Stable, storable, and reusable asymmetric catalyst: A novel La-linked- BINOL complex for the catalytic asymmetric Michael reaction [10]

Full text of DOI:10.1021/ja001036u

6506
J. Am. Chem. Soc. 2000, 122, 6506-6507
Table 1. Catalytic Asymmetric Michael Reaction Promoted by
(R,R)-La-M-linked-BINOL Complexes
Stable, Storable, and Reusable Asymmetric Catalyst:
A Novel La-linked-BINOL Complex for the Catalytic
Asymmetric Michael Reaction
Yun Sik Kim, Shigeki Matsunaga, Jagattaran Das,
Akihiro Sekine, Takashi Ohshima, and Masakatsu Shibasaki*
Graduate School of Pharmaceutical Sciences
The UniVersity of Tokyo, Hongo, Bunkyo-ku
Tokyo 113-0033, Japan
entry
M
temp. (°C)
time (h)
yield (%)a
ee (%)b
1
2
3
4
Li
Na
K
H (4)
0
0
24
24
24
45
21
41
16
53
35
43c
54c
85
ReceiVed March 23, 2000
-20
0
The development of efficient methods to facilitate the recovery
and reuse of asymmetric catalysts remains an important goal in
organic chemistry. To address this issue, intensive efforts have
been devoted to develop soluble and insoluble polymer-supported
asymmetric catalysts,1 which often result in lower enantioselec-
tivities or efficiencies than non-polymer-supported catalysts with
a few excellent exceptions.2,3 However, only a few reusable non-
polymer-supported homogeneous asymmetric catalysts4 have been
accomplished due to the difficulty of recovery for recycling. In
addition, reusable asymmetric Lewis acid catalysts2b-d,4 are quite
rare because of their high reactivity to moisture despite their
usefulness in catalysis.5 Therefore, an increasingly important
objective within this area is the development of highly stable
asymmetric Lewis acid catalysts that can be recovered and reused.
Recently, we reported the oxygen-containing linked-BINOL 5,
which effectively stabilized the Ga-Li-complex against ligand
exchange with a nucleophile under the reaction conditions.6 In
terms of catalyst stability itself, however, group XIII metal
complexes such as Al and Ga complexes are unsatisfactory. We
expected that rare earth metals could be useful as a Lewis acidic
center metal in the linked-BINOL complex. This should lead to
much more stable complexes, because of not only its stability
against moisture but also its longer ionic radius so that the oxygen
in the linker would coordinate to the rare earth metal in contrast
to the Ga-Li-linked-BINOL complex.7 We report here that the
La-linked-BINOL complex 4 is indeed a highly stable, storable
and reusable homogeneous catalyst for the asymmetric Michael
reaction. This reaction is notable not only for its high enantiose-
lectivity and the synthetic utility8 of its products but also for its
facility in handling of the catalyst.
a Isolated yield. b Determined by HPLC analysis. c The mirror image
enantiomer was formed.
Table 2. Catalytic Asymmetric Michael Reaction Promoted by
Stock (R,R)-La-linked-BINOL Complex 4
(R,R)-La-linked-BINOL 4
(10 mol %)
1 + 2
storage time (week)a
8 3
DME, rt, 72 h
0
1
2
3
4
yield (%)b
ee (%)c
94
>99
93
>99
94
>99
94
>99
95
>99
a (R,R)-La-linked-BINOL complex 4 was stored under air. b Isolated
yield. c Determined by HPLC analysis.
efficient method for enantioselective carbon-carbon bond forma-
tions. Although efficient catalytic asymmetric Michael reactions
have been achieved by our group8b,9 and by others,10-12 there is
still a big demand for improvement in terms of generality and
stability of the catalyst, for instance, AlLibis(binaphthoxide)
complex (ALB)8b is applicable only to cyclic enones and it is
also moisture-sensitive. In an attempt to prepare an efficient linked
BINOL complex, lanthanum was chosen as a Lewis acidic center
metal. Asymmetric Michael reaction of 2-cyclohexen-1-one (1)
with dibenzyl malonate (2) was examined with monometallic (La
only) and heterobimetallic (La-Li, La-Na, La-K) complexes.
The results are summarized in Table 1. The best result was
obtained using alkali-metal free La-linked-BINOL complex 4
(entry 4), in which lanthanum metal should work as a Lewis acid
and lanthanum naphthoxide moiety should work as a Brønsted
base to promote the reaction. After optimization of the reaction
conditions, we finally found that the use of DME as a solvent
afforded 3 in 94% yield and >99% ee even at room temperature
(Table 2, 0 week). As we expected, the novel La-linked-BINOL
complex 4 was very stable even under air and storable over a
In recent years, the catalytic asymmetric Michael reaction
promoted by chiral metal complexes has been recognized as an
(1) For a recent review, see: ComprehensiVe Asymmetric Catalysis;
Jacobsen, E. N., Pfaltz, A., Yamamoto, H., Eds.; Springer: New York; 1999;
Chapter 38.
(2) For examples of recycling of the polymer-supported asymmetric
catalysts itself, see: (a) Nozaki, K.; Itoi, Y.; Shibahara, F.; Shirakawa, E.;
Ohta, T.; Takaya, H.; Hiyama, T. J. Am. Chem. Soc. 1998, 120, 4051. (b)
Annis, D. A.; Jacobsen, E. N. J. Am. Chem. Soc. 1999, 121, 4147. (c) Sellner,
H.; Seebach, D. Angew. Chem., Int. Ed. 1999, 38, 1918. (d) Heckel, A.;
Seebach, D. Angew. Chem., Int. Ed. 2000, 39, 163. (e) Vachal, P.; Jacobsen,
E. N. Org. Lett. 2000, 2, 867.
(3) For a example of recycling of the polymer-supported metal and the
chiral ligand independently, see: Kobayashi, S.; Endo, M.; Nagayama, S. J.
Am. Chem. Soc. 1999, 121, 11229.
(4) (a) Mart´ınez, L. E.; Leighton, J. L.; Carsten, D. H.; Jacobsen, E. N. J.
Am. Chem. Soc. 1995, 117, 5897. (b) Tokunaga, M.; Larrow, J. F.; Kakiuchi,
F.; Jacobsen, E. N. Science 1997, 277, 936.
(5) For a recent review, see: Lewis Acid Reagents. A Practical Approach;
Yamamoto, H., Ed.; Oxford University Press: New York; 1999.
(6) Matsunaga, S.; Das, J.; Roels, J.; Vogl, E. M.; Yamamoto, N.; Iida, T.;
Yamaguchi, K.; Shibasaki, M. J. Am. Chem. Soc. 2000, 122, 2252.
(7) The X-ray data of LiCl free Ga-Li-linked-BINOL complex showed
that there was no coordination between oxygen in the linker and gallium. See
ref 6.
(8) (a) Magnus, P.; Sear, N. L.; Kim, C. S.; Vicker, N. J. Org. Chem. 1992,
57, 70. (b) Shimizu, S.; Ohori, K.; Arai, T.; Sasai, H.; Shibasaki, M. J. Org.
Chem. 1998, 63, 7547. (c) Ohori, K.; Shimizu, S.; Ohshima, T.; Shibasaki,
M. Chirality, in press.
13
long time. The complex 4 was easily prepared from La(O-i-Pr)3
(9) For a review, see: Shibasaki, M.; Sasai, H.; Arai, T. Angew. Chem.,
Int. Ed. Engl. 1997, 36, 1236.
(10) For recent reviews, see: (a) ComprehensiVe Asymmetric Catalysis;
Jacobsen, E. N., Pfaltz, A., Yamamoto, H., Eds.; Springer: New York; 1999;
Chapter 31. (b) Tye, H. J. Chem. Soc., Perkin Trans. 1 2000, 275.
(11) For a representative example of the catalytic asymmetric Michael
reactions of enones with malonates, see: Yamaguchi, M.; Shiraishi, T.; Hirama,
M. J. Org. Chem. 1996, 61, 3520.
(12) For representative examples of other catalytic asymmetric Michael
reactions, see: (a) Feringa, B. L.; Pineschi, M.; Arnold, L. A.; Imbos, R.; de
Vries, A. H. M. Angew. Chem., Int. Ed. Engl. 1997, 36, 2620. (b) Takaya,
Y.; Ogasawara, M.; Hayashi, T.; Sakai, M.; Miyaura, N. J. Am. Chem. Soc.
1998, 120, 5579. (c) Corey, E. J.; Noe, M. C.; Xu, F. Tetrahedron Lett. 1998,
39, 5347. (d) Evans, D. A.; Rovis, T.; Kozlowski, M. C.; Tedrow, J. S. J.
Am. Chem. Soc. 1999, 121, 1994. (e) Ji, J.; Barnes, D. M.; Zhang, J.; King,
S. A.; Wittenberger, S. J.; Morton, H. E. J. Am. Chem. Soc. 1999, 121, 10215
and references therein.
(13) La(O-i-Pr)3 can be purchased from Kojundo Chemical Laboratory Co.,
Ltd., 5-1-28, Chiyoda, Sakado-shi, Saitama 350-0214, Japan (Fax: +(81)-
492-84-1351).
10.1021/ja001036u CCC: $19.00 © 2000 American Chemical Society
Published on Web 06/21/2000

Communications to the Editor
J. Am. Chem. Soc., Vol. 122, No. 27, 2000 6507
Table 4. Catalytic Asymmetric Michael Reaction with Catalyst
Recycling
(R,R)-La-linked-BINOL 4
(10 mol %)
1 + 2
8 3
DME, 4 °C, 110 h
cycle
1
2
3
4
yield (%)a
ee (%)b
82
>99
94
>99
68
99
50
98
a Isolated yield. b Determined by HPLC analysis.
Figure 1. Stock air-stable powdered (R,R)-La-linked-BINOL complex
4, which has no deliquescent property.
reaction of acyclic enones such as 20 with 2 (97%, 78% ee) and
10 (95%, 74% ee).15 In addition, the Michael reaction of 23 with
24 gave 25 in 97% yield and 75% ee, where a newly formed
chiral center was induced by the Michael donor moiety. To the
best of our knowledge no efficient catalytic Michael reaction of
eight- and nine-membered ring enones with malonates has been
reported to date, and this is the first example of a Michael reaction
where the catalyst shows such broad generality.
Table 3. Catalytic Asymmetric Michael Reactions Promoted by
(R,R)-La-linked-BINOL 4
Finally, we were very pleased to demonstrate that complex 4
could be recovered from the reaction mixture due to the large
difference of solubility between the complex 4 and product 3 and
reused. The successful implementation of this strategy is shown
in Table 4. After completion of the reaction, the complex 4 was
precipitated by addition of pentane at 0 °C. Supernatant liquid,
which contained product 3 and only trace amounts of 4, was
simply separated via cannula. The residual precipitate was dried
under reduced pressure to afford the powdered complex 4 again.16
The recovered complex 4 was reused several times. Although
slight loss of activity was observed, the recovered complex 4
promoted the Michael reaction to afford the desired product 3 in
very high ee even after the fourth use.
In conclusion, we have successfully developed the stable,
storable, and reusable La-linked-BINOL complex 4 for the
asymmetric Michael reaction (up to >99% ee) with broader
generality compared to any reported catalysts. Even after 4 weeks
storage, this powdered complex 4 was highly effective in the
catalytic asymmetric Michael reaction. Furthermore, recovering
and recycling of the complex 4 has been accomplished, and the
recovered complex 4 still gave the desired product in very high
ee even after fourth cycle.
â-dicarbonyl temp. time
yielda eeb
(h) produce (%) (%)
entry enone compounds
(°C)
1
2
6
6
2
10
2
4
4
rt
4
rt
rt
4
4
rt
85
85
72
85
72
84
85
85
96
120
56
56
36
12
13
3
85
96
94
98
95
84
96
97
82
61
97
95
97
>99
>99
>99
>99
>99
98
3
4
1
1
2
3
5
1
1
7
10
11
2
10
10
2
2
10
24
14
15
16
17
18
19
21
22
25
6c
7
>99
>99
99
82
78
74
75
8
7
8
9c
10
11
9
4
20
20
23
-40
-40
-30
12
13d
Acknowledgment. We thank CREST and JSPS for the financial
support.
a Isolated yield. b Determined by HPLC analysis. c The reaction was
carried out in DME/THF (9:1). d 24 was added dropwise over 24 h.
Supporting Information Available: Experimental procedures for the
preparation of the La-linked-BINOL complex 4, general procedures for
the Michael reaction and spectral and analytical data for all new
compounds (PDF). This material is available free of charge via the Internet
at http://pubs.acs.org.
and 1.0 equiv of linked-BINOL 5, which were mixed in THF
followed by removal of solvent under reduced pressure to afford
4 as a pale-yellow powder (Figure 1). This air-stable complex 4
is storable without any care at ambient temperature for at least 4
weeks. As shown in Table 2, no change in catalytic activity, in
terms of both chemical yield and ee, was observed using stock
catalyst. After 4 weeks storage, 3 was obtained in 95% yield and
>99% ee. Although the structure of complex 4 has not yet been
unequivocally elucidated, we propose on the basis of the previous
structural investigation on Ga-Li-linked-BINOL that the complex
4 would be monomer.14
Having succeeded in developing a novel stable and storable
catalyst, we examined the scope and limitations of different
substrates. As shown in Table 3, the complex 4 promoted the
Michael reaction of a variety of cyclic enones (n ) 0-4) with
various malonates to afford Michael adducts15 with good to
excellent ee’s. The complex 4 was also effective for the Michael
JA001036U
(14) Computational optimization of a model compound (La-linked-biphenol
complex) using molecular mechanics calculation followed by ab initio
calculation indicated that the distance between lanthanum and oxygen in the
linker should be 2.62-2.69 Å, which is nearly equal to that between lanthanum
and phenolic oxygen (2.58-2.65 Å). It seems reasonable to consider that the
oxygen atom in the linker would also function as coordinative moiety and
thus linked-BINOL would function as pentadentate ligand toward La, thus
making the La-linked-BINOL complex 4 unusually stable. See Supporting
Information for details and references of the calculation.
(15) The absolute configurations of 3, 12-17, and 25 have already been
determined. See Supporting Information. The absolute configurations of 18,
19, 21, and 22 were tentatively determined on the basis of the previous results.
See ref 9.
(16) The recovered powdered complex 4 contained ∼10-15% of 3.