Highly enantioselective epoxidation of cis-olefins by chiral dioxirane [23]

Full text of DOI:10.1021/ja003049d

J. Am. Chem. Soc. 2000, 122, 11551-11552
11551
Chart 1
Highly Enantioselective Epoxidation of cis-Olefins by
Chiral Dioxirane
Hongqi Tian, Xuegong She, Lianhe Shu, Hongwu Yu, and
Yian Shi*
Department of Chemistry, Colorado State UniVersity
Fort Collins, Colorado 80523
ReceiVed August 16, 2000
Table 1. Asymmetric Epoxidation of Olefins Catalyzed by Ketone 2a
Asymmetric epoxidation of olefins presents a powerful strategy
for the synthesis of enantiomerically enriched epoxides.1-3 For
the epoxidation of unfunctionalized cis-olefins, chiral salen and
porphyrin complexes give very high enantioselectivities. Jacob-
sen’s Mn salen catalyst is particularly successful and practical.
Dioxiranes generated in situ from chiral ketones have been shown
to be highly enantioselective for the asymmetric epoxidation of
trans-olefins and trisubstituted olefins.4-6 However, highly enan-
tioselective epoxidation of cis-olefins using chiral dioxiranes still
remains a challenging problem. Herein we wish to report our
preliminary efforts on this subject.
Recently, we reported that the fructose-derived ketone 1 is an
effective epoxidation catalyst and gives high ee values for a
variety of trans-olefins and trisubstituted olefins (eq 1).6 However,
epoxidation of cis-olefins using this ketone led to rather poor
enantioselectivity.6c For example, a 39% ee was obtained for cis-
â-methylstyrene, giving the (1R,2S) epoxide as the major enan-
(1) For recent reviews on highly enantioselective epoxidation of allylic
alcohols, see: (a) Johnson, R. A.; Sharpless, K. B. In Catalytic Asymmetric
Synthesis; Ojima, I., Ed.; VCH: New York, 1993; Chapter 4.1. (b) Katsuki,
T.; Martin, V. S. Org. React. 1996, 48, 1.
(2) For recent reviews on metal catalyzed highly enantioselective epoxi-
dation of unfunctionalized 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. (c) Katsuki, T. Coord. Chem. ReV. 1995, 140, 189.
(d) Mukaiyama T. Aldrichim. Acta 1996, 29, 59.
(3) For a recent review on asymmetric epoxidation of electron-deficient
olefins, see: Porter, M. J.; Skidmore, J. Chem. Commun. 2000, 1215.
(4) For general leading references on dioxiranes see: (a) Adam, W.; Curci,
R.; Edwards, J. O. Acc. Chem. Res. 1989, 22, 205. (b) Murray, R. W. Chem.
ReV. 1989, 89, 1187. (c) Curci, R.; Dinoi, A.; Rubino, M. F. Pure Appl. Chem.
1995, 67, 811. (d) Clennan, E. L. Trends Org. Chem. 1995, 5, 231. (e) Adam,
W.; Smerz, A. K. Bull. Soc. Chim. Belg. 1996, 105, 581. (f) Denmark, S. E.;
Wu, Z. Synlett 1999, 847.
(5) For leading references on asymmetric epoxidation mediated by chiral
ketones see: (a) Curci, R.; Fiorentino, M.; Serio, M. R. J. Chem. Soc., Chem.
Commun. 1984, 155. (b) Curci, R.; D’Accolti, L.; Fiorentino, M.; Rosa, A.
Tetrahedron Lett. 1995, 36, 5831. (c) Denmark, S. E.; Forbes, D. C.; Hays,
D. S.; DePue, J. S.; Wilde, R. G. J. Org. Chem. 1995, 60, 1391. (d) Brown,
D. S.; Marples, B. A.; Smith, P.; Walton, L Tetrahedron 1995, 51, 3587. (e)
Yang, D.; Yip, Y. C.; Tang, M. W.; Wong, M. K.; Zheng, J. H.; Cheung, K.
K. J. Am. Chem. Soc. 1996, 118, 491. (f) Yang, D.; Wang, X.-C.; Wong,
M.-K.; Yip, Y.-C.; Tang, M.-W. J. Am. Chem. Soc. 1996, 118, 11311. (g)
Song, C. E.; Kim, Y. H.; Lee, K. C.; Lee, S. G.; Jin, B. W. Tetrahedron:
Asymmetry 1997, 8, 2921. (h) Adam, W.; Zhao, C.-G. Tetrahedron: Asym-
metry 1997, 8, 3995. (i) Denmark, S. E.; Wu, Z.; Crudden, C. M.; Matsuhashi,
H. J. Org. Chem. 1997, 62, 8288. (j) Wang, Z.-X.; Shi, Y. J. Org. Chem.
1997, 62, 8622. (k) Armstrong, A.; Hayter, B. R. Chem. Commun. 1998, 621.
(l) Yang, D.; Wong, M.-K.; Yip, Y.-C.; Wang, X.-C.; Tang, M.-W.; Zheng,
J.-H.; Cheung, K.-K. J. Am. Chem. Soc. 1998, 120, 5943. (m) Yang, D.; Yip,
Y.-C.; Chen, J.; Cheung, K.-K. J. Am. Chem. Soc. 1998, 120, 7659. (n) Adam,
W.; Saha-Moller, C. R.; Zhao, C.-G. Tetrahedron: Asymmetry 1999, 10, 2749.
(o) Wang, Z.-X.; Miller, S. M.; Anderson, O. P.; Shi, Y. J. Org. Chem. 1999,
64, 6443. (p) Carnell, A. J.; Johnstone, R. A. W.; Parsy, C. C.; Sanderson,
W. R. Tetrahedron Lett. 1999, 40, 8029. (q) Armstrong, A.; Hayter, B. R.
Tetrahedron 1999, 55, 11119.
a All reactions were carried out with olefin (0.5 mmol), ketone
(0.075-0.15 mmol), Oxone (0.89 mmol), and K2CO3 (2.01 mmol) in
DME/DMM (3:1, v/v) (7.5 mL) and buffer (0.2 M K2CO3-AcOH,
pH 8.0) (5 mL) at -10 or 0 °C unless otherwise stated. The reactions
were stopped after 3.5 h. b The epoxides were purified by flash
chromatography and gave satisfactory spectroscopic characterization.
c With 0.075 mmol of ketone at -10 °C. d With 0.10 mmol of ketone
at -10 °C. e With 0.075 mmol of ketone at 0 °C. f With 0.15 mmol of
ketone at -10 °C. g With 0.15 mmol of ketone at 0 °C. h Enantiose-
lectivity was determined by chiral GC (Chiraldex G-TA). i Enantiose-
lectivity was determined by chiral HPLC (Chiralcel OJ). j Enantiose-
lectivitywasdeterminedbychiralHPLC(ChiralcelOB). k Enantioselectivity
was determined by chiral HPLC (Chiralpak AD). l Enantioselectivity
was determined by chiral HPLC (Chiralcel OD). m The absolute
configurations were determined by comparing the measured optical
rotations with the reported ones. n The epoxide was reduced to 1-(2-
naphthyl)propanol with LiAlH4, and the absolute configuration was
determined by comparing the measured optical rotation of the alcohol
with the reported one (ref 10). o The epoxide was reduced with LiAlH4
to the corresponding homopropargyl alcohol, and the absolute config-
uration was determined by a correlation of the resulting alcohol with
a prepared authentic sample by a different route.
tiomer. Spiro transition states A and B are likely to be the two
major competing transition states (Chart 1).6c The low ee obtained
suggests that the ketone catalyst does not provide the necessary
structural environment to sufficiently differentiate between the
phenyl and methyl groups of the olefin in these two transition
states (Chart 1).
During the course of our continuing studies of structural effects
of ketones on catalysis, ketone 2, a nitrogen analogue of 1, was
10.1021/ja003049d CCC: $19.00 © 2000 American Chemical Society
Published on Web 11/02/2000

11552 J. Am. Chem. Soc., Vol. 122, No. 46, 2000
Communications to the Editor
designed and investigated for epoxidation. Although our initial
Chart 2
studies showed that ketone 2 did not give as high enantioselec-
tivity as 1 for trans-olefins, its behavior toward cis-olefins was
strikingly different. When the epoxidation of cis-â-methylstyrene
was carried out with 15 mol % ketone 2 at -10 °C, (1R,2S)-cis-
â-methylstyrene oxide was obtained in 87% yield with 91% ee.
Furthermore, like other dioxirane-mediated epoxidations, the
epoxidation of cis-â-methylstyrene was found to be stereospecific,
with no trans-â-methylstyrene oxide formed during the reaction,
transition states (Chart 2). The determination of the absolute
configurations of some selected epoxides (Table 1, entries 1-8)
showed that groups with a π system preferred to be proximal to
the spiro oxazolidinone. It appears that spiro C is favored over
D for substrates containing a π system. A clear mechanistic
understanding awaits further investigation.
In summary, we report a highly enantioselective epoxidation
for cis-olefins using chiral ketone 2 as catalyst and Oxone as
oxidant. High ee values have been obtained for a number of cyclic
and acyclic cis-olefins. The epoxidation was stereospecific with
no isomerization observed in the epoxidation of acyclic systems.
The source of the enantioselectivity is not known at this time.
The results described show that chiral dioxiranes can also
epoxidize cis-olefins in addition to trans-olefins and trisubstituted
olefins in a high degree of enantioselectivity. Ketone 2 reveals a
promising structural element required for the ketone to induce
the high enantioselectivity for the epoxidation of cis-olefins, which
provides a basis for further optimization of the ketone structure
to enhance both enantioselectivity and catalytic activity.
1
as judged by H NMR and GC assays of the crude reaction
mixture.
Encouraged by this result, we investigated the asymmetric
epoxidation of a variety of cis-olefins to explore the generality
of ketone 2. The enantiomeric excesses were generally high for
a variety of acyclic and cyclic cis-olefins conjugated with
aromatics (Table 1, entries 1-6). The epoxidation of cis-1-
cyclohexyl-1-propene with ketone 2 resulted in the formation of
the (2R,3S)-2-cyclohexyl-3-methyloxirane in 67% ee, indicating
that a conjugated aromatic group is beneficial for the enantiose-
lectivity. The epoxidation of acyclic enynes was also found to
be both highly enantioselective and stereospecific, providing cis-
epoxides with high ee values (Table 1, entries 7 and 8).7 High ee
values were obtained for 3,3-ethylenedioxycycloalkenes as well
(Table 1, entries 9-11). The enantioselectivity for trans-olefins
and trisubstituted olefins was found to be substrate dependent
(Table 1, entries 12-15).8
Acknowledgment. We are grateful for the generous financial support
from the General Medical Sciences of the National Institutes of Health
(GM59705-02), the Arnold and Mabel Beckman Foundation, the Camille
and Henry Dreyfus Foundation, Alfred P. Sloan Foundation, DuPont,
Eli Lilly, GlaxoWellcome, and Merck.
The high ee values obtained with ketone 2 for cis-olefins are
rather intriguing. Spiro C and D are the two most plausible
(6) For examples of asymmetric epoxidation mediated by fructose-derived
ketones see: (a) Tu, Y.; Wang, Z.-X.; Shi, Y. J. Am. Chem. Soc. 1996, 118,
9806. (b) Wang, Z.-X.; Tu, Y.; Frohn, M.; Shi, Y. J. Org. Chem. 1997, 62,
2328. (c) Wang, Z.-X.; Tu, Y.; Frohn, M.; Zhang, J.-R.; Shi, Y. J. Am. Chem.
Soc. 1997, 119, 11224. (d) Frohn, M.; Dalkiewicz, M.; Tu, Y.; Wang, Z.-X.;
Shi, Y. J. Org. Chem. 1998, 63, 2948. (e) Wang, Z.-X.; Shi, Y. J. Org. Chem.
1998, 63, 3099. (f) Zhu, Y.; Tu, Y.; Yu, H.; Shi, Y. Tetrahedron Lett. 1998,
39, 7819. (g) Tu, Y.; Wang, Z.-X.; Frohn, M.; He, M.; Yu, H.; Tang, Y.; Shi,
Y. J. Org. Chem. 1998, 63, 8475. (h) Wang, Z.-X.; Cao, G.-A.; Shi, Y. J.
Org. Chem. 1999, 64, 7646. (i) Warren, J. D.; Shi, Y. J. Org. Chem. 1999,
64, 7675. (j) Frohn, M.; Zhou, X.; Zhang, J.-R.; Tang, Y.; Shi, Y. J. Am.
Chem. Soc. 1999, 121, 7718. (k) Shu, L.; Shi. Y. Tetrahedron Lett. 1999, 40,
8721.
(7) For leading references on asymmetric epoxidations of enynes using
(salen)Mn(III) see: (a) Lee, N. H.; Jacobsen, E. N. Tetrahedron Lett. 1991,
32, 6533. (b) Brandes, B. D.; Jacobsen, E. N. J. Org. Chem. 1994, 59, 4378.
(c) Hamada, T.; Irie, R.; Katsuki, T. Synlett 1994, 479. (d) Sasaki, H.; Irie,
R.; Hamada, T.; Suzuki, K.; Katsuki, T. Tetrahedron 1994, 50, 11827. (e)
Hamada, T.; Daikai, K.; Irie, R.; Katsuki, T. Synlett 1995, 407.
(8) Up to 40% ketone could be recovered after extracting the aqueous layer
of the reaction mixture with CH2Cl2-EtOAc, and purification by flash
chromatography. The recovered ketone gave a similar conversion and ee for
the epoxidation.
Supporting Information Available: Experimental procedures for the
preparation of ketone 2 and for the asymmetric epoxidation reaction and
the characterization of the ketone and epoxides along with the GC and
HPLC data for the determination of the enantiomeric excess of the
epoxides (PDF). This material is available free of charge via the Internet
at http://pubs.acs.org.
JA003049D
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