Ru-salen catalyzed asymmetric epoxidation: Photoactivation of catalytic activity

Article abstract of DOI:10.1055/s-1999-2780

(ON⁺)(Salen)ruthenium(II) complex 1 was found to be an efficient catalyst for the epoxidation of conjugated olefins under sunlight coming through windows or incandescent light. The most suitable terminal oxidant was 2,6-dichloropyridine N-oxide 2. All the examined conjugated olefins showed high enantioselectivity greater than 80% ee, irrespective of their substitution pattern.

Full text of DOI:10.1055/s-1999-2780

LETTER
1157
Ru-Salen Catalyzed Asymmetric Epoxidation: Photoactivation of Catalytic
Activity
Tsuyoshi Takeda, Ryo Irie, Yo Shinoda, Tsutomu Katsuki*
Department of Molecular Chemistry, Graduate School of Science, Kyushu University 33, Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan
Fax +81 (92) 642 2607
Received 14 April 1999
iliaries for Mn-catalyzed asymmetric oxidations. In order
Abstract: (ON+)(Salen)ruthenium(II) complex 1 was found to be an
to expand ruthenium chemistry, we synthesized
efficient catalyst for the epoxidation of conjugated olefins under
(ON+)(salen)ruthenium(II) complex [(ON)Ru-salen com-
sunlight coming through windows or incandescent light. The most
plex] 1 and examined asymmetric epoxidation of 6-aceta-
mido-2,2-dimethyl-7-nitrochromene as a substrate in the
presence of various oxidants (Table 1). Although the
chemical yields of the epoxide 3 were poor, all the reac-
tions proceeded with good to high enantioselectivity.
However, enantioselectivity of the reaction decreased as
the reaction time was elongated (entries 2 and 3). This
suggested that (ON)Ru-salen complex 1 was decomposed
during the reaction and the generated Ru-species cata-
lyzed epoxidation with the lower enantioselectivity.12)
suitable terminal oxidant was 2,6-dichloropyridine N-oxide 2. All
the examined conjugated olefins showed high enantioselectivity
greater than 80% ee, irrespective of their substitution pattern.
Key words: asymmetric epoxidation, (ON+)(salen)ruthenium(II)
complexes, conjugated olefins, Ru-salen complex, photoactivation
Chiral metallosalen complexes (hereafter referred to as
M-salen complexes) show versatile asymmetric catalysis
for a wide range of chemical transformations, especially
for oxene, nitrene, carbene transfer reactions.1) Thus far,
chiral Mn-salen complexes have been recognized as the
best catalyst for asymmetric epoxidation of conjugated
olefins. However, good substrates for this reaction are
mostly limited to conjugated cis-di-, tri- and some tetra-
substituted olefins.2) Epoxidation of conjugated monosub-
stituted olefins such as styrene requires low reaction tem-
perature as low as -78 °C.3) Recently chiral dioxiranes
have been found to be excellent oxidants for enantioselec-
tive oxidation of trans-di and tri-substituted olefins.4)
Still, there is no general methodology for enantioselective
epoxidation. In the course of our study on the catalysis of
chiral M-salen complexes, we found that the stereochem-
istry of M-salen- catalyzed epoxidation is affected by the
metal ion, chiral ligand, and solvent used.5) On the other
hand, Ru complexes are well known to serve as catalysts
for oxidation6) and some chiral Ru-complexes such as Ru-
porphyrin,7) Ru-Schiff base complex,8) desymmetric Ru-
Schiff base complex,9) and Ru-bisamide complex,10) have
already been used for asymmetric epoxidation. Although
enantioselectivity so far obtained with these complexes
are moderate, some interesting phenomena have been ob-
served: i) epoxidation of p-nitrostyrene with Ru-Schiff
base complex shows good enantioselectivity of 80% ee,
though epoxidation of styrene is moderate (58% ee).8,11)
ii) Differing from M-salen and M-porphyrin catalyzed ep-
oxidations, trans-b-methylstyrene shows better enanti-
oselectivity than cis-b-methylstyrene in the epoxidation
using Ru-bisamide complex (62 and 25% ee, respective-
ly).10) These results suggest that the scope of metal-cata-
lyzed asymmetric epoxidation could be further expanded
by taking the advantage of ruthenium chemistry.
On the other hand, Hirobe et al. have reported that 2,6-
dichloropyridine N-oxide 2 is an excellent terminal oxi-
dant for Ru-porphyrin-catalyzed epoxidation.13) There-
fore,
we
examined
the
epoxidation
of
dihydronaphthalene14) in ether using N-oxide 2 as the ox-
We have revealed that the salen ligands bearing chiral bi-
naphthyl unit as a chiral element are excellent chiral aux-
idant and found that the chemical yield of the epoxide was
Synlett 1999, No. 07, 1157–1159 ISSN 0936-5214 © Thieme Stuttgart · New York

1158
Katsuki, T.
LETTER
considerably improved (Table 2). However, the enanti- ether proceeded smoothly. With respect to enantioselec-
oselectivity again decreased with the formation of ketone tivity and chemical yield, benzene and ether were consid-
4 which was the rearrangement product of the epoxide, as ered to be the solvents of choice. In particular, benzene
the reaction proceeded. Decrease of enantioselectivity was a good solvent for the reaction yielding an acid-sen-
was considered to be attributable to the decomposition of sitive epoxide such as 1,2-epoxy-3,4-dihydro-naphtha-
(ON)Ru-salen complex 1 and to the enantiomer-differen- lene (vide infra). The reaction in a polar solvent such as
tiating rearrangement.15) This suggested that the Ru-spe- acetone or ethyl acetate which more solubilized the cata-
cies generated in the reaction or (ON)Ru-salen complex 1 lyst was very slow.
itself served as a Lewis acid catalyst (c.f., entries 2 and
The epoxidations of various olefins under the optimized
conditions are summarized in Table 3. As we expected,
3).15)
epoxidation of cis-disubstituted olefins proceeded with
high enantioselectivity. The sense of asymmetric induc-
tion by 1 was the same as that by Mn-salen complex bear-
ing the same salen ligand as 1. Although the reactions in
ether generally showed slightly better enantioselectivity,
higher chemical yields were attained in benzene. This is
probably because the decomposition of epoxides was
slower in benzene than in ether (c.f., Table 2, entry 3 and
Table 3, entry 5). No formation of ketone was observed in
the epoxidation of other olefins, except for styrene: For-
mation of only a trace amount of the aldehyde is detected
by TLC analysis in the reaction of styrene. Despite this,
enantioselectivity of the reactions gradually decreased es-
pecially in the ether, suggesting the slow decomposition
of the catalyst under the reaction conditions (entries 6 and
7). The decomposition of the catalyst seemed to be sup-
pressed by use of benzene as a solvent (entries 8 and
9).17,18) The unexpected good results were, however, ob-
served when we examined the epoxidation of trans-di-
Furthermore, the present reaction was found to be accel-
erated by exposition to sunlight. The reaction in the dark
and mono-substituted olefins which are poor substrates
was slow and showed slightly lower enantioselectivity
for Mn-salen-catalyzed epoxidation (entries 8, 11, and
(entry 4). The light around 450 nm accelerated the reac-
13). It is also noteworthy that epoxidations of trans- and
tion most effectively (Scheme 1). This suggests that the
electron-transfer from ruthenium ion to a ligand and the
cis-b-methylstyrenes proceeded to exclusively give trans-
and cis-epoxides respectively, though the formation of a
trace amount (<0.5%) of the isomerized cis- and trans-ep-
subsequent ligand dissociation is responsible for this pho-
to-acceleration.16) Exposure of the reaction medium to
oxides was detected by GLC analysis (entries 6-9 and 10).
UV-light provided the complex products.
Epoxidation of trans-stilbene also gave the corresponding
trans-epoxide exclusively (entry 11).
Preparation of the complex 1 and the typical experimental
procedure for the epoxidation using it as a catalyst are de-
scribed below. The equipments made with Pyrex glass
were used through these reactions.
Preparation of Ru-salen complex 1:19) NaH (60% disper-
sion in mineral oil, 44 mg, 1.1 mmol) was weighed into
a flask and washed with dry hexane (3 x 1.0 ml). DMF
(5 ml) was added to the flask with stirring, followed by
salen-H2 (413.5 mg, 0.50 mmol). Hydrogen evolved and
the mixture turned to a clear red solution. After 30 min, a
solution of Ru(NO)Cl3·H2O (191.6 mg, 0.75 mmol) in
Scheme 1
DMF (5 ml) was added. The mixture was stirred at 110 °C
for 48 h. The resulting opaque red-brown mixture was
concentrated under high vacuum. The residue was dis-
solved in CH2Cl2 and washed with H2O. The CH2Cl2 layer
was dried over Na2SO4, then evaporated to dryness. The
residue was recrystallized from CH2Cl2/CH3CN to give 1
[Ru(salen)(NO)(Cl)] as red-brown crystals (160 mg,
32%).
We next examined the effect of solvent using dihy-
dronaphthalene as a substrate under incandescent light
(100V, 60W). The reaction proceeded smoothly in a less
polar solvent such as benzene or ether. Although complex
1 and N-oxide 2 are soluble to benzene, their solubility to
ether is small and the reaction in ether was carried out un-
der the suspended conditions. Despite this, the reactions in
Synlett 1999, No. 07, 1157–1159 ISSN 0936-5214 © Thieme Stuttgart · New York

LETTER
Ru-Salen Catalyzed Asymmetric Epoxidation: Photoactivation of Catalytic Activity
1159
Acknowledgement
The authors are grateful to Professor Y. Matsuda and Dr. T. Kojima,
this Department, for allowing us to use xenon short arc lamp (Type
UXL 500D-0, USHIO) and color filter glass (V-42 & UV-D33S,
TOSHIBA), and helpful discussions. Financial supports from a
Grant-in-Aid for Scientific Research from the Ministry of Educati-
on, Science, Sports, and Culture, Japan, and from Asahi glass foun-
dation are gratefully acknowledged.
References and Notes
(1) Katsuki, T. Coord. Chem. Rev. 1995, 140, 189-214.
(2) a) Katsuki, T. J. Mol. Cat. A: Chem. 1996, 113, 87-107. b) Ito,
Y. N.; Katsuki, T. Bull. Chem. Soc. Jpn. 1999, 72, 603-619.
(3) a) Palucki, M.; Pospisil, P. J.; Zhang, W.; Jacobsen, E. N. J.
Am. Chem. Soc. 1994, 116, 9333-9334. b) Palucki, M.;
McCormick, G. J.; Jacobsen, E. N. Tetrahedron Lett. 1995,
35, 5457-5460.
(4) a) Yang, D.; Yip, Y. C.; Tang, M. W.; Wong, M. K.; Zheng,
J. H.; Cheung, K. K. J. Am. Chem. Soc. 1996, 118, 491-492.
b) Tu, Y.; Wang, Z.-X.; Shi, Y. J. Am. Chem. Soc. 1996, 118,
9806-9807.
(5) Katsuki, T. J. Synth. Org. Chem., Jpn. 1995, 53, 940-951.
(6) Trost, B. M. eds. "Comprehensive Organic Synthesis," Vol.7,
Pergamon Press, Oxford (1991)
(7) Lai, T.-S.; Zhang, R.; Cheung, K.-K.; Kwong, H.-L.; Che, C.-
M. Chem. Commun. 1998, 1583-1584.
(8) Kureshy, R. I.; Khan, N. H.; Abdi, S. H. R. J. Mol. Cat. A:
Chem. 1995, 96, 117-122.
(9) Kureshy, R. I.; Khan, N. H.; Abdi, S. H. R.; Bhatt, A. K. J.
Mol. Cat. A: Chem. 1995, 96, 33-40.
(10) End, N.; Pfaltz, A. Chem. Commun. 1998, 589-590.
(11) Unfortunately, the correct reaction temperature of this
reaction was not given.
(12) The rearrangement of the epoxide giving a ketone was not
detected in this reaction.
(13) Higuchi, T.; Ohtake, H.; Hirobe, M. Tetrahedron Lett. 1989,
30, 6545-6548.
(14) We changed the substrate from 6-acetamido-2,2-dimethyl-7-
nitrochromene to dihydronaphthalene which provides the
more Lewis acid-sensitive epoxide, to clarify the effect of the
Lewis acidity of Ru-complexes to the reaction.
(15) See the succeeding communication.
(16) It has been reported that flash photolysis of Ru(TPP)(NO)Cl
presumably generates a transient species, Ru(TPP)Cl:
Lorkovic, I. M.; Miranda, K. M.; Lee, B.; Bernhard, S.;
Schoonover, J. R.; Ford, P. C. J. Am. Chem. Soc. 1998, 120,
11674-11683. However, we can not remove the possibility
that the chloro ligand in 1 dissociates under the present
reaction conditions from the result obtained in the asymmetric
cyclopropanation described in the succeeding
Epoxidation
of
6-acetamido-2,2-dimethyl-7-nitro-
chromene: (ON)Ru-salen complex 1 (2.0 mg, 2 mol%)
was added to a solution of the substrate (26.2 mg, 0.1
mmol) in benzene (1.0 ml). To the solution was added 2,6-
dichloropyridine N-oxide (16.4 mg, 0.1 mmol) and the
whole mixture was stirred for 20 h at room temperature
under incandescent light (100V, 60W). The mixture was
directly submitted to column chromatography (SiO2, hex-
ane/AcOEt = 8/2 to 7/3) to give the corresponding ep-
oxide (16.5 mg, 59%). The enantiomeric excess of the
epoxide was determined by HPLC analysis using optical-
ly active column (DAICEL CHIRALCEL OJ, hexane/2-
propanol = 1/1).
communication.
(17) This is probably due to that benzene absorbs unnecessary UV
light.
(18) Absorption maximums in UV-vis spectrum of 1 in benzene
(2.0 x 10-4 M): lmax = 456 nm (e 5.5 x 103); lmax = 535 nm
(e 2.7 x 103). The broad signal at 535 nm became gradually
weak with the exposure of complex 1 under the reaction
conditions to incandescent light. This probably suggests that
the decomposition of complex 1 was brought about on
irradiation.
In conclusion, we were able to find a general methodology
for the epoxidation of conjugated olefins. Further study is (19) Odenkirk,W.; Rheingold, A.L.; Bosnich, B. J. Am. Chem. Soc.
1992, 114, 6392-6398.
now proceeding in our laboratory.
Article Identifier:
1437-2096,E;1999,0,07,1157,1159,ftx,en;Y07499ST.pdf
Synlett 1999, No. 07, 1157–1159 ISSN 0936-5214 © Thieme Stuttgart · New York