A R T I C L E S
Kitamura et al.
addition of H2 to 6 is endergonic and reversible and that the
olefin/Rh-H migratory insertion, 7 f 8, constitutes the
turnover-limiting step.10 In addition to this standard pathway,
alternative dihydride-unsaturate mechanisms have also been
postulated.11,12 The formation of a RhH2 may precede the
olefin-Rh interaction, depending on the chiral ligands and
reaction conditions.
Reactions using Ru(II) complexes, particularly with a BINAP
ligand,3,13 find a wider synthetic scope, allowing asymmetric
hydrogenation of a range of functionalized olefins including
enamides,14,15 R,â- and â,γ-unsaturated carboxylic acids,16 and
allylic and homoallylic alcohols,17 as well as various function-
alized ketones.18,19 The mechanism of the Ru-catalyzed hydro-
genation, however, is multifarious, and only limited cases have
been fully clarified.20-24 Most interestingly, the Rh and Ru
complexes with the same chiral diphosphine often exhibit an
opposite sense of asymmetric induction in the hydrogenation
of (Z)-R-(acylamino)acrylic acids or esters.3,25 As illustrated in
Figure 2, in the presence of (S)-9, a cationic (S)-BINAP-Rh
complex, (Z)-R-(benzamido)cinnamic acid or the methyl ester
is hydrogenated in ethanol to give the R hydrogenation product
in 93-100% ee,3 whereas the acetamido ester (Z)-1 in methanol
gives the R product in 61% ee. In contrast, (S)-BINAP-Ru
complexes afford the S enantiomers in up to 92% ee.15,26 For
example, (S)-10 hydrogenates (Z)-1 or the simple, nonphenylated
substrate 3 to give the S products equally in 90% ee.
Figure 1. Catalytic cycle of the Rh(I)-catalyzed hydrogenation of N-
acylated dehydroamino esters. â-Substituents are omitted for clarity.
olefin migratory insertion in the dihydride 7 followed by the
reductive elimination of the five-membered organorhodium
hydride 8 gives the saturated product and 5. The overall cis
hydrogenation using a single H2 molecule is achieved by the
cis olefin insertion in 7 and the hydride transfer in 8 with
retention of the configuration.7 When a C2 chiral diphosphine
is used, the enamide complex 6 could be a mixture of
diastereomers, depending on the enantioface selection. The
overall enantioselectivity is determined by multiplication of the
equilibrium concentrations of the stereoisomers and their relative
reactivities. Remarkably, the less stable, minor isomer of 6 has
proved more reactive toward H2, overwhelming the relative
stabilities, to afford the major enantiomer.8,9 A recent detailed
theoretical study suggests the possibility that the oxidative
(10) Landis, C. R.; Hilfenhaus, P.; Feldgus, S. J. Am. Chem. Soc. 1999, 121,
8741-8754.
(11) (a) Ojima, I.; Kogure, T.; Yoda, N. J. Org. Chem. 1980, 45, 4728-4739.
(b) Sinou, D. Tetrahedron Lett. 1981, 22, 2987-2990. (c) Harthun, A.;
Kadyrov, R.; Selke, R.; Bargon, J. Angew. Chem., Int. Ed. Engl. 1997, 36,
1103-1105. (d) Kuwano, R.; Ito, Y. J. Org. Chem. 1999, 64, 1232-1237.
(12) (a) Gridnev, I. D.; Yamanoi, Y.; Higashi, N.; Tsuruta, H.; Yasutake, M.;
Imamoto, T. AdV. Synth. Catal. 2001, 343, 118-136. (b) Gridnev, I. D.;
Higashi, N.; Asakura, K.; Imamoto, T. J. Am. Chem. Soc. 2000, 122, 7183-
7194.
(13) BINAP ) 2,2′-bis(diphenylphosphino)-1,1′-binaphthyl.
(14) (a) Noyori, R.; Ohta, M.; Hsiao, Yi; Kitamura, M.; Ohta, T.; Takaya, H. J.
Am. Chem. Soc. 1986, 108, 7117-7119. (b) Kitamura, M.; Hsiao, Yi;
Noyori, R.; Takaya, H. Tetrahedron Lett. 1987, 28, 4829-4832. (c)
Kitamura, M.; Hsiao, Yi; Ohta, M.; Tsukamoto, M.; Ohta, T.; Takaya, H.;
Noyori, R. J. Org. Chem. 1994, 59, 297-310.
(5) (a) Brown, J. M.; Chaloner, P. A. Tetrahedron Lett. 1978, 19, 1877-1880.
(b) Brown, J. M.; Chaloner, P. A. J. Chem. Soc., Chem. Commun. 1978,
321-322. (c) Brown, J. M.; Chaloner, P. A. J. Chem. Soc., Chem. Commun.
1980, 344-346. (d) Brown, J. M.; Murrer, B. A. Tetrahedron Lett. 1980,
21, 581-584. (e) Brown, J. M.; Chaloner, P. A. J. Am. Chem. Soc. 1980,
102, 3040-3048. (f) Brown, J. M.; Murrer, B. A. J. Chem. Soc., Perkin
Trans. 2 1982, 489-497. (g) Brown, J. M.; Parker, D. Organometallics
1982, 1, 950-956. (h) Brown, J. M.; Chaloner, P. A.; Morris, G. A. J.
Chem. Soc., Chem. Commun. 1983, 664-666. (i) Brown, J. M.; Maddox,
P. J. J. Chem. Soc., Chem. Commun. 1987, 1276-1278. (j) Brown, J. M.;
Guiry, P. J.; Wienand, A. In Principle of Molecular Recognition; Buck-
ingham, A. D., Legon, A. C., Roberts, S. M., Eds.; Blackie: Glasgow,
1993; pp 79-107.
(15) Kitamura, M.; Yoshimura, M.; Tsukamoto, M.; Noyori, R. Enantiomer
1996, 1, 281-303.
(16) Ohta, T.; Takaya, H.; Kitamura, M.; Nagai, K.; Noyori, R. J. Org. Chem.
1987, 52, 3174-3176.
(17) Takaya, H.; Ohta, T.; Sayo, N.; Kumobayashi, H.; Akutagawa, S.; Inoue,
S.; Kasahara, I.; Noyori, R. J. Am. Chem. Soc. 1987, 109, 1596-1597,
4129.
(18) (a) Noyori, R.; Ohkuma, T.; Kitamura, M.; Takaya, H.; Sayo, N.;
Kumobayashi, H.; Akutagawa, S. J. Am. Chem. Soc. 1987, 109, 5856-
5858. (b) Kitamura, M.; Ohkuma, T.; Inoue, S.; Sayo, N.; Kumobayashi,
H.; Akutagawa, S.; Ohta, T.; Takaya, H.; Noyori, R. J. Am. Chem. Soc.
1988, 110, 629-631. For reviews, see: (c) Ohkuma, T.; Kitamura, M.;
Noyori, R. In Catalytic Asymmetric Synthesis, 2nd ed.; Ojima, I., Ed.;
VCH: Weinheim, 2000; Chapter 1.4.1. (d) Geneˆt, J. P. In Reductions in
Organic Synthesis: Recent AdVances and Practical Applications (ACS.
symposium series 641); Abdel-Magid, A. F., Ed.; American Chemical
Society: Washington, DC, 1996; Chapter 2.
(19) For asymmetric hydrogenation of simple, unfunctionalized ketones, see:
Noyori, R.; Ohkuma, T. Angew. Chem., Int. Ed. 2001, 40, 40-73.
(20) Ashby, M. T.; Halpern, J. J. Am. Chem. Soc. 1991, 113, 589-594.
(21) Ohta, T.; Takaya, H.; Noyori, R. Tetrahedron Lett. 1990, 31, 7189-7192.
(22) Wiles, J. A.; Bergens, S. H.; Young, V. G. J. Am. Chem. Soc. 1997, 119,
2940-2941.
(23) Wiles, J. A.; Bergens, S. H. Organometallics 1998, 17, 2228-2240.
(24) Wiles, J. A.; Bergens, S. H. Organometallics 1999, 18, 3709-3714.
(25) (a) Miyashita, A.; Takaya, H.; Souchi, T.; Noyori, R. Tetrahedron 1984,
40, 1245-1253. (b) Kawano, H.; Ikariya, T.; Ishii, Y.; Saburi, M.;
Yoshikawa, S.; Uchida, Y.; Kumobayashi, H. J. Chem. Soc., Perkin Trans.
1 1989, 1571-1575. (c) Noyori, R.; Ikeda, T.; Ohkuma, T.; Widhalm, M.;
Kitamura, M.; Takaya, H.; Akutagawa, S.; Sayo, N.; Saito, T.; Taketomi,
T.; Kumobayashi, H. J. Am. Chem. Soc. 1989, 111, 9134-9135. (d) Lubell,
W. D.; Kitamura, M.; Noyori, R. Tetrahedron: Asymmetry 1991, 2, 543-
554.
(6) DIPAMP ) 1,2-bis[(o-methoxyphenyl)phenylphosphino)]ethane. CHIRA-
PHOS ) 2,3-bis(diphenylphosphino)butane.
(7) For the participation of protic solvents at the Rh hydride stage, see: Bakos,
J.; Karaivanov, R.; Laghmari, M.; Sinou, D. Organometallics 1994, 13,
2951-2956.
(8) Many chiral diphosphine ligands that form a λ configurated Rh complex
yield (S)-R-amino acids, whereas phosphines that form the δ structure yield
the R enantiomers as major products. See: (a) Fryzuk, M. D.; Bosnich, B.
J. Am. Chem. Soc. 1977, 99, 6262-6267. (b) Fryzuk, M. D.; Bosnich, B.
J. Am. Chem. Soc. 1978, 100, 5491-5494. (c) Vineyard, B. D.; Knowles,
W. S.; Sabacky, M. J.; Bachman, G. L.; Weinkauff, D. J. J. Am. Chem.
Soc. 1977, 99, 5946-5952.
(9) For possible discrepancies inferred from the geometries of chiral diphos-
phines, see: (a) Burk, M. J.; Feaster, J. E.; Nugent, W. A.; Harlow, R. L.
J. Am. Chem. Soc. 1993, 115, 10125-10138. (b) Robin, F.; Mercier, F.;
Ricard, L.; Mathey, F.; Spagnol, M. Chem.-Eur. J. 1997, 3, 1365-1369.
(c) Marinetti, A.; Kruger, V.; Buzin, F.-X. Tetrahedron Lett. 1997, 38,
2947-2950. (d) Imamoto, T.; Watanabe, J.; Wada, Y.; Masuda, H.;
Yamada, H.; Tsuruta, H.; Matsukawa, S.; Yamaguchi, K. J. Am. Chem.
Soc. 1998, 120, 1635-1636. (e) Yamanoi, Y.; Imamoto, T. J. Org. Chem.
1999, 64, 2988-2989. (f) Marinetti, A.; Jus, S.; Geneˆt, J.-P. Tetrahedron
Lett. 1999, 40, 8365-8368.
(26) Ikariya, T.; Ishii, Y.; Kawano, H.; Arai, T.; Saburi, M.; Yoshikawa, S.;
Akutagawa, S. J. Chem. Soc., Chem. Commun. 1985, 922-924.
9
6650 J. AM. CHEM. SOC. VOL. 124, NO. 23, 2002