Technology Spotlights

Highly Selective Ubaphox Iridium Catalysts for Asymmetric Hydrogenations

Chiral phosphine-oxazoline as P-N ligands were found to be highly versatile and effective for a number of enatioselective catalytic reactions.1 A great contribution of these ligands lies on the iridium catalyzed asymmetric hydrogenations. The P-N iridium catalysts are air-stable, easy to handle and do not require the presence of coordinating groups adjacent to the olefin in contrast with most rhodium and ruthenium complexes. One of the pioneers in this field, Prof. Andreas Pfaltz at the University of Basel has shown that cationic BArF iridium complexes of threonine derived phosphinite oxazoline ligands Ubaphox (also known as ThrePHOX) are excellent for asymmetric hydrogenation of a range of substrates (Scheme 1).2-5

 

Asymmetric hydrogenation of alkenes with [Ir(cod)Ubaphox][BArF].

Scheme 1: Asymmetric hydrogenation of alkenes with [Ir(cod)Ubaphox][BArF].

The catalysts proved to promote distinctly higher enantioselectives for most test substrates typically used for asymmetric hydrogenations in relation to previously reported P-N ligands. Common aryl substituted olefins ranged from 85 to 99% ee, 2-alkyl and 2-aryl-4H-chromenes were converted with even greater control into chiral chromanes, plant metabolites with a wide spectrum of biological activities. Chiral amines and N-protected indoline derivatives, other motifs often found in bioactive synthetic and natural products, were obtained in good enantiomeric excess from their corresponding enamines and indoles. Furthermore, the catalysts were recently reported to be effective on asymmetric hydrogenation of substituted tetrahydrofuranes and suitable for recycling when propylene carbonate is applied as solvent.6,7

 

Iridium Ubaphox catalyst from Solvias

Scheme 2: Iridium Ubaphox catalyst from Solvias available from Aldrich Chemistry
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For a complete list of Solvias ligands and complexes available from Aldrich Chemistry click here


References

  1. Helmchen, G.; Pfaltz, A. Acc. Chem. Res. 2000, 33, 336
  2. Menges, F.; Pfaltz, A. Adv. Synth. Catal. 2002, 344, 40
  3. Nanchen, S.; Pfaltz, A. Chem. Eur. J. 2006, 12, 4550
  4. Valla, C.; Baeza, A.: Menges, F.; Pfaltz, A. Synlett 2008, 20, 3167
  5. Blakenstein, J.; Pfaltz, A. Angew. Chem. Int. Ed. 2001, 40, 4445
  6. Kim, H.; Kasper, A. C.; Moon, E. J.; Park, Y.; Wooten, C. M.; Dewhirst, M. W.; Hong, J. Org. Lett. 2009, 11, 89
  7. Bayardon, J.; Holz, J.; Schäffner, B.; Andrushko, V.; Verevkin, S.; Preetz, A.; Börner, A. Angew. Chem. Int. Ed. 2007, 46, 5971