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trend, reduced ee values of 78 and 66% were determined for
the [F]- and [CF3]-derivatives (G and H, respectively). Optical
rotation measurements on A, D, and E prepared from (S)-1-Me
established the preferential formation of the R alkane.
The hydrogenation of the less hindered alkene, I, was also
explored. Such substrates are more challenging for enantiose-
lective hydrogenation because the steric differentiation required
for facial discrimination is subtle. Quantitative hydrogenation of
I with (S)-1-Me was observed in less than 1 h, but the
enantiomeric excess was poor (39%). Performing the hydro-
genation at 0 °C did little to improve enantioselectivity (41%
ee). Using the more hindered and less active tert-butyl-
substituted cobalt precatalyst (S)-2-CM produced 44% yield of
the alkane at 22 °C in 24 h. In this case, optical rotation
measurements established the preferential formation of the S
alkane. Isomerization to 3-methyl-1H-indene accompanied
hydrogenation and accounted for the balance of the material.
Gratifyingly, I was obtained in 96% ee, representing, to the
best of our knowledge, the highest enantiomeric excess ever
observed with any catalyst for this substrate. Improving the
conversion by using increased pressures of H2 is currently
under study.
(3) Pfaltz, A.; Blankenstein, J.; Hilgraf, R.; Hormann, E.; McIntyre, S.;
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Menges, F.; Schonleber, M.; Smidt, S. P.; Wustenberg, B.;
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In summary, asymmetric hydrogenation of prochiral alkenes
has been observed using enantiopure C1-symmetric bis(imino)-
pyridine cobalt precatalysts. Importantly, the presence of
coordinating functionalities on the olefin is not required for
high enantiomeric excess. High activity was related to the
suppression of competing cyclometalation, a key feature for
future catalyst design.
(13) Trovitch, R. J.; Lobkovsky, E.; Bill, E.; Chirik, P. J.
Organometallics 2008, 27, 1470.
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B. M.; Trovitch, R. J.; Bart, S. C.; Tondreau, A. M.; Lobkovsky, E.;
Milsmann, C.; Bill, E.; Wieghardt, K.; Chirik, P. J. Inorg. Chem. 2009,
48, 4190.
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A. D.; Budzelaar, P. H. M.; Gal, A. W. Angew. Chem., Int. Ed. 2001, 40,
4719. (b) Gibson, V. C.; Humphries, M. J.; Tellmann, D. F.; Wass, A.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Complete experimental details; the H NMR spectrum of (S)-
1
1-H; and crystallographic data for (R)-1-Cl, (S)-1-Cl, (S)-1-
Me, (S)-1-CM, (R)-1-CM, (R)-2-CM, and (R)-2-Cl. This
material is available free of charge via the Internet at http://
(17) (a) Knijnenburg, Q.; Hetterscheid, D.; Kooistra, T. M.;
Budzelaar, P. H. M. Eur. J. Inorg. Chem. 2004, 1204. (b) Humphries,
M. J.; Tellmann, K. P.; Gibson, V. C.; White, A. J. P.; Williams, D. J.
Organometallics 2005, 24, 2039.
AUTHOR INFORMATION
■
Corresponding Author
(18) Bowman, A. C.; Milsmann, C.; Bill, E.; Lobkovsky, E.;
Notes
Weyhermuller, T.; Wieghardt, K.; Chirik, P. J. Inorg. Chem. 2010,
̈
The authors declare no competing financial interest.
49, 6110.
(19) (a) Bart, S. C.; Chlopek, K.; Bill, E.; Bouwkamp, M. W.;
Lobkovsky, E.; Neese, F.; Wieghardt, K.; Chirik, P. J. J. Am. Chem. Soc.
2006, 128, 13901. (b) Knijnenburg, Q.; Gambarotta, S.; Budzelaar, P.
H. M. Dalton Trans. 2006, 5442.
(20) At 45 °C, the conversion of 1-Me to 1-CM reached 10% after
24 h.
(21) Baar, C. R.; Levy, C. J.; Min, E. Y. J.; Henling, L. M.; Day, M.
W.; Bercaw, J. E. J. Am. Chem. Soc. 2004, 126, 8216.
(22) See the Supporting Information for details.
(23) Bell, S.; Wuestenberg, B.; Kaiser, S.; Menges, F.; Netscher, T.;
Pfaltz, A. Science 2006, 311, 642.
ACKNOWLEDGMENTS
■
We thank the National Science Foundation and the Deutsche
Forschungsgemeinschaft for a Cooperative Activities in
Chemistry between US and German Investigators Grant. S.M.
and S.P.S. thank the Natural Sciences and Engineering
Research Council of Canada for post- and predoctoral
fellowships, respectively. Z.R.T. thanks the US−UK Fulbright
Commission and AstraZeneca for a fellowship. We also thank
Dr. Bastian Theis for preliminary experiments and Dr. Christina
Kraml of Lotus Separations for optical rotation measurements.
REFERENCES
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