile molecular solvents.2,3 The use
of non-volatile ionic liquids, or molten salts, in chemical
synthesis and catalysis has received attention in recent years;2
however, the utility of these compounds remains largely
unexplored. We report the first catalytic epoxidation system that
makes use of a room-temperature ionic liquid as the solvent.
Alkenes and allylic alcohols have been oxidized to their
corresponding epoxides using the well studied methyltrioxo-
rhenium (MTO) catalyst and urea hydrogen peroxide (UHP)
with the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoro-
borate, [emim]BF4, as the solvent.
Numerous low-melting ionic salts are known, including
chlorocuprates, halogenoaluminates, and alkylphosphonium,
N-alkylpyridinium, alkylammonium, and N,NA-alkylimidazo-
lium cations with various anions.2 Chlorocuprates are not
suitable for catalytic oxidations because of their oxygen
sensitivity, and halogenoaluminates are difficult to work with
because of their instability in the presence of water. We chose to
use the 1-ethyl-3-methylimidazolium cation because: (i) its
synthesis is facile,† and (ii) it is neither oxygen nor water
Table 1 Epoxidations of various substrates with MTO–peroxide in
a
[emim]BF4
2
sensitive (but is hygroscopic). Furthermore, with BF4 as the
counteranion, the salt melts at 15 °C.4
The discovery by Herrmann in 1991 of MTO’s ability to
catalyze the epoxidation of olefins has led to a wealth of
information regarding the efficiency and versatility of this
catalyst.5 This discovery has also fueled many efforts aimed at
developing ‘environmentally friendly’ oxidation systems, as the
only byproduct of the MTO–peroxide system is water. The
creation of two active oxygen-transfer complexes, a monoper-
oxo- and a diperoxorhenium species, during the catalytic cycle
has been well established, as have the mechanisms of oxygen
transfer (Scheme 1). We refer the reader to the many reviews
that have been written on the subject of MTO-catalyzed
oxidations, for further information.6–9
Urea hydrogen peroxide has been shown to be a water-free
peroxide source for MTO-catalyzed epoxidations.10 The only
disadvantage, thus far, to using UHP is that it is insoluble in
organic solvents, such as chloroform, methylene chloride, and
acetonitrile. Hence, the MTO–UHP system in organic media is
heterogeneous.
Scheme 1
DOI: 10.1039/b001661f
Chem. Commun., 2000, 1165–1166
This journal is © The Royal Society of Chemistry 2000
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was obtained. It is also interesting to note the poor conversion in
the reaction of 1-decene (entry 10). Better conversions for this
reaction have been obtained with other MTO–peroxide epox-
idation systems;11–13 however, after 72 h, we observed that the
reaction mixture was still intensely yellow, indicating that the
catalyst is still active and, apparently, highly stable in this ionic
medium. The poor conversion of 1-decene may be the result of
heterogeneity in the solution; 1-decene was the least soluble
substrate in the ionic liquid. The results in Table 1 also indicate
that the time required for the epoxidation of these substrates is
quite comparable to that required for previously reported
results.5,11,12 Additionally, as shown in previous work,10 the use
of UHP eliminates the epoxide ring-opening that is commonly
observed when using aqueous hydrogen peroxide. This is
because the urea that is produced during the consumption of
UHP modulates the pH of the solution and prevents acid-
catalyzed ring opening.
progress includes the determination of equilibrium constants K1
and K2 (Scheme 1) in [emim]BF4 and characterization of the
kinetics. We also hope to expand the use of this oxidation
solution to substrates other than olefins, such as amines,
alcohols, hydrocarbons, and aromatics.
We are grateful to the National Science Foundation
(CAREER Grant CHE-9874857), the Arnold and Mabel
Beckman Foundation for a Young Investigator Award to M. M.
A. O., and the University of California Toxic Substances
Research and Teaching Program (UCTSR&TP) for financial
support of this research. We would like to thank Professor
Thomas Welton for helpful discussions.
Notes and references
† The 1-ethyl-3-methylimidazolium cation is readily synthesized from
1-methylimidazole and bromoethane. Subsequent metathesis with sodium
tetrafluoroborate gives the desired ionic liquid, which is purified by passage
through neutral alumina.14 The conversion in the metathesis step is rather
important, as bromide competes with alkenes for oxidation to hypo-
bromite.15
As shown in Table 1, two equivalents of UHP were used
relative to the substrate. Although two equivalents of oxidant
leads to faster epoxidation rates, only a single equivalent is
required, as evidenced by experiments with 1,5-cyclooctadiene
using one and two equivalents of UHP per double bond, which
yield the same product distributions.
1 R. T. Baker and W. Tumas, Science, 1999, 284, 1427.
Remaining reactants and products are both easily removed
from the reaction mixture via extraction with diethyl ether,
which is immiscible with the ionic liquid used in this work. This
method of removing reactants and products is also advanta-
geous because MTO, the peroxorhenium species, and the urea
byproduct are insoluble in diethyl ether. Careful evaporation of
the ether extracts gives the reactant–product mixture, which can
then be analyzed by NMR and GC–MS.
2 T. Welton, Chem. Rev., 1999, 99, 2071.
3 M. J. Earle, P. B. McCormac and K. R. Seddon, Green Chem., 1999, 1,
23.
4 J. S. Wilkes and M. J. Zaworotko, J. Chem. Soc., Chem. Commun.,
1992, 965.
5 W. A. Herrmann, R. W. Fischer and D. W. Marz, Angew. Chem., Int. Ed.
Engl., 1991, 30, 1638.
6 W. A. Herrmann and F. E. Kuhn, Acc. Chem. Res., 1997, 30, 169.
7 C. C. Romao, F. E. Kuhn and W. A. Herrmann, Chem. Rev., 1997, 97,
In summary, the advantages of this oxidation system are
numerous: (i) urea hydrogen peroxide and MTO are completely
soluble in [emim]BF4, giving a homogeneous oxidation solu-
tion. (ii) The oxidation solution is nearly water-free, so
conversion of the substrates yields only the epoxides and not the
diols. (iii) Left-over reactants, if any, and products are easily
separated from the oxidation solution by extraction with an
immiscible solvent. (While it is true that molecular solvents
have been used for isolation of the reactants/products in this
work, one can easily imagine a large-scale system in which the
reactants and products are distilled from the reaction mixture,
thereby completely eliminating the use of organic solvents.)
(iv) Most rates of epoxidation in this system are at least
comparable to previously published data. Current work in
3197.
8 J. H. Espenson, Chem. Commun., 1999, 479.
9 G. S. Owens, J. Arias and M. M. Abu-Omar, Catal. Today, 2000, 55,
317.
10 W. Adam and C. M. Mitchell, Angew. Chem., Int. Ed. Engl., 1996, 35,
533.
11 J. Rudolph, K. L. Reddy, J. P. Chiang and K. B. Sharpless, J. Am. Chem.
Soc., 1997, 119, 6189.
12 A. K. Yudin and K. B. Sharpless, J. Am. Chem. Soc., 1997, 119,
11536.
13 C. Coperet, H. Adolfsson and K. B. Sharpless, Chem. Commun., 1997,
1565.
14 T. Welton, personal communication, 1999.
15 J. H. Espenson, O. Pestovsky, P. Huston and S. Staudt, J. Am. Chem.
Soc., 1994, 116, 2869.
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