2264
J . Org. Chem. 1996, 61, 2264-2265
Sch em e 1a
A BMP D-Yttr iu m Isop r op oxid e Com p lex:
High ly Efficien t Ch ir a l Lew is Acid Ca ta lyst
for Asym m etr ic Silylcya n a tion
Atsushi Abiko* and Guo-qiang Wang
Kao Institute for Fundamental Research, Kao Corporation,
Ichikai-machi, Haga-gun, Tochigi, 321-34, J apan
Received February 9, 1996
Excellent enantioselectivity has been achieved for
various chiral Lewis acid-catalyzed asymmetric reactions;
however, the major disadvantage of these processes is
the low turnover number of the catalyst.1 Usually, 5-20
mol % of the catalyst is required, and few catalysts are
effective below 1 mol % of the catalyst loading, which
makes chiral Lewis acid catalysts unattractive for practi-
cal use. We have discovered that a 1:1 complex of (R,R)-
BMPD [1,3-bis(2-methylferrocenyl)propane-1,3-dione] (1)
and yttrium isopropoxide, prepared in situ, is a remark-
able catalyst for asymmetric silylcyanation. In the
presence of 0.2 mol % of the catalyst, the reaction of
benzaldehyde and TMSCN proceeded in 95% yield with
87% ee. The turnover number of 500 for this catalyst is
remarkably high for chiral Lewis acid-catalyzed asym-
metric reactions.
a
Key: (a) t-BuLi, MeI, ether; (b) H3O+, 90% from 2; (c) MnO2,
HCN, MeOH, 85%; (d) NaOH, EtOH, 97%; (e) CF3CH2OH, DCC,
DMAP, 90%; (f) MeMgBr, THF; (g) MnO2, benzene, 75% from 3;
(h) KH, THF, 5, then recrystallization, 80%.
alkylation with iodomethane and then hydrolysis af-
forded (R)-2-methylformylferrocene (3)6 in 90% yield,
which was shown to be 94%ee by HPLC analysis of the
corresponding methyl ester 47 on a chiral column (Chiral-
cel OD, with Hex:i-PrOH ) 10:1). The Claisen condensa-
tion of 68 to lead 1 was problematic but was achieved
efficiently (in 80% yield) with the use of the trifluoroethyl
ester 5 and potassium hydride in THF. Pure (R,R)-1,[
mp 150-151 °C; [R]23D -75.8 (c ) 0.80, CHCl3); FAB MS
m/z 468 (M+). Anal. Calcd for C25H24O2Fe2: C, 64.14;
H, 5.17. Found: C, 63.93; H, 5.23] was obtained after
recrystallization from dichloromethane and hexane to
remove the meso-isomer (Scheme 1).
After a brief survey of the catalytic activity of the metal
complexes derived from BMPD for asymmetric reactions,
we have found that the complex prepared in situ from
9,10
the ligand and Y5(O)(O-i-Pr)13
catalyzed the silylcy-
anation of benzaldehyde with remarkable efficiency (eq
1).11 The optimized reaction procedure for the asym-
Although 1,3-dicarbonyl compounds have been among
the most popular ligands for metals,2 their chiral modi-
fication has received little attention. Most of the known
chiral 1,3-dicarbonyl ligands are derived from camphor,
and a limited number of synthetic applications of their
metal complexes have been reported. Only a few asym-
metric catalytic reactions of a metal-chiral 1,3-dicarbo-
nyl compound complex have exhibited good enantiose-
lectivity, such as the use of Eu(hfc)3 and related complexes
as catalysts for the hetero Diels-Alder reaction3 and the
use of Cu(hfc)2 as a catalyst for asymmetric cyclopropa-
nation.4 BMPD, a new chiral 1,3-dicarbonyl compound,
was designed to affect asymmetric induction on the basis
of the C2 symmetry of the ligand and of the chiral
ferrocene for various asymmetric catalytic reactions.
Retrosynthetic analysis shows that 1 could be prepared
by the condensation of an enantiopure acetylferrocene
and an ester of enantiopure ferrocenecarboxylic acid of
the same sense, both of which could be obtained from the
same formylferrocene. Thus, (R)-2-methylformylfer-
rocene was prepared using Kagan’s protocol.5 ortho-
Metalation of the chiral acetal 2 with t-BuLi followed by
metric silylcyanation using 1 mol % of the catalyst is as
follows (Table 1 entry 1). A solution of Y5(O)(O-i-Pr)13
(12.3 mg, 0.01 mmol) and (R,R)-BMPD (23.4 mg, 0.05
mmol, 1 equiv compared to yttrium) in CH2Cl2 (2 mL)
was stirred at room temperature for 1 h. One fifth of
the benzaldehyde (106 mg, 1.0 mmol) was added at room
temperature, and the reaction was stirred for 3 min.
Then the reaction mixture was cooled to -78 °C, and
TMSCN (150 mg, 1.5 mmol) was added. The remaining
benzaldehyde (424 mg, 4 mmol) and TMSCN (600 mg,
6.0 mmol)12 were added in three portions in every 20 min.
(6) Schlo¨gl, K.; Fried, M.; Falk, H. Monatsh. Chim. 1964, 95, 576.
(7) Falk, H.; Schlo¨gl, K.; Steyrer, W. Monatsh. Chim. 1966, 97, 1029.
(8) Marquarding, D.; Burghard, H.; Ugi, I.; Urban, R.; Klusacek, H.
J . Chem. Res. Synop. 1977, 82.
(9) (a) Poncelet, O.; Sartain, W. J .; Hubert-Pfalzgraf, L. G.; Folting,
K.; Caulton, K. G. Inorg. Chem. 1989, 28, 263. (b) Bradley, D. C.;
Chudzynska, H.; Frigo, D. M.; Hammond, M. E.; Hursthouse, M. B.;
Mazid, M. A. Polyhedron 1990, 5, 719. The commercial “Y(O-i-Pr)3”
(purchased from Soekawa Chemical Co.) was usually a mixture of an
oligomeric Y(O-i-Pr)3 and Y5(O)(O-i-Pr)13 with a variable ratio, from
which pure Y5(O)(O-i-Pr)13 was obtained by recrystallization from
i-PrOH. The complex prepared from the oligomeric Y(O-i-Pr) 3 afforded
the same results, but the complex formed rather slowly (14 h at room
temperature).
(1) Yamamoto, H.; Maruoka, K. In Catalytic Asymmetric Synthesis;
Ojima, I., Ed.; VCH Publishers: Weinheim, 1993; pp 413-440.
(2) Comprehensive Coordination Chemistry; Wilkinson, G., Ed.;
Pergamon Press: New York, 1987; Vol. 2, Chapter 15.4.
(3) (a) Bednarski, M.; Danishefsky, S. J . Am. Chem. Soc. 1983, 105,
3716. (b) Bednarski, M.; Maring, C.; Danishefsky, S. Tetrahedron Lett.
1983, 24, 3451.
(4) Matlin, S. A.; Lough, W. J .; Chan, L.; Abram, D. M. H.; Zhou, Z.
J . Chem. Soc., Chem. Commun. 1984, 1038.
(5) Riant, O.; Samuel, O.; Kagan, H. B. J . Am. Chem. Soc. 1993,
115, 5835.
(10) With 5 mol % of the isolated Y(BMPD)3 complex, the silylcy-
anation of benzaldehyde afforded silylated cyanohydrin in 75% yield
with 25% ee after 14 h at -78 °C. Y(BMPD)3 was prepared by the
reaction of YCl3‚nH2O and 3BMPD in the presence of KOH (3 equiv)
in aqueous EtOH in 85% yield: mp 260 °C dec; [R]23 -61.1 (c ) 0.90,
D
CHCl3); FAB MS m/z 1491 (M+); 1H NMR δ 6.20 (s, 3H).
(11) North, M. Synlett 1993, 807.
0022-3263/96/1961-2264$12.00/0 © 1996 American Chemical Society