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Synthesis and Application of Chiral N ‐Heterocyclic Carbene–Oxazoline Ligands: Iridium‐Catalyzed Enantioselective Hydrogenation
Author(s) -
Nanchen Steve,
Pfaltz Andreas
Publication year - 2006
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200501500
Subject(s) - iridium , oxazoline , carbene , chemistry , deprotonation , asymmetric hydrogenation , enantioselective synthesis , catalysis , medicinal chemistry , salt (chemistry) , polymer chemistry , organic chemistry , ion
Two libraries of enantiomerically pure imidazolium salts bearing an oxazoline unit were synthesized. Deprotonation of the imidazolium salts and complexation of the resulting oxazoline–carbene ligands to iridium( I ) was achieved in one step by mixing the imidazolium salts with NaO t Bu and [(η 4 ‐cod)IrCl] 2 in THF at room temperature. The air‐stable complexes were purified by flash chromatography. All complexes were analyzed by two‐dimensional (2D) NMR methods and one compound from each family was characterized by X‐ray structure analysis. The two libraries of iridium complexes were successfully tested in the asymmetric hydrogenation of unfunctionalized and functionalized olefins. Enantioselectivities of up to 90 % ee were obtained with trans ‐α‐methylstilbene. Upon complexation of imidazolium salt 15 p with R 1 = phenyl, CH bond activation of the phenyl ring gave rise to iridium( III ) complex 17 , which was fully characterized by NMR spectroscopy and X‐ray structure analysis. Complex 17 proved to be catalytically inactive in the hydrogenation.