Synthesis and Application of Chiral N-Heterocyclic Carbene-Oxazoline Ligands: Iridium-Catalyzed Enantioselective Hydrogenation

• Published in: Chemistry : a European journal Vol. 12; no. 17; pp. 4550 - 4558
• Main Authors: Nanchen, Steve, Pfaltz, Andreas
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 02.06.2006; WILEY‐VCH Verlag; Wiley
• Subjects: asymmetric catalysis; carbene ligands; Catalysis; Chemistry; Chemistry, Multidisciplinary; Combinatorial Chemistry Techniques; Crystallography, X-Ray; enantioselectivity; Hydrocarbons - chemistry; Hydrogenation; Imidazoles - chemical synthesis; Imidazoles - chemistry; iridium; Iridium - chemistry; Ligands; Magnetic Resonance Spectroscopy; Methane - analogs & derivatives; Methane - chemistry; Organometallic Compounds - chemistry; Oxazoles - chemistry; Physical Sciences; Science & Technology; Stereoisomerism; enantioselectivity; iridium; ALKENES; RHODIUM; carbene ligands; COMPLEXES; OLEFINS; asymmetric catalysis; ASYMMETRIC HYDROGENATION; hydrogenation
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200501500

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 NaOtBu and [(η4‐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 R1 = phenyl, CH 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. Chiral iridium(oxazoline‐carbene) complexes (Ir‐D and Ir‐F), containing a six‐membered chelate ring and different substituents at the oxazoline and the N‐heterocyclic carbene, were synthesized (see picture). They were evaluated as catalysts in the enantioselective hydrogenation of unfunctionalized and functionalized olefins. The best enantiomeric excesses were generally obtained with Ir‐F complexes.