9570
J. Am. Chem. Soc. 1997, 119, 9570-9571
opposite enantiomer of the ligand cannot be easily synthesized
because of the unavailability of the appropriate carbohydrate
starting materials. This limitation renders the catalyst useless
when an opposite enantiomer of the product is desired.
The most important advantage of chiral phosphinite ligands
over the corresponding phosphine ligands is the easiness of
preparation. The synthesis of phosphinites by reacting the corre-
sponding alcohols with chlorophosphines in the presence of an
organic base is very convenient, and the yields are usually quan-
titative. From a practical standpoint, it is of substantial interest
to develop highly effective chiral phosphinite ligands for asym-
metric catalysis. In this paper, we report the synthesis, charac-
terization, and application of a novel class of highly effective
chiral phosphinite ligands that contain a spiro backbone.
Noyori and co-workers suggested that the highly skewed
position of the naphthyl rings in BINAP was the determining
factor for the ligand to be so effective in asymmetric catalytic
reactions.8 A comparison of the structure of BINAP with that
of the less-effective BINAPO [2,2′-bis(diphenylphosphinoxy)-
1,1′-binaphthyl] ligand6a reveals two possible reasons for the
difference in their effectiveness as chiral ligands in homogeneous
catalysis. (1) The oxygen atoms in BINAPO increase the
distance between the chiral binaphthyl moiety and the PPh2
groups and therefore decrease the influence of the binaphthyl
functionality on the stereopositions of the phenyl rings of the
PPh2 group. Consequently there is less control of stereoselec-
tivity in the catalyst-substrate interaction. (2) The presence
of the C-O-P bond in BINAPO substantially increases the
flexibility of the backbone and consequently decreases the
enantioselectivity of the catalyst.
Novel Spiro Phosphinite Ligands and Their
Application in Homogeneous Catalytic
Hydrogenation Reactions
Albert S. C. Chan,* Wenhao Hu, Cheng-Chao Pai, and
Chak-Po Lau
Union Laboratory of Asymmetric Synthesis
and Department of Applied Biology
and Chemical Technology
The Hong Kong Polytechnic UniVersity, Hong Kong
Yaozhong Jiang,* Aiqiao Mi, Ming Yan, Jian Sun,
Rongliang Lou, and Jingen Deng
Union Laboratory of Asymmetric Synthesis
Chengdu Institute of Organic Chemistry
Academia Sinica, Chengdu, China
ReceiVed March 26, 1997
The homogeneous catalytic asymmetric hydrogenation of
prochiral olefins is one of the most important advancements in
modern organic synthesis.1 For the asymmetric hydrogenation
of prochiral enamides, the use of rhodium catalysts containing
chiral phosphine ligands has been found to be the most
successful. The successful asymmetric hydrogenation of (Z)-
2-acetamido-3-(3-methoxy-4-acetoxyphenyl)acrylic acid with
Rh(DIPAMP)+ catalyst was developed by Knowles et al. to be
a commercial process for the production of L-Dopa.2 More
In our recent pursuit of the design and synthesis of novel
chiral ligands, we have found an excellent opportunity both to
test these hypothesis and to develop a class of highly effective
chiral phosphinite ligands. Our new chiral phosphinite ligands,
namely 1(R),5(R),6(R)-1,6-bis(diphenylphosphinoxy)spiro[4.4]-
nonane (abbreviated R-spirOP, 1) and 1(S),5(S),6(S)-1,6-bis-
(diphenylphosphinoxy)spiro[4.4]nonane (abbreviated S-spirOP,
2) are based on the use of a rigid spiro backbone which mimics
the binaphthyl rings in BINAP in its influential state (skewed
position). Since the spiro backbone is totally rigid, the small
(1)
recently, the development of Rh(BINAP),3 Ru(BINAP),4 and
Rh(Duphos)5 catalysts have attracted much attention. In
contrast, the development of chiral phosphinite ligands for
asymmetric hydrogenation has been less successful.6 So far
no chiral phosphinite ligand has been found to give the same
high enantioselectivity as the best chiral phosphine ligands.
Recently Selke et al. developed a series of aryl 4,6-O-(R)-
benzylidene-2,3-bis(O-diphenylphosphino)-â-D-glucopyrano-
sides and found them to be effective chiral ligands for rhodium
catalysts in the asymmetric hydrogenation of dehydroamino acid
derivatives.7 The disadvantage of this type of natural-product-
based ligand is its chirality being limited by mother nature: the
(1) (a) Noyori, R. Asymmetric Catalysis in Organic Synthesis; Wiley:
New York, 1994; pp 16-94. (b) Brunner, H. Synthesis 1988, 645. (c)
Bosnich, B., Ed.; Asymmetric Catalysis; Martinus Nijhoff Publishers:
Dordrecht, The Netherlands, 1986, pp 19-31. (d) Knowles, W. S. J. Chem.
Educ. 1986, 63, 222.
amount of conformational flexibility in the C-O-P bond in 1
(or 2) is not expected to cause too many problems. This
rationale was strongly supported by our experimental results.
Compounds 1 and 2 were conveniently prepared through the
reaction of chlorodiphenylphosphine with the corresponding
spiro diols 3 and 4.9
(2) Knowles, W. S. Acc. Chem. Res. 1983, 16, 106.
(3) (a) Miyashita, A.; Yasuda, A.; Takaya, H.; Toriumi, K.; Ito, T.;
Souchi, T.; Noyori, R. J. Am. Chem. Soc. 1980, 102, 7932. (b) Miyashita,
A.; Takaya, H.; Souchi, T.; Noyori, R. Tetrahedron 1984, 40, 1245.
(4) (a) Ohta, T.; Takaya, H.; Kitamura, M.; Nagai, K.; Noyori, R. J.
Org. Chem. 1987, 52, 3174. (b) Noyori, R. Science 1990, 248, 1194. (c)
Noyori, R.; Takaya, H. Acc. Chem. Res. 1990, 23, 345.
(5) (a) Burk, M. J.; Feaster, J. E. J. Am. Chem. Soc. 1992, 114, 6266.
(b) Burk, M. J.; Feaster, J. E.; Harlow, R. L. Tetrahedron: Asymmetry
1991, 569. (c) Burk, M. J.; Gross, M. F.; Martinez, J. P. J. Am. Chem. Soc.
1995, 117, 9375.
(2)
(3)
(6) (a) Grubbs, R. H.; DeVries, R. A. Tetrahedron Lett. 1977, 1879. (b)
Hayashi, T.; Tanaka, M.; Ogata, I. Tetrahedron 1977, 295. (c) Sugi, Y.;
Cullen, W. R. Chem. Lett. 1979, 39. (d) Bakos, J.; Toth, I.; Marko, L. J.
Org. Chem. 1981, 46, 5427. (e) Bakos, J.; Toth, I.; Heil, B. Tetrahedron
Lett. 1984, 25, 4965. (f) Johnson, T. H.; Rangarajan, G. J. Org. Chem.
1980, 45, 62. (g) Johnson, T. H.; Klein, K. C.; Thomas, S. J. Mol. Catal.
1981, 12, 37. (h) Jackson, W. R. Aust. J. Chem. 1982, 35, 2069.
(7) (a) Selke, R.; Schwarze, M.; Baudisch, H.; Grassert, I.; Michalik,
M.; Oehme, G.; Stoll, N.; Costisella, B. J. Mol. Catal. 1993, 84, 223. (b)
Kumar, A.; Oehme, G.; Roque, J. P.; Schwarze, M.; Selke, R. Angew. Chem.,
Int. Ed. Engl. 1994, 33, 2197.
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