1958
M. Kanai et al.
PAPER
Scheme 2
and to develop an asymmetric catalyst based on this con-
cept are currently ongoing.
Asao, N.; Kii, S.; Hanawa, H.; Maruoka, K. Tetrahedron
Lett. 1998, 39, 3729. (e) Monomer formation using bulky
phenoxy ligands: Maruoka, K.; Ooi, T.; Yamamoto, H. J.
Am. Chem. Soc. 1989, 111, 6431.
Typical Procedure (Table 1, Entry 1)
A hexane solution of Me3Al (49 L, 0.05 mmol) was added to a sus-
pension of 1 (28 mg, 0.055 mmol) in CH2Cl2 at r.t. and the resulting
solution was stirred for 1 h. After the solution was cooled to –20 °C,
5a (102 L, 1 mmol) and allyltrimethylsilane (238 L, 1.5 mmol)
were added and the reaction temperature was maintained at –20 °C
for 30 min. The reaction temperature was gradually raised to 4 °C
and after 4 h, 2 M HCl aq in MeOH (1:1) was added for desilylation.
The standard aqueous work-up and purification by silica gel column
chromatography (EtOAc–hexane, 1:19) gave 6a in 84% yield.
(4) For examples of Lewis acidity enhancement by ligand-
defined metal geometry, see: (a) Nelson, S. G.; Kim, B.-K.;
Peelen, T. J. J. Am. Chem. Soc. 2000, 122, 9318.
(b) Denmark, S. E.; Griedel, B. D.; Coe, D. M.; Schnute, M.
E. J. Am. Chem. Soc. 1994, 116, 7026; and references cited
therein.
(5) Fleischer, E. B.; Gebala, A. E.; Levey, A.; Tasker, P. A. J.
Org. Chem. 1971, 36, 3042.
(6) (a) pKa values in DMSO–CF3SO2NH2 (9.7), PhSO2NH2
(16.1), CH3OH (29.0) (b) Bordwell, F. G. Acc. Chem. Res.
1988, 21, 456.
(7) These calculations were performed using the UNIVERSAL
forcefield (v. 1.02) performed on Cerius 2 4.0 (Molecular
Simulations Inc.): (a) Rappé, A. K.; Casewit, C. J.; Colwell,
K. S.; Goddard, W. A. III; Skiff, W. M. J. Am. Chem. Soc.
1992, 114, 10024. (b) Casewit, C. J.; Colwell, K. S.; Rappé,
A. K. J. Am. Chem. Soc. 1992, 114, 10035. (c) Casewit, C.
J.; Colwell, K. S.; Rappé, A. K. J. Am. Chem. Soc. 1992, 114,
10046; An N-methyl analog, instead of 4, was used in these
calculations for simplification.
Selected spectral data of ligand 1
1H NMR (CDCl3, 500 MHz): = 1.05 (s, 3 H), 3.17 (d, J = 7.0 Hz,
6 H), 5.61 (t, J = 7.0 Hz, 3 H)
13C NMR (DMSO-d6, 126 MHz): = 15.89, 38.4, 46.32, 118.5 (q,
J = 33 Hz)
19F NMR [CDCl3, 470 MHz, CF3CO2H used as the external stan-
dard ( = 0 ppm)]: = –76.9.
Acknowledgement
(8) The chemical yield decreased when catalyst loading of less
than 5 mol% was used, <50% with 2 mol% and no reaction
with 1 mol%.
Financial support was provided by RFTF of Japan Society for the
Promotion of Science and PRESTO of Japan Science and Techno-
logy Corporation (JST).
(9) Trifluorotoluene (CF3C6H5), toluene, and acetonitrile gave
the product in 10%, 50%, and 0% yield, respectively.
(10) Unfortunately, the desired allylation did not proceed from
aliphatic aldehydes or , -unsaturated aldehydes. Cyclic
trioxanes were the major products from primary and
secondary alkyl substituted aldehydes. No reaction occurred
from pivalaldehyde and , -unsaturated aldehydes.
(11) Hamashima, Y.; Sawada, D.; Nogami, H.; Kanai, M.;
Shibasaki, M. Tetrahedron 2001, 57, 805; and references
cited therein.
(12) Side-reaction pathways mediated by a reagent-derived
Lewis acidic silicon are problematic, especially in the case
of catalytic enantioselective reactions: Carreira, E. M. In
Comprehensive Asymmetric Catalysis, Vol. 3; Jacobsen, E.
N.; Pfaltz, A.; Yamamoto, H., Eds.; Springer: Heidelberg,
1999, Chap. 29; the possibility that the Lewis acidic silicon
of 8 is the actual catalyst in the present case is unlikely due
to the fact that the control catalyst 4–Al did not promote the
reaction.
References
(1) Lewis Acids in Organic Synthesis; Yamamoto, H., Ed.;
Wiley–VCH: Weinheim, 2000.
(2) (a) Mikami, K.; Kotera, O.; Motoyama, Y.; Sakaguchi, H.;
Maruta, M. Synlett 1996, 171. (b) Ishihara, K.; Kubota, M.;
Yamamoto, H. Synlett 1996, 265. (c) Marx, A.; Yamamoto,
H. Angew. Chem. Int. Ed. 2000, 39, 178. (d) Ishihara, K.;
Hiraiwa, Y.; Yamamoto, H. Synlett 2001, 1851; and
references cited therein.
(3) For other interesting strategies to enhance the catalyst
activity of Lewis acid complexes, see: (a) Lewis acid-
Brönsted acid combination: Ishihara, K.; Yamamoto, H. J.
Am. Chem. Soc. 1994, 116, 1561. (b) See also: Corey, E. J.;
Shibata, T.; Lee, T. W. J. Am. Chem. Soc. 2002, 124, 3808.
(c) Lewis base coordination: Denmark, S. E.; Wynn, T. J.
Am. Chem. Soc. 2001, 123, 6199. (d) Bimetallic system:
Synthesis 2002, No. 14, 1956–1958 ISSN 0039-7881 © Thieme Stuttgart · New York