J. Am. Chem. Soc. 1996, 118, 11311-11312
11311
Highly Enantioselective Epoxidation of
trans-Stilbenes Catalyzed by Chiral Ketones
Dan Yang,* Xue-Chao Wang, Man-Kin Wong,
Yiu-Chung Yip, and Man-Wai Tang
Department of Chemistry, The UniVersity of Hong Kong
Pokfulam Road, Hong Kong
Figure 1.
ReceiVed August 1, 1996
Table 1. Asymmetric Epoxidation of trans-Stilbene with Catalysts
1-8a
Catalytic asymmetric epoxidation of unfunctionalized olefins
has been a great challenge in organic synthesis.1 For cis-
olefins,2 trisubstituted olefins,2d,3 tetrasubstituted olefins,4 and
styrenes,2d,5 highly enantioselective catalysts based on metal-
oxo complexes have been developed. However, there is no
general and effective method available for catalytic asymmetric
epoxidation of trans-olefins.6,7 Here, we report on a series of
novel chiral ketone catalysts that give the highest enantiose-
lectivities reported to date for epoxidation of trans-stilbenes.
In addition, using chiral ketones as probes, we provide convinc-
ing experimental evidence for a spiro transition state of dioxirane
epoxidation.8
We recently reported that C2 symmetric chiral dioxirane 1a,
generated in situ from chiral ketone 1 and Oxone, is selective
for asymmetric epoxidation of trans-olefins and trisubstituted
olefins (33-87% ee).7 The X-ray structure of ketone 1 revealed
that the two naphthalene rings were located on the opposite faces
of the keto group.7 As shown in Figure 1, H-3 and H-3′ are
closer to the dioxirane group than other atoms on the chiral
binaphthalene unit and may therefore be the steric sensors in
the oxygen transfer process.9,10 We expected that, by increasing
steric bulkiness at the 3 and 3′ positions, the resulting chiral
ketones would have better enantioselectivity than ketone 1.11
New chiral ketones 2-8 were thus designed and synthesized.12
The results for asymmetric epoxidation of trans-stilbene with
catalysts 1-8 (10 mol %) are summarized in Table 1. Several
trends were observed. (1) As the size of the steric sensor
a All epoxidation reactions were carried out at room temperature with
0.1 mmol of trans-stilbene, 0.01 mmol of catalyst, 0.5 mmol of Oxone,
1.55 mmol of NaHCO3, 1.5 mL of CH3CN, and 1 mL of aqueous
Na2‚EDTA solution (4 × 10-4 M). b Optical purity: 98% ee. All chiral
ketones were recovered in over 80% yield without loss of catalytic
activity and chiral induction. c Isolated yield. d Determined by circular
1
dichroism spectroscopy. e Determined by H NMR using chiral shift
reagent Eu(hfc)3. f Yield based on recovered trans-stilbene (50%
conversion). g Not completed.
(1) For recent reviews on catalytic asymmetric epoxidation of unfunc-
tionalized olefins, see: (a) Jacobsen, E. N. In Catalytic Asymmetric
Synthesis; Ojima, I., Ed.; VCH: New York, 1993; Chapter 4.2. (b) Collman,
J. P.; Zhang, X.; Lee, V. J.; Uffelman, E. S.; Brauman, J. I. Science 1993,
261, 1404.
(2) (a) Jacobsen, E. N.; Zhang, W.; Muci, A. R.; Ecker, J. R.; Deng, L.
J. Am. Chem. Soc. 1991, 113, 7063. (b) Lee, N. H.; Muci, A. R.; Jacobsen,
E. N. Tetrahedron Lett. 1991, 32, 5055. (c) Deng, L.; Jacobsen, E. N. J.
Org. Chem. 1992, 57, 4320. (d) Palucki, M.; McCormick, G. J.; Jacobsen,
E. N. Tetrahedron Lett. 1995, 36, 5457. (e) Irie, R.; Noda, K.; Ito, Y.;
Katsuki, T. Tetrahedron Lett. 1991, 32, 1055. (f) Hosoya, N.; Hatayama,
A.; Irie, R.; Sasaki, H.; Katsuki, T. Tetrahedron 1994, 50, 4311.
(3) (a) Brandes, B. D.; Jacobsen, E. N. J. Org. Chem. 1994, 59, 4378.
(b) Fukuda, T.; Irie, R.; Katsuki, T. Synlett 1995, 197.
(4) Brandes, B. D.; Jacobsen, E. N. Tetrahedron Lett. 1995, 36, 5123.
(5) (a) Palucki, M.; Pospisil, P. J.; Zhang, W.; Jacobsen, E. N. J. Am.
Chem. Soc. 1994, 116, 9333. (b) Naruta, Y.; Ishihara, N.; Tani, F.;
Maruyama, K. Bull. Chem. Soc. Jpn. 1993, 66, 158.
(6) (a) Lee, N. H.; Jacobsen, E. N. Tetrahedron Lett. 1991, 32, 6533.
(b) Chang, S.; Lee, N. H.; Jacobsen, E. N. J. Org. Chem. 1993, 58, 6939.
(c) Chang, S.; Galvin, J. M.; Jacobsen, E. N. J. Am. Chem. Soc. 1994, 116,
6937. (d) Li, A.-H.; Dai, L.-X.; Hou, X.-L.; Huang, Y.-Z.; Li, F.-W. J.
Org. Chem. 1996, 61, 489. (e) Aggarwal, V. K.; Ford, J. G.; Thompson,
A.; Jones, R. V. H.; Standen, M. C. H. J. Am. Chem. Soc. 1996, 118, 7004.
(7) Yang, D.; Yip, Y.-C.; Tang, M.-W.; Wong, M.-K.; Zheng, J.-H.;
Cheung, K.-K. J. Am. Chem. Soc. 1996, 118, 491.
(8) For excellent reviews on dioxirane chemistry, see: (a) Adam, W.;
Curci, R.; Edward, J. O. Acc. Chem. Res. 1989, 22, 205. (b) Murray, R. W.
Chem. ReV. 1989, 89, 1187.
became larger (from H to Cl to Br to I, see entries 1-4; from
H to Me to MOM to acetal to TMS, see entries 1 and 5-8),
enantioselectivity was first increased and then decreased. This
implies that ketones with the appropriate size of steric sensor
are desirable. (2) While Cl is smaller in size than Me, higher
ee was obtained with chloro ketone 2 than methyl ketone 5,
which suggests that the presence of electronegative atoms on
the steric sensor is also important. (3) Among ketones 1-8,
acetal ketone 7 was found to be the most reactive one.
It is interesting to note that trans-stilbene has a phenyl group
and a hydrogen atom on one side of the double bond. When
encountering trans-stilbene, C2 symmetric chiral dioxirane (R)-
1a has two possible orientations (favored and disfavored based
on steric considerations) under either a spiro or a planar
transition state (Figure 2).13,14 The favored orientation has the
phenyl group of trans-stilbene positioned away from the
naphthalene rings of the dioxirane. When the steric sensors at
the 3 and 3′ positions become larger up to certain size (e.g.,
from H to Br), there is little increase of steric interaction in the
favored orientation, whereas steric interaction is significantly
increased in the disfavored orientation, thereby giving higher
enantioselectivity.15 However, when the steric sensors become
(9) In the X-ray structure of ketone 1, the distance between H-3 or H-3′
and the keto group is ca. 5 Å, approximately the length of a phenyl ring.
(10) The structure of chiral dioxirane (R)-1a was created using the Chem
3D program on the basis of the coordinates from the X-ray structure of
ketone 1.
(11) When the C2 symmetric chiral element was changed from 1,1′-
binaphthyl-2,2′-dicarboxylic acid to 6,6′-dinitro-2,2′-diphenic acid, similar
ee values were observed. Unpublished results.
1
(12) All new compounds were characterized by H and 13C NMR, IR,
HRMS, and LRMS (see Supporting information).
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