Catalytic asymmetric epoxidation of α,β-unsaturated ketones promoted by lanthanoid complexes

Full text of DOI:10.1021/ja964037o

J. Am. Chem. Soc. 1997, 119, 2329-2330
2329
Catalytic Asymmetric Epoxidation of
r,â-Unsaturated Ketones Promoted by Lanthanoid
Complexes
Scheme 1. Preparation of Chiral Lanthanoid-BINOL
Derivative Catalysts
Masahiro Bougauchi, Shizue Watanabe, Takayoshi Arai,
Hiroaki Sasai, and Masakatsu Shibasaki*
Faculty of Pharmaceutical Sciences, UniVersity of Tokyo
Hongo, Bunkyo-ku, Tokyo 113, Japan
8
hydroperoxide (CMHP). As expected, the reaction of 3 with
ReceiVed NoVember 22, 1996
9
TBHP (2 equiv) in the presence of LSB (10 mol %), generated
from (R)-BINOL, in THF at room temperature for 10 h was
found to give 4 with 83% ee and in 92% yield. Unfortu-
nately however LSB as well as ALB and GaSB was not useful
for many other enones. Undeterred, we considered the pos-
sibility that an alkali metal free lathanum complex of the type
known to be effective for catalytic asymmetric Michael reac-
tions might also be useful in this case. The catalytic
suspension of the La-BINOL complex was prepared from La-
3
(O-i-Pr) and (R)-BINOL (1 molar equiv) in the presence of
MS 4A (Scheme 1). As expected it was found that treatment
Catalytic asymmetric epoxidations are one of the most
1
10
11
important asymmetric processes. In 1980 Sharpless et al.
reported a stoichiometric asymmetric epoxidation of allylic
alcohols, a method which was later improved to a catalytic
2
a
2
b
process.
Moreover, catalytic asymmetric epoxidations of
unfunctionalized olefins using salen-manganese complexes
3a
12
have been reported independently by Jacobsen et al., Katsuki
3
b
3c,4
et al., and Mukaiyama et al.
In striking contrast to these
1
3
successful achievements, an efficient catalytic asymmetric
epoxidation of enones with broad generality has not been
developed. Quite recently, Enders and co-workers reported a
very impressive and practical asymmetric epoxidation of enones
using a stoichiometric amount of N-methylpseudoephedrine as
a chiral source. In this Communication we report an efficient
catalytic asymmetric epoxidation of enones using lanthanoid
complexes, which is the first example of a catalytic process
applicable to various enones.
We have succeeded in developing several kinds of hetero-
bimetallic chiral catalysts such as LnM3tris(binaphthoxide)
complexes (LnMB), AlMbis(binaphthoxide) complexes (AMB),
and GaMbis(binaphthoxide) complexes (GaMB) (Ln ) lantha-
noid, M ) alkali metal, A ) aluminium, Ga ) gallium, B )
BINOL (1) or its derivatives).7 Among them, LSB, ALB, and
GaSB (L ) lanthanum, S ) sodium, L ) lithium) have been
found to be very useful for catalytic asymmetric Michael
reactions.7 We envisioned that these chiral catalysts would be
useful for the asymmetric epoxidation of enones using hydro-
peroxides such as tert-butyl hydroperoxide (TBHP) and cumene
1
4
5
15
of 3 with TBHP (1.5 equiv) in the presence of 10 mol % of
the La-BINOL complex (La-1) in THF at room temperature
1
0
11
16,17
for 0.5 h afforded 4 with 62% ee in 90% yield.
Moreover, the use of CMHP instead of TBHP improved the
6
18,19
asymmetric epoxidation, giving 4 with 83% ee in 93% yield
(5 mol % of La-BINOL complex). In marked contrast to LSB
this type of chiral lanthanum catalyst was found to be applicable
to a range of enone substrates. Thus, 5 was converted to 6
with 86% ee and in 93% yield, and 7 was transformed to 8
with 85% ee in 85% yield (Table 1, entries 1, 4, and 6). After
several attempts, we were pleased to find that the use of (R)-
3-hydroxymethyl-BINOL (2) instead of 1 substantially improved
the catalytic asymmetric epoxidations (Table 1, entries 2, 3, 5,
Namely, 4, 6, and 8 were obtained with 91, 94,
and 83% ee, respectively, and in excellent yields.
15
10
1
1
10
11
2
0-22
and 7).
1
1
In contrast to the results presented above, the enones shown
in Table 1 (entries 8, 9, 10, and 11) were best converted to the
corresponding epoxides by using the ytterbium complex gener-
1
3
14
ated from Yb(O-i-Pr)3, (R)-2, and MS 4A in THF at 40 °C
(
1) (a) Noyori, R. Asymmetric Catalysis In Organic Synthesis; John Wiley
Sons: New York, 1994. (b) Catalytic Asymmetric Synthesis; Ojima, I.,
Ed.; VCH: New York, 1993.
2) (a) Katsuki, K.; Sharpless, K. B. J. Am. Chem. Soc. 1980, 102, 5974-
976. (b) Hanson, R.; M. Sharpless, K. B. J. Org. Chem. 1986, 51, 1922-
&
(8) For the mechanism of epoxidations using hydroperoxides, see: Reed,
K. L.; Gupton, J. T.; Solarz, T. L. Synth. Commun. 1989, 19, 3579-3587.
(9) For obtaining anhydrous TBHP in toluene, see: Hill, J. G.; Rossiter,
B. E.; Sharpless, K. B. J. Org. Chem. 1983, 48, 3608-3611.
(10) The absolute configurations were determined by transformation to
the authentic samples (for 4, 6, 8, 10, and 14) or determined by the Mosher
method after transforming to the corresponding â-hydroxyketones (for 12
and 16). See: Marsman, B.; Wynberg, H. J. Org. Chem. 1979, 44, 2312-
2314.
(11) The ees of the epoxy ketones were determined by chiral stationary
phase HPLC.
(12) (a) Sasai, H.; Arai, T.; Shibasaki, M. J. Am. Chem. Soc. 1994, 116,
1571-1572. And also, see: (b) Zhang, F-Y.; Yip, C-W.; Chan, A. S. C.
Tetrahedron: Asymmetry 1996, 7, 2463-2466.
(
5
1
925.
(
3) (a) Zhang, W.; Loebach, J. L.; Wilson, S. R.; Jacobsen, E. N. J. Am.
Chem. Soc. 1990, 112, 2801-2803. (b) Irie, R.; Noda, K.; Ito, Y.;
Matsumoto, N.; Katsuki, T. Tetrahedron Lett. 1990, 31, 7345-7348. (c)
Yamada, T.; Imagawa, K.; Nagata, T.; Mukaiyama, T. Chem. Lett. 1992,
2
231.
(
4) Other examples of asymmetric epoxidations, see: Tu, Y.; Wang, Z-X.;
Shi, Y. J. Am. Chem. Soc. 1996, 118, 9806-9807 and references cited
therein.
(
5) (a) Juli a´ , S.; Masana, J.; Vega, J. C. Angew. Chem., Int. Ed. Engl.
980, 19, 929-931. (b) Kroutil, W.; Mayon, P.; Lasterra-S a´ nchez, M. E.;
1
Maddrell, S. J.; Roberts, S. M.; Thornton, S. R.; Todd, C. J.; T u¨ ter, M.
Chem. Commun. 1996, 845-846 and references cited therein. (c) Helder,
R.; Hummelen, J. C.; Laane, R. W. P. M.; Wiering, J. S.; Wynberg, H.
Tetrahedron Lett. 1976, 1831-1834. (d) Colonna, S.; Gaggero, N.;
Manfredi, A.; Spadoni, M.; Casella, L.; Carrea, G.; Pasta, P. Tetrahedron
(13) Purchased from Kojundo Chemical Co., Ltd., 5-1-28, Chiyoda,
Sakato, Saitama 350-02, Japan (Fax: +81-492-84-1351).
(14) MS 4A was used after drying at 180 °C for 3 h under reduced
pressure. Although the role of MS 4A is not clear at present, the reactions
are accelerated by their addition.
1
988, 44, 5169-5178. (e) Colonna, S.; Manfredi, A.; Annunziata, R.;
(15) Based on the amount of Ln(O-i-Pr)3.
Gaggero, N. J. Org. Chem. 1990, 55, 5862-5866. (f) Baccin, C.; Gusso,
A.; Pinna, F.; Strukul, G. Organometallics 1995, 14, 1161-1167. (g)
Kumar, A.; Bhakuni, V. Tetrahedron Lett. 1996, 37, 4751-4754. (h) For
an impressive catalytic asymmetric epoxidation of cinnamate esters, see:
Jacobsen, E. N. Deng, Li.; Furukawa, Y.; Martinez, L. E. Tetrahedron 1994,
(16) The use of other lanthanoids gave less satisfactory results.
(17) In the case of the Michael reactions reported in 12a, we observed
that a 1:1 ratio of La(O-i-Pr)3 and BINOL gave almost the maximum ee.
(18) Pure CMHP was obtained by the method in Purification of
Laboratory Chemicals, 3rd ed.; Perrin, D. D., Armarego, W. L., Eds.;
Pergamon Press: New York, 1988.
5
0, 4323-4334.
6) Enders, D.; Zhu, J.; Raabe, G. Angew. Chem., Int. Ed. Engl. 1996,
5, 1725-1728.
(
(19) The use of THF gave the best result (cf.: toluene, 62% ee in 96%
yield; Et2O, 47% ee in 78% yield; CH2Cl2, 45% ee in 93% yield).
(20) In the case of Ln-2 catalyst, a 1:1.25 ratio of Ln(O-i-Pr)3 and 2
gave the maximum ee.
3
(7) (a) Sasai, H.; Arai, T.; Satow, Y.; Houk, K. N.; Shibasaki, M. J. Am.
Chem. Soc. 1995, 117, 6194-6198. (b) Arai, T.; Sasai, H.; Aoe, K.;
Okamura, K.; Date, T.; Shibasaki, M. Angew. Chem., Int. Ed. Engl. 1996,
(21) Synthesized from (R)-BINOL in four steps. See Supporting
Information.
3
5, 104-106. (c) Arai, T.; Yamada, Y. M. A.; Yamamoto, N.; Sasai, H.;
Shibasaki, M. Chem. Eur. J. 1996, 2, 1368-1372 and references cited
(22) The use of (R)-3, 3′-dialkyl-BINOL (alkyl ) CH3 or CH2CH3), and
therein.
(R)-3, 3′-dihydroxymethyl-BINOL gave less satisfactory results.
S0002-7863(96)04037-1 CCC: $14.00 © 1997 American Chemical Society

2330 J. Am. Chem. Soc., Vol. 119, No. 9, 1997
Communications to the Editor
Table 1. Catalytic Asymmetric Epoxidations Using Alkali Metal
Free Lanthanoid Complexes
3
Figure 1. Influence of the ratio of Ln(O-i-Pr) and 1 on yield and ee.
Left: epoxidation of 3 catalyzed by La-1 catalyst. Right: epoxidation
of 9 catalyzed by Yb-1 catalyst.
a
Absolute configurations were determined to be (RS, âR). See ref
1
0.
for 1 h (Scheme 1, Yb-2).20 That is, treatment of 9 with TBHP
1.5 equiv) in the presence of 5 mol %15 of Yb-2 in THF at
Figure 2. Asymmetric amplification in the epoxidation of 9 catalyzed
by Yb-1 catalyst.
the orientation of the enone.26 Such a mechanism, analogous
to those for enzymatic methods, may explain why various
epoxides can be synthesized with good enantiomeric excesses
even at room temperature.
(
10
room temperature for 96 h was found to give 10 with 94%
1
1
ee in 83% yield. Gratifyingly, 12, 14, and 16 were obtained
with 88, 88, and 91% ee, respectively, and in excellent yields.
In contrast, the use of either Yb-1 catalyst or La-CMHP system
afforded 10 with less satisfactory results.2
3,24
It seems likely
The general procedure is as follows: A THF solution of (R)-
BINOL (2.5 mL, 0.25 mmol) or (R)-3-hydroxymethyl-BINOL
(3.1 mL, 0.31 mmol) was added to a THF suspension (25 mL)
of MS 4A (1.0 g). To this stirred suspension was added La-
that the difference in ionic radius between lanthanum and
ytterbium as well as the difference in Lewis acidities accounts
for the observed center metal effects.
1
3
Although we have not succeeded in determining the structure,
it was found that an almost 1:1 ratio of Ln(O-i-Pr)3 (Ln ) La
or Yb) and 1 gave the maximum enantiomeric excesses (Figure
(O-i-Pr)3 (0.25 mmol, 1.25 mL) or Yb(O-i-Pr) (0.25 mmol,
3
2.5 mL) in THF at room temperature. After stirring for 0.5-1
h at room temperature (for La) or 40 °C (for Yb), to the resulting
THF suspension was added the enone (5.0 mmol) and a toluene
solution of the hydroperoxide (2.1 mL, 7.5 mmol) at room
temperature. After stirring for the time shown in Table 1 at
room temperature, the reaction mixture was treated with
25
13
1
).
The C-NMR spectrum of La-1 was quite obscure,
suggesting that both the chiral Yb-1 catalyst and the La-1
catalyst exist as oligomers. Moreover, we have succeeded in
obtaining asymmetric amplification of the catalytic asymmetric
epoxidation (Figure 2). We believe that the oligomeric structure
of these lanthanoid-BINOL catalysts may play a key role in
these catalytic asymmetric epoxidations of enones. That is, a
Ln-alkoxide moiety in the catalysts appears to act as a Brøsted
base, activating a hydroperoxide moiety so as to make possible
a Michael reaction, and at the same time another Ln metal ion
seems to act as a Lewis acid, both activating and controlling
saturated NH Cl aqueous. The epoxide was further purified
4
by flash chromatography on silica gel.
In conclusion, we have succeeded in developing an efficient
and general method for the catalytic asymmetric epoxidation
of enones using lanthanoid complexes. It is noteworthy that
this catalytic asymmetric epoxidation can be carried out at room
temperature using 1-8 mol % of a chiral catalyst to give
epoxides with good enantiomeric excesses. Further studies are
in progress.
(
23) In the case of Yb catalysis, the use of CMHP caused decrease in
yields and ees of the epoxides obtained.
(24) Using Yb-1 catalyst, the following ees and yields were obtained
1
(
(
TBHP, 5 mol % of the catalyst). 10, 88% ee (94% yield); 12, 83% ee
84% yield); 14, 53% ee (87% yield); 16, 67% ee (83 % yield).
Supporting Information Available: Experimental procedures, H,
C NMR, IR, and mass spectral data for the products (27 pages). See
13
(
25) The structure i is tentatively proposed. The unequivocal structural
any current masthead page for ordering information and Internet access
instructions.
determination is currently under investigation.
JA964037O
(26) Considering the mechanism of epoxidations using a hydroperoxide,
the activation of an enone by the same lanthanoid atom appears to be
precluded.