activation modes of metal catalysis and organocatalysis.6À11
These catalysts could be easily tuned by modifying or
replacing the metal complex and the organocatalyst unit.
Hydrogen bonding of (thio)urea derivatives has been
widely applied in the catalyst design as one of the impor-
tant noncovalent interactions.12 Although (thio)ureaphos-
phine ligands have been reported as metal-P,O(S)-type
catalysts and self-assembling supramolecular catalysts
in asymmetric catalysis, there are no effective metal/
bisphosphine-thiourea catalytic systems with thiourea as
an activating and directing group in asymmetric hydrogena-
tion.2j,13,14 Thus, we designed a novel catalyst involving a
metal species coordinated by a chiral ferrocenyl bisphos-
phine15 as the scaffold and a tunable thiourea12 as the
hydrogen bond donor (Scheme 1). Based on previous
reports,6À15 we envisioned that the incorporation of the
thiourea group could activate substrates and provide an
excellent chiral environment in catalysis. The ligand was
easily prepared from readily accessible Ugi’s amine
(Scheme 1).16
Rhodium catalyzed asymmetric hydrogenation of ole-
fins is a powerful method to construct versatile chiral
molecules.17 However, highly enantioselective hydrogena-
tion of nitroalkenes remains a challenge. Compared with
biocatalytic reduction,18 transfer hydrogenation,19,20 and
conjugate addition,21 direct hydrogenation is potentially
more practical if high enantioselectivities and turnovers
could be achieved. Only recently we reported the first
example catalyzed by Rh-Josiphos with modest to good
enantioselectivities (82À96% ee) at a 1.5À3 mol % catalyst
loading under 50 atm of H2.22 Since thioureas have been
approved for the activation of the nitro group in organo-
catalysis, we reasoned that asymmetric hydrogenation of
nitroalkenes should be ideal to test our new catalysts.
Indeed, this novel strategy proved to be efficient in
this transformation: up to 98% ee and 98% conversion
were obtained with a 0.25 mol % catalyst loading under
5 atm of H2.
Scheme 1. Ligand L8 Design, Synthesis, and X-ray Structure
(All hydrogen atoms are omitted for clarity)
(10) Selected examples of ferrocenyl phosphine ligands with “second
interaction”: (a) Ito, Y.; Sawamura, M.; Hayashi, T. J. Am. Chem. Soc.
1986, 108, 6405–6406. (b) Hayashi, T.; Kawamura, N.; Ito, Y. J. Am.
Chem. Soc. 1987, 109, 7876–7878. (c) Hayashi, T.; Yamamoto, A.; Ito,
Y.; Nishioka, E.; Miura, H.; Yanagi, K. J. Am. Chem. Soc. 1989, 111,
6301–6311. (d) Kimmich, B. F. M.; Landis, C. R.; Powell, D. R.
Organometallics 1996, 15, 4141–4146.
(11) (a) Selected example of ion paired ligands: Ohmatsu, K.; Ito, M.;
Kunieda, T.; Ooi, T. Nat. Chem. 2012, 4, 473–477. (b) Selected example
of Lewis acid catalysis with a pendant Lewis basic site: Takamura, M.;
Funabashi, K.; Kanai, M.; Shibasaki, M. J. Am. Chem. Soc. 2000, 122,
6327–6328.
(12) Selected reviews: (a) Doyle, A. G.; Jacobsen, E. N. Chem. Rev.
2007, 107, 5713–5743. (b) Hydrogen Bonding in Organic Synthesis; Pihko,
P. M., Ed.; Wiley-VCH: Weinheim, Germany, 2009. Recent reviews of
(thio)urea organocatalysts: (c) Zhang, Z.; Schreiner, P. R. Chem. Soc. Rev.
2009, 38, 1187–1198. (d) Takemoto, Y. Chem. Pharm. Bull. 2010, 58,
593–601.
(13) Selected reviews: (a) Phosphorus(III) ligands in homogeneous
catalysis: Design and synthesis; Kamer, P. C., van Leeuwen, P. W., Eds.;
Wiley: Chichester, 2012. (b) Zhang, W.-H.; Chien, S. W.; Hor, T. S. A. Coord.
Chem. Rev. 2011, 255, 1991–2024. (c) Lam, F. L.; Kwong, F. Y.; Chan,
A. S. C. Chem. Commun. 2010, 46, 4649–4667. (d) Mellah, M.; Voituriez,
A.; Schulz, E. Chem. Rev. 2007, 107, 5133–5209.
(14) Recent examples of self-assembly of supramolecular catalysts
with (thio)ureaphosphine ligands: (a) Lang, K.; Park, J.; Hong, S.
Angew. Chem., Int. Ed. 2012, 51, 1620–1624. (b) Park, J.; Lang, K.;
Abboud, K. A.; Hong, S. Chem.;Eur. J. 2011, 17, 2236–2245. (c) Breuil,
P.-A. R.; Patureau, F. W.; Reek, J. N. H. Angew. Chem., Int. Ed. 2009,
48, 2162–2165. (d) Sandee, A. J.; van der Burg, A. M.; Reek, J. N. H.
Chem. Commun. 2007, 864–866. Recent example of P,O-bidentate
coordinated ureaphosphine ligands: (e) Meeuwissen, J.; Detz, R. J.;
Sandee, A. J.; de Bruin, B.; Reek, J. N. H. Dalton Trans. 2010, 39, 1929–
1931.
At the beginning of our study, we examined the catalytic
activity of L8 with different metal sources (Table 1, entries
(19) Recent examples: (a) Chen, L.-A.; Xu, W.; Huang, B.; Ma, J.;
Wang, L.; Xi, J.; Harms, K.; Gong, L.; Meggers, E. J. Am. Chem. Soc.
2013, ASAP, DOI: 10.1021/ja403777k. (b) Cai, X.-F.; Chen, M.-W.; Ye,
Z.-S.; Guo, R.-N.; Shi, L.; Li, Y.-Q.; Zhou, Y.-G. Chem.;Asian J. 2013,
8, 1381–1385. (c) Schneider, J. F.; Lauber, M. B.; Muhr, V.; Kratzer, D.;
Paradies, J. Org. Biomol. Chem. 2011, 9, 4323–4327. (d) Martin, N. J. A.;
Ozores, L.; List, B. J. Am. Chem. Soc. 2007, 129, 8976–8977. (e) Martin,
N. J. A.; Cheng, X.; List, B. J. Am. Chem. Soc. 2008, 130, 13862–13863.
(20) Recent examples: (a) Tang, Y.; Xiang, J.; Cun, L.; Wang, Y.;
Zhu, J.; Liao, J.; Deng, J. Tetrahedron: Asymmetry 2010, 21, 1900–1905.
(b) Soltani, O.; Ariger, M. A.; Carreira, E. M. Org. Lett. 2009, 11, 4196–
4198. (c) Czekelius, C.; Carreira, E. M. Org. Process Res. Dev. 2007, 11,
633–636. (d) Czekelius, C.; Carreira, E. M. Org. Lett. 2004, 6, 4575–
4577. (e) Czekelius, C.; Carreira, E. M. Angew. Chem., Int. Ed. 2003, 42,
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(21) (a) Selected review: Berner, Otto M.; Tedeschi, L.; Enders, D.
Eur. J. Org. Chem. 2002, 2002, 1877–1894. (b) Recent example in metal
catalysis: Wang, Z.-Q.; Feng, C.-G.; Zhang, S.-S.; Xu, M.-H.; Lin, G.-Q.
Angew. Chem., Int. Ed. 2010, 49, 5780–5783. (c) Recent example in
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(15) Selected reviews: (a) Chiral Ferrocenes in Asymmetric Catalysis:
Synthesis and Applications; Dai, L.-X., Hou, X.-L., Eds.; Wiley-VCH:
ꢁ
ꢁ
Weinheim, Germany, 2010. (b) Gomez Arrayas, R.; Adrio, J.; Carretero, J. C.
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N.; Hamada, Y.; Matsumoto, A.; Kawakami, S.; Konishi, M.; Yamamoto,
K. Bull. Chem. Soc. Jpn. 1980, 53, 1138–1151.
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M. l.; Faber, K.; Bommarius, A. S. Org. Lett. 2011, 13, 2540–2543. (b)
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