Similar good results were obtained for the phenylalanine
precursor 4 (entries 8–13, up to 98% ee). Conversely, in the
case of dimethyl itaconate (5), the classical monodentate
phosphite ligand 2c performed with best results (97% ee, entry
19). This again proves that there exists no ideal catalyst for all
substrates.
Ed., 2006, 45, 1599; (f) M. Weis, C. Waloch, W. Seiche and B. Breit,
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Leeuwen and J. N. H. Reek, Angew. Chem., Int. Ed., 2003, 42,
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¨
Leeuwen and J. N. H. Reek, J. Am. Chem. Soc., 2004, 126, 4056;
In conclusion, a new class of supramolecular PhanePhos-
analogous ligands formed through self-assembly via hydrogen-
bonding between peptidyl side chains was successfully applied
in rhodium-catalyzed asymmetric hydrogenation. Both the
reactivity and enantioselectivity observed are comparable to
the best results obtained with common monodentate or bi-
dentate Rh–phosphite catalysts,1 and match the results
achieved by PhanePhos-type ligands.6,19
(d) J. N. H. Reek, M. Roder, P. E. Goudriaan, P. C. J. Kamer, P.
¨
W. N. M. van Leeuwen and V. F. Slagt, J. Organomet. Chem.,
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Taken together, our experimental findings support the con-
clusion that helical secondary structures are important ele-
ments for significant catalyst activity and enantioselectivity. In
summary, salient features of these catalysts, such as their ease
of preparation from readily accessible and modular building
blocks, the inherent combinatorial possibilities of this supra-
molecular approach, as well as the excellent levels of stereo-
control, render these ligand systems attractive for practical
asymmetric synthesis.
H. Jiao, D. Michalik, J. Holz, R. Paciello, B. Breit and A. Borner,
¨
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11 For other phosphorus-containing peptidyl ligands which have been
applied in catalysis, see: (a) S. R. Gilbertson and X. Wang, J. Org.
Chem., 1996, 61, 434; (b) S. R. Gilbertson, X. Wang, G. S. Hoge,
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This work was supported by the Fonds der Chemischen
Industrie, DFG (Int. Research Training Group GRK 1038),
the Alfried Krupp Award for young university teachers of the
Krupp Foundation (to BB), and BASF.
Notes and references
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ꢁc
This journal is The Royal Society of Chemistry 2008
846 | Chem. Commun., 2008, 844–846