Enantioselective epoxidation of conjugated Z-olefins with newly modified Mn(salen) complex

Article abstract of DOI:10.1246/cl.2007.46

Chiral Mn(salen) complex 3 bearing a 1-ethyl-1-methyl-propyl group at C3, C3′, C5, and C5′ was found to induce higher asymmetry in the epoxidation of conjugated Z-olefins, especially less nucleophilic ones, in the presence of 4-phenylpyridine N-oxide than

Full text of DOI:10.1246/cl.2007.46

46
Chemistry Letters Vol.36, No.1 (2007)
Enantioselective Epoxidation of Conjugated Z-Olefins
with Newly Modified Mn(salen) Complex
Hiromichi Egami, Ryo Irie, Ken Sakai, and Tsutomu Katsukiꢀ
Department of Chemistry, Faculty of Science, Graduate School, Kyushu University 33,
Hakozaki, Higashi-ku, Fukuoka 812-8581
(Received September 19, 2006; CL-061083; E-mail: katsuscc@mbox.nc.kyushu-u.ac.jp)
Table 1. Asymmetric epoxidation using J-type catalystsa
Chiral Mn(salen) complex 3 bearing a 1-ethyl-1-methyl-
propyl group at C3, C30, C5, and C50 was found to induce higher
asymmetry in the epoxidation of conjugated Z-olefins, especially
less nucleophilic ones, in the presence of 4-phenylpyridine N-
oxide than complex 1 bearing a t-butyl group at the same
carbons. The higher enantioselectivity was considered to be
attributable to Me-in-plane conformation of the 1-ethyl-1-meth-
ylpropyl group at C3 and C30.
R1 O R2
R1
R2
3, aq. NaOCl
R1 = unsaturated group
PPNO
Temp. Time Yieldb
Entry Sub.
% ee
(% ee)c (% ee)d
/ꢁC
/h
/%
1
2
3
4
5
4
4
5
6
7
4
4
0
0
4
0.5
4
12
82
61
53
89
88e
88e
75f
86f(86g) 79(85h)
4
88e
86i(88g,j
89f
92m
)
4
96k
96(96j)l
8
92(94j)
In 1986, Kochi et al. reported that Mn(salen) complexes
served as catalysts for epoxidation of olefins1 and subsequently
Jacobsen’s and our groups independently reported chiral Mn-
(salen)-catalyzed asymmetric epoxidation in 1990.2 Although
many chiral Mn(salen) complexes have been synthesized from
then on, they are roughly classified into two groups, complexes
developed by Jacobsen (J-type) and by us (K-type). They differ
in the presence or absence of a chiral substituent at C3 and C30:
K-type catalysts are constructed based on the diastereomeric
strategy and show higher asymmetric induction and wider scope
of applicability, while J-type catalysts are more easily available.3
Thus, it is still of value to improve asymmetric induction by
J-type catalysts. Although asymmetric induction by Mn(salen)
complexes is governed by many factors, steric repulsion between
the olefinic substituent and the substituent at C3 or C30 plays
a crucial role.3,4 Second generation K-type catalysts bear a 2-
phenylnaphthyl group at C3 or C30 and direct the 2-phenyl group
close to an incoming olefin,5 while J-type catalysts generally
bear t-butyl (t-Bu) groups that locate rather away from the olefin,
at C3 and C30. Recently, Bu et al. reported that replacement of
the t-Bu group with a 1,1-dimethylpropyl (t-Pen) group im-
proved asymmetric induction by J-type complexes, because
the t-Pen group exerts better steric effectiveness than the t-Bu
group due to the presence of the free rotational ethyl group in
t-Pen group.6 However, the improvement of asymmetric induc-
tion by complex 2 does not seem as large as expected, probably
because the t-Pen group at C3 and C30 can take two conformers,
the desired (R1 = Et) and the undesired (R2 = Et), almost
equally (Figure 1).
(trans=cis ¼ 1=5)n (trans=cis ¼ 1=11:5)m
6
7
8
9
0
0
24
48
34
50
92e
77o
94p,q
83o,q
(trans=cis ¼ 10=1)n (trans=cis ¼ 7:3=1)o
8
10
rt
4
93
97q(71r)l 93q(58r)j,s
(trans=cis ¼ 1:8=1, 88t)n(trans=cis ¼ 2=1, 81t)s
aAll the reactions were carried out in dichloromethane in the pres-
ence of PPNO by using NaOCl as the terminal oxidant, unless
otherwise mentioned. bIsolated yield. cThe highest % ee reported
for asymmetric epoxidation using 1 as a catalyst. dThe reactions
with complex 2 as catalyst. The % ees were taken from Ref. 6.
eDetermined by HPLC analysis (DAICEL CHIRALCEL OB-H).
fThe reaction was carried out by us using 1 purchased from Aldrich,
as the catalyst according to the reported procedure. gTaken from
Ref. 3a. hThe reaction was carried out in the presence of pyridine
i
j
N-oxide. Taken from Ref. 11. The reaction was carried out in the
absence of 4-PPNO. kDetermined by HPLC analysis (DAICEL
l
CHIRALCEL AS-H). Determined by HPLC analysis of the cis-
epoxide (DAICEL CHIRALCEL OJ-H). mTaken from Ref. 12. nThe
present study. oTaken from Ref. 13. pDetermined by HPLC analysis
(DAICEL CHIRALCEL OD-H). qThe number stands for ee of the
trans-epoxide. rThe number stands for ee of the cis-epoxide. sTaken
t
from Ref. 14. The number stands for the face selectivity. Face
selectivity = eetrans ꢂ %trans + eecis ꢂ %cis.
Ph
OTBDMS
n
CO2Et
Ph
CO2Et
4; n = 2
5; n = 1
6; n = 3
7
8
9
10
On the other hand, we recently found that an (ON)Ru(salen)
complex possessing a 1-ethyl-1-methylpropyl (t-Hex) group at
C3, C30, C5, and C50 served as an efficient catalyst for chemose-
lective aerobic oxidation of primary alcohols even in the pres-
ence of activated secondary alcohols.7 The t-Hex groups at C3
and C30 have been proved or by NOE experiment to adopt
Me-in-plane (Mip) conformation. We anticipated that Mn(salen)
complex 3 bearing the same salen ligand would induce much
larger asymmetry than complexes 1 and 2, because the t-Hex
groups at C3 and C30 would also adopt Mip conformation
(R1, R2 = Et, R3 = Me, Figure 1) and show much larger steric
effectiveness than t-Pen.
R4
R5
R6
R2 R3
3'
R1
R2
N
R1
N
R3
Mn
O
O
O
R
O
L O
R
Mn
L
3
R5
R4
N
N
R
R
R6
Rs
1: R= t-Bu, L= Cl
2: R= t-Pen, L= Cl
3: R= t-Hex, L= Cl
RL
-
11: R= t-Hex, L= ClO4
Figure 1. J-type catalysts and the possible conformation of 3-
and 30-substituents in the corresponding O=Mn(salen) species.
Copyright ꢀ 2007 The Chemical Society of Japan

Chemistry Letters Vol.36, No.1 (2007)
47
tioselectivity of the epoxidation using J-type catalyst could be
further enhanced by introducing suitably designed 3,30-substitu-
ents.
Financial support from CREST, Japan Science and
Technology Agency (JST), Banyu Pharmaceutical Co., and
Nissan Chemical Industries is gratefully acknowledged. H. E.
acknowledges Research Fellowships of the Japan Society for
the Promotion of Science for Young Scientists.
3
11
Figure 2. ORTEP diagrams for the X-ray structures of 3 and
11. The t-Hex group at C5 in 3 and the perchlorate ion in 11
show orientational disorder. The perchlorate ion and hydrogen
atoms are omitted for clarity.
This paper is dedicated to Professor Teruaki Mukaiyama on
the occasion of his eightieth birthday.
Thus, we synthesized complex 3 in a conventional manner2,7
and examined epoxidation of dihydronaphthalene 4 in the pres-
ence of 4-phenylpyridine N-oxide (PPNO) (Table 1).8 The enan-
tioselectivity of the epoxidation of 4 using 1 as catalyst under the
same conditions was 75% ee.9 Bu et al. have reported that epox-
idation with 2 shows 79% ee at 4 h (Entry 2).6,10 The epoxidation
with 3 showed the highest ee of 88%. It is noteworthy that the
enantioselectivity did not vary during the reaction (Entries 1
and 2). On the other hand, enantioselectivity of epoxidation of
indene 5 with 3 was almost identical to that with 1 (Entry 3).
However, the epoxidation of 6 with 3 showed much better enan-
tioselectivity than that with 1 (Entry 4). Epoxidation of 7 also
showed a similar trend (Entry 5). We inferred from these results
that 3 could exert better catalytic performance, as coplanarity of
the double bond and aromatic substituent becomes smaller, that
is, olefins become less nucleophilic. In order to ascertain this
assumption, we further examined the epoxidation of (2E,4Z)-
2,4-dienoates 8 and 9. Indeed, 3 showed much better enantiose-
lectivity in the epoxidation of these olefins than 1 (Entries 6 and
7). Epoxidation of enyne 10 was also better effected by 3 than by
1 (Entry 8). Steric effectiveness of t-Bu, t-Pen, and t-Hex groups
is mainly caused by van der Waals repulsion and is limited in
a short range. Thus, their steric effectiveness can be fully exerted
only when a substrate comes close to the substituents. As
substrate becomes less nucleophilic, the transition state (TS) of
the oxidation shifts to a later stage and the substrate is more
accessible to the substituents at TS. This general discussion
agrees with the above results.
The proposal that Mn(salen)-catalyzed epoxidation in
the presence of PPNO proceeds through a PPNO-bound oxo
Mn(salen) species has been accepted.3 In order to prove our
assumption on the conformation of the 3,30-Hex groups, we
performed an X-ray analysis of 3 and cationic Mn complex 11
coordinated by methanols at the apical positions15 (Figure 2).
Complex 11 was prepared as the model compound of the
O = Mn(salen)-PPNO adduct. Many Mn(salen)Cl complexes
take a square pyramidal configuration, in which the salen ligands
mostly adopt a planar or stepped conformation. However, the
salen ligand of 1 has been reported to adopt an umbrella-shape
(US) conformation.11 The salen ligand of 3 also adopted a dis-
torted US conformation, but it is noteworthy that one of the
3,30-t-Hex groups in 3 took Et-in-plane conformation, reflecting
their steric effectiveness.16 In contrast, 11 possessed octahedral
configuration and both the 3 and the 30-t-Hex groups were found
to adopt the desired Mip conformation (Figure 2).17,18
References and Notes
1
K. Srinivasan, P. Michaud, J. K. Kochi, J. Am. Chem. Soc.
1986, 108, 2309.
2
a) W. Zhang, J. L. Loebach, S. R. Wilson, E. N. Jacobsen, J.
Am. Chem. Soc. 1990, 112, 2801. b) R. Irie, K. Noda, Y. Ito,
N. Matsumoto, T. Katsuki, Tetrahedron Lett. 1990, 31, 7345.
a) E. N. Jacobsen, in Catalytic Asymmetric synthesis, ed. by
I. Ojima, VCH Publishers, New York, 1993, pp. 159–202. b)
T. Katsuki, Coord. Chem. Rev. 1995, 140, 189. c) T. Katsuki,
J. Mol. Catal. A: Chem. 1996, 113, 87. d) E. M. McGarrigle,
D. G. Gilheany, Chem. Rev. 2005, 105, 1564.
T. Katsuki, Adv. Synth. Catal. 2002, 344, 131.
H. Sasaki, R. Irie, T. Hamada, K. Suzuki, T. Katsuki, Tetra-
hedron 1994, 50, 11827.
3
4
5
6
7
8
A. Gaquere, S. Liang, F.-L. Hus, X. R. Bu, Tetrahedron:
Asymmetry 2002, 13, 2089.
H. Egami, H. Shimizu, T. Katsuki, Tetrahedron Lett. 2005, 46,
783.
A
typical procedure: complex 3 (2 mmol) and PPNO
(0.02 mmol) were weighed into a Schlenk tube, followed by
addition of dry CH2Cl2 (1 mL) and olefin (0.1 mmol). The mix-
ture was cooled to the specified temperature described in
Table 1 and aqueous NaOCl (0.44 M, pH ¼ 11:35, 1.2 mL)
pre-cooled to the specified temperature was added. The whole
mixture was stirred at the temperature for the time given in
the Table, and worked up in a conventional manner. The
enantiomer excesses of the products were determined as
described in the footnotes to Table 1.
9
It has been reported that the ee of the resulting epoxide increas-
es with the reaction time, because of enantiomer-differentiat-
ing C–H oxidation of the epoxide under the condition using
a J-type catalyst: F. J. Larrow, E. N. Jacobsen, J. Am. Chem.
Soc. 1994, 116, 12129.
10 The epoxidation proceeded with higher enantioselectivity of
85% ee in the presence of pyridine N-oxide. However, whether
C–H oxidation occurred under this condition or not was not
described in the paper.
11 P. J. Pospisil, D. H. Carsten, E. N. Jacobsen, Chem. Eur. J.
1996, 2, 974.
12 E. N. Jacobsen, W. Zhang, A. R. Muci, J. R. Ecker, L. Deng,
J. Am. Chem. Soc. 1991, 113, 7063.
13 S. Chang, N. O. Lee, E. N. Jacobsen, J. Org. Chem. 1993, 58,
6939.
14 O. N. Lee, E. N. Jacobsen, Tetrahedron Lett. 1991, 32, 6533s.
15 Complex 11 was prepared by treating 3 with AgClO4. Single
crystals of 3 and 11 were obtained by recrystallization from
MeOH–CH3CN and MeOH–i-PrOH, respectively.
16 CCDC No. 618956.
17 CCDC No. 616680.
18 In general, the ligand of octahedral Mn(salen) complexses
adopts a planar or stepped conformation.
In conclusion, we were able to synthesize a new Mn(salen)
complex 3 bearing a t-Hex group at C3, C30, C5, and C50 and to
achieve highly enantioselective epoxidation with it in the pres-
ence of PPNO. Moreover, this study demonstrated that the enan-
Published on the web (Advance View) December 16, 2006; doi:10.1246/cl.2007.46