Photo-controlled Lewis acidity: Chiral (ON)Ru-salen catalyzed hetero Diels-Alder reaction and kinetic resolution of racemic epoxides

Article abstract of DOI:10.1055/s-1999-2781

(ON⁺)(Salen)ruthenium(II) complex 1 serves as a chiral Lewis acid catalyst for asymmetric hetero Diels-Alder reaction and for kinetic resolution of racemic epoxides when the reactions were carried out under sunlight coming through windows or incandescent light.

Full text of DOI:10.1055/s-1999-2781

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LETTER
Photo-controlled Lewis Acidity: Chiral (ON)Ru-Salen Catalyzed Hetero Diels-
Alder Reaction and Kinetic Resolution of Racemic Epoxides
Jun Mihara, Tetsuya Hamada, Tsuyoshi Takeda, Ryo Irie, 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
We next examined the reaction using 1 as the catalyst un-
Abstract: (ON+)(Salen)ruthenium(II) complex 1 serves as a chiral
der the direct sunlight coming through windows and
Lewis acid catalyst for asymmetric hetero Diels-Alder reaction and
found that the reaction was accelerated and both the enan-
for kinetic resolution of racemic epoxides when the reactions were
tioselectivity and chemical yield were improved (entry 5).
On the other hand, the reaction in the dark was very slow
and the enantioselectivity was decreased to a considerable
extent (entry 6). This suggests that photo-activation is es-
sential for the present reaction. The reaction was also ex-
amined in various solvents and it was found that the
highest enantioselectivity of 79% ee and the moderate
chemical yield were achieved when t-butyl methyl ether
(TBME) was used as a solvent (entry 10). The similar re-
sult was obtained in the reaction under incandescent light.
carried out under sunlight coming through windows or incandescent
light.
Key words: (ON+)(salen)ruthenium(II) complex, photo-activation,
Lewis acid catalysis, hetero Diels-Alder reaction, kinetic resolution
of racemic epoxides
In some metal complexes such as ruthenium bipyridine
complex, it is well known that electron transfer from a
central metal ion to its ligand and the subsequent ligand-
dissociation is promoted by exposure of the complex to
light.1) This electron transfer process has attracted the
chemist’s attention and has been extensively studied.2) On
the other hand, ruthenium complexes are well known to
catalyze the various types of reactions and many useful
chiral ruthenium catalysts have been developed to date.3)
It is naturally expected that electron transfer process af-
fects the catalysis of ruthenium complexes, especially,
their Lewis acidity. We recently found that the metallo-
salen complexes bearing the salen ligands which carry bi-
naphthyl unit as its chiral element serve as useful catalysts
for various asymmetric reactions.4) In this context, we
synthesized (R,S)-(ON+)(salen)ruthenium(II) complex
[(R,S)-(ON)Ru-salen complex] 1 and found that it served
as an efficient catalyst for asymmetric epoxidation when
the reactions were carried out under incandescent light, as
described in the preceding communication. On the other
hand, Bosnich et al. have reported that an achiral (ON)Ru-
salen complex serves as a catalyst for Diels-Alder reac-
tion.5) These results prompted us to explore the potential
of chiral (ON)Ru-salen complexes as chiral Lewis acid
catalysts. Thus, we examined (R,S)-(ON)Ru-salen-cata-
lyzed hetero Diels-Alder reaction.6) According to the Bos-
nich’s report, cationic (R,S)-(ON)Ru-salen complex 2 was
first examined as a catalyst7) for the reaction of Danishef-
sky’s diene and benzaldehyde but the reaction was slug-
gish (Table 1, entry 1). We next examined the reactions
using (R,S)-(ON)Ru-salen complex 1. Complex 1 showed
modest catalytic activity and moderate enantioselectivity
of 64% ee in the reaction at room temperature on the lab-
oratory bench exposed to scattered sunlight coming
through windows (entry 2). The reactions using complex-
es 3 and 4 as catalysts were also examined under the same
conditions but the diminished enantioselectivities of 53
and 42% ee were observed, respectively (entries 3 and 4).
The reaction at 0 °C increased enantioselectivity to 83%
ee (Table 2, entry 1). Under the conditions, reactions of
other aldehydes also proceeded with good enantioselec-
tivity (entries 2 and 3).
To explore the general utility of (ON)Ru-salen complexes
as Lewis acid catalysts, we examined kinetic resolution of
Synlett 1999, No. 07, 1160–1162 ISSN 0936-5214 © Thieme Stuttgart · New York

LETTER
Chiral (ON)Ru-Salen Catalyzed Hetero Diels-Alder Reaction and Kinetic Resolution
1161
smaller relative rates. The acceleration by sunlight com-
ing through windows was also essential for this reaction.
The reaction did not occur in the dark (entry 3). On the
other hand, it has been reported that an oxo (salen)manga-
nese(V) complex shows higher Lewis acidity than the cor-
responding (salen)manganese(III) complex.9) Therefore,
we examined kinetic resolution in the presence of 2,6-
dichloropyridine N-oxide but no reaction was observed
under the conditions (entry 4).10) Complexes 3 and 4
showed poor catalytic activity and enantiomer-differenti-
ation (entries 5 and 6).
Kinetic resolution of indene oxide which is the more acid
sensitive proceeded smoothly in ethyl acetate with high
relative ratio of 30 (Eq. 1). We also examined the rear-
rangement of cyclohexene oxide but no reaction occurred.
This results may suggest that the substrates of the present
reaction are limited to aryl or alkenyl epoxides.
racemic epoxides.8) We first examined resolution of race-
mic 1,2-epoxy-3,4-dihydronaphthalene (Table 3). The re-
action using (R,S)-(ON)Ru-salen complex 1 as the
catalyst in ethyl acetate proceeded smoothly and showed
excellent enantiomer differentiation of krel = 49 (entry 1).
The reaction in ether also proceeded smoothly with slight-
ly better differentiation of krel = 61 (entry 2). The reactions
in acetone, acetonitrile, and dichloromethane showed the
Equation 1
Typical procedures of ruthenium-catalyzed hetero Diels-
Alder reaction and kinetic resolution of racemic epoxides
were described below. The equipments made with Pyrex
glass were used through these reactions.
Synlett 1999, No. 07, 1160–1162 ISSN 0936-5214 © Thieme Stuttgart · New York

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J. Mihara et al.
LETTER
Hetero Diels-Alder reaction of benzaldehyde and 1-meth-
oxy-3-trimethylsilyloxy-1,3-butadiene (Danishefsky´s di-
ene): In a 5 ml round-bottomed flask were placed
complex 1 (2.0 mg, 2.0 µmol) and t-butyl methyl ether un-
der nitrogen atmosphere and the mixture was cooled to
0 °C. To this solution were successively added benzalde-
hyde (10.2 µl, 0.1 mmol) and 1-methoxy-3-trimethylsily-
loxy-1,3-butadiene (19.5 µl, 0.1 mmol). The mixture was
stirred for 2 days at 0 °C under incandescent light (100 V,
60 W). Trifluoroacetic acid (10 ml) was added to the reac-
tion mixture and stirred for 5 min. To this solution was
added several drops of pyridine and the mixture was con-
centrated in vacuo. The residue was chromatographed on
silica gel (hexane/ethyl acetate = 1/0 to 8/2) to give (S)-2-
phenyl-2,3-dihydro-4H-pyran-4-one (4.6 mg, 26%). The
optical purity of the product was determined to be 83% ee
by HPLC analysis using optically active column (DA-
ICEL CHIRALCEL OD-H, hexane/2-propanol = 9/1,
flow rate = 0.5 ml/min).
Acknowledgement
Financial supports from a Grant-in-Aid for Scientific Research
from the Ministry of Education, Science, Sports, and Culture, Ja-
pan, is gratefully acknowledged.
References and Notes
(1) Juris, A.; Balazani, V.; Barigelletti, F.; Campagna, S.; Belser,
P.; Zelewsky, A. V. Coord. Chem. Rev. 1988, 84, 84-277.
(2) Ford, P. C.; Bourassa, J.; Miranda, K.; Lee, B.; Lorkovic, I.;
Boggs, S.; Kudo, S.; Laverman, L. Coord. Chem. Rev. 1998,
171, 185-202.
(3) Noyori, R. "Asymmetric Catalysis in Organic Synthesis" John
Wiley & Sons, New York, 1994.
(4) 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.
(5) Bosnich et al. has been reported Diels-Alder reaction using
achiral cationic (NO)Ru-salen as a catalyst: Odenkirk, W.;
Rheingold, A. L.; Bosnich, B. J. Am. Chem. Soc. 1992, 114,
6392-6398.
(6) a) For asymmetric hetero Diels-Alder reactions, see:
Waldmann, H. Synthesis 1994, 535-551. b) For the Cr-salen-
catalyzed reaction, see: Schaus, S. E.; Brånalt, J.; Jacobsen, E.
N. J. Org. Chem. 1998, 63, 403-405.
(7) For preparation of (ON)Ru-salen complex 1, see the preceding
communication. Cationic (ON)Ru-salen complex 2 was
synthesized from (ON)Ru-salen complex 1 and AgSbF6
according to the reference 5.
(8) For kinetic resolution of epoxides, see: a) Asami, M. J. Synth.
Org. Chem., Jpn. 1996, 54, 188-199. b) Tokunaga, M.;
Larrow, J. F.; Kakiuchi, F.; Jacobsen, E. N. Science 1997, 277,
936-938.
(9) Yamashita, Y.; Katsuki, T. Synlett 1995, 829-830.
(10) As described in the preceding communication, the produced
epoxide in the epoxidation of dihydronaphthalene suffered
from the undesirable decomposition under the reaction
conditions. This is probably attributable to that the produced
epoxide is coordinated to the Lewis acidic ruthenium ion and,
therefore, the epoxide is much more subject to the
decomposition.
(11) The mechanism of the photo enhancement of the Lewis
acidity is not clear at present but the photo-induced ligand
dissociation is considered to be responsible for this
phenomenon as described in Synlett 1999, 1160
Kinetic resolution of racemic 1,2-epoxy-3,4-dihy-
dronaphthalene: To a solution of racemic 1,2-epoxy-3,4-
dihydronaphthalene (32.2 mg, 220 mmol) in diethyl ether
(2.2 ml) was added 1-bromonaphthalene (30.6 ml) as an
internal standard. An aliquot (200 ml) of this solution was
taken out of the flask as a zero point, concentrated and an-
alyzed by 1H NMR to determine the peak areas of the ep-
oxide and the internal standard. To the remnant solution
was added complex 1 (4.0 mg, 4.0 mmol) and the mixture
was stirred for 72 h at room temperature. The mixture was
passed through a pad of silica gel (pentane/diethyl
ether = 1/1) to remove the complex. The eluate was con-
centrated on a rotary evaporator. The amounts of the un-
reacted epoxide (46%) and ketone (48%) were determined
by 1H NMR analysis. The optical purity of the unreacted
epoxide was determined to be 99.0% ee by HPLC analysis
using optically active column (DAICEL CHIRALCEL
OB-H, hexane/2-propanol = 50/1, flow rate = 0.5 ml/
min).
In conclusion, we were able to disclose an interesting
Lewis acidic nature of (ON)Ru-salen complexes which is
activated by sunlight or incandescent light.11) Further
study is now proceeding in our laboratory.
Article Identifier:
1437-2096,E;1999,0,07,1160,1162,ftx,en;Y07599ST.pdf
Synlett 1999, No. 07, 1160–1162 ISSN 0936-5214 © Thieme Stuttgart · New York