Novel cerium(III)-(R)-BNP complex as a storable chiral Lewis acid catalyst for the enantioselective Hetero-Diels-Alder reaction

Article abstract of DOI:10.1246/cl.2003.608

A novel Ce(III)-(R)-BNP complex was conveniently prepared from CAN and (R)-BNP-Na, then successfully used as a homogeneous catalyst for the asymmetric hetero-Diels-Alder reaction affording, after acidic work-up, 6-substituted 5,6-dihydro-4H-pyran-4-one de

Full text of DOI:10.1246/cl.2003.608

608
Chemistry Letters Vol.32, No.7 (2003)
Novel Cerium(III)–(R)-BNP Complex as a Storable Chiral Lewis Acid Catalyst for the
Enantioselective Hetero-Diels–Alder Reaction
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Tetsuji Hayano, Toshiaki Sakaguchi, Hiroshi Furuno, Masaaki Ohba, Hisashi Okawa, and Junji Inanaga
Institute for Materials Chemistry and Engineering (IMCE), Kyushu University, Hakozaki, Higashi-ku, Fukuoka 812-8581
yDepartment of Chemistry, Faculty of Sciences, Kyushu University, Hakozaki, Higashi-ku, Fukuoka 812-8581
(Received April 14, 2003; CL-030316)
Anovel Ce(III)–( R)-BNP complex was conveniently pre-
pared from CAN and (R)-BNP–Na, then successfully used as
a homogeneous catalyst for the asymmetric hetero-Diels–Alder
reaction affording, after acidic work-up, 6-substituted 5,6-dihy-
dro-4H-pyran-4-one derivatives with high enantioselectivities
(up to 94% ee). Amechanistically interesting positive nonlinear
effect was also observed.
Scheme 1. Preparation of the Ce–(R)-BNP complex 1.
temperature.6 The results are summarized in Table 1. All reac-
tions proceeded under homogeneous conditions and, in the case
of the aromatic aldehydes (2a–i), generally high enantioselec-
tivities (80–94% ee) were obtained. The high performance of
the catalyst 1 clearly indicates that the structure of 1 is different
from that of the Ce[(R)-BNP]3 complex previously prepared
from CeCl3. It should be noted that the catalyst 1 is very stable
with respect to maintaining its original activity even after two
years (Entry 8). The relatively low ee observed in the reaction
of an aliphatic aldehyde (2j) suggests the importance of some
p–p interaction between the aromatic ring of the aldehydes
and that of the catalyst in the transition state.
While utilization of chiral rare earth metal complexes as
Lewis acid catalysts for various asymmetric reactions has cur-
rently received much attention,1 the central metal ions of such
complexes are rather limited to scandium, yttrium, or the hea-
vier lanthanides like ytterbium because of their stronger Lewis
acidities compared with the lighter lanthanides. Also, there have
been no reports that structurally defined chiral cerium com-
plexes functioned as an effective Lewis acid catalyst though
cerium is one of the most economical rare earth elements.2 Re-
cently, we demonstrated that some of the Ln[(R)-BNP]3 com-
plexes (Ln = La–Lu, Y, and Sc; BNP = 1,10-binaphthyl-2,20-
diyl phosphate) prepared from LnCl3 and (R)-BNP–Na effec-
tively catalyzed the asymmetric hetero-Diels–Alder reaction
of aldehydes 2 with the Danishefsky’s diene 3 at room
temperature.3;4 In the above reaction, the Yb[(R)-BNP]3 com-
plex was found to be the most effective, having a 70% ee
(77% yield) for the reaction of benzaldehyde (2a). When 2,6-lu-
tidine was used as an additive, the reaction proceeded under
homogeneous conditions, improving both the optical and chem-
ical yields,3b and a remarkably high asymmetric amplification
was also observed in the reaction.3c On the other hand, the cor-
responding Ce[(R)-BNP]3 complex turned out to be the least ef-
fective for the same transformation (5% ee, 40% yield). The low
catalytic activity of the Ce(III) complex prompted us to exam-
ine the corresponding Ce(IV) complex because it can be a
stronger Lewis acid by having a capacity of four chiral ligands.
Therefore, we tried to prepare a chiral Ce(IV) complex from
ammonium cerium(IV) nitrate (CAN) and (R)-BNP–Na. Unfor-
tunately, the Ce(IV) complex could not be produced but the cor-
responding Ce(III) complex 1 was generated (Scheme 1).5 We
further found that 1 works as an effective homogeneous catalyst
for the hetero-Diels–Alder reaction.
Figure 1 shows the linear and the positive nonlinear effects
observed during the reaction of 4-ethylbenzaldehyde (2c) with
3.1a,8 For example, when the optically impure catalyst prepared
by mixing the Ce–(R)-BNP and the Ce–(S)-BNP complexes in
Table 1. The Ce(III)–(R)-BNP-catalyzed asymmetric hetero-
Diels–Alder reaction of 2 with 3
Entry
R
2/4
Yield/%a
Ee/%b,c
1
2
3
4
5
6
7
8d
9
Ph
2a/4a
2b/4b
2c/4c
2d/4d
2e/4e
2f/4f
2g/4g
2h/4h
2i/4i
91
96
95
61
75
46
91
93
80
36
80
88
93
93
94
93
85
88
92
61
4-MePh
4-EtPh
4-i-BuPh
4-t-BuPh
4-Biphenylyl
4-FPh
4-ClPh
4-BrPh
n-Pr
The Ce/P ratio of 1 was analyzed to be ca. 1:3.2 using an
inductively coupled plasma (ICP) mass spectrometer, which
suggests the formation of Ce(III) species. The electron spectro-
scopy for chemical analysis (ESCA) study of 1 also supported
the trivalent cerium structure, and it was finally confirmed based
on its magnetic susceptibility measurement.5
10
2j/4j
aIsolated yield. bDetermined by the HPLC analysis using
chiral column (Daicel CHIRALCEL OD for 4a–e,g–i or
CHIRALPAK AD for 4f,j). The absolute configuration of
c
4a was determined to be R by comparison with the sign of
the optical rotation in the literature,7 and these of 4b–i and
4j were tentatively assigned to be R and S, respectively, based
on the retention times on the HPLC analyses and the optical
Using various aldehydes 2, the complex 1-catalyzed asym-
metric hetero-Diels–Alder reaction was carried out at room
d
rotations. The catalyst 1 stored for two years was used.
Copyright Ó 2003 The Chemical Society of Japan

Chemistry Letters Vol.32, No.7 (2003)
609
9:1 (80% ee) and 4:1 (60% ee) ratios was used, ee’s of 90% and
81% for 4c were produced, respectively. These reactions pro-
ceeded under heterogeneous conditions because the mixing of
the (R)- and (S)-catalysts produced precipitates, the amounts
of which were dependent on their mixing ratios. Therefore,
the precipitate of the heterogeneous mixture 5 was separated
by centrifugation, and both the soluble complex 69 and the in-
soluble complex 79 were independently used for the reaction.
As anticipated from our previous finding on the Yb(BNP)3-cat-
alyzed reaction,3c the soluble complex 6 showed a high asym-
metric amplification. Unexpectedly, however, the insoluble
complex 7 exhibited considerable activity and brought about
an almost linear relationship. More interestingly, the (R)-rich-
BINOLs 9 obtained from 7 by the LiAlH4 reduction showed
uniformly lower ee values than those of the products obtained
using catalyst 7, whereas the reduction of complex 6 afforded
the almost enantiopure (R)-BINOL 8. These behaviors are much
different from those previously observed ones with the optically
impure Yb[(R)-BNP]3–2,6-lutidine system.3c We do not have a
clear explanation for these phenomena at the moment but rapid
exchange of the chiral ligands of 7 must occur in order to realize
such a new-type asymmetric amplification. The existence of a
small amount of nitrate ion derived from CAN seems to play
an important role in such a ligand exchange reaction and also
in increasing the solubility of the catalyst. Actually, when the
previously prepared Ce[(R)-BNP]3 was used in the presence
of nitrate ions for the reaction of 2a with 3, the reaction mixture
became clear and the enantioselectivity increased from 5% ee to
73% ee (with NH4NO3) and 68% ee (with NaNO3).
This work was supported by a Grant-in-Aid for Scientific
Research on Priority Areas from the Ministry of Education, Cul-
ture, Sports, Science and Technology, Japan, the Japan Society
for the Promotion of Science, and also by the Kyushu Univer-
sity P&P Programs ‘Green Chemistry’ (to J.I.). We are grateful
to Professor H. Kanno of National Defense Academy in Japan,
and Mr. S. Watanabe of Shimadzu Corporation, Japan for the
measurement of Raman spectrum and the ESCAof 1, respec-
tively. Special thanks are due to Tosoh Analysis and Research
Center, Japan for providing us with the ICP data.
References and Notes
1
2
3
4
For recent reviews, see: a) J. Inanaga, H. Furuno, and T.
Hayano, Chem. Rev., 102, 2211 (2002). b) S. Kobayashi,
M. Sugiura, H. Kitagawa, and W. W.-L. Lam, Chem.
Rev., 102, 2227 (2002). c) K. Mikami, M. Terada, and H.
Matsuzawa, Angew. Chem., Int. Ed. Engl., 41, 3554 (2002).
For the enantioselective reactions using chiral cerium cata-
lysts prepared in situ, see: a) S. Kobayashi, T. Hamada, S.
Nagayama, and K. Manabe, Org. Lett., 3, 165 (2001). b)
G. Desimoni, G. Faita, S. Filippone, M. Mella, M. G.
Zampori, and M. Zema, Tetrahedron, 57, 10203 (2001).
a) J. Inanaga, Y. Sugimoto, and T. Hanamoto, New J.
Chem., 19, 707 (1995). b) T. Hanamoto, H. Furuno, Y.
Sugimoto, and J. Inanaga, Synlett, 1997, 79. c) H. Furuno,
T. Hanamoto, Y. Sugimoto, and J. Inanaga, Org. Lett., 2,
49 (2000).
For the Sc(BNP)3-catalyzed asymmetric Michael addition
of O-alkylhydroxylamines to conjugated enones, see: a) H.
Sugihara, K. Daikai, X. L. Jin, H. Furuno, and J. Inanaga,
Tetrahedron Lett., 43, 2735 (2002). b) X. L. Jin, H.
Sugihara, K. Daikai, H. Tateishi, Y. Z. Jin, H. Furuno,
and J. Inanaga, Tetrahedron, 58, 8321 (2002). c) X. L.
Jin, H. Sugihara, K. Daikai, H. Tateishi, Y. Z. Jin, H.
Furuno, and J. Inanga, Tetrahedron, 59, 877 (2003).
In conclusion, we succeeded for the first time, in develop-
ing a chiral storable cerium(III) complex 1 that can work as an
efficient Lewis acid catalyst and found a new type of asym-
metric amplification for the 1-catalyzed hetero-Diels–Alder re-
action. Further investigations on the precise structure of the
complex and mechanism of the asymmetric amplification are
now in progress.
16
5
6
Data of 1: [a]D ꢀ490:8 (c 1.00, CHCl3). Anal. calcd for
C60H36CeO12P3ꢁ3H2Oꢁ0.5NaNO3: C, 56.37; H, 3.31; N,
0.55. Found: C, 56.24; H, 3.50; N, 0.39. ICP spectrometry
analysis: Ce, 10; P, 7.0 (calculated data for the composition
ratio, Ce/P 1:3.2). Effective magnetic moment (rt):
m
eff, 2.35 mB.
For some recent examples of the highly efficient catalytic
asymmetric hetero-Diels–Alder reaction with 3, see: a) S.
Kii, T. Hashimoto, and K. Maruoka, Synlett, 2002, 937. b)
B. Wang, X. Feng, Y. Huang, H. Liu, X. Cui, and Y. Jiang,
J. Org. Chem., 67, 2175 (2002). c) J. Long, J. Hu, X. Shen,
B. Ji, and K. Ding, J. Am. Chem. Soc., 124, 10 (2002).
M. Bednarski, C. Maring, and S. Danishefsky, Tetrahedron
Lett., 24, 3451 (1983).
7
8
For some reviews on the positive nonlinear effect, see: a) C.
Puchot, O. Samuel, E. Dun˜ach, S. Zhao, C. Agami, and H.
B. Kagan, J. Am. Chem. Soc., 108, 2353 (1986). b) N.
Oguni, Y. Matsuda, and T. Kaneko, J. Am. Chem. Soc.,
110, 7877 (1988). c) D. G. Blackmond, Acc. Chem. Res.,
33, 402 (2000). d) R. Noyori, S. Suga, H. Oka, and M.
Kitamura, Chem. Rec., 1, 85 (2001). e) H. B. Kagan, Synlett,
2001, 888.
9
The ICP-MS data for 6: Ce, 8.4; P, 5.8 (the calculated Ce/P
ratio is 1:3.1). For 7: Ce, 11; P, 7.2 (Ce/P = 1:3.0).
Figure 1. The relation between optical purities of the cata-
lyst and ee’s of the reaction product.
Published on the web (Advance View) June 17, 2003; DOI 10.1246/cl.2003.608