Optically active cationic cobalt(III) complexes: Highly efficient catalysts for enantioselective hetero Diels-Alder reaction

Article abstract of DOI:10.1246/cl.2000.824

Optically active β-ketoiminato cobalt(III) complexes were developed for use as effective catalysts of the enantioselective hetero Diels-Alder reaction of aryl and alkyl aldehydes with 1-methoxy-[3-(tert-butyldimethylsilyl)oxy]-1,3-butadiene. In the presen

Full text of DOI:10.1246/cl.2000.824

824
Chemistry Letters 2000
Optically Active Cationic Cobalt(III) Complexes:
Highly Efficient Catalysts for Enantioselective Hetero Diels–Alder Reaction
Satoko Kezuka, Tsuyoshi Mita, Natsuki Ohtsuki, Taketo Ikeno, and Tohru Yamada
Department of Chemistry, Faculty of Science and Technology, Keio University, Hiyoshi, Kohoku-ku, Yokohama 223-8522
(Received May 9, 2000; CL-000449)
Optically active β-ketoiminato cobalt(III) complexes were
First of all, a variety of optically active β-ketoiminato
cobalt complexes (Figure 1) was examined for the asymmetric
hetero Diels–Alder reaction of o-fluorobenzaldehyde (Table 1).
These complexes A, B and C were respectively prepared from
the corresponding optically active 1,2-diaryl-1,2-ethylenedi-
amines and 1,3-dicarbonyl compounds. When the optically
active cobalt(II) complex B was employed as the catalyst, the
yield and the enantioselectivity of the product were slightly
improved compared with complex A (Entries 1 and 2). On the
contrary, both the activity and enantioselectivity were unex-
pectedly decreased when complex C was used. For improve-
ment of the catalytic activity, a variety of axial ligands that
enhance the Lewis acidity was next examined. The cobalt(III)
complex A-Cl was prepared by the ligand-exchange procedure5
from the corresponding cobalt(III) acetate complex6 and lithi-
um chloride. The cobalt(III) complexes A-Br and A-I7 were
obtained by oxidation of the cobalt(II) complex A with Br2 and
I2, respectively. In the presence of complexes A-Cl and A-Br,
the optical yield of the product was rather decreased whereas
the reaction smoothly proceeded (Entries 4 and 5). When the
cobalt(III) complex A-I was employed, the reaction time was
shortened and the optical yield of the product was maintained
(Entry 6). Furthermore, the cationic cobalt(III) complex A-
OTf, synthesized by treatment of the corresponding cobalt(III)
complex A-I with AgOTf,8 was proved to be a highly active
catalyst for the hetero Diels–Alder reaction. In the presence of
5 mol% complex catalyst A-OTf, the reaction was completed in
12 h with 79% ee (Entry 7). The addition of MS4A to the reac-
tion mixture improved both the yield and optical yield of the
developed for use as effective catalysts of the enantioselective
hetero Diels–Alder reaction of aryl and alkyl aldehydes with 1-
methoxy-[3-(tert-butyldimethylsilyl)oxy]-1,3-butadiene. In the
presence of 5 mol% cationic cobalt(III) triflate complexes
derived from optically active 1,2-bis(3,5-dimethylphenyl)-1,2-
ethylenediamine, the hetero Diels–Alder reaction of p-nitro-
benzaldehyde proceeded in 94% yield with 94% ee.
Among the various carbon–carbon bond-forming reactions,
the enantioselective hetero Diels–Alder reaction has emerged
as an important target in synthetic organic chemistry1 because
carbon–carbon and carbon–oxygen bonds are both formed to
afford six-membered pyran derivatives in a stereoselective
manner. In a previous paper, it was reported that the hetero
Diels–Alder reactions of ortho-substituted aryl aldehydes with
electron-rich dienes proceeded in the presence of a catalytic
amount of the optically active β-ketoiminato2 cobalt(II) complex
to afford the corresponding dihydropyran-4-ones with moderate
yields and good enantioselectivities.3 In order to achieve high
performance in the catalytic enantioselective reaction, the
detailed design of the optically active β-ketoiminato complex
catalysts should be required, e.g., for increasing their catalytic
activities, cobalt complex catalysts of high oxygenophilicity
should be developed. The corresponding cobalt(III) halides and
cationic cobalt(III) complexes are expected to be employed as
highly active Lewis acid catalysts. The optically active β-
ketoiminato cobalt(III) iodide complexes were already prepared
and their structures were determined by X-ray analysis,4 but they
have never been used for asymmetric reactions. In this commu-
nication, we would like to report that the cobalt(III) derivatives
were synthesized and employed as efficient catalysts for the
highly enantioselective hetero Diels–Alder reaction of aryl and
alkyl aldehydes with electron-rich dienes.
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
825
References and Notes
1
product, and the cationic cobalt(III) triflate complex B-OTf, pre-
pared from the complex B-I, remarkably accelerated the hetero
Diels–Alder reaction to produce the corresponding dihydropy-
ran-4-one with high enantioselectivity (Entries 8, 9 and 10).
The highly active cationic cobalt(III) triflate complexes A-
OTf and B-OTf were successfully applied as effective chiral cat-
alysts for the enantioselective hetero Diels–Alder reaction of var-
ious aldehydes with the electron-rich diene.9 As shown in Table
2, the hetero Diels–Alder reactions of the o-fluoro- and o-
chloro-benzaldehydes proceeded smoothly to afford the corre-
sponding dihydropyran-4-ones in high yield10 with high enan-
tioselectivity (Entries 1 and 2). By using the cationic cobalt(III)
complex B-OTf as a Lewis acid catalyst, aryl aldehydes substi-
tuted by an electron-withdrawing group at the para-position
reacted with Danishefsky’s diene and was completely consumed
within 7–24 h. The enantioselectivities in these reactions of the
p-nitro-, p-trifluoromethyl-, and p-chlorobenzaldehydes were
94% ee, 91% ee, and 91% ee, respectively (Entries 3–5). The
hetero Diels–Alder reaction of benzaldehyde itself also proceed-
ed to produce the corresponding pyranone derivative in 83%
yield with 90% ee11 (Entry 6). In the presence of a catalytic
amount of the cationic cobalt(III) triflate complex A-OTf, the
hetero Diels–Alder reaction of 3-phenylpropionaldehyde and
octanal smoothly proceeded to afford the corresponding dihy-
dropyran-4-ones with high enantioselectivity (90% ee and 88%
ee, Entries 7 and 8, respectively).
S. E. Schaus, J. Brånalt, and E. N. Jacobsen, J. Org. Chem., 63,
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and 3421 (1995). The corresponding cobalt(II) complex cata-
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Mukaiyama, Inorg. Chim. Acta, 296, 86 (1999).
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It was reported that the Co(II) complexes with salen-type ligands
showed better enantioselectivity in the hetero Diels–Alder reac-
tion than the corresponding Co(III) complexes; L.-S. Li, Y. Wu,
Y.-J. Xia, and Y.-L. Wu, Tetrahedron: Asym., 9, 2271 (1998).
Preparation of cobalt(III) triflate complex: To a solution of the
cobalt complex B-I (259.9 mg, 0.295 mmol) in dichloromethane
(10.0 mL) was added AgOTf (84.4 mg, 0.329 mmol) in
dichloromethane (1.0 mL). The mixture was stirred for 1.5 h and
the resulting AgI was filtered off. The crude product was puri-
fied by reprecipitation from dichloromethane/hexane to afford
the cationic cobalt(III) complex B-OTf (231.4 mg) in 87% yield
as a green solid.
2
3
4
5
6
7
8
9
Typical procedure: To a solution of the cobalt complex B-OTf
(22.9 mg, 5.0 mol%) in dichloromethane (0.5 mL) in the pres-
ence of molecular sieves 4A was added p-nitrobenzaldehyde
(75.4 mg, 0.50 mmol) in dichloromethane (1.0 mL). A solution
of the diene (200 µL, 0.97 mmol) in dichloromethane (1.0 mL)
was then added at –78 °C. The mixture was stirred for 7 h at –78
°C, followed by treatment with trifluoroacetic acid (0.2 mL) at rt
for 5 h. After neutralization with sat. NaHCO3 solution, a stan-
dard workup and chromatography on silica gel afforded 2,3-
dihydro-2-(4-nitrophenyl)-4H-pyran-4-one (102.5 mg) in 94%
yield. The optical yield of the product was determined by HPLC
analysis (Daicel Chiralcel OD-H, IPA 10% in hexane) to be 94%
ee.
10 Aryl aldehydes with ortho-halide or orhto-alkoxide reacted with
dienes faster than that with the corresponging aldehydes of para-
substitution. A detailed study of these observations is currently
under way.
11 (S)-2-Aryl-dihydropyran-4-ones were obtained corresponding to
the (S,S)-cobalt complex catalysts. 2-(p-Nitrophenyl)dihydropy-
ran-4-one: R. F. Lowe and R. J. Stoodley, Tetrahedron Lett., 35,
6351 (1994). 2-Phenyldihydropyran-4-one: E. J. Corey, C. L.
Cywin, and T. D. Roper, Tetrahedron Lett., 33, 6907 (1992).
It is noted that the cationic cobalt(III) triflate complexes
with optically active β-ketoiminato ligands effectively catalyzed
the hetero Diels–Alder reaction of various aryl and alkyl aldehy-
des with Danishefsky’s diene to afford the corresponding dihy-
dropyran-4-one derivatives in high yield with high enantioselec-
tivity. Further applications of the present cationic cobalt(III)
complexes as chiral Lewis acid catalysts are in progress.