C O M M U N I C A T I O N S
Table 1. Rhodium-Catalyzed Enantioselective Hydrogenation of
Methyl (Z)-R-Acetamidocinnamate (5a) under 1 atm of H2 by Using
under the same reaction conditions with a high enantioselectivity
(Table 2, run 10). Separately, when hydrogenation of 5a was
Pseudorotaxane Molecules (1), Prepared in situ from 2 and 3, as
Chiral Ligandsa
investigated by using only the crown ether 2a as a chiral ligand, it
proceeded sluggishly with a lower enantioselectivity (Table 2, run
1). In addition, the hydrogenation did not proceed smoothly even
when 2 equiv of 2a to the rhodium complex was used as a chiral
ligand. These results indicate that only the rhodium complex 4a
coordinated to phosphine and phosphite moieties of the pseudoro-
taxane skeleton works as a good catalyst for enantioselective
hydrogenation of enamides. To the best of our knowledge, this is
the first successful example of the use of the pseudorotaxane
molecule as a chiral ligand for homogeneous transition-metal-
catalyzed asymmetric reactions.
a
All reactions of 5a (0.50 mmol) in CH2Cl2 (3 mL) under 1 atm of H2
in the presence of a rhodium complex, generated in situ from [Rh(cod)2]PF6
Supporting Information Available: Experimental procedures and
spectroscopic data (PDF). This material is available free of charge via
the Internet at http://pubs.acs.org.
b
(
0.005 mmol), 2 (0.005 mmol), and 3 (0.005 mmol), at 25 °C. Determined
by H NMR. Determined by HPLC.
1
c
Scheme 4
References
(
1) For a recent review, see: Kay, E. R.; Leigh, D. A.; Zerbetto, F. Angew.
Chem., Int. Ed. 2007, 46, 72 and references therein.
(
2) For selected reviews, see: (a) Amabilino, D. B.; Stoddart, J. F. Chem.
ReV. 1995, 95, 2725. (b) Takata, T.; Kihara, N.; Furusho, Y. AdV. Polym.
Sci. 2004, 171, 1. (c) Badjic, J. D.; Nelson, A.; Cantrill, S. J.; Turnbull,
W. B.; Stoddart, J. F. Acc. Chem. Res. 2005, 38, 723.
(
3) For recent selected examples, see: (a) Oku, T.; Furusho, Y.; Takata, T.
Angew. Chem., Int. Ed. 2004, 43, 966. (b) Kihara, N.; Motoda, S.;
Yokozawa, T.; Takata, T. Org. Lett. 2005, 7, 1199. (c) Sasabe, H.; Kihara,
N.; Mizuno, K.; Ogawa, A.; Takata, T. Tetrahedron Lett. 2005, 46, 3851.
(d) Sasabe, H.; Kihara, N.; Mizuno, K.; Ogawa, A.; Takata, T. Chem.
Lett. 2006, 35, 212. (e) Tachibana, Y.; Kawasaki, H.; Kihara, N.; Takata,
T. J. Org. Chem. 2006, 71, 5093.
some pseudorotaxanes prepared from a variety of wheel and axle
moieties (2 and 3). Typical results are shown in Table 1. A high
enantioselectivity was achieved only by the combination of 2a and
(4) For recent examples, see: (a) Leung, K. C. F.; Mendes, P. M.; Nagonov,
S. N.; Northrop, B. H.; Kim, S.; Patel, K.; Flood, A. H.; Tseng, H.-R.;
Stoddart, J. F. J. Am. Chem. Soc. 2006, 128, 10707. (b) Hou, H.; Leung,
K. C.-F.; Lanari, D.; Nelson, A.; Stoddart, J. F.; Grubbs, R. H. J. Am.
Chem. Soc. 2006, 128, 15358. (c) Williams, A. R.; Northrop, B. H.; Chang,
T.; Stoddart, J. F.; White, A. J. P.; Williams, D. J. Angew. Chem., Int.
Ed. 2006, 45, 6665.
3a, and the use of other axles (3b and 3c) in place of 3a did not
work successfully (Table 1, runs 1-3). On the other hand, when a
crown ether bearing an optically active oxazoline moiety 2b was
employed in place of 2a, hydrogenation proceeded, but with only
a low enantioselectivity (Table 1, run 4). In these cases, pseudoro-
taxane molecules (1b-d) were formed from the corresponding 2
and 3 (Scheme 4), but no formation of new rhodium complexes
was detected by the addition of cationic rhodium complex [Rh-
(5) (a) Horie, M.; Suzuki, Y.; Osakada, K. J. Am. Chem. Soc. 2004, 126,
3
684. (b) Tokunaga, Y.; Kawai, N.; Shimomura, Y. Tetrahedron Lett.
2
007, 48, 4995.
(
6) (a) Kihara, N.; Tachibana, Y.; Kawasaki, H.; Takata, T. Chem. Lett. 2000,
506. (b) Oku, T.; Furusho, Y.; Takata, T. Org. Lett. 2003, 5, 4923. (c)
Tachibana, Y.; Kihara, N.; Takata, T. J. Am. Chem. Soc. 2004, 126, 3438.
7) For recent reviews, see: (a) Breit, B. Angew. Chem., Int. Ed. 2005, 44,
6816. (b) Sandee, A. J.; Reek, J. N. H. Dalton Trans. 2006, 3385.
8) (a) Breit, B.; Seiche, W. J. Am. Chem. Soc. 2003, 125, 6608. (b) Weis,
M.; Waloch, C.; Seiche, W.; Breit, B. J. Am. Chem. Soc. 2006, 128, 4188.
2 6
(cod) ]PF to the solution of 1b-d. In the former cases, the distance
(
(
between phosphine and phosphite moieties is too far to be
coordinated to the rhodium atom. In the latter case, the coordination
ability of the oxazoline moiety may be too weak to be coordinated
to the rhodium atom. In addition, when secondary amine 3d was
used in place of its ammonium salt 3a, the reaction hardly proceeded
(c) Chevallier, F.; Breit, B. Angew. Chem., Int. Ed. 2006, 45, 1599.
(9) (a) Jiang, X.-B.; Lefort, L.; Goudriaan, P. E.; de Vries, A. H. M.; van
Leeuwen, P. W. N. M.; de Vries, J. G.; Reek, J. N. H. Angew. Chem.,
Int. Ed. 2006, 45, 1223. (b) Kuil, M.; Soltner, T.; van Leeuwen, P. W. N.
M.; Reek, J. N. H. J. Am. Chem. Soc. 2006, 128, 11344.
(Table 1, run 5).
(
10) (a) Reetz, M. T.; Sell, T.; Meiswinkel, A.; Mehler, G. Angew. Chem.,
Typical results of enantioselective hydrogenation of other (Z)-
Int. Ed. 2003, 42, 790. (b) Reetz, M. T.; Li, X. Angew. Chem., Int. Ed.
enamides (5) by using 1a as a chiral ligand under the optimal
reaction conditions are shown in Table 2.19 In addition to the
trisubstituted (Z)-R-acetamidocinnamates (5) (Table 2, runs 1-9),
hydrogenation of methyl 2-acetamidoacrylate (5j) proceeded smoothly
2005, 44, 2959. (c) Reetz, M. T.; Meiswinkel, A.; Mehler, G.; Angermund,
K.; Graf, M.; Thiel, W.; Mynott, R.; Blackmond, D. G. J. Am. Chem.
Soc. 2005, 127, 10305. (d) Reetz, M. T.; Fu, Y.; Meiswinkel, A. Angew.
Chem., Int. Ed. 2006, 45, 1412.
(11) Machut, C.; Patrigeon, J.; Tilloy, S.; Bricout, H.; Hapiot, F.; Monflier, E.
Angew. Chem., Int. Ed. 2007, 46, 3040.
(
12) (a) Takacs, J. M.; Reddy, D. S.; Moteki, S. A.; Wu, D.; Palencia, H. J.
Am. Chem. Soc. 2004, 126, 4494. (b) Takacs, J. M.; Hrvatin, P. M.; Atkins,
J. M.; Reddy, D. S.; Clark, J. L. New J. Chem. 2005, 29, 263.
Table 2. Rhodium-Catalyzed Enantioselective Hydrogenation of
Methyl (Z)-R-Acetamidocinnamates (5) under 1 atm of H2 by Using
Pseudorotaxane Molecule (1a), Prepared in situ from 2a and 3a,
as a Chiral Liganda
(
13) Furusho, Y.; Sanno, R.; Oku, T.; Takata, T. Bull. Korean Chem. Soc.
2004, 25, 1641.
(
(
(
14) See Supporting Information for experimental details.
15) Saito, M.; Nishibayashi, Y.; Uemura, S. Organometallics 2004, 23, 4012.
16) For recent examples, see: (a) Mobian, P.; Banerji, N.; Bernardinelli, G.;
Lacour, J. Org. Biomol. Chem. 2006, 4, 224. (b) Makita, Y.; Kihara, N.;
Nakakoji, N.; Takata, T.; Inagaki, S.; Yamamoto, C.; Okamoto, Y. Chem.
Lett. 2007, 36, 162.
(
(
(
17) The most characteristic evidence for the formation of the pseudorotaxane
skeleton is the large downfield shifts of the signals of the benzylic
1
protonsof ammonium salts in H NMR.
18) For a recent example, see: Giacomina, F.; Meetsma, A.; Panella, L.; Lefort,
L.; de Vries, A. H. M.; de Vries, J. G. Angew. Chem., Int. Ed. 2007, 46,
1497 and references therein.
19) When [Rh(cod) ]BF was used as a catalyst in place of [Rh(cod) ]PF ,
2
4
2
6
a
All reactions of 5 (0.50 mmol) in CH2Cl2 (7 mL) under 1 atm of H2 in
similar reactivity and enantioselectivity were observed under the reaction
conditions.
the presence of a rhodium complex, generated in situ from [Rh(cod)2]PF6
(
0.005 mmol), 2a (0.005 mmol), and 3a (0.005 mmol), at 0 °C for 12 h.
Determined by H NMR. Determined by HPLC. In the absence of 3a.
b
1
c
d
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J. AM. CHEM. SOC.
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