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Metal-Catalyzed Asymmetric Epoxidations of Terminal Olefins Using Hydrogen Peroxide as the Oxidant.

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DOI: 10.1002/anie.200602044
Asymmetric Epoxidations
Metal-Catalyzed Asymmetric Epoxidations of Terminal
Olefins Using Hydrogen Peroxide as the Oxidant
Isabel W. C. E. Arends*
Keywords:
asymmetric synthesis · epoxidation ·
hydrogen peroxide · ligand design · metal catalysis
Asymmetric
epoxidation of alkenes
has the potential of creating two chiral
centers in one step and is therefore a
pivotal reaction in drug synthesis
(Scheme 1). State-of-the-art oxidation
Scheme 1. Asymmetric epoxidations using
H2O2 as oxidant.
technologies should be characterized
by high atom economy and selectivity,
broad substrate scope, use of environmentally benign oxidation reagents, and
sufficient catalyst stability and productivity. Moreover, for industrial applications, systems which are simple and easy
to work up are important. In terms of
atom efficiency, after oxygen, aqueous
hydrogen peroxide would be the oxidant
of choice since it produces water as the
sole by-product and can therefore be
classified as the ultimate green oxidant.
Furthermore, oxidation with aqueous
hydrogen peroxide is cheap, safe, and
easy to operate.[1]
Many research efforts have been
dedicated to the development of chiral
metal catalysts that can perform asymmetric epoxidation. Most well-known is
probably the Sharpless method, based
on titanium tartrate complexes, for ep-
[*] Dr. I. W. C. E. Arends
Department of Biotechnology
Delft University of Technology
Julianalaan 136
2628 Delft (The Netherlands)
Fax: (+ 31) 15-278-1415
E-mail: i.w.c.e.arends@tudelft.nl
6250
oxidizing allylic alcohols.[2] The Jacobsen–Katsuki method uses Mn(salen)
complexes and provides an efficient
method for unfunctionalized and particularly cis olefins.[3, 4] However, these
methods require the use of alkyl hydroperoxides, hypochlorite, or other nonatom-efficient reagents as the oxidant.
For electron-deficient olefins the Julia–
Colonna method, which uses aqueous
H2O2 as the oxidant and polypeptides as
the catalyst, is available and can be
applied on an industrial scale.[5] But in
general, there is still a significant number of alkenes that cannot be converted
to epoxides with stereochemical control.
In particular, the asymmetric oxidation
of terminal nonactivated olefins remains
an area to be developed. The challenge,
therefore, is to develop catalysts which
can perform asymmetric epoxidations
with a wider scope by using H2O2 as
oxidant.
The main obstacle when using hydrogen peroxide is the high activity of
many later and first-row transition metals in the decomposition thereof, the socalled catalase reaction. For example,
Ru and Mn complexes will decompose
H2O2 rapidly. Furthermore, complexes
of early transition metals, for example,
Ti, Mo, and V complexes, are often
hampered by their instability in the
presence of hydrogen peroxide and/or
water. Therefore, until recently, results
obtained in the asymmetric epoxidation
with H2O2 and chiral metal catalysts
(notably Mn and Ru) have been rather
disappointing.[6]
An alternative methodology has
been developed by Shi and others[7, 8]
using chiral ketone-derived organocatalysts. The sugar-derived system developed by Shi is able to epoxidize trisub-
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
stituted trans alkenes and certain cis
alkenes with good to excellent stereocontrol.[7] In most cases potassium peroxomonosulfate (KHSO5), commonly
denoted as oxone, is used as the oxidant.
Moreover, hydrogen peroxide can be
used as the oxidant in the presence of
acetonitrile (the so-called Payne reagent).[9] In this case acetonitrile not
only serves as a solvent but also as a
reagent to react with H2O2 to form the
peroxyimidic acid; an example is given
in Table 1. This technology has been
scaled up for industrial purposes, but the
major disadvantage is that it requires up
to 15 mol % of catalyst and is not
selective for terminal olefins.
Another noteworthy development is
the use of cell-free monooxygenases as
biocatalysts. Recently flavin-dependent
styrene monooxygenases were coupled
to a formate-driven regeneration system, whereby the major obstacle of
providing the reducing equivalents was
overcome.[10] Volumetric product formation rates of around 1 g l 1 h 1 were
reported.[10b] Various styrene derivatives
were converted into the essentially optically pure (S)-epoxides.
The recent reports of Katsuki and
Beller in which Ti-[11, 12] and Ru-based[13]
complexes were used for asymmetric
epoxidation with H2O2, have however
changed the state of the art in the area of
metal-catalyzed asymmetric epoxidations. Especially for titanium in combination with reduced salen-type ligands,
excellent stereochemical control was
achieved for terminal olefins using hydrogen peroxide as the oxidant.[11, 12]
Also with ruthenium, which is notorious
for its catalase activity, excellent yields
and enantioselectivities were obtained
for cis and activated olefins.[13] In TaAngew. Chem. Int. Ed. 2006, 45, 6250 – 6252
Angewandte
Chemie
Table 1: Comparison of metal catalysts for the asymmetric epoxidation of alkenes using aqueous
H2O2.
Substrate
Cat.
Cat.
loading
[mol %]
Equivalents
30 % aqueous
H2O2
Yield [%]
Epoxide
ee [%]
Epoxide
Ref.
1[a]
2[b]
3[c,d]
1
5
5
1.05
1.5
3
90
47
85
93
82
59
[11]
[12]
[13a]
1[a]
2[b]
1
5
1.05
1.5
70
25
82
55
[11]
[12]
1[a]
2[b]
1
5
1.05
1.5
99
87
99
96
[11]
[12]
3[c]
4[e]
5
15
3
4
95
93
72
92
[13a]
[9]
[a] Room temperature, 12–48 h, CH2Cl2 as solvent. [b] 25 8C, 6–24 h, CH2Cl2 as solvent. [c] Room
temperature, 12 h, 2-methylbutan-2-ol as solvent. [d] Acetic acid (20 mol %) was added. [e] 0 8C,
12 h, CH3CN/2.0 m K2CO3 in 4 C 10 4 m EDTA.
ble 1 an overview is given of the performance of titanium and ruthenium complexes as catalysts with hydrogen peroxide. The structures of the complexes are
given in Scheme 2.
For titanium, only 1 mol % of catalyst 1 and 1.05 equivalents of H2O2 are
required to obtain high yields and selectivities. 1,2-Dihydronaphthalene is an
activated cis olefin, which obviously
gives the best results (yield and enantioselectivity of over 98 %) using little or
no excess of hydrogen peroxide. However, also for simple styrene, 93 % ee can
be achieved by using the chiral titanium
complex 1. What is most striking is the
result obtained for 1-octene. For this
simple nonactivated olefin a reasonable
82 % ee could be attained. Catalyst 1 is a
chiral di-m-oxo titanium half-reduced
salen complex. It was prepared in 60 %
yield using an in situ Meerwein–Ponndorf–Verley reduction. This complex
adopts a homochiral (aR,S,D,aR,S,D)
configuration, and it maintains its dimeric structure in methanol for over
24 hours. This finding is exceptional
because the corresponding salen com-
Scheme 2. State-of-the-art catalysts for asymmetric epoxidation using H2O2.
Angew. Chem. Int. Ed. 2006, 45, 6250 – 6252
plex, which is completely ineffective as
an epoxidation catalyst with aqueous
H2O2, immediately dissociates into the
monomeric [Ti(salen)] species in methanol. For catalyst 1 in the presence of
hydrogen peroxide, the active species in
solution is most likely a monomeric
peroxotitanium species. It is speculated
that this peroxotitanium species is activated by an intramolecular hydrogen
bond with the amine proton as indicated
in Scheme 3.
Scheme 3. Putative peroxotitanium species
activated by hydrogen bonding.
However, catalyst 1 has an elaborate
structure, and its synthesis is rather
specific for this particular ligand. For
catalysis, construction of a more flexible
and easy-to-synthesize ligand is important. Therefore, a range of the more
accessible [Ti(salan)] complexes were
tested as well.[12] Catalyst 2 (Scheme 2)
is less robust (5 mol % instead of
1 mol % of catalyst is required) and
selective than catalyst 1. However, its
ease of synthesis and tunability offer a
large advantage. Moreover, catalyst 2
can be conveniently synthesized in situ
from Ti(OiPr)4 and the salan ligand with
almost comparable enantioselectivities.[13] Results in Table 1 for catalysts 1
and 2 were obtained using dichloromethane as the solvent. However, results
with titanium catalyst 1 in toluene or
ethyl acetate gave equally good results.[11]
Many research groups have studied
ruthenium for asymmetric oxidations
owing to its rich complexation chemistry. In most cases, non-atom-efficient
oxidants were used.[14, 15] Recently, Beller and co-workers introduced a new
approach towards optimizing activity
and selectivity in Ru-catalyzed asymmetric epoxidations. The Ru catalyst 3
was the best in a whole series of [Ru(pybox)] and [Ru(pyboxazine)] complexes, which all contained pyridinedicarboxylate (pydic) as the second ligand.[13] The strategy that was followed
by the group of Beller was to study
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
6251
Highlights
ruthenium complexes with a combination of two meridional ligands, which
provided the ability to tune both the
activity and the asymmetric induction of
the catalyst separately. The original
catalyst [Ru(pybox)(pydic)] was reported by Nishiyama et al. for the epoxidation of trans-stilbene in the presence of
PhI(OAc)2 as the oxidant.[16] The pydic
ligand turned out to be essential to
suppress the catalase activity of ruthenium, and still in this case three equivalents of H2O2 were required. The enantioselectivity could then be optimized by
screening a range of pybox and pyboxazine ligands. The latter could be synthesized easily from b-amino acid derivatives. The best results with complex
3 were obtained with trans-disubstituted
styrene and reached up to 84 % ee (not
shown).[13b] Furthermore, although the
results for ruthenium complex 3 were
obtained using 5 mol % of catalyst, the
authors reported that a similar level of
efficiency could be reached with
0.5 mol % of catalyst.[13a]
It was suggested that the corresponding ruthenium dioxo complex is
the active catalyst.[13] Electrospray ionization mass spectrometry of the reaction mixture showed molecular ions
corresponding to both mono-oxo and
dioxo complexes. However, the monooxo complex turned out to be quite
stable and not active in oxygen transfer.
Regarding the origin of the enantioselectivity, it is believed that p–p interactions between the ligand and the
substrate are the dominating factor for
the asymmetric induction. The ruthenium system is therefore also extremely
6252
www.angewandte.org
promising because it provides numerous
handles for optimizing the enantioselectivity in epoxidation reactions with
H2O2. At this point it is not clear
whether it will also be able to perform
the asymmetric epoxidation of nonactivated olefins.
In conclusion, the state of the art in
asymmetric epoxidation catalysts using
aqueous hydrogen peroxide as the oxidant is represented by [Ti(salalen)] and
[Ti(salan)] complexes. For the first time,
asymmetric catalytic epoxidation for a
nonactivated terminal olefin has been
demonstrated. However, the Ti catalysts
are closely rivaled by chiral pyridine
dicarboxylate Ru complexes. Obviously,
for industrial applications, higher turnover frequencies and higher chiral inductions for terminal olefins are required. Therefore, these systems are still
prone to improvement, but the major
disadvantages in terms of greenness
have been overcome.
[7]
[8]
[9]
[10]
[11]
[12]
Published online: September 4, 2006
[13]
[1] I. W. C. E. Arends, R. A. Sheldon, Top.
Catal. 2002, 19, 133 – 141.
[2] R. A. Johnson, K. B. Sharpless in Catalytic Aymmetric Synthesis (Ed.: I. Ojima), VCH, New York, 1993, chap. 4.1.
[3] E. N. Jacobsen in Catalytic Aymmetric
Synthesis (Ed.: I. Ojima), VCH, New
York, 1993, chap. 4.2.
[4] T. Katsuki, Adv. Synth. Catal. 2002, 344,
131 – 147.
[5] For a review, see: D. R. Kelly, S. M.
Roberts, Biopolymers 2006, 84, 74 – 89.
[6] Selected examples for asymmetric epoxidations of styrene with H2O2 : a) R. M.
Stoop, A. Mezzetti, Green Chem. 1999,
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
[14]
[15]
[16]
1, 39 – 41; b) C. Bolm, D. Kadereit, M.
Valacchi, Synlett 1997, 687 – 688; c) C.
Bolm, N. Meyer, G. Raabe, T. WeyhermFller, E. Bothe, Chem. Commun. 2000,
2435 – 2436; d) R. I. Kureshy, N. H.
Khan, S. H. R. Abdi, S. T. Patel, R. V.
Jasra, Tetrahedron: Asymmetry 2001, 12,
433 – 437.
Y. Shi, Acc. Chem. Res. 2004, 37, 488 –
496.
D. Yang, Acc. Chem. Res. 2004, 37, 497 –
505.
L. Shu, Y. Shi, Tetrahedron 2001, 57,
5213 – 5218.
a) F. Hollmann, P.-C. Lin, B. Witholt, A.
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Lang, B. Witholt, A. Schmid, Angew.
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Y. Sawada, K. Matsumoto, S. Kondo, H.
Watanabe, T. Ozawa, K. Suzuki, B.
Saito, T. Katsuki, Angew. Chem. 2006,
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a) M. K. Tse, C. DHbler, S. Bhor, M.
Klawonn, W. MIgerlein, H. Hugl, M.
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Angew. Chem. Int. Ed. 2006, 45, 6250 – 6252
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