Synthesis and application of chiral N-heterocyclic carbene-oxazoline ligands: iridium-catalyzed enantioselective hydrogenation.
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 [(eta(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-alpha-methylstilbene. Upon complexation of imidazolium salt 15 p with R(1) = 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.
Department of Chemistry, University of Basel, St-Johanns Ring 19, 4056 Basel, Switzerland. andreas.pfaltz@unibas.ch
Steve
S
Nanchen
Andreas
A
Pfaltz
D002384:Catalysis; D020650:Combinatorial Chemistry Techniques; D018360:Crystallography, X-Ray; D006838:Hydrocarbons / Q000737:chemistry; D006865:Hydrogenation; D007093:Imidazoles / Q000138:chemical synthesis / Q000737:chemistry*; D007495:Iridium / Q000737:chemistry*; D008024:Ligands; D009682:Magnetic Resonance Spectroscopy; D008697:Methane / Q000031:analogs & derivatives* / Q000737:chemistry; D009942:Organometallic Compounds / Q000737:chemistry*; D010080:Oxazoles / Q000737:chemistry*; D013237:Stereoisomerism
D006838:Hydrocarbons; D007093:Imidazoles; D008024:Ligands; D009942:Organometallic Compounds; D010080:Oxazoles; C030011:carbene; D007495:Iridium; D008697:Methane