H. Tsuruta et al. / Tetrahedron Letters 46 (2005) 2879–2882
2881
CO Me
2
CO Me
2
References and notes
2 or 3 (1 mol%)
Ph
Ph
+
H
2
3
MeOH, rt
NHCOR
NHCOMe
3 atm
1. For representative reviews, see: (a) Knowles, W. S. Angew.
Chem., Int. Ed. 2002, 41, 1998; (b) Noyori, R. Angew.
Chem., Int. Ed. 2002, 41, 2008; (c) Ohkuma, T.; Kitamura,
M.; Noyori, R.; In Catalytic Asymmetric Synthesis; Ojima,
I., Ed.; 2nd ed. VCH: New York, 2000; pp 1–110; (d)
Noyori, R.; Kitamura, M.; Ohkuma, T. Proc. Natl. Acad.
Sci. U.S.A. 2004, 101, 5356.
2: 95% ee
3: 94% ee
Scheme 4. Asymmetric hydrogenation of methyl (Z)-a-acetamidocin-
namate catalyzed by 2 and 3.
2. For recent review, see: Tang, W.; Zhang, X. Chem. Rev.
2003, 103, 3029.
3. (a) Halpern, J.; Riley, D. P.; Chan, A. S. C.; Pluth, J. J.
J. Am. Chem. Soc. 1977, 99, 8055; (b) Brown, J. M.;
Chaloner, P. A. Tetrahedron Lett. 1978, 1877; (c) Brown,
J. M.; Chaloner, P. A. J. Am. Chem. Soc. 1980, 102, 3040;
(d) Chan, A. S. C.; Pluth, J. J.; Halpern, J. J. Am. Chem.
Soc. 1980, 102, 5952; (e) Landis, C. R.; Halpern, J. J. Am.
Chem. Soc. 1987, 109, 1746; (f) Gridnev, I. D.; Higashi,
N.; Asakura, K.; Imamoto, T. J. Am. Chem. Soc. 2000,
122, 7183; (g) Giernoth, R.; Heinrich, H.; Adams, N. J.;
Deeth, R. J.; Bargon, J.; Brown, J. M. J. Am. Chem. Soc.
2000, 122, 12381; (h) Gridnev, I. D.; Yasutake, M.;
Higashi, N.; Imamoto, T. J. Am. Chem. Soc. 2001, 123,
5268; (i) Gridnev, I. D.; Yamanoi, Y.; Higashi, N.;
Tsuruta, H.; Yasutake, M.; Imamoto, T. Adv. Synth.
Catal. 2001, 343, 118; (j) Gridnev, I. D.; Higashi, N.;
Imamoto, T. J. Am. Chem. Soc. 2001, 123, 4631; (k)
Gridnev, I. D.; Imamoto, T. Acc. Chem. Res. 2004, 37,
633.
4. (a) Landis, C. R.; Hilfenhaus, P.; Feldgus, S. J. Am. Chem.
Soc. 1999, 121, 8741; (b) Landis, C. R.; Feldgus, S. Angew.
Chem. Int., Ed. 2000, 39, 2863; (c) Feldgus, S.; Landis, C.
R. J. Am. Chem. Soc. 2000, 122, 12714; (d) Feldgus, S.;
Landis, C. R. Organometallics 2001, 20, 2374; (e) Drexler,
H.-J.; Zhang, S.; Sun, A.; Spannenberg, A.; Arrleta, A.;
Preetz, A.; Heller, D. Tetrahedron: Asymmetry 2004, 15,
2139; (f) Gridnev, I. D.; Imamoto, T. Acc. Chem. Res.
2004, 37, 633; (g) Shimizu, H.; Ishizaki, T.; Fujiwara, T.;
Saito, T. Tetrahedron: Asymmetry 2004, 15, 2169.
5. (a) d,k-Rule: Fryzuk, M. D.; Bosnich, B. J. Am. Chem.
Soc. 1978, 100, 5491; (b) Samuel, O.; Couffignal, R.;
Lauer, M.; Zhang, S. Y.; Kagan, H. B. Nouv. J. Chem.
1981, 5, 15; (c) Kagan, H. B. In Asymmetric Synthesis
Using Organometallic Catalysis; Wilkinson, G., Stone, F.
G. A., Abel, E. W., Eds.; Pergamon: Oxford, 1982; Vol. 8,
pp 463–498.
H2
H2
3
2
Me
Me
P
P
MeOH
+
MeOH
Rh
O
O
Me
Me
H
H
_
X
4a (X = PF6)
4b (X = SbF6)
Scheme 5. Structure of active catalyst formed in situ in course of
asymmetric hydrogenation catalyzed by 2 and 3.
Inspection of the Figures 1and 2 shows that the solid-
state conformations of the chelate cycles in 2 and 3 are
opposite. The norbornadienerhodium complex 2 has d-
type conformation of the chelate cycle with quasi-equa-
torial phenyls and quasi-axial o-tolyl-substituents. On
the other hand, the chelate cycle in the cyclooctadienyl
complex 3 apparently has the k-conformation; the
o-tolyl-substituents are quasi-equatorial and the phenyls
are quasi-axial.
As expected, both complexes 2 and 3 provided the same
sense and order of enantioselection when applied as pre-
catalysts in asymmetric hydrogenations of dehydro-
amino acids (Scheme 4). Notably high eeꢀs (comparable
to those reported for DIPAMP) have been observed
demonstrating that even the smallest possible difference
in the structure of aryl substituents in a C2-symmetric
diphosphine ligand is sufficient to secure high
enantioselctivity.14
It is clear that after the activation of a precatalyst
(hydrogenation of the coordinated diene), the active cat-
alytic species (4) generated from 2 and 3 (Scheme 5) dif-
fer only by counter-ion, and it is well known that the
influence of the counter-ion on the course of asymmetric
hydrogenation is minimal, if any. Hence, the different
conformations of the same chelate cycle observed for 2
and 3 demonstrate convincingly that the solid-state
structures of the conformationally flexible Rh complexes
cannot be reliably used as a lead in discussing the mech-
anism of enantioselection, as it has been accepted previ-
ously. Detailed analysis of the conformational equilibria
in solution for each important intermediate is required.
Such studies are underway in our research group.
6. (a) Quadrant rule: Koenig, K. E.; Sabacky, M. J.;
Bachman, G. L.; Christopfel, W. C.; Barnstoff, H. D.;
Friedman, R. B.; Knowles, W. S.; Stults, B. R.; Vineyard,
B. D.; Weinkauff, D. Ann. N.Y. Acad. Sci. 1980, 333, 1 6;
(b) Knowles, W. S. Acc. Chem. Res. 1983, 16, 106.
7. Twist of coordinated diene: Kyba, E. P.; Davis, R. E.;
Juri, P. N.; Shirley, K. R. Inorg. Chem. 1981, 20,
3616.
8. Imamoto, T.; Watanabe, J.; Wada, Y.; Masuda, H.;
Yamada, H.; Tsuruta, H.; Matsukawa, S.; Yamaguchi, K.
J. Am. Chem. Soc. 1998, 120, 1635.
9. Vineyard, B. D.; Knowles, W. S.; Sabacky, G. L.;
Bachman, G. L.; Weinkauff, D. J. J. Am. Chem. Soc.
1977, 99, 5946.
´
´
´
10. Bakos, J.; Toth, I.; Heil, B.; Szalontai, G.; Parkanyi, L.;
Fulo¨p, V. J. Organomet. Chem. 1989, 370, 263.
¨
11. Wada, Y.; Imamoto, T.; Tsuruta, H.; Yamaguchi, K.;
Gridnev, I. D. Adv. Synth. Catal. 2004, 346, 777.
Supplementary data
12. Single crystals of 2 suitable for X-ray crystallographic
analysis were grown by recrystallization from THF.
Crystal data for 2 (C39H44F6P3RhO (2ÆTHF)): M =
838.59, T = 106 K, monoclinic space group C2 (no. 5), a =
Supplementary data associated with this article can be
and 3.
˚
15.039(4), b = 11.823(3), c = 20.373(6) A, b = 97.257(3)°,