9870
J. Am. Chem. Soc. 2000, 122, 9870-9871
Table 1. Catalytic Enantioselective Isomerization of Allylic
Alcohols (5% [Rh(cod)2]BF4, 5% (+)-1, THF, 70 °C)
Enantioselective Isomerization of Allylic Alcohols
Catalyzed by a Rhodium/Phosphaferrocene Complex
Ken Tanaka, Shuang Qiao, Mamoru Tobisu,
Michael M.-C. Lo,1 and Gregory C. Fu*
Department of Chemistry
Massachusetts Institute of Technology
Cambridge, Massachusetts 02139
ReceiVed July 7, 2000
The highly enantioselective isomerization of allylic amines to
enamines, catalyzed by a Rh+/BINAP complex (eq 1), represents
one of the most noteworthy accomplishments in the field of
asymmetric catalysis, due to its early discovery and to its industrial
utility.2-5 Unfortunately, comparable success has not been
achieved for the corresponding isomerization of readily available
allylic alcohols (eq 2), despite the obvious usefulness of this
a Isolated yield, average of two runs. b Reaction temperature: 100
°C.
process; to the best of our knowledge, 53% ee is the highest
enantioselectivity reported to date (with a Rh+/BINAP catalyst).6
During the past few years, we have been pursuing the design
and the development of new families of chiral ligands, based on
planar-chiral heterocycles.7 In this Communication, we establish
that a Rh+/planar-chiral phosphaferrocene complex can catalyze
the enantioselective isomerization of allylic alcohols with good
levels of enantioselectivity (eq 3).
In a previous report, we described the synthesis of C1-
symmetric phosphaferrocene ligand 1 and its application to the
rhodium-catalyzed hydrogenation of dehydroamino acids, a test
reaction for our ligand design.7c Having thus established the
potential of our design, we turned our attention to an unsolved
problem, the catalytic enantioselective isomerization of allylic
alcohols. We were pleased to discover that Rh+/phosphaferrocene
complexes serve as effective catalysts for this process. Optimiza-
tion experiments revealed that the level of enantioselection is
-
dependent on the counterion and on the solvent (BF4 and THF
are the best, respectively, among those that we have examined),
but not on the temperature.
As illustrated in Table 1, a variety of allylic alcohols undergo
isomerization with good selectivity in the presence of 5% [Rh-
(cod)2]BF4/(+)-1. Rearrangement of the methyl-substituted cin-
namyl alcohol derivative occurs with relatively modest ee (entry
1), whereas reaction of the corresponding isopropyl-substituted
derivative proceeds with significantly enhanced stereoselection
(83% ee, entry 2).8 We have determined that Z allylic alcohols
typically isomerize with higher enantiomeric excess than do E
allylic alcohols (e.g., entry 2 vs entry 3). Electronic effects on
the level of asymmetric induction appear to be small, as indicated
by entries 2, 4, and 5. Importantly, the process is not limited to
olefins that bear an aromatic substituent, e.g., Rh+/1 isomerizes
a cyclohexyl/methyl substituted allylic alcohol with good selectiv-
ity (entry 6).
(1) Correspondence concerning the X-ray crystal structure should be
directed to M. M.-C. Lo.
(2) (a) Tani, K.; Yamagata, T.; Otsuka, S.; Akutagawa, S.; Kumobayashi,
H.; Taketomi, T.; Takaya, H.; Miyashita, A.; Noyori, R. J. Chem. Soc., Chem.
Commun. 1982, 600-601. (b) For an overview, see: Noyori, R. Asymmetric
Catalysis in Organic Synthesis; Wiley: New York, 1994; Chapter 3.
(3) Takasago International Corporation annually manufactures ∼3700 tons
of (-)-menthol and related terpenes through the Rh+/BINAP-catalyzed
enantioselective isomerization of allylic amines: Akutagawa, S. In Chirality
in Industry; Collins, A. N., Sheldrake, G. N., Crosby, J., Eds.; Wiley: New
York, 1992.
We have applied the Rh+/1-catalyzed enantioselective isomer-
ization process to the first asymmetric synthesis of carboxylic
acid 4, which has served as a key intermediate in racemic
syntheses of 7-hydroxycalamenene (5) and 7-hydroxycalamenenal
(4) For the catalytic enantioselective isomerization of cyclic allylic ethers,
see: (a) Hiroya, K.; Kurihara, Y.; Ogasawara, K. Angew. Chem., Int. Ed.
Engl. 1995, 34, 2287-2289. (b) Hiroya, K.; Ogasawara, K. J. Chem. Soc.,
Chem. Commun. 1995, 2205-2206. (c) Frauenrath, H.; Reim, S.; Wiesner,
A. Tetrahedron: Asymmetry 1998, 9, 1103-1106. (d) Brunner, H.; Prom-
mesberger, M. Tetrahedron: Asymmetry 1998, 9, 3231-3239. (e) Faitg, T.;
Soulie, J.; Lallemand, J.-Y.; Mercier, F.; Mathey, F. Tetrahedron 2000, 56,
101-104.
(6) Tani, K. Pure Appl. Chem. 1985, 57, 1845-1854. The isomerization
proceeds in modest yield (47%).
(7) (a) Dosa, P. I.; Ruble, J. C.; Fu, G. C. J. Org. Chem. 1997, 62, 444-
445. (b) Lo, M. M.-C.; Fu, G. C. J. Am. Chem. Soc. 1998, 120, 10270-
10271. (c) Qiao, S.; Fu, G. C. J. Org. Chem. 1998, 63, 4168-4169. (d) Rios,
R.; Liang, J.; Lo, M. M.-C.; Fu, G. C. Chem. Commun. 2000, 377-378.
(8) Replacement of the isopropyl group with a bulkier tert-butyl group leads
to sluggish isomerization (12% yield, 80% ee after 48 h at 100 °C).
(5) For the catalytic enantioselective isomerization of an unfunctionalized
alkene, see: Chen, Z.; Halterman, R. L. J. Am. Chem. Soc. 1992, 114, 2276-
2277.
10.1021/ja002471r CCC: $19.00 © 2000 American Chemical Society
Published on Web 09/23/2000