Optically active β-ketoiminato cationic cobalt(III) complexes: Efficient catalysts for enantioselective carbonyl-ene reaction of glyoxal derivatives

Article abstract of DOI:10.1021/ol015980m

(formula presented) Optically active β-ketoiminato cationic cobalt(III) complexes were synthesized as effective Lewis acid catalysts for the enantioselective carbonyl-ene reaction. In the presence of a catalytic amount of cobalt(III) hexafluoroantimonate

Full text of DOI:10.1021/ol015980m

ORGANIC
LETTERS
2
001
Vol. 3, No. 12
937-1939
Optically Active â-Ketoiminato Cationic
Cobalt(III) Complexes: Efficient
Catalysts for Enantioselective
Carbonyl-Ene Reaction of Glyoxal
Derivatives
1
Satoko Kezuka, Taketo Ikeno, and Tohru Yamada*
Department of Chemistry, Faculty of Science and Technology, Keio UniVersity,
Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan
yamada@chem.keio.ac.jp
Received April 13, 2001
ABSTRACT
Optically active â-ketoiminato cationic cobalt(III) complexes were synthesized as effective Lewis acid catalysts for the enantioselective carbonyl-
ene reaction. In the presence of a catalytic amount of cobalt(III) hexafluoroantimonate derived from the optically active 1,2-diphenyl-1,2-
ethanediamine, the carbonyl-ene reaction with a variety of alkenes and glyoxal derivatives smoothly proceeded to afford the corresponding
homoallylic alcohols in good-to-high yields with high enantioselectivities.
The enantioselective carbonyl-ene reaction promoted by a
catalytic amount of Lewis acid is an intriguing route to some
optically active homoallylic alcohols. The carbonyl-ene
as Lewis acid catalysts; various centered metals, the design
of chiral ligands, and their combination have been tried for
achieving a high catalytic ability and high enantioselectivity
1
3
reaction is one of the most convenient methods for carbon-
in the carbonyl-ene reaction.
Optically active â-ketoiminato cobalt complexes derived
4
carbon bond formation without any pretreatment such as
enolization, and the resulting homoallylic alcohol could be
further transformed into more functionalized products by
taking advantage of its carbon-carbon double bond. In
addition, like the Diels-Alder reaction, it is assumed that
the carbonyl-ene reaction is a 6π-electron electrocyclic
from the corresponding 1,3-dicarbonyl compounds and
optically active 1,2-diaryl-1,2-ethanediamines were also
(3) Maruoka, K.; Hoshino, Y.; Shirasaka, T.; Yamamoto, H. Tetrahedron
Lett. 1988, 29, 3967. Mikami, K.; Terada, M.; Nakai, T. J. Am. Chem. Soc.
1989, 111, 1940. Mikami, K.; Terada, M.; Nakai, T. J. Am. Chem. Soc.
1990, 112, 3949. Mikami, K. Pure Appl. Chem. 1996, 68, 639. Evans, D.
A.; Burgey, C. S.; Paras, N. A.; Vojkovsky, T.; Tregay, S. W. J. Am. Chem.
Soc. 1998, 120, 5824. Evans, D. A.; Tregay, S. W.; Burgey, C. S.; Paras,
N. A.; Vojkovsky, T. J. Am. Chem. Soc. 2000, 122, 7936. Hao, J.; Hatano,
M.; Mikami, K. Org. Lett. 2000, 2, 4059. Qian, C.; Huang, T. Tetrahedron
Lett. 1997, 38, 6721. Qian C.; Huang, T. Tetrahedron: Asymmetry 2000,
11, 2347. For reviews on the carbonyl-ene reaction, see: Mikami, K.;
Terada, M.; Shimizu, M.; Nakai, T. J. Synth. Org. Chem. Jpn. 1990, 48,
292. Mikami, K.; Shimizu, M. Chem. ReV. 1992, 92, 1021.
2
reaction. For these reasons, a variety of metal complexes
with optically active ligands have been widely investigated
(
1) Snider, B. B. Acc. Chem. Res. 1980, 13, 426 and references therein.
(2) A zwitterionic intermediate was also proposed for the Lewis acid
catalyzed carbonyl-ene reaction: Snider, B. B.; Rodini, D. J.; Kirk, T. C.;
Cordova, R. J. Am. Chem. Soc. 1982, 104, 555. Snider, B. B.; Phillips, G.
B. J. Org. Chem. 1983, 48, 464.
1
0.1021/ol015980m CCC: $20.00 © 2001 American Chemical Society
Published on Web 05/17/2001

employed as effective Lewis acid catalysts for enantiose-
lective hetero Diels-Alder reactions. In the presence of 5
cationic cobalt(III) complexes with various counteranions
were prepared by treatment of the iodo cobalt(III) complex
1b with the corresponding silver salts, and then they were
subjected to the carbonyl-ene reaction. The cobalt(III) triflate
complex 1c, one of the most effective catalysts for the hetero
Diels-Alder reaction, afforded the carbonyl-ene product
in only 9% yield and with 2% ee (entry 3). The correspond-
ing cobalt(III) tetrafluoroborate 1d and cobalt(III) hexafluo-
rophosphate 1e effectively catalyzed the carbonyl-ene reac-
tion to obtain the corresponding homoallylic alcohol in good
yields with good enantioselectivities (entries 4 and 5).
Screening of the counteranions revealed that the hexafluo-
roantimonate cobalt(III) complex 1f could be significantly
employed as a highly reactive cationic cobalt(III) complex
and that the resulting homoallylic alcohol with 88% ee was
afforded in 90% yield (entry 6).
5
mol % cationic cobalt(III) complexes, the reaction of
aromatic or aliphatic aldehydes with electron-rich dienes
smoothly proceeded to afford the corresponding pyranone
derivatives in good-to-high yields with high enantioselec-
6
b
6
tivities. In this communication, we report that the cationic
cobalt(III) complex with the optically active â-ketoiminato
ligand was efficiently employed as a Lewis acid catalyst for
the enantioselective carbonyl-ene reaction of various terminal
alkenes with glyoxal derivatives (eq 1).
It could be assumed that the cobalt(III) complexes in which
counteranions were completely separated from the centered
cobalt effectively catalyzed the present carbonyl-ene reaction
as effective Lewis acids. These series of catalytic activities
8
could be correlated to the acidity of the conjugate acid of
the counteranion in the cobalt(III) complexes; the cobalt-
First of all, a variety of â-ketoiminato cobalt(II) and cobalt-
III) complex catalysts were examined for the asymmetric
(III) complex whose counteranion derived the stronger
(
conjugate acid achieved the higher yield of the carbonyl-
ene product. For instance, the carbonyl-ene product was
carbonyl-ene reaction of phenylglyoxal and R-methylstyrene
7
(
Table 1). The cobalt(II) complex 1a and the corresponding
obtained with low ee catalyzed by cobalt(III) trifluo-
iodo cobalt(III) complex 1b could scarcely catalyze the
carbonyl-ene reactions (entries 1 and 2). To improve the
catalytic activity of the cobalt complex, the corresponding
9
romethanesulfonate (H
acid (CF SO
III) hexafluoroantimonate (H
acid (HF-SbF ), -27.9) afforded the resulting homoallylic
0
value of trifluoromethanesulfonic
3
3
H), -14.1), whereas the corresponding cobalt-
value of hexafluoroantimonic
(
0
5
alcohol with high ee.
The highly active catalyst, cationic cobalt(III) hexafluo-
roantimonate complex 1f, was successfully applied to the
enantioselective carbonyl-ene reaction of various alkenes
with the glyoxal derivatives (Table 2). The reaction with
benzyl glyoxylate smoothly proceeded to afford the corre-
sponding homoallylic alcohol with good enantioselectivity
Table 1. Various Cationic Cobalt Complex Catalysts for
Asymmetric Carbonyl-Ene Reaction
(entry 1). When the carbonyl-ene reaction of phenylglyoxal
with R-methylstyrene was tried, the enantioselectivity of the
resulting product was increased to 88% ee (entry 2). The
carbonyl-ene reaction of phenylglyoxal with various alkenes
was then attempted in the presence of a catalytic amount of
cobalt(III) hexafluoroantimonate 1f. The R-methylstyrenes
substituted with 4-methyl and 4-fluoro smoothly reacted with
phenylglyoxal to afford the corresponding homoallylic al-
cohols in high yields with high enantioselectivities (entries
3
and 4). The reaction of isopropenylnaphthalene was
completed in 48 h, and the optical yield of the corresponding
product was 89% ee (entry 5). In the presence of a catalytic
(4) The ketoiminato cobalt(II) complexes effectively catalyzed the
enantioselective borohydride reductions of ketones, imines, and R,â-
unsaturated carboxamides and the enantioselective cyclopropanation of
styrene derivatives: Nagata, T.; Yorozu, K.; Yamada, T.; Mukaiyama, T.
Angew. Chem., Int. Ed. Engl. 1995, 34, 2145. Sugi, K. D.; Nagata, T.;
Yamada, T.; Mukaiyama, T. Chem. Lett. 1997, 493. Yamada, T.; Ohtsuka,
Y.; Ikeno, T. Chem. Lett. 1998, 1129. Yamada, T.; Ikeno, T.; Sekino, H.;
Sato, M. Chem. Lett. 1999, 719. Ikeno, T.; Sato, M.; Yamada, T. Chem.
Lett. 1999, 1345.
(
5) Yamada, T.; Kezuka, S.; Mita, T.; Ikeno, T. Heterocycles 2000, 52,
1
041.
1938
Org. Lett., Vol. 3, No. 12, 2001

Table 2. Asymmetric Carbonyl-Ene Reaction of Various
Alkenes
Figure 1. Values (ppm) obtained for (R)-MTPA esters of secondary
alcohols.
In summary, the cationic cobalt(III) hexafluoroantimonate
complex with an optically active â-ketoiminato ligand
effectively catalyzed the asymmetric carbonyl-ene reaction
of various alkenes with glyoxal derivatives to afford the
corresponding homoallylic alcohols in high yields with high
enantioselectivities.
amount of the cobalt(III) hexafluoroantimonate complex 1f,
the carbonyl-ene reaction of 2,4,4-trimethyl-1-pentene, 2,3-
dimethyl-1-butene, and methylenecyclohexane with phe-
nylglyoxal proceeded, and the corresponding products were
obtained with high enantioselectivities (entries 6, 7, and 8,
1
0
respectively). It should be considered that two geometric
isomers could be produced; however, the regioselectivity in
the present carbonyl-ene reaction was totally superior (>99:
Figure 2. Reasonable explanation for the enantioselection in the
carbonyl-ene reaction catalyzed by the (S,S)-cobalt complex.
1, NMR analysis).
The absolute configuration of the obtained product was
determined by NMR analysis of the corresponding MTPA
esters (eq 2)1 and it was confirmed that the homoallylic
1
Supporting Information Available: Experimental pro-
cedures and spectral data for compounds 4a-g, and 6a. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OL015980M
(7) Among a number of solvents (CHCl3, CH2Cl2, PhF, PhCH3, Et2O,
CH3CN, and hexane), CHCl3 was the most suitable solvent to afford the
corresponding homoallylic alcohol in high yield and with high enantiose-
lectivity. The enantioselectivity at 0 °C was lower than that at -20 °C.
Although the enantioselectivity at -40 °C was slightly improved, the
reaction rate was decreased.
alcohol of the (S)-configuration corresponding to the (S,S)-
cobalt complex catalyst was obtained (Figure 1). The absolute
configuration of the resulting homoallylic alcohol was in
(8) Olah, G. A.; Prakash, G. K. S.; Sommer, J. Superacids; John Wiley
Sons: New York, 1985.
&
(
9) Fabre, P. L.; Devynck, J.; Tremillon, B. Chem. ReV. 1982, 82, 591.
Grondin, J.; Sagnes, R.; Commeyras, A. Bull. Soc. Chim. Fr. 1976, 1779.
(10) For the present carbonyl-ene reaction, mono- and trisubstituted
alkenes were not reactive enough to the glyoxal derivatives.
accord with the postulated mechanism for the corresponding
hetero Diels-Alder reaction (Figure 2).12
(11) Ohtani, I.; Kusumi, T.; Kashnan, Y.; Kakisawa, H. J. Am. Chem.
Soc. 1991, 113, 4092.
(
6) (a) Kezuka, S.; Mita, T.; Ohtsuki, N.; Ikeno, T.; Yamada, T. Chem.
(12) The absolute configuration of the resulting pyranone derivatives in
the cobalt-complex-catalyzed hetero Diels-Alder reaction was explained
by the semiempirical method; ref 6b.
Lett. 2000, 824. (b) Kezuka, S.; Mita, T.; Ohtsuki, N.; Ikeno, T.; Yamada,
T. Bull. Chem. Soc. Jpn., in press.
Org. Lett., Vol. 3, No. 12, 2001
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